Separation and recovery of critical metal ions using ionic liquids

Abstract

Separation and purification of critical metal ions such as rare-earth elements (REEs), scandium and niobium from their minerals is difficult and often determines if extraction is economically and environmentally feasible. Solvent extraction is a commonly used metal-ion separation process, usually favored because of its simplicity, speed and wide scope, which is why it is often employed for separating trace metals from their minerals. However, the types of solvents widely used for the recovery of metal ions have adverse environmental impact. Alternatives to solvent extraction have been explored and advances in separation technologies have shown commercial establishment of liquid membranes as an alternative to conventional solvent extraction for the recovery of metals and other valuable materials. Liquid membrane transport incorporates solvent extraction and membrane separation in one continuously operating system. Both methods conventionally use solvents that are harmful to the environment, however, the introduction of ionic liquids (ILs) over the last decade is set to minimize the environmental impact of both solvent extraction and liquid membrane separation processes. ILs are a family of organic molten salts with low or negligible vapour pressure which may be formed below 100°C. Such liquids are also highly thermally stable and less toxic. Their ionic structure makes them thermodynamically favorable solvents for the extraction of metallic ions. The main aim of this article is to review the current achievements in the separation of REE, scandium, niobium and vanadium from their minerals, using ILs in either solvent extraction or liquid membrane processes. The mechanism of separation using ILs is discussed and the engineering constraints to their application are identified.

Solvent extraction of gold using ionic liquid based process

“…The idea of complexation is to modify the properties such as the interaction between the ions/complexes with the adsorptive materials in adsorption process 39 . Furthermore, the complex formation and pH variation were also reported to enhance the separation in other mentioned methods [34][35][36][37][38][39][40][41] . Various separation methods for ionic species are summarized in Table 1.…”

“…Some alternative methods can be chosen to separate ionic compounds including the most common solvent extraction 34 , ion exchange 35 , electrophoresis 36 , chromatography 37 , magnetic field 38 , adsorption 39 , membrane separation 40 , and also IL separation 41 . Each separation method has been studied separately to improve the performance.…”

Rare earth element enrichment using membrane based solvent extraction

“…Solvent extraction is based on fundamental physical chemistry of solvent-solute interaction, activity factors of the solutes in the pure phases, aqueous complexation and complex adduct interactions 27 . Most separation and purification process of REE use solvent extraction techniques using two immiscible liquids (solvents) to obtain two liquid phases with different metal ions concentration 26,27 .…”

“…Basic extractants remove metallic ions from aqueous solutions as anionic complexes by anion exchange. Typical basic extractants are long chain amines 26 .…”

“…Liquid membrane typically traps solvent in a module and pass the metal through the solvent with minimal loss of solvent. Ionic liquids have also been developed to substitute solvents for liquid membrane application 26 .…”