The chemistry of tricalcium aluminate hexahydrate relating to the Bayer industry

Abstract

TCA forms when lime is added to Bayer liquors and, as such, its formation is important to the Bayer process. More specifically, tricalcium aluminate hexahydrate (TCA) is used within the Bayer process as a filter aid during green liquor clarification or as a desilicating agent during pre-desilication/digestion, where it is able to remove silica from the system with minimal loss of soda. This review paper examines the properties of TCA of interest to the Bayer industry, with an emphasis on those methods used for the formation of TCA and the effect of different reaction conditions on TCA morphology and crystal size. Literature results indicate TCA can be prepared by reaction of CaO or Ca(OH) 2 in sodium aluminate solutions. Examination of the TCA/Ca(OH) 2 or TCA/CaCO 3 phase diagrams indicates TCA is best prepared at low temperatures, low caustic concentrations and low CO 3 2− concentrations. The presence of organic compounds in the reaction mixture can affect the TCA particle size and synthesis of TCA in the presence of selected organic compounds is used to prepare TCA of a closely controlled particle size. The preparation of TCA by decomposition of certain calcium aluminates (e.g. C 2AH 8, C 4AH 13, C 4ACO 2H 11) and the reactions of TCA occurring under conditions of relevance to the Bayer industry (decomposition in caustic, thermal decomposition, hydrogarnet formation) are also discussed.