Sustainably Mining Rare Earth Elements From Fertilizer Byproduct

By PENN STATE UNIVERSITY NOVEMBER 6, 2021

Penn State engineers received a National Science Foundation grant to recover rare earth elements from phosphogypsum, a fertilizer byproduct stored indefinitely in open dumps and pumped into designated lakes, shown here.

Despite their name, rare earth elements are not actually that rare. The 17 metallic elements are ubiquitous in nature and are becoming even more common in technology, as a critical component of microchips and more. The “rare” description pertains to how difficult they are to extract into a useable form. The normal technique to pull them from composite minerals is typically energy intensive and produces significant carbon emissions, and a large portion of rare earth elements are lost in waste from other industrial processes. 

To develop a more sustainable process that can retrieve rare earth elements from phosphogypsum, a byproduct of fertilizer production, Penn State researchers were awarded a four-year, $571,658 National Science Foundation grant as part of a collaboration with Case Western Reserve University and Clemson University totaling $1.7 million in funding. Each university is independently funded to pursue a specific aspect of the project, but the project is centrally coordinated by researchers at Case Western Reserve. Lauren Greenlee, associate professor of chemical engineering, is leading the Penn State effort with co-principal investigator Rui Shi, assistant professor of chemical engineering. 

“Today, an estimated 200,000 tons of rare earth elements are trapped in unprocessed phosphogypsum waste in Florida alone,” Greenlee said, explaining that phosphogypsum is piped to ditches and ponds for indefinite storage. “This source of rare earth elements is presently untapped due to challenges associated with radioactive species and the difficulty of separating the individual elements. The vision for this project is to discover new separation mechanisms, materials and processes to recover valuable resources, including rare earth elements, fertilizers and clean water, from waste streams of the fertilizer industry, paving the way for a sustainable domestic supply of rare earth elements and a sustainable agriculture sector.”

Greenlee also noted that the United States relies largely on international sources for rare earth element supplies, and the COVID-19 pandemic has caused lengthy delays in the supply chains. 

“It’s a significant problem that’s compounded by the economic, environmental, and security complexities of obtaining and using rare earth elements internationally,” Greenlee said.  

Phosphogypsum is formed when phosphate rock is processed into fertilizer, and contains small amounts of naturally occurring radioactive elements, such as uranium and thorium. Because of this radioactivity, the byproduct is stored indefinitely, and improper storage can contaminate soil, water and the atmosphere. To harvest the rare earth elements trapped in phosphogypsum, the researchers propose a multistage process using engineered peptides capable of precisely identifying and separating out the rare earth elements through a specialized membrane. 

“Individual rare earth elements have similar sizes and identical formal charges, so traditional membrane separation mechanisms are insufficient,” Greenlee said. “A key technical goal of this research is to discover the mechanisms that underpin peptide-ion selectivity and leverage those mechanisms to design a new class of highly selective membranes.”

Case Western Reserve researchers Christine Duval, principal investigator and assistant professor of chemical engineering, and Julie Renner, co-principal investigator and assistant professor of chemical and biomolecular engineering, will develop the molecules to latch to specific rare earth elements. Their design will be guided by computational modeling work by Rachel Getman, principal investigator and associate professor of chemical and biomolecular engineering at Clemson. Once the peptides are developed, Greenlee will investigate how they work in water solutions, while Shi will use systems analysis tools, including techno-economic analysis and life cycle assessment, to evaluate the environmental impacts and economic feasibility of the proposed rare earth elements-recovery system under various design and operating conditions.

“What are the overall sustainability implications of this process?” Shi asked. “We want to navigate away from the current environmental impacts to be more sustainable, and we can do that by translating the fundamental research and laboratory-scale results to systems-level environmental and economic impacts. Then, we can integrate the sustainability results back into design to guide future research targets while advancing rare earth element recovery and phosphogypsum processing.”

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

“For our project, the hypothesis is that water molecules associated with the peptides binding 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 examine 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’ll continue to iterate to ensure we understand how the molecules work together.”