Electroleaching-Electrodeposition For recovery of Precious metals from electrical / electronic equipments – EE4Precious

EE4Precious aims to develop a soft chemistry process for the recovery of precious metals (Ag, Au, Pd, Pt) in waste electrical and electronic equipment (WEEE) especially printed circuit boards. The proposed process is electrochemical, with anodic leaching of waste and deposition of pure metal to the cathode. The solvents used are ionic solvents, with coordinating properties and high electrochemical stability, which can effectively replace aqueous solutions containing harmful compounds. The project, based on fundamental and applied aspects, aims to:
- Understanding the behaviour of metals in electrolyte through a molecular macroscopic scale (WP1) approach. Thus, WP1 covers the macroscopic properties of electrolytes (viscosity, conductivity, etc.), the study of molecular-scale interactions between ionic species, and metal speciation;
- the study of the reactivity and transport of solvate metal species (WP2), aimed at determining the optimal composition of the electrolyte, and to characterize the electrochemical behaviour of metals, in correlation with data from WP1;
- the design of a fully monitored/piloted laboratory cell based on a mathematical model incorporating the results obtained in WP 1 and 2 (WP3). Critical aspects of the process can thus be established;
- the application to the treatment of synthetic and then real waste, provided by the industrial partner Terra Nova Développement (WP4) and the study of the durability of the electrolyte. The process life cycle analysis and CAPEX/OPEX estimates should confirm its industrial and environmental potential.

Two types of electrolytes allowing to leach Au, Pd and Ag with quantitative faradic yields were defined at the beginning of the project: the C4C1Im(NTf2)0.78Cl0.22 ionic liquid mixture and Ethaline, a deep eutectic solvent (DES) composed of choline chloride (ChCl) and ethylene glycol (EG). Further works were conducted in Ethaline, which has more advantageous transport properties (low viscosity, high conductivity) and has the advantage of being much less expensive than ionic liquids melt. The feasibility of electroleaching-electrodeposition (EE) coupling was established in 100 mL cells for pure Au and Pd. Methods for the determination of the various metals concentrations by ICP-OES, developed in ionic solvents, make it possible to check the efficiency of the process.
We then focused on the study of metal-ethaline interactions and the determination of the fundamental data necessary for the development of the process. Procedures for the acquisition of reliable data on the physico-chemical properties and kinetic characteristics of electrochemical systems have been defined (repeatability, cross-validation between CEA/IJL partners). Thus, the physico-chemical properties of Ethaline (density, viscosity, conductivity, diffusion coefficient) have been determined, particularly in relation to its water content. The ongoing structural analyses by SAXS/WAXS will allow to interpret these macroscopic data in terms of intra and intermolecular interactions and ionicity. Studies using Raman spectroscopy and UV-visible spectrophotometry have established the speciation of Pd and Au leached in Ethaline, and have highlighted the slow dismutation of Au(I) formed. Current NMR-PFG and NOESY analyses will provide a better understanding of intra- and intermolecular interactions between metals and ionic solvent. Regarding the study of the electrochemical reactivity of metals, to date the Ag system, chosen as a model, is perfectly characterized in Ethaline in terms of transport properties and electrochemical kinetics.

Future works will cover two aspects: the acquisition of fundamental data, to be continued (WP1-2) and the implementation of the laboratory-scale process (WP3-4). The energy efficiency, environmental impact and economic viability of the process will serve as the basis for its optimization. Thus, the replacement of ethylene glycol is currently being studied, in order to reduce the environmental impact and cost of the process while maintaining a similar chemical reactivity to metals. The impact on the physico-chemical properties of DES, the metal-DES interactions and the feasibility of the process is ongoing.
Two aspects of the process will then be studied in particular:
- the chemical stability of the Au leached and its potential impact on the process;
- the selectivity of the process and the composition of the metallic film deposited at the cathode (alloys, pure metals) with regard to the economic recovery/cost of the process/its environmental impact (solvent recycling, LCA).
Laboratory-scale implementation will be conducted in parallel with the following methodology:
- Design of an electrochemical reactor using a model integrating the reactive kinetics and chemical balances of metal-DES systems. Performance assessment will be conducted on synthetic waste;
- application to mixtures of precious metals from the pre-treatment of real waste by TND;
- a study of the life cycle and recycling of the solvent used to carry out the life cycle analysis of the process.

