Ion-pair extraction and targeted separation of highly hydrated nickel sulfate

The extraction separation of nickel from sulfuric acid media represents a pivotal yet challenging process in hydrometallurgy, particularly for recycling complex secondary resources. Conventional cation-exchange extraction systems often struggle with the elemental similarity, typically requiring complex pretreatment or strict pH control. This work presents a novel ion-pair extraction strategy employing 2-(5-octyl-1H-pyrazol-3-yl) pyridine (denoted as L2) for the targeted separation of nickel sulfate. This extractant exhibits exceptional selectivity following the order Ni > Co > Fe > Zn > Mn ≈ Al. Especially, Ni extraction remains essentially independent of pH and the extraction efficiency of nickel always remains above 95% over the pH range 0~2.0. Even at a Ni/Al molar ratio of 1:100, aluminum co-extraction remains below 1%, demonstrating exceptional selectivity. Spectroscopic analyses reveal that L2 extracts nickel from sulfate media via an ion-pair mechanism, where sulfate ions act as outer-sphere counter-anions to stabilize the cationic complex Ni(L2)2(H2O)22+. The introduction of di-(2-ethylhexyl) phosphoric acid (P204) as a modifier effectively suppresses third-phase formation by displacing sulfate as the charge-balancing species and enhances solubility of the extracted species. This ion-pair extraction strategy enables direct nickel recovery under highly acidic conditions, offering an efficient pathway for recycling secondary resources.