Use of electrochemical techniques for the study of solubilization processes of cerium–oxide compounds and recovery of the metal from molten chlorides
Introduction
Among the main applications of molten salts, one of the most promising is their use as reaction media for extractive metallurgy. In particular, molten chlorides are good reaction media for performing selective solubilization or precipitation in chemical reactions, and their use provides a promising route for the treatment of raw materials. Moreover, molten salts have proved to be suitable media for metal electrowinning and electrorefining. The accumulated knowledge concerning their high-temperature electrochemistry leads to openings for the deposition of metals in the solid state and in alloys. Their possibilities lie in the fact that, because of their variety, one can always find a solvent whose chemical and electrochemical characteristics and melting point are suitable to carry out a given process.
Over recent years a new field of the molten salts has been developed, the use of these media for pyrochemical separation processes as a promising option in the nuclear fuel cycle for the future [1], [2], [3], [4], [5]. The reason for this interest is mainly due to the progress in the assessment of new concepts for transmutation and the corresponding fuel cycles [6]. In order to assess the feasibility of such pyrochemical separation, several processes have been developed for the recovery of minor actinides from spent metallic, nitride and oxide nuclear fuels, and high level radioactive liquid wastes [7], [8].
One of the most important steps in the pyrometallurgical reprocessing is the electrorefining in molten chlorides. In this step, spent metal fuel is anodically dissolved into molten chlorides, and the minor actinides are selectively recovered at the cathodes. The determination of thermodynamic data of solutions as well as the electrochemical behaviour of the elements are of crucial importance for the understanding of the process and the design of the separation cell.
The chemical behaviour of fission products, in nuclear oxide or metal fuels, is another important topic in nuclear reprocessing. The pyrochemical reprocessing method for oxide fuels differs from that used for the metal fuel. One prospective process for treating spent oxide fuels involves dissolving some of the products in a molten chloride using chlorinating reagents.
Among the fused salts that can be considered as solvents for these processes, alkali molten chlorides are particularly attractive. Hence, obtaining basic chemistry data of actinides and lanthanides in molten halogenide salts is a major concern. This work presents a study on the chemical and electrochemical properties of cerium trichloride, which, as well as the other lanthanide elements, is a difficult fission product to separate from actinides due to their similar chemical properties. The study has been carried out in two molten chlorides with different oxoacidity properties, the eutectic LiCl–KCl and the equimolar mixture CaCl2–NaCl.
Few studies have been conducted relating to the chemical and electrochemical behaviour of cerium in molten salts. The LiCl–KCl eutectic, the equimolar mixture NaCl–KCl, the eutectic LiF–KF–NaF (FLINAK) and room temperature AlCl3-1-methyl-3-ethylimidazolium chloride are the most used for this kind of study, and some differences have been found in the stability and the electrochemical properties of cerium ions in these media. For example Ce(IV) is unstable in alkali molten chlorides; Ce(IV) is reduced by Cl− ion upon dissolution in the LiCl–KCl [9]. However, cerium(IV) is stable in molten fluorides such as FLINAK [10], and in the AlCl3–MeEtimCl at 313 K, where the half-life of Ce(IV) was approximately 8 days [11].
The standard potential of the redox couple Ce(III)/Ce(0) and its temperature dependence has been determined in the eutectics LiCl–KCl [12], [13], [14], [15], [16], [17] and KCl–CsCl [18]. Chemical reactions between Ce(III) and oxide ions were also investigated in molten alkali chlorides by potentiometry with a stabilised zirconia membrane electrode, and different behaviour was observed depending on the media. In the eutectic LiCl–KCl at 723 K, addition of oxide ion to CeCl3 solutions does not cause the precipitation of Ce2O3, but causes that of CeOCl [13], whereas in the equimolar mixture NaCl–KCl at 1000 K, Combes et al. [19] observed the formation of soluble cerium oxychloride (CeO+) and precipitated cerium oxide (Ce2O3).
