Sandia Applies for Patent to Mine Rare Earth Metals from Coal Ash

Scientists at Sandia National Laboratory (SNL) have patented a new method for extracting rare earth metals from coal ash using water, carbon dioxide, high pressure, and citric acid from limes. If developed, this process would serve two purposes, says Guangping Xu, a senior technical staff member at the laboratory.

First, it's a new way to clean up the residue left by burning coal, of which there is a lot. There are 3 billion tons of coal ash distributed throughout the country, and the U.S. will generate about 100 million additional tons of waste each year. Second, the extraction of rare earth metals or elements is a national security issue, as the vast majority of these valuable metals used in the United States are imported from China.

Xu said rare earths are made up of 17 elements, including cerium, promethium, and thulium, and are utilized for their magnetic, electrical and luminescent properties. They are used to make components ranging from electronics and communications equipment to wind turbines, he said. A document on the patent application said the U.S. imported about $160 million of rare earth elements in 2019.

"These elements make the products you buy and rely on every day better," said Mark Rigali, a leading Sandia technician involved in the study. "Your LED TVs have more vibrant colors, and they have a lot of applications in many weapons systems, so they're critical to national security."

Scientists at the lab have been working on this project for about two years, and the lab applied for a patent last year that is still pending. This research is funded in part by the lab's Directed Research and Development Program.

If the results of the study eventually lead to a product, Xu said, he envisions a container that coal can be filled and mixed with water and citric acid. Then, when carbon dioxide and high pressure are added, the components mix and form a slurry from which heavy metals can be extracted.

This process yields cleaner coal, both before and after combustion. Similar methods currently in existence for extracting elements from coal ash use industrial solvents that become hazardous waste, while the patented method uses an environmentally benign acid found in lime. It is also believed that Sandia's method will be more effective in separating and removing metals.

This allows the coal industry to reduce remediation costs," the researchers said. The method means you can clean up your coal ash and mine rare earth metals at the same time."

Even as the country works to transition from coal to more renewable energy, Xu says, "Coal will remain a source of power for the national grid 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 of that. You still need other sources of electricity, and our process makes coal cleaner."

The Sandia National Laboratories is one of three National Nuclear Security Administration research and development laboratories in the United States, managed and operated by the National Technology and Engineering Solutions of Sandia.

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