Sandia National Laboratories scientist Guangping Xu adds coal ash to a citric acid mixture. This solution will be fed into a reactor, operating at about 70 times atmospheric pressure, where supercritical carbon dioxide aids citric acid in extracting rare-earth metals. (Photo by Rebecca Lynn Gustaf courtesy Sandia National Laboratories and Guangping Xu) Public domain
By Sunny Lewis for Maximpact
ALBUQUERQUE, New Mexico, November 29, 2021 (Maximpact.com Sustainability News) – A harmless food-grade solvent is being used to extract valuable rare-earth metals from environmentally hazardous coal waste, reducing the amount of coal ash without damaging the environment and at the same time increasing supplies of a scarce and critically important resource.
Coal ash is an unwanted but widely present residue of burning coal for power, while rare-earth metals are used for a variety of high-tech equipment from smartphones to submarines.
In the United States, the new separation method, which uses carbon dioxide, water and food-grade citric acid, like that found in lemon juice, is the subject of a Sandia National Laboratories patent application.
“This technique not only recovers rare-earth metals in an environmentally-harmless manner but would actually improve environments by reducing the toxicity of coal waste dotting America,” said Guangping Xu, lead Sandia researcher on the project, from his lab in Albuquerque, New Mexico.
Coal ash can contain arsenic, boron, cadmium, chromium, lead, and mercury and rare earth elements, a group of 17 chemical elements of the periodic table including 15 lanthanides plus yttrium and scandium.
They find numerous applications in clean energy production, health care, oil refining and electronics. They are the key ingredients for magnets for wind turbines and other devices, batteries for electric vehicles, phosphors for lighting, catalysts for petroleum refining.
Due to their critical roles, both the European Commission and the U.S. Department of Energy consider rare earth elements as critical raw materials. But China dominates the rare earth element supply chain of mining, separation, refining and manufacturing, producing more than 85 percent of the rare earth raw materials and oxides.
“Harmless extraction of rare-earth metals from coal ash not only provides a national source of materials essential for computer chips, smartphones and other high-tech products – including fighter jets and submarines – but also makes the coal ash cleaner and less toxic, enabling its direct reuse as concrete filler or agricultural topsoil,” Xu explained.
If widely adopted, the new method could make coal ash into another commercially viable product.
The waste product of coal combustion for electricity, today coal ash is commonly re-used as structural fill or fill for abandoned mines; as a top layer on unpaved roads; as an ingredient in concrete, wallboard, and in school running tracks; as an agricultural soil additive; and as “cinders” to be spread on snowy roads.
Nontoxic Chemical Separates Rare Earths From the Ash
The most common acids used as chemical separators in mining – nitric, sulfuric or phosphoric acids – also are able to extract rare-earth metals from coal ash but produce large amounts of acid waste, leaving the environment in worse shape than before, Xu explained. “Environmentally harmful acids would raise clean-up costs beyond economic feasibility in the United States.”
The Sandia process uses citric acid as a carrier for the rare-earth metals so they separate from coal ash, the host material.
Citric acid is named for citrus fruit, in which it is prevalent. Lemon juice contains citric acid but also contains other chemical compounds. Pure citric acid is common in nature and is also a common food additive, prized for its flavor and preservative properties.
The process of extracting rare earth metals from coal ash is facilitated by using supercritical carbon dioxide solvent. Xu’s Sandia colleague Yongliang Xiong suggested citric acid, a commonly-used and environmentally-friendly chemical for holding metals in solution.
Xu found that in less than one day, at 158 degrees Fahrenheit (70 degrees Celsius) and 1,100 pounds per square inch pressure (about 70 times ordinary atmospheric pressure), his citric acid method extracted 42 percent of the rare-earth metals present in coal waste samples.
Chinese mines, where 95 percent of the world’s resources of rare-earth metals are located, achieve less efficient separation while using environmentally damaging methods, Xu explains.
“Theoretically, an American company could use this technique to mine coal and coal byproducts for rare-earth metals and compete with Chinese mining,” said Xu.
Detoxifying coal ash for reuse alone should be worth the effort, Xu said.
For decades, utilities have dumped coal ash into unlined ponds and landfills where the toxins leak into groundwater. According to the coal industry’s own data, more than 95 percent of the coal ash ponds in the United States are unlined.
Almost all of them are contaminating groundwater with toxins above levels that the U.S. Environmental Protection Agency advises are safe for drinking water.
Legal and technical experts from the public interest law firm Earthjustice, the Environmental Integrity Project, and partner organizations located and analyzed the industry’s data disclosures, publishing the results in July.
This culvert carries water contaminated with coal ash, August 6, 2021, Rudry, Wales (Photo by Ben Salter) Creative Commons license via Flickr
More than 550 units at 265 coal-fired plants reported groundwater monitoring data. Based on that data, they found that 91 percent of these plants are contaminating groundwater with toxic substances at levels exceeding federal safe standards.
“If we don’t detoxify and reuse the coal ash, then it will be abandoned in ponds and landfills and cost billions of dollars to clean up over the long term,” Xu said.
To help make that outcome less likely, he explained, “We expect tests of our extraction techniques at larger volumes and on a variety of coal-based sources in the near future.”
Carbon Sequestration Possible
This technology could open a new avenue for carbon dioxide reutilization and sequestration, said Xu’s Sandia colleague Mark Rigali, who, with Xu, is exploring the use of citric acid and supercritical carbon dioxide to mine metals from oil and gas shales that are often rich in metals.
“Using existing oil and gas fracking wells, the citric acid and supercritical carbon dioxide can be used cost-effectively to mine metals while disposing of carbon dioxide below ground,” Rigali said.
Subsurface storage of the carbon dioxide should keep it from entering the atmosphere and contributing to climate change, Rigali said.
The work is supported by Sandia’s Laboratory Directed Research and Development program, which invests in high-risk, potentially high-payoff activities that enable national security missions and advance the frontiers of science and engineering.
Sandia National Laboratories is a multimission laboratory operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration.
Sandia Labs has research and development responsibilities in nuclear deterrence, global security, defense, energy technologies and economic competitiveness, with main facilities in Albuquerque, New Mexico, and Livermore, California.