Aluminum is the third most abundant element (8%) in the Earth's crust, exceeded by oxygen (47%) and silicon (28%). Earth scientists generally agree that aluminum, like most other elements, was formed during the birth of our solar system through successive collisions of hydrogen atoms under conditions of high temperatures and pressures. Because of its strong affinity to oxygen, aluminum never occurs as a metal in nature but is found only in the form of its compounds, such as \Talumina\t. This strong affinity to oxygen also explains why it withstood all attempts to prepare it in its elemental form until well into the 19th century. The metal's name is derived from alumen, the Latin name for alum. In 1761 the French chemist Guyton de Morveau proposed the name alumine for the base in \Talum\t, and in 1787 Antoine \TLavoisier\t identified alumine as the oxide of a then-undiscovered metal. In 1807 Sir Humphry \TDavy\t assigned the name alumium to the metal and later agreed to change it to aluminum. Shortly thereafter, the name aluminium was adopted to conform with the -ium ending of most elements, and this spelling is now in general use throughout the world, except in the United States (where the second i was dropped in 1925) and Italy (where alluminio is used). PHYSICAL PROPERTIES Aluminum, symbol Al, is a silvery-white metal in Group IIIA of the periodic table. Its atomic number is 13, its atomic weight 26.9815. It is ductile, nonmagnetic, and an excellent conductor of heat and electricity. The density of aluminum at 20 deg C is 2.699 g/cu cm (0.1 lb/cu in); it melts at 660.24 deg C and boils at 2,450 deg C. The role aluminum plays in human physiology is not known. Although the metal is ingested through food and water, most of it is believed to be excreted. Aluminum has been detected in the brain cells of \TAlzheimer\t's patients, but it is not known whether the metal's presence is a cause or an effect of the disease. Aluminum is widely used in many kinds of products because a combination of properties gives it special advantages over other materials. Lightness and Strength Perhaps the best-known quality of aluminum is its light weight. Its specific gravity is only 2.7; it is thus only about one-third as dense as iron, copper, or zinc. Despite its light weight, it can easily be made strong enough to replace heavier and more costly metals in thousands of applications. Aluminum \Lalloy\ls have the highest strength-to-weight characteristics of any commercial metal. The strength of the various aluminum alloys increases, with little change in ductility, as temperatures drop into the cryogenic range. At temperatures as high as 200 deg C (392 deg F), certain alloys remain remarkably strong. Resistance to Corrosion Aluminum and its various alloys are highly resistant to corrosion. When exposed to air, the metal develops a thin film of Al(2)O(3) almost immediately. The reaction then slows, however, because the film seals off oxygen, preventing further oxidation or chemical reaction. The film is colorless, tough, and nonflaking. Few chemicals can dissolve it. Some aluminum alloys are better suited to certain corrosive environments than others. Many different alloys and claddings are available to meet the requirements of specific corrosive environments. Electrical and Thermal Conductivity Aluminum's electrical alloy has the highest conductivity per pound of any commercially sold conductor. Because aluminum is only one-third as dense as copper, it supplies about twice the conductivity per pound. For this reason more than 90% of the transmission and distribution lines in the United States are made from aluminum. Aluminum is an excellent conductor of heat as well. It is about 1.8 times as thermally conductive as copper by weight, depending on the alloy, and about 9 times as conductive as stainless steel. For this reason it is widely used in automobile radiators; cooling coils and fins; heat exchangers in the chemical, petroleum, and other industries; and heater fins in baseboard and other types of heaters. Reflectivity and Emissivity Aluminum is an excellent reflector of all forms of radiated energy. This characteristic is commonly put to work in building insulation, including roofing materials. Because it reflects about 90% of radiated heat, aluminum is effective at keeping heat out or in. Aluminum foil can also be used to jam radar by reflecting it. Aluminum gives off as radiant energy only a small percentage (about 7%) of the heat that it does take on. It gives up most of its heat by conduction and convection. This characteristic, known as low emissivity, is especially valuable in installations where aluminum reflects most of any radiant energy rather than absorbing it and