Clarifying complex redox behavior of zirconium in molten LiCl-KCl salt

So far, the electrochemical behavior of Zr in molten LiCl-KCl salt has been contradictory and controversial in the literature due to its electrochemical complexity. Prof. Jong-Il Yun’s group has resolved the start button for this contradiction for the first time on the basis of a unique comproportionation reaction of Zr in molten LiCl-KCl salt. The research improves the electrochemical understanding of Zr and contributes to efficient Zr recovery processes.

Article  |  Fall 2018

Spent nuclear fuel, including cladding material, contains about 40% of hafnium-free zirconium. By recycling hafnium-free zirconium, not only can the amount of radioactive waste be significantly reduced, but also the hafnium-free zirconium can be efficiently recovered for its further use in the nuclear industry.

For the development of spent fuel recycling process based on the pyroelectrochemical method, it is fundamental to clearly understand the redox mechanism of zirconium in the molten LiCl-KCl salt. However, understanding the complex redox reactions of zirconium is still contradictory and controversial due to the lack of experimental evidence.

Han Lim Cha, PhD candidate, and Prof. Jong-Il Yun of the Department of Nuclear and Quantum Engineering clarified the controversial redox mechanism of zirconium based on the comproportionation reaction between Zr(IV) and Zr(0) in molten LiCl-KCl eutectic for the very first time. The comproportionation between Zr(0) and Zr(IV) is a unique chemical reaction of Zr, by which soluble intermediate Zr chloride ions are formed with brown color in molten LiCl-KCl.

According to earlier publications, Zr(II) was assigned as an intermediate product of zirconium chloride by the comproportionation reaction without direct experimental evidence. Therefore, the researchers cautiously prepared the intermediate zirconium product characterized by X-ray photoelectron spectroscopy (XPS). The XPS measurements revealed Zr(III) instead of Zr(II), which was described in the literature as an intermediate zirconium chloride ion from the comproportionation reaction between Zr(0) and Zr(IV). The XPS result was checked and confirmed using square wave voltammetry (SWV).

 

The authors also found that the current of two oxidation peaks on the cyclic voltammogram (CV) of LiCl-KCl-ZrCl₄ system is drastically altered with tungsten and zirconium working electrodes and by the presence of Zr(IV) in the LiCl-KCl system. Taking into account the characteristic CVs, accurate reaction mechanisms of zirconium can be provided.

 

Overall, the authors identified the three major redox peaks (one reduction and two oxidations) of zirconium in the cyclic voltammogram. The three redox peaks are more closely associated with the expected redox reaction of zirconium unlike other literatures. The research will help to develop a method for minimizing the co-deposition of zirconium during electrorefining and to allow an efficient zirconium recovery process from spent nuclear fuel cladding.

This research was published in Electrochemistry Communications (doi.org/10.1016/j.elecom.2017.10.010) and featured in Advances in Engineering, an online magazine introducing the latest cutting-edge research articles (https://advanceseng.com/redox-behaviors-zirconium-molten-licl-kcl-eutectic-salt-comproportionation-reaction/).