Chapter 3

Literature Review

3.1 Waste Ferrogenous Materials of Blast Furnace:

Blast furnace is a counter current reactor where the raw materials consisting of ferrogenous material and coke is charged from the top and air at higher pressure is sent from the bottom .at the end hot metal and slag is obtained from the taphole while gas enriched with carbon monoxide is obtained from uptakes and down comer. As has been made clear that even the most efficient of the modern blast furnace would produce an effluent gas containing a significant proportion of carbon monoxide which could not be used for iron oxide reduction. The actual CO content may vary around 20-30% by volume. This has a calorific value of nearly 900 kcal/m3. The quantity of gas produced depends upon the amount of fuel burnt. For one tone of coke burnt nearly 4000 m3 of effluent gas may be produced. Hence a blast furnace requiring 1000 t of coke per day would generate nearly 4*106 m3 of gas with a total energy content of 3600 * 106 kcal which is nearly equivalent to 500 T of coke. Although various uses of blast furnace gas, including recovery of by-product, have been possible to use the gas for any other purpose other than just a lean fuel.

The effluent gas from the furnace cannot directly be used as a fuel since a substantial quantity of dust from the burden is also discharged along with. It may lead to accumulation of dust and wear in the equipment using the gas. The gas is, therefore , cleaned before its use and in so doing the sensible heat of the gas is invariably lost. The chemical heat of the cleaned gas is what is utilized.

Neglecting the large lumps blown out occasionally when the furnace slips the dust particles carried in the gas may vary in size from nearly 6 mm down to few microns. the average dust content may vary in the range of 7-30 g/m3.In general cleaning is carried out in three stage viz coarse,

semi-fine and fine cleaning. A typical scheme is shown in Fig. The coarse cleaning is done in dust catcher and cyclones in dry condition. The dust content of the coarse cleaned gas is nearly 5-10 g/m3. The semi fine cleaning is carried out in scrubbers, ventury washers, cyclone separators, centrifugal disintegrators,feld washers or even in electrostatic precipitators or at times by high speed rotary disintegrators. The dust content is thereby reduced down to 0.01 g/m3. The semi fine and fine cleaning is carried out either in wet or dry condition. Wet methods are generally preferred to dry methods for their better efficiency and smooth working.

3.1.1Uptake,Downcomer and Bleeder:

Vertical pipes called uptakes brought the effluent gases from furnace are normally four in number. Two adjacent uptakes are joined together to form one single duct and the two such ducts, thus formed, are connected

to form only one duct which carries the gas downward in to the dust catcher. Two down coming pipe or duct is called down comer.

A bleeder valve is a safety device, which opens automatically or is opened, to release extra pressure developed inside the furnace and eliminate the danger of explosion. Bleeder valves are either counterweighted or operated by air cylinders.

The uptakes and the down comers are steel pipes and are lined from inside with firebricks or a monolithic cast able cement applied over a wire mesh that is anchored from inside to the shell.

3.1.2Dust Catcher

A conventional dust-catcher for blast furnace gas comprises a pressure vessel, with a gas inlet and outlet dome , a separation chamber and a lower dust hopper , and a diffuser pipe extending axially through the gas inlet and outlet dome into the separation chamber.

The blast furnace gas is led by the down comer directly in to a dust catcher, which is as the name implies, removes as much of the flue dust as is possible. The dust catcher operates on the principle that the dust is contained in the gas by virtue of its velocity, and hence , if the velocity is zero the dust particles will be eliminated freo the gas.

The efficiency of the dust catcher is usually 60-75 %. It is also a practice to install two dust catcher in series, on some furnaces, to effect proper coarse cleaning of the gas and consequently to reduce burden on the semi fine gas cleaning device.

3.1.3Semi-Fine or Primary Cleaning

The gas from the dust catcher is led in to the primary or semi-fine gas cleaning system.

3.1.3.1Scrubbers or Spray Towers

It consist of a tall cylinder with conical top and bottom. The gas enters in this at a point near the bottom of the cylinder and leaves through a centrally located outlet at the top.Water is fed from the top in the form of , one or more rows of water sprays to cover the entire cross-section of the cylindrical shell.Inside the cylindrical shell several layers of wooden packing with perforation are fitted to break up the gas flow in to small jets and thereby intimate contact of gas and water. The dust contained in the gas stick to the wooden packing’s and is washed down with water.

The coarse cleaned gas from the dust catcher enters the scruber.Due to sudden increase in the volume velocity is decreased. As a result some of the coarse particle settles at the bottom itself as it becomes wet. finer dust is washed down from the top.

The slurry that formed collects at the bottem and is continuously from there into a thickener for separating water from the solid for recirculation.

3.1.3.2 Ventury Washer

Water is sprayed in a vertical unit to wet the moving gas.Two sets of water sprays are used, one at right angle to the direction of gas flow and the other at an obtuse angle.

3.2 Agglomeration of Iron Ores

Agglomeration is used to combine the resulting fine particles into durable clusters. The iron concentrate is balled in drums and heated to create a hardened agglomerate. Agglomerates may be in the form of pellets, sinter, briquettes, or nodules. The purpose of agglomerating iron ore is to improve the permeability of blast furnace feed leading to faster gas-solid contact in the furnace . Agglomerating the ore prior to being sent to blast furnaces reduces the amount of coke consumed in the furnace by increasing the reduction rate.

