Kinetics of ion exchange in polyphase systems including crystallizing substances

Abstract

Kinetics of ion exchange has been investigated in triphase systems: (1) strong-base anion-exchanger AV-17 × 8 in the CO 3 2− form-CaCI 2 solution-CaCO 3 precipitate; and (2) sulfonate cation-exchanger KU-2 × 8 in the Ca 2+ form-Na 2CO 3 solution-CaCO 3 precipitate, and in quadriphase systems: (1) AV-17 × 8 in the CO 3 − form-CaSO 4 · 2H 2O precipitate-CaSO 4 saturated solution-CaCO 3 precipitate; and (2) AV-17 × 8 in the CO 3 2− form-KU-2 × 8 in the Ca 2+ form-Na 2CO 3 solution-CaCO 3 precipitate. It has been shown that in both tri- and quadri-phase systems, the rate of cation exchange is lower than that of anion exchange. The amount of the precipitate crystallizing on the surface of resin beads depends on the type of ion-exchange resin, its granulation and on the ratio of resin components in the case of quadriphase systems with a mixed bed. By varying this ratio and the granulation of the ion-exchangers it is possible to accomplish the precipitation mostly in the anion- or cation-exchanger. Under certain conditions the precipitation takes place mainly in the solution phase. The mathematical model of ion-exchange process accompanied by precipitation on the surface of the resin beads has been proposed. The rate of the ion-exchange process has been assumed to be controlled by diffusion within the porous layer of the precipitate fixed on the surface of the bead. As an example of practical application of the results obtained the method of simultaneous regeneration and separation of mixed bed resins exhausted in a water demineralization process has been proposed.