Investigative Studies toward the Extraction and Recovery of Praseodymium From Ionic Liquids Using Supercritical Carbon Dioxide
Recycle of rare earth elements requires the extraction and recovery of these metals from consumer products such as rechargeable batteries, electric motor magnets, and aircraft engines once the rare earths have been oxidatively dissolved. One approach to this is to combine the metal extraction and recovery steps into a continuous process. Such a scheme would involve direct-dissolution and ligand-assisted supercritical fluid extraction of the metal, followed by deposition of the extracted metal complex into an ionic liquid where it can be electrochemically reduced to its metallic state. Whereas supercritical fluid extraction of rare earth metals has been recently studied, deposition of rare earth metal-ligand complexes into an ionic liquid is not well known for many rare earth metal-ligand systems. In these studies we measured the solubility of several Praseodymium complexes in scCO2. We also measured the solubility of the Praseodymium complexes in ionic liquids with different physical properties of interest including polarizability, viscosity, and conductivity. The ionic liquids studied are from the pyrrolidinium family and include n-butyl-n-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMPy TFSI). Upon completion of these experiments, we added an additional modifier to the ionic liquid solution in the form of a chelating agent and carbon dioxide. Overall, the additions of the various chelating agents and BMPy TFSI result in large changes to viscosity, conductivity, and metal complexation properties of the solutions. Here we report the results of our findings and discuss the possibility of tuning the extractive and reductive properties of the solutions by picking ionic liquids and chelating agents with physical properties conducive to the desired metal recovery scheme.