## 3 Results and Discussion
At elevated solution temperatures above 30 \({}^{\circ}\)C, further dehydration leads to the formation of WO\({}_{3}\)\(\cdot\)H\({}_{2}\)O or H\({}_{2}\)WO\({}_{4}\) (yellow tungstic acid). At low current densities, the temperature of the circulating anolyte and catholyte remained between 25 and 28 \({}^{\circ}\)C, resulting in the formation of a gel-like white tungstic acid. In contrast, higher current densities led to electrolyte heating above 30-35 \({}^{\circ}\)C, promoting the formation of dispersed particles of yellow tungstic acid. Furthermore, during sodium ion removal from solutions containing Na\({}_{2}\)WO\({}_{7}\)-type compounds, the transient formation of intermediate hydrated tungstate species, such as H\({}_{2}\)WO\({}_{7}\), is also possible. Figure 6 shows that as sodium ions are removed from the anolyte and the pH of the sodium tungstate solution decreases pH to the formation of polyoxotungstates and tungstic acid in the pH range of 8 to 7--the resistance of the electrodialysis cell increases. This is attributed to a reduction in the electrical conductivity of the tungstate solution. As the acidity further increases, with the pH decreasing from approximately 7 to 1.42, the cell resistance decreases in a stepwise manner. This also supports the occurrence of compositional changes in the anolyte during the electrodialysis process. One of the key indicators determining the feasibility of using the electrodialysis of sodium tungstate solutions for tungstic acid production and for reducing the consumption of inorganic acids in tungsten processing technologies is the energy consumption per unit mass of tungstate. Based on the results of electrodialysis experiments conducted at various current densities, the dependence shown in Figure 7 was obtained. With an increase in both anode and cathode current densities above 200 A/m\({}^{2}\), the specific energy consumption per unit mass of sodium tungstate increases significantly. At the same time, the proportion of energy directed toward the electrode reactions--namely, the formation of tungstic acid and oxygen in the anode compartment, and alkali and hydrogen in the cathode compartment--decreases from 61.5-69.4% to 24.8-30.3%. As the current density rises, electrical losses due to heat generation and electrolyte heating also increase. Figure 6: Dependence of solution pH on total cell resistance of electrodialysis at da = 666.7 A/m\({}^{2}\). Process efficiency can be enhanced by reducing the interelectrode gap and implementing bipolar electrolyzers equipped with titanium electrodes featuring unilateral platinum coating. However, scaling up the process is limited by the available size of platinum-coated electrodes. Currently, insoluble titanium-based anodes with a platinum layer thickness of 1 to 2.5 microns and plate dimensions ranging from 50 \(\times\) 100 mm to 310 \(\times\) 500 mm are manufactured in the Russian Federation. The resulting tungstic acid solutions were evaporated in a SNOL drying oven at a temperature of 120 \({}^{\circ}\)C to obtain solid precipitates of tungstic acid and sodium paratungstate. A photograph of the obtained samples is presented in Figure 8.  
The phase composition of the tungstic acid powders was analyzed using an X-ray diffractometer X'Pert MPD PRO (PANalytical) and a JED-2300 Analysis Station (JEOL). The results of the structural and elemental analyses are presented in Figures 9 and 10. During the electrodialysis of sodium tungstate solutions at pH 4-5, a small amount of insoluble white precipitate of sodium paratungstate may form. A micrograph and corresponding spectrogram of this compound are presented in Figure 10. Sodium paratungstate is separated from the tungstic acid solution by filtration. Figure 8: Photographs of solid samples obtained by evaporating tungstic acid solutions at 120 \({}^{\circ}\)C: yellow tungstic acid and sodium paratungstate. Figure 7: Dependence of energy consumption during electrodialysis of Na\({}_{2}\)WO\({}_{4}\) on anodic current density.