This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
Aluminum never occurs free in nature, but no other metal known, not excepting iron, occurs in such abundance in its compounds, nor is any other metal so widely distributed over the earth. Its combination with oxygen, silicon, alkalies and acids forms clay, koalin, emery, mica, feldspar and many of the precious stones. It is obtained mostly from Bauxite, an oxide known as alumina, first found near Baux, in France. This is dissolved in a fused bath of cryolite (a fluoride of sodium and aluminum) by the heat of an electric current. This heat fuses the cryolite, which in turn dissolves the alumina, in which condition the electrolytic action of the current decomposes the alumina, but does not change the cryolite. The liberated metal sinks to the bottom of the vessel and is siphoned off when it accumulates. The process is continuous so long as alumina is supplied and the current is maintained.