Thermodynamic and kinetic study of (NH4)2SO4 crystallization in the presence of CaSO4

The laser method was used to study the effect of CaSO4 on the crystalline solubility, metastable zone width, thermodynamics, and kinetics of an (NH4)2SO4 solution. The results showed that (NH4)2SO4 solubility decreased with an increase in CaSO4 concentration, and the dissolution process was a heat-absorbing reaction. With an increase in the CaSO4 concentration, the metastable zone width of (NH4)2SO4 increased and then decreased. When the solution was unsaturated with CaSO4, it was dominated by liquid impurities, that prolong the nucleation time of the grains. When the solution was supersaturated, it was dominated by solid impurities, leading to heterogeneous nucleation and promoting nucleation. The nucleation kinetics and activation energy of (NH4)2SO4 were calculated using the self-consistent Nývlt-like equation and the classical 3D nucleation theory, respectively, and the results showed that the nucleation mode was not affected by the saturation temperature. Furthermore, an increase in CaSO4 concentration was more conducive to nucleation steadily, and the nucleation constant decreased with an increase in the cooling rate. The parameter F1/F was independent of the saturation temperature, and it was a solute-solvent property. In addition, the solid-liquid interfacial energy decreases with increasing saturation temperature and increases and then decreases with increasing CaSO4 concentration.