Contents
Syllabus ................................................................................................................................................... 1
Module: 1 ............................................................................................................................................... 1
Lecture: 1 ............................................................................................................................................ 1
HEAVY AND FINE CHEMICALS ......................................................................................................... 1
OVERVIEW ................................................................................................................................... 1
Classification ........................................................................................................................... 1
UNIT OPERATION AND UNIT PROCESS.................................................................................... 3
Module: 2 ............................................................................................................................................... 7
Lecture: 2 ............................................................................................................................................ 7
CARBON DIOXIDE ............................................................................................................................ 7
INTRODUCTION ........................................................................................................................... 7
SOURCES OF CO2 ......................................................................................................................... 8
MANUFACTURE ........................................................................................................................... 8
METHODS OF RECOVERY ............................................................................................................ 9
1. Girbotol amine process ................................................................................................. 10
2. Sodium carbonate process ............................................................................................ 10
3. Potassium carbonate process ....................................................................................... 11
PURIFICATION ........................................................................................................................... 12
1. Purification of low % CO2 containing gas ...................................................................... 12
2. Purification of high % CO2 containing gas ..................................................................... 12
PROPERTIES ............................................................................................................................... 13
USES .......................................................................................................................................... 13
Module: 2 ............................................................................................................................................. 14
Lecture: 3 .......................................................................................................................................... 14
OXYGEN AND NITROGEN .............................................................................................................. 14
INTRODUCTION ......................................................................................................................... 14
Oxygen .................................................................................................................................. 14
Nitrogen ................................................................................................................................ 15
MANUFACTURE ......................................................................................................................... 18
Linde's process (O2 and N2) ................................................................................................... 18
PROPERTIES ............................................................................................................................... 24
Oxygen .................................................................................................................................. 24
Nitrogen ................................................................................................................................ 24
USES .......................................................................................................................................... 24
Oxygen .................................................................................................................................. 24
Nitrogen ................................................................................................................................ 24
Module: 2 ............................................................................................................................................. 25
Lecture: 4 .......................................................................................................................................... 25
HYDROGEN .................................................................................................................................... 25
INTRODUCTION ......................................................................................................................... 25
MANUFACTURE ......................................................................................................................... 26
1. Electrolytic Process ....................................................................................................... 26
2. Lane process or steam hydrogen process ..................................................................... 28
Module: 2 ............................................................................................................................................. 30
Lecture: 5 .......................................................................................................................................... 30
HYDROGEN (Continued) ................................................................................................................ 30
3. Steam Hydrocarbon Process ......................................................................................... 30
4. Liquefaction of coke oven gas or coal gas..................................................................... 33
5. Bosch Process ................................................................................................................ 34
PROPERTIES ............................................................................................................................... 34
USES .......................................................................................................................................... 34
Module: 2 ............................................................................................................................................. 36
Lecture: 6 .......................................................................................................................................... 36
AMMONIA ..................................................................................................................................... 36
INTRODUCTION ......................................................................................................................... 36
MANUFACTURE ......................................................................................................................... 36
(a) Haber and Bosch Process .......................................................................................... 36
(c) Modified Haber Bosch process ................................................................................. 41
PROPERTIES ............................................................................................................................... 45
USES .......................................................................................................................................... 45
Module: 2 ............................................................................................................................................. 46
Lecture: 7 .......................................................................................................................................... 46
ACETYLENE .................................................................................................................................... 46
INTRODUCTION ......................................................................................................................... 46
MANUFACTURE ......................................................................................................................... 46
1. From calcium carbide .................................................................................................... 47
2. From paraffin hydrocarbons by pyrolysis (Wulff process) ............................................ 49
3. From natural gas by partial oxidation (Sachasse process) ............................................ 51
PROPERTIES ............................................................................................................................... 53
USES .......................................................................................................................................... 53
Module: 3 ............................................................................................................................................. 54
Lecture: 8 .......................................................................................................................................... 54
SODIUM CHLORIDE ....................................................................................................................... 54
INTRODUCTION ......................................................................................................................... 54
SOURCES OF SODIUM CHLORIDE .............................................................................................. 54
MANUFACTURE ......................................................................................................................... 55
1. Solar Evaporation .......................................................................................................... 55
2. Artificial Evaporation..................................................................................................... 57
3. Freezing Method ........................................................................................................... 58
PROPERTIES ............................................................................................................................... 58
USES .......................................................................................................................................... 58
Module: 3 ............................................................................................................................................. 60
Lecture: 9 .......................................................................................................................................... 60
SODIUM CARBONATE.................................................................................................................... 60
INTRODUCTION ......................................................................................................................... 60
MANUFACTURE ......................................................................................................................... 60
1. Leblanc process ............................................................................................................. 60
2. Solvay's ammonia soda process .................................................................................... 62
Module: 3 ............................................................................................................................................. 70
Lecture: 10 ........................................................................................................................................ 70
SODIUM CARBONATE (continued) ................................................................................................ 70
3. Dual process .................................................................................................................. 70
4. Electrolytic process ........................................................................................................... 72
PROPERTIES ............................................................................................................................... 74
USES .......................................................................................................................................... 74
Module: 3 ............................................................................................................................................. 75
Lecture: 11 ........................................................................................................................................ 75
SODIUM BICARBONATE ................................................................................................................ 75
INTRODUCTION ......................................................................................................................... 75
MANUFACTURE ......................................................................................................................... 75
PROPERTIES ............................................................................................................................... 77
USES .......................................................................................................................................... 78
Module: 3 ............................................................................................................................................. 79
Lecture: 12 ........................................................................................................................................ 79
SODIUM HYDROXIDE..................................................................................................................... 79
INTRODUCTION ......................................................................................................................... 79
TYPE OF CELLS ........................................................................................................................... 79
Hooker cells ........................................................................................................................... 80
Nelson cell ............................................................................................................................. 81
The Castner Kellner cell......................................................................................................... 82
Membrane cell ...................................................................................................................... 83
Module: 3 ............................................................................................................................................. 85
Lecture: 13 ........................................................................................................................................ 85
SODIUM HYDROXIDE (Continued) ................................................................................................ 85
MANUFACTURE ......................................................................................................................... 85
1. Using Diaphragm cell .................................................................................................... 85
Module: 3 ............................................................................................................................................. 89
Lecture: 14 ........................................................................................................................................ 89
SODIUM HYDROXIDE (Continued) ................................................................................................ 89
2. Lime soda process ......................................................................................................... 89
PROPERTIES ............................................................................................................................... 92
USES .......................................................................................................................................... 92
Module: 3 ............................................................................................................................................. 93
Lecture: 15 ........................................................................................................................................ 93
CHLORINE ...................................................................................................................................... 93
INTRODUCTION ......................................................................................................................... 93
MANUFACTURE ......................................................................................................................... 93
1. Using diaphragm cells ................................................................................................... 93
2. Deacon’s method .......................................................................................................... 93
3. Other methods .............................................................................................................. 94
PROPERTIES ............................................................................................................................... 94
USES .......................................................................................................................................... 95
Module: 4 ............................................................................................................................................. 96
Lecture: 16 ........................................................................................................................................ 96
NITRIC ACID ................................................................................................................................... 96
INTRODUCTION ......................................................................................................................... 96
MANUFACTURE ......................................................................................................................... 96
1. From Chile saltpeter or nitrate ..................................................................................... 96
2. Arc process or Birkeland and eyde process .................................................................. 98
3. Ostwald's process or Ammonia oxidation process ....................................................... 99
PROPERTIES ............................................................................................................................. 105
USES ........................................................................................................................................ 106
Module: 4 ........................................................................................................................................... 107
Lecture: 17 ...................................................................................................................................... 107
SULFURIC ACID ............................................................................................................................ 107
INTRODUCTION ....................................................................................................................... 107
MANUFACTURE ....................................................................................................................... 108
1. The lead chamber process .......................................................................................... 108
Module: 4 ........................................................................................................................................... 113
Lecture: 18 ...................................................................................................................................... 113
SULFURIC ACID (continued) ........................................................................................................ 113
2. The contact process for sulfuric acid .......................................................................... 113
PROPERTIES ............................................................................................................................. 119
USES ........................................................................................................................................ 120
Module: 4 ........................................................................................................................................... 121
Lecture: 19 ...................................................................................................................................... 121
HYDROCHLORIC ACID .................................................................................................................. 121
INTRODUCTION ....................................................................................................................... 121
MANUFACTURE ....................................................................................................................... 122
1. Synthesis from Hydrogen and Chlorine ...................................................................... 122
2. The Salt–Sulfuric acid process ..................................................................................... 125
3. As by-product from chemical processes ..................................................................... 126
4. From incineration of waste organics........................................................................... 126
5. From hydrochloric acid solutions ................................................................................ 127
PROPERTIES ............................................................................................................................. 127
USES ........................................................................................................................................ 127
Module: 4 ........................................................................................................................................... 129
Lecture: 20 ...................................................................................................................................... 129
PHOSPHOROUS ........................................................................................................................... 129
INTRODUCTION ....................................................................................................................... 129
PHOSPHATE ROCK ................................................................................................................... 130
YELLOW PHOSPHORUS ........................................................................................................... 132
RED PHOSPHORUS .................................................................................................................. 134
PROPERTIES ............................................................................................................................. 135
USES ........................................................................................................................................ 136
Module: 4 ........................................................................................................................................... 137
Lecture: 21 ...................................................................................................................................... 137
PHOSPHORIC ACID ...................................................................................................................... 137
INTRODUCTION ....................................................................................................................... 137
MANUFACTURE ....................................................................................................................... 137
1. Using phosphate rock and blast furnace .................................................................... 137
2. Using phosphate rock and electric furnace ................................................................ 140
3. Oxidation and Hydration of phosphorous .................................................................. 142
4. Wet process or from sulfuric acid and phosphate rock .............................................. 143
PROPERTIES ............................................................................................................................. 149
USES ........................................................................................................................................ 149
Module: 5 ........................................................................................................................................... 151
Lecture: 22 ...................................................................................................................................... 151
CEMENT INDUSTRIES .................................................................................................................. 151
INTRODUCTION ....................................................................................................................... 151
CLASSIFICATION ...................................................................................................................... 153
Module: 5 ........................................................................................................................................... 157
Lecture: 23 ...................................................................................................................................... 157
CEMENT CLASSIFICATION (Continued) ....................................................................................... 157
MANUFACTURE OF PORTLAND CEMENT ................................................................................ 158
Significance of constituents ................................................................................................ 159
Module: 5 ........................................................................................................................................... 161
Lecture: 24 ...................................................................................................................................... 161
CEMENT MANUFACTURE ............................................................................................................ 161
MANUFACTURE ....................................................................................................................... 161
PROPERTIES ............................................................................................................................. 170
Module: 5 ........................................................................................................................................... 171
Lecture: 25 ...................................................................................................................................... 171
CEMENT (Continued) .................................................................................................................. 171
CHEMICAL COMPOSITION ....................................................................................................... 171
PHYSICAL REQUIREMENT ........................................................................................................ 171
SETTING AND HARDENING OF CEMENT ................................................................................. 172
USES ........................................................................................................................................ 174
Module: 6 ........................................................................................................................................... 175
Lecture: 26 ...................................................................................................................................... 175
CERAMIC INDUSTRIES ................................................................................................................. 175
INTRODUCTION ....................................................................................................................... 175
CLASSIFICATION ...................................................................................................................... 175
RAW MATERIAL ....................................................................................................................... 176
PROPERTIES ............................................................................................................................. 177
USES ........................................................................................................................................ 177
Module: 6 ........................................................................................................................................... 178
Lecture: 27 ...................................................................................................................................... 178
WHITEWARES .............................................................................................................................. 178
1. Whitewares ..................................................................................................................... 178
classification ............................................................................................................................ 178
Manufacture ........................................................................................................................... 179
Properties ................................................................................................................................ 181
Uses ......................................................................................................................................... 181
Module: 6 ........................................................................................................................................... 182
Lecture: 28 ...................................................................................................................................... 182
CLAY PRODUCTS AND REFRACTORIES ......................................................................................... 182
2. STRUCTURAL CLAY PRODUCTS ........................................................................................ 182
PROPERTIES ............................................................................................................................. 183
USES ........................................................................................................................................ 183
3. REFRACTORY MATERIALS ................................................................................................ 184
CLASSIFICATION ...................................................................................................................... 184
MANUFACTURE ....................................................................................................................... 186
PROPERTIES ............................................................................................................................. 188
USES ........................................................................................................................................ 191
Module: 6 ........................................................................................................................................... 193
Lecture: 29 ...................................................................................................................................... 193
SPECIALIZED CERAMIC PRODUCTS AND VITREOUS ENAMEL ..................................................... 193
4. SPECIALIZED CERAMIC PRODUCTS .................................................................................. 193
5. VITREOUS ENAMEL ......................................................................................................... 195
MANUFACTURE ....................................................................................................................... 195
PROPERTIES ............................................................................................................................. 196
USES ........................................................................................................................................ 196
Module: 7 ........................................................................................................................................... 197
Lecture: 30 ...................................................................................................................................... 197
GLASS INDUSTRIES ...................................................................................................................... 197
INTRODUCTION ....................................................................................................................... 197
TYPES OF GLASSES ................................................................................................................... 197
Module: 7 ........................................................................................................................................... 203
Lecture: 31 ...................................................................................................................................... 203
MANUFACTURE OF GLASS .......................................................................................................... 203
RAW MATERIAL ....................................................................................................................... 203
MANUFACTURE ....................................................................................................................... 204
Module: 7 ........................................................................................................................................... 207
Lecture: 32 ...................................................................................................................................... 207
GLASS (Continued) ...................................................................................................................... 207
MANUFACTURE (Continued) ................................................................................................... 207
PROPERTIES ............................................................................................................................. 209
Module: 8 ........................................................................................................................................... 211
Lecture: 33 ...................................................................................................................................... 211
FERTILIZER ................................................................................................................................... 211
INTRODUCTION ....................................................................................................................... 211
TYPES OF SOIL ......................................................................................................................... 211
PLANT NUTRIENTS .................................................................................................................. 212
FUNCTION OF NUTRIENT ........................................................................................................ 212
NEED OF FERTILIZER ................................................................................................................ 215
CLASSIFICATION ...................................................................................................................... 215
Module: 8 ........................................................................................................................................... 220
Lecture: 34 ...................................................................................................................................... 220
AMMONIUM PHOSPHATE........................................................................................................... 220
INTRODUCTION ....................................................................................................................... 220
MANUFACTURE ....................................................................................................................... 221
PROPERTIES ............................................................................................................................. 223
USES ........................................................................................................................................ 224
Module: 8 ........................................................................................................................................... 225
Lecture: 35 ...................................................................................................................................... 225
SUPERPHOSPHATE ...................................................................................................................... 225
INTRODUCTION ....................................................................................................................... 225
MANUFACTURE ....................................................................................................................... 226
PROPERTIES ............................................................................................................................. 232
USES ........................................................................................................................................ 232
Module: 8 ........................................................................................................................................... 233
Lecture: 36 ...................................................................................................................................... 233
TRIPLE SUPERPHOSPHATE .......................................................................................................... 233
INTRODUCTION ....................................................................................................................... 233
MANUFACTURE ....................................................................................................................... 234
PROPERTIES ............................................................................................................................. 238
USES ........................................................................................................................................ 238
Module: 9 ........................................................................................................................................... 239
Lecture: 37 ...................................................................................................................................... 239
UREA............................................................................................................................................ 239
INTRODUCTION ....................................................................................................................... 239
MANUFACTURE ....................................................................................................................... 240
PROPERTIES ............................................................................................................................. 244
USES ........................................................................................................................................ 245
Module: 9 ........................................................................................................................................... 246
Lecture: 38 ...................................................................................................................................... 246
CALCIUM AMMONIUM NITRATE ................................................................................................ 246
INTRODUCTION ....................................................................................................................... 246
MANUFACTURE ....................................................................................................................... 246
PROPERTIES ............................................................................................................................. 249
USES ........................................................................................................................................ 249
Module: 9 ........................................................................................................................................... 250
Lecture: 39 ...................................................................................................................................... 250
AMMONIUM CHLORIDE .............................................................................................................. 250
INTRODUCTION ....................................................................................................................... 250
MANUFACTURE ....................................................................................................................... 251
1. Direct reaction ............................................................................................................ 251
2. Duel salt process ......................................................................................................... 252
USES ........................................................................................................................................ 254
Module: 9 ........................................................................................................................................... 256
Lecture: 40 ...................................................................................................................................... 256
AMMONIUM SULFATE ................................................................................................................ 256
INTRODUCTION ....................................................................................................................... 256
MANUFACTURE ....................................................................................................................... 257
PROPERTIES ............................................................................................................................. 261
USES ........................................................................................................................................ 261
Module: 10 ......................................................................................................................................... 262
Lecture: 41 ...................................................................................................................................... 262
POTASSIUM CHLORIDE................................................................................................................ 262
INTRODUCTION ....................................................................................................................... 262
MANUFACTURE ....................................................................................................................... 262
PROPERTIES ............................................................................................................................. 264
USES ........................................................................................................................................ 264
Module: 10 ......................................................................................................................................... 265
Lecture: 42 ...................................................................................................................................... 265
POTASSIUM SULFATE .................................................................................................................. 265
INTRODUCTION ....................................................................................................................... 265
MANUFACTURE ....................................................................................................................... 265
1. Mannheim process ...................................................................................................... 265
2. Recovery from natural complex salts ......................................................................... 267
PROPERTIES ............................................................................................................................. 268
USES ........................................................................................................................................ 268
Module: 11 ......................................................................................................................................... 269
Lecture: 43 ...................................................................................................................................... 269
PAINT INDUSTRIES ...................................................................................................................... 269
INTRODUCTION ....................................................................................................................... 269
CLASSIFICATION OF PAINTS .................................................................................................... 269
Module: 11 ......................................................................................................................................... 273
Lecture: 44 ...................................................................................................................................... 273
PAINT INDUSTRIES (continued) ................................................................................................... 273
CONSTITUENTS OF PAINTS ...................................................................................................... 273
Module: 11 ......................................................................................................................................... 278
Lecture: 45 ...................................................................................................................................... 278
PAINT INDUSTRIES (continued) ................................................................................................... 278
MANUFACTURE ....................................................................................................................... 278
SETTING OF PAINT ................................................................................................................... 281
REQUIREMENT OF A GOOD PAINT .......................................................................................... 282
PAINT FALIURE ........................................................................................................................ 283
PROPERTIES ............................................................................................................................. 284
1
Syllabus
Curriculum of the subject is divided into eleven modules and 45 lectures.
Module No. Lecture
Numbers
Topics to be covered
Module No. 1 1 Overview
Introduction, classification of chemical industries,
heavy and fine chemicals
Module No. 2 2 – 7 Industrial Gases
Introduction, manufacture and uses of carbon
dioxide, nitrogen, oxygen, hydrogen, ammonia,
acetylene.
Module No. 3 8 – 15 Sodium compounds
Sources, uses and preparation of sodium chloride.
Manufacture, properties and uses of sodium
carbonate, sodium bicarbonate sodium hydroxide
and chlorine.
Module No. 4 16 – 21 Mineral acids
Manufacture, properties and uses of nitric acid,
sulfuric acid, hydrochloric acid, phosphorus and
phosphoric acid
Module No. 5 22 – 25 Cement Industries
Raw materials, manufacturing method, types of
cement
Module No. 6 26 – 29 Ceramic Industries
Raw materials, manufacturing methods and
properties of white wares, clay products, refractories.
Module No. 7 30 – 32 Glass Industries
Raw materials, manufacture of glass, types of glass
Module No. 8 33 – 37 Phosphorus based agrochemicals
Introduction of fertilizers. Synthesis, properties and uses
of ammonium phosphate, super phosphate, triple
super phosphate.
Module No. 9 38 – 40 Nitrogen fertilizers
Introduction, manufacture & properties of urea,
ammonium chloride, calcium ammonium nitrate
(CAN), ammonium sulfate
Module No. 10 41 - 42 Potassium fertilizers
Introduction manufacture and properties of potassium
chloride and potassium sulfate
Module No. 11 43 – 45 Paint Industries
Introduction, types, manufacture and properties of
paints
Jay Shri Harsiddhi Mataji Module 1 Lecture: 1 Overview
Dr. N. K. Patel
N P T E L
1
Module: 1
Lecture: 1
HEAVY AND FINE CHEMICALS
OVERVIEW
Chemical industries are basically divided into two groups.
First which produces simple compounds from the locally available large
amount of raw materials usually they are very large industries and the product
manufacture are purified to the extent that they can be used as raw material for
other industries or they are directly marketed as a consumer goods. In general they
are heavy chemical industries.
On the other hand certain industries deal with speciality chemicals and they
are making small quantity of product having better quality which is sold into market
as finished good. They are called as fine chemical industries.
Classification
The materials used or produced in the chemical industries are classified in the
following manner.
1. Quantity of production and consumption
a) Heavy chemicals
Those dealt in large quantity normally crude or less purified chemicals.
E.g. mineral acid, NaOH, Na2CO3 etc.
b) Fine chemicals
They are complete purified substances and produced in limited quantity.
E.g. speciality solvent, perfumes, medicines etc.
2. Chemical composition
a) Organic compound
Compounds having carbon atom in the main structure of the molecule is
called organic compound.
E.g. hydrocarbons, phenols, carboxylic acid etc.
Module 1 Lecture: 1 Overview
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b) Inorganic compound
They are the compounds which do not have carbon in the main structure.
E.g.Na2CO3, K2Cr2O7, MgCl2
c) Polymers
They are the macromolecular mass compounds made from covalent
bonding of repeating structured units which may be natural, synthetic or semi
synthetic. E.g. polystyrene, polyvinylchloride etc.
3. Based on availability
a) Natural compounds
Compounds which are available in nature or produced or extracted from
plant and animals are referred as natural products. Due to large utilization & limited
production the natural source is depleting. E.g. coal, petroleum etc.
b) Synthetic products
Men made compounds are referred as synthetic products. They may be
synthesized using natural product or they are synthesized completely using other
type of synthetic materials, but the main target or such product is that must be
suited to direct applications.
4. Based on application
a) Catalyst
A substance, usually used in small amounts relative to the reactants, that
either increases or decreases the rate of a reaction without being consumed in the
process. If consumed than it should regenerative at the end of process. E.g. AlCl3,
MnO2, Pt etc.
b) Bulk drug
Bulk drug is the active substance used in a drug formulation. It becomes an
active ingredient of a finished dosage form of the drug, but the term does not
include intermediates used in the synthesis of such substances. E.g. Pantoprazole,
Bisacodyl etc.
c) Resin
Resin is a natural or synthetic compound which begins in a highly viscous
state and hardness with treatment.
E.g. Urea formaldehyde, epoxy, polyester etc.
Module 1 Lecture: 1 Overview
Dr. N. K. Patel
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d) Dyes and Pigments
A dye or a dyestuff is usually a coloured organic compound or mixture that
may be used for imparting colour to a substrate such as cloth, paper, plastic or
leather in a reasonably permanent fashion.
Pigments are defined as colouring agents that are practically insoluble in the
application medium, whereas dyes are colouring agents that are soluble in the
application medium.
Many organic pigments and dyes have the same basic chemical structure.
The insolubility required in pigments can be obtained by excluding solubilizing
groups, by forming insoluble salts (lake formation) of carboxylic or sulfonic acids, by
metal complex formation in compounds without solubilizing groups, and particularly
by incorporating groups that reduce solubility (e.g. amide groups).
e) Solvent
A liquid in which substances (or solutes) are dissolved to form a solution is
called as solvent.
E.g. Benzene, THF, DMF, DMSO etc.
f) Miscellaneous
All other compounds which do not cover in above class are called as
miscellaneous.
E.g. fertilizer, glass etc.
UNIT OPERATION AND UNIT PROCESS
Activities of chemical manufacturing plant are broadly covered under the
label of conversion of raw materials into useful products. In some cases the product
are used as starting materials for further modification and thus the product may not
be termed as end product but is called as intermediate. In another cases the
products are ready for marketing known as finished product. But still some of the
finished products may be used for physical blending or combination with other
materials and binders particularly in pharmaceutical industries.
Form the above discussion materials which are used in chemical industries
can be classified into following categories.
Raw materials
They are naturally occurring material or not produced at the manufacturing
unit and are procured from outside the manufacturing plant.
Module 1 Lecture: 1 Overview
Dr. N. K. Patel
N P T E L
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Intermediate
They are undergoes some processing and further proceed for modification
Finished product
Product which are ready for marketing or sale
By product
It is useful material generated with main product. Also known as co-product
Waste
Do not have any commercial value. May be discarded after giving some
treatments regarding control of pollution.
Further, any commercial manufacture or production unit of chemicals have
combination of series of physical and chemical changes of raw materials or
intermediates or finished product. Ultimately comprehensive utilization of material for
improvement in chemical properties, modification of chemicals, maximize the yield
and conversion, utilization of waste products etc.
For the systematic study of chemical process industries the physical and
chemical changes which are important for the manufacturing processes have been
classified as unit operation and unit processes respectively
Thus,
Chemical Process Industries = Unit operation + Unit process
Unit operation
Major physical changes occur which are useful to chemical industries are
known as unit operation. In majority of cases, operations are to be done to set up
the condition to carry out chemical changes. Thus very important classification of
various changes useful to chemical industries was needed to be done.
Unit operations shall be broadly classified as follows.
1. Fluid flow processes : Fluids transportation, filtration, solids
fluidization
2. Heat transfer processes : Evaporation, condensation
3. Mass transfer processes : Gas absorption, distillation, extraction,
adsorption, drying
Module 1 Lecture: 1 Overview
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N P T E L
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4. Thermodynamic processes : Gas liquefaction, refrigeration
5. Mechanical processes : Solids transportation, crushing and
pulverization, screening and sieving
6. Combination : Mixing
7. Separation : Distillation, extraction
Unit process
Useful chemical transformations with or without physical changes occurs in
the chemical industries are called as unit process e.g. halogenations, oxidation,
reduction, alkylation and acylation etc.
The study of these processes includes
The basic knowledge of chemistry and mechanism of particular chemical
reaction
Design of equipment for the reaction
Optimization of reaction parameter
However, still the condition and parameter for carrying unit process in plant
level may differ from product to product. But the regularities emerged from the study
of a particular process can be useful in setting up condition for the manufacture of
new chemical which may include one or more such unit processes.
E.g. In the unit process nitration
Reaction is almost exothermic
Physicochemical principles of equilibrium and chemical kinetics are similar
Material of construction of plant and equipment for the process can be
predicted
The principles of widely varying sequence of making up a chemical process
do not depend upon the nature of the materials being worked upon and other
characteristic of the system under study. If the step of process is recognized, the
process can be designed in such a way that each step to be used can be studied
individually.
In both unit operations and unit processes the similarities within any unit
operation or unit process are separated and studied; thus drawing attention to the
like qualities of a given physical change or chemical change. Finally these results
help to understanding the process, establishment of reaction parameter and
reactor design. This is the scientific and engineering approach. The ultimate study by
Module 1 Lecture: 1 Overview
Dr. N. K. Patel
N P T E L
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this method of the technical changes culminated in chemical engineering formulas
and laws for using the classified observations in each unit operation or unit process.
These formulas and laws are the tools for the industrial chemist uses in designing or
operating a chemical plant.
In conclusion, Both physical and chemical changes have been useful not
only to fundamental concept but also to provide the technical detail as well as
smoothen the manufacturing process at optimized reaction condition at low cost.
Module: 2 Lecture: 2 Carbon dioxide
Dr. N. K. Patel
N P T E L
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Module: 2
Lecture: 2
CARBON DIOXIDE
INTRODUCTION
Carbon dioxide (CO2) is composed of two oxygen atoms covalently bonded
to a single carbon atom. It is a trace gas with a concentration of 0.039% by volume
in atmospheric air.
In the seventeenth century, Jan Baptist Van Helmont observed that during
burning of charcoal in the closed vessel, the mass of the resulting ash was much less
than that of the original charcoal. His explanation was that the rest of the charcoal
had been transmuted into an invisible substance termed as "gas" or "wild spirit"
Carbon dioxide‘s properties were studied by Joseph Black in 1750. He found
that limestone could be heated or treated with acids to yield a gas (fixed air). He
observed that gas was denser than air and supported neither flame nor animal life.
Black also found that when bubbled through an aqueous solution of lime, it would
precipitate calcium carbonate. Based on this phenomena he illustrate that CO2 is
produced by animal respiration and microbial fermentation. Joseph Priestley, in 1772
invented the soda water preparation by dripping sulfuric acid on chalk in order to
produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of
water in contact with the gas. Humphry Davy and Michael Faraday first liquefied
CO2 at elevated pressure in 1823. While in 1834 Charles Thilorier solidifies CO2, in
pressurized container of liquid carbon dioxide.
In higher animals, the carbon dioxide travels in the blood from the body's
tissues to the lungs where it is breathed out. CO2 is an end product in organisms that
obtain energy from breaking down sugars, fats and amino acids with oxygen as part
of their metabolism, in a process known as cellular respiration. This includes all plants,
animals, many fungi and some bacteria. During photosynthesis, plants, algae, and
Cyanobacteria absorb carbon dioxide, light, and water to produce carbohydrate
energy for themselves and oxygen as a waste product. However, in darkness,
photosynthesis cannot occur, and during the resultant respiration small amounts of
carbon dioxide are produced.
Carbon dioxide is also produced by combustion of coal or hydrocarbons, the
fermentation of liquids and the breathing of humans and animals. In addition, it is
emitted from volcanoes, hot springs, geysers and other places where the earth‘s
Module: 2 Lecture: 2 Carbon dioxide
Dr. N. K. Patel
N P T E L
8
crust is thin; and is freed from carbonate rocks by dissolution. CO2 is also found in
lakes at depth under the sea, and commingled with oil and gas deposits.
SOURCES OF CO2
By burning of carbonaceous materials
C + O2 CO2 (10 to 18% Pure) ΔH = - 23.16kcals
In the production of H2 by steam water gas 16% pure CO2 is obtained.
In manufacture of alcohol (ethanol) by the fermentation process. 99.9 % pure
CO2 is obtained.
In calcinations of CaCO3 40% CO2 is obtained
1000°C
CaCO3 CaO + CO2 (40%)
MANUFACTURE
Raw materials
Coke or coal
Air
Reaction
C + O2 CO2 (10 to 18% Pure) ΔH = - 23.16 kcals
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Coke
250 psig Steam to
turbine driver
Water From Coolers
Reactivator
CO2 to Purification
Ethanolamine Solution
CO2 free flue gas
Figure: Manufacture of Carbon dioxide from Coke
Flue gas Scrubbers Absorber
Heat Exchange
Cooler
Cooler
Blower Pumps
Flue Gase
12-18% CO2
Reboiler
Girbotol recovery unit
Exhaust steam from turbine
Water Boiler
Module: 2 Lecture: 2 Carbon dioxide
Dr. N. K. Patel
N P T E L
9
Coke, coal, fuel or gas is burned under a standard water-tube boiler for the
production of 200-250psig steam. The flue gases containing 10-18% CO2 are taken
from the boiler at 3450C and passed through two packed towers where they are
cooled and cleaned by water. After passing through the scrubbing towers, the
cooled flue gases pass through a booster blower and into the base of the
absorption tower in which CO2 is absorbed selectively by a solution of ethanolamines
passing countercurrent to the gas stream. CO2 free flue gases are exhausted to
atmosphere from top of the tower. The CO2 bearing solution passes out of the
bottom of the absorption tower are sprayed from the top of a reactivation tower.
Where CO2 is stripped from the amine solution by heat and the reactivated solution
returns through the heat exchanger equipment to the absorption tower. CO2 and
steam pass out through the top of the reactivation tower into a gas cooler in which
the steam condenses and returns to the tower as reflux. CO2 gas is stripped out at
the pressure of about 300 psig. If liquid or solid CO2 is desired, it may be further
purified for odour removal before compression.
Energy economics
All the pumps and blowers and turbine are driven by high pressure steam
from the boiler. The low-pressure exhaust steam is used in the reboiler of the recovery
system and the condensate returns to the boiler. Although there is some excess
power capacity provided in the high-pressure steam for driving other equipment,
such as compressors in CO2 liquefaction plant, all the steam produced by the boiler
is condensed in the recovery system. This provides a well-balanced plant in which
few external utilities are required and combustion conditions may be controlled to
maintain efficient operation.
METHODS OF RECOVERY
The processes most commonly used for recovery of carbon dioxide are
1. Ethanolamine process
2. Sodium carbonate process
3. Potassium carbonate process
All the processes are in commercial use and choice of suitable process will
depend on the individual conditions. In all the process CO2 is recovered by
absorption-desorption. First CO2-bearing gases are passed countercurrent to a
solution that removes the CO2 by absorption and retains it until desorbed by heat
in a separate piece of equipment. Due to relatively low water solubility of CO2,
water alone is not used as a absorption medium. Alkali carbonate and
ethanolamine solutions are used due to the higher solubility of CO2 with the
absorbing medium.
Module: 2 Lecture: 2 Carbon dioxide
Dr. N. K. Patel
N P T E L
10
1. Girbotol amine process
It was developed by the Girdler Corporation of Louisville, Kentucky. The
various operation used in the process are discussed earlier during the manufacture
of CO2 from coke. The process uses aqueous solutions of an mono-, di- or tri-
ethanolamine as absorption medium.
2OHC2H4NH2 + H2O + CO2 (OHC2H4NH3)2CO3
The operation are depends on the reversible nature of the above reaction.
Forward reaction proceeds at low temperatures (650C) and absorbs CO2 from the
gas in the absorber. The amine solution, rich in CO2, passes out of the bottom of the
tower and through heat exchanger, where it is preheated by hot, lean solution
returning from the re-activator. Then solution passes countercurrent to a stream of
CO2 and steam, which strips CO2 out of the solution. As the solution reaches to
bottom of the tower, where heat is supplied by a steam heated or direct fired
re-boiler, it has been reactivated. This hot solution (1400C) passes out of the tower,
through the heat exchanger and cooler, and returns to the absorber tower. In the
case of flue gases containing oxygen, small side stream of solution is passed through
re-distillation unit, where the oxidation products are removed and the distilled amine
is returned to the process.
Advantages
Complete removal of carbon dioxide
Regeneration up to 100% with moderate steam consumption is possible
Higher absorption of CO2 in the solution
Lower operating cost
2. Sodium carbonate process
Na2CO3 + H2O + CO2 2NaHCO3
Recovery of pure carbon dioxide from gases containing other diluents, such as
nitrogen and carbon monoxide, is based on the reversibility of the above reaction.
This reaction proceeds to the right at low temperatures and takes place in the
absorber where the CO2 bearing gases are passed countercurrent to sodium
carbonate solution. CO2 absorption rate depends up on temperature, pressure,
partial pressure of CO2 in the gas, and solution strength. Reverse reaction will
proceed when heat is applied and is carried out in lye boiler. A heat exchanger
serves to preheat the strong lye as it approaches the boiler and cool the weak lye
returning to the absorber. Additional weak lye cooling is accomplished in lye cooler
to permit the reaction to proceed further to the right in the absorber. CO2 gas and
water vapour released from the solution in the boiler pass through steam condenser
where the water condenses out and returns to the system. The cool CO2 proceeds
to the gas holder and compressors.
Module: 2 Lecture: 2 Carbon dioxide
Dr. N. K. Patel
N P T E L
11
Engineering aspects
Absorber
Absorber is constructed by a carbon-steel filled with coke, raschig rings, or
steel turnings. The weak solution is spread from top of the bed and contacts the gas
intimately on the way down. In another variation tower filled with sodium carbonate
solution and allow the gas to bubble up through the liquid. Later provides better gas
and liquid contact but high power is required to force the gas through the tower.
Lye boiler
The lye boiler may be a direct fired boiler or a steam heated boiler. The
separation efficiency may be increased by adding a tower section with bubble-cap
trays. For better efficiency and conversion, series of absorbers are used and
designed to re-circulate the lye over it and only 20-25% of solution flowing over this
tower passes through the lye boiler.
3. Potassium carbonate process
As potassium bicarbonate has more solubility than its corresponding sodium
salt, it provides better absorption of CO2 than other process. Operation and
equipment layout of process are similar to sodium carbonate process.
Variations of the potassium carbonate process have come into commercial
use in recent years.
Hot potassium carbonate process
Absorbent solution flows directly from the lye boiler to the absorber without
cooling. This process used for removing CO2 from NH3 synthesis gas mixtures, and
from natural gas. These gas streams are treated at 250 psig, or higher pressure which
increases the partial pressure of CO2 so that the hot K2CO3 solution (20-30%) will
absorb substantial amount of CO2 at 1100C. The solution sends to the CO2 stripping
tower operating at or near atmospheric pressure. Part of the absorbed CO2 flashes
out of the solution as it enters the stripping tower, and the balance is stripped
from the solution by steam. The overall energy requirements for CO2 recovery by
the hot carbonate process are lower than for other processes when the gases being
scrubbed have high carbon dioxide partial pressures.
Use of additives
This variation has been developed by Vetrocoke in Italy. Use of various
additives like amino acids, arsenic trioxide, and selenium and tellurium oxides in hot
potassium carbonate absorbent solution which increase CO2 absorption rate, and
decrease the steam required for stripping CO2 from the solution. The Vetrocoke
processes have also employed air stripping for removing CO2 from additive
Module: 2 Lecture: 2 Carbon dioxide
Dr. N. K. Patel
N P T E L
12
containing hot potassium carbonate solutions in cases in which CO2 is not recovered
as a pure gas.
PURIFICATION
Carbon dioxide obtained in the impure state can be purified by different
ways. There are two main categories for purification of carbon dioxide.
1. Purification of low % CO2 containing gas.
2. Purification of high % CO2 containing gas.
1. Purification of low % CO2 containing gas
Block diagram of manufacturing process
Diagram with process equipment
Animation
18% hot CO2 gas passes through exchanger to lower the temperature. Then it
is passes through a scrubber in which the water is percolated from the top to
remove SO2 and dust particles. Then the gas passes through two packed towers
where the gas is scrubbed with Na2CO3 solution and absorbed in it to form NaHCO3
solution in second tower. Solution is heated in heat exchanger to remove absorbed
carbon dioxide. This carbon dioxide is then cooled in cooler and stored.
2. Purification of high % CO2 containing gas
Block diagram of manufacturing process
Diagram with process equipment
Impure CO2
Na2CO3
SO2 & Dust
Figure: Purification of Low % CO2 containing gases
H2O
99% stored
CO2
H2O
Steam
Heat Exchanger
Sc
rub
be
r
Packed Tower 1
Packed Tower 2
Hot 99% CO2
Cooler
Heat Exchanger
Module: 2 Lecture: 2 Carbon dioxide
Dr. N. K. Patel
N P T E L
13
Animation
Gases are first compressed to 80psi pressure and passes through a scrubber to
remove organic matters with KMnO4. The gas is then dehydrated using silica gel or
activated alumina or conc. H2SO4 by passing through dehydration tower. Then the
gas passes through an oil scrubber to remove bad odour of gas. Now the gas is,
compressed in two stages, 80 psi to 300 psi and 300 psi to 900 psi for getting
compressed gas or liquid respectively. For liquid CO2 the temperature is brought
down much below 31.1°C. After compression by cooling of CO2, the liquid is stored
at -10° C temperature. If the liquid CO2 is passes through an expansion tank and
pressure is released then the solid CO2 is formed at -40° C temperature.
PROPERTIES
Molecular formula : CO2
Molecular weight : 44.01gm/mole
Appearance : Colourless gas
Odour : Odourless gas
Boiling point : -570C
Melting point : -780C
Density : 1. 977kg/m3 @ 1atm and 00C
Solubility : Soluble in water
USES
As solid CO2 in refrigeration process
Liquid CO2 is needed in carbonated.
Used in creating inert atmosphere.
As fire extinguisher
Gaseous CO2 used as a neutralizing agent
Gaseous CO2 is the basic raw material for production of Na2CO3, NaHCO3
Impure CO2
KMnO4
Organic matter
Conc. H2SO4
CompressorS
cru
bb
er
Dehydrationtower
Oil
Sc
rub
be
r
Co
ole
r
Ex
pa
ns
ion
tan
k
Solid CO2
Figure: Purification of high % CO2 containing gases
Compressors
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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Module: 2
Lecture: 3
OXYGEN AND NITROGEN
INTRODUCTION
Oxygen
Oxygen (O2) composed of two atoms of the element at (O) bind to form
dioxygen, a very pale blue, odorless, tasteless diatomic gas. Diatomic oxygen gas
constitutes 20.8% of the volume of air. It is necessary to sustain global life.
Oxygen is the highly reactive nonmetallic element that readily forms
compounds or oxides with almost all other elements. Oxygen is a strong oxidizing
agent and has the second-highest electronegativity after fluorine than of all the
elements. By mass, after hydrogen and helium, oxygen is the third-most abundant
element in the universe. Free oxygen is too chemically reactive to appear on Earth
without the photosynthetic action of living organisms, which use the energy of
sunlight to produce elemental oxygen from water. Elemental O2 only began to
accumulate in the atmosphere after the evolutionary appearance of these
organisms, roughly 2.5 billion years ago.
As larger constituent by mass of water, oxygen comprises most of the mass of
living organisms. Elemental oxygen is produced by cyanobacteria, algae and
plants, and is used in cellular respiration for all complex life. Oxygen is toxic to
anaerobic organisms, which were the dominant form of early life on Earth until O2
began to accumulate in the atmosphere.
Oxygen was independently discovered by Carl Wilhelm Scheele and Joseph
Priestley in 1773 and 1774 respectively, but work was first published by Priestley.
Antoine Lavoisier named as oxygen in 1777, whose experiments with oxygen helped
to discredit the then-popular phlogiston theory of combustion and corrosion.
Oxygen is produced industrially by fractional distillation of liquefied air, use of
zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water
and other means.
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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Nitrogen
Nitrogen (N2) is a colorless, odorless, tasteless, and mostly inert diatomic gas at
standard conditions, constituting 78.09% by volume of Earth's atmosphere. Nitrogen
occurs in all living organisms, primarily in amino acids, proteins and in the nucleic
acids (DNA and RNA). The human body contains about 3% by weight of nitrogen,
the fourth most abundant element after oxygen, carbon, and hydrogen.
Nitrogen was discovered by Daniel Rutherford in 1772, who called it noxious
air or fixed air. He also explains that nitrogen does not support combustion. At the
same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, referred
it as burnt air or phlogisticated air. Antoine Lavoisier referred nitrogen as inert gas
and as "mephitic air" or azote, in which animals died and flames were extinguished.
English word nitrogen entered the language in 1794.
The extremely strong bond in elemental nitrogen causing difficulty for both
organisms and industry in breaking the bond to convert the nitrogen into useful
compounds, but large amounts of useful energy released when the compounds
burn, explode, or decay back into nitrogen gas.
Analysis of Air
Air mainly consist of two gases oxygen and nitrogen, which are practically
considered to constitute 1/5 and 4/5 of air by volume respectively. The list of various
gases present in air by weight percent is as under
Name of the gas % by weight in air
Oxygen 20.99
Nitrogen 78.01
Carbon dioxide 0.03 - 0.07
Argon 0.94
Hydrogen 0.01
Neon 0.0015
Helium and Krypton 0.01 - 0.02
Except CO2 the concentration of all the gases listed above are present in air
are constant. However water vapours and traces of ozone and iodine are present in
air in variable amounts. Also, composition of air also depends on altitude and
distance to sea, in neighbourhood of industry, built up urban areas, nearby volcanic
phenomena. Other gases such as CO, H2S and NO2 are also present in air.
Kinetics and theory of gases
According to kinetic theory a gas consists of swiftly moving molecules moving
in a haphazard manner. During the movement some molecules collide with one
another, some others move away from one another, these phenomena leaving an
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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average distance between the molecules. At lower pressure the average distance is
large, and at high pressure the molecules are brought near to one another.
In all the three state of matter, molecules have two tendencies i.e. Attraction
tendency and Repulsive tendency. The repulsive tendency is most predominant in
gases, and least prominent in solids. While in most of liquids the attractive tendency
in molecules is more than the repulsive tendency, so that the molecules remain
together, but the attracting tendency is still less than in comparison of solids there so
liquid is in fluid state. In the gases as the temperature raises the repulsion tendency
increases, and pressure remaining constant the average distance between
molecules increases i.e. the volume increases. Hence with fall of temperature the
distance diminishes and, the molecules come closer together. Thus it becomes
apparent that the average distance of molecules will progressively diminish with fall
of temperature, and rise of pressure. Change of average distance of gas is
quantitatively expressed by PV/T = constant
Critical temperature
When by decreasing the distance the molecules of a gas are brought close
together the gas assumes the liquid form provided the repulsive tendency has been
diminished beyond a certain point known as critical temperature which is different
for different gases.
Critical temperature is the temperature below which any gas can be
liquefied by increasing the pressure. Above the critical temperature any gas cannot
be liquefied by compression.
Critical pressure
Above critical temperature the gas will never liquefy under any pressure. The
minimum pressure under which gas liquefies at the critical temperature is called as
critical pressure.
Therefore air should be cooled at very high pressure and low temperature for
cooling purpose. The liquid form is obtained when the kinetic energy and the
potential energy of the substance is approximately equal.
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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The critical temperature and critical pressure of some gases are as follows.
Sr.
No.
Gases Critical temperature
(0C)
Critical pressure
(atm.)
1. Ethylene +9.5 50.65
2. Methane -82.85 45.6
3. Nitrogen -147.13 33.49
4. Hydrogen -239.9 12.8
5. Oxygen -118.75 49.7
6. Acetylene +35.5 61.55
7. Ammonia +132.5 112.3
8. Carbon monoxide -138.7 34.6
9. Carbon dioxide +31.3 72.9
Liquefaction of air by Joule - Thomson effect
CO2 free air is compressed to 200atm and is cooled by water. The condensed
water is removed by passing through activated alumina. Then air is passed through
inner coil of heat exchangers. The valve with nozzle is provided at the end of the
inner coil. Then gas is allowed to suddenly expand by opening the valve, which
result in decrease of temperature of air. After expanding the cold air goes out
through the outer coil, is then recompressed to 200atm pressure, cooled by water
and then again allowed to transverse the inner coil. The temperature of the
Water
Water
Piston
Compressed Air
Nozzle
Liquid Air
Figure: Liquefaction of Air by Joule Thomson effect
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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incoming air further falls due to the presence of cold air in the outer coil. Now as the
cooled air suddenly expands through the nozzle, the air suffers cooling, the
temperature becomes lower than in the first operation. The colder air now passes
through the outer coil producing an atmosphere of lower temperature. Hence when
the cooled compressed air passes repeatedly through the inner coil and
subsequently undergoes Joule-Thomson effect, the temperature of the air further
drops. In this way progressive cooling takes place until the temperature of air falls
below the critical temperature of oxygen and nitrogen. When this happens air
undergoes liquefaction in the inner coil, so on opening the valve liquid air falls in the
container. A part of liquid air evaporates, through the outer coil, maintaining the low
temperature below the critical temperature.
MANUFACTURE
Oxygen in pure condition is obtained as a byproduct in the manufacture of
H2 by electrolytic process is described in Module: 2, Lecture: 4. Oxygen and nitrogen
are usually separated by rectification of liquid air.
Linde's process (O2 and N2)
The first rectification of N2 and O2 using Joule Thomson effect was carried out
by Linde in 1906. After six year Claude rectified them by combined effect of external
work and internal work in cooling the air to liquefaction point.
Raw materials
Basis: 1000kg Oxygen (95%)
Air = 3600Nm3
Steam = 1750kg
Cooling water = 5000kg
Electricity = 450-480KWH
Manufacture
Block diagram of manufacturing process
Process equipment
Animation
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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The distillation tower is specially designed bubble cap tray double columns
arranged one above another. The two distillation columns are having intermediate
distillation dome for effective separation of liquid enriched with O2. The column feed
is liquefied air at 200atm pressure introduced at the bottom of the column. Since the
boiling point of O2 (-183OC) and N2 (-195OC) are very low, column does not require
any external heating. Distillation take place only due to release of vacuum. Thus a
number of recycling from lower column to upper column and lower column to
dome is required. The construction of dome includes number of internal pipes so that
distillate of the lower column collides to the roof and is returned back to the column
as reflux. The compressed air which arrives from the first section of the plant
which acts as the heating fluid in the heater at the base of the enrichment
column. The same air, always contained within a tube, passes out from the Iower
column of the tower only to reenter it higher up after the pressure to which it is
subjected is reduced by means of a valve, resulting in the lowering of its
temperature. Nitrogen with a small oxygen impurity collects at the top of the
enrichment column, and after expansion to atmospheric pressure; this nitrogen is
sent to back as the reflux in the rectification column situated above. The liquid
which collects in the heater at the base of the enrichment column is fed, after
expansion to atmospheric pressure onto a suitable plate of the rectification column.
Only after number of recycling, liquid with 82% concentration of O2 is taken in
outer part of dome. This liquid goes to further rectification in upper column where it is
refluxed with N2 rich liquid coming from lower column. The final separation in the
Cold Gaseous Nitrogen
Compressed Air
Figure: Manufacturing of Nitrogen & Oxygen by Linde's Process
Water
Water
Compressed Air
Nozzle
Liquid Air
Piston
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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upper column takes place which has less number of trays. Gaseous N2 is the top
product of the column and the bottom product is liquid O2.
Claude process
In Claude process, progressive cooling of compressed air is done by external
work and Joule - Thomson effect. 70% of air is cooled by external work and 30% by
Joule - Thomson effect.
Two variants of Claude process
Molecular sieve variant
Cooling of air is brought about primarily by expansion with the performance
of work. Therefore, there is no need to equip the plant with cycles that use
refrigerants or make use of very high pressures which are employed when free
expansion is used, in order to produce cooling.
Evaporation to diffusion
Cooling of the air can be adopted such as causing liquids which can
evaporate to diffuse air. It is then safe to reabsorb them when necessary.
Kinetics and thermodynamics
Liquefied air is subjected to rectification to separate the oxygen and nitrogen
components present in it. In liquid state both are miscible in all ratios and they do not
form azeotropic mixtures; so they could not separate by boiling the solution.
Operation of a rectification column
liquid L2, temperature Tn-1, vapour V2
Tn-1
liquid L, temperature Tn, vapour VTn
liquid L1, temperature Tn+1, vapour V1
Tn+1
V2
V1
V
L2
L
L1
Pn+1
Pn
Pn-1
Figure: Section of a plate rectification column
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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Rectification is carried out in a 'plate column', which is tall cylindrical structure
inside which repeated condensations and evaporations take place on plates, which
lead to a continuous change in the composition of the binary system throughout the
length of the column. This is continued until one of its pure components exists at top
of the column and the other at the bottom of the column.
For better understanding of rectification of binary mixture a small section of
column is shown in figure which is formed by three plates: an intermediate plate Pn.
and two collateral plates Pn-1 and Pn+1 which are arranged below and above the
plate Pn respectively.
V1, V, and V2 be the vapours which, as they pass toward the top of the
column, leave the plates Pn-1, Pn, and Pn+1 respectively
L1, L, and L2 be the liquids which, as they pass down the column, descend
from the plates Pn-1, Pn, and Pn+1 respectively
Tn+1, Tn and Tn-1 be the temperatures of the plates Pn-1, Pn, and Pn+1 respectively
The rectification of liquid mixture is exclusively on the basis of heat exchange
of the different fraction present in liquid form as well as vapour form. As shown in the
diagram plate Pn is considered as the reference plate having temperature Tn and
liquid composition L, vapour composition V. As pressure is released the more volatile
component i. e. N2 is evaporated out partly and goes to the upper plate Pn+1. The
composition of liquid L2 is having less concentration of N2 at temperature Tn-1.
Similarly liquid below the reference plate is Pn-1 has higher concentration of O2 and
vapour V1 having higher composition of O2 at temperature Tn+1.
Thus the separation of more volatile component N2 in vapour form and low
volatile component O2 in the liquid form is achieved.
Finally:
The liquids which fall down from the plates toward the heater in the base of
the column (L2, L, L1) become progressively richer in the less volatile
component (oxygen)
The vapours which rise toward the top of the column (V1, V, V2) gradually
become enriched in the more volatile component (nitrogen)
Subsequently, in a column fitted with a suitable number of plates, O2 is
obtained in pure state at the base of the column and the N2 is obtained in a
practically pure state at the top of the column.
For, perfect operation of a rectification column always requires that:
The liquid should always be introduced onto a plate which supports a
liquid of the same composition as that of the feedstock liquid
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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Part of the distillate from the top of the column is recycled in the form of a
'reflux' with the aim of repeated washing on all of the plates which refine the
vapours moving towards the top of the column
Condensation medium
The separation of liquid air into nitrogen and oxygen is not as simple as the
fractionation of any other binary mixture. Because the separation is carried out at
very low temperature around -2000C. It is difficult to find a suitable medium for the
refluxing of a liquid air distillate at this temperature. The only possible media for the
condensation of the reflux would be liquid helium or liquid hydrogen, the use of
which is clearly unacceptable on both economic and operational grounds.
Engineering aspects
Two-section fractionating tower
Designing a fractionating tower consisting of two columns which are
arranged one above the other is the economically acceptable solution. The upper
column is about twice the height of the lower column, and both of them are fitted
with plates spaced at intervals. The average numerical ratio of the repartitioning
between the two columns is 42-25.
The upper column has all the requisites of a rectification column, while the
lower column functions as a simple enrichment column. As a bottom reboiler, the
lower column has a boiler with a curved base. There is no condenser at the top of
the rectification column, and it is closed by means of a gently curved cover with an
outlet aperture.
Fundamentally, one is concerned with two columns, one being situated
above the other, working at different pressures
Lower column operating at 6atm
Upper column operates only slightly above atmospheric pressure
The heat exchanger provided between the two columns acts as a condenser
with respect to the lower column and a boiler with respect to the upper column
precisely as a result of the two different pressures which appertain in the two
compartments.
More precisely: the upper column is supplied with a feedstock of a
composition which is proportionate with that of the liquid situated on the plate
where the feedstock is let in, and receives a suitable reflux at the top, while the
lower column is fed almost normally but is not refluxed, and , instead of leading to
practically pure components, it produces a liquid which is enriched in the oxygen at
the bottom and in the nitrogen at the top of the column.
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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The shape of the plates differs according to the type of plant in which they
operate.
Linde method
Every plate is made up from two metal plates which are separated from one
another by a certain spacing and perforated with very small apertures in the lower
plate and quite large holes in the upper plate.
Claude method
The column plants using claude‘s method is strips of thin steel plate wound
into a spiral with separation of the order of tenth of a millimeter between the spirals.
On account of the capillarity due to the small apertures in the lower half of
the Linde plates and the small cavities between the spirals of the Claude plates, the
Linde's type plate
Claude's type plate
Module: 2 Lecture: 3 Oxygen and Nitrogen
Dr. N. K. Patel
N P T E L
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down-flows of the liquids are retarded, thereby favouring perfect contact between
the descending liquids and the rising vapours. Material of constriction should be
chosen in such a way that can resist very low temperature.
