Electrochemical Reduction of Tungsten Oxide in Molten Chlorides

Abstract

The direct electrochemical reduction of titanium oxide to titanium metal in molten CaCl2 salt has been proven by the FFC Cambridge process. [1] Subsequently, the process has been applied to a number of refractory metals. However, there are limitations to the FFC Cambridge process. For example, the current efficiency of the process is quite low, 10-40%, to achieve sufficiently low oxygen content, 0.3%, in the final titanium product. [2] This could be due to a number of reasons, such as the increasing concentration of O2- ions in the melt as the reduction process proceeds. The metal oxide electrode has an inherent pore structure and sponge-type substrate electrodes can be used with a range of pore sizes, which has the advantage of high surface area and access of the melt to the oxide. However, when the metal oxide is reduced, oxide ions accumulation in the pores can significantly change the potential needed for the reduction, as shown in the Littlewood [3] predominance diagram. It can also result in the formation of other unwanted metal phases, such as those that include the salt’s metal, as it reacts with the oxide ions. Oxide ion build up close to the electrode surface can ultimately bring the reduction process to a halt, leaving the inner parts of the metal oxide unreduced. 1. Chen, G.Z., D.J. Fray, and T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride. Nature, 2000. 407(6802): p. 361-364. 2. Schwandt, C., D.T.L. Alexander, and D.J. Fray, The electro-deoxidation of porous titanium dioxide precursors in molten calcium chloride under cathodic potential control. Electrochimica Acta, 2009. 54(14): p. 3819-3829. 3. Littlewood, R., Diagrammatic Representation of the Thermodynamics of Metal‐Fused Chloride Systems. Journal of the Electrochemical Society, 1962. 109(6): p. 525.