Extraction of Rare Earth Elements from Phosphogypsum Using Mineral Acids: Process Development and Mechanistic Investigation

A hydrometallurgical process is developed to leach yttrium, neodymium, and dysprosium from phosphogypsum using three mineral acids, namely, nitric acid, hydrochloric acid, and sulfuric acid over a wide range of conditions. Hydrochloric acid is selected as the best leachant, resulting in 94.6% neodymium and 86.1% dysprosium leaching efficiency under the optimum operating conditions of 45 °C, 2.5 M, and 29.8 mL/g L/S ratio. A systematic investigation using statistical analyses and empirical modeling was performed to determine the best leachant and optimum operating conditions that result in high rare earth element (REE) leaching reported in the literature. Thorough characterization techniques, thermodynamic modeling, and kinetic analysis are utilized to elucidate the leaching mechanism. The results indicate that leaching efficiency is highly correlated with phosphogypsum solubility. The occurrence modes of rare earth elements in phosphogypsum are determined using time-of-flight secondary-ion mass spectrometry (ToF-SIMS), which shows that rare earths are present both as isomorphous Ca2+ substitutions and as separate phases such as oxides and sulfates trapped inside the phosphogypsum crystal. Hence, achieving a high leaching yield requires the destruction of the phosphogypsum lattice through acid attack and dissolution.