Selective Removal of Soluble F From F/P-Bearing CaSO₄·2H₂O Slurry by CaCl₂

A novel method for selectively removing F from F/P-containing CaSO₄·2H₂O slurry using CaCl₂ as the precipitant is reported. Experimental results indicate that under conditions of 25°C, a Ca/F ratio of 2.5, and acidic pH (0.9-4.0), the F removal and P retention performance is optimal, where the removal rate of soluble F increases from 67.93% to 95.15%, while the loss rate of soluble P rises from 0% to 46.67%. At pH ≥ 6.0, the P loss rate exceeds 60%. Thermodynamic equilibrium calculations were conducted to reveal the equilibrium concentration distributions of F and P components and precipitation behavior of F and P at varying pH. At pH ≥ 4.4, binary co-precipitation of CaF₂ and CaHPO₄ occurs, hindering F removal while retaining P. At pH = 5.6-11.8, ternary co-precipitation of CaF₂, Ca₃(PO₄)₂, and CaHPO₄ further complicates selective F removal. In contrast, at pH = 0.9-4.0, the method effectively removes F while preserving P, aligning closely with the experimental results.