The separation of nickel(II) from base metal ions using 1-octyl-2-(2′-pyridyl)imidazole as extractant in a highly acidic sulfate medium

In this study, 1-octyl-2-(2′-pyridyl)imidazole (OPIM), along with dinonylnaphthalene sulfonic acid (DNNSA) as a synergist, was investigated as a potential selective extractant of Ni2 + from base metals in a solvent extraction system using 2-octanol/Shellsol 2325 (4:1) as a diluent and modifier, respectively. The separation of Ni2 + from the borderline and hard acids; Co2 +, Cu2 +, Zn2 +, Fe2 +, Fe3 +, Mn2 +, Mg2 + and Ca2 + was carried out at a pH range of 0.5–3.5 in sulfate and sulfate/chloride media. The separation of Ni2 + from Co2 + in 0.001 M solutions was achieved with an optimised concentration of 0.025 M OPIM along with 0.02 M DNNSA to the tune of a ∆pH½ ≈ 1.6. The extraction system further proved the rejection of 0.001 M Fe3 +, Mn2 +, Mg2 + and Ca2 +. The absence of DNNSA resulted in very low extraction efficiencies of the Ni2 + metal ions in the pH range under investigation, thus highlighting the role of the synergist (DNNSA). The three-stage counter-current extraction of Ni2 +, at the optimised pH of 1.89, from a synthetic mixture of Ni2 +, Co2 + and Cu2 +, yielded 99.01 (± 1.79)%. The total co-extracted Cu2 + was 48.72 (± 0.24)% of the original quantity in the mixture, and it was 19.85 (± 0.28%) for Co2 +. The co-extracted Cu2 + was scrubbed off from the loaded organic phase at pH ≈ 8.5 by using an ammonium buffer, while co-extracted Co2 + was selectively and quantitatively stripped with H2SO4 at pH 1.64. The total recovery of Ni2 + by stripping at pH 0.32 was 94.05 (± 1.70)%. The underlying chemistry in this extraction system was studied through solid state/solution studies for the complexation of the ligand with the base metals, and it is apparent from the qualitative/semi-quantitative data that the separations achieved are of kinetic origin rather than thermodynamic, nor are they being driven by stereochemical preferences. Therefore, 1-octyl-2-(2′-pyridyl)imidazole can be effectively utilized alongside DNNSA as a co-extractant in the separation of Ni2 + from base metals in acidic sulfate and sulfate/chloride media.