Recovering copper from e-waste: recyclable precipitation versus solvent extraction with carbon emission assessment

Abstract

As the demand for copper continues to rise, so too does the need for sustainable methods for its recovery from waste streams. Taking inspiration from phenolic oxime reagents used in solvent extraction, the development of two recyclable ligands that act as selective precipitants for the recovery of copper from aqueous mixed-metal acidic solutions is reported. Switching the mode of action from traditional solvent extraction to precipitation eliminates the need for an organic solvent, fulfilling an important principle of green chemistry. The ligand designs feature ditopic phenolic oxime or pyrazole units that result in metal coordination at two sites, thereby promoting the formation of infinite coordination polymers that precipitate from solution. Complete copper recovery from single-metal solutions of CuSO₄ and from mixed-metal solutions that contain nickel, zinc, cobalt and iron is demonstrated, under mildly acidic conditions. The copper was recovered from the loaded precipitates by washing with dilute sulfuric acid, and the ligands reused directly for multiple cycles without loss of performance. Furthermore, 96% of the copper present in a leachate solution derived from waste printed circuit boards was recovered using the phenolic pyrazole ligand. The carbon emissions of this process were also estimated by life cycle assessment and compared with those generated from the recovery of copper by ACORGA M5910, a typical phenolic oxime solvent extractant, with the precipitation process found to be more environmentally benign.