A Nitrate-Chloride Brønsted Acid Strategy for Sustainable Recovery of Rare Earth Elements and Iron from Mining Tailings

Iron (Fe)-rich tailings generated during rare earth element (REE) processing constitute a strategically important secondary resource. However, their utilization is hindered by the refractory nature of REE-bearing mineral phases and the environmental footprint of conventional mineral acid leaching. This study presents a sustainable hydrometallurgical strategy employing an in situ Brønsted acidic medium generated from aluminum nitrate nonahydrate (Al(NO3)3·9H2O) and sodium chloride (NaCl) for the recovery of REEs and Fe from Fe-rich tailings containing 48 wt % Fe and 7 wt % total REEs. Alkali activation at 250 °C for 30 min induced mineralogical reconstruction by decomposing goethite and REE phosphate phases into thermodynamically less stable Fe and REE oxides, thereby enhancing their chemical reactivity and accessibility to proton-assisted dissolution. The synergistic combination of alkali activation and leaching enabled almost complete dissolution of the roasted material under optimized conditions (Al(NO3)3·9H2O:NaCl = 1:0.5, liquid-to-solid ratio = 100 mL g–1, 75 °C, 4 h), achieving nearly quantitative dissolution of both Fe and REEs. Kinetic modeling demonstrated that the leaching process is predominantly governed by interfacial chemical reactions (R2 = 0.91), with intraparticle diffusion becoming rate-limiting only at later stages, confirming that mineralogical destabilization substantially lowers the activation barrier for dissolution. The nitrate-chloride Brønsted acid system effectively replaces concentrated mineral acids while maintaining high leaching efficiency, significantly reducing process hazards and environmental impacts.