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Reduction of Metals

The circumstances under which the metals are found in nature are exceedingly diverse, some being found in a native state or alloyed with other metals, as gold, silver, bismuth, and some others; some combined with arsenic, as cobalt, nickel, etc.; but by far the most abundant forms in which the metals are to be found are combinations with oxygen and sulphur. There are few of the metals that do not exist naturally in the state of oxides, which are either free or else combined with acids, forming salts. The majority of the metals exist also in nature combined with sulphur. The native compounds of the metals are termed ores, and the metal is said to be mineralized by the substance to which it is united. The several processes of reduction, or extracting the metal, must of course be regulated by the composition of the ores in which it is contained. When the metal exists only in an oxidized condition, the ore is heated in contact with the fuel, by which carbon is supplied in abundance for its reduction. The carbon combines with the oxygen and the metal is set free. Should the mineralizing substance be anything else than oxygen, carbon, no matter how intense the heat, could produce no effect upon the ore. Native sulphurets, etc., for this reason are not acted upon by carbon; and in order to reduce the metal from its sulphuret, the ores of lead, zinc, copper, etc., are first reduced to powder and heated to redness in a current of air by the oxygen, of which the sulphur is converted into sulphurous and sulphuric acid, while the metal is oxidized. This process is termed calcination. A great part of the sulphuric acid formed is carried off with the current of air, and the remaining product is a sulphate of the metal. When the salt so formed is deoxidized by contact with the fuel, the excess of oxide, abandoning its oxygen, yields an equivalent quantity of metal, which, however, would be impure and of inferior quality, having dissolved a portion of the sulphuret reproduced by the reduction of the sulphur from the sulphuric acid. It is therefore necessary to get rid of that residual portion of the sulphuric acid before the deoxidizing process commences, and this is effected by mixing up a quantity of lime with the calcined mass. The lime decomposes the metallic sulphate, combines with the sulphuric acid, and sets the oxide free; and when the deoxidizing flames of the reverberatory furnace pass over the calcined mass, the metallic oxide being reduced yields a pure metal, while the sulphate of lime, by losing its oxygen, is brought to the state of sulphuret of calcium, and remains a slag upon the surface. For the processes by which iron is reduced, see CAST IRON; CALCINATION; etc.