PROPERTIES
Oxygen
Molecular formula : O2
Molecular weight : 32gm/mole
Appearance : Colourless gas
Odour : Odourless
Boiling point : -182.950C
Melting point : -218.790C
Density : 1.429gm/L (00C,101.325kPa)
Solubility : Sparingly soluble in water
Nitrogen
Molecular formula : N2
Molecular weight : 28gm/mole
Appearance : Colourless gas
Odour : Odourless gas
Boiling point : -195.790C
Melting point : -2100C
Density : 1.251gm/L (00C,101.325kPa)
Solubility : Slightly soluble in water
USES
Oxygen
It is used to produce oxyacetylene flame to cutting and welding the metals
Used in L. D. process for steel production
Used for artificial respiration in case of patients
Used for mountain climbers and high attitude aero planes flights
Nitrogen
Used in manufacture of synthetic ammonia, nitric acid
Used in manufacture organic nitrates like propellants and explosives,
Synthetically produced nitrates are key ingredients of industrial fertilizers
Used in producing nitrogen oxide.
Applied to create inert atmosphere.
Module: 2 Lecture: 4 Hydrogen
Dr. N. K. Patel
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Module: 2
Lecture: 4
HYDROGEN
INTRODUCTION
Hydrogen (H2) is colourless, odourless, tasteless, non-toxic, nonmetallic, highly
combustible diatomic gas. Atomic hydrogen is found rare on Earth because it
readily forms covalent compounds with most elements, water and organic
compounds. Hydrogen plays an important role in acid base chemistry.
The most common isotope of hydrogen is protium (1H) with a single proton
and no neutrons. As the only neutral atom with an analytic solution to the
Schrödinger equation, the study of the energetics and bonding of the hydrogen
atom played a key role in the development of quantum mechanics.
Robert Boyle produced hydrogen by reaction between iron filings and dilute
acid in 1671. Henry Cavendish identified hydrogen gas as a discrete substance in
1766. He named the gas from a metal-acid reaction as "flammable air". The name
hydrogen was given by Antoine Lavoisier in 1783, when he and Laplace reproduced
Cavendish's finding that water is produced when hydrogen is burned. Antoine-
Laurent de Lavoisier produced hydrogen by reacting flux of steam with metallic iron
through an incandescent iron tube heated in a fire.
Anaerobic oxidation of iron by the protons of water at high temperature can
be schematically represented by the set of following reactions
Fe + H2O FeO + H2
2 Fe + 3 H2O Fe2O3 + 3 H2
3 Fe + 4 H2O Fe3O4 + 4 H2
Hydrogen was first liquefied by James Dewar in 1898 by using regenerative
cooling in the vacuum flask. He produced solid hydrogen the next year. Deuterium
was discovered in December 1931 by Harold Urey, and tritium was prepared in 1934
by Ernest Rutherford, Mark Oliphant, and Paul Harteck. Heavy water, which consists
of deuterium in the place of regular hydrogen, was discovered by Urey's group in
1932. François Isaac de Rivaz built the first internal combustion engine powered by a
mixture of hydrogen and oxygen in 1806.
Module: 2 Lecture: 4 Hydrogen
Dr. N. K. Patel
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MANUFACTURE
The various method used for production of hydrogen gas are as follows.
1. Electrolytic process
2. Lane process or iron steam process
3. Steam hydrocarbon process
4. Liquefaction of coal gas and coke oven gas
5. Bosch process or water gas-steam process
1. Electrolytic Process
Pure hydrogen along with oxygen is manufactured by electrolytic process. It
is also obtained as a by-product in the production of caustic soda by electrolysis of
aqueous solution of sodium chloride as discussed in Module: 3, Lecture: 9. Heavy
water may be prepared on a large scale by burning deuterium separated from
hydrogen obtained by electrolysis.
Reactions
In acidulated water
H2SO4 2H+ + SO4-2
At cathode
2H+ + 2H2O 2[H3O]+
[H3O]+ + e- H+ + H2O
H+ + H+ H2
At anode
SO4-2 SO4 + 2e-
SO4 + H2O H2SO4 + O-2
O-2 + O-2 O2
In KOH solution
KOH K+ + OH¯
At cathode
K+ + e- K
K + H2O KOH + H+
H+ H+ H2
At anode
2OH¯ H2O + O-2
O-2 + O-2 O2
In Ba(OH)2 solution
Ba(OH)2 Ba+2 + 2OH¯
At cathode
Ba+2 + 2e- Ba
Module: 2 Lecture: 4 Hydrogen
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Ba + H2O Ba(OH)2 + H2
At anode
2OH¯ 2OH + 2e-
2OH¯ O-2 + H2O
O-2 + O-2 O2
Manufacture
Animation
Construction
Both unipolar and bipolar cells are used for electrolysis. In case of unipolar
cells, iron sheets and nickel coated iron sheets are used as cathodes and anodes
respectively. Both anodes and cathodes are placed close to one another to
prevent the loss of voltage. Asbestos diaphragm is placed between anode and
cathode compartment.
In case of bipolar cells the same sheet act as both anode and cathode as
the electrodes are connected in series. The cell is partitioned by vertical iron sheet;
the anode side of the sheet is electroplated with nickel. Vertical asbestos sheet as
diaphragm is placed between the anode side and cathode side of former sheet.
The same sheet acts as anode of one cell and cathode of the cell behind it. The
anode and cathode compartments are connected to two separate horizontal
pipes by means of standpipes, to lead away oxygen and hydrogen respectively. 250
cells are connected in series, and current of 10000amp is passed to operate the
cells. The purity of H2 and O2 are 99.95% and 99.6% respectively.
H2
+-
Gas Collecting Bells
Diaphrams
Electrodes
Figure: Manufacturing of Hydrogen by Electrolytic Process
H2O2
H2O2
Isolation and Packing
Module: 2 Lecture: 4 Hydrogen
Dr. N. K. Patel
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Working
Pure hydrogen can be manufacture by electrolysis of brine or water. Since
water is non-conductor of electricity and can be made conductor by addition of
small quantities of pure H2SO4 or KOH or Ba(OH)2. 2:1 volume of H2(g) and O2(g) are
simultaneously liberated at cathode and anode respectively
Produced hydrogen contains small quantity of oxygen, which can be
removed by passing the gas over the catalyst gently heated palladium asbestos.
The gas obtained by electrolysis is very pure and used for hydrogenation of oils.
Engineering aspect
The energy consumption is high due to resistance cause by the bubbling. This
is somewhat mitigated by conducting electrolysis under pressure. The decomposition
voltage is 1.23 volts, so that the evolution of 2gms of hydrogen 53.6amp.hour is
necessary. Hence to generate 1000litre of H2 at 180C and 1atm pressure 2.8KWH are
required because of overvoltage of the electrode and ohmic resistance of the
electrolyte and diaphragm
2. Lane process or steam hydrogen process
Raw material
Iron
Steam
Reaction
Fe3O4 + CO + H2 Fe +2FeO + CO2 + H2O
Fe + 2FeO + 2H2O Fe3O4 + 2H2
Manufacture
Heat
Exchanger
Contact
Material
Purge Steam
Stack Valve
Process Steam
Reheating Air
Charging door
Combustion Air
Cleaning door
Hydrogen outlet
Super Heater
Figure: Manufacturing of Hydrogen by Lane Process
Module: 2 Lecture: 4 Hydrogen
Dr. N. K. Patel
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Animation
Iron oxide and water gases are charged to fire bricks lined generator which is
heated externally by burning of producer gas or other gaseous fuel. The first action is
endothermic and the second action is exothermic. The temperature is maintained in
both actions at 6500C, first by heating the iron oxide and second by passing
superheated steam through the iron forming iron oxide by the heat of the reaction
and that of the steam. The reducing gas is passed through the heated iron oxide
from the bottom of the generator. The spent gas coming out of the generator is
burnt, and the hot products of combustion are sent through the super heater which
is a chamber filled with checker work. The time cycle is 20 minutes, then excess of
steam is passed through the super heater from the bottom and then passes through
the mass of iron from above, the hydrogen goes out through the pipe at the bottom
of the generator. The time cycle of passing steam is 10 minutes. The hot hydrogen
from the generator is cooled by passing through a cooler. The excess of steam
condenses and dissolves the H2S, which is practically completely removed. The gas is
then scrubbed with weak NaOH solution to remove CO2 and then mixed with steam
is passed over heated iron oxide catalyst to convert CO to CO2. The gas is then
scrubbed with weak NaOH solution for the second time; pure hydrogen gas is thus
produced.
Kinetics
The production of hydrogen depends upon the exothermic reaction
between red hot iron and steam. The continuity of production with the help of same
mass of iron is maintained by reducing with water gas, the iron oxide produced by
the iron-steam reaction, and repeating the cycle of oxidation and reduction. In
actual practice the iron oxide is not completely reduced, and the water gas is not
completely oxidized.
Engineering aspects
The iron mass must have a large exposed surface, and should be resistant to
disintegration. Such mass of iron was produced by calcining FeCO3 (spathic iron ore)
and then reducing the iron oxide to spongy iron having a large surface.
Module: 2 Lecture: 5 Hydrogen
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Module: 2
Lecture: 5
HYDROGEN (Continued)
3. Steam Hydrocarbon Process
Catalytic steam-hydrocarbon reforming began commercial operation in 1930
and by 1965 most of the hydrogen and synthesis gas mixture are produced by this
method.
Gaseous hydrocarbons (methane and ethane) and low boiling liquid
(propane, butane) and other normally liquid hydrocarbons up to octane are
reacted with steam over nickel catalyst at 650-9500C to produce carbon oxides and
hydrogen.
Manufacture
Raw material
Gaseous hydrocarbon/liquid hydrocarbon up to octane
Air
Catalyst
Characteristics of process hydrocarbon
Sulfur content
Sulfur will poison the nickel catalyst. There so total sulfur content should be
kept less than 5ppm.
Unsaturated hydrocarbons content
It should be free from unsaturated hydrocarbons because they tend to
deposit carbon on the reforming catalyst, causing both loss of activity and physical
deterioration. Hydro-desulfurization processes which are used for removal of sulfur
compounds will hydrogenate the unsaturated hydrocarbons.
Used in the vapour phase
Suitable procedure of vaporization should be adopted, if liquid hydrocarbons
such as natural gasoline and light petroleum naphtha are used.
Reactions
CH4 + H2O CO + 3H2 ΔH = - 48900cal
CH4 + 2O2 CO2 + 2H2O ΔH = 191800cal
Module: 2 Lecture: 5 Hydrogen
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Overall reaction
12CH4 + 5H2O + 5O2 29H2 + 9CO + 3CO2 ΔH = 20400cal
From propane
C3H8 + 3H2O 3CO + 7H2 ΔH = -129270cal
Manufacture
Coke oven gas free from sulfur compounds is scrubbed with water under
pressure and weak alkali solution to remove CO2. The gas is then liquefied by cooling
and compression to liquefy the gaseous hydrocarbons mainly methane. The residual
gas is chilled with liquid nitrogen under high pressure to remove nitrogen, when pure
hydrogen is obtained.
Methane rectified from liquid hydrocarbon or extracted from natural gas may
be used. Finely divided nickel supported on carrier of silicate used as a catalyst. The
temperature of the endothermic reaction is maintained at 8150C by partial
combustion of methane in presence of oxygen. Oxygen should be taken in such
amount so that the reaction becomes exothermic.
In case of propane as the raw material the reaction is endothermic. The
temperature is maintained at 8500C either by external heating or internal
combustion as in case of methane.
Whatever the starting material the mixture of CO, H2 and CO2 is mixed with
steam and then passed through the iron oxide shift converter at the optimum
temperature of 450°C.
For high-purity hydrogen, the primary reaction product is reacted
catalytically with additional steam to oxidize carbon monoxide to carbon dioxide.
The carbon dioxide is then removed in order to produce high-purity hydrogen.
In past few decades two noteworthy developments occurred
An increase in operating pressure from 250 to 600 psig
Large scale pressure application of pressure reforming (15atm) of petroleum
naphtha
Kinetics, thermodynamics and other factors
The following factors should be considered before design of the steam
hydrocarbon plant for hydrogen manufacture.
a) Operating pressure
CH4 + 2H2O CO2 + 4H2
Module: 2 Lecture: 5 Hydrogen
Dr. N. K. Patel
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Higher pressure increases the overall CO2 removal process and efficient heat
recovery from the reaction product stream. The equilibrium content of methane
remaining in the hydrogen varies directly with square of the operating pressure at a
given temperature and steam hydrocarbon ratio, so that as the operating pressure is
increased, this adverse effect must be compensated for by increasing either
operating temperature or steam-hydrocarbon ratio, or both.
b) Steam-hydrocarbon ratio
At atmospheric pressure, hydrocarbons may be reacted to produce
hydrogen with less than 0.1% residual methane by using 2mole of steam per atom of
carbon in the hydrocarbon, and carrying out the reaction at temperatures above
8700C. As the operating pressure is increased, the steam hydrocarbon ratio is
generally increased to between 3 and 4mole of steam per atom of carbon in the
hydrocarbon.
c) Space velocity
The space velocity that can be used will depend on the operating pressure
and temperature, hydrocarbon feed composition, activity of the catalyst, steam
hydrocarbon ratio and tube diameter. Typical space velocities for a plant reforming
natural gas at 10atm pressure, with 3:1 steam-methane ratio, and an operating
temperature of 9000C, would be from 3000 to 4000, based on hydrogen production
(750 to 1000 based on methane input), using 4inch diameter tubes.
d) Product gas temperature at furnace outlet
It is important factor in determining the amount of unreacted methane. The
exit gases will have a composition corresponding to the equilibrium composition at
temperature only 0-100C below the actual gas temperature. Since the equilibrium
constant for the steam methane reforming reaction increases from 170 at 8000C to
520 at 8500C, threefold increase in 500C, the influence of temperature on product
gas composition is evident. The maximum allowable tube surface temperature, the
tube diameter, the space velocity, and the type of furnace will all influence the
outlet product gas temperature.
e) Flue gas temperature
It should be from 9000C to 10400C, and the flue gas will be used to preheat
the furnace feed streams, and to generate steam.
f) Tube surface temperature
Chromium-nickel steel tube should be selected for high temperature and
high pressure operating condition. The life of the tube shall be more than five years.
Module: 2 Lecture: 5 Hydrogen
Dr. N. K. Patel
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g) Carbon monoxide removal
CO is reacted with steam to produce H2 and CO2. The product gases from
the reforming furnace at 7600C - 9250C are cooled to about 3700C and passed over
an iron oxide-chromium oxide catalyst for conversion of CO to CO2. Cooling carried
out by direct quenching with steam or water, or indirectly in a waste heat boiler. In
contrast with the steam reforming reaction, the equilibrium constant of the carbon
monoxide conversion reaction increases with decreasing temperature, going from 9
at 4250C to 207 at 2050C.
CO + H2O CO2 + H2
Several "low-temperature" CO conversion catalysts are commercially
available e.g. Copper, Chromium and Zinc oxides. They permit CO conversion
reaction to be carried out at temperatures of 1750C - 2000C, so that it is possible to
reduce the CO content from 15% down to 0.2% in single step. In contrast with the
iron oxide chromium oxide high-temperature catalysts which are only slightly
deactivated by small amounts of sulfur compounds, the low temperature catalysts
suffer permanent loss of activity at higher than normal temperature and are
poisoned by very small amounts of sulfur compounds, so that extra precautions
must be taken for that the feed gases, steam, and catalysts in the reforming and
high temperature shift units are sulfur free.
h) Hydrogen purity
The effect of impurities in the hydrogen must be evaluated against the
production costs incurred in minimizing them.
4. Liquefaction of coke oven gas or coal gas
Hydrogen, ethylene, paraffins, and nitrogen were separated from coke oven
gas by liquefaction process using Claude process, in which cooling is produced by
allowing compressed gas to do external work combined with internal work.
The olefin and paraffin fractions are separately fractionated to recover pure
ethylene and methane respectively. Ultimately the mixture of nitrogen and
hydrogen is liquefied to get liquid nitrogen and gaseous pure hydrogen.
The coke oven gas was first purified from H2S, HCN, NH3, CO2 and light oil. The
gas was compressed to 250 to 300psi and then first scrubbed in a pressure bubble
cap tower with dilute ammonia to remove CO2, and HCN. Then the scrubbing was
done with water to remove ammonia, then the gas was scrubbed with a petroleum
oil solvent to remove the light oil, and finally the gas was washed with an alkali
solution to remove the remaining CO2.
Module: 2 Lecture: 5 Hydrogen
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The gas is then dried and further compressed and subjected to progressive
cooling to remove ethylene and other olefins first, then methane and other gaseous
paraffin. Then the gas is further compressed and cooled so that nitrogen is liquefied
and hydrogen remains in the gaseous condition having very high purity.
Sr.
No.
Name of gas Critical temperature
(0C)
Critical pressure
(atm.)
1. Ethylene +9.5 50.65
2. Methane -82.85 45.6
3. Nitrogen -147.13 33.49
4. Hydrogen -239.9 12.8
5. Oxygen -118.75 49.7
6. Acetylene +35.5 61.55
1000 tons of coal yields 5 to 6 million cu. ft. of hydrogen, 3.5 to 4 million cu. ft.
of CH4 and 250,000 cu. ft. of ethylene.
5. Bosch Process
The process is same as modified Haber Bosch process and it is discussed in
ammonia manufacture Module: 2, Lecture: 6. In preparing hydrogen, only water gas
instead of mixture of water gas and producer gas (3:1 volume of H2 and N2) are
used.
PROPERTIES
Molecular formula : H2
Molecular weight : 2.0gm/mole
Appearance : Colourless gas
Odour : Odourless
Boiling point : -252.870C
Melting point : -259.140C
Density : 0.08988gm/L (0°C, 101.325kPa)
Solubility : Slightly soluble in water
USES
In fertilizer industries to produce NH3 which is converted into (NH4)2SO4, urea
and HNO3
In hydrogenation of oils to make fats or in hardening of fatty oils
In hydrogenating coal, low temperature carbonization tar and water gas to
produce gasoline
In hydrogenating water gas to produce methanol
In production of HCI, which is used in large quantity in industries
Module: 2 Lecture: 5 Hydrogen
Dr. N. K. Patel
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For filling in metrological balloons which are essential for upper air observation
to guide the air flights
In making oxy-hydrogen flame used for melting of platinum, quartz and in
auto welding of lead
In producing an inert media and in making tungsten filaments for electric
lamps, mixture of nitrogen and hydrogen is used
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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Module: 2
Lecture: 6
AMMONIA
INTRODUCTION
Ammonia (NH3) or azane is a compound of nitrogen and hydrogen. It is a
colourless gas with a characteristic pungent smell. Ammonia contributes significantly
to the nutritional needs of terrestrial organisms by serving as a precursor to food and
fertilizers. Although in wide use, ammonia is both caustic and hazardous.
It is one of the most important nitrogenous material. It is a base from which all
the nitrogen containing compounds are derived. Mostly is produced synthetically,
but during some chemical processes obtained as by product. Either directly or
indirectly, ammonia is a building-block for the synthesis of many pharmaceuticals
and is used in many commercial cleaning products.
Gaseous ammonia was first isolated by Joseph Priestley in 1774 and was
termed as "alkaline air". Claude Louis Berthollet ascertained its composition in 1785.
The Haber-Bosch process to produce ammonia from the nitrogen in the air
was developed by Fritz Haber and Carl Bosch in 1909 and patented in 1910. Prior to
the availability of cheap natural gas, hydrogen as a precursor to ammonia
production was produced via the electrolysis of water or using the chloralkali
process.
MANUFACTURE
(a) Haber and Bosch Process
Raw materials
Basis: 1000kg of NH3 (85% yield)
Hydrogen = 210kg
Nitrogen = 960kg
Catalyst = 0.2kg
Power = 850KWH
Fuel gas for compressors = 3800Kcal
Cooling water = 12,500kg
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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Sources of raw material
Nitrogen
Nitrogen is taken form air as discussed in Lecture: 3 (Module: 2)
Hydrogen
Hydrogen can be synthesized from any feed stock listed in the table
Feed stock Process or techniques to produce H2
Natural gas Partial oxidation / steam reforming
Coke oven gas Partial oxidation/ low temperature separation
Fuel oil or low sulfur heavy stock Partial oxidation
Coal Partial oxidation
Water Electrolysis
Catalyst
Most widely used catalyst for ammonia synthesis is iron with added promoters
e.g. oxides of aluminium, zirconium or silicon at about 3% concentration and
potassium oxide at about 1%. These prevent sintering and make the catalyst more
porous. Iron catalysts lose their activity rapidly, if heated above 520°C. Also, is
deactivated by contact with copper, phosphorous, arsenic, sulfur and CO.
Purification of raw gases
The Liquid nitrogen wash is mainly used to purify and prepare ammonia
synthesis gas within fertilizer plants. It is usually the last purification step upstream of
ammonia synthesis.
The liquid nitrogen wash has the function to remove residual impurities like
CO, Ar and CH4 from a crude hydrogen stream and to establish a stoichiometric
ratio H2/N2 = 3:1. Carbon monoxide must be completely removed, since it is
poisonous for the ammonia synthesis catalyst. Ar and CH4 are inert components
enriching in the ammonia synthesis loop. If not removed, a syngas purge or
expenditures for purge gas separation are required.
If partial oxidation of coal, heavy oil or residue oil were selected as feedstock
basis for ammonia production then liquid nitrogen wash is typically arranged to
downstream of the scrubbing process.
Traces of water, carbon dioxide, solvent (methanol) are removed in the
adsorber station. Center piece of the liquid nitrogen wash is the so called ―coldbox‖.
The process equipment of the cryogenic separation is installed close-packed in the
coldbox, which is covered with a metal shell. The coldbox voidage is filled with
insulation material (perlite) to prevent heat input.
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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Raw hydrogen and HP nitrogen are fed to the liquid nitrogen wash unit. Both
streams are cooled down against product gas. Feeding raw hydrogen to the
bottom of the nitrogen wash column and some beforehand condensed liquid to the
top. Trace components are removed and separated as fuel gas. To establish the
desired H2/N2 ratio, HP nitrogen is additionally admixing inside and outside the
coldbox.
Reaction
N2(g) + 3H2(g) 2NH3(g) ΔH = - 22.0kcals
Manufacture
The method was first developed by Haber and Bosch therefore known as
Haber and Bosch Process. The manufacture of ammonia is carried out by passing
mixture of pure hydrogen and nitrogen in the proportion of 3:1 by volume under
pressure (100-1000atm depending on conversion required). Both the gases are sent
through filter to remove compression oil and additionally through the high
temperature guard converter in which CO and CO2 are converted to CH4, and also
removal of traces of H2O, H2S, P and As. The relatively cool gas is added along the
outside of converter tube walls to provide cooling. Carbon steel is used as material
of construction for pressure vessel and internal tubes.
Figure: Purification of raw gases
Feed gas (cold)
LP-N2
for stripping
LP-N2
Purified gas
Fuel gas
HP-N2
Steam
Absorber unit Cold box
He
at
ex
ch
an
ge
r
Ab
so
rb
er
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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The preheated gas flows next through the inside of the tube which contains
promoted iron catalyst at 500-5500C. The NH3 product, with 8-30% conversion
depending on a process conditions, is removed by condensation, first with water
cooling and then NH3 refrigeration. The unconverted N2-H2 mixture is recirculated to
allow an 85-90% yield.
Block diagram of manufacturing process
Diagram with process equipment
Animation
Water
Water
Heating coil
Liquid Ammonia
Liquid Ammonia
Water
Wa
ter
Separators
Filter
NH3 GasL
iqu
id N
H3
Recycled Gas 300 atm
300 atm 1 Vol N2 + 3 Vols H2
Figure: Manufacturing of Ammonia by Haber Process
compresor
Co
nd
en
se
r
Condenser
Co
nd
en
se
r
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
N P T E L
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Cost is greatly influenced by the pressure, temperature, catalyst, purity of raw
materials and most importantly heat recovery and reuse. For achieving quality
material at lower cost modification in Haber and Bosch Process are initiated.
(b) Modern method/ Killogg ammonia process
Raw material
Natural gas
Air
Reaction
2CH4 + O2 2CO + 4H2
2CO + O2 2CO2
N2 + 3H2 2NH3
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
In the process natural gas is used for production of nitrogen and hydrogen.
The purified nitrogen and hydrogen is thus reacted to give ammonia gas. In
commercial production sulfur free natural gas is mixed with steam in the volume
H2
Reactor for organic sulphur hydrogenation
Naturalgas
Natural gas heater
H2O
Hydrogen sulphide adsorber
Heatexchanger
Natural gas
Air
Tube- type furnace(methane convertor)
Furnace
Air
Shaft methaneconvertor
2nd - stage CO
convertor
Steam boiler
Ste
am
b
oile
r
Heatexchanger
Air cooler
Steamturbine
1st - stage CO convertor
CO2
regenerator
Methanator
COabsorber
Tu
rbo
c
om
pre
ss
or
wit
h g
as
tu
rbin
e
NH3
NH3
primaryseparator
Co
ld h
ea
t e
xc
ha
ng
er
Ammoniacooler
Secondaryseperator
Air Cold
Ho
t h
ea
t e
xc
ha
ng
er
Ste
am
bo
ile
rw
ate
r h
ea
ter
Pla
nt
sy
nth
es
is c
olu
mn
Figure: Manufacturing of Ammonia By Kellogg Process
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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based ratio of 3.7:1 and compressed to 40atm. The mixture is preheated with the
recycled flue or effluent gases and fed into the furnace. At 800-8500C in the
presence of iron catalyst promoted with other metal oxides conversion of methane
takes place with the formation of CO. The residual gas is mixed with air and fed into
shaft converter to get complete conversion. The waste heat is utilized for the steam
generation and ethanolamine which are used in CO2 and H2S removal. The exit gas
containing poison was regenerated in the methanator at 280-3500C which ultimately
used for heating the feed water.
Purified N2 and H2 mixture was compressed to 300atm at 320 to 3800C in the
presence of catalyst converted to NH3. 14-20% conversion per pass was achieved.
NH3 condensed and separated from exit gas, whereas unconverted N2 and H2 gases
were recycled along with the fresh gases.
Ammonia synthesis is being exothermic the process requires an effective
temperature control system at every stage of reaction.
(c) Modified Haber Bosch process
The manufacture of ammonia may be carried out by the partial oxidation of
hydrocarbon derived from naphtha, natural gas or coal by oxygen enriched air in
the presence of catalyst. CO is removed by passing through ammonical solution of
cuprous formate. The remaining N2 and H2 gas are utilized for the manufacture of
ammonia by Haber process.
Modified Haber Bosch process has following steps
a) Manufacture of reactant gases
b) Purification
c) Compression
d) Catalytic reaction
e) Recovery of ammonia and recycle of reactant gases
a) Manufacture of reactant gases
Water gas as source of H2 is prepared from coke and steam at 10000C-
14000C. It is cooled and purified by passing through lime and iron oxide coated
wood shavings.
C + H2O CO + H2 ∆H = -38900cal
Producer gas is prepared by passing air through heated coke or coal bed
at10000C-14000C. Resulting CO2 passed through the hot bed of the fuel which
reduced it to carbon monoxide, the nitrogen of the air remains mixed with CO. The
gas is cooled and purified. In both the cases sensible heat of the gases is utilized by
raising steam in waste heat boiler
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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C + 1/2O2 CO ∆H = -28900cal
b) Purification
Both water gas and producer gas are mixed in such a ratio so that after
purification concentration of nitrogen and hydrogen by volume becomes one is to
three (1: 3). The cold mixed gas is mixed with excess of steam, then the hot gases are
passed through horizontal converters containing catalyst consisting of iron oxide
promoted with Cr2O3 and CeO2. The exothermic conversion of CO to CO2 by steam
is carried out at an optimum temperature 4500C by the heat of reaction.
CO + H2 + H2O CO2 + 2H2O ∆H =98,000cals.
The hot mixture of CO2, H2, N2 and CO is cooled by passing through the heat
exchanger then the cooled gas is stored. CO2 is removed by any one method which
is described (Module: 2, Lecture: 2) as method of recovery of CO2
The gases after removal of CO2, are compressed to 200atm pressure, cooled,
and treated in a pressure tower with ammonical solution of cuprous formate
(HCOOCu) which absorbs CO. The resultant gas is mixture of H2 and N2 (3vol: 1vol).
The cuprous formate solution after stripping of carbon monoxide is recycled back to
the process.
c) Compression
The purified N2 and H2 mixture at 200atm pressure is further compressed to
300atm pressure mixed with recycling gas at the same pressure and passed through
oil filters. The compressed gas mixture is then cooled by cold water followed by
refrigeration by liquid ammonia. The recycling gas in the mixed gas contained some
ammonia. This ammonia is liquefied by pressure and refrigeration hence before
allowing the gas mixture to enter into the converter, the liquid ammonia is
separated.
d) Catalytic reaction
The gas mixture then passes into the converter which is made of nickel,
vanadium, chromium steel having 7ft. height and 21 inches diameter. The seamless
cap having 3 inch wall thickness is held by bolts of nickel steel. The converter is fitted
with double coil acting as heat interchanger through the inner tube of which cold
gas mixture passes, and through the outer tube of which passes the hot outgoing
gas mixture. At the base of the coil there is resistance coil for electrical heating. In
the converter there is the contact catalyst chamber consist of three concentric
tubes which contain the granular catalyst.
The compressed gas enters through the inner coil of the heat interchanger.
After passing through the interchanger the gas is heated electrically by the
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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resistance coil and then goes up 1st catalyst chamber, and then down through the
2nd, and lastly up through the last. It then enters the outer coil of the central heat
exchanger, gives up the heat to the incoming gas, and then goes out of the
converter from the top.
e) Recovery of ammonia and recycle of reactant gases
The mixed outgoing gas containing19% NH3 and rest N2 and H2 going out of
the converter is cooled by cold water in the condenser. Major portion of ammonia
liquefies. The liquid NH3 is separated and the unconverted gas mixture containing
some unliquefied NH3 is compressed to 300atm pressure and then mixed with fresh
compressed gas mixture and recycled. A part of the recycled gas is rejected from
time to time to prevent the accumulation of argon and methane. The temperature
in the contact chamber is 5500C.
Kinetics and thermodynamics
N2(g) + 3H2(g) 2NH3(g) ΔH = - 22.0kcals
The highest yield of above reaction can be obtained at high pressure and
low temperature which can be expressed as follows
√
Where, the equilibrium constant is an inverse function of the absolute
temperature
∆F= -RT ln Kp = -19000 + 9.92T ln T + 1.15 X 10-3T2 - 1.63 X 10-6T3 - 18.32T
The reaction is exothermic and similar to oxidation of SO2 is favoured by low
temperature from equilibrium stand point but reaction kinetics dictate a
compromise temperature at some higher value like 500 - 5500C in single stage
convertor.
The cost of high pressure reaction system is higher so multistage operation as
used with SO2 oxidation is not economically feasible for ammonia production.
The design problem thus reduces to an optimization of space velocity based
on the following considerations.
The fraction of NH3 (x) in the exit gas decreases with increase in flow rate or
space velocity by equation
x = fV-n
Module: 2 Lecture: 6 Ammonia
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Where,
n<1 if bed is at correct temperature and mass transfer rates are improved
n>1 where bed is at too low temperature because of high velocity gas
cooling
The space time yield (Y) is
( )( )
Y = V.V-n = V(1-n)
In addition to very high space velocity, cooling the bed will increases the cost
of NH3 recovery because x is lower and also increases the pumping cost hence
based on these considerations an optimized cost is calculated.
Catalyst development
Iron oxide promoted by alkali is widely used as catalyst or nonferrous metal
oxides such as K2O (1-2%) and Al2O3 (2-5%). The iron oxide is fused in an electric
furnace and the promoters added. The solidified mass is ground to desired particle
size. The iron oxide is reduced to porous iron in the start-up phases of operation in the
synthesis reactor. There is a maximum operating temperature of about 6200C, above
which the catalyst fuses.
A promoted iron catalyst has recently been developed in Europe (Mont Cenis
process) which allows for very low temperature (4000C) and low pressure operation
(100atm). The life of the catalyst is not firmly established.
Process design modifications
The pressure affects conversion, recirculation rates and refrigeration of the
process. The various process used with different process parameter are as follows
Very high pressure (900-1000atm, 500-6000C, 40-80% conversion) — Claude,
Du pont, L‘air liquide
High pressure (600atm, 50000C, 15-25% conversion) — Casale
Moderate pressure (200-300atm, 500-5500C, 10-30% conversion) — Haber
bosch, Kellogg, Fauser, Nitrogen Engineering Corporation
Low pressure (100atm, 400-4250C, 8-20% conversion)
Mont Cenis: uses a new type of iron catalyst promoted iron cyanide.
The modern trend is towards lower pressure and increased recirculation loads
because of the relatively high cost of pressure vessels. The large single train plants
Module: 2 Lecture: 6 Ammonia
Dr. N. K. Patel
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using centrifugal compressors and having capacities as high as 1000 tons/day from
a single reactor at low production cost are used widely.
PROPERTIES
Molecular formula : NH3
Molecular weight : 17.031gm/mole
Appearance : Colourless gas
Odour : Strong pungent
Boiling point : -33.340C
Melting point : -77.730C
Density : 681.9kg/m3 at −33.30C (liquid)
Solubility : Soluble in water
USES
Ammonia is major raw material for fertilizer industries
It is used during the manufacture of Nitro compounds, Fertilizers e.g. urea,
ammonium sulfate, ammonium phosphate etc.
It is also used in manufacture of Nitric acid, Hydroxylamine, Hydrazine, Amines
and amides, and in many other organic compounds
It is also used in heat treating, paper pulping, as explosives and refrigerants
Module: 2 Lecture: 7 Acetylene
Dr. N. K. Patel
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Module: 2
Lecture: 7
ACETYLENE
INTRODUCTION
Acetylene (C2H2) is colorless gas used as a fuel and a chemical building
block. As an alkyne, acetylene is unsaturated because its two carbon atoms are
bonded together in a triple bond having CCH bond angles of 1800. It is unstable in
pure form and thus is usually handled as a solution. Pure acetylene is odorless, but
commercial grades usually have a marked odor due to impurities.
In 1836 acetylene identified as a "new carburet of hydrogen" by Edmund
Davy. The name "acetylene" was given by Marcellin Berthelot in 1860. He prepared
acetylene by passing vapours of organic compounds (methanol, ethanol, etc.)
through a red-hot tube and collecting the effluent. He also found acetylene was
formed by sparking electricity through mixed cyanogen and hydrogen gases.
Berthelot later obtained acetylene directly by passing hydrogen between the poles
of a carbon arc.
MANUFACTURE
Acetylene manufacture by following processes
1. From calcium carbide
2. From paraffin hydrocarbons by pyrolysis (Wulff process)
3. From natural gas by partial oxidation (Sachasse process)
Nowadays acetylene is mainly manufactured by the partial oxidation of
natural gas (methane) or side product in ethylene stream from cracking of
hydrocarbons. Acetylene, ethylene mixture is explosive and poison Zigler Natta
catalyst. There so acetylene is selectively hydrogenated into ethylene, usually using
Pd-Ag catalysts.
Acetylene was the main source of organic chemicals in the chemical industry
until 1950. It was first prepared by the hydrolysis of calcium carbide, a reaction
discovered by Friedrich Wöhler in 1862.
CaC2 + 2H2O Ca(OH)2 + C2H2
Module: 2 Lecture: 7 Acetylene
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Calcium carbide production requires extremely high temperatures, ~20000C,
necessitating the use of an electric arc furnace.
Also hydrocarbon cracking is carried out in an electric arc furnace. In which
electric arc provides energy at very high flux density so that reaction time can be
kept at a minimum. There so the design of the electro-thermal furnace is one of the
important factors.
In one design (Huels process) gaseous feedstock enters the furnace
tangentially through a turbulence chamber, then passes with a rotary motion
through pipe in which the arc is passed between a bell shaped cathode and anode
pipe. The rotary motion of the gas causes the arc to rotate and thus reducing
fouling. The arc is operated at 8000kw D.C. at 7000volts and 1150amp cathodes are
said to last 800hours while anodes only 150hours.
In other design, fresh hydrocarbon and recycle gas are fed to the arc. The
effluent reaction gases are quenched and purified. 35%w purified acetylene along
with 17%w ethylene and 10%w carbon black, H2 and other products in minor amount
is obtained in one pass through furnace.
The difference is that the arc is rotated by means of an external magnetic
coil, and quenching is carried by propane and water in 1st and 2nd step respectively.
Some propane cracking improves the yield of acetylene. The propane quench
cools the arc gases to 10950C in 0.0001 to 0.0004 sec while the water quench cools
the mixture to 3000C in 0.001 to 0.003 sec. Power consumption is 12.36kwhr/kg of
pure acetylene. 21-22%v acetylene is obtained in the product gases.
1. From calcium carbide
Raw materials
Basis: 1000 cu ft. acetylene
Calcium carbide (85%) = 100kg
Water = 815kg
Sources of raw material
Calcium carbide is manufactured from lime and coke in 60:40 ratio in electric
furnace at 2000-21000C temperature.
Reaction
CaC2 + 2H2O Ca(OH)2 + C2H2 ΔH = - 32.5kcals
Manufacture
Block diagram of manufacturing process
Module: 2 Lecture: 7 Acetylene
Dr. N. K. Patel
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Diagram with process equipment
Animation
There are two methods for the manufacture of acetylene from calcium
carbide
Wet process
Dry process
In the wet process, calcium carbide is added to large quantity of water
releasing acetylene gas and calcium hydrate as residue. Later is discharged in the
form of lime slurry containing approximately 90% water.
In the dry process, in order to eliminate the waste of calcium hydrate equal
amount of water is added to CaC2 (1:1 ratio) in a generator. The heat of reaction
(166 Btu/ft3 of acetylene) is used to vaporize the excess water over the chemical
equivalent, leaving a substantially dry calcium hydrate which is suitable for reuse as
a lime source. The temperature must be carefully controlled below 1500C at 15psi
pressure throughout the process because the acetylene polymerizes to form
benzene at 6000C and decomposes at 7800C. Further with air-acetylene mixture
explodes at 4800C.
The crude acetylene gas containing traces of H2S, NH3 and phosphine (PH3)
form generator is either scrubbed with water and caustic soda solution or sent to
purifier where the impurities are absorbed by the use of iron oxide or active chlorine
compounds. The dry gas is fed to cylinders or sent to manufacturing units.
Safety and handling
Acetylene is not especially toxic but when generated from calcium carbide it
can contain toxic impurities such as traces of phosphine and arsine. It is also highly
Gas holder
Water
Acetylene
Waste
Cacium Hydroxide
Calcium carbide
Figure: Manufacture of Acetylene by using Calcium carbide
Water
Scrubber
Dryer
Generator
Module: 2 Lecture: 7 Acetylene
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flammable. Concentrated or pure acetylene can easily react in an addition-type
reaction to form number of products like benzene, vinyl acetylene etc. These
reactions are exothermic and unlike other common flammables do not require
oxygen to proceed. Consequently, acetylene can explode with extreme violence if
the absolute pressure of the gas exceeds about 200kPa (29 psi). The safe limit for
acetylene is 101kPag or 15 psig. That so it is shipped and stored by dissolving in
acetone or dimethylformamide (DMF), contained in a metal cylinder with a porous
filling.
2. From paraffin hydrocarbons by pyrolysis (Wulff process)
Raw materials
Basis: 1000kg acetylene (100%)
Natural gas = 262000Sef
Steam (600psig) = 26308kg
Electricity =140kWH
Cooling water =25000gal
Process water =200gal
Solvent (make up) =2.95kg
Reaction
C4H10 C2H2 + C2H4 + CO + H2
C2H4 C2H2 + H2
2CH4 C2H2 + 3H2 ΔH = + 96.7kcals
Manufacture
Block diagram of manufacturing process
Sta
bili
zer
Str
pp
ing
Co
loum
n
Dimethyl formamide
Solvent containing
diacetylene
Acetylene
Solvent to purification and re-use
Off Gas
Dilute Steam
Fuel Gas
Hydrocarbon feed
Air
Stack Gas
Cooling Water
Excess Fuel Gas
Tar
Tar
Figure: Manufacture of Acetylene by Wulff Process
Furnace
Electrostatic
PrecipitatorTar trap
Compressor
Ab
so
rbe
r
Ab
so
rbe
r
Boiler
Module: 2 Lecture: 7 Acetylene
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Diagram with process equipment
Animation
In the Wulff process acetylene may be produced by thermal decomposition
of hydrocarbons such as methane, ethane, propane, butane, ethylene etc.
Pyrolysis is carried out in the Wulff regenerative furnace which is a rectangular
steel box filled with refractory bricks checker work. Before the hydrocarbon feed is
sent to the chamber, it is diluted with steam (up to 1:8 ratios). The feed is carried
through the chamber at sub-atmospheric pressure by virtue of a large vacuum
pump. Which reduced the residence time as little as 0.03 sec; cracked gas leaves
the chamber at about 370°C. The maximum temperature in the furnace just after
the heating cycle approaches 1315°C. The furnace is operated in four minutes cycle
in which the checker work is first heated for one minute and then feed gas pyrolyzed
for one minute. The same sequence of operation is then done in reverse direction
through the furnace. To facilitate reversal of the gas flow, fuel gas burners and
hydrocarbon feed pipes are located on each side of the combustion chamber. To
allow continuous flow of cracked gases to the purification train, two furnaces are
usually operated on staggered cycles. Cooled cracked gases from the chamber
are then further quenched in a tar trap, where steam and various tars are removed.
The gas is compressed to atmospheric pressure, passed through a knock-out forum
and electrostatic precipitator, and sent to the recovery system.
Usually diacetylene and acetylene are separated by absorption in DMF
(dimethyl formamide). By proper adjustment of solvent ratio and temperature,
diacetylene may be removed in the first scrubbing column. In the acetylene
absorber small quantities of ethylene, CO2 and higher acetylenes are also absorbed.
Most of the acetylene-free off-gases used for various heat exchanging operation
like used as fuel for the steam boilers, for combustion chamber heating etc.
Acetylene rich solution is sent to stabilizer, where less soluble components are
removed by stripping. Acetylene is then removed from the solvent in a second
stripping column. The solvent is readied for reuse by stripping out high boilers by
blowing with off gas from the acetylene absorber-followed by rectification.
Usually, off-gas from the acetylene recovery system is used as fuel for heating
the combustion chamber. The volume of off-gas is much more than is required for
fuel, so it may be either recycled to the furnace or used as a raw material for some
other operation.
Yield of acetylene (98.5 to 99.3 % purity) varies with the hydrocarbon feed
stock used. Average yields for the once-through process are 22.5 kg per 100kg
Module: 2 Lecture: 7 Acetylene
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methane, 38.6 kg per 100kg ethane, and 35.5kg per 100kg propane. The off-gas is
principally ethylene, carbon monoxide, hydrogen and methane.
Kinetics and thermodynamics
The principal conditions which must be considered are as follows
Energy requirement to convert hydrocarbons to acetylene is very high and
which can be supplied by very high temperature. For paraffinic feedstock, the heat
of formation for a gram atom carbon in acetylene decreases with the increasing
length of the chain and it increases in the case of olefinic hydrocarbons. The heat
of formation is of the order of magnitude required for the dissociation of steam.
2CH4 + 174,000 Btu C2H2 + 3H2
The formation of acetylene begins at a relatively high temperature; in the
case of methane occurs around 8150C. The temperature required decreases with
the increase in the number of carbon atoms of the hydrocarbon feed.
The decomposition of hydrocarbons to carbon and hydrogen begins at
relatively low temperatures. E.g. decomposition of methane occurs at 4500C. Hence,
the decomposition into the elements proceeds in competition with the formation of
acetylene. However, the rate of acetylene formation is greater than that of the
decomposition reaction. There so care should be taken that the hydrocarbon feed
must reach at relatively high temperatures (above 6750C) in the shortest possible
time and then the attained equilibrium must be immediately quenched to about
2850C in order to preserve the acetylene formed. The time interval for the reaction
should be of the order of milliseconds.
3. From natural gas by partial oxidation (Sachasse process)
Raw materials
Basis: 1000kg acetylene (99.5%) plus 340000 Cu ft. off gas (345 Btu/Cu ft.)
Natural gas = 190000 Sef
Oxygen (95%) = 5400kg
Solvent = 2.3kg
Power = 15000kWH
Steam = 4535.9kg
Water (cooling) = 22710liter
Reaction
CH4 + 2O2 CO2 + 2H2O
2CH4 C2H2 + 3H2 ΔH = + 79.8 kcals
Module: 2 Lecture: 7 Acetylene
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Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Acetylene may be produced from a variety of hydrocarbon feed stocks
(natural gas, LPG, naphtha, fuel oil, even crude oil) by high-temperature cracking.
Heat for the cracking operation is developed by partial oxidation of the feed stock
with oxygen. The heat evolved cracks the excess hydrocarbon to acetylene. After
rapid quenching with water, the acetylene is separated from the gas stream by
absorption-desorption in a suitable solvent. The process is known as Sachasse
process using natural gas as raw material.
Natural gas (1mole) and low purity oxygen (0.65moles 95%O2) are preheated
separately to 5100C and fed to a specially designed burner.
The converter is vertical cylindrical unit built in three sections
Mixing chamber
Flame room
Quench chamber
After rapid and through mixing of oxygen and methane in the mixing
chamber, the gases are fed to the flame room through the portal in a burner block
designed to prevent back travel or blow-off. The heat of combustion heats the
gases to 15500C to allow cracking of the excess methane to acetylene. The
residence time is 0.001 to 0.01 seconds. The decomposition of acetylene is
prevented by rapid quenching of the resulting gases with water to 380C. The cooled
Rec
tify
ing
Co
lum
n
Oxygen
Natural Gas
Water
Recycle Water
Water
Soot Filter
Soot
Off Gas
Acetylene
PolymerSolvent
Cooler
Str
ipp
er
Filter
Figure: Manufacturing of Acetylene from Natural Gas
Re
ac
tor
Preheater
Preheater
Module: 2 Lecture: 7 Acetylene
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effluent gases on the dry basis contain 8% acetylene, 54% H2, 26% CO, 5% CH4, 4%
CO2 and 3% N2 and higher acetylenes. These gases are run to a filter where using
carbon black, acetylene of 99.5% or higher purity is produced (23.5kg/1000kg of
acetylene is separated and purified in a manner as described for the Wulff process).
PROPERTIES
Molecular formula : C2H2
Molecular weight : 26.04gm/mole
Appearance : Colourless gas
Odour : Odourless gas
Boiling point : -840C (sublimation point)
Melting point : -80.80C @1.27atm
Density : 1.097kg/m3
Solubility : Soluble in acetone and DMF
It is transported under high pressure in acetone soaked on porous material
packed in steel cylinders
It is lighter than air
It is somewhat poisonous in nature
It burns with luminous flame and forms explosive mixture with air
USES
In the chemical manufacture of acrylonitrile, vinyl chloride, vinyl acetate,
acrylates etc.
In manufacture of acetaldehyde, trichloroethylene, acetic acid, polyvinyl
alcohol, perchloroethylene etc.
In manufacture of propagryl alcohol, butyrolactone, vinyl pyrrolidine etc.
In metallurgy industries for welding and cutting
Module: 3 Lecture: 8 Sodium chloride
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Module: 3
Lecture: 8
SODIUM CHLORIDE
INTRODUCTION
Sodium chloride (NaCl), also known as salt, common salt, table salt or halite,
is an ionic compound. Salt is most widely distributed inorganic compound
throughout the world. It is responsible for the salinity of the ocean and of the
extracellular fluid of many multi-cellular organisms.
It is a part of human food and life without salt is probably impossible. Other
living beings, such as animals, also require salt for their growth. In India, about 70% of
the salt is consumed by human being and rest 30% is used in the manufacture of
chemicals.
Salt is the basic raw material for the caustic soda and chlorine, soda ash
(sodium carbonate), sodium sulfate, hydrochloric acid etc. Salt is also used in a
large number of other industries, such as hydrogenation of oil, manufacture of soap,
dyes, textile, food processing etc.
SOURCES OF SODIUM CHLORIDE
1. Sea Water
As the India has one of the largest seashore in the world, salt manufacture
sites are spread throughout the country. Main salt manufacturing centers are
Gujarat, Maharashtra, Tamilnadu, Kerala, Andhra Pradesh, Karnataka, Orissa
and West Bengal. About 70% of the total salt production comes from sea
water.
2. Salt Lakes
There are two important salt lakes in India. Sambhar lake in Rajasthan and
Chilka lake on eastern coast. Sambhar lake produce more than 2.5 lakh tones
of common salt every year.
Module: 3 Lecture: 8 Sodium chloride
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3. Sub Soil Water
It contains more salt than the sea water that is why, is becoming an important
source of salt in the country. Leading salt manufacture sites form sub soil
water are Kharagoda, Didwana, Dharangadhra and Tucticorin.
4. Rock Salt
Rock salt is used during religious festivals mainly produced in Mandi (Himachal
Pradesh).
MANUFACTURE
Salt obtained from above sources 1, 2, or 3 is in solution or liquid form. This
form is called as brine. The various methods used for concentrating the brine
solutions are
1. Solar Evaporation
2. Artificial Evaporation
3. Freezing method
1. Solar Evaporation
It is the cheapest and best method of manufacturing salt from the brines. This
method has widely been used in India.
Block diagram of manufacturing process
Diagram with process equipment
Figure: Manufacturing of Sodium Chloride by Solar Evaporation
Grainer pan
Sodium Chloride
Recirculating Brine
Calcium Sulfate
Lake Salt /Sub soil Salt/ Sea water
Redissolution
Tank
Dryer
Screens
FlasherField Solar evaporator
Sun Graveller
Centrifuge
Module: 3 Lecture: 8 Sodium chloride
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Animation
Sea brine (3-3.5°Be) is first conveyed to a reservoir through channels to store
brine, remove suspended impurities and to concentrate the brine from original, 3-
3.5°Be to about 10°Be by solar energy. The reservoir is usually kept at a certain
height, where brine flows under gravity. The 10°Be brine is again concentrated to 25°
Be brine by solar evaporation by passing it to condensers through the channels. Due
to evaporation of water from brine the solution gradually concentrates and different
impurities separate out at different concentration as follows
At 7.5°Be ferrous iron present separates out as ferric oxide
At 10°Be, calcium carbonate precipitates out
At12-25°Be, calcium sulfate precipitated out
At the 250Be brine from the condensers is now passed on to the crystallizes,
where salt crystallizes from 25.4°Be to 30°Be and other impurities also start separating.
Salt form the solar evaporation method may be purified by dissolving it in
purified brine or water then crystallize in grainer which is open pan having 15-20ft
width, 150-200ft length and 2ft depth. Beneath the submerged coils is a system of
reciprocating rakes for the salt removal. Evaporation takes place in grainer at 95-
1000C. Flat hopper shaped crystals formed on the surface and then fall to bottom of
the grainer, where the crystal grow further before removed by rack system. The wet
crystal are centrifuged, dried and screened. 99.98% NaCl can be obtained, if the
incoming brine treated properly.
The mother liquor (bittern) is separated for the recovery of other by products.
The main constituents of bittern are NaCl, MgCl2, MgSO4, KCl and Br2.
Major engineering problem
The factors which influence solar evaporation are as under
Absorption of solar energy
Air humidity
Temperature
Wind velocity
Suitable dyes or black sand are used to increase the rate of absorption of
solar energy and thereby increase the evaporation rate. Suitable soil stabilization is
necessary for the open brine condensers and crystallizers to reduce seepage and
increasing their bearing strength for mechanical harvesting of salt crystals. In
modern salt farms soil is stabilized by blending the soil with salt, gypsum, bentonite
Module: 3 Lecture: 8 Sodium chloride
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and lined with bricks, bituminous plaster and plastic films. Heavy mechanical salt
harvesters are used for gathering salts.
2. Artificial Evaporation
Raw material
Saturated brine = 3450kg
Soda ash (58%) = 3.5kg
Caustic soda (50%) = 0.375kg
Steam = 1135kg (for triple effect evaporator)
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
In cold countries, where solar evaporation is not possible on a large scale,
artificial evaporation method is used. Combination of solar evaporation as well as
artificial evaporation methods is also used in France and Germany.
BrineMud
Sodium Chloride
Hydrogen Sulfide
Brine
Air
Caustic Soda
Soda ash
Brine
Cl2
Aerator
Settling
Tank
MixerFilter
Purified Brine
Washer
Dryer
Screens
Figure: Manufacturing of Sodium Chloride by artificial evaporation
Multiple
effect
Evaporator
Module: 3 Lecture: 8 Sodium chloride
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Artificial evaporation was carried out in open pans but it is now carried out in
vacuum pans, known as vacuum evaporation methods. It this method, brine is
boiled under reduced pressure in vacuum pan in order to get cubical grains of salt.
Brine is first aerated to remove H2S. Addition of chloride removes the last
traces of H2S and oxidize ferrous ion to ferric ion. Then brine sent to settling tank
where it is treated with dilute solution of caustic soda and soda ash to remove most
of calcium, magnesium and ferric ions. Purified brine is pumped to the vacuum
pans, where calcium sulfate is removed as a result of counter current flow and
hydraulic washing with brine. The vacuum pan evaporators are usually triple effect
evaporators made of cast iron steel sheets and copper tubes. Salt slurry is
continuously drawn from each evaporator through the salt leg at the bottom of
which brine is feed so that the salt slurry is washed by incoming brine, thus washing
back the impurities into the pans. The salt slurry is then conveyed to a cone shaped
tank from where it passes to feed tank for dewatering and drying. The filtered and
partially dried salt from the feed tank finally goes to a rotary drier for final drying. The
lumps of the dried salt are removed from fine dry crystals by passing through a
scalping screen. The salt is then conveyed to storage bins, where it is screened, sized
and packed.
Free flowing table salts are made by blending 0.5-2% magnesium carbonate,
hydrated calcium silicate or tricalcium phosphate with the salt. Iodized salt after
blending contains 0.01% potassium iodide, 0.1% sodium carbonate as stabilizer and
0.1% sodium thiosulfate.
3. Freezing Method
In some countries, salt is also manufactured by freezing the brine, but it is not
a common method.
PROPERTIES
Molecular formula : NaCl
Molecular weight : 58.44gm/mole
Appearance : White crystal
Odour : Odourless
Boiling point : 14130C
Melting point : 8010C
Density : 2.165gm/mL
Solubility : Soluble in water
USES
In chlor –alkali industries
In manufacture of chemical like caustic soda and chlorine, soda ash, sodium
sulfate, hydrochloric acid etc.
Module: 3 Lecture: 8 Sodium chloride
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In manufacture of soap, dyes,
Used in textile, food processing, pharmaceutical industries
High way ice and snow removal
Used in fire extinguisher
Used in house hold food preparation.
Module: 3 Lecture: 9 Sodium carbonate
Dr. N. K. Patel
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Module: 3
Lecture: 9
SODIUM CARBONATE
INTRODUCTION
Sodium carbonate (Na2CO3) also known as washing soda or soda ash, is a
sodium salt of carbonic acid. Most commonly occurs as a crystalline heptahydrate,
which readily effloresces to form a white powder, the monohydrate. Sodium
carbonate is domestically well known as a water softener. It can be extracted from
the ashes of many plants. It is synthetically produced in large quantities from salt and
limestone in a process known as the Solvay process.
Soda ash is the most important high tonnage, low cost, reasonably pure,
soluble alkali available to the industries as well to the laboratory.
MANUFACTURE
Sodium carbonate is manufactured by following process.
