Molecular mechanisms induced by phase modifiers used in hydrometallurgy: consequences on transfer efficiency and process safety

It is a matter of strategic independence for many countries to urgently find processes that take into account environmental and economic issues when recycling critical metals. Liquid–liquid (L/L) extraction is a promising method for recovering rare-earth elements from electrical and electronic waste. However, an optimized process on an industrial scale has not yet been established. One of the main reasons is the lack of fundamental knowledge. Therefore, designing a cost-effective and adaptive formulation is still beyond the scope of possibilities. This requires deciphering the molecular forces that control ion transfer beyond the classical supramolecular complexation and developing predictive models compatible with the design and control needs of recycling processes. In all liquid/liquid processes, the high loading of the organic solvent with metal salts/acids or extractant can sometimes lead to a third phase formation. Phase modifiers are often added to the solvent phase in order to prevent the formation of this third phase. However, the effect of these additives on the extraction efficiency as well as their mechanisms of action are still poorly understood. The phase modifiers used in industrial processes are mainly fatty alcohols, called “lipotropes”. In this paper, we study a new class of molecules opening new possibilities beyond the commonly used phase modifiers (i.e., n-octanol). These are the “hydrotropic” molecules. We first show the role of a model hydrotrope (PnP) in preventing the third phase formation for different extraction systems. We also show that the role of modifiers can be understood according to three molecular mechanisms: as co-solvent, as co-surfactant and by preferential solvation. The dominant molecular effect can be identified and quantified by combining surface tension and neutron scattering experiments. In the case of phase modifiers that are hydrotropes, the co-solvent or co-surfactant effect is dominant. In the case of “lipotropes”, the preferential solvation mechanism is emphasized. Finally, the consequences of these mechanisms on the extraction efficiency and selectivity are discussed

