Aluminum oxide

UsesAluminum oxide is used for the separation of both inorganic anions and acidic organic molecules such as acidic amino acids, aromatic acids and carboxylic acids. It is essential in protein extraction as a component in the preparation of tissues. It is also used as a grinding or blending agent. Aluminum oxide is a source of aluminum in reactions, an abrasive agent, and as a refractory material. Also Aluminum oxide can act as a catalyst in a variety of reactions such as the claus process and in the dehydration of alcohols to alkenes. In single crystals of aluminum oxide intrinsic diffusion occurs in a high temperature region.

PreparationPure alumina, needed to produce aluminum by the Hall process, is made by the Bayer process. The starting material is bauxite (Al2O3 nH2O). The ore contains impurities, such as, SiO2, Fe2O3, TiO2, and Na2O. Most impurities are removed following treatment with caustic soda solution. Bauxite is dissolved in NaOH solution. Silica, iron oxides and other impurities are filtered out of the solution. CO2 is then bubbled through this solution. This precipitates out hydrated alumina, which is heated to remove water and produce Al2O3. These impurities are removed. Calcinations of bauxite produce alumina of abrasive and refractory grades. Activated aluminas of amorphous type, as well as the transition aluminas of γ, η, χ, and ρ forms, are obtained from various aluminum hydroxides, such as, α- and β-trihydrates, α-monohydrate and alumina gel. Such chemicals are obtained from bauxite by the Bayer process also.

DefinitionA mineral form of aluminiumoxide, Al2O3. It crystallizesin the trigonal system and occurs aswell-developed hexagonal crystals. Itis colourless and transparent whenpure but the presence of other elementsgives rise to a variety ofcolours. Ruby is a red variety containingchromium; sapphire is ablue variety containing iron andtitanium. Corundum occurs as arock-forming mineral in both metamorphicand igneous rocks. It ischemically resistant to weatheringprocesses and so also occurs in alluvial(placer) deposits. The secondhardest mineral after diamond (it hasa hardness of 9 on the Mohs’ scale), itis used as an abrasive.