PRECIPITATION OF BAYER ALUMINA TRIHYDRATE SUITABLE FOR THE MANUFACTURE OF CERAMIC GRADE ALUMINA OF IMPROVED GRINDABILITY

摘要

Alpha alumina characterized by monocrystals of a small size, e.g. smaller than 5 microns, is produced from alumina trihydrate precipitated from pregnant Bayer solution by adding thereto alumina trihydrate seed crystals having a particle size such that at least 80-90 percent by weight of the seed crystals have a particle size smaller than 325 mesh. The seed crystals are added to the pregnant Bayer solution in a relatively large amount, such as an amount greater than 70 grams per liter. Desirably, the precipitation of the alumina trihydrate from the Bayer solution is carried out at a relatively high temperature, such as at a temperature in the range 165 DEG -175 DEG F. The resulting precipitated alumina trihydrate, characterized by a relatively low fluorine and residual soda (Na2O) content, is separated and calcined, such as at a temperature of about 1,250 DEG C. to produce an alpha alumina product characterized by monocrystals having a size smaller than about 5 microns. The resulting produced alpha alumina is readily reduced in size by grinding, such as by impact grinding or by ball milling, to produce a superior ceramic grade alumina.