Extractive metallurgy is the practice of extracting metal from ore, purifying it, and recycling it.
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 necassary 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, 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:
Ferrous metallurgy, which includes reduction of iron ore into iron, and further refinement and alloying with other metals to make steel.
Non Ferrous metallurgy, which includes all other metals. This can be further broken down into:
Precious metals. The recovery of gold and silver and the platinum group metals.
Base metals. The recovery of lead, zinc, copper, and nickel.
Light metals. The recovery of magnesium, aluminium, tin, and titanium.
Minor elements. The recovery of arsenic, selenium, bismuth, tellurium, and antimony.
Industrial minerals. Major examples include clays, sands, silicates, and heavy mineral sands
Coal. The benficiation and treatment of coal.
Gems and precious minerals. The recovery of emeralds, diamonds, sapphires, rubies, and others.
Mineral processing involves the use of physical processes to manipulate ore particle size, and concentrate valuable minerals using the processes of separation, based on such properties of the ore, as density, chemical composition, electrostatic, magnetic or fluorescence properties. A good example of a separation process is froth flotation.
Also of interest to the mineral processor is the separation of mineral solids from water and aqueous solutions by thickening, filtering and drying.
Pyrometallurgy involves the treatment of ores at high temperature to convert ore minerals to raw metals, or intermediate compounds for further refining. Roasting, smelting and converting are the most common pyrometallurgical processes.
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 mineral are more reluctant 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.
The created in these operations sulfur dioxide and carbon dioxide are major pollutants.
Hydrometallurgy involves the use of aqueous solutions to extract metals or compounds from their ores. Some of the hydrometallurgical processes include leaching, precipitation of insoluble compounds, pressure reduction.
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.