Purdue researchers develop new rare earth separation process Metal Tech News Weekly Edition – May 13, 2020
Coal-fired power plants have been inadvertently creating deposits of rare earth elements while producing low-cost electricity for American consumers. Eyeing the untapped potential in the ashes of decades of burning coal, researchers at Purdue University have developed a sustainable way to extract and separate these tightly interlocked elements into the metals needed for modern technology.
And any efficient and environmentally friendly process developed to separate REEs found in coal ash could be used to extract these elements from recycled materials and newly mined ore in the United States.
Rare earths – the collective name for 17 different elements – are not as uncommon as their name suggests. In fact, they are all more plentiful than silver and some are nearly as common as copper and nickel. They, however, seldom appear in economically viable concentrations. Instead, they tend to be widely dispersed.
"Rare earth elements occur in ores in very small concentrations, a few thousand parts per million. Extensive processes of mining, grinding, extraction, and purification are needed to transform the ores to very high purity (approximately 99.9% minimum) rare earth metals required for commercial applications," said Linda Wang, a professor of chemical engineering at Purdue.
Once purified, the 17 individual rare earths have seemingly mystical magnetic, electrical, and luminescent properties that are being used in a wide range of modern technological and industrial applications – from permanent magnets in electric vehicles and wind turbines, to speakers and other components in smartphones and computers, to catalysts in the chemical industry.
China, however, has monopolized the rare earths market and since the late 1990s has accounted for more than 90% of global production, on average.
"For example, after China reduced the export quotas in 2010, the costs of rare earth magnets for one wind turbine increased from $80,000 to $500,000," said Wang. "After China relaxed the export restrictions 18 months later, the prices returned to lower levels than in 2010."
China has been able to maintain its control over rare earth mining and separation due to lower environmental standards and labor costs.
More information on rare earths and China's REE strategy can be read at A tale of two rare earth countries in the Jan. 15 edition of Metal Tech News.
"We currently have one dominant foreign source for these metals and if the supply were to be limited for any reason, it would be devastating to people's lives," said Wang. "It's not that the resource isn't available in the U.S., but that we need a better, cleaner way to process these rare earth metals."
The chemistry professor has developed such a process in the labs at Purdue and a Florida-based company is looking to apply it on a commercial scale.
Rare earth paradox
The U.S. is in a paradoxical position of having rich reserves that could provide a domestic supply of rare earths for decades but does not have an economically viable method of separating these increasingly important technological elements while also meeting its stringent environmental standards.
Enormous domestic reserves of the 17 rare earths are found in the coal that has been mined in America for decades. While natural coal deposits may not make a very good REE mine, the burning of this power plant fuel works like a concentrator that leaves behind higher grades of these technology metals in the ash.
"Coal ash is rich in rare earth elements, as rich as some of the ore deposits," said Wang. "The United States produces about 129 million tons of coal ash every year."
In recent years, nearly half of this coal ash is recycled as fill for abandoned mines; an ingredient in concrete; gypsum in sheetrock and other wallboard; and an agricultural soil additive.
With decades of coal-fired electrical generation, however, billions of tons of REE-bearing coal ash have simply been buried or stored in containment facilities in the U.S.
Coal ash is not the only American source of rare earths, there is also huge potential of domestic ore and urban mining.
"About 60% of rare earth metals are used in magnets that are needed in almost everyone's daily lives. These metals are used in electronics, airplanes, hybrid cars and even windmills," Wang said.
The U.S. currently does not have the facilities to extract the rare earths from these products at the end of their usefulness. Because of this, the magnets and other REE containing components often find their way into landfills or on a ship to China for recycling.
This lack of separation capacity has also limited the mining of rare earths in the U.S. The one REE mining operation on American soil, the Mountain Pass Mine in California, must ship its concentrates to China for processing.
A better, cleaner process
Even when rare earths are found in mineable concentrations – whether in heaps of coal ash, recycled magnets or naturally occurring geological deposits – separating the individual elements is extremely difficult because they have the same ionic charge and are very similar in size.
The traditional method of separating these rare earths into the individual metals, known as solvent extraction, involves a long and complex process that uses numerous chemicals.
"Typically, old technologies from the 1950s are used for separation and purification," said Wang. "They usually require 1,800 different extraction stages in series and in parallel for purification. Such processes can be hazardous, costly and inefficient."
Looking for a way to economically separate the rare earths from coal ash stored in the U.S. without the complexity and chemicals that come with solvent extraction, Wang has developed new techniques that could separate rare earth elements first from other impurities and then from each other in just a few steps.
The Purdue chemistry professor is using chromatography, which involves the use of substances that cause the various components of a mixture to flow at different speeds, to separate the notoriously tightly interlocked rare earth elements.
She and her team have refined the process, known as ligand-assisted displacement chromatography, to produce rare earths with purities topping 99%, while also capturing more than 99% of the available rare earths.
The ligands the Wang-led Purdue team are using to achieve these results are readily available and low-cost. The primary material used for this purpose is titania, a naturally occurring oxide of titanium.
"Using titania sorbents is what makes this innovation unique. They are robust and inexpensive, making the processes efficient and affordable. We are the first group in the world who developed this technology," said Wang. "Additionally, the byproducts of our process include silica gel, aluminum oxide, and other metal oxides of commercial value, making the overall process profitable and economical."
Joe Pekny, a Purdue professor of chemical engineering, said this process developed by his colleague provides a sustainable and economical path for the U.S. to re-enter the rare earth metals market.
"What's exciting is that the U.S. has the rare earth metals to meet the growing demands of the U.S. market and other markets around the globe and reduces our dependence on foreign sources," he said. "Linda's method replaces a very inefficient process and replaces it with an earth-friendly, safe extraction process."
Out of the lab
The feasibility of chromatography REE separation has been proven at laboratory scale and Wang is confident of the Purdue team's ability to scale this process up to commercial production.
"We have had success in many challenging chromatographic separations, including the purification of medical isotopes, sugars, amino acids, chiral drugs, insulin, polymers, and many others. Thus, we are confident that we can produce high purity REEs from coal ash," she said.
Hasler Ventures, a Florida-based company founded in partnership with Purdue, has secured first rights from the Purdue Research Foundation to commercialize the patented ligand-assisted chromatography rare earth technology.
In addition to the potential of pulling rare earths from the ashes of coal-fired power plants, Hasler Ventures sees the recycling and mined ore as potential circular and sustainable sources of domestic REEs. Which, in turn, would break America's dependence on China for these critical elements.
"It's a supply chain challenge with wide implications on the U.S. economy and national security. We have a critically needed product and one dominant source for this product," said Dan Hasler, founder of Hasler Ventures. "This new patented process promises to enable U.S. independence from the China near monopoly."