Burden Distribution

The blast furnace working on the counter current principle. The descending solid charge meets a current of ascending gases and reduction of iron takes place during some passes. The production rate of blast furnace is determined by :
Blast furnace - bell charging system
Bell charging uptake

1. The rate of reduction of iron oxide
2. The rate of heating of the burden.

Burden distribution is a generic term used to denote radial ore or coke distribution as well as the particle size distribution in a blast furnace. It consists of different sizes of coke and ore, as also sinter, pellets and limestone, with different physical properties.

● These materials are charge from top of stock column, which is quite difficult. So that the entire charge have vertical and horizontal cross sections of the furnace would offer equal resistance to the gas flow.

Factor Affecting Distribution :

A number of factor affect the distribution inside the furnace and their careful control can be minimise the unevenness of gas flow and uniformity of temperature also reduction of the charge. That are :

● Design of the blast furnace and it's charging device
1. Angle and size of the big bell.
2. Additional mechanical devices used for obtained better distribution.
3. Speed of lowering of large bell.

● Inconsistencies in physical properties of the charge materials (deficiencies caused by this should be eliminated by improving quality of the burden.
1. Size range of the various charge materials.
2. Angle of repose of raw materials and other physical characteristics of the charge.
3. Density of charge materials.

● Level, system and sequence of charging programming of revolving the distributor (conditions determining major means of blast furnace process control from top)
1. Distribution of the charge on the big bell.
2. Height of the big bell from the stock like that is charge level in the furnace throat.
3. Order and proportion of charging of various raw materials.

Angle and size of the big bell :
Blast furnace - bell system
Scheme of location of counter type

The bells are arranged at an angle of 50 to 55° from the horizontal which allows the rapid discharge, minimism the possibilities of build up wet materials and regular distribution. The size of the bell should be the throat diameter of the furnace. Too much or too narrow gap between the bell corner and throat wall tends to segregate the sizes and pervents normal distribution.

Stockline level :

The stock line control the nature of segregation in the furnace burden, which allow to go down or come up with respect to the designed optimum level of materials. The lowing of the stock line to the point against wall helps the fine segregating of charging and any size of materials. At the higher the angle of inclination become steeper and the hopper formed in center becomes deeper and the crest move away from the wall.

Speed of the lowering the big bell :

The speed of lowering and raising of the large bell is of considerable the faster rate of drop, tends to throw the materials toward the wall there by alters the contour as well as distribution.

Density of Charge material :

The density of three important raw material are iron ore, limestone and the coke, which are quite different. The heaviest is iron ore with around 5 to 6g/cc, and lighter is the coke with 1.5g/cc and the limestone is intermediate with a value of 3.0 to 3.5g/cc.

Size of Charge Particles :

The wider range of sizes in the chare is more prone to segregation and crest formation, which is common practice to screen off the fines. Even sinter and pellet are also screened to removed the fraction -5mm fraction. The size ranges as charge in the modern furnace are :
More reducible ores                          +10 to 37mm
Less reducible ores.                           +10 to 25mm
Sinter.                                                   + 5to 50mm
Pellets.                                                  + 9 to 16mm
With -5mm fraction in the burden as maximum of 5%.

Distribution of Charge on Big bell :

The modern blast furnace have using belt conveyers instead of buckets car. The charging arrangements required a oval shape hopper for the small bell and the material is thrown from the skip at an angle of 45° to the bell control line. The actual distribution of the raw material on the large bell prior to their dropped inside, affects the distribution in the furnace. The early distribution using a eccentric chute in place of small bell. The extent of mixing is possible by the :

Blast furnace - operation of charging
Operation of charging

1. Both bells closed skip discharing the charge in the small belt hopper.
2. Big bell closed , small bell open to allow the charge to fall in the big bell hopper.
3. Small bell closed, big bell opened to allow the charge to be dropped inside the furnace.
4. Both bells closed.

Order of Charging :

The charging sequence, in relation to the level of charging and size of the charge, has to be adjusted so as to have maximum utilization of gas, ensuring smooth performance of the furnace.

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