Controlled partial neutralization of amphoteric ion exchange resin for improved metals separation

Controlled partial neutralization of an amphoteric aminomethylphosphonium functional ion exchange resin (Lewatit TP-260) was investigated. The particular goal of controlled neutralization was to replace H+ in the acidic phosphonium groups with Na+ ions but leave the basic amino groups protonated. In this manner, metals' sorption is enhanced and undesired pH changes during the metal separation process are avoided. The conjugate bases of organic weak acids (sodium formiate, sodium acetate and sodium citrate), sodium phosphate and NaOH were studied as neutralizing agents. The organic acid salts and sodium phosphate were all found to be suitable. Lengthening the neutralization cycle and using more concentrated neutralization agents were found to yield higher extents of neutralization and more uniform profiles along a resin bed. Concentrated NaOH also neutralized the amine group from the resin. With dilute NaOH, desired extent of neutralization was not achieved within a reasonable time. The effect of partial neutralization on metal sorption was tested with dynamic single column experiments. 5.5 BV of Ag-NaCl solution could be purified from Ca, Mg, Pb and Zn when the resin was partially neutralized with 2.0 M NaAc. With the acid form resin, the impurity metals broke through at 1.0 BV.