Aluminium oxide, Al2O3, is the most stable form of aluminium in normal conditions. In nature it is found as the mineral gibbsite (hydrargillite). It is also a common intermediate in the process of alumina production by the alkaline method.
It is extracted from bauxite by the Bayer process, in which molten bauxite is electrolysed in a solution of aluminium hydroxide and molten cryolite - another aluminium compound. It is used as an alloy and in a wide variety of products including cars, electronics, construction materials and a wide range of other industrial uses.
There are several ways that aluminium can be extracted from its ore. One of the most common is electrolysis, where the molten aluminium is syphoned out at intervals and new aluminium oxide added at the top of the cell.
The process is not cheap, because it requires large amounts of energy and huge currents to run the cell. It does have an environmental advantage, however - it can be recycled and reused as an alloy, which reduces the need for virgin aluminium to be produced.
Another way of extracting aluminium is via anodising. This essentially involves etching an aluminium article with dilute sulphuric acid, to remove the existing oxide layer and make it the anode in an electrolysis of the same solution.
Anions have an important influence on the polycondensation processes of aqua ions of Al(III) and on the evolution of hydroxides in solutions. Sulfate ions hinder formation of Al13 complexes and completely suppress it when the concentration is high.