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Aluminium is a silvery white member of the boron group of chemical elements. Ithas the symbol Al, and its atomic number is 13. It is not soluble in water undernormal circumstances The chief ore of aluminium is bauxite.Aluminium is remarkable for the metals low density and for its ability to resistcorrosion due to the phenomenon of passivation. Structural components made fromaluminium and its alloys are vital to the aerospace industry and are important inother areas of transportation and structural materials. The most useful compoundsof aluminium, at least on a weight basis, are the oxides and sulfates.Aluminium is a soft, durable, lightweight, ductile and malleable metal withappearance ranging from silvery to dull gray, depending on the surface roughness.Aluminium is nonmagnetic and does not easily ignite.. The yield strength of purealuminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness ofsteel. It is easily machined, cast, drawn and extruded.Corrosion resistance can be excellent due to a thin surface layer of aluminium oxidethat forms when the metal is exposed to air, effectively preventing furtheroxidation. The strongest aluminium alloys are less corrosion resistant due togalvanic reactions with alloyed copper. This corrosion resistance is also oftengreatly reduced when many aqueous salts are present, particularly in the presence ofdissimilar metals.Production and refinementBauxite, a major aluminium ore. The red-brown colour is due to the presenceof iron minerals.
Aluminium forms strong chemical bonds with oxygen. Compared to most othermetals, it is difficult to extract from ore, such as bauxite, due to the energy requiredto reduce aluminium oxide (Al2O3). For example, direct reduction with carbon, as isused to produce iron, is not chemically possible, since aluminium is a strongerreducing agent than carbon. There is an indirect carbothermic reduction possible byusing carbon and Al2O3, which forms an intermediate Al4C3 and this can furtheryield aluminium metal at a temperature of 1900–2000°C. This process is still underdevelopment. This process costs less energy and yields less CO2 than theHall-Héroult process, the major industrial process for aluminiumextraction. Aluminium oxide has a melting point of about 2,000 °C. Therefore, itmust be extracted by electrolysis. In this process, the aluminium oxide is dissolvedin molten cryolite with calcium fluoride and then reduced to the pure metal. Theoperational temperature of the reduction cells is around 950 to 980 °C . Cryolite isfound as a mineral in Greenland, but in industrial use it has been replaced by asynthetic substance. Cryolite is a chemical compound of aluminium and sodiumfluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refiningbauxite in the Bayer process of Karl Bayer.The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrousaluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminiumoxide are carbon. Once the refined alumina is dissolved in the electrolyte, its ions are free to movearound. The reaction at the cathode is:Al3+ + 3 e− → AlHere the aluminium ion is being reduced. The aluminium metal then sinks to the bottom and istapped off, usually cast into large blocks called aluminium billets for further processing.At the anode, oxygen is formed:2 O2− → O2 + 4 e−To some extent, the carbon anode is degraded by the oxygen. The anodes in a reduction cell musttherefore be replaced regularly, since they are consumed in the process. The cathodes do erode,mainly due to electrochemical processes and metal movement. After five to ten years, dependingon 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 lotof energy, but alternative processes were always found to be less viableeconomically and/or ecologically. The worldwide average specificenergy consumption is approximately 15±0.5 kilowatt-hours perkilogram of aluminium produced (52 to 56 MJ/kg). The most modernsmelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Comparethis to the heat of reaction, 31 MJ/kg, and the Gibbs free energy ofreaction, 29 MJ/kg.) Reduction line currents for older technologies aretypically 100 to 200 kiloamperes; state-of-the-art smelters operate atabout 350 kA. Trials have been reported with 500 kA cells.The Hall-Heroult process produces aluminium with a purity of above99%. Further purification can be done by the Hoope process. The processinvolves the electrolysis of molten aluminium with a sodium, barium andaluminium fluoride electrolyte. The resulting aluminium has a purity of99.99%. General useAluminium is the most widely used non-ferrous metal. Global production of aluminium in2005 was 31.9 million tonnes. It exceeded that of any other metal except iron (837.5 milliontonnes). Forecast for 2012 is 42–45 million tons, driven by rising Chinese output.Aluminium is almost always alloyed, which markedly improves its mechanical properties,especially when tempered. For example, the common aluminium foils and beverage cans arealloys of 92% to 99% aluminium. The main alloying agents are copper, zinc, magnesium,manganese, and silicon (e.g., duralumin) and the levels of these other metals are in the range of afew percent by weight.
