Separation of Li(I), Co(II), Ni(II), Mn(II), and Fe(III) from hydrochloric acid solution using a menthol-based hydrophobic deep eutectic solvent

The process of recovering valuable materials from lithium-ion batteries is important because of the economic and environmental problems associated with these batteries. In this study, a comprehensive process based on the combination of a hydrophobic deep eutectic solvent (HDES) for the extractive separation of Li(I), Co(II), Ni(II), Mn(II), and Fe(III) and subsequent recovery was developed. For the first time, Aliquat 336 and environmentally friendly L-menthol were combined to form HDESs. The solid–liquid equilibrium phase diagram of the Aliquat 336 and L-menthol mixture was analysed to determine the hydrogen bond donor and hydrogen bond acceptor molar ratios corresponding to eutectic points. The physicochemical properties of the two HDESs (density, refractive index, and viscosity) were determined. Thereafter, the extraction properties of a more convenient HDES corresponding to a 3:7 Aliquat 336/L-menthol molar ratio were investigated to determine the optimal conditions for metal separation. One mol/L HCl was added to the metal solution, and then HCl up to 3 mol/L and LiCl up to 5 mol/L were added, resulting in 99% recovery rates for each of Fe(III), Mn(II), and Co(II) for each extraction step. The stripping of each metal to the aqueous phase with an efficiency of up to 99% allows the reuse of both the metal and extractant. The Ni(II) separation via precipitation leads to a concentrated hydrochloric solution of Li(I), and thus, both metals can be reused. In the final process, Aliquat 336/L-menthol-based HDES was used as the only extractant for efficient metal separation, without using volatile and toxic organic solvents.