Use of electrochemical techniques for the study of solubilization processes of cerium–oxide compounds and recovery of the metal from molten chlorides

Abstract

This work presents a study on the chemical and electrochemical properties of CeCl 3 in two molten chloride mixtures with different oxoacidity properties, the eutectic LiCl–KCl at 723 K and the equimolar CaCl 2–NaCl melt at 823 K. The E– pO 2− (potential–acidity) diagram for Ce–O compounds stable in both melts has been constructed by combining both theoretical and experimental data. The stable oxidation states of Ce have been found to be (III) and (0) in both melts; Ce(IV) is only stable in the form of solid CeO 2. The standard potential value of the Ce(III)/Ce(0) system has been determined by potentiometry, giving values −3.154±0.006 V and −3.036±0.009 V versus Cl 2/Cl − in the eutectic LiCl–KCl at 723 K and the equimolar CaCl 2–NaCl melt at 823 K, respectively. On the other hand, from the calculated activity coefficient values it was possible to say that the Ce(III) ions form stronger complexes with the chloride ions in the alkaline melt. This is probably due to the smaller amount of free chloride ions in the calcium molten mixture. Identification of the Ce–O compounds that are stable in the melts as well as the determination of their solubility products was easily carried out by potentiometric titration using an yttria stabilised zirconia membrane electrode (YSZME). The results indicated that Ce 2O 3 is a strong oxobase and that CeOCl is a solid stable compound in the melts studied. CeO 2 is also a stable compound that can exist under oxobasic conditions. The best chlorinating conditions could be extracted from the comparison of the E– pO 2− diagram corresponding to the Ce–O compounds and that of some chlorinating mixtures. Experimental solubilization tests allowed us to verify the thermodynamic chlorinating predictions. Moreover the electrochemical behaviour of CeCl 3 solutions was also investigated. 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 on metallic substrates such as tungsten and molybdenum. The results showed that the Ce(III)/Ce(0) system is quasi-reversible with values of the charge transfer rate constant, k°, and transfer coefficient, α, around 10 −3.7 cm s −1 and 0.4, respectively. The diffusion coefficient of Ce(III) ions was also determined by different electrochemical techniques, obtaining a value in the order of 1×10 −5 cm 2 s −1. The validity of the Arrhenius law was also verified by plotting the variation of the logarithm of the diffusion coefficient versus 1/ T.