Process Description and flow diagram of Coal Based DRI Plant

Process Description and flow diagram of Coal Based DRI Plant

Description of Sponge Iron Manufacturing Process 

Most of the plants in India use DRI process—a solid state direct reduction process by which iron ore is reduced to sponge without phase change. Raw material mix-iron ore, dolomite and coal are fed to one end of rotary Kiln as well as some fine coal also fed from discharge end and product sponge iron along with char is taken out from the other end.

 Apart from this, primary and secondary airs are supplied to the Kiln to initiate the combustion and reaction processes. The reaction takes place at high temp (1050˚C to 1065˚C). Coal plays a dual role in the Kiln. Part of coal is used as fuel to supply the desired heat so as to take the raw materials to the desired temp. But main role of coal is to supply carbon in the reduction process. Dolomite is used as sulphur scavenger which finally comes out with the char. Char contains ash of coal and other impurities of iron ore. 

The reactions inside the kiln are given below

 C + O2 = CO2 

CO2 + C = 2CO 

3Fe2O3 + CO = 2Fe3O4 + CO2 

Fe3O4 + CO = 3FeO + CO2 

FeO + CO = Fe (product) + CO2 

Plants in India use wide variety of raw material and coal which has direct bearing on the process. Again some plants don’t use iron ore directly. They make iron pellets before feeding it to rotary kiln. 

Few highlighting points of the selected plant are given below to understand the process better:

 • Other than oxides of Fe, iron ore also contains impurities like gangue material, sulphur, phosphorous and moisture. The quality of ore is determined by the iron percentage as it has direct bearing on the yield. The best quality feed has the following characteristics

• Fe (total): 65% or more; 
• LOI 1% 
• SiO2 + Al2O3 (Gangue material): 5% 
• Sulphur: 0.01% 
• Phosphorous: 0.05% 
• Moisture: 1% 
• Size: 3 - 18 mm (no crushing required, can be fed to kiln directly)

• As mentioned earlier Dolomite is used here as sulphur scavenger—it doesn’t have any role in the main stoichiometric reaction. Dolomite typically contains MgO (20%), CaO (28%) and acid soluble (7%). The size of dolomite is 2 - 8 mm hence no crushing is required. 

• The reducing agent is here non-coking coal. Coal size is 0-22 mm  hence it requires crushing before feeding into kiln. the good quality should be as bellow to get better result. 

Coal Properties	Indian Coal	South African
Coal Fixed Carbon %	43	55
Volatile Matter %	31	30
Moisture	8	12
Ash	26	15
GCV (Kcal/Kg)	5360	6540

Because of the huge length of the kiln and to maintain the temperature profile, 45-55% coal is injected from the discharge end. 

• Sponge iron produced by the process will be the high grade which contains 80% Fe, 12% FeO and 8% gangue material. After separation of sponge iron in the magnetic screen/pulley, the remains are collected as char. Char has good amount of fixed carbon (25% - 30%) and moderate GCV (2500 - 2600 Kcal/Kg). Char is sold outside as low grade fuel which is subsequently used in brick kiln or large boiler. Out of the total kiln discharge, 15% to 20% is the char generation.