Separation of Quaternary Conjugate Salt Systems by Fractional Crystallization

A method is presented that synthesizes fractional crystallization separation processes to obtain pure solids from conjugate salt solutions. The method, which includes a procedure to generate phase diagrams by using solubility products, explains how to calculate concentrations of all components in a system given a point on a Jänecke projection. Both multivalent ions and species that exhibit complex-ionic equilibrium are considered. For solutions from which only simple salts crystallize, three separation classes are identified. The classification is based on the composition of the feed, the characteristics of the phase diagram, and the product salts. Class I separates compatible salts, Class II incompatible salts, and Class III two compatible salts and one incompatible salt. Process paths and design equations for each class are also presented. Guidelines proposed select the separation sequence and the stream compositions based on the effects of the design variables on stream flows and on total annual cost. The method is applicable to systems in which hydrates and double salts are present. An example given shows how to obtain simple salts from these systems.