Relative equilibrium potentials of Gd, Nd, Ce, and La in molten LiCl–KCl

On the electrochemical formation of Pu–Al alloys in molten LiCl–KCl

Behavior of a boron-doped diamond electrode as an oxygen evolution electrode material was investigated at 773 K in molten LiCl–KCl (58.5:41.5 mol%), LiCl–KCl (75:25 mol%), LiCl–CaCl2 (64:36 mol%), LiCl–NaCl–CaCl2 (52.3:13.5:34.2 mol%) containing oxide ion. In molten LiCl–KCl systems, the BDD electrode is stable and its stability does not depend on the concentration of oxide ion and the melt composition. In molten LiCl–CaCl2 and LiCl–NaCl–CaCl2, the BDD electrode is less stable than in molten LiCl–KCl systems.KeywordsOxygen gas evolutionInert anodeMolten saltsMetal oxides

1. Introduction Recycling spent Nd magnets is important for guaranteeing a stable rare earth (RE) supply in Japan. As a new recycling process of Nd magnets, the authors have studied a one-step method using molten salt and alloy diaphragms [1-3], where Dy or Nd is selectively permeated by controlling the potential of the alloy diaphragms. This technique is based on the rapid electrochemical formation of RE–iron group (IG) alloys in molten salts. To investigate the mechanism of the rapid alloy formation, the authors recently conducted in situ energy-dispersive X-ray diffraction measurement for the electrochemical alloying and de-alloying of Dy–Ni in molten LiCl–KCl [4,5]. Through these results, new insights into Dy–Ni alloying and de-alloying were obtained. However, the reason why the electrochemical formation of RE–IG alloys proceed so rapidly is still unclear. Therefore, we investigated the mechanism by comparing Ni with the other metal, Cu, which has a similar crystal structure (fcc) to Ni, and its alloy formation rate with RE is relatively high [6]. In this study, in situ X-ray fluorescence (XRF) measurement of Dy–Cu alloying and de-alloying in molten LiCl–KCl was performed and the alloying/de-alloying behaviors of Dy–Cu and Dy–Ni were compared. 2. Experimental The experiments were conducted in a similar manner to the previous studies [4,5] at the BL28B2 beamline of SPring-8. 35.0 g of LiCl–KCl (44 : 56 wt% eutectic composition, 35.0 g) was set in a graphite crucible inside a glovebox with an Ar atmosphere. Pre-electrolysis was conducted before the experiment. An electric furnace with windows was fixed on the transition stage. Electrolysis was conducted in Pyrex vessel at 723 K in a dry Ar atmosphere. After potential calibration by the Li+/Li equilibrium potential using a Mo electrode, DyCl3 was added to the melt up to 0.5 mol%. A Cu plate (5 mm × 15 mm × 0.1 mmt), an Ag+/Ag, and the graphite crucible were used as the working, reference, and counter electrodes, respectively. After the measurement of cyclic voltammetry, the potentiostatic electrolysis was conducted using the same Cu plate for 180 min at 0.5 V vs. Li+/Li and then, potential was shifted to 1.5 V vs. Li+/Li and kept for 30 min. XRF measurements were started simultaneously with electrolysis. The incident beam size was 0.1 mm × 2.0 mm and irradiated to the Cu plate in the molten salt. The take-off angle was set at 5°. Transmitted X-rays were consecutively detected with the integration time of 120 s. 3. Results and Discussion The X-ray fluorescence peaks of Dy Kα1, Kα2, and Kβ1 were confirmed. Fig 1 shows peak area–time data for X-ray fluorescence of Dy Kα1 during Dy–Cu alloying and de-alloying. The peak area increased as the alloying progressed, which seemed to correspond to the formation and growth of Dy–Cu alloy layer. The peak area slightly decreased during de-alloying. Compared to Dy–Ni alloying/de-alloying [5,6], the intensities of fluorescence peaks were weak. These results suggest that the was alloy formation rate is smaller in the case of Cu than that of Ni. Acknowledgement This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Synchrotron radiation studies were performed at the BL28B2 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal No. 2022B1338).

