Extractive metallurgy
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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 minerals. Metal-oxide compounds must be reduced to liberate the desired metal. Many metals exist as sulfide minerals. There are two methods of reduction: electrolytic and chemical.
Chemical reduction or smelting is the process of heating the ore with combustion agents and purifying agents to separate the pure metal from the waste products. 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.
Electrolytic reduction involves passing a large current through a molten metal oxide or molten metal oxide solution. This is how aluminium is electrolysed from bauxite dissolved in molten cryolite.
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
Mineral processing involves the use of physical processes to manipulate ore particle size, and concentrate valuable minerals using processes such as density, froth flotation, electrostatic, magnetic or fluorescence.
Also of interest to the mineral processor is the separation of mineral solids from water and aqueous solutions by thickening, filtering and drying.
Pyrometallurgy
Pyrometallurgy involves the treatment of ores at high temperature to convert ore minerals to raw metals, or intermediate compounds for further refining. Smelting and roasting are the two most common pyrometallurgical processes.
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, being liberated as sulfur dioxide gas, leaving an oxidized mineral. The metal oxides can then be purified 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. Other materials are often added as flux, aiding the melting of the oxide ores.
The created sulfur dioxide and carbon dioxide are major pollutants.
Hydrometallurgy
Hydrometallurgy involves the use of aqueous solutions to extract metals or compounds from their ores.