The behaviour of selected rare-earth elements during the conversion of phosphogypsum to calcium sulphide and residue

Phosphogypsum (PG) is a large hazardous waste from fertiliser and phosphoric acid industries from which useful products including rare-earth elements (REEs) can be recovered depending on the treatment process. Its conversion to calcium sulphide (CaS) which was achieved at 95% followed by the formation of S, CaCO₃ and residue is one of the plausible treatment processes leading to economic and environmental benefits. This study aimed at monitoring selected REEs behaviour during the conversion of (PG) to (CaS). The concentrations of REEs in the raw PG, the produced CaS and the obtained residue were determined after digestion (microwave and traditional acid leaching) using ICP-OES. The effect of CO₂ and H₂S used in the process of forming CaCO₃ and S from PG on the concentrations of REEs was also investigated. Microwave digestion proved to be more effective than traditional acid leaching in the recovery of REEs. Microwave digestion using 3 mL HNO₃ + 1 mL HCl was more effective than 1 mL HNO₃ + 3 mL in REEs recovery. CaS contained the highest amount of Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, La and Y with values of 2646, 476, 2255, 320, 60.5, 376, 79.8, 1.24, 476, 1185 and 318 µg/g respectively. Based on these findings, the residue could be further processed to recover REEs despite less than 40% decrease in concentration for the majority of REEs observed due to the use of H₂S and CO₂. CO₂ was found to be more suitable as fewer REEs were leached as compared to H₂S. All things considered, the obtained residue could be a good secondary source of REEs as it is easier to leach, retained good amount of REEs and lesser impurities.