Solvent extraction is widely used for selective separation of metals from solutions. Ionic liquids are showing potential for this purpose. To date, little research has focused on design, operation and optimization of solvent extraction flowsheets using ionic liquids. This work addresses this gap in knowledge, aiming to support development, design and optimization of such solvent extraction processes. In this work, a general flowsheet simulation model is developed and applied for the case of cobalt–nickel separation using ionic liquid trihexyl(tetradecyl)phosphonium chloride ([P66614]Cl). All components are treated as distributing between the two phases and are modelled using distribution coefficient models derived from published experimental data and ab initio computational results. The rate of mass transfer between the two phases is calculated using a mass transfer model. Simulation results are shown to be generally in good agreement with published experimental results.