A.M. Martínez

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 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. The E–pO2− (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 CeO2. 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 Cl2/Cl− in the eutectic LiCl–KCl at 723 K and the equimolar CaCl2–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 Ce2O3 is a strong oxobase and that CeOCl is a solid stable compound in the melts studied. CeO2 is also a stable compound that can exist under oxobasic conditions. The best chlorinating conditions could be extracted from the comparison of the E–pO2− 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 CeCl3 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 cm2 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.

Electrochemical behaviour of magnesium ions in the equimolar CaCl2NaCl mixture at 550 °C

Abstract The electrochemical behaviour of magnesium ions was studied in molten CaCl 2 NaCl at 550 °C by cyclic voltammetry, chronoamperometry and chronopotentiometry. We observed that the electrode process of the reduction of Mg(II) on a tungsten electrode is quasireversible. The values of the transfer coefficient, α, and the charge-transfer rate constant, k 0 , were calculated by means of the logarithmic analysis of the convoluted curves. Those values were compared with the ones obtained from the steady-state current-potential curves. On the other hand, the diffusion coefficient of the Mg(II) ions, has also been determined by different electrochemical techniques, giving values ranging from 7.2 to 9.9 × 10 −6 cm 2 s −1 .

A chemical and electrochemical study of titanium ions in the molten equimolar CaCl2+NaCl mixture at 550°C.

Abstract The chemical and electrochemical properties of solutions of titanium chlorides in the fused CaCl 2 +NaCl equimolar mixture were studied at 550°C using different working electrodes. We determined the stability range of the various oxidation states of titanium and calculated the standard potentials of the Ti(III)/Ti(II) and Ti(II)/Ti(0) redox couples, the solubility products of the titanium oxides, the kinetic parameters of the electrochemical systems at the different electrodes and the diffusion coefficients of the electroactive species. We have also studied the titanium electrodeposition process by potential step measurements which indicated instantaneous nucleation of titanium at tungsten substrates. All these data allowed us to build-up the potential-pO 2− diagram which summarizes the properties of Ti-O compounds in the melt studied and can be used (together with the E-pO 2− diagram of chlorinating gaseous mixtures) to predict operating conditions for the process of industrial production of metallic titanium from molten salt systems.

Oxoacidity reactions in equimolar molten CaCl2-NaCl mixture at 575 °C

Abstract The behaviour of an yttria-stabilized zirconia electrode in the CaCl2-NaCl melt has been investigated at 575 °C. The response of the electrode is Nernstian and permits its use in the titration of Mg(II) with oxide ions. The solubility product of MgO was also determined. We have calculated the equilibrium constant of the carbonate ion dissociation as well as those corresponding to the systems HCl H 2 O and Cl 2 O 2 , finding that molten CaCl2-NaCl is more acidic than the LiCl-KCl eutectic melt and the BaCl2-CaCl2-NaCl (23.5:24.5:52 mol%) melt. On the other hand, the mixture can be purified with respect to oxide ions by gaseous reagents such as HC1 and Cl2.

Electrochemical study of the properties of iron ions in ZnCl2 + 2NaCl melt at 450°C

The electrochemical properties of Fe(II) in the ZnCl2 + 2NaCl melt at 450°C have been studied using a glassy carbon electrode. Iron oxidation states II and III have been shown to exist in the mixture and the stability of metallic iron has been confirmed. The kinetics of the electrodeposition and electrocrystallization of iron were studied finding that the process is quasi-reversible. In the same way, the Fe(III)Fe(II) exchange was found to be quasi-reversible. The values of the kinetic parameters, ko and α for both reactions were obtained. Mass transport towards the electrode is a simple diffusion process. The diffusion coefficient was found ((1.3 ± 0.4) × 10−5 cm2 s−1. Potential step measurements indicate instantaneous nucleation and growth of iron (I against t12 followed by diffusion control (Cottrell equation).

