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• Description

  The oxide of aluminum is Al2O3. The natural crystalline mineral is called corundum, but the synthetic crystals used for abrasives are designated usually as aluminum oxide or marketed under trade names. For other uses and as a powder, it is generally called alumina. It is widely distributed in nature in combination with silica and other minerals and is an important constituent of the clays for making porcelain, bricks, pottery, and refractories.
  
  The crushed and graded crystals of alumina when pure are nearly colorless, but the fine powder is white. Off colors are due to impurities. American aluminum oxide used for abrasives is at least 99.5% pure, in nearly colorless crystals melting at 2050°C. The chief uses for alumina are for the production of aluminum metal and for abrasives, but it is also used for ceramics, refractories, pigments, catalyst carriers, and in chemicals.
  Aluminum oxide crystals are normally hexagonal and are minute in size. For abrasives, the grain sizes are usually from 100 to 600 mesh. The larger grain sizes are made up of many crystals, unlike the single-crystal large grains of SiC. The specific gravity is about 3.95, and a hardness is up to 2000 Knoop.
  There are two kinds of ultrafine alumina abrasive powder. Type A is alpha alumina with hexagonal crystals with particle size of 0.3 μm, density 4.0, and hardness 9 Mohs, and type B is gamma alumina with cubic crystals with particle size under 0.1 μm, specific gravity of 3.6, and a hardness 8. Type A cuts faster, but type B gives a finer finish. At high temperatures, gamma alumina transforms to the alpha crystal. The aluminum oxide most frequently used for refractories is the beta alumina and hexagonal crystals heat-stabilized with sodium.
  Activated alumina is partly dehydrated alumina trihydrate, which has a strong affinity for moisture or gases and is used for dehydrating organic solvents, and hydrated alumina is alumina trihydrate.;

• History

  The hexagonally closest packed α-Al2O3 modification is the only stable oxide in the Al2O3 –H2O system. Corundum is a common mineral in igneous and metamorphic rocks. Red and blue varieties of gem quality are called ruby and sapphire, respectively. The lattice of corundum is composed of hexagonally closest packed oxygen ions forming layers parallel to the (0001) plane. Only two-thirds of the octahedral interstices are occupied by aluminum ions. The structure may be described roughly as consisting of alternating layers of Al and O ions. The corundum structurewas determined in the early 1920s .;

• Physical properties

  Regular aluminum oxide is white and has a chemical composition of approximately 95 % Al2O3, 1.5 % SiO2, less than 0.5 % Fe2O3, and 3 % TiO2. Because of very slow cooling in the Higgins furnace, the crystals of alumina are coarse, averaging 10 – 15mm in diameter.;

• Classifications

  Four grades can be distinguished according to the content of calcined aluminium:
  (i) Standard calcined aluminas with a sodium content of between 3000 and 7000 ppm wt. Na₂O.
  (ii) Intermediate calcined alumina with a sodium content of between 1000 and 3000 ppm wt. Na₂O. The sodium content of this grade has been lowered by modifying the conditions of the precipitation of gibbsite or of the calcination.
  (iii) Low-sodium calcined aluminas with a sodium content of between 300 and 1000 ppm wt. Na₂O. These aluminas are usually obtained by washing the precursor or by the extraction of sodium as a volatile compound with the mineralizer during calcination.
  (iv) High-purity aluminas with an extra-low sodium content below 100 ppm wt. Na₂O. These aluminas obtained from an aluminum hydroxide produced by a process other than the Bayer process. The main applications for calcined aluminas are as feedstocks for refractories, glass and enamel, tiles and porcelain, and advanced ceramics. The diversity of applications for calcined aluminas can be explained by the wide range of properties: refractoriness, sinterability, chemical inertness in both oxidizing and reducing atmosphere and in both acid and alkaline media, hardness, wear and abrasion resistance, dimensional stability, high thermal conductivity, electrical resistivity, low dielectric loss and high permittivity, and high ionic conductivity in the case of beta-alumina.;

• Uses

  Aluminum Oxide (Alumina) is the most widely used oxide, chiefly because it is plentiful, relatively low in cost, and equal to or better than most oxides in mechanical properties. Density can be varied over a wide range, as can purity-down to about 90% alumina-to meet specific application requirements. Alumina ceramics are the hardest, strongest, and stiffest of the oxides. They are also outstanding and with electrical resistivity and dielectric strength, are resistant to a wide variety of chemicals, and are unaffected by air, water vapor, and sulfurous atmospheres. However, with a melting point of only 2039°C, they are relatively low in refractoriness and at 1371°C retain only about 10% of room-temperature strength. In addition to its wide use as electrical insulators and its chemical and aerospace applications, the high hardness and close dimensional tolerance capability of alumina make this ceramic suitable for such abrasion-resistant parts as textile guides, pump plungers, chute linings, discharge orifices, dies, and bearings.;

• Industrial Manufacture

  Monocrystalline aluminum oxide is a very high purity abrasive, produced directly from bauxite in a single-stage fusion . The preferred method employs a furnace feed consisting of bauxite, pyrite (FeS2) or sulfur, carbon, and iron borings. When subjected to fusion in the Higgins batch furnace, two immiscible liquids are formed as with regular aluminum oxide. However, in the present case very slowcooling of the upper liquid results in essentially pure, individual crystals of Al2O3 in a matrix of sulfides. After the pig has cooled and been crushed, the matrix is removed chemically and mechanically. This treatment releases alumina crystals in the range of sizes required by the industry.;

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