TRANSIENT ELECTROCHEMICAL TECHNIQUES FOR STUDYING ELECTRODEPOSITION OF NIOBIUM IN FUSED NACL-KCL

The mechanism of the electroreduction of niobium chlorides dissolved in fused NaCl-KCl was determined by transient electrochemical techniques at temperatures ranging from 670 to 870 degrees C. At temperatures <750 degrees C it is shown that the stable oxidation states of niobium ions are Nb(V), Nb(IV), and Nb(III); reduction of Nb(III) gives rise to metallic niobium. At higher temperatures, Nb(V) tends to decompose into Nb(IV) with chlorine evolution. Introducing fluoride ions stabilizes the higher oxidation states. For a molar ratio F/Nb, r(F) > 2, Nb(V) is stable over the whole temperature range and the direct reduction Nb(IV) --> Nb is obtained as the Nb(III) species is no longer stable. The standard potentials of the redox reactions are calculated for various concentrations of fluoride ions. Presence of fluoride ions has a beneficial effect on the quality of the niobium deposit, a ratio r(F) similar to 2 is enough for obtaining coherent deposits: