Electrochemical Reduction Behavior of Boron in Molten LiCl–KCl–KBF4
Authors: Katasho Yumi , Oishi Tetsuo 
Published: December 2020

1. Introduction

The consumption and demand of Nd-Fe-B magnets, so-called neodymium magnets, is increasing year by year. Since the rare earth elements such as Nd can be obtained from sparse mineral resources, the technology for the recycling system of Ne-Fe-B magnets are highly required. The authors have been developing a technique to separately recover Nd and Dy from used Ne-Fe-B magnets using alloy diaphragm and molten salt [1].
Although the electrochemical behavior of Nd and Dy was intensively investigated, the Ne-Fe-B magnet contains B as well as Fe, and the behavior of B in molten salt has not been clarified yet. Therefore, in this study, we investigated the behavior of B in LiCl–KCl eutectic melts at 723 K. The LiCl–KCl eutectic melt was chosen because a high separation ratio of Nd and Dy was obtained in our past experiments [2].

2. Experimental

The experiments were conducted in LiCl–KCl (44:56 wt%) eutectic melt in a dry Ar atmosphere at 723 K inside a glove box with an electronic furnace. 300 g of LiCl–KCl was vacuum dried at 473 K for more than 24 hours before elevating the temperature. As working electrodes, Ni, and Ag wire (φ1 mm) electrodes were used for cyclic voltammetry. Ni and Ag plate electrodes (5 mm×15 mm×0.1 mm(t)) were used for electrodeposition. A glassy carbon rod was used as a counter electrode. A reference electrode was an Ag+/Ag electrode, which was prepared by immersing an Ag wire in a LiCl−KCl melt containing 1 mol% of AgCl set in a Pyrex glass tube. The potential of the reference electrode was calibrated by the Li+/Li potential obtained by open circuit potential measurement on a Mo wire (φ1 mm) electrode just after electrodepositing Li metal. All the potential hereafter is described in reference to this Li+/Li potential.
Cyclic voltammetry was conducted at KBF4 concentrations of 0, 0.5 and 2.0 mol%. Potentiostatic and galvanostatic electrolysis was conducted at KBF4 concentrations of 2.0 mol%. After the electrolysis, the plate electrodes were rinsed by distilled water, and then characterized by SEM and XRD.

3. Results and Discussion

3-1. Cyclic Voltammetr
y

Cyclic voltammograms (CV) obtained by Ni and Ag electrodes are shown in Fig.1(a) and (b), respectively. In the case of Ni electrode, cathodic current was observed at around 1.2 V shown “a” in the figure. The couple of peaks “b” and “c” at around 0.0 V correspond to Li metal deposition and dissolution, respectively. In addition to current peak “a”, a small current peak “e” appeared at 1.8 V, which seems to correspond to a formation of Ni-B compounds. Except for the peak “e”, essentially the same results were obtained in the case of Ag electrode. In both cases, the peak current at around 1.2 V increased with the concentration of KBF4 (shown as “a” in the figures). This peak can correspond to the reduction of B(III) to B(0), although further investigation is needed to confirm it.

3-2. Characterization of Products Prepared by Potentiostatic and Galvanostatic Electrolysis

Fig. 2 shows a microscope image and a surface SEM image of the deposit on the Ni plate electrode obtained by potentiostatic electrolysis at 1.2 V, Q= −50 C. The granules with a diameter of c.a. 1-2 μm were observed. From XRD measurement, the diffraction peaks coincided to Ni, Ni2B, and NiCl2 was confirmed. Weak peaks that might correspond to B were partially observed.
Next, the galvanostatic electrolysis was conducted at −9.8 mA cm−2, Q= −15 C for a Ni and an Ag plate electrode, respectively. In the case of Ni electrode, Ni, Ni2B, NiCl2 and KCl were detected by XRD analysis. This is almost the same result as the potentiostatic electrolysis at 1.2 V. Ag was selected as the substrate because no alloys of Ag and B exist in phase diagram. From the XRD results, only clear diffraction peaks coincided to Ag was confirmed. Other peaks were also detected, but they did not correspond to elemental B or other expected compounds. From the observation by sight, some deposits were actually formed. Therefore, the deposit can be amorphous B. In the future, further analysis and experiments are needed to clarify the behavior of B on Ag electrode.

Acknowledgement

This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

[1] T. Oishi, H. Konishi, T. Nohira, M. Tanaka and T Usui, Kagaku Kogaku Ronbunshu, 36, 299–303 (2010).
[2] T. Oishi, M. Yaguchi, Y. Katasho and T. Nohira, Rare metal technology 2020, 151–156 (2020).

Figure 1