Ammonia-free recovery of rare earth elements from ion-adsorption clays using sodium citrate

This study presents an ammonia-free hydrometallurgical flowsheet for the recovery of rare earth elements (REEs) from ion-adsorption clays using sodium citrate (Na 3 Cit) as a green lixiviant. Desorption tests demonstrated that 0.15 M Na 3 Cit at pH 6.4 achieves the highest total REE (TREE) extraction (1300 mg kg −1 ), outperforming conventional ammonium sulfate while minimizing dissolution of Al, Fe, and other matrix elements. Mechanistic analysis supported by zeta potential measurements shows that Na 3 Cit operates through ligand-assisted desorption coupled with mild ion exchange, enabling efficient REE release under near-neutral conditions without structural degradation of the clay. Two precipitation pathways were evaluated for REE recovery from the citrate leachate. Oxalic acid precipitation at pH 2.5–3.5 resulted in near-quantitative REE recovery (>95%) and produced high-purity mixed REE oxalates containing 95–97 wt% total REO with minimal gangue incorporation. STEM–EDS mapping confirmed the formation of uniform nanoscale REE-oxalate crystallites. In contrast, a two-step carbonate precipitation process (acidic activation followed by CaCO 3 /Na 2 CO 3 neutralization) achieved high TREE removal but generated mixed REE–CaCO 3 solids with significantly lower purity (40–55 wt% REO), consistent with micron-scale Ca-rich crystallite formation observed by STEM. This work establishes sodium citrate as an effective, environmentally benign alternative to ammonium-based lixiviants for ionic-clay processing. When coupled with oxalic acid precipitation, the process enables high REE recovery with exceptional selectivity and product purity, providing a viable foundation for sustainable, ammonium-free REE extraction from ion-adsorption clays. • Sodium citrate enables high-selectivity REE desorption at near-neutral pH. • Oxalic acid yields >95% REE recovery with >97 wt% REO product purity. • Carbonate precipitation recovers REEs but forms Ca-rich, lower-purity solids. • Zeta potential confirms ligand-assisted REE desorption by citrate. • Integrated citrate–oxalate flowsheet offers an ammonia-free REE recovery route.