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Bauxite is the most important aluminium ore. It consists largely of the minerals gibbsite Al(OH)3, boehmite γ-AlO(OH), and diaspore α-AlO(OH), together with the iron oxides goethite and hematite, the clay mineral kaolinite and small amounts of anatase TiO2. It was named after the village Les Baux-de-Provence in southern France, where it was first discovered in 1821 by geologist Pierre Berthier.
Lateritic bauxites (silicate bauxites) are distinguished from karst bauxites (carbonate bauxites). The early discovered carbonate bauxites occur predominantly in Europe and Jamaica above carbonate rocks (limestone and dolomite), where they were formed by lateritic weathering and residual accumulation of intercalated clays or of clayey dissolution residues of the limestone.
The lateritic bauxites occur in many countries of the tropical belt. They were formed by lateritization (see laterite) of various silicate rocks such as granite, gneiss, basalt, syenite and shale. Compared with iron-rich laterites, the formation of bauxites demands even more intense weathering conditions with a very good drainage. This enables dissolution of kaolinite and precipitation of gibbsite. Zones with highest aluminium content are frequently located below a ferruginous surface layer. The aluminium hydroxide in the lateritic bauxite deposits is almost exclusively gibbsite.
In 2005, Australia was the top producer of bauxite with almost one-third world share, followed by Guinea, Brazil and China, reports the British Geological Survey.
Bauxite is strip mined (surface mining) because it is found at the surface, with little or no overburden. Approximately 95% of the world's bauxite production is processed into aluminium. Bauxites are typically classified according to their intended commercial application: metallurgical, abrasive, cement, chemical and refractory.
Bauxites are heated in pressure vessels with sodium hydroxide solution at 150-200 °C through which aluminium is dissolved as aluminate (Bayer process). After separation of ferruginous residue (red mud) by filtering, pure gibbsite is precipitated when the liquor is cooled and seeded with fine grained aluminium hydroxide. Gibbsite is converted into aluminium oxide by heating. This is molten at approx. 1000 °C by addition of cryolite as a flux and reduced to metallic aluminium by a highly energy-consumptive electrolytic process (the Hall-Héroult process).
According to The Recycler's Handbook By The EarthWorks Group (1990), Earth's bauxite supplies will run out in 200-300 years. However Lester Brown has suggested bauxite could run out within 69 years based on an extremely conservative extrapolation of 2% growth per year.
Aluminocrete or Alcrete
Aluminocrete or Alcrete is a type of soil deposit, rich in aluminium. It is generally found in tropical regions, where the parent material is rapidly weathered because of high rainfall and high ambient temperatures. In these conditions, more mobile elements are leached out of the soil, leaving the relatively insoluble Al3+ ions behind. A hard, nearly impermeable crust, called a duricrust, may form on or near the surface of such soils. Duricrusts of aluminium-rich soils are called alcrete, or aluminocrete. Alcrete deposits with extremely high concentrations of aluminium are known as bauxite.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Bauxite". A list of authors is available in Wikipedia.|