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aluminum (əlo͞oˈmĭnəm), called in British countries aluminium (ălˌyo͞omĭnˈēəm), metallic chemical element; symbol Al; at. no. 13; at. wt. 26.98154; m.p. 660.37℃; b.p. 2,467℃; sp. gr. 2.6989 at 20℃; 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.
a chemical element in Group III of Mendeleev’s periodic table. Atomic number, 13; atomic mass, 26.9815. Silvery white lightweight metal. Aluminum has only one stable isotope, Al-27.
Background. The word aluminum derives from alumen, the Latin name used as early as 500 B.C. for aluminum oxides, which were used as mordants for fabric coloring and leather tanning. In 1825 the Danish scientist H. C. Oersted obtained relatively pure aluminum by treating anhydrous A1C13 with an amalgam of potassium and then distilling the mercury. The first industrial method for the production of aluminum was worked out in 1854 by the French chemist H. E. Sainte-ClaireDeville. His method consisted of reducing double chloride of aluminum and of sodium Na3AlCl6 with metallic sodium. Similar to silver in color, aluminum was at first very expensive. From 1855 Jo 1890 only 200 tons of aluminum were produced.
The modern method of obtaining aluminum was worked out simultaneously in 1886 by C. Hall in the USA and P. Héroult in France, working independently. Their method involves the electrolysis of alumina dissolved in molten cryolite.
Distribution in nature. In its distribution in nature, aluminum occupies third place after oxygen and silicon and first place among metals. The content of aluminum in the earth’s crust is 8.80 percent of the mass. Aluminum is not found in a free state because of its chemical activity. Several hundred minerals of aluminum are known, primarily aluminosilicates. Bauxite, alunite, and nepheline have industrial value. Nepheline rocks have a smaller alumina content than bauxite, but through their extensive processing such important byproducts as soda, potash, and sulfuric acid are obtained. A method for the extensive processing of nepheline ores was developed in the USSR. Deposits of nepheline ores, unlike bauxite, are very substantial in the USSR and offer virtually unlimited possibilities for the development of the aluminum industry.
Physical and chemical properties. Aluminum exhibits a valuable complex of properties: low density, high thermal conductivity, high electrical conductivity, high plasticity, and good resistance to corrosion. Aluminum readily lends itself to forging, punching, rolling, and drawing and can be easily welded by gas, electric current, and other types of welding. The aluminum lattice is cubic face-centered, with the parameter a = 4.0413 A. The properties of aluminum, as of all metals, depend to a great degree on its purity. The properties of pure (99.996 percent) aluminum are density (at 20°C), 2,698.9 kg/m3; melting point, 660.24°C; boiling point, about 2500°C; coefficient of thermal expansion (20° to 100°C), 23.86 x 10”6; thermal conductivity (at 190°C), 343 watts (W) per min • °K (0.82 calories (cal) per cm • sec • °C); specific heat (at 100°C), 931.98 joules (J) per kg • °K (0.226 cal/g • °C); electrical conductivity compared to copper (at 20° C), 65.5 percent. Aluminum has a low value for yield strength, 50–60 meganewtons (MN) per m2; low hardness, 170 MN/m2, according to Brinell; and high plasticity, up to 50 percent. Cold rolling aluminum increases its strength to 115 MN/m2, with hardness increasing to 270 MN/m2 and relative elongation decreasing to 5 percent (1 MN/m2 ≈ 0.1 kgf/mm2). Aluminum takes a high polish, can be anodized, and is almost as reflective as silver, reflecting up to 90 percent of luminous energy. Because of its great affinity with oxygen, aluminum exposed to the air becomes covered with a thin but highly resistant layer of the oxide A12O3. This layer protects the metal from any further oxidation and is the cause of the high corrosion resistance of aluminum. The durability and protective power of this oxide film are greatly reduced in the presence of admixtures of mercury, sodium, magnesium, copper, and so on. Aluminum is highly resistant to atmospheric corrosion and the effects of sea and fresh water; it scarcely reacts with concentrated or highly diluted nitric acid, organic acids, or food products.
