Improved rare earth element recycling using a sustainable diglycolamide-based hydrophobic eutectic solvent

Abstract

The sustainable recycling of rare earth elements (REEs) is essential for environmental preservation due to their significant role in facilitating green technologies. These elements are crucial for the ecological transition, especially in renewable energy systems. Hydrometallurgical techniques, such as liquid–liquid extraction, are commonly used but often rely on a large volume of environmentally harmful volatile solvents. Growing environmental concerns are encouraging industries to reduce their reliance on these compounds, challenging the traditional extraction methods. Neoteric systems, such as deep eutectic solvents (DESs), offer promising alternatives with advantageous physical properties and, in some cases, better extraction performance. Despite their potential, they remain underutilized because they require further optimization and understanding. This study focuses on the hydrophobic eutectic solvent (HES) N,N,N′,N′-tetraoctyldiglycolamide (TODGA) and decanoic acid for the extraction of REEs from a nitric acid leachate. The results are complemented by a systematic comparative study between the hydrophobic eutectic solvent and the traditional solvent equivalent for REE extraction. The HES exhibits enhanced extraction performance and achieves extraction efficiencies for the lanthanides with distribution coefficients ranging from 30 to 20 000. Under the conditions considered, the extraction efficiencies are greater than 96%, demonstrating selectivity against iron, with a SF_Dy/Fe > 10 000. UV-vis-near infrared measurements support the hypothesis of a similar extraction mechanism to that in the conventional reference solvent, attributing the higher obtained extraction efficiency to the greater concentration of TODGA in the HES, without the need for a phase modifier to prevent phase separation. The eutectic mixture also exhibits lower volatility (16 times less than the conventional system studied) as measured by thermal gravimetric analysis (TGA), improving the operational safety and environmental sustainability of the extraction process. A life cycle assessment further demonstrates this improvement, showing more favourable results for the HES system to purify 1 kg of neodymium (Nd).