Characteristics: A Silvery white, crystalline solid, aluminum is lightweight, ductile and malleable. Aluminum forms a microscopically thin (~4 nm thick) protective oxide coating on exposure to air or water, which makes it highly resistant to ordinary corrosion. Aluminum has about one-third the density and stiffness of steel, and about two-thirds the thermal and electrical conductivity of copper. It is easily machined, cast, drawn and extruded. Tensile strength when annealed is 6800 psi, cold-rolled is 16,000 psi.
Attacked by concentrated and dilute solutions of HCl, hot concentrate H2SO4 and perchloric acid, also violently by strong alkalis. Rapidly oxidized by water at 180 oC. Not attacked by dilute or cold concentrated H2SO4 and concentrated nitric acid.
Derivation: From bauxite by Bayer process and subsequent electrolytic reduction by Hall-Héroult process. There are several processes for obtaining ultrapure aluminum: (a) electrolytic (three-layer); (b) zone refining; and (c) chemical refining. More efficient processes are the Alcoa and Toth processes; these require much less electric power than the Hall-Héroult process; Another method, using no electricity, involves heating a mixture of aluminum ores with a coal-derived fuel in a closed furnace; this yields AlOSi alloys, which are not only useful as such, but can be refined to pure aluminum.
Applications: Aluminum is one of the most widely used elements on earth, for transportation (cars, aircraft, spacecraft, trains, marine craft), packaging, construction, household appliances, consumer electronics, and even in paint in powdered form. Aluminum, evaporated in a vacuum, forms a coating that is highly reflective to both visible light and radiant heat. These coatings soon form a thin layer of the protective oxide and do not deteriorate as do silver coatings. They are used to coat telescope mirrors and to make decorative paper, packages, and toys. Aluminum is usually alloyed with small amounts of copper, magnesium, silicon, manganese, or other elements to improve its mechanical properties.
Hazard: Fine powder and dusts are flammable and form explosive mixtures in air. If powder or dusts come into contact with certain metal oxides (such as iron oxide or copper oxide) a thermite reaction can be initiated with an ignition source. Exposure limits: (respirable) 1 mg/m3.
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