| Metallurgy-Aluminium production |
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Alumina is obtained treating mineral bauxite with caustic soda under moderate hydrothermal conditions. The purpose of this treatment is to extract the Al2O3 from the mineral. The solubility of Al2O3 in NaOH is temperature dependent; most other components of the bauxite are quite inert in the process; and the silica that does dissolve subsequently forms a nearly insoluble compound. These features permit formation of a sodium aluminate solution, physical separation of the impurities, and precipitation of pure Al(OH)3 from the cooled solution.
The process begins with the preparation of the bauxite by blending to obtain an uniform composition, and then by grinding. The bauxite is grounded while suspended in a portion of the process solution. This slurry is mixed with the balance of the heated NaOH solution, and then treated in a digester vessel at pressure far above the atmospheric one.
The produced slurry is removed from the digester and the solid bauxite residues are separated from the sodium aluminate solution in two steps so that the coarse fraction is processed separately from the fine. Both residue fractions are washed and discarded.
After several steps, aluminaAl2O3 is obtained and used as raw material for aluminium production in Hall-Héroult electrolytic cells.
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