SOLID LIQUID EXTRACTION OF RARE EARTHS FROM AQUEOUS SOLUTIONS: A REVIEW

Rare Earths (REs) demand is constantly increasing in the global market because new technological applications exploit these materials for their unique properties. Since natural resources are located just in focused areas, lots of worldwide countries are arranging solutions to recover REs from end-of-life products. Currently each EU citizen produces about 17 kg of Waste Electrical and Electronic Equipment (WEEE) per year. These wastes are rich in precious and strategic metals and, in many cases, are characterized by higher REs contents than those of natural minerals. Accordingly, recycling can be considered a valuable opportunity: this perspective is known as "urban mining”. From a technological point of view, recycling of WEEE and recovering of metals can be divided into three major steps: disassembly, upgrading and refining. Regarding the refining, different methods have been proposed. Among the others, hydrometallurgical method has been reported to be one of the most interesting. Some of the few disadvantages are related to the step of metal ions removal from aqueous solution. REs separation has always represented indeed a critical challenge in traditional chemical processing because of the similar chemical and physical properties of these elements. Solvent extraction is usually the most used process to separate individual REs but solid-liquid extraction (SLE) presented as an alternative. This paper reviews the different SLE extraction systems proposed for REs separation with particular focus on the different kinds of solid sorbents used (resins, nanotubes, clays and modified clays, membranes, silica and ion-imprinted polymers) evaluating advantages and critical issues of the different systems.