Analyzing Phase Changes of Rare Earth Element-Chelate Complexes for Supercritical Fluid Extraction

Supercritical carbon dioxide extraction has been investigated as a greener extraction method for many applications. Use of carbon dioxide reduces or eliminates the need for organic solvents and the associated environmental concerns. The properties of supercritical fluid, such as low viscosity and surface tension, have been shown to be conducive for extractions. This research focuses on the extraction of rare earth elements (REE) out of coal fly ash using food-grade organic acids as chelating agents for the target metals and supercritical carbon dioxide as the solvent. The chelators include acetic, citric, and lactic acids. For process evaluation and design, more information is needed about the solubility of the REE-chelator complexes in carbon dioxide at different supercritical temperatures and pressures. Some complexes are more desirable than others, so it is useful to be able to separate by solubility if the pressure and temperature can be tuned.

The purpose of using phase analyzer is to collect a wide range of data about phase diagrams without needing large numbers of experiments. The instrument is called the SFT Phase Monitor II and is designed to be primarily used with carbon dioxide as the solvent. The sample is placed in a capillary tube, lowered into the phase analyzer, and the sample chamber pressurized with carbon dioxide to supercritical conditions. The sample chamber has a viewing port and camera to observe dissolution and crystallization, and software for continuous data collection. The first trials will be with naphthalene to check the instrument function against known phase diagrams and to create an internal lab standard for future checks. Then, tests on the three solid acids will follow to determine the range of temperatures and pressures for their solubility (outside of complexes). Lastly, REE- chelator complexes will be synthesized to test each specific solubility by element and chelator.