The develop ment of these processes to the extent of their adoption in commercial practice became possible only during the second world war, when a general shortage of ore made it imperative to use all available raw materials.Out of the above processes sintering on grate and pelletising are widely used in preference to others because of their technical and economic suitability for agglomeration of iron ore fines on a large scale.

3.2.1 Briquetting

Briquetting essentially consist of pressing of ore fines, with or without a binder, in to a block or briquette of some suitable size and shape, and then subjecting it to a hardening process. A wide range of organic and inorganic binders like tar,pitch,cereal products, sodium silicate, ferrous sulphate, magnesium chloride limestone, cement , bentonite etc. have tried with varying success.Although briquetting was used commercially during and after the second war it has practically been abandoned since 1960. On a small scale briquetting does offer some advantages over other processes.

Recently hot briquetting has also been developed in U.S.A by pressing ore fines at 800-1050 0C under normal or slightly reducing condition.

In the U.S.S.R hardened briquettes have been produced by using hydrated lime and mollases as binders and hardening carried out at about 3000C in an atmosphere of cabon dioxide.

3.2.2 Nodulising

Nodulising is the process of consolidating fine mineral concentrates into lumps by kneading with a binder substance and occasionally by employing heat or chemical reactions. In nodulising process flue dust, pyrite residue or fine ore concentrate along with some carbonaceous material like tar passed through a rotary kiln heated by gas or oil. The feed travels countercurrent to the gases. The temperature inside the kiln is just sufficient to soften the ore but not high enough to fuse the ore.

The nodules varied considerably in composition and were too dense, slaggy and lacked the required porosity and hence this process could not find great favor.It has been largely replaced by sintering and pelletising.

3.2.3 Vacuum Extrusion

The process of vacuum extrusion otherwise used in ceramic industry,was adopted for producing iron ore compacts in the 1950 s on a small scale. Moist ore with or without bentonite as a binder is fed in dearing chamber and extuded into a cylindrical product which is cut in to desired small sizes. The product is dried and fired before use in blast furnace.

3.2.4 Pelletising:

Pelletization is one of the agglomeration processes which converts the fines into pellets of suitable size. Binders are important for holding the fine particles together during the pelletization process. Either organic or

inorganic binders can be used. Organic binders burn or volatilize during movement of the flame front.

3.2.4.1 Mechanism Of Pellet Formation:

Ball Formation – Surface tension of water & gravitational force creates pressure on Particles, so they coalesce together & form nuclei which grow in size into ball.

Green Pelletizing:Theory Of Ball Formation:

The forces responsible for the agglomeration of ore fines are surface tension andcapillary action of water and gravitational forces of particles due their rotation in balling unit.

Pelletizing in Disc:Green pellets with a size range of 8-16 mm are prepared in balling drum or discs. Discs are preferred to produce quality green pellets as these are easy to control operation with minimum foot space. The disc is an inclined pan of around 5 to 7.5 meters diameter rotating at around 6 to 8 rpm. The inclination of disc is around 45o and it can be adjusted in the off-line between 45o to 49o. The pre wetted mix is fed into the disc at a controlled rate. Ore fines are lifted upwards until the friction is overcome by gravity and the material rolls down to the bottom of the disc. This rolling action first forms small granules called seeds. Growth occurs in the subsequent revolutions of the disc by the addition of more fresh feeds and by collision between small pellets. As the pellet grows in size, they migrate to the periphery and to the top of the bed in the discs, until they overflow the rim. Pellet growth is controlled by the small amount of water sprayed in the disc and the adjustment in the disc rotational speed.

3.2.4.2 ADVANTAGES OF PELLETS:

Iron ore pellet is a kind of agglomerated fines which has better tumbling

index as compared to that of parent ore and can be used as a substitute for

the same. Iron ore pellets are being used for long in blast furnaces in

many countries where lump iron ore is not available. In India, the

necessity of pelletisation is realized because of several reasons and

advantages. The excessive fines generated from the iron ore mining and

crushing units for sizing the feed for blast furnace and sponge iron ore

plants are mostly un-utilized. Pelletisation Technology is the only route

that is going to dominate the Indian steel industry in future. Pellets have;

Good Reducibility:

Because of their high porosity that is (25-30%), pellets are usually

reduced considerably faster than hard burden sinter or hard natural

ores/lump ores.

Good Bed Permeability:

Their spherical shapes and containing open pores, gives them good

bed permeability. Low angle of repose however is a drawback for

pellet and creates uneven binder distribution.

High uniform Porosity (25-30%):

Because of high uniform porosity of pellets, faster reduction and high

metallization takes place.

Less heat consumption than sintering.

Approx. 35-40% less heat required than sintering.

Uniform chemical composition & very low LOI:

The chemical analysis is to a degree controllable in the concentration

processing within limits dictated by economics. In reality no LOI

makes them cost effective.

Easy handling and transportation.

Unlike Sinter, pellets have high strength and can be transported to long

distances without fine generation. It has also good resistance to disintegration.

3.2.5 Sintering :