Bauxite

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Bauxite hérault

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Bauxite with unweathered rock core. C 021

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2005bauxite

Bauxite is a sedimentary rock with a relatively high aluminum content. It is the world's main source of aluminum and gallium. Bauxite consists mostly of the aluminum minerals gibbsite (Al(OH)₃), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)), mixed with the two iron oxides goethite (FeO(OH)) and hematite (Fe₂O₃), the aluminum clay mineral kaolinite (Al₂Si₂O₅(OH)₄) and small amounts of anatase (TiO₂) and ilmenite (FeTiO₃ or FeO.TiO₂). Because it is a mixture of minerals, bauxite itself is a rock, not a mineral. Bauxite is typically found in topsoil in various tropical and subtropical regions and is extracted through open-pit mining.

Formation and sources

Bauxite is formed by the thorough weathering of many different kinds of rocks. Rich sources of bauxite usually contain a specific blend of minerals, including various aluminum oxides and hydroxides. The primary sources of bauxite can be found in countries with tropical or subtropical climates, significant among these are Australia, Brazil, India, and Guinea. The process of bauxite formation is linked to intense leaching of silicate-bearing rocks under tropical conditions, leaving behind oxides and hydroxides of aluminum and other metals, while soluble materials are washed away.

Extraction and refining

The extraction of aluminum from bauxite is a two-step process. First, bauxite is processed to produce alumina (aluminum oxide) through the Bayer process. In this process, bauxite is mixed with caustic soda (sodium hydroxide) and heated under pressure. The alumina dissolves, leaving behind impurities which are then removed. Following this, the alumina is subjected to an electrolytic process called the Hall-Héroult process, where it is dissolved in molten cryolite and then electrolytically reduced to pure aluminum metal.

Uses

Aluminum produced from bauxite is used in a wide range of products from airplanes and vehicles to kitchen utensils and foil. It is also used in construction materials, electrical transmission lines, and as an adsorbent or catalyst. Gallium, a byproduct of the alumina refining process, is used in electronics, such as in the manufacture of semiconductors and LED lights.

Environmental impact

The mining of bauxite can have a significant environmental impact. Open-pit mining, the method most commonly used to extract bauxite, can lead to deforestation, soil erosion, and loss of biodiversity. Additionally, the process of refining bauxite into alumina produces red mud, a highly alkaline waste product that can cause pollution if not managed properly. Efforts are ongoing to find more sustainable mining practices and to reduce the environmental footprint of bauxite extraction and processing.

See also

• Aluminum
• Mining
• Environmental impact of mining
• Sustainable mining