The synthesis method of fractional crystallization processes for the separation of salts belonging to reciprocal salt pairs is presented based on a flow network model constructed between thermodynamic states. The state nodes represented in the model include feeds, products, intermediate products, and solid–liquid equilibrium states. Flows between the states are represented by arcs within the flow model. A systematic procedure is given for identification of the solid–liquid equilibrium states that could potentially serve as operating points. This identification is essential to the construction of the flow network and reduces the problem to a simple mathematical form. The advantages of the method lie in its capability to consider general process flow pattern, handle systems forming double salt, deal with several temperatures as potential operation conditions, and handle several feeds and products. Examples are given to illustrate the procedure.