Abstract: An improved process for the causticisation of Bayer liquors in an alumina refinery, the process including the steps of reacting lime with aluminate ions in a Bayer liquor within a primary reactor under controlled conditions of low to moderate temperature (between 70-80° C.) and agitation, to form substantially only a hydrocalumite species and hydroxyl ions; and a secondary reactor wherein the hydrocalumite species formed is subjected to heating in contact with a Bayer liquor under controlled conditions so as to cause the hydrocalumite species to react with the liquor to form calcium carbonate, aluminate ions and hydroxyl ions, whereby a causticised Bayer liquor is obtained and wherein the efficiency of lime utilization is substantially increased and/or alumina losses are minimized.

Abstract: A process for removing anionic impurities from a caustic aluminate solution having aqueous tetrahydroxy aluminate ions is described. A caustic aluminate solution is obtained such that anionic impurities are partially or substantially substituted into tetrahydroxy aluminate ions to form substituted aluminate ions. A suitable calcium compound, such as quicklime, is reacted in a conventional slaker (10) with a slaking solution, for example water, and stored in an agitated storage tank (12). The resulting slurry is pumped to a reaction vessel (14) where it is reacted with a caustic aluminate solution under appropriate conditions such that the calcium compound is converted into tricalcium aluminate (TCA) precipitate. Upon reaction substituted aluminate ions are substantially incorporated within the TCA precipitate to form a substituted TCA precipitate enabling the anionic impurities to be removed from the solution via the substituted TCA precipitate for disposal or recovery.