Electrochemical Formation of Elemental Boron in LiCl–KCl–KBF4 at 723 K

Significance 

The increasing global popularity and growing use of green vehicles like hybrid vehicles have significantly increased the consumption of neodymium magnets (Nd-Fe-B magnets). The trend is projected to increase further in the coming decades. Therefore, through application in green vehicles, Nd-Fe-B magnates are expected to play a critical role in reducing the consumption of fossil fuels, the leading producer of greenhouse gases. Generally, Nd-Fe-B magnets consist of light and heavy rare earth (RE) elements like Dy and Nd metals. Whereas light RE elements are distributed worldwide, heavy RE elements are generally scarce and unevenly distributed. Therefore, there is a need to develop cost-effective and efficient Nd-Fe-B magnets recycling technologies to guarantee the stable supply of RE elements.

An electrochemical process utilizing alloy diaphragms and molten salts is a feasible recycling process for recovering the individual elements from Nd-Fe-B magnets. It is possible to separate and recover Nd and Dy from the magnate wastes since the permeation rate through the alloy diaphragm is dependent on the electrolytic conditions and the type of the RE elements. However, if boron is dissolved from the magnet into the molten salt, it can form elemental boron or boron compounds on the surface of the alloy diaphragms, which could interfere with the reaction of the RE elements on the alloy diaphragms. Thus, the electrochemical behavior of B in molten salts is worth investigating.

In addition, an investigation of the behavior of B in molten salts is important from the viewpoint of developing a new production process for elemental boron. Among the available elemental boron producing methods, molten salt electrolysis offers a simple process for low-cost and efficient production of elemental boron without contamination by reductants. The electrochemical behaviors of B(III) ions in different molten salt electrolysis have been studied. In most of these studies, molten salt electrolysis has only been performed at high temperatures exceeding 823 K. However, obtaining elemental boron through low-temperature molten salt electrolysis has remained an important challenge. If it becomes feasible, the molten salt electrolysis process could become a low-cost and energy-saving process.

On this account, Dr. Yumi Katasho and Dr. Tetsuo Oishi from the National Institute of Advanced Industrial Science and Technology (AIST) investigated the electrochemical behavior of B(III) ions in molten salt. In their approach, LiCl–KCl–KBF4 melt at a relatively low temperature of 723 K was utilized due to its two main advantages: it is the most intensively studied as for the process using alloy diaphragms and the most suitable for studying the elemental boron production at low temperature because it has a low eutectic point (625 K). The authors started by performing cyclic voltammetry (CV) in the presence and absence of B(III) ions, followed by potentiostatic electrolysis at different potentials. The obtained samples were analyzed and the elemental B was identified via different characterization techniques like STEM. Their work is currently published in the Journal of the Electrochemical Society.

The researchers showed the B(III) ions were reduced to B(0) at both 1.5 V (vs. Li+/Li,) potential and potential that are more negative. After potentiostatic electrolysis, spherical electrodeposits were observed at 1.1 – 1.5 V (vs. Li+/Li,). These electrodeposits were typical elemental amorphous boron as per the images of different characterization techniques (STEM/EDX etc.). The products were about 85 wt% boron, while the efficiency of the elemental boron electrodeposition system was 96.2%, which is relatively high value in this field. One of the reasons for the high current efficiency in our study could be the low residual current due to impurities such as moisture, because the electrolysis in our study was performed in a clean atmosphere.

In a nutshell, Dr. Yumi Katasho and Dr. Tetsuo Oishi successfully studied the electrochemical behavior of B(III) ions in LiCl–KCl–KBF4 at 723 K. The products formed included spherical electrodeposits with 90 wt% purity B. The results further suggested that the presence of B(III) ions in the melt led to adverse effects on the Nd-Fe-B magnets recycling process, as indicated by the reduction of B(III) ions. The boron electrodeposition process was, however, efficient. In a statement to Advances in Engineering, the authors said that it is also a potential process for cost-effective and less-energy intensive production of elemental boron.

Reference

Katasho, Y., & Oishi, T. (2021). Electrochemical Formation of Elemental Boron in LiCl–KCl–KBF4 at 723 KJournal of the Electrochemical Society, 168, 122503.

Go To Journal of the Electrochemical Society

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