On the origin of bi-modal bubble size distributions in the absence of frother

Several authors have noted bi-modal bubble size distributions (BSDs) in flotation systems at low frother or inorganic salt concentrations. The origin appears to be related to bubble–bubble interactions. The present work examines interactions among bubbles produced at a capillary using high-speed imaging. The study provides visual evidence of coalescence-related and wake-related mechanisms creating fine bubbles and bi-modal distributions. Four coalescence mechanisms are identified: coalescence-induced break-up, droplet formation and collision, liquid jet formation and collision, liquid jet disruption to droplets and collision; and two wake-related events: distortion and break-up of trailing bubble, and premature detachment. Comparing to the fine/coarse mode ratio in flotation systems (ca. 1/10) the possible relevant mechanisms are suggested. Knowing that frothers and certain inorganic salts act to retard coalescence, the origin of the bi-modal BSD is argued to be coalescence-related.