Aluminium Hydroxide (CAS No: 21645-51-2)

Aluminium hydroxide, Al(OH)3, archaically called hydrate of alumina or alumina trihydrate (Al2O3·3H2O), is found in nature as the mineral gibbsite (also known as hydrargillite) and its three, much more rare polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO(OH), and aluminium oxide, Al2O3, differing only by loss of water. These compounds together are the major components of the aluminium ore bauxite. Freshly precipitated aluminium hydroxide forms gels, which is the basis for application of aluminium salts as flocculants in water purification. This gel crystallizes with time. Aluminium hydroxide gels can be dehydrated (e.g., using water-miscible non-aqueous solvents like ethanol) to form an amorphous aluminium hydroxide powder, which is readily soluble in acids. Aluminium hydroxide powder which has been heated to an elevated temperature under carefully controlled conditions is known as activated alumina and is used as a desiccant, an adsorbent, in gas purification, as a Claus catalyst support, water purification, and an adsorbent for the catalyst during the manufacture of polyethylene by the Sclairtech process.

Properties
Gibbsite has a typical metal hydroxide structure with hydrogen bonds. It is built up of double layers of hydroxyl groups with aluminium ions occupying two-thirds of the octahedral holes between the two layers.

Aluminium hydroxide is amphoteric. It dissolves in acid, forming [Al(H2O)6]3+ (hexaaquaaluminium) or its hydrolysis products. It also dissolves in strong alkali, forming [Al(OH)4] (tetrahydroxidoaluminate).

Uses
Annual production is some 100 million tonnes,over 90% of which is converted to aluminium oxide (alumina) that is used in the manufacture of aluminium metal.
The major other uses of aluminium hydroxide is as a feedstock for the manufacture of other aluminium compounds: specialty calcined aluminas, aluminium sulfate, polyaluminium chloride, aluminium chloride, zeolites, sodium aluminate, activated alumina, aluminium nitrate.