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.