An acoustic agglomeration method for segregation of micro– to nano–bubbles for the flotation of ultrafine particles

Microbubbles, typically defined as bubbles with diameters less than 100 µm, are effective in recovering fine particles by froth flotation. However, they present challenges due to their low buoyancy, poor separation kinetics and potential loss to the tailings. This study explored the use of acoustic agglomeration to enhance the buoyancy of microbubbles and concentrated on designing a microbubble reactor for potential applications in ultrafine particle separation. Microbubbles are generated by a venturi tube as an alternative to conventional acoustic cavitation methods. The bubble behavior and its impact on bubble separation from the liquid were visualized using a high-speed camera to provide both microscopic and macroscopic perspectives. It was found that the microbubbles undergo agglomeration facilitated by primary and secondary acoustic radiation forces (ARF) within an acoustic standing wave field, resulting in the formation of bubble clusters that migrate towards standing wave nodes and experience attractive forces between neighboring bubbles. This process increased the volume of the bubble aggregates, thereby enhancing their buoyancy. For instance, the size of the microbubble aggregates expanded from 50 µm to 650 µm, which subsequently reduced the flotation time from several minutes to less than 1 s. A continuous reactor integrated with a microbubble generator for froth flotation of fine particles was proposed based on the results to reduce the acoustic wave amplitude for acoustic agglomeration and leverage the bubble crowding effect. It was found that microbubbles were completely segregated from the downward flow to the tailing by the acoustic agglomeration technique.