Chemical compounds are substances that are formed by the combination of two or more chemical elements with the absorption or release of energy. The number of atoms, and thus the weight, of each element in a compound is a characteristic of the compound and is a whole number. Compounds differ from mixtures or alloys in this respect and in their properties, which differ from those of the pure elements. As a result of its strong chemical affinity (tendency to react chemically) aluminium does not occur in the pure form in nature but only in compounds - with properties completely different to those of aluminium. Aluminium compounds are also produced artificially.
Naturally occurring aluminium compounds
They are a constituent part of rocks in the Earth"s crust, such as feldspars and micas as well as their products of weathering, clay and bauxite (the amount of aluminium in the Earth"s crust is eight per cent by weight). The most important naturally occurring aluminium compounds are listed below in alphabetical order:
- Alum (used since antiquity as medication and a colour binder).
- Aluminium hydroxides (also: aluminium oxide hydrides, compounds with oxygen and hydrogen); the main chemical variations boehmite and bayerite are the main constituents of bauxites (from which aluminium oxide is obtained using the Bayer process).
- Aluminium oxide (a compound with oxygen), also called "alumina", the starting point for the extraction of aluminium and constituent of many precious stones.
- Aluminium sodium silicate (compound with sodium and silica), the material of the blue semi-precious stone lapis lazuli.
- Cryolite, a compound of aluminium with sodium and fluorine; a mineral that is used for fused-salt electrolysis (mainly in a synthetically produced form).
Synthetic aluminium compounds
Synthetic aluminium compounds, which are produced artificially in chemical processes, sometimes in large quantities, find use in different industrial processes as well as in medication. The most important ones are listed below in alphabetical order:
- Aluminium chloride (a compound with chlorine), colourless crystals, readily soluble in water. Most widely used catalyst (substance that speeds up chemical reactions but which is itself left unchanged by the reaction) in the production of organic substances, for example of medication, dyes and fragrances or in the cracking of crude oil into petrol and heavy oils.
- Aluminium fluoride (compound with fluorine), a white powder, melt additive in fused-salt electrolysis.
- Aluminium hydroxides for medication.
- Aluminium soaps (compounds with fatty and resin acids), for example aluminium palmitate (for thickening paints, impregnating agents) or aluminium stearate (for primers, lubricating greases, impregnation of building materials and textiles).
- Aluminium sulphate (compound with sulphur and oxygen), a white powder, produced by dissolving aluminium hydroxide in concentrated sulphuric acid at 100 °C). Used as surface filler and colour binder in paper, tanning agent and seed-dressing agent as well as for treating waste water and drinking water (reacts with lime, which is also added, to form lumps of aluminium hydroxide, which sink to the bottom and take dirt particles with them). Starting material for the production of nearly all synthetic aluminium compounds.