Most metals found in the Earth's crust exist as oxide and sulfide minerals. These compounds must be reduced to liberate the desired metal. There are two methods of reduction: electrolytic and chemical.
Chemical reduction can be carried out in a variety of processes, including reductive smelting - the process of heating an ore with reducing agent (often, coke or charcoal) and purifying agents to separate the pure molten metal from the waste products. Some other processes for chemical reduction include autoclave hydrogen reduction and converting. The latter though does not produce the pure metal, therefore requiring further treatment of its product.
Electrolytic reduction involves passing a large current through a molten metal oxide or an aqueous solution of the metal's salt. For example, aluminium is electrolysed from bauxite dissolved in molten cryolite via the Hall-Héroult process.
Prior to reduction, it is often necessary to separate metal compounds to exclude co-reduction of different metals and contamination of the product. There is a great variety of separation processes: roasting, oxidative smelting, converting, amalgamation, leaching and many others.
Extractive metallurgical technologies are divided into mineral processing, hydrometallurgy and pyrometallurgy areas. Extractive metallurgical and mineral dressing operations are divided into:
A roasting process is used to extract metals from sulfide ores: in this process the ore is heated in the presence of oxygen and the sulfur is oxidised and driven off as sulfur dioxide. Some metals in this process remain in the sulfide form, while other metals are turned into an oxide form. The desired metal may be in either product.
Oxidative smelting and converting are similar to the roasting process, but differ slightly in the way that the processes' temperatures are high enough to promote melting of materials. Some minerals are more resistant to oxidation, so they remain in the sulfide form, while other minerals are completely oxidized and form compounds with additives, often called flux. Molten sulfides and oxide compounds split in two layers because of the different specific weights.
Leaching is a process for chemical dissolution of the desired minerals in aqueous solutions. Due to the difference in the dissolution rates, it is possible to separate the compounds of different metals. Often, some oxidative reagents need to be added to promote leaching.
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