Efficient extraction of impurities from phosphogypsum during crystal regulation of α‐hemihydrate gypsum

Abstract 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 CaCl 2 –HCl solutions under mild conditions, efficient removal of P impurities (leaching efficiency of 97.78%) and synchronous preparation of high‐strength α‐CaSO 4 ·0.5H 2 O (α‐HH) (compressive strength of 35.2 MPa) from PG were achieved during the phase transition from CaSO 4 ·2H 2 O (DH) to α‐HH, which was a reaction of DH dissolution, followed by α‐HH crystallization. The co‐crystalline P was fully released during the dissolution process, which was necessary to efficiently eliminate P impurities. HCl dissolved the released P and transformed them into the protonated specie (H 3 PO 4 ) with less similarity to SO 4 2− , which prevented the recombination of the released P with gypsum during the crystallization process. Furthermore, α‐HH morphology and size were controlled by seeding in the mixed solutions. The formation of regular large α‐HH 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 α‐HH was identified as high‐strength gypsum with 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.