Household aluminium foilAluminium-bodied Austin "A40 Sports" (circa 1951)Aluminium slabs being transported from a smelterSome of the many uses for aluminium metal are in: Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles, etc.) as sheet, tube, castings, etc. Packaging (cans, foil, etc.) Construction (windows, doors, siding, building wire, etc.) A wide range of household items, from cooking utensils to baseball bats, watches. Street lighting poles, sailing ship masts, walking poles, etc. Outer shells of consumer electronics, also cases for equipment e.g. photographic equipment. Electrical transmission lines for power distribution MKM steel and Alnico magnets Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs. Heat sinks for electronic appliances such as transistors and CPUs. Substrate material of metal-core copper clad laminates used in high brightness LED lighting. Powdered aluminium is used in paint, and in pyrotechnics such as solid rocket fuels and thermite. Aluminium can be reacted with hydrochloric acid or with sodium hydroxide to produce hydrogen gas. A variety of countries, including France, Italy, Poland, Finland, Romania, Israel, and the former Yugoslavia, have issued coins struck in aluminium or aluminium-copper alloys.
Some guitar models sports aluminium diamond plates on the surface of the instruments, usually either chrome or black. Kramer Guitars and Travis Bean are both known for having produced guitars with necks made of aluminium, which gives the instrument a very distinct sound. Sustainability of Aluminium in Buildings After prospectors mine bauxite deposits, the ore is processed to separate the component elements of aluminum oxide and iron oxide. The Bayer process, developed in 1888 by an Austrian chemist, is used to filter out the iron oxide and precipitate the aluminum oxide, resulting in a fine white powder, which is pure aluminum oxide, also called alumina. With a melting point of 2,054 degrees Celsius, alumina is extremely hard and has many commercial uses. It can be used: o As an abrasive o In cutting tools o To purify water o To make ceramics o To make building materials The primary use of alumina, however, is to extract aluminum. Until 1886, chemists were able to extract only small amounts of aluminum by displacing it with more reactive metals, which made it commercially unviableAlumina or aluminum oxide, Al2O3, chemical compound with m.p. about 2,000°Cand sp. gr. about 4.0.It is insoluble in water and organic liquids and very slightly soluble in strong acidsand alkalies.Alumina occurs in two crystalline forms. Alpha alumina is composed of colorlesshexagonal crystals with the properties given above; gamma alumina is composed ofminute colorless cubic crystals with sp. gr. about 3.6 that are transformed to thealpha form at high temperatures.Alumina powder is formed by crushing crystalline alumina; it is white when pure.Alumina is widely distributed in nature. Combined with silica and other minerals itoccurs in clays, feldspars, and micas. It is the major component of bauxite andoccurs in an almost pure form as corundum. Alumina is commerciallyimportant. A major use is in the production of aluminum metal. Itis also used for abrasives; corundum and emery are widely used, as are artificially
prepared alumina abrasives. Trade names for alumina abrasives include Alundumand Aloxite. Alumina is also used in ceramics, in pigments, and in the manufactureof chemicals. Clays containing alumina are used in porcelain, pottery, and bricks.Pure alumina is used in making crucibles and other refractory apparatus. Hydratedalumina is used in mordant dyeing to make lake pigments; it is also used inglassmaking, in cosmetics, and in medicine as an antacid.Any of several forms of aluminum oxide, Al2O3, occurring naturally as corundum, ina hydrated form in bauxite, and with various impurities as ruby, sapphire, andemery, used in aluminum production and in abrasives, refractories, ceramics, andelectrical insulation. Also called aluminum oxide.