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 α-hemihydrate gypsum (α-HH), simultaneously removing P impurities from PG. The mechanisms and effects of pH and sodium citrate on α-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 dissolution-recrystallization. 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 α-HH. This work provides a theoretical foundation for the high-value, clean utilization of PG. • α-HH was synthesized in a relatively mild Na 2 SO 4 solution under atmospheric pressure. • Co-crystallized P was released during the recrystallization process of PG. • α-HH with smaller specific surface areas reduced the adsorption of phosphates. • The simultaneous removal of P impurities during α-HH synthesis was achieved.