Excavating every coal ash dump in North Carolina could cost up to $10 billion, according to estimates by the N.C. Utilities Commission, but it’s possible the coal ash itself could foot part of the bill.
While state lawmakers and environmental authorities are fixated on coal ash as a hazard, some are beginning to think of the waste as a resource. Rare earth elements – key ingredients of technologies like smart phones, electric car batteries and bulletproof glass – are found in abundance in coal ash and could be mined for profit, according to Duke University researchers.
Helen Hsu-Kim, an environmental engineering professor at Duke University, and her student Ross Taggart study the chemicals that make up coal ash from regions throughout the United States and have found that ash from the Appalachia region contains more rare earth elements than other regions.
Coal ash is held in ponds or above-ground landfills at 14 Duke Energy power plants throughout North Carolina. The N.C. Department of Environmental Quality classifies all 14 sites as hazardous enough to recommend that they be excavated.
Duke Energy says it’s unlikely that extraction of rare earth elements will be a solution to its coal ash in North Carolina. Dawn Santoianni, a spokeswoman for Duke Energy, notes that the state has given the company 18 months to make coal ash storage sites safer before they will be reevaluated and possibly excavated to move the ash to more secure locations. And, Santoianni says, the technology to extract metals from coal ash on a large scale is just not ready yet.
But both the U.S. Department of Energy and The Electric Power Research Institute, an independent research organization supported by the industry and others, are spurring efforts to extract metals from coal ash. EPRI discovered that coal ash contained metals in the 1990s, using similar methods as Hsu-Kim. Like Duke Energy, it stopped work on the topic once researchers encountered difficulty “scaling-up” the process enough to be profitable.
Ken Ladwig, supervisor of EPRI research in coal combustion residuals and products, said the increasing value of the rare earth elements has renewed interest in mining coal ash at the institute. EPRI is evaluating the “state of the science” and again looking at how leaching coal ash can be done in large quantities. By early 2017, EPRI will determine what technologies could make the coal ash mining process workable at a factory scale.
The technology “has a lot of promise,” Ladwig said, “if we can make it work economically.”
Rare earth elements
Rare earth elements, as Hsu-Kim explains, “are not actually rare, just hard to find” in high enough quantities that can be profitable. On paper, they are found in very low numbers in almost any rock. On the periodic table, you can find the metals known as lanthanides on the row second from the bottom, elements No. 57 lanthanum through No. 71 lutetium. One of these elements, No. 61 promethium, can only be made in the lab, so it is not found in rare earth element mines or coal ash.
Though the natural lanthanides are 15 different elements, they and No. 39 yttrium and No. 21 scandium make up the rare earth elements.
In a rare earth mine, the elements are found in veins where tiny streams of water trickling through a rock left the elements behind. The veins are small and make up very little of the rock they are found in.
Rock containing rare earth veins must be dug up and crushed, often along with rocks that don’t contain rare earths at all. One problem is that rocks that do not have rare earth veins will usually have radioactive elements, like thorium or lead. The only rare earth element mine in the U.S., in southern California, was shut down because thorium got into the water supply.
Rare earth elements are found in coal ash because of how we use the coal it comes from. There are no rare earth veins in coal, but when it is burned, the most common ingredient in the coal, carbon, becomes the energy powering our homes. The ash that is left over concentrates the glass and metals that make up only a small part of the coal.
Taggart, the student who works with Hsu-Kim, says the glass and metal in burning coal become smoke but cool off and become solid ash as they go up a flue shoot. He and Hsu-Kim have found that the rare earth elements and other metals are encased in pellets of the glass.
To extract the metal, you first have to get through the glass. The Duke researchers do this by dissolving, or leaching, coal ash in strong acids, which separates the glass from the metal in the lab.
Hsu-Kim and Taggart measured metals found in the acid that digested Appalachian coal ash and discovered it had more rare earth elements than coal ash from elsewhere in the U.S. To them, that shows coal ash can be a resource. Referring to recent government proposals to dig up coal ash, Taggart said, “If you have to excavate it anyway, you can do something with it.”
Extraction and pollution
But the Duke scientists understand that laboratory experiments are not enough to build a business around extracting metals from coal ash in North Carolina. The next step, they say, is to come up with a way to separate rare earth elements from other metals.
Hsu-Kim and her students are working with the Department of Energy to isolate each of the 15 rare earth elements after they are freed from coal ash. She is developing filters which can separate the elements by their size and their magnetic properties. The rare earth elements are very similar in these respects, so it will take time to perfect a method to screen for a single element.
Another challenge is the potential pollution that could arise when the rare earth elements are singled out. Metals such as arsenic are also found in coal ash, which is why it is considered an environmental hazard. Toxic metals are also encased in the glass pellets and are also released after they are dissolved in acid.
Toxic byproducts are a problem in conventional rare earth mines, too, but as Hsu-Kim emphasized, toxic metals in coal ash are already an environmental hazard, unlike the thorium and lead found with rare earths that have yet to be dug up. The waste acid used to concentrate the toxic metals would need to be carefully disposed.
Today, about 45 percent of coal ash is used in making concrete and roadways. Ladwig of EPRI and Hsu-Kim believe that leaching coal ash for metals will not impact its ability to be used as a building material, though both acknowledge that until leached coal ash is used in construction they cannot know for sure.
The Duke University lab extracts metals with expensive, dangerous acids to most accurately measure how many there are. Hsu-Kim says that a cheaper procedure, combined with the right separation method, could make mining coal ash worthwhile for firms like Duke Energy. Her lab and others across the country have begun testing their ideas for separation with support of the Department of Energy, while the EPRI looks into existing technologies in other industries.
If these efforts lead to commercial extraction of rare earths, Hsu-Kim says her research shows Appalachian coal ash from coal-fire power plants in the southeast U.S., including North Carolina would be the best candidate.
“The Department of Energy is investing $20 million into research on extraction technologies for coal wastes, and there is literally billions of dollars’ worth of rare earth elements contained in our nation’s coal ash,” she said. “If a program were to move forward, they’d clearly want to pick the coal ash with the highest amount of extractable rare earth elements.”
Stephen Ginley: 919-829-4520, email@example.com