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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.