Direct reduction (blast furnace)
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Direct reduction is the fraction of iron oxide reduction that occurs in a blast furnace due to the presence of coke carbon, while the remainder – indirect reduction – consists mainly of carbon monoxide from coke combustion.
It should also be noted that many non-ferrous oxides are reduced by this type of reaction in a blast furnace. This reaction is therefore essential to the operation of historical processes for the production of non-ferrous metals by non-steel blast furnaces (i.e. blast furnaces dedicated to the production of ferromanganese, ferrosilicon, etc., which have now disappeared).
Direct-reduction steelmaking processes that bring metal oxides into contact with carbon (typically those based on the use of hard coal or charcoal) also exploit this chemical reaction. In fact, at first glance, many of them seem to use only this reaction. Processes that historically competed with blast furnaces, such as the Catalan forge, have been assimilated into this reaction. But modern direct reduction processes are often based on the exclusive use of reducing gases: in this case, their name takes on the exact opposite meaning to that of the chemical reaction.
For blast furnaces, direct reduction corresponds to the reduction of oxides by the carbon in the coke. However, in practice, direct reduction only plays a significant role in the final stage of iron reduction in a blast furnace, by helping to reduce wustite (FeO) to iron. In this case, the chemical reaction can be trivially described as follows:[1]
FeO + C → Fe + CO, consuming 155.15 kJ/mol
However, "in the solid state, there is virtually no reaction in the absence of gases, even between finely ground iron ore and coal powders. In other words, it seems certain that the reaction takes place via gases".[2] This means that direct reduction most probably corresponds to the following chain of reactions:[1]
FeO + CO → Fe + CO2, producing 17.45 kJ/mol (reduction by CO)
CO2 + C ⇌ 2 CO, consuming 172.45 kJ/mol (Boudouard reaction)