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Pyrometallurgy uses high temperatures to transform metals and their ores. These transformations may produce pure metals, or intermediate minerals or alloys, suitable as feed for other refining or commercial applications.
Some pyrometallurgical processes are autogenous, and so the energy required to heat the minerals comes from the exothermic reaction of the minerals in the process and no further energy is required.
Roasting
Solid-gas reactions are used to chemically transform minerals.
As an example, roasting is used to remove sulfur from sulfide ores. The ore is heated in an oxidizing atmosphere such as air. Sulfides are oxidized by the oxygen in the air, with the sulfur liberated as sulfur dioxide gas, leaving an oxidized mineral. ex. 2ZnS + 3O2 --> 2ZnO + 2SO2
The sulfur dioxide produced in the process is a significant air pollutant. For example that from smelting around Swansea significantly blighted the environment in that area during the Industrial Revolution. Concern about the production of carbon dioxide is only a recent worry, following the identification of the greenhouse effect.
Smelting
The metal oxides can then be smelted by heating with coke or charcoal (forms of carbon), a reducing agent that liberates the oxygen as carbon dioxide leaving a refined mineral.
Carbonate ores are also smelted with charcoal, but are sometimes need to be calcined first.
Other materials may need to be added as flux, aiding the melting of the oxide ores and assisting in the formation of a slag, as the flux reacts with impurities, such as silicon compounds.
Smelting usually takes place at a temperature above the melting point of the metal, but processes vary considerably according to the ore involved and other matters.
See also
"Pyrometallurgy"