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Ionometallurgy can be considered as an alternative to conventional hydrometallurgy for precious metals recovery, reducing process hazards and toxic waste production. More specifically, ionic solvents are of particular interest for electrometallurgy processes, since being intrinsically conductive and stable in a broad range of potentials.At the edge of ionic solvents, Deep Eutectic Solvents (DES) are better candidates than regular ionic liquids, being much less expensive and greener, since they are often partially bio-sourced and they exhibit a higher biodegradability, as for betaine, choline-based DES. We recently demonstrated that palladium (Pd) and gold (Au) can be quantitatively leached and recovered in a single electrochemical reactor using Ethaline, a DES composed of choline chloride (ChCl) as hydrogen bond acceptor and ethylene glycol (EG) as hydrogen bond donor, typically in a 1:2 molar ratio. However, if ChCl can be considered as a “green” reactant, this is not the case for EG. Comparable DES with a lower toxic nature than Ethaline can be obtained by replacing EG by other glycols e.g. propylene glycol (PG), widely used in cosmetics and pharmacology: the resulting DES with PG is Propeline. The present paper deals with the potential of this less known DES in the recovery of precious metals.Because the change in the hydrogen bond donor with PG leads to a modification of the DES bulk properties, the first part of this work deals with the determination of Propeline density, viscosity, conductivity and electrochemical stability, which are properties of interest for electrochemical processes. The influence of water content on these properties was thoroughly investigated. Values of the above property parameters are compared to those obtained with Ethaline as a reference DES. In a second part, we present the performances of Propeline for the selective electrochemical leaching of Ag, Pd and Au. The performance of Propeline in leaching was evaluated in ambient atmosphere, i.e. in the presence of water at percent levels. Leaching efficiencies could be studied after thorough development of analytical procedures dedicated to elemental analysis e.g. ICP-EOS in DES. The speciation of leached metals was determined by use of cross-linked analysis, namely UV-vis, and EXAFS/XANES spectroscopic techniques, in both cases with comparison with those in Ethaline. Systems (leached metal species-DES) were then thoroughly studied by electrochemical methods. In particular, diffusion coefficients of the solvated metal species were determined by electrochemical transient and stationary techniques, in the aim of leached metal recovery by electrochemical deposition.

The development of technologies that allow us to recycle or reuse waste electrical and electronic equipment (WEEE) is paramount in order to create an environmentally sustainable economy. It is also noteworthy that the percentage of some metals in WEEE is larger than in their natural sources, thus generating economic interest in their recycling [1]. Notably, the concentration of precious metals (e.g. Au, Ag, Pd, Pt) in High Tech end-of-life products like smartphones or tablets can be ten to twenty-five times higher than in ores. Generally, precious metals sludges produced by pyrometallurgical primary treatments are treated using multi-step complex hydrometallurgical processes, that have some important limitations for workers or installation safety and remain far from sustainable development considerations. Besides, these pyrometallurgical processes dilute many “minor”, but critical metals (Sn, Sb, Ta…) in the multimetallic outgoing fraction, so that they cannot be recovered in a viable way.In this context, the promising properties of ionic solvents might be an opportunity to design new metal recovery processes being more respectful of Green Chemistry principles with less wastewater generation, less reactants used and less hazards associated. We will present the potentialities of coupling electrochemical leaching and deposition (EL-ECD) in a single cell using ionic solvents for the recovery of precious metals (Au, Pt, Pd, Ag) from rich-metallic waste issued from WEEE primary treatment. The influence of anionic part of various ionic solvents on metals electrochemical behavior was studied by electrochemical techniques. Metal speciation and kinetics parameters associated to electrochemical reactions were determined in most promising electrolytes, by spectroscopic and electrochemical transient/stationary techniques respectively. The performances of the process, namely leaching and electrodeposition efficiencies, were established in ambient atmosphere. Faradaic yields, recovery rates and selectivity for mono and bi-metallic synthetic waste were studied, thanks to the development of a large set of analytical procedures dedicated to elemental analysis in ionic solvents (ICP-OES). The feasibility of using this kind of electrolytes for Sn recovery from the solders used in PCB will also be presented. The effects of varying and mixing anions, temperature and atmosphere (Ar and air) were investigated. The results demonstrate certain conditions under which the oxidation and deposition of Sn in ILs are reversible, indicating the possibility to recover Sn in ionic solvents. The selectivity towards Pb and Cu was evaluated.[1] Marra A., Belgiorno V. 2018. Glob. Nest. J., 20, 679-694

