An ion-exchange process with thermal regeneration XI. The Ca/Na selectivity behavior of weakly acidic resins

Abstract

Calcium-sodium selectivity data for a number of carboxylic acid resins were obtained under constant solution conditions with the object of determining the effects of variations in resin structure. As weakly acidic resin ionization is pH dependant, plots of selectivity versus fraction ionized were obtained from measurements over a range of pH values. Calcium retention was found to be favoured by higher levels of resin ionization, greater degrees of crosslinking, by deliberate building in of chelating residues, and in commercial resins by acrylic over methacrylic acid types. In experimental composite resins comprising micro particles of active material embedded in an inert matrix, no difference in behavior of the two acid types was discernible, but the methacrylic variety is preferred for low calcium retention because of its more weakly acidic nature. The enhanced performance of a snake-cage acidic resin containing polyether and of a copolymer system containing ether groups in this regard was attributed to their greater swelling tendencies, arising from the incorporation of hydrophilic groups. In another copolymer system, however, the presence of amide groups was found to approximately halve the high affinity of acrylic acid resins for calcium, in particles which had normal swelling properties, and a reasonable high exchange capacity of 8.4 meq/g.