Effects of Process Conditions on the Fluidised Cathode Electrochemical Reduction of Tungsten Oxide in Molten LiCl-KCl Eutectic

The direct electrochemical reduction of titanium oxide to titanium metal has been evinced by the FFC Cambridge process. [1] Numerous studies have been carried out to assess the performance and applicability of the process to the reduction of titanium, other refractory metals and metal alloys. However, there are some limitations to the FFC Cambridge process. For example, at least in the case of titanium, the current efficiency is quite low (10-40% to achieve 0.3% oxygen content in the final titanium product). [2]

There are two main reasons that hinder the reduction process: oxide product build-up in the electrolyte close to the surface and within the pores of the precursor, resulting in anomalous electro-migration of the oxygen vacancies, creating a barrier to the reduction process; and limited diffusion of the electrolyte within the precursor porous matrix. [3]

Thus, innovative new designs of the process are needed. The fluidised cathode process [4] offers a solution to these problems. Figure 1 is a schematic of the electrolytic cell used. The process is potentially applicable to the range of metals and molten salts systems and has been exemplified for the reduction of WO₃ in LiCl-KCl eutectic. However, there is still much to be investigated to establish a full understanding and optimise the process. This paper discusses experimental findings of the effects the fluidisation rate and the void fraction have on the fluidised cathode applied to the Li-K-W-O-Cl system.

Figure 2. is a chronoamperogram of the process at different rates of fluidisation (argon flow rate). Coulometric analyses have been carried out to assert the extent of its effect. Electrochemical impedance spectroscopy (EIS) studies have also been conducted to establish the effects of the fluidisation rate and the void fraction (concentration of WO₃ in the melt) on the diffusion properties between the working and counter electrodes.

Applications to more technologically relevant systems such as the electrochemical reduction of uranium oxide to uranium, as being investigated in the EPSRC REFINE consortium programme, are also discussed.

References:

[1] G. Z. Chen, D. J. Fray and T. W. Farthing, Nature, 407(6802), 361 (2000).

[2] C. Schwandt, D. T. L. Alexander and D. J. Fray, Electrochim. Acta, 54(14), 3819 (2009).

[3] E. Krasicka-Cydzik, ECS Trans., 50(11), 39 (2013).

[4] R. Abdulaziz, L. D. Brown, D. Inman, S. Simons, P. R. Shearing and D. J. L. Brett, Electrochem. Commun., 41(0), 44 (2014).