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Ionic Liquids as innovative organic phase for solvent extraction and separation of lanthanoids: a comparative study of conventional and synergistic systems

Final Report Summary - INNOVILLN (Ionic Liquids as innovative organic phase for solvent extraction and separation of lanthanoids: a comparative study of conventional and synergistic systems)

1. Context of the research
Lanthanoids (Ln) are a series of 14 widely used metals (in permanent magnets, fluorophor lamps, screens, batteries, etc.). They belong to the EU critical material list because, nowadays, China is dominating the ore exploitation. This monopoly has impulsed studies on actual recycling processes that make use of volatile and flammable organic solvents, to improve i) extraction efficiencies of individual Ln, ii) synergistic separation factors of Ln mixtures and iii) environmental aspects.
To this aim, this fundamental project focused on the replacement of the undesirable organic solvents by ionic liquids (ILs), a new class of innovative solvents, displaying favorable properties. In particular, ILs are non-volatile, non-flammable and most of the time, offer tremendous extraction efficiencies as compared to organic solvents. However, the chemical mechanisms at work in ILs are still not fully understood and, at the beginning of this project, almost nothing was known about synergistic metal extraction and separation in ILs, while these are well-assessed efficient phenomena in traditional organic solvents. This project thus aimed at an understanding of the laws ruling Ln synergistic extraction and separation in ILs and at a comparison with traditional organic phases.
2. Results obtained with in the project
Benzene and chloroform were chosen as representatives of widely used organic solvents. The focus was set on four ILs (named as C1CnimTf2, n= 4, 6, 8, 10). Overall, six different well-known extracting compounds (three being neutral, three being weak acids) were individually investigated, in both types of solvents (organic and IL). Then, this list of extracting compounds was used to form five different synergistic couples, studied in the two types of solvents. Up to seven lanthanoids were used. From this wealth of results, general rules within the IL phase C1C4imTf2N could be derived:
1- Whatever the extracting compound, its extracting ability in the IL phase is always largely above that in the traditional organic solvent. Within a given chemical family (i.e. neutral –calixarene- or acidic -β-diketone), compounds presenting very different extracting efficiencies in organic solvents display rather close efficiencies in the IL.
2- Turning to synergistic couples, again, lanthanoid extraction is always more efficient than in organic solvents.
3- However, acidic/neutral synergistic couples do not provide improved separation factors between two lanthanoids in IL as compared to organic solvents and, in some cases, they even display no separation at all.
The first two results are clear advantages of the IL compared to organic solvents, while the last one might be considered as rather disappointing at first glance.
Second, some experiments were performed in order to understand part of the chemical reasons leading to these results. It was found that the IL does not interact with extracting agents, by contrast to chloroform, for example. Interactions may lead to a decreased availability of the extracting compound in view of Ln extraction and separation. However, the difference between the IL and chloroform could not be quantified and is most probably only part of the explanation for results #1 and #2. In case of organic solvents, it is well-known that a decisive parameter for efficient extraction and high separation factor is the aqueous acidic ability of the extracting compound to be used, because this may vary to large extend according to its chemical structure, while the organic acidic ability is always negligible and has thus no impact. We have shown that C1C4imTf2N induces non-negligible and almost similar IL acidic abilities for all the compounds tested. This fact is closely related to results #1 and #3. Finally, changing from C1C4imTf2N to C1C10imTf2N allows annihilating IL acidic abilities.
3. Conclusions and perspectives
General rules for the individual and synergistic extraction and separation of lanthanoids in C1C4imTf2N could be derived. Comparisons with traditional organic solvents highlight the clear advantages of the IL in view of individual extractions of lanthanoids. By contrast, the separation of lanthanoids by use of the IL is not improved and can be even made nill. Some data could not be exploited, owing to a lack of time and a reduced stay of the fellowship beneficiary.
A very interesting point is that extracting compounds that are not efficient in organic solvents can become very efficient in IL. This means that the already existing compound library patiently elaborated in organic solvents is largely extended in IL, taking advantage of rejected compounds that could present favorable efficiencies in IL phases. Moreover, the search for enhanced extraction or separation by synthesyzing new compounds, a classical approach that is relevant in organic solvents, seems rather vain in ILs. The major parameter to play with in view of enhanced extraction and separation might well be focusing on the design of new ILs instead.
Recycling of used technological objects could benefit from the absence of Ln separation by use of ILs. In these objects, the relative proportions of the lanthanides they contain are actually ideally suited for an identical fabrication. In this respect, separating Ln to later on, mixing them again, is not necessary.

General information. This project has been supervised by Dr. Isabelle Billard (France). The fellowship beneficiary is Dr. Maria Atanassova-Petrova (Bulgaria).