1. Leblanc process.
2. Solvay‘s ammonia soda process.
3. Dual process (modified Solvay‘s process)
4. Electrolytic process.
1. Leblanc process
The process has only historical importance, because is now been replaced
completely by Solvay process or modified by Solvay process.
Raw materials
Basis: 1000kg Sodium carbonate (98% yield)
Common salt = 1126kg
Sulfuric acid = 945kg
Lime stone = 963kg
Coke = 463kg
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Module: 3 Lecture: 9 Sodium carbonate
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Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reactions
NaCl + H2SO4 NaHSO4 + HCl
NaHSO4 + NaCl Na2SO4 + HCl
Na2SO4 + 4C Na2S + 4CO
Na2S + CaCO3 Na2CO3 + CaS
(Black ash sludge)
CaS + H2O + CO2 CaCO3 + H2S
CaS + H2S Ca(HS)2
Ca(HS)2 + CO2 + H2O CaCO3 + 2H2S
H2S + O H2O + S
Manufacture
Mixer
Pulverizer
Mixer
Crusher
Leaching
NaCl
Hot gas
Water
Flue gases
Na2CO3
HCl
Concentrated H2SO4
Lime Stone Coke
Salt Cake
Furnace
Black ash Rotary
Furnace
Cooler
Calcination tower
Figure: Manufacturing of Sodium Carbonate by Lablance process
Tower
Water
NaCl
Furnace
Open pan Evaporator
Water in
Water out
Module: 3 Lecture: 9 Sodium carbonate
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Block diagram of manufacturing process
Diagram with process equipment
Animation
Common salt is first mixed with the conc. H2SO4 in equivalent quantities and
heated in a cast iron salt cake furnace by flue gases from adjacent coal of fire.
NaHSO4 along with HCl gas is formed. HCl is passed to tower packed with coke and
is absorbed through a spray of water comes down in the tower. The paste of
NaHSO4 is taken out and heated to a high temperature on the hearth of a furnace
along with some more common salt. NaHSO4 is thus converted into sodium sulfate,
known as salt cake.
The salt cake is broken or pulverized, mixed with coke and limestone and
charged into black ash rotary furnace consisting of refractory lined steel shells. The
mass is heated by hot combustion gases entering at one end and leaving at the
others. The molten porous gray mass thus formed known as black ash is separated
from the calcium sludge and then crushed and leached with water in absence of
air in a series of iron tank.
The extract containing Na2CO3, NaOH, and other impurities is sprayed from
the top of a tower in counter current to flow of hot gases from the black ash
furnace. The sodium carbonate thus obtained is concentrated in open pans and
then cooled to get sodium carbonate. The product is calcined to get soda ash
which is re-crystallized to Na2CO3.10H2O. The sludge containing mostly CaS is left
behind as alkali waste.
The liquor remaining after removal of first batch of soda ash crystals is purified
and then causticized with lime to produce caustic soda.
Recovery of sulfur from alkali waste
Alkali waste is charged into cylindrical iron vessels arranged in series and CO2
delivered from lime kilns is passed through it, the H2S gas thus obtained is then
conduced together with a regulated amount of air in a Claus kiln containing iron
oxide as catalyst. The exothermic reaction proceeds without further external heat.
Recovered sulfur is used in the manufacture of sulfuric acid.
2. Solvay's ammonia soda process
Raw materials
Basis: 1000kg sodium carbonate
Salt = 1550kg
Limestone = 1200kg
Module: 3 Lecture: 9 Sodium carbonate
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Coke = 90kg
Ammonia as a catalyst = 1.5kg (Loss)
High pressure steam = 1350kg
Low pressure steam = 1600kg
Cooling water = 40000 - 60000kg
Electric power = 210KWH
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reactions
CaCO3 CaO + CO2 ΔH = + 43.4kcals
C(s) + O2 (g) CO2 (g) ΔH = - 96.5kcals
CaO(s) + H2O (l) Ca(OH)2 (aq) ΔH = - 15.9kcals
NH3(aq) + H2O(l) NH4OH(aq) ΔH = - 8.4kcals
2NH4OH + CO2 (NH4)2CO3 + H2O ΔH = - 22.1kcals
(NH4)2CO3 + CO2 + H2O 2NH4HCO3
NH4HCO3 + NaCl NH4Cl + NaHCO3
2NaHCO3 Na2CO3 + CO2 + H2O ΔH = + 30.7kcals
2NH4Cl + Ca(OH)2 2NH3 + CaCl2 + 2H2 ΔH = + 10.7kcals
Overall reaction
CaCO3 + 2NaCl Na2CO3 + CaCl2
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Process equipment
Animation
Module: 3 Lecture: 9 Sodium carbonate
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Ammonia is dissolved in a salt solution and ammoniate brine solution is
allowed to react with CO2 which is obtained by calcining lime stone with coke. A
precipitate of NaHCO3, thus obtain is then calcined to produce high purity Na2CO3.
Preparation and purification of brine
Saturated solution of NaCl is used. Brine contains impurities such as calcium,
magnesium and iron compounds. To remove calcium sulfate, magnesium and iron
salts sodium carbonate and sodium hydroxide are added. The precipitated
carbonates and hydroxide are removed by filtration. Sometimes sulfate are
removed with BaCl2 or the hot brine is treated with OH¯ and CO3-2 ions. The calcium,
magnesium and iron salts from saturated brine may be precipitated by dilute
ammonia and CO2 in a series of washing towers. The brine is purified by allowing it to
settle in vats, as a result of which precipitated CaCO3, MgCO3, Mg(OH)2 and iron
hydroxide settle down and pure brine solution is pumped to the ammonia absorber
tower, where it dissolve NH3 with the liberation of heat.
Brine Tank
Ammonia absorbing tank
Cold water
NH3 (+ CO2)Saturation tank
Settling Tankfor ammonical brine
Ammonical brine
Gas outlet to absorbers
Cooling coils
CO2
Filter Calcination tower
CaO
Ca(OH2)
Va
cc
um
fi
lte
r
Ca
rbo
na
tio
n t
ow
er
Na2CO3pump
Figure: Manufacturing of Sodium Carbonate by Solvay's Process
Water
Water out
Am
mo
nia
re
co
ve
ry
tow
er
Water out
Water in
Module: 3 Lecture: 9 Sodium carbonate
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Ammoniation of brine
The purified brine is allowed to percolate down the ammonia tower in which
ammonia gas is passed through the bottom in a counter current fashion. The brine
solution thus takes up the necessary amount of ammonia and liberates heat. The
gas which escapes solution in the tank is absorbed by the brine falling down the
tower. Some carbon dioxide is also absorbed by ammonia, as a result of which
some insoluble carbonate is also precipitated. The ammoniated brine is allowed to
settle, coded to about 30°C and pumped to the carbonating tower.
Carbon dioxide formation
Limestone is calcined to get CO2 in a lime kiln filled with coke. As a result of
burning of coke necessary heat required for the decomposition of lime stone is
generated. CaO obtained from the lime kiln is converted into slaked lime and
pumped to the ammonia recovery tower.
Carbonation of ammonium brine
CO2 from the lime kiln is compressed and passed through the bottom of
carbonating tower down which ammoniated brine percolates. Carbonating towers
operated in series with several precipitation towers are constructed of cast iron
having 22-25meter height, 1.6-2.5meter in diameter. During the precipitation cycle,
the temperature is maintained about 20-25°C at the both ends and 45-55°C at the
middle by making use of cooling coils, provided at about 20ft above the bottom.
The tower gradually becomes flooded as sodium bicarbonate cakes on the cooling
coils and shelves. The cooling coils of the foulded tower are shut off. Then the fresh
hot ammoniated brine is fed down the tower in which NaHCO3 are dissolved to form
ammonium carbonate solution. The solution containing (NH4)2CO3, unconverted
NaHCO3 is allowed to fall down a second tower, called making tower. The making,
towers are constructed with a series of boxes and sloped baffles. Ammoniated brine
and CO2 gas (90-95%) from the bicarbonate calciner is recompressed and pumped
to the bottom of the making tower. The ammonium carbonate first reacts with CO2
to form ammonium bicarbonate and the latter reacting with salt, forms sodium
bicarbonate. The heat of exothermic reaction is removed by cooling coils.
Filtration
NaHCO3 slurry is then filtered on a rotary vacuum filter which helps in drying of
bicarbonate and in recovering ammonia. The filter cake after removal of salt and
NH4Cl by washing with water, sent to a centrifugal filter to remove the moisture or
calcined directly. During washing, about 10% NaHCO3 also passes into filtrate. The
filtrate containing NaCl, NH4Cl, NaHCO3 and NH4HCO3 is treated with lime obtained
from lime kiln to recover NH3 and CO2.
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Calcination
NaHCO3 from the drum filter is calcined at about 200°C in a horizontal
calciner, which is either fired at feed end by gas or steam heated unit. The heating
being through the shell parallel to the product, which prevent the formation of
bicarbonate lumps.
The hot soda ash form the calciner is passed through a rotary cooler and
packed in bags. The exit gases (CO2, NH3, steam etc.) are cooled and condensed
to get liquid ammonia; the rich CO2 gas is cooled and returned to the carbonating
tower. The product from the calciner is light soda ash. To produce dense soda ash,
sufficient water is milled with it to form more mono hydrate Na2CO3.H2O and the
mixture is recycled.
Recovery of ammonia
The ammonia is recovered in strong ammonia liquor still, consisting of two
parts. The parts above and below the lime inlet is called as heater and lime still
respectively. The filtrate obtained from washing of NaHCO3 from the pressure type
rotary filter is fed into the heater, where free ammonia and carbon dioxide are
driven off by distillation. Dry lime or milk of lime (slaked lime) obtained from lime kiln is
fed through the lime inlet and mixed with the liquor from the heater. As the liquor
flows down the column, calcium chloride and calcium sulfate are formed and NH3
gas is released.
NH4Cl + Ca(OH)2 CaCl2 + 2NH3 + H2O
(NH4)2SO4 + Ca(OH)2 CaSO4 + 2NH3 + 2H2O
The liquor from the bottom of the lime still is free from ammonia and contains
unreacted NaCl and largely CaCI2, which is disposed off. The liquor is, therefore
allowed to settle in settling ponds and the clear liquid is evaporated till the salt
separates out and is sold as such for calcium chloride or further evaporated.
Kinetics and thermodynamics
The overall reaction shows that salt and calcium carbonate are the only raw
materials which are continuously supplied in the process and that produce sodium
carbonate and calcium chloride
CaCO3 + 2NaCl Na2CO3 + CaCl2
Overall reaction of ammoniation of brine and then treatment of carbon
dioxide to ammoniated brine is as under
2NaCl + 2H2O +2NH3 + 2CO2 2NaHCO3 + 2NH4Cl
Module: 3 Lecture: 9 Sodium carbonate
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The above reaction shows the role of ammonia and carbon dioxide in the
process, and also determines the yield of the final product. There so conditions
favourable to it are precisely defined. For these the reaction is divided in to two
steps.
2NH3 + 2CO2 +2H2O 2NH4HCO3 ---- (a)
2NaCl + 2NH4HCO3 2NaHCO3 + 2NH4Cl ---- (b)
Reaction (a) is undoubtedly favoured by low temperature because it requires
the dissolution of gas in water, is displaced to right by virtue of the fact that reaction
(b), which utilizes the product by subtracting it from (a) is displaced in the same
direction. Consequently, it is the precipitation of NaHCO3 according to (b) which is
the driving force behind the entire method.
The solubilities of the salts at various temperature is as under
Temperature Solubility in gm/litre
NaCl NH4HCO3 NH4Cl NaHCO3
00C 357 120 298 69
200C 358.5 217 374 95.4
300C 359 269 467 109
Above data indicate that precipitation fortunately tends to take place
preferentially with satisfactory yields. On the basis of data and common ion effect
on precipitation of salts, physicochemical conditions most suitable for the forward
step of reaction (b) which causes precipitation of NaHCO3 are as under
To maintain lowest possible temperature in order to lower the solubility off
sodium bicarbonate
To maintain the greatest possible concentration of one or both the salts
appearing on the product side of reaction (b) with the aim of lowering still
further solubility of sodium bicarbonate.
These conditions are nevertheless discerningly applied because they serve to
bring about appreciable increase in the yields of NaHCO3 and permit the most
effective use of most costly reagent NH4HCO3 in reaction (b).
Attention is paid to the fact that, if precipitation temperature is always kept
low, the sodium bicarbonate separates in a microcrystalline form which is with
difficult to filter and it is soluble during subsequent washing on the filter, increase
requirement of NaCl.
Module: 3 Lecture: 9 Sodium carbonate
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Experimentally, the conditions which are most effectively reconcile the
physicochemical aspect of precipitation of sodium bicarbonate economically are
as under
284gm/liter (≈4.9mole/liter) of NaCl reacting with 76gm/liter (≈4.5mole/liter) of
NH3 instead of equimolecular solution of two reagents
Relatively high temperature (60-650C) at the start so as to allow the formation
of well-developed NaHCO3 crystallization seeds and increasing the volume of
these seeds to decrease the solubility of salt with gradual cooling.
Major engineering problem
Absorption units
The absorption units should be constructed to permit the downward travel of
growing sodium bicarbonate crystals. This is done by having each unit simulate a
very large single bubble cap with down sloping floors. The absorption is carried out in
towers filled with liquid. Hence CO2 must be compressed. Due to the compression
the partial pressure and solubility of CO2 increased at the end of carbonating cycle.
Making tower
Sodium bicarbonate formed in the making tower is drawn off as a suspension,
it is necessary to ensure that the precipitated sodium bicarbonate is easily filterable
and efficiently washable. It is carried out by regulating the temperature and
concentration in the making tower. During the precipitation cycle, the temperature
gradient is maintained at 200C at the both ends and 450C in the middle and fine
crystals of sodium bicarbonate are allowed to grow. The temperature is increased
from 200C to 45-550C by heat of reaction and reduced by using coils.
Development of suitable calcining equipment
Moist sodium bicarbonate will cake on sides of kiln, preventing effective heat
transfer through shell. Kiln must be equipped with heavy scraper chain inside and
wet filter cake must be mixed with dry product to avoid caking. These problems can
be avoided by using fluidized bed calciner.
Filtration unit
Filtration should be carried out by using vacuum on the drum filter. It helps in
drying the bicarbonate and in recovering ammonia
Ammonia recovery
Ammonia recovery costs 4-5 times that of Na2CO3 inventory so losses must be
kept low. By proper choice of equipment design and maintenance, losses are less
than 0.2% of recycle load or 0.5%/kg product or 1kg/ton of sodium carbonate.
Module: 3 Lecture: 9 Sodium carbonate
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Waste disposal
Large quantities of CaCl2-NaCl liquor is generated during the process. The
uses of these liquor is to be find out or dispose it as waste.
Module: 3 Lecture: 10 Sodium carbonate
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Module: 3
Lecture: 10
SODIUM CARBONATE (continued)
3. Dual process
Raw materials
Basis: 1000kg Sodium carbonate
Crystalline Salt = 1260kg
Ammonia = 325kg
High pressure steam = 1350kg
Low pressure steam = 100kg
Cooling water = 50000 - 80000kg
Electric power = 450KWH
Co-product (NH4Cl) = 620kg
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
Reactions
C + O2 CO2
NH3 + H2O NH4+OH-
CO2 + H2O HCO3- + H+
CO2 + OH- HCO3-
Na+ + Cl- + NH4+ + HCO3
- NH4+Cl- + NaHCO3
2NaHCO3 Na2CO3 + CO2 + H2O
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module: 3 Lecture: 10 Sodium carbonate
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The liquor from carbonation tower, containing ammonium chloride,
unreacted NaCl and traces of sodium carbonate is ammoniated in ammonia
absorber. The ammoniated liquor is sent to a bed of washed salt in salt dissolver. The
resulting liquor is gradually cooled to 00C in refrigerating tank unit, resulting into
crystallize out ammonium chloride. The slurry containing ammonium chloride is
thickened and NH4Cl is centrifuged and dried. In this process ammonium chloride is
obtained as co-product. These is the principal modification of dual process in which
ammonium chloride is recovered as co-product rather than liberation of the
contained ammonia for recycle as in the Solvay process.
The liquor obtained after separation of NH4Cl is charged to series of
carbonation towers in which CO2 is passed from bottom in the counter current flow
of liquor. The resulting sodium bicarbonate is thickened into thickener and
centrifuged. It is then calcined into sodium carbonate.
Major engineering problem
Salt purification
Solid salt which is used to obtain better crystallization yields of NH4Cl cannot
be purified as with brine feeds in Solvay process. Only purification method is
mechanical washing and dewatering.
Brine Tank
Ammonia absorbing tank
Cold water
Saturation tank
Settling Tankfor ammonical
brine
Ammonical brine
Cooling coils
CO2
Carbonation tower
Saltdissolver
Refrigeration tank
CrystalizerS
ep
era
tor
NH4ClNaHCO3
ThicknerCentrifuge
Sodium carbonate
Gas outlet to absorbers
Figure: Manufacturing of Sodium Carbonate by Dual Process
Module: 3 Lecture: 10 Sodium carbonate
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Corrosion
Ammonium chloride solution is quite corrosive to equipment involved in
crystallization and solids recovery. So, corrosion resistant material or rubber-lined units
are preferred.
Refrigeration Cost
Actual refrigeration cost is variable but to maintain the temperature around
00C, the electric requirements are still double than Solvay's operation.
Choice of process
Advantage of Solvay process
Less electric power
Less corrosion problem
Use of low grade brine
Not a problem of disposal of co-product
Does not require ammonia plant
Disadvantage of Solvay process
Higher salt consumption
Waste disposal of CaCl2-brine stream
Higher investment in ammonia recovery units than crystallization unit of NH4Cl
More steam consumption
Higher capacity plant set up require for economic break even operation
(100 v/s 55tons/day)
NH4Cl will be used as mixed fertilizer ingredient which minimizes the disposal
problem of Duel process.
Plant location
One ton of soda ash production requires 8 tons of brine. As the salt sources
are the key factor and they are less widely distributed than limestone or coal. There
so plant should be located nearby the salt sources.
4. ELECTROLYTIC PROCESS
Raw materials
Basis: 1000kg Sodium carbonate (98% yield)
Salt = 563kg
Carbon dioxide = 424kg
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Module: 3 Lecture: 10 Sodium carbonate
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Reactions
NaCl Na+ + Cl¯ 2H2O + 2e¯ H2 + OH¯
At cathode
2H2O + 2e¯ H+ + 2OH¯
Na+ + OH¯ NaOH
2NaOH + CO2 Na2CO3 + H2O
At Anode
Cl¯ - e¯ Cl
Cl¯ + Cl¯ Cl2
Manufacture
Animation
Electrolytic cell consists of a perforated steel tube having a thin lining of
asbestos on the inside. The steel tube acts as the cathode and is suspended in an
outer steel tank. Brine is placed inside the cathode tube and a graphite rod is
immerged in it acts as anode. When an electric current is passed, the salt solution
undergoes electrolysis and its ions pass through the diaphragm as a result of
electrical migration. Hydrogen and caustic soda are formed at the cathode and
chlorine at the anode. Hydrogen gas is allowed to escape through an opening
provided at the top of the cell. Chlorine liberated at the anode is led away through
Cl2H2 H2
Spent Brine
Steam
Sodium Carbonate
Sodium Carbonate
Steam & CO2
Brine
Asbestos
Figure: Sodium Carbonate by Electrolytic Process
Module: 3 Lecture: 10 Sodium carbonate
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a pipe and compressed into steel cylinders. The space between the cathode and
outer tank is kept full of steam and Carbon dioxide.
Sodium ions pass through the asbestos and reach the cathode, where H+
ions and OH¯ Ions are formed as a result of reduction of water. Hydrogen escapes
through an opening at the top and Na+ ions combine with OH¯ ions to form
caustic soda. Sodium hydroxide is reacted with pressurized CO2 yielding Sodium
carbonate which is collected from bottom of the cell.
PROPERTIES
Molecular formula : Na2CO3
Molecular weight : 105.978gm/mole
Appearance : White crystalline solid
Odour : Odourless
Boiling point : 16330C
Melting point : 8510C
Density : 2.54gm/mL (Anhydrous)
Solubility : Soluble in water
99%sodium carbonate (58%Na2CO3) is known as light soda ash (solid density
1.86). Dense soda ash has solid density of 1.91. Both grads (lightly and dense) are
granular. Na2CO3. 10H2O is known as washing soda.
USES
Widely used in the manufacture of glass,
Used in manufacture of sodium bicarbonate, caustic soda,
Used in soap, pulp and paper, textiles industries
Used in petroleum and dyes industries
Used in foods, leather and water softening industries.
As a photographic film developing agent
As an electrolyte
As a washing soda in household uses.
Module: 3 Lecture: 11 Sodium bicarbonate
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Module: 3
Lecture: 11
SODIUM BICARBONATE
INTRODUCTION
Sodium bicarbonate (NaHCO3) is also known as backing powder. The ancient
Egyptians used natural deposits of natron, a mixture consisting mostly of sodium
carbonate decahydrate, and sodium bicarbonate. The natron was used as a
cleansing agent like soap.
In 1791 Nicolas Leblanc, produced sodium carbonate. In 1846, John Dwight
and Austin Church, established the first commercial plant to develop baking soda
from sodium carbonate and carbon dioxide.
MANUFACTURE
Sodium bicarbonate is manufactured in the carbonation tower, same as used
in Solvay‘s Process.
Raw material
Basis: 1000kg of Sodium bicarbonate (98% yield)
Sodium carbonate = 643.74kg
Carbon dioxide = 267.28kg
Water = 109.40kg
Sources of raw material
Sodium carbonate can be manufactured by Solvay‘s process, dual process
or electrolytic process as described in Module: 3, Lecture: 9
CO2 shall be obtained from any one source as described in Module: 2,
Lecture: 2
Reaction
Na2CO3 + CO2 + H2O 2NaHCO3
Manufacture
Block diagram of manufacturing process
Module: 3 Lecture: 11 Sodium bicarbonate
Dr. N. K. Patel
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Diagram with process equipment
Animation
Saturated solution of soda ash is passed from the top of a carbonating tower.
Compressed CO2 gas is admitted in counter currently from the bottom of the tower.
The temperature of the tower is maintained at 40°C by the cooling coils, provided
above the bottom of the tower. The suspension of bicarbonate formed is removed
from the bottom of the tower, filtered and washed on a vacuum rotary drum filter.
After centrifugation, the product is dried on continuous belt conveyor at 70°C to get
99.9% pure sodium bicarbonate.
Sodium bicarbonate is not manufactured by refining the crude NaHCO3
obtained from Solvay's process in the vacuum drum filters due to following reasons
It is very difficult to dry completely
Loss of ammonia during the process
NaHCO3 is not as pure as obtained during Solvay's process
Small traces of ammonia in the products makes it unfit for many applications
Saturated solutionof soda ash
Cooling coils
CO2
Ca
rbo
na
tio
n t
ow
er
Gas outlet to absorbers
Filter
Centrifuge
Sodiumbicarbonate
Washer
Drum filter
Hot air in
Hot airout
Figure: Manufacturing of Sodium Bicarbonate
Water In
Water Out
Water
Module: 3 Lecture: 11 Sodium bicarbonate
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BAKING POWDERS
Sodium bicarbonate is primarily used in cooking (baking), as a leavening
agent for producing aeration and lightness in breads and cakes. NaHCO3 often
impetrated an unpleasant taste or even a yellowish colour due to alkalinity of the
sodium carbonate formed, the search for better reagents continued.
Baking powders consist of a dry mixture of sodium bicarbonate with one or
more chemicals capable of completely decomposing it. The principal backing soda
used is monocalcium phosphate monohydrate, anhydrous monocalcium
phosphate, sodium acid pyrophosphate, sodium aluminum sulphate, tartaric acid
and the acid tartrates. However, monocalcium phosphates have widely been used.
Filler or drying agent, like starch or flour, is added to the active ingredients for better
distribution throughout the dough and to serve as diluents or preventive of the
reaction until water and heat are applied.
The actions of different, backing powders can be represent, by the following
equations
Na2Al2(SO4)4 + 6NaHCO3 6CO2 + 4Na2SO4 + 2Al(OH)3
3CaH4(PO4)2 .H2O + 8NaHCO3 8CO2 + Ca3(PO4)2 + NaHPO4 + 11H2O
KH2PO4 + NaHCO3 CO2 + KNaHPO4 + H2O
NaH2PO4 + NaHCO3 CO2 + Na2HPO4 + H2O
Na2H2P2O7 + 2NaHCO3 2CO2 + 2Na2HPO4 + H2O
KHC4H4O6 + NaHCO3 KNaC4H4O6 + CO2 + H2O
Most baking powders are so made that contain 26-29% NaHCO3 and enough
of acid ingredients to decompose the bicarbonate and 14-15% CO2. The rest of
powder, 20-40%, consists of corn starch of flour.
PROPERTIES
Molecular formula : NaHCO3
Molecular weight : 84.01gm/mole
Appearance : White crystal
Odour : Odourless
Boiling point : 8510C
Melting point : 3000C
Density : 2.20gm/mL (liquid)
Solubility : Soluble in water
Module: 3 Lecture: 11 Sodium bicarbonate
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USES
Sodium bicarbonate is used in cooking as baking powder
Due to amphortic properties is used in neutralization of acids and bases in
laboratory and swimming pool.
Heating of it release CO2, which is used to smother small fire.
Medical uses in treatment of chronic renal failure, renal tubular acidosis,
urinary alkalization for the treatment of aspirin overdose and uric acid renal
stone etc.
Used for personal hygiene product like toothpaste, mouth wash, deodorant
Used as a supplement for athletes in speed based events like small to middle
distance running. But the overdose causes serious gastrointestinal irritation.
As a cleaning agent
As a bio-pesticide to control fungus growth
As a cattle feed supplement
In preparation of carbonated water
Module: 3 Lecture: 12 Sodium hydroxide
Dr. N. K. Patel
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Module: 3
Lecture: 12
SODIUM HYDROXIDE
INTRODUCTION
Sodium hydroxide (NaOH), also known as lye and caustic soda is a highly
caustic metallic base which is a white solid available in pellets, flakes, granules, and
as 50% saturated solution.
Caustic soda and chlorine are produced as co-products by the electrolysis of
brine. In India 80% caustic soda and more than 95% of chlorine produces by
electrolysis of brine.
During electrolysis chlorine is liberated at the anode and caustic soda along
with hydrogen is produced at the cathode. Various commercial cells have been
developed in order to keep the anode and cathode products separate from one
another.
TYPE OF CELLS
Cells which are used for production of caustic soda are
1. Diaphragm cell
2. Mercury cathode cell
3. Membrane cell
Diaphragm cells
Diaphragm cells are two types.
(1) Submerged Cells
Cathodes remain submerged in this type of cell. Graphite is universally used
as anode. The liquid in the cathode compartment is at low-level in order to prevent
the back flow of OH¯ ions by diffusion. E. g. Hooker and Townsend cells
(2) Dry Diaphragm Cells
The diaphragm cells contain a porous asbestos diaphragm which permits a
flow of brine from the anode to cathode and prevents the mixing of anode product
and cathode products. Graphite is used as an anode. Electrolysis starts with dry or
empty cathode compartment. E. g. Nelson, Gibbs and Vorce cells
Module: 3 Lecture: 12 Sodium hydroxide
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Submerged cell
Hooker cells
Animation
Construction
Hooker cells are cubic in shape with capacity from 10000amp to 55000amp.
It has concrete cover at the base from which flat blades of the graphite projects
upward and act as anodes which is supported vertically by a layer of lead cast
concrete base. 90 anodes, each has measuring of 46 X 16 X 3 cm are used. The
cathode consisting of flat steel fingers are supported horizontally from the side steel
frame extending inwards, from two sides so as to fit between the rows of anode
blades. Concrete cover has inlet for brine and exit pipe for chlorine gas. This
concrete cover also projects the cast lead forming the condenser to the anodes
from attack by cell liquor. The cathode assembly has hydrogen and caustic off
takes and the cathode connection. The cathode is directly covered with asbestos
and forms the diaphragm, which is completely submerged. Diaphragm is applied
by dipping the cathode into a bath of asbestos slurry and the asbestos is drawn into
the screen by applying a vacuum to the hydrogen outlet.
Caustic Soda and Brine
H2
Cathode Conductor
Graphite Anodes
Anode Conductor
Brine
Cl2
Concrete Cell Cover
Asbestos Coated Steel Mesh Cathodes
Concrete cell Base
Cast In Lead
Sight Glass
Figure: Hooker Diaphragm Cell
Module: 3 Lecture: 12 Sodium hydroxide
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Working
A feed of brine between anode and cathode compartment maintains the
separation of anode products from cathode products. The brine passed into the
anode compartment of the cell through the concrete cover and liberated chlorine
at the anode escapes through the cell cover. Hydrogen liberates at the steel
cathodes and the weak brine containing caustic soda is withdrawn through the
hollow rectangular channel frames at the side.
Dry/Porous diaphragm cells
Nelson cell
Reactions
NaCl Na+ + Cl¯ ΔH = + 97.2kcals
2H2O + 2e¯ H2 + OH¯ ΔH = + 68.3kcals
At cathode
2H2O + 2e¯ H+ + 2OH¯
Na+ + OH¯ NaOH ΔH = + 112.0kcals
At Anode
Cl¯ - e¯ Cl
Cl¯ + Cl¯ Cl2
Animation
Caustic Soda Solution
Asbestos Diaphragm
Perforated Steel Cathode
Steam Inlet
Hydrogen
Brine Chlorine
Graphite Anode
Figure: Manufacturing of Caustic Soda & Chlorine by Diaphragm cell
Module: 3 Lecture: 12 Sodium hydroxide
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Nelson cell consists of a perforated steel tube having a thin lining of asbestos
on the inside. The steel tube acts as the cathode and is suspended in an outer steel
tank. Brine is placed inside the cathode tube and a graphite rod is immerged in it.
The graphite rod acts as anode. The brine undergoes electrolysis by passing current
and ions of salt are passing through the diaphragm due to electrical migration.
Sodium ions pass through the asbestos and reach the cathode, where H+ ions and
OH¯ Ions are formed as a result of reduction of water. Hydrogen escapes through
an opening at the top and Na+ ions combine with OH¯ ions to form caustic soda,
which is collected at the bottom of the outer tank. Hydrogen and caustic soda are
formed at the cathode and chlorine at the anode. Hydrogen gas is escape through
outlet provided at the top of the cell, while caustic soda is collected at bottom and
withdrawn from time to time. Chlorine liberated at the anode is led away through a
pipe and compressed into steel cylinders.
The space between the cathode and outer tank is kept full of steam, which
acts in two ways.
It heats the electrolyte and thus reduces its resistance
Keeps the pores of the asbestos diaphragm clear which make migration of
ions easy.
Mercury cathode cells
The method of electrolysis using mercury cathode was first introduced by
Castner and Kellner in 1892.
The Castner Kellner cell
Animation
A B C
Mercury
H
Eccentric Wheel
Cathode
Anode
Caustic SodaCl2
H2
Cl2
Partition
BrineBrine
Partition
-
+
C D
+
Figure: Castner Cell
Module: 3 Lecture: 12 Sodium hydroxide
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Castner Kellner cell consists of large rectangular tank with a layer of the
mercury at the bottom and divided into three compartments by the state partition
which does not touch the bottom of cell. Movement of eccentric wheel H comforts
the circulation of mercury from one compartment to another. Each of the side
compartments called A, A is fitted with graphite anodes dipping in brine, whereas a
series of iron roads suspended in the middle compartment act as cathodes. The
compartment contains a dilute solution of soda.
When the electric current is passed, the electrolysis of brine takes in the outer
compartment A, A. Chlorine is liberated at the anode and is led away through an
exit provided at the top. Sodium ions are discharged at the mercury layer which
acts as cathode by induction. It should be noted that H+ ion will not be discharged
because of high over potential over the mercury.
Na+ + e¯ Na (At cathode)
The liberated sodium atoms dissolved in the mercury to from a sodium
amalgam which comes into the central compartment due to the rocking motion
given to the cell by eccentric wheel H. In the compartment the Hg layer acts as an
anode. As a result of electrolysis of NaOH solution present in central compartment,
OH¯ ions and Na+ ions are formed. The OH¯ ions move to the mercury anode and
after getting discharged react with the sodium atom presents in the amalgam to
form sodium hydroxide. At the same time, the H+ ions furnished by slight dissociation
of water get discharged as hydrogen which escapes through exit above the middle
compartment, the caustic soda solution is sufficiently concentrated(above 20%) it is
removed periodically and concentrated to get fused caustic soda.
Membrane cell
AnodeCathode
Cl2 H2
NaOH
Figure: Membrane Cell
Membrane
Module: 3 Lecture: 12 Sodium hydroxide
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Animation
Membrane cell used a semi-permeable membrane to separate the anode
and cathode compartments. Membrane is porous chemically active plastic sheet
that allow Na+ ion to pass but rejects the OH¯ ions. While in diaphragm cells, back
migration of ion is controlled by the rate of flow of fluids through the diaphragm and
this is regulated by careful control of liquid level in the compartments.
Several polymers have been developed as membrane. Du Pont has
developed per sulfonic acid polymer (Nafion) while Ashai uses a multiple layer
membrane of per fluorosulfonic acid polymer. The purpose of membrane is to
exclude OH¯ and Cl¯ ions from the anode chamber, thus making the product far
lower in salt than that from diaphragm cell.
A membrane cell 20 times larger than diaphragm is being offered in 1981.
Such a cell unit can produce 240 ton of chlorine per year and power consumption is
satisfactory reduced below either mercury or diaphragm cells. A bipolar cell unit is
capable of producing 20,000 ton per year with a current density of 4 KA/M2.
Combination plant using the output of the membrane cells as fed to
diaphragm cells might result in considerable cost reduction. Such combinations
have been used with mercury cell output feeding the diaphragm cells.
Advantages
More concentrated brine can be used
Purer and concentrated products (28% NaOH containing 50ppm of NaCl, 40%
NaOH product) are produced.
Saving of energy and transportation cost
Low production cost
Disadvantages
Readily clogged of membrane
Pretreatment of brine is required to remove calcium and magnesium salts
Module: 3 Lecture: 13 Sodium hydroxide
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Module: 3
Lecture: 13
SODIUM HYDROXIDE (Continued)
MANUFACTURE
1. Using Diaphragm cell
Raw materials
Basis: 1000kg of Caustic soda (76%), 879kg Cl2, 274.7m3 H2
Salt = 1600kg
Sodium carbonate = 29.2kg
Sulfuric acid = 100.5kg
Steam = 10060kg
Electricity = 1197kJ
Refrigeration = 910kg
Direct labour = 20work-h
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Sodium carbonate can be manufactured by Solvay‘s process, dual process
or electrolytic process as described in Module: 3, Lecture: 8
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18
Reaction
NaCl Na+ + Cl¯ ΔH = + 97.2kcals
2H2O + 2e¯ H2 + OH¯ ΔH = + 68.3kcals
At cathode
2H2O + 2e¯ H+ + 2OH¯
Na+ + OH¯ NaOH ΔH = + 112.0kcals
At Anode
Cl¯ - e¯ Cl
Cl¯ + Cl¯ Cl2
Module: 3 Lecture: 13 Sodium hydroxide
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Manufacture
Brine purification
Brine purification is essential for getting pure caustic soda and to decrease
clogging of the cell diaphragm by insoluble hydroxides formed during electrolysis.
Brine contains impurities such as calcium, magnesium and iron compounds. These
impurities are removed by adding lime and soda ash, when insoluble carbonates
and hydroxides are precipitated. Sometimes sulfate are removed with BaCl2 or the
hot brine is treated with OH¯ and CO3-2 ions. After the treatment brine is allow for
settling to separate the impurities and then neutralized with hydrochloric acid. The
saturated brine containing 324gms/liter of NaCl is fed to the cell at 600C. The
electrolysis is carried out in diaphragm cells; each cell usually required 3.0-4.5 volts. A
number of them are put in series to increase the voltage of a given group.
Brine Electrolysis
Animation
Brine electrolysis is carried out with an anode current density of 0.07amp/cm2.
Na+ ions formed by electrolysis are moved to the cathode, where H+ ions and OH¯
ions are also formed as a result of reduction of water. On the other hand Cl¯ ions are
directed towards the anode, where they lose one electron each and form chlorine
molecules which liberate as chlorine gas at the anode. Since the discharge
potential of chlorine ions is lower than that of OH¯ ions, Cl¯ ions are discharge at the
anode and OH¯ ions are remain in solutions. Similarly the discharge potential of Na+
Caustic Soda Solution
Asbestos Diaphragm
Perforated Steel Cathode
Steam Inlet
Hydrogen
Brine Chlorine
Graphite Anode
Figure: Manufacturing of Caustic Soda & Chlorine by Diaphragm cell
Module: 3 Lecture: 13 Sodium hydroxide
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is greater than H+ ions hence H+ ions are discharge at the cathode, while Na+ ions
remain in the solutions.
2NaOH +Cl2 NaCl + NaClO +H2O
Chlorine attack caustic soda solution even in the cold, resulting into sodium
chloride and hypochlorite. To preclude the reaction, it is necessary that NaOH and
Cl¯ once formed do not come in direct contact with one another.
Evaporation and salt separation
About 10 to 15% caustic soda solution along with some unconverted NaCl is
obtained after electrolysis. The decomposition efficiency of the cells being in the
range of only 50%, about half of NaCl remains unconverted and is recovered by
reason of its low solubility in caustic soda solutions after concentrations. Hence the
weak caustic soda solution is first concentrated to 50% in a double or triple effect
evaporator so that NaCl completely separated which is recycled. The liquid
obtained from the salt separator is 50% caustic soda solution containing 2% NaCl and
0.1 to 0.5% NaCl on a dry basis.
Final Evaporation
50% NaOH solution is concentrated in huge cast iron pot on open fire.
Approximately 99% water is removed and molten caustic soda is formed at 5000C to
6000C. Now a days these pots are replaced by dowtherm heated evaporators for
caustic evaporation about 50%.
Another method of dehydrating 50% caustic soda is the precipitation of
NaOH.H2O by adding ammonia which also succor to purify the caustic soda. If 50%
caustic soda is treated with anhydrous ammonia in pressure vessels in a counter
current manner, free flowing anhydrous crystals of NaOH separate out from the
resulting aqua ammonia.
The hot anhydrous caustic is treated with sulfur to precipitate iron and then
allowed to settle. Then a centrifugal pump is lowered by crane in the molten NaOH
and the liquid is pumped out in to thin steel drums.
Purification of caustic soda
50% caustic soda solution still contains impurities such as colloidal iron, NaCl
and NaClO. Iron is removed by treating caustic with 1% by weight of 300mesh
CaCO3 and filtering the resulting mixture through a filter on CaCO3 per coat. Sodium
chloride and hypochlorite are removed by dropping the 50% caustic solution
through a column of 50% NH4OH.
Module: 3 Lecture: 13 Sodium hydroxide
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Chlorine Drying
The hot chlorine evolved from the anode compartment contains much water
vapour. Therefore it is cooled to condense most of the water vapour and further
dried in the sulfuric acid scrubber. A stoneware tower or stainless steel tower with
acid proof packing should be used for drying. The dried CI2 is compressed between
35 to 80 psi by one of the following temperature pressure combination.
High pressure (9-10atm), water cooling
Medium pressure (2-3atm), refrigeration at -200C
Low pressure (3-10 cm Hg ), refrigeration at -400C
Rotary compressors with H2SO4 seals have been used for liquefaction process.
The heat of compression is progressively removed by water and finally by
refrigeration to about -290C, when all the chlorine should be liquefied. It is further
cooled -450C and the liquid chlorine is led to a steel storage tank and then filled in
steel cylinder of 50-100 kg capacity for sale.
Hydrogen
Hydrogen evolved at the cathode is either burnt for boiler fuel or used as
hydrogen source.
Module: 3 Lecture: 14 Sodium hydroxide
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Module: 3
Lecture: 14
SODIUM HYDROXIDE (Continued)
2. Lime soda process
Raw materials
Basis: 1000kg Sodium hydroxide
Sodium carbonate = 1360kg
Lime = 75kg
Water = 1000kg
Steam = 1225kg
Fuel = 13000000 Btu
Electricity = 19KWH
Sources of raw material
Sodium carbonate can be manufactured by Solvay‘s process, dual process
or electrolytic process as described in Module: 3, Lecture: 9
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reaction
Na2CO3 + Ca(OH)2 NaOH + CaCO3
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module: 3 Lecture: 14 Sodium hydroxide
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Series of causticizer tank or agitator are added with 20% solution of soda ash
(made with weak liquor from a previous stage) and milk of lime or slaked lime. The
causticizer tanks containing steam line to heat the mixture to 80–900C are either
fitted with mechanical stirrer or compressed air as substitute of agitator.
After the equilibrium the liquid is allowed to settle for 2-3hour.The clear liquid
containing about 10% NaOH is drawn by a swing pipe. The sludge is washed in
counter current manner with the washing of the previous operation. The causticising
process is completed in the series of three agitators. The mixture of NaOH and
CaCO3 from the last agitator is charged to the first Door thickener, which consist of a
large shallow cylindrical tank into which the slurry is fed at the center. The over flow
liquid from the first thickener is filtered and filtrate containing 10-11% NaOH is
evaporated to 50 % solution in a triple effect vacuum evaporator as described
earlier. The solid CaCO3 is gradually settles to the bottom. The lime sludge from the
bottom of the first thickener is washed with the water. The filtrate from the next
operation is also added to the second thickener, where the liquor is treated with
excess of weak soda solution. The overflow from the second thickener is used as a
weak liquor to make soda ash solution. The lye suspension from the second agitator
is settled in the second thickener is filtered through rotary drum vacuum filter and
passed to a third thickener where it is finally washed with fresh water to remove any
traces of NaOH. The slug of the filter cake (CaCO3) is return in the lime kiln to from
lime. The caustic soda (11 % strength) contains small amount of NaCl and Na2CO3.
Soda Ash
Dissolving Tank
Causticizers
Milk of lime
Hot Water
Thickeners
Weak liquor from filter
Filtrate
Weak liquor to 1 st
dissolving tank
Figure: Manufacturing of Caustic Soda by Lime Soda Process
Filter
ThickenersSludge
Dilute caustic solution storage
Module: 3 Lecture: 14 Sodium hydroxide
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Kinetics and thermodynamics
Concentration of reagents favourable to the process
Na2CO3 + Ca(OH)2 NaOH + CaCO3
The equilibrium constant of above reaction is
( ) ---- (1)
Since calcium carbonate and calcium hydroxide are only slightly soluble,
their solutions are always saturated and concentration of two components in the
solution is therefore constant. Equation (1) can be written as
---- (2)
The yield of NaOH is given by,
---- (3)
Upon dividing the terms in the fraction by the concentration of hydroxide,
then (3) becomes
---- (4)
The ratio which appears in the denominator of (4), when use is made of (2) is
equal to the other ratio [NaOH]/K'c. On the basis of this (4) becomes
---- (5)
That is
---- (6)
It is seen from (6) that the yield of NaOH is high when the concentration of the
same hydroxide at equilibrium is low, i.e. when starting concentration of sodium
carbonate is small.
In practice it is necessary to work with stating solutions which are not too
dilute in order to avoid excessive cost of concentrating the produced caustic soda
solutions. Generally, solution containing 12-14% of sodium carbonate are used.
Module: 3 Lecture: 14 Sodium hydroxide
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Effect of temperature
Temperature effects on both equilibrium yield and rate of reaction.
PROPERTIES
Molecular formula : NaOH
Molecular weight : 39.997gm/mole
Appearance : White, waxy, opaque crystal
Odour : Odourless
Boiling point : 13880C
Melting point : 3180C (Decompose)
Density : 2.13gm/mL
Solubility : Soluble in water
It is hygroscopic in nature
USES
It is an important heavy chemical and occupies among the basic chemicals
position equal in importance to sulfuric acid and ammonia.
It is used in soap, rayon, dyes, paper, drugs, foods, rubber, textiles, chemicals,
bleaching, metallurgy and petroleum industries.
Module: 3 Lecture: 15 Chlorine
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Module: 3
Lecture: 15
CHLORINE
INTRODUCTION
Chlorine (Cl) is a chemical element having atomic number 17. It is the second
lightest halogen after fluorine. The element forms diatomic molecules under STP,
called dichlorine. It has the highest electron affinity and the third highest
electronegativity of all the elements; for this reason, chlorine is a strong oxidizing
agent.
Chlorine gas was obtained by Jan Baptist van Helmont in1630. The synthesis
and characterization of elemental chlorine occurred in 1774 by Carl Wilhelm
Scheele, who called it "dephlogisticated muriatic acid air," having thought he
synthesized the oxide obtained from the hydrochloric acid. Because acids were
thought at the time to necessarily contain oxygen, a number of chemists, including
Claude Berthollet, suggested that Scheele's dephlogisticated muriatic acid air must
be a combination of oxygen and the yet undiscovered element, and Scheele
named this new element within this oxide as muriaticum. In 1809, Joseph Louis Gay-
Lussac and Louis-Jacques proved that this newly discovered gas was the simple
element which was reconfirmed by Sir Humphry Davy in 1810, who named it
chlorine, from the Greek word chlōros meaning "green-yellow."
Chlorine can be manufacture by several methods such as electrolysis,
Deacon‘s, heating of auric acid and platonic chloride. All methods except
electrolysis are costly. So, chlorine is largely manufacture by electrolysis process
MANUFACTURE
1. Using diaphragm cells
Chlorine can be obtained as co-product during the manufacture of caustic
soda by electrolysis process as discussed in Module: 3, Lecture: 9.
2. Deacon’s method
Hydrochloric acid is partially oxidizes to chlorine by heating of HCl gas with
oxygen (air) at 400-4500C in presence of porous earthenware impregnated CuCl2 as
catalyst.
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4HCl + O2 2Cl2 + 2H2O
OR
2CuCl2 2CuCl + 2Cl2
4CuCl + O2 2Cu2OCl2
Cu2OCl2 + 2HCl 2CuCl2 + H2O
Cl2 mixed with unconverted HCI and system is washed with cold water and
dried with conc. H2SO4. This is an old method for manufacture of chlorine and is not
in used now.
3. Other methods
Pure chlorine can also be prepared by heating Auric chloride (AuCl3)or
platonic chloride (PtCl4)in a hard glass tube.
175OC 190OC
2AuCl3 2AuCl + 2Cl2 Au + 3Cl2
375OC 600OC
PtCl4 PtCl2 + Cl2 Pt + 2Cl2
PROPERTIES
Molecular formula : Cl2
Molecular weight : 70.906gm/mole
Appearance : Yellow green gas
Odour : similar to house hold bleach
Boiling point : -340C
Melting point : -1010C
Vapour density : 2.48 (v/s air)
Vapour pressure : 4800mmHg (200C)
In upper atmosphere, chlorine containing molecules such as chlorofluoro-
carbons have been implicated in ozone depletion.
Elemental chlorine is extremely dangerous and poisonous for all life forms
It is necessary to most forms of life, including humans, in form of chloride ions.
It is the only acidic gas which turns damp blue litmus red and bleaches it to
white.
It is two and a half times heavier than air. It becomes a liquid at −34 °C.
The affinity of chlorine for hydrogen is so great that the reaction proceeds
with explosive violence in presence of light
Module: 3 Lecture: 15 Chlorine
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USES
Used for producing safe drinking water.
Chlorinated compounds are used mostly for sanitation, pulp bleaching and
textile processing.
Used for the manufacture of chlorates and it is important in organic chemistry,
forming compounds such as chloroform, carbon tetrachloride, polyvinyl
chloride, and synthetic rubber.
Used in dyestuffs, petroleum products, medicines, antiseptics, insecticides,
foodstuffs, solvents, paints and plastics.
As an oxidizing agent and in substitution reactions.
In paper and pulp, solvents, explosives, plastics, pesticides and sanitation
As a common disinfectant, chlorine compounds are used in swimming pools
to keep them clean and sanitary.
Module 4 Lecture: 16 Nitric acid
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Module: 4
Lecture: 16
NITRIC ACID
INTRODUCTION
Nitric acid (HNO3), also known as aqua fortis (strong water) and spirit of niter,
is a highly corrosive strong mineral acid. The pure compound is colourless, but older
samples are yellowish in colour due to the accumulation of oxides of nitrogen.
Commercially available nitric acid having concentration of 68% HNO3, while the
solution containing more than 86% HNO3, is referred to as fuming nitric acid.
Depending on the amount of nitrogen dioxide present, fuming nitric acid is further
characterized as white fuming nitric acid or red fuming nitric acid, at concentrations
above 95%.
First Nitric acid was mentioned in Pseudo-Geber's De Inventione Veritatis
which is prepared by calcining a mixture of saltpetre (Niter KNO3), alum and sulfuric
acid. Also, described by Albertus in the 13th century and by Ramon Lull, who
prepared it by heating niter and clay and called as "eau forte" (aqua fortis).
Glauber invent the process to obtain HNO3 by heating niter with strong sulfuric
acid. In 1776 Lavoisier showed that it contained oxygen, and in 1785 Henry
Cavendish determined its precise composition and synthesized it by passing a
stream of electric sparks through moist air.
MANUFACTURE
Nitric acid is manufactured by three methods.
1. From Chile saltpetre or nitrate
2. Arc process or Birkeland and eyde process
3. Ostwald's process or Ammonia oxidation process
1. From Chile saltpeter or nitrate
It is the first commercial process of manufacture of nitric acid from sodium
nitrate extracted from Chile saltpeter. The process is now become obsolete since
second decade of nineteenth century.
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
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Raw materials
Basis: 1000kg Nitric acid (95% yields)
Sodium Nitrate = 1420kg
Sulfuric acid = 1638kg
Sources of raw material
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18.
Sodium nitrate can be obtained from caliche ore. Also, it is manufactured by
neutralization of soda ash with nitric acid as well by reaction of ammonium nitrate
and sodium hydroxide.
Reaction
NaNO3 + H2SO4 NaHSO4 + HNO3
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Cast Iron Retort
Furnace
NaNO3 + H2SO4
HNO3
Conc. HNO3
Cooled Silica Pipes
Water or Dil. HNO3
StonewareBalls
Dil. HNO3
Figure: Manufacture of nitric acid from chile saltpetre or nitrate
Water out
Water in
Module 4 Lecture: 16 Nitric acid
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Equal weight of sodium nitrate (or potassium nitrate) and sulfuric acid is
charged to cast iron retort having outlet provided at bottom to take out solution of
sodium bisulfate. The reactants are heated to about 2000C by the hot furnace
gases. The furnace gases are produced by combustion of coal in the furnace. Then
the vapour of nitric acid are cooled and condensed in water cooled silica pipes.
The cooled acid is collected in stoneware receiver. The un-condensed vapours are
scrubbed with water in absorption tower which is packed with stone ware balls and
cooled by cold water. The dilute acid is re-circulated till it becomes concentrated.
The residual sodium bisulfate is removed by outlet provided at the bottom of retort.
2. Arc process or Birkeland and eyde process
Raw materials
Basis: 1000kg Nitric acid (98% yield)
Air = 198kg
Water = 145kg
Reaction
N2 + O2 2NO ΔH = + 43.2 kcals
2NO + O2 2NO2 ΔH = - 26.92 kcals
4NO2 + 2H2O + O2 4HNO3
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Na2CO3
SolutionWater
50 % Nitric Acid
Absorption Towers
Figure: Manufacturing of Nitric Acid by Arc Process
Soda Tower
Na2CO3 Tower
Air
Cooling water
Electric Arc Furnace
Ox
ida
tio
n
To
we
r
Boiler
Module 4 Lecture: 16 Nitric acid
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Animation
Air freed from CO2 and moisture is passed through electric arc chamber
having two copper electrodes which are continuously circulated by cold water and
are connected with AC dynamo. A powerful electromagnet placed at right angles
to the electrodes spreads the arc in the form of a disc. The chamber is also provided
with inside suction pumps for rapid circulation of air across the flame through holes
of refractory fire work. Nitrogen and oxygen of air combines at 20000C temperature
to form nitric oxide. The hot exit gases (10000C) leaving the chamber is passed
through tube fire boiler for steam generation. The temperature of gases leaving the
boiler is significantly reduced up to 1500C. The gases are allowed to pass through
oxidation chambers made of iron and lined inside with acid proof stone. Here, nitric
oxide is further oxidizing to nitrogen peroxide in presence of air. The exit gases from
oxidation towers are passed through series of absorption tower filled with broken
quartz through which cold water or dilute nitric acid is continuously sprayed from
top. The gases which enter from the base of 1st tower are leave at the top.
Continuous counter current flow of gases in each tower is maintained by centrifugal
fan. The 3rd tower is fed with cold water and the dilute nitric acid is collected at the
base is re-circulated to the top of the preceding tower. 50% HNO3 is obtained at the
base of 1st tower. The gases leaving the last absorption tower contains traces of
nitrogen oxides. The gases are allowed to pass through two wooden towers which
are sprayed down by dilute solution of soda ash. The solution at the base of sodium
carbonate tower is evaporated to collect crystal of sodium nitrate.
Engineering aspects
The conversion of NO to HNO3 was carried out by means of oxidation and
hydration processes which is same as product obtained from oxidation of ammonia
Reason for obsolesce
High electrical energy consumed. There were enormous amounts of gas in
circulation compared to low concentration of NO which was formed (about 2%) on
account of the fact that high temperature also promote the reverse dissociation
reaction.
3. Ostwald's process or Ammonia oxidation process
Raw Materials
Basis: 1000kg nitric acid (100%)
Ammonia = 290kg
Air = 3000Nm3
Platinum = 0.001kg
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
N P T E L
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Water = 120000kg
Steam credit = 1000kg @ 200psig
Power = 10-30KWH
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
Reaction
Major reactions
4NH3 + 5O2 4NO + 6H2O ΔH = - 216.6kcals ---- (1)
2NO+O2 2NO2 ΔH = - 27.1kcals ---- (2)
Side reactions
4NH3 + 3O2 2N2 + 6H2O ΔH = - 302.7kcals ---- (3)
2NH3 N2 + 3H2 ΔH = + 26.7 kcals ---- (4)
2NH3 + 2O2 N2O + 3H2O ΔH = - 65.9kcals ---- (5)
4NH3 + 6NO 5N2 + 6H2O + 432.25kcal ΔH = - 431.9kcals ---- (6)
Nitrous oxide oxidation and absorption
2NO+O2 2NO2 ΔH = - 27.1kcals ---- (7)
3NO2 + H2O 2HNO3+ NO ΔH = - 32.2kcals ---- (8)
2NO2 N2O4 ΔH = - 13.9kcals ---- (9)
2NO2 + H2O HNO3 + HNO2 ΔH = - 27.7kcals ---- (10)
HNO2 H2O + NO + NO2 ---- (11)
Manufacture
Nitric acid is made by the oxidation of ammonia, using platinum or platinum-
10% rhodium as catalyst, followed by the reaction of the resulting nitrogen oxides
with water.
