Tungsten Chemistry in Alkali Chloride Melts

Abstract

Speciation of tungsten in alkali chloride melts (based on NaCl-2CsCl, NaCl-KCl and 3LiCl-2KCl mixtures) was studied between 550 and 750◦C using electronic absorption spectroscopy. OnlyW(IV) and W(V) chloro and oxychloro species can be stabilized under the conditions studied. Tungsten(IV) chloride ions are very sensitive to oxide/hydroxide impurities present in the melt. Anodic dissolution ofWmetal at anodic current densities in the range of 0.005 - 0.1 A/cm2 produces only [WCl6]2− ions that can be electrochemically (on a glassy carbon anode) oxidized to [WCl6]−. Small amounts of oxide ions present in the melt result in a gradual conversion of W(IV) chloro species intoW(IV) oxychloro species. In the presence of O2 in the atmosphere [WCl6]2− is oxidized into the tungsten(V) species [WOCl5]2−. Dissolution of tungsten hexachloride, WCl6, in an NaCl-2CsCl melt initially yields [WCl6]2− (due to disproportionation). Reaction of metallic tungsten with Pd(II)-containing melts results in the formation of [WCl6]2− species. The main spectroscopic parameters of [WCl6]2− and [WCl6]− complex ions were calculated.