Carbonation-NH₄Cl Roasting-Metathesis Conversion of Phosphogypsum to Calcium Sulfate Whiskers

Global phosphogypsum (PG) discharge exceeds 300 million tons annually, and its massive stockpiling or marine disposal poses serious threats to the ecological environment. To address the need for high-value utilization of PG, this study proposes a short-route process of "carbonation conversion-ammonium chloride roasting-metathesis" for the preparation of high-purity calcium sulfate whiskers. After PG pre-neutralization with lime milk to pH 8.5, ammonium carbonate is used as the carbonation agent to convert CaSO₄·2H₂O to CaCO₃ with a conversion of 97.4%, while the released fluoride is immobilized as CaF₂. The CaCO₃-bearing slag is roasted with NH₄Cl at 600 °C for 60 min (CaCO₃ reaction efficiency 97.2%), and the resulting CaCl₂ solution and (NH₄)₂SO₄ solution are used as the calcium and sulfur sources for the subsequent metathesis crystallization. Under optimized conditions (Ca 66.58 g/L, S 53.30 g/L, pH ≈ 6, 50-60 °C, 0.3 g/L Mg²⁺), calcium sulfate dihydrate (CaSO₄·2H₂O) whiskers with a length/diameter aspect ratio above 60 and a purity above 98 wt% are obtained; XRD (PDF #33-0311) and elemental analyses confirm the dihydrate crystalline phase and a strong preferred orientation along the [010] axis. Overall recoveries of Ca (PG → CaCl₂ solution) and S (PG → (NH₄)₂SO₄ solution) are 94.3% and 97.4%, respectively, and the process reduces the residual solid waste from 200 g of PG to only 21 g of F-enriched residue, i.e., an 89.5% solid-waste reduction. The ammonium salt medium is internally recycled, forming a closed loop of Ca, S and NH₄⁺.