Optimization of Supercritical Fluid Extraction of Rare Earth Elements from Complex Ores Using a Tributyl Phosphate-Nitric Acid Adduct

Abstract

This study introduces a new approach to extracting rare earth elements (REEs) from a Canadian ore concentrate, employing supercritical fluid extraction (SCFE) with supercritical carbon dioxide (sc-CO₂) as the solvent and a tributyl phosphate-nitric acid (TBP-HNO₃) adduct as the chelating agent. Addressing the environmental and safety concerns of traditional extraction methods, this research explores an eco-friendly and efficient SCFE technique, enhanced by a preliminary NaOH cracking step, to achieve nearly complete extraction efficiency of REEs. Characterization of the ore pre- and postextraction was thoroughly carried out using X-ray diffraction (XRD), scanning electron microscopy energy dispersion spectroscopy (SEM-EDX), and Raman Spectroscopy, revealing significant alterations in the mineralogical structure that facilitate the SCFE process. Focusing on the distribution and accessibility of REEs in feed concentrate, NaOH cracked samples, and SCFE residue, this investigation reveals the predominant presence of REE-bearing minerals in the initial and cracked samples, particularly within zircon structures. A notable transformation of iron from hematite to magnetite, absent in the feed but present in postprocessing samples, suggests a reduction process facilitated by high-temperature NaOH cracking. The findings emphasize the complexity of REE extraction from mineral matrices and the potential of integrating SCFE with NaOH cracking for improved results. The study optimized the operational parameters for NaOH cracking and SCFE, demonstrating their crucial role in maximizing REE efficiencies. An empirical model was used to quantify how these parameters influence extraction efficiency, providing insights into the SCFE process mechanisms and identifying optimal conditions. Our findings highlight the potential of SCFE as a sustainable alternative for REE extraction from primary resources with complex matrices. By significantly reducing hazardous waste and potentially utilizing atmospheric CO₂, this method aligns with global sustainability goals. This research not only contributes to advancing REE extraction technologies but also highlights the importance of exploring green chemistry solutions in critical material recovery for future technologies.