Electrolytic Separation of Iodine from LiCl–KCl–LiBr–LiI Melt and Recovery of Iodine Gas with Copper

Abstract

IntroductionPyrochemical reprocessing technology for metal fuels has advantages such as high MA transmutation efficiency [1], solvent stability against heat and radiation, and batch actinide recovery without additional processes [2]. By incorporating this into the fast reactor fuel cycle, it is possible to establish an MA recovery and conversion system that can flexibly respond to a wide range of fast reactor deployment scenarios and Pu supply and demand scenarios, while maximizing the effectiveness in reducing the toxicity of the wastes. For the spent molten chloride generated by pyrochemical reprocessing, we plan to reduce the amount of solidified high-level waste by converting FP elements recovered by electrolysis into oxides and including them in glass. Concerning iodine, one of the long half-life FPs, we aim to separate and recover it by electrolysis. In this study, we present the results of an attempt to separate and recover iodine gas electrolytically generated from LiCl–KCl–LiI–LiBr by reaction with copper outside the molten salt.ExperimentalThe experiments were carried out in eutectic LiCl–KCl (LiCl:KCl = 58.8:41.2 mol%, 100 g) at 723 K after the addition of LiI and LiBr. A Au plate or a glassy carbon rod was used for the working electrode. An Al wire was used as the counter electrode, and an Al–Li alloy electrode prepared electrochemically in the melt beforehand was used as the reference electrode [4]. The potential was calibrated by Li+/Li potential measured at a Ni electrode. To collect the generated I2 as CuI, a Cu mesh was suspended using a Mo wire at the top of the cell.Results and DiscussionIodine recovery by Cu mesh were conducted using the melt containing 0.3 mol% LiI and LiBr. The Au plate electrode was used as the working electrode, and electrolysis was performed at 3.2 V vs. Li+/Li for 1 hour. After the electrolysis, the Cu mesh was left at the top of the cell for 50 minutes to fully react with I2 gas. A photograph of the obtained Cu mesh (Fig. 1) shows that most of the Cu mesh became white. Furthermore, as shown in Fig. 2, an XRD pattern of the white area is consistent with copper iodide (CuI), indicating that the Cu mesh reacted with I2 gas to form CuI.Since it was found that the I2 gas generated by electrolysis can be recovered as CuI, the effects of electrode and potential were investigated. Electrolysis was performed in the melt containing 1.0 mol% LiI and LiBr, using either the Au plate electrode or the glassy carbon rod electrode as the working electrode. The potentials were 3.2 V for the Au electrode and 3.2 V and 3.3 V for the glassy carbon electrode, and the amount of electricity was unified at 100 C. The Cu mesh was removed 30 minutes after electrolysis, and the weight change of the Cu mesh before and after electrolysis was measured. The recovery efficiency at the glassy carbon electrode was 63.9 % at 3.2 V and 86.5 % at 3.3 V. When the glassy carbon was used, I3 − is easily generated, and the following reactions are considered to partially proceed at the cathode and anode.(Cathode) I3 − + 2 e− → 3 I− (Anode) 3I− → I3 − + 2 e− This is called the shuttle effect and is the cause of lowering the current efficiency. The lower recovery efficiency at 3.2 V compared to 3.3 V is thought to be due to the smaller total current, which means that the current used for I3 − generation became relatively larger, resulting in a larger shuttle effect. The Au electrode showed the highest recovery efficiency of 92.9% among the experiments conducted in this study. This result indicates that I3 − generation is less likely to occur at the Au electrode. However, 0.0085 g of the Au electrode was anodically dissolved.From the above results, it was found that although a Au electrode is a promising electrode when only the efficiency of I2 gas generation is considered, the electrode itself dissolves. Therefore, the use of a glassy carbon electrode at relatively positive potentials is most promising for practical purposes.AcknowledgementA part of this study is a result of the MEXT Nuclear Energy Systems R&D Project "Development of Flexible MA Recovery and Conversion Technology".