Ion-adsorbed REE clays: Swelling challenges and future solutions

Abstract

The extraction of rare earth elements (REE) has experienced a significant transition from primary sources to secondary and tertiary (alternative) sources. Ion-adsorbed clay (IAC) deposits have attracted considerable attention due to their abundant presence of sought-after elements. Nonetheless, the recovery of rare earth elements faces several challenges. In-situ recovery or leaching is regarded as the most environmentally friendly method and a prospective alternative to conventional soil excavation, shaft, and open-pit mining activities. However, this technique can induce clay swelling, which subsequently diminishes permeability to flow. The injection of a leaching solution, primarily composed of water, destabilizes the clay interlayers, contributing to this phenomenon. A common solution is the use of inhibitors such as magnesium and ammonium salts. Nonetheless, their usage threatens the environment's sustainability and is costly for large-scale utilization, requiring other options. Deep eutectic solvents (DES) have recently gained popularity due to their great tunability, low cost, and environmental friendliness. As such, researchers have argued that it has demonstrated some positive qualities such as more practical preparation methods, higher purity, lower cost, less corrosive, and biodegradability, specifically in the extraction and separation of metals. Therefore, this review analyses the up-to-date mechanics of REE extraction, state-of-the-art appraisal of in situ leaching in ion adsorbed clays, timely assessment of clay swelling with conventional inhibitors, and progressively study DES as alternative solvents for rare earth element recovery and clay swelling inhibitors in ion adsorbed clay deposits. Additionally, this review presents conductor-like screening model for real solvents (COSMO-RS) as a suitable tool to screen potential DES to minimize swelling. A future study is proposed to investigate the interaction between DES, IACs, and REEs at the molecular and nanoscale levels through molecular simulations to ensure optimal recovery efficiency. Predictive algorithms such as COSMO-RS can be utilized to evaluate, modify, and design more environmentally friendly DES. The synthesized DES can then be used to extract REEs from IACs. Additionally, besides screening, COSMO-RS can be employed to analyze the thermodynamics and interactions between REEs and DES.