In a blast furnace, which is generally used to obtain pure iron from its ore, the following reactions take place.
C + O2 --> CO2 + Heat
C + CO2 --> 2CO
Fe2O3 + 3CO --> 2Fe + 3CO2
Fe2O3 + 3C --> 2Fe + 3CO
Of these only the reaction between oxygen and carbon is exothermic. This reaction is responsible for increasing the temperature of the blast furnace so that the other reactions between the carbon compounds and iron oxide can take place. It is essential to increase the temperature of the furnace to levels near 700 degree Celsius for iron oxide to react and form iron.
In a blast furnace a mixture of iron oxide (Fe2O3), coke (carbon derived from coal), and limestone are heated to produce elemental iron (Fe) plus residual slag.
External heat is used to preheat the air introduced into the bottom of the blast furnace but the main source of heat in the furnace is the burning of the coke with oxygen.
C + O2 --> CO2 + heat
In addition, the CO2 reacts with some of the coke to produce carbon monoxide (CO). The carbon monoxide then reacts with the iron ore to reduce it to iron.
C + CO2 --> 2 CO
Fe2O3 + 3CO --> 2 Fe + 3 CO2