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Rare earth elements (REE), a group of elements comprising scandium, yttrium, and the Lanthanides, are used in a wide range of applications from electronics and magnets, to the handling of nuclear materials. The focus of this project is to produce gadolinium for the management of nuclear waste. gadolinium (Gd) is widely used in nuclear applications as a neutron poison, meaning that gadolinium readily absorbs neutrons which could cause fissions in other isotopes. Gd is found in small concentrations within the Earth’s crust and is often found alongside other ores. Global production of pure Gd is estimated to be four hundred tonnes per year and is mostly produced in China.

The United States relies heavily on a steady supply of REE and, therefore, has a vested interest in increasing domestic production of REE. One promising domestic source of REE is coal fly ash. The U.S. currently produces millions of tons of coal fly ash per year. Preliminary results from batch extractions using organic acids in supercritical CO₂ have shown this method to be promising for yield and REE selectivity. This project aims to develop the means for continuous extraction of REE from coal fly ash. The fine particle size of coal fly ash makes many typical forms of filtration ineffective. However, there are many components of coal fly ash, such as silicon, aluminum, iron, and calcium, which could be separated from supercritical CO₂ by density.

Cyclones are regularly used in industry for the physical separation of mixtures through centrifugal force and differences in density. Less dense particles remain near the top of the vessel while denser particles fractionate to the bottom. There is very little existing data on the use of supercritical fluids in cyclone separation. One key issue in the design of a cyclone for this application is that the CO₂ must remain in the supercritical state to avoid the components of the coal fly ash reattaching to each other. To produce supercritical CO₂, a currently existing supercritical fluid extractor will be modified to pump supercritical CO₂ into a reactor vessel connected to the cyclone. Outlet pressure of the system will be carefully regulated to maintain the supercritical state and the appropriate flow through the system to achieve velocities necessary for the cyclone.