Engineers to sustainably mine rare earth elements from fertilizer byproduct

UNIVERSITY PARK, PA. - Despite their name, rare earth elements are not really rare. The 17 metallic elements are ubiquitous in nature and are increasingly used in technology as critical components of microchips and more. The "rare" description refers to how difficult it is to extract them into a usable form. The normal technique of extracting them from composite minerals is typically energy intensive and causes significant carbon emissions, and a large proportion of the rare earth elements are lost to wastes from other industrial processes.

To develop a more sustainable process that can recover rare earth elements from phosphorus gypsum, a by-product of fertilizer production, Penn State researchers were awarded a four-year National Science Foundation grant of 571,658 as part of a collaboration with Case Western Reserve University and Clemson University $ 1.7 million in grants. Each university is independently funded to pursue a particular aspect of the project, but the project is centrally coordinated by researchers from the Case Western Reserve. Lauren Greenlee, associate professor of chemical engineering, leads Penn State's efforts with co-research director Rui Shi, assistant professor of chemical engineering.

"Today, in Florida alone, an estimated 200,000 tons of rare earth metals are trapped in unprocessed phosphogypsum waste," Greenlee said, explaining that phosphogypsum is directed into trenches and ponds for indefinite storage. “This source of rare earth elements is currently untapped due to challenges associated with radioactive species and the difficulty of separating each element. The vision for this project is to discover new separation mechanisms, materials and processes to recover valuable resources, including rare earths, fertilizers and clean water, from waste streams of the fertilizer industry and thus pave the way for a sustainable domestic supply of rare earths and sustainable agriculture. "

Greenlee also noted that the United States relies heavily on international sources for rare earth supplies and the COVID-19 pandemic has created long delays in supply chains.

"It is a significant problem that is exacerbated by the economic, environmental and safety complexities of international procurement and use of rare earths," said Greenlee.

Phosphogypsum is produced when phosphate rock is processed into fertilizers and contains small amounts of naturally occurring radioactive elements such as uranium and thorium. Because of this radioactivity, the by-product is stored indefinitely and improper storage can contaminate soil, water and the atmosphere. In order to harvest the rare earth elements enclosed in phosphogypsum, the researchers propose a multi-stage process in which engineered peptides are used that are able to precisely identify and separate the rare earth elements through a special membrane.

"Individual rare earth elements have similar sizes and identical formal charges, so conventional membrane separation mechanisms are not enough," said Greenlee. "An important technical goal of this research is to discover the mechanisms underlying peptide ion selectivity and to use these mechanisms to develop a new class of highly selective membranes."

Case Western Reserve researchers Christine Duval, senior researcher and assistant professor of chemical engineering, and Julie Renner, assistant director of studies and assistant professor of chemical and biomolecular engineering, will develop the molecules that bind to certain rare earth elements. Her design is guided by computational modeling by Rachel Getman, Principal Researcher and Associate Professor of Chemistry and Biomolecular Engineering at Clemson. Once the peptides are developed, Greenlee will study how they work in water solutions, while Shi will use systems analysis tools, including techno-economic analysis and life cycle analysis, to assess the environmental impact and economic feasibility of the proposed rare earth recovery system under various construction - and operating conditions.

“What is the overall impact of this process on sustainability?” Asked Shi. “We want to move away from current environmental impacts in order to be more sustainable, and we can do this by transferring basic research and results on a laboratory scale to ecological and economic impacts at the system level. Then we can reintegrate the sustainability results into the design to guide future research goals while promoting the extraction of rare earth elements and the processing of phosphogypsum. "

The proposed project will also complement other Penn State research, including working with naturally occurring protein molecules to extract grouped rare earth elements from other industrial waste sources.

"Our project hypothesizes that water molecules associated with the peptides that bind to the rare earth elements reorganize, and we can precisely control that reorganization to be more efficient based on the individual rare earth element," Greenlee said, noting that her team will study the interactions at the atomic level by using X-ray absorption spectroscopy to validate how the molecules exchange atoms as they bind. "With modeling and experimentation, we will continue to iterate to make sure we understand how the molecules work together."

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