Distribution of phosphates in molten salts and molten slags and implications of phosphorus and REE recovery

Abstract

The recovery of phosphorus and rare earth elements (REEs) from mine tailings, sludges, and other industrial waste streams is crucial due to the growing global demand and resource scarcity. This study thoroughly investigates the distribution behaviour of phosphates in molten salt and silicate slag systems, with implications for P and REE recovery. Seven samples of sodium and potassium phosphate-sulfate-silica compositions, including a REE-doped composition (La, Nd), were treated at three different temperatures: 1000 °C, 1100 °C, and 1200 °C in sealed silica ampoules for an equilibration time of 24 hours. SEM–EDS analysis confirmed the formation of distinct salt, silica, and in some cases, silicate slag phases. Phosphorus and sulfur preferentially partitioned into the salt phase, while silica remained as solid phase. REEs were enriched in phosphate-rich regions. Phase diagrams were established using FactSage to support and verify experimental results, showing that all salt compositions occur in the fully molten region across the investigated temperature range. The results depict the thermodynamic stability of phosphate and REE species in high-temperature systems and their presence in phosphate-rich phases for efficient recovery.