aluminium
The extraction of aluminium is a two-stage process. In the first stage bauxite is crushed to extract aluminium oxide (alumina). In the second stage pure aluminium is extracted from the alumina by electrolysis.
Lightweight, silver-white, ductile and malleable, metallic element, atomic number 13, relative atomic mass 26.9815, melting point 658°C/1,216°F. It is the third most abundant element (and the most abundant metal) in the Earth's crust, of which it makes up about 8.1% by mass. It is non-magnetic, an excellent conductor of electricity, and oxidizes easily, the layer of oxide on its surface making it highly resistant to tarnish.
Pure aluminium
Aluminium is a reactive element which forms stable compounds, so a great deal of energy is needed in order to separate aluminium from its ores, and the pure metal was not readily obtainable until the middle of the 19th century. Commercially, it is prepared by the electrolysis of alumina (aluminium oxide), which is obtained from the ore
bauxite. In its pure state aluminium is a weak metal, but when combined with elements such as copper, silicon, or magnesium it forms alloys of great strength.
Uses Aluminium is widely used in the shipbuilding and aircraft industries because of its light weight (specific gravity 2.70). It is also used in making cooking utensils, cans for beer and soft drinks, and foil. It is much used in steel-cored overhead cables and for canning uranium slugs for nuclear reactors. Aluminium is an essential constituent in some magnetic materials; and, as a good conductor of electricity, is used as foil in electrical capacitors. A plastic form of aluminium, developed in 1976, which moulds to any shape and extends to several times its original length, has uses in electronics, automobiles, and building construction. |
History Aluminium was first discovered by Humphry Davy in 1807 and first produced in 1827 by Hans Oersted, who fused potassium with the anhydrous chloride of aluminium in a closed crucible, obtaining the metal in the form of a gray powder. Afterwards Friedrich Wöhler improved this method and succeeded in procuring the metal in a purer form in fused globules and in determining its specific gravity. In 1854 Henri Sainte-Claire Deville tried the same process, but replaced the potassium by sodium, and the ‘silver made from clay’ of the Paris exhibition of 1855 drew much attention to the question of its economical production. However, no easy way could be found to produce it, and the shiny new metal became more precious than gold. Napoleon is said to have had a set of aluminium cutlery made for his most honoured guests. |
Commercial production The method now used for its commercial production is the electrolysis of alumina. An iron pot, lined with carbon, is charged with cryolite and heated to about 800°C/1,470°F by the electric current. For the electrolysis, a bundle of carbon rods is used as the anode, while the pot itself forms the cathode. The oxygen liberated combines with the carbon of the anode to form carbon dioxide, while the aluminium falls to the bottom of the vessel. More alumina is added and the process continued, the molten metal being drawn off from time to time. |
Other uses Aluminium is used in the Thermite process for the extraction of high-melting metals from their oxides. Aluminium oxide is used in the manufacture of refractory bricks and as an adsorption filter in chromatography. Aluminium hydroxide is used as a mordant in dyeing and aluminium chloride as a catalyst in organic reactions. Aluminium sulphate is the most widely used chemical in water treatment worldwide. |
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