Aluminum is the most abundant metal in the Earth's crust and the second most produced metal after iron and steel, permeating nearly every corner of modern life. Construction and infrastructure is a major outlet: aluminum-alloy window and door profiles, curtain-wall panels, ceiling framing and roofing systems are light and rust-free, and are widely used in high-rise buildings and bridges. Transportation is another pillar: from bicycle frames, automobile wheels and body panels, to the carbodies of urban rail trains and subway cars, and further to the skin and wing structures of civil airliners, the demand to reduce weight and save energy keeps pushing aluminum consumption higher. Packaging is almost dominated by aluminum—beverage cans, foil containers, pharmaceutical blister packs and food flexible packaging exploit aluminum’s non-toxicity, good barrier properties, light weight and easy formability. In the power industry, high-voltage transmission lines use steel-reinforced aluminum conductors that are far lighter than copper for the same conductivity, allowing longer tower spans. Aluminum cooking pots, radiators, heat sinks for computers and LEDs, as well as aluminum-alloy bicycles and mountaineering gear, are everyday examples. Aluminum is everywhere not only because of its abundance, but also because it naturally forms a dense oxide film on its surface and so resists corrosion, and it can be die-cast, extruded and recycled, with an extremely high recycling rate.
Aluminum oxide (Al2O3) is the main constituent of corundum; its single crystals are ruby and sapphire. Next to diamond, it is the hardest mineral, and is used as abrasive, sandpaper grit and ceramic cutting-tool inserts. High-purity ultrafine alumina is the substrate material for LEDs, the coating for lithium-battery separators, and the bioceramic for artificial joints; it is also the direct feedstock for electrolytic aluminum production. Aluminum hydroxide (Al(OH)3) decomposes endothermically on heating and loses water; it is a very widely used inorganic flame retardant and smoke suppressant in plastics, rubber and wire and cables, and is also used as a paper filler and as an antacid ingredient. Alum—potassium aluminum sulfate, KAl(SO4)2—has traditionally been used as a flocculant for water purification, in food leavening and in leather tanning, and externally in traditional Chinese medicine as an astringent; in modern drinking-water and wastewater treatment it is also commonly used in the form of aluminum sulfate or polyaluminum chloride as a flocculant. In addition, high-purity alumina is used as a polishing micro-powder and as a mild abrasive in toothpaste, and aluminum hydroxide gel is a common antacid ingredient in treatments for gastric hyperacidity.
