Reduction of Metals

the process of producing metals from ores using reduction reactions. The reduction of metals was originally understood to be the reactions used to obtain metals from their oxides by using substances having greater affinity for oxygen than the metal. The simplest example is the production of iron from its protoxide:

FeO + C = Fe + Co

This reaction takes place, in particular, in blast furnaces.

The possibility of reducing metals is determined by the free energy of the reaction

MeO + R = Me + RO

where MeO is the metal oxide and R is the reducing agent. If in this reaction (at constant temperature and pressure) the total free energy for Me and RO is less than for MeO and R, the process proceeds from left to right, with formation of metal. The process is facilitated if the final product, which is metal, is present in the dissolved state (solid or liquid), since dissolution is accompanied by a decrease in free energy. This explains why, in the reduction of metals, some particularly stable oxides yield the corresponding alloys as end products. Thus, the reduction of metals requires the presence of a definite thermodynamic stimulus. In addition, great importance also attaches to the kinetic conditions of reduction, which are determined by crystallochemical changes (in the case of solid oxides), the mechanism of the chemical reactions at the phase boundaries, and the mass-transfer conditions for the reagents—for example, diffusion.

In a more general chemical sense, the reduction of metals consists of the addition of electrons to an atom or group of atoms. Therefore, reduction of metals also includes processes in which metals are obtained at a cathode by electrolysis of salt melts or solutions—for example, in the case of copper:

Cu⁺⁺+ 2e = Cu

where e is an electron.

In technology the most important examples of such processes are the production of aluminum by electrolysis of alumina from a melt and the production of copper from aqueous solutions of CuSO₄. In nonferrous metallurgy, reduction of metals is carried out in the production of metals from sulfides, chlorides, and other compounds. Since the electrons given off by the reducing agent are necessary for reduction, reduction processes are inseparably connected with oxidizing processes.