From the ashes

Dr Linda Wang received a gold award from the prestigious Purdue Foundry — an alumni offshoot of Purdue University in Indiana, United States — last year, for some honest-to-goodness alchemical sorcery (aka science).

She has developed a method to filter and extract rare earth elements (REE) from waste coal ash with purity levels and yields above 95 per cent. Using a method of chromatographic technique that involves the application of inexpensive titanium oxide sorbents, Wang has been able to separate valuable lanthanides, as well as usable byproducts such as silica and aluminium oxide.

The US has 13 million tonnes of rare earth deposits, but produces nearly 130 million tonnes of waste coal ash per year, less than half of which is recycled. The US is largely reliant on China for its rare earth elements, which are produced expensively and wastefully using old-fashioned purification techniques.

“It has been known for a while that coal ash contains REE elements however an economic solution has been difficult to develop,” Wang explained. “My lab focuses on developing practical, scalable separation solutions for complex feed mixtures.”

Wang hopes her discovery could be used to make the US a domestic player in the burgeoning REE market, which is in the midst of a boom due to rising global demand for parts in renewable technologies. Elements such as yttrium, europium, terbium and cobalt are particularly pertinent to the EV (electric vehicle) sector, renewable batteries, photodiodes and mobile phone, tablet and laptop parts.

Currently, China controls around 90 per cent of the global supply of such materials, holding an essential monopoly; one of the best known sources for heavy rare earths is located in clay pits in Jiangxi Province in southern China.

The US is estimated to be hoarding over 1.5 billion tonnes of coal waste ash in total, a potential mountain of minerals; Wang hopes that if applied on an industrial scale, the US —  along with other coal-rich countries — could become less dependent on rare earth elements.

“It would make the market more competitive and would reduce the threat of one or two countries controlling the market,” Wang explained. “For example, after China reduced export quotas in 2010, the cost of rare earth magnets for one wind turbine increased significantly.

“After China relaxed the export restrictions 18 months later, the prices returned to lower levels than in 2010. It’s highly desirable to develop the capacity to produce REEs in the US and to become independent of foreign suppliers.”

The project is in early stages but has already attracted industry attention; theoretically, there’s nothing to suggest separation couldn’t work on a large scale, a possibility that is currently being explored by Wang and her team at Purdue. The extraction process itself is not selective to specific elements, and depends on the type of coal ash used.

Wang has been in discussions with mining industry stakeholders and plans to maintain engagement while she works on refining the technology. There is particular interest in seeking alternatives to current separation techniques, which she cites as inefficient and wasteful.

“Current separation technologies produce large amounts of chemical waste, which cannot be economically recycled,” she explained. “One of the 10 most polluted sites in the world is a manmade lake in China, where the waste effluents from REE extractions are stored.”

The biggest economic advantages of the product are associated with separation efficiency. Capital expenditure is lowered due to the reduction in processing equipment that comes with the chromatographic approach.

Recycling of solvents, a relatively small footprint, use of inexpensive and readily available materials, and the generation of valuable byproducts are also considered advantageous side benefits.

“In developing the process we kept in mind the barriers that have hindered other attempts at doing this separation; additionally, we have focused on developing a process that characterizes and illuminates the intrinsic properties of the separation process, which allows for rapid scaling and feedstock flexibility. Finally, the process was also designed to produce a series of high-value co-products that increases the economic viability of the process.”

Wang’s project is not the only academic mission to extract rare earth metals from coal byproducts, however. In 2016, a University of Kentucky professor Rick Honaker, in association with Virginia Tech and West Virginia University, secured phase one funding for a pilot project part-funded by a grant from the US Department of Energy. Amounting to nearly $US1 million, the research funding was used to created a patent pending process called hydrophobic-hydrophilic separation, or HHS.

In August this year, he secured a further $7.5 million in phase two funding for the creation of a pilot plant, and in November recovered over 80 per cent rare earth elements from a coal ash feed source (and 98 per cent rare earth oxides).

“We are excited to be selected for a phase two award, which will allow us to move forward with developing and testing our process circuitry in continuous mode at two Kentucky operating preparation plants,” Honaker said in a University of Kentucky newsletter.

“In phase one, we were able to produce concentrates containing over 50 per cent total rare earth elements from Kentucky coal sources which included a significant amount of critical elements like neodymium and yttrium.

“Successful completion of phase two work will substantially advance the process toward commercialisation and providing significant economic value to coal operations and serving a critical need for the nation in the supply of critical REEs.”

When it comes to coal ash extraction, it looks like things might be heating up.

This article also appears in the December 2017 edition of Australian Mining. 

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