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Bauxite is a
sedimentary rock Sedimentary rocks are types of Rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at the Earth's surface, followed by Cementation (geology), cementation. Sedimentation is the coll ...
with a relatively high
aluminium Aluminium (aluminum in American English, American and Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a density lower than those of other common Metal, metals, at ap ...
content. It is the world's main source of aluminium and
gallium Gallium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...
. Bauxite consists mostly of the aluminium
mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...

mineral
s gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)), mixed with the two
iron oxide Iron oxides are chemical compounds composed of iron and oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, ...
s goethite (FeO(OH)) and haematite (Fe2O3), the aluminium clay mineral kaolinite (Al2Si2O5(OH)4) and small amounts of anatase (TiO2) and
ilmenite
ilmenite
(FeTiO3 or FeO.TiO2). Bauxite appears dull in luster and is reddish-brown, white, or tan in color.

In 1821 the French people, French
geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, although backgrounds in physics, ...
Pierre Berthier
Pierre Berthier
discovered bauxite near the village of Les Baux-de-Provence, Les Baux in Provence, southern France.


Formation

Numerous classification schemes have been proposed for bauxite but, , there was no consensus. Vadász (1951) distinguished Laterite, lateritic bauxites (silicate bauxites) from karst bauxite ores (carbonate bauxites): * The carbonate bauxites occur predominantly in Europe, Guyana, Suriname, and Jamaica above carbonate rocks (limestone and Dolomite (mineral), dolomite), where they were formed by lateritic weathering and residual accumulation of intercalated clay layers – dispersed clays which were concentrated as the enclosing limestones gradually dissolved during chemical weathering. * The lateritic bauxites are found mostly in the countries of the tropics. They were formed by lateritization of various Silicate#Silicate rock, silicate rocks such as granite, gneiss, basalt, syenite, and shale. In comparison with the iron-rich laterites, the formation of bauxites depends even more on intense weathering conditions in a location with very good drainage. This enables the dissolution of the kaolinite and the precipitation of the gibbsite. Zones with highest aluminium content are frequently located below a Iron oxide, ferruginous surface layer. The aluminium hydroxide in the lateritic bauxite deposits is almost exclusively gibbsite. In the case of Jamaica, recent analysis of the soils showed elevated levels of cadmium, suggesting that the bauxite originates from recent Miocene Volcanic ash, ash deposits from episodes of significant volcanism in Central America.


Production and reserves

Australia is the largest producer of bauxite, followed by China. Increased aluminium recycling, which has the advantage of lowering the cost in electric power in producing aluminium, will considerably extend the world's bauxite reserves. In November 2010, Nguyen Tan Dung, the prime minister of Vietnam, announced that Vietnam's bauxite reserves might total 11,000 Megatonne, Mt (11 trillion kg); this would be the largest in the world.


Processing

Bauxite is usually surface mining, strip mined because it is almost always found near the surface of the terrain, with little or no overburden. , approximately 70% to 80% of the world's dry bauxite production is processed first into alumina and then into aluminium by electrolysis. Bauxite rocks are typically classified according to their intended commercial application: metallurgical, abrasive, cement, chemical, and refractory. Usually, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution at a temperature of . At these temperatures, the
aluminium Aluminium (aluminum in American English, American and Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a density lower than those of other common Metal, metals, at ap ...
is dissolved as sodium aluminate (the Bayer process). The aluminium compounds in the bauxite may be present as gibbsite(Al(OH)3), boehmite(AlOOH) or diaspore(AlOOH); the different forms of the aluminium component will dictate the extraction conditions. The undissolved waste, bauxite tailings, after the aluminium compounds are extracted contains iron oxides, silica, calcia, Titanium dioxide, titania and some un-reacted alumina. After separation of the residue by filtering, pure gibbsite is precipitated when the liquid is cooled, and then seeded with fine-grained aluminium hydroxide. The gibbsite is usually converted into aluminium oxide, Al2O3, by heating in rotary kilns or fluid flash calciners to a temperature in excess of . This aluminium oxide is dissolved at a temperature of about in molten cryolite. Next, this molten substance can yield metallic aluminium by passing an electric current through it in the process of electrolysis, which is called the Hall–Héroult process, named after its American and French discoverers. Prior to the invention of this process, and prior to the Deville process, aluminium ore was refined by heating ore along with elemental sodium or potassium in a vacuum. The method was complicated and consumed materials that were themselves expensive at that time. This made early elemental aluminium more expensive than gold.


Source of gallium

Bauxite is the main source of the rare metal
gallium Gallium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...
. During the processing of bauxite to Aluminium oxide, alumina in the Bayer process, gallium accumulates in the sodium hydroxide liquor. From this it can be extracted by a variety of methods. The most recent is the use of ion-exchange resin. Achievable extraction efficiencies critically depend on the original concentration in the feed bauxite. At a typical feed concentration of 50 ppm, about 15 percent of the contained gallium is extractable. The remainder reports to the red mud and aluminium hydroxide streams.


See also

*Bauxite, Arkansas *Rio Tinto Alcan *United Company RUSAL *MS Bulk Jupiter


References


Further reading

*Bárdossy, G. (1982): ''Karst Bauxites: Bauxite deposits on carbonate rocks''. Elsevier Sci. Publ. 441 p. *Bárdossy, G. and Aleva, G.J.J. (1990): ''Lateritic Bauxites''. Developments in Economic Geology 27, Elsevier Sci. Publ. 624 p. *Grant, C.; Lalor, G. and Vutchkov, M. (2005) ''Comparison of bauxites from Jamaica, the Dominican Republic and Suriname''. Journal of Radioanalytical and Nuclear Chemistry p. 385–388 Vol.266, No.3 *Hanilçi, N. (2013). ''Geological and geochemical evolution of the Bolkardaği bauxite deposits, Karaman, Turkey: Transformation from shale to bauxite''. Journal of Geochemical Exploration


External links


USGS Minerals Information: Bauxite
* {{Authority control Sedimentary rocks Aluminium minerals Bauxite mining, Articles containing video clips Regolith Weathering