Kinetics of the precipitation of calcium sulfate dihydrate in a desalination unit

Abstract

The kinetics of calcium sulfate dihydrate spontaneous precipitation in supersaturation solutions for temperatures ranging between 20 and 70°C was investigated. The quartz microbalance technique (QMC) was used. From measurements of the induction period for gypsum nucleation, interfacial tension was found to be 48.5 and 46.2 mJ.m2 at 20 and 70°C, respectively. The dependence of the induction period on temperature determined the activation energy, which ranged from 71 to 51 kJ.mol-1 for the supersaturation range between 2.5 and 11. From the dependence of the induction period on temperature and supersaturation, it was possible to distinguish between the homogeneous and heterogeneous nucleation mechanisms. It was also observed that the induction period depends highly on the lattice cation/anion molar ratio. This dependence is more important for the lower supersaturations. In all cases, the DRX technique showed that the formed solid phase was solely the calcium sulfate dihydrate. Finally, this work demonstrates that the QMC is a very attractive instrument to identify precipitation mechanisms and to measure the amount of the deposited crystals.