Hydrotropes formulated for sustainable metal extraction processes

L’extraction liquide-liquide (L­L) est la technologie principale de séparation employée dans les procédés hydrométallurgiques pour le recyclage des métaux stratégiques nécessaires à l’économie circulaire. La mise en œuvre industrielle du recyclage repose sur le contrôle du transfert d’espèces entre une solution concentrée d’électrolytes contenant les cations métalliques à extraire sélectivement et une solution de tensioactif lipophile associé à un solvant non miscible à l’eau et des « modificateurs de phase ». Une limitation des procédés d’extraction L­L tel qu’utilisés actuellement est la formation de la 3ème phase. De plus, ils induisent un lourd impact environnemental en raison de l’utilisation de volumes élevés de réactifs et l’emploi intensif de solvants organiques non respectueux de l’environnement. Une des stratégies pour répondre à ces problématiques est d’utiliser des systèmes à base d’hydrotropes.Les hydrotropes sont une famille de molécules utilisés pour des applications en biochimie analytique, pharmaceutique et en cosmétique. Ces molécules n’ont jamais été étudiées dans le cadre du recyclage des métaux. Cette thèse est consacrée à la compréhension et la mise en œuvre d’hydrotropes pour l’extraction des métaux, ainsi qu’à l’identification des forces motrices mises en jeu.Ce travail décrit via la démarche « iénaïque », associant les approches supramoléculaire et colloïdale, ce qui se passe lorsque l’on remplace respectivement le diluant, le modificateur de phase et même l’extractant par des hydrotropes. Deux types d’hydrotropes sont étudiés : des hydrotropes qui sont des tensioactifs neutres courts et des hydrotropes électrolytes comme le salicylate de sodium. Dans chaque cas, la détermination des diagrammes de phases et de la nanostructuration des phases sont des préalables nécessaires à la compréhension des forces moléculaires à l’origine des transferts mesurés. L’utilisation des techniques de fluorescence de rayons X, de diffusion de rayons X et des neutrons, de tensiométrie interfaciale ainsi que de calorimétrie ont été déterminantes pour la compréhension des mécanismes sous-jacents à l’extraction hydrotropique.Au prix d’une augmentation de complexité des schémas procédés liés à la solubilité de l’hydrotrope dans les phases aqueuses, nous démontrons que l’emploi d’hydrotropes à la place du diluant ou même à la place de l’extractant, compris par la décomposition « iénaïque », amènent à un gain d’un ordre de grandeur en intensification de procédé et/ou en volume d’effluents produits, ouvrant la voie à l’extraction « raisonnée » des métaux en vue de leur recyclage depuis la mine urbaine.Liquid-liquid extraction (LLE) is the main separation technology used in hydrometallurgical processes for the recycling of strategic metals needed for a circular economy. The industrial implementation of recycling relies on the control of the transfer of species between a concentrated solution of electrolytes containing the metal cations to be selectively extracted and a solution of lipophilic surfactant associated with a water-immiscible solvent and “phase modifiers”. A limitation of LLE processes as currently used is the formation of the 3rd phase. In addition, they induce a heavy environmental impact due to the use of high volumes of reagents and the intensive use of non-environmentally friendly organic solvents. One possible strategy to overcome these problems is by using hydrotrope-based systems.Hydrotropes are a family of molecules used for applications in analytical biochemistry, pharmaceuticals and cosmetics. These molecules have never been studied in the context of metal recycling. This thesis is devoted to the understanding and implementation of hydrotropes for metal extraction, as well as to the identification of the driving forces involved.This work uses the “ienaics” approach to measure and understand what happens when the diluent, the phase modifier and even the extractant are replaced by hydrotropes, respectively. Two types of hydrotropes are studied: hydrotropes that are short neutral surfactants and electrolyte hydrotropes such as sodium salicylate. In each case, the determination of the phase diagrams and the nanostructuration of the phases are necessary prerequisites to understand the molecular forces at the origin of the measured transfers. The use of X-ray fluorescence, X-ray and neutron scattering, interfacial tensiometry and calorimetry techniques have been decisive for the understanding of the mechanisms underlying hydrotropic extraction.At the cost of an increase in complexity of the process schemes related to the solubility of the hydrotrope in the aqueous phases, we demonstrate that the use of hydrotropes instead of the diluent or even instead of the extractant, understood by the “ienaics” decomposition, leads to a gain of an order of magnitude in process intensification and/or in volume of effluents produced, opening the way to the “reasoned” extraction of the metals for their recycling from the urban mine

