Cleaner processing techniques for rubidium extraction and recovery: A critical review

Rubidium (Rb), an important strategic rare alkali metal, has received significant attention in the last few years due to its applications in biomedical research, solar cells, atomic clocks, electronics, aerospace, quantum computing, laser materials and chemical industry. However, the extraction and processing of this metal are complex, since mineral resources containing rubidium are very scarce and are characterised by low rubidium content. For this reason, several technologies have recently been put forward for cleaner recovery processes of rubidium from various resources. Rb compounds are typically co-produced from silicate minerals, such as pollucite or lepidolite, when they are processed for the production of Li and Cs by-products. Seawater and salt lakes have been identified as promising and more sustainable alternative sources This approach could avoid the highly chemical- and energy-intensive routes associated with the processing of minerals containing Rb. As can be observed from the published literature, researchers have employed various techniques such as solvent extraction, membrane sorption processes (hybrid mode), precipitation and adsorption to recover Rb. Very few reviews of Rb extraction have been published in peer-reviewed journals, meaning that Rb has received less attention than other alkali metals. In the last few years, hybrid processes involving membrane sorption have attracted attention, but this aspect has not been covered in previously published reviews. A more in-depth examination of membrane separation technology and other separation/recovery techniques is needed for selective Rb recovery from different resources, with a particular focus on the potential for recovery of Rb via membrane sorption, ion exchange and adsorption processes. Hence, this state-of the-art review covering the period 2010–2024 (up to December 2024) highlights the progress of Rb recovery technologies, addresses the challenges involved, and suggests future avenues for the extraction of rubidium. We also discuss the fact that these cleaner routes, based on the valorisation of waste streams (e.g. the mineral processing stages of lithium production from lake and seawater brines), face thermodynamic barriers due to low Rb concentrations, thus requiring the development of advanced materials (e.g. with high selectivity separation factors over other dominant monovalent ions such as Na or K). Such thermodynamic barriers currently limit both the technical and economical feasibility of these new, cleaner alternatives for Rb recovery.