The anodic formation of an oxide film on MoSi2 in molten MgCl2–NaCl–CaCl2 containing oxide ions has been investigated based on our former study, and the compatibility as an inert anode in the melt is discussed in contradistinction in molten LiCl–KCl. A small anodic current flew continuously during potentio-static electrolysis, and gas bubble generation was seen. The weight of MoSi2 was changed by the electrolysis, and an oxide film consisting of SiO2 and MgSiO3 was formed by the electrolysis above 1.9 V (vs. Mg/Mg2+). The current contributions estimated from the weight change and the film thickness indicate that the reaction other than the Mo dissolution and the oxide film formation enlarged with the increase in the electrolysis duration and the raise in the electrolysis potential. The current contribution other than the Mo dissolution and the oxide film formation became above 90%, which suggests MoSi2 is promising as an inert anode. A SiO2 film was formed on MoSi2 in molten LiCl–KCl containing oxide ions, but most of the current was consumed for the Mo dissolution and the oxide film formation. It is considered that the formation of MgSiO3 influences the anodic behavior of MoSi2.

Preparation of Co–Sn alloys by electroreduction of Co(II) and Sn(II) in molten LiCl–KCl

We proposed a new separation and recovery process for RE metals from Nd magnet scraps using molten salt electrolysis. The present study focused on the electrochemical formation of RE–Ni (RE = Dy, Nd, Pr) alloys in a molten LiCl–KCl system at 723 K. Cyclic voltammetry was conducted using a Ni electrode in molten LiCl–KCl–RECl3 systems at 723 K. In the negative scan for the DyCl3 added system, a large cathodic current was observed from 0.70 V (vs. Li+/Li) as a result of the formation of Dy–Ni alloys. In contrast, large cathodic currents as results of the formation of Nd–Ni and Pr–Ni alloys were observed from 0.60 V. On the basis of these results, an alloy sample was prepared by potentiostatic electrolysis at 0.65 V for 1 h using a Ni plate cathode in a molten LiCl–KCl–DyCl3–NdCl3–PrCl3 system. The mass ratio of Dy/Nd+Pr in the alloy sample was found to be 50 by ICP-AES. Finally, a Dy permeation experiment was conducted with a new electrolytic cell.

Relative activities of rare earths in molten LiCl-KCl are important for predicting partitioning of rare earth fission products during electrochemical processing of used nuclear fuel. In such processing, metallic spent fuel is loaded into an anode basket and polarized to electrorefine U or U/TRU. TRU is an acronym for transuranic actinides. The electrolyte is commonly comprised on eutectic LiCl-KCl with 5-10 wt% UCl3. Given this electrolyte composition, cathodic deposition first favors U then TRU then rare earth metals. The precise sequence of deposition is dependent upon equilibrium potentials, which are unique for each metal. The Nernst equation can be used to calculate the equilibrium potential for each metal under the condition in which deposition of the metal has initiated. For each metal, the Nernst equation requires a standard potential, activity of the metal in the salt as a chloride, and activity of the metal in its reduced state. Even under the most simplifying case in which each metal reduces to a pure solid with activity of one, there is insufficient data from the literature to build a verified order of rare earth metal activity. Some reported potentials are reported as apparent standard potentials, meaning that they assume constant equilibrium potential as metal chloride concentration approaches zero. However, Bagri and Simpson reported that equilibrium potentials for rare earth chloride salts do not in fact approach a constant value in the direction of infinite dilution. In a series of variable concentration experiments, Gd/Gd3+, Nd/Nd3+, Ce/Ce3+, and La/La3+ were compared in LiCl-KCl at 773 K. The observed sequence of activity was LaCl3 > CeCl3 > NdCl3 > GdCl3. But that sequence disagrees with standard apparent reduction potentials reported by Zhang (LaCl3 > NdCl3 > CeCl3 > GdCl3). Given that the potential differences are very small and reference electrodes were used in the measurements made by Bagri and Simpson, a new experiment was devised and executed to yield a more definitive activity sequence for these key rare earth metals. The experiment does not involve use of a reference electrode. Rather, it involves relative potential measurements between Gd, Nd, Ce, and La metal immersed in eutectic LiCl-KCl at temperatures ranging from 723 to 823 K. The salt was initially spiked with a small concentration of LaCl3 and allowed to equilibrate. Salt samples were taken and concentrations of Gd, Nd, Ce, and La were measured using ICP-MS. Results of these experiments will be reported and compared to calculated standard potentials based on standard state Gibb’s free energy of formation for each rare earth chloride. These results should definitively establish the order or rare earth activity in LiCl-KCl, eliminating error induced by reference electrodes.

