Effect of Supersaturation on Competitive Nucleation of CaSO4 Phases in a Concentrated CaCl2 Solution

Nucleation is a combined result of thermodynamics and kinetics. Calcium sulfate dihydrate (DH), α-calcium sulfate hemihydrate (α-HH), and calcium sulfate anhydrite (AH) all have the potential to precipitate when the solution is supersaturated with them at the same time. The effect of supersaturation on CaSO4 phase's occurrence was investigated in a 3.00 m CaCl2 solution at 91 °C to better understand the kinetics of competitive nucleation. In a wide range of supersaturations, DH and α-HH precipitated concomitantly without AH forming. The molar fraction of α-HH in the precipitate increased gradually from 47.23% to a summit of 98.91% when the supersaturation (SHH) varied from 1.90 to 3.76 and then followed a dramatic decrease to 18.77% with a further increase in SHH to 6.26. The predominant nucleation shifted from α-HH at the lower supersaturations to DH at the higher supersaturations. The crystallization rate evolution with supersaturation was responsible for the growth and decline of DH and α-HH in the precipitate.