Dictionary:

aluminum

  (ə-lū'mə-nəm)

A silvery-white, ductile metallic element, the most abundant in the earth's crust but found only in combination, chiefly in bauxite. Having good conductive and thermal properties, it is used to form many hard, light, corrosion-resistant alloys. Atomic number 13; atomic weight 26.98; melting point 660.2°C; boiling point 2,467°C; specific gravity 2.69; valence 3.

[ALUMIN(A) + –(I)UM.]

The metallic element aluminum is the third most plentiful element in the earth's crust, comprising 8% of the planet's soil and rocks (oxygen and silicon make up 47% and 28%, respectively). In nature, aluminum is found only in chemical compounds with other elements such as sulphur, silicon, and oxygen. Pure, metallic aluminum can be economically produced only from aluminum oxide ore.

Metallic aluminum has many properties that make it useful in a wide range of applications. It is lightweight, strong, nonmagnetic, and nontoxic. It conducts heat and electricity and reflects heat and light. It is strong but easily workable, and it retains its strength under extreme cold without becoming brittle. The surface of aluminum quickly oxidizes to form an invisible barrier to corrosion. Furthermore, aluminum can easily and economically be recycled into new products.

Background

Aluminum compounds have proven useful for thousands of years. Around 5000 B.C., Persian potters made their strongest vessels from clay that contained aluminum oxide. Ancient Egyptians and Babylonians used aluminum compounds in fabric dyes, cosmetics, and medicines. However, it was not until the early nineteenth century that aluminum was identified as an element and isolated as a pure metal. The difficulty of extracting aluminum from its natural compounds kept the metal rare for many years; half a century after its discovery, it was still as rare and valuable as silver.

In 1886, two 22-year-old scientists independently developed a smelting process that made economical mass production of aluminum possible. Known as the Hall-Heroult process after its American and French inventors, the process is still the primary method of aluminum production today. The Bayer process for refining aluminum ore, developed in 1888 by an Austrian chemist, also contributed significantly to the economical mass production of aluminum.

In 1884, 125 lb (60 kg) of aluminum was produced in the United States, and it sold for about the same unit price as silver. In 1995, U.S. plants produced 7.8 billion lb (3.6 million metric tons) of aluminum, and the price of silver was seventy-five times as much as the price of aluminum.

Raw Materials

Aluminum compounds occur in all types of clay, but the ore that is most useful for producing pure aluminum is bauxite. Bauxite consists of 45-60% aluminum oxide, along with various impurities such as sand, iron, and other metals. Although some bauxite deposits are hard rock, most consist of relatively soft dirt that is easily dug from open-pit mines. Australia produces more than one-third of the world's supply of bauxite. It takes about 4 lb (2 kg) of bauxite to produce 1 lb (0.5 kg) of aluminum metal.

Caustic soda (sodium hydroxide) is used to dissolve the aluminum compounds found in the bauxite, separating them from the impurities. Depending on the composition of the bauxite ore, relatively small amounts of other chemicals may be used in the extraction of aluminum. Starch, lime, and sodium sulphide are some examples.

Cryolite, a chemical compound composed of sodium, aluminum, and fluorine, is used as the electrolyte (current-conducting medium) in the smelting operation. Naturally occurring cryolite was once mined in Greenland, but the compound is now produced synthetically for use in the production of aluminum. Aluminum fluoride is added to lower the melting point of the electrolyte solution.

The other major ingredient used in the smelting operation is carbon. Carbon electrodes transmit the electric current through the electrolyte. During the smelting operation, some of the carbon is consumed as it combines with oxygen to form carbon dioxide. In fact, about half a pound (0.2 kg) of carbon is used for every pound (2.2 kg) of aluminum produced. Some of the carbon used in aluminum smelting is a byproduct of oil refining; additional carbon is obtained from coal.

Because aluminum smelting involves passing an electric current through a molten electrolyte, it requires large amounts of electrical energy. On average, production of 2 lb (1 kg) of aluminum requires 15 kilowatt-hours (kWh) of energy. The cost of electricity represents about one-third of the cost of smelting aluminum.

