Investigation on\nthe Mechanism of Efficient Removal\nof Phosphorus and Fluorine Impurities from Phosphogypsum by an In\nSitu Recrystallization Purification Method

Phosphogypsum, a solid waste produced in the wet phosphoric\nacid\nprocess, can lead to severe environmental pollution. Recently, the\nresource utilization of phosphogypsum as a raw material for cement\nretarders and other products has received widespread attention, but\nit is limited by the high cost of purifying impurities, such as phosphorus\nand fluorine, contained in phosphogypsum. Herein, a novel method for\npurifying phosphorus and fluorine in phosphogypsum by in situ recrystallization\nwas developed and has been employed in industrial application successfully.\nFurther, the purification process parameters and mechanism were investigated\nbased on a series of characterizations. The experimental results indicate\nthat the removal efficiencies of phosphorus and fluorine achieve 86.4%\nand 95.71% under the optimized conditions, respectively. Significantly,\nduring the recrystallization and washing process of fresh phosphogypsum,\nCaSO<sub>4</sub>·0.5H<sub>2</sub>O dissolves and then transforms\ninto larger rhombic CaSO<sub>4</sub>·2H<sub>2</sub>O particles.\nMeanwhile, the impurities, such as water-soluble phosphorus ([H<sub>3</sub>PO<sub>4</sub>, H<sub>2</sub>PO<sub>4</sub><sup>–</sup>, HPO<sub>4</sub><sup>2–</sup>, and Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub>), eutectic phosphorus (CaHPO<sub>4</sub>·2H<sub>2</sub>O), and water-soluble fluorine (F<sup>–</sup>, Na<sub>2</sub>SiF<sub>6</sub>, and K<sub>2</sub>SiF<sub>6</sub>), carried\nin fresh phosphogypsum are transferred from the solid phase to the\nliquid phase, thereby achieving purification of phosphogypsum. This\nresearch offers robust theoretical backing and practical guidance\nfor the effective purification of phosphogypsum.