When reducing iron ore in the form of iron oxide by using solid carbon as the reducing agent, the carbon is oxidized into carbon monoxide and carbon dioxide. The chemical reaction represented by Eq. 1 in the figure occurs among solid carbon, carbon monoxide and carbon dioxide.

Curve 1 in the figure shows the equilibrium relationship for this reaction, which is called Boudouard's equilibrium. When the gas concentration of carbon monoxide is below this curve, the reaction proceeds in the direction for forming carbon monoxide, this being called the carbon solution reaction or solution-loss reaction. In the neighborhood of 1,200K, carbon dioxide that has been formed by the reduction of iron oxide is changed into carbon monoxide by this reaction, making it possible to maintain the reducing capability of the gas. On the other hand, when the gas concentration of carbon monoxide is in the region above curve 1, a carbon deposition reaction occurs according to equilibrium theory; that is, carbon monoxide is dissociated into carbon dioxide and carbon, and carbon is deposited. However, due to its extremely slow speed, this reaction does not practically proceed at lower temperatures and low carbon monoxide concentrations. Carbon deposition actually occurs in the region where metallic iron coexists to provide strong catalytic action, and in the region of higher temperature and high carbon monoxide concentration. This region corresponds to the shaded part in the drawing. The figure also shows the regions of magnetite, wustite, and iron metal that coexist in a stable manner with carbon monoxide and carbon dioxide. It also shows that, at temperatures above 1,000K, reduction from magnetite to metallic iron can be achieved with a composition at which carbon monoxide and carbon dioxide are in equilibrium with each other.

Examination of the foregoing equilibrium theory makes it possible to decide whether a desirable reaction is possible and which conditions need to be met to obtain such a reaction. For practical control of a reaction, however, the mechanism that controls the reaction rate should be clarified and the heat and mass transfer should be analyzed on the basis of reaction rate theory and transport phenomena.