A stage extraction process is depicted in Figure. In such systems, a stream containing a weight fraction Y_in of a chemical enters from the left at a mass flow rate of F₁. Simultaneously, a solvent carrying a weight fraction X_in of the same chemical enters from the right at a flow rate of F_2. Thus, for stage i, a mass balance can be represented as

F₁Y_i-₁ + F₂X_i+₁ = F₁Y_i + F₂X_{i? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??}(P a)

At each stage, an equilibrium is assumed to be established between Y_i and X_i as in

K = X_i/Y_{i? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?}(P b)

Flow w F Kost -1 Jout -Flow F