Molten chlorides can be used for niobium electrowinning and electrorefining. Apart from the technological value, the information concerning the kinetics of the electrode reactions can also be used for verifying theoretical predictions concerning the nature of heterophase electrochemical reactions. In the present study a variety of non-stationary electrochemical techniques (chronoamperometry, chronopotentiometry, linear, cyclic and square-wave voltammetry) and in situ electronic absorption spectroscopy measurements were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. Niobium, glassy carbon or tungsten working electrodes encapsulated in beryllium oxide were used for the electrochemical measurements and all potentials were measured relative to chlorine reference electrode. Niobium-containing melts were prepared by a specially developed method of dry chlorination of metallic niobium with subsequent absorption of the reaction products by fused salts. The experiments were conducted between 700 and 850 0C. Analysis of cyclic voltammetry and spectroscopic data showed that soluble niobium(V) species represented the highest oxidation state of niobium in NaCl-KCl melts. Formation of Nb(V) from NaCl-KCl-NbCln (n=3.8-4.1) is a quasireversible process accompanied by simultaneous disproportion of Nb(V) into Nb(IV) and chlorine gas. Cathodic reduction of niobium(IV)-containing melts on the tungsten electrode at -1.55÷-1.6 V resulted in the formation of niobium metal. When the glassy-carbon working electrode was used the electoreduction process took place at more negative potentials due to formation of niobium carbides. The character of cyclic voltammograms recorded in niobium containing melts depended on the scan rate: this dependence is caused by superimposition of the following electrode reactions: Nb(IV) → Nb(III), Nb(IV) → Nb(0) and Nb(III) → Nb(0) due to closeness of Nb(III)/Nb(0) and Nb(IV)/Nb(0) formal standard potentials. Linear voltammetry measurements showed that intermediate niobium(III) species can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts. It was also found that the major product of niobium anodic dissolution in chloride melts is niobium(III) species, NbCl63-. The increase of current density and time of electrolysis in galvanostatic dissolution or increasing the potential applied during potentiostatic anodic dissolution lead to formation of Nb(IV) species, NbCl62-.

Niobium speciation in chloride melts: Electrochemistry and spectroscopy

254th National Meeting (2017)

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