Cobalt–nickel separation from acidic thiocyanate leach solutions by emulsion liquid membranes (ELMs) using TOPO as carrier

An ELM process using tri-noctylphosphine oxide (TOPO) to extract cobalt from the zinc plant acidic thiocyanate leach solutions containing cobalt and nickel has been presented. ELM consists of a diluent (kerosene), a surfactant (SPAN 80), and a carrier (TOPO). 6 M NH3 solution has been used as stripping solution. In this work, the zinc plant copper cake (CINKUR Co., Turkey), leached with hydrochloric acid, was subjected to an initial separation process in order to eliminate some interfering metallic ions. After initial separation processes, ammonium thiocyanate and water were added to remaining leach solution. Thus, acidic thiocyanate leach solutions used in ELM process were obtained. Then, the important parameters governing the permeation of cobalt and their effect on the separation process have been studied. These parameters are membrane type and composition, mixing speed, pH and thiocyanate concentration of feed solution, surfactant and extractant concentrations, NH3 concentration of the stripping solution, phase and treatment ratios. The parameters mentioned were investigated and optimum conditions were determined. It is possible to selectively extract 99% of cobalt at the optimum conditions. The separation factors of cobalt with respect to nickel, based on initial feed concentration, have experimentally found to be of as high as 480 for about equimolar Co–Ni feed solutions.