Fluorine Transformation Along the Chain from Phosphate Rock to Phosphogypsum and Phosphogypsum-Based Cemented Backfill

Fluorine in phosphogypsum (PG) poses a persistent leaching risk, and PG-based cemented paste backfill (CPB) offers a practical option for in situ stabilization. In this study, fluorine transformation from phosphate rock (PR) to PG and further to CPB was quantified using acid dissolution for total fluorine (TF) determination and sequential extraction for fluorine speciation. Pearson correlation analysis was used to examine statistical relationships among fluorine fractions. TF decreased progressively during wet-process phosphoric acid production, PG stockpiling, and CPB preparation. However, the proportion of mobile fluorine increased from PR to CPB, indicating incomplete reversal of process-induced fluorine activation. CPB immobilized most mobile fractions, increasing the residual fraction to 88.83–97.17% of TF and achieving fluorine transfer efficiencies of 93.59–94.75%. Speciation results showed marked decreases in water-soluble fluorine (Ws-F) and exchangeable fluorine (Ex-F) in CPB, while Pearson correlation analysis further suggested strong statistical associations among Ws-F, Ex-F, and other fractions, suggesting potential interconversion pathways under alkaline, Ca-rich CPB conditions. Nevertheless, a Ws-F-dominated mobile fraction remained in CPB, accounting for 2.83–11.19% of TF, and governed the long-term leaching risk. Therefore, assessing CPB based on process-oriented fluorine speciation, rather than merely on TF content, is critical for optimizing PG-based CPB formulations and achieving the safe reutilization of PG.