radiating it into the interior. Workability Aluminum has a face-centered cubic crystalline structure similar to that of tin and gold. As a consequence, it is very workable. Aluminum is as weldable as steel, at twice the rate of steel. It is also highly amenable to \Tbrazing\t, soldering, and cold welding (pressure joining; see \Twelding and soldering\t). THE ALUMINUM INDUSTRY The aluminum industry, founded in 1854, is the newest of the nonferrous metal industries. In the United States, commercial production began in 1859 at a cost of $17 for a pound. Not until the late 1880s was a method found to bring prices down and permit aluminum to be used in a wide range of applications. The aluminum industry is now worldwide. Bauxite, the Source of Aluminum Most aluminum produced today is made from \Tbauxite\t. First discovered in 1821 near Les Baux, France (from which its name is derived), bauxite is an ore rich in hydrated aluminum oxides, formed by the weathering of such siliceous aluminous rocks as feldspars, nepheline, and clays. During weathering the silicates are decomposed and leached out, leaving behind a residue of ores rich in alumina, iron oxide, titanium oxide, and some silica. In general, economically attractive ores contain at least 45% alumina and no more than 5% to 6% silica. Most of the large bauxite deposits are found in tropical and subtropical climates, where heavy rainfall, warm temperatures, and good drainage combine to encourage the weathering process. Because bauxite is always found at or near the surface, it is mined by open-pit methods. It is then crushed if necessary, screened, dried, milled, and shipped for processing. Guinea, Brazil, Jamaica, the \TUSSR\t, Suriname, and Yugoslavia lead the world in bauxite production. Assuming that 4 tons of bauxite are required to produce 1 ton of aluminum, bauxite reserves known today are large enough to supply the world with aluminum for several hundred years at current production levels. Other ores that might be used when the high-grade bauxite deposits are exhausted include kaolin, anorthosite, and alunite. Early History of Extraction Processes Although proof of the existence of aluminum as a metal did not exist until the 1800s, clays containing the metallic element were used in Iraq as long ago as 5300 BC to manufacture high-quality pottery. Certain other aluminum compounds such as the "alums" were used widely by Egyptians and Babylonians as early as 2000 BC. Despite these early uses of the "metal of clay," however, it was almost 4,000 years before the metal was freed from its compounds, which made it a commercially usable metal. Credit for first separating aluminum metal from its oxide goes to the Danish physicist Hans Christian \TOersted\t. In 1825 he reported to the Royal Danish Academy that he accomplished this by heating anhydrous aluminum chloride with potassium amalgam and distilling off the mercury. His product was so impure, however, that he did not succeed in determining its physical properties beyond observing a metallic luster. In 1845, after many years of experimentation, Friedrich \TWohler\t succeeded--by substituting potassium for the amalgam--in producing globules of aluminum large enough to allow the determination of some of its properties. In 1854, Henri Sainte-Claire Deville substituted sodium for the relatively expensive potassium and, by using sodium aluminum chloride instead of aluminum chloride, produced the first commercial quantities of aluminum in a small plant near Paris. Bars and various objects made of this metal were shown at the Paris Exposition in 1855, and the ensuing publicity was in large measure responsible for launching the industry. Several plants using essentially the same process were subsequently built in France and England, but none survived for long, in part because of the lack of an economic source of electricity. The invention of the dynamo in 1866 paved the way for the development of modern aluminum processing. In 1886 Charles Martin Hall of Oberlin, Ohio, and Paul L. T. Heroult of France, both 22 years old at the time, discovered and patented almost simultaneously the process by which alumina is dissolved in molten cryolite and decomposed electrolytically. This reduction process, generally known as the Hall-Heroult process, has survived many attempts to supplant it; it remains the only method by which aluminum is produced in commercial quantities today. Two years after Hall and Heroult made their discovery, Karl Joseph Bayer, a German chemist, developed a process that improved the method for making alumina from bauxite ores low in silica content, a step necessary before the Hall-Heroult process could be applied. This achievement completed the foundation