Bauxite Mines


Bauxite is named after the French town of Les Baux-de-Provence, where the first deposit was discovered. In 1821, French geologist Pierre Berthier discovered a rock, or rather a clay-like deposit, that was quite unusual. It is a mixture of various aluminium compounds such as gibbsite (hydrargillite) γ-Al(OH)3, boehmite γ-AlO(OH) and diaspore α-AlO(OH). The clay-like consistency comes from the clay mineral kaolinite. The colour, brownish, reddish or beige, is contributed by small amounts of iron oxides such as haematite Fe2O3 and goethite FeO(OH). It is simply a residual rock formed by the dissolution of clay-rich rock, leaving behind the non-water-soluble components. In tropical climates, particularly intense weathering and increased drainage occur, leading to the dissolution or leaching of non-aluminium-containing compounds.

Bauxite is usually very soft, not a rock but more like clay, and often forms thick deposits. It is therefore usually mined in open casts, with the material being transported onto a conveyor belt by a bucket wheel excavator.

The extraction of aluminium from bauxite is unusual in that, unlike other metals, it is not smelted but extracted in a chemical solution process in combination with electric current. It is heated in pressure vessels at 150 to 200 °C in caustic soda, whereby aluminium dissolves as aluminate. When cooled with the addition of fine aluminium hydroxide as a crystallisation seed, pure gibbsite precipitates. This is converted into aluminium oxide Al2O3 by annealing. With the addition of cryolite as a fluxing agent, it is melted at around 1000 °C and reduced to metallic aluminium in electrolysis cells. The power requirement is considerable, so aluminium plants have mostly been built where cheap electricity is available in large quantities.

Aluminium is not stable in the atmosphere, but oxidises immediately when exposed to atmospheric oxygen. However, this oxide layer seals the aluminium underneath and prevents further oxidation.