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Bauxite tailings
Bauxite tailings, residue or alumina refinery residues (ARR) is a by-product in the production of alumina (aluminium oxide), 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 its generation make it an important waste products and consequentially that issues with its storage are reviewed and that 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 2014 was approximately 108 million tonnes resulting in the generation of about 135 million tonnes of bauxite tailings/residue.〔Annual statistics collected and published by World Aluminium. http://www.world-aluminium.org/statistics/alumina-production/〕〔K Evans, E. Nordheim and K. Tsesmelis, "Bauxite Residue Management", Light Metals, 63-66(2012).〕
==Production==
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 1100oC 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 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 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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