(Digital Presentation) Electrochemical Reduction of Oxygen-Containing Tungsten Compounds on a Liquid Gallium Cathode in an Eutectic Melt of Calcium and Sodium Chlorides
Authors: Medvezhynska Olha , Omel'chuk Anatoliy 
Published: November 2022

This paper presents the results of studies of electrochemical reduction of oxygen-containing tungsten compounds (WO3, CaWO4) in a molten eutectic mixture of calcium and sodium chlorides. In contrast to the known methods of direct electrochemical reduction of refractory metal oxides, it is proposed to reduce the initial oxygen-containing tungsten compounds in powder form on a liquid gallium cathode without resorting to pre-pressing and sintering operations.

In the temperature range 600-1000 °C, at which the electrochemical reduction of these compounds takes place, gallium does not evaporate, does not interact with the components of the melt, or with the initial compounds of tungsten or pure tungsten. The liquid gallium cathode not only provides reliable contact with highly dispersed initial tungsten powder compounds and their uniform polarization but also protects the reduced metal from contamination by products of electrolysis and electrolyte. In addition, gallium, due to its lower specific weight than tungsten oxide, protects WO3 from unwanted interactions with calcium chloride, which causes the formation of volatile tungsten oxochlorides and the loss of the initial tungsten trioxide. Due to the large difference in crystallization temperatures of the molten electrolyte (over 500 °C) and the gallium cathode (~ 30 °C), as well as the difference in specific weights of the electrochemical cell components, fine tungsten powder isolated during electrolysis is easily separated from melt and gallium.

During the reduction of tungsten trioxide at the beginning of electrolysis on the surface of the gallium cathode, sodium and calcium cations, which are part of the electrolyte mixture, are discharged. In the range of reduction temperatures (700-800 °C) the discharge potentials of sodium and calcium cations are almost the same. As the gallium cathode is saturated with isolated metals (sodium and calcium), tungsten trioxide is reduced

WO3 + 3nM → W + 3MnO, where М = Na, Ca,

and n = 2 when sodium is involved in the reduction and n = 1 when the reducing agent is calcium. The formed metal oxides (MnO) react with tungsten trioxide, forming a mixture of tungstates of the appropriate metals

WO3 + MnO → MnWO4

Both oxides and tungstates of calcium and sodium have a lower specific weight than gallium, so as they accumulate, they float to the interface: cathode/melt, where at the appropriate values of the electrode potential, also participate in electrode processes

MnWO4 + 6е− → W + MnO + 3О2−

MnO + 2е− → nM + О2−.

Reduced tungsten settles to the bottom of the gallium cathode container, and calcium and sodium contribute to the reduction of tungstates.

The results of voltammetric studies of oxidized and unoxidized monocrystalline tungsten, tungsten cathode, on the surface of which is applied a film of WO3, gallium cathode with tungsten starting compounds (WO3, CaWO4) in a molten eutectic mixture of sodium and calcium chlorides indicate in favor of the fact that this process takes place in stages in several stages. This is also supported by the results of electrolysis under potentiostatic conditions at different potentials of the cathode relative to the unpolarized tungsten electrode. The mixture of tungsten and calcium tungstate is the reduction product at potentials lower than the reduction potentials of sodium and calcium tungstates. For example, with a reduction potential of −1.3 V, the reduction products of both tungsten trioxide and calcium tungstate contain no more than 2.0% of tungsten, the rest is calcium tungstate. Reduction at a potential of −2.0 V provides a product with a tungsten content of not less than 20.0%. Finally, the reduction at a potential equal to or greater than −2.4 V at the cathode produces tungsten without the content of extraneous phases. The degree of extraction of tungsten from tungsten trioxide is not less than 65.0%, from calcium tungstate is about 72.8%.

The morphology of reduced tungsten was studied by scanning electron microscopy. The resulting tungsten powder has the form of dendritic crystals with a size of several tens of micrometers.