Intensified metal purification by solvent extraction with ionic liquids in milliflow reactors

High-purity metals are essential for many modern applications such as electric vehicles, solar cells, wind turbines, and smartphones. Metals are, however, not readily available in the necessary purity and metal production processes typically require a variety of different operations. Among such operations is solvent extraction, a technique commonly used for the purification and separation of metallic species based on differences in their distribution over two immiscible liquid phases.

Ensuing the continual search for more sustainable solvent extraction processes, the use of ionic liquids has become increasingly popular. Ionic liquids are solvents that consist entirely out of ions and are characterised by a negligible volatility, low flammability, and intrinsic electrical conductivity. Such properties make them inherently safer and more environmentally friendly than the conventional, molecular solvents commonly used in the industry. Moreover, because of their modular and ionic character, ionic liquids are highly tunable and can be tailor-made for a particular application. An increased process efficiency and/or selectivity can thus often be achieved. Despite their advantages, the practical applications of ionic liquids are, however, far from widespread. The relatively high price of ionic liquids makes substitution of cheap, molecular chemicals hard to justify from an economic point of view. In addition, ionic liquids generally display a high viscosity which can hinder mechanical manipulations and slow mass and heat transfer rates.

In recent years, the use of milli- or microfluidic reactors has been proposed as a possible answer to these drawbacks. Millifluidic technology involves the manipulation of fluids in channels with small dimensions, typically less than 5 mm. The use of such small reactor dimensions is generally accompanied by improved mass and heat transfer rates, increased specific interfacial areas, a reduced energy consumption, and an improved process control. In combination with the small reactor volume, an intensified solvent use can thus be expected and the use of ionic liquids may very well become more economically viable. However, thus far only few research efforts have covered metal separations using ionic liquids in milli- or microfluidic reactors.

Over the course of this PhD, the combination of ionic liquids and millifluidic reactors was explored for the solvent extraction separation and purification of metals. A first part of the work covers the development of two new solvent extraction processes using undiluted ionic liquids. The investigated case studies, namely precious metal separations and germanium purification are industrially relevant. Precious metals find use in car exhaust catalysts and electronics, while germanium is used in fibre-optics and photovoltaic cells. The first solvent extraction process involves the separation of gold and palladium from copper and iron rich solutions using an undiluted quaternary ammonium bromide ionic liquid and may be applicable to the recycling of waste electrical and electronic equipment. The second separation procedure concerns the recovery of germanium from a synthetic zinc refinery residue leachate containing zinc, iron, copper, and arsenic as contaminants using an undiluted quaternary ammonium hydrogensulphate ionic liquid.

The second part of the work deals with the application and evaluation of the developed separation procedures in millifluidic reactors. Overall, the undiluted ionic liquids were found to be compatible with millifluidics and improved mass-transfer rates, manifested by rapid, second-scale metal extractions, were achieved. The improved process control also proved useful as enhanced extraction selectivities were achieved by exploitation of differences in extraction rate and precise contact time modulation. An evaluation and comparison of the hydrodynamic and mass transfer performance of diluted and undiluted ionic liquids eventually paved the way towards the development of an integrated setup combining various metallurgical operations into a single millifluidic device.

Following the use of ionic liquids incorporating various anions, the final part of the work involved a more in depth study of the ionic liquid metathesis or anion-exchange process. To monitor the progression of the anion exchange process, wavelength dispersive X-ray fluorescence (WDXRF) was explored. The metathesis of ionic liquids was subsequently investigated in a counter-current lab-scale mixer-settler. Significant improvements in product conversion, reagent consumption, and waste generation were achieved.