Fundamental studies of the mechanism of electrode reactions in molten chlorides are, as yet, sparse, and there is some divergence in the literature values. In the eutectic LiCl–KCl and the equimolar mixture NaCl–KCl, the electroreduction of cerium trichloride at solid cathodes (W or Mo) occurs in only one electrochemical step close to the alkaline electrodeposition [9], [15]. A simultaneous deposition of Ce and alkali metals occurred when a constant current density was applied [15], [20]. According to the results of Sokolovskii and Smirnov [21], it seems to be that the addition of LiF to the LiCl–KCl melt until 10% (w/w), does not change the cathodic behaviour of CeCl3.
The diffusion coefficient of Ce(III) was determined (in 1964) by Sokolovskii et al. [22] using a chronopotentiometric method, some years later (in the 1980s) by Mottot and coworkers [13], [14] using different electrochemical techniques, and more recently (in 1998) by Iizuka [23] in the temperature range between 673 and 823 K, also by chronopotentiometry. Empirical equations for the temperature dependence of the diffusion coefficient were also developed [22], [23], and the activation energy for diffusion and the diffusion coefficient was discussed in connection with cerium ionic radii and the stability of its complex ions [23].
In addition to the studies in chloride systems, Lin and Hussey [11] carried out an electrochemical and spectroscopic study of cerium in the AlCl3–MeEtimCl molten salt. They found that the Ce(IV)/Ce(III) electrode reaction is quasireversible at GC, Pt and W electrodes, and the kinetic parameters such as the intrinsic rate constant k0, the charge transfer coefficient α and the diffusion coefficient DCe(III) are given.
The purpose of our investigation was to determine basic data of cerium trichloride in two molten chlorides with different intrinsic acidities, the eutectic LiCl–KCl melt and the equimolar mixture CaCl2–NaCl. The methodology used is based on the construction and comparison of the E–pO2− diagrams, for cerium and the chlorinating gaseous mixtures in both melts [24], [25], [26], [27]. In order to build up these diagrams the following data are needed: (i) standard potential values of the different electrochemical systems which were established by emf measurements, and (ii) information about the stability of the different metal oxides and oxichlorides.
The present work also deals with the electrochemical behaviour of cerium trichloride solutions. Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry and chronoamperometry, were used in order to study the reaction mechanism and the transport parameters of electroactive species.
Section snippets
Preparation and purification of the melt
The chloride mixtures CaCl2–NaCl or LiCl–KCl (analytical-grade) were melted in a 100 cm3 Pyrex, alumina or glassy carbon (GC) crucible placed in a quartz cell inside a Taner furnace. A West 3300 programmable device controlled the temperature of the furnace to ±2 °C. The working temperature was measured with a thermocouple protected by an alumina tube inserted into the melt. All handling of the salts was carried out in a glove box mBraun Labstar 50 under an argon atmosphere.
The mixture was fused
Stable oxidation states of cerium in the melts studied
The stable oxidation states of cerium were identified by different electrochemical techniques, i.e. cyclic voltammetry, chronopotentiometry and square wave voltammetry.
Typical cyclic voltammograms obtained in the eutectic LiCl–KCl melt and in the equimolar CaCl2–NaCl mixture containing Ce(III) ions (Fig. 1a and b), showed a single group of signals, A/A′, in the potential range close to the lower limit of the melt (B/B′, electrodeposition of liquid lithium and sodium, respectively). The shape of
Conclusions
The chemical and electrochemical properties of CeCl3 in two molten chloride mixtures with different oxoacidity properties, the eutectic LiCl–KCl at 723 K and the equimolar CaCl2–NaCl melt at 823 K, have been studied, with basically similar results being obtained in both cases. One of the biggest differences is the solvation activity coefficient values of CeCl3 species, which indicates that Ce(III) ions form stronger complexes with the chloride ions in the eutectic LiCl–KCl than in the equimolar
Acknowledgements
The authors thank ENRESA (Spain) for financial support (CIEMAT–ENRESA and CIEMAT–Universidad de Valladolid agreements). A.M.M. wishes to thank the “Secretarı́a de Estado de Educacion, Universidades, Investigacion y Desarrollo” (Spain) for a post-doctoral grant. Some aspects of the work were part of the UE PYROREP FIKW-CT-2000-00049 project.
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Present address: Department of Materials Technology and Electrochemistry, Sem Sælands vei 6, 7491 Trondheim, Norway.