Bauxite tailings

Bauxite tailings, bauxite residue or alumina refinery residues (ARR) is a by-product in the production of alumina (aluminium oxide). Alumina is the principal raw material used in the manufacture of aluminium metal and also widely used in the manufacture of ceramics, abrasives and refractories. The scale of production makes the waste product an important one, and issues with its storage are reviewed and every opportunity is explored to find uses for it. Over 95% of the alumina produced globally is through the Bayer process; for every tonne of alumina produced, approximately 1 to 1.5 tonnes of bauxite tailings/residue are also produced. Annual production of alumina in 2015 was approximately 115 million tonnes resulting in the generation of about 150 million tonnes of bauxite tailings/residue.

There are over 60 manufacturing operations across the world using the Bayer process to make alumina from bauxite ore. Bauxite ore is mined, normally in open cast mines, and transferred to an alumina refinery for processing. To extract the alumina, the soluble part of the bauxite ore is dissolved using sodium hydroxide under conditions of high temperature and pressure. The insoluble part of the bauxite (the residue) is removed, giving rise to a solution of sodium aluminate, which is then seeded and allowed to cool and aluminium hydroxide precipitates from the solution. Though some of the aluminium hydroxide is then returned and used to seed the next batch, the remainder is calcined (heated) at over 1000^oC in rotary kilns or fluid flash calciners to produce aluminium oxide (alumina). The alumina content of the bauxite used is normally about 50% but ores with a much wider range of alumina contents can be used; the aluminium compound may be present as gibbsite (Al(OH)₃), boehmite (AlOOH) or diaspore (AlOOH). The tailings/residue invariably has a high concentration of iron oxide which gives the product a characteristic red colour. A small residual amount of the sodium hydroxide used in the process remains with the tailings, causing the material to have a high pH/alkalinity, normally >12. Various stages in the solid/liquid separation process are introduced to recycle as much sodium hydroxide as possible from the residue back into the Bayer Process in order to make the process as efficient as possible and reduce production costs. This also lowers the final alkalinity of the tailings making it easier to handle.

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