Waste-to-Resource: Rare Earth Element Separations From Geological Residuals

Coal feedstock for fly ash

Critical metals such as rare earth elements are essential materials for modern technologies across many sectors, including consumer electronics, alternative energy, transportation, and defense industries. Despite their broad and growing importance, the market for these elements is vulnerable to global supply and political disruptions. For example, the production of rare earth elements (REEs), defined as the 15 stable lanthanides plus yttrium and scandium, is monopolized by China. Trade restrictions and growing global demand for these materials have created great instability in the REE supply market, underscoring the need for domestic production and source diversification of critical metals of national importance. As such, metals-enriched waste streams and other unconventional feedstocks have garnered tremendous interest as part of a larger domestic resource diversification strategy.


Solid wastes and other geological residuals such as acid mine drainage, electronic wastes, and combustion residuals can be enriched in critical metals and have received attention as promising alternative feedstocks. Oftentimes these secondary feedstocks are wastes generated during the production of energy, fuel, and other material resources. Because these feedstocks are geographically dispersed or modestly enriched relative to ores at conventional mines, traditional large scale hydrometallurgical processes often are not economical or feasible for implementation as distributed production facilities. New technologies are needed to overcome the hurdles in implementing metals production from unconventional sources at the moderate to small scale.


Research activities in the Hsu-Kim group has focused on separation and purification of rare earth elements from coal combustion ash and other energy-related residuals. This work includes methods to extract the REE from solid wastes and the development of purification methods from complex and relative dilute feedstocks. Membrane-based separation technologies are the focus of our research activities. We employ fundamental principles in metal-chelation chemistry and ion transport as a means to understand factors controlling extraction and to improve separation efficiencies.

This work is supported by the National Science Foundation and Department of Energy.

Selected publications on REE recovery from coal ash:


Middleton, A., and H. Hsu-Kim. Separation of Rare-Earth Elements by Supported Liquid Membranes: Impacts of Soluble Iron, Aluminum, and pH in Low-Grade Feedstocks.” ACS ES&T Engineering, 3, 1197–1204. https://doi.org/10.1021/acsestengg.3c00060

Middleton, A.; Park, D.M.; Jiao, Y.; Hsu-Kim, H. (2020). Major element composition controls rare earth element solubility during leaching of coal fly ash and coal by-products. Intl. J. Coal Geology. DOI: 10.1016/j.coal.2020.103532

Park, D.; Middleton, A.; Smith, R.; Deblonde, G.; Laudal, D.; Theaker, N.; Hsu-Kim, H.; Jiao, Y. (2020). A biosorption-based approach for selective extraction of rare earth elements from coal byproducts. Separation and Purification Technology. 241, 116726. DOI: 10.1016/j.seppur.2020.116726

Smith, R.C.; Taggart, R.K.; Hower, J.C.; Wiesner, M.R.; Hsu-Kim, H. (2019). Selective Recovery of Rare Earth Elements from Coal Fly Ash Leachates Using Liquid Membrane Processes. Environ. Sci. & Technol. 53, 4490-4499.DOI: 10.1021/acs.est.9b00539

Taggart, R.K.; Rivera, N.A.; Levard, C.; Ambrosi, J.-P.; Borschneck, D.; Hower, J.C.; Hsu-Kim, H. (2018). Differences in bulk and microscale yttrium speciation in coal combustion fly ash. Environmental Science: Processes & Impacts. 20, 1390-1403. DOI: 10.1039/c8em00264a (open access)

King, J.F.; Taggart, R.K.; Smith, R.C.; Hower, J.C.; Hsu-Kim, H. (2018). Aqueous Acid and Alkaline Extraction of Rare Earth Elements from Coal Combustion Ash. International Journal of Coal Geology. 195: 75-83. DOI: 10.1016/j.coal.2018.05.009

Taggart, R.K., Hower, J.C.; Hsu-Kim H. (2018). Effects of Roasting Additives and Leaching Parameters on the Extraction of Rare Earth Elements from Coal Fly Ash. International Journal of Coal Geology. 196, 106-114. DOI: 10.1016/j.coal.2018.06.021

Mutlu, B.K; Cantoni, B.; Turolla, A; Antonelli, M.; Hsu-Kim, H; Wiesner, M.R. (2018). Application of Nanofiltration for Rare Earth Elements Recovery from Coal Fly Ash Leachate: Performance and Cost Evaluation. Chemical Engineering Journal. 349: 309-317. DOI: 10.1016/j.cej.2018.05.080

Taggart, R.K.; Hower, J.C.; Dwyer, G.S.; Hsu-Kim, H. (2016). Trends in the rare earth element content of U.S.-based coal combustion fly ashes. Environ. Sci. & Technol. 50(11), 5919-5926. DOI: 10.1021/acs.est.6b00085 (open access).