Identifying Regions-of-Interest and Extracting Gold from PCBs Using MHz HIFU

Increased digitalization and technological development raises the demand for rare and precious metals (RPM). Due to their rarity, mining RPMs from the earth is becoming increasingly difficult. Traditional urban mining methods to recover RPMs from printed circuit boards (PCB) need to separate the RPMs from non-metallic substances, e.g. plastic. This separation requires toxic substances and causes unwanted and toxic by-products and emissions. The ability to identify regions-of-interest on PCBs, i.e. the gold pads, and to extract RPMs from only the desired areas would reduce the need for toxic substances. In this study, a single 12 MHz high-intensity focused-ultrasound transducer was used to 1) image a PCB to locate the gold pads, and 2) to subsequently induce inertial cavitation to remove gold from three extraction areas on the selected gold pad. The sonication was performed in water without additional chemicals. Gold removal was verified by imaging the pad with a coded-excitation scanning acoustic microscope (f_c = 375 MHz). Average areas and volumes of the three extraction regions were A = (12.2 ± 0.5)·10³ μm² and V = (18 ± 2)·10³ μm³, respectively. The total amount of removed gold and nickel (from beneath the gold plating) from all three extraction areas was estimated to m_Au,tot = (570 ± 20) ng and m_Ni,tot = (440 ± 30) ng. This study constitutes a first step towards more environmentally friendly, non-toxic urban mining of RPMs.