Effect of HIFU frequency on gold removal efficiency from e-waste

The depletion of mineable gold reserves, paired with the increase in demand for electronics, underlines the need for sustainable gold recycling methods. Owing to its high precious metal content, electronic waste (e-waste) is a promising source for gold recovery. Conventional approaches, such as pyro- and hydrometallurgy, remain environmentally taxing, producing harmful emissions and toxic byproducts. This study advances a recently reported ultrasound-based, green technology for recovering gold from discarded printed circuit boards (PCBs). The method uses localized cavitation in water, driven by high-intensity focused ultrasound (HIFU), to mechanically remove gold from PCB edge connectors. In this work, the influence of ultrasound frequency on removal efficiency is investigated using three custom-built HIFU transducers operating at 4.2, 7.3, and 11.8 MHz. Gold removal at 4.2 MHz was found to be 4.6 and 3.8 times more efficient than at 11.8 MHz and 7.3 MHz, respectively. Direct comparison of the erosion marks to the pressure fields revealed that complex frequency-dependent cavitation cloud dynamics are likely responsible for the increase in removal efficiency at the lowest frequency employed. The improved efficiency indicates potential for scale-up, although industrial viability will require further optimization of throughput and thermal management.