Speciation of Molybdenum and Tungsten in Molten Chlorides: A Spectroelectrochemical Study

Speciation of tungsten and molybdenum in alkali chloride melts (based on NaCl-2CsCl, NaCl-KCl and 3LiCl-2KCl mixtures) was studied between 450 and 750 oC using electronic absorption spectroscopy. Only W(IV) and W(V) chloro- and oxychlorospecies can be stabilised under conditions studied. Tungsten(IV) chloride ions are very sensitive to oxide/hydroxide impurities present in the melt. Anodic dissolution of W metal at anodic current densities in the range of 0.005-0.1 A/cm2 produces only WCl6 2- ions that can be electrochemically oxidised to WCl6 -. Small amounts of oxide ions present in the melt result in a gradual conversion of W(IV) chloro-species into W(IV) oxychloro-species. In the presence of O2 WCl6 2- is oxidised into tungsten(V) species WOCl5 2-. Molybdenum forms Mo(III), (IV) and (V) species. MoCl6 2- complex ions in chloride melts can be obtained by reacting MoO2 with HCl, or by electrochemical oxidation of Mo(III) (MoCl6 3- → MoCl6 2-) on a glassy carbon electrode.