Leaching foams - Reducing the impact of metal mining

The recovery of a wide variety of metals from WEEE (waste electrical and electronic equipment), also known as "e-waste", is a major challenge in the preservation of natural resources and, at the same time, in the treatment of a large quantity of waste. Hydrometallurgy, based on leaching solutions containing oxidants or acids to extract metals, traditionally used in the mining industry for ore processing, is also a promising route for recovering metals from WEEE (such as metals from the Pt, Au, Ag, Cu group). Furthermore, by carefully selecting leachants, it is possible to achieve a selective process maximizing the recovery of precious metals while minimizing the extraction of accessory elements. Nevertheless, hydrometallurgical processes can generate large volumes of polluting effluents such as cyanides or sulfuric acid.

To solve this problem, environmentally-friendly processes are needed to treat WEEE. These can be based on more environmentally-friendly leachants (formic acid or potassium persulphate) or bioleaching methods based on micro-organisms, possibly combined with chelating agents. A recently investigated approach proposes another way of reducing the quantities of reagents and effluents by developing aqueous leaching foams. These foams consist of an assembly of bubbles stabilized by surface-active molecules, and contain at least 75% gas phase. They are already used in a variety of processes, from wastewater treatment to mineral flotation, because of their ability to concentrate particles, ions and certain molecules such as proteins. The high gas/liquid ratio makes it possible to target a 75-99% reduction in liquid volume compared with solution-based processes, leading to substantial gains in terms of reagent and effluent volumes. In addition, the surfactant can be combined with other functions, such as that of a ligand, to improve extraction selectivity. The gas phase can also be controlled to adjust its oxidizing potential, using different oxygen partial pressures or ozone.

The study presented in this article illustrates a proof of this concept, by dissolving metallic copper with a foam consisting of air, dilute hydrochloric acid and non-ionic surfactants of the alkyl polyoxyethylene ether type. The oxygen in the air bubbles reacts very quickly with the copper metal to dissolve it in the presence of non-oxidizing acid, in contrast to the control experiments carried out without foam. The use of an anionic surfactant acting as a ligand for cupric ions promotes this phenomenon.

These results would pave the way for more environmentally-friendly processes, by making it possible to treat the large volumes of electronic waste generated annually and currently under-valued.

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