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Aluminium (or aluminum) is a chemical element in the boron group with symbol Aland atomic number 13. It is a silvery white, soft, ductile metal. Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal, in theEarth's crust. It makes up about 8% by weight of the Earth's solid surface. Aluminium metal is so chemically reactive that native specimens are rare and limited to extremereducing environments. Instead, it is found combined in over 270 different minerals.^[5] The chief ore of aluminium is bauxite.

Aluminium is remarkable for the metal's low density and for its ability to resist corrosiondue to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas oftransportation and structural materials. The most useful compounds of aluminium, at least on a weight basis, are the oxides and sulfates.

Despite its prevalence in the environment, aluminium salts are not known to be used by any form of life. In keeping with its pervasiveness, aluminium is well tolerated by plants and animals.^[6] Owing to their prevalence, potential beneficial (or otherwise) biological roles of aluminium compounds are of continuing interest.

Characteristics

Etched surface from a high purity (99.9998%) aluminium bar, size 55×37 mm

Physical

Aluminium is a relatively soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. A fresh film of aluminium serves as a good reflector (approximately 92%) of visible lightand an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.^[7]Aluminium has about one-third the density and stiffness of steel. It is easily machined,cast, drawn and extruded.

Aluminium atoms are arranged in a face-centered cubic (fcc) structure. Aluminium has astacking-fault energy of approximately 200 mJ/m².^[8]

Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper's density. 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 (10milliteslas).^[9]

Chemical

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.^[7] This corrosion resistance is also often greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.

Owing to its resistance to corrosion, aluminium is one of the few metals that retain silvery reflectance in finely powdered form, making it an important component of silver-colored paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3,000–10,000 nm (far IR) regions; in the 400–700 nm visible range it is slightly outperformed by tin and silver and in the 700–3000 (near IR) by silver,gold, and copper.^[10]

Aluminium is oxidized by water to produce hydrogen and heat:

2 Al + 3 H₂O → Al₂O₃ + 3 H₂

This conversion is of interest for the production of hydrogen. Challenges include circumventing the formed oxide layer which inhibits the reaction and the expenses associated with the storage of energy by regeneration of the Al metal.^[11]

Isotopes

Main article: Isotopes of aluminium

Aluminium has many known isotopes, whose mass numbers range from 21 to 42; however, only ²⁷Al (stable isotope) and ²⁶Al (radioactive isotope, t_1/2 = 7.2×10⁵ y) occur naturally. ²⁷Al has a natural abundance above 99.9%. ²⁶Al is produced from argon in theatmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartzin 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 10⁶year time scales.^[12] 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 substantial²⁶Al production. After falling to Earth, atmospheric shielding drastically reduces 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 someasteroids after their formation 4.55 billion years ago.^[13]

Production and refinement

Bauxite, a major aluminium ore. The red-brown colour is due to the presence of ironminerals.

Aluminium forms strong chemical bonds with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the high reactivity of aluminium and the high melting point of most of its ores. For example, direct reduction with carbon, as is used to produce iron, is not chemically possible because aluminium is a stronger reducing agent than carbon. Indirect carbothermic reduction can be carried out using carbon and Al₂O₃, which forms an intermediate Al₄C₃ and this can further yield aluminium metal at a temperature of 1900–2000 °C. This process is still under development; it requires less energy and yields less CO₂ than the Hall-Héroult process, the major industrial process for aluminium extraction.^[19] Electrolytic smelting of alumina was originally cost-prohibitive in part because of the high melting point of alumina, or aluminium oxide, (about 2,000 °C(3,600 °F)). Many minerals, however, will dissolve into a second already molten mineral, even if the temperature of the melt is significantly lower than the melting point of the first mineral. Molten cryolite was discovered to dissolve alumina at temperatures significantly lower than the melting point of pure alumina without interfering in the smelting process. In the Hall-Héroult process, alumina is first dissolved into molten cryolite with calcium fluoride and then electrolytically reduced to aluminium at a temperature between 950 and 980 °C (1,740 to 1,800 °F). Cryolite is a chemical compound of aluminium and sodium fluorides: (Na₃AlF₆). Although cryolite is found as a mineral in Greenland, its synthetic form is used in the industry. The aluminium oxide itself is obtained by refining bauxite in the Bayer process.

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, it disassociates and its ions are free to move around. The reaction at the cathode is:

Al³⁺ + 3 e⁻ → Al

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:

2 O²⁻ → O₂ + 4 e⁻

To some extent, the carbon anode is consumed by subsequent reaction with oxygen to form carbon dioxide. The anodes in a reduction cell must therefore 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, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

World production trend of aluminium

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-hoursper 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 kiloamperes; state-of-the-art smelters operate at about 350 kA. Trials have been reported with 500 kA cells.^{[citation needed]}

The Hall-Heroult process produces aluminium with a purity of above 99%. Further purification can be done by the Hoope process. The process involves the electrolysis of molten aluminium with a sodium, barium and aluminium fluoride electrolyte. The resulting aluminium has a purity of 99.99%.^[20]^[21]

Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Aluminium production consumes roughly 5% of electricity generated in the U.S.^[22] Smelters tend to be situated where electric power is both plentiful and inexpensive, such as the United Arab Emirates with excess natural gas supplies and Iceland and Norway with energy generated from renewable sources. The world's largest smelters of alumina are People's Republic of China, Russia, and Quebec andBritish Columbia in Canada.^[22]^[23]^[24]

Aluminium spot price 1987 2012

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 (before being overtaken by China in 2007).^[23]^[25] Australia produced 68 million tonnes of bauxite in 2010. The Australian deposits have some refining problems, some being high in silica, but have the advantage of being shallow and relatively easy to mine.^[26]