Efficient removal of impurities from phosphogypsum during preparation of α-hemihydrate gypsum

In this study, we achieved deep removal of co-crystalline phosphorus (P) impurities and synchronous preparation of high-value-added products from PG by phase transformation under mild conditions. The co-crystalline P was released entirely during the dissolution of phase transition from dihydrate gypsum (DH) to α-hemihydrate gypsum (α-HH) in Na2SO4 solutions, which was necessary to efficiently eliminate P impurities. Adding H2SO4 to Na2SO4 solutions dissolved the released P impurities and made them exist mainly in H3PO4 form which was challenging to chemically adsorb or bind to calcium sulfate in the presence of SO42− during α-HH crystallization. After regulating the phase transformation and P species in Na2SO4–H2SO4 solutions, the P leaching efficiency reached 83.8 %. Moreover, α-HH shape and size were controlled by seeding in the mixed solutions. The formation of large short-columnar α-HH, which was identified as high-strength gypsum, suppressed the physical adsorption of P on the product surface and further increased the leaching efficiency to above 97 %. This work pioneered a cost-effective approach for clean and high-value utilization of industrial gypsum residues.