This paper describes the optimization of a solvent extraction plant used for the enrichment of uranium by redox chemistry and chemical exchange. The basic unit of the plant is a standard mixer-settler where the organic and aqueous phases are in chemical equilibrium. Repeated contact of the phases transfers small amounts of U-235 into the organic phase and equally small amounts of U-238 into the aqueous phase. The mixer-settler units, all of the same size, are grouped into a countercurrent train. The plant or cascade as a whole consists of a number of trains. Each of these consists of mixer-settlers of different sizes, and the number of stages in the trains varies; The optimization procedure is a four-stage process designed to minimize the whole plant size.