Element recovery from acidic waste streams

The world is facing a rising demand for resources, as a result of both the expanding human population and increasing affluence across the globe. Consequently, some nations have started to declare specific resources as critical due to their economic importance, while being at risk of supply shortage. One such critical raw material is scandium (Sc), a light-weight rare earth metal. Although not widely used yet, its unique physicochemical properties offer advantages for the transport and energy sectors that could lead to disruptive changes. For instance, Sc could foster large scale additive manufacturing of ultra-light car bodies for aircraft or other vehicles. Furthermore, it is crucial for commercial solid oxide fuel cells, which represent one of the best available technologies for hydrogen-based electricity supply. Unfortunately, the production of Sc is limited by the availability of concentrated ores. Hence, although an estimated 6 Mt of Sc could be available, the annual production is only about ~20 t. Notably, Sc was found in comparably high concentrations in bulk industrial wastes, such as bauxite residues and white pigment acid waste from production of aluminium and titanium oxides respectively. Overall, this makes Sc a prime example for the development of novel hydrometallurgical strategies that allow for sustainable element recovery. Membrane processes are an attractive option for this purpose, as they can provide ion-selective separation that complements established procedures, while generally having a low material footprint. In this thesis, two advanced membrane procedures were selected to investigate their potential for Sc recovery in a realworld scenario. Therefore, based on European $\textrm{TiO}_2$ acid waste, acid resistant nanofiltration and liquid membrane extraction using polymer inclusion membranes were extensively researched. The conducted studies included the manufacturing and optimisation of tailor-made nanofiltration and polymer inclusion membranes, culminating in the development of a process cascade for Sc recovery, that was eventually piloted on cubic metre scale. The results presented here may contribute to the establishment of future Sc supply from secondary streams. However, they are also expected to have broader implications beyond Sc recovery, potentially extending to other critical raw materials. Thus, this work advances the role of membranes in hydrometallurgy and may ultimately facilitate strategies for a more sustainable supply of resources.