Dictionary:
a·lu·mi·num (ə-lū'mə-nəm)
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| Britannica Concise Encyclopedia: aluminum |
For more information on aluminum, visit Britannica.com.
| How Products are Made: How is aluminum made? |
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
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.
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.
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]
| Sci-Tech Encyclopedia: Aluminum |
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 Is22s22p63s23pl. 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.
| Dental Dictionary: aluminum |
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.
| US History Encyclopedia: Aluminum |
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.
| Columbia Encyclopedia: aluminum |
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.
| Veterinary Dictionary: aluminum |
A chemical element, atomic number 13, atomic weight 26.982, symbol Al.
| Wikipedia: Aluminium |
Aluminium (
ˌæljʊˈmɪniəm (help·info), al-yoo-MIN-ee-əm) or aluminum (
/əˈluːmɪnəm/ (help·info), ə-LOO-mi-nəm, see spelling below) is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is the most abundant metal in the Earth's crust, and the third most abundant element therein, after oxygen and silicon. It makes up about 8% by weight of the Earth's solid surface. Aluminium is too reactive chemically to occur in nature as a free metal. Instead, it is found combined in over 270 different minerals.[4] The chief source of aluminium is bauxite ore.
Aluminium is remarkable for its ability to resist corrosion due to the phenomenon of passivation and for the metal's low density. Structural components made from aluminium and its alloys are vital to the aerospace industry and very important in other areas of transportation and building. Its reactive nature makes it useful as a catalyst or additive in chemical mixtures, including being used in ammonium nitrate explosives to enhance blast power.
Contents |
Aluminium is a soft, durable, lightweight, malleable metal with appearance ranging from silvery to dull grey, depending on the surface roughness. Aluminium is nonmagnetic and nonsparking. It is also insoluble in alcohol, though it can be soluble in water in certain forms. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.[5] Aluminium has about one-third the density and stiffness of steel. It is ductile, and easily machined, cast, drawn and extruded.
Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.[5] This corrosion resistance is also often greatly reduced when many aqueous salts are present, particularly in the presence of dissimilar metals.
Aluminium atoms are arranged in a face-centred cubic (fcc) structure. Aluminium has a stacking-fault energy of approximately 200 mJ/m².[6]
Aluminium is one of the few metals that retain full silvery reflectance in finely powdered form, making it an important component of silver paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3000–10000 nm (far IR) regions, while in the 400–700 nm visible range it is slightly outdone by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.[7]
Aluminium is a good thermal and electrical conductor, by weight better than copper. Aluminium is capable of being a superconductor, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss.[8]
Aluminium has nine isotopes, whose mass numbers range from 23 to 30. Only 27Al (stable isotope) and 26Al (radioactive isotope, t1/2 = 7.2 × 105 y) occur naturally; however, 27Al has a natural abundance of 99.9+ %. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of 26Al to 10Be has been used to study the role of transport, deposition, sediment storage, burial times, and erosion on 105 to 106 year time scales.[9] Cosmogenic 26Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further 26Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.[10]
In the Earth's crust, aluminium is the most abundant (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon).[11] Because of its strong affinity to oxygen, however, it is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the Earth's crust, are aluminosilicates. Native aluminium metal can be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes.[12] It also occurs in the minerals beryl, cryolite, garnet, spinel and turquoise.[11] Impurities in Al2O3, such as chromium or cobalt yield the gemstones ruby and sapphire, respectively. Pure Al2O3, known as Corundum, is one of the hardest materials known.[11]
Although aluminium is an extremely common and widespread element, the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite (AlOx(OH)3-2x). Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions.[13] Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica but the primary mining areas for the ore are in Ghana, Indonesia, Jamaica, Russia and Surinam.[14] Smelting of the ore mainly occurs in Australia, Brazil, Canada, Norway, Russia and the United States. Because smelting is an energy-intensive process, regions with excess natural gas supplies (such as the United Arab Emirates) are becoming aluminium refiners.
Although aluminium is the most abundant metallic element in the Earth's crust, it is rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III, emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold.[15][16] The Washington Monument was completed, with the 100 ounce (2.8 kg) aluminium capstone being put in place on December 6, 1884, in an elaborate dedication ceremony. It was the largest single piece of aluminium cast at the time. At that time, aluminium was as expensive as silver.[17] Aluminium has been produced in commercial quantities for just over 100 years.
Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide (Al2O3). For example, direct reduction with carbon, as is used to produce iron, is not chemically possible, since aluminium is a stronger reducing agent than carbon. However there is an indirect carbothermic reduction possible by using carbon and Al2O3 which forms an intermediate Al4C3 and this can further yield aluminum metal at a temperature of 1900–2000°C. This process is still under development. This process costs less energy and yields less CO2 than the Hall-Héroult process.[18] Aluminium oxide has a melting point of about 2,000 °C (3,632 °F). Therefore, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. The operational temperature of the reduction cells is around 950 to 980 °C (1,796 °F). Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a chemical compound of aluminium, sodium, and calcium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville process was the predominant refining technology.)
The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the refined alumina is dissolved in the electrolyte, its ions are free to move around. The reaction at the cathode is:
Here the aluminium ion is being reduced. The aluminium metal then sinks to the bottom and is tapped off, usually cast into large blocks called aluminium billets for further processing.
At the anode, oxygen is formed:
This carbon anode is then oxidized by the oxygen, releasing carbon dioxide:
The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process.
Unlike the anodes, the cathodes are not oxidized because there is no oxygen present, as the carbon cathodes are protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes and metal movement. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.
Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Compare this to the heat of reaction, 31 MJ/kg, and the Gibbs free energy of reaction, 29 MJ/kg.) Reduction line currents for older technologies are typically 100 to 200 kA; state-of-the-art smelters[19] operate at about 350 kA. Trials have been reported with 500 kA cells.
Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Smelters tend to be situated where electric power is both plentiful and inexpensive, such as South Africa, Ghana, the South Island of New Zealand, Australia, the People's Republic of China, the Middle East, Russia, Quebec and British Columbia in Canada, and Iceland.[20]
In 2005, the People's Republic of China was the top producer of aluminium with almost a one-fifth world share, followed by Russia, Canada, and the USA, reports the British Geological Survey.
Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina.[21] Australia produced 62 million tonnes of bauxite in 2005. The Australian deposits have some refining problems, some being high in silica but have the advantage of being shallow and relatively easy to mine.[22]
Aluminium is 100% recyclable without any loss of its natural qualities. Recovery of the metal via recycling has become an important facet of the aluminium industry.
Recycling involves melting the scrap, a process that requires only five percent of the energy used to produce aluminium from ore. However, a significant part (up to 15% of the input material) is lost as dross (ash-like oxide).[23] The dross can undergo a further process to extract aluminum.
Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public awareness.
In Europe aluminium experiences high rates of recycling, ranging from 42% of beverage cans, 85% of construction materials and 95% of transport vehicles.[24]
Recycled aluminium is known as secondary aluminium, but maintains the same physical properties as primary aluminium. Secondary aluminium is produced in a wide range of formats and is employed in 80% of the alloy injections. Another important use is for extrusion.
White dross from primary aluminium production and from secondary recycling operations still contains useful quantities of aluminium which can be extracted industrially.[25] The process produces aluminium billets, together with a highly complex waste material. This waste is difficult to manage. It reacts with water, releasing a mixture of gases (including, among others, hydrogen, acetylene, and ammonia) which spontaneously ignites on contact with air;[26] contact with damp air results in the release of copious quantities of ammonia gas. Despite these difficulties, however, the waste has found use as a filler in asphalt and concrete.[27]
AlH is produced when aluminium is heated in an atmosphere of hydrogen. Al2O is made by heating the normal oxide, Al2O3, with silicon at 1,800 °C (3,272 °F) in a vacuum.[28]
Al2S can be made by heating Al2S3 with aluminium shavings at 1,300 °C (2,372 °F) in a vacuum.[28] It quickly disproportionates to the starting materials. The selenide is made in a parallel manner.
AlF, AlCl and AlBr exist in the gaseous phase when the tri-halide is heated with aluminium. Aluminium halides usually exist in the form AlX3, where X is F, Cl, Br, or I.[28]
Aluminium monoxide, AlO, has been detected in the gas phase after explosion[29] and in stellar absorption spectra.[30]
Fajans' rules show that the simple trivalent cation Al3+ is not expected to be found in anhydrous salts or binary compounds such as Al2O3. The hydroxide is a weak base and aluminium salts of weak acids, such as carbonate, cannot be prepared. The salts of strong acids, such as nitrate, are stable and soluble in water, forming hydrates with at least six molecules of water of crystallization.