Chemical and electrochemical behaviour of chromium in molten chlorides

The chemical and electrochemical behaviour of chromium ions in the equimolar CaCl2 NaCl mixture at 550°C has been studied by electrochemical techniques (i.e. cyclic and square wave voltammetry, chronopotentiometry, chronoamperometry and potentiometry) using various substrates (Mo, W and glassy carbon) as working electrodes. Some of the results have been compared to data obtained in the eutectic LiClKCl mixture at 450°C. The stable oxidation states of chromium species were found to be the same in both melts. However, in the case of the CaCl2 NaCl melt and when using W substrates, a new electrochemical wave appears, which has been related to an enhancement of the oxidation of trace amounts of O 2 ions on the W material by the presence of Cr ions. Kinetic parameters characterising the mass and charge transfer of the electrochemical reduction and oxidation of Cr(II) ions as well as of the Cr(III) reduction process were also obtained. Chronoamperometric studies did not show evidence of nucleation phenomena in the case of the equimolar CaCl2 NaCl mixture when using any of the substrates. However, in the eutectic LiClKCl it has been shown that nucleation of metallic chromium, especially on glassy carbon substrates, controls the electroreduction process. The electrochemical behaviour of some chromium oxo-anions, i.e. Cr2O7 and CrO4 , has also been studied. The results show that both complexes lead to the same stable Cr(VI) species in the melt, whose reduction at glassy carbon electrodes implies a single one-electron process giving Cr(V) ions which are present in the melt as soluble species. Potentiometric titration of both Cr(III) and Cr(II) solutions were performed by using an oxide selective ion electrode, which consist of a membrane of ZrO2 stabilised with yttrium oxide (YSZME). The values of pKs for the different CrO compounds stable in the melt, i.e. CrO, Cr2O 2 and Cr2O3, were obtained.

A comparative study of the electrochemistry of tin ions in the ZnCl2 + xNaCl mixtures at 450°C

The electrochemistry of Sn(II) was studied with voltammetry, chronopotentiometry and chronoamperometry at tungsten (W) and glass carbon (GC) electrodes in the acidic and basic zinc chloride + sodium chloride (ZnCl2 + 2NaCl, 2ZnCl2 + 3NaCl2 + NaCl) molten salts at 450°C. The voltammograms showed peaks attributed to the presence of three oxidation states of tin, i.e. 0, II and IV, in all the mixtures. Both Sn(IV)/Sn(II) and Sn(II)/Sn systems increase their oxidation power when the amount of ZnCl2 increases in the melt, and this seem to arise from differences in the solvation of the metal ions in basic and acidic melts. The electrodeposition of tin was uncomplicated at W but on GC was complicated by nucleation. In addition the voltammograms corresponding to the electrochemical Sn(IV)/Sn(II) exchange have a better definition on GC than on W, on which some waves were accompanied by current oscillations due to the formation of volatile SnCl4. The two electrochemical exchanges were found to be quasi-reversible, and the values of the kinetic parameters, k0 and α for both reactions were obtained. Mass transport towards the electrode is a simple diffusion process and the diffusion coefficient DSn(II) was calculated.

Electrochemical behaviour of indium ions in molten equimolar CaCl2-NaCl mixture at 550°C

The stability of indium chloride and oxide as well as the electrochemical behaviour of indium ions have been studied in the equimolar CaCl2–NaCl melt at 550 ∘C by X-ray diffraction (XRD) and different electrochemical techniques, using molybdenum and tungsten wires as working electrodes. Voltammetric and chronopotentiometric studies showed signals attributed to the presence of three oxidation states of indium, i.e. 0, i and iii. The standard potential of the redox couples, as well as the solubility products of indium oxides have been determined, showing that In(iii) ions are completely reduced to monovalent indium by the indium metal according to the reaction: In (Ш) + 2 In ↔ 3 In () and that In2O is a strong oxide donor according to the reaction: In2O(s)→ 2 In() + O2- These results have allowed the construction of E-pO2− equilibrium diagrams summarising the properties of In–O compounds. The electrodeposition of indium was uncomplicated at Mo and W electrodes. Very good adherence of liquid indium to the electrode materials was observed, with the formation of Na–In alloys at highly reducing potentials, and there was no evidence of indium dissolution into the melt. Moreover, the voltammograms corresponding to the electrochemical In(iii)/In(i) exchange were well defined. The two electrochemical steps were found to be quasi-reversible, and the values of the kinetic parameters, ko and α, for both reactions, as well as the diffusion coefficients, DIn(III) and DIn(I) were calculated.