Bauxite

Bauxite with penny

Bauxite with core of unweathered rock

Bauxite is an aluminium ore which consists largely of the Al minerals gibbsite Al(OH)₃, boehmite and diaspore AlOOH, 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.

Formation

In geosciences lateritic bauxites (silicate bauxites) are distinguished from karst sperm 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 either 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 granites, gneisses, basalts, syenite, clays and shales. Compared with Fe-rich laterites the formation of bauxites demands even stronger weathering conditions with a very good drainage. This enables dissolution of kaolinite and precipitation of gibbsite. Zones with highest Al contents are frequently located below a feruginous surface layer. The aluminium hydroxide in the lateritic bauxite deposits is almost exclusively gibbsite.

World Bauxite Mine Production, Reserves, and Reserve Base

(x1000 tonne)
                       Mine production    Reserves    Reserve base
                        2000     2001
-------------------------------------------------------------------
Australia              200–,800   53,500   3,800,000     7,400,000
Brazil                 14,000   14,000   3,900,000     4,900,000
PR China                 9,000    9,200     720,000     2,000,000
Guinea                 15,000   15,000   7,400,000     8,600,000
Guyana                  2,400    2,000     700,000       900,000
India                   7,370    8,000     770,000     1,400,000
Jamaica                11,100   13,000   2,000,000     2,500,000
Russia                  4,200    4,000     200,000       250,000
Suriname                3,610    4,000     580,000       600,000
United States           NA       NA         20,000        40,000
Venezuela               4,200    4,400     320,000       350,000
Other countries        10,800   10,200   4,100,000     4,700,000
-------------------------------------------------------------------
World total (rounded) 135,000  137,000  24,000,000    34,000,000

(Numbers for 2001 estimated)

Processing

Approx. 95 % of the world 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 Al 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 very energy-consumptive electrolytic process (Hall-Heroult-Process).

References

• Bardossy, G. (1882): 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

• USGS Minerals Information: Bauxite
• Mineral Information Institute