A novel electrodialysis process has been developed for the recovery of rare-metal elements from their EDTA complexes. The electrodialyzer used for the experiments was composed of five compartments divided by a cation-exchange membrane, C, and an anion-exchange membrane, A. The compartments were set in the order of Anode, Feed, Reaction, Strip, and Cathode compartments, and were partitioned with the A, C, C and A membranes, respectively. The Feed solution, the Reaction solution, and the Strip solution, each of which flowed through the corresponding compartment, contained CuCl
2, rare metal-EDTA complex, and HCl solution, respectively. The electrodialysis for the recovery of rare-metal elements was expected to proceed as follows: When voltage is applied to the electrodialyzer, Cu
2+ should proceed from the Feed compartment to the Reaction compartment, and act as the substitute for rare-metal elements in the EDTA complexes. The resultant free ions of rare-metal elements move to the Strip compartment, and are recovered.
In the experiments, Co, La, Gd, Y, and Ni were selected as rare-metal elements. In the electrodialysis experiment for the Co-EDTA system, Cu
2+ from the Feed compartment was immediately substituted for Co, and part of the resultant Co
2+ moved to the Strip compartment. No permeation of Cu-EDTA and free Cu
2+ was observed during the dialysis. The flux of Co
2+ increased with an increase in current density. For the systems of La-EDTA, Gd-EDTA, and Y-EDTA, the permeation behavior of metal ions was almost the same as that for the Co-EDTA system. For the Ni-EDTA system, however, Cu
2+as well as Ni
2+ proceeded to the Strip compartment: This suggests that for the Ni-EDTA system the metal substitution reaction is much slower than that for the other metal elements. Moreover, the substitution reaction rates of Cu for the rare-metal elements in the EDTA complexes were measured spectroscopically and analyzed with due consideration of dinuclear complex intermediates. The rate constants were smaller in decreasing order-La-EDTA, Gd-EDTA, Y-EDTA, and Ni-EDTA and were correlated with the stability constants for EDTA.
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