Solvent extraction of metals: Role of ionic liquids and microfluidics

Microfluidic technology has attracted great interest across industry and academia. Its engineering characteristics, through miniaturization, can enhance mass- and heat transfer rates together with allowing operation at high concentrations. Combining this technology with a green designer solvent is one of the most recent advances in separation processes. Ionic liquids have negligible volatility and flammability and have an exceptionally large chemical diversity space, which these days can be better utilised through solvent modelling. Ionic liquids have been demonstrated to increase the efficiency and selectivity of extraction by orders of magnitude. Different types of microfluidic devices have been designed until now, and among those, the segmented flow with alternate regular slugs is the most prominent. Helical coiling can further intensify the internal recirculation by convection, which is the motor of the advanced mass transfer. This is done by liberating Dean forces. A device that leverages such mass transfer intensification in the best possible way is the Coiled flow inverter (CFI) (Saxena Nigam, 1984) [1]. The coil periodicity is just 4 turnings, and then the winding direction is inversed, e.g. changed from clockwise to counter-clockwise, and this is repeated multiple times. The CFI extraction performance is typically much better than for a straight and a non-inverted helical capillary. Separation of metals using liquid-liquid extraction methodology is an important research subject of large economical relevance. The common types of equipment in metal extraction have some disadvantages such as long mixing time and huge plant footprint for the coalescence of the multi-phase, which might take very long due to emulsion formation. In this regard, microfluidic devices and ionic liquids provide an alternative as more compact, more efficient, and faster technology. This review shall help researchers to understand the recent improvement in metal extraction processes, and what the addition of disruptive technology can add to an industrial transformation.