Scientists look to coal for REE solution

As security concerns mount over the nearly total reliance of the United States on foreign sources of rare earth elements, mostly from China, researchers in public and private laboratories across the country are aggressively searching for ways to spur domestic production of these critical minerals in the face of rapidly growing global demand.

Rare earths, which in reality are plentiful in natural settings, can be found most everywhere. But the prohibitive financial and environmental costs of extracting the 15 elements in the lanthanide series, along with yttrium and scandium, from the earth in significant quantities have given these minerals this misleading moniker.

REEs are used today in manufacturing an increasing array of emerging and green technologies, from cell phones and defense systems to wind turbines and solar systems.

Working at the behest of Congress and the U.S. Department of Energy, the National Energy Technology Laboratory is funding the efforts of researchers from North Carolina to North Dakota who are investigating a readily available and surprisingly abundant source of REEs – our nation's vast coal reserves and coal byproducts.

The United States has the world's largest coal reserves and in 2017, coal use accounted for about 30% of the electric power generated in the United States, according to the U.S. Energy Information Administration, even though gas-fired power generation has increased significantly in the past decade. When coal is burned, REEs are retained and enriched in the fly ash and, as a result, fly ash has long been considered a potential resource for REEs. About half of the vast quantities of fly ash being produced today is beneficially reused, primarily in construction. The remainder, however, is stored, mostly in landfills and impoundments.

Working under recent authorizations from Congress, DOE's NETL has awarded millions of dollars in grants to more than 25 groups of scientists to develop innovative ways to identify, extract and separate REE from coal and coal byproducts, such as fly ash.

The NETL and collaborating researchers since have demonstrated methods by which the REEs can be extracted from coal and coal byproducts, including acid mine drainage.

"While searching for a reliable supply of these vital elements, we need to research all viable technologies to determine which are the most efficient and cost-effective," said Jason Hissam, NETL federal project manager.

Recent progress

The NETL reported Feb. 11 that researchers at the University of Kentucky have developed a technology for extracting REEs from coal that has shown to be viable.

Plasma, which is distinct from liquid, gaseous and solid states of matter, is formed by striking a gas with enough energy that gas molecules are ionized. During the past century, thermal plasma treatment saw applications in torch welding/cutting, spray coating, metal synthesis, extractive metallurgy, refining metallurgy, hazardous waste destruction and more.

The UK scientists researched using low-temperature plasma to pretreat coal-based materials resourced from West Kentucky No. 13 and Fire Clay mines located in Kentucky. Surface area measurements found that plasma treatment provided increased surface area and pore volume, which made other processes more effective at recovering REEs. This novel technology integrated with traditional leaching and extraction processes was demonstrated to effectively recover REEs from coal samples, the NETL said.

Low-temperature plasma treatment was found to provide heavy REE leaching performance improvements on the low-density, higher carbon content fractions of the West Kentucky No. 13 coal, and high-temperature oxidation provided exceptionally high REE recovery for all fractions of both the Fire Clay and West Kentucky No. 13 coarse refuse materials.

"The higher recovery of rare earth elements using this method shows promise," Hissam added. "It's another potential tool at our disposal as we search for optimal ways to secure a reliable domestic supply of these crucial minerals. Moving forward, the challenge for NETL and its partners in academia is refinement and discovering how to make this process economically attractive by industry."

In January, the NETL also highlighted innovations by Ohio State University researchers that have shown potential to deliver a supply of REEs.

The OSU scientists demonstrated that a conceptual three-stage trap-extract-precipitate process can successfully extract REEs from coal mine drainage. The TEP process relies on the use of environmentally benign industrial by-products to trap the REEs and an organic chelating agent to recover the REEs from the mine drainage.

This approach generates lower post-extraction waste and minimizes the associated environmental impacts when compared with other REE extraction techniques. The TEP process retains more than 99% of the REEs and produces solids that contain more than 7 weight by percentage (70,000 parts per million) total REEs.

Early findings at Duke

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 sparked considerable debate.

