THE CHEMISTRY OF TRICALCIUM ALUMINATE HEXAHYDRATE RELATING TO THE BAYER INDUSTRY

TCA forms when lime is added to Bayer liquors and, as such, its format ion is important to the Bayer process. More specifically, tricalcium a luminate hexahydrate (TCA) is used within the Bayer process as a filte r aid during green liquor clarification or as a desilicating agent dur ing 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 diffe rent reaction conditions on TCA morphology and crystal size. Literatur e 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 TC A/CaCO2 phase diagrams indicates TCA is best prepared at low temperatu res, low caustic concentrations and low CO32- concentrations. The pres ence of organic compounds in the reaction mixture can affect the TCA p article 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 aluminate s (e.g. C(2)AH(8), C(4)AH(13), C(4)ACO(2)H(11)) and the reactions of T CA occurring under conditions of relevance to the Bayer industry (deco mposition in caustic, thermal decomposition, hydrogarnet formation) ar e also discussed.