High-Pressure Carbonation of Phosphogypsum for Calcium Carbonate Preparation and Crystal Modification Regulation

Phosphogypsum (PG) was used as a calcium source for preparing calcium carbonate (CaCO₃) through NH₄Cl leaching followed by high-pressure carbonation. The effects of NH₄Cl concentration, liquid-to-solid mass ratio, temperature, and leaching time on Ca²⁺ extraction were investigated, and the effects of CO₂ pressure, carbonation time, and NH₃·H₂O dosage on Ca²⁺ conversion were evaluated. The optimal conditions were an NH₄Cl concentration of 1.5 mol/L, a liquid-to-solid mass ratio of 60:1, a leaching temperature of 25 °C, a leaching time of 60 min, a CO₂ pressure of 1 MPa, a carbonation time of 10 min, and 12 vol% NH₃·H₂O addition. Under these conditions, the Ca²⁺ leaching rate and conversion rate reached 81.25% and 97.36%, respectively. The product obtained without organic additives was mainly spherical vaterite with partial particle agglomeration. Based on the optimized process, aspartic acid, glutamic acid, ethanol, and glycerol were introduced to regulate CaCO₃ crystallization. Appropriate additive dosages further improved Ca²⁺ conversion, promoted calcite as the dominant polymorph, and produced well-dispersed spherical CaCO₃ particles. Among the tested additives, glutamic acid and glycerol showed the strongest effects on crystal morphology regulation.