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.
Highlights
The European Commission published a report in July 2010 identifying a number of raw materials as ‘‘critical’’ to Europe’s economy [1]. ‘‘Critical’’ raw materials are those for which the supply into the European Union (EU) is under threat due to the rapid increase in demand and current geopolitical situations
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, 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
The main aim of this article is to review the current achievements in the separation of rare-earth elements (REEs), scandium, niobium and vanadium from their minerals, using ILs in either solvent extraction or liquid membrane processes
Summary
The European Commission published a report in July 2010 identifying a number of raw materials as ‘‘critical’’ to Europe’s economy [1]. ‘‘Critical’’ raw materials are those for which the supply into the European Union (EU) is under threat due to the rapid increase in demand and current geopolitical situations. The European Commission published a report in July 2010 identifying a number of raw materials as ‘‘critical’’ to Europe’s economy [1]. ‘‘Critical’’ raw materials are those for which the supply into the European Union (EU) is under threat due to the rapid increase in demand and current geopolitical situations. Niobium and the rare-earth elements (REEs) are into this classification because of their low substitution rate and their key role in important economic sectors. Up to 30% of the energy required is employed in separation processes such as solvent extraction [2, 3]. The latest developments in extractive technologies of vanadium, niobium, tantalum and REEs are presented
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