Selective Evaporation of Components of Molten LiCl–RbCl–CsCl–SrCl2–BaCl2 Mixtures under Reduced Pressure

Abstract

Evaluation has been made of high-temperature vacuum distillation of multicomponent LiCl-based molten mixtures containing the compounds of fission product imitators (alkali and alkali-earth metal chlorides) with a total concentration of about 5 mole percent. Experiments have been conducted under different conditions (temperatures, times, cells). It has been found that the vacuum distillation of the electrolyte and its volatile components (LiCl, RbCl, CsCl) in the temperature interval 750–900°C at pressure P = 1–2 Pa proceeds rapidly and nearly completely if vapor is continuously evacuated from the open surface of the melt. The volatility and evaporation rate (degree of distillation) of the multicomponent melt rise rapidly with temperature mostly owing to its volatile components (LiCl, RbCl, CsCl). The separation ratio (the vapor/melt concentration (in mol %) ratio) for BaCl₂ and SrCl₂, which were hardly volatile in all molten mixtures studied was on the order of 0.001–0.0001. Obviously, the volatility of these chlorides under distillation conditions is much lower than that of alkali metal chlorides. By the end of distillation, the concentration of dichlorides in the molten electrolyte sharply rises (up to several tens of percent) compared to their concentration in the starting melt (0.5–1.8 mol %) depending on the degree of distillation of much more volatile chlorides LiCl, CsCl, and RbCl. Conclusions have been drawn about the degree of distillation, evaporation selectivity of molten mixture components, and relative volatilities of different chlorides in order to substantiate the efficiency of electrolyte distillation from “metallization” products (by metallization is meant uranium dioxide reduction by metallic lithium). Findings mentioned above may be useful in developing promising techniques for spent fuel processing using salt distillation.