The 17 rare earth elements are crucial components in various technologies, such as cell phones, computers, TVs, and vehicles. Unfortunately, the supply of these elements is limited and geographically concentrated, mainly in China, leaving the global market vulnerable to political and economic factors. Additionally, the mining of REEs involves environmentally unsustainable practices, leading to soil erosion, water pollution, and deforestation, among other negative impacts. Despite these concerns, the demand for these elements is expected to continue to rise due to the growth of technology and the increasing focus on sustainability. Thus, it is crucial to find ways to ensure a sustainable supply chain while minimizing the negative environmental impact of REE mining.
The team headed by Young-Shin Jun, who serves as a professor of energy, environmental, and chemical engineering in the McKelvey School of Engineering at Washington University in St. Louis, has successfully devised a proof-of-concept method for extracting rare earth elements (REEs) from coal fly ash. Coal fly ash is a finely ground waste product that results from the combustion of coal.
“We wanted to use a greener process to extract REEs than traditionally more harmful processes,” Jun said. “Since the coal has already been used, this process is ultimately a pathway toward reduction and remediation of waste products.”
Young-Shin Jun, along with her former doctoral student Yaguang Zhu, has developed a pioneering method for extracting REEs from coal fly ash, which is typically considered waste. This groundbreaking process involves using supercritical fluids, which are substances that exist in a state between a liquid and a gas due to high pressure and temperature. The team’s research demonstrated that common supercritical fluids, such as nitrogen, carbon dioxide, and air, were highly efficient at extracting REEs while also separating impurities.
According to the team’s report published in RSC Sustainability, the potential value of REEs that could be recovered from coal fly ash in the United States alone is estimated to be over $4 billion annually. Through their experiments, they discovered that supercritical carbon dioxide significantly reduced the concentration of impurities in the final product. In the end, their process produced REE products with up to 6.47% concentration, compared to the initial coal fly ash concentration of only 0.0234%.
“The uniqueness of our work is not only using the supercritical CO2, but also showing that supercritical air and nitrogen, with much lower temperature and pressure than those required for CO2, can extract REE effectively,” said Jun, who leads the Environmental NanoChemistry Laboratory.
“We can use lower temperatures and pressures with nitrogen or air to extract the rare earth elements from coal fly ash, which means lower energy cost. Of course, the supercritical CO2 works best, but supercritical air or nitrogen could do a much better job compared with traditional high temperature boiling with acids and organic solvents for REE extraction.”
The innovative extraction process developed by Jun’s team involves two distinct stages. First, metal ions, including REEs and impurities, are extracted from coal fly ash and then react with nitric acid to form metal nitrates. In the second step, tributyl phosphate (TBP) is used to react with the metal nitrates. Supercritical carbon dioxide, nitrogen, or air is then introduced to form complexes with REEs, which can be extracted from the coal fly ash.
Following the extraction process, the team employed a multistage stripping process that significantly reduced impurities while collecting REEs. Furthermore, the nitric acid and TBP used during extraction can be recycled multiple times without losing efficiency, which helps to mitigate disposal concerns.
This method is noteworthy in that it eliminates the need for high-temperature roasting of raw materials, which is required in traditional extraction processes. Additionally, there is no need for strong acids or large amounts of toxic organic solvents to extract REEs, which also generate waste products.
“Supercritical fluid is considered as a greener solvent, is less invasive to the environment and allows us to extract REE directly from solid waste without leaching and roasting raw materials, so less energy is required for our new process, which also produces less waste,” Jun said. “We are seeking a more environmentally benign process for critical element recycling and recovery from materials previously considered to be waste.”
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