Application of Magnesium Sulfate in In-Situ Leaching of Rare Earth Elements: Mechanisms, Performance and Environmental Implications
Ion-adsorption clay deposits are among the most important sources of heavy rare-earth elements (REEs), particularly in southern China, where extraction is commonly performed by in situ leaching (ISL) using ammonium salts. Although ammonium sulfate and related lixiviants achieve high extraction efficiencies via ion-exchange mechanisms, their large-scale use has raised significant environmental concerns, including ammonium contamination, soil degradation, and groundwater pollution. In recent years, magnesium sulfate (MgSO₄) has emerged as a potential alternative lixiviant with lower environmental impact. This review critically analyzes the application of MgSO₄ in the in-situ leaching of REE-bearing ion-adsorption clays, focusing on ion-exchange mechanisms, leaching performance, hydrodynamic behavior, and environmental implications. The thermodynamic and geochemical aspects governing Mg²⁺–REE exchange reactions are examined together with laboratory, column, and pilot-scale studies. Comparative analyses with conventional ammonium-based lixiviants are discussed in terms of extraction efficiency, selectivity, reagent consumption, and environmental footprint. The review also identifies key research gaps related to process optimization, hydrogeological modeling, and large-scale implementation. Overall, MgSO₄-based leaching systems represent a promising pathway toward more sustainable rare-earth extraction, although further technological and environmental validation is required before full-scale industrial deployment.