Browsing by Author "Pudas, Topi"

This thesis contributes to the ongoing development of a novel, environmentally friendly e-waste recycling technology. We utilize high-intensity focused ultrasound to locally extract gold from the surface of printed circuit boards via cavitation erosion. Acoustic cavitation erosion is the phenomenon in which the acoustically driven violent collapse of gas bubbles in liquid cause damage to nearby solids. Bubble collapse is preceded by its dramatic growth, which is driven by the rarefactive phase of the acoustic wave. In this work, I investigate the effect of ultrasound frequency on the efficiency of gold extraction. Gold extraction experiments were conducted with three custom-built transducers, with different resonant frequencies [4.2, 7.3, 11.8] MHz. The geometries of the transducers are identical, as were the electrical driving parameters. With each transducer, a sequence of gold extraction experiments was conducted with an increasing number of acoustic bursts (ranging from 100k to 1.9M). The results demonstrate that the lowest frequency (4.2 MHz) is 3.8 and 4.5 times more efficient at extracting gold compared to [7.3, 11.8] MHz, respectively. This dramatic improvement is likely due to larger cavitation bubbles associated with lower frequencies. Larger bubbles in the cavitating zone would be expected to undergo more bubble coalescence due to a higher gas volume ratio. Since the energy of bubble collapse increases with bubble size, increased bubble coalescence should augment the energy of bubble collapse. These results provide valuable insights relating to cavitation research and will guide the ongoing development of our novel e-waste recycling technology.