Rare earth elements (REEs) are a group of 17 metal elements found in almost every technology, including cell phones, televisions, computers and nearly every component of vehicles. The demand for these elements increases every year, but the supply is geopolitically limited and exploited using environmentally unsustainable practices.
Young-Shin Jun, a professor of energy, environmental and chemical engineering at Washington University in St. Louis, and her team created a proof-of-concept solution: extracting rare earth elements from fly ash, a fine, powdery waste from coal combustion.
"We want to extract rare earth elements using processes that are more environmentally friendly than those that are traditionally more harmful," Jun said. "Since coal is already being used, this process is ultimately a pathway to reduce and remediate waste.
Jun and her former doctoral student, Yaguang Zhu, now a postdoctoral scholar at Princeton University, developed the novel extraction process that uses a supercritical fluid, typically used for decaffeinated coffee, to recover these much-needed rare earth elements from materials that would otherwise be discarded in landfills. A supercritical fluid is a substance with a temperature and pressure above its critical point, and its properties are between liquids and gases. Jun's team reports that the U.S. produces more than 790,000 tons of fly ash annually, and the potential value of rare earth elements that can be extracted from U.S. fly ash is estimated at more than $400 million per year.
Their work, which appeared in RSC Sustainability, showed for the first time that ordinary and accessible supercritical fluids, including carbon dioxide, nitrogen and air, are capable of extracting rare earth elements and separating impurities very efficiently. In addition, by experimenting with fly ash, they found that supercritical carbon dioxide reduced the concentration of impurities in the final rare earth product. Ultimately, their final product contains up to 6.47% rare earth elements, compared to 0.0234% in the original fly ash source.
"The uniqueness of our work lies not only in the use of supercritical carbon monoxide 2, but also in the fact that supercritical air and nitrogen, at a much lower temperature and pressure than those required for CO2, can efficiently extract rare earths," said Jun, head of the Environmental Nanochemistry Laboratory.
"We can use lower temperatures and pressures with nitrogen or air to extract rare earth elements from coal ash, which means lower energy costs. Of course, supercritical carbon monoxide 2 works best, but supercritical air or nitrogen can do better than traditional high-temperature boiling for rare earth extraction with acids and organic solvents.
Jun's team's extraction process consists of two steps: first, metal ions in fly ash, including rare earth elements and impurities, leach from coal ash and react with nitric acid to form metal nitrates; Secondly, metal nitrates react with tributyl phosphate (TBP). They found that through supercritical carbon dioxide, nitrogen or air, rare earth elements form compounds that can be extracted from coal fly ash.
After extraction, their multi-stage stripping process collects rare earths and reduces impurity concentrations. The nitric acid and TBP used in this process can be completely recovered multiple times without sacrificing efficiency, minimizing their disposal problems.
Jun's method also eliminates the need to roast raw materials at extremely high temperatures or greater than 500°C, as well as the extraction of rare earth elements with strong acids and large amounts of toxic organic solvents, which also become waste in traditional extraction processes.
"Supercritical fluids are considered a more environmentally friendly solvent and less invasive to the environment, allowing us to extract rare earth elements directly from solid waste without leaching and roasting feedstock, so our new process requires less energy, which also produces less waste," Jun said. "We are seeking a more environmentally friendly process for the recycling of key elements and recycling from materials that were previously considered waste.