Any metallurgical operation in which metal is separated by fusion from those impurities with which it may be chemically combined or physically mixed, such as in ores. The smelting process is distinct from roasting, sintering, fire refining and other pyrometallurgical operations. The two most important operations are reduction smelting which produces molten metal and molten slag and matte smelting which produces molten matte and molten slag. Slag is a waste product and matte is the resulting metallic sulphide mixture containing the valuable minerals (e.g., copper, lead, nickel, etc.), ready for refining.