Highlights

• Researchers developed a four-step modular system to recover rare earth elements and critical metals from municipal solid waste incineration ash with over 95% recovery efficiency.
• The system uses Eutectic Freeze Crystallization, sulfide, alkaline, and oxalate precipitation techniques to selectively extract metals.
• Citrate plays a crucial role as both a leaching and protecting agent, enabling selective and sequential metal recovery.

---

This study, led by Yinghao Wen (opens in a new tab), co-authored by Emily L. Tribby (opens in a new tab) and Yuanzhi Tang (opens in a new tab) all of Georgia Institute of Technology, addresses the challenge of critical mineral recovery in the context of the global transition to a zero-carbon economy. The authors hypothesize that an optimized, modular system can effectively concentrate and recover rare earth elements (REEs) and other valuable metals from waste sources, such as municipal solid waste incineration ash, thereby improving recovery efficiency and economic viability.

The study employs a four-step modular system:

1. Eutectic Freeze Crystallization (EFC) to pre-concentrate metals, reduce reaction volume, and enhance downstream separation.
2. Sulfide Precipitation for recovering copper (Cu) and zinc (Zn).
3. Alkaline Precipitation for recovering aluminum (Al) and iron (Fe).
4. Oxalate Precipitation for recovering REEs.

The system achieved over 95% recovery efficiency and greater than 98% product purity for all metals. The researchers found that using citrate as a leaching and protecting agent played a pivotal role. Thermodynamic modeling demonstrated that citrate forms metal-citrate complexes that prevent premature precipitation of target metals, allowing for selective and sequential recovery when competing precipitating agents are carefully introduced.

Findings and Implications

The study underscores the potential of EFC as an energy-efficient pre-treatment step in metal recovery processes. It also highlights citrate's dual function in metal recovery: acting as both a leaching agent and a protector against undesired reactions. This system provides a scalable, efficient framework for the selective recovery of critical minerals from diverse feedstocks, enhancing resource sustainability.

The recent output was published in SSRN (opens in a new tab).

Limitations

• Feedstock-Specific Results: The study focused on municipal solid waste incineration ash, which may limit its generalizability to other waste streams.
• Economic Analysis: While the system improves efficiency, the study lacks a detailed cost-benefit analysis of implementing the technology on an industrial scale.
• Scale-Up Challenges: The performance of the system under real-world, large-scale conditions requires further validation.
• Environmental Considerations: The environmental impact of the reagents, including citrate and precipitating agents, was not extensively explored.

Conclusion

This study provides a promising pathway for intensifying and selectively recovering REEs and other critical metals, addressing a key bottleneck in the global supply chain for critical minerals. Future research should focus on validating the system across diverse feedstocks, scaling up the process, and conducting a comprehensive economic and environmental analysis.