The use of organophosphorus extractants as a component of hydrophobic deep eutectic solvents (HDES) for the processing of spent lithium‑iron phosphate batteries

Abstract

Hydrometallurgical processes for managing industrial waste have attracted significant attention due to tightening global standards on toxic emissions. Among these processes, the use of non-volatile hydrophobic deep eutectic solvents (HDESs) has emerged as a promising approach to optimising extraction processes. In this study, HDESs incorporating tributyl phosphate (TBP) and di(2-ethylhexyl)phosphoric acid (D2EHPA) were investigated in the context of the extraction and separation of elements found in lithium‑iron phosphate batteries, including lithium, aluminium, iron and copper. The physical properties of the HDESs, such as density, viscosity and refractive index, were characterized and the interactions between their components were analysed using infrared spectroscopy. Considering the different classes of extractants represented by D2EHPA and TBP, extraction efficiency for target metals was evaluated across a range of hydrochloric acid concentrations (1–10 mol/L). Optimal conditions for re-extraction (stripping) were identified for extractant regeneration and the production of individual metal ion solutions. Results demonstrated that Fe³⁺ ions could be extracted with an efficiency exceeding 99% across the majority of acidity levels, while Al³⁺ ions exhibited similar efficiency from a pH of 1.4. In contrast, Cu²⁺ ions showed limited extraction (<5%) at lower pH values but the level of extraction increased to 50% at pH 1.9 and above. Leveraging these findings, a sequential extraction scheme is proposed for Al³⁺, Cu²⁺, Fe³⁺ and Li⁺ from their mixture, based on a gradual reduction in solution acidity.