Rare Earth Elements Solvent Extraction
Rare Earth Elements (REEs) are elements found within the Earths crust and have been primarily used as catalysts, colorants and phosphors in commercial applications. However, they have been gaining increasing attention due to their critical role in green and clean technology applications such as electric and hydrogen cars, renewable energy, and advanced materials. Solvent extraction (SX) is an important tool for the separation and purification of REEs and their compounds.
The solvent extraction process is quite complex and depends on a combination of factors such as the stability of the aqueous and organic phases, the nature and properties of the REEs being extracted, and the type of solvent system used. Solvents generally used in SX processes are aliphatic and aromatic hydrocarbons, halogenated hydrocarbons, diols and triols, aliphatic ethers and esters, and halogenated aliphatics and ethers.
SX is the most commonly used method for the extraction of REEs from aqueous solutions. It consists of several steps including contact, extraction, strip, back-extraction and purification. In the contact step, an aqueous solution is mixed with an organic solution containing an appropriate REE-selective extractant or mixture of extractants. This is followed by an extraction step wherein the REEs are transferred from the aqueous to the organic phase. Next, the organic phase is transferred to a stripping column, wherein the REEs are separated from the extractant. The aqueous solution is then sent to a back-extraction column, wherein the target REEs are transferred from the organic to the aqueous phase. Finally, the aqueous phase is sent to a purification step to remove unwanted impurities.
The primary challenge with the solvent extraction of REEs is their selective extraction and recovery. To address this, systematic research studies have been carried out to develop novel extractants and new process configurations. Most of the studies have been focused on aqueous-organictwo-phase systems featuring hydrocarbon-containing or halogenated hydrocarbon-based extractants. Such systems yield satisfactory performance for the extraction of light REEs (LREEs), such as lanthanum and cerium, whereas the extraction of heavy REEs is less successful due to their lack of selectivity.
Recently, new systems based on the use of imidazolium and ammonium-based ionic liquids (ILs) as extractants have been proposed and are currently gaining increasing attention. These systems offer a number of advantages compared to hydrocarbon-based systems, such as a higher selectivity for the extraction of both light and heavy REEs and a reduction in environmental and operational risks. Furthermore, the use of ILs enables the enrichment of rare earth elements from low grade resources, thus reducing the cost.
Despite the recently developed IL-based REE extraction systems, there is still much room for improvement. Studies are currently underway to increase the efficiency and selectivity of these systems and to further reduce the environmental and operational risks. In addition, research is being conducted to reduce the cost of REE extraction processes and make them more economically feasible.
In conclusion, solvent extraction is an important tool for the separation and purification of rare earth elements and their compounds. The extraction process is complex and depends on a combination of factors including the stability of the liquids, the properties of the target REEs, and the type of solvent used. Hydrocarbon-based systems have been successfully used for the extraction of light REEs, however, more recently, systems based on the use of ionic liquids have been proposed, offering a number of advantages. Despite the recent progress, there is still much room for improvement, and research is currently underway to enhance the efficiency and selectivity of these systems and to further reduce the cost of REE extraction processes.
