Vanadium Electrorefining in NaCl–KCl–VCl2 Melts

Abstract

Molten salt mixtures can be effectively employed for electrowining and electrorefining of relatively chemically active rare refractory metals having negative reduction potential. One of such metals is vanadium. However quality of the metal produced (impurities content and size of crystals in the deposit) and current efficiency in a standard vanadium electrorefining process does not fully correspond to the requirements. The current work was carried out aiming to determine the optimal conditions of operating industrial vanadium electrolysis baths. The experiments on vanadium electrorefining were carried out in a semi-industrial electrolyser made of stainless steel. Crude vanadium metal was loaded in an anodic basket made of molybdenum. The experiments were performed in dry purified argon atmosphere, and glassy-carbon or nickel crucibles were used to contain the melt. Vanadium-containing melts were prepared by dry chlorination of vanadium metal with subsequent absorption of chlorination products by fused salt mixtures. To stabilize vanadium(II) ions, the molten electrolyte was kept in contact with vanadium for 12–18 hours. Mean oxidation state of vanadium in the melt determined oxidimetrically was always close to two and didn’t depend on vanadium concentration. The method of mathematical planning of experiments was used to determine optimal conditions of electrolysis. Current efficiently was chosen as a response factor. Variable parameters and their intervals were chosen on the basis of the preliminary experiments and the results of kinetic studies. Current density, vanadium concentration and specific quantity of electricity were selected as variables. Temperature had no effect on the current efficiency and increasing temperature only resulted in rising vapor pressure of the melt. Therefore, temperature was not included into variable parameters. Anodic current density was kept essentially constant between 0.1 and 0.15 A/cm2. Such anodic current density allowed maintaining average oxidation state of vanadium ions close to two in the course of the experiments. The 23 full factor experiment was performed and the regression equation was obtained using the experimental results. All variable parameters were significant and the equation obtained was adequate. The influence of the quadratic term was insignificant. The response function was then obtained in the form of a dependence of the cathodic current efficiency on current density, vanadium concentration and specific quantity of electricity passed. The most suitable parameters of vanadium electrorefining were determined using this equation and the experiment under the optimal conditions was carried out, Figure. The total amount of impurities in the refined product was below 0.09 wt. % and the current efficiency equaled to 0.96 g/A∙h. Figure. High-purity electrolytic vanadium Figure 1