The Manufacturing
Process

Aluminum manufacture is accomplished in two phases: the Bayer process of refining the bauxite ore to obtain aluminum oxide, and the Hall-Heroult process of smelting the aluminum oxide to release pure aluminum.

The Bayer process

• First, the bauxite ore is mechanically crushed. Then, the crushed ore is mixed with caustic soda and processed in a grinding mill to produce a slurry (a watery suspension) containing very fine particles of ore.
• The slurry is pumped into a digester, a tank that functions like a pressure cooker. The slurry is heated to 230-520°F (110-270°C) under a pressure of 50 lb/in² (340 kPa). These conditions are maintained for a time ranging from half an hour to several hours. Additional caustic soda may be added to ensure that all aluminum-containing compounds are dissolved.
• The hot slurry, which is now a sodium aluminate solution, passes through a series of flash tanks that reduce the pressure and recover heat that can be reused in the refining process.
• The slurry is pumped into a settling tank. As the slurry rests in this tank, impurities that will not dissolve in the caustic soda settle to the bottom of the vessel. One manufacturer compares this process to fine sand settling to the bottom of a glass of sugar water; the sugar does not settle out because it is dissolved in the water, just as the aluminum in the settling tank remains dissolved in the caustic soda. The residue (called "red mud") that accumulates in the bottom of the tank consists of fine sand, iron oxide, and oxides of trace elements like titanium.
• After the impurities have settled out, the remaining liquid, which looks somewhat like coffee, is pumped through a series of cloth filters. Any fine particles of impurities that remain in the solution are trapped by the filters. This material is washed to recover alumina and caustic soda that can be reused.
• The filtered liquid is pumped through a series of six-story-tall precipitation tanks. Seed crystals of alumina hydrate (alumina bonded to water molecules) are added through the top of each tank. The seed crystals grow as they settle through the liquid and dissolved alumina attaches to them.
• The crystals precipitate (settle to the bottom of the tank) and are removed. After washing, they are transferred to a kiln for calcining (heating to release the water molecules that are chemically bonded to the alumina molecules). A screw conveyor moves a continuous stream of crystals into a rotating, cylindrical kiln that is tilted to allow gravity to move the material through it. A temperature of 2,000° F (1,100° C) drives off the water molecules, leaving anhydrous (waterless) alumina crystals. After leaving the kiln, the crystals pass through a cooler.

The Hall-Heroult process

Smelting of alumina into metallic aluminum takes place in a steel vat called a reduction pot. The bottom of the pot is lined with carbon, which acts as one electrode (conductor of electric current) of the system. The opposite electrodes consist of a set of carbon rods suspended above the pot; they are lowered into an electrolyte solution and held about 1.5 in (3.8 cm) above the surface of the molten aluminum that accumulates on the floor of the pot. Reduction pots are arranged in rows (potlines) consisting of 50-200 pots that are connected in series to form an electric circuit. Each potline can produce 66,000-110,000 tons (60,000-100,000 metric tons) of aluminum per year. A typical smelting plant consists of two or three potlines.

• Within the reduction pot, alumina crystals are dissolved in molten cryolite at a temperature of 1,760-1,780° F (960-970° C) to form an electrolyte solution that will conduct electricity from the carbon rods to the carbon-lined bed of the pot. A direct current (4-6 volts and 100,000-230,000 amperes) is passed through the solution. The resulting reaction breaks the bonds between the aluminum and oxygen atoms in the alumina molecules. The oxygen that is released is attracted to the carbon rods, where it forms carbon dioxide. The freed aluminum atoms settle to the bottom of the pot as molten metal.

  The smelting process is a continuous one, with more alumina being added to the cryolite solution to replace the decomposed compound. A constant electric current is maintained. Heat generated by the flow of electricity at the bottom electrode keeps the contents of the pot in a liquid state, but a crust tends to form atop the molten electrolyte. Periodically, the crust is broken to allow more alumina to be added for processing. The pure molten aluminum accumulates at the bottom of the pot and is siphoned off. The pots are operated 24 hours a day, seven days a week.

• A crucible is moved down the potline, collecting 9,000 lb (4,000 kg) of molten aluminum, which is 99.8% pure. The metal is transferred to a holding furnace and then cast (poured into molds) as ingots. One common technique is to pour the molten aluminum into a long, horizontal mold. As the metal moves through the mold, the exterior is cooled with water, causing the aluminum to solidify. The solid shaft emerges from the far end of the mold, where it is sawed at appropriate intervals to form ingots of the desired length. Like the smelting process itself, this casting process is also continuous.

