Effect of Fluorine and Phosphorus Impurities in Phosphogypsum on Microstructure and Mechanism of α-Type Hemihydrate Gypsum Crystals

In this study, phosphogypsum (PG) is simulated by doping fluorine and phosphorus ions in an analytically pure reagent of gypsum dihydrate. The influence of fluorine and phosphorus impurity and content on the dehydration reaction process of phosphogypsum and its crystalline micromorphology is assessed during the preparation of α-type gypsum hemihydrate in the reversed-phase microemulsion system and its mechanism. The results show that when the fluorine content increases from 0 to 1.0 mol L⁻¹ (ωNaF = 1.0 mol L⁻¹), the dehydration process of dihydrate gypsum will be greatly slowed down. Scanning electron microscopy (SEM) analysis showed that even a small amount of F− (ωNaF = 0.2 mol L⁻¹) can significantly inhibit the formation of α-type hemi-hydrated gypsum. When ωH₃PO₄ = 0.10 mol L⁻¹, the water of crystallization content in the solid phase of the sample decreased to 5.24% after 90 min, which is significantly lower than during the same period of the benchmark group. However, there is a threshold value for the effect of phosphorus on the microscopic morphology of the α-type gypsum hemihydrate crystals, when ωH₃PO₄ ≤ 0.04 mol L⁻¹, the crystal morphology is basically unaffected. Moreover, when ωH₃PO₄ continued to increase, the defects on the crystal surface increased.