Rare Earth Element Recovery from Coal Fly Ash

Rare earth elements (REE) are metallic elements that consist of fifteen of the lanthanides, scandium, and yttrium. Despite their name, REE are not rare. REE are found in common minerals, albeit at low concentrations. REE have large radii and high charges, meaning that they cannot fit into most mineral structures and there are few geological environments that support them. REEs have a wide range of applications such as magnets, wind turbines, optical materials, and electronics. Depending on the literature source, China controls 90-100% of the REE on the market. Like many metals, improper disposal of REE can be harmful to the environment through leaching into soil and water.

In this research, coal fly ash is explored as a source of REE within the U.S. Fly ash is abundant in many locations around the U.S. and has been shown to contain trace amounts of REE. Extracting REE from fly ash would address issues of supply control and price, as well as reduce leaching of those metals into the environment. Researchers at Sandia National Laboratories have developed a process to extract REE from coal fly ash using water, supercritical CO₂, and small organic acid chelators. The goal of this project is to determine if and how this process can be economically feasible at the large-scale.

Experiments will be conducted with increasing levels of fidelity to the Sandia process. Coal fly ash will be extracted with water and water plus chelator at different temperatures and atmospheric pressure. Then, experiments will be conducted at elevated pressures with nitrogen or carbon dioxide in a batch or continuous supercritical fluid reactor. These experiments will give insight to the effects of temperature, kinetics, thermodynamics, and residence time on chelators.