Publication :
ElectroLeaching-ElectroChemical Deposition (EL-ECD) of gold and palladium in a deep eutectic solvent (DES), Journal of Environmental Chemical Engineering, 10(3), 108004, doi.org/10.1016/j.jece.2022.108004

Conference presentations :
1. Coupling electrochemical leaching and electrodeposition in ionic solvents for critical and precious metals recovery, S. Legeai, EUCHEMSIL 2022, 5-10 Juin 2022, Patras, Grèce
2. Electroleaching and electrodeposition in ionic solvents for the recovery of silver from urban mines, C. Bertoloni, EUCHEMSIL 2022, 5-10 Juin 2022, Patras, Grèce
3. Mass transport in Ionic Solvents during electrodeposition of gold and palladium, ILMAT6, 22-26 Novembre 2021, Obernai, France
4. Ionométallurgie : apport des solvants ioniques à l'extraction des métaux de la mine urbaine, S. Legeai, Webinaire GDRPromethee, 9 Décembre 2021

EE4Precious (Electroleaching-Electrodeposition For the recovery of Precious metals from waste electrical and electronic equipments) is a collaborative research project involving 3 academic partners (IJL, LRGP, CEA) and an industrial partner (TND) of complementary skills in urban mining area. Precious metals (PM: Au, Pt, Pd) from primary or secondary resources are currently produced through complex extraction processes using corrosive, hazardous reagents, and with emissions of toxic gases. EE4Precious aims at developing a soft chemical alternative for recovery of these strategic metals from waste electrical and electronic equipments (WEEE) with a focus on printed circuit boards (PCBs). With a growing production, WEEE are far metal-richer than primary resources. They represent an enormous economic potential and an environmental asset for the preservation of resources. As demonstrated by the consortium, effective separation of metals can be achieved in an electrochemical cell, with anodic leaching and deposition of pure metal at the cathode (EE process). For PM, ionic liquids (IL), coordinating solvents with high electrochemical stability, can effectively replace aqueous solutions of harmful complexants, as shown by the recovery of Pt from spent fuel cells by EE in a mixture of LI by IJL and CEA.
EE4Precious relates to a more complex PM-containing waste, resulting from a PCB treatment process developed by TND allowing the elimination of base metals and organic matter. The EE process will require optimal selectivity of the two EE reactions to avoid degradation of the IL and to obtain pure PM. For this purpose, the composition of the IL will be optimized by correlating the study of the interactions of metal ions with the IL environment and the mechanisms / kinetics of the electrode processes. A pilot cell will then be designed and tested for industrial development.
The project, based on fundamental and applied aspects, is organized in 4 WP in addition to WP0 for project management and communication. First, a macroscopic to molecular scale approach will allow a deep understanding of the behavior of metals in the IL environment. WP1 thus covers the macroscopic properties of IL media (viscosity, conductivity, etc.), the study of interactions at the molecular level between ionic species, and the speciation of metals, using advanced analytical and spectroscopic techniques (SAXS, NMR -HOESY, EXAFS-XANES). The inter-correlation of the three data sources will lead to physically significant laws for the properties of the IL phase. The reactivity and the transport capacity of metallic species solvated by IL form WP2, aiming to determine the optimal mixtures of IL in terms of composition, and to characterize the electrochemical behavior of systems (metal-IL), in correlation with the data from WP1.
Taking advantage of the two data sources above, a mathematical model will be developed (WP3). Critical aspects of the process can thus be established, and a fully monitored / piloted laboratory cell will be designed, comprising a sacrificial anode with a compact bed containing the waste and a flat cathode for the recovery of metals. The metal extraction methodology will be defined for optimal selectivity and yields with synthetic mixtures, then with the real waste prepared in WP4. The viability of EE4Precious and its integration into the WEEE treatment chain will be studied under the leadership of the industrial partner (WP4), as well as the durability and purification of the LI. Finally, life cycle analysis of the EE process and CAPEX / OPEX estimation must confirm its industrial and environmental potential in hydrometallurgy.