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Aluminum (in Canada and Europe, aluminium), symbol Al, the most abundant
metallic element in the earth's crust. The atomic number of aluminum is
13; the element is in group 13 (IIIa) of the periodic table (see
Periodic Law).
Hans Christian Oersted, a Danish chemist, first isolated aluminum 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 specific
gravity of aluminum and show its lightness. In 1854 Henri Sainte-Claire
Deville, in France, obtained the metal by reducing aluminum chloride
with sodium. Aided by the financial backing of Napoleon III, Deville
established a large-scale experimental plant and displayed pure aluminum
at the Paris Exposition of 1855.
II PROPERTIES
Aluminum is a lightweight, silvery metal. The atomic weight of aluminum is
26.9815; the element melts at 660° C (1220° F), boils at 2467° C (4473°
F), and has a specific gravity of 2.7. Aluminum is a strongly
electropositive metal and extremely reactive. In contact with air,
aluminum rapidly becomes covered with a tough, transparent layer of
aluminum oxide that resists further corrosive action. For this reason,
materials made of aluminum 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 aluminum) is heated, the
aluminum rapidly removes the oxygen from the iron; the heat of the
reaction is sufficient to melt the iron. This phenomenon is used in the
thermite process for welding iron (see Welding).
The oxide of aluminum is amphoteric-showing both acidic and basic
properties. The most important compounds include the oxide, hydroxide,
sulfate, and mixed sulfate compounds (see Alum). Anhydrous aluminum
chloride is important in the oil and synthetic-chemical industries. Many
gemstones-ruby and sapphire, for example-consist mainly of crystalline
aluminum oxide.
III OCCURRENCE
Aluminum is the most abundant metallic constituent in the crust of the
earth; only the nonmetals oxygen and silicon are more abundant. Aluminum
is never found as a free metal; commonly as aluminum silicate or as a
silicate of aluminum mixed with other metals such as sodium, potassium,
iron, calcium, and magnesium. These silicates are not useful ores, for
it is chemically difficult, and therefore an expensive process, to
extract aluminum from them. Bauxite, an impure hydrated aluminum oxide,
is the commercial source of aluminum 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
alumina, or aluminum oxide, would dissolve in fused cryolite (Na3AlF6)
and could then be decomposed electrolytically to a crude molten metal. A
low-cost technique, the Hall-Héroult process, is still the major method
used for the commercial production of aluminum, although new methods are
under study. The purity of the product has been increased until a
commercially pure ingot is 99.5 percent pure aluminum; it can be further
refined to 99.99 percent.
USES A given volume of aluminum 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
aluminum useful in the construction of aircraft, railroad cars, and
automobiles, and for other applications in which mobility and energy
conservation are important. Because of its high heat conductivity,
aluminum is used in cooking utensils and the pistons of
internal-combustion engines. Aluminum has only 63 percent of the
electrical conductance of copper for wire of a given size, but it weighs
less than half as much. An aluminum 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 aluminum conductors are now used to transmit
electricity at 700,000 V or more.
The metal is becoming increasingly important architecturally, for both
structural and ornamental purposes. Aluminum 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. Aluminum becomes stronger and retains its toughness as it gets
colder and is therefore used at cryogenic temperatures. Aluminum 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,
aluminum 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. Aluminum'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, castability, or resistance to corrosion
or high temperatures. Some new alloys can be used as armor plate for
tanks, personnel carriers, and other military vehicles.
PRODUCTION In 1886 the world production of aluminum was less than 45 kg
(less than 100 lb), and its price was more than $11 per kg (more than $5
per lb). In 1989, by contrast, the estimated world production of primary
aluminum was 18 million metric tons and an estimated 4 million metric
tons was produced in the United States alone, whereas the price of
aluminum was less than $2 per kg. U.S. consumption, by major markets,
consisted of containers and packaging, 31 percent; building and
construction, 20 percent; transportation, 24 percent; electric
equipment, 10 percent; consumer durables, 9 percent; and miscellaneous,
6 percent. In 1989, recycled aluminum accounted for over 20 percent of
total aluminum consumption in the United States.
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