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                           And /or

Fe₂O₃ + 3 H₂ = 2 Fe + 3 H2O

As shown in above reactions, carbon monoxide and hydrogen are reducing gases. These reducing gases can be obtained by controlled combustion of natural gas in the gas- based processes. In the coal based process gasification of the solid coal, according the Boudouard reaction produces carbon monoxide.

C + CO₂ --> 2CO

As maintained earlier direct reduction is the conversion of oxide feedstock to the metallic state with such conversion taking place entirely in the solid state (at no time does the material become molten). The carbon monoxide (CO) produced as above reduces iron oxide to metallic iron.

However, the reduction form oxide to metal does not occur in one step, but by gradual removal of oxygen giving, rise to various intermediary oxides. The reduction sequence can be expressed as follows:

                                                  Hemitite     Magnetite    Wystite   Iron

For Sulphur removal in the ore, dolomite/ limestone is used and the reactions are as follows:

FeS+ CaO+ CO-> Fe + CaS + CO₂

Reaction

The reaction occurring inside the bed of iron ore and coal are heterogeneous in nature. Two sets of reaction take place.

            2nd stage: Between iron ore particles and CO.

Each particle is considered as a porous. Reaction between the particle and the gaseous reactant can be visualized to occur in 5 successive stages

Reaction 1

Diffusion of gaseous reactant through the film surrounding the particle to the surface of the solid.

Reaction  2

Penetration and diffusion of the reactant through the blanket of the reacted outer layer to the surface of the unreacted core.

Reaction  3

Reaction of gaseous reactant with solid at this reaction surface.

Reaction  4

Diffusion of gaseous products through the reacted layer to the exterior surface of the solid.

Reaction  5

Diffusion of gaseous products trough the gas film back to the main body of the gas.

Since the reaction takes place in several steps and each in succession, any of them having the least rate could be the rate controlling. Be it diffusion or reaction, the rate should be primarily a function of the following three parameters.

a)         Concentration of reactants at active interface temperature.

b)         Active area of reaction.

The reaction time or time for completion or reaction varies inversely to the rate.

The rotary kiln process chosen for the unit considers reduction of iron ore (5-15 mm) with solid carbonaceous material like coal heated up to a temperature of 950-10000C in rotary kiln and then cooled in a rotary cooler with external water cooling system. The products are screened and magnetically separated. Sponge Iron being magnetic gets attracted and gets separated form non- magnetic char.

The iron ore and coal crushed and screened to respective sizes are fed to rotary kiln through feed nozzle in pre-determined ratio. The rotary kiln is slightly inclined at an angle 1.5 deg and rotated by AC variable drive at a  variable speed ranging form 0.2-1.0 rpm. Due to inclination and rotary motion of the kiln, the material moves form feed end to discharged end . the fine coal is  injected form the discharge end to maintain the required temperature and the carbon concentration in the bed. The kiln has shell air fans mounted on the top which blow air in the respective zones to maintain the required temperature profile. The material and the hot gases move in the counter current direction, as a result the iron ore gets pre- heated and gradually reduced by the time it reaches the discharge end. The hot reduced material is transferred to rotary cooler via the transfer chute. The cooler is also inclined and rotates at a variable speed of 0.3 to 1.2 rpm driven by an AC variable drive. Water is sprayed on top of shell to cool indirectly the inside material. The material gets cooled to 80 deg C and discharged on the belt conveyor but the double pendulum value, which acts as a seal for prevention of atmospheric air into the kiln cooler system. The cooler discharge is sent to the product separation system where material is screened to various size fractions and magnetically separated. Gases flowing in counter current direction to material go to the dust settling chamber where heavier particles settle down and latter dust particles are arrested in pollution control equipment like an ESP.

The gases which flow in the counter current direction of the material go to the dust settling chamber (DSC) where the heavier particles settle down. These particles are continuously removed by the wet scrapper. The gases then pass to the after burner chamber (ABC) where the residual carbon or CO is burned by the excess air available. The gases are of high temperature and have a lot of heat energy which can be utilized for the power generation through the waste heat recovery boiler. The hot gases after the heat recovery boiler gets cooled to 2000C, the gases then are passed through an ESP and the clean gases are let of to the atmosphere at 80 0C through the chimney.