Efficient extraction of impurities from phosphogypsum during crystal regulation of alpha-hemihydrate gypsum

Phosphogypsum (PG) is a solid waste generated during the "wet phosphoric acid process." The phosphorus (P) impurities and low-value-added recycling products seriously constrain PG re-utilization, which results in its massive accumulation and thus occupies large land areas and poses a severe pollution threat to the environment. In this study, by regulating gypsum crystal and P species in CaCl2-HCl solutions under mild conditions, efficient removal of P impurities (leaching efficiency of 97.78%) and synchronous preparation of highstrength alpha-CaSO4 center dot 0.5H(2)O (alpha-HH) (compressive strength of 35.2 MPa) from PG were achieved during the phase transition from CaSO4 center dot 2H(2)O (DH) to alpha-HH, which was a reaction of DH dissolution, followed by alpha-HH crystallization. The co-crystalline Pwas fully released during the dissolution process, whichwas necessary to efficiently eliminate P impurities. HCl dissolved the released P and transformed them into the protonated specie (H3PO4) with less similarity to SO4 (2-), which prevented the recombination of the released P with gypsum during the crystallization process. Furthermore, alpha-HH morphology and size were controlled by seeding in the mixed solutions. The formation of regular large aHH crystals with a low-specific surface area significantly weakened the surface adsorption of P in solutions and further increased the P leaching efficiency. More importantly, the large stumpy a-HHwas identified as high-strength gypsumwith high added value. This work would provide innovative guidance to efficiently remove impurities from gypsum and pioneer a cost- effective approach for clean and high-value utilization of industrial gypsum residues.