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Hardness of water
Hardness in water is that characteristic , which prevents the lathering of soap
Due to presence of salts of Ca, Mg, and other heavy metals
Sample of hard water , when treated with soap ( Na or K salt of higher fatty acid,
Oleic , palmitic or stearic) does not produce lather but on the other hand
forms a white scum or precipitate . This is due to insoluble soaps of Ca and Mg .
Water which does not produce lather with soap solution but forms white curd
called Hard water
Water which lathers easily on shaking with soap solution is called soft water
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Hardness of water
Water which does not produce lather with soap solution but forms white curd
called Hard water
Water which lathers easily on shaking with soap solution is called soft water
2 17H35 a a l2+ ( 17H35 )2 a +2 a l
2 17H35 a g 4+ ( 17H35 )2 g +2 a l
odium tearate
(soap)
Hardness
In soluble
In solubleodium tearate
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Temporary or carbonate Hardness
Is caused by dissolved bicarbonates of Ca, and Mg and other heavy
metals and carbonate of ion.
This hardness can be destroyed by mere boiling of water, when
bicarbonates decomposed yielding carbonates or hydroxides which are
deposited as crust at the bottom of vessel
( 3)2
t
3 + 2 + 2
( 3)2
t( )2 + 2 2
In soluble
In soluble
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Permanent or non carbonate Hardness
Is caused by dissolved chlorides and sulphates of Ca, and Mg , iron
and other heavy metals ion.
This hardness can not be destroyed by boiling of water
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Concentration of hardness as well as non hardness constituting
ions are usually expressed in terms of equivalent amount of CaCO3
This mode permits multiplication and division of concentration
and this choice is due to the Molecular weight of CaCO3: = 100;Equivalent weight = 50
And CaCO3 is most insoluble in water and can be precipitated in
Water treatment
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Units of Hardness
1. Parts per million (ppm)1 part of CaCO3 equivalent hardness per 10
6parts of water
2. Milligrams per liter (mg/L): Number of mg of CaCO3 equivalenthardness present per liter of water1mg /L = 1 mg of CaCO3 equivalent hardness of 1L of water
but 1L water weights = 1Kg = 1000 g = 1000X1000mg = 106 mg
Hence 1mg/L = 1 ppm
3. Clarkes degree (oCl): number of grains (1/7000 lb) of CaCO3equivalent hardness per gallon (10 lb) of water
Or parts of CaCO3 equivalent hardness per 70000 parts of water
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4. Degree French (oFr): parts of CaCO3 equivalent hardness per
105parts of water
1 oFr = 1 parts of CaCO3 equivalent hardness per 105parts of
water
5. Milli equivalent per liter(meq/L): Number of milli equivalents
of hardness present per liter
1 meq/L = 1meq of CaCO3per L of water
= 10-3
X 50 g of CaCO3 eq per liter= 50 mg /L of CaCO3 eq = 50 ppm
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Hardness of water
The equivalents of CaCO = [Mass of hardness producing substance X chemical equivalent of CaCO ]
Chemical equivalent of Hardness producing substance
The equivalents of CaCO = [Mass of hardness producing substance X 0]
Chemical equivalent of Hardness producing substance
Ca(HCO ) : Molar Mass: 6 chemical equivalent = 8
Mg(HCO ) : MM = 6 C.E. =
CaSO
: MM = 6 C.E. = 68
MgSO : MM = 0 C.E. = 60
CaCl : MM = C.E. = .
MgCl : MM = 9 C.E. = .
MgCO : MM = 8 C.E. =
CaCO : MM = 00 C.E. = 0
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Disadvantages of Hard Water
In domestic :
Washings :
Wastage of lot of soap, white sticky precipitate adheres on the
fabric giving spots and streaks. Irons salts could cause for
staining of cloth
Bathing : White mass deposits on the bath tub and body, cleaning
quality soap is depressed
Cooking :boiling point of water is elevated due to the presence
of ions. More fuel and time is required. Pulses, beans and peas
do not cook properly with hard water. Salty material depositson cooking utensil. Un-pleasent taste of tea or coffee if prepared
using hard water
Drinking : Bad effect on our digestive system.
Possibility of forming calcium oxalate crystals in urinary tract is
increased
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Disadvantages of Hard Water
In industrial use :
Textile industry : Wastage of lot of soap, white sticky precipitate
adheres on the fabric giving spots and streaks. These fabrics when
dyed do not provide exact shade of color
Irons salts could cause for staining of cloth.
Sugar industry : Water containing sulphate, nitrite, alkali carbonate
refining causes difficulties in the crystalization of sugar
Dyeing industry : Ca, Mg, and Fe salts in hard water may react with
costly dyes forming undesirable precipitates. It could give impure
shades of color and spots on the fabric
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Pharmaceutical industry: Undesirable products will be obtained if
hard water is used in drug manufacturing industry
Paper industry : Ca, Mg, salts in hard water tend to react with
chemicals and other material employed to provide smooth and
shining finish to paper. Iron salt could give undesired color to thepaper.
