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I was wondering if you have any information about how aluminium is extracted when mining and the process that occurs so the aluminium can be used.
Question Date: 2002-10-20
Answer 1:

Aluminum is naturally occurring in Bauxite, a heterogeneous material composed primarily of one or more aluminum hydroxide minerals, plus various mixtures of silica, iron oxide, and other impurities. It is most commonly formed in deeply weathered rocks. In some locations, deeply weather volcanic rocks, usually basalt, form bauxite deposits. The world supply of aluminum comes from:

Australia 38%
Papua New Guinea 13%
Jamaica 11%
Brazil 9%
India 5%
Other 24 %

About 110 million tones of aluminum were produced in 1994. Australia produces most of the world's aluminum. The bulk of world bauxite production (approximately 85%) is used for the manufacture of alumina (aluminum oxide) in a process known as the Bayer process. The process is far more efficient when the ore is reduced to a very fine particle size prior to reaction. This is achieved by crushing and milling the pre-washed ore.

1. Extraction
The hydrated aluminum oxide is selectively removed from the other (insoluble) oxides by transferring it into a solution of sodium hydroxide (caustic soda) to form a saturated solution of sodium aluminate. Insoluble impurities, called red mud because of the iron in it, settle to the bottom. This is then sent to a heated pressure digester. Thw conditions within the digester (concentration, temperature and pressure) vary according to the properties of the bauxite ore being used. After the extraction stage the liquid part (containing the dissolved Al2O3) must be separated from the insoluble bauxite residue and purified and filtered. The mud is thickened and washed so that the caustic soda can be removed and recycled.

2. Precipitation
Sizable amounts of hydrated aluminum (Al2O3 3H2O) crystals are added to the solution in the precipitators as seeding crystals to hasten crystal separation. Crystalline aluminum trihydrate is extracted from the digestion liquor by hydrolysis.

3. Calcination
This precipitate of aluminum hydrate crystals Al(OH)3 is filtered out washed and heated to 980 C. Free water and water that is chemically combined are driven off in these fluidized-bed calciners leaving pure alumina (aluminum oxide (Al2O3)) for the aluminum smelting process. (This fine powder is still half aluminum half oxygen by weight, bonded so firmly that neither chemicals nor heat alone can separate them.

Aluminum's melting process:
Here is a description from Encyclopaedia Britannica about this process:

In a modern smelter, alumina is dissolved in reduction potsdeep, rectangular steel shells lined with carbon that are filled with a molten electrolyte consisting mostly of a compound of sodium, aluminum, and fluorine called cryolite. By means of carbon anodes, direct current is passed through the electrolyte to a carbon cathode lining at the bottom of the cell. A crust forms on the surface of the molten bath. Alumina is added on top of this crust, where it is preheated by the heat from the cell (about 950 C [1,750 F]) and its adsorbed moisture driven off. Periodically the crust is broken, and the alumina is fed into the bath. In newer cells, the alumina is fed directly into the molten bath by means of automated feeders. The results of electrolysis are the deposition of molten aluminum on the bottom of the cell and the evolution of carbon dioxide on the carbon anode. About 450 grams (1 pound) of carbon are consumed for every kilogram (2.2 pounds) of aluminum produced. About 2 kilograms of alumina are consumed for each kilogram of aluminum produced.

Answer 2:

Aluminum is mostly found in the ore bauxite, which is Al2O3 with lots of iron compounds (that make it red) and lots of sand. When these are removed (by a long process of washing and chemical separation) the pure white Al2O3 is then electrochemically reduced to make pure aluminum.

To do this, one needs a solvent for the aluminum oxide. A pair of inventors (Charles Martin Hall from the USA and Pauk Herault from France) found that cryolite, which has the formula Na3AlF6, when melted can dissolve Al2O3, and into this, one can place carbon electrodes. When an electrical potential is provided to the two electrodes, pure aluminum deposits on the cathode (the electrode with the negative charge).

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