The actual metallurgical operation of breaking up the chemical combination of metal with oxygen and with other elements in ores is done by heat in furnaces and is called the dry process; or is done by making a solution of the metallic compounds and breaking up these by chemical re-agents, known as the wet process or leaching.
Smelting, as used in the dry process, is the operation of fusing ores by heat in suitable furnaces. This is an essential step of metal extracting, and its meaning is usually broadened to include all steps of the process of heat extraction of metals.
Reduction is the separation of a metal from its oxide, though usually applied to the separation of a metal from any ore, whether oxide or not.
Roasting is the heating with free access of air in order to change the ore partly or entirely to an oxide for reduction later on.
The processes of extracting most of the common metals from their ores are complicated because of the several chemical changes necessary to extract the metal in a sufficiently pure state for use. The different chemical compositions of ores, their various grades due to the relative per cents of metal and of other ingredients contained, and the existence frequently of two or more metals in the same ore mixture, make the treatment of most ores very varied and complex. By far the greater quantity of metals is produced by the dry process, and iron is very easily produced because its oxide ores are abundant and are smelted by the single process of reduction.
The treatment of ores of copper, zinc, lead, tin, nickel and antimony is more difficult. The object of each step in the processes of extracting a metal from its ores is to simplify the ore compound. This is done, as has been outlined in part, by (1) separating the ore from the gangue (par. 49), (2) driving away certain ingredients by a heat short of fusion (par. 50), and (3) fusing the ore one or more times in the presence of certain re-agents called fluxes which combine with the non-metallic substances and allow the liberated metal to separate and settle in an impure state. A metal thus obtained in the impure state must be refined before it is used.