Cadmium Refining in Chloride Melts Using a Bipolar-Electrode Electrolyzer

Abstract

The kinetics and mechanism of the formation of subvalent cadmium complexes in the Cdo – CdCl2 – ZnCl2 – NH4Cl system were studied spectroscopicaly. The results indicate that metallic cadmium reacts with Cd2 2+ and Cd2 3+ complex cations. The formation of Cd2 2+ and Cd2 3+ is evidenced by the absorption bands around 270 and 335 nm, respectively, in the electronic spectrum of the melt. The variation of the intensity of absorption bands during the reaction between Cd2+ and metallic cadmium was used to evaluate the kinetics of Cd2 2+ and Cd2 3+ formation. The rate constants for the formation of Cd2 2+ and Cd2 3+ complex cations evaluated from the slop of plot of lg(Ao - A∞) versus time are 4.22∙10-3 and 3.50∙10-3 s-1 , respectively. The effect of the electrolysis conditions on the rate of cadmium exchange between the electrodes was examined. The anode current efficiency of cadmium dissolution was found to increase with current density and temperature and to exceed the theoretically predicted value, which was attributed to the formation of subvalent cadmium compounds. The present results may be helpful in assessing parameters of cadmium electrorefining. A method was proposed for electrolytic Cd refining in which electrolysis is performed in molten cadmium chloride containing Cd in lower oxidation states. The technique for the preparation of the electrolyte was described. To prevent impurity transfer from the anode to cathode, bipolar Cd electrodes were used. The distribution ratios evaluated for typical metallic impurities in Cd show that high-purity Cd can be obtained in a single electrolysis cycle.