Byproducts/Waste

Alumina, the intermediate substance that is produced by the Bayer process and that constitutes the raw material for the Hall-Heroult process, is also a useful final product. It is a white, powdery substance with a consistency that ranges from that of talcum powder to that of granulated sugar. It can be used in a wide range of products such as laundry detergents, toothpaste, and fluorescent light bulbs. It is an important ingredient in ceramic materials; for example, it is used to make false teeth, spark plugs, and clear ceramic windshields for military airplanes. An effective polishing compound, it is used to finish computer hard drives, among other products. Its chemical properties make it effective in many other applications, including catalytic converters and explosives. It is even used in rocket fuel—400,000 lb (180,000 kg) is consumed in every space shuttle launch. Approximately 10% of the alumina produced each year is used for applications other than making aluminum.

The largest waste product generated in bauxite refining is the tailings (ore refuse) called "red mud." A refinery produces about the same amount of red mud as it does alumina (in terms of dry weight). It contains some useful substances, like iron, titanium, soda, and alumina, but no one has been able to develop an economical process for recovering them. Other than a small amount of red mud that is used commercially for coloring masonry, this is truly a waste product. Most refineries simply collect the red mud in an open pond that allows some of its moisture to evaporate; when the mud has dried to a solid enough consistency, which may take several years, it is covered with dirt or mixed with soil.

Several types of waste products are generated by decomposition of carbon electrodes during the smelting operation. Aluminum plants in the United States create significant amounts of greenhouse gases, generating about 5.5 million tons (5 million metric tons) of carbon dioxide and 3,300 tons (3,000 metric tons) of perfluorocarbons (compounds of carbon and fluorine) each year.

Approximately 120,000 tons (110,000 metric tons) of spent potlining (SPL) material is removed from aluminum reduction pots each year. Designated a hazardous material by the Environmental Protection Agency (EPA), SPL has posed a significant disposal problem for the industry. In 1996, the first in a planned series of recycling plants opened; these plants transform SPL into glass frit, an intermediate product from which glass and ceramics can be manufactured. Ultimately, the recycled SPL appears in such products as ceramic tile, glass fibers, and asphalt shingle granules.

The Future

Virtually all of the aluminum producers in the United States are members of the Voluntary Aluminum Industrial Partnership (VAIP), an organization that works closely with the EPA to find solutions to the pollution problems facing the industry. A major focus of research is the effort to develop an inert (chemically inactive) electrode material for aluminum reduction pots. A titanium-diboride-graphite compound shows significant promise. Among the benefits expected to come when this new technology is perfected are elimination of the greenhouse gas emissions and a 25% reduction in energy use during the smelting operation.

Where to Learn More

Books

Altenpohl, Dietrich. Aluminum Viewed from Within: An Introduction into the Metallurgy of Aluminum Fabrication (English translation). Dusseldorf: Aluminium-Verlag, 1982.

Russell, Allen S. "Aluminum." McGraw-Hill Encyclopedia of Science & Technology. New York: McGraw-Hill, 1997.

Periodicals

Thompson, James V. "Alumina: Simple Chemistry—Complex Plants." Engineering & Mining Journal (February 1, 1995): 42 ff.

Other

Alcoa Aluminum. http://www.alcoa.com/ (March 1999).

Reynolds Metals Company. http://www.reynoldswrap.com/gbu/bauxitealumina/ (April 1999).

[Article by: Loretta Hall]

A metallic chemical element, symbol Al, atomic number 13, atomic weight 26.98154, in group 13 of the periodic system. Pure aluminum is soft and lacks strength, but it can be alloyed with other elements to increase strength and impart a number of useful properties. Alloys of aluminum are light, strong, and readily formable by many metalworking processes; they can be easily joined, cast, or machined, and accept a wide variety of finishes. Because of its many desirable physical, chemical, and metallurgical properties, aluminum has become the most widely used nonferrous metal. See also Periodic table.

