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.