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
N P T E L
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The process involves four steps
1. Catalytic oxidation of ammonia with atmospheric oxygen to yield nitrogen
monoxide
2. Oxidation of the nitrogen monoxide product to nitrogen dioxide or dinitrogen
tetroxide
3. Absorption of the nitrogen oxides to yield nitric acid
4. Concentration of nitric acid
Compressed air is mixed with anhydrous ammonia, fed to a shell and tube
convertor designed so that the preheater and steam heat recovery boiler-super
heater are within the same reactor shell. The convertor section consists of 10-30
sheets of Pt-Rh alloy in the form of 60-80 mesh wire gauge packed in layers inside the
tube. Contact time and of the gas passes downward in the catalyst zone
2.5 X 10-4sec and are heated at 8000C.
Product gases from the reactor which contain 10-12% NO, are sent through
heat recovery units consisting of heat recovery boiler, super heater and quenching
unit for rapid cooling to remove large fraction of product heat, and into the oxidizer-
absorber system. Air is added to convert NO to NO2 at the more favourable
temperature (40-500C) environment of the absorption system. The equipment in the
Vaporizer
Compresed preheated air
Water cooling
Figure: Manufacturing of Nitric acid from by oxidation of ammonia
Convertor
Compressor
Catalyst Recovery
Filter
57-60 % HNO3 solution for use or Concentrated to 95% HNO3
Air
NH3
storage
Heat Recovery
Boiler
Superheater
Converter 8000C
Tail gas heater
Steam Economiser
Ab
so
rpti
on
T
ow
er
Ox
ida
tio
n
To
we
r
Water cooling
Water Condense
Eapander
Turbine
Process Steam
Exhaust Gas
Make up Water
Air
Air
Tail Gas
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
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absorption train may be series of packed or sieve tray vertical towers or a series of
horizontal cascade absorbers. The product from this water absorption system is 57-
60% HNO3 solution which can be sold as or concentrated as follows
Concentration by H2SO4
Rectification with 93% H2SO4 (660Be) in silicon-iron or stoneware tower
produces concentrated nitric acid and 70% H2SO4 which can be re-evaporated to
93% H2SO4 or used as it is.
Concentration by Mg(NO3)2
Magnesium nitrate solution containing 70-75% Mg(NO3)2 is fed to dehydrating
tray along with dilute HNO3 from the absorption tower. The salt solution acts as an
extractive distillation agent, removing water at 1000C or higher, thus allowing
rectification with azeotropic formation. The dilute Mg(NO3)2 solution re-concentrated
by evaporation
Advantages
Operating cost is half compare to H2SO4 process
Acid quality and yield improved
Disadvantage
Increase in 70% capital expenditure
Engineering aspects
Thermodynamics and kinetics
4NH3 + 3O2 2N2 + 6H2O ΔH = - 302.64kcal ---- (12)
4NH3 + 6NO 5N2 + 6H2O ΔH = - 432.25kcal ---- (13)
2NO N2 + O2 ΔH = - 43kcal ---- (14)
All the above exothermic reaction takes place in more or less extent.
Reaction 12 and 13 occurs with decrease in enthalpy with increase in number of
moles followed by increase in entropy.
4NH3 + 5O2 4NO + 6H2O
Ammonia oxidation reaction has an extremely favourable equilibrium
constant so that one step, high temperature converter design may be used.
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
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Further, ammonia air mixture exhibit explosion limits. At STP it is 15.6%
ammonia, while temperature above 6000C and 1atm pressure, the limit is lowered to
10.5%
The following condition should be fulfilled to convert NH3 into NO
Explosion limit
The explosion limits are avoided by employing quantity of air such that the
amount of ammonia mixed with it is less than 10.5vol% of total volume.
Thermodynamics
The thermodynamics of competing reactions (12) and (13) are rendered
unfavourable by working above 5000C, while the reaction (14) are not favoured if
the process is carried out under 12000C
Kinetics
Kinetics of reaction (1) is speeded up by use of catalyst. This is also done by
preventing any reduction in the velocity of the reaction brought about by presence
of inert gas nitrogen in the reaction zone.
Reaction kinetics in ammonia oxidation stage
Rate of reaction is directly proportional to system pressure
Alloying of platinum with rhodium improves yield at given set of conditions
Reaction to form NO is favoured by increasing temperature until an optimum
is reached which increases with higher velocities. This results from the
prevention of back diffusion of NO into higher NH3 concentration region. If this
occurs the following reaction is quite probable and should be avoided for
high NO yield.
4NH3 + 6NO 5N2 + 6H2O
Rate of NO formation very nearly corresponds to diffusional transport of
ammonia molecules to the catalyst surface
There is slight equilibrium advantage to operation at atmospheric pressure.
This is more than offset by increased capacity in a given reactor volume with
subsequent catalyst and reactor savings when operating high pressures (3-8atm.)
Oxidation of nitrogen oxide does not have as large equilibrium constant.
There so, the reaction predominates in water and absorption portions of the process,
which operates at low temperature at 40-500C. All the nitrogen oxide liberated on
absorption of NO2 must be reoxidized in absorption tower
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
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Absorption of nitrogen oxides into water
Following design criteria should be considered
Rate of abortion depends on concentration of NO2 in gas phase. In absorber
where concentration of NO2 is greater than 5%, the controlling reaction is
solution of N2O4 accompanied by hydrolysis of HNO3 and HNO2.
Low temperature is beneficial for absorber operation efficiency
Increasing pressure favours physical absorption rate and shift chemical
equilibrium to produce higher acid strength
Process design modification
Most plants operate at higher pressure (3-8atm) rather than complete
atmospheric pressure. Some operates at a combination of 1atm pressure oxidation
and high pressure absorption. Very high pressure is limited due to cost of pressure
vessel.
Advantages and disadvantages of elevated pressure are as follows
Advantages
Higher acid strength
Lower investment cost
Higher reaction rate and lower volume in both oxidation and absorption
equipment
Disadvantages
Lower oxidation yield
Higher power require if recovery units are not specified
Higher catalyst loss unless good catalyst recovery procedure are not used
Catalyst for oxidation of ammonia
Platinum/rhodium alloy containing 10% rhodium is the only industrially viable
catalyst. Rhodium not only improves the catalytic properties of platinum but also
improves mechanical and anti-abrasive properties of material under the operating
condition such as to counter the severe corrosion and oxidation atmosphere. 4–10 %
of rhodium used in Pt/Rh supported catalyst. Higher efficiencies and smaller platinum
losses can be achieved by knitted gauzes.
The metallic alloy catalyst is prepared into very fine threads of diameter
0.05mm which are woven into meshes with more than 1000stiches/cm2. Two to four
or even more of these meshes are placed on top of one another inside the reactors
when these are put into operation.
Catalyst threads are smooth, bright and less active at initial stage, as the time
progresses they becomes dull and wrinkled whereupon their activity rises to the
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
N P T E L
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maximum. Finally they become spongy with activity falling off. When it is in most
active state, ammonia oxidation yields up to 98% of NO are obtained.
Ammonia conversion efficiency is a function of pressure and temperature. As
the pressure increases, higher temperatures are needed to obtain the high
conversion efficiency. An increased flow rate and the presence of several layers of
the catalyst help to minimize undesirable side reactions. However, high flow rates
increase the catalyst loss which leads to search for non-platinum catalysts for
ammonia oxidation. The most prospective non-platinum catalysts are based on
oxides of Co, Fe or Cr.
Catalyst poison
Sulfates, H2S, chlorides, Arsenic and its oxide, Si, P, Pb, Sn and Bi are
permanently poisoning the catalyst. These elements lead to the formation of
inactive compounds in the wires resulting in decreasing of the catalytic activity.
Traces of acetylene, ethylene, Cr, Ni and Fe temporarily reduce the conversion
efficiency which can be restored by treatment with HCl. There so air should be freed
from all above impurities along with suspended particles of lubricants, fats, fine dust
and abrasive powder. Also, suspension of Fe2O3 from ammonia is removed. For that
efficient filtration system along with magnetic separators are provided.
PROPERTIES
Physical Properties
Molecular formula : HNO3
Molecular weight : 63.013gm/mole
Appearance : Colourless liquid
Odour : Pungent
Boiling point : 1210C (68% HNO3 solution)
Melting point : -420C
Density : 1.5129gm/mL (liquid)
Solubility : Miscible with water in all proportions
The impure nitric acid is yellow due to dissolved oxides of nitrogen, mainly
NO2.
It has a corrosive action on skin and causes painful blisters.
Chemical Properties
Acidic properties: It is a strong monobasic acid and ionization in aqueous
solution.
Oxidizing properties: It acts as a powerful oxidizing agent, due to the
formation of nascent oxygen.
Action on metals: It reacts with almost all the metals, except noble metals, like
Pt and Au. The metals are oxidized to their corresponding positive metal ions
Module 4 Lecture: 16 Nitric acid
Dr. N. K. Patel
N P T E L
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while HNO3 is reduced to NO, NO2. N2O, NH2OH or NH3, depending upon the
conditions such as temperature, nature of metal and concentration of the
acid.
Nitric acid has ability to separate gold and silver.
USES
As a starting material in the manufacture of nitrogen fertilizers such as
ammonium nitrate, ammonium phosphate and nitrophosphate. Large
amounts are reacted with ammonia to yield ammonium nitrate.
Weak acid are used to digest crude phosphates.
As a nitrating agent in the preparation of explosives such as TNT,
nitroglycerine, cellulose polynitrate, ammonium picrate
In manufacture of organic intermediates such as nitroalkanes and
nitroaromatics.
Used in the production of adipic acid.
Used in fibers, plastics and dyestuffs industries
Used in metallurgy and in rocket fuel production
As the replacement of sulfuric acid in acidulation of phosphate rock.
Module 4 Lecture: 17 Sulfuric acid
Dr. N. K. Patel
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Module: 4
Lecture: 17
SULFURIC ACID
INTRODUCTION
Sulfuric acid (H2SO4) is a highly corrosive strong mineral acid. It is a colorless to
slightly yellow viscous liquid which is soluble in water at all concentrations. It is one of
the most important heavy industrial chemicals due to it has a number of large-scale
uses particularly in the phosphate fertilizer industry. About 60 % of the sulfuric acid
produced is utilized in fertilizer manufacture.
Sulfuric acid was called "oil of vitriol" by Medieval. The study of vitriol began in
ancient times. Sumerians had a list of types of vitriol that classified according to
substance's colour.
Johann Glauber prepared sulfuric acid by burning sulfur together with
saltpeter (potassium nitrate, KNO3), in the presence of steam in the 17th century.
Decomposition of saltpeter followed by oxidation produces SO3, which combines
with water to produce sulfuric acid. Joshua Ward used the method for the first large-
scale production of sulfuric acid in 1736.
John Roebuck, produce less expensive and stronger sulfuric acid in lead-lined
chambers in 1746. The strength of sulfuric acid by this method is 65%. After several
refinements, this method, called the "lead chamber process" or "chamber process",
remained the standard for sulfuric acid production for almost two centuries.
The process was modified by Joseph Louis Gay-Lussac and John Glover which
improved concentration to 78%. However, the manufacture of some dyes and other
chemical processes require a more concentrated product. Throughout the 18th
century, this could only be made by dry distilling minerals in a technique similar to
the original alchemical processes.
Pyrite (iron disulfide, FeS2) was heated in air to yield iron(II) sulfate, FeSO4,
which was oxidized by further heating in air to form iron(III) sulfate, Fe2(SO4)3, which,
when heated to 4800C, decomposed to iron(III) oxide and sulfur trioxide, which
could be passed through water to yield sulfuric acid in any concentration. But the
production expenses are very high.
Module 4 Lecture: 17 Sulfuric acid
Dr. N. K. Patel
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More economical process i.e. the contact process was patented by
Peregrine Phillips in 1831. Today, nearly all of the world's sulfuric acid is produced
using this method.
MANUFACTURE
The Industrial manufacture of sulfuric acid is done mainly by two processes
1. The Lead Chamber process
2. The Contact process
1. The lead chamber process
The Lead Chamber process for the manufacture of sulfuric acid dates back
about 200 years. Although less efficient than the contact process, it is still of
considerable commercial importance.
Raw Materials
Basis: 1000kg Sulfuric acid (98% yield)
Sulfur = 400kg
Air = 399kg
Reaction
S + O2 SO2 ΔH = - 70.9kcals
4FeS2 + 11O2 2Fe2O3+ 8SO2
SO2 + NO2 SO3 + NO
SO3 + H2O H2SO4 ΔH = - 92.0kcals
NO + O2 2NO2 ΔH = - 27.12kcals
NaNO3 + H2SO4 NaHSO4 + HNO3
2HNO3 + 2SO2 2SO3 + H2O + NO + NO2
NO + NO2 + 2H2SO4 2NO.HSO4 + H2O
2ON.O.SO2OH + H2O H2SO4 + NO2 + NO
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module 4 Lecture: 17 Sulfuric acid
Dr. N. K. Patel
N P T E L
109
Sulfur dioxide is obtained by burning sulfur or by roasting pyrites. There are two
function of burner
1. To oxidize sulfur to maximum extent
2. To produce and constant supply of gas containing maximum concentration
of SO2
The burner of the furnace should expose large surface of melted sulfur and
should be provided secondary air in order to burn sublimed burner. This is necessary
due to low heat of combustion and high vapour pressure of sulfur. At about 4000C,
pyrite (FeS2) decompose in to FeS and sulfur vapour, the later oxidized to SO2 in
presence of excess air. The residual FeS also oxidizes to Fe2O3 and SO2. Iron oxide
(Fe2O3) slightly catalyzed oxidation of SO2 to SO3. Burner gas should contain sufficient
oxygen for carry out further oxidation of SO2 to SO3.
The burner gases which contain SO2, N2, O2 and dust or fine particle of pyrites
are passed through dust chamber followed by Cottrell electrical precipitator or
centrifugal separator in order to remove dust or fine particle of ore. Dust chambers
are provided with horizontal shelves or baffles followed by filtration through crushed
coke or similar material.
Now, burner gases are passed through niter oven made of cast iron in which
equimolecular proportion of NaNO3 and H2SO4 is heated. Resulting nitric acid reacts
with SO2 to give mixture of nitric oxide (NO) and nitrogen dioxide (NO2) which are
carried with burner gases.
Nitrated Acid
Chamber Acid
Chamber I Chamber II Chamber III
Nitre Pot
Burners
Conc. Acid Chamber Acid
Conc. H2SO4
To Chimney
Ga
y -
lu
ss
ac
To
we
r
Watrer Spray
Figure: Manufacturing of Sulfuric acid by Chamber process
Module 4 Lecture: 17 Sulfuric acid
Dr. N. K. Patel
N P T E L
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In modern plant oxides of nitrogen are produced by passing mixture of
ammonia and air through heated platinum gauze acting as catalyst (same as
manufacture of HNO3 by ammonia oxidation process)
After passing burner gases to dust chamber and niter oven, they pass through
5 meter square and 10 -15meter high Glower tower which is packed with flint stone,
quartz, tile or acid resisting bricks. The packing in the tower is loosely stacked at the
bottom to facilitate mixing of hot gases. The hot burner gases passes up this tower is
at 450 - 6500C and dilute H2SO4 from the lead chamber and nitrosyl sulfuric acid from
Gay-Lussac tower are made to trickle down the Glower tower by means of sprayers.
Here, burner gases are cooled down to 70-800C, dilute chamber acid is
concentrated up to 78% and nitrosyl sulfuric acid (nitrous vitriol) is denitrated by
action of water.
The tower acid is drawn off from the bottom of the tower and collected in the
container called acid egg. The acid from base of Glower tower is cooled to 400C by
air coolers.
The mixture of SO2, Oxides of nitrogen and air is then passed to series of
rectangular vessels made of lead (lead chamber) having 15-45 meter length,
6-7 meter width and 7 meter length. The number of chambers depends upon the
size of plant, but usually they are 3 to 6 in number. The chambers are arranged in
two parallel rows. Steam from low pressure boiler or pure filtered water is sprayed
from top of the chamber. Mixture of gases is converted into H2SO4 having 65-70%v
strength is collected at the bottom of the chamber. Dilute sulfuric acid obtained in
any of the chamber is called chamber acid. A part of chamber acid is pumped to
Glower tower, and the rest is sent for concentration.
The unabsorbed remaining gases contain oxides of nitrogen and SO2 from
lead chamber are then passed through Gay-Lussac tower at the top of which
Glower acid is sprayed to recover oxides of nitrogen.
The oxides of nitrogen recovered in the form of nitroso sulfuric acid are
pumped to Glower tower to again regenerate oxides of nitrogen.
When pyrite is used as raw material, the chamber acid may contain arsenious
oxide (from pyrite), lead sulfate from lead chamber are removed by treatment of
H2S and dilution of acid respectively. Dilute acid may be further concentrated into
Glower tower.
Kinetics and thermodynamics
2NO+O2 2NO2 ∆H0 =-27.118kcals
Module 4 Lecture: 17 Sulfuric acid
Dr. N. K. Patel
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Above reaction is rate controlling step in the chamber process. The
exothermic forward reaction is favoured by decrease in temperature. As the
reaction proceeds with decrease in volume, the formation of NO2 would be
favoured by increase in pressure. It has been observed that the rate of oxidation is
slow at ordinary temperature and rate is proportional to the square of the absolute
pressure. At lower temperature, the production of chamber acid has been found to
be greater. All these facts are in good agreement with the fact that the oxidation of
NO to NO2 is the rate controlling step in this process.
The dilution of nitrosyl sulfuric acid within the Glover tower leads to its
decomposition and nitrous fumes produced catalyze the synthesis of sulfuric acid
when they come in contact with sulfur dioxide and water.
2HSO4.NO + H2O 2H2SO4 + NO + NO2 ---- (1)
NO + NO2 + SO2 + H2O H2SO4 + 2NO ---- (2)
Reaction (2) can be shown in chain as follow
NO + NO2 N2O3 + H2O 2HNO2 + SO2 H2SO4 + 2NO
Reaction (2) can be repeated cyclically by the partial reoxidation of the nitric
oxide produced by excess air which forms part of the sulfurous gas coming from the
combustion chamber.
2NO + 1/2 O2 NO + NO2 ---- (3)
Reaction (2) and (3) mainly occur in chambers following the Glover tower
until the SO2 has been exhausted.
The recovery of nitrous gases is important task of Gay Lussac towers but it is
difficult. A reverse reaction of reaction (1) is taking place here. This is in effect, a
typical equilibrium reaction which is particularly sensitive to the mass action effect
by water
2H2SO4 + NO + NO2 2HSO4.NO + H2O
or reversible reaction
2HSO4.NO + H2O 2H2SO4 + NO + NO2
Above reaction is displaced to the right in the Glover tower where water is
relatively abundant and to the left in the Gay Lussac tower which is supplied with
sulfuric acid which is transformed into nitrosyl sulfuric acid by absorbing
equimolecular mixture of NO and NO2 on account of its high concentration (78%).
Module 4 Lecture: 17 Sulfuric acid
Dr. N. K. Patel
N P T E L
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Reason for obsolesce
As discussed above, overall reaction consisting of number of partial reactions
which takes place in liquid phase, the development of surfaces which are covered
in this liquid is a factor of fundamental importance in promoting the synthesis of
sulfuric acid. Maximum strength of sulfuric acid obtained by chamber process is 78%.
However, in manufacture of some dyes and chemical processes require more
concentrated H2SO4. There so, the process is largely replaced by contact process.
Module 4 Lecture: 18 Sulfuric acid
Dr. N. K. Patel
N P T E L
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Module: 4
Lecture: 18
SULFURIC ACID (continued)
2. The contact process for sulfuric acid
Almost all sulfuric acid is manufactured by the contact process.
Raw Materials
Basis: 1000kg sulfuric acid (100%)
Sulfur dioxide or pyrite (FeS2) = 670kg
Air = 1450-2200Nm3
Sources of raw material
The sources of sulfur and sulfur dioxide are as follows
Sulfur from mines
Sulfur or hydrogen sulfide recovered from petroleum desulfurization
Recovery of sulfur dioxide from coal or oil-burning public utility stack gases
Recovery of sulfur dioxide from the smelting of metal sulfide ores
2PbS + 3O2 2PbO + 2SO2
Isolation of SO2 from pyrite
Reactions
S + O2 SO2 ΔH = - 71.2kcals
2SO2 + O2 2SO3 ΔH = - 46.3kcals
SO3 + H2O H2SO4 ΔH = - 31.1kcals
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module 4 Lecture: 18 Sulfuric acid
Dr. N. K. Patel
N P T E L
114
Steps in the Contact Process
The steps in this process are as follow.
1. Burning of sulfur
2. Catalytic oxidation of SO2 to SO3
3. Hydration of SO3
1. Burning of sulfur
Burning of sulfur in presence of dry air is carried out in sulfur pyrite burner. As
SO2 is needed for the catalytic oxidation and prevention of corrosion, dry air is used
in the combustion process. If sulfur contains carbonaceous impurities, the molten
material has to be filtered to avoid poisoning the catalyst and forming water from
burning hydrogen.
2. Catalytic oxidation of SO2 to SO3
When using sulfur from sources 1 and 2, purification of the SO2 gas is normally
not needed. Other sources of SO2 require wet scrubbing followed by treatment of
the gas with electrostatic precipitators to remove fine particles. The catalyst used is
vanadium pentoxide (V2O5) and the pressure is 1.2-1.5 atmospheres. The
temperature has to be kept around 4500C. If it rises above 4500C, the equilibrium is
displaced away from SO3. Temperature should reach around 4500C for the catalyst
to be activated. This process is strongly exothermic. The catalytic reactor is designed
as a four-stage fixed-bed unit. The gas has to be cooled between each step. Four
passes, together with "double absorption, described below, are necessary for overall
conversion of 99.5-99.8% (three passes, 97-98%). The temperature rises to over 6000C
with the passage of the gas through each catalyst bed. The doubled absorption
consists of cooling the gases between each bed back to the desired range by
sending them through the heat exchanger and then back through the succeeding
beds. Between the third and fourth beds, the gases are cooled and sent to an
AirSulphurpyritesburner
Dustchamber
Scrubber DryingTower
ArsenicPurifier
Cooling leadpipe
Steam Water Conc. H2SO4
Tyndallbox
Preheater
AbsorberOleum
WasteGases
100 % H2SO4
Figure: Manufacturing of Sulfuric acid by Contact process
SO3
SO2 + O2 + N2 (suphurous gases)
H.E.-1
H.E.-2
H.E.-3
Hot gases
Module 4 Lecture: 18 Sulfuric acid
Dr. N. K. Patel
N P T E L
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absorption tower. This is to shift the equilibrium to the right by absorbing SO3. The
gases are then sent to the heat exchanger to warm them to 410-4300C and then on
to the fourth catalyst bed.
3. Hydration of SO3
After the catalytic oxidation process, the resulting SO3 is hydrated by
absorption in packed towers filled with 98-99% sulfuric acid. This is the H2SO4
azeotrope of minimum total vapour pressure. The catalytic oxidation has to proceed
in high yield to avoid air pollution problems. SO2 has a low solubility in 98% H2SO4. At
lower acid concentrations, sulfuric acid and SO3 form a troublesome mist and at
higher concentrations emissions of SO3 and H2SO4 vapour become significant. The
absorption acid concentration is kept within the desired range by exchange as
needed between the H2SO4 in the drying acid vessel that precedes the combustion
chamber with the H2SO4 in the absorption tower. The acid strength can be adjusted
by controlling the streams of H2SO4 to give acid of 91 to 100% H2SO4 with various
amounts of added SO3 and water. The conversion of sulfur to acid is over 99.5%.
Kinetics and thermodynamics
The crucial step is the oxidation of SO2 to SO3. At normal conditions, the
equilibrium lies far to the left and the amount of SO3 formed is very small. To improve
the yield of SO3, the reaction is carried out at around 4500C and 1.5-1.7atm pressure
in presence of V2O5 or Pt as catalyst.
2SO2 + O2 2SO3 ∆H = - 46.98kcal
These conditions are chosen by applying Le Chatelier's principle as explained
below.
Effect of temperature
Since the forward reaction is exothermic, at higher temperatures the
backward reaction i.e., the dissociation of SO2 is more favoured. However, at very
low temperature, the rate of combination of SO2 and O2 is very slow and at higher
temperature of about 4500C, the rate of formation of SO3 is high and rate of
decomposition of SO3 is minimum. Hence, the temperature range which best meets
kinetics and thermodynamics requirements for high yield in the synthesis of SO3 is
located in between 4000C to 5000C, with optimum temperature at about 4500C.
Effect of pressure
In the forward reaction i.e. formation of sulfur trioxide, the number of moles of
gaseous components is decreasing.
Δng = (2) - (2+1) = -1
Module 4 Lecture: 18 Sulfuric acid
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The formation of SO3 takes place with decrease in volume and hence
increase in pressure is expected to increase the rate of formation of SO3, i.e., rate of
forward reaction. However, it has been observed that there is no appreciable
change in the yield at higher pressures. Also, higher pressure will increase the rate of
corrosion of iron tower used in the process. Hence pressure of 1.5-1.7atm is usually
satisfactory.
The equilibrium constant in terms of partial pressure is given by
The amount of SO3 at various concentrations of SO2 and O2 can be estimated
by using partial pressures of the gases. If a and b are the moles of SO2 and O2
respectively and X is the mole of SO3 formed at equilibrium, then Kp is given by,
( )
( ) ( )
Rate of contact reactions
The main steps involved in the rate of contact reactions in heterogeneous
catalyst are as follows
Diffusion of the reacting gases to the catalyst
Adsorption of the gases on the surface of the catalyst
Chemical reactions taking place on the surface of the catalyst
Desorption of the reaction products from the surface of the catalyst
Diffusion of the reacted molecules away from the catalyst
It has been observed that the rate of oxidation of SO2 on the surface of
platinum catalyst is proportional to the pressure of SO2 and inversely proportional to
the square root of the pressure of SO3. The rate is independent of the pressure of
oxygen. The energy of activation on platinum surface is about 10kcal/mole as
against an activation energy of about 23-34kcal/mole for the promoted vanadium
catalyst. Maximum value of 34kcal/mole has actually been observed for pure V2O5
catalyst. The rate of oxidation of SO2 to SO3, on the surface of vanadium catalyst
largely depends upon the pressure of oxygen and weakly upon the pressure of SO2.
The rate of oxidation is determined by the following three steps.
Rate of absorption of reacting gases ( SO2 and O2) on the surface of the
catalyst
Chemical reactions between absorbed SO2 and O2 on the surface of the
catalyst
The rate of desorption of SO2 from the surface
Module 4 Lecture: 18 Sulfuric acid
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The actual reactions are very complicated, as they involve a series of
reactions between the gas, the catalyst, the promoters and the carriers.
Comparison of vanadium and platinum catalyst
Aspect Vanadium catalyst Platinum catalyst
Conversion Higher Lower and decrease with use
Investment Initially less, 5% replacement is
required per year
High, Lower life and highly
fragile
Catalyst poisoning Relatively immune to poison Poisoned, especially by
arsenic
Handling of SO2 Less (7-8%) High (8-10%)
Requirement per
1000kg
(100% acid)/day
14kg catalyst mass containing
7-8%V2O5
189gms
Operation of multistage convertor
SO3
Access ports
SO2 + O2 + N2 (sulfurous gases)
H.E.-1
H.E.-2
H.E.-3
Figure: Multistage reactor for the conversion of SO2 into SO3
Module 4 Lecture: 18 Sulfuric acid
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The apparatus in which SO2 is converted into SO3 is as shown in figure. It is
designed so as to achieve high rate of conversion along with highest possible
thermodynamic yields. The convertor is subdivided into several compartments
having separate layers of catalytic mass supported by meshes.
In four compartment reactor, upon entering the reactor from top , the
sulfurous gases have been heated to about 4000C by heat exchange carried out
earlier on the sulfurous gases themselves, the added air or the mixture of them are
heated up to about 6000C where upon they react. The rate of reaction is high but
the yield does not exceed 75%.
Upon leaving the first compartment the temperature of the partially
converted gases is lowered by 1000C in the gas-gas heat exchanger (HE-1), and
they are returned to the converter where, in correspondence with the temperature
of the catalytic bed in the second compartment, they are brought up to about
5000C and react to form further SO3 from SO2. The rate of reaction is lower but the
yield goes up to 85%.
The gases are again sent out of the reactor and their temperature is reduced
again by 1000C by means of heat exchanger (HE-2). Then returned to third
compartment where yields raised up to 95% by passing through the catalytic bed at
4800C. The rate of reaction is further lowered, but now only small amounts of gas to
be converted into SO3.
After lowering the temperature third time by external heat exchange (HE-3),
the gases are passed back to the reactor where they undergo on the catalytic bed
in the fourth compartment, final conversion at about 4500C, which gives yield of 98-
99%.
Major engineering problems
Design of multistage catalytical convertor for highly exothermic reaction.
Earlier two stage converter is used but nowadays the design of three or four
stages rather than conventional two stage operation are developed.
To optimize space velocity in catalyst chamber because it deals with
pumping cost or fixed charges of reactor
Thin catalyst beds of 30-50cm height used to avoid above difficultties. Yield
can drop due to longitudinal mixing if the convective gas velocity through
the bed is low
Removal of heat of absorption of SO3 in acid. Pipe coolers with water dripping
over external surface have been replaced by cast iron pipe with internal fins
to promote better heat transfer.
Pressure drop must be low, so, 8cm stacked packing is often used.
Module 4 Lecture: 18 Sulfuric acid
Dr. N. K. Patel
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PROPERTIES
Physical Properties
Molecular formula : H2SO4
Molecular weight : 98.08gm/mole
Appearance : Water white slightly viscous liquid
Boiling point : 2900C
Melting point : 100C
Density : 1.840gm/mL (liquid)
Solubility : Miscible with water in all proportions
Viscosity : 26.7cP (200C)
Aqueous sulfuric acid solutions are defined by their H2SO4 content in weight-
percent terms.
Anhydrous (100%) sulfuric acid sometimes referred to as ―monohydrate,‖
which means that it is the monohydrate of SO3.
Dissolve any quantity of SO3, forming oleum (―fuming sulfuric acid‖).
The physical properties of sulfuric acid and oleum are dependent on H2SO4
and SO3 concentrations, temperature, and pressure.
Chemical Properties
1. Dehydrating agent
Has a great affinity for water and the reaction is extremely exothermic.
A large amount of heat is produce due to formation of mono and dehydrates
(H2SO4.H2O and H2SO4.2H2O) on mixing acid with water. So while preparing
dilute solutions of H2SO4 the acid should be added to water slowly with
constant stirring. Never add water to the acid.
Used for drying almost all gases, except NH3 and H2S.
Its corrosive action on skin is also due to dehydration of skin which then burns
and produces itching sensation.
Due to dehydrating property, it chars sugar to give carbon.
C12H22O11 12C + 11H2O
Also, paper, starch, wood etc. are charred by conc. H2SO4 due to the
removal of water. It is also used in removing water from various substances
such as oxalic acid and formic acid.
COOH-COOH H2O + CO + CO2
2. Oxidising agent
Gives O2 on strong heating, hot conc. H2SO4 also acts as an oxidising agent.
3. Pickling agent
Finds application in pickling in which layers of basic oxides are removed
before electroplating, enameling, galvanizing and soldering.
Module 4 Lecture: 18 Sulfuric acid
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4. Acidic nature
Strong dibasic acid and forms two series of salts with alkalis. These are
bisulfates (HSO4-) and sulfates (SO4
-2).
USES
The largest single use is in the fertilizer industry.
Mostly in production of phosphoric acid, which in turn used to manufacture
fertilizers such as triple superphosphate, mono and diammonium phosphates
Used for producing superphosphate and ammonium sulfate.
Used as an acidic dehydrating reaction medium in organic chemical and
petrochemical processes involving such reactions as nitration, condensation,
and dehydration, as well as in oil refining, in which it is used for refining,
alkylation, and purification of crude-oil distillates
In the inorganic chemical industry e.g. in the production of TiO2 pigments,
hydrochloric acid, and hydrofluoric acid
In the metal processing industry e.g. for pickling and descaling steel, for
leaching copper, uranium, and vanadium ores in hydrometallurgical ore
processing, and in the preparation of electrolytic baths for nonferrous-metal
purification and plating
Certain wood pulping processes in the paper industry require sulfuric acid,
used in textile and chemical fiber processes and leather tanning
In manufacture of explosives, detergents and plastics
In production of dyes, pharmaceuticals
Module 4 Lecture: 19 Hydrochloric acid
Dr. N. K. Patel
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Module: 4
Lecture: 19
HYDROCHLORIC ACID
INTRODUCTION
Hydrochloric acid (HCl), also known as muriatic acid, is a solution of hydrogen
chloride in water. HCl exists in solid, liquid, and gaseous states and is water soluble in
all proportions.
The first hydrochloric acid was prepared through heating common salt and
sulfuric acid by Benedictine Monk and Basil Valentine in 15th century. Also, Libavius
prepared free hydrochloric acid by heating salt in clay crucibles in 16th century.
In the 17th century, Johann Rudolf Glauber used NaCl and H2SO4 for the
preparation of sodium sulfate in the Mannheim process, releasing hydrogen chloride
gas as a by-product. Joseph Priestley prepared pure HCl in 1772, and chemical
composition includes hydrogen and chlorine was proven by Humphry Davy in 1818.
Demand for alkaline substances increased during the Industrial Revolution in
Europe, Nicolas Leblanc developed cheap large-scale production of sodium
carbonate (soda ash). Using common salt, sulfuric acid, limestone and coal which
release HCl as a by-product. Until the British Alkali Act 1863 and similar legislation in
other countries, the excess HCl was vented to air. After the passage of the act,
waste gas is absorbed in water, producing hydrochloric acid on an industrial scale.
In the twentieth century, the Leblanc process was effectively replaced by the
Solvay process without hydrochloric acid by-product. Since hydrochloric acid was
already fully settled as an important chemical in numerous applications, the
commercial interest initiated other production methods, some of which are still used
today. After the year 2000, hydrochloric acid is mostly made by absorbing by-
product hydrogen chloride during a chemical manufacturing process such as
chlorination of hydrocarbons.
Since 1988, hydrochloric acid has been listed as a Table II precursor under the
1988 United Nations convention against illicit traffic in narcotic drugs and
psychotropic substances because of its use in the production of heroin, cocaine,
and methamphetamine.
Module 4 Lecture: 19 Hydrochloric acid
Dr. N. K. Patel
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MANUFACTURE
HCl is manufactured by various methods as follows
1. Synthesis from hydrogen and chlorine
2. From salt and sulfuric acid
3. As by-product from chemical processes
4. From incineration of waste organics
5. Hydrochloric acid solutions
1. Synthesis from Hydrogen and Chlorine
There is large demand in the market for water white acid. Such acid is
obtained by synthetic method, and most of the plants are based on this process.
Raw materials
Basis: 1000kg of Hydrochloric acid (98% yield)
Hydrogen = 28.21kg
Chlorine = 999.21kg
Sources of raw material
Both hydrogen and chlorine can be obtained during electrolysis of brine for
manufacturing of NaOH as described in Module: 3, Lecture: 13.
Also, hydrogen can be synthesized from any one methods of following which
are described in detail in Module: 2, Lecture: 4.
1. Lane process or iron steam process
2. Steam hydrocarbon process
3. Liquefaction of coal gas and coke oven gas
4. Bosch process or water gas-steam process
Reaction
H2 + Cl2 2 HCl ΔH = - 43.9kcals
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module 4 Lecture: 19 Hydrochloric acid
Dr. N. K. Patel
N P T E L
123
The plant consists of combustion chamber of structural carbon or lined with
silica bricks provided with cooling device which may consist even of cold-water
circulation in the shell. To ensure all the chlorine reacts with hydrogen, excess of 10%
hydrogen compare to chlorine is charged from the bottom of combustion chamber.
Also, care should be taken that the combustion chamber and length of ducting
which leads the gas to absorber should be sufficiently specious, otherwise
hydrochloric acid will contain free chlorine. The burning of hydrogen is started by
igniting the burner with an external air-hydrogen torch. Dry chlorine is passed into the
combustion chamber, where hydrogen burns in an atmosphere of chlorine to
produce HCl. The exothermic nature of the direct combination of both gases (H2
and Cl2) is such as to raise the temperature of the reagents, and the reaction
products to a point where they are incandescent. The reaction is carried out at
24000C with greenish flame. The gases are always kept above dew point to avoid
corrosion. The combustion chamber is then cooled externally by water and gas tight
lid is fitted at the top of the reactor which suddenly opens to allow the gases to
escape in case of emergency. Hydrochloric acid gas is cooled absorbed in water
or dilute HCl solution by passing through cooler and absorber through the
connecting pipe. The strength of acid produced is generally 32-33 %. The heat of
absorption of HCl in water is removed by spray of cold water outside the absorber.
The solution of HCl flows into a storage tank.
Anhydrous hydrogen chloride
Hot gases originating from combustion chamber are passing over anhydrous
CaCl2 or washing them with 98% sulfuric acid and then cooled and compressed to
60atm pressure. The cooled and compressed gas having 99.9% purity is filled in steel
cylinders.
Cold waterinlet
Chlorine
Hydrogen
Hydrogenburning
in chlorine
Co
mb
us
tio
nc
ha
mb
er
Dilute acid or wateradded under control
Exit for exhaust
gasExhaust fan
Water outlet
Hydrochloric acid
storage tank
Figure: Manufacturing of Hydrochloric acid from hydrogen and chlorine combustion
Module 4 Lecture: 19 Hydrochloric acid
Dr. N. K. Patel
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In another process, absorb the combustion gas into water and distilled it to
36% concentration of HCl. If one is to obtain 97% HCl at the top of the column. The
35% acid is cooled to -120C and aqueous liquid containing 50% HCl is left to
condense, while residual gases, when they have been de nebulized as compressed
to 60atm are of purity exceeding 99.5%.
Thermodynamics and kinetics
H2 + Cl2 2HCl ΔH = - 44kcal
Above exothermic reaction is much favoured by both by large evolution of
energy and the product gas leaving the chamber, thereby circumventing the fact
that equilibrium would otherwise be attained. The very fact that equilibrium is not
established also precludes the large increase in temperature from having a negative
effect on the yield of highly exothermic reaction.
On account of the existence of large energy barrier to the reaction, mixture
of molecular H2 and Cl2 is stable at ambient temperatures and in absence of
suitable wavelengths. Photons with frequencies which are able to furnishing the
activation energy can be produced by creating an electrical spark in a mixture of
molecular H2 and Cl2 or by first burning mixture of H2 with air and then gradually
replacing air with chlorine.
The initiation, propagation and termination of the chain reactions are as
follows
Initiation
Cl2 + hυ 2Cl•
Propagation
Cl• + H2 HCl + H+
H• + Cl2 HCl + Cl•
Termination
Cl• + Cl• Cl2 + heat
H• + H• H2 + heat
H• + Cl• HCl +heat
A large amount of heat is developed both from chain propagation reactions
and from chain termination processes, the continued renewal of the chain
propagators by thermal route is ensured over the long term. In brief reaction
between hydrogen and oxygen to produce hydrogen chloride is a chain reaction
with a high quantum yield.
Module 4 Lecture: 19 Hydrochloric acid
Dr. N. K. Patel
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Engineering aspect
The combustion chamber and ducting to absorber should be sufficiently
specious for avoiding wall effect. From physicochemical point of view if the walls of
reactor in which chain reactions takes place by their varying nature, development,
shape and orientation as to affect the chain carriers is called wall effect. In the
present case wall tends to interrupt the process by promoting the chain breaking
reaction (termination reactions). Physico chemically, chain terminators act as a third
body in a system which already consists of the reactant bodies.
2. The Salt–Sulfuric acid process
The reaction between NaCl and sulfuric acid occurs in two endothermic
stages.
Raw materials
Basis: 1000kg Hydrochloric acid
Sodium Chloride = 3206kg
Sulfuric acid = 2688kg
Sources of raw material
Sodium chloride can be obtained from sea water, salt lake and sub –soil
water as described in Module: 3, Lecture: 8.
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18
Reaction
NaCl + H2SO4 NaHSO4 + HCl
NaCl + NaHSO4 Na2SO4 + HCl
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module 4 Lecture: 19 Hydrochloric acid
Dr. N. K. Patel
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Salt (NaCl) and sulfuric acid are charged to the furnace. It is desirable to
keep one of the components in the reaction mixture in a liquid form in both steps.
The first step is carried out at the lower temperature compare to second step. Even
so, for liquefaction of NaHSO4, which is required to carry out in second step, material
is heated up to 4000C. Sodium sulfate in form of sludge is collected from the bottom
of the furnace. The product and unconverted sulfuric acid is sent to further
processing in which recovery of sulfuric acid and nitric acid in cooling tower and
absorber respectively.
3. As by-product from chemical processes
Over 90% of the hydrogen chloride produced as a by-product from various
chemical processes. The crude HCl generated in these processes is generally
contaminated with impurities such as unreacted chlorine, organics, chlorinated
organic and entrained catalyst particles. A wide variety of techniques are
employed to treat these HCl streams to obtain either anhydrous HCl or hydrochloric
acid. Some of the processes in which HCl is produced as by-product is the
manufacture of chlorofluorohydrocarbons, manufacture of aliphatic and aromatic
hydrocarbons, production of high surface area silica, and the manufacture of
phosphoric acid and esters of phosphoric acid.
4. From incineration of waste organics
Environmental regulations regarding the disposal of chlorine-containing
organic wastes have motivated the development of technologies for burning or
paralyzing the waste organics and recovering the chlorine values as hydrogen
chloride. Several catalytic and non-catalytic processes have been developed to
treat these wastes to produce hydrogen chloride.
HClStorage
H2SO4
Salt
Fuel
Na2SO4
Reactor
H2SO4
Cooler
Absorber
OffgasesH2O
Sc
rub
be
r
H2O
Figure: Manufacturing of Hydrochloric acid from salt and sulfuric acid
Co
ole
r
Module 4 Lecture: 19 Hydrochloric acid
Dr. N. K. Patel
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5. From hydrochloric acid solutions
Gaseous hydrogen chloride is obtained by partially stripping concentrated
HCl acid using an absorber–desorber system. The stripper is operated at a pressure
of 100–200kPa (1–2atm) for improved recovery of HCl. The overhead vapors
consisting of 97% HCl and 3% H2O is cooled to remove most of the water as
concentrated HCl, and the residual water vapor is removed by drying the gas with
sulfuric acid.
PROPERTIES
Molecular formula : HCl
Molecular weight : 36.5gm/mole
Appearance : Colourless liquid
Odour : Pungent
Boiling point : -850C
Melting point : -1140C
Density : 1.179gm/mL (35.2% HCl )
Solubility : Extremely soluble in water
Water solubility depends on temperature. At 760mmHg (1atm) pressure 1liter
of water dissolves 525.2 liters of HCl at 00C (46.15%w of HCl) and at 180C,
451.2 liter of HCl are dissolved (42.34%w of HCl).
Forms azeotropic mixture with water, containing 20.24% HCl which boils at
1100C.
Commercially available in 27.9%, 31.5% and 35.2%wt HCl solution in water.
Anhydrous HCl is available in steel cylinders because completely dry HCl is not
very reactive. But dry HCl often reacts only in the presence of catalysts.
Solution of hydrogen chloride in a polar solvent is strong acid and, therefore,
an aggressive reagent.
USES
Hydrogen chloride and the aqueous solution, muriatic acid, find application
in many industries.
Anhydrous HCl is consumed for its chlorine value, whereas aqueous
hydrochloric acid is often utilized as a non-oxidizing acid.
Used in metal cleaning operations, chemical manufacturing, petroleum well
activation, and in the production of food and synthetic rubber.
Used for the manufacture of chlorine and chlorides, e.g. Ammonium chloride
used in dry cell.
In the manufacture of glucose from corn starch.
For extracting glue from bones and purifying boneblack.
A saturated solution of zinc chloride in dilute HCl is used for cleaning metals
before soldering or plating.
Module 4 Lecture: 19 Hydrochloric acid
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It is also used in medicine and as laboratory reagent.
Aqua regia used for dissolving metal
Module: 4 Lecture: 20 Phosphorous
Dr. N. K. Patel
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Module: 4
Lecture: 20
PHOSPHOROUS
INTRODUCTION
Phosphorus is the chemical element that has the symbol P and atomic
number 15. As phosphorus was the 13th element to be discovered and can be used
in explosives, poisons and nerve agents is referred as "the Devil's element". It is
essential for life. It is a component of DNA, RNA, ATP, and also the phospholipids that
form all cell membranes.
Hennig Brand discovered phosphorous during his experiment on urine in 1669.
Robert Boyle was the first to use phosphorus to ignite sulfur-tipped wooden splints,
forerunners of our modern matches, in 1680.
Due to its high reactivity, phosphorus is never found as a free element on
Earth. Phosphorus as a mineral is present in its maximally oxidized state, as inorganic
phosphate rocks from which it can be extracted out. Phosphorous which is primarily
extracted from calcium phosphate rocks consider as an expensive mineral but, with
the increasing demand of phosphorous products like phosphoric acid, synthetic
fertilizer and phosphate salts. The situation demands modification in extraction
method as well as product manufacture.
Elemental phosphorus exists in two major forms
White phosphorus
Red phosphorus,
White phosphorus was first made commercially, for the match industry in the
19th century, by distilling off phosphorus vapour from precipitated phosphates, mixed
with ground coal or charcoal, which was heated in an iron pot, in retort. The
precipitated phosphates were made from ground up bones that had been
degreased and treated with strong acids. Carbon monoxide and other flammable
gases produced during the reduction process were burnt off in a flare stack. This
process became obsolete when the submerged arc furnace for phosphorus
production was introduced to reduce phosphate rock. The electric furnace method
allowed production to increase to the point where phosphorus could be used in
weapons of war.
Module: 4 Lecture: 20 Phosphorous
Dr. N. K. Patel
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PHOSPHATE ROCK
Phosphate rock is a natural mineral found as a geological deposit on a large
scale in the form of sedimentary rocks containing various amounts of calcium
phosphates. India alone is estimated to have about 140 million tons of rock
phosphate deposits, most of which are however, of low grade and with substantial
impurities unsuitable for the production of phosphate fertilizers.
Phosphate rock is used as a phosphatic fertilizer after grinding called
phosphorite or mineral phosphate, or as a primary source of phosphorus. Finely
ground rock (60-100mesh size) phosphate can be applied directly to the acidic soil.
For neutral or alkaline soils acid-treated rock phosphate (like superphosphate) is
used. Powdered rock phosphate is free-flowing and is easily amenable to handling
and storage. Crops like rubber, tea, coffee, apples and fruit plantations of oranges
are suitable for direct application of rock phosphate.
Francolite (calcium carbonate-fluorapatite) of formula [Ca5(PO4,CO3)3(F,OH)
is the most predominant mineral of phosphate. Four kinds of phosphate rocks are
recognized: hard rock phosphate, soft rock phosphate, land pebble phosphate and
river pebble phosphate, with the phosphorus content varying from 2 to 21 %.
Phosphate rock contains phosphorous in an apatite form which is water
insoluble. The citrate solubility can vary from 5 to 17 % of the total phosphorus,
depending on the chemical nature of the rock and the size to which it is ground.
The efficiency of the ground rock phosphate can be increased by
Mixing with soluble phosphorus and fertilizers
Mixing with elemental sulfur or sulfur-producing compounds
Using phosphate solubilizing micro-organisms
More than 90% of rock phosphate is used for production of superphosphate
and phosphoric acid. Less than 8 % is used directly as soil fertilizer and about 2 % as
animal and poultry feed.
Purification
Calcium phosphate is obtained after removal of various impurities present in
phosphate rock. The up grading of ore and removal of impurities is carried out by
Floatation of phosphate rock.
Clays (kaolinite, illite, smectites and attapulgite), quartz and other silicates
(feldspars), carbonates (calcite and dolomite), secondary phosphates (phosphates
bearing iron and aluminum) and iron oxides (geothite, hematite and magnetite) are
the common impurities which are associated with phosphate rock.
Module: 4 Lecture: 20 Phosphorous
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Phosphate ore beneficiation is done by many methods. Froth floatation is a
widely used technique in the phosphate industry. Froth floatation is generally
employed with siliceous ores when other less expensive or less complicated
techniques fail to produce phosphate concentrates suitable for chemical
processing. Prior to its conditioning for floatation, the floatation feed of phosphate
rocks is delimed.
In the floatation of phosphate ores, apatite particles are generally directly
transferred to the froth fraction (direct floatation) by using anionic collectors such as
fatty acids. The anionic collectors selectively attach themselves to the phosphate
particles, render them hydrophobic and lift them to the surface by the froth and air
bubbles formed. The mineral bearing froth may simply overflow the cells or paddles
or may be skimmed off. Quartz and other silicates are removed from the bottom of
the floatation cells.
A second stage of floatation may be required to remove silica from the
phosphate-rich float by cationic collectors (usually amines), when silica is floated
and the phosphate particles settle to the underflow.
A selective floatation of carbonates from phosphate rock is rather difficult
owing to the similarity in the physicochemical properties of the carbonate and
phosphate minerals. Several treatments have been proposed, including floatation,
calcination, acid washing, magnetic separation and heavy media separation for
the removal of free carbonates from the phosphates.
Uses
The most important use of phosphate rock is in fertilizers. Table is a compilation
of phosphate-rock treatment processes.
Module: 4 Lecture: 20 Phosphorous
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Table: Phosphate-rock processing, products and byproducts
Process Raw materials
and reagents
Main products
and derivatives
By- products
Acidulation Phosphate rock,
Sulfuric acid,
phosphoric acid,
hydrochloric acid
ammonia,
potassium chloride
Superphosphate,
phosphoric acid (wet
process) triple
super phosphate , mono
ammonium phosphate,
diammonium phosphate,
Fluorine
compounds
vanadium,
uranium (limited)
Electric-furnace
reduction
Phosphate rock,
siliceous flux, coke
(for reduction),
electrical energy,
condensing water
Phosphorus, phosphorus
pentoxide and halides,
phosphoric acid, triple
superphosphate, various
Na,K,NH4,Ca salts; mono
potassium phosphate
Fluorine
compounds,
CO, slag
(for RR ballast
aggregate,
fillers,
ferrophosphorus
Calcium
metaphosphate
Phosphate rock,
phosphorus, air or
oxygen, fuel
Calcium metaphosphate Fluorine
compounds
Calcination or
defluorination
Phosphate rock
silica, water or
steam, fuel
Defluorinated phosphate Fluorine
compounds
YELLOW PHOSPHORUS
Raw materials
Basis: 1000kg Phosphorus
Calcium phosphate = 6804kg
Sand = 2018kg
Coke = 1202kg
Carbon electrode consumption = 22.68kg
Electricity = 13000kWH
Reaction
2Ca3(PO4)2 + 10C + 6SiO2 CaSiO3 + P4 + 10CO
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module: 4 Lecture: 20 Phosphorous
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Rock phosphate is crushed as fine powder, so that complete transformation
to phosphorous pentoxide (P2O5) and finally element phosphorous is possible. The
powdered rock phosphate is mixed with sand and coke powder in the required
proportion and charged into electric furnace having electrodes at bottom. The
base of furnace is heated with carbon blocks as the temperature increases due to
electrical heating. Reaction of rock phosphate with sand starts at about 11500C
resulting into calcium silicate and P2O5. Further increasing the temperature to 15000C
carbon particle reacted with P2O5 there by phosphorous and carbon monoxide
formed which is collected from top outlet. Residual calcium silicate settles down at
bottom in form of slag which is taken out time to time from the outlet provided at
bottom of the furnace.
Product gases which is mixture of phosphorous and carbon monoxide is
cooled in a water cooler thereby phosphorous solidify and carbon monoxide gas is
separated. Purification of phosphorous is carried out by melting it and treating with
chromic acid (mixture of K2Cr2O7 and H2SO4). The carbon and silicon impurities are
removed due to oxidation. Pure phosphorous which is pale yellow colour is washed
with water before it is casted into sticks.
Yellow Phosphorous
Phosphate Rock
Figure: Manufacturing of Yellow Phosphorous
Electrostatic Precipitator
Sc
ree
n
A
C
Hopper
Molten Slag
Coke and Sand
Fines to Waste
Electric Furnace
Cooler
CO
Grinder
Sinterer
Module: 4 Lecture: 20 Phosphorous
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Recovery of the products
The fumes emerging from the furnace are first freed from any mineral and
other fine reagents which may have been carried up at less than 3000C so that the
phosphorous is not condensed out. The remaining gases consist of phosphorous, CO
and SiF4, are sent to a bottom of tower in which water is sprayed from two different
heights. Care should be taken that temperature should not drop below 600C. The
phosphorous condenses out but does not solidify, is collected under water which
reacts with the SiF4 gas, converting into metasilicic and fluorosilicic acids.
SiF4 + 3H2O H2SiO3 + 2H2SiF6
The fluorinated components are subsequently recovered from the solution.
CO which is completely freed from phosphorous and fluorine compounds is then
cooled, dried and subsequently used as a fuel.
The slags consisting of CaSiO3, which are produced in the furnace and
subsequently discharged from outlet provided at base of furnace, are good
additives for cements, air-port runway construction and antiskid conglomerates.
The liquid phosphorous, after decolouration with activated carbon is filtered
and solidify to yellow phosphorous. It is stored under water.
Kinetics and thermodynamics
It is important to ensure that three component i.e. phosphate minerals, silica
and coke are thoroughly and homogeneously mixed with one another. To do this if
they are obtained from flotation processes, the phosphate mineral must be
agglomerated or converted into nodules after addition of a small amount of
quartzite in rotating furnaces which are heated by utilizing the combustion of
carbon monoxide, which is formed during the process, in conjugation with that of
fuel oil.
The arc resistance furnace is responsible for providing the energy, by the
conversion of electricity into thermal energy, required for the above endothermic
reaction which requires 5894kcal/kg.
India has abundant supply of calcium phosphate, salt and coke, but the
manufacture of phosphorus largely depends upon the production of cheap electric
power.
RED PHOSPHORUS
Raw material
Yellow phosphorous
Module: 4 Lecture: 20 Phosphorous
Dr. N. K. Patel
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Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Batch process
Yellow phosphorous obtain from earlier process is taken into iron pot provided
with safety outlet and thermometer jackets. Conversion of yellow phosphorous to
red phosphorous is exothermic reaction and strict maintenance of temperature in
range 2400C to 2500C in required as conversion do not take place below 2400C and
above 2500C accident chances are there. After complete conversion product is
washed with NaOH solution as yellow phosphorous is soluble but red phosphorous is
not soluble in NaOH
Continuous process
In the process liquid white phosphorous is maintained at boiling point for
5-6hrs to achieve 35 to 50% conversion. The product is taken into screw conveyer in
which unreacted phosphorous vaporizes which is recrystallize and recycled. The red
phosphorous is of high purity and therefore doesn't require further purification.
PROPERTIES
Molecular formula : P
Molecular weight : 30gm/mole
Appearance : White, red and black solid
Odour : Irritating odour
Boiling point : 280.50C
Red Phosphorous
Figure: Manufacturing of Red Phosphorous
Temp. 2400C
Ashpit
Burning Phosphorus
Iron safety tube
Thermometer
Fire brick setting
Coke Furnace
Casr iron pot
Charged with white or yellow
phosprus
Washer
Water
Co
ole
r
Washer
Na2CO3
Dryer
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Melting point : 44.20C
Density : 2.2-2.34gm/ml
Solubility : Insoluble in water and soluble in carbon disulfide
White phosphorus is a highly reactive, waxy, white-yellow, transparent solid
with acrid fumes. It emits a weak green glow (luminescence) in the presence
of oxygen. White phosphorus ignites spontaneously in air.
