(239a) Cyclone Design for Post-Extraction Separation of Coal Fly Ash from Supercritical Carbon Dioxide-Chelator Solvent Systems

In the processing of spent nuclear fuels, radioactive materials in storage tanks are concentrated into the sludge fraction. Sludges are then vitrified into glass waste cannisters for storage at geologic waste repositories. Higher concentrations of fissile materials in the sludge leads to more fissile material in the final vitrification products and, ultimately, lower waste volumes. Processing of the higher-fissile-content sludges requires special care to avoid nuclear criticality issues. This is done by adding neutron poisons, such as iron or manganese. Gadolinium, a lanthanide already used as a burnable absorber in nuclear reactors and fuels, is a particularly good neutron poison. Less volume of added poison means maintenance of safe conditions and less overall waste to prepare, vitrify, and store. Among the feasibility aspects to be tested for using Gd for radioactive tank waste treatment is the availability of tons of Gd, a rare earth element (REE).

Gd is typically found in ores and other materials at parts per million concentrations. Nearly all of the U.S. supply of REE, including Gd, is sourced from China. This critical material supply chain concern has led to investigations of domestic sources of REE. One potential strategy is the extraction of REE from coal fly ash using solutions of organic acids in supercritical carbon dioxide. Batch reactions have shown promising results in terms of yields and REE selectively. A hurdle for scale up to continuous operation is the separation of the spent fly ash from the supercritical fluid while maintaining supercritical conditions so that the extracted REE solutions do not reattach to the fly ash upon CO₂ conversion to the gas phase. The very fine particle sizes of the coal fly ash make simple filtration, as is done in continuous supercritical fluid extraction of biomass materials, ineffective.

In this study, the design of cyclone separators was undertaken to take advantage of the particle density differences between the ashes and the supercritical fluids. Key design parameters are extraction vessel outlet pressure to maintain supercritical conditions and provide enough pressure difference to achieve the needed fluid velocities, and size and number of cyclones to achieve the needed throughput and fines removal. To date, little research has been done on the use of supercritical carbon dioxide as the carrier phase in cyclones, warranting investigation on the impacts of unique supercritical fluid properties, compared to liquids or gases, on cyclone separator performance.