Ionic liquids (ILs) are of high interest as alternative solvents in solvent extraction applications and metal processing. Their negligible vapor pressure and low flammability make them safer and more convenient to handle than volatile organic solvents. Furthermore, their structure can be modified and functionalized to incorporate metal extracting groups and to tune their physical properties. In this thesis we used smart IL design to provide new innovative solutions to the recycling of critical metals from end-of-life products. Recycling of critical metals is important to guarantee a sustainable long-term supply, diminish the impact on the environment and to diminish the geopolitical dependence on certain countries. An important advantage of recycling is the fact that these metals are already present in the correct ratios in consumer products, but innovative recycling technologies must be developed to recover these metals efficiently without the creation of additional waste. The development of greener and more selective metal processing techniques is therefore at the core of this thesis.New IL-based recycling processes were developed for lamp phosphor waste and NdFeB permanent magnets. These consumer products have the highest recycling potential when it comes to the recovery of rare earths. Using the unique properties of ionic liquids, we developed processes which are more efficient, use less chemicals and produce less waste than classic hydrometallurgical processes. We also worked on the synthesis of new classes of ionic liquids, with strongly acidic extractants incorporated in their structure, designed to dissolve and/or extract metal ions. We have demonstrated that ionic liquid technology can overcome many problems encountered in classic solvent extraction and hydrometallurgy. The ionic liquids and processes that were developed in this thesis can be used as a toolbox to tackle future issues, because we took care to understand the underlying fundamentals which explain the often unexpected behavior of metals in ionic liquids. We therefore also worked on developing a general theory to explain and predict the effect of metal salts and acids on the (thermomorphic) behavior and mutual solubility of biphasic IL/water systems. This general theory, based on the principles of the Hofmeister series, can be used for the rational synthesis of ionic liquids as well as for the design of IL-based solvent extraction systems.
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