ALUMINUM - Materials for Civil and Construction Engineers - page 140

Aluminum has many favorable characteristics and a wide variety of ap-
plications. The advantages of aluminum are that it (Budinski 1996)

has one-third the density of steel

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has good thermal and electrical conductivity

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has high strength-to-weight ratio

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can be given a hard surface by anodizing and hard coating

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has alloys that are weldable

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will not rust

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has high reflectivity

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can be die cast

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is easily machined

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has good formability

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is nonmagnetic

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is nontoxic

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Aluminum's high strength-to-weight ratio and its ability to resist corrosion
are the primary factors that make aluminum an attractive structural engineer-
ing material. Although aluminum alloys can be formulated with strengths
similar to steel products, the modulus of elasticity of aluminum is only about
one-third that of steel. Thus, the dimensions of structural elements must be in-
creased to compensate for the lower modulus of elasticity of aluminum.

4.1 Aluminum Production

Aluminum production uses processes that were developed in the 1880s.
Bayer developed the sodium aluminate leaching process to produce pure
alumina Hall and Héroult, working independently, developed an
electrolytic process for reducing the alumina to pure aluminum. The
essence of the aluminum production process is shown in Figure 4.3.
The production of aluminum starts with the mining of the aluminum
ore, bauxite. Commercial grade bauxite contains between 45% and 60%
alumina. The bauxite is crushed, washed to remove clay and silica mate-
rials, and is kiln dried to remove most of the water. The crushed bauxite
is mixed with soda ash and lime and passed through a digester, pressure
reducer, and settling tank to produce a concentrated solution of sodium
aluminate. This step removes silica, iron oxide, and other impurities from
the sodium aluminate solution. The solution is seeded with hydrated alu-
mina crystals in precipitator towers. The seeds attract other alumina crys-
tals and form groups that are heavy enough to settle out of solution. The
alumina hydrate crystals are washed to remove remaining traces of impu-
rities and are calcined in kilns to remove all water. The resulting alumina
is ready to be reduced with the Hall-Héroult process. The alumina is
melted in a cryolite bath (a molten salt of sodium-aluminum-fluoride).
An electric current is passed between anodes and cathodes of carbon to

1 Al 2 O 3 2 .