Aluminium

I

Introduction

Aluminium, symbol Al, the most abundant metallic element in the Earth's crust. The atomic number of aluminium is 13; the element is in group 13 (IIIa) of the periodic table.

Hans Christian Oersted, a Danish chemist, first isolated aluminium in 1825, using a chemical process involving potassium amalgam. Between 1827 and 1845, Friedrich Wöhler, a German chemist, improved Oersted's process by using metallic potassium. He was the first to measure the relative density of aluminium and show its lightness. In 1854 Henri Sainte-Claire Deville, in France, obtained the metal by reducing aluminium chloride with sodium. Aided by the financial backing of Napoleon III, Deville established a large-scale experimental plant and displayed pure aluminium at the Paris Exposition of 1855.

II

Properties

Aluminium is a lightweight, silvery metal. The atomic weight of aluminium is 26.9815; it melts at 660° C (1220° F), boils at 2467° C (4473° F), and has a relative density of 2.7. Aluminium is a strongly electropositive metal and extremely reactive. In contact with air, aluminium rapidly becomes covered with a tough, transparent layer of aluminium oxide that resists further corrosive action. For this reason, materials made of aluminium do not tarnish or rust. The metal reduces many other metallic compounds to their base metals. For example, when thermite (a mixture of powdered iron oxide and aluminium) is heated, the aluminium rapidly removes the oxygen from the iron; the heat of the reaction is sufficient to melt the iron. This phenomenon is used in the Thermit process for welding iron.

The oxide of aluminium is amphoteric—showing both acidic and basic properties. The most important compounds include the oxide, hydroxide, sulphate, and mixed sulphate compounds. Anhydrous aluminium chloride is important in the oil and petro-chemical industries. Many gemstones—ruby and sapphire, for example—consist mainly of crystalline aluminium oxide.

III

Occurrence

Aluminium is the most abundant metallic constituent in the crust of the Earth; only the non-metals oxygen and silicon are more abundant. Aluminium is found commonly as aluminium silicate or as a silicate of aluminium mixed with other metals such as sodium, potassium, iron, calcium, and magnesium, but never as a free metal. The silicates are not useful ores, for it is chemically difficult, and therefore expensive, to extract aluminium from them. Bauxite, an impure hydrated aluminium oxide, is the commercial source of aluminium and its compounds.

In 1886 Charles Martin Hall in the United States and Paul L. T. Héroult in France independently and almost simultaneously discovered that aluminium oxide, or alumina, would dissolve in fused cryolite (Na₃AlF₆) and could then be decomposed electrolytically to a crude molten metal. The Hall-Héroult process, is still the major method used for the commercial production of aluminium, although new methods are under study. The purity of the product has been increased until a commercially pure ingot is 99.5 per cent pure aluminium; it can be further refined to 99.99 per cent.

IV

Uses

A given volume of aluminium weighs less than one-third as much as the same volume of steel. The only lighter metals are lithium, beryllium, and magnesium. Its high strength-to-weight ratio makes aluminium useful in the construction of aircraft, railway carriages, and motor vehicles, and for other applications in which mobility and energy conservation are important. Because of its high heat conductivity, aluminium is used in cooking utensils and the pistons of internal-combustion engines. Aluminium has only 63 per cent of the electrical conductance of copper for wire of a given size, but it weighs less than half as much. An aluminium wire of comparable conductance to a copper wire is thicker but still lighter than the copper. Weight is particularly important in long-distance, high-voltage power transmission, and aluminium conductors are now used to transmit electricity at 700,000 volts or more.

The metal is becoming increasingly important architecturally, for both structural and ornamental purposes. Aluminium siding, storm windows, and foil make excellent insulators. The metal is also used as a material in low-temperature nuclear reactors because it absorbs relatively few neutrons. Aluminium becomes stronger and retains its toughness as it gets colder and is therefore used at cryogenic temperatures. Aluminium foil 0.018 cm (0.007 in) thick, now a common household convenience, protects food and other perishable items from spoilage. Because of its light weight, ease of forming, and compatibility with foods and beverages, aluminium is widely used for containers, flexible packages, and easy-to-open bottles and cans. The recycling of such containers is an increasingly important energy-conservation measure. Aluminium's resistance to corrosion in salt water also makes it useful in boat hulls and various aquatic devices.

A wide variety of coating alloys and wrought alloys can be prepared that give the metal greater strength, or resistance to corrosion or high temperatures. Some new alloys can be used as armour plate for tanks, personnel carriers, and other military vehicles.