Aluminium hydride, (AlH3)n, can be produced from trimethylaluminium and an excess of hydrogen. It burns explosively in air. It can also be prepared by the action of aluminium chloride on lithium hydride in ether solution, but cannot be isolated free from the solvent. Alumino-hydrides of the most electropositive elements are known, the most useful being lithium aluminium hydride, Li[AlH4]. It decomposes into lithium hydride, aluminium and hydrogen when heated, and is hydrolysed by water. It has many uses in organic chemistry, particularly as a reducing agent. The aluminohalides have a similar structure.
Aluminium hydroxide may be prepared as a gelatinous precipitate by adding ammonia to an aqueous solution of an aluminium salt. It is amphoteric, being both a very weak acid, and forming aluminates with alkalis. It exists in various crystalline forms.
Aluminium carbide, Al4C3 is made by heating a mixture of the elements above 1,000 °C (1,832 °F). The pale yellow crystals have a complex lattice structure, and react with water or dilute acids to give methane. The acetylide, Al2(C2)3, is made by passing acetylene over heated aluminium.
Aluminium nitride, AlN, can be made from the elements at 800 °C (1,472 °F). It is hydrolysed by water to form ammonia and aluminium hydroxide. Aluminium phosphide, AlP, is made similarly, and hydrolyses to give phosphine.
Aluminium oxide, Al2O3, occurs naturally as corundum, and can be made by burning aluminium in oxygen or by heating the hydroxide, nitrate or sulfate. As a gemstone, its hardness is only exceeded by diamond, boron nitride, and carborundum. It is almost insoluble in water. Aluminium sulfide, Al2S3, may be prepared by passing hydrogen sulfide over aluminium powder. It is polymorphic.
Aluminium iodide, AlI3, is a dimer with applications in organic synthesis. Aluminium fluoride, AlF3, is made by treating the hydroxide with HF, or can be made from the elements. It consists of a giant molecule which sublimes without melting at 1,291 °C (2,356 °F). It is very inert. The other trihalides are dimeric, having a bridge-like structure.
When aluminium and fluoride are together in aqueous solution, they readily form complex ions such as [AlF(H2O)5]2+, AlF3(H2O)3, and [AlF6]3−. Of these, [AlF6]3− is the most stable. This is explained by the fact that aluminium and fluoride, which are both very compact ions, fit together just right to form the octahedral aluminium hexafluoride complex. When aluminium and fluoride are together in water in a 1:6 molar ratio, [AlF6]3− is the most common form, even in rather low concentrations.
Organometallic compounds of empirical formula AlR3 exist and, if not also polymers, are at least dimers or trimers. They have some uses in organic synthesis, for instance trimethylaluminium.
The presence of aluminium can be detected in qualitative analysis using aluminon.
Aluminium is the most widely used non-ferrous metal.[31] Global production of aluminium in 2005 was 31.9 million tonnes. It exceeded that of any other metal except iron (837.5 million tonnes).[32] Forecast for 2012 is 42–45 million tons, driven by rising Chinese output.[33] Relatively pure aluminium is encountered only when corrosion resistance and/or workability is more important than strength or hardness. A thin layer of aluminium can be deposited onto a flat surface by physical vapour deposition or (very infrequently) chemical vapour deposition or other chemical means to form optical coatings and mirrors. When so deposited, a fresh, pure aluminium film serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation.
Pure aluminium has a low tensile strength, but when combined with thermo-mechanical processing, aluminium alloys display a marked improvement in mechanical properties, especially when tempered. Aluminium alloys form vital components of aircraft and rockets as a result of their high strength-to-weight ratio. Aluminium readily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon (e.g., duralumin). Today, almost all bulk metal materials that are referred to loosely as "aluminium," are actually alloys. For example, the common aluminium foils are alloys of 92% to 99% aluminium.[34]
Some of the many uses for aluminium metal are in:
Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (ANSI) or by names indicating their main alloying constituents (DIN and ISO).
The strength and durability of aluminium alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminium a bad reputation. (See main article)
One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure will eventually occur under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life.
Another important property of aluminium alloys is their sensitivity to heat. Workshop procedures involving heating are complicated by the fact that aluminium, unlike steel, will melt without first glowing red. Forming operations where a blow torch is used therefore requires some expertise, since no visual signs reveal how close the material is to melting. Aluminium alloys, like all structural alloys, also are subject to internal stresses following heating operations such as welding and casting. The problem with aluminium alloys in this regard is their low melting point, which make them more susceptible to distortions from thermally induced stress relief. Controlled stress relief can be done during manufacturing by heat-treating the parts in an oven, followed by gradual cooling—in effect annealing the stresses.
