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Separation chemistry: a step toward greener metal recycling processes

Chemists from the CEA, CNRS, ENSCM and the Université de Montpellier, working together at the Institute of Separative Chemistry in Marcoule (ICSM) as part of a French-German collaborative project, together with the Max Planck Institute of Colloids and Interfaces (Potsdam) and Universität Regensburg (Germany), have successfully explained how extractant molecules behave, as used in metal recycling and decontamination, as well as in drug purification and the production of biodiesel. Their discovery solves an enigma that has puzzled chemists since the 1960s: molecular aggregates that are both polymorphic and elusive influence the efficiency with which the molecules of interest are extracted. Understanding the mechanisms involved should enable us to optimize the processes used, by taking advantage of the synergy found in these detergent extractants. The industrial uses of this approach have already been highlighted in three publications, including ACS Nano, and in a patent application[1], all between December 2019 and February 2020.

Published on 25 February 2020

Liquid-to-liquid extraction is the basic process used in hydrometallurgy[1] for recycling metals and decontaminating solvents (to recover molecules that can be re-used or to decontaminate them). Until now, the "recipes" used in chemical processes have been based on operating feedback and theories that are only partly understood. No physical and chemical predictive models have been available to optimize experiments, particularly in the case of using several extractants together


Representation at the nanometric scale of the interface between two liquids (orange and blue). Based on their theory, the research team demonstrated the behavior and synergy of the extractant molecules used to extract metals from a solution: using different molecules, these extractants self-assemble into a diversity of dynamic aggregates capable of extracting a greater quantity of metal salts more efficiently. When only one type of extractant is used, the ability to form aggregates is more limited, and extraction is less efficient. The experimental set-up is the only one of its kind in the world and is currently operational at the SCARCE joint laboratory (CEA/NTU) in Singapore.

This is an unofficial adaptation of an article that appeared in an ACS publication. ACS has not endorsed the content of this adaptation nor the context in which it is used.

The teams at the Institute of Separative Chemistry in Marcoule (CEA/CNRS/ENSCM/Université de Montpellier) and their foreign partners have successfully identified and explained the mysterious "synergy" that operates between extractants, a phenomenon that has been known to exist since the 1960s but which has never been explained until now[1]. The researchers call this an "ienaics" approach[2]. It provides new insight into the physical and chemical interactions that occur beyond immediate neighbors in a solution. To this end, experiments and measurements that are ten times more precise than any published to date[3] were carried out using the instrumented bench assembled at Marcoule and deployed at the SCARCE laboratory in Singapore (see second image below), to rigorously quantify, for the first time, the efficiency of the extractant molecules and the synergy between them.

At the same time, the CEA, in partnership with CNRS and the Universities of Regensburg and Montpellier, patented this new method, covering the combination of conventional extractants mixed with non-extractant molecules in the hydrotrope class, a family of chemicals that have not, to date, been used in recycling, but which present synergistic behavior that had not been identified until now.

Last, another paper[4] confirms the potential industrial uses of the ienaics approach, by applying it as a predictive model for fluid viscosity in nuclear hydrometallurgy, something that has been a major obstacle to ramping up recycling processes, in addition to the turbidity of degreasing detergents.

These early days for ienaics have focused on improving the processing of heavy and strategic metals by extraction. In particular, it has been used to reduce the quantities of fluid used and, therefore, the amount of effluent produced. All this promises a new boost for hydrometallurgy processes, for example, for recycling wind turbine magnets in France and other EU Member States. Within the next ten years or so, advances in the technology are expected to result in more efficient and ecologically acceptable extraction processes at biofuel refineries and in cheaper purification methods for molecules for use in the pharmaceutical industry.

About the REE-CYCLE project: the REE-CYCLE project (Rare Earth Element reCYCling with Low Harmful Emissions) was funded by the European Research Council (ERC) and, since 2013, brought together several teams working on fundamental research and R&D at the CEA. At the same time, the international "RECYCLING" laboratory, backed by CNRS, brought together the ICSM at the Max Planck Institute of Colloids and Interfaces (Potsdam) and the Universität Regensburg. Since 1998, this bilateral French-German network developed between CNRS and the DFG (German Research Foundation) has brought together German and French researchers who have co-published their work in the field of chemical preparations using unconventional fluids and the non-electric specific effects of complex ions.

[1] Patent No.EP20305039, submitted on 01/17/2020 by the CEA, CNRS and Universität Regensburg

[2] Technique used to extract metals which includes a step in which the metal is placed in solution to purify it. 

[3] The desire to explain the increased selectivity seen when two extractants are combined in certain solvents but not in others is one of the questions that led to the Institute of Separative Chemistry in Marcoule (ICSM) being set up by its supervisory bodies (CEA/CNRS/École Nationale Supérieure de Chimie de Montpellier/Université de Montpellier). The first paper presenting a comprehensive theory was published in ACS Nano 2019, Volume 13, 13745−13758 "Synergistic Solvent Extraction is Driven by Entropy" on December 24, 2019

[4] Ienaics, from the Greek verb meaning "to go, to move" and the root from which the word "ion" derives, is a sub-discipline of Nanoscience dealing with the liquid-to-liquid transfer of species influenced by colloidal forces in the absence of an external field.

[5] In the journal Physical Chemistry Chemical Physics,

[6] In EPJ Nuclear Sci. Technol. 6, 3 (2020), DOI: 10.1051/epjn/2019055, the journal highlighted this paper given the interesting research carried out and its potential applications.

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