for a commercially feasible aluminum industry. Bayer Process: Bauxite to Alumina The Bayer Process of separating alumina from the bauxite ore was patented in 1888 and is still used today. The process begins when bauxite is pulverized by being mixed with soda ash and lime in a ball mill. Water is added to turn the mixture into a slurry, which is drained from the ball mills into tanks or digesters. In these tanks, which are heated by the injection of live steam, the alumina contained in the slurry is liquefied like sugar in ordinary water, then poured into settling tanks. Solids--largely sand, iron, and other elements that do not dissolve--move downward while a coffee-colored liquor remains on top. Cleared of all solids, the liquid is pumped into large, open-topped vats, or precipitators, up to six stories high. There the liquid is agitated, and minuscule alumina crystals begin to form. Agitation causes the crystals to adhere to each other as they slowly sink to the bottom of the precipitators, and they become slightly larger than grains of sugar. They are then pumped into settling tanks and washed again to remove the soda ash and lime solution that was added at the beginning of the process. The final step is to drive off the remaining moisture by passing the alumina, which now resembles white mud, through kilns that heat it to more than 1,000 deg C (1,830 deg F). The sugarlike alumina, now dry and about 99% pure, pours out of the lower end of the tilted kiln and is stored in silos, ready to go into the reduction cells to make the metal. Hall-Heroult Process: Alumina to Aluminum Although the original concept of the Hall-Heroult process has not changed, improvements have been made in a continuing effort to lower production costs in both equipment and materials. The electrolytic, or smelting, process (see \Telectrolysis\t) takes place in reduction cells or pots, of which there may be 1,000 or more in a modern plant. Rectangular in shape, the reduction cells are made of steel with a carbon lining. Generally, two rows of carbon anodes, or electrodes, are suspended overhead from bus bars, which carry the electric power. Cells are filled with molten cryolite maintained at a temperature of about 980 deg C (1,800 deg F). Direct electrical current is passed through the cryolite from the suspended carbon anodes to the cathodes, or bottoms of the carbon lining of the cells, and on into the collector bars, or plates, that are embedded in the bottom of the carbon lining. The energy causes a crust to form over the top of the molten cryolite. Alumina is then added to the crust from supply bins above the cells. The electric current passing through the crust drives off the oxygen atoms in the alumina, leaving the aluminum atoms in a molten state. They collect as molten aluminum at the bottoms of the cells. The oxygen atoms adhere to the carbon anodes and gradually erode them. Spent anodes are removed and replaced on a regular schedule. The process is kept operating continuously by periodically breaking the crusts and adding alumina to the cells. At regular intervals, the molten aluminum is siphoned off from the cells into a crucible lined with high-refractory brick. It is then either transported to a holding furnace, from which are poured the various forms of ingots, or it may be alloyed before being poured into rolling ingots. Reduction cells are placed on long lines, called potlines, that are connected electrically. The cells operate on from four to six volts; electrical current loads range from 50,000 to 150,000 amperes. Since the process requires direct current, reduction plants located in countries that produce alternating current must include rectifier stations to convert the alternating current to direct current. A single reduction cell can produce about 900 kg (2,000 lb) of 99.5%-pure metal every 24 hours. The impurities consist of traces of silicon and iron not removed by the reduction process. A 99.99%-pure aluminum can be produced from the basic aluminum. The superpure metal is used in the petroleum, electronics, and jewelry industries; however, most aluminum today is produced in alloy forms. The range of useful alloys is constantly being increased through research. Alloys: Aluminum to Aluminum Products Although pure aluminum has good working and forming properties and high ductility, it has low mechanical strength. Before being used in many applications, it must be strengthened by alloying, strain \Thardening\t (cold working), or precipitation hardening (heat treatment). Aluminum alloys are generally divided into two basic types, casting alloys and wrought alloys. Aluminum casting alloys most frequently contain silicon, magnesium, copper, zinc, or nickel, alone