BAUXITE

Bauxite is the ore from which most of the world’s primary aluminium metal production is ultimately sourced and takes its name from the village of Les Baux in Provence, southern France, near where in 1821 it was first discovered by the French geologist Pierre Berthe. Bauxite typically contains a mixture of one or more aluminium hydroxide (hydrated alumina) minerals as well as some impurities, which can vary widely, that together determine the conditions under which different types of bauxite are processed and their value. In industrial production of primary aluminium metal, bauxite is first ‘refined’ by a chemical leach process using caustic soda (sodium hydroxide), named after its 19th century inventor Carl Josef Bayer, to produce alumina (aluminium oxide), which in turn is the feed for production of primary aluminium metal by electrolytic reduction of the alumina in a molten bath of natural or synthetic cryolite (sodium hexafluoroaluminate), known as the Hall-Héroult process.

 

Bauxite, which is typically reddish brown in colour (owing to the presence of iron oxide) and of a dull to earthy lustre, can be classified in different ways: by the geology of the deposit (karst (carbonate) or lateritic (silicate)); the country or region in which it occurs (for example, Jamaican or European); or its intended application (while it is estimated over 85% of bauxite is used to produce alumina, some bauxite has other uses as abrasives or in refractory, cement or chemicals production). Bauxite can also be classified according to its mineralogy, of which there are three types: gibbsite, which contains alumina trihydrate; and the polymorphs boehmite and diaspore, which contain alumina monohydrates. The key difference between the former and the latter is that gibbsite is processed at lower temperature and pressure (±145⁰C, ±5 bar) than boehmite or diaspore (≥250⁰C, ≥38 bar), which requires less expensive equipment and energy input into the refining process and so ceteris paribus lowers capital and operating costs.

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Among the impurities invariably occurring in bauxite, by far the most important is silica, which can be present in different forms. Some reactive crystalline forms of silica complicate the alumina refining process, making it more difficult to recover the alumina contained in bauxite, as well as increasing quantities of caustic soda consumed in the refining process to produce alumina. Any mass of silica reacting in the alumina refining process will cause an approximately equal loss of caustic soda and of alumina to red mud residues, the waste product from alumina refining, with the result that reactive silica content is a key element in establishing the commercial value of any bauxite.

 

Bauxite can also contain potentially valuable by-product metals, which can be accumulated in and recovered from the refining process. For example, bauxite is the main source of gallium, a metal used mainly in the electronics manufacturing industry. However, since the market for gallium is very small, few alumina refineries take steps to recover it.

Most of the world’s bauxite resources occur in tropical, or former tropical, regions where rocks containing aluminium silicates have been subject to intense chemical weathering over many millions of years to yield the ores that are mined today. Bauxite most often occurs in sedimentary layers, typically of 4-6m, at or near surface with little overburden and is usually extracted by open-pit strip mining methods.