Numeric transition stands on the utilization of sophisticated devices like smartphones, computers or server, which contain an important concentration of critical, rare, or precious metals. So on, metal production from primary ores or from end-of-life devices will necessary increase in the next decades. Currently, precious metals production imply many pyrometallurgical and hydrometallurgical steps and a refining process to obtain pure metals. This refining step is similar using e-waste or primary ores for production, and presents hazardous drawbacks including harmful reagent utilization (cyanide or strong acids) and important wastewater generation. The association of ionic solvents (Ionic Liquids (IL) or Deep Eutectic Solvents (DES)) to electrochemistry could represent a greener solution to this application. These solvents have a low vapor pressure and are non-flammable which considerably reduce fire hazards or worker’s inhalation. Furthermore, they have a great stability (thermal and electrochemical) and act like efficient complexing agents, two very useful properties for precious metals electroleaching without any solvent degradation. These properties have been already used to recover different metals in IL and DES at lab1 or pilot scale2.Recently, CEA Liten and Jean Lamour Institute obtained promising results using a single step electroleaching-electrodeposition (El-Ed) process using ionic liquids for Pt recovery from a membrane electrode assembly (MEA)3.In present work, we evaluate ionic solvent relevance for developing an El-Ed process for gold and palladium electrochemical recovery. We present leaching and deposition performances in IL-mixture and DES for gold and palladium, and electrochemical characteristics which are required for this process. For some ionic liquids, we obtained a gold electroleaching rate comparable to conventional cyanide solution meanwhile DES exhibit faster leaching kinetics. For these promising electrolytes, we focused on the electrodeposition steps (limiting step), and studied metals and ionic species mass transport using Walden plot, PFG-NMR and electroanalytical techniques. Finally, we discussed about relevant experimental parameters (additives, temperature, concentration) to improve mass transport in these solvents. References:1G. R. T. Jenkin, A. Z. M. Al-Bassam, R. C. Harris, A. P. Abbott, D. J. Smith, D. A. Holwell, R. J. Chapman and C. J. Stanley, Minerals Engineering, 2016, 87, 18–24.2A. P. Abbott, G. Frisch and K. S. Ryder, Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2008, 104, 21.3M. Balva, S. Legeai, N. Leclerc, E. Billy and E. Meux, ChemSusChem, 2017, 10, 2922–2935.

Nowadays, Waste Electrical and Electronic Equipment (WEEE) are rising all over the world. Furthermore, e-waste (mobiles, tablets…) could contain some important concentration of critical, rare, or precious metals. So on, metal valorization from end-of-life electronic appliances is one of the most important challenge regarding raw materials depletion and supply but also waste generation. Currently, precious metals recovery from end-of-life products imply pyrometallurgical and hydrometallurgical processes. However, hydrometallurgical processes used to refine precious metals presents some hazardous drawbacks (harmful reagents for the human life and the earth like cyanide or strong acids, wastewater generation). The use of Ionic Liquids (IL) or Deep Eutectic Solvents (DES) could represent a solution more respectful of Green chemistry principles to this application. Indeed, these solvents have a low vapor pressure and are non-flammable which considerably reduce hazards. Furthermore, they have a great stability (thermal and electrochemical) and should act like efficient complexing agents, two very useful properties for precious metals leaching without any solvent degradation. Therefore, IL and DES are already used at the pilot stage to recover metals [1][2][3].Recently, CEA Liten and Jean Lamour Institute obtained promising results on a single step electroleaching-electrodeposition (El-Ed) process using ionic liquids for Pt recovery from a membrane electrode assembly (MEA)[4].However similar process for precious metals contained in WEEE is not straightforward. In present work, we evaluate ionic solvent relevance for developing an El-Ed process for gold and palladium recovery. We present here electrochemical behavior and leaching and deposition potential data in IL-mixture and DES for gold and palladium, which are required for this process. Promising ionic liquids tested exhibit gold electroleaching rate comparable to in the same magnitude of conventional cyanide solution meanwhile DES exhibit faster leaching kinetics. For these promising electrolytes, we present other relevant properties including electrochemical stability, density, viscosity, and conductivity to further optimize leaching step and recovery steps.1.A.P. Abbott, G. Frisch, K. S. Ryder, Annual Reports Section « A » (Inorganic Chemistry), 104 (2008) 21. 2.ANR program, « Strategic metal recycling in Ionic Liquids by EXtraction and Electrodeposition process», 20143.https://www.seren-ag.com/rem-magnet-recycling/4.M. Balva, S. Legeai, N. Leclerc, ChemSusChem, 10 (2017) 2922 2935.

