Coal tailings could solve United States' need for rare earth elements - Business - Engineering

Extracting rare earth elements, used in many technologies, is a difficult job. Coal tailings, the mining byproduct shown here, offer a solution. Credit: Alyssa Stone/Northeastern University

Rare earth elements are an easy-to-find, hard-to-refine resource critical for everything from magnets and electronics to batteries and catalysts for chemical reactions. Since the 1980s, a race has been on between the United States and China for dominance of the rare earth element market—and the United States is losing.

New research out of Northeastern University published in Environmental Science & Technology has discovered a new way to extract rare earth elements out of coal tailings, the cast-off soil and rock left behind by coal mining. Using a chemical treatment and a specially designed microwave reactor to control the temperature, the researchers have doubled the extraction levels previously possible.

How rare are rare earths?

Rare earth elements, often just called rare earths, aren't really all that rare, according to the Science History Institute. Rather, in nature, they appear mixed with other elements, sometimes radioactive ones, making them difficult to refine.

Damilola Daramola, an assistant professor in the departments of chemistry and chemical biology, and chemical engineering at Northeastern, says rare earth elements are often made accessible as byproducts of other mining operations. For his purposes, that means coal.

Burning the carbon in coal leaves behind an ash of inorganic compounds, often called fly ash, which Daramola says has been well-studied for rare earths. But coal tailings, the mountains of unwanted detritus left behind by coal mining, remain an underused resource, partly due to the ready availability of fly ash and partly due to the fact that there is simply all that organic material still left to sift through in the tailings, he continues.

By pretreating the coal tailings in a solution of water and sodium hydroxide, or lye, and then bathing them in nitric acid while controlling the temperature of the reaction through a specialized microwave reactor, Daramola and his team found that they could extract rare earths two to three times more efficiently than in the standard methods used on coal byproducts.

"That combination, of being able to thermally stimulate the material," alongside the treatment with sodium hydroxide, Daramola says, "well, it turns out that what you're doing is actually changing the solid structure of this material" and allowing for the extraction of valuable rare earths.

Daramola and Northeastern Ph.D. student Lawrence Ajayi, the first author of the study, say that the rare earths they extracted have a diverse range of applications. Neodymium, Ajayi gives as one example, is "the third-most abundant element in the tailings, and it happens, also, to be very important for energy," being found in high-powered magnets and the motors for both electric vehicles and wind turbines.

Lawrence Ajayi, a Northeastern Ph.D. student and the first author on the paper, says that "we have so many abandoned mines that we could take advantage of" in the United States. The global rare earths market is currently dominated by China, but the billions of tons of tailings scattered around the U.S. could prove to be a hidden resource. Photo by Alyssa Stone/Northeastern University

Obstacles to wider adoption

But just because their method leads to more efficient harvesting doesn't mean that microwave reactors will start cropping up all around the country, they say. There remain many questions left to answer, both logistical and chemical.

One reason is simply the technology. Microwave reactors of the kind used by Daramola and Ajayi are expensive and not yet widespread. Another is a question of scale.

"One barrier is having more information about the solid-state structure of these mine tailings. That information is very dispersed, very spotty," Daramola says. He describes a patchwork of mining operations around the United States, some active, some long abandoned.

Pennsylvania alone, where the tailings for their experiment were harvested, contains an estimated 2 billion tons of coal tailings, according to their paper.

In "the U.S., for example, we have so many abandoned mines that we could take advantage of," Ajayi says. But another big issue, he continues, is that "coal tailings have different mineral compositions depending on where you source it from." So just how effective their acid-plus-microwave method would be on tailings from another part of the U.S. remains an open question, Daramola says.

Additionally, any large operation would want to conduct additional extractions from the tailings. Rare earths aren't the only thing in mining refuse. There are often radioactive elements, magnesium, iron and other potentially valuable minerals. Daramola says that an industrial-scale operation would need to take all these factors into account.

This is all on top of the environmental and social policy questions, which could be significant, Daramola continues. Once you've bathed your coal tailings in lye and acid, something has to be done with those liquid waste products and with whatever materials are left over from the tailings that you no longer want.

Rare earth security

Daramola says that China controls the vast majority of rare earths currently available, perhaps as much as 90%, not because there is a higher concentration of them in China, but because of the Chinese government's wholesale investment in the enterprise. By 2015, China had consolidated its mining operations into six huge, largely state-owned companies.

Rare earths remain vital to the production of current technologies. They are used everywhere in modern life, from the electromagnets in cell phones to medical imaging equipment and electric vehicle batteries.

China's impact on the global rare earths market has long been visible; by lowering the price of their rare earths, they can, and have, put companies in other countries out of business, Daramola notes.

Now Daramola says that there is a push to find alternative sources for rare earths, ones not so reliant on a foreign power. One of those alternative sources could well be the billions of tons of currently unused tailings scattered across the U.S., he says. 

Provided by Northeastern University 

by Noah Lloyd, Northeastern University

edited by Lisa Lock, reviewed by Robert Egan 

Source: Coal tailings could solve United States' need for rare earth elements