The employing of pure calcium sulfate extracted from phosphogypsum for composing highly pure ammonium sulfate and calcium carbonate

Abstract

ABSTRACT The phosphoric acid wet process waste stream provided the phosphogypsum (PG). Phosphogypsum waste is environmentally harmful and economically valuable. However, processing phosphogypsum to produce (NH4)2SO4 and CaCO3 can solve these environmental and economic issues. ICP-OES, XRD, SEM-EDS, FTIR, and TGA were used to characterize PG, purified CaSO4, (NH4)2SO4, and CaCO3. Herein, PG is treated with different HNO3 concentrations and 0.5M NH4NO3 to eliminate most impurities for gaining a highly purified CaSO4. The factors affecting CaSO4 purification were studied. These aspects include HNO3 and NH4NO3 concentration, the S/L ratio, time spent on treatment, temperature, and grain size. According to the findings, the ideal working conditions for extracting 98.71% of CaSO4 from PG are as follows: 149 µm grain size, 0.5M NH4NO3, 2M HNO3, 1/4 S/L, 30 minutes reaction time, 300 rpm, and 80°C. Using a pure CaSO4 suspension as the raw material, CO2, and a mechanical stirring method, the (NH4)2SO4 solutions crystallized by heating and CaCO3 precipitate were investigated through several trials. The preparation method was investigated by changing variables such as the NH4 +/Ca2+ molar ratio, reaction time, CO2 flow rate, and temperature. Finally, (NH4)2SO4 (98.25%) and CaCO3 (98.5%) are produced from the slurry of pure CaSO4.