Smelting is a form of
extractive metallurgy; its main use is to produce a
metal from its
ore. This includes
iron extraction (for the production of
steel) from iron ore, and
copper extraction and other base metals from their ores. Smelting uses heat and a chemical
reducing agent, commonly a fuel that is a source of
carbon such as
coke, or in earlier times
charcoal, to change the
oxidation state of the metal ore. The carbon or
carbon monoxide derived from it removes
oxygen from the ore to leave the metal. The carbon is thus oxidized, producing
carbon dioxide and
carbon monoxide. As most ores are impure, it is often necessary to use
flux, such as
limestone, to remove the accompanying rock
gangue as
slag.
Plants for the electrolytic reduction of aluminium, while not using carbon, are also generally referred to as smelters.
Smelting involves more than just "melting the metal out of its ore". In most ores, the metal is tightly combined with other elements, such as oxygen (as an oxide) or sulfur (as a sulfide). With the exception of mercury oxide, which decomposes at about 500&_160;°C (932&_160;°F), these compounds will resist temperatures much higher than those that can be attained in a wood- or coal-burning furnace. Smelting therefore requires providing suitable reducing substances that will combine with those oxidizing elements, freeing the metal.
Historically, the first smelting processes used carbon (in the form of charcoal) to reduce the oxides of tin (cassiterite, SnO2), copper (cuprite, CuO) and lead (Lead(II) oxide, PbO), and eventually iron (hematite, Fe2O3) according to the overall reactions
