Transformation of anhydrous calcium sulphate into calcium sulphate dihydrate in aqueous solutions

Abstract

The kinetics of transformation of anhydrous calcium sulphate (AH) to calcium sulphate dihydrate (DH) in aqueous solutions was investigated using well-defined mixtures of AH and DH seed crystals to initiate the process. The progress of transformation was followed by recording the changes both in the liquid and in the solid phases. The transformation process was found to be solution-mediated, which was supported by a mathematical model proposed in this work. The model takes into account the experimentally determined initial solution calcium concentration and mass concentration of the solid phases as well as the rate constants and mechanisms of the processes involved in the transformation: dissolution of AH, dissolution of DH and crystal growth of DH. The kinetics of these processes were studied separately in the temperature range 10–40°C and the rate laws were found to be parabolic, linear and parabolic, respectively. The activation energies obtained for the temperature-dependent rate constants, E a(AH) D=61.0 kJ mol −1, E a(DH) D=24.0 kJ mol −1 and E a(DH) G=64.0 kJ mol −1, support the assumption that the AH dissolution and the DH growth were controlled by processes at the crystal surface and that the DH dissolution was most probably a diffusion controlled process.