From mining to magnet manufacturing, the process for refining rare-earth elements is complex and intensive. The supply chain for such critical materials is dominated by China - and so is the oxalic acid needed for the separation and purification stages.
To move toward a U.S. supply chain for rare-earth element recovery, researchers from Lawrence Livermore National Laboratory (LLNL), the University of Illinois Urbana-Champaign and the University of Kentucky established a novel microbial platform that produces oxalic acid and purifies rare-earth elements. With further optimization and scaleup, the bio-based process is expected to be comparable in cost to the commercial chemical method, and it provides a separate, independent source for the important acid. The work was published in Nature Communications.
"What makes this collaboration especially powerful is that it tackles two supply-chain challenges at once - rare-earth elements and oxalic acid, a critical processing chemical in its own right," said LLNL scientist and author Yongqin Jiao. "By directly coupling microbial production with rare-earth element recovery and validation, our teams demonstrated a new, integrated pathway for strengthening domestic critical materials supply chains."
Oxalic acid binds to rare earth elements and selectively transforms them from a solution to a solid. In doing so, it separates them from other "junk" metals like zinc, which stay dissolved in the solution.
Few companies in the U.S. make oxalic acid, and it can take six months of lead time to receive an order.
In response to a DARPA Environmental Microbes as a Bioengineering Resource (EMBER) solicitation, the team at LLNL identified this bottleneck and discussed it with colleagues at Illinois - who have been working with engineered yeast strains to produce similar products for years - to explore potential U.S.-based bioengineered alternatives.
"Engineering a low-pH tolerant yeast Issatchenkia orientalis for oxalic acid production has greatly simplified the biomanufacturing process and made the entire rare-earth element recovery process potentially economically viable," said Illinois professor and corresponding author Huimin Zhao. "By leveraging our expertise in metabolic engineering of this low-pH tolerant yeast for organic acid production, we were able to quickly create a yeast strain capable of producing more than 40 grams per liter of oxalic acid and use the fermentation broth directly for rare-earth element precipitation with over 99% efficiency."
To test the effectiveness of the microbial platform, the team needed realistic samples of rare-earth elements. University of Kentucky researchers processed the ore to provide them.
"This work provides a crucial validation of bio-oxalic acid under realistic ore-processing conditions and establishes a strong foundation for its integration into industrial rare-earth extraction and purification flowsheets," said author Rick Honaker of the University of Kentucky.
At LLNL, the scientists quantified and characterized how the oxalic acid performed compared to chemically-produced acid that they purchased. They found it comparable in every way at separating the rare earths.
The innovative platform also cuts out a potentially costly process: extracting the oxalic acid from the spent media after the yeast grow and secrete it. After removing the yeast, the rest of the solution mixes directly with the ore leachate solution containing the rare-earth elements. This results in rare-earth precipitation and purification and newly acid-free growth media that is ready to be reused.
"This work is a great example of what becomes possible when synthetic biology and chemical process engineering are tightly integrated," said LLNL scientist and author Dan Park. "Illinois' metabolic engineering expertise and LLNL's rare-earth separation and validation capabilities came together as a truly multidisciplinary team, enabling an end-to-end solution - from biological production to materials recovery."
While the bio-based oxalic acid excels in the rare-earth separation process, the yeast currently yields a small amount of acid compared to the sugar it consumes. To make the entire process more commercially viable, that yield will need to increase. The team is hard at work on improvements to do just that.
This project was funded by the DARPA EMBER program.