The low melting point of aluminium alloys has not precluded their use in rocketry; even for use in constructing combustion chambers where gases can reach 3500 K. The Agena upper stage engine used a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region.
Compared to copper, aluminium has about 65% of the electrical conductivity by volume, although 200% by weight. Traditionally copper is used as household wiring material. In the 1960s aluminium was considerably cheaper than copper, and so was introduced for household electrical wiring in the United States, even though many fixtures had not been designed to accept aluminium wire. In some cases the greater coefficient of thermal expansion of aluminium causes the wire to expand and contract relative to the dissimilar metal screw connection, eventually loosening the connection. Also, pure aluminium has a tendency to creep under steady sustained pressure (to a greater degree as the temperature rises), again loosening the connection. Finally, Galvanic corrosion from the dissimilar metals increased the electrical resistance of the connection.
All of this resulted in overheated and loose connections, and this in turn resulted in fires. Builders then became wary of using the wire, and many jurisdictions outlawed its use in very small sizes in new construction. Eventually, newer fixtures were introduced with connections designed to avoid loosening and overheating. The first generation fixtures were marked "Al/Cu" and were ultimately found suitable only for copper-clad aluminium wire, but the second generation fixtures, which bear a "CO/ALR" coding, are rated for unclad aluminium wire. To adapt older assemblies, workers forestall the heating problem using a properly-done crimp of the aluminium wire to a short "pigtail" of copper wire. Today, new alloys, designs, and methods are used for aluminium wiring in combination with aluminium termination.
Ancient Greeks and Romans used aluminium salts as dyeing mordants and as astringents for dressing wounds; alum is still used as a styptic. In 1761 Guyton de Morveau suggested calling the base alum alumine. In 1808, Humphry Davy identified the existence of a metal base of alum, which he at first termed alumium and later aluminum (see Etymology section, below).
The metal was first produced in 1825 (in an impure form) by Danish physicist and chemist Hans Christian Ørsted. He reacted anhydrous aluminium chloride with potassium amalgam and yielded a lump of metal looking similar to tin.[36] Friedrich Wöhler was aware of these experiments and cited them, but after redoing the experiments of Ørsted he concluded that this metal was pure potassium. He conducted a similar experiment in 1827 by mixing anhydrous aluminium chloride with potassium and yielded aluminium.[36] Wöhler is generally credited with isolating aluminium (Latin alumen, alum), but also Ørsted can be listed as its discoverer.[37] Further, Pierre Berthier discovered aluminium in bauxite ore and successfully extracted it.[38] Frenchman Henri Etienne Sainte-Claire Deville improved Wöhler's method in 1846, and described his improvements in a book in 1859, chief among these being the substitution of sodium for the considerably more expensive potassium.
(Note: The title of Deville's book is De l'aluminium, ses propriétés, sa fabrication (Paris, 1859). Deville likely also conceived the idea of the electrolysis of aluminium oxide dissolved in cryolite; however, Charles Martin Hall and Paul Héroult might have developed the more practical process after Deville.)
Before the Hall-Héroult process was developed, aluminium was exceedingly difficult to extract from its various ores. This made pure aluminium more valuable than gold.[39] Bars of aluminium were exhibited at the Exposition Universelle of 1855,[40] and Napoleon III was said[citation needed] to have reserved a set of aluminium dinner plates for his most honoured guests.
Aluminium was selected as the material to be used for the apex of the Washington Monument in 1884, a time when one ounce (30 grams) cost the daily wage of a common worker on the project;[41] aluminium was about the same value as silver.
The Cowles companies supplied aluminium alloy in quantity in the United States and England using smelters like the furnace of Carl Wilhelm Siemens by 1886.[42] Charles Martin Hall of Ohio in the U.S. and Paul Héroult of France independently developed the Hall-Héroult electrolytic process that made extracting aluminium from minerals cheaper and is now the principal method used worldwide. The Hall-Heroult process cannot produce Super Purity Aluminium directly. Hall's process,[43] in 1888 with the financial backing of Alfred E. Hunt, started the Pittsburgh Reduction Company today known as Alcoa. Héroult's process was in production by 1889 in Switzerland at Aluminium Industrie, now Alcan, and at British Aluminium, now Luxfer Group and Alcoa, by 1896 in Scotland.[44]
By 1895 the metal was being used as a building material as far away as Sydney, Australia in the dome of the Chief Secretary's Building.