USES
Red phosphorus is used in fireworks, smoke bombs and pesticides.
Black phosphorus no significant commercial uses due to least reactivity.
White phosphorus and zinc phosphate are mainly used as a poison for rats.
Used in making incendiary bombs, tracer bullets and for producing smoke
screen
Used in fertilizers, which provides phosphate as required for all life and is often
a limiting nutrient for crops.
Used in the manufacture of PCl3, PCl5, P2O5 and phosphorus bronze
Organophosphorus compounds used in detergents, pesticides and nerve
agents, and matches
Phosphorus is one of the most essential mineral in the body and is ranked
second to calcium. However, the deficiency of phosphorous is relatively rare
About 80% of all phosphorus is present in human body in the form of calcium
phosphate in the teeth and bones.
It also participates in several vital functions of the body, such as energy
metabolism, synthesis of DNA and the absorption and utilization of calcium.
Phosphorus plays a role in facilitating optimal digestion.
It helps in the normal functioning of the kidneys and ensures proper discharge
of wastes.
Adequate levels in body is essential to maintain normal brain functions
Phosphorus helps maintain a good hormonal equilibrium.
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Module: 4
Lecture: 21
PHOSPHORIC ACID
INTRODUCTION
Phosphoric acid (H3PO4), also known as orthophosphoric acid or
phosphoric(V) acid, is a mineral (inorganic) acid. Orthophosphoric acid molecules
can combine with themselves to form a variety of compounds which are also
referred to as phosphoric acids.
Amongst the mineral acids, phosphoric acid stands an special status as it is
used for specialty application including anticorrosive and food industry out of
number of processes available only the latest are discussed below which includes
applications of electric furnace, blast furnace and process including oxidation and
hydration of phosphorous or the wet process which uses sulfuric acid and rock
phosphate.
The continuous process of phosphoric acid production uses liquid white
phosphorous at the boiling condition for 5 to 6hrs so that about 35 - 50 % white
phosphorous is converted to red phosphorous. The hot red phosphorous is taken in a
screw conveyer which along with inert gases gives a solid pure red phosphorous as
product. Red phosphorous finds the application in manufacturing of matchsticks,
chlorides of phosphorous as PCl3, PCl5, phosphorous oxide as P2O5 and phosphor
bronze etc.
MANUFACTURE
The modern manufacturing methods of phosphoric acid are following:
1. Using phosphate rock and blast furnace
2. Using phosphate rock and electric furnace
3. Oxidation and hydration of phosphorous
4. Wet process or from sulfuric acid and phosphate rock
1. Using phosphate rock and blast furnace
The blast furnace process was widely used in the first three decades of 20th
century. Resulting phosphoric acid can be used in manufacturing of insecticide,
pesticides, detergents etc. but not for fertilizers.
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Raw materials
Basis: 1000kg phosphoric acid (100%)
Phosphate rock = 2290kg
Sand (Silica) = 6800kg
Coke = 3175kg
Briquette binder = 227kg
Air = 450000ft3
Reactions
Ca3(PO4)2 + 3SiO2 + 5C 2P + 5CO + 3CaSiO3
2P + 5CO + 5O2 P2O5 + 5CO2
P2O5 + 3H2O 2H3PO4 85-90% yield
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Phosphate rock is pulverized and mixed with coke powder and binder is
compressed to 5000 psi resulting into the briquettes. Briquettes are dried and
charged along with sand and additional coke powder from top of the blast furnace.
The preheated air (1000 – 11000C)is charged from bottom of the blast furnace via
tuyere. A tuyere is cooled copper conical pipe numbering 12 in small furnace and
up to 42 in large furnace through which hot air is blown in to the furnace. Preheated
air leads to burning of briquettes giving temperature rise up to 13700C. The coke acts
Briqueticpress
pressure 5000 psi
Dustcollector
Hotblaststove
Compressedair
Steamboiler
Phosphoric acid
SteamTo phosphorous plant
Phosphporous vapour
Sand
CokeBinder
Phosphate rock
SlagFerro
phosphorous
Hy
dra
ter
Water
Figure: Manufacturing of Phosphoric acid using blast furnace
BlastFurnace
Co
ttre
llp
rec
ipit
ato
r
Water
Air
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as reducing agent as well as fuels. About 760kg of coke is consumed in reduction of
phosphate rock to phosphorous and remaining generates heat by combustion with
air. Reaction is completed in the furnace itself producing P2O5 and calcium silicates
as slag. The product gases also contain carbon monoxide and nitrogen along with
dust particles. For purification, it is passed through cyclone separator and
phosphorous condenser. Thus, P2O5 and elemental phosphorous are separated out.
Hot P2O5 gases are cooled in the heat exchanger. Therefore, superheated steam is
produced and a part of gas is taken into regenerative blast furnace. As a result the
entire phosphorous and phosphorous pentoxide is cooled and purified before taken
into hydrating towers. Purification of phosphoric acid includes removal of arsenic by
hydrogen sulfide treatment followed by filtration.
Engineering aspects
Blast furnace
Blast furnace is made of high temperature resistant refractories brick. Blast
furnace have accessories of hot blast stove for supply of compressed preheated air
having temperature 1000 – 11000C, briquette press for preparation of briquettes of
Phosphate rock and coke, dust collector (cyclone separator) for removal of dust
particles from product stream. Also, two outlets for removal of slag and
ferrophosphorous are provided at the bottom of the furnace.
The top of the blast furnace is closed as it operates at high top pressure. There
are two different systems are used for charging of briquettes, coke and silica. One is
having double bell system which is often equipped with movable throat armour and
other is bell less top as shown in figure.
Also, there are two construction techniques to support the blast furnace as
shown in figure.
Module: 4 Lecture: 21 Phosphoric acid
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Animation
One utilized lintel or support ring at the bottom of the shaft upon which the
higher level of the furnace rests. The other is free standing construction requiring
and independent support for the blast furnace top and gas system. The required
expansion both thermal as well as pressure for the installation is below for the lintel
i.e. in bosh/belly area in lintel type furnace, while compensator for expansion in the
free standing furnace is at the top.
2. Using phosphate rock and electric furnace
Raw materials
Basis: 1000kg phosphoric acid (100%)
Phosphate rock = 2225kg
Sand (silica) = 680kg
Coke breeze = 400kg
Caron electrode = 8kg
Air = 100000ft3
Electricity = 4070KWH
Reactions
Ca3(PO4)2 + 3SiO2 + 5C 2P + 5CO + 3CaSiO3 ΔH = - 364.8 kcals
2P + 5CO + 5O2 P2O5 + 5CO2
P2O5 + 3H2O 2H3PO4 87-92% yield ΔH = - 44.9 kcals
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Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
The phosphate rock is reduced to elemental phosphorous by the action of
coke and heat in the presence of sand in electric arc furnace subsequent oxidation
of phosphorous gives phosphorous pentoxide which on hydration gives the product
phosphoric acid.
Phosphate rock after proper grinding and primary purification is taken into
sintering oven where it is nodulized and granulized so that fast oxidation of the
separated phosphorous takes place. Temperature of 10950C is maintained in electric
furnace so that maximum amount of elemental phosphorous extracted out and
oxidation takes place. Since fluoride of phosphorous and calcium are the common
impurity which reacts with sand giving flourosilicates as the slag.
The gases from the furnace, phosphorous and carbon monoxide are
removed by the suction process and the oxidation product P2O5 is taken into
hydration column which gives P2O5 to H3PO4 at about 850C. Purification of
phosphoric acid is carried out by H2S to remove Arsenic, H2SO4 to remove calcium
salts and Silica to remove fluorides. All the byproducts are removed before
concentrating the acid and filtering it as final product.
Sintering&
Sizing
Hy
dra
ter
Sand
Cokebreeze
Phosphaterock
SizeFerro
phosphorous
Water
Air
Water CO2
H2S / H2O
Phosphoric acid 85 %
Water
Sandfilter
Figure: Manufacturing of Phosphoric acid using Electric furnace
ElectricFurnace2400 oF
Co
ttre
llp
rec
ipit
ato
r
Purifier
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Advantages of electric arc furnace over other methods
The advantage of the electric furnace process lies in its ability to use a low-
grade phosphate rock, provided the major impurity is silica. Also, iron and aluminum
oxides are not objectionable as in the wet process. Siliceous rocks containing 24%
phosphorus as P2O5 are acceptable. The by-product carbon monoxide, is used as a
fuel for calcination.
3. Oxidation and Hydration of phosphorous
Raw materials
Basis: 1000kg phosphoric acid (100%)
Phosphorus = 300kg
Air = 46000ft3
Steam = variable
Water = variable
Reactions
2P + 2½O2 P2O5
P2O5 + 3H2O 2H3PO4 (94 – 97% yield)
Manufacture
At the locations away from phosphate rocks mines from purified elemental
phosphorous is oxidized and hydrated to give phosphoric acid. In the manufacturing
process molten phosphorous is sprayed into combustion chamber along with
preheated air and superheated steam. Combustion of phosphorous increases the
temperature up to 19800C. Furnace design depends on the requirement with
respect to quantity and quality. They are made of acid proof structural bricks,
graphite, carbon and stainless steel.
Block diagram of manufacturing process
Diagram with process equipment
Animation
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The gases from furnace which mainly contains P2O5, steam, N2 and small
quantity of oxygen is taken into a hydration column where counter current mixing of
dilute phosphoric acid and the gases gives the product concentrated H3PO4 of 75%
to 85% concentration. Remaining acid is trapped into packed column or
electrostatic precipitator.
4. Wet process or from sulfuric acid and phosphate rock
The wet process is according to the acids (sulfuric acid, nitric acid or
hydrochloric acid) used to decompose the phosphate rock. The process using
sulfuric acid is the most common among all particularly for producing fertilizer grade
phosphoric acid.
The wet process phosphoric acid, also called as green acid. Depending upon
the hydrate forms of calcium sulfate produced during the wet process, it is classified
as anhydrate, hemi hydrate and dihydrate. The hydrate form is controlled mainly by
temperature and acid concentration.
Phosphorous
Air
Steam
Water
COMBUSTIONCHAMBER
Se
pe
rato
rC
oo
ler
85% Phosphoric acid
Cy
clo
ne
se
pe
rato
r
Gla
ss
wo
ol
filt
er
Vent
Hy
dra
ter
Figure: Manufacturing of Phosphoric acid by Oxidation and Hydration
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Anhydrite (n=0) process is not commercially viable due to high temperatures
requirement and the higher rate of corrosion.
The dihydrate process is the most popular due to simplicity and ability to use
a wide variety of phosphate rocks in the process. Hemihydrate processes produce
phosphoric acid of a relatively high concentration without the need of the
concentration step. There is also some attentiveness in the two-stage processes that
involve crystallization in the hemi-hydrate form followed by recrystallization in the
dihydrate form, with or without filtration or centrifugation.
Raw materials
Basis: 1000kg phosphoric acid (100%)
Phosphate rock = 1635kg
Sulfuric acid = 1360kg
Reaction
Ca10F2(PO4)6 + 10H2SO4 + 20H2O 10CaSO4.2H2O + 2HF + 6H3PO4
Manufacture
There are two processes i.e. dihydrate and hemihydrates (CaSO4.2H2O and
CaSO4.1/2H2O) are used for production of phosphoric acid.
Aspect Dihydrate process Hemihydrate process
Strength of sulfuric acid 78% 95%
Operating temperature Below 800C 1000C
Resistance to material Less High
Digestion time High Short
P2O5 content in product 33% 38%
Quantity of calcium sulfate High Small
Sulfuric acid on reaction with phosphate rock along with precipitation of
calcium sulfate results into the formation of phosphoric acid. The process is simple
and requires grinding of phosphate rock reacting with dilute phosphoric acid so that
melt is produced which in a reactor as mixed with concentrated sulfuric acid for 4 to
8hrs in the temperature range of 75-800C. Lot of air is required to control the
temperature. Resulting gases includes HF and P2O5 which in the absorption tower is
separated and finally treated to give fluorosilicates and dilute phosphoric acid. The
main product in the liquid form which is phosphoric acid and calcium sulfate is
filtered and washed. Thus, gypsum and phosphoric acid are separated and after
minor purification the phosphoric acid is concentrated into the evaporator.
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Block diagram of manufacturing process
Diagram with process equipment
Animation
Throughout the plant, corrosion resistant materials of construction must be
used. The most common ones are structural carbon or nickel alloy for evaporator
heat exchangers; rubber or carbon-brick for reactor linings; polyester-fiber glass in
pipes, ducts, and small vessels. Yield of phosphoric acid based on phosphorus
content of raw material is 95%
Phosphate rock
Sulphuric acid
Tank Reactor
To Fluorine Scrubber
Cooling Air
FilterFeedTank
Water
AirHot
WaterSuction
Gypsum To
WasteProduct acid
Titing Pan Filter
Vaccum
Recycled Weak Phosphoric Acid
Figure: Manufacturing of Phosphoric acid by Wet process
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Kinetics and thermodynamics
The average retention time in the reactor system is about eight hours. The
direct contact between phosphate rock and sulfuric acid are prevented, so that
maximum phosphorous is extracted from the rock and formation of easily filterable
crystal of gypsum. A high concentration of free sulfuric acid would result in the
formation of a coating of calcium sulfate on the phosphate rock, thereby blocking
further reaction. On the other hand, a high concentration of calcium ions in the
slurry would increase the amount of phosphate co- crystallized with gypsum. In order
to maintain a uniform composition of slurry, the incoming stream of sulfuric acid and
phosphate rock is mixed and agitated as rapidly and as completely as possible to
ensure homogeneity. Most of the modern plants use about 96 % pure sulfuric acid. Its
mixing with dilute phosphoric acid generates heat which is used to evaporate the
water and volatilize fluorine compounds (mainly SiF, and HF).
Three methods of cooling are generally in use: (a) blowing air on to the slurry,
(b) blowing air across the slurry, and (c) flash cooling under vacuum.
The fumes emerging from the reactors and the digesters are sent to the
fluorine recovery unit, while the suspension of the digesters are filleted in the first
compartment of a continuous filter, which yields phosphoric acid containing about
33% of P2O5.
Engineering aspects
Selection of phosphate rock
The plant is designed in such a way that can be used for blend of rocks from
different sources. The plants have extra capacity for grinding, filtration and slurry
handling systems to take care of variation in the rock composition. As phosphate
rock is a complex raw material that affects plant operation in numerous ways, a
thorough chemical and mineralogical evaluation of the quality should be made
before selecting a phosphate rock or changing the source to another. However, a
trial run in a pilot plant is needed for complete evaluation of the rock. The
phosphate rock used in the process is of as high a grade as possible, usually ranging
from 30 to 35 % P2O5 and 1 to 4 % iron and aluminum.
Before attack by acids, mineral must be crushed so that 60 – 70 % of it passes
through 200 mesh sieve. Crushed mineral is calcined to remove organic impurities,
because the presence of organic substances promotes the formation of foams
which makes it difficult to filter off calcium sulfate. The purified mineral is again
crushed to powder form by milling and is premixed first with recycled phosphoric
acid and then fed with sulfuric acid. The recycle phosphoric acid contributes to
attack on the mineral and disperse both heat of reaction and heat of dilution of
H2SO4, thereby facilitating crystallization of the calcium sulfate. Also, these steps
favours both precipitation, of readily filterable chalk consisting of minutes crystals of
Module: 4 Lecture: 21 Phosphoric acid
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CaSO4.2H2O or CaSO4.1/2H2O which act as crystallization nuclei, as well as digestion
in suitable containers, after the attach of reaction mixture.
Grinding of rock materials
Wet or dry grinding of Phosphate rocks is required depending upon the raw
materials. Fine grinding is required in case of multi-compartment digesters and poor
sulfate control. Most of the old plants use dry grinding, and a majority of the new
plants use wet grinding with a ball mill or ring roller mills having air classification. Slurry
containing 62 to 70% solid is produced. Wet grinding requires about 30 to 40% less
power and there is no atmospheric pollution by dust. The main disadvantages of this
process are that the balls and the mill lining wear out faster, and the amount of
recycled waste water that may be required in phosphoric acid production is
reduced. Also, it is necessary to maintain the ratio of solids while grinding.
Handling and storage of phosphate rock
Phosphate rock are stored in dry conditions and protected against rain, wind
and freezing weather. Relatively coarse rocks can be stored in piles. To ensure the
constant supply, the storage capacity should ideally be 1.5 times the largest
shipment.
Amount of sulfuric acid
As the by-product acid may contain some impurities, most phosphoric acid
plants have on site facilities for producing sulfuric acid from sulfur or pyrites. Sulfuric
acid of 93 to 98% concentration is used. The sulfuric acid requirement is calculated
assuming its amount required to combine with calcium present in the rock to form
calcium sulfate. Also, considering that about 15% of fluorine combines with calcium
oxide to form calcium fluoride, the sulfuric acid requirement is calculated. For a high
grade phosphate rock, H2SO4 required is 2.5 tons per ton of phosphorus; and for low
grade rocks, it is 3.15 tons per ton of phosphorus.
Filtration of gypsum
Filtration is carried out as efficiently and economically as possible. All modern
plants use continuous horizontal vacuum filters. The other widely used filters are tilting
pans, rotary filters, rotary table filters and belt filters. Some of the product acid is
recycled to the digestion step to control the percentage of solids in the slurry, which
is normally 35 to 45 %.
Filters are characterized by their surface area and the rate of rotation (in a
rotary filter) or the rate of travel (in a belt filter). The filtration rate is also affected by
the size and shape of gypsum crystals which, in turn, are decided by the type of
phosphate rock, crystal shape modifiers, control of reaction conditions, sulfate
concentration, slurry re-circulation, phosphoric acid concentration, etc.
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The filtration rate is also affected by temperature, concentration, viscosity of
the acid, the desired recovery, the amount of vacuum, the design of the filter and
insoluble impurities in the rock like clay. The normal filtration rates reported in the
production are 2 to 18 tons/m2/day.
Purification
Phosphoric acid which is intended for use in fertilizers does not require any
purification but for chemical and food products it must be purified. The block
diagram of purification processes is as follows.
Sludge disposal
The sludge usually contains gypsum, fluosilicates, iron and phosphate
compounds. Acids containing sludge can be used for onsite fertilizer (triple
H3PO4
(Impure)
Adjustment to pH = 2
Adjustment to pH = 5
Reductions
Adjustment to pH = 8.5
Precipitation of anhydrous and hydrated
CaSO4
Filtration
H3PO4
Purified
Na2SiF6
BaCO3
As2S3, BaSO4, PbS, etc.
Al(OH)3, Fe(OH)3, Mn(OH)2, MnO(OH) etc.
Removal of precipitates
NaOH or Na2CO3
H2S
Fe
Ca(OH)2
H2SO4
Figure: Purification of Phosphoric acid
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superphosphate) production. Most of the phosphorus (as P2O5) in the sludge is
citrate-soluble but not water-soluble.
The sludge may be used in the production of non- granular mono-ammonium
phosphate which is used as an intermediate in the production of compound
fertilizers. There is no economical method for using sludge solids where phosphate
fertilizers are sold on the basis of water solubility. Precipitation after concentration to
54 % phosphorus (as P2O5) is slow and never so complete as more precipitate will
form on standing.
PROPERTIES
Molecular formula : H3PO4
Molecular weight : 97.994gm/mole
Appearance : White solid or colourless viscous liquid above 420C
Boiling point : 1580C (decompose)
Melting point : 42.350C (anhydrous)
29.320C (hemihydrate)
Density : 1.885gm/mL (liquid)
1.685gm/mL (85% solution)
2.030gm/mL (crystal at 250C)
Solubility : Soluble in water
Viscosity : 147cP (100%)
USES
Used for preparation of hydrogen halides
Used as a "rust converter", by direct application to rusted iron, steel tools, or
surfaces. It converts reddish-brown iron(III) oxide, Fe2O3 (rust) to black ferric
phosphate, FePO4
Food-grade phosphoric acid is used to acidify foods and beverages such as
various colas.
Used in dentistry and orthodontics as an etching solution, to clean and
roughen the surfaces of teeth where dental appliances or fillings will be
placed.
As an ingredient in over-the-counter anti-nausea medications that also
contain high levels of sugar (glucose and fructose).
Used in many teeth whiteners to eliminate plaque.
Used as an external standard for NMR and HPLC
As a chemical oxidizing agent for activated carbon production
As the electrolyte in phosphoric acid fuel cells and is used with distilled water
(2–3 drops per gallon) as an electrolyte in oxyhydrogen (HHO) generators.
Also, used as an electrolyte in copper electro polishing for burr removal and
circuit board planarization.
As a flux by hobbyists (such as model railroaders) as an aid to soldering.
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As common wet etching agent in compound semiconductor processing,
Hot phosphoric acid is used in micro fabrication to etch silicon nitride (Si3N4). It
is highly selective in etching Si3N4 instead of SiO2, silicon dioxide.
As a cleaner by construction trades to remove mineral deposits, cementitious
smears, and hard water stains.
As a chelant in some household cleaners aimed at similar cleaning tasks.
Used in hydroponics pH solutions to lower the pH of nutrient solutions.
As a pH adjuster in cosmetics and skin-care products.
As a dispersing agent in detergents and leather treatment.
As an additive to stabilize acidic aqueous solutions within specified pH range
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Module: 5
Lecture: 22
CEMENT INDUSTRIES
INTRODUCTION
Cement is broadly described as material having adhesive and cohesive
property with capacity to bond the material like stone, bricks, building blocks etc.
Cement is a binder, a substance that sets and hardens independently, and can
bind other materials together. Cements are inorganic material that shows the
cementing properties of setting and hardening when mixed with water. Cement is
prepared from calcareous (Ca) material and argillaceous (Al + Si) material.
Cement has property of setting and hardening under water by virtue of
chemical reaction of hydrolysis and hydration. Therefore, cements are generally
divided into two types hydraulic and non-hydraulic that is on the basis of their setting
and hardening pattern. Hydraulic cements harden because of hydration, chemical
reactions that occur independently of the mixture's water content; they can harden
even underwater or when constantly exposed to wet weather. The chemical
reaction that results when the anhydrous cement powder is mixed with water
produces hydrates that are not water-soluble. Non-hydraulic cements must be kept
dry in order to retain their strength. Portland cement is example of hydraulic cement
material while ordinary lime and gypsum plaster are consider as example of non-
hydraulic cement.
Cement is used for structural construction like buildings, roads, bridges, dam
etc. The most important use is the production of mortar and concrete the bonding
of natural or artificial aggregates to form a strong building material that is durable in
the face of normal environmental effects.
Both cement and concrete are different, because the term cement refers to
the material used to bind the aggregate materials of concrete. Concrete is a
combination of a cement and aggregate.
In the last couple of decades of eighteenth century, modern hydraulic
cements began to be developed due to fulfill following requirement
For finishing brick buildings in wet climates
Development of strong concretes
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Hydraulic mortars for masonry construction of harbor works, etc., in contact
with sea water
As the good quality building stone became expensive and construction of
prestige buildings from the new industrial bricks, and to finish them with a stucco to
imitate stone became the common practice. Hydraulic lime was favored for this, but
the need for a fast set time encouraged the development of new cements.
James Parker developed cement from clay minerals and calcium carbonate
and patented as Roman cement in 1796. It was made into a mortar with sand, set in
5–15 minutes. The success of "Roman Cement" led other manufacturers to develop
competing products by burning artificial mixtures of clay and chalk.
In the first decade of nineteenth century, it was proved that the "hydraulicity"
of the lime was directly related to the clay content of the limestone from which it
was made first by John Smeaton and then by Louis Vicat. Vicat produce artificial
cement by burning of chalk and clay into an intimate mixture in 1817. Also, James
Frost produced "British cement" in a similar manner around the same time, and
patented in 1822. At the same time Portland cement, was patented by Joseph
Aspdin in 1824.
"Setting time" and "early strength" are important characteristics of cements.
Hydraulic lime, "natural" cements, and "artificial" cements all rely upon their belite
content for strength development. Belite develops strength slowly. Because they
were burned at temperatures below 1250°C, they contained no alite, which is
responsible for early strength in modern cements. In early 1840s the first cement to
consistently contain alite was made by William, who is son of Joseph Aspdin. This
was what we call today "modern" Portland cement. Vicat is responsible for
establishing the chemical basis of these cements, and Johnson established the
importance of sintering the mix in the kiln.
William Aspdin's innovation has high manufacturing costs but the product set
reasonably slowly and developed strength quickly, thus opening up a market for use
in concrete. The use of concrete in construction grew rapidly from 1850 onward, and
was soon the dominant use for cements. Thus Portland cement began its
predominant role.
But in the early 1930s it was discovered that, Portland cement had a faster
setting time it was not durable especially for highways. These leads to development
of some specialty cement based on the application and requirement of strength
and setting time.
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CLASSIFICATION
Based on source of cement
1. Natural cement
2. Artificial cement
1. Natural cement
Natural cement is obtained by burning and crushing of 20-40% clay,
carbonate of lime and small amount of magnesium carbonate. It is brown in colour
and best variety known as Roman cement. The natural cement resembles very
costly element hydraulic lime and sets very quickly and strongly as compare to
artificial cement. It finds very limited application
2. Artificial cement
Artificial cement is obtained by burning of calcareous mixture at very high
temperature. Mixture of ingredients should be intimate and they should be in correct
proportion. Calcined product is known as Clinker. A small quantity of gypsum added
to clinker and pulverized to fine powder is known as cement or ordinary cement or
normal setting cement. After setting, this cement closely a variety of sandstone
which is found in abundance in Portland in UK. Therefore, it is also known as Portland
cement.
Based on broad sense cement
1. Natural cement
2. Puzzolana cement
3. Slag cement
4. Portland cement
1. Natural cement
It is prepared from naturally occurring lime stone by heating it to a high
temperature and subsequently pulverizing it. During heating both siliceous and
calcareous material are oxidized and combined to give calcium silicates and
calcium aluminates.
2. Puzzolana cement
It is the material which when mixed with lime without heating gives hydraulic
cement. They mainly contains silicates of aluminum, iron and calcium natural
Puzzolana which is found in deposits of volcanic ash consist of glassy material and
simple mixing and grinding gives the cement. Similarly slaked lime also gives
Puzzolana cement but they are the cement of ancient time and at present hardly
used.
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3. Slag cement
It is made by mixing blast furnace slag and hydrated lime. Furnace slag
largely contains silicates of calcium and aluminum which is granulated by pouring it
into cold water. Later it is dried and mixed with hydrated lime and the mixture is
finally powdered to increase the rate of setting. Accelerator like clay, salt or caustic
soda may be added.
4. Portland cement
It is refine powder of calcined product of clay and lime stone. It has
controlled composition and therefore setting property. It is named after the paste of
cement with water which resembled in colour and hardness to the Portland stone.
Based on the application, appearance and constituent of cement
1. Acid resistance cement
2. Blast furnace cement
3. Coloured cement
4. White cement
5. Rapid hardening cement
6. High alumina cement
7. Puzzolana cement
8. Hydrophobic cement
9. Expanding cement
10. Low heat cement
11. Quick setting cement
12. Sulfate resisting cement
1. Acid resistance cement
It is composed of
Acid resistant aggregates like quartz
Additives such as Na2SiF6
Aqueous solution of sodium silicate or sodium glass
Sodium fluorosilicate accelerates the hardening process of soluble glass and
increase the resistance to acid.
Soluble glass (water solution of sodium or potassium silicate) is used as binding
material.
The cement has poor water resistance and fails when attacked by water or
weak acids. By adding 0.5% linseed oil or 2% ceresit, its resistance to water is
increased and cement is known as acid and water resistance cement.
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It is used in acid resistant and heat resistant coatings or insulations in chemical
industry.
2. Blast furnace cement
The cement is prepared from slag obtained from blast furnace. Slag is the
waste product in manufacturing of pig iron and contains the basic elements of
cement like alumina, lime and silica. Clinkers of cement are ground with 60-65% slag.
The properties are same as ordinary cement except less strength in early days. It
requires longer curing periods.
3. Coloured cement
It can be obtained by intimately mixing mineral pigments of desired colour
with ordinary cement. The amount of colouring material may vary from 5 to 10 %. If
it exceeds 10 %, the strength of cement is affected. Chromium oxide gives green
colour, while cobalt imparts blue colour. Iron oxide in different proportions gives
brown, red or yellow colour. Manganese dioxide is used to produce black or brown
coloured cement.
Coloured cements are widely used for finishing of floors external surfaces,
artificial marble, window sill slabs, textured panel faces, stair treads etc.
4. White cement
It is a variety of ordinary cement having white colour. It is prepared from
colourless oxides of iron, manganese or chromium. For burning of this cement, oil
fuel is used instead of coal. It should not set earlier than 30 minutes. It should be
carefully transported and stored in closed containers only. It is more costly than
ordinary cement because of specific requirements imposed upon the raw materials
and the manufacturing process.
It is used for floor finish, plaster work ornamental work etc.
5. Rapid hardening cement
The cement is slightly costly than ordinary cement. Initial and final setting
times of it are the same as those of ordinary cement. But it attains high strength in
early days due to following facts
Very fine grinding
Burning at high temperatures.
Increased lime content in cement composition.
Advantage
As it sets rapidly, construction work may be carried out speedily.
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Increased the frequency of use of formwork of concrete, due to possible
earlier removal
Structural members constructed with this cement may be loaded earlier.
Requires short period of cutting
It obtains strength in a short period. Compressive strength at the end of
one day is about 115 kg/cm2 and after 3 days is about 210 kg/cm2. Similarly
tensile strength at the end of one day is about 20 kg/cm2 and that after
3 days is about 30 kg/cm2
It is light in weight
Allows higher permissible stresses in the design. It therefore results in economic
design.
6. High alumina cement
It is produced by grinding clinkers formed by calcining bauxite (ore of
Aluminium) and lime. It is specified that total alumina content should not less than
32% and the ratio by weight of alumina to lime should be between 0.85 and 1.30.
Advantage
Can withstand high temperatures
Initial setting time is more than 3 hours. Final setting time is about 5 hours.
Therefore, it allows more time for mixing and placing operations
Evolves great heat during setting, hence, not affected by frost
Resists the action of acids in a better way
Sets quickly and it attains compressive strength of about 400 kg/cm2 after 1
day and that after 3 days is about 500 kg/cm2
Its setting action mainly depends on the chemical reactions and hence, it is
not necessary to grind it to fine powder
Disadvantage
Extreme care is to be taken to see that it does not come in contact with
even traces of lime or ordinary cement.
It cannot be used in mass construction as it evolves great heat.
It is costly.
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Module: 5
Lecture: 23
CEMENT CLASSIFICATION (Continued)
7. Puzzolana cement
Puzzolana is a volcanic powder. It is found in Italy near Vesuvius. It resembles
surkhi which is prepared by burning bricks made from ordinary soils. It can also be
processed from shales and certain types of clays. Puzzolana material should be
used in between10 to 30%.
Advantage
Evolves less heat during setting
Possesses higher tensile strength
Imparts higher degree of water tightness
Attains comprehensive strength with age
Can resist action of sulfates
Imparts plasticity and workability to mortar and concrete prepared from it.
Offers great resistance to expansion
It is cheap
Disadvantages
Compressive strength in early days is less
Possesses less resistance to erosion and weathering action
This cement is used to prepare mass concrete of lean mix and for marine
structures. It is also used in sewage works and for laying concrete under water.
8. Hydrophobic cement
It contains hydrophobic admixtures such as acidol, naphthelene soap,
oxidized petroleum etc., which decrease the wetting ability of cement grains and
form a thin film around cement grains. When water is added to hydrophobic
cement, the absorption films are torn off the surface and they do not in any way,
prevent the normal hardening of cement. However, in initial stage, the gain in
strength is less as hydrophobic films on cement grains prevent the interaction with
water. However, its strength after 28 days is equal to that of ordinary Portland
cement.
When hydrophobic cement is used, the line fine pores in concrete are
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uniformly distributed and thus the frost resistance and the water resistance of such
concrete are considerably increased.
9. Expanding cement
It is produced by adding an expanding medium like sulfoaluminate and
establishing agent to ordinary cement. Hence this cement expands whereas other
cements shrink.
It is used for the construction of water retaining structures and for repairing
the damaged concrete surfaces.
10. Low heat cement
Considerable heat is produced during the setting action of cement. It
contains lower percentage of tricalcium aluminate (C3A) and higher percentage of
dicalcium silicate (C2S) which reduce the amount of heat produced.
This type of cement possesses less compressive strength. Initial setting time is
about one hour and usual setting time is about 10 hours. It is mainly used for mass
concrete work.
11. Quick setting cement
It is produced by adding a small percentage of aluminium sulfates and by
finely grinding the cement. Percentage of gypsum or retarder for setting action is
also greatly reduced. Addition of aluminium sulfate and fineness of grinding
accelerate the setting of cement. The setting action of cement starts within five
minutes addition of water and it becomes hard like stone in less than 30 minutes.
Mixing and placing of concrete should be completed within very short period. This
cement is used lay concrete under static water or running water.
12. Sulfate resisting cement
In this cement percentage of tricalcium aluminate is kept below 5 to 6%
which increase in resisting power against sulphates.
This cement is used for structures which are likely to be damaged by severe
alkaline conditions such as canal linings, culverts, syphons etc.
MANUFACTURE OF PORTLAND CEMENT
Raw materials
Basis: 1000kg of cement
Clay = 100-300kg
Limestone = 1200-1300kg
Gypsum = 30-50kg
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Coal = 250-400kg
Water = 3000kg
Electricity = 80kWH
The most commonly used composition on % basis by mass for the Portland
cement manufacturing is given below
Component % range by mass
Lime ( CaO) 60-69
Silica (SiO2) 17-25
Alumina (Al2O3) 3-8
Iron oxide (Fe2O3) 2-4
Magnesium oxide (MgO) 1-5
Sulfur trioxide (SO3) 1-3
Alkali Oxide (Na2O + K2O) 0.3-1.5
Significance of constituents
Lime
Lime is also defined as non-hydraulic cement mainly consisting of calcium
oxide and small amount of magnesium oxide. It is prepared by calcining the lime
stone (CaCO3) at temperature that it will slake, when brought in contact with water.
It is principal constituent of cement. Proper amount of lime is important as excess of
it reduces the strength as well as lesser amount also reduces the strength and makes
its quick setting. Lime is mainly used for white washing of mortar for joining bricks,
metallurgy and glass industries.
Silica
It imparts strength to cement.
Alumina
It works as an accelerator and makes the cement quick settling. However
excess of alumina makes the cement unsound.
Gypsum (Calcium sulfate)
It retards the setting action of cement but enhances the initial setting time.
Iron oxide
It provides colour, strength and hardness of cement.
Magnesia
If present in small amount impart hardness and colour to cement
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Sulfur trioxide
If present in small amount it imparts soundness to cement but excess of it is
undesirable
Alkalis
Most of the alkalis present in raw materials are carried away by the flue gases
during heating and cement contains only a small amount of alkalis. If present in
excess causes the efflorescent to cement.
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Module: 5
Lecture: 24
CEMENT MANUFACTURE
MANUFACTURE
It involves the following steps
1. Mixing of raw material
2. Burning
3. Grinding
4. Storage and packaging
1. Mixing of raw material
Mixing can be done by any one of the following two processes
(a) Dry process
(b) Wet process
a) Dry Process
Block diagram of manufacturing process
Diagram with process equipment
Jaw Crusher Jaw Crusher
Calcareous materials
Argillaceousmaterial
Bin
Mixer
Pulverizer
Rotary kilnHot air
out
Hot air in
Clinker for grinding
Packaging and storage
Gypsum
Figure: Manufacturing of Cement by Dry Process
BinBin Silo
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Animation
Lime stone or chalk and clay are crushed into gyratory crusher to get 2-5 cm
size pieces. Crushed material is ground to get fine particle into ball mill or tube mill.
Each material after screening stored in a separate hopper. The powder is mixed in
require proportions to get dry raw mix which is stored in silos (storage tank) and kept
ready to be fed into the rotary kiln. Raw materials are mixed in required proportions
so that average composition of the final product is maintained properly.
b) Wet process
Block diagram of manufacturing process
Diagram with process equipment
Animation
Raw materials are crushed, powdered and stored in silos. The clay is washed
with water in wash mills to remove adhering organic matter. The washed clay is
stored separately. Powdered lime stone and wet clay are allowed to flow in channel
and transfer to grinding mills where they are intimately mixed and paste is formed
known as slurry. Grinding may be done either in ball mill or tube mill or both. Then
slurry is led to correcting basin where chemical composition may be adjusted. The
slurry contains 38-40% water stored in storage tank and kept ready for feeding to a
rotary kiln.
Water
Jaw Crusher Jaw Crusher
Calcareous materials
Argillaceousmaterial
Bin
Mixer
Pulverizer
Rotary kilnHot air out
Hot air in
Clinker for grinding
Packaging and storage
Gypsum
Figure: Manufacturing of Cement by Wet Process
BinBin Silo
Water
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Comparison of dry process and wet process
Criteria Dry process Wet process
Hardness of raw material Quite hard Any type of raw material
Fuel consumption Low High
Time of process Lesser Higher
Quality Inferior quality Superior quality
Cost of production High Low
Overall cost Costly Cheaper
Physical state Raw mix (solid) Slurry (liquid)
The remaining two operations burning and grinding are same for both the
process.
2. Burning
Burning is carried out in rotary kiln which rotating at 1-2 rpm at its longitudinal
axis. Rotary kiln is steel tubes having diameter in between 2.5-3.0meter and length
varies from 90-120meter. The inner side of kiln is lined with refractory bricks. The kiln is
rested on roller bearing and supported columns of masonry or concrete in slightly
inclined position at gradient of 1 in 25 to 1 in 30. The raw mix or corrected slurry is
injected into the kiln from its upper end. Burning fuel like powdered coal or oil or hot
gases are forced through the lower end of the kiln so long hot flame is produced.
Due to inclined position and slow rotation of the kiln, the material charged
from upper end is moving towards lower end (hottest zone) at a speed of
15meter/hour. As gradually descends the temperature is rises. In the upper part,
water or moisture in the material is evaporated at 4000C temperature, so it is known
as drying zone.
In the central part (calcination zone), temperature is around 10000C, where
decomposition of lime stone takes place. After escapes of CO2, the remaining
material in the forms small lumps called nodules.
CaCO3 CaO + CO2
The lower part (clinkering zone) have temperature in between 1500-17000C
where lime and clay are reacts to yielding calcium aluminates and calcium silicates.
This aluminates and silicates of calcium fuse to gather to form small and hard stones
are known as clinkers. The size of the clinker is varies from 5-10mm.
2CaO + SiO2 Ca2SiO4 (dicalcium silicate (C2S))
3CaO + SiO2 Ca3SiO5 (tricalcium silicate (C3S))
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3CaO + Al2O3 Ca3Al2O6 (dicalcium aluminate (C2A))
4CaO + Al2O3 + Fe2O3 Ca4Al2Fe2O10 (tetracalcium aluminoferrite(C4AF))
As clinkers are coming from burning zone, they are very hot. The clinkers are
cooled down by air admitting counter current direction at the base of rotary kiln.
Resulting hot air is used for burning powdered coal or oil and cooled clinkers are
collected in small trolleys or in small rotary kiln.
3. Grinding
Cooled clinkers are ground to fine powder in ball mill or tube mill. 2-3%
powdered gypsum is added as retarding agent during final grinding. So that,
resulting cement does not settle quickly, when comes in contact with water. After
initial set, cement - water paste becomes stiff, but gypsum retards the dissolution of
tri-calcium aluminates by forming tricalcium sulfoaluminate which is insoluble and
prevents too early further reactions of setting and hardening.
3CaO.Al2O3 + xCaSO4.7H2O 3CaO.Al2O3.xCaSO4.7H2O
4. Storage and packaging
The ground cement is stored in silos, from which it is marketed either in
container load or 50kg bags.
Pretreatments to raw material
Wet process
Cement manufacture by wet process used either chalk or lime stone as one
of the raw material. Following treatment should be given to them before its use. The
remaining procedure after the treatment is same for both.
Chock should be finely broken up and dispersed in water in a wash mill. The
clay is also broken up and mixed with water in wash mill. The two mixtures are now
pumped so as to mix in predetermined proportions and pass through a series of
screens. The resulting cement slurry flows into storage tanks.
Limestone should be blasted, then crushed, usually in two progressively
smaller crushers (initial and secondary crushers), and then fed into a ball mill with the
clay dispersed in water. The resultant slurry is pumped into storage tanks.
Impurity profile of raw materials
The amount of different components in Portland cement as oxides is
tabulated in table: 1which shows that CaO and SiO2 by far constitute the major part
of the final product.
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About one-third of the raw meal mass can be attributed to Loss on Ignition
(LOI), which is almost exclusively due to the calcination of the CaCO3 used as a
precursor for forming CaO. This corresponds to the fact that the raw meal contains
about 75 wt% of CaCO3.
The mass loss in the calcination process corresponds to a raw meal to cement
clinker ratio of about 1.5, if the raw meal is dry when fed into the kiln system. The raw
meal composition stated in table: 1 is usually obtained by blending limestone and
clay (clay being rich in Si, Fe and Al oxides). If needed, correctives like sand and iron
ore can be added to the raw meal in order to achieve the correct composition.
In order to ensure the proper quality of the final product, the amount of
certain minor components is limited. Column 4 in table: 1 shows some general upper
limits for certain elements, but the exact amount that can be allowed depends on a
wide range of factors such as what the cement will be used for, the amount of other
impurities, production facilities and so on, which is why the acceptable amount must
be determined from case to case. The limits stated in table: 1 cannot be exceeded
significantly, and in many cases it is actually desirable to be well below these limits.
Components Content in clinker Content in raw meal Impurity limits
Wt. % Wt. % Wt. %
CaO 63.8-70.1 ~43
SiO2 19.7-24.3 ~14
Al2O3 3.8-6.8 ~4
Fe2O3 1.3-1.6 ~5
MgO 0.0-4.5 5
SO3 0.2-2.1 4.5
K2O 0.3-1.8 0.8 as (NaO2)e*
Na2O 0.0-0.3 0.8 as (NaO2)e*
Mn2O3 0.0-0.7 0.5
TiO2 0.2-0.5
P2O5 0.0-0.3 0.2
CO2 0.0-0.8
H2O 0.0-1.1
Cl2 0.1
LOI 0.1-1.6 ~34 3
*(NaO2)e, the effective amount of alkali, is calculated as 0.658(%K2O) + %Na2O.
Table :1 Composition of Portland cement clinker and raw meal and impurities limit
If the raw materials used in this process contain sulfide, can lead to emissions
of SO2 from the preheater tower. SO2 emissions are most often caused by the
oxidation of pyritic sulfide, which occurs between 300 and 6000C. Of the formed SO2,
around 50% is often said to be emitted from the preheater. However, large variations
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in this number have been observed, with the circulation of CaO from the calciner
given as the main reason for this phenomenon.
Also, the chlorine level in raw material should be below 0.1% , if it will exceed
then free chlorine will accumulate in rotary kiln.
K2O and Na2O, known as the alkalis have been found to react with the
reactive silica found in some aggregates, the products of the reaction causing
increase in volume leading to disintegration of the concrete. The increase in the
alkalis percentage has been observed to affect the setting time and the rate of the
gain of strength of cement.
SO3 form low percentage of cement weight. SO3 comes from the gypsum
added (2-6% by weight) during grinding of the clinker, and from the impurities in the
raw materials, also from the fuel used through firing process.
MgO, present in the cement by 1-4%, which comes from the magnesia
compounds present in the raw materials. MgO by 5%, to control the expansion
resulted from the hydration of this compound in the hardened concrete. When the
magnesia is in amorphous form, it has no harmful effect on the concrete.
Other minor compounds such as TiO2, Mn2O3, P2O5 represent < 1%, and they
have little importance.
The upper and lower limit of impurities present in lime stone is tabulated in
table: 2
Impurity Typical range Impurity Typical range
Low High Unit Low High Unit
Silica (as SiO2) 0.1 2 w/w% Copper 1 30 mg/kg
Alumina (as Al2O3) 0.04 1.5 w/w% Fluoride 5 3000 mg/kg
Iron (as Fe2O3) 0.02 0.6 w/w% Lead 0.5 30 mg/kg
sulphur (as CaSO4) 0.01 0.5 w/w% Mercury 0.02 0.1 mg/kg
Carbonaceous matter 0.01 0.5 w/w% Molybdenum 0.1 4 mg/kg
Manganese (as MnO2) 20 1000 mg/kg Nickel 0.5 15 mg/kg
Antimony 0.1 3 mg/kg Selenium 0.02 3 mg/kg
Arsenic 0.1 15 mg/kg Silver 0.2 4 mg/kg
Boron 1 20 mg/kg Tin 0.2 15 mg/kg
Cadmium 0.1 1.5 mg/kg Vanadium 1 20 mg/kg
Chromium 3 15 mg/kg Zinc 3 500 mg/kg
Table: 2 Impurities often found in limestone
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Engineering aspects
Cyclone preheater
The raw materials are preheated or calcined in preheater or series of
cyclones before entering to the rotary kiln. A preheater, also called as suspension
preheater is a heat exchanger in which the moving crushed powder is dispersed in a
stream of hot gas coming from the rotary kiln. Common arrangement of series of
cyclones is shown in figure.
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The heat transfer of hot kiln gases to raw meal is takes place in co-current. The
raw materials are heated upto 8000C within a less than a minutes. About 40% of the
calcite is decarbonated during the heat transfer.
The quality and quantity of fuel used in the kiln can be reduced by
introducing a proportion of the fuel into preheater. 50 – 65 % of the total amount of
fuel is introduced into preheater or precalciner which is often carried out by hot air
ducted from cooler.
The fuel in the precaliner is burnt at relatively low temperature, there so heat
transfer to the raw meal is very efficient. The material has residence time in the
hottest zone of a few seconds and its exit temperature is about 9000C, 90 – 95% of
calcite is decomposed. Ash from the fuel burn in the precalciner is effectively
incorporated into mix.
Advantages of precalination
Decrease the size of kiln
Decrease in capital cost
Increase in rate of material passes to the kiln.
Decrease in rate of heat provided which ultimately lengthens the life of
refractory lining
Less NOx is formed, since much of the fuel is burnt at a low temperature, and
with some designs NOx formed in the kiln may be reduced to nitrogen.
Rotary Kiln
Rotary kiln is a tube, sloping at 3 – 4 % from the horizontal and rotating at 1 – 4
revolution/minute into which material enters at the upper end and then slides, rolls
and flows counter to the hot gas produced by a flame at the lower or front end.
The kiln is lined with refractory bricks. The type and size of the bricks may vary
depending up on the length of rotary kiln and the maximum temperature
employed. Further, arranging the bricks in a ring requires perfect closing of the ring
which is difficult, time consuming and expensive. Two types of the joints, the radial
and axial joints are used for bricks. The redial joints are between the brick in each
ring and axial joints are between the successive rings. The bricks are coated with
thin layer of clinker for extending the life as well as insulation.
The rotary kiln used which precalciner is 50 – 100 meter long having length to
diameter (L/D) ratio between 10 to 15. The kiln having very small L/D ratio ensures
rapid clinker formation and quick reaction run without recrystallization phenomena.
Due to this higher hydraulic activity of cement is achieved
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Conveyors
The following types of conveyors are used during the cement manufacturing
process.
Belt conveyor
Bucket conveyor
Screw conveyor
Roller conveyor
Animation
Belt conveyor
Belt conveyor is used for transportation of raw material form storage to the
initial crushing devises mostly jaw crusher. Belt conveyor consists of two or more
pulleys, with a continuous loop of material or the conveyor belt which rotates about
them. Either one or both of the pulleys are powered, moving the belt and the
material on the belt forward. The powered pulley is called the drive pulley while the
unpowered pulley is called the idler.
Bucket conveyor
Bucket conveyor are used for transportation of crushed material and clay to
mixing zone in cement industries. A bucket conveyor, also called a grain leg, is a
mechanism for carrying the bulk materials vertically. It consists of buckets to contain
the material, a belt to carry the buckets and transmit the pull, means to drive the
belt and accessories for loading the buckets or picking up the material, for receiving
the discharged material, for maintaining the belt tension and for enclosing and
protecting the elevator.
Screw conveyor
A screw conveyor or auger conveyor is a mechanism that uses a rotating
helical screw blade, called a "flighting", usually within a tube, to move liquid or
granular materials. Screw conveyors are often used horizontally or at a slight incline
as an efficient way to move semi-solid materials. Screw conveyor are used for
transportation of material for storage to homogeneous siloes.
Roller conveyor
Roller conveyors are line restricted device and consist of rollers mounted
between two side members. Bearings are usually incorporated in the idlers to cut
down the mechanical losses. An unpowered gravity roller conveyor is set at an
appropriate incline and goods move down it by gravity. In power unit normally an
electric motor drive the rollers via chains or belt, providing controlled movement of
goods. They are generally used for transportation of packed material.
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PROPERTIES
Cement-modification improves the properties of certain silt clay soils that are
unsuitable for use in subgrade construction. The objectives may be to
decrease the soil‘s cohesiveness (plasticity), to decrease the volume change
characteristics of expansive clay, to increase the bearing strength of a weak
soil, or to transform a wet, soft subgrade into a surface that will support
construction equipment.
Tricalcium Silicate (C3S): Hardens rapidly and is largely responsible for initial
set and early strength. In general, the early strength of Portland cement
concrete is higher with increased percentages of C3S.
Dicalcium Silicate (C2S): Hardens slowly and contributes largely to strength
increases at ages beyond 7 days.
Tricalcium Aluminate (C3A): Liberates a large amount of heat during the first
few days of hardening and together with C3S and C2S may somewhat
increase the early strength of the hardening cement. It affects setting time.
Tetracalcium Aluminoferrite (C4AF): Contributes very slightly to strength gain.
However, acts as a flux during manufacturing. Contributes to the colour
effects that makes cement gray.
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Module: 5
Lecture: 25
CEMENT (Continued)
CHEMICAL COMPOSITION
According to IS: 269-1975, composition of ordinary Portland cement should
satisfy the following conditions
Ratio of percentage of lime to that of silica, alumina and iron oxide shall be in
between 0.66 - 1.02 which is calculated by the formula as follow
Ratio of percentage of alumina to that of iron oxide shall not be less than 0.66
Weight of insoluble residue shall not exceed 2%
Weight of magnesia shall be less than 6%
Total sulfur content shall not be more than 2.75%
Total loss on ignition shall not exceed 4%.
PHYSICAL REQUIREMENT
Setting time
Initial : Not less than 30 minutes
Final : Not more than 600 minutes
Compressive strength
Compressive strength of 1:3 cement mortar cube of cement and sand.
3 days : Not less than1.6 kg/mm2
7 days : Not less than 2.2 kg/mm2
Soundness
It expresses the expansivity of the cement set in 24 hours between 250C and
1000C.
Un-aerated cement : Maximum 10mm
Aerated cement : Maximum 5mm
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Fineness
It should not less than 215m2/kg
SETTING AND HARDENING OF CEMENT
The setting and hardening of cement is due to hydration and hydrolysis of its
constituents. The hydration products of cement are cementious but not cement. The
heats of hydration of cement constituents decides the formulation of cement for
different applications as well as useful in preventing the water in cement paste from
freezing in winter and in accelerating the setting and hardening processes. If heat
liberated is not dissipated rapidly particularly in large constructions like dam serious
stress cracking may occur. Further, the knowledge of reaction speed is important,
because it determines the time of setting and hardening.
On hydration, the cement constituents generally give rise to hydrated
calcium silicate (CSH) obtained as poorly crystallized gels (3CaO.3SiO23H2O)
commonly known as tobermorite gels having structural resemblance to the mineral
tobermorite found in Tobermory, Scotland .
When cement is gauged with water, the C3A, C3S and C3SF phases reacts
very rapidly and gauging water becomes saturated with Ca(OH)2 formed in
hydration reactions. The C2S phase hydrated rather slowly. The initial settling is
attributed to the reaction of C3A, C3S and C3AF.
When the cement powder comes into contact with the water in the paste,
two phenomena takes place
Setting: After 25 hours
Hardening: After one year, but proceeds to completion only after decades
The reactions take place when cement first comes into contact with water,
are as follows
C3A + 6H2O C3A.6H2O ---- (1)
C3AF + nH2O CF.(n-6)H2O + C3A.6H2O ---- (2)
C3S + H2O C2S + C.H2O ---- (3)
The fluxes are therefore the first components to be hydrated, with evolution of
large amount of heat. The alite react after few hours of contact with water.
During the second stage setting process stops and the hardening process
starts. Hardening involves both the reaction in which tetracalcium aluminate hydrate
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is formed and the hydration of the original belite after dicalcium silicate formed in
reaction (3) has been hydrated
C3A.6H2O + C.H2O + 6H2O C4A.13H2O ---- (4)
C2S + nH2O C2S.nH2O ---- (5)
Reaction (5) takes place between setting and hardening, while the same
reaction is much slower in case of belite present in the clinker, and components
manufactured from the cement attain their definitive compactness and mechanical
strength after one year.
Overall, the water in the cement paste, which initially served to soak it and to
make it fluid, is consumed by the cement at certain rate, depending on its structural
characteristics, which bring hydrolysis of cement constituents, act as water of
crystallization via hydration reaction and in promotion of colloidal phenomena
Reaction (4) tends to occur on its own at the expense of the free lime in the
surroundings as the lime which is formed in reaction (3) is insufficient, at least at the
start, to furnish large amounts of calcium hydroxide demanded by high rate of
reaction (1) which leads to the promoter (C3A.6H2O) of reaction (4) itself.
However, when there is shortage of free lime, decalcification of dicalcium
silicate occurs into compounds which are poorer in lime (C3S2 and CS type) which
by hydration more rapidly than C2S to form. To avoid these serious irregularities in the
hardening and setting of the cement, it is necessary to preclude the environment
from becoming a large consumer of C.H2O by improving it from the C3A.6H2O which
is formed by reaction (1)
C3A.6H2O + 3CaSO4.2H2O + 19H2O C3A.3CaSO4.31H2O ---- (6)
Gypsum is capable of combining with tricalcium aluminate hydrate as in
reaction (6). The calcium sulfoaluminate which is formed concurrently with
C4A.13H2O from reaction (4) indirectly stabilize the C2A, and moreover can be lead
via reaction (5) to mass getting the highest possible mechanical properties. For,
these reasons, gypsum is considered as an essential additive with the aim of
regularizing the setting and hardening of cement.
Further, the setting properties of cement depend on the proportions of
alumina and ferric oxide in it, greater proportion of these bringing about
acceleration of setting process. The setting time to the cement is controlled by
grinding about 2-5% of gypsum with the cement clinker. The setting time is not
directly influence by gypsum; a small increase beyond limit, it may produces large
increase in setting time, also large amount of it leads to cracking of set cement due
Module: 5 Lecture: 25 Cement industries
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to expansion. Plaster of Paris and anhydrite also retard the setting of cement but
they are more vigorous in their action.
Among the substances that affect the setting time of Portland cement
containing gypsum as retarder, some retard the process (sugar) and some
accelerate the process (CO2, Alkali carbonates &chlorides and alkaline-earth metals
except Ca, NaOH, KOH etc.), some retard the set at low concentrations and
accelerate the process at high concentrations (CaCl2, NH4Cl, FeCl3 etc.)
