Bauxite
‘
|
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 TiO
2. It was named after the village Les Baux-de-Provence in southern France, where it was first discovered in 1821.
In geosciences 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 either of intercalated clays or of clayey dissolution residues of the limestone. The economic relevance of the European deposits has strongly declined.
The lateritic bauxites occur in many countries of the tropical belt. They where 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.
(x1000
tonne)
Mine production Reserves Reserve base
2000 2001
---
Australia 200â€",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)
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 alumminium by a very energy-consumptive electrolytic process (Hall-Heroult-Process).
*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
*
USGS Minerals Information: Bauxite*
Mineral Information Institute
