Feasibility of the electrochemical way in molten fluorides for separating thorium and lanthanides and extracting lanthanides from the solvent

An alternative way of reprocessing nuclear fuel by hydrometallurgy could be using treatment with molten salts, particularly fluoride melts. Moreover, one of the six concepts chosen for GEN IV nuclear reactors (Technology Roadmap - http://gif.inel.gov/roadmap/) is the molten salt reactor (MSR). The originality of the concept is the use of molten salts as liquid fuel and coolant. During the running of the reactor, fission products, particularly lanthanides, accumulate in the melt and have to be eliminated to optimise reactor operation. This study concerns the feasibility of the separation actinides-lanthanides-solvent by selectively electrodepositing the elements to be separated on an inert (Mo, Ta) or a reactive (Ni) cathodic substrate in molten fluoride media. The main results of this work lead to the conclusions that:

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The solvents to be used for efficient separation must be fluoride media containing lithium as cation.

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Inert substrates are suitable for actinide/lanthanide separation; nickel substrate is more suitable for the extraction of lanthanides from the solvent, owing to the depolarisation occurring in the cathodic process through alloy formation.

     

The preparation of corrosion-resistant layers by the electrolytic deposition of tantalum on nickel and stainless steel

The deposition of tantalum by electrolysis in molten fluorides is applied to stainless steel and nickel. The deposit is made at 800°C and is controlled by measuring the cathode potential. Only with deposits on nickel is surface alloy formation observed. This effect is not observed in the case of deposits on stainless steel.     

Electrodeposition of carbon films from molten alkaline fluoride media

The electrodeposition of carbon films from carbonate ions (CO₃²⁻) in molten alkaline fluorides (LiF/NaF) was investigated in the 700-800 °C temperature range using cyclic voltammetry and chronopotentiometry. The cathodic peak in the cyclic voltammogram indicates that CO₃²⁻ ions are reduced in a one-step process: CO₃²⁻+4e⁻→C+3O²⁻. Deposits of amorphous carbon were obtained by potentiostatic electrolysis and analysed by several physical techniques: X-ray diffraction, Raman spectroscopy, scanning electron microscopy coupled with energy dispersive spectroscopy.     

Electrochemical reduction of Gd(III) and Nd(III) on reactive cathode material in molten fluoride media

The electrochemical reduction of two lanthanides (neodymium Nd and gadolinium Gd) was investigated in the 800–950 °C temperature range on nickel and copper electrodes. These materials react with lanthanides (Ln) to form intermetallic compounds. The formation mechanism of the alloys was determined by coupling electrochemical techniques and Scanning Electron Microscopy (SEM) after electrolyses runs; this also allowed the identification of the binary compounds formed. In addition, from the electrochemical results, we calculated the Gibbs energies of Nd/Ni, Gd/Ni, Gd/Cu and Nd/Cu.     

Electrochemical extraction of europium from molten fluoride media

This work concerns the extraction of europium from molten fluoride media. Two electrochemical ways have been examined: (i) the use of a reactive cathode made of copper and (ii) the co-deposition with aluminium on inert electrode, leading to the formation of europium–copper and europium–aluminium alloys, respectively, as identified by SEM-EDS analysis. Cyclic voltammetry and square wave voltammetry were used to identify the reduction pathway and to characterise the step of Cu–Eu and Al–Eu alloys formation. Then, electrochemical extractions using the two methodologies have been performed with extraction efficiency around 92% for copper electrode and 99.7% for co-reduction with aluminium ions.     

Preparation of oxygen evolving electrodes with long service life under extreme conditions

Among the numerous base metals tested for DSA® type electrodes (e.g., titanium and its alloys, zirconium, niobium etc.), tantalum is a potentially excellent substrate owing to its good electrical conductivity and corrosion resistance, and the favourable dielectric properties of its oxide. Nevertheless, a DSA® type electrode fabricated on a tantalum substrate would be very expensive due to the high cost of the metal. To prepare an anode combining the excellent properties of tantalum at reasonable price, a new material has been developed in our laboratory. This consists of a common base metal (e.g., Cu) covered with a thin tantalum coating. This tantalum layer was obtained by molten salt electroplating in a LiF–NaF–K2TaF7 melt at 800°C. Thus, an anode of the type Metal/Ta/Ta2O5–IrO2 with a surface load of 22gm-2 IrO2, submitted to the severe test conditions used in this work, exhibits a standardized lifetime tenfold greater than one made with ASTM grade 4 titanium base metal. Thus, this type of electrode might be advantageously employed as an oxygen evolution anode in acidic solutions.     

Electrochemical study of the Eu(III)/Eu(II) system in molten fluoride media

The electrochemical behaviour of the Eu(III)/Eu(II) system was examined in the molten eutectic LiF–CaF₂ on a molybdenum electrode, using cyclic voltammetry, square wave voltammetry and chronopotentiometry. It was observed that EuF₃ is partly reduced into EuF₂ at the operating temperatures (1073–1143 K). The electrochemical study allowed to calculate both the equilibrium constant and the formal standard potential of the Eu(III)/Eu(II) system. The reaction is limited by the diffusion of the species in the solution; their diffusion coefficients were calculated at different temperatures and the values obey Arrhenius’ law. The second system Eu(II)/Eu takes place out of the electrochemical window on an inert molybdenum electrode, which inhibits the extraction of Eu species from the salt on such a substrate.     

Neodymium and gadolinium extraction from molten fluorides by reduction on a reactive electrode

This work describes the electrochemical extraction on a reactive cathode (Cu, Ni) of two lanthanides Ln (Ln = Nd and Gd) from molten LiF–CaF₂ medium at 840 and 920 °C for Nd and 940 °C for Gd. Extraction runs have been performed and the operating conditions (cathodic material and temperature) optimized. The titration of the Nd and Gd concentrations in the melt during extraction used square wave voltammetry. At the end of each run, the residual Ln content was checked by ICP-AES; the extraction efficiencies of the two lanthanides were found to be more than 99.8% on both reactive substrates.     

Formation of corrosion-resistant layers by electrodeposition of refractory metals or by alloy electrowinning in molten fluorides

The electrolytic treatment of less resistant metals such as iron, copper and nickel with tantalum or niobium has been carried out in K₂TaF₇-LiF-NaF or K₂NbF₇-LiF-NaF solutions in the 550 to 1050°C temperature range. Two kinds of experiments have been used.

1. At lower temperatures, electroplating with pure tantalum and niobium on inert cathodes was performed. The electrodeposition mechanism of each metal was studied and coherent electroplates were prepared which were tested in electrocatalytic applications.
2. At higher temperatures (850–1050°C), using nickel cathodes, intermetallic compounds were obtained at more positive potentials than that for pure electrodeposition (Ta₂Ni, TaNi, TaNi₂, TaNi₃, NbNi, NbNi₃). The electrowinning of stable TaNi₃ and NbNi₃ layers was carried out by the metalliding process which makes these materials resistant to corrosion in various media. Further, a study of the kinetics of growth of the diffusion layer allowed a diffusion parameter to be determined which was in agreement with other results obtained by conventional methods.

     

Electrochemical alloying of nickel with niobium in molten fluorides

The metalliding of nickel with niobium was carried out by electrolysis in moten fluorides in the temperature range 850 to 1050° C. Several alloy layers were prepared and analysed. The composition of the bulk of the layer corresponds to the NbNi₃ compound but it is assumed that during the metalliding process a thin layer of NbNi is formed at the surface of the electrode. Electrochemical measurements were related to the kinetics of layer growth; these allowed the determination of the diffusion coefficient and the Arrhenius linear relationship of this parameter with the temperature.