Oxidation–reduction processes in halide and oxohalide niobium containing melts. Part I: Interaction of fluoride–chloride and oxofluoride–chloride Nb(V) melts with niobium oxides

The interaction between the K2NbF7–KCl–NaCl melt widely used for niobium electrolysis and niobium oxides of higher (Nb2O5) and lower (NbO2 and NbO) oxidation states was studied by linear sweep voltammetry and analysis of the dissolution time dependences. Three monooxofluoride complexes, NbOF4−, NbOF2−5 and NbOF63−, may form at the mole ratio of Nb2O5:K2NbF7=1:3. No significant difference was found in the electrochemical behaviour of the complexes, but their discharge potentials become more negative with increasing number of fluorine anions in the complex. Due to the interaction of K2NbF7 with NbO2 and NbO, oxofluoride Nb(v) and fluoride Nb(iv) complexes form simultaneously in the KCl–NaCl melt. Thus, the content of the latter increase until the molar ratios in the melt become NbO2dissolv.:K2NbF7init.=1:2 and NbOdissolv.:K2NbF7init.=1:3 and decrease with further dissolution of oxides in the melt. Dissolution of lower niobium oxides in melts containing oxofluoride Nb(v) complexes was also studied.