USES
In the production of concrete, it plays a crucial role in setting and hardening
the concrete.
On being mixed with other aggregates, Portland cement begins to serve a
dual purpose. First, it provides for the concrete products to be workable when
wet and second, it provides them to be durable when dry.
It is also brought into usage in mortars, plasters, screeds and grouts as a
material which can be squeezed into gaps to consolidate the structures.
Civil (piers, docks, retaining walls, silos, warehousing, poles, pylons, fencing)
Building (floors, beams, columns, roofing, piles, bricks, mortar, panels, plaster)
Transport (roads, pathways, crossings, bridges, sleepers, viaducts, tunnels,
stabilization, runways, parking)
Agriculture (buildings, processing, housing, feedlots, irrigation)
Water (pipes, culverts, kerbing, drains, canals, weirs, dams, tanks, pools)
Used by the retaining walls and the precast concrete block walls as a major
component to build a strong foundation of concrete.
By mixing it with water, Portland cement literally turns into a plastic stone and
thereby it can be used for purposes and in places where stone was to be
used and that too by keeping within the financial limits.
Concrete casing, made by utilizing Portland cement, they can be effectively
protect the surface from air, water or corrosion.
Due to its ability to prevent corrosion, it is also put to use in ships, tanks and
bunkers.
It may be moulded to obtain a hard and fire-proof material which may be
employed in designing buildings, shop floors, reservoirs and other foundations.
Any structure that is meant to support huge amounts of weight will bring
Portland cement into use. These structures range from ground floors of multi-
storey buildings to bridge floors and from bridge spans to dams.
A blaze or an overwhelming fire may leave a structure completely burnt but
with the use of Portland cement, this can be prevented.
Module: 6 Lecture: 26 Ceramic industries
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Module: 6
Lecture: 26
CERAMIC INDUSTRIES
INTRODUCTION
Ceramic is an inorganic, nonmetallic solid prepared by the action of heat
and subsequent cooling. Ceramic materials may have a crystalline or partly
crystalline structure, or may be amorphous (e.g. glass). Because most common
ceramics are crystalline, the definition of ceramic is often restricted to inorganic
crystalline materials, as opposed to the non-crystalline glasses.
The earliest ceramics were pottery objects made from clay, either by itself or
mixed with other materials, hardened in fire. Later ceramics were glazed and fired to
create a coloured, smooth surface. Ceramics now include domestic, industrial and
building products and art objects. In the 20th century, new ceramic materials were
developed for use in advanced ceramic engineering; e.g., in semiconductors.
The word "ceramic" comes from the Greek word Keramos means burnt stuff.
Earlier the term ceramic was applied to products made from natural earth material
that was not exposed to heat. But nowadays the silicate mainly used in the
construction industries and prepared by burning the clay products are called as
ceramics.
CLASSIFICATION
A broad sense classification divides the ceramic products in to two classes
1. Heavy clay products e.g. bricks, roofing tiles, drain tiles, hollow tiles, stoneware
and refractories
2. Pottery products e.g. chinaware, wall tiles, electric insulation
Ceramic may also be classified as porous and non-porous. The porosity is
depends on particle size, moulding pressure and temperature of vitrification.
Further, ceramic may be classified based on the method of production and
its uses into following classes.
1. Whiteware
2. Structural clay products
3. Refractory material
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4. Special ceramic products
5. Vitreous enamel
RAW MATERIAL
The raw materials for ceramics are divided into following groups.
a) Plastics material such as clay
b) Fluxes such as feldspar
c) Non-plastics materials such as silica
a) Clay
Clay gives the main body to the ceramics. The advantage of using clay are it
is plastic when mixed with water becomes hard after drying and finally it becomes
irreversibly solid after firing. Clay is chosen according to the requirements of
particular products and is often blended.
Impurities in common clay incorporate specific qualities as follows.
Iron oxide in common clay gives red colour to the burnt material
Lime, magnesia, iron oxide and alkali oxides act as flux which lowers the
fusion point of clay
Silica increases its porosity and refractory nature, while decreases shrinkage
Clay containing very little and good deals of silica known as fire clays
b) Feldspar
Feldspar is general name given to the group of minerals. Flux materials like
feldspar (Na2OAl2O3.6SiO2) which is easily melting material decreases the melting
point of sand or quartz present in the ceramic body. So, that after firing glass like
material is obtained called as vitrified material, which is highly impervious and stable
to the environment. Fluxes are used for adding vitrifications, reducing porosity, to
increase the strength of cold articles and for bonding. Feldspar is used as fluxing
constituent in ceramic formulations along with clay. The common fluxing agents
which lower the temperature are borax, boric acid, soda ash, sodium nitrate,
potassium carbonate, calcined bones, lead oxides, lithium & barium minerals.
Type of feldspar
Potash feldspar K2O.Al2O3.6SiO2
Soda feldspar Na2O.Al2O3.6SiO2
Lime feldspar CaO.Al2O3.6SiO2
Module: 6 Lecture: 26 Ceramic industries
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c) Silica or quartz
The non-plastic or leading admixture like sand or quartz gives strength to the
body. It is incorporated in ceramic formulation to reduce shrinkage and cracking
which is occurs during drying and firing.
PROPERTIES
The physical and chemical properties of ceramic depend upon component
present in it.
The strength is mainly controlled by the factors like temperature, size and
shape, composition, surface condition and microstructure
Mechanically they are brittle and highly resistant to compression
Oxides and carbides which give high chemical and physical stability
Electrical and magnetic properties are due to composition itself. E.g. oxides
are generally bad conductors where the non-oxides are semiconductors and
ceramics with transition metal ions shows the magnetic properties.
Transparency depends upon the crystal lattice of ceramic which in turn is
dependent of composition, crystal structure, extent of polarization etc.
USES
Ceramic is one of the oldest materials used in construction with the time
quality and decoration has been added to its property and therefore they are at
present used in following
Construction and decoration as bricks and tiles
Metallurgy as construction material of furnace
Chemical products as stoneware and porcelain
In drainage of water
In sanitation
The small uses includes pottery work, specialty ceramic like peuzo electric
and insulation material in electrical connections
Therefore, we conclude that ceramics deals with manufacture and technical
characteristics and raw material for article building.
Module: 6 Lecture: 27 Whitewares
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Module: 6
Lecture: 27
WHITEWARES
Whiteware are made by mixing china clay, feldspar and flint (SiO2) with or
without Whiteware having good strength, translucency and very low porosity can be
obtained by firing the products at 1450-15000C. Whitewares contain refractory body
and glassy coating known as the glaze. There is a corresponding variation in the
degree vitrification due to different amount of fluxes.
1. WHITEWARES
They are available in number of special types such as floor tiles, resistant to
abrasion, glazed or unglazed, impervious to stain penetration and wall tiles.
CLASSIFICATION
a) Earthenware
It is also known as semi-vitreous dinnerware which is porous, non-translucent
and soft glaze.
b) Chinaware
It is a vitrified translucentware with a medium glaze which resists abrasion and
used for non-technical application.
c) Porcelain
It is vitrified translucentware with a hard glaze which resists abrasion to a
maximum degree and may include chemical, insulating and dental porcelain.
d) Sanitaryware
It was made from clay which is porous but nowadays vitreous composition is
used. Prefired and sized vitreous grog is include with triaxial composition
e) Stonewares
They are the oldest ceramicwares which is used before porcelain. It is known
as crude porcelain but its raw materials are of poor grade and not well fabricated.
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MANUFACTURE
Block diagram of manufacturing process
Diagram with process equipment
Animation
It involves three steps
Body of ware
Glaze
Decoration
Body of ware
Kaolin and feldspar are reduced to fine powder then suspended in the form
of cream. Then excess water is removed by filter press. By use of resulting the paste,
article can be obtained after moulding, are dried slowly and fired to get
porousware which is known as bisque.
Glaze
Porous article is covered with glaze to get water tight article which is done by
melting it over the surface of the body. Quartz, feldspar, boric oxide are the
constituent of the glaze which are finely ground and mixed with water which forms
slip then it is fired to higher temperature to produce smooth and glossy surface. The
glaze must be chose in such a way that resist the reagents and must have same co-
efficient of expansion as the body; otherwise the vessel which is exposed to change
of temperature will crack.
Decoration
Design may be painted on the body before glazing using metal oxide or
painted upon the glaze using coloured glass and article is fired so, the pigment melts
into glaze. It painted upon glaze then known as over glazed.
Kaolin
Felspar
Water
Water
SlurryDecoration
White Wares
Gaze Article
Furnace
Reactor
Filter Press
Dilution Tank
Figure: Manufacturing of White Wares
Water
GrinderCasting
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Applications
Dishes
Crucibles
Spark plug
Electrical insulator
Laboratory equipment
Glazing
It is mixture of glass forming materials which is in form of fine powder e.g. lead
silicates, borosilicate etc. If mixture free from iron and colouring pigments then forms
colourless glaze.
Metal oxides
The following metal oxides are used for producing the colour to glaze.
Iron oxide: Brown colour
Iron oxide and lime: Cream colour and yellowish tint
Copper oxide: Green colour
Cobalt blue: Blue colour
Requisite of glazing
To provide smooth, glossy surface to treated materials
To protect the surfaces from the environmental or atmospheric actions
To produce decorative effect
To make the surface impervious to liquid, water etc.
To improve appearance and durability of the article
Method of glazing
a) Salt glazing
b) Liquid glazing
a) Salt glazing
Sodium chloride is used to get glossy film over the earthenwares. Sodium
chloride is throwing to furnace where articles are in red hot condition. Due to heat
salt volatilize and react with silica which makes glossy film of sodium chloride.
b) Liquid glazing
Liquid glazing is much superior then salt method in which fine powder of glaze
mixture, colouring pigments are mixed with water to form a colloidal solution known
as slip glaze. The article is then burnt in kiln at low temperature. Then it is dipped into
glaze slip so the glaze materials fill up the pores of articles. To fuses and forming thin
glossy film, articles are fired into the kiln. Care should be taken that firing articles
Module: 6 Lecture: 27 Whitewares
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does not comes in contact with direct fire, otherwise soot & dust should discoloured
the article. Delicated articles should be burnt in muffle furnace which is heated
externally while articles are kept inside.
PROPERTIES
A traditional ceramic used to make pottery and porcelain. While, whiteware
ceramics often have a glassy structure.
Any of a broad class of ceramic products that are white to off-white in
appearance and frequently contain a significant vitreous, or glassy,
component.
Imperviousness to fluids, low conductivity of electricity, chemical inertness,
and an ability to be formed into complex shapes. These properties are
determined by the mixture of raw materials chosen for the products, as well
as by the forming and firing processes employed in their manufacture
USES
Whitewares find application in spark plugs, electrical insulators, laboratory
equipment, crucibles, dishes, and high-class potteries. Including products as diverse
as fine china dinnerware, lavatory sinks and toilets, dental implants, and spark-plug
insulators
Module: 6 Lecture: 28 Clay products and Refractories
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Module: 6
Lecture: 28
CLAY PRODUCTS AND REFRACTORIES
2. STRUCTURAL CLAY PRODUCTS
Block diagram of manufacturing process
Diagram with process equipment
Animation
Unglazed pipes, tiles, terracotta and building bricks are manufactured from
locally clays without glaze, which contain the fluxes needed for binding. For building
and face bricks, red burning clay is used, which is cheap and durable.
Bricks can be manufactured from
Soft mud process
Stiff mud process
Dry press process
Soft mud process
The clay mixture contain 30% water is moulded in moulds. This mould bricks
are dried in tunnel drier. While common bricks are burnt in scove kiln, in which coal is
used as fuel. The kiln is built from the green bricks with the outside walls daubed or
scoved with clay. It is a variation of undraft type of kiln. After burning the kiln is
completely dismantled.
Raw Clay
Clay Product
Dryer
Screening
Firing
Figure: Manufacturing of Clay Products
Water
GrindingS
oa
kin
g
Ma
tura
tio
n
MouldingMixing
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Stiff and mud process
Clay mixture contains 12-15% water, which is just enough to stick together. The
clay is forced out through a die in a screw or auger machine. The extruded clay bar
passes long or short belt conveyor on to a cutting table on which a frame with
number of wires which automatically cuts the bar into appropriate lengths. These
bricks may be repressed to make face bricks.
Dry process
The water content of the clay is reduced to 4-7% which make the clay non-
plastic. The brick unit is moulded at high temperature.
PROPERTIES
Depending on the content of the soil, clay can appear in various colours,
from a dull gray to a deep orange-red.
Clays exhibit plasticity when mixed with water in certain proportions, when
dry, it becomes firm and when fired in a kiln, permanent physical and
chemical changes occur which converted clay into a ceramic material.
Because of these properties, clay is used for making pottery items, both
utilitarian and decorative.
Different types of clay, when used with different minerals and firing conditions,
producing earthenware, stoneware, and porcelain.
USES
Clay is the starting raw material for manufacturing bricks, tiles, terracotta,
pottery, earthenwares, sewer, drain pipes, and covers for electrical cables
Clay is one of the oldest building materials on Earth, among other ancient,
naturally-occurring geologic materials such as stone and organic materials
like wood.
Between one-half and two-thirds of the world's population, in traditional
societies as well as developed countries, still live or work in a building made
with clay as an essential part of its load-bearing structure. Also a primary
ingredient in many natural building techniques
Used to create adobe, cob, cordwood, and rammed earth structures and
building elements such as wattle and daub, clay plaster, clay render case,
clay floors and clay paints
A traditional use of clay as medicine goes back to prehistoric times. Kaolin
clay and attapulgite have been used as anti-diarrheal medicines
Used where natural seals are needed, such as in the cores of dams, or as a
barrier in landfills against toxic seepage
Clay tablets were used as the first known writing medium, inscribed
with cuneiform script through the use of a blunt reed called a stylus.
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Clays sintered in fire were the first form of ceramic, Bricks, cooking pots, art
objects, dishware, and even musical instruments such as the ocarina can all
be shaped from clay before being fired.
Used in many industrial processes, such as paper making, cement production,
and chemical filtering.
Used in the manufacture of pipes for smoking tobacco.
3. REFRACTORY MATERIALS
Refractories are ceramic materials that can withstand unusually high heat as
well as abrasion and the corrosive effects of acids and alkalis. They are used in
furnaces, stills for the cracking of petroleum, ceramic kilns, boilers, incinerators,
electrolytic cells for aluminum production etc. Refractories are an important
constituent of nuclear reactors. Jet engines would not last very long without
refractory parts.
Fire clay is an important raw material for refractories. In recent years non-clay
refractories of alumina, zirconia, silicon carbide, chromia, magnesite, graphite and
other less common materials are developed. The cost of these refractories is much
higher than that of fireclay. However, their use in critical parts of a furnace will keep
it in operating condition for longer periods, with less time taken out for repairs.
CLASSIFICATION
Acid refractories
Basic refractories
Neutral refractories or special refractories
Acid refractories
The prime ingredient for acid refractory is silica. Acid refractory have high
temperature loadbearing capacity and are used in the arched roofs of steel and
glass making furnaces at temperatures as high as 16500C. At this temperature small
portion of the brick will actually exist as a liquid. Alumina should be kept at 0.2 to
1.0% by weight because it adversely effect on the performance of these refractories.
These refractory materials are resistant to acid slags that are rich in silica. But they
are readily attacked by basic slags composed of CaO and / or MgO, there so
contact with these oxides should be avoided.
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Sr.
No.
Acid refractories Fusion
temperature
1. Silica(SiO2) 17000C
2. Aluminium silica
(46% Al2O3 + 54% SiO2)
17800C
3. Alumina (Al2O3) 20500C
4. Silmanite
(63%Al2O3 + 37%SiO2)
19000C
Basic refractories
The main constituent of basic refractories is magnesia (MgO), along with
calcium, chromium and iron compounds. The presence of silica is harmful for use in
high temperature performance; Basic refractories are especially resistant to attack
by slags containing high concentrations of MgO and CaO, and find extensive use in
some steel making open hearth furnaces.
Sr. No. Basic refractories Fusion temperature
1. Magnesia 22000C
2. Bauxite 18000C
3. Dolomite 15000C
Special refractories or neutral refractories
The special refractories are relatively expensive due to use of high-purity oxide
materials and very little porosity. Alumina, silica, magnesia, beryllia (BeO), zirconia
(ZrO2) and mullite (3AI2O3.2SiO2), as well as carbide compounds, in addition to
carbon and graphite are used for manufacture of special refractories. Silicon
carbide (SiC2) has been used for electrical resistance elements, as a crucible
material, and in internal furnace components. As carbon and graphite are
susceptible to oxidation at temperatures in excess of about 8000C, they find limited
application in spite of refractory characteristics.
Sr. No. Neutral refractories Fusion temperature
1. Chromite 21800C
2. Graphite 30000C
3. Silicon carbide 27000C
4. Zirconia 22000C
Silica and high alumina refractories
Refractories are obtained from ores of silica or those of silica and alumina.
After mining or chemical production and calcining, refractory materials are crushed,
ground and prepared to size. They are then mixed with other materials and shaped
as bricks. Bricks are used for lining-melting and other applications. Bricks of acid,
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basic and neutral refractories having different shapes are available for furnace
construction. Bricks during construction work are boned and cushioned with the help
of a mortar consisting of chrome, silicon carbide, silica and alumina. Brick work may
be coated with thin mortar for further protecting the same during furnace operation.
MANUFACTURE
Composition of silica and basic refractory bricks
Sr.
No.
Type of Brick Silica
(SiO2)
Alumina
(Al2O3)
Lime
(CaO)
Magnesia
(MgO)
Iron
oxide
(Fe2O3)
Chromic
oxide
(Cr2O3)
Other
Oxides
1. Silica 95-97 0.2-1.2 1.8-3.5 ---- 0.3-0.9 ---- 0.05-0.3
2. Basic
Chrome 3-6 15.33 ---- 14-19 11-17 30-45 1-2
Magnesite 3-6 0.4-2.0 1-5 85-95 0.5-4.0 ---- 0.5-1.0
Forsterite 33-39 ---- ---- 47-55 9-11 ---- 3-4
Composition of fireclays and high alumina bricks
Sr.
No.
Type of Brick Silica
(SiO2)
Alumina
(Al2O3)
Titania
TiO2
Other
oxides
1. Fireclay
Super duty 49-53 40-44 2.0-2.5 3-4
Semi-Silica 72-80 18-24 1.0-1.5 1.5-2.5
Medium duty 57-70 25-36 1.3-2.1 4-7
Low duty 60-70 21-32 1.0-2.0 5-8
2. High Alumina
60% Alumina 31-37 57.5-62.5 2-3.3 3-4
80% Alumina 11-15 77.5-82.5 3-4 3-4
90% Alumina 8-9 89-91 0.4-0.8 1-2
99% Alumina 0.5-1.0 98-99 Trace 0.6
Block diagram of manufacturing process
Clay
Refractory Bricks
Screening
Dryer
Figure: Manufacturing of Refractory
Rotary kiln
Hot air out
Hot air in
Water
Dry PressGrinding
Mixing
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Diagram with process equipment
Animation
It involves the following steps
Crushing
The clay is crushed to 25 mm in size.
Grinding
The crushed materials are ground in suitable grinding machine down to 200
mesh size.
Screening
Screening is carried out to separate fine and coarse materials. The desired
size material is used for brick making and oversize is recycled to grinding machine.
Mineral-dressing
Mineral dressing is used to purify the raw materials and producing better
refractories. Purification is carried out by settling, magnetic separation and by
chemical methods.
Storage
Pure materials are stored in storage bins using bucket elevators,.
Mixing
Proper distribution of plastic material throughout the mass is necessary for
easy moulding.
Moulding
It can be accomplished either manually or mechanically at high pressure.
Refractories of low density and low strength can be produce by hand-moulding.
While mechanical-moulding produces refractories of high density and strength.
Deairing is essential in order to increase the density and strength of refractory by
mechanical moulding.
Deairing
It is carried out by
Allowing air inside the void space in the refractory to go out by decreasing
the rate of pressure application and release
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Double-pressing the material viz. first pressed and allowed to crack. Then it is
repressed again so as to close the voids.
Applying vacuum through vents in the moulds
Drying
Drying is carried out to remove the moisture from refractories. Drying is carried
out in tunnel dryer usually very slowly and under well-set conditions of humidity and
temperature, depending upon the type of refractory. Rack cars are placed in a
long tunnel and heat is supplied by steam, which passes through the tunnel.
Firing
To stabilize and strengthen the structures of refractories firing is carried out in
tunnel kilns or shaft kilns or rotary kilns. The bricks are generally fired at a temperature
as high as or higher than their arc temperature.
The firing temperature of different bricks is as follows
High-fired super duty bricks : 14800C
Kaolin bricks : 17000C
Some basic bricks : 18700C
PROPERTIES
The following properties need to be considered while selecting a refractory
material.
Refractoriness
Refractoriness is indicated in terms of pyrometric cone equivalent (PCE) value
which indicates the temperature of softening the refractory. PCE should be well
above the operating temperature. As inner end of the refractory wall is at higher
temperature compare to the outer end, therefore, unless the brick melts away
completely, it can often be used to withstand a temperature higher than its
softening temperature. The outer end will be at a lower temperature and still in a
solid state, thus giving strength.
Strength
Strength under the combined effect of temperature and load is an important
factor, particularly in taller furnaces, the refractory has to support a heavy load.
Refractory must be strong enough to resist physical wetting away and to take load.
Refractoriness under load is an important consideration because usually a refractory
fails at a lower temperature when subjected to load because some crystals which
have become fluidic act as a lubricant and deformation becomes easier when
subjected to load.
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Specific gravity
Specific gravity reflects true porosity of refractory. Porosity increases with
increase in bulk density. It is also a criterion while buying the raw material because
raw materials of higher specific gravity will give fewer bricks or article per unit weight.
Eventually increases the production cost. Refractory material in powdered form is
used for making furnace lining or for repairs. Lesser weight of a material with lower
specific gravity will be required for this purpose. Materials of lower specific gravity
are also preferable because bricks in the lower part of a tall structure will not be
subjected to a heavy load
Porosity and slag permeability
If the refractory has open pores the gases and slags will enter the material
more easily and to a greater depth and may react and reduce the life of the
refractory.
Thermal expansion
In furnace design, allowance has to be made for thermal expansion. This
becomes a very important factor if the refractory is subjected to rapid changes in
temperature as a furnace door which is, on opening, is suddenly exposed to air at
room temperature. Within a single brick in the wall there may be a temperature drop
of a few hundred degrees from the hot face to the cold face. This causes
differences in thermal expansion in different sections of the brick. There may be
changes in the internal structures of the material with the result that there is
expansion in volume. All these result in the development of high internal stresses.
Thermal conductivity
It is important factor if the refractory is subjected to rapid changes in
temperature. Thermal conductivity should be low so that the heat of the furnace
may not be lost. Exceptions to this rule are coke over walls, mulle furnaces and
retorts which is heated form outside. They should be made of refractories of high
thermal conductivity. To prevent heat loss sometimes a refractory is backed by an
insulate material like asbestos. The life of refractories is shorter which are constantly
maintained at high temperature.
Electrical conductivity
It should be low for electrical furnaces. Except graphite, all other refractories
are bad conductors of electricity. The electrical resistivity of the refractories should
drop rapidly with increase in temperature.
Chemical composition
The chemical composition of the refractory should be such that the
surrounding do not chemically attack the refractory and corrode it.
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Characteristics of refractories
Can withstand high temperatures without being fused
Crucibles and furnace sides and bottoms containing molten metal are made
up of refractories
Use as ladles for pouring metal into the mould
Constitute furnace walls and roof and thus minimize heat losses
Properties of various refractory bricks
Sr.
No.
Type Class Properties
1. High Alumina 50% Alumina to
90% Alumina
High refractoriness which is increasing with
alumina content
High mechanical strength at high temperatures
Excellent to fair resistance to spilling
Greater resistance to corrosion
2. Fireclay Low, medium
and high duty
Good spilling resistance and thermal insulation
value
Fair resistance to fluxes and acid slags
Lower resistance to basic slags and fluxes
3. Fireclay Semi-Silica Rigidity under load at high temperatures
Resistance to structural spilling
Volume stability
Resistance to volatile alkalis or fumes
4. Silica Super duty
Conventional
High refractoriness and resistance to abrasion
High mechanical strength at high temperatures
Greater thermal conductivity as compared to
high duty fireclay brick, at high temperatures
High resistance to corrosion by acid slags
Fair resistance to attack by oxides of lime,
magnesia and iron
Readily attacked by basic slags and fluorine
Poor resistance to spilling at low temperatures
5. Chrome fired
magnesite
Chemically
bonded
Better resistance to spilling
High resistance to corrosion by basic slags
Mechanical strength and stability of volume at
high temperatures
6. Chrome fired High-resistance to corrosion by basic and
moderately acid slags and fluxes
Basic slags do not adhere to chrome bricks
Absorbed iron oxide may damage expansion
Possess thermal conductivity lower than that of
magnesite brick but higher than fireclay brick
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7. Magnesite fired
-Magnesia
88-90%
-above 90%
-Chemically
bonded
metal encased
Extremely high refractoriness and high thermal
conductivity
Poor resistance to slags containing high % silica
Great resistance to corrosion by basic slags
Chemically boned and metal encased bricks
have marked resistance to spilling
8. Forsterite-Fired Excellent strength at high temperature
High refractoriness
Attacked by acid slags
Fair resistance to most basic slags
Good corrosion resistance to alkali compounds
USES
Application and selection of different refractories
Sr.
No.
Type of refractory Name of the furnace used for melting ferrous metals
/alloys
1. Fire bricks Bottom of induction furnace
Sub-hearth of open hearth furnace
Doors of open hearth furnace
Spout of open hearth furnace
Spout of direct arc furnace (acidic)
2. Silica brick Side walls and roof of direct arc furnace
Roof of open hearth furnace
3. Magnesite-chrome brick Melting zone of basic cupola
Direct arc furnace roof (for Ni and Cu melting)
4. Dolomite Backing of open hearth furnace
Side walls of direct arc furnace(basic)
5. Chrome magnesite brick Side walls of direct arc furnace (basic)
Side walls of open hearth furnace
Blocks and ends of open hearth furnace
6. High Alumina fireclay Ladle refractories - as lining nozzle, stopper etc.
7. Magnesite Hearth and side walls of direct arc furnaces for
melting nickel and copper
Sub - hearth of direct arc furnace for melting steel
(basic)
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Refractories used in various furnaces
Sr.
No.
Furnace Refractory material
1. Iron blast furnace
Entire lining consists of fireclay but of different
qualities
Hearth and Bosh Coarsely ground fireclay
Shaft or stock Medium ground fireclay
Top Finely ground fireclay
Hot blast stoves Porous fireclay bricks
Hot blast main Fireclay
Bustle pipe Fireclay
2. Acid open-hearth Furnace
Portions above the working floor Silica bricks
Regenerative walls Fireclay bricks
3. Basic Open-hearth Furnace
Roof Silica bricks
Side walls Silica bricks
Hearth Dolomite or magnesite
4. Acid Bessemer Converter
Body Ganister
Tyres Fireclay
5. Basic Bessemer Converter Calcined dolomite or magnesite
6. Basic Electric Furnace
Roof Silica bricks
Bottom and sides A layer of fireclay bricks next to shell and upon
this a layer of magnesite
7. Cupola Fireclay lining
8. Reheating Furnace
Roof Silica bricks
Hearth Chromite or magnesite bricks
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Module: 6
Lecture: 29
SPECIALIZED CERAMIC PRODUCTS AND VITREOUS ENAMEL
4. SPECIALIZED CERAMIC PRODUCTS
Based on the specific requirements/application ceramic material having
special properties are developed and they are as follow
a) Ceramic composites
Materials with different combinations of properties are required in specific
applications like underwater, transportation and aerospace. E.g. in construction of
aircrafts, structural materials should have low densities. At the same time, they should
be strong, stiff, and resistant to abrasion, impact and corrosion. Ceramic composite
like cermet should fulfill such demands. Cermet is intimate mixture of ceramic and
metallic components in the form of powder, which are compacted and sintered.
Cemented carbide is probably the most common cermet which is composed of
extremely hard particles of a refractory carbide ceramic like tungsten carbide or
titanium carbide embedded in a matrix of a metal like cobalt or nickel which
increases toughness of the carbide particles. These carbides are widely used as
cutting tools for hardened steels. Cermet is also used in linings for brakes and
clutches and it is also used as heat shields, rocket nozzles and ram jet chambers in
aerospace application.
Ceramic metal composites can be produce by bonding of the two materials
which are heated below the melting point of individual components. Ceramic
catalyzes the conversion of metal into its oxide. As the metal oxide is formed, crystals
of the oxide grow into the crystal structure of the ceramic materials. Thus the bond
becomes strong between the two phases of the resultant cermet. Metals like Pt, Au,
and Ag exhibit the best bonding with the ceramic phase like Alumina, Magnesia,
Silica, Zirconia, or Beryllia. Their applications are gold coated ceramic wafers for
semiconductor chips, zirconia-lined steel for corrosion resistant uses, and ceramic
capped gold dental crowns. The process is also used for gem setting in gold.
b) Ferroelectric ceramics
Dielectric materials like insulators are known as ferroelectrics, which show
spontaneous polarization in the absence of an electric field e.g. Barium titanate
(BaTiO3) which have very high dielectric constants at relatively low applied field
frequencies and permanent electric dipoles. Another example of ferroelectrics is
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potassium dihydrogen phosphate (KH2PO4), Rochelle salt (sodium potassium
tartarate- NaK.C4H4O4.4H2O), KH2AsO4, NaTaO3, KTaO3, LiTiO3 etc.
BaTiO3 have piezoelectricity. For piezoelectric application poly crystalline
BaTiO3 is used which is cooled through the curie temperature in the presence of
strong electric field. This imparts permanent orientation to the dipoles in the resulting
ceramic materials.
Piezoelectric materials convert electrical energy into mechanical strains or
vies versa when employed in transducer. They are also employed in phonograph,
pick-ups, microphones and sonar detectors. They are employed in ultrasonic
generators which are used for mixing of powders and paints, homogenisation of
milk, aging of clays and the emulsifying of liquids etc.
High amount of voltage is generated because pressure is applied on
piezoelectric material. The spark that can be drawn from such a high voltage in
used for ignition in gas lighter, cooking stoves and cigarette lighters.
c) Ferromagnetic ceramic
Ferrites are an important class of compounds which having magnetic
properties. It formed by mixing of oxides of Iron with other metals. But they are poor
electrical conductors. Soft ferrites can be easily magnetized and demagnetized.
They are used in transformers, in capacitors, as microwave devices in
communicating radio signals, as memory devices in computers and tape recorders.
While hard ferrites are permanent magnets made from oxides of iron, barium and
strontium. Hard ferrites are used in motors and loudspeakers.
d) Ceramic biomaterials
Ceramics have many advantages as biomaterials. They are light weight,
more wear resistant and not attacked by enzymes and biochemical in the human
body. Ceramics are used in making artificial teeth, bone joints. It is also used in filling
the gaps in damaged bones which is facilitated by the similarity between natural
bone and calcium phosphate ceramics.
e) High alumina ceramic
They are mechanically strong dense and special ceramic material. They
possess good wear resistance, corrosion resistance and dimensional stability. So they
are used in insulators for electrostatic precipitators. It is also used in linings for mining
chutes and slides and in making of precision machine parts.
f) Sensors
When ceramic material is exposed to water vapour and certain gases it
transforms non-electrical signals into electrical ones. Humidity sensors are made from
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mixture of titanium oxide and magnesium chromates while gas sensors usually
ceramic semiconductors made from oxides of titanium, iron, tin, silver and zinc.
g) Ceramic superconductors
Ceramics which had been used as insulating materials could be better than
conductors. They are high temperature superconductors and show zero electrical
resistance. It can be prepared by fusing an oxide of barium and copper. Thus if
cables are made of superconducting material, one-fifth of the loss of electricity
during its transmission through aluminium or copper wires is avoided.
Super conductors can repel magnet and also applied in super-fast
magnetically levitated trains. It is also used in magnetic resonance imaging (MRI),
which is a modern diagnostic tool.
h) Ionic conduction
Ceramics materials are made into sodium-sulfur batteries which are used in
electric cars. Ions become mobile at high temperatures and able to carry electrical
charge across them is known as ionic conductivity.
5. VITREOUS ENAMEL
Besides flux and refractories, oxidizing agents such as pyrolusite, red lead and
nitre are included for the formation of enamel. In the formation of enamel colouring
agents, floating agents and free electrolytes are required. Lead oxide, boric acid,
potassium and sodium oxide are easily fusible compounds at lower temperature so
considered as fluxes. Refractories include feldspar; quartz and clay contribute to the
acidic part of the melt and give body to the glass. The basic part is supplied by the
flux.
MANUFACTURE
Raw materials
The raw materials used for the manufacture of enamel are feldspar quartz,
kaolin quartz and feldspar contribute to the hardness and resistance to the action of
acid of the enamel.
Kaolin lends plasticity
Fluxes: boric acid, borax
Flux as well as oxidation agents: Red lead and lead carbonate
Opacifiers: TiO2 SnO2, ZrO2, fluorspar, cryolite
Floating agent: Clay and gums
Colouring agents: Oxides, elements or frits
Electrolytes: borax, Na2CO3, MgSO4 and MgCO3
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Manufacture
The manufacture of enamel glass (frit) is similar to the first stage of
manufacture of ordinary glass. The finely powdered raw materials are mixed in
proper proportioned and charged into a melting furnace. After the batch has been
uniformly melted, the melt is poured into a quenching tank to granulate it. The cold
water shatters the melt to innumerable pieces, which are called frit. The frit is then
ground into ball mill with porcelain balls where plastic clay is added to prevent the
separation of water from the powdered material. Than colouring agents and
opacifiers are used for milling. After milling the product is discharged and in the form
of thick enamel slip.
Articles of high carbon steels and of cast iron can be enameled. But before
the application surface of these objects should be cleaned thoroughly of all foreign
matters so that the coating of enamel may adhere well.
Enamel may be applied to the metal by slushing, brushing or spraying. e.g.
Iron sheet, or iron pieces are coated by dipping or slushing. In slushing enamel slips is
poured over the metal surface to allow the excess run off.
PROPERTIES
Vitreous enamel is opaque seldom transparent, coloured or colourless flux. It is
easily fusible material
USES
It is used as protective or decorative agent to coat the surface of glass,
porcelain and metals particularly iron sheets.
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Module: 7
Lecture: 30
GLASS INDUSTRIES
INTRODUCTION
When silica or quartz is heated up to 16500C it melts to a colourless liquid
which on cooling gives glass. This fused mass is highly sensitive to temperature
change therefore it requires special heat treatment so that ordinary glass can be
manufacture which is much stable to temperature change. The glass of various
commercial qualities is prepared by heating sand or quartz along with metal oxide
or carbonates.
TYPES OF GLASSES
1. Soda-lime or soft glasses
The raw materials are silica (sand), calcium carbonate and soda ash. Their
approximate composition is Na2O.CaO.6SiO2. About 90% of all glasses produced
belong to soda lime glass. The low cost, low melting point soda-lime glass has
resistant to de-vitrification and to water. However, they have poor resistance to
common reagents like acids.
Uses: They are used as window glass, electric bulbs, plate glass, bottles, jars, building
blocks and cheaper tablewares, where high temperature resistance and chemical
stability are required.
2. Potash-lime or hard glasses
Silica (sand), calcium carbonate and potassium carbonate are the basic raw
material for potash lime glass. Their approximate composition is K2O.CaO.6SiO2. They
possess high melting point, fuse with difficulty and have good resistance to acids,
alkalis and other solvents compare to ordinary glasses.
Uses: These glasses are costlier than soda-lime glasses and are used for chemical
apparatus, combustion tubes, etc., which are to be used for heating operations.
3. Lead glass or Flint glass
Instead of calcium oxide, lead oxide is fused with silica. As much as 80% of
lead oxide is incorporated for dense optical glasses. In addition, K2O is used instead
of sodium oxide. So, its approximate composition is K2O.PbO.6SiO2. Lead glass is
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more expensive than ordinary lime-soda glass, but it is much easier to shape and to
work with. Lead glass has a lower softening temperature and higher refractive index
than soda glass. It has excellent electrical properties. It is bright, lustrous and
possesses high specific gravity (3 to 3.3).
Uses: Lead glasses are used for high quantity table wares, optical lenses, neon sign
tubing, cathode ray tubes, electrical insulators and in the art objects because of
their high luster. High lead content glasses are used for extra dense optical glasses,
for windows and shields to protect personnel from X-rays and gamma rays in
medical and atomic energy fields respectively.
4. Borosilicate glass or Pyrex glass or Jena glass
It is the most common of the hard glasses of commerce which contain
virtually only silica and borax with a small amount of alumina and still less alkaline
oxides. Borosilicate glass has the following composition.
Component SiO2 B2O3 Al2O3 K2O Na2O
Percentage 80.5 13 3 3 0.5
Boron and aluminium oxides substitutes Na2O and CaO used the lime-soda
glasses which results in a glass of low thermal coefficient of expansion, and high
chemical resistance. Borosilicate glasses have a very much higher softening point
and excellent resistivity to shock.
Uses: They are used in pipelines for corrosive liquids, gauge glasses, superior
laboratory apparatus, kitchenwares, chemical plants, television tubes, electrical
insulators etc.
5. 96% Silica glass
It is produced and shaped as typical borosilicate glass, having dimensions
bigger than desired. The heat treatment to the article, separate the glass into two
layers, one consisting mainly of silica and the other of the alkali oxides and borates.
Then article is dipped in hot acid which dissolves away the alkali oxides and boron
oxide layer, leaving behind a fine porous structure consisting of about 96% silica, 3%
B2O3 and traces of other materials. This glass is then washed carefully and annealed
to 12000C. The shrinkage of about 14% takes place and hard firm shape is produced
which is almost gaslight. The translucent 96% glass, if it is so desired heated to a
higher temperature and made almost transparent or clear.
It is expensive than other types of glasses. The expansion coefficient is very
low which accounts for its high resistance to thermal shot. The softening temperature
is about 15000C. They possess high chemical resistance to most corrosive agents.
They are corroded by only HF, hot H3PO4 and concentrated alkaline solutions.
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Uses: They are used where high temperature resistance is required and articles can
be safely used at temperature up to 8000C. They are used for the constructed
chemical plants, laboratory crucibles, induction furnace linings, electrical insulators.
6. 99.5% silica glass or Vitreosil
It is produced by heating SiO2 to its melting point (1,7500C). Because of
absence of fluxing agents, it is extremely difficult to get rid of the bubbles. Shaping of
the glass is difficult due to high viscosity at its working temperature. The final product
is translucent. It has high softening temperature about 16500C, compare to 96% silica
glass. Its thermal expansion is very low. Due to their opaque nature, they tend to be
mistaken for pipe when dirty and are, therefore, often broken accidentally.
If vitreosil glass is heated for long periods above its melting point, it finally
becomes transparent and is then known as ―clear silica glass‖. It has considerable
transmission properties e.g. 1mm of this material allows no less than 93% of light to
pass corresponding figure for good optical glass is only 6%.
Uses: uses are similar to 96% silica glass. It is exposed for the construction of pipelines
for hot concentrated acid. Clear silica glass is used mainly for plant ware, chemical
laboratory wares, electrical insulating materials, and in electrical heat furnaces.
7. Alumino-silicate glass
They possess exceptionally high softening temperature and having the typical
constituent as follow
Component SiO2 Al2O3 B2O3 MgO CaO Na2O & K2O
Percentage 55 23 7 9 5 1
Uses: it is used for high pressure mercury discharge tubes chemical combustion tube,
certain domestic equipment etc.
8. Safety glass
Thin layer of vinyl plastic is introduced between two or three flat sheets of
glass and the whole is subjected to slight pressure. It is then heated till the glass layers
and plastic layers merge into one another to give a sandwich. On cooling the glass
becomes quite tough. When such a glass breaks it does not fly into pieces as the
inner plastic layer tends to hold back the broken pieces of the glass.
Uses: It is mostly used in automobile and aero plane industries as wind shield.
9. Optical or Crookes glasses
They contain phosphorus and lead silicate, together with a little cerium oxide,
is capable of absorbing harmful UV light. Very careful manufacturing process of
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heating the molten mass for prolonged time secured the homogeneity of the glass.
In general optical glasses have low melting points and are relatively soft. Their
chemical resistant and durability are appreciably lower than those of ordinary
glasses.
Uses: Used for manufacture of lenses.
10. Polycrystalline glass or Pyroceram
It is the most recent development of producing glass by adding one or more
nucleating agents to a special or convectional glass batch. Then it is shaped into
desired form and subjected to controlled heat treatment.
The nucleating agents induced the formation of a large number of sub-
microscopic crystalline which act as centers for further crystal growth. Crystalline
glass is not ductile, but it has much greater impact strength than ordinary glass. It
exhibits high strength and considerable hardness and can be formed and shaped
into articles by any methods of manufacturing.
11. Toughened glass
It is made by dipping articles still hot in an oil bath, so that certain chilling
takes place. There so, outer layers of the articles shrink and acquire a state of
compression; while the inner layers are in a state of tension. Such a glass is more
elastic and capable of withstanding mechanical and thermal shocks. When such a
glass breaks, it does not fly but is reduced to fine powder.
Uses: It is used for window shields of fast moving vehicles like cars, trucks, aeroplane;
window shields of furnaces, automatic opening doors and large show cases.
12. Insulating glass
It is a transparent unit prepared by using two or more plates of glass
separated by 6-13 mm thick gap, field up with dehydrated air and then thematically
sealing around the edges. This provides a high insulation against heat. Thus, if such a
glass is used for separating apartments, it does not transmit heat and consequently
the apartments will remain cool during summer and warm during winter.
Uses: It is used as thermal insulating materials
13. Wired glass
It is formed by embedding a wire mesh at the center of the glass sheet during
casting due to this when glass breaks it do not fall into splinters. Additionally, it is
more fire resistant than ordinary glass.
Uses: It is used mainly for making fire-resisting doors, windows, skylights, roofs
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14. Laminated glass
It is made by pressing or bonding together two or more sheets /plates of glass
with one or more alternating layer of bonding material like plastic resin, asphalt or
synthetic rubber.
The essential qualities of the laminated glass are
It is shatter-proof, i.e. its pieces do not fly off when suddenly broken.
It is shock-proof, i.e. it can with stand sudden changes of temperature and
pressure without breaking.
A bullet-resistant laminated glass is manufactured by pressing together
several layers of glass with vinyl resins in alternate layers. Ordinary, thickness of such
glass varies from 12.7 mm - 76.5 mm. Even thicker types are made for specific uses.
Uses: As safety glass in aircrafts, automobiles, helicopters, submarines. Bullet resistant
lamination glass finds application in making automobile wind screens, looking
windows etc.
15. Glass wool
It is a fibrous wool-like material composed of intermingled fine threads or
filaments of glass which is completely free from alkali. Glass filaments are obtained
by forcing molten mass of glass through small orifice of average diameter of 0.005 -
0.007mm continuously which is sent to rapidly revolving drum resulting in wool like
form. It has low electrical conductivity and eight times higher tensile strength than
steel. It does not absorb moisture and it is completely heat proof.
Uses: It is employed for heat insulation purpose, e.g., insulation of metal pipe lines,
motors, vacuum cleaners, walls and roofs of houses. Being resistant to chemicals,
glass wool is used for filtration of corrosive liquids like acids and acidic solution. It is
used for electrical and sound insulation. It is also employed in air filter as dust filtering
material. It is also used for manufacturing fiber-glass, by blending with plastic resins.
16. Photosensitive Glass
It is UV sensitive high alumina soda lime glass. The positive in UV region on
glass is developed by thermal treatment only at 540-550°C. The desired photo
activity in UV region can be obtained by admixture of high alumina soda lime glass
with small amounts of Cu2O NaCN.SnO2 and abeitic acid in appropriate amounts. A
blue colour is promoted by NaCN absence of tin oxide. In presence of tin oxide an
impression in red is observed. By manipulation the ingredients in glass, brown and
yellow images can also be possible. A potash alumina glass mixed with LiSiO3, cerium
and silver, salts in appropriate proportions have also been used as photosensitive
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glass.
17. Photochromic glass
Large number of microscopic particles of silver halides trapped in the three
dimensional silicate networks in fixed concentration. On exposure light, temporary
colour centers consisting of silver particles only are produced and these add quickly
producing total darkness. The intensity of darkness depends upon the concentration
of silver. Because reversible darkening is controlled by the radiations in the UV region
quite abundant in day light, the photo blackening does not occurs markedly in the
lamp light night.
18. Fiber glass
Fiber glass is nothing but molten glass process mechanically to a flexible
thread of filament. A hot platinum nozzle filled with molten glass forces out the fluid
in the form of a thin continuous thread which when caught by a rapidly moving disc
gets converted into fiber through elongation and twist given by the disc fabrics
made of glass are bad conductors of heat and electricity and are noninflammable.
Hence articles made of fiber glass are fire proof.
Uses: Such type of glass is used in textiles and reinforcing and can be spun into yarn,
gathered into a mat, and made into insulation and a great variety of other products
may be with it.
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Module: 7
Lecture: 31
MANUFACTURE OF GLASS
RAW MATERIAL
The raw material in manufacture of glass may be selected from the following.
Sand, soda ash, calcium oxide, fled spar, borax, magnesia, zinc, alumina,
lead oxide, manganese oxide, selenium metal, broken glass, fluxes, colouring agent,
reducing agent, oxidizing agent etc.
Oxide should satisfy following conditions
Every oxygen atom must be attached with 2-4 cations e.g. SiO2, B2O3, GeO2,
P2O5 and As2O5
The oxygen polyhedral must share the corner position and not the edge.
At least three corners of each tetrahedron must be share.
The oxides used for glass manufacture are classified into following groups
a) Network former
b) Network modifier
c) Intermediate glass formers
d) Oxidizing agent
e) Refining agent
f) Cullet
g) Colouring agent
a) Network former
These are oxides of elements which are surrounded by four oxygen atoms in
the tetrahedral chain forming glass.
b) Network modifier
These are large diameter elements having higher co-ordination number. On
simple melting they do not give glass but in presence of other network forming
oxides they can give glassy products easily. The important network modifiers are
oxides of alkali metal, alkaline earth metals, lead, zinc etc.
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c) Intermediate glass formers
They do not give glass on melting but in presence of some network formers
using their co-ordination number they start giving glass. E.g., Oxides of aluminum,
silica
d) Oxidizing agent
Material like sodium nitrate or certain peroxides are used to reduce the colour
of impurities like iron oxides and manganese oxide
e) Refining agent
To reduce or to eliminate quantity of air bubbles from molten glass refining
agents like arsenic oxide or small amount of feldspar is added to glass.
f) Cullet
Waste or broken glass species are called cullet. In normal glass production
33% of charge is broken glasses. Recycling of cullet increases the rate of production.
g) Colouring agent
Metal oxide is added as colourant during manufacture of colour glasses e. g.
oxides of chromium and iron give green glass while copper and cobalt give blue
glass.
MANUFACTURE
Tank Furnace
Gas Air
Lime stone + Cullets
Hot gases
Hot gases
Hot flame at 18000C
Figure : Manufacture of Glass
Proportioning Tank
Soda ash+
Sand
Cooler
Hot end Cool end
Finishing
Forming & Shaping Anealing
Glass
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Block diagram of manufacturing process
Diagram with process equipment
Animation
The manufacturing process of glass consisting of the following four steps
1. Melting the charge
2. Fabrication of the article
3. Annealing the article formed
4. Finishing treatments
1. Melting of the charge
Amount of raw materials for the batch are calculated from the chemical
composition of individual components. Minor ingredients are weighed accurately
and given a preliminary mixing with one of the dry batch ingredients before adding
to the main charge then to the batch mixer which is a revolving drum provided with
blades to lift and spread the material uniformly. After proper mixing of ingredient it is
charged into the furnace.
Two types furnaces are used for glass melting
a) Pot furnace
b) Tank furnace
a) Pot furnace
In pot furnace, glass is melted in open or covered pots (closed pots) of fire
clay placed inside the combustion chambers of the furnace fired directly with coal
(used in bangle industry) or producer gas (used tableware manufacture). Pot
furnaces are generally used for small scale melting and fabrication by hand, for the
production of glass bangles, table wares, lamp wares, thermos-flask etc.
b) Tank furnace
In this process, cross flame regenerative type of gas or oil used. The port is
arranged along the side of the tank above the glass level those on any one side is
alternatively incoming and outgoing ports.
Manufacturing large quantities of a particular type of glass tank furnace is
used. E.g. manufacture of sheet glass container ware, lamp shells and resistance
glasses, continuous tank furnaces are generally used.
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The regeneration system consists of chambers filled with open brickwork
situated on either side of the furnace, through which hot waste gases and air
required for combustion pass alternatively at regular intervals of about 30 minutes.
The flame acts directly upon the raw batch and molten glass. The temperature
inside the furnace is generally kept at 870-9850C. The molten glass kept at a constant
level by continuous charging rate which is equal to the rate of withdrawal from the
furnace. After withdrawal from the furnace, slow cooling of molten glass is carried to
minimize permanent strain. The higher the temperature used for reheating, lesser will
be time to remove the strain.
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Module: 7
Lecture: 32
GLASS (Continued)
MANUFACTURE (CONTINUED)
2. Fabrication of an article
The molten glass is fabricated to require size and shape by either by hand or
by machine. Hand fabrication is adopted for small production and machine
fabrication is adopted for large scale production.
Following are the different ways fabrication
a) Blowing
b) Casting
c) Drawing
d) Pressing
e) Rolling
f) Spinning
a) Blowing
Blow pipe of diameter is about 12mm and its length about 180cm is used for
blowing purpose. One end of the blow pipe is dipped in the molten mass of glass
and lump of about 5 kg weight is taken out. This lump of glass will then lengthen to
some extent by its own weight. The operator then blows vigorously from other end of
blow pipe. The same can also be done with the heat of an air compressor. This
blowing causes the molten mass to assume the shape of cylinder. It is then heated
for few seconds and is blown again. The blowing and heating are continued till the
cylinder of required size is formed. It is then placed on an iron plate and it is
disconnected from blow pipe. The cylinder is then cut vertically by the diamond
which is falls into a thin plate by gravity.
b) Casting
The molten glass is poured in moulds and it is allowed to cool down slowly,
large pieces of glass of simple design can be prepared by this method. It is also
adopted to prepared mirrors and lenses.
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c) Drawing
Simply pulling the molten glass either by hand or mechanical equipment is
carried out. In the process, an iron bar is dipped sideways in the molten mass of
glass. Then it is lifted up horizontally and in doing so, it catches up a sheet of molten
glass. The sheet is then allowed pass over a large rotating roller. The roller helps the
molten glass to spread in the sheet.
d) Pressing
In this process, the molten glass is pressed into moulds. The pressure may be
applied by hand or by mechanical means. This process is adopted for ornament
article and hollow glass articles.
e) Rolling
There are two methods of rolling.
In one method, the molten mass of glass is passes between heavy iron rollers
and flat glass plate of uniform thickness is obtained.
In another method the molten mass of glass is poured on a flat iron casting
table and it is then turned flat with the help of a heavy iron roller.
f) Spinning
The molten glass is spun at high speed to a very fine size. This glass has tensile
strength equal to that of mild steel. It does not fade, decay or shrink. It is not
attached by acids, fire and vermin. It is very soft and flexible. It is used for providing
insulation against electricity and sound.
3. Annealing
Glass articles, after being manufactured, are to be cooled down slowly and
gradually. This process of slow and homogeneous cooling of glass articles is known
as annealing of glass.
Annealing of glass is a very important process. If glass articles are allowed to
cool down rapidly, the superficial layer of glass cools down first, as glass is a bad
conductor of heat. The inter portion remains comparatively hot and it is, therefore in
a state of strain. Hence such glass article breaks to pieces under very slight shocks or
disturbances
Following are the methods of annealing
a) Flue Treatment
b) Oven treatment
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a) Flue treatment
This method is useful for large scale production. In the process long flue is used
which is constructed in such a way that there is gradual decrease in temperature
from one end of it to the other. The red-hot articles of glass are allowed to enter at
the hot end of flue and they are slowly moved on travelling bands. They become
cool when they reach the cool end of flue.
b) Oven treatment
This method is useful for small scale production. The red-hot glass articles are
placed in ovens, in which arrangement is made to control the temperature. After
articles are placed in ovens, the temperature is slowly brought down.
4. Finishing treatments
a) Bending
b) Cutting
c) Opaque making
d) Silvering
a) Bending
Glass may be bent into desired shape by placing it in ovens in which
temperature can be regulated. Glass in the form of rods, sheets or tubes is placed in
such ovens and heated. It is then bent when it is suitable heated.
b) Cutting
Glass is cut in required sizes with the help of diamond or rough glasses or small
wheels of hardened steel
c) Opaque making
Glass can also be made opaque or impervious to light. It is done by grinding
the glass surface with emery. It can also be achieved chemically by the application
of hydrofluoric acid.
d) Silvering
This process consists in applying a very thin coat of tin on the surface of glass.
Silver is deposited on this layer of tin. A suitable paint is then applied to give
protection, against atmospheric effects.
PROPERTIES
Glass has excellent properties amongst various material of construction. To
summarize the good properties of glass can be considered with respect to hardness,
transparency, refractive index, dispersion of light, low expansion of coefficient,
insulation, thermal conductivity, chemical inertness etc.
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The properties of glass are mainly governed by factors such as composition of
the constituent state of surface, thermal treatment conditions, dimensions of
specimens etc.
Absorbs, refracts or transmits light
Can take up a high polish and may be used as substitute for very costly germ
Has no definite crystalline structure
Has no sharp melting point
Affected by alkalis
Excellent electrical insulator at elevated temperatures due to the fact the
glass can be considered as an ionic liquid.
Available in beautiful colours.
Behaves as more solid than most solids in the sense that it is elastic. But when
that elastic limit is exceeded, it fractures instead of deforming.
It can be blown, drawn or pressed. But it is difficult to cast in large pieces.
Extremely brittle.
Usually not affected by air or water.
It is not easily attacked by ordinary chemical reagents.
Possible to intentionally alter some of its properties such as fusibility, hardness
refractive power etc.
The glasses may be cleaned colourless, diffused and stained.
It is possible to weld pieces of glass by fusion.
It is transparent and translucent. The transparency is the most used
characteristic of glass and it is due to the absence of free electrons. For the
same reason. It is works as a good insulator.
When heated, it becomes soft and transformed into a mobile liquid, which on
cooling formed into articles of desired shape.
It is possible to manufacture glass lighter than cork or softer than cotton or
stronger than steel. The strength glass however is considerably affected by
foreign inclusions, internal defect are cords or chemically heterogeneous
areas.
Not easily affected by air/oxidizing agent
Highly stable against acid but affected easily by alkalis.
When fused with excess of fusion mixture, glass decomposes into silicates and
carbonates of metal.
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Module: 8
Lecture: 33
FERTILIZER
INTRODUCTION
Fertilizers are the products that improve the levels of the available plant
nutrients and/or the chemical and physical properties of the soil, thereby directly or
indirectly enhancing the growth, yield and quality of the plant. Fertilizers are
compounds used to promote plants growth. They are usually applied either through
the soil, for uptake by plant roots, or uptake through leaves. Fertilizers can be
organic matter or inorganic chemicals or minerals. They can be naturally occurring
compounds such as peat or mineral deposits, or manufactured through natural
processes or chemical processes.
