Solubility of tricalcium aluminate in synthetic spent Bayer liquor

Abstract

Tricalcium aluminate hexahydrate (3CaO·Al2O3·6H2O, TCA) was synthesized via a reaction between slaked lime and sodium aluminate solution; the resulting solids were confirmed to be 3CaO·Al2O3·6H2O by X-ray diffraction. The TCA solubility was then determined over a range of industrially relevant conditions by measuring calcium concentration in solution when at equilibrium with excess solids. It was observed that the solubility increased with increasing sodium carbonate concentration. Slight shifts in the XRD pattern were observed when the solids were exposed to the carbonate-containing liquor. From the analysis of existing thermodynamic data, the dominant equilibrium reaction at the conditions tested was determined to be 3CaO·Al2O3·6H2O + 3CO32- + 4H+ = 3CaCO3(aq) + 2Al(OH)4- + 4H2O. From the experimental results and activity coefficient estimates, the relationship between the reaction equilibrium constant (K) and temperature (T) in the range of 308- 368 K is described by the relation log K = 80.56 - 0.105T.