Hydrotropes formulated for sustainable metal extraction processes

L’extraction liquide-liquide (L­L) est la technologie principale de séparation employée dans les procédés hydrométallurgiques pour le recyclage des métaux stratégiques nécessaires à l’économie circulaire. La mise en œuvre industrielle du recyclage repose sur le contrôle du transfert d’espèces entre une solution concentrée d’électrolytes contenant les cations métalliques à extraire sélectivement et une solution de tensioactif lipophile associé à un solvant non miscible à l’eau et des « modificateurs de phase ». Une limitation des procédés d’extraction L­L tel qu’utilisés actuellement est la formation de la 3ème phase. De plus, ils induisent un lourd impact environnemental en raison de l’utilisation de volumes élevés de réactifs et l’emploi intensif de solvants organiques non respectueux de l’environnement. Une des stratégies pour répondre à ces problématiques est d’utiliser des systèmes à base d’hydrotropes.Les hydrotropes sont une famille de molécules utilisés pour des applications en biochimie analytique, pharmaceutique et en cosmétique. Ces molécules n’ont jamais été étudiées dans le cadre du recyclage des métaux. Cette thèse est consacrée à la compréhension et la mise en œuvre d’hydrotropes pour l’extraction des métaux, ainsi qu’à l’identification des forces motrices mises en jeu.Ce travail décrit via la démarche « iénaïque », associant les approches supramoléculaire et colloïdale, ce qui se passe lorsque l’on remplace respectivement le diluant, le modificateur de phase et même l’extractant par des hydrotropes. Deux types d’hydrotropes sont étudiés : des hydrotropes qui sont des tensioactifs neutres courts et des hydrotropes électrolytes comme le salicylate de sodium. Dans chaque cas, la détermination des diagrammes de phases et de la nanostructuration des phases sont des préalables nécessaires à la compréhension des forces moléculaires à l’origine des transferts mesurés. L’utilisation des techniques de fluorescence de rayons X, de diffusion de rayons X et des neutrons, de tensiométrie interfaciale ainsi que de calorimétrie ont été déterminantes pour la compréhension des mécanismes sous-jacents à l’extraction hydrotropique.Au prix d’une augmentation de complexité des schémas procédés liés à la solubilité de l’hydrotrope dans les phases aqueuses, nous démontrons que l’emploi d’hydrotropes à la place du diluant ou même à la place de l’extractant, compris par la décomposition « iénaïque », amènent à un gain d’un ordre de grandeur en intensification de procédé et/ou en volume d’effluents produits, ouvrant la voie à l’extraction « raisonnée » des métaux en vue de leur recyclage depuis la mine urbaine.Liquid-liquid extraction (LLE) is the main separation technology used in hydrometallurgical processes for the recycling of strategic metals needed for a circular economy. The industrial implementation of recycling relies on the control of the transfer of species between a concentrated solution of electrolytes containing the metal cations to be selectively extracted and a solution of lipophilic surfactant associated with a water-immiscible solvent and “phase modifiers”. A limitation of LLE processes as currently used is the formation of the 3rd phase. In addition, they induce a heavy environmental impact due to the use of high volumes of reagents and the intensive use of non-environmentally friendly organic solvents. One possible strategy to overcome these problems is by using hydrotrope-based systems.Hydrotropes are a family of molecules used for applications in analytical biochemistry, pharmaceuticals and cosmetics. These molecules have never been studied in the context of metal recycling. This thesis is devoted to the understanding and implementation of hydrotropes for metal extraction, as well as to the identification of the driving forces involved.This work uses the “ienaics” approach to measure and understand what happens when the diluent, the phase modifier and even the extractant are replaced by hydrotropes, respectively. Two types of hydrotropes are studied: hydrotropes that are short neutral surfactants and electrolyte hydrotropes such as sodium salicylate. In each case, the determination of the phase diagrams and the nanostructuration of the phases are necessary prerequisites to understand the molecular forces at the origin of the measured transfers. The use of X-ray fluorescence, X-ray and neutron scattering, interfacial tensiometry and calorimetry techniques have been decisive for the understanding of the mechanisms underlying hydrotropic extraction.At the cost of an increase in complexity of the process schemes related to the solubility of the hydrotrope in the aqueous phases, we demonstrate that the use of hydrotropes instead of the diluent or even instead of the extractant, understood by the “ienaics” decomposition, leads to a gain of an order of magnitude in process intensification and/or in volume of effluents produced, opening the way to the “reasoned” extraction of the metals for their recycling from the urban mine

Importance of weak interactions in the formulation of organic phases for efficient L/L extraction of metals

International audienceRecent experimental studies demonstrate the need to take into account weak interactions in the understanding of solvent extraction processes. This well-established industrial technology now beneficiates of a supramolecular approach, complementary to the traditional analysis based on coordination chemistry. In this article, we focus on the integration of a colloidal approach in the analysis of solvent extraction systems: organic phases employed are complex fluids, in which extracting molecules self-assemble into reverse aggregates. We detail the available analytical tools employed towards characterization of these organic phases, and emphasize the recent results in aggregation driven extraction. All experimental data is discussed in light of theoretical approaches which propose adequate thermodynamic models and shed light on the importance of entropy on the phenomena. Diluent effects or synergism have been successfully rationalized, efficient new formulations based on a physico-chemical analysis have been proposed, and the door is now open for further development at industrial scale

Diluent effects on the stability range of w/o micellar systems and microemulsions made with anionic extractants

