Burden Preparation

Burden Quality

In order to obtain smooth and hard driving operation the burden charge in the furnace ideally possess these properties

Physical :

1. A close size range with minimum of fines.
2. An ability to withstand the physical stresses incurred on being transported to the furnace, charged to the hopper and the bells and finally in the furnace.
3. Non decrepitating nature.
4. An ability to withstand mildly reducing conditions at lower temperature without breaking.
5. A good bulk reducibility so as to obtain closely equilibrium condition between solid and gas phases in the stack.
6. Low swelling tendency during reduction.
7. A high softening temperature with a narrow temperature range of fusion.

Chemical :

1. A high percentage of iron and low gangue contents.
2. A low percentage of silica, alumina, etc and a low alumina silica ratio.
3. Proper overall chemistry of the burden to ensure adequate desulphurisation of metal and absorption of coke ash in slag.
4. Proper overall chemistry to ensure clean slag and metal separation at minimum temperature and free flow of both slag and metal from the furnace on opening the corresponding outlets.
Blast furnace
Blast furnace 

The pretreatment or processings, which includes the treatment of ores, coke and limestone. Major attention is focussed on to the treatment of iron ores alone are known as burden preparation.

Benification of the Iron ores :

In the bonification process, the iron minerals must by physically liberated from the other components of ore. The phenomenon of liberation is achieved by comminution that is crushing and grinding. Properties like magnetic susceptibility, grain size and shape, density, electrical conductivity, etc are important in this respect. Each side must be judged not only in terms of its inherent quality but also in relation to local condition and the distance it will have to be subsequently transported to reach the blast furnace.

Benification Methods :

In these treatment the low grade ore has to be first crushed and perhaps ground to such as extent as to liberate the iron mineral from the gangue at the optimum cost.

1. Magnetic separation at either high or low intensity magnetic fields.
2. Gravity separation using either a heavy fluid medium or by jigging, tabling or the use of spirals.
3. Flotation.
4. Electrostatic separation.
5. Magnetic roasting followed by a low intensity magnetic separation.
6. Although it is not the aim but some degree of concentration is achieved by washing and by calcination.

Drying :

● The increasing the proportion of dried ore in or otherwise wet charge has resulted in decreasing the coke rate. The economy is chiefly attribute to the better screening of dry ores and it's consequences improved distribution in the burden.

● Wet ores are sticky and interfere with screening . Moreover it sticks to the charging bell and thereby interferes with distribution. The dried employed are generally rotary type and are heated by burning blast furnace gas. These kilns are generally inclined at about 5°to the horizontal. Ovedryiing of the the ore has been found to results  in increased disintegration of ore particle in to  fines and dust which has to be screened off suitably.

Calcination / Roasting :

Iron is ultimate produce by reducing iron oxide present in the ore. The carbonated or sulphide type of ores can be used for iron making only after calcination or roasting. Normally a good amount of coaly matter or sulphur contained in the ore also get eliminated during calcining. In the past calcining was fairly practised where carbonate and hydroxide ores was the only source but now a days it completely replaced by sintering and pelletizing.

Blending :

Blending means mixing up of two or more types of materials from two or more sources to even out variation in physical and or chemical quality to obtained a more uniform material of desired qualities over an extended periods. Iron ore blend may be made up from coke breeze, flue dust or limestone etc.

Product of Ore Processing :

It may be treatment as follows :
1. Sized lumps suitable for direct charging in the furnace.
2. Ore fines and slimes after screening the lumps or produced deliberately for subsequent agglomeration.
3. Upgraded or concentrate, usually in the from of fines, for agglomeration may be as fine as 80 to 350% mesh.

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