Many navies use an aluminium superstructure for their vessels, however, the 1975 fire aboard USS Belknap that gutted her aluminium superstructure, as well as observation of battle damage to British ships during the Falklands War, led to many navies switching to all steel superstructures. The Arleigh Burke class was the first such U.S. ship, being constructed entirely of steel.
In 2008 the price of aluminium peaked at $1.45/lb in July but dropped to $0.7/lb by December.[45]
The earliest citation given in the Oxford English Dictionary for any word used as a name for this element is alumium, which British chemist and inventor Humphry Davy employed in 1808 for the metal he was trying to isolate electrolytically from the mineral alumina. The citation is from the journal Philosophical Transactions of the Royal Society of London: "Had I been so fortunate as to have obtained more certain evidences on this subject, and to have procured the metallic substances I was in search of, I should have proposed for them the names of silicium, alumium, zirconium, and glucium ."[46][47]
Davy had settled on aluminum by the time he published his 1812 book Chemical Philosophy: "This substance appears to contain a peculiar metal, but as yet Aluminum has not been obtained in a perfectly free state, though alloys of it with other metalline substances have been procured sufficiently distinct to indicate the probable nature of alumina. "[48] But the same year, an anonymous contributor to the Quarterly Review, a British political-literary journal, in a review of Davy's book, objected to aluminum and proposed the name aluminium, "for so we shall take the liberty of writing the word, in preference to aluminum, which has a less classical sound."[49]
The -ium suffix conformed to the precedent set in other newly discovered elements of the time: potassium, sodium, magnesium, calcium, and strontium (all of which Davy had isolated himself). Nevertheless, -um spellings for elements were not unknown at the time, as for example platinum, known to Europeans since the sixteenth century, molybdenum, discovered in 1778, and tantalum, discovered in 1802. The -um suffix is consistent with the universal spelling alumina for the oxide, as lanthana is the oxide of lanthanum, and magnesia, ceria, and thoria are the oxides of magnesium, cerium, and thorium respectively.
The spelling used throughout the 19th century by most U.S. chemists ended in -ium, but common usage is less clear.[50] The -um spelling is used in the Webster's Dictionary of 1828. In his advertising handbill for his new electrolytic method of producing the metal 1892, Charles Martin Hall used the -um spelling, despite his constant use of the -ium spelling in all the patents[43] he filed between 1886 and 1903.[51] It has consequently been suggested that the spelling reflects an easier to pronounce word with one fewer syllable, or that the spelling on the flier was a mistake. Hall's domination of production of the metal ensured that the spelling aluminum became the standard in North America; the Webster Unabridged Dictionary of 1913, though, continued to use the -ium version.
In 1926, the American Chemical Society officially decided to use aluminum in its publications; American dictionaries typically label the spelling aluminium as a British variant.
Most countries spell aluminium with an i before -um. In the United States, the spelling aluminium is largely unknown, and the spelling aluminum predominates.[52][53] The Canadian Oxford Dictionary prefers aluminum, whereas the Australian Macquarie Dictionary prefers aluminium.
The International Union of Pure and Applied Chemistry (IUPAC) adopted aluminium as the standard international name for the element in 1990, but three years later recognized aluminum as an acceptable variant. Hence their periodic table includes both.[54] IUPAC officially prefers the use of aluminium in its internal publications, although several IUPAC publications use the spelling aluminum.[55]
Despite its natural abundance, aluminium has no known function in living cells and presents some toxic effects in elevated concentrations. Its toxicity can be traced to deposition in bone and the central nervous system, which is particularly increased in patients with reduced renal function. Because aluminium competes with calcium for absorption, increased amounts of dietary aluminium may contribute to the reduced skeletal mineralization (osteopenia) observed in preterm infants and infants with growth retardation. In very high doses, aluminium can cause neurotoxicity, and is associated with altered function of the blood-brain barrier.[56] A small percentage of people are allergic to aluminium and experience contact dermatitis, digestive disorders, vomiting or other symptoms upon contact or ingestion of products containing aluminium, such as deodorants or antacids. In those without allergies, aluminium is not as toxic as heavy metals, but there is evidence of some toxicity if it is consumed in excessive amounts.[57] Although the use of aluminium cookware has not been shown to lead to aluminium toxicity in general, excessive consumption of antacids containing aluminium compounds and excessive use of aluminium-containing antiperspirants provide more significant exposure levels. Studies have shown that consumption of acidic foods or liquids with aluminium significantly increases aluminium absorption,[58] and maltol has been shown to increase the accumulation of aluminium in nervous and osseus tissue.[59] Furthermore, aluminium increases estrogen-related gene expression in human breast cancer cells cultured in the laboratory.[60] These salts' estrogen-like effects have led to their classification as a metalloestrogen.
