Efficient Separation of Co²⁺ from Ni²⁺ in Hydrochloric Acid Solution Using a Novel Ionic Liquid of [N₈₈₈]₂[SO₄]

In this study, five novel ionic liquids (ILs), namely [N888][Cyanex 272], [N888][P507], [N888][Cl], [N888][NO3], and [N888]2[SO4], were synthesized through an organic acid–base reaction, followed by separation using rotary evaporation. Their structures were confirmed by 1H and 13C NMR spectroscopy, showing full consistency with theoretical predictions and no impurity peaks. Under identical extraction conditions, the ILs exhibited the following separation efficiency order for Co2+ from Ni2+: [N888]2[SO4]>[N888][Cl] > [N888][P507] > [N888][Cyanex 272]>[N888] [NO3]. Density functional theory (DFT) calculations revealed that these differences in separation performance arose from variations in the ionic radii of the IL anions. With a simple single-stage extraction under optimized conditions, [N888]2[SO4] enabled near-complete separation of Co2+ from Ni2+, achieving an extraction efficiency of 99.5 ± 2.14% and a separation factor approaching + ∞. Thermodynamic analysis confirmed that the extraction process was endothermic and nonspontaneous. The Co2+ -loaded IL-based extraction phase was efficiently stripped using diluted sulfuric acid, with a stripping efficiency of 99.3 ± 0.57%. Using sodium hydroxide as a precipitant, the solid powder of Co(OH)2 was obtained from the Co2+-containing aqueous phase. Co(OH)2 was characterized qualitatively and quantitatively by XRD and ICP-OES, and its morphology was also analyzed using SEM. The recycled IL-based extraction phase, after activation with dilute sulfuric acid, was analyzed via FT-IR spectroscopy. This regenerated IL-based extraction phase was reused for the separation of Co2+ from Ni2+, showing no degradation in performance after three cycles. Based on these findings, a process flowchart was designed for the efficient preparation of Co(OH)2 from minerals or secondary resources using the [N888]2[SO4]-based extraction phase.