or in various combinations. Silicon improves the fluidity and castability of molten aluminum; copper and zinc harden the alloy and increase its strength; magnesium improves corrosion resistance, strength, and machinability; and nickel improves dimensional stability and high-temperature strength. The mechanical properties of aluminum casting alloys vary not only with composition but also as a function of casting conditions and subsequent heat treatment, if any. Heat-treated alloys are generally stronger and more ductile than others. Wrought alloys are alloys that have been mechanically worked after casting. Working operations include \Lforg\ling, rolling, drawing, and extruding. Alloying elements (magnesium, silicon, copper, and others) usually make pure aluminum stronger and harder but also render it less ductile and more difficult to fabricate. Working and heat treatments change these alloys' structure, which in turn determines their corrosion resistance and mechanical properties. Wrought alloys are divided into two basic classes: non-heat-treatable and heat-treatable alloys. The former rely on the hardening effect of such alloying elements as manganese, silicon, iron, and magnesium for their initial strength. They are further strengthened by various degrees of cold working. Heat-treatable alloys, which contain elements such as copper, magnesium, zinc, and silicon, are strengthened by heat treatment and artificial aging, but they may also be cold worked after an initial heat treatment. New Uses for Aluminum Aluminum is increasingly used to conserve energy both in home heating and cooling and in the transportation industry. Aluminum storm doors and windows, insulation backed with aluminum foil, and aluminum siding are excellent insulators. Because vehicle weight is a significant determinant of automobile gas mileage, substituting aluminum for heavier metals in cars saves fuel. Use of aluminum in the manufacture of a car has increased from an average of 38 kg (84 lb) in 1975 to 61 kg (135 lb) in 1983. Drive shafts, suspension parts, and wheels are among the latest car components to be made with aluminum. The container and packaging industry is by far the largest user of aluminum, consuming about 30% of total U.S. production. Most of the aluminum used by the industry has gone into the production of aluminum cans. Several new packaging concepts, however, have recently shown potential for replacing the standard can and other container forms. Milk and other beverages can now be packaged in "aseptic" containers that are made of paper-plastic film-aluminum foil lamination. Liquids in aseptic containers need not be refrigerated and can be kept for a considerable time without spoiling. The retort pouch, originally developed for military field rations, is a flexible package made of an aluminum laminate. Food contaminants cannot enter the sealed pouch, and the contents remain edible for years without refrigeration. Production World production of primary aluminum at the end of the 1980s was about 17 million metric tons (18.7 million U.S. tons) annually. The United States is the world's largest aluminum producer, followed by the \TUSSR\t, Canada, and Australia. Together, these four nations manufacture half the total world production. The United States is also the world's largest aluminum consumer, using over 6.5 million metric tons (7 million U.S. tons) annually. The U.S. aluminum industry consumes 1% of the nation's energy, largely in the form of electricity. The amount of electricity used per pound of metal in smelting, which accounts for about two-thirds of the industry's total energy consumption, has decreased steadily, and today the average is about 7 kWh. Aluminum can be recycled for less than 5% of the energy required for producing virgin metal. U.S. aluminum-can recycling provides over half the aluminum used in making new cans, and recycling of aluminum scrap equals almost half of total aluminum production. H. D. Chambliss Bibliography: Altenpohl, D. G., Aluminum Viewed from Within (1981); Ammen, C. W., Casting Aluminum (1985); Bakker, M., Wiley Encyclopedia of Packaging Technology (1986); Burkin, R. R., Production of Aluminum and Alumina (1987); Dorre, E., et al., Aluminum Processing, Properties, and Applications (1984); Gitelman, H. J., Aluminum and Health (1989); Graham, Ronald, The Aluminum Industry and the Third World (1982); King, F., Aluminum and Its Alloys (1987); King, R. G., Surface Treatment and Finishing of Aluminum (1988); Pampillo, C., and Biloni, H., Aluminum Transformation Technology and Applications (1980); Peck, M. J., ed., World Aluminum Industry in a Changing Era (1988); Sinia, R. J., Aluminum in Packaging (1973); Valeton, I., Bauxites (1972).