REMOVAL OF CADMIUM BY ANION EXCHANGE IN A WET PHOSPHORIC ACID PROCESS. PART 1: CHEMISTRY

ABSTRACT The removal of cadmium ions by anion exchange from 55-65 w% H3PO4 at 90°C in the presence of small amounts of halides was investigated. Iodide appears to be the most effective additive. Cd distribution coefficients, K = [CD]/[Cd] (w/w), of a few thousand were obtained at equilibrium concentrations of a few hundred ppm iodide in the solution. The stability constants of CdIi. complexes in 55-65 w% H3PO4 are much higher than in diluted aqueous solutions and the negatively charged CdIi. complexes have a high affinity for the amine groups of the resin. Stability constants Bi. of CdIi. complexes in concentrated phosphoric acid were determined by using differential pulse polarography. A tentativedescription of the complexation and ion-exchange equilibria assuming adsorption of only one negatively charged Cd species(CdI 3 −) or CdI4 2−) is presented. The removal of I- from the phosphoric acid solution exchange was also investigated. Iodide distribution coefficients of about 300 w/w were obtained for a weekly basic resin at an iodide loading of 0.22 mol/kg dry resin.