(400aq) Use of Supercritical Fluid Extraction in the Life Cycle of Mining Processes

Mining processes are critical to the present and future of energy security on both domestic and global scales. Extraction processes are a vital part in the life cycle of most mining processes. Specifically, extraction of rare-earth elements (REE's) from ore and reclamation of mining sites from contaminants will play a vital role in ensuring domestic energy security in the future. The use of REE's is of critical importance to many industries in the United States, and worldwide, because of their use in electronics, energy, and defense applications. Current extraction of REE's from mine tailings is possible; however, it is costly and inefficient as a result of the use of harsh solvents, downstream separations, and toxic waste that is generated. PFAS (per- and polyfluoroalkyl substances) compounds have contaminated many different matrices. Remediation of aquifers and other water sources have been one of the main focuses for research. Another major affected matrix exists in the solid form, for example landfills and soil. Mining processes have been found to pollute the environment with PFAS compounds, and their removal from the environment in the future will become a vital process. A fluid is described as supercritical when the fluid is raised to a temperature and pressure that are above its critical temperature and pressure. When a fluid is supercritical, the fluid begins to have the diffusivity of a gas and the density of a liquid. Carbon dioxide (CO₂) is an abundant, low-cost, and non-toxic fluid that has been used in supercritical fluid extraction (SFE) for decades. CO₂ has a critical temperature of 31^oC and a critical pressure of 74 bar. SFE with carbon dioxide has been shown to be effective in extracting nonpolar solids, and many PFAS compounds fit this profile. SFE with CO₂ as a solvent can be used to extract REE-oxides from ore with the use of a co-solvent such as tri-butyl phosphate (TBP). SFE with CO₂ has also been shown to be capable of removing a variety of PFAS compounds from various matrices. This extraction allows for a "concentration" of solid waste as well as the remediation of the solid matrices that have been contaminated. In this work, we evaluate the effects of temperature, pressure, and appropriate time scales on the ability for SFE with CO₂ to extract REE's from ore and desorb PFAS from granular activated carbon.