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, alpha-hemihydrate gypsum (alpha-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 alpha-HH was controlled by adding F and Al compound regulators, and the mechanical properties of alpha-HH were improved. The difference of alpha-HH synthesized by PG with different qualities was analyzed, and the mechanism of alpha-HH morphology was studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energydispersive spectroscopy (EDS). Results showed that in the wet-process phosphoric acid solution, alpha-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 alpha-HH products met the requirements of alpha 50 grade in JC/T 2038-2010 alpha-high-strength gypsum in China and have a good commercial application prospect. Moreover, in the phosphoric acid solution, the average diameter of the alpha-HH crystal increased from 8.1 mu m to 54.8 mu m, the average aspect (L/D) ratio decreased from 32.65 to 0.52, and the induction period and growth period of the alpha-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 (200), (020), and (400) was inhibited, and the morphology of the alpha-HH crystal changed from whisker to short column. When the dosage of the regulator was 1.1 %, the 2 h bending strength of the alpha-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 alpha-HH products from PG.