aluminum

The Columbia Encyclopedia, 6th ed.

aluminum

aluminum (əlōō´mĬnəm), called in British countries aluminium (ăl´yōōmĬn´ēəm), metallic chemical element; symbol Al; at. no. 13; at. wt. 26.98154; m.p. 660.37°C; b.p. 2,467°C; sp. gr. 2.6989 at 20°C; valence +3.

Aluminum is a silver-white metal with a face-centered cubic crystalline structure. It is a member of Group 13 of the periodic table. It is ductile, malleable, and an excellent conductor of heat and electricity. The pure metal is soft, but it becomes strong and hard when alloyed. Although less conductive than copper wire of the same diameter, aluminum wire is often used for high-tension power transmission because it is lighter and cheaper. Although it is chemically very reactive, aluminum resists corrosion by the formation of a self-protecting oxide coating. It is rapidly attacked by alkalies (such as lye) and by hydrochloric acid.

Although it is the most abundant metal in the earth's crust (about 8% by weight), aluminum does not occur uncombined but is an important constituent of many minerals, including clay, bauxite, mica, feldspar, alum, cryolite, and the several forms of aluminum oxide (alumina) such as emery, corundum, sapphire, and ruby. Commercially, aluminum is prepared by the Hall-Héroult process, which consists essentially of the electrolysis of alumina prepared from bauxite and dissolved in fused cryolite. In an electric furnace an iron tank lined with carbon serves as the cathode and large blocks of carbon serve as the anode; the electric current generates enough heat to keep the cryolite melted. Molten aluminum collects at the bottom of the tank, and oxygen is liberated at the anode. The anode is consumed as it combines with the oxygen to form carbon dioxide.

Aluminum foil is used as a wrapping material. Aluminum powder is used in paints. A mixture of powdered aluminum and iron oxide, called thermite, is used in welding because of the large amount of heat liberated when it is ignited. The development of methods for coloring aluminum led to its use in jewelry, on wall surfaces, and in colored kitchenware. Important alloys of aluminum include duralumin, aluminum bronze, and aluminum-magnesium; they are used extensively in aircraft and other industries.

Although the metal was not isolated until the 19th cent., use of aluminum compounds originated in antiquity. The Romans used various aluminum compounds as astringents; they called these alum. Sir Humphry Davy and other chemists in the early 19th cent. recognized aluminum as the metal and alumina as its oxide. H. C. Oersted succeeded in obtaining impure aluminum in 1825, but Friedrich Wöhler had greater success and is usually credited with its first isolation, in 1827. H. E. Sainte-Claire Deville first prepared inexpensive pure metal in 1854 and set about perfecting a process for its commercial production. However, it was not until 1886 that the process by which aluminum is produced today was discovered independently by C. M. Hall, a student at Oberlin College, and Paul Héroult, a French metallurgist. The process depends critically on the availability of cheap hydroelectric power.

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