Metal recycling is essential for conserving our limited mineral resources and promoting a sustainable society. Unlike mining natural ores, metal recycling offers numerous benefits, such as reducing soil pollution, lowering energy consumption, and preventing resource depletion. At HORIBA, we had the pleasure of interviewing Professor Mikito Ueda from the Faculty of Engineering at Hokkaido University in December, 2022. Professor Ueda's work focuses on the electrolytic purification of metals at lower temperatures using ionic liquids, aiming to enhance energy efficiency in the metal recycling process.
Professor Mikito Ueda, the Faculty of Engineering, Hokkaido University with a HORIBA XGT-9000.
Professor Ueda explains that his research is based on electrochemistry, particularly using non-aqueous liquids as electrolytes. In the non-aqueous solutions, metals can be electrodeposited that cannot be obtained in aqueous solutions, unlike conventional aqueous solutions. His team is primarily focused on electroplating aluminum and recycling aluminum through electrorefining, using ionic liquids.
Generally, aluminum electrorefining is carried out using inorganic ionic liquids (molten salts) at temperatures exceeding 1000°C. This process consumes a significant amount of electrical power and has a considerable environmental impact. Additionally, such high temperatures restrict the materials available for the electrolytic bath to expensive options like bricks or quartz. Professor Ueda's team aims to achieve the same purification process at lower temperatures, enabling the use of more common materials like Pyrex or PTFE and simplifying the industrial process.
Their research has successfully achieved aluminum electrodeposition at around 150°C and even at room temperature using ionic liquids. This breakthrough allows for aluminum plating on plastics with conductive surfaces, producing lightweight while looking like aluminum. Although the lower-temperature process is less efficient in terms of production volume, it offers significant energy savings and reduces environmental impact.
Research on ionic liquids containing organics began in the 1970s, with many researchers exploring their applications. In the late 1970s, organic ionic liquids started being used for aluminum electrodeposition. In Professor Ueda's lab, they use a mixture of aluminum chloride and 1-ethyl-3-methylimidazolium chloride. Interestingly, these solid powders become liquid when mixed at room temperature without heating.
His team has applied the ionic liquids for electrodepositing not only pure aluminum but also aluminum alloys. They have successfully created aluminum-chrome, aluminum-nickel, and aluminum-tungsten alloys using various metal chloride additives.
Their technology also extends to aluminum recycling. Low-grade recycled aluminum contains impurities and has limited usages. Professor Ueda's team aims to use his team’s electrolysis technology with the ionic liquids to purify recycled aluminum, making it suitable for a wider range of applications. This is particularly important given the challenges of importing aluminum and the increasing need for high-purity recycled materials.
Professor Ueda found the HORIBA micro-XRF, the XGT-9000, at a 2019 conference. Seeking a wider visualization of elemental distribution than previous methods, he found the XGT-9000 ideal for visualizing the entire electrode surface before and after electrorefining.
The XGT-9000’s mapping function reveals element distributions on electrode surfaces, providing valuable insights into the dissolution and deposition processes. Unlike SEM-EDX, which has limited analysis areas, the XGT-9000 can scan entire surfaces, significantly enhancing their research capabilities.
In addition to aluminum electrodeposition research, Professor Ueda's team has been working on producing high-purity sodium from used sodium-sulfur secondary batteries. They now aim to further purify this high-purity sodium to produce sodium used in the sodium flux method of gallium nitride crystal growth. Gallium nitride is a promising material for power semiconductors with the potential to reduce CO2 emissions. High purity sodium is required to produce gallium nitride crystals with low defect density.
The research team has successfully removed trace amounts of calcium, potassium, and other metallic impurities from sodium, and continues to work on the next challenge of removing trace amounts of oxygen from sodium. In collaboration with HORIBA, Prof. Ueda discovered the optimal conditions for detecting low concentrations of oxygen in sodium. HORIBA's EMGA-820* Oxygen and Nitrogen Analyzer and a special vessel for handling sodium samples make it possible to accurately analyze oxygen in sodium without exposure to air.
*This product has been discontinued, and its features have been integrated into a new model EMGA-Expert.
Professor Mikito Ueda, with a HORIBA EMGA-820.
While ionic liquids containing organic compounds offer room-temperature operation, they have drawbacks such as higher costs and lower conductivity compared to inorganic ionic liquids. Professor Ueda's team continues to explore various ionic liquids and additives to optimize their electrolytic purification techniques for different metals.
The team’s current activity is the production of high purity sodium using ionic liquids. This low-temperature process enables the production of high-purity sodium while saving energy, and the technology can contribute to the crystal growth of high-quality gallium nitride. Professor Ueda's innovative research holds the promise of more efficient and environmentally friendly metal recycling processes in the future.
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