Crystal Conversion\nof Calcium Sulfate Hemihydrate\nin the H<sub>2</sub>SO<sub>4</sub>–H<sub>3</sub>PO<sub>4</sub>–H<sub>2</sub>SiF<sub>6</sub> System

The byproduct gypsum from the dihydrate\nwet-process phosphoric\nacid production presents a complex composition that limits its potential\nfor high-value utilization. The hemihydrate–dihydrate (HH–DH)\nprocess produces concentrated phosphoric acid while reducing gypsum\nimpurities through a two-step process involving hemihydrate reaction\nand recrystallization. Crucially, residual acids from the hemihydrate\nstage may influence the recrystallization step. Here, we systematically\ninvestigate the effects of H<sub>3</sub>PO<sub>4</sub>, H<sub>2</sub>SO<sub>4</sub>, and H<sub>2</sub>SiF<sub>6</sub> on the hydration\nkinetics and morphology of CaSO<sub>4</sub>·2H<sub>2</sub>O \nduring this process. Our results showed that over a range of acid\nconcentrations, an increase in the concentration of H<sub>3</sub>PO<sub>4</sub> delays the conversion of CaSO<sub>4</sub>·0.5H<sub>2</sub>O, H<sub>2</sub>SO<sub>4</sub> accelerates the conversion, and H<sub>2</sub>SiF<sub>6</sub> has a negligible effect on the hydration rate.\nThe presence of these acids changed the supersaturation of the solution,\nimpacting the crystallization of CaSO<sub>4</sub>·2H<sub>2</sub>O. Furthermore, both H<sub>2</sub>SO<sub>4</sub> and H<sub>2</sub>SiF<sub>6</sub> increase crystal defects and reduce crystalline grain\nsize, and decreasing the concentrations of H<sub>2</sub>SO<sub>4</sub> and H<sub>2</sub>SiF<sub>6</sub> in gypsum hemihydrate resulted\nin crystals with large aspect ratios. This study provides detailed\ninsights into the hydration mechanism of the HH–DH process,\nwhich proceeds by a dissolution–recrystallization mechanism,\nelucidating the role of residual acid, and provides guidance for the\ncontrol of conditions in the hemihydrate conversion process based\non the effect of acid concentration, which provides valuable guidance\nfor optimizing production.