Bauxite

Bauxite with core of unweathered rock

Numerous classification schemes have been proposed for bauxite but, as of 1982, there was no consensus.[5]

Vadász (1951) distinguished lateritic bauxites (silicate bauxites) from karst bauxite ores (carbonate bauxites):[5]

• The carbonate bauxites occur predominantly in Europe, Guyana, Suriname, and Jamaica above carbonate rocks (limestone and 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 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 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 Miocene volcanic ash deposits from episodes of significant volcanism in Central America.

World bauxite production in 2005

One of the world's largest bauxite mines in Weipa, Australia

Australia is the largest producer of bauxite, followed by Guinea and China.[6] 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.

2018 Bauxite production and reserves (kilotonnes)[7][8]

Country	Production	Reserves
World	327,000	30,000,000
Australia	110,000	6,000,000
Guinea	82,000	7,400,000
China	60,000	1,000,000
Brazil	35,000	2,600,000
Indonesia	23,000	1,200,000
India	22,000	660,000
Jamaica	7,700	2,000,000
Russia	6,100	500,000
Kazakhstan	5,800	160,000
Vietnam	4,000	3,700,000
Saudi Arabia	4,000	200,000
Greece	1,800	250,000
Guyana	1,700	850,000
Other countries	9,000	3,740,000

Bauxite being loaded at Cabo Rojo, Dominican Republic, to be shipped elsewhere for processing; 2007

Bauxite is usually strip mined because it is almost always found near the surface of the terrain, with little or no overburden. As of 2010, approximately 70% to 80% of the world's dry bauxite production is processed first into alumina and then into aluminium by electrolysis.[9] 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 150 to 200 °C (300 to 390 °F). At these temperatures, the aluminium 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, 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 1,000 °C (1,830 °F). This aluminium oxide is dissolved at a temperature of about 960 °C (1,760 °F) 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.[10]

As a bulk cargo, Bauxite is a Group A cargo that may liquefy if excessively moist.[11] Liquefaction and the Free surface effect can cause the cargo to shift rapidly inside the hold and make the ship unstable, potentially sinking the ship. One such vessel suspected to be sunk due to this issue was the MS Bulk Jupiter in 2015.[12] One method which can demonstrate this effect is the Can test, in which a sample of the material is placed in a cylindrical can and struck against a surface many times.[13] If a moist slurry forms in the can, then there is a likelihood for the cargo to liquefy; although conversely, even if the sample remains dry it does not conclusively prove that it will remain that way, or that it is safe for loading.

Bauxite is the main source of the rare metal gallium.[14]

During the processing of bauxite to 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.[15] 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.[15] The remainder reports to the red mud and aluminium hydroxide streams.[16]

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

• 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. ISBN 0-444-98811-4
• 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

Wikimedia Commons has media related to Bauxite.

• USGS Minerals Information: Bauxite
• Mineral Information Institute
• "Bauxite" . New International Encyclopedia. 1905.