(Digital Presentation) Electrochemical Reduction of Pressed Tungsten (VI) Oxide on a Solid Electrode in a Melt of Calcium and Sodium Chlorides

Abstract

This report presents the results of research on the electrochemical reduction of tungsten (VI) oxide in a molten electrolytic mixture of CaCl2-NaCl of eutectic composition. The electrochemical behavior of tableted WO3 on a solid tungsten electrode was studied by voltammetry. Analysis of the obtained products was carried out by X-Ray diffraction and electron microscopy.During the electrochemical reduction, pressed cylindrical samples were placed on a tungsten disk, which served as a cathode. A silica cloth impregnated with a CaCl2-NaCl eutectic melt was placed on top of the tableted sample in order to prevent a short circuit in the cell. The silica cloth and tableted tungsten oxide were pressed to the cathode by the anode. The anode was made of low-porous graphite, and the current was supplied to it with a molybdenum wire. The electrode assembly prepared in this way was placed in a crucible with molten chlorides of calcium and sodium.Analysis of the obtained results indicates that reduction processes occur in pressed tungsten oxide (Fig. 1, a). According to the results of X-Ray phase analysis, in the surface layer of pressed tungsten (VI) oxide, which was in contact with the cathode, when the amount of electricity (Q) twice greater than that required for the complete recovery of loaded tungsten trioxide was passed, the following phases were detected: W, CaWO4 and a small amount of NaxWO3. Having studied the entire volume of the tableted sample, the following phases were recorded: NaxWO3 (main phase) and CaWO4, W (Fig. 1, b). The formation of phases of sodium and calcium tungstates is explained by the interaction of tungsten trioxide with the components of the melt. Passing electricity through the pressed sample causes electrochemical reactions to occur with the formation of tungsten metal, which grows from the cathode/WO3/electrolyte interface into the sample volume.During galvanostatic electrolysis (i=0.4 A/cm2) of a pressed sample of tungsten (VI) oxide with a mass of 0.43 g (r=0.4 cm, h=0.174 cm) and a duration of 6 hours (Qpractical/Qtheoretical = 6), the degree of extraction of tungsten from tungsten trioxide is not less than 97.48%, and the current yield is about 29.06%. The X-Ray diffraction pattern of the obtained product under such conditions confirms the presence of only a monophase of crystalline tungsten (Fig. 1, b). Figure 1