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Most metals found in the earth's crust exist in oxide compounds. Metal-oxide compounds must be reduced to liberate the desired metal. Some metals exist in sulphide form. There are two methods of reduction: electrolytic and chemical.
Chemical reduction or smelting is the process of heating the ore with combustion agents and purifying agents to separate the pure metal from the waste products. A roasting process is used to purify sulphide ores: in this process the ore is heated in the presence of air. Electrolytic reduction involves passing a large current through a molten metal oxide or molten metal oxide solution. This is how aluminum is electrolysed from bauxite dissolved in molten cryolite.