Reactions and speciation of technetium and rhenium in chloride melts: a spectroscopy study

Abstract

The reactions of rhenium metal, rhenium oxides (ReO₂ and ReO₃) and technetium dioxide with chlorine and hydrogen chloride were investigated in LiCl–KCl, NaCl–CsCl and NaCl–KCl based melts between 450 and 720 °C. Reaction progress was followed using in situ electronic absorption spectroscopy, with the spectra measured between 200 and 1100 nm. Samples of the quenched melts were analysed by EXAFS spectroscopy. Rhenium metal and rhenium oxides react with Cl₂ or HCl yielding hexachlororhenate(IV) species, although the reaction of rhenium metal with HCl is extremely slow even at 700 °C and no reaction between rhenium metal and Cl₂ was observed below 500 °C. Chlorination of technetium dioxide in molten salts has been studied here for the first time. TcO₂ reacts with HCl producing [TcCl₆]²⁻. However, the reaction of TcO₂ with chlorine results in the oxidation of technetium to pertechnetate which is retained in the melt and production of a volatile solid, which is possibly a mixture of Tc(IV) chloride and Tc(VII) oxychloride. This is the first reported investigation into Tc speciation in molten salts.