Bauxite

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File:Bauxite with unweathered rock core. C 021.jpg
Bauxite with core of unweathered rock

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.

Bauxite formation

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.

Production trends

File:2005bauxite.PNG
Bauxite output in 2005

In 2007, Australia was the top producer of bauxite with almost one-third world share, followed by China, Brazil, Guinea and Jamaica. Although aluminum demand is rapidly increasing, known reserves are sufficient to meet the needs for a considerable time. Increased aluminum recycling, which has the advantage of lowering the energy costs of production, will help extend Bauxite reserves.

(x1000 tonne, Numbers for 2007 estimated)
Country Mine production Reserves Reserve base
2006 2007
Australia 62,300 64,000 5,800,000 7,900,000
People's Republic of China 21,000 32,000 700,000 2,300,000
Brazil 21,000 24,000 1,900,000 2,500,000
Guinea 14,500 14,000 7,400,000 8,600,000
Jamaica 14,900 14,000 2,000,000 2,500,000
India 12,700 13,000 770,000 1,400,000
Russia 6,600 6,000 200,000 250,000
Venezuela 5,500 5,500 320,000 350,000
Suriname 4,920 5,000 580,000 600,000
Kazakhstan 4,800 4,900 360,000 450,000
Greece 2,450 2,400 600,000 650,000
Other countries 5,460 6,800 3,400,000 4,000,000
World total (rounded) 178,000 190,000 25,000,000 32,000,000

Source: U.S. Geological Survey, Mineral Commodity Summaries, January 2008

Processing

File:CaboRojoDRBauxite.jpg
Bauxite being loaded at Cabo Roho, Domincan Republic, to be shipped elsewhere for processing; 2007.

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).

References

  • Bardossy, G. (1982): Karst Bauxites. Bauxite deposits on carbonate rocks. Elsevier Sci. Publ. 441 p.
  • Bardossy, G. and Aleva, G.J.J. (1990): Lateritic Bauxites. Developments in Economic Geology 27, Elsevier Sci. Publ. 624 p. ISBN 0-444-98811-4

External links


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