Tracking velocity of multiple bubbles in a swarm

This communication describes a new technique to identify and track individual bubbles moving in a bubble swarm covering bubble diameters from 0.2 to 5 mm and gas hold-up from 2% to 30%, conditions relevant to mineral flotation systems. The technique employs a 2D column, slot-type spargers, a digital high-speed camera and image analysis software developed for tracking multiple moving objects. The camera collects sequences of images recorded at 2 ms intervals at 1280x 1024 pixel size. The software allows bubble geometric properties to be measured in the image sequence and compiled into a data structure. To track each bubble, a shape and proximity criterion is applied on consecutive pictures to identify each bubble. The bubble trajectory is reconstructed from the data structure; up to 60,000 matching bubbles can be tracked per test. The bubble tracking measurements revealed a velocity-size relationship with some of the following features: faster moving bubbles speed up slower moving ones whether larger or smaller; a large population of slow moving fine bubbles slows all bubbles; surfactant type affects velocity, equivalent size bubbles in Polyglycol moving slower than in n-Pentanol. Practical implications in flotation are discussed. (C) 2010 Elsevier B.V. All rights reserved.