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Summary
A long slightly inclined (2.5%) to the horizontal slowly rotating kiln is employed to carry out the reduction. The charges are fed from that end which is at a higher level it travel under gravity aided by the rotating motion through several heating zones & the reduced charge comes out from the other end of the kiln. This is a solid phase and gaseous reaction and no liquid phase occurs inside the kiln.
Details
For the direct reduction of iron ore the main furnace used is rotary kiln. The rotary kiln is the refractory lined cylindrical vessel on which blowers are mounted & the air pipes, to provide combustion air to the inside of the kiln. During lite up there is a start up burner at the kiln discharge end which is oil fired to heat up the rotary kiln. To ignite the injected coal from outlet of the kiln. When the desired temp. is reached, Iron ore, coal & lime stone or dolomite are fed from kiln feed end and also coal is injected from from kiln discharge end.
The start up burner which uses oil for initial heat up is withdrawn after getting the ignition temperature of the injected coal. Now the coal if the desired temp. for reduction process is obtained , iron ore with dolomite is fed from inlet of the kiln. Since the kiln has a downward slope of 2.5% from the feed end side to discharge end side, its rotation causes the iron ore & coal to mix and travel through the system. The rotational speed of the kiln is adjustable as per the feed rate & percentage of metallization. The % age of inclination, rotational speed the length of the kiln and time the material is exposed to the reducing agent CO, the temperature all to be taken into consideration the kiln has three functions.
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It is a heat exchanger |
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It is a vessel for chemical reaction |
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It is conveyor of solids |
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Degree of metallization indicated the degree of reduction to iron oxide to iron metallic. |
Fr the Better the heat transfer & better the metallization
The following reaction take place inside the kiln.
C+O2=CO2;
CO2+C=2CO;
The above reaction is known as bound ward reaction which is reversible.
Fe2O3+CO=Fe3O4+CO2;
Fe3O4+CO=FeO+CO2;
FeO+CO=Fe (Metallic) +CO2;
However the reduction from oxide to metal does not occurs in one step but by a gradual removal of oxygen giving rise to various intermediate oxides the reduction chain may be expressed as follows:
2 Fe2O3 ------------------------- 2 Fe3O4 ------------------- 6 FeO -------------6 Fe
Hematite Magnetite Wustite Iron
The degree of reduction between each stage is as follows:
11% 22% 67%
3Fe2O3--------------------- 2Fe3O4-------------------------- 6FeO--------------6 Fe
The byproduct of above reaction is carbon die oxide (CO2).
This Co2 reacts with carbon from coal to produce CO.
CO2 + C = 2 CO.
The above reaction is known as bound ward reaction which is reversible.
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