Investigation of the use of foams for silver leaching using the thiosulfate‑copper(II)-ammonia system in the context of e-waste recycling

Due to its physical properties, metallic silver is present in numerous electronic wastes. Its recycling requires selective extraction, which involves leaching of silver as the first step. This work focusses on leaching silver with Cu(II)/NH₃/S₂O₃²⁻ which has been widely used for gold leaching. As recently shown for the leaching of copper, using foams whose aqueous phase consists of leaching chemicals is a promising way to reduce the environmental footprint, by improving the metal oxidation caused by the fast transfer of O₂ from bubble to bubble. In this work, metallic silver samples are dissolved by foams made of Cu(II)/NH₃/S₂O₃²⁻ solution with bubbles composed of O₂-N₂ mixtures. The main problem of the thiosulfate route is its degradation during metal oxidation hence it requires using large quantities of this reactant. The results obtained for our leaching foams show that the quantity of silver leached per quantity of thiosulfate used is about three times greater in comparison with a solution, which would bring a new approach to this problem. Moreover, we investigate the role of the bubble size and the gas composition (dioxygen partial pressure). Besides we find that the dissolved silver is inhomogeneously distributed between the foam column and the bottom solution, with an accumulation of 90 % of silver inside the foam, hence opening an interesting perspective for an easy separation of silver upon leaching. Comparing several surfactants, we show that only non-ionic surfactant polyoxyethylene oleyl ether, Brij O10, shows satisfying results, while dodecyl trimethyl ammonium chloride most likely binds with silver complexes and triggers a quick collapse of the foams.