Corundum is crystalline aluminium oxide Al2O3. Several types of corundum are used as abrasives. Basically, we distinguish between fused and sintered corundum according to its method of production. Fused corundum or electrocorundum include white, brown and normal corundum. Special forms of single crystal and zircon corundum are available to meet special requirements. Sintered bauxite and sol-gel corundum belong to the group of sintered corundum materials.
Manufacture
In principle, the following methods are distinguished in the production of corundum. These determine its properties to a considerable extent:
• melting raw bauxite in an electric furnace for the fabrication of normal, brown and mono crystalline corundum,
• melting pure alumina (Al2O3) in electric furnaces for the manufacture of white corundum, pink corundum and red corundum,
• sintering of ground raw bauxite to produce sintered bauxite corundum and
• sintering aluminium hydroxide gel to produce sol-gel corundum.
Bauxite serves as the raw material in the manufacture of fused corundum in electric furnaces (Fig. 3-2). Bauxite as it appears in nature contains up to 25 % impurities such as iron oxide Fe2O3, iron hydroxide Fe(OH)3, titanium oxide TiO2 and silicic acid, aluminium hydroxide Al(OH)3 and aluminium oxide hydrates. For the production of white corundum, impurities are removed from the bauxite by means of the Bayer method, so that ultimately pure alumina (Al2O3) is added to the melting furnace. Brown and normal corundum is made by melting bauxite. The melting process takes place in an electric arc furnace while adding coke and iron. Under the electric arc, corundum is melted at a temperature of over 2000 °C. Melting times vary according to the method used (billet/Higgins furnace 15-24 hours, tilting furnaces 3-5 hours) and according to the size of the furnace. Tilting
furnace technology is enjoying increasing popularity, as it offers both advantages in performance as well as a high degree of flexibility.