Laundry: Waste of soap; Iron salt could give undesired color to the
cloth
Concrete making: Water containing chlorine and sulphates couldaffect
Hydration of cement which eventually affects the strength of
concrete
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Disadvantages of Hard Water
In steam generation in boilers:To generate stream , boilers are extensively used. If
hard water used in boilers, many troubles arises due to
the salt content
i) Scale and sludge formation
ii) Corrosion
iii) Priming and foaming
iv) Caustic embrittlement
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In Boiler, water evaporates continuously and salt concentration
Increases progressively.
When salt concentration reaches its saturation point, they
precipitate out of water.
If precipitate is loose and slimy, it is called Sludge
If the precipitated matter forms hard , adhearing crust/coating on the inner
walls of boiler, it called Scale
Sludge and Scale
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Scale and sludge formation
Sludge: It can be easily scrapped off with wire brush
It formed at colder part of the boiler
It collects in the area where flow rate is slow or bends
Scale: These are formed by the salts which has greater solubility
in hot water than cold water
MgCO3,MgCl2, CaCl2,MgSO4
Disadvantages_ Sludge
Sludges are poor conductors; wastage of heat that generated
IfSludges formed with scale, former gets trapped in the latter
and both get deposited as scales
Excessive sludge formation disturbs the working boiler . It settle
In the region of poor water circulation such as pipe connection,
Plug opening, gauge-glass connection: choking of the pipes
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Scale and sludge formation
Disadvantages_ Scale
1. Wastage of fuel
Scales are low thermal conductivity: rate of heat transfer fromboiler to water is low: To get excess heat, fuel consumption is high
2. Lowering the boiler safety
Over heating results boiler material softener and weaker and it
distorts boiler tube
3. Decrease in effeciency
Scales sometimes deposits on valves and condenser of the
boiler. It chokes them partially
4. Danger of explosionWhen thick scale crack due to uneven expansion, water
comes out and touches over heated iron plates. This results
high stream supply subsequently generates high pressure
which may even cause explosion
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Sludge and Scale
Ca(HCO ) CaCO H O CO
Formation of Scales is due to
1. Decomposition of CaHCO3
ScaleCaCO3 scale is soft. This type scale forms in low pressure
Boilers.
However, in high pressure boilers, CaCO3 is soluble
CaCO H O Ca(OH) CO
soluble
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Sludge and Scale
Formation of Scales is due to
2. Deposition of CaSO4
Solubility of CaSO4in water decreases with rise of
Temperature.
At 15oC, 3200 ppm,
At 55oC, 55 ppm
At 320oC 27 ppm
Hence, CaSO4
gets precipitated as hard Scale at heated
Portions of the Boiler
This is main cause of scales in high pressure Boilers
CaSO4 scale is quite adherent and difficult to remove
soluble
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Sludge and Scale
Formation of Scales is due to
3. Hydrolysis of Mg salts
Dissolved Mg salts undergo hydrolysis at high temperature
In side the boiler forming MgOH2precipitate as soft Scale
MgCl2 + 2 2 Mg( )2 + 2 Cl
Scale
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Sludge and Scale
Formation of Scales is due to
4. Presence of silica (SiO2)
Even small amount of silica in water deposits as Calcium
silicate or Magnesium silicate
Ca(SiO3) and Mg(SiO3)
These stick very firmly and very difficult to remove
Source of silica in water: sand filter
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Removal of Scale
1. With the help of scraper or piece of wood or wire brush
If they are loosely adhering
2. Thermal shocks ( heating and sudden cooling with cold water)
if they are brittle
3. Dissolving scales by using chemicals
if they are adherent and hard
CaCO3 scales: 5-10% HCl
CaSO4 Scales: EDTA (which forms soluble complexes)
4. By frequent blow down operationif they are loosely adhering
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Prevention of Scales Formation
1. External treatment : Using softening water
2. Internal treatment (sequestration):
Dissolved salt ions are complexed with chemicals to give
soluble salts
a. Precipitate the scale forming impurities n the form of sludge
which can be removed by blow-down operation
b. Convert to dissolved compounds which stays in water but do not cause
any harm
Blow down operation:
Partial removal of hard water through top at the bottom of the boiler
Addition of fresh softened water to make up after blow-down
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1. Colloidal conditioning: In low pressure boiler scale can be
Avoided by using kerosene, tannin, agar-agar gel etc.Scale gets coated to give non sticky and loose deposits and thiscan be removed by blow-down operation
2. Phosphate conditioning: In high pressure boiler, scale can
be avoided by adding sodium phosphate
3 CaCl2 + 2 a3P 4 Ca3(P 4)2 + aCl
Soft sludgeHard scale
Soft, non-adherent sludge of calcium and magnesium
phosphates can be removed by blow-down operation
NaH2PO4 (acidic); Na2HPO4 (weakly alkaline);
Na3PO4 (alkaline)
Choice of salt depends upon the alkalinity of boiler-feed water
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3. Carbonate conditioning: In low pressure boilers, Na2CO3
addition avoids scale formation
CaSO4 is converted to CaCO3
CaSO Na CO CaCO Na SO
scale Loose sludgeCan be removed by blow down
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To be continued in next class
4. Calgon conditioning
5. Treatment with sodium Aluminate
6. Electric conditioning
7. Radioactive conditioning
8. Complexometric method