Fertilizers typically provide, the three major plant nutrients (phosphorous,
nitrogen and potassium), the secondary plant nutrients (sulfur, calcium and
magnesium), and sometimes trace elements (or micronutrients) with the role in plant
nutrition: chlorine, boron, manganese, zinc, iron, copper, and molybdenum.
TYPES OF SOIL
a) Virgin Soil
It is the portion of the soil or land in which plants have not grown since long
time. It is always fertile and good crop may be yielded due to elements present in it
have not been used up by the plants as food.
b) Exhausted Soil
It is the soil on which crop and after crop, especially of the same variety has
been raised. The plants keep on absorbing the same elements for the soil and hence
the soil becomes impoverished or an unproductive soil
Factors affecting the fertility of soil are as follows
Nitrogen, phosphorus, potassium and other mineral salts
The amount of fixed nitrogen
pH value of the soil
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PLANT NUTRIENTS
Plants require following nutrient elements for their growth
a) Natural nutrients
Carbon, hydrogen and oxygen are derived from air and water and so these
are called natural nutrients.
b) Primary nutrients
Nitrogen, phosphorus and potassium are consumed in large amounts by the
plants for their growth and so these are called primary nutrients.
c) Secondary nutrient
Calcium, magnesium and sulfur which occur to a limited extent in all soils, are
called secondary nutrients.
d) Micronutrients
Zinc, boron, copper, manganese, chlorine, iron and molybdenum are
required in little amount by the plants and so these are called micro nutrients.
More than sixty elements are found in the composition of various plants.
Among those elements carbon, oxygen and hydrogen make up the principal part of
plant matter, nitrogen, phosphorus, potassium, magnesium, sulfur, calcium and iron
are utilized in the formation of plant tissues and heighten the growth of plants.
Air provides CO2 as a source of carbon require for photosynthesis while water
provides/transport various mineral to the plants.
Nitrogen, potassium and phosphorus containing minerals are principally
important for normal plant life. These elements stimulate processes of metabolism in
the plant cells, growth of the plant and especially its fruits, increase the content of
valuable plant components such as starch of potatoes, sugars of beets, fruits and
berries, proteins of grains and increase resistance to frost, drought and diseases.
FUNCTION OF NUTRIENT
The role of various nutrients in the plant growth is as follows
a) Nitrogen
Nitrogen supplied as nitrate ion or ammonium salt is one of the important
constituent for synthesis of amino acids which are then converted into proteins and
enzymes. Proteins thus formed make part of the protoplasm, while enzymes act as
catalysts for various reactions taking place in the plants. Nitrogen is also a special
constituent of the chlorophyll, without which photosynthesis is not possible. Nitrogen
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makes up 16-18% of the plant protein and constitutes 1-4% of the dry weight of
plants and is required in large amounts of any of the plant nutrients.
Sources of nitrogen
Atmosphere
Organic nitrogen compounds formed in the soil by recurring natural processes
Fertilizers
The natural sources are not sufficient for adequate plant growth and so
artificial nitrogen compounds in the form of fertilizers are added to the soil.
b) Phosphorus
It is required in much lesser amounts than nitrogen. Most soils contain
phosphate in the form of complex calcium phosphate, aluminium and iron
complexes and organic compounds. Such sources are insoluble and so the plants
can make very little use of them.
Phosphates are involved in the respiratory and photosynthetic processes
which provide energy in some of the plants metabolic processes without which the
plant could not live. The need of phosphorus is also necessary for the health of the
plant as it is constituent of nucleic acids, phytins and phospholipids. It is also found in
seeds and fruits. The phosphorus has also been found to contribute to the formation
of the reproductive parts in the early life of the plant.
c) Potassium
It is necessary for healthy growth of plants and cannot be replaced even by
closely related elements as sodium and lithium. In the plant, it either occurs as a part
of the anion of organic acid or as a soluble inorganic salt in the tissues. It contributes
to formation and movement of carbohydrates in plant. Deficiency of potassium
quickly reduces the carbohydrate contents. The potassium content of plants ranges
from about 0.5-2.5% of the dry weight.
d) Magnesium
Magnesium carried out the phosphates which are important for the formation
of phospholipids and in the synthesis of nucleoproteins. Magnesium is also a mineral
constituent of chlorophyll and makes up 2.7% of the weight of chlorophyll.
Deficiency of magnesium is removed by the naturally occurring magnesium salt
present in soil. Dolomitic limestone is used to supplement the natural supply.
e) Calcium
Calcium acts as a plant nutrient and soil amendment to correct soil acidity. It
is found as plant constituent in the cell walls of leaves in the form of calcium
pectate. Calcium is closely associated with the growth of the flowers. The deficiency
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of calcium also prevents normal development of buds and tips. Calcium is also
found in cell sap either in the ionic form or as salts of organic acids. Application of
calcium to the soils corrects the soil's acidity rather than supplying a nutrient.
f) Sulfur
It is present in many proteins in the form of methionine and cystine which
contain 21.5 and 26.7% sulfur respectively. A deficiency of sulfur decreases the plant
growth accompanied by extensive yellowing of green parts. The sulfur needs of the
plants are small and supplied by soil compounds, from industrial gases that distribute
sulfur compounds, or from sulfates supplied in fertilizers.
g) Iron
Iron is used in certain respiratory enzyme systems by plants, mainly, catalyse,
cytochrome and peroxidase. A deficiency of iron causes leaves to turn white and
growth to cease. Iron deficiency is noted in the growth of citrus and in crops such as
soyabeans and peanuts.
h) Boron
It is required in extremely small amounts by plants. Its function is obscure, but
accumulation of carbohydrates and water soluble amino compounds in plants
efficient in boron suggests that boron is of some importance in protein synthesis.
i) Zinc
It is involved in enzyme systems in the plant, particularly carbonic anhydrase
and carboxylase.
j) Manganese
It is found in active regions of the plant and acts as an oxidising agent for iron.
Deficiencies of manganese usually occur in organic soils and in alkaline or highly
acidic soils.
k) Copper
Copper is associated with some of the plant enzyme systems, such as
polyphenol oxidase and ascorbic acid oxidase. Deficiencies are generally
associated with organic soils.
l) Chlorine
It is the most recent addition to the essential nutrient list. It has been observed
that the deficiency of chlorine can cause wilt chlorosis (yellowing of green plants)
and necrosis. Chlorine in small amounts also stimulates growth of crops like barley,
alfalfa and tobacco.
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m) Molybdenum
It is associated with the functioning of one or more of the plant enzyme
systems, especially nitrate reducing enzymes.
n) Non-Essential Elements
Certain non-essential elements, such as sodium, silicon, aluminium etc. are
also found in plants. No evidence has yet been found that meets any of the
requirements for essentiality.
NEED OF FERTILIZER
It can be defined as the quantity of additional nutrients required by a
particular crop to increase its growth to the optimal level in a given soil. After
repeated cultivation the soil reached to a stage where it becomes less productive, if
supply of the above nutrients is not provided. Thus, in order to make up this
deficiency, certain elements in the form of their compounds have to the added to
the soil to make it reproductive. These substances are known as fertilizers.
Fertilizing the land is essential due to following reason
Provide nutrient essential for growth and better yield.
Maintain the pH of the soil in the vicinity of 7-8 to facilitate optimum growth
Provide food supplement to the plants
CLASSIFICATION
Classifications of fertilizer are as under
1. Based on their chemical composition
Organic products: Produced out of wastes of animal husbandry (stable
manure, slurry manure, etc.), plant decomposition products (compost, peat,
etc.), or products from waste treatment (composted garbage, sewage
sludge, etc.).
Mineral fertilizers: Contains inorganic or synthetically produced organic
compounds.
Synthetic soil conditioners: It‘s main function is to improve the physical
properties of the soil.
2. Based on their nutrient content
Micronutrient fertilizers: Containing nutrients required in small quantities by
plants.
Straight fertilizers: Containing one primary nutrient, and
Compound fertilizer: Also known as complex or multi-nutrient fertilizers. It
contains several primary nutrients and sometimes micronutrients.
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3. Based on the physical state
Solid fertilizer: Packed in bags
Liquid fertilizers: Packed in containers
4. Based on the source
Fertilizers can be obtained from natural and artificial sources
a) Natural organic fertilizers
Animal matter: Powdered dry fish and red dry blood from the slaughter house
are important nitrogenous fertilizers.
Farm yard manures: Typical farmyard manure consists of cow dung, sheep
dung and human excretions.
Guano: Guano is a classic example of complete fertilizer and it is a mixture of
bird 's excrement, fish refuge and fish hones.
Plant matter: Oil cakes from cotton seed meal; linseed meal and caster cake
belong to this class and contain 7%, 5.5% and 6% of nitrogen respectively.
b) Natural inorganic fertilizers
Rock Phosphates: Finely divided rock phosphate, although insoluble in water,
weathers rapidly and may be used directly. Bone metal is another source
which supplies phosphorus but phosphorus is exclusively supplied by the
artificial sources.
Chile Saltpetre: Chilean deposits would not last for more than 250 years, even
at present about 83% of the world's requirements of NaNO3 come from
artificial sources.
Potassium Salts: Natural potassium sources are wood ash (containing 5-6%
potash) and waste materials of sugar beet crops.
These natural organic and inorganic fertilizers are not sufficient to
make the soil productive, as they can no wholly meet the demand. Hence
fertilizers are made artificially.
c) Artificial fertilizers
One of the major problems for modern fertilizer industry is to determine
the most effective and economical materials for supplying the nutrients. These
may be developed under three groups, according to the nature of the
element.
Phosphorus fertilizer
Nitrogenous fertilizer
Potassium fertilizer
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Nitrogen is usually supplied either in the nitrate or in the ammonical form. For
most agricultural situations it makes little difference which form is used, because the
ammonical form is converted rapidly to nitrates in the soil by the action of nitrifying
bacteria. In special cases, e.g., in case of low soil temperature, nitrification may be
delayed: For example, urea requires a little longer time to reach nitrate stage,
because it must first hydrolyze to the ammonical form.
Phosphates are supplied in calcium phosphate or ammonium phosphate
forms to the plant. Dicalcium phosphate is the principal phosphate constituent in
nitric phosphate and ammoniated super phosphate.
Potash as source of potassium is available in natural deposits in low cost. So
selection of potassium fertilizer is not a great problem compare to nitrogen and
phosphorous fertilizers.
Sulfur is supplied as ammonium sulfate and potassium sulfate, and in the
calcium sulfate found in one form of superphosphate. Micronutrients are usually
supplied as soluble sulfate
Straight and mixed fertilizers
Straight fertilizers are sometimes used in special situations, like giving the plant
an additional supply of nitrogen after it is used up, thereby reducing leaching loss
and ensuring an adequate supply of nitrogen throughout the growth period. Potash
is often added to pastures as a straight material. Phosphate is also applied straight,
but too much lesser extent than in mixed fertilizers. The amount of nitrogen in straight
form is more than six times as that of phosphate or potash.
More than one nutrient is required for most of crops application. The farmer
could purchase single nutrient materials and apply them separately or mix them
together before application. Mixed fertilizers are prepared by mixing appropriate
amounts of ammonium salts, superphosphate and potassium salts.
Granulation
Prior 1920, fertilizer was generally finely divided, with some lumps formed due to
high moisture contents of the product. It became more and more difficult to
produce fertilizers of acceptable physical properties for ammonium nitrate, urea
and ammonium phosphates. So, various methods were developed to increase the
particle size of such fertilizers, in order to reduce moisture absorption by reducing
surface area and to minimize caking by reducing the number of contact points
between the particles. In 1935, Nitrophoska first prepared a multi-nutrient fertilizer in
granular form containing a granulated mixture of ammonium nitrate, diammonium
phosphate and potassium chloride. After that the development of products range
from semi-granular mixed fertilizers to granular and to urea and ammonium nitrate
prills have been carried out.
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Ideal granular fertilizers have homogeneous, spherical particles in the size range
of 10-14mesh. Modern granular fertilizers are satisfactory products which also reduce
fertilizer cost by the use of high analysis, hygroscopic materials. Farmers now also
demand granular fertilizer because of its attractive appearance, freedom from dust
and ease of handling.
Bulk blending
In the early 1950's most of the emphasis was on the granulation of mixed
fertilizers of ammoniated superphosphate types. Some granulation of fertilizers based
on ammonium phosphate was also carried out, often with ammonium sulfate and
potassium chloride. In 1955, another method known as bulk blending was
introduced for making granular mixed fertilizers. This is the simple mechanical mixing
of high analysis, granular materials giving a product of high analysis and good
physical properties. Materials used in this technique are granular ammonium sulfate
or ammonium nitrate, triple superphosphate or ammonium phosphate and
potassium chloride.
Requisite as fertilizer
The chief requisites of a fertilizer are
Must be soluble in water
The element present in the compound must be easily available to the plant
Should be cheap
Should not be toxic to plant
Should be stable for long time
Should maintain the pH of the soil in the vicinity of 7 to 8.
The most important factor of fertilizers is the movement of water in the soil.
Application of lime opens the pores of the soil and enables a free circulation of
water.
Fertilizer ratio
Commercial fertilizer has specific ratio of nutrients, or fertilizer ratio or plant
food ratio. It is the ratio of the number of fertilizer units in a given mass of fertilizer
expressed in the order N, P and K. Thus, it is the ratio of two or more nutrient
percentages to one another. For instance, a fertilizer with 5-10-15 grade has 1-2-3
ratio, whereas a fertilizer with 10-20-20 grade has 1-2-2 ratio. Fertilizer ratio is also
defined as the relative proportion of primary nutrients in a fertilizer grade divided by
the highest common denominator for the grade. For example, the grade 16-12-20
has a ratio of 4-3-5 of N, P and K, respectively.
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Fertilizer benefits
The fertilizer benefits to the human environment are as follows.
a) Farming efficiency improvement
The farmer's income can be increased by the application of fertilizers. If the
use of economic optimum levels of fertilizer is consistent, negative consequences
are minimized.
b) Improvement of soil quality with adequate fertilization
The aggregating action from enhanced root proliferation and a greater
amount of decaying residues have reportedly made the soil more friable, tillable
and water retentive.
c) Crop quality improvement
The mineral, protein and vitamin contents of crops can be improved by
balanced fertilization.
d) Water conservation
Plants well nourished by fertilizers, use water efficiently through their
expanded root system, thereby reducing water evaporation losses and conserving
this natural resource.
Efficient fertilizer use is the key to sustained productivity. A well-fertilized soil
gives a dense canopy, which protects the soil from erosion, absorbs more carbon
dioxide and gives out more oxygen. Future agricultural strategies should aim at
minimizing leaching, erosion, volatilization losses of chemical fertilizers and organic
manures, and prevention of over fertilization.
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Module: 8
Lecture: 34
AMMONIUM PHOSPHATE
INTRODUCTION
Ammonium phosphate ((NH4)3PO4) also known as ammonium
orthophosphate is the salt of ammonia and phosphoric acid. It consists of
ammonium cations and phosphate anion. It is water soluble and the aqueous
solution on boiling losses ammonia.
Ammonium orthophosphates are applied to soil either directly, or as a
solution, or in a suspension form, depending on the proportion of insoluble
phosphates present in the soil. Ammonium phosphates refer to a generic class of
phosphorus fertilizers and are manufactured by reacting anhydrous ammonia with
orthophosphoric acid or super phosphoric acid. These are either in solid or liquid
form.
There are two major types of ammonium phosphate which are
monoammonium phosphate (MAP, NH4H2PO4) and diammonium phosphate (DAP,
(NH4)2HPO4) and these can be inter-converted by changing ammonia or phosphoric
acid as needed. Mono-ammonium phosphate is manufacture by reacting ammonia
with phosphoric acid, centrifuging and drying in a rotary dryer. While diammonium
phosphate requires two-stage reactor system in order to prevent loss of ammonia.
The granulation process followed by neutralization is completed in rotary dryer,
which is heated by a furnace using fuel.
Two grades of ammonium phosphate are available
1. Monoammonium phosphate (MAP)
Anhydrous ammonia added to liquid phosphoric acid gives monoammonium
phosphate (MAP). It is a fertilizer or fertilizer intermediate with high P2O5 content of
about 55% and nitrogen content 11-12%.
2. Diammonium phosphate (DAP)
With more ammonia, technical grade diammonium phosphate (DAP)
containing 16 to 18% nitrogen and 20 to 21 % phosphorus (46% P2O5) is formed.
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MANUFACTURE
Raw Materials
Basis: 1000kg of Diammonium phosphate
Ammonia = 200kg
Phosphoric acid = 465kg
Electricity = 200MJe
Fuel = 525MJ
Direct labour = 0.5work-hr
Reactions
NH3 + H3PO4 NH4H2PO4
NH3 + NH4H2PO4 (NH4)2HPO4
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
Phosphoric can be manufacture by electric arc furnace, blast furnace or wet
process as described in Module: 4, Lecture: 21.
Manufacture
H3PO4
Ammonmium phosphategranules
Re
ac
tor
1
Re
ac
tor
2
Dry
er
Gra
nu
lato
r
Screen
Figure: Manufacturing of Ammonium Phosphate
Liquid Ammonia
Anhydrous Ammonia
H3PO4
Re
ac
tor
3
H3PO4
KCl
Ammonia recycled to process
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Block diagram of manufacturing process
Diagram with process equipment
Animation
The two principal steps are
a) Neutralization
b) Granulation
a) Neutralization
Quantities of phosphoric acid and ammonia in the neutralization step are
different form mono ammonium phosphate (MAP) and diammonium phosphate
(DAP). To manufacture mono ammonium phosphate, ammonia to phosphoric acid
ratio is 0.6 in the neutralizer and then 1.0 in the granulator. While for diammonium
phosphate, the ratios are 1.4 and 1.0 in the neutralizer and granulator respectively.
Phosphoric and ammonia are added to the first of three continuous mixed
reactors, anhydrous ammonia is added beneath the slurry level in the first neutralizer
in an amount equivalent to 80% neutralization. Further ammonia is added in the 2nd
and 3rd tanks to obtain conversion to the diammonium salt if a higher nitrogen
containing fertilizer is needed.
The exothermic reaction heats the slurry nearly to the boiling point (130°C).
Unreacted and excess NH3 vapor is collected from the top of each tank and
recharged below the liquid level for reducing NH3 losses (less than 3%). The hot slurry
containing about 16 to 20% water is pumped into the granulator, where more
ammonia is added to increase the molar ratio to approximately 2.0.
b) Granulation
Slurry from the third neutralized is mixed with KCl and absorbed in a bed of dry
recycle fertilizer moving through a rotating drum granulator. This provides a tumbling
action to coal recycle material with a slurry film.
A rotary adiabatic drier reduces the moisture to less than 1%, with 10 minute
contact time with air initially at 1500C. Dried product is separated into three fractions
on a double deck screen. A portion of the product from the deck of the lower
screen is sent to bagging operations. The balance, together with pulverized oversize
and fines, is returned to the granulator. The weight ratio of recycle to product is 6: 1-
15: 1 depending on the grade produced.
Module: 8 Lecture: 34 Ammonium phosphate
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Powder ammonium phosphate
Powdered ammonium phosphate is still in use because of its high phosphorus
content (as P2O5).
In addition, a group of fertilizers, such as ammonium phosphate-sulfates,
ammonium phosphate-chloride and ammonium phosphate-nitrate are produced
by a number of processes involving the neutralization of ammonia with a mixture of
phosphoric acid and plant waste acids like sulfuric acid, nitric acid or hydrochloric
acid. These fertilizers are free flowing and non-hygroscopic (or less hygroscopic)
compared to the individual components, and have been successfully used in many
types of soils.
Major engineering problem
Ammonia losses
Ammonia loss should be kept below 3%. It can be achieved by using
multistage reactors along with efficient recycling mechanism of collecting the
vapour of ammonia from top of the neutralization tower and recycle back to the
process.
Corrosion
Use of corrosion resistance material like SS316 for hot acid and fumes ducts,
carbon steel for granulation, drying and screening
PROPERTIES
Molecular formula : NH4H2PO4
Molecular weight : 115.03gm/mole
Appearance : White crystal
Odour : Odourless
Melting point : 1900C
Density : 1.803gm/mL
Solubility : Moderately soluble in water
pH : 4-4.5
Ammonium phosphate fertilizers are highly soluble in water and fast acting in
soil to give nitrogen and phosphorus in a chemical combination.
Storage properties and the ease of granulation depend on the amount of
impurities, which form a gel like structure (mainly aluminum and iron
phosphates). This gel promotes granulation and serves as a conditioner to
prevent caking even at moderately high moisture levels. A small proportion of
phosphate rock added to phosphoric acid before ammoniation improves the
granulation.
The standard commodity grade of diammonium phosphate is 18-46-0.
Module: 8 Lecture: 34 Ammonium phosphate
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Pure and completely soluble ammonium phosphates are used mainly as
liquid fertilizers.
DAP is unstable at temperatures above 1500C while monoammonium
phosphate remains stable even at much higher temperatures.
These two fertilizers usually form a part of concentrated compound fertilizers
and are rarely used individually in their pure states.
USES
Used as a high effective non-chloride N, P compound fertilizer in agriculture. It
contains totally 73% fertilizer elements (N+P2O5), and may be used as a basic
raw material for N, P and K compound fertilizer
In flame-proofing, plant nutrient solutions
Used in manufacturing of yeast, vinegar, yeast foods, and bread improvers
Used in buffer solutions and in analytical chemistry
Used as a fire prevention agent for fabric, timber and paper, as well as a fire
prevention coating, and dry powder for fire extinguisher.
For food grade, it is mainly used as a fermentation agent, nourishment agent.
Module: 8 Lecture: 35 Superphosphate
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Module: 8
Lecture: 35
SUPERPHOSPHATE
INTRODUCTION
Superphosphate, Ca(H2PO4)2, also refereed as single superphosphate (SSP), is
the first chemically manufactured commercial fertilizer. John B Lawes named as
superphosphate. Single superphosphate is produced as a combination of rock
phosphate and concentrated sulfuric acid. Approximately equal amounts of the
two ingredients are thoroughly mixed, dried and cured. Chemically, SSP contains
monocalcium phosphate and calcium sulfate. The hardened mass is either ground
or granulated.
The monocalcium phosphate of single superphosphate dissolves in the soil
moisture and the roots absorb phosphoric acid in that form. The rest of the solution of
monocalciumphosphate precipitates in the soil pores and forms different phosphate
compounds which are water-insoluble and do not leach out. A compound like
dicalcium phosphate dissolves in carbonic acid in water and becomes available to
plants, but the insoluble tri-calcium phosphate remains fixed in the soil. Where soil is
markedly acidic i.e., rich in active iron and aluminum monocalcium phosphate gets
converted into insoluble phosphate compounds
2Ca(H2PO4)2 + Fe2O3 2FePO4 + 2CaHPO4 + 3H2O
Because iron and aluminum phosphates are insoluble, phosphorus does not
available to the plant. That why SSP does not use in acidic soils unless it is limed.
If single superphosphate is applied just before sowing, plants get enough
supply of phosphorus at their critical growing stages. Single superphosphate is not
suitable for top dressing because of its slow movement.
Sometimes, single superphosphate is mixed with lime or dolomite in order to
increase its effectiveness. The production of single superphosphate is on the rise in
tropical countries, like India.
SSP compared to various sulfur-containing fertilizers like DAP and TSP,
significantly increases the grain yield of many agronomical important crops like
wheat, chickpea and groundnut. SSP is compatible to mix with (NH4)3PO4, (NH4)2SO4,
NH4Cl, KCl and K2SO4. Mixtures of SSP with materials containing free lime or CAN or
Module: 8 Lecture: 35 Superphosphate
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urea should not be stored for long as they cause reversion of water-soluble
phosphate.
In order to get the maximum benefits, single superphosphate should be
applied to soils deficient in phosphorus as well as sulfur. The time, place and the
quantity of application are critical.
SSP is the principal phosphate fertilizer due to
The production process is simple, requires little skill and small investment
It sets a standard of comparison for other phosphate fertilizers
It supplies two secondary nutrient elements, namely, sulfur and calcium
Despite these advantages, single superphosphate has a low phosphorus
content (16 to 22 % P2O5), and 6 to 10% moisture content which sometimes make SSP
production uneconomical.
MANUFACTURE
Raw Materials
Basis: 1000kg Superphosphate (den process)
Calcium phosphate = 625kg
Sulfuric acid = 320kg
Water = 90kg
Electricity = 0.2kWH
Labour = 0.1man-hr
Sources of raw material
Phosphate rock can be obtained and purified as described in Module: 4,
Lecture: 20.
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18.
Reactions
Ca3(PO4)2 + 2H2SO4 + 4H2O CaH4(PO4)2 + 2(CaSO4.2H2O)
CaF2 + H2SO4 + 2H2O CaSO4.2H2O + 2HF
4HF + SiO2 SiF4 + 2H2O
3SiF4 + 2H2O SiO2 + 2H2SiF6
Overall reaction
CaF2.3Ca3(PO4)2 + 7H2SO4 + 3H2O 3CaH4(PO4)2.H2O + 2HF + 7CaSO4
Module: 8 Lecture: 35 Superphosphate
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Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
The manufacture of superphosphate involves following steps
a) Preparation of phosphate rock
The quality of the product is determined by the grade of phosphate rock. Fine
grinding is demanded due to low reactivity of the rock. Phosphate rock is finely
ground up to the size of less than 100 meshes. Grinding of phosphate rock to fine
powder has following advantages
Increase the rate of reaction
Less sulfuric acid is needed
A higher grade of product in better condition is obtained.
It is difficult to make SSP from igneous apatite. Up to a point, the presence of
aluminum and iron compounds can be tolerated, though they reduce the solubility
Figure: Manufacturing of Superphosphate by Continuous-den process
Ground phosphate rock
Sulphuric acidWater
Cone
Mixer
Pug mill
continuous den
Cutter
Conveyor
Conveyor
Water
We
t S
cru
bb
er
Exhaust
Drain
Limestone bed
BulkStorage
Pulverizer
Bagging and
StorageSuper phosphate
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of phosphorus in water. The increase in the ratio of CaO:P2O5 raises the consumption
of sulfuric acid per unit of P2O5 and decreases the grade. Silica has no adverse
effect and higher chloride content in the phosphate rock is acceptable.
b) Mixing with acid
A finely ground (less than 100 meshes) phosphate rock is mixed with sulfuric
acid in a cone mixer. The commercial concentrated sulfuric acid (77 to 98 %) is
diluted to around 68 to 75% before reacting with the rock.
c) Curing and drying of the original slurry by completion of the reactions
The fluid material from the cone mixer goes to a den where it solidifies owing
to a continued reaction and crystallization of monocalcium phosphate. The
superphosphate is removed from the den after 0.5 to 4 hours. It is still at temperature
of about 100°C and plastic in nature.
d) Excavation, milling, and bagging of the finished product
The product from the den is sent to storage piles for final curing of 2 to 6
weeks. During curing, the free acid, moisture and the unreacted rock content
decreases, whereas the available water-soluble phosphorus content increases. As
the reaction approaches completion during curing, the material hardens and cools.
The cured product is crushed in a hammer mill or cage mill to a size of about 6 mesh.
e) Granulation
When granular superphosphate is required, the product is granulated before
or after curing. Granulation before curing is advantageous as it requires less steam or
water. After granulation, the product is dried in a direct contact drier and screened.
Super phosphate is manufactured by
1. Batch process
2. Continuous process
1. Batch - den process
Rock phosphate and sulfuric acid in correct quantities are added to a pan
mixer of 1 to 2 tons capacity. After mixing for 2 minutes, the fluid slurry is discharged
into a box den which has 10 to 40 ton capacity. When the den is filled completely
after 1 hour, it is moved slowly to a mechanical cutter which removes thin slices of
product to the conveyor. Some plants have two dens, which are used alternatively.
This set up gives a production rate of 40 tons per hour.
Advantages
If only igneous rock is available, batch mixing are preferred due to precise
control of mixing conditions available and den can be made tight enough to
Module: 8 Lecture: 35 Superphosphate
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contain the very fluid slurry.
For a small phosphate source in a remote place, batch process can be built.
Although newer plants use continuous processes, some plants still conduct
these operations stepwise. All plants first pulverize the rock with modern pulverizing
and air-separation equipment, most rock is ground so that 70 to 80% of particles are
passed through 200mesh screen.
2. Continuous-den process
Finely ground phosphate rock is fed by a weigh feeder into a double-conical
mixer, where it is thoroughly mixed with metered quantities of sulfuric acid. The acid
and water are fed into the cone mixer tangentially to provide the necessary mixing
with the phosphate rock. The sulfuric acid is dilute with water in the cone to the
concentration of 51°Be. The heat of dilution of the sulfuric acid serves to maintain
proper reaction temperature, and excess heat is dissipated by evaporation of extra
water added. The rate of water addition and acid concentration may be varied to
control product moisture. The fresh superphosphate is discharged from the cone
mixer into a pug mill, where additional mixing takes place and the reaction starts.
From the pug mill the superphosphate drops onto the den conveyor, which has a
very low travel speed to allow about 1hr for solidifying before reaching the cutter.
The cutter slices the solid mass of crude product so that it may be conveyed to pile
storage for "currying" or completion of the chemical reaction, which takes 10-20 day
to reach P2O5 availability acceptable for plant food. The conveyers den is enclosed
so that fumes do not escape into the working area. These fumes are scrubbed will
water sprays to remove acid and fluoride before being exhausted to the
atmosphere. Scrubber water is neutralized by passing through the limestone bed.
Granulation
Both granulation and drying of the SSP are carried out in the same piece of
equipment. The powdered superphosphate enters to rotary drum granulator, where
it is mixed and granulated with recycled fines (recycle ratio: > 0.6). Granulation is
controlled by adjusting the water content and temperature of the product in the 1st
part of the rotary drum. Drying is achieved near the exit from the granulator and
involves adding more sulfuric acid and ground limestone (about 60 kg/ton SSP). The
heat produced by the reaction dries the product and the calcium sulfate formed
encapsulates the product in such a way that caking is avoided during final curing
and storage of the product. No P2O5 reversion is noticed. Product is sized using a
conventional system of screens and crushers such that 90% of the product is
between 1 and 4 mm in size. Final curing of the product occurs during storage for
less than two weeks.
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Nitric and mixed acid acidulation of phosphate rock
Block diagram of manufacturing process
Diagram with process equipment
Animation
First use of nitric and mixed acid acidulation of phosphate rock was carried
out in Europe. It is desirable, since nitrogen has an essential value as plant food and
can be resold at its purchase price. Also, this saves sulfur. Simple acidulation of
phosphate rock with nitric acid produces the hydroscopic superphosphate, since it
contains calcium nitrate.
There are various commercial modification in the process is carried out
In one, the phosphate rock is extracted by mixed nitric and sulfuric acids,
followed by ammonization, drying. In another method, mixed nitric and phosphoric
acidulation followed by the conventional steps and others use nitric acid alone for
acidulation. These processes, as well as conditioning against moisture absorption as
practiced for ammonium nitrate, have led to an extension of this acidulation with
nitric acid. Nitrophosphate is also gaining importance particularly in European
countries. Phosphate rock is decomposed with nitric acid plus small amount of
Ground phosphate rock
Sulphuric acidWater
Cone
Mixer
Pug mill
continuous den
Cutter
Conveyor
Conveyor
Water
We
t S
cru
bb
er
Exhaust
Drain
Limestone bed
BulkStorage
Pulverizer
Bagging and
StorageSuper phosphate
HNO3
Figure: Manufacturing of Superphosphate by nitric and mixed acid acidulation
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phosphoric acid. The resulting slurry is ammoniated and carbonated and, if desired,
combined with potassium salts and spray-dried to yield a uniform palletized product.
Special grades
A technical variation among superphosphates is the Kotka superphosphate,
a mixture of superphosphate and phosphate rock. It is so named because it was
originally made in Kotka, Finland. It needs little curing and the free acid content is
low. Its effectiveness is equal to fully acidulated superphosphate and raw phosphate
rock applied separately.
Another special grade superphosphate is serpentine superphosphate, a
product obtained by mixing 20% serpentine (a mineral consisting of magnesium
silicate) with 80% single superphosphate. Serpentine supplies magnesium to crops
and improves the physical properties of superphosphate by reaction with free acid.
For serpentine superphosphate to be effective, SSP must contain at least 16%
phosphorus (as P2O5,) soluble in neutral ammonium citrate, of which at least 93 % is
water-soluble.
‗Enriched superphosphate‘ is essentially a mixture of single superphosphate
and triple superphosphate made by acidulation of phosphate rock with a mixture of
sulfuric and phosphoric acids. The grade contains 25 to 35 % phosphorus (as P2O5)
and is useful for application in sulfur deficient areas.
Handling and storage
Powder SSP is not free flowing and has the tendency to cake. Granulated SSP
can be easily handled and uniformly distributed in the field without any problem.
Due to the presence of free acid, single superphosphate is normally bagged
in polyethylene lined HDPE woven bags. Polypropylene woven bags can also be
used.
Kinetics
Ca3(PO4)2 + 2H2SO4 + H2O Ca(H2PO4)2.H2O + 2CaSO4 ΔH = -108.44kcal
The above reaction takes place in two stages. In the first stage, sulfuric acid
reacts with the phosphate rock, forming phosphoric acid and calcium sulfate. In the
second step, phosphoric acid reacts with more phosphate rock, forming
monocalcium phosphate. The first step occurs readily, while the second stage takes
several days.
Since most phosphate rock is fluorapatite, fluorides react with sulfuric acid to
give hydrogen fluoride, which reacts with silica to form silicon tetra fluoride as well as
fluorosilicates.
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4HF + SiO2 SiF4 + 2H2O
3SiF4 + 2H2O SiO2 + 2H2SiF6
HF acid reacts with silica to form fluosilicic acid results in to incomplete
removal of fluorine. An excess of sulfuric acid is consumed by such impurities in the
phosphate rock (as CaCO3 Fe2O3 and CaF2). The product increase in weight over
the 70-750bpl (bone phosphate of line) by phosphate rock used as much as 70%,
resulting in superphosphate with 16 to 20% available P2O5
Cost
The costs of bagging, transportation and storage of SSP are high, because
the mass of SSP required is more than twice that for TSP. Hence small plants of SSP
are economically better suited to serve small local markets.
PROPERTIES
Molecular formula : CaH4(PO4)2
Molecular weight : 234.05gm/mole
Elemental analysis : 16% P2O5 (7%P), 12% S, 21% Ca, 4% phosphoric acid
Appearance : White, Gray or brown granular
Odour : Odourless
Boiling point : 2030C
Melting point :1090C
Density : 2.22gm/ml
Solubility : Solubility in water, HNO3 and HCl
Angle of repose : 260
Critical humidity : 93.7% at 300C
USES
It is the principal carrier of phosphate, the form of phosphorus usable by
plants, and is one of the world's most important fertilizers.
it is low cost source of phosphorous in a wide range of pasture and cropping
situations
Generally mixed with sulfate of ammonia and muriate of potash, but can be
blended with other fertilizers
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Module: 8
Lecture: 36
TRIPLE SUPERPHOSPHATE
INTRODUCTION
Triple superphosphate (TSP) is the more concentrated fertilizer than ordinary
superphosphate, containing from 44 to 51% of available P2O5 or nearly three times
the amount in the regular superphosphate. Owing to the high concentration of
phosphorus, TSP is used widely in the production of high analysis compound fertilizers.
Triple superphosphate, also known as concentrated superphosphate,
contains 45 to 50% monocalcium or water-soluble phosphate and 17 to 20% lime. Its
concentrated form is cheaper to transport, store and apply when compared with
the dilute form.
TSP is manufactured by adding phosphoric acid to rock phosphate,
producing mainly water-soluble monocalcium phosphate with no calcium sulfate. In
most processes, a large percentage of fluorine remains in the product, probably as
fluorosilicate or calcium fluoride. When triple superphosphate is used as a fertilizer,
the yield from short season crops like cereals, potato and some vegetables is
markedly higher. This fertilizer lets a weak root system establish itself firmly and
supports the crop to stand during the growing period.
Advantages
It is a highly concentrated straight phosphate fertilizer.
It has a low-cost source.
Its manufacture requires small capital investment and low-skilled manpower.
Disadvantages
Its total nutrient content is lower than that of ammonium phosphate.
Its acidic character deteriorates storage bags.
It is not suitable for blending with urea as it causes the latter to deteriorate
The manufacture of triple super phosphate involves following steps
a) Reaction
b) Denning or Den process step
c) Storage and curing
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d) Granulation
a) Reaction
The reaction is carried out in a cone-mixer. A similar process known as
Kuhlman process uses a mixer that has a high-speed stirrer in a cylindrical vessel. 95
to 98% of the ground rock phosphate, passed through 100 mesh sieve, is mixed with
phosphoric acid (1kg rock phosphate of 34% P2O5 is mixed with 2.6kg acid). The acid
is of commercial grade with P2O5 content of 52%.
b) Den step
The den step, of TSP is faster (10-30min) than that for SSP (30-120min). The
mixture from the reaction vessel goes to the den where it solidifies owing to
continued reaction and crystallization of monocalcium phosphate.
c) Storage and curing step
Product from den step is stored in piles for curing. Curing requires 3 to 6 weeks,
depending on the quality of raw materials. During curing, the free acid, moisture
and unreacted rock contents decrease and the available phosphorus and the
water soluble P2O5 increase. Fluorine compounds evolved in minor quantities which
are scrubbed to prevent atmospheric pollution.
After storage and curing, TSP is ground to a 6 mesh screen (3.3 mm). This
material is called run off pile TSP or ROP-TSP and is used for making compound
fertilizers by agglomeration granulation.
d) Granulation process
Granulation is preferred due to powder form have dusty nature and caking
quality when moist. The milled and screened TSP is conveyed to a drum granulator
where water is sprayed and steam is spurge underneath the bed to wet the
material. The wet granules are dried in the rotary drier. The dried granules are
screened, and the oversized and the fines are returned to the granulator. The dust
and fumes from the drier are scrubbed or removed by the dust filter.
MANUFACTURE
Raw Materials
Basis: 1000kg Triple superphosphate
Phosphate rock = 386kg
Phosphoric acid = 540kg
Power = 40kWH
Steam = 20kg
Labour = 0.3man-hr
Fuel = 140000Btu
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Sources of raw material
Phosphate rock can be obtained and purified as described in Module: 4,
Lecture: 20.
Phosphoric can be manufacture by electric arc furnace, blast furnace or wet
process as described in Module: 4, Lecture: 21.
Reaction
CaF2 + 3Ca3(PO4)2 + 14H3PO4 10Ca(H2PO4)2 + 2HF
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Phosphoric Acid
Phosphate Rock
Water
Steam
Figure: Manufacturing of Triple Superphosphate
Cooler
Screen
Cy
clo
ne
se
pe
rato
r
WaterWater
ExhaustExhaust
WasteWaste
Coarse
Bulk Storage
BaggingShipping
We
t S
cru
bb
er
We
t S
cru
bb
er
Acid Preheater
Mill
Granulator
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Finely ground phosphate rock and 62% H3PO4 are charged continuously to
the granulator, where reaction and granulation take place. Fines from the product
screen are recycled to the granulator, and the moisture and temperature required
for proper granulation are maintained by addition of water and/or steam. The
granulator is a cylindrical vessel rotating about a horizontal axis and has in overflow
dam at the discharge end.
The phosphoric acid is fed uniformly under the bed of material through a
perforated pipe. When wet-process phosphoric acid is used, it is also necessary to
provide an acid pre-heater. The granules overflow the dam into the rotary cooler,
where they are cooled and dried slightly by counter current flow of air. The exhaust
gases from the cooler pass through the cyclone, where dust is collected and
returned to the granulator as recycle, the cooled product is screened, the coarse
material being milled and returned, along with the fines, to the granulator.
The product is then conveyed to bulk storage, where the material is cured for
1 to 2 weeks, during which a further reaction of acid and rock occurs, which,
increases the availability of P2O5 as plant food, the exhaust gases from the
granulator and cooler are scrubbed with water to remove silicofluorides.
The cost per unit of P2O5 is higher as compared to ordinary superphosphate,
because of more capital investment and additional labour and processing.
However, this is balance to the great extent by the ability to use the lower-grade,
cheaper phosphate rock to make the phosphoric acid. There are also substantial
savings on handling, bagging, shipping, and distributing.
Granulation
The Den process or direct slurry granulation process may prepare triple
superphosphate either in a granular or non-granular form. The granular form of TSP is
preferred for direct application or blending and the non-granular form for making
compound fertilizers.
Advantages of the direct slurry granulation process
Product is available at a lower cost
Generate denser and stronger granules
Use of conventional granulation equipment
Disadvantages
Uses phosphate rocks which have a short reaction time, resulting in a greater
loss of soluble phosphorus (as P2O5) due to an incomplete reaction or higher
ratio of phosphoric acid
Granular triple superphosphate is produced directly rather than from the
powder fertilizer.
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Ex-den granulation
In this process, phosphate rock is further ground and the den retention time is
larger (25 to 45 minutes instead of 10 to 30 minutes). The product from the den
directly goes to granulation instead of to curing. The granulated product is dried to
get a product with 4 to 6% moisture. Further reactions take place during storage.
The product is much easier to granulate than cured TSP because of the
plasticity and heat content. This requires less recycle, water and steam, resulting in
the saving of power and manpower.
Advantages direct granulation process
Low cost
Dense and strong granules
Interchangeability of granulation equipment with that for other ammonium
phosphates.
Disadvantages of the direct granulation process
Short reaction time makes the un-reactive rocks unsuitable
Greater loss of soluble P2O5 due to incomplete reaction.
Jacobs-Dorrco process
Ground phosphate rock and phosphoric acid (38 to 40% P2O5) are fed into
steam heated reaction vessels. The overall reaction time is 30 minutes and the
reaction temperature is 90°C. The thick slurry is fed into a rotary drum granulator with
a high proportion of recycle time. The moist granules are dried and screened and
the product size material sent to storage.
A process, very similar to Jacobs-Dorrco process, is used in Europe. However,
spraying the slurry onto a cascading curtain of granules at the feed end of
cocurrent rotary dryer combines granulation and drying.
Leyshorr and Mangat suggested use of an aging conveyer for transportation
of granules from the granulator to the dryer. Evaporation during transportation in
conveyer makes the granules less sticky in the drying operation. Ultimately, reduces
clogging in the feeding chutes. The oversized and the undersized granules, after
screening, are crushed and recycled. The recycle ratio is 8:1 and 12:1 for the rotary
drum dryer and the granulation respectively. This lower ratio for the rotary drum dryer
is caused by moisture evaporation due to a counter-current sweep of the air.
Handling and storage
TSP in powder form is not free flowing and has a tendency to cake on
storage. The granulated product has excellent handling and storage characteristics,
and is free flowing. The material is packed in polyethylene film lined HDPE bags.
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PROPERTIES
Molecular formula : Triple superphosphate
Molecular weight : 252.07gm/mole
Appearance : White or gray granules
Solubility : water soluble
Density : 1.089gm/ml
pH : 2.5-3.0 (aqueous solution)
P2O5 (water soluble) : 42.5% minimum
Free phosphoric acid : 3% by wt. (Max.)
Angle of repose : 450
Bulk density : 800-881kg/m3
Moisture : 12.0% by wt. (Max.)
USES
TSP can be used in wastewater treatment to reduce lead levels
As a low cost source of phosphorus, TSP is the main substitute for single
superphosphate in cropping blends.
It is used mainly in blends with DAP and MAP
Source of phosphorus in situations where no nitrogen is required e.g. good
fallow after clover dominant pasture.
In horticultural blends where the phosphorus content needs boosting
Module: 9 Lecture: 37 Urea
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Module: 9
Lecture: 37
UREA
INTRODUCTION
Urea (NH2CONH2) or carbamide is an organic compound has two —NH2
groups joined by a carbonyl (C=O) functional group. Urea serves an important role
in the metabolism of nitrogen containing compounds by animals and is the main
nitrogen containing substance in the urine of mammals.
Urea was first discovered in urine in 1727 by Herman Boerhaave, though this
discovery is often credited to Hilaire Rouelle.
Friedrich Wöhler synthesized urea from an inorganic precursor in 1828. It was
the first time that the molecule found in living organisms could be synthesized in the
laboratory without biological starting materials. Due to this discovery, Wöhler is
considered as the father of organic chemistry by many scientists.
Urea has the highest nitrogen content ava*ilable in a solid fertilizer (46%). It is
easy to produce as prills or granules and easily transported in bulk or bags with no
explosive hazard. It dissolves readily in water. It leaves no salt residue after use on
crops and can often be used for foliar feeding.
Urea is an acceptable fertilizer for rice and preferable to nitrates for flooded
rice because of the reduction of nitrates to N2O and/or nitrogen (in anaerobic
conditions) which is lost to the atmosphere. Also, rice can utilize the ammonium form
of nitrogen efficiently. Hydrolysis and nitrification (in aerobic conditions) are rapid in
tropical, sub-tropical and warm climates
Urea can be sprayed on leaves and can also be mixed with insecticides or
herbicides for soil application. A urea ammonium nitrate mixture with herbicide is
also used for weed control.
Disadvantages
When applied to a bare soil surface, urea hydrolyzes rapidly result into loss of
significant quantity of ammonia by volatilization. Such losses vary from soil to
soil and are greater for urea in a pellet form rather than in solution form.
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It is phytotoxic due to rapid hydrolysis of urea in soils can cause injury to the
seedlings by ammonia,
The fertilizer grade urea may contain toxic biuret which is formed during urea
manufacture by an excessive temperature rise. Above 2% concentration of
biuret in urea is harmful to plants.
Feed grade urea is sometimes referred to by the number 262 which is the
product of its nitrogen content (42%) multiplied by 6.25, the latter being the factor
used by chemists to convert nitrogen to its protein equivalent.
MANUFACTURE
Raw materials
Basis: 1000kg prilled urea
Item Once Through Partial recycle Total Recycle
NH3 1150kg 880kg 600kg
CO2 1470kg 910kg 770kg
Power 210kWH 165kWH 145kWH
Steam 1800kg 2000kg 2400kg
Cooling water 120000kg 70000kg 110000kg
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
CO2 shall be obtained from any one source as described in Module: 2,
Lecture: 2
Reaction
CO2 + 2NH3 NH2COONH4 ΔH = - 37,021 Kcal
NH2COONH4 NH2CONH2 + H2O ΔH = + 6.3 kcals
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module: 9 Lecture: 37 Urea
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Urea is always made in an ammonia plant because it produces CO2 as by
product, which can be used directly without further treatment.
Two reactions are involved in the manufacture of urea. First ammonium
carbonate is formed under pressure by highly exothermic reaction between carbon
dioxide and ammonia followed by the endothermic decomposition reaction. While
the former reaction under pressure, reaches to almost completion and the
decomposition reaction incomplete. Unconverted carbon dioxide and ammonia,
along with un decomposed carbamate, must be recovered and reused. The
synthesis is further complicated by the formation of a dimer called biuret,
NH2CONHCONH2.H2O which must be kept low because it adversely affects the
growth of some plants.
Liquid ammonia, gaseous carbon dioxide and recycle materials charged in
the heat exchanger-reactor at the pressure of 14MPs at 170 - 1900C to form
carbamate, with most of the heat of reaction carried away as useful process steam.
The carbamate decomposition reaction is both slow and endothermic. The mixture
of unreacted reactants and carbamate flows to the decomposer. The
stoichiometric ratio of CO2/NH3 conversion to urea is essentially about 55%, but by
using an excess of CO2 (or NH3) the equilibrium can be driven as high as 85%. The
reactor must be heated to force the reaction to proceed. CO2 is introduced at
process pressure followed by stripper. All the unreacted gases and undecomposed
carbamate to be removed from the product, the urea must be heated at lower
pressure (400kPa). The reagents are reacted and pumped back into the system.
Evaporation and prilling or granulating produces the final product. Overall, over 99%
of both CO2 and NH3 are converted to urea, making environmental problems to
minimum. Carbamate is highly corrosive to both ordinary and stainless steel, but with
oxygen present, 300 series stainless steel resist it very well, so some air is introduced
along with CO2 reagent to reduce system corrosion.
Liquid Ammonia
Pump
Compressor
CO2
Molten Mass
(NH3 + CO2 + H2O)
Synthesis tower
Inn
er
Cu
p(r
ea
cti
on
ch
am
be
r)Steam
Dis
till
ati
on
T
ow
er
Condensate
Condensate
Condensate
Steam
Ev
ap
ora
tor
To Vaccum
Air
Air
Prilling Tower
ConveyorUrea
Tank for the molten mass
Figure: Manufacturing of Urea
Ev
ap
ora
tor
Module: 9 Lecture: 37 Urea
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Developments in urea process technologies
Item Unit Process A B c D
Ammonia Kg 570 570 570 570
CO2 Kg 740 740 740 740
Steam Kg 900 800 660 790
Power KWH 140 140 16 21
Water m3 3.1 2.4 3.1 3
The raw material and utilities requirement for different processes for synthesis
and purification of urea are tabulated as earlier.
a) Montedison's IDR process
Montedison's process employing two specially designed stripping columns.
Ammonia and CO2 are used as the stripping agent in 1st and 2nd column
respectively. The reactor constructed in two sections having perforated trays and
also a down comer meant for circulation solution. High NH3 to CO2 ratio results in
increased conversion efficiency and lower carbamate recycle duty of the plant.
Excess NH3 is removed by CO2 stripping instead of distillation as practiced in
conventional total recycle processes, minimizing the energy requirement.
b) TEC-ACES process
This is typically CO2 stripping process employing higher ratio (4:1) of NH3 to
CO2, and higher synthesis pressure leading to high conversion efficiencies as
compare to total recycle process. Stripping is carried out in a two stage stripper
constructed of special steel. The upper part of the stripper is a tray column for the
removal of excess ammonia whereas the lower part is a falling film exchanger for
the stripping action.
c) Stamicarbon stripping process
Consumption of steam is decrease by employing a pool condenser of new
design featuring high resistance time and direct heat exchange between
condensing vapours from stripper and the stripped urea solution; and an evaporator
of improved design which allows better utilization of multiple effect principle in heat
transfer.
d) Ammonia casale's SRR process
Split reaction recycle (SRR) process of ammonia casale is specifically
developed for revamping plants based on stripping technology of either
snamprogetti or stamicarbon and includes installation of secondary high pressure
section consisting of feed pump, reactor, supplementary decomposer and
Module: 9 Lecture: 37 Urea
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separator which extend the urea formation reaction. The operating conditions are
same as traditional ones. The new secondary section added to the synthesis loop
can be prefabricated on skid mounted units and can be erected at site without any
modification on the layout of the existing synthesis section.
Granulation
Now a day, granular urea has gained importance since it minimize air
pollution and granules has higher strength larger sizes and is more compatible with
other granular fertilizers.
Following commercial processes are available for granulation of urea:
Pan granulation and falling curtain granulation process of Tennessee Valley
Authority (TVA)
High temperature pan granulation (GTPG) process of Norsk Hydro.
Fluidized bed granulation process of Hydro Agri Licensing & Engineering.
Fluidized bed granulation process of TEC.
Major Engineering problems
Autoclave variables
The objective of autoclave reaction is to produce the optimum economic
yield. The conditions which affects rate of reactions are temperature, pressure,
NH3/CO2 ratio and feed rate. The urea production rate can be varied as follows
Increase with increasing pressure
Increase with temperature to maximum at 175-1800C, then falls of sharply. The
operating pressure should be above the dissociation pressure (dissociation
pressure is 180atm at 190°C) for the carbamate.
Use no excess ammonia.
Reasons for not operating at maximum temperature and pressure without
excess ammonia
Increased pressure increases capital and operating cost of compression and
reaction equipment.
At higher temperature urea decomposed to biuret, which is detrimental to
germinating seeds and toxic to animals.
The above process conditions enhance corrosion rates to machinery
Carbamate decomposition and recycle
It is optimized by short residence times in a stripping column operating at low
pressure and high temperature. Later should be below 1100C if hold up time
Module: 9 Lecture: 37 Urea
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exceeds 1-2 seconds to avoid biuret formation. Use of millisecond contact time in a
flash evaporator allows 1400C operating temperatures in the high recycle design.
Main difference in competing processes is in the recycle design. Since
conversion is only 40-50% per pass, the unreacted off gases must be recirculated or
used economically elsewhere. Recompression of off gases is virtually impossible
because of corrosion and formation of solid carbamate in compressors. A solution is
formed and pumped into the autoclave.
Production of granular urea (Prilling)
Problem again is biuret formation. Vacuum drying of 80% urea to > 99% and
spraying to air cooled and solidify must be done just above the melting point of
urea and with a minimum residence time.
Heat dissipation in the autoclave
The exothermic heat of reaction can be removed by coils, wall cooling, or by
adding excess reactant to provide sensible heat pick up.
Corrosion
It can be minimized by use of the corrosion resistant metals and maintaining
the proper reaction conditions. High cost silver or tantalum liners are used in the
autoclaves with titanium, stainless (321SS) and aluminum alloys used in other parts of
the plant. Minimum temperature and pressure with excess NH3 are desirable to
reduce the severe corrosion rates.
PROPERTIES
Molecular formula : CH4N2O
Molecular weight : 60.06gm/mole
Appearance : White granules
Odour : Odourless
Bulk density : 673-721kg/m3
Angle of repose : 300
Melting point : 132-1350C
Density : 1.32gm/ml
Solubility : Solubility in water, ethanol, glycerol
Moisture : 1% by wt. (Max.)
It is highly soluble in water and practically non-toxic (LD50 is 15 gm/kg for rat).
Dissolved in water, it is neither acidic nor alkaline. As soon as urea dissolves in the soil,
it forms around it a zoning layer of high pH and ammonia concentration turning the
soil to be acidic and toxic at the same level. Urea is high moisture absorbent
therefore it should be stored in sealed and well enclosed bags.
Module: 9 Lecture: 37 Urea
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USES
As a fertilizer
As a protein food supplements for ruminant
As an ingredient in the manufacture of resins, plastics, adhesive, coatings
Textiles anti-shrink agents and ion exchange resins
In melamine production
It is an intermediate in the manufacture of ammonium sulfamate, sulfamic
acid and pthalocyanines.
Module: 9 Lecture: 38 Calcium ammonium nitrate
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Module: 9
Lecture: 38
CALCIUM AMMONIUM NITRATE
INTRODUCTION
Calcium ammonium nitrate (CAN) is a nitrogenous fertilizer produced by
treating ammonium nitrate solution with powdered limestone. It is a white to grey
chalky powder, with the colour depending on the limestone used in the
manufacturing process. Made with dolomitic limestone, the fertilizer contains 20%
nitrogen, 6 % calcium and 4 % magnesium. If the quantity of limestone is smaller than
that of used ammonium nitrate, the nitrogen content can go up to 28 %. CAN is
preferred to ammonium nitrate in acid soils. The most common grade of CAN
contains about 21% nitrogen, corresponding to 60% ammonium nitrate.
Calcium nitrate contains 15.5% nitrogen and its manufacturing process
involves reaction of lump limestone with concentrated nitric acid, addition of
ammonia to neutralize excess of acid, evaporation of the resulting solution, and
prilling or flaking the melt. The resulting product is a double salt, Ca(NO3)2NH4NO3
called calcium ammonium nitrate and is more useful than the single salt calcium
nitrate.
