Direct extraction of Ni/Co from Ca/Mg-rich sulfate aqueous solutions with a novel solvent extraction system of D2EHPA–MSL255

MSL255 was synthesized as a pyridyl ligand for Ni/Co extraction. • D2EHPA–MSL255 gave β(Ni/M) > 249 and β(Co/M) > 41 over impurities. • Over 99% of Ca, Mg and Mn was removed with minimal Ni loss. • Co and Ni were stepwise stripped at different acidities. • FT-IR, MS and DFT revealed selective Ni/Co coordination. Complex acidic Ni/Co liquors from low-grade ores and secondary resources usually contain abundant impurity cations, making selective Ni/Co enrichment highly challenging. Industrial routes for producing mixed sulfide precipitate (MSP) or mixed hydroxide precipitate (MHP) are technically mature, but often suffer from long flowsheets, Ni/Co losses, and impurity-rich intermediates requiring further purification. Direct solvent extraction offers a promising alternative owing to its high selectivity, efficient separation, and easy automation. However, conventional acidic organophosphorus extractants, such as D2EHPA, PC-88 A, and Cyanex 272, suffer from limited Ni/Co selectivity due to severe impurity co-extraction. Moreover, some reported synergistic systems remain constrained by insufficient acid resistance, limited loading capacity, or poor industrial applicability. To address these issues, a neutral ligand, MSL255, integrating two pyridyl groups with a hydrophobic aromatic ether framework, was designed and synthesized. When combined with D2EHPA, the optimized formulation provides pronounced synergistic enhancement, yielding synergistic factors (SF) of 87.2 for Ni(II) and 23.3 for Co(II), while impurity ions exhibit SF < 1. Correspondingly, the separation factors (β) reach β(Ni/M) > 249 and β(Co/M) > 41. The system removes more than 99% of Ca, Mg, and Mn with minimal Ni loss, and displays a clear stepwise stripping sequence in which Co is released at lower acidity whereas Ni requires higher acidity for complete stripping. Spectroscopic characterization together with DFT/ESP analysis confirms that MSL255 stabilizes Ni/Co–extractant adducts while weakening the coordination of impurity ions, thereby enhancing selectivity. Overall, the D2EHPA–MSL255 system provides a low-acidity, high-selectivity strategy for Ni/Co enrichment from Ca–Mg–Mn sulfate media.