Because of its potentially toxic effects, aluminium's use in some antiperspirants, dyes (such as aluminum lake), and food additives is controversial. Although there is little evidence that normal exposure to aluminium presents a risk to healthy adults,[61] several studies point to risks associated with increased exposure to the metal[62]. Aluminium in food may be absorbed more than aluminium from water.[63] Some researchers have expressed concerns that the aluminium in antiperspirants may increase the risk of breast cancer,[64] and aluminium has controversially been implicated as a factor in Alzheimer's disease.[65] The Camelford water pollution incident involved a number of people consuming aluminium sulphate. Investigations of the long-term health effects are still ongoing, but elevated brain aluminium concentrations have been found in post-mortem examinations of victims who have later died, and further research to determine if there is a link with cerebral amyloid angiopathy has been commissioned.[66]
According to The Alzheimer's Society, the overwhelming medical and scientific opinion is that studies have not convincingly demonstrated a causal relationship between aluminium and Alzheimer's disease.[67] Nevertheless, some studies, such as those on the PAQUID cohort[68], cite aluminium exposure as a risk factor for Alzheimer's disease. Some brain plaques have been found to contain increased levels of the metal[69]. Research in this area has been inconclusive; aluminium accumulation may be a consequence of the disease rather than a causal agent. In any event, if there is any toxicity of aluminium, it must be via a very specific mechanism, since total human exposure to the element in the form of naturally occurring clay in soil and dust is enormously large over a lifetime.[70][71] Scientific consensus does not yet exist about whether aluminium exposure could directly increase the risk of Alzheimer's disease.[67]
Aluminium is primary among the factors that reduce plant growth on acid soils. Although it is generally harmless to plant growth in pH-neutral soils, the concentration in acid soils of toxic Al3+ cations increases and disturbs root growth and function.[72][73][74]
Most acid soils are saturated with aluminium rather than hydrogen ions. The acidity of the soil is therefore a result of hydrolysis of aluminium compounds.[75] This concept of "corrected lime potential"[76] to define the degree of base saturation in soils became the basis for procedures now used in soil testing laboratories to determine the "lime requirement"[77] of soils.[78]
Wheat's adaptation to allow aluminium tolerance is such that the aluminium induces a release of organic compounds that bind to the harmful aluminium cations. Sorghum is believed to have the same tolerance mechanism. The first gene for aluminium tolerance has been identified in wheat. It was shown that sorghum's aluminium tolerance is controlled by a single gene, as for wheat.[79] This is not the case in all plants.
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| H | He | ||||||||||||||||||||||||||||||||||||||||
| Li | Be | B | C | N | O | F | Ne | ||||||||||||||||||||||||||||||||||
| Na | Mg | Al | Si | P | S | Cl | Ar | ||||||||||||||||||||||||||||||||||
| K | Ca | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn | Ga | Ge | As | Se | Br | Kr | ||||||||||||||||||||||||
| Rb | Sr | Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd | In | Sn | Sb | Te | I | Xe | ||||||||||||||||||||||||
| Cs | Ba | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg | Tl | Pb | Bi | Po | At | Rn | ||||||||||
| Fr | Ra | Ac | Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Uub | Uut | Uuq | Uup | Uuh | Uus | Uuo | ||||||||||
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This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
| Translations: Aluminium |
idioms:
Français (French)
n. - aluminium, d'aluminium, en aluminium
Deutsch (German)
n. - Aluminium
Ελληνική (Greek)
n. - (χημ.) αργίλιο, αλουμίνιο
idioms:
Italiano (Italian)
alluminio, di alluminio
Português (Portuguese)
n. - alumínio (m) (Quím.)
idioms:
idioms:
Español (Spanish)
n. - aluminio, de aluminio
Svenska (Swedish)
n. - aluminium
中文(简体)(Chinese (Simplified))
铝
idioms:
中文(繁體)(Chinese (Traditional))
n. - 鋁
idioms:
idioms:
العربيه (Arabic)
(الاسم) المنيوم, محتوي على الألمنيوم
עברית (Hebrew)
n. - חמרן, אלומיניום (יסוד, LA, מס' אטומי 31)
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