Bauxite has been the basic material for the extraction of aluminium on a large industrial scale since the very beginning. However, one has to first separate out aluminium oxide because the other metal oxides contained in the bauxite would contaminate the aluminium. In the early days of aluminium production, lime and soda were used. In 1887 and 1892 the Austrian chemist Karl Joseph Bayer patented a process which first became established after his death in 1904. Today, it is used almost exclusively worldwide.
- Digestion: bauxite is finely ground and mixed with caustic soda. This mixture is then agitated in an autoclave (a vessel that can be heated and is used for chemical reactions under pressure) at pressures up to 40 bar and at temperatures between 100 and 320 °C for up to several hours depending on the composition of the bauxite. The caustic soda dissolves the aluminium oxide out of the bauxite by combining with it to form sodium aluminate. The undissolved constituents of the bauxite form so-called "red mud".
- Separation from red mud: most of the red mud settles to the bottom of large containers. The remaining lye is filtered to remove remaining red mud and pumped as a clear, tea-coloured liquid into huge settling tanks that hold up to 4500 cubic metres of solution.
- Precipitation: by cooling the lye to about 60-70 °C, the liquid becomes supersaturated in aluminium hydroxide, which with continual agitation (for example with compressed air) causes aluminium hydroxide, an aluminium compound, to crystallise out as a solid. The precipitation process is accelerated by adding powdered aluminium hydroxide ("seeding"), but still takes some 20 to 60 hours or more.
- Calcination (using heat to drive off water present in combined form): at approximately 1100 °C in fluidised-bed furnaces (where the alumina hydrate is kept in suspension by compressed air) the water molecules split away from the aluminium hydroxide and leave behind pure aluminium oxide (with less than 0.1 per cent of impurities).
In Germany, approx. 0.5 tonnes of red mud is produced during the production of a tonne of aluminium oxide. Possible ways of using or recycling red mud have been investigated, for example as a building material, but without any market success.