Exchange in Countercurrent Columns

Ion-exchange processes can be differentiated by assignment to one of two groups Group I replacement of ions of one kind by ions of another, for example, purification of nonelectrolytes or weak electrolytes firom strong electrolytes, softening and demineralization of water, recovery of substances from solutions, etc. Group II separation of mixtures of dissolved electrolytes. 

The problems related to the first group are most easily solved. In the sample cases listed, passing of solution through a resin bed results in the replacement of the electrolyte solution ions by resin ions. Depending on the nature of the exchange equilibrium isotherm and the kinetic peculiari- 

As a rule in these processes that can be classified as ion replacement or recovery operations, the conditions are selected to assure a highly convex sorption isotherm. With the development of such a sorption isotherm, the lull recovery of the ions of interest does not require sizable multiplication of the separation reached with one theoretical plate. In view of this, the application of countercurrent columns has been initiated to facilitate further the resolution of these relatively simple problems. 

A more complex field of ion-exchange application is associated with the separation of mixtures of substances that react similarly with ion-exchange resins. In this case the separation is based on differences in resin selectivity toward the mixture s components. These differences are characterized by the magnitude of the equilibrium separation coefficient which can be represented for a pair of ions as follows  

The particular problem of separating mixtures of substances with similar properties when the equilibrium separation coefficient, a, does not differ greatly from unity can be solved only by using effective columns. The application of countercurrent operations in these processes is still confined to laboratory or semi-industrial experimental installations. 

---