Deep removal of phosphorus and synchronous preparation of high-strength gypsum from phosphogypsum by crystal modification in NaCl-HCl solutions

Phosphogypsum (PG) is a bulk solid waste that is generated during the production of wet-process phosphoric acid. Presently, the impurities (especially co-crystalline phosphorus impurities) and low value-added recycling products restrict the utilization of PG. In this study, the complete deconstruction of co-crystalline phosphorus encapsulated in gypsum lattices was realized through phase transformation from PG to α-hemihydrate gypsum (α-HH) in NaCl solutions, which was the key to efficiently eliminating phosphorus impurities from PG. Simultaneously, the addition of HCl into NaCl solutions could fully dissolve the phosphates released from gypsum lattices and distinctly increase the phosphorus leaching efficiency. Moreover, synergistic regulation of crystal morphology and particle size of α-HH was obtained in NaCl-HCl solutions by crystal seed induction, and regular large-grained α-HH with small specific surface areas was prepared, which weakened the phosphorus adsorption on the product surfaces and further improved the leaching efficiency. Importantly, the prepared α-HH were high-strength gypsum with high added value. Consequently, deep phosphorus removal (>99%) and synchronous preparation of high-strength gypsum (>30 MPa) with high added value from PG was achieved by regulating the phase transition from PG to α-HH and controlling the α-HH morphology and size by crystal seed induction in mixed solutions of NaCl and HCl under mild conditions. This work provides instructive guidance for efficient impurity removal and high value-added utilization of industrial byproduct gypsum at low cost.