Kinetics and morphology of formed gypsum

Calcium sulfate dihydrate scale was electrochemically deposited from supersaturated solutions on gold electrodes (probe) of an electrochemical quartz crystal microbalance (QCM). From measurements of the induction period for the gypsum nucleation, the interfacial tension was calculated. The dependence of the induction period on temperature allowed determining the activation energy. 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. The critical size rc represents the minimum size of a stable nucleus. Particles smaller than rc dissolve or evaporate and particles larger than rc continue to grow. It was observed that rc decreased with increasing temperature for the heterogeneous nucleation process, but it had a constant value for the homogeneous nucleation process. From the SEM photos, it was observed that long induction times allow the growth of larger crystals while a shorter induction period permits smaller crystals and that the lattice cation/anion molar ratio did not effect the size of the crystals that were formed. Finally, this work demonstrated that the QCM is a very attractive instrument to identify precipitation mechanisms and to measure the amount of deposited crystals.

Experimental work was carried out to study the thermodynamics and kinetics of calcium sulphate at various conditions by using the QCM technique. By using the experimental data relative to the homogeneous nucleation mechanism, the dependence of the induction time on temperature allowed the determination of the activation energy for gypsum nucleation, while the dependence on supersaturation allowed the estimation of the interfacial tension between the gypsum and aqueous solution. In addition to the degree of supersaturation and temperature, the induction time generally decreased with increasing and decreasing lattice cation/anion molar ratios, and the critical radius rc decreased with increasing temperature. The limit between the heterogeneous and homogeneous nucleation was estimated in an interval of supersaturation ranging between 3.5 and 4.