Sandia applies for patent to clean coal ash, mine rare-earth metals
Domestic source of elements important for commerce and national security
Scientists at Sandia National Laboratories are using limes for a lot more than spritzing up gin and tonics.
The lab has applied for a patent on a new method of extracting rare-earth metals from coal ash using water, carbon dioxide, high pressure and citric acid from limes.
The process, if widely developed, would serve two purposes, said Guangping Xu, a senior technical staff member at Sandia whose research led to the pending patent.
First, it’s a new way to clean up the residue left from burning coal, which is plentiful. Xu said there is estimated to be 3 billion tons of coal ash spread across the country, and the U.S. will produce about 100 million additional tons of it a year. Secondly, the extraction of rare-earth metals, or elements, is a national security issue because the vast majority of those valuable metals used in America are imported from China.
Xu said those 17 elements — which include cerium, promethium and thulium — are needed for their magnetic, conductive and florescent properties. They are used in making anything from electronics and communication equipment to wind turbines, he said. A Sandia document on the patent application said in 2019 the country imported about $160 million of those metals.
“They make a lot of the products you buy and depend on everyday better,” said Mark Rigali, a principal member of technical staff at Sandia involved in the research. “Better colors in your LED TV, and they have a large variety of military applications in many weapons systems, so they are very important to national security.”
Sandia scientists have been working on the project for about two years, and the lab applied last year for the patent that is still pending. The research
was funded in part through the Laboratory Directed Research and Development Program.
Xu said if the research results ultimately lead to a product, he envisions a receptacle of sorts where coal can be loaded and mixed with water and citric acid. Then, when carbon dioxide and high pressure are added, the ingredients mix together and form a slurry, from which the heavy metals can be extracted.
The process leads to cleaner coal, either before combustion or after, Xu said. He said similar methods that exist today to extract the elements from coal ash use industrial solvents that become hazardous waste, as opposed to their method that uses an environmentally benign acid found in limes. It’s also believed that the Sandia
method will be more efficient at separating and removing the metals, Xu said.
That, the researchers said, allows the coal industry to recoup some of its cost for remediation.
“You need a way to clean your coal ash,” Xu said. “With our method, you can clean your coal ash and at the same time be mining rare-earth metals.”
Even though the country is trying to transition away from coal power to more renewable sources of energy, Xu said that coal will still be a source of electricity on the country’s power grids for years to come.
“Even though we want to transition to clean energy, it’s not very easy,” he said. “You still need a lot of energy and renewables are only a small part. You still need electricity from other sources,” he said. “Our process makes coal cleaner.”