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 Bayer Process Chemistry
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The process of producing pure alumina from bauxite (the Bayer Process) has changed very little since the first plant was opened in 1893. The Bayer process can be considered in three stages:

Extraction

The aluminium-bearing minerals in bauxite - Gibbsite, Böhmite and Diaspore - are selectively extracted from the insouble components (mostly oxides) by dissolving them in a solution of sodium hydroxide (caustic soda):

Gibbsite: Al(OH)3 + Na+ + OH- ---> Al(OH)4- + Na+
Böhmite and Diaspore: AlO(OH) + Na+ + OH - + H2O ---> Al(OH)4- + Na+

Depending on the quality of the ore it may be washed to beneficiate it prior to processing. The ore is crushed and milled to reduce the particle size and make the minerals more available for extraction. It is then combined with the process liquor and sent in a slurry to a heated pressure digester.

Conditions within the digester (concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high Gibbsite content can be processed at 140oC. Processing of Böhmite on the other hand requires between 200 and 240oc. The pressure is not important for the process, as such but is defined by the steam pressure during the actual process conditions. At 240oC the pressure is approximately 35 atmospheres (atm).

Although higher temperatures are often theoretically advantageous there are several diadvantages including corrosion problems and the possibility of oxides other than alumina dissolving into the caustic liquor.

After the extraction stage the insoluble bauxite residue must be separated from the Aluminium-containing liquor by a process known as settling. The liquor is purified as much as possible through filters before being transferred to the precipitators. The insoluble mud from the first settling stage is thickened and washed to recover the caustic soda, which is then recycled back into the main process.

Precipitation

Crystalline aluminium trihydroxide (Gibbsite), conveniently named "hydrate", is then precipitated from the digestion liquor:

Al(OH)4- + Na+ ---> Al(OH)3 + Na+ + OH-

This is basically the reverse of the extraction process, except that the product's nature is carefully controlled by plant conditions, including seeding or selcetive nucleation, precipitation temperature and cooling rate. The "hydrate" crystals are then classified into size fractions and fed into a rotary or fluidised bed calcination kiln. Undersize particles are fed back into the precipitation stage.

Calcination

"Hydrate", is calcined to form alumina for the aluminium smelting process. In the calcination process water is driven off to form alumina:

2Al(OH)3 ---> Al2O3 + 3H2O

The calcination process must be carefully controlled since it dictates the properties of the final product.


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