Characterization of 14-Crown-4 Ethers for the Extraction of Lithium from Natural Brines: Synthesis, Solubility Measurements in Supercritical Carbon Dioxide, and Thermodynamic Modeling

Abstract

Improving lithium mining methods is key to furthering lithium’s use as an energy metal. Current mining methods from brines are slow and inefficient and have significant environmental impacts, providing an avenue of research toward developing more effective and less environmentally damaging methods. Supercritical fluid extraction has been evaluated in earlier research for the extraction of metals such as uranium, mercury, and several alkali and earth alkaline metals from aqueous solutions but not for lithium. This study documents the synthesis and preliminary testing of two lithium extractants for possible application in a supercritical carbon dioxide fluid extraction process to recover lithium from natural brines. Based on their high solubility in supercritical carbon dioxide and their desirable structural properties, two 14-crown-4 ethers with different functional groups were selected. In this work, solubility measurements and thermodynamic modeling were carried out to characterize their solubility behavior in the range of P = 100–300 bar with T = 60 °C and T = 85 °C. The main conclusion from this work is the experimental observation of sufficiently high solubilities of the synthesized crown ethers in supercritical carbon dioxide. The highest measured solubility of M14C4, the nonfluorinated 14-crown-4 ether, was 0.30 mol/L at 60 °C and 205 bar, and the highest measured solubility of F14C4, the fluorinated 14-crown-4 ether, was 0.27 mol/L at 60 °C and 285 bar. Another major conclusion was the observed higher solubility of M14C4 than F14C4. We also showed that equation of state modeling provides insight into the factors affecting the crown ether solubility in supercritical carbon dioxide.