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.