Crystal Conversion of Calcium Sulfate Hemihydrate in the H₂SO₄–H₃PO₄–H₂SiF₆ System

The byproduct gypsum from the dihydrate wet-process phosphoric acid production presents a complex composition that limits its potential for high-value utilization. The hemihydrate–dihydrate (HH–DH) process produces concentrated phosphoric acid while reducing gypsum impurities through a two-step process involving hemihydrate reaction and recrystallization. Crucially, residual acids from the hemihydrate stage may influence the recrystallization step. Here, we systematically investigate the effects of H3PO4, H2SO4, and H2SiF6 on the hydration kinetics and morphology of CaSO4·2H2O during this process. Our results showed that over a range of acid concentrations, an increase in the concentration of H3PO4 delays the conversion of CaSO4·0.5H2O, H2SO4 accelerates the conversion, and H2SiF6 has a negligible effect on the hydration rate. The presence of these acids changed the supersaturation of the solution, impacting the crystallization of CaSO4·2H2O. Furthermore, both H2SO4 and H2SiF6 increase crystal defects and reduce crystalline grain size, and decreasing the concentrations of H2SO4 and H2SiF6 in gypsum hemihydrate resulted in crystals with large aspect ratios. This study provides detailed insights into the hydration mechanism of the HH–DH process, which proceeds by a dissolution–recrystallization mechanism, elucidating the role of residual acid, and provides guidance for the control of conditions in the hemihydrate conversion process based on the effect of acid concentration, which provides valuable guidance for optimizing production.