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 el ectrolysis and niobium oxides of higher (Nb2O5) and lower (NbO2 and NbO) ox idation states was studied by linear sweep voltammetry and analysis of the dissolution time dependences. Three monooxofluoride complexes, NbOF4-; NbOF 52- and NbOF63-, may form at the mole ratio of Nb2O5 : K2NbF7 = 1 : 3. No s ignificant difference was found in the electrochemical behaviour of the com plexes, 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 si multaneously 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 dissoluti on of oxides in the melt. Dissolution of lower niobium oxides in melts cont aining oxofluoride Nb(V) complexes was also studied.