International audienceHere we present a series of complete phase prisms for water, an organic diluent and di-(2-ethylhexyl) phosphoric acid (HDEHP), one of the most widely used double-branched lipophilic surfactants in hydrometallurgy. Partial or total titration with sodium hydroxide evidence that the mole fraction of the counter-cation “Z” is the variable that controls the packing and spontaneous curvature of the curved film formed by this extractant. Penetrating solvents such as toluene and iso-octane and the non-penetrating solvent dodecane as well as common hydrotropes acting as co-solvents, are considered. The three classical cuts of the phase prism are shown. The regions for which liquid–liquid extraction is possible are determined, as well as the location of the liquid crystals at the origin of the often observed third-phase formation. It is shown that profoundly different trends are obtained when replacing the common solvents currently used in hydrometallurgical processes with hydrotropes

Microfluidic Study of Synergic Liquid-Liquid Extraction of Rare Earth Elements

International audienceA microfluidic technique is associated to X-ray fluorescence in order to investigate the origin of the so-called synergy effect observed in liquid-liquid extraction of rare earth elements (REE) when special combinations of two extractants-one solvating and one ionic-are used.The setup enables kinetic studies by varying the two phases' contact time. Results obtained are compared to those obtained using standard batch extraction method with equal contact time. We then determine variation of free energies of transfer for five rare earth elements present in solution together with a non-target ion (Fe$^{3+}$) at different pH. Analysis of the effect of temperature, as well as of surface charge density of the coexisting cations, allow separating electrostatic from complexation effects. We finally show that all non-linear (synergic) effects are quadratic in mole fraction. This demonstrates that in-plane mixing entropy of the bent extractant film, in the first nanometer around rare earth ions, is the determining term in the synergy effect. Surprisingly, even when the third phase is present, free energies of transfer could still be measured in the diluted phase, which is reported for the first time, to our knowledge. We hence show that the extractive power of the dense third phase is stronger than conventional reverse aggregates in equilibrium with excess water

Microstructure-efficiency relationship in liquid-liquid extraction

International audienceIt is a matter of strategic independence for Europe to urgently find processes taking account of environmental and economic issues, when mining and recycling rare earth elements. Separation and recycling of rare earths from electronic waste is important for the success of present and future carbon-free technologies. Hydrometallurgical separation based on nanoscience is one of the first technologies allowing the take-off of circular economy. Liquid-liquid extraction is a promising method for retrieving rare earths from electronic waste. However, an optimized process on an industrial scale has not been established. One major reason is the lack of fundamental knowledge, therefore designing a cost-efficient, adaptive and predictive formulation is still out of scope of possibilities. Emulsification and demulsification processes in extraction devices are only efficient when the coexisting phases are located between binodal tie-lines in the Winsor II regime. Most extraction processes are based on the combination of an extractant with a diluent. The main disadvantage of these processes is the formation of viscous emulsions known as third phase accident. This occurs when processes are intensified by increasing solute and extractant concentration. Our objective is to develop the fundamental understanding involved in the process’ complex fluids (experimental and theoretical) concerning liquid-liquid extraction of REE and furthermore to use it to design new, cost-effective and environment-friendly recycling processes.A new and promising approach has been recently proposed using Ultra Flexible MicroEmulsions (UFME) which are characterized by an Ornstein-Zernike scattering often observed for weak extractants. These surfactant-free self-assembly is based on the usage of hydrotropic co-solvents instead of the classical extractant/diluent couple. Co-solvents as well as hydrotropes quench the formation of third phases. A systematic comparison of the extracting power of a given formulation by the classical solvent-based, modifier enhanced co-solvent based and the new possible UFME route is now necessary. This requires measuring with enough precision the free energy of transfer of ions along the lines in the quaternary phase diagram. This is only achievable by using a newly developed liquid-liquid extraction microfluidic device coupled to X-ray fluorescence microanalysis. Our contribution towards a more complete understanding in this matter is the analysis and comparison of the phase behavior, extracting efficiency and selectivity of such systems as well as the correlation of these findings with the “ienaics” approach by identifying the molecular driving forces favoring or quenching the transfer

Phase diagrams and microstructures of aqueous short alkyl chain polyethylene glycol ether carboxylate and carboxylic acid triblock surfactant solutions

International audienc