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32 BAUXITE

Bauxite waste disposal Bauxite miners find highly varied solutions to waste disposal. Michael Schwartz* explains Managing the physical products from bauxite mining in the aftermath of extraction imposes varied demands. The mining may have taken place as shallow as two feet below the surface or, as in the case of several bauxite mining operations in the Nothern Urals, about 1km underground. Even this latter figure was overtaken in April 2015 with the launch of Russia’s deepest mine; Rusal’s startup complex at the CheremoukhovskayaGloubokaya bauxite mine in the North Urals will descend 1,550m. Needless to say, when a mine closes there will be socially important issues – and they are as different from one another as the original extractive techniques. In shallow workings, strip mining means the removal of topsoil and the consequent dust which emerges from soft rock as a fine clay, as well as the bauxite itself, which also generates considerable dust. Ground water pollution from clay and bauxite fines is a problem as well: The Malaysian government at one point imposed a three-month ban on bauxite exports to lower the then-existent pollution levels. Underground bauxite operations present the typical combination of issues for an underground metal ore mine, where water management is prominent. After the bauxite has been sent to the

alumina refinery to recover the aluminium oxide
(Al2O3) by the Bayer process, it typically has a grade of 45-55% Al2O3 (although it can be as low as 40% and as high as 60%). In addition, 
there can be an amount of silica with a ratio to the Al2O3 typically not lower than eight, but usually within the range of 10, 12, 13, 15 and higher. Yet another component of the bauxite is iron oxide, which can be present in various forms, e.g, hydrated, not hydrated, Fe 3+ (frequently) and Fe 2+(more seldom). Other impurities comprise organic matter TiO2, minerals
containing sulphur, chromium and others. The role of digestion… Key to the Bayer process is bauxite digestion in the recycled solution, containing asodium hydroxide. This process is accomplished within atmospheric tanks, continuous autoclaves
- varying in size from approximately 20m3 through to 70m3 and ranging as high as 300m3 - or tubular digesters. During digestion the alumina is extracted to the solution, as are the remaining minerals, including iron, titanium and calcium, as well as some alumina which have reacted with the silica within the aluminosilicates and which will stay in the

solid phase as a leach residue called red mud. …and red mud This red mud is separated from the solution via the multi-stage counter current decantation circuit, ie, red mud thickening and washing. Older plants for this process comprise five-ten sequential washing thickeners, while more modern plants use two-three stages followed by
filtration. The slurry produced via older circuits comprises 30-50% solids, while modern circuits involve filter cake with 3845%
moisture. The typical composition of red mud solids is about 40% Fe2O3, 9-16% Al2O3, 4-12% SiO2 and about 4%
Na2O plus other impurities depending on the bauxite composition. The liquid phase of the red
mud slurry - has 3-10 g/l Na2O including sodium hydroxide, sodium aluminate, sodium carbonate, sulphates and other salts. Red mud research Stockpiling and the comprehensive utilisation of red mud formed the subjects of a recent review by two lecturers at Chongqing University, China. Writing in Materials, Dong-Yan Liu and Chuan-Sheng Wu of the College of Civil Engineering,

*Correspondent March/April 2016

BAUXITE Michael.indd 1

Aluminium International Today

3/16/16 12:33 PM

Aluminium International Today March/April 2016  
Aluminium International Today March/April 2016  
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