Effect of pH and sodium citrate on the preparation of α-CaSO4.0.5 H2O and the leaching of impurity phosphorus from phosphogypsum

Phosphogypsum (PG) is a common by-product of the wet phosphoric acid leaching process. However, the phosphorus (P) impurities in PG restrict its use in construction materials and other fields. This study used liquid-phase crystallization in sodium sulfate solution under atmospheric pressure to synthesize alpha-hemihydrate gypsum (alpha-HH), simultaneously removing P impurities from PG. The mechanisms and effects of pH and sodium citrate on alpha-HH crystallization and P leaching were analyzed using FTIR, XPS, specific surface area, and density functional theory (DFT). The results show that PG completely releases lattice-bound co-crystallized P during dissolutionrecrystallization. Short columnar products with smaller specific surface areas reduce the physical adsorption of phosphates on the crystal surface. As the solution pH increases, phosphates are more likely to replace SO42-with HPO42-, re-entering the crystal lattice. As pH and sodium citrate concentrations increase, citrate ions adsorb onto the product surface, leading to a gradual decrease in the crystal aspect ratio. At pH = 3 and sodium citrate concentration of 12 %, large crystal short-columnar products can be produced, with a P leaching efficiency of 80 % in the acid-washed alpha-HH. This work provides a theoretical foundation for the high-value, clean utilization of PG.