## 5 Discussion  
### Chemical Form in Cathode Deposit  
ZrO\({}_{2}\) was represented in all XRD patterns, which were obtained from tests 1-3. It was estimated that the ZrO\({}_{2}\) phase was formed by the oxidation of Zr while preparing and conducting XRD analysis in the air atmosphere outside the glove box, because the O\({}_{2}\) concentration was maintained below 1 ppm in the glove box. The precipitate was attributed to the Zr metal detached from the cathode surface in test 1 and 2. In test 3, it was found that ZrCl and Zr were co-deposited on the cathode. It has been known that Zr has two soluble states, Zr\({}^{4+}\) and Zr\({}^{2+}\), and two insoluble states, ZrCl and Zr, in molten LiCl-KCl salt [14; 15; 16; 17; 18]. According to the cyclic voltammogram of ZrCl\({}_{4}\) in LiCl-KCl at 773 K, three peaks arise at the cathodic side and the corresponding reactions are represented in order of reduction tendency as follows [15; 16]:  
\[\mathrm{Zr}^{4+}+2\mathrm{e}=\mathrm{Zr}^{2+} \tag{1}\]  
\[\mathrm{Zr}^{4+}+3\mathrm{e}+\mathrm{Cl}^{-}=\mathrm{ZrCl} \tag{2}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Origin of Precipitates**} & \multicolumn{4}{c}{**Concentration (ppm)**} \\ \cline{2-5}  & **Nb** & **Fe** & **Co** & **Ni** \\ \hline Bottom of the cell in test 1 & N/D & 12.01 & N/D & 15.55 \\ Bottom of the cathode basket in test 2 & 0.24 & 17.36 & 0.15 & 23.22 \\ Bottom of the cathode basket in test 3 & 1.26 & 27.77 & 0.03 & 14.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Concentrations of Nb, Fe, Co, and Ni in the precipitates after the tests 1–3. Figure 5: (**a**) Cathode deposit in test 3; (**b**) salt ingot obtained from the cathode basket in test 3; (**c**) XRD pattern of the cathode deposit. \[\text{ZrCl}+\text{e}=\text{Zr}+\text{Cl}^{-} \tag{3}\]  
If Zr metal is deposited on the cathode during the electrorefining, that can be explained by two kinds of Zr reduction mechanism under the presence of Zr\({}^{4+}\) in the chloride electrolyte as follows:  
1. \(\text{Zr}^{2+}\) generation from Zr\({}^{4+}\) by Equation (1), which is followed by a disproportionate reaction where Zr\({}^{2+}\) produces Zr\({}^{4+}\) and Zr metal as shown in Equation (4); \[2\text{Zr}^{2+}=\text{Zr}^{4+}+\text{Zr}\] (4)
2. \(\text{ZrCl}\) generation from Zr\({}^{4+}\) by Equation (2), which is followed by a ZrCl consumption reaction to produce Zr metal by Equation (3). In the voltammogram, cathodic peaks related to the reduction reactions 2 and 3 were clearly recognized regardless of ZrCl\({}_{4}\) concentrations [16]. The cathodic peak related to the reduction Equation (1) (R1) seemed to be distinct in high ZrCl\({}_{4}\) concentrations, whereas R1 had a flat shape in low ZrCl\({}_{4}\) concentrations. Moreover, as scan rate decreased, R1 could not be distinguished as a peak shape, and the reduction current was very low. This indicated that Zr\({}^{4+}\) tended to be directly reduced to ZrCl by Equation (2) in a low concentration of ZrCl\({}_{4}\) such as 0.5 wt. %, equivalent to the concentration employed in this study. Therefore, in tests 1 and 2, it was estimated that Zr metal was produced not by the first but by the second mechanism associated with Equations (2) and (3). Additionally, in order to cause the Zr metal formation by Equations (3), it was determined that a sufficiently low electrode potential was applied to the cathode. Meanwhile, cell designs utilized in the electrorefining did not include a stirring system and even had a cathode basket. These factors hinder mass transfer and cause large overpotential to the cathode, which could be evidence of the estimation that Equation (3) occurred at the cathode in tests 1 and 2. In addition, the color of molten salt could be another reason that Equation (1) did not occur at the cathode. Based on the literature, it was reported that the molten salt was shown to have a brown color if Equation (1) progressed [14; 27]. In tests 1-3, all salt ingots had a white color, as shown in Figures 3-5.