Recovery of pure metal sulfate solutions from a synthetic sulfuric acid leachate of base-rare earth metals representing spent Ni-MH batteries through selective precipitation, stepwise ionic-liquid/solvent extraction and stripping

Nickel-metal hydride (Ni-MH) batteries contain cobalt, manganese, nickel, and rare earth elements. To meet the demand for these critical metals, it is essential to recover them from secondary resources. In this study, separation experiments were conducted to recover pure cobalt, manganese and nickel solutions from a synthetic sulfuric acid leachate of spent Ni-MH batteries. The synthetic solutions employed in this study contained Co(II), Fe(III), Mn(II), Ni(II), Zn(II), Ce(III), La(III), and Nd(III). First, Fe(III) was removed by precipitation as Fe(OH)₃ at room temperature. Second, an ionic liquid prepared by reacting Aliquat 336 and Cyanex 272 was used to selectively extract Zn(II) and REEs(III) from the filtrate. Third, Co(II) and Mn(II) were simultaneously extracted using saponified Cyanex 272, leaving pure Ni(II) in the raffinate. Fourth, after stripping of Co(II) and Mn(II) from the loaded Cyanex 272, Mn(II) was selectively extracted over Co(II) with D2EHPA. The optimal conditions for each separation step were determined by analyzing the effects of various parameters. As a result, a process consisted of precipitation and solvent extraction was proposed to recover pure cobalt, manganese and nickel sulfate solutions from the synthetic sulfuric acid leachate of spent Ni-MH batteries. Moreover, all separation processes were carried out at room temperature, reducing energy consumption.