Utilization of phosphogypsum to synthesize α-hemihydrate gypsum in H3PO4–H2O solution

Phosphogypsum (PG) is a kind of industrial solid waste with high impurity content and low utilization rate. In this study, α-hemihydrate gypsum (α-HH) was synthesized from PG in H3PO4–H2O solution (wet-process phosphoric acid solution and phosphoric acid solution) under atmospheric pressure. The crystal morphology of α-HH was controlled by adding F and Al compound regulators, and the mechanical properties of α-HH were improved. The difference of α-HH synthesized by PG with different qualities was analyzed, and the mechanism of α-HH morphology was studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy-dispersive spectroscopy (EDS). Results showed that in the wet-process phosphoric acid solution, α-HH synthesized from white PG (by H3PO4–H2SO4 two-step method) had better whiteness, purity, and mechanical properties compared with that from industrial PG, and the performance indexes of the α-HH products met the requirements of α50 grade in JC/T 2038–2010 α-high-strength gypsum in China and have a good commercial application prospect. Moreover, in the phosphoric acid solution, the average diameter of the α-HH crystal increased from 8.1 µm to 54.8 µm, the average aspect (L/D) ratio decreased from 32.65 to 0.52, and the induction period and growth period of the α-HH crystal were prolonged with the increase in regulator dosage. F− and Al3+ ions were preferred to adsorb on the crystal faces. The growth of the crystal parallel to c-axis (2 0 0), (0 2 0), and (4 0 0) was inhibited, and the morphology of the α-HH crystal changed from whisker to short column. When the dosage of the regulator was 1.1 %, the 2 h bending strength of the α-HH product was 6.56 MPa, and the drying compressive strength was 52.4 MPa. This study provides a new technique and method for the synthesis of high-strength α-HH products from PG.