Theory of blast furnace

The main chemical reactions in the Iron-Blast furnace are oxidation of carbon by air in front of the tuyeres to give CO₂ plus heat (fig 1):

C + O₂ = CO₂...(1)

Endothermic reaction of the CO₂ with carbon to produce CO is according to Boudouard reaction(fig 2):

CO₂ + C = 2CO...(2)

There are there forms of iron oxide: hematite (Fe₂O₃), magnetite (Fe₃O₄) and wustite (FeO, non-stoichiometric compound).These oxides are reduced in stages. Their reaction with CO, the equilibrium CO/CO₂ ratios and CO-utilization factors Ƞ_CO at 900 ⁰C are given below:

The CO-utilization factor is:

% Ƞ_CO =100%CO₂/%CO + %CO₂

The reactions, their CO/CO2 ratios and utilization factors at 900 ⁰C are as following (table1):

3Fe₂O₃ + CO = 2Fe₃O₄ + CO₂...(3)

Fe₃O₄ + CO = 3FeO + CO₂...(4)

FeO + CO = Fe + CO₂...(5)

Table 1

Reactions	Equilibrium at 900 0C	Equilibrium at 900 0C
	CO/CO2	ȠCO,%
3Fe2O3 +CO = 2Fe3O4 + CO2	0	ca.100
Fe3O4 + CO = 3FeO + CO2	025	80
FeO + CO = Fe + CO2	2.3	30

Equilibrium gas composition for Reactions 2,4 and 5are represented in Fig3.This figure shows that above 960 K the gas produced by the above reactions are sufficiently strong in CO to reduce wustite to iron but below this temperature it is not.Similarly the gas produced by Boudouard reaction can reduce to wustite at all temperatures above about 940 K and Fe₂O₃. to Fe₃O₄ at any temperature.

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