Medvezhynska O. Electrochemical reduction of oxygen-containing compounds of tungsten in melts based on sodium and calcium chlorides

The dissertation is aimed at solving an urgent task of modern science and technology: the development of the scientific foundations of a new commercial process for obtaining tungsten from natural raw materials, which will allow to exclude from the technological cycle of tungsten production the stage of its processing into tungsten trioxide and to abandon the use of hydrogen as a reducing agent. In contrast to known scientific and technical solutions, oxygen-containing compounds of tungsten (WO3 and CaWO4 ) are proposed to be reduced by the method of high-temperature electrolysis in a molten eutectic mixture CaCl2 –NaCl on a liquid gallium cathode. Such a cathode provides not only a reliable contact with the original tungsten compounds and their uniform polarization, but also protects the reduced tungsten from oxidation and interaction with the electrolyte mixture. The difference in the specific masses of the initial tungsten trioxide and gallium avoids the undesirable interaction of WO3 with the molten electrolyte mixture, which results in the formation of by-products: calcium tungstate and tungsten oxochlorides. The reduction of oxygen-containing compounds of tungsten on both W and Ga cathodes proceeds through a series of successive electrochemical transformations resulting in the formation of complex oxide compounds of calcium and sodium with tungsten of various degrees of oxidation. Recovery occurs not only due to electric current, but also due to sodium and calcium, which are formed during electrolysis. A necessary condition for the recovery of oxygen-containing compounds of tungsten is the provision of a compatible discharge of the cations of the molten electrolyte mixture at the cathode. Since the electrochemical reduction of tungsten trioxide in the eutectic melt of sodium and calcium chlorides occurs through the intermediate stage of calcium tungstate formation, it is advisable to use calcium tungstate rather than WO3 as the starting compound. Favorable conditions for the recovery of oxygen-containing tungsten compounds are electrolysis using a liquid gallium cathode in both galvanostatic and potentiostatic conditions, and it is advisable to use the original tungsten compounds in a finely dispersed state. Under the conditions that the amount of electricity passed through the electrolysis cell corresponds to the amount of electricity theoretically necessary for the complete recovery of the loaded initial tungsten compound, the average degree of tungsten extraction from WO 3 is 65%, the yield is current ~ 65%,the particle size of the obtained tungsten powder is ~3.6·10 -2 μm. The degree of extraction of W from CaWO4 is ~ 75%, the current output is ~ 76%, the size of the tungsten powder is ~3.5·10 -2 μm. It was noted that with increasing temperature, the degree of their crystallinity and the size of tungsten powder particles increase. The optimal recovery temperature is 800°C. The specific electricity consumption is estimated at 3.6–4.0 Wh/g of tungsten. It is shown that oxygen-containing compounds of tungsten can be recovered in the solid state on solid electrodes. Samples of initial tungsten compounds without prior heat treatment are better recovered. The specific consumption of electricity during recovery on a solid electrode is estimated at 7.8 Wh/g of tungsten. The resulting tungsten powder has an average crystallite size of ~ 4.2·10-2 μm. The interaction of WO3 and CaWO4 with a molten eutectic mixture of CaCl2 –NaCl in the temperature range of 600–800°С was investigated. It was found that the equilibrium content of tungsten in the salt phase is the same in both cases and naturally increases with increasing temperature according to the same law. The interaction of WO 3 with the melt proceeds through the stage of formation of calcium tungstate. The obtained results give reason to consider that the interaction of the mentioned oxygen-containing compounds of tungsten produces homogeneous salt phases of the ternary system CaCl2 –NaCl–CaWO4 , the content of which calcium tungstate in the temperature range of 700÷800°С is satisfactorily described by the dependence ln S = 8.331 – 14057/T. The phase equilibria of the CaCl2 –NaCl–CaO ternary system in the area enriched with CaCl2 and NaCl were investigated; the equilibrium concentration of calcium oxide and the composition of phases that exist in an equilibrium state at different temperatures were determined. At a constant ratio of the components [NaCl]:[CaCl2]=1.06 (mol), in the melts of the ternary system CaCl2–NaCl–CaO,the equilibrium content of calcium oxide reaches 12.0mol%, while their crystallization temperature does not exceed 550°C. Mixtures of the specified composition allow electrochemical reduction of oxygen-containing compounds of tungsten in a wide temperature range (from 550 to 1000°C) with a high content of both calcium and sodium chlorides (not lower than 40mol%) and calcium oxide (up to 12.0mol%).