Ammonium nitrate is first prepared by the reaction of ammonia and nitric
acid. Ammonium nitrate so obtained contains some un-reacted nitric acid which is
neutralized by adding calcium carbonate (obtained as a by-product, in the
manufacturing of ammonium sulfate) on cooling grains of calcium ammonium
nitrate separates out.
The granules of calcium ammonium nitrate are finally coated with thin layer
of soap stone powder, which; acts as a protective coating and prevents the
absorption of moisture during storage and transportation CO2 is obtained as a
byproduct.
MANUFACTURE
Raw materials
Basis: 1000kg of CAN
Ammonia = 70kg
Nitric acid = 810kg
Lime stone or dolomite = 425kg
Module: 9 Lecture: 38 Calcium ammonium nitrate
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Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
Nitric acid shall be synthesized from Ostwald‘s process as described in
Module: 4, Lecture: 16.
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reaction
NH3 + HNO3 NH4NO3
CaCO3 + 2HNO3 Ca(NO3)2 + CO2 + H2O
_____________________________________________________________
CaCO3 + NH3+ 3HNO3 Ca(NO3)2 + NH4NO3 + CO2+H2O
Manufacture
Block diagram of manufacturing process
Reactor Vaporizer
Evaporator
Molten Ammonium nitrate 96%
Fines
Mix
er
Lime StonePowder
Gra
nu
lati
on
T
ow
er
ConveyerElevator
Sorting Screen
Crushing Mill
Air
Bag FilterCooling Drum
Air cooler
Elevator
SortingScreen
Lime stonePowder
Coating drum
Belt conveyer
Nitrochalk
Figure: Manufacturing of Calcium Ammonium Nitrate
Bin
HNO3NH3
Mix
er
Module: 9 Lecture: 38 Calcium ammonium nitrate
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Diagram with process equipment
Animation
CAN is produced by mixing quickly concentrated ammonium nitrate solution
with ground or powdered calcitic or dolomitic limestone. Both prilling and
granulation technologies are used to produce CAN.
Prilling process
Ammonium nitrate solution is premixed with ground limestone just before
prilling. Prill towers of 30 to 50m height are employed. 1 to 3 % China clay, kieselghur
or calcined fuller's earth is used to condition the prilled CAN. The mean particle size
of CAN formed is 2 to 2.5 mm.
Granulation process
The various methods used for granulation are
Pug mill process
Drum process
Cold spherodizer process
Fluid bed process
Calcium ammonium nitrate is produced by granulating concentrated
ammonium nitrate solution with pulverized limestone or dolomite in a granulator.
Ammonium nitrate solution is prepared by reacting preheated ammonia with nitric
acid in a neutralizer. Ammonia is preheated to 850C by vapours from the neutralizer
which also preheats nitric acid to about 650C. Ammonium nitrate liquor of 82-83%
concentration which is produced in the neutralizer is concentrated to 92-94% in a
vacuum concentrator heated with steam and stored in a tank.
Concentrated ammonium nitrate is pumped and sprayed into the granulator
which is fed with weighed quantity of limestone powder and recycle fines from the
screens. The hot granules are dried in a rotary drier by hot air.
Dried hot granules are screened and fines and oversize recycled. Granules of
proper size are cooled in a rotary cooler by air and coated with soapstone dust in a
coating drum. The final product is sent to storage.
Comparison of granulation processes
Pan granulation is difficult to handle as the pan is very sensitive to factors such
as heat and material balance. Irregular shape of the product is obtained. The other
processes need additives and their melt concentrations are also different. For
example, a spherodizer needs ammonium sulfate or magnesium sulfate while a fluid
Module: 9 Lecture: 38 Calcium ammonium nitrate
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bed requires magnesium nitrate. In the pug mill process, 0.3 to 0.5 % sulfate as
ammonium sulfate is added to improve hardness. The melt concentrations by weight
of ammonium nitrate for these processes are as follows: fluid bed 98 to 99%, pug mill
94.5 to 95.5% and drum 93.5 to 94.5 %.
Handling and storage
CAN is better to store in air conditioned silos below 30°C. CAN is normally
bagged in polyethylene-lined jute or HDPE bags
PROPERTIES
Molecular formula : 5Ca(NO3)2.NH4NO3.10H2O
Molecular weight : 1080.71gm/mole
Appearance : White granular
Odour : Odourless
Melting point :1690C (approximately)
Density : 1.725gm/ml (200C)
Solubility : Solubility in water
CAN is a granulated nitrogenous fertilizer that supplies nitrogen to plants in a
balanced and secure manner. The combination of ammonium nitrogen and nitrate
nitrogen makes CAN a special product with neutral pH. The excellent granulation
and specific surface coating has very good spreading properties
USES
CAN is a valuable source of nitrogen. As a fertilizer it can be applied for all
types of soil and all plants.
It is a nitrogen fertilizer supplying nutritive elements (N as NH4+ and NO3
-, Mg
and Ca as carbonates). It is suitable for blending with other granulated
fertilizers
Commonly used on fruit, process and vegetable crops
Module: 9 Lecture: 39 Ammonium chloride
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Module: 9
Lecture: 39
AMMONIUM CHLORIDE
INTRODUCTION
Ammonium chloride (NH4Cl) is white crystalline salt highly soluble in water.
Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of the
natural, mineralogical form of ammonium chloride. The mineral is commonly formed
on burning coal dumps, due to condensation of coal derived gases. It is also found
around some types of volcanic vent. It is used as a flavouring agent in some types of
liquor ice. It is the product from the reaction of hydrochloric acid and ammonia.
Several methods are used to produce ammonium chloride. The most
important is the dual salt process (modified Solvay process) wherein ammonium
chloride and sodium carbonate are produced simultaneously using common salt
and anhydrous ammonia as the principal starting materials. When ammonium
chloride is mixed with phosphorous and potassium fertilizers, a large amount of soil
calcium is lost as its conversion into soluble calcium chloride causes it to leach out
easily.
Ammonium chloride is used as fertilizer. A coarse form of it is preferred to the
powdered form for direct application. Its crystals are used in compound fertilizers. As
a fertilizer, ammonium chloride has an advantage in that it contains 26% nitrogen,
which is higher than that found in ammonium sulfate (20.5%). In terms of per unit cost
of nitrogen, ammonium chloride is relatively cheaper than ammonium sulfate and
has some agronomic advantages for rice. Nitrification of ammonium chloride is less
rapid than that of urea or ammonium sulfate. Therefore, nitrogen losses are lower
and yields, higher.
However, ammonium chloride is a highly acid forming fertilizer and the
amount of calcium carbonate required to neutralize the acidity is more than the
fertilizer itself, Further, it has lower nitrogen content and higher chloride content
compared to urea and ammonium nitrate, making it harmful to some plants.
Like ammonium sulfate, ammonium chloride can be applied to wet land
crops. In terms of the agronomic suitability, it is generally rated as equal to other
straight nitrogenous fertilizers.
Module: 9 Lecture: 39 Ammonium chloride
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Ammonium chloride is used as a fertilizer for rice and some other crops in a
limited way since, it may increase the residual chloride content of some soils. It is not
ideal for chilies, potatoes and tobacco as the added chlorine affects the quality
and storability of these crops.
MANUFACTURE
Ammonium chloride is manufactured by two processes
1. Direct reaction
2. Duel salt process
1. Direct reaction
The direct reaction process for manufacture of ammonium chloride is not
widely used.
Raw materials
Basis: 1000kg Ammonium chloride
Ammonia = 323kg
Hydrochloric acid (50%) = 215kg
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
HCl can be manufactured by any one methods as described in Module: 4,
Lecture: 19.
Reaction
NH3 + HCl NH4Cl
Manufacture
Gaseous ammonia is bubbled into 30 %hydrochloric acid solution in a reactor.
The reaction is controlled by addition of water. The resulting solutions are then
reacted with ammonia. The slurry from the saturator is centrifuged and the crystals
are washed with water and dried with warm air in a manner to that used in
ammonium sulfate.
Engineering aspects
It is necessary to ensure that the reactors have an acid resistant lining and
they must not be operated above certain temperatures during the drying phase as
NH4Cl to tends dissociate. In practice, the salt is dried by circulating air or under low
pressure.
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2. Duel salt process
Raw materials
Basis: 1000kg of ammonium chloride
Ammonia (30%) = 1000kg
CO2 = 840kg
NaCl = 1115kg
Water = 350kg
Reaction
NH3 + H2O + CO2 NH4HCO3
NH4HCO3 + NaCl NaHCO3 + NH4Cl
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
CO2 shall be obtained from any one source as described in Module: 2,
Lecture: 2.
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Manufacture
The most widely used process for producing ammonium chloride is the salting
out process for soda ash manufacture or modified Solvay's process.
In the process, 30% solution of ammonia is treated with carbon dioxide in a
carbonating tower to form ammonium carbonate. The ammonium bicarbonate as it
is formed, reacts with sodium chloride to give sodium bicarbonate and ammonium
chloride. The bicarbonate is separated by filtration, washed and calcined to
produce sodium carbonate.
Block diagram of manufacturing process
Diagram with process equipment
Animation
Module: 9 Lecture: 39 Ammonium chloride
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In the modified Solvay‘s process, ammonium chloride in the solution, after
separation of the sodium bicarbonate, is salted out by ammoniating the solution,
cooling below 150C and adding washed sodium chloride.
The precipitated ammonium chloride is centrifuged, washed and dried. The
fine crystals can be granulated by roll compaction. Large ammonium chloride
crystals of 2 to 3 mm size have been developed by cooling, nucleation and
crystallization, under closely controlled conditions in specially designed vessels.
The slurry from the crystallizer is centrifuged, washed and dried to about 0.25%
free moisture in a rotary drier at 1050C. After the removal of ammonium chloride, the
liquor is reammoniated to start a new cycle of operations.
As the demand for soda ash is comparatively lower than that for nitrogen
fertilizers, ammonium chloride from this source is not likely to meet the nitrogen
fertilizer needs.
Handling and storage
Crystalline ammonium chloride is free flowing and non-abrasive and does not
Ammonia Gas
Ammonia Tower
CoolingTower
AmmoniatedBrineTank
Ammonium Chloride Slurry Tank
Centrifuge
Water
Salt
Salt Washer
Chilled Brine
Salt Reator
Slurry Pump
Concentrator
Drier
MotherLiquorTank
NH4ClProduct
Conveyer
Ammonium Chloride Product
Bicarbonatemother liquor
Tank
Soda ash productconveyer
Soda ashcalciner
Bicarbonate slurry tank
Bicarbonateslurry tank
Carbonating Tower
CO2 Gas
Salt centrifuge
Figure: Manufacturing of Ammonium Chloride by dual salt process
Module: 9 Lecture: 39 Ammonium chloride
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have any problem in handling and storage. As it is susceptible to caking at high
humidity and has slightly acidic reaction, ammonium chloride has to be bagged in
HDPE or jute bags lined with polyethylene film.
NH4Cl as fertilizer
Advantages
Its low cost, as it is often directly available as a by-product from important
industries such as the Solvay soda industry and potassium sulfate industry
The fact that it combats certain plant diseases and prevents others
Disadvantages
Incompatibility of Cl- ions with the physiology of many plants
The corrosive action which it exhibits owing to the high degree of hydrolysis
that it undergoes
Difficult to store as it has tendency to cake.
The pronounced acidic behaviour of ammonium chloride can be countered
by mixing it with Ca(OH)2 and calcium cyanamide.
PROPERTIES
Molecular formula : NH4Cl
Molecular weight : 53.491gm/mole
Appearance : White solid, hygroscopic
Odour : Odourless
Melting point : 3380C(decomposes)
Density : 1.527gm/ml
Solubility : Solubility in water, alcohol
USES
Used as fertilizer.
Used to produce low temperatures in cooling baths. Ammonium chloride
solutions with ammonia are used as buffer solution.
It is an ingredient in fireworks, safety matches and contact explosives.
Used in a ~5% aqueous solution to work on oil wells with clay swelling
problems.
It is also used as electrolyte in zinc carbon batteries.
Uses in hair shampoo, in the glue that bonds plywood, and in cleaning
products. In hair shampoo, it is used as a thickening agent in ammonium
based surfactant systems, such as ammonium lauryl sulfate.
Used in the textile and leather industry in dyeing, tanning, textile printing and
to luster cotton
Used as a flux in preparing metals to be tin coated, galvanized or soldered. It
works as a flux by cleaning the surface of work pieces by reacting with the
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metal oxides at the surface to form a volatile metal chloride. For this purpose,
it is sold in blocks at hardware stores for use in cleaning the tip of a soldering
iron and can also be included in solder as flux
It is used as food additive.
Module: 9 Lecture: 40 Ammonium sulfate
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Module: 9
Lecture: 40
AMMONIUM SULFATE
INTRODUCTION
Ammonium sulfate containing 21% nitrogen is another important nitrogenous
fertilizer.
It occurs naturally as the mineral mascagnite and offers many advantages as
a fertilizer, such as low hygroscopicity, good physical properties, excellent chemical
stability, good agronomic effectiveness and long shelf life.
Ammonical nitrogen is fixed in the soil in an exchangeable form until nitrated
by nitrifying bacteria. The ammonical nitrogen of ammonium sulfate does not leach
out easily. Ammonium sulfate is an acid forming fertilizer, and hence used in neutral
or alkaline soils. In its free flowing form, it is directly applied to the soil or blended with
other granular materials. Ammonium sulfate also supplies sulfur, which is an essential
nutrient for plants. It is a quick-acting fertilizer. It is resistant to leaching as it gets
adsorbed on the soil colloids, clay and humus, and replaces calcium. This adsorbed
ammonium salt is converted to nitrate by nitrifying bacteria for use by growing
plants.
It can be obtained as a by-product or may be manufactured synthetically.
Ammonium sulfate is obtained as a byproduct; in the steel industry in which NH3
(another by-product) from coke ovens is absorbed in sulfuric acid. (NH4)2SO4 is also
manufactured by reacting synthetic ammonia with sulfuric acid.
(NH4)2SO4 is obtained from waste streams of chemical and metallurgical
industries e.g. ammonia leaching of ores, production of pigments and synthetic
fibers, manufacture of caprolactam produce by product; solutions containing
ammonium sulfate.
Flue gases are another source of the ammonium sulfate. The metallurgical
smelters and coal burning power plants liberate large quantities of SO2 that pollute
the atmosphere. SO2 is collected and converted into sulfuric acid, (NH4)2SO4 is then
produced by passing ammonia gas through 60% sulfuric acid placed in lead lines
vats at about 60°C the crystals of ammonium sulfate separate out on cooling.
2NH4OH + H2SO4 (NH4)2SO4 + 2H2O
Module: 9 Lecture: 40 Ammonium sulfate
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MANUFACTURE
a) As by-product from caprolactam
b) Recovery from cock oven
c) Direct neutralization
d) Gypsum process
a) As By-product from caprolactam
(NH4)2SO4 solution is formed during the manufacture of caprolactam. The
waste liquor containing 35% solution of ammonium sulfate is concentrated, and
ammonium sulfate, is recovered by crystallization, centrifuging and drying.
b) Recovery from coke oven gas
Cleaned coke even gases passed into an absorption column, counter current
to the recirculating stream of saturated solution of (NH4)2SO4. 96-98% sulfuric acid is
introduced into the stream at another point. The neutralization takes place and the
effluent solution containing (NH4)2SO4 is passed to the crystallizer, where, crystals of
(NH4)2SO4 are separated out on cooling. The crystals are dried after separation by
filtration or by centrifuge. The mother liquor overflowing from the crystallizer is
acidified and recycled to the absorber.
In an old saturation process ammonium sulfate is produced by passing coke
oven gas and sulfuric acid into a vat containing a saturated solution of (NH4)2SO4
and then setting out the crystal.
c) Direct neutralization or Synthetic Manufacture
Raw materials
Basis: 1000kg of ammonium sulfate
Ammonia (30%) = 800kg
Sulfuric acid (90%) = 900kg
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18.
Reaction
2NH3 + H2SO4 (NH4)2SO4
Ammonium sulfate made with manufactured ammonia is called synthetic
ammonium sulfate. Both saturator's and crystallizers are employed in the synthetic
Module: 9 Lecture: 40 Ammonium sulfate
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manufacture of ammonium sulfate. The heat of reaction of anhydrous ammonia
and concentrated sulfuric acid obtained by contact process is very high. Hence
water evaporated from the crystallizer must be returned either by means of a
condenser or by addition of water.
Ammonia and sulfuric acid are introduced via the slurry recycle line, wherein
they react and superheat the recycling slurry. The slurry is subsequently flashed in the
upper chamber at a reduced pressure (550 – 580mm of Hg). The exothermic heat of
reaction is removed by evaporating water either present in the feed acid or added
to the system for temperature control of the process. The loss of water in this zone
supersaturates the slurry which recirculates to the lower suspension vessel via an
internal pipe and comes into contact with small crystals and nuclei. This induces
further crystal growth in terms of size rather than in number. The slurry is recycled by a
thermal syphon and/ or by an external pump. This type of crystallizer is generally
known as 'Krystal' or 'Oslo unit'.
During operation the pH control is required to be maintained within close
limits (3.0 to 3.5), otherwise, thin crystals result. The excessive acidity promotes an
overgrowth of crystals in the pipelines. A higher pH or a lower acidity leads to inferior
crystals which are difficult to wash and store and may cause ammonia losses as well.
In another type of reduced pressure crystallizer with a draft tube battle unit,
growing crystals are brought to the surface of the flashing slurry. At this surface, super
saturation induces maximum crystal growth, and sufficient nuclei are present to
minimize the scale formation inside the unit.
Several types of atmospheric pressure units are preferred to a vacuum
crystallizer because of their simplicity and lower capital cost. Ammonia is added via
a jet-type mixer or a sparger tube. In another design, a simple absorption column
incorporating a few large slotted bubble-hoods is used. In some other cases, a single
vessel is employed for both reaction and crystallization and the heat of reaction is
removed by evaporation of water. There are designs where separate vessels for
reaction and crystallization are used for easy operation and closer control. An
optimum balance between the cooling air energy and the yield of crystals is
obtained when the crystallization temperature is in the range of 63 to 66°C.
In most cases, the product is recovered from ammonium sulfate slurry by
continuous or automatic batch type centrifuge. The product is washed with water
and very dilute ammonia and spin dried again before drying.
For small output, top-feed filters are used since the product can be
separated, washed and dried in single equipment. Ammonium sulfate liquor is
corrosive and wetted parts of the equipment are made of stainless steel or rubber
lined mild steel. To improve the shape and size, modifiers are used, such as trivalent
Module: 9 Lecture: 40 Ammonium sulfate
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metallic salts. Small amounts of phosphoric acid or arsenic compounds are added
as corrosion inhibitors.
d) Gypsum process
Raw materials
Ammonia
CO2
Gypsum
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
CO2 shall be obtained from any one source as described in Module: 2,
Lecture: 2
Reaction
2NH3 + H2O + CO2 (NH4)2CO3
(NH4)2CO3 + CaSO4 (NH4)2SO4 + CaCO3
CaCO3 CaO + CO2
Manufacture
Liquorammonia
CO2
Ab
so
rpti
on
to
we
r
Finely crushedgypsum
Re
ac
tor
Water
Vacuumfilter
Cry
sta
l e
va
po
rato
r
25 % (NH4)2SO4 Solution
Crystalline(NH4)2SO4
Rotary dryer
Hot air in
Hot airout
Figure: Manufacturing of Ammonium Sulphate
Module: 9 Lecture: 40 Ammonium sulfate
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Block diagram of manufacturing process
Diagram with process equipment
Animation
Ammonia is manufactured by Haber process and CO2 is manufactured by
heating limestone. Ammonia is absorbed in water and carbonated at the pressure
of about 2.1kg/cm2 in towers packed with aluminum rings. The prepared liquor
strength corresponds to approximately 170gms of ammonia and 225gms of CO2 per
liter. In another method jet absorbers are used for preparing ammonia solution and
ammonium carbonate liquor in conjunction with a carbonating tower.
Natural gypsum or anhydrite, when used, is ground so that about 90% of the
material passes through 120 mesh sieve. When the byproduct gypsum of phosphoric
acid plant is used, the impurities are removed by repulping the filter cake prior to
washing and dewatering on a drum or disc filter.
Now proper proportion of finely ground gypsum or anhydrite is fed into the
aqueous solution of ammonium carbonate in large tanks, whereby calcium
carbonate is precipitated gradually as a result of double decomposition.
Reactions of ammonium carbonate and gypsum solutions are carried out in a
series of wooden vessels or mild steel vessels having steam coils and agitators to give
a total retention time of 4 to 6 hours. CO2 and NH3 are passed until all the gypsum is
converted into CaCO3. The slurry produced is filtered and the calcium carbonate
cake washed and dewatered. The solution is evaporated and the crystals are
centrifuged and dried in a rotary drier at 1200C. CaCO3 obtained as by product is
used as a raw material for the manufacture of cement.
Action of (NH4)2SO4 as fertilizer
It reacts with lime present in the soil to for ammonium hydroxide which is
oxidized by air with the help of nitrifying bacteria into nitrous acid. Later is then
converted into nitrites. The nitrous acid and nitrites also undergo oxidation by means
of air in presence of nitrifying bacteria and form nitric acid and nitrate.
Handling and storage
Crystalline ammonium sulfate is free flowing and does not normally pose any
problem in handling and storage. However, it generally contains some powdered
material which causes caking, especially under high humidity.
Due to its susceptibility to caking and slight acidity, ammonium sulfate is
normally bagged in polyethylene lined gunny bags or high density polyethylene
(HDPE) woven sacks.
Module: 9 Lecture: 40 Ammonium sulfate
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PROPERTIES
Molecular formula : (NH4)2SO4
Molecular weight : 132.14gm/mole
Appearance : Fine white hygroscopic granules or crystals
% of nitrogen : 20-21%
Bulk density : 720-1040kg/m3
Melting point : >2800C
Specific gravity : 1.769
Angle of repose : 320
Solubility : Solubility in water and insoluble in alcohol, ether,
acetone
Moisture : 1% wt. (Max.)
USES
Most commonly used in fertilizers. It is often used in combination with other
materials, such as urea, to make dry fertilizers.
It is a good source of nitrogen for cotton, rice, wheat, barley, maize, sorghum,
sugar cane and fiber crops.
It is also used as a general purpose food additive, dough conditioner and
food for yeast.
In medicine, ammonium sulfate plays an important role in the development
of vaccines. The DTaP vaccine, for example, which protects children from
diphtheria, tetanus and whooping cough, uses ammonium sulfate in the
purification process
Used in rice and jute cultivation
Module: 10 Lecture: 41 Potassium chloride
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Module: 10
Lecture: 41
POTASSIUM CHLORIDE
INTRODUCTION
Potassium chloride (KCl) is a metal halide salt composed of potassium and
chlorine. It is odorless and has a white or colorless vitreous crystal appearance, with
a crystal structure that cleaves easily in three directions. Potassium chloride crystals
are face centered cubic. "Muriate of potash" is name which is occasionally
association with its use as a fertilizer containing 60% plant food as K2O.
Potash varies in colour from pink or red to white depending on the mining and
recovery process used. White potash or soluble potash is usually higher in analysis
and is used primarily for making liquid starter fertilizers. It occurs naturally as the
mineral sylvite and in combination with sodium chloride as sylvinite.
MANUFACTURE
Raw Materials
Basis: 1000kg of Potassium chloride (Muriate)
Sylvinite = 2510kg
Steam = 1250kg
Water =170-200m3
Electricity = 180MJ
Direct labour = 4-5work-hr
Potash mineral or brine
Potash mineral or sylvinite contain potassium chloride and sodium chloride
Manufacture
Potassium chloride is obtained by following methods
1. Leaching process
2. Flotation process
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Leaching process
The solubility of potassium chloride and sodium chloride increases as the
temperature increases. Individually both are much more soluble in hot water than in
cold water. But in solutions saturated with both sodium and potassium chloride, the
concentration of NaCl diminishes as temperature rises, while the concentration of
potassium chloride increases. These solubility characteristics are used to extract
potassium chloride from sylvinite. Crushed ore is mixed with sufficient quantity of
recycle brine which is already saturated with NaCl and heated almost to hilling to
dissolve KCl. The KCl rich brine on clarification and then cooling by vacuum
evaporation produces KCl crystals which are centrifuged, washed, dried and
packed. The filtrate (brine) is recycled for leaching more ore.
Flotation process
Block diagram of manufacturing process
Diagram with process equipment
Animation
Floatation process for extraction of potassium chloride is much cheaper than
leaching process and hence is used more extensively in the industry. In the process
the ore is crushed to +10 mesh size then washed to remove clay slimes. To render it
inert to amines, washed crushed ore is treated first with starch or mannogalactan
gums and then with an amine acetate which selectively coats KCl particles. Air is
then bubbled through the slurry. The air bubbles attach themselves to the coated
particles and float them to the surface while the uncoated particles sink. The floated
KCl is centrifuged, dried and packed.
Water
Crusher
Sylvinite
Water
StarchAmine acetate
AirUncoated particlesKCl
Re
ac
tor
Flotationchamber
Ce
ntr
ifu
ge
Dry
er
Water
Figure: Manufacturing of Pottasium Chloride by Flotation Process
Ab
so
rbe
r
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Handling and storage
The crystalline potassium chloride is free flowing and does not normally pose
any problem in handling and storage. Potassium chloride is imported as bulk cargo
and transported to NPK fertilizer and mixing plants in open trucks or in bags.
It is stored in bulk in closed storage yards. Caking occurs in presence of
impurities when humidity is high and the mass tends to become like rock. Retrieval
from such storage may pose problems and sometimes explosives may have to be
used.
PROPERTIES
Molecular formula : KCl
Molecular weight : 74.55gm/mole
Appearance : white crystalline solid
Odour : Odourless
Boiling point : 14200C
Melting point : 7700C
Density : 1.984gm/ml
Solubility : Soluble in glycerol and water, slightly soluble in alcohol,
insoluble in ether
Moisture : 0.5% by wt. (Max.)
USES
As a fertilizer
Used as thickeners, stabilizer, mineral salts, gelling agents and acidity
regulator in food
Used as a salt replacer in foods, for recovery of potassium in the human body,
It's used in brewing, as a salt substitute (as salt free, sodium free, and low
sodium products), gelling agent, and in reduced sodium breads.
Used in various pharmaceutical preparations to correct potassium deficiency.
It used as a substitute for table salt in the diet of people with cardiovascular
disorders, in administration of the potassium ion, and as a constituent of
Ringer's solution.
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Module: 10
Lecture: 42
POTASSIUM SULFATE
INTRODUCTION
Potassium sulfate (K2SO4) also called sulfate of potash, arcanite, or archaically
known as potash of sulfur is a non-flammable white crystalline salt which is soluble in
water. The chemical is commonly used in fertilizers, providing both potassium and
sulfur.
It is known since 14th century, and was studied by Glauber, Boyle and
Tachenius. In the 17th century, it was named arcanuni or sal duplicatum, as it was a
combination of an acid salt with an alkaline salt. It was also known as vitriolic tartar
and Glaser's salt or sal polychrestum Glaseri after its first used in medicine by
Christopher Glaser.
Potassium sulfate contains 48 to 54% potassium (as K2O) and supplies 17-20 %
of sulfate. Potassium sulfate is the second largest tonnage of potassium compound
and is primarily used as a fertilizer.
Potassium sulfate can be made either by the Mannheim process where
potassium chloride is reacted with sulfuric acid, or, made from natural complex salts
like kainite or langbeinite.
MANUFACTURE
Potassium sulfate can be manufacture by two processes
1. Mannheim process
2. Recovery from natural complex salts
1. Mannheim process
Raw materials
Potassium chloride
Sulfuric acid
Reaction
KCl + H2SO4 KHSO4 + HCl
KHSO4 + KCl K2SO4 + HCl
Module: 10 Lecture: 42 Potassium sulfate
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Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
The Mannheim process was originally developed for sodium sulfate
production. For making potassium sulfate, sodium chloride is replaced with
potassium chloride.
Potassium chloride reacts with sulfuric acid during the slow mixing of the
ingredients in the gas heated Mannheim furnace consisting of cast iron muffle with
rotating plough which helps to agitate the mixture. Hydrochloric acid produce
during the reaction is cooled and absorbed into water to produce 33% hydrochloric
acid as a byproduct. The precipitated potassium sulfate fertilizer is cooled, filtered
and the lumps are crushed and granulated.
Potassium sulfate is twice as costly as potassium chloride. Granulation adds
further to its cost. Potassium sulfate contains over 50% potassium (as K2O) and less
than 1 % chlorine.
KClH2SO4
WaterK2SO4
Fil
tra
tio
n t
ow
er
Dry
er
Ne
utr
iliz
er
Figure: Manufacturing of Pottasium Sulfate
Water
Ab
so
rbe
r
HCl
Cold air out
Hot air in
Module: 10 Lecture: 42 Potassium sulfate
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2. Recovery from natural complex salts
Potassium sulfate occurs naturally as complex salts. The basic reactions
leading to potassium sulfate from kainite are by transformation of kainite to
schoenite followed by water leaching.
Natural resources
The mineral form of potassium sulfate, arcanite, is relatively rare. Natural
resources of potassium sulfate are minerals abundant in the Stassfurt salt. These are
co crystallizations of potassium sulfate and sulfates of magnesium calcium and
sodium.
The minerals of potassium sulfate are
Kainite, MgSO4·KCl·H2O
Schönite, K2SO4·MgSO4·6H2O
Leonite, K2SO4·MgSO4·4H2O
Langbeinite, K2SO4·2MgSO4
Glaserite, K3Na(SO4)2
Polyhalite, K2SO4·MgSO4·2CaSO4·2H2O
Minerals like kainite, from which potassium sulfate can be separated,
because the corresponding salt is less soluble in water. Kainite MgSO4·KCl·H2O can
be combined with a solution of potassium chloride to produce potassium sulfate.
Process of recovery of potassium sulfate from kainite consists of four basic
elements, and they are
Preparation of the ore and floatation
Production of schoenite and its recovery
Leaching of schoenite to potassium sulfate
Liquor treatment
Other processes involve addition of sylvite to kainite, langbeinite or kieserite.
The reactions are as follows
Mixing of kainite with sylvite
Mixing of sylvite with kieserite and other magnesium salts
A Russian Kalush plant method of potassium sulfate production uses potash
ores as the starting material. The Carpathian ore contains about 9% potassium and
15% clay. The ore is leached with hot synthetic kainite solution in a dissolution
chamber. The langbeinite, polyhalite and halite remain un dissolved and are
discarded. The overflow from the dissolution chamber is directed to a Dorr Oliver
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settler where clay is settled and directed to a washer and discarded. The slurry of
schoenite is filtered and the crystals, leached with water, to produce potassium
sulfate crystals.
Potassium sulfate from langbeinite is produced by mixing large amounts of
muriate of potash, sylvite and langbeinite.
4KCl + K2SO4.2MgSO4 3K2SO4 + 2MgCl2
2KCl + 2(K2SO4.2MgSO4) 3(K2SO4.MgSO4) + MgCl2
The langbeinite ore is separated from sylvite and Halite by selective washing,
froth floatation and heavy media separation. Langbeinite must be powdered and
mixed with potassium chloride solution to get crystalline potassium sulfate and brine.
The crystals are centrifuged or filtered, dried and classified to the required size.
Handling and storage
The crystalline potassium sulfate is free flowing and does not normally pose
any problem in handling and storage. It is imported as bulk cargo and transported
to NPK fertilizer mixing plants and dealers in bulk or in bags. It is stored in bulk in
closed storage yards.
PROPERTIES
Molecular formula : K2SO4
Molecular weight : 174.26gm/mole
Appearance : white solid
Odour : Odourless
Boiling point : 16890C
Melting point : 10690C
Density : 2.66gm/ml
Solubility : Soluble in water, slightly soluble in glycerol, insoluble in
acetone, alcohol, CS2
USES
Potassium sulfate is used as fertilizer particularly in chloride sensitive crops like
tobacco, grapes and potato which require chloride free potassium fertilizers. These
three crops, being major crops, account for about 7% of the total potash
consumption. For best results, potassium sulfate should contain at least 50 % potash
by weight.
Used as a flash reducer in artillery propellant charges.
It reduces muzzle flash, flareback and blast overpressure
The crude salt is also used in the manufacture of glass.
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Module: 11
Lecture: 43
PAINT INDUSTRIES
INTRODUCTION
Paints are stable mechanical mixtures of one or more pigments which impart
desired colour and to protect the film from penetrating radiation, such U. V. rays. The
pigments and the extenders are carried or suspended in drying oils called vehicle.
Which is a film forming material, to which other ingredients are added in varying
amount e .g. linseed oil, tung oil, castor oil, tall oil etc. Boiled Linseed oil is prefered to
unboil oil because it develops a good drying power and requires only two days for
drying. The drying time is reduced further by adding driers to the paint. Driers act to
promote the process of film formation and hardening. Thinners maintain the
uniformity of the film through a reduction in the viscosity of the blend.
The purpose of paint may be protective or decorative or both and can be
applied on a metal or wood surface. It is applied by brushing, dipping, spraying, or
roller coating.
The important varieties of paints are emulsion paints, latex paints, metallic
paints, epoxy resin paints, oil paints, water paints or distempers etc.
CLASSIFICATION OF PAINTS
On the basis of their applications, paints can be classified as
a) Exterior house paints
Generally have constituents such as pigment (ZnO, TiO2, white lead etc.),
extenders (talc, barytes, clay etc), vehicle (e.g. boiled linseed oil) and thinners (e.g.
mineral spirit, naphtha etc.) Coloured pigments for light tint are also added in
varying amount.
b) Interior wall paints
It is prepared by mixing pigments (e.g. white and colored pigments), vehicle
(e.g. varnish or bodied linseed oil) and resins (e.g. emulsified phenol formaldehyde
resins and casein)
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c) Marine paints
Also known as antifouling paint and can be prepared by mixing various
ingredients such as pigments (ZnO and venetian red), resin (shellac), driers
(manganese lineolate), vehicle (coal tar), diluents (pine oil), toxic
components(cuprous oxide and mercuric oxide) and small amount of bees wax.
d) Emulsion paints
These paints are highly durable, impermeable to dirt, resistant to washing,
rapidly drying, contain water as thinner and can be easily cleaned. It contain an
emulsion of alkyds, phenol formaldehyde etc.(vehicle) in water pigments and
extenders are also added to get other desirable properties.
e) Chemical resistant paints
Consist of baked oleo resinous varnishes, chlorinated rubber compositions,
bituminous varnishes and phenolic dispersion as chemical resistant materials in paint
formulations.
f) Fire resistant paints
These paints impart a protective action on the article being coated through
easy fusion of the pigments and other paint ingredients giving off fume on heating,
they do not support combustion. It consist of borax, zinc borate, ammonium
phosphate synthetic resins etc as anti-fire chemicals.
g) Luminous paints
Consist of phosphorescent paint compositions such as pigment (sulfides of
Ca, Cd and Zn dispersed in spirit varnish), vehicle (chlorinated rubber, styrol etc.)
and sensitizer for activation in UV region.
h) Latex paints
These paints usually contain
Protein dispersion: Prepared by soyabean proteins or casein in aqueous
ammonia solution for about an hour at room temperature
Pigments: ZnS,TiO2 etc dispersed in water
Extenders: clay, talc, MgSiO3, BaSO4 etc.
Preservatives: Penta chlorophenol
Antifoaming agent: Pine oil
Plasticizer: Tributylphosphate
Latex: Prepared from a butadiene styrene copolymer in water.
All these ingredients well stirred in water, screened, again stirred and packed.
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i) Aluminum paints
Used as heat reflecting paints and consist of pigment (aluminum powder)
and vehicle (spirit varnishes) and cellulose nitrate lacquers.
j) Metal paints
Applied on the metal surfaces or bodies for protection and decoration and
are of two types
Barrier coating
Protective barrier is formed between the surface coated and its surroundings.
These consist of pigment, vehicle, anticorrosive agents (e.g. zinc or chrome yellow),
resins (e.g. alkyds, epoxy, polyamides, chlorinated rubbers and silicones) etc. Alkyd
resists weathering of metals, epoxy and polyamides form tough film resistant to
chemicals. Chlorinated rubbers resist action of soaps, detergents and strong
chemicals and silicons are added as heat resistant and water repellents.
Galvanic coating
Protection is provided by self-undergoing of galvanic corrosion. e.g. Zinc
coating (Galvanization) on steel.
Before applying metal paints it is important to clean thoroughly the surface to
be coated. Moreover, paint should be applied over a primer such as red lead by a
high pressure spray gun.
k) Cement paints
It is prepared by mixing white cement with colouring matter or pigments,
hydrated lime and fine sand as inert filler. They are available in the form of powder
of particular colour.
The dispersion medium may be water or oil, depending upon the purpose of
coating. Water and linseed oil are used as dispersion medium for stone/brick
structure and for coating of corrugated metal surfaces respectively. Before applying
cement paint a primer coat is applied which consist of a dilute solution of sodium
silicate and zinc sulfate.
Cement paints have marked water proofing capacity, give a stable and
decorative film and do not require fresh application even in four to five years, if
coated even on rough surface.
l) Distempers
Distempers are water paints consisting of pigments which may be white as
well as coloured (e.g. Reimann‘s green), extenders (e.g. chalk powder, talc), binders
(e.g. casien or glue) and dispersion medium water. These water paints have good
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covering power, easy applicability, and smooth, pleasant looking durable film. The
major disadvantage of these is the porous nature of the film which is not moisture
proof.
In general the paints are known for their gloss, adhesion as well as chemical
and mechanical properties. They are suitable for the interior decoration as well as
painting.
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Module: 11
Lecture: 44
PAINT INDUSTRIES (continued)
CONSTITUENTS OF PAINTS
1. Pigments
Pigments are various inorganic or organic insoluble substances which are
widely used in surface coatings. The most important properties of pigments are
opacity, good covering power, mixing ability with oil, chemical inertness, non toxicity
or low toxicity, high hiding power, high tinting strength and reasonable cost. They
protect the film by reflecting the destructive ultra violet light, to strengthen the film.
Pigments are classified as follows
a) Natural or mineral pigments: e.g. talc mica, chalk, clays, iron ores, barytes,
diatomaceous earth etc.
b) Synthetic or chemical pigments: e.g. white lead, zinc oxide, lithopone,
titanium oxide, and many other organic and inorganic colours
c) Reactive pigments: Those pigments which react with drying oils or their fatty
acids and form soaps are called reactive pigments. e.g. zinc oxide, red lead,
titanium dioxide etc
d) Organic dyes: Toners (insoluble organic dyes) used directly as pigments
because of their durability and colouring power. Lakes, which are organic
dyes on an inorganic adsorbent (such as clay), have also been used in many
colours. Para red, toluidine toner, Hansa yellow G (lemon yellow) etc. are
important lakes. Clay, barite, aluminum hydroxide etc. are well known
inorganic adsorbents. Both toners and lakes are ground in oil or applied like
any other pigment.
Various pigments used for making paints are
White: White lead, titanium dioxide, zinc oxide, lithopone.
Red: Red lead, iron oxides, cadmium reds, rouge etc.
Blue: Ultramarine, cobalt blues, iron blues etc.
Chromium oxide: Chromium oxide, chrome green, phthalocyanine green.
Yellow: Litharge, lead or zinc cnromates, ochre etc.
Black: Carbon black, lamp black, furnace black etc.
Orange: Basic lead ·chromate, cadmium orange etc.
Brown: Burnt umber, burnt sienna etc.
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Metallics: Copper powder, zinc dust, aluminium etc.
Metal protective pigments: Red lead, blue lead, zinc and basic lead etc.
2. Extenders or Fillers
They are used for decreasing the cost of paint and to supplement the
pigment in increasing the covering and weathering power of the film. Extenders
improve consistency, levelling and setting of the paint. E.g. Talc, china clay, gypsum,
silica, barite, glass flakes, asbestos and anhydrite etc.
3. Film forming materials
The vehicle or film forming materials plays dual role as carriers for the pigments
and as formers of protective films. Reactive oils containing olefinic unsaturation are
used as vehicles. These are usually called drying and semidrying oils, depending on
degree of unsaturation. E.g. Linseed oil, soyabean oil, tung oil, talc oil, castor oil,
varnishes, casein, fish oil etc. These oils form a protective film through oxidation and
polymerization of the unsaturated constituents of the drying oil.
Drying oil is thus a film forming component which upon exposure to oxygen has
the property of drying to hard, firm, non-sticky film through oxidation involving
organic peroxides as the chain initiators.
The various properties of drying oil which are used to decide the grade of
paints are
Specific gravity: It lies between 0.93 to 0.97
Refractive index: It lies between 1.48 to 1.51
Saponification value: It lies between 183 to 187
Iodine value: For common drying oils close to 90 – 120, while semi drying oils
have the value close to 90
Linseed oil is light yellow in colour but becomes colourless after oxidative
purification. After drying it sets to a hard glossy film.
Linseed oil is used in four different grades by paint and pigment manufacturers.
Refined oil
Boiled linseed oil
Heat bodied linseed oil (stand oil)
Blown linseed oil
Refined linseed oil
Depending on the application, refined linseed oil may be obtained by acid
treatment or alkali treatment.
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In acid refining, the oil is agitated with concentrated H2SO4 (1% of the total oil
by weight) for about an hour, and allowed to stand for about 24 hours for allowing
the coagulated colouring matter and mucilage to settle down. The clear oil in
carefully siphoned off and washed with water to remove excess of acid and stored
after the separation of water.
In alkali treatment, the slightly warm oil is agitated vigorously with 10% NaOH
and allowed for setting out the precipitate. The oil is carefully siphoned off, heated
to expel moisture and finally decolourised with bleaching clay and filtering presses.
Refined linseed oil is mainly used in the manufacture of varnishers.
Boiled linseed oil
It is obtained by adding small quantities of the oxides and acetates of Co, Mn
and Pb to hot linseed oil, during heating the oil thickens with darkening of colour. This
change in colour is referred to as boiling. Boiled linseed oil provides durability to the
paint.
The heat bodied linseed oil or stand oil
Linseed oil is heated alone at elevated temperatures. Which increase its
viscosity due to partial polymerization and attain a state called as bodied. The
same can also be done by exposing the linseed oil to sun light for many hours.
This oil is used mainly in making printing inks and enamels.
Blown linseed oil
It is obtained by blowing air through linseed oil to make it bodied. During
blowing oxidation and polymerization take place at unsaturated positions. Blown oil
undergoes hardening much faster than the heat bodied oil.
In manufacture of interior paints blown linseed oil is used.
In addition, tung oil and soyabean oil are extensively used as drying oils. When
properly treated tung oil dries with extreme rapidity forming hard, dense and tough
film, which is more durable and less penetrable than that formed by linseed oil.
Tung oil is used in making water proof paints. The Soyabean oil is used in
making interior paints.
4. Driers
Initially PbO was used as a drier, but the modern driers are Co, Mn, Pb, Zn,
resinoleate, linoleate and naphthenates etc. They dissolve in the hot oil and the
drying time has been much reduced. They are usually mixed with hot boiled linseed
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oil. Too rapid drying is not desirable because of some unwanted effects, e.g, the film
suffers from wrinkling.
The drier acts as catalyst and promotes the oxidation polymerization process
which accelerates drying of the film. The unsaturated drying oils polymerize by
reaction mechanism which involves a peroxide intermediate.
5. Thinners or Diluents
In order to dissolve film forming material and to thin concentrated paints for
better handling as well as brushing thinner is added. It is also used to suspended
pigments. Diluents or thinners may include aliphatic or aromatic naphtha fractions or
many contain turpentine. Solvent such as turpentine in spite of its low volatility,
maintains the fluidity of the freshly applied film for reasonable period of time.
6. Lacquer
Lacquer is a liquid coating composition containing the basic film forming
ingredients cellulose esters or ethers and plasticizers, without or with resin. Lacquers
employ aliphatic chemicals, such as ethers, esters, ketones and alcohols to provide
the desired controlled volatility. By virtue of evaporation of solvent, they are also
called non-convertible coating. When a pigment is added to a clear lacquer, it is
called lacquer enamel or pigmented lacquer.
7. Anti-skinning agent
Certain anti-skinning agents are also added to the paint in order to prevent
gelling and skinning of the finished product before application of the paint by
brushing, spraying or dipping. e.g. Polyhydroxyphenols.
8. Plasticizers
Plasticizers, low melting solids or liquids of low volatility which provide
elasticity to the film and thus prevent cracking of the paint. Chemically, plasticizers
are mostly esters. Triphenyl phosphate, dibutylphthalate and castor oil etc are used
as plasticizers.
9. Resins
Resins are required for water base paints contain no oils and depend on
vinyl acetate, acrylic or butadiene styrene polymer resin as the film forming
materials.
Varnishes are also, used in the form of natural or synthetic resins. Examples of
natural resins are copal or rosin, while that of synthetic resins are urea formaldehyde,
acrylate, vinyl or silicone resins. Laquers also contain nitrocellulose as the resin
constituent.
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10. Binders
Binders act to fix the paint on the coated surface and provide tough,
tenacious and glossy film on the surface being painted.
Binders may be of following types.
(a) Oil modified alkyd resins or polymers forming vehicle with the drying oil
These may be oxidizing alkyd resin (used for house paints, interior paints, air
drying under coats etc.) alkyd and cellulose resins (used for making low temperature
backing under coats) and alkyd and silicon resins (used for superior chemical and
heat resistant coats).
(b) Resin acting as vehicle and not containing any drying oil or alkyd resin
Examples are phenoplast (used for making thermosetting under coats),
Polyesters (used for making chemical and discolouration resistant glossy film) and
acrylonitrile copolymers, butadiene copolymers etc. (used for making emulsion
paints, fire resistant as well as corrosion resistant coats and interior decoration paints
etc.)
11. Extenders
Extenders such as clay, talc, barytes etc are added to the paint mix in order to
prevent the cracking of the film when dry.
12. Other Compounds
Water based paints also require dispersing agents (e.g. casein), antifoam
agent (e.g. pine oil) and preservative (e.g. chlorophenol).
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Module: 11
Lecture: 45
PAINT INDUSTRIES (continued)
MANUFACTURE
Raw material
Paint Formulations
White Gloss Enamels (Solvent Based)
Sr.
No. Ingredients
% by
Weight
1 Titanium Dioxide (Rutile) 29.3
2 Calcium Carbonate 1.4
3 Long Oil alkyd (70% NV) 52.2
4 Mineral Turpentine Oil (MTO) 13.3
5 Dipentene 2.1
6 Methyethylketoxime 0.1
7 Cobalt octoate (6%) 0.1
8 Zirconium octoate (18%) 0.3
9 Calcium octoate (3%) 1.2
Red oxide Primer (Solvent Based)
Sr.
No. Ingredients
% by
Weight
1 Red Iron Oxide 35.9
2 Zinc Chrome 10
3 Talc 3
4 Whiting 1.5
5 Lecithin 0.4
6 Medium Oil Alkyd (70% NV) 36.4
7 Mineral Turpentine Oil (MTO) 11.4
8 Cobalt Octoate (6%) 0.4
9 Lead Octoate (18%) 1
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Exterior Emulsion Paint (Water Based)
Sr. No. Ingredients % by
Weight
1 Water 5.2
2 Non Ionic surfactant 1.1
3 Anti-foam 0.2
4 Ammonium polyacrylate solution (2%) 9.6
5 Titanium dioxide (Rutile) 20.5
6 Talc 8.4
7 Whiting 9.2
8 Propylene glycol 2.1
9 Pine oil 0.3
10 Preservative 0.2
11 Acrylic emulsion (46%) 43
12 Ammonia 0.2
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
Pigment
Vehicle
Other raw material
Mix
er
Ball mill
Mix
er
Ba
tch
ti
nti
ng
Packaging & storage
Figure: Manufacturing of Paint
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Required amount of ingredient along with pigments and vehicles are mixed in
mixer which is usually high speed disperser. The basic operation in the manufacture
of paint is the dispersion of pigment particles (often mixed with extenders) in a paint
vehicle or medium to produce uniform, stable system. The process of pigment
dispersion essentially consists of wetting, dispersion and stabilization of pigments and
extenders in vehicle. The dispersion involves breaking down bigger aggregates and
agglomerates to smaller units, wetting of these units and particles by the paint
medium and stabilization of the resulting dispersion.
After grinding, the mill base is mixed with other paint ingredients, i.e. vehicle
and other additives and if necessary with tinting agents in mixer. Tests such as
degree of dispersion (fineness of grind), viscosity etc. are carried out for finished
liquid paint. The straining of paint is done to remove contaminants and it generally
utilize metal or synthetic fiber gauge (screen). The paint passes through the hopper
of the filling machine where it is filled into cans or drums, labeled and packed.
Grinding mill
All the grinding mills generally utilize application of shear, attrition and impact
to effectively break down pigment agglomerates and aggregates and provide
subsequent dispersion of smaller units.
The grinding mills widely used in paint industry are
Ball mill
Pebble mill
Attritor
Sand mill
Bead mill
Basket mill
High speed disc disperser etc.
Ball mill
Ball mills are primarily used for fine grinding is consists of a cylinder mounted
on a horizontal axis and rotated at specific speed. Steel balls are used as grinding
media which is placed inside the cylinder. The grinding action of ball mill embodies
combination of impact, shear and attrition.
Pebble mill
Ceramic lining is provided inside the cylinder where ball mills are steel lined.
Pebble mill are charged with balls of steatite, alumina or porcelain. Pebble mills are
slower than the all steel mills in reaching degree of dispersion.
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Attritor
Attritor is faster than ball mill and utilizes less space and is high efficient. Attritor
consist internally agitated media in which the grinding chamber is static and the
grinding media is stirred or agitated by help of rotating shaft to achieve dispersion
where grinding media are steatite balls, steel balls and natural pebbles.
Sand Mill
Conventional vertical sand mills were invented to get around the batch size
limitations of ball mills. The sand mill consists of a high speed rotor (impeller) with disks
mounted on it at regular intervals in a cylinder. The space between the rotor and
cylinder is filled with grinding media. The pre mixed pigment slurry is pumped in at
the bottom of the cylinder and rises through the grinding media which is vigorously
agitated due high speed rotation of the impeller. Dispersion of pigment takes place
as a result of shear as it rises through the cylinder.
Bead Mill
It operates on the same principle as the sand mill using beads. (generally
glass beads).
Basket mill
The Basket mill is comprised of two shafts. The main shaft is the basket or
media agitator shaft and the second is the batch agitator shaft. The basket mill is a
submersible milling unit where it will achieve particle size reduction without the use of
hard to clean pumps, hoses, and tanks. The basket mill allows a greater amount of
material to pass through the milling chamber.
High speed disperser
High speed disperser consists of a vertical shaft having high shear disc
mounted at the end of the shaft. The disc rotates at very high speed (up to 5000
rpm) and creates a radial flow pattern within a stationary mix vessel. The disc
creates a vortex that pulls in the contents of the vessel to the blades sharp edges.
The disc surfaces then mechanically tear apart pigment particles thereby reducing
their size, and at the same time dispersing them. High speed dispersers are normally
used for pre mixing process, as dispersers for soft pigments and as thinning mixers.
SETTING OF PAINT
When the paint is applied on the surface of the metal or wood, the oil present
in it forms a protective film of dried oil. The film is formed through oxidation in
presence of air and polymerization of the unsaturated constituents of drying oil. The
drier present in the paint accelerates the drying of the protective film through
oxidation and polymerization and thus acts as a catalyst for these reactions.
Pigments strengthen the film and protect it by reflecting the destructive ultraviolet
light, while extenders increase the covering power and weathering of pigments and
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thus improve the consistency, levelling and setting. The cracking aspects are
reduced by adding some oils as plasticizers.
REQUIREMENT OF A GOOD PAINT
Pigment should be opaque to ensure good covering power.
Should be chemically inert to secure stability and hence long life.
Should have a good colour and high hiding power.
Should be weather resistant.
Should have good washability and metal anti corrosive property.
Its consistency should be suitable to appreciation by the types of application
such as brushing or roller coating.
The individual requirements are met by proper choice of pigments,
extetenders and drying oils.
Pigment volume concentration
Various requirements, such as gloss, washability, durability and reflectance,
rheological properties are largely controlled by pigment volume concentration
(PVC), which is defined as,
PVC =
The PVC range for various paints as follows
Sr.
No. Paint PVC range
1 Flate paint 50 – 70%
2 Semigloss paints 35 – 45%
3 Gloss paints 25 – 35%
4 Exterior house paints 28 – 36%
5 Metal primers 25 – 40%
6 Wood primers 35 – 40%
The gloss decreases as the PVC increases. This is due to the fact that when
volume of pigment increases relative to the nonvolatile vehicle, gloss
decreases until the finish or gloss of the paint becomes flat.
With increase in PVC, adhesion as well as durability both decreases. If volume
of pigment increases as compared to the volume of binder, the film will lose
cohesion. The paint will be in powdered form and obviously will have little
durability.
When extenders are added, the PVC increases and gloss decreases.
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PAINT FALIURE
The failure of paints may be due to various causes. Various terms used to explain
the paint failure are as follows.
Chalking
It is progressive chalking or powdering of the paint film and is used by
destructive oxidation of the oil after drying of the paint on the surface.
Erosion
Very rapid chalking is called as erosion.
Flaking or peeling
Poor attachment of the paint on the surface to be coated is called flaking or
peeling and caused by the presence of dirt of grease on the surface or water
entering from below the paint.
Alligatoring
If the center portion remains attached to the surface and the portion around
the center peels off, a term alligatoring is employed.
Checking
Fine surface cracking is called as checking and is due to the absence of
plasticizers in the paint.
Blistering
Appearance of blisters on the coat applied to wood is generally known as
blistering.
Blooming
Appearance of dull patches on the surfaces called blooming.
Blinding
Discoloration of the film is known as blinding.
Cissing
Refusal of some portions of the surface to be painted is called cissing.
Paint failures can be avoided by
Careful mixing of the constituents or ingredients in specified proportions.
Proper processing of the surface to be coated before the paint is applied.
Using a primer coat before the application of the paint.
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PROPERTIES
Sr.
No.
Property Example
1 Appearance Gloss/ Matt/ Semi-gloss
2 Application Method By Brush, Roller or Sprayer
3 Drying Time Fast dry/ Slow dry
4 Adhesion Adhesion to substrates/ Existing coating/
Intercoating
5 Mechanical
Characteristics
Hardness/ Flexibility
6 Resistance Ultra-violet/ Chemical/ Abrasion/ Fungus/ Algae
7 Outdoor Durability Gloss retention/ Color / Ultra-violet
8 Storage Stability Settling tendency/ Viscosity stability
Special applications of the paints
Paints are extensively used as acid resisting coats.
Oil bound paints or distempers are widely used for interior decoration of walls.
Coal tar products dissolved in mineral spirits have been used as protective
coatings of pipes under the name bituminous paints.
Ship bottoms are protected by antifouling paints which are prepared by
mixing iron oxide, mercuric oxide and copper resinate dispersed in tung oil
(vehicle).
Paint with damp resisting properties can be prepared by mixing paraffin wax, rosin.
Bitumen and gutta parcha dispersed in tung oil (vehicle).
