Critical metal recovery from e-waste in concentrated ionic media using ultrasound†

Abstract

With an increase in the amount of e-waste being generated worldwide, there is greater demand for sustainable recycling techniques to recover components and technology critical metals (TCMs) that would otherwise be discarded. Current methods for solder removal are inefficient, produce harmful gases and by-products. This work aims to use catalytic etchants in concentrated ionic media, to improve the sustainability of recycling techniques. The viscosity of these solutions is often perceived as a limiting factor for ion mobility, so ultrasonic agitation has been used to improve mass transport. Cyclic voltammetry and linear sweep voltammetry have been used to investigate the redox behaviour of tin and lead in solder, and how ultrasound can overcome passivation and improve the dissolution of these metals. Ultrasound results in a linear response between the slope of the LSV and solution conductivity for tin, suggesting a migration-controlled mechanism, however passivation still occurred with lead, showing that there is still some diffusion control. A waste printed circuit board was etched using FeCl₃ catalyst in a choline chloride and ethylene glycol DES (ChCl : 2EG) but no major components were removed after 30 minutes of sonication at room temperature. The use of a choline chloride and water in a 1 : 10 molar ratio removed most of the components, along with some gold coating, under the same conditions. The additional water content in the brine improved the fluidity of the solution, enabling dissolution of the solder and copper under-layer, freeing the gold. Cavitation effects including acoustic streaming and jetting work in tandem to aid metal removal.