Propeline: a green alternative to Ethaline for electrometallurgy of precious metals ?
Résumé
The economical viability of WEEE (Waste from Electrical and Electronic Equipment) recycling is mainly based on the recovery of copper and precious metals. In this context, we aim at developing a green process for the recovery of silver, palladium, and gold from rich printed-
circuit boards (cell phones, labtops…). The recovery will be conducted after the necessary stages of non-metallic materials removal and the beneficiation of less precious metals such as copper and nickel, carried out by Terra Nova Development (TND), our industrial partner.
The process envisaged is based on electrometallurgy in “Deep eutectic solvents” (DES), a technology that can be considered as green alternative to conventional hydrometallurgy for precious metals recovery. Indeed, DES have a low reactivity and volatility, making them far
less noxious to environment than corrosive liquors employed in conventional processes (cyanides, aqua regia). In particular, the DES named Ethaline (choline chloride-ethylene glycol mixture) has been widely used in electrometallurgy, because of its acceptable viscosity and
the presence of substantial Cl- concentration leading to a high solubility of numerous metallic compounds. However, if choline chloride (ChCl) can be considered as a “green” reactant, ethylene glycol (EG) is known to be harmful to men and animals in case of repeated exposure or inhalation periods. 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, with costs comparable to those of EG: The resulting DES is named Propeline.
We will present 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 will present the performances of Propeline for the 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.