Researchers from Duke University measured the content of REE in samples of coal ash representing every major coal source in the United States. The results, published online May 26, 2016 in the journal "Environmental Science and Technology," showed that coal from the Appalachian Mountains contains the highest concentrations of rare earth elements.

Extraction tests offer answers

Researchers from Duke University and the University of Kentucky, testing multiple types of coal fly ash samples, found that "acid leaching of high calcium-containing fly ashes from Powder River Basin coals yielded the highest recoveries of low-grade REEs (near 100% of total REE contents)," according to a report in the July 1, 2018 edition of the International Journal of Coal Geology. The test samples represented coals from three major U.S. coal basin sources.

Though the Appalachian Basin-based coal ashes contained the highest concentrations of rare earth elements, they required more complex leaching methods to first decompose the chemical structure of the ash before acid leaching REEs from it.

Miner takes notice

As researchers move toward an economic breakthrough, at least one mining company is taking a closer look at coal as a more expedient path to commercial REE production.

Don Bubar, president and CEO of Toronto-based Avalon Advanced Materials Inc., told Metal Tech News recently that his company is working with researchers in southern Illinois on developing a viable method for producing REEs from coal and coal by-products.

"We're looking at a new REE project to extract the minerals from a coal mine lease in southern Illinois. There are a lot of coal sites in the United States with significant levels of REEs that offer potential recovery," Bubar said, who heads a junior mining company that owns a large REE project in Northwest Territories.

The company signed a binding letter of intent in October with Coal Strategy Advisors LLC to earn up to a 50% interest in Will Scarlett, a closed coal mine site where previous geochemical sampling found elevated levels of rare earths and other metallic elements such as cobalt, nickel, lithium, manganese and zinc in mine waste materials. Since signing the LOI, Avalon has been focused on defining an appropriate process flowsheet to recover a mixed rare earth oxide product from an accumulation of precipitates created through historical lime treatment of acid mine drainage to neutralize its acidity. Previous sampling indicates these precipitates contain total rare earth concentrations in the order of 900 ppm.

Avalon said Feb. 6 it is now evaluating a low-cost rare earth recovery process involving dissolution of the precipitates followed by an innovative proprietary new treatment process to recover the rare earth oxides.

"Most of the costs associated with mining rare metals is in getting REEs into acid solution. In these sites of coal waste, the REE is already in an acid solution," Bubar said in a recent interview. "That's the opportunity. There's lots of scope with new technology to come up with new extractions methods."

Other make breakthroughs

Last May, the National Coal Council reported some of the recent advances in extraction methods and understanding in the sector. Among them:

• University of Kentucky – Produced small quantities of 80 wt. percent total REEs on a dry basis and more than 98 wt. percent REOs. The scientists also built a pilot plant in late 2018 that currently produces a few grams per day of REO concentrate. The products were derived by processing leachate collected from a coarse refuse area.

• West Virginia University - Achieved lab recovery of nearly 100% REEs from coal acid mine drainage sludge as 5-6 wt. percent mixed REE concentrates and have commissioned an extraction bench/pilot-scale facility for recovery of REEs from acid mine drainage feedstock.

• University of North Dakota – Identified some 80-95 wt. percent of the REE content in lignite coals is organically associated, primarily as coordination complexes as opposed to mineral forms typically found in the older/higher-rank coals.

• Physical Sciences Inc. – In cooperation with the University of Kentucky, the University of Wyoming, and others, achieved >30 wt. percent mixed REE pre-concentrates from coal-based materials. PSI also built a micro-pilot facility that produced >15 wt. percent concentrates of mixed REEs from post-combustion ash resulting from burning East Kentucky fire clay coal in a power plant boiler. PSI also built a pilot facility in Sharon, Pa. that was set to begin operations in mid-2019.

• On the industrial front, Crazy Horse Coal developed an automated technology for mining from deep formations that would otherwise be unrecoverable by other mining methods. The technology may prove useful for the recovery of strategic and critical elements from geology that are too deep to mine.