Clean ion-exchange technologies. 3. Temperature-enhanced conversion of potassium chloride and lime milk into potassium hydroxide on a carboxylic ion exchanger

This paper reports the results obtained by studying the ion-exchange synthe sis of potassium hydroxide from lime milk and carboxylic resin Lewatit CNP 80 in the K-form. Carboxylic resin has been shown to have several advantage s in comparison with, for example, sulfonate ion exchanger, due to its high er selectivity toward Ca2+, which substantially enhances at elevated temper ature. The increase of temperature from 293 to 333 K enhances the sorption of Ca2+ versus K+ that allows achieving far higher KOH concentration in the solution phase. The values of equilibrium separation factor, or, for Ca2+- K+ exchange have been determined at 293 and 333 K by varying the equivalent fraction of Ca2+ in the solution phase from 0.04 to 0.7. A remarkable incr ease of ct values has been observed at higher temperatures and at low calci um content in the solution phase. The regeneration of the resin (conversion from Ca- to K-form) has been carried by using the mixtures of potassium ch loride and potassium sulfate. The regeneration process under these conditio ns is accompanied by the ion-exchange isothermal supersaturation of calcium sulfate, which forms a stable supersaturated solution in the resin bed. Af ter leaving the column, CaSO4 crystallizes spontaneously, which allows reus e of the regenerating solution following the complete elimination of the Ca 2+ admixture with a small amount of K2CO3. The precipitates of calcium sulf ate and calcium carbonate are the only wastes produced in the process. The complete regeneration of the resin has been shown to require a nearly 70-fo ld excess of the regenerant; nevertheless, the decrease in the degree of re sin regeneration from 100 to 80% allows reduction of the amount of the rege nerant by more than 2 times. The use of incompletely regenerated resin for the synthesis of KOH does not decrease remarkably the efficiency of the pro cess. The flow sheet of the proposed process is presented and discussed.