Aluminum is the most abundant metallic element on the Earth and Moon but is never found free in nature. The element is widely distributed in plants, and nearly all rocks, particularly igneous rocks, contain aluminum in the form of aluminum silicate minerals. When these minerals go into solution, depending upon the chemical conditions, aluminum can be precipitated out of the solution as clay minerals or aluminum hydroxides, or both. Under such conditions bauxites are formed. Bauxites serve as principal raw materials for aluminum production.

Aluminum is a silvery metal having a density of 1.56 oz/in.³ at 68°F (2.70 g/cm³ at 20°C). Naturally occurring aluminum consists of a single isotope, ²⁷₁₃Al. Aluminum crystallizes in the face-centered cubic structure with edge of the unit lattice cube of 4.0495 angstroms (0.40495 nanometer). Aluminum is known for its high electrical and thermal conductivities and its high reflectivity.

The electronic configuration of the element is Is²2s²2p⁶3s²3p^l. Aluminum exhibits a valence of +3 in all compounds, with the exception of a few high-temperature monovalent and divalent gaseous species.

Aluminum is stable in air and resistant to corrosion by seawater and many aqueous solutions and other chemical agents. This is due to protection of the metal by a tough, impervious film of oxide. At a purity greater than 99.95%, aluminum resists attack by most acids but dissolves in aqua regia. Its oxide film dissolves in alkaline solutions, and corrosion is rapid.

Aluminum is amphoteric and can react with mineral acids to form soluble salts and to evolve hydrogen.

Molten aluminum can react explosively with water. The molten metal should not be allowed to contact damp tools or containers.

At high temperatures aluminum reduces many compounds containing oxygen, particularly metal oxides. These reactions are used in the manufacture of certain metals and alloys.

Applications in building and construction represent the largest single market of the aluminum industry. Millions of homes use aluminum doors, siding, windows, screening, and down-spouts and gutters. Aluminum is also a major industrial building product. Transportation is the second largest market. Many commercial and military aircraft have become virtually all-aluminum. In automobiles, aluminum is apparent in interior and exterior trim, grilles, wheels, air conditioners, automatic transmissions, and some radiators, engine blocks, and body panels. Aluminum is also found in rapid-transit car bodies, rail cars, forged truck wheels, cargo containers, and in highway signs, divider rails, and lighting standards. In aerospace, aluminum is found in aircraft engines, frames, skins, landing gear, and interiors, often making up 80% of a plane's weight. The food packaging industry is a fast-growing market.

In electrical applications, aluminum wire and cable are major products. Aluminum appears in the home as cooking utensils, cooking foil, hardware, tools, portable appliances, air conditioners, freezers, and refrigerators, and in sporting equipment such as skis, ball bats, and tennis rackets.

There are hundreds of chemical uses of aluminum and aluminum compounds. Aluminum powder is used in paints, rocket fuels, and explosives, and as a chemical reductant.

A widely used metallic element and the third most abundant of all the elements. Aluminum is a principal component of many compounds used in antacids, antiseptics, astringents, and styptics. Aluminum hydroxychloride is the most commonly used agent in antiperspirants.

For more information on aluminum, visit Britannica.com.

Aluminum, the most useful of the nonferrous metals, was first isolated in metallic form in 1825 by Hans Christian Oersted in Denmark. The metal remained a laboratory curiosity until 1854, when Henri Sainte-Claire Deville discovered a process using metallic sodium as a reductant that led to the first commercial production of aluminum. The price of the metal fell from $545 per pound in 1852 to $8 in 1885, and uses for the lightweight metal began to increase greatly. Emperor Napoleon III of France, for example, considered outfitting his army with lightweight aluminum armor and equipment, but the price of the metal remained too high for widespread use.

In 1886, an American, Charles Martin Hall, and a Frenchman, Paul Héroult, independently discovered that aluminum could be produced by electrolyzing a solution of aluminum oxide in molten cryolite (sodium aluminum fluoride). The electrolytic process won immediate acceptance by the commercial industry and in 2002 remained the sole commercial method used for making aluminum.

