Phosphogypsum Two-Step Ammonia-Carbonation Resulting in Ammonium Sulfate and Calcium Carbonate Synthesis: Effect of the Molar Ratio OH−/Ca2+ on the Conversion Process

We describe herein a detailed study on the effect of molar ratio MR = n(OH−)/n(Ca2+) on the Phosphogypsum (PG) waste ammonia-carbonation process. The use of an aqueous NH3 solution/gaseous CO2 mixture as an alternative and environmental medium compared with other mediums [(NH4)2CO3 for example] is proposed for CO2 capture and PG recycling. The main purity, reaction time, and production efficiency of the obtained products were systematically studied for the different molar ratios using several physicochemical techniques: XRD, XRF, FTIR, SEM/EDX, pH, and electrical conductivity meters. For economic reasons, the molar ratio was varied to obtain values close to those of the stoichiometry, MR (2 ≤ MR ≤ 4). The obtained results show that the MR presents an important effect on the conversion process. In parallel, a comparative study between the conversion process of the synthetic gypsum (SG) and PG shows that the two samples are similar. All these findings revealed that PG waste could be used as a substituent to the SG in the general application of this last useful product.