Interaction of Niobium-Containing Chloride Melts with Niobium Metal

Growing application of niobium and its alloys in electronics, chemical technology, aerospace and nuclear industry leads to increasing world demands for high-purity metallic niobium. Electrolytic refining in molten chlorides is one of the most prospective methods of high-purity niobium production. Prolonged contact of niobium-containing melts with metallic niobium can be employed for stabilizing necessary niobium species in the electrolytic baths. A complex of independent physical and chemical methods of analysis was used to study the mechanism of niobium interaction with niobium chloro-species in fused salts. In the current study, we found that upon contacting metallic niobium with a melt containing higher oxidation state niobium ions, the average oxidation state of niobium in the salt phase, mass of the metal and potential difference between niobium and an indicator electrodes decreases. After a certain period of time, these parameters tend to achieve certain constant values. This moment corresponds to the stationary state of the system. Analysis of the experimental data showed that the mixture of niobium (III) and (IV) ions is the final product of interaction of niobium-containing chloride melts with niobium metal and therefore, a mixture of niobium ions in two oxidation states is present in equilibrium with the metal. The average oxidation state of niobium in the obtained melts (after contacting with Nb metal) increases by increasing the niobium concentration. This observation correlates with the results of cathodic current efficiency measurements during niobium electrorefining. Thermodynamic calculations were performed using niobium red-ox and equilibrium electrode potentials in NaCl-KCl based melts at 700 0C. The results confirmed that equilibrium oxidation state of niobium in the fused chloride can increase by increasing the total concentration of niobium in the melt.