A low cost environmentally benign CO2 based hydrometallurgical process

Abstract

Green process technologies involving the use of liquid and supercritical carbon dioxide have shown much promise as sustainable extraction and reaction media in many applications such as homogeneous and heterogeneous catalysis, nanotechnology, polymer synthesis, organic transformations, and the hydrometallurgical processing of zinc. Commercialization of these technologies has been limited by the high capital equipment and utility costs of CO₂ based chemical processes. Incorporation of CO₂-philic groups such as perfluoroethers, perfluoroalkyls, perfluoracrylates, and siloxanes in molecules has led to organic materials capable of transporting otherwise insoluble polar and organometallic compounds into carbon dioxide at relatively moderate pressures. Combining solvating extractants and carbon dioxide has shown promise in commercial and developmental zinc recovery operations; however, transport of the metal species into carbon dioxide has been limited by the solubility of the charged organometallic compounds formed. Here we show that the use of a CO₂ soluble counterion such as perfluorocarboxylic acid will enhance the solubility of organometallic compounds in carbon dioxide provided the molecules are designed to strike a balance between low cohesional energy and high free volume. Following these simple thermodynamic rules, we have identified CO₂ soluble organometallic complexes that readily dissolve in carbon dioxide at a low pressure and temperature of 860 psi and 25 °C, respectively . Even though this study focuses on the design of highly CO₂ soluble organometallic compounds for hydrometallurgical processes, we surmise that our guidelines can be employed for the development of CO₂-philic compounds useful in applications such as catalysis, synthesis of nanoparticles, and organic synthesis in general.