Liquid-liquid extraction of rare earth elements using systems that are more environmentally friendly: Advances, challenges and perspectives

•Alternative liquid–liquid techniques for rare earth extraction are reviewed.•Properties and green characteristics of alternative methods are assessed.•Comparisons with traditional solvent extraction method are presented.•Cloud point extraction demonstrates analytical pre-concentration method potential.•Aqueous biphasic systems demonstrate potential for large scale implementation. The rare earth elements, a group of metals consisting of the lanthanide series, together with yttrium and scandium, are increasingly important in high technology industries worldwide. Given the need to expand production of these elements, there is a search for processes to obtain them that are less environmentally damaging, since the rare earths production chain is associated with several environmental problems and concerns. Solvent extraction, one of the important steps involved in the obtaining of rare earth elements, is receiving much attention in the scientific and industrial community, with a renewed search for approaches that avoid the use of organic solvents. Among the various options, cloud point extraction and the use of aqueous two-phase systems are alternatives to the use of traditional organic solvents. This review considers these two more environmentally friendly techniques and their use for the extraction and/or separation of rare earth elements. The issues addressed include their potential to replace traditional liquid–liquid extraction, in terms of the efficiency, toxicity, and biodegradability of the components forming the systems, as well as their potential applications and future perspectives.