Solubility of Cerium in (NaCl–KCl)–CeCl₃ Melts at 850°C

Abstract—Molten mixtures of alkali and polyvalent metal chlorides have found wide application in the electrolytic refining of a number of metals, for example, plutonium. The current output in such processes depends on many factors, among which the most significant ones are the corrosion resistance of products made of ceramic materials in contact with melts and the solubility of the metal to be cleaned in melts containing its chloride. For example, for refining electrolysis performed to purify liquid cerium in a molten equimolar mixture of sodium and potassium chlorides with an initial cerium trichloride concentration of 3 mol %, the maximum current efficiency is 63%. A significant part of the losses is shown to be explained by the precipitation of alkali metal and the dissolution of metallic cerium. There is no information in the literature on the solubility of a (polyvalent) metal in molten salt compositions containing its chloride. Therefore, the purpose of this work is to determine the solubility of metallic cerium in the (NaCl–KCl)–CeCl₃ melt at an electrolysis temperature of 850°C. An installation has been created to determine the solubility of cerium with sampling of a salt melt without access for oxidants. This is achieved by pouring a salt melt from a glass, where the melt is saturated with cerium, into an outer glass without opening the device. The solubility of cerium is shown to decrease sharply with a decrease in the cerium trichloride concentration and to be approximated by the equation N_Ce = 1.67 × 10^–5x³ – 9.62 × 10^–4x² + 4.50 × 10^–2x, where N_Ce is the concentration of dissolved cerium in mol % and x is the cerium trichloride concentration in the melt in mol %.