Recovery of rare earth elements from phosphogypsum using subcritical water extraction

Phosphogypsum (PG) is generated in massive amount in the production of fertilizer. It has been indicated that PG is a promising secondary source for rare earth elements (REEs). This study examined the leaching of REEs, including yttrium (Y), lanthanum (La), cerium (Ce) and neodymium (Nd) from PG utilizing subcritical water extraction (SWE). Hydrochloric acid (HCl) was selected as the leaching reagent, and the leaching efficiency increased with increasing HCl concentration and decreasing solid to liquid ratio (S/L). Complete leaching of REEs (100.36% of Y, 97.50% of La, 100.06% of Ce and 96.58% of Nd) was achieved under experimental conditions of 0.1 mol/L HCl, 10 g/L, 100 °C, initial pressure of 10 kg/cm², and 5 min. The total energy consumption in SWE was 4.2 × 10⁶ kJ/kg. Leaching efficiency of Y, La, and Nd slightly increased as temperature increased to 125 °C, and decreased when further increased to 150 °C. The behavior was due to solubility of anhydrite as affected by temperature and pressure under subcritical conditions, and the enhanced mass transfer. From comparisons, SWE could be an efficient and green technology for the recovery of REEs from PG.