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  Sponge Iron Manufacturing Process In Rotary Kiln
 
Sponge iron or direct reduced iron (DRI) is used as a raw material in electric arc as well as induction furnace for the manufacture of steel. It is being produced in lumps or pellets form, compacted and briquette form. It has a honeycomb structure with small pores. Used as a substitute of iron scrap and has a known and uniform chemical composition. Direct reduction processes are classified based on the reductant namely gaseous and solid. In India no DRI plant uses liquid fuel. All gas based plants use natural gas. Most solid reductant process use coal as reductant due to abundantly available non-coking coal. The different coal based processes include retort/shaft, tunnel kiln, rotary kiln, Lurgi etc. The process to be adopted is the rotary kiln process using low-grade coal to reduce sponge grade iron ore. The locally available raw material, possibly to setup low capacity kilns and the lowest capital cost per installed capacity has prompted the promoter to adopt this process.
 SL/RN Process
The SL/RN Process is the most widely used coal based reduction Technology the world.

The iron ore, coal and flux charged into the rotary kiln are rapidly heated up by the combustion of coal volatiles released in the charge using the specially designed RN inderbed air injection system. The reduction of iron ore in the solid follows at about 10000 C with carbon monoxide (CO) generated converting the iron ore into metallic iron. The reduction and gasification reaction which takes place in the kiln are :

The coal is burnt in the Kiln to form
C + O2 CO2
The carbon dioxide formed in the process is converted to carbon monoxide by means of the carbon in the reductant.
C + O2 2 CO
Some carbon monoxide is formed by the following reaction:
C + ½ O2 = CO …………………………… (1)
The above reaction takes place with the evolution of heat, as it is exothermic.
The reduction of iron ore by carbon monoxide, beginning with the ferric oxide takes place in three stages, at temperatures above 5700 C:
  • Stage 1: In this, Hematite is transferred to magnetite by the following equation:
    3 Fe2 O3 + CO = 2 Fe3O4 + CO2 …………………. (2)
    The above reaction is exothermic and evolves heat.
     
  • Stage 2: In this stage, Magnetite (Fe3 O4) is changed to FeO by the following reaction.
    Fe3O4 + CO = 2 FeO + CO2……………… (3)
    The above reaction is endothermic and absorbs heat. It may be noted that FeO is unstable below 5700 C and decomposes as follows:
    4 FeO CO` = 2FeO + CO2…………………. (4)
     
  • Stage 3 in this stage, FeO is changed to iron (Fe) by the following reactions:
    FeO + CO = Fe + CO2 ……………………… (5)
    This reaction is also exothermic and generates heat. By summing up reactions 2, 3 and 4, we get:
    Fe2O3 +3 CO =2 Fe+ 2 CO2
The above reduction and gasification reactions take place simultaneously inside the kiln chamber and are controlled by burning off the surplus carbon monoxide with additional air blow in the kiln atmosphere.

At the discharge end for the kiln a central burner is installed which is used in initial preheating by fuel oil, pulverized coal can also be injected through burner if required.

To maintain uniform reduction zone temperature by burning combustibles from the bed, air is blown by shell mounted fans through burner tubes spaced uniformly along the length of kiln.
 
 Process Flow Of Sponge Iron Making
 
 
 
 

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