Understanding of electrochemical behaviors of niobium in molten LiCl–KCl eutectic for pyrochemical decontamination process

Abstract

In this study, the electrochemical behaviors of niobium were examined with 1.0 wt% of NbCl5 in the molten LiCl–KCl at 450 °C via different electrochemical techniques. A chronoamperometry was performed to prepare solutions containing the selected oxidation state of the Nb ion, and the color of the solution was compared to the literature data. Cyclic voltammetries were performed with a combination of different scan ranges and scan rates using an electrochemical cell prepared by LiCl–KCl–NbCl5 salt mixture and an inert tungsten electrode. Five oxidation and four reduction peaks were found, and possible redox reactions attributing to each peak were identified based on the rate of change of the current density as a function of the scan rate. It was found that Nb(V) can be deposited as a metallic Nb via Nb(IV), Nb(III), and Nb(II), and the deposition of the metallic Nb is perturbed by the formation of insoluble nonstoichiometric Nb chloride compounds on the electrode surface. Three irreversible oxidation peaks were caused by the oxidation of metallic Nb and insoluble Nb chlorides, which competed with each other, but the dissolution of metallic Nb became dominant as the scan rate increased. X-ray diffraction analysis was performed to examine the possibility of a chemical reduction of Nb(V) to Nb(IV), but no evidence was found, which indicated the initial concentration of Nb(V) was preserved in all the experiments. It was also found that the different pyrochemical processes would desirable to separate Nb for the purpose of salt purification as well as volume reduction of radioactive waste, depending on the electrochemical behaviors for other elements that can be simultaneously dissolved with Nb.