The outer electron shell of the aluminum atom consists of three electrons and is structured 3s23p. Under normal conditions, aluminum is trivalent in compounds, but at high temperatures it can be monovalent, forming so-called sub-compounds. The subhalides of aluminum, A1F and A1C1, are stable only in a gaseous state, in a vacuum, or in an inert atmosphere. With an increase in temperature, they disintegrate (disproportionate) into pure Al and A1F3 or AICI3 and therefore can be used to obtain ultrapure aluminum. On incandescence, finely ground or powdered aluminum burns energetically in air. During combustion of aluminum in an oxygen current, a temperature of over 3000°C is reached. The ability of aluminum to combine actively with oxygen is used in the reduction of metals from their oxides. During dark-red incandescence, fluorine energetically reacts with aluminum to form A1F3. Chlorine and liquid bromine react with aluminum at room temperature, iodine during heating. At high temperatures, aluminum reacts with nitrogen, carbon, and sulfur, forming aluminum nitride (A1N), carbide (Al4C3), and sulfide (Al2S3). Aluminum does not react with hydrogen. Aluminum hydride (AlH3) is obtained by an indirect method. Of great interest are the double hydrides of aluminum and the elements of Groups I and II of the periodic table, known as alumohydrides (chemical composition MeHn×n AlH3). Aluminum readily dissolves in alkalies, releasing hydrogen and forming aluminates. Most of the aluminum salts dissolve well in water. Because of hydrolysis, solutions of aluminum salts show an acid reaction.
Preparation. In industry aluminum is obtained by electrolysis of alumina Al2O3, dissolved in molten cryolite Na3AlF6 at a temperature of about 950°C. Electrolytic cells of three basic constructions are used: (1) electrolytic cells with continuous self-baking anodes and a lateral current feed; (2) the same but with an upper current feed; (3) electrolytic cells with prebaked anodes.
The electrolytic bath is an iron casing, lined with a thermally and electrically insulating material—firebrick—and with carbon plates and blocks. The working volume is filled with molten electrolyte containing 6–8 percent alumina and 94–92 percent cryolite, usually with the addition of A1F 6and about a 5–6 percent mixture of magnesium and potassium fluorides. The bottom of the bath serves as a cathode, while the baked carbon blocks dipped in electrolyte or the compacted self-baking electrodes serve as the anode. During current passage, molten aluminum appears on the cathode, accumulating on the bottom of the bath, while oxygen appears on the anode and combines with the carbon anodé to form CO and CO2. Alumina, the chief source material, has to meet high standards of purity and particle size. The presence of oxides of elements more electropositive than aluminum leads to contamination of the aluminum. When the content of alumina is sufficiently high the bath operates normally at 4–4.5 volts (V). The baths, in groups of 150–160, are connected in series to a source of a direct current. Modern electrolyzers work at a current intensity of up to 150 kilo amperes (kA). Aluminum is usually removed from the bath with a vacuum ladle. Molten aluminum of 99.7 percent purity is poured into containers. Ultrapure aluminum (99.9965 percent) is produced by electrolytic refining of primary aluminum by the so-called three-layer method, which decreases the impurity content of admixtures of Fe, Si, and Cu. Studies of electrolytic refining of aluminum by using organic electrolytes have indicated the theoretical possibility of obtaining aluminum of 99.999 percent purity with a relatively low expenditure of energy, but so far this method has low productivity. Zone melting or distillation of the subfluoride are used for thorough refining of aluminum.
Electrolytic production of aluminum involves the dangers of electrical shock, burns, and gas poisoning. To prevent accidents, the baths are well insulated, and workers wear dry cloth boots and special clothes. The air is well ventilated. Constant inhalation of powdered aluminum and its oxide can result in aluminosis of the lungs. Workers employed in the aluminum industry often suffer from catarrhs of the upper respiratory organs (rhinitis, pharyngitis, laryngitis). The limit of permissible concentration of the dust of metallic aluminum, its oxide, and its alloys is 2 mg/m3.
Applications. The combined physical, mechanical, and chemical properties of aluminum result in its wide application in virtually all fields of technology, especially in the form of alloys. In electrical technology, aluminum successfully replaces copper, especially in the production of large conductors—for example, in overhead lines, high-voltage cables, busbars of distribution structures, and transformers. The electrical conductivity of aluminum approaches 65.5 percent of the conductivity of copper, and aluminum is more than three times lighter than copper. In a cross section providing the same conductivity, the mass of an aluminum conductor is one-half the mass of a copper conductor.
Ultrapure aluminum is used in the production of electrical condensers and rectifiers, whose action is based on the ability of the oxide film of aluminum to allow an electric current to pass in only one direction. Ultrapure aluminum refined by zone melting is used to synthesize semiconductor contacts of the type AIIIBv used in the production of semiconductor devices. Pure aluminum is used in the production of various mirrors and reflectors. Aluminum of high purity (in plating and aluminum paint) is used to protect metal surfaces against atmospheric corrosion. Having a relatively low absorption cross section for neutrons, aluminum is used as a construction material in nuclear reactors.
