Greener Route for Recovery of High-Purity Lanthanides from the Waste of Nickel Metal Hydride Battery Using a Hydrophobic Deep Eutectic Solvent

Abstract

A hydrophobic deep eutectic solvent (HDES) based on trioctylphosphine oxide and decanoic acid was successfully exploited to separate lanthanides from battery acid leachates. Subsequently, chemical conditions were defined for stripping the metal ions to the aqueous phase, quantitative separation of La and Ce, and their recoveries as La2(C2O4)3 and Ce(OH)4. To achieve a higher yield and greenness, the process was optimized in relation to the acid used for waste leaching (HNO3, H2SO4, and CH3SO3H), the mass ratio of the aqueous to hydrophobic phase (WP:HP), experimental conditions for the back-extraction of lanthanides (Ln) to an aqueous phase, and their recovery as precipitates. Leaching with 2.0 mol L–1 HNO3 was more selective for Ln in relation to the transition metals and further yielded better extraction and separation with HDES. High-extraction efficiencies (96% La and 98% Ce) were achieved at a WP:HP of 1:8 with separation factors of 763 for La/Ni and 1149 for Ce/Ni. Ln was stripped from the HDES phase with 4.0 mol L–1 HCl solution, and the metals were recovered as Ce(IV) hydroxide and La(III) oxalate. The extraction mechanism was proposed, and the metal recoveries at each step of the procedure were evaluated. The global recoveries were 86.8% La and 97.6% Ce, and the solids were obtained with >99.6% purity, as characterized by energy-dispersive X-ray spectrometry coupled with electron scanning microscopy and inductively coupled plasma optical emission spectrometry analysis. The recycling and reuse of HDES for at least five extraction cycles without affecting recovery or selectivity were demonstrated, increasing the environmental friendliness of this approach. This proposal stands out from the environmental and economic perspectives owing to the sustainable recovery of critical raw materials from a secondary source using an alternative solvent with minimum separation steps.