Stability of a boron-doped diamond electrode in molten chloride systems

The electrochemical formation of Tb-Ni alloys was investigated in a molten LiCl–KCl–TbCl3 (0.50 mol% added) at 723 K. Open-circuit potentiometry was conducted using a Ni electrode after electrodepositing Tb metal at 0.20 V (vs. Li+/Li) for 300 s. There were four potential plateaus at (a) 0.66 V, (b) 0.80 V, (c) 0.95 V and (d) 1.56 V, respectively. Alloy samples were prepared by potentiostatic electrolysis at 0.60 and 0.70 V at 723 K. The alloy phase was identified as only TbNi2. Anodic dissolution of Tb from the formed TbNi2 was conducted at 0.90, 1.20 and 1.60 V, respectively. In the sample obtained at 0.90 V for 3 h, the existence of TbNi3 was identified by the XRD. Phase of the sample obtained at 1.20 V for 3 h was TbNi5. The sample obtained at 1.60 V for 3 h was Ni. Alloy samples were prepared by potentiostatic electrolysis at 0.50–0.80 V for 1 h using Ni plate cathodes in a molten LiCl–KCl containing TbCl3 (0.50 mol%) and NdCl3 (0.50 mol%). The highest mass ratio of Tb/Nd in the alloy sample was 56 at 0.70 V.

Phase formation and acoustic emission during electrochemical incorporation of lithium into aluminium from molten LiCl—KCl

The effect of Cu and Li additions to the intermetallic alloy Fe-40at.pctAl on the corrosion performance in an LiCl-55wtpctKCl molten eutectic salt was studied by means of electrochemical impedance spectroscopy, transmission line modeling (TLM), and cathodic polarization. The tests were done at 723 K, 773 K, and 823 K (450 °C, 500 °C, and 550 °C), for 60 and 720 minutes. The element additions could improve the corrosion resistance of Fe-40at.pctAl in molten LiCl–KCl, while TLM could characterize and quantify the interfacial processes in hot corrosion. The polarization curves helped to establish the possible cathodic reactions in the experimental conditions.

Optical absorption and fluorescence properties of trivalent lanthanide chlorides in high temperature molten LiCl–KCl eutectic

When a type-A zeolite is immersed in molten LiCl–KCl eutectic salt, the alkali and halogen elements in the salt phase are absorbed in the vacancies of the zeolite in the form of ions. Recently, it was revealed that I– in molten LiCl–KCl–KI salt exhibited greater absorption in the zeolite than Cl–, although Br– in molten LiCl–KCl–KBr salt did not show such absorption behavior. In the present study, molecular dynamics calculations were performed to understand the difference in the absorption behavior of I– and Br– in a type-A zeolite by calculating the dependences of the formation enthalpy of the zeolite containing several alkali halides on the composition of the halogen elements. From these calculations, it appeared that the enthalpy is negative in a Cl–I mixture system, indicating that when a few I– ions are absorbed in a zeolite cage, more I– is absorbed than that expected from the I–/(I–+Cl–) ratio in the salt, i.e., I– is preferentially absorbed. In contrast, no such negative potential appeared in the ...

Electrochemical synthesis of Ni–Sn alloys in molten LiCl–KCl

In situ X-ray diffraction analysis of electrochemical Dy–Ni alloying in molten LiCl–KCl