International audience

Petroleum industries continuously consume catalysts on very large scales. The recycling of spent catalysts is thus of major economic and environmental importance and its first step consists of the characterization of the valuable metal content. Wavelength dispersive X-ray fluorescence (WDXRF) analysis is compared with inductively coupled plasma atomic emission spectrometry (ICP-AES) for the analysis of five samples of spent hydrodesulphurization (HDS) and hydrodemetallization (HDM) catalysts. The elements are considered for their economic interest (Co, Ni, Mo, and V) or for the problems that can arise when they are present in the sample in significant quantities (Al, As, P, Fe). First, the systematic comparison of the analysis of known synthetic samples was performed. The originality here is that the samples were first beaded with lithium tetraborate (Li2B4O7) for WDXRF analysis and then dissolved in hot HCl 6M for ICP-AES measurements. With this processing, we were able to clearly identify the origin of analytical problems when they arose. Second, the semi-quantitative protocol of WDXRF is compared with the quantitative procedure. Finally, the analysis of the spent catalysts is presented and the information gained by the systematic comparison of ICP-AES and WDXRF is shared. The interest of the simultaneous determination by the two techniques when such complicated heterogeneous matrices are involved is clearly demonstrated.

International audience

International audience; The platinum nanoparticles used as catalyst in Proton Exchange Membrane Fuel Cells (PEMFCs) represent around 46% of the cells total price and is one of the limitations for their large scale commercialization. The recycling of the platinum catalyst represents a good way to limit the production cost of PEMFCs. Usual recovery routes for platinum-containing catalysts are pyro-hydrometallurgical processes, combining a calcination step and an aqua regia treatment, generating fumes and NOx emission. An electrochemical recovery route by coupling electrochemical leaching and electrodeposition in ionic liquids (ILs) is proposed here, more environmentally friendly, performed in "soft" temperature conditions and without any gases emission. Imidazolium chloride ILs were selected for their complexing ability against platinum and their good electrochemical stability. They were used either pure or diluted in a low coordinating IL, BMIMTFSI (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide). The electroleaching of massive platinum was first studied in several chloride-based ILs. BMIMCl (1-butyl-3-methylimidazolium chloride) appeared to be the best of leaching media tested, as passivation phenomena were observed in other ILs. BMIMCl + BMIMTFSI melts were then tested, containing different BMIMCl amounts. A marked increase of the leaching rate with BMIMCl content is observed up to 0.1 M. For contents higher than 0.5 M, the leaching rate is comparable with the one obtained in pure BMIMCl. The study of the electrochemical recovery of leached platinum revealed that its electrodeposition is impossible in the electrochemical window of pure BMIMCl, due to its high chloride content. On the contrary, platinum electrodeposition was successful in BMIMCl + BMIMTFSI ILs melts for BMIMCl content up to 0.1 M. ILs melts were then successfully used for platinum recovery from fuel cell electrodes. By further modulating the composition of the melt and the electrode potential control mode, it also appeared possible to simultaneously leach and electrodeposit platinum in a single cell process.

International audience; Proton Exchange Membrane Fuel Cells (PEMFC) are made of a membrane electrode assembly (MEA), constituted by two electrodes separated by a polymer membrane. The electrodes consist of a carbon substrate on which platinum nanoparticles are immobilized with a platinum loading ranging from 0.15 to 4 mg.cm-² [1]. The amount of platinum used in MEA represents 25 % of the MEA's price. With the future emergence of PEMFC, platinum used in MEA should represent a very important industrial market - approximately 20 % of platinum production- for platinum recovery. At the moment, only extractive pyro-hydro-metallurgical processes are known for the recovery of platinum contained in PEMFC [2]. Typically, in these processes, MEA are calcinated to eliminate the carbon substrate and the polymer membrane. Ashes obtained from this calcination are then leached by aqua regia, in order to recover the platinum. Calcination step leads to a high consumption of energy and induces the use of a fume treatment system. Moreover, the use of aqua regia, a highly corrosive solution, induces NOx emission and tight working conditions. It is then very important to find a new treatment pathway which can be more friendly environmental.The aim of this work is to develop an electrochemical route for platinum recovery from MEA using ionic liquids (ILs). ILs are interesting solvents for hydrometallurgical treatments purposes due to their low vapor pressure and their chemical stability leading to safer/greener processes. As the platinum of MEA's electrodes is already immobilized on carbon substrates, the leaching of platinum can be achieved by electrochemical dissolution. The leached platinum can then be recovered by electrodeposition. For this purpose, ILs are particularly promising due to their wide electrochemical window allowing the electrodissolution and the electrodeposition of numerous metals [3].Our results show that simultaneous electroleaching and electrodeposition of platinum is achievable in ionic liquids melts containing chloride ions. The leaching rates are comparable to those obtained by chemical dissolution in aqua regia without the use of any oxidant in the ionic liquid medium. The influence of applied potential or current was studied, as well as the influence of temperature and chloride content. The leaching efficiency and rate were evaluated by Atomic Absorption Spectrometry. By modulating the composition of the ionic liquid medium, we show that it is possible to recover platinum by electrodeposition at the counter electrode simultaneously to its electroleaching. This single cell process was then successfully applied to the recovery of platinum from MEA's electrodes, in ambient atmosphere.

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