Leaching of Bastnasite Ore in a Type IV Deep Eutectic Solvent (EG-FeCl₃): Taguchi Optimization and Mechanistic...

The extraction of rare earth elements (REE) from complex mineral matrices typically relies on hydrometallurgical routes involving strong inorganic acids, which pose significant environmental and operational challenges, including silica gel formation. In this study, a sustainable leaching approach was developed for the recovery of Ce, La, and Nd from calcined bastnasite ore using a nonaqueous type IV deep eutectic solvents (DES) composed of ethylene glycol (EG) and iron-(III) chloride (FeCl₃). A systematic optimization was conducted using the Taguchi L32 orthogonal array design to evaluate the effects of temperature, solid-to-liquid (S/L) ratio, FeCl₃ concentration, leaching time, and stirring speed. Statistical analysis and stepwise regression modeling identified temperature as the most critical parameter governing extraction efficiency, with the regression models exhibiting high predictive accuracy (R ² > 0.87). Molarity and S/L ratio did not exhibit significant effects individually; however, their efficacy became pronounced in conjunction with temperature. The highest total light REE (LREE) extraction efficiency was found to be 62.8% under the conditions of 50 °C, 1 M FeCl₃, S/L ratio of 0.05 g/mL, and 16 h. Furthermore, the dissolution mechanism was elucidated using FTIR spectroscopy, which provided direct evidence of an in-situ acidification process. Spectral analysis confirmed that ferric ions coordinate with ethylene glycol to form iron-glycolate complexes, releasing protons that drive the dissolution of REE oxides, with water generated as a stoichiometric byproduct. These findings demonstrate that the EG-FeCl₃ system offers a competitive, simplified, and water-free alternative for processing REE ores, mitigating the limitations of conventional aqueous methods.