Reactive crystallization of CaCl2 and Na2SO4 in the presence of acoustic cavitation

Abstract

The present work explores the formation of calcium sulphate (CaSO4) by reactive crystallization of calcium chloride (CaCl2) and sodium sulphate (Na2SO4) as a potential pretreatment approach to reduce scaling on the heat exchange surfaces. The investigation comprises of a comparative study of crystallization by conventional stirring and ultrasound. Degree of supersaturation of calcium sulphate formed has been determined which is useful to predict the nature of nucleation i.e., either homogeneous or heterogeneous. Induction time, equilibrium time, kinetics of the reaction and activation energy required for formation CaSO4 have been determined along with the crystal characteristics. It has been demonstrated that, while homogeneous nucleation occurs, there is a significant reduction in induction time, achievement of faster equilibrium and 72% reduction in the particle size due to the application of ultrasound in comparison to silent conventional stirring. It has been also elucidated that ultrasound lowers the activation energy for the formation of calcium sulphate with faster rate of precipitation by impacting nucleation rate. TGA and XRD analysis confirmed the formation of calcium sulphate dihydrate (CaSO4•2H2O) or gypsum. In summary, ultrasound played a vital role in intensifying the precipitation of CaSO4 and formation of smaller crystals compared to silent conditions.