Impact of alkali roasting and leaching conditions on the recovery of rare earth elements from iron-rich mine tailings: A study using deep eutectic solvents

Reprocessing of mine tailings has gained significant attention as a sustainable approach for recovering critical minerals, including rare earth elements (REEs). While hydrometallurgical methods using mineral acids are widely studied and recognized as effective, their environmental impact remains a major concern. This study explores deep eutectic solvents (DESs) as eco-friendly alternatives for recovering REEs from Fe-rich Australian tailings. Six different DES formulations were evaluated for their leaching efficiency, and the impact of alkali roasting on recovery efficiency was assessed. Additionally, the performance of the most effective DES was compared against the commonly used mineral acids—H₂SO₄, HCl, and HNO₃. The findings identified alkali roasting with KOH at 250 °C for 30 minutes, followed by leaching with a DES composed of choline chloride (ChCl) and p-toluenesulfonic acid (PTSA) in a 2:1 molar ratio, as the most efficient approach. This method achieved up to 95 % leaching efficiency for total rare earth elements (TREEs) and Fe, along with 94 % material dissolution—significantly outperforming other DES formulations and the three mineral acids. The superior performance of the ChCl:PTSA was attributed to KOH roasting, which enhanced REEs liberation by breaking down complex mineral matrices into simpler oxides. Furthermore, KOH roasting required less energy than conventional roasting due to its lower temperature and shorter duration. The DES-based leaching process using ChCl:PTSA offered a promising green alternative for REEs recovery from Fe-rich tailings, providing high efficiency under mild operating conditions while reducing environmental impact.