Aluminium Oxide - Production

Production

Aluminium hydroxide minerals are the main component of bauxite, the principal ore of aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite (Al(OH)₃), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)), along with impurities of iron oxides and hydroxides, quartz and clay minerals. Bauxites are found in laterites. Bauxite is purified by the Bayer process:

Al₂O₃ + 3 H₂O → 2 Al(OH)₃

Except for SiO₂, the other components of bauxite do not dissolve in base. Upon filtering the basic mixture, Fe₂O₃ is removed. When the Bayer liquor is cooled, Al(OH)₃ precipitates, leaving the silicates in solution. The solid is then calcined (heated strongly) to give aluminium oxide:

2 Al(OH)₃ → Al₂O₃ + 3 H₂O

The product alumina tends to be multi-phase, i.e., consisting of several phases of alumina rather than solely corundum. The production process can therefore be optimized to produce a tailored product. The type of phases present affects, for example, the solubility and pore structure of the alumina product which, in turn, affects the cost of aluminium production and pollution control.

Known as alundum (in fused form) or aloxite in the mining, ceramic, and materials science communities, alumina finds wide use. Annual world production of alumina is approximately 45 million tonnes, over 90% of which is used in the manufacture of aluminium metal. The major uses of specialty aluminium oxides are in refractories, ceramics, and polishing and abrasive applications. Large tonnages are also used in the manufacture of zeolites, coating titania pigments, and as a fire retardant/smoke suppressant.