Large-capacity aluminum containers are used to store and transport liquid gases (such as methane, oxygen, and hydrogen), nitric and acetic acids, pure water, hydrogen peroxide, and edible oils. Aluminum is widely used for equipment and devices in the food industry, for wrapping food (as a foil), and for the production of various consumer goods. There has been a sharp increase in the use of aluminum in building trim and architectural, transport, and sports structures.
In metallurgy aluminum, aside from the alloys in which it is the main component, is one of the most common additives of alloys based on Cu, Mg, Ti, Ni, Zn, and Fe. Aluminum is used for the deoxidation of steel before casting and in processes for obtaining certain metals by aluminothermy. Sintered aluminum powder, having a high thermal stability at temperatures over 300°C, has been produced from aluminum by powder metallurgy.
Aluminum is used in the production of explosive substances (ammonal, “alumotol”). Various aluminum compounds also have wide application.
The production of and the use of aluminum are constantly growing, considerably exceeding the production growth rate of steel, copper, lead, and zinc.
REFERENCESBeliaev, A. I., G. E. Vol’fson, G. I. Lazarev, and L. A. Firsanova. Poulchenie chistogo aliuminiia. [Moscow], 1967.
Beliaev, A. I., N. B. Rappoport, and L. A. Firsanova. Elektrometallurgiia aluminiia. Moscow, 1953.
Beliaev, A. I. “Istoriia aluminiia.” In Trudy In-ta istorii estestvtvoznaniia i tekhniki, vol. 20. Moscow, 1959.
Fridliander, I. N. Aliuminii i ego splavy. Moscow, 1965.
IU. I. ROMAN’KOV
Geochemistry of aluminum. The geochemical properties of aluminum are determined by its great affinity for oxygen (in minerals aluminum forms oxygen octahedrons and tetrahedrons), its constant valence (3), and the weak solubility of most of its natural compounds. In endogenic processes—when magma thickens and igneous rocks are formed—aluminum forms a crystal lattice of feldspar, mica, and other aluminosilicate minerals. In the biosphere aluminum is a very poor migrant and is found rarely in organisms or in the hydrosphere. In humid climates, where disintegrating remnants of rich flora may form many organic acids, aluminum in the form of mineral-organic colloidal compounds migrates in soil and water. Aluminum is absorbed by the colloids and accumulates in the lower soil layers. The bond of aluminum and silicon is partially broken, and in some tropical regions mineral hydroxides of aluminum—boehmite, diaspore, and gibbsite—are formed. The greater part of aluminum, however, goes to form aluminosilicates—kaolinite, beidellite, and other argillaceous minerals. Its weak mobility is the cause of its residual accumulation in the weathered crust of the humid tropics. As a result, eluvial bauxites are formed. In past geological epochs, bauxites were also accumulated in lakes and in the coastal zone of tropical seas (for example, the sedimentary bauxites of Kazakhstan).In steppes and deserts, where there are few living substances and where the water is neutral and alkaline, aluminum migrates but slightly. The most extensive migration of aluminum takes place in volcanic regions, where the highly acid river water and underground water rich in aluminum can be found. In places where the acidic waters mix with alkaline seawater (at the mouths of rivers, etc.), aluminum is precipitated and forms bauxite deposits.
A. I. PEREL’MAN
Aluminum in the organism. Aluminum enters the composition of animal and plant tissues; in the organs of mammals from 10 −3 to 10 −5 percent of the bulk was found to consist of alumina. Aluminum accumulates in the liver, the pancreas, and the thyroid gland. In plants the aluminum content varies from 4 mg per kilogram of dry substance (potato) to 46 mg per kilogram (yellow turnip); in products of animal origin, from 4 mg (honey) to 72 mg (beef) per kilogram of dry substance. The human diet for 24 hours contains 35–40 mg of aluminum. Some organisms are known to be concentrators of aluminum—for example, the lycopodium (Lycopodiacae), whose ash content reaches 5.3 percent aluminum, and mollusks (Helix and Lithorina), whose ash content is 0.2– 0.8 percent aluminum. By forming insoluble compounds with phosphates, aluminum disturbs the feeding of plants (the phosphates are absorbed by the roots) and animals (the phosphates are absorbed by the intestine).
REFERENCEVoilnar, A. O. Biologicheskaia rol’ mikroelementov v organizme zhivotnykh i cheloveka, 2nd ed. Moscow, 1960. Pages 73–77.
V. V. KOVAL’SKII