Column separation of nickel and cobalt using a microencapsulated extractant

•Adsorption of metals correlated well with the Langmuir adsorption isotherm equation.•Metal complex formation and its diffusion in the microcapsule pores were key factors.•Reducing feed flow rate and increasing column height enhanced the metal separation.•Separation of Ni and Co achieved in five repetitions of adsorption–desorption step

The adsorption of nickel, cobalt, copper and zinc onto microcapsules containing 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA) was investigated. Through scanning electron microscopy imaging of the microcapsule-surface and neutralization titration estimation of the amount of EHPNA extractant remaining in the microcapsule, the microencapsulated extractant was characterized. The equilibrium of adsorption for each of the metals was measured and applied to a Langmuir adsorption isotherm, and from this and the metal adsorption rates, it was estimated that both the formation of a metal complex and its diffusion through the pores of the microcapsule contribute to the overall adsorption rate of the metals. Furthermore, the separation ability of the metals was found to be enhanced by reducing the flow rate of the feed solution or increasing the height of the packed column. By repeating the adsorption–desorption of a Ni–Co mixture five times, the Ni and Co were effectively separated.