Key Takeaways & Executive Findings
- •• Mn(II) reduction on tungsten electrode is quasi-reversible and diffusion-controlled, with kinetic parameters determined at 973–1048 K. • Controlled-potential electrolysis at −1.5 V for 8 h reduced MnCl2 impurity from 0.62 wt.% to 0.037 wt.%, achieving 94.14% removal efficiency. • Removal of Mn(II) impurity significantly improved magnesium purity from 98.59% to 99.94% and promoted formation of large globules. • The proposed method is green and effective for separating metal ion impurities in molten salt electrolytes for high-purity magnesium production.
Abstract
The electrochemical separation of Mn(II) impurity from molten NaCl−KCl−MgCl2 was systematically investigated to facilitate the electrolytic production of high-purity magnesium. The reduction of Mn(II) to Mn metal on tungsten electrode was a quasi-reversible process controlled by diffusion. The apparent standard potential and exchange current density of Mn(II)/Mn(0) electrode reaction were determined at temperatures ranging from 973 to 1048 K. Solid Mn metal generated during electrolysis aggregated into irregular clumps and adsorbed some needle-like MgO, imposing a detrimental effect on both the aggregation and the purity of magnesium metal. After electrolysis at −1.5 V in molten NaCl−KCl−MgCl2−0.62wt.%MnCl2 for 8 h, the concentration of MnCl2 impurity decreased to 0.037 wt.%, achieving a removal efficiency of 94.14%. When direct electrolysis was performed in molten NaCl−KCl−MgCl2−0.62wt.%MnCl2, the obtained magnesium metal was small blocks with a caviar-like appearance, and the purity was just 98.59%. In contrast, a large globule of magnesium metal was obtained when electrolysis was performed in the purified electrolyte, and its purity was improved to 99.94%. The controlled-potential electrolysis proposed in this work has been verified to be a green and practically effective method to separate the metal ion impurities from molten electrolyte for high purity magnesium extraction.
1. Introduction
Magnesium is well-known for its excellent properties and has been widely applied in the automotive, aerospace, 3C (computer, communication and consumer electronics) and other industrial fields in the world [1−4]. The global magnesium market is growing at a compound annual growth rate of about 5%, and is forecasted to reach 1.6×106 t by 2027 [5]. The electrolytic technology based on the molten salt electrolysis of anhydrous magnesium chloride or carnallite [6], is one of the two main techniques for the industrial production of metallic magnesium on a worldwide scale due to the significant advantages such as continuous operation, relatively low energy consumption, large-scale production, and more environmental friendliness [7]. Since MgCl2 has an inexhaustible supply from brines, the electrolytic technology is considered to have greater application prospects in the future.
The impurities, especially the metal ion impurities such as Fe, Ni, Mn, Cu, and Al ions, are detrimental to both the magnesium purity and current efficiency [8]. The elimination of ionic impurities is necessary for the electrolytic magnesium production. On the other hand, electrorefining in molten salts is practicable for the recycling of magnesium metal, which is regarded as a crucial technology to expand the usage of magnesium in the future and fits well with the concept of green chemistry regarding the circular economy and near zero-emission [9]. There is no doubt that some other metal impurities are contained in the magnesium scrap, and their contamination of magnesium metal should be properly controlled in the electrorefining process. Therefore, the investigation on the electrochemical behavior of impurity elements is of great importance to both the electrolytic production of magnesium metal and the electro-recycling of magnesium scrap.
The electrochemical behaviors of Fe, Ni, Mn, Cu and Al have been extensively explored in melts consisting of alkali metal chlorides [10−18]. However, the electrolysis and electrorefining of magnesium are usually conducted in the melts containing MgCl2 as a main ingredient, such as the NaCl−KCl−MgCl2 system or the NaCl−CaCl2−MgCl2 system. The reported results may not be entirely accurate or applicable to these systems.
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Zhi-wen ZHAO, Zheng ZENG, Yan-ping WANG, Pei TANG, Chang JIANG, Zhong-sheng HUA (2025). Electrochemical separation of Mn(II) impurity from molten salt electrolyte for magnesium electrolysis. SinoTechIntel Verified Research. https://doi.org/10.1016/S1003-6326(25)67012-9
Research & Educational Purpose Only:The translations, structured abstracts, analytical annotations, and data reports provided by SinoTechIntel are intended exclusively for academic research, internal corporate R&D, and educational benchmarking. They do not constitute formal engineering, chemical safety, legal, or professional advice.
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Frequently Asked Questions
What is the main objective of this study?
The main objective is to systematically investigate the electrochemical separation of Mn(II) impurity from molten NaCl−KCl−MgCl2 electrolyte to facilitate the production of high-purity magnesium.
How was Mn(II) impurity removed from the molten salt?
Mn(II) impurity was removed via controlled-potential electrolysis at −1.5 V for 8 hours, which reduced the MnCl2 concentration from 0.62 wt.% to 0.037 wt.%, achieving a removal efficiency of 94.14%.
What effect did Mn(II) impurity have on magnesium purity?
The presence of Mn(II) impurity resulted in magnesium metal with a caviar-like appearance and a purity of only 98.59%. After removal, the purity improved to 99.94% and the metal formed large globules.
What is the significance of this research for magnesium production?
This research provides a green and practically effective method for separating metal ion impurities from molten salt electrolytes, which is crucial for producing high-purity magnesium and improving current efficiency in industrial electrolysis.
What are the key findings regarding the electrochemical behavior of Mn(II)?
The reduction of Mn(II) to Mn on a tungsten electrode is a quasi-reversible process controlled by diffusion. The apparent standard potential and exchange current density were determined at temperatures from 973 to 1048 K.
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