Hall's invention led to the formation of the Pittsburgh Reduction Company in 1888. This company, now known as Alcoa (for Aluminum Company of America), initially produced fifty pounds of aluminum per day, becoming by the turn of the twentieth century the world's largest producer of aluminum, a position it still enjoys in 2002. A more diverse aluminum industry developed in Europe. Within ten years, firms operated in Switzerland, Germany, Austria, France, and Scotland—all having obtained rights to Héroult's patents to make the metal. By 1900 total world production was about 7,500 short tons; American production was 2,500 tons.

The advent of the airplane in World War I greatly increased demand for the lightweight metal. In 1918 the primary capacity in the United States had grown to 62,500 short tons; world production amounted to 143,900 tons. Steady growth of the aluminum industry continued, and in 1939 the United States produced 160,000 tons of the 774,000 tons produced worldwide. The airplane became a key factor in waging World War II, and aluminum production throughout the world tripled; in the United States it grew sixfold. Another major period of growth in the industry took place during the Korean War, when the United States produced almost half of the world total of 3,069,000 tons. In 1972 total world production of aluminum came to some 12 million tons, but the American share, produced by twelve companies, had dropped to 34 percent, or 4,122,000 tons. By 2000, the aluminum industry in the United States operated more than three hundred plants in thirty-five states, employed more than 145,000 people, and produced an average of 11.5 million tons of aluminum annually.

Aluminum is the most abundant metallic element in the earth's crust. It is made from the mineral bauxite (hydrated aluminum oxide), which is found in plentiful supply throughout the tropical areas of the world. Five countries, Jamaica, Surinam, Guyana, Guinea, and Australia, mined about 61 percent of the world's supplies in 1972, with the remainder coming from twenty-two other countries. At the end of the twentieth century, the U.S. aluminum industry relied to a roughly equivalent degree on production from domestic ore materials (34.3 percent of production in 2000), imported ingots and mill products (33.5 percent), and recycled scrap materials (32.2 percent).

The great growth in the use of aluminum metal indicates its versatility. It has a unique combination of useful properties: lightness, good thermal and electrical conductivity, high reflectivity, malleability, resistance to corrosion, and excellent tensile strength in alloyed form. It is extensively employed in building and construction, where each new house uses almost four hundred pounds of the metal for such items as windows, doors, and siding. Another major market is transportation: the average automobile uses almost eighty pounds of aluminum, and truck and railroad car bodies use aluminum extensively because each pound of weight saved permits an extra pound of revenue-producing payload. The aerospace industries are also large consumers of aluminum. There are many electrical applications because it is one-third as heavy and roughly two-thirds as conductive as copper. Applications for the metal are also growing rapidly for containers and packaging, where it is used in cans, foil, and frozen-food containers. Indeed, the metal's versatility suggests countless possible applications.

Bibliography

Van Horn, Kent R., ed. Prepared by engineers, scientists, and metallurgists of Aluminum Company of America. Aluminum. Vol. 2, Design and Application. Metals Park, Ohio: American Society for Metals, 1967.

—Kenneth B. Higbie/C. W.

A chemical element, atomic number 13, atomic weight 26.982, symbol Al.

• a. acetate — a preparation of aluminum subacetate and glacial acetic acid, used for its antiseptic and astringent action on the skin. Called also Burow's solution.
• a. binding agents — usually includes aluminum carbonate and hydroxide. See phosphate binders.
• a. chloride — a deliquescent, crystalline powder used topically as an astringent solution and antiperspirant.
• a. equivalent — a radiological measurement expressing the thickness of aluminum that produces the same attenuation of the x-ray beam as the thickness of the material being examined.
• a. factory — prime source of fluorine pollution of pasture.
• a. filter — inserted in the window of x-ray tubes to filter out x-rays of long wavelength; reduces potentially harmful and unnecessary radiation.
• a. hydroxide, a. phosphate — aluminum preparations, available in suspension, as a gel, or in dried form, used as an antacid in the treatment of peptic ulcer in humans and gastric hyperacidity and in phosphate binders.
• a. poisoning — pollution of pasture occurs from dust from factories handling aluminum products and in acid rain near such industrial works; contributes to nutritional deficiency of phosphorus by interfering with phosphorus absorption. Bodies of water which receive drainage from soils rich in aluminum may experience fish kills in circumstances in which the amount of aluminum is increased.
• a. sulfate — see alum.