A basic mathematical model, mainly focused on economic assessment is directly applied to a counter-current, stage-wise, solvent-selective extraction system. A standard extraction model, based on the modeling of a mixer-settler process, is used as point of departure. This system is used to extract a dissolved compound in a process stream. Thereafter sensible operating and economic parameters are integrated into it. An adequate handling of these parameters allows a practical preliminary optimization process modeling. The economic assessment model can be applied from one to "n" stages in any industrial stage-wise process unit. An optimum number of stages are determined based on solvent extraction and economic assessment.