Rare Earth Element Extraction from Coal

Rare earth elements (REEs) are critical for modern technology, powering everything from electronics and renewable energy systems to defense applications. Traditional mining of REEs is resource-intensive and concentrated in a few regions worldwide, raising concerns about environmental impact and supply chain stability. As a promising alternative, researchers have turned to coal waste, specifically coal fines the fine-grained by-products of coal mining and processing. Coal fines have been found to contain valuable REEs, presenting a new opportunity to convert mining waste into a high-demand resource.

Extracting REEs from coal fines isn’t straightforward; it requires specialized chemicals, known as lixiviants, that dissolve the REEs and make them easier to separate from the surrounding materials.

Why Coal Fines Are Considered for REE Extraction

Coal fines are small particles of coal by-products that contain organic and mineral matter, which often includes traces of REEs like yttrium, dysprosium, and neodymium. These fines are a cost-effective alternative to traditional mining for several reasons:

1. Availability: Coal fines are abundant as a waste product from the coal industry.
2. Economic Opportunity: Extracting REEs from coal fines not only utilizes waste but could also help reduce the environmental footprint of mining activities.
3. Potential for Recovery: Preliminary research indicates that coal fines contain REE concentrations high enough to justify extraction efforts under optimized conditions.

However, the challenge lies in isolating REEs from a complex mixture of organic materials and minerals within coal fines. Different lixiviants are employed to achieve this, each varying in effectiveness, cost, and environmental impact.

Types of Lixiviants for REE Extraction from Coal Fines

Lixiviants can be classified into several categories based on their chemical properties and extraction mechanisms:

1. Organic Acids (e.g., Citric Acid and EDTA)
2. Traditional Ion-Exchange Lixiviants (e.g., Ammonium Sulfate)
3. Mineral Acids (e.g., Hydrochloric Acid and Sulfuric Acid)

Each type of lixiviant interacts differently with the REEs in coal fines, and their effectiveness can vary depending on the pretreatment methods used, such as ashing.

Ashing in Enhancing REE Extraction

Ashing is a process in which coal fines are heated to around 550°C, reducing organic matter and leaving behind a mineral-rich residue. This step is critical for improving REE extraction, as it frees up REEs from the coal’s organic matrix, making them more accessible to lixiviants.

• Impact on Recovery Rates: Studies show that ashing can increase REE recovery rates by nearly fivefold in some cases, particularly with mineral acids. This improvement occurs because ashing breaks down complex organic compounds that can otherwise trap REEs, making them accessible for dissolution by lixiviants.
• Compatibility with Lixiviants: Ashing particularly enhances the performance of mineral acids like sulfuric acid and hydrochloric acid, which can more easily penetrate and react with the mineral components in ashed coal. While organic acids like citric acid and EDTA also see improved efficiency after ashing, the gains are less dramatic.

Future Directions in REE Extraction from Coal Fines

While extracting REEs from coal fines holds promise, the commercial viability of this process depends on overcoming several challenges:

1. Optimization of Lixiviant Concentrations: Finding the right balance between lixiviant concentration and pH can help maximize REE recovery while minimizing costs and co-extraction of unwanted elements.
2. Reduction of Environmental Impact: Developing protocols to safely handle and dispose of mineral acid waste is essential. Organic acid-based lixiviants may offer an eco-friendlier alternative if their extraction efficiency can be improved.
3. Selective Extraction Techniques: Improving the selectivity of lixiviants or integrating them with other separation technologies, such as solvent extraction or membrane filtration, could reduce the need for post-extraction processing.
4. Application of Beneficiation Techniques: Pretreatments like ashing and perhaps even more advanced methods of beneficiation could further enhance the accessibility of REEs, making coal fines an increasingly viable REE source.
   
   

Conclusion

Extracting REEs from coal fines using lixiviants is an innovative approach to addressing the growing demand for rare earth elements. By repurposing coal waste, this method has the potential to reduce mining’s environmental footprint while providing a new REE source. Although mineral acids currently offer the highest extraction rates, the development of more selective, eco-friendly lixiviants is key to making coal-based REE extraction economically viable and environmentally sustainable. With continued research, coal fines may soon play a valuable role in diversifying the global supply of REEs.

References

Yang, J., Montross, S. & Verba, C. Assessing the Extractability of Rare Earth Elements from Coal Preparation Fines Refuse Using an Organic Acid Lixiviant. Mining, Metallurgy & Exploration 38, 1701–1709 (2021). https://doi.org/10.1007/s42461-021-00439-2