Comparison of Organic Extractants for Rare Earths Elements Extraction from a Columbite Leach Residue

Abstract Rare earth elements (REEs), the 15 lanthanides plus yttrium (Y) and scandium (Sc), are critical metals for energy transition and digital technologies. They are essential for permanent magnets, catalysts, phosphors, and advanced ceramics. Secondary sources such as mining residues often contain REEs along with impurities such as iron (Fe), thorium (Th), and uranium (U), which complicate selective recovery. In this context, the present work focuses on evaluating the selectivity and efficiency of different extractant systems for REEs in a real columbite leachate residue. The precipitation step was evaluated using calcium oxide (CaO), sodium hydroxide (NaOH), and ammonium hydroxide (NH 4 OH) at pH 1.0 to assess initial impurity removal. Five commercial extractants with distinct functional groups— bis (2-ethylhexyl) phosphoric acid (D2EHPA), 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC88A, also known as HEHEHP), bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), a mixture of trialkylphosphine oxides (Cyanex 923), and tri - n -butyl phosphate (TBP)—(10% v/v in kerosene) were compared by varying pH (−0.5 to 2.0), extractant concentration (v/v), and the aqueous-to-organic ratio (A/O). Preliminary precipitation tests performed with calcium oxide (CaO), sodium hydroxide (NaOH), and ammonium hydroxide (NH 4 OH) at pH 1.0 indicated partial removal of Fe, Nb, Th, and U from the aqueous phase. Although these reagents promoted partial precipitation of impurities, they were not selective for REEs, confirming that additional purification steps were required prior to oxalic acid precipitation. D2EHPA exhibited the best overall performance, achieving 98.6–99.2% extraction of heavy rare earth elements (HREEs) at pH 1.5–2.0 and 80% for light rare earth elements (LREEs) at pH 2.0, albeit with coextraction of Th, U, Nb, and Fe. Under selected baseline conditions (A/O = 2:1, pH −0.5, 10% v/v), REE extraction reached 21.7% (HREE) and 15.8% (LREE), with impurity extraction of 55.5%. These results highlight the intrinsic difficulty of impurity control in real sulfate matrices and demonstrate that Route A serves as a diagnostic screening stage for extractant selection and process parameter definition, providing a foundation for subsequent multi-stage purification routes. Graphical Abstract