Bubble size is crucial for determining flotation efficiency. Fine bubbles can be cost-effectively generated using a multi-orifice sparger with oscillatory air supply. Sparger configuration is defined by the orifice size, the plate thickness and the chamber volume. To date, the effect of sparger configuration on bubble size with oscillatory air supply is not clear yet. To facilitate the control of bubble size formed with oscillatory air supply, the present work investigated the dependence of bubble size on sparger configuration. It was found that bubble size was positively correlated with the chamber volume and the orifice size, while a nonlinear relationship was observed with the plate thickness. Besides, it was found that flotation recovery decreased over increasing bubble size when changing the sparger configuration. The results indicated that sparger configuration exhibited a significant effect on flotation performance via influencing bubble size when oscillatory air supply was applied.

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This paper examines how the gas supply affects the size of bubbles formed at a submerged orifice. Recent studies showed that switching the air supply pattern from steady to oscillatory using a fast-switching solenoid valve could significantly decrease bubble diameter. However, the previous studies have only investigated the response of the ultimate bubble diameter to the features of oscillatory air supply. The subprocesses of bubble formation under the oscillatory air supply remain unclear. In the present work, the formation dynamics of a bubble generated from a submerged orifice with the oscillatory air supply were quantitatively studied for the first time. It was found that replacing steady air supply with oscillatory air supply could both drive bubble detachment at an earlier stage and depress bubble coalescence after detachment. More specifically, the oscillatory air supply drove bubbles to detach at an earlier stage with the bubble diameter decreased by 14.74% at a maximum, while bubble diameter was decreased by up to 85.87% owing to coalescence suppression. The results revealed that the decreased bubble diameter under the oscillatory air supply was mainly attributed to suppressed coalescence after detachment. The underlying mechanism was also discussed.

The size of bubbles generated from a porous sparger is at least an order of magnitude larger than the pore diameter under the steady air pattern. Recent studies have shown that the bubble size can be significantly reduced when the steady air flow is replaced by an oscillatory pattern. However, the effectiveness of oscillatory air flow on reducing bubble size under different sparger characteristics is yet to be studied. This work fundamentally investigates the response of bubble size to sparger characteristics under an oscillatory air pattern by segregating the bubble formation subprocesses into bubble detachment and consequent coalescence. The results show that bubble size significantly decreased with hydrophilic plates regardless of contact angle, owing to depressed coalescence, while no impact was observed for a hydrophobic plate. The influence of the oscillatory air pattern on decreasing bubble size was weakened as the chamber volume was increased, and above a critical volume the bubble-formation process became similar with that under a steady air pattern. An optimum plate thickness was obtained for bubble generation by avoiding weeping, and meanwhile taking full advantage of the momentum force by the oscillatory airflow. The outcomes show that the oscillatory air pattern in determining bubble formation closely depends on the sparger characteristics, which should be appropriately determined.