GB350561A - - Google Patents

Abstract

An abrasive consisting of crystalline a -alumina substantially free from impurities, b -alumina, and voids and having a density of at least 3,97 is obtained by melting impure alumina with carbon to obtain complete reduction of impurities, removing reduction products as well as any alkaline-earth impurities and remelting the purified alumina to form the required crystals. In carrying out the process, a mixture of calcined bauxite, 1-2 per cent of ferric or ferrous sulphide, and sufficient carbon for reduction of impurities, conversion of the iron sulphide to metal and aluminium sulphide, and for reduction of 2-4 per cent of the alumina to metal or carbide is fused in a tilting furnace. The solidified slag obtained is exposed to air and moistened to reduce it to powder, which is washed, screened, dried, and passed through a magnetic separator to remove iron. After calcination to remove residual oxidizable impurities, the powder is fed slowly into an electric furnace and fused under conditions of superheat. The resulting ingot, due to slow cooling, develops very large crystals and consists substantially of a -alumina. The dense product having less than 2 per cent of voids is unaffected by heat treatment, has higher strength and impact resistance values, and is highly stable in vitrified bonds. It may also be used with low-temperature bonds such as shellac or phenolic resins without prior heat treatment. By remelting the product with about 1 per cent of an alkali metal or compound thereof, or by introducing the latter into the aluminous melt, a porous weak-grained product may be obtained.