Extraction of Cobalt, Nickel, Magnesium, Manganese, Zinc, and Calcium from Chloride Solutions Using Trioctyl(alkyl)phosphonium Chloride Ionic Liquids

Trioctyl(alkyl)phosphonium chloride ionic liquids ([P_888n][Cl], n = 8, 14, and 16) were synthesized, characterized, and investigated for the extraction of Co, Ni, Mn, Mg, Zn, and Ca from chloride solutions. The three ionic liquids were very effective for the extraction of cobalt (over 95%) from solutions containing 1 g/L of Co(II) and 4 M HCl or NaCl. Equilibrium cobalt extraction was attained in less than 10 min at 25 °C using the most viscous ionic liquid [P₈₈₈₁₆][Cl]. Based on a speciation diagram for cobalt–chloride species and ultraviolet–visible spectrometric analysis of the phases, it was concluded that Co(II) extraction involved the extraction of the neutral species CoCl₂. Only at high chloride concentration, the anionic exchange mechanism involving $\mathrm{C}\mathrm{o}{\mathrm{C}\mathrm{l}}_4^{2-}$ was the most dominant. The stripping of the loaded ionic liquids can be carried out with water, and the stripped ionic liquids can be recycled up to five times maintaining their extraction effectiveness. In all of the conditions tested, the selectivity of [P_888n][Cl] ionic liquids for the extraction of cobalt over nickel was great, with a separation factor over 25,000 for 5M HCl solutions. Furthermore, very good selectivity for Co(II) over Mg(II) and Ca(II) extraction was also obtained. Conversely, Zn(II) can be selectively extracted over Co(II) and Mn(II) using only diluted [P₈₈₈₁₄][Cl].