Appalachian coal ash richest in rare earth elements

May 27, 2016

National Science Foundation, Environmental Research and Education Foundation, American Coal Ash Association DURHAM, N.C. -- A study of the content of rare earth elements in U.S. coal ashes shows that coal mined from the Appalachian Mountains could be the proverbial golden goose for hard-to-find materials critical to clean energy and other emerging technologies.

In the wake of a 2014 coal ash spill into North Carolina's Dan River from a ruptured Duke Energy drainage pipe, the question of what to do with the nation's aging retention ponds and future coal ash waste has been a highly contested topic.

One particularly entrepreneurial idea is to extract so-called "critical" rare earth elements such as neodymium, europium, terbium, dysprosium, yttrium and erbium from the burned coal. The Department of Energy has identified these globally scarce metals as a priority for their uses in clean energy and other emerging technologies. But exactly how much of these elements are contained in different sources of coal ash in the U.S. had never been explored.

Researchers from Duke University measured the content of rare earth elements in samples of coal ash representing every major coal source in the United States. They also looked at how much of these elements could be extracted from ash using a common industrial technique.

The results, published online on May 26 in the journal Environmental Science and Technology, showed that coal from the Appalachian Mountains contains the most rare earth elements. However, if extraction technologies were cheap enough, there are plenty of rare earth elements to be found in other sources as well.

"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," said Heileen Hsu-Kim, the Mary Milus Yoh and Harold L. Yoh, Jr. Associate Professor of Civil and Environmental Engineering at Duke.

"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, and our work is the first comprehensive study to begin surveying the options," Hsu-Kim said.

The researchers took coal ash samples from power plants located mostly in the American Midwest that burn coal sourced from all over the country, including the three largest sources: the Appalachian Mountains, southern and western Illinois, and the Powder River Basin in Wyoming and Montana. The content of rare earth elements was then tested using hydrofluoric acid, which is much stronger and more efficient than industrial methods, but is too hazardous to use on a large scale.

The results showed that ash collected from Appalachian Mountain coal has the highest amount of rare earth elements at 591 milligrams per kilogram (or parts per million). Ash from Illinois and the Powder River Basin contain 403 mg/kg and 337 mg/kg, respectively.

The researchers then used a common industrial extraction technique featuring nitric acid to see how much of the rare earth elements could be recovered. Coal ash from the Appalachian Mountains saw the lowest extraction percentages, while ash from the Powder River Basin saw the highest. Hsu-Kim thnks this might be because the rare earth elements in the Appalachian Mountain coal ash are encapsulated within a glassy matrix of aluminum silicates, which nitric acid doesn't dissolve very well.

"One reason to pick coal ash from the Appalachian Mountains would be for its high rare earth element content, but you'd have to use a recovery method other than nitric acid," said Hsu-Kim, who also holds an appointment in Duke's Nicholas School of the Environment. "For any future venture to begin an extraction program, the recovery method will need to be tailored to the specific chemistry of the coal ash being used."

The Duke researchers also tried "roasting" the coal ash with an alkali agent before dissolving it with nitric acid. Even though the process hadn't been optimized for recovery purposes, the tests showed a marked improvement in extraction efficiency.

"The reagents we used are probably too expensive to use on an industrial scale, but there are many similar chemicals," said Hsu-Kim. "The trick will be exploring our options and developing technologies to drive the costs down. That way we can tap into this vast resource that is currently just sitting around in disposal ponds."
-end-
This work was supported by the National Science Foundation (CBET-1510965, OISE-12-43433), the Environmental Research and Education Foundation and the American Coal Ash Association.

The study is available free online at http://dx.doi.org/10.1021/acs.est.6b00085.

"Trends in the Rare Earth Element Content of U.S.-Based Coal Combustion Fly Ashes," Ross K. Taggart, James C. Hower, Gary S. Dwyer and Heileen Hsu-Kim. Environmental Science & Technology, May, 26 2016. DOI: 10.1021/acs.est.6b00085

Duke University

Related Clean Energy Articles from Brightsurf:

Hydrogen embrittlement creates complications for clean energy storage, transportation
Hydrogen is becoming a crucial pillar in the clean energy movement, and developing safe and cost-effective storage and transportation methods for it is essential but complicated, because hydrogen can cause brittleness in several metals including ferritic steel.

Researchers forecast COVID-19 pandemic could delay clean energy transition
Traveling restraints and shelter-in-place orders that grounded planes and emptied streets during the first wave of the COVID-19 pandemic brought greenhouse gas emissions down and air quality up.

Navigating the clean energy transition during the COVID-19 crisis
In a Commentary published April 29 in the journal Joule, energy and climate policy researchers in Switzerland and Germany provide a framework for responsibly and meaningfully integrating policies supporting the clean energy transition into the COVID-19 response in the weeks, months, and years to come.

Gas storage method could help next-generation clean energy vehicles
A Northwestern University research team has designed and synthesized new materials with ultrahigh porosity and surface area for the storage of hydrogen and methane for fuel cell-powered vehicles.

New material developed could help clean energy revolution
Researchers developed a promising graphene-carbon nanotube catalyst, giving them better control over hugely important chemical reactions for producing green technology and clean energy

The uncertain role of natural gas in the transition to clean energy
A new MIT study examines the opposing roles of natural gas in the battle against climate change -- as a bridge toward a lower-emissions future, but also a contributor to greenhouse gas emissions.

As China rapidly adopts clean energy, use of traditional stoves persists
Old habits are hard to break. A McGill-led study of replacement of traditional wood and coal burning stoves with clean energy in China suggests that, without a better understanding of the reasons behind people's reluctance to give up traditional stoves, it will be difficult for policies in China and elsewhere in the world to succeed in encouraging this shift towards clean energy.

Bionic catalysts to produce clean energy
A biohybrid material that combines reduced graphene oxide with bacterial cells offers an eco-friendly option to help store renewable energy.

Speeding up the journey towards clean energy through photocatalyst optimization
Osaka University researchers studied the photocatalytic activity of oxyhalide materials and were able to demonstrate a relationship between parameters measured by time-resolved microwave conductivity (TRMC) and oxygen generation.

A step for a promising new battery to store clean energy
Researchers have built a more efficient, more reliable potassium-oxygen battery, a step toward a potential solution for energy storage on the nation's power grid and longer-lasting batteries in cell phones and laptops.

Read More: Clean Energy News and Clean Energy Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.