• University of Idaho researchers combined microwave pretreatment with ultrasound-assisted leaching to extract rare earths from Idaho soil, achieving a 17.9% yield and a 51% emissions reduction, but at a high cost of $1,563 per kilogram.
• The combined method increased leaching rates to 0.128–0.133% per minute compared to 0.048% for ultrasound alone, primarily improving extraction speed rather than total recovery from monazite and xenotime minerals.
• Citric acid accounted for 52% of process costs, and with modest extraction yields, the study represents an incremental improvement rather than a breakthrough for domestic rare earth supply chains.

---

In a new study published in Hydrometallurgy, lead author Rebecca M. Brown of the University of Idaho (opens in a new tab), working with collaborators Cody Steven, Amin Mirkouei, and David Reed, investigated whether combining microwave pretreatment with continuous ultrasound-assisted leaching could improve recovery of rare earth elements (REEs) from surface soil deposits in Idaho containing monazite and xenotime minerals. Their results suggest modest but meaningful improvements.

Using citric acid—an organic acid intended to mimic environmentally friendly bio-derived leaching agents—the researchers demonstrated that the combined method increased the leaching rate, reduced emissions by roughly 51% compared with ultrasound-assisted leaching alone, and achieved a maximum mixed rare earth extraction yield of 17.9% under optimal conditions (900 W microwave treatment for five minutes followed by two hours of ultrasound). The work reflects growing interest in lower-impact rare-earth extraction technologies as the United States and allied countries seek to build domestic supply chains. However, the results also highlight the difficulty of replacing conventional extraction methods: yields remain limited, and the techno-economic analysis estimated costs at roughly $1,563 per kilogram of leached rare earth elements, with more than half of that cost tied to purchasing citric acid.

How the Study Worked

Researchers collected REE-rich surface soil from the Diamond Creek area near Salmon, Idaho, where rare earths are primarily hosted in monazite and xenotime minerals. The team tested three process pathways:

• microwave pretreatment alone
• ultrasound-assisted leaching alone
• a combined microwave + ultrasound approach

Citric acid served as the leaching agent because it represents a potential biologically derived organic acid that could reduce environmental impacts compared with strong mineral acids.

To evaluate real-world feasibility, the team also conducted techno-economic analysis (TEA) and life-cycle assessment (LCA). Scanning electron microscopy (SEM) images revealed cracking and structural changes in monazite grains after microwave exposure—likely caused by thermal stress. These fractures may improve leaching efficiency by allowing acid to penetrate deeper into the mineral particles.

What the Results Really Mean

The most important outcome was faster extraction, not dramatically higher recovery.

The combined approach increased the leaching rate to approximately 0.128–0.133% per minute, compared with 0.048% per minute for ultrasound-only leaching. This means the process can reach comparable extraction levels in less time.

However, the total recovery remained relatively modest at 17.9%, indicating that much of the rare earth content remained locked in the mineral matrix. Environmental impacts were also mixed. While emissions dropped substantially compared with ultrasound treatment alone, the life-cycle analysis showed higher impacts than some alternative leaching approaches, including certain mineral-acid or bioleaching systems reported in prior studies.

For policymakers and industry observers, the message is clear: the technique represents incremental process improvement, not a breakthrough extraction technology.

Limitations and What Comes Next

The study was conducted at laboratory scale using a single soil feedstock, meaning results may vary significantly across other deposit types. Economic results were also heavily influenced by the price of citric acid, which accounted for roughly 52% of the total estimated process cost.

The research was funded through an IGEM grant from the Idaho Department of Commerce, with technical support from Idaho Strategic Resources, though the authors reported no competing financial interests.

Future research will need to focus on:

• improving overall rare earth recovery rates
• lowering organic acid input costs
• testing bio-generated leaching agents
• validating results through pilot-scale experiments

For investors and supply-chain planners watching the Western rare earth buildout, the takeaway is straightforward: this study contributes useful process insights but remains far from delivering a scalable mine-to-magnet extraction solution.

Citation: Brown RM, Steven C, Mirkouei A, Reed D. Combined microwave and ultrasound-assisted leaching process for the extraction of rare earth elements. Hydrometallurgy. 2026;242:106685. DOI: 10.1016/j.hydromet.2026.106685.