R22 Electrochemistry in the blast furnace

The production of iron in blast furnaces has been of major economic importance for centuries. Illustration R22 shows the processes involved. The reduction of iron ore (oxides of iron(II), iron(III), silicon and other metals)
to iron with coke can be regarded as an example of a redox reaction. A blast furnace is designed to realise the following overall reaction:

For the sake of investigating the energetics of this reaction, it can be regarded as taking place in the presence of water. Under such conditions the reaction can be presented by the following half-reaction equations :

Although the reduction of iron(II) and iron(III) oxides and the oxidation of coke are self-sustaining reactions, the activation energy threshold for both is high. In reality the processes are high temperature gas-solid reactions taking place in the absence of water. In view of the high activation energy threshold, the temperatures in the reaction zones of the blast furnace need to be very high.
Such high temperatures are achieved by blowing hot air over the coke, which is thereby initially oxidised exothermically to carbon dioxide:

The carbon monoxide reduces the iron oxides exothermically, the iron is formed dropping to the bottom of the furnace, where the temperature is high enough to melt it.

A pool of molten iron forms on the bottom of the furnace. Formation of a molten slag results from the limestone (which is included in the charge together with the iron ore and coke) dissociating to form calcium oxide and carbon dioxide

and then combining with the silicon oxide and impurities from the ore. This slag trickles down to the bottom and, being less dense, forms a layer on top of the molten iron.
The iron and slag are tapped off every few hours. A modern furnace makes 3000 tons of iron a day.