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Aluminium | |||
These alloys form vital components of aircraft and rockets. When aluminium is evaporated in a vacuum it forms a coating that reflects both visible light and radiant heat. These coatings form a thin layer of protective aluminium oxide that doesn't deteriorate as silver coatings do. Coating telescope mirrors is another use of this metal.
Some of the many uses for aluminium are in
Its oxide, alumina, is found naturally as corundum, emery[?], ruby, and sapphire and is used in glass making. Synthetic ruby and sapphire are used in lasers for the production of coherent light[?].
Aluminium oxidizes very energetically and as a result has found use in solid rocket fuels and thermite.
Ancient Greeks and Romans used this metal as dyeing mordant[?] and as an astringent to bind wounds, and alum is still used as a styptic. In 1761 Guyton de Morveau[?] proposed calling the base alum alumine.
Recovery of this metal from scrap (via recycling) has become an important component of the aluminium industry. A common practice since the early 1900s, aluminium recycling isn't new. It was, however, a low-profile activity until the late 1960s when recycling of aluminium beverage cans[?] finally placed recycling into the public consciousness. Sources for recycled aluminium include automobiles, windows and doors, appliances, containers and other products.
Aluminium is a reactive metal and cannot be extracted from its ore, bauxite (Al2O3), through reduction with carbon. Instead it is extracted by electrolysis — the metal is oxidized in solution and then reduced again to the pure metal. The ore must be in a liquid state for this to occur. However, bauxite has a melting point of 2000°C, which is too high a temperature to achieve economically. Instead, the bauxite for many years was dissolved in molten cryolite, which lowers the melting point to about 900°C. But now, cryolite has been replaced by an artificial mixture of aluminium, sodium, and calcium fluorides. This process still requires a great deal of energy, and aluminium plants usually have their own power stations nearby.
The electrodes used in the electrolysis of bauxite are both carbon. Once the ore is in the molten state, its ions are free to move around. The reaction at the negative cathode is
Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.
The positive anode oxidizes the oxygen of bauxite, which then reacts with the carbon electrode to form carbon dioxide:
This cathode must be replaced often because it turns into carbon dioxide. Despite the cost of electrolysis, aluminium is a cheap and widely used metal. Aluminium can now be extracted from clay, but this process isn't economical.
Cosmogenic Al-26 was first applied in studies of the Moon and meteorites. Meteorite fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial Al-26 production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further Al-26 production, and its decay can then be used to determine the meteorite's terrestrial age.
In 1990 the IUPAC adopted aluminium as the standard international name for the element. Aluminium is also the name used in French, Dutch, German, and Swedish; Italian uses alluminio, Portuguese alumínio and Spanish aluminio. (The use of these words in these other languages is one of the reasons IUPAC chose aluminium over aluminum.) In 1993, IUPAC recognized aluminum as an acceptable variant, but still prefers the use of aluminium.
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