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Aluminum, 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.
Hans Christian Orstead, 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.
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 .
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. 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.
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.
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.
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.
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.