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Collision and attachment of particles on bubbles in flotation systems

Contact persons: T. D. Karapantsios [karapant@chem.auth.gr], F. Peleka [peleka@chem.auth.gr] and D. Michalentzaki [dmichalentzaki@hotmail.com]

 

When during a flotation process a solid particle approaches closely a bubble, interaction in the immediate vicinity of the bubble surface leads either to an attachment or to a driftage of the particle. It has been found that two essential kinds of interaction may occur: (a) colliding (impact), in which the bubble surface is strongly deformed and an extended thin liquid film is formed between the particle and the bubble surface, and (b) sliding across the bubble surface with weak deformation and point contact between the particle and the bubble without formation of an extended thin liquid film (Figure 1).

In this study, a special experimental set-up has been built to investigate the particle/bubble interaction (Figure 2). Instead of having many bubbles rising and the solid particles floating more or less freely in the liquid, the situation is simplified and reversed. That is, there is just a single bubble standing at a fixed position (extending out of the tip of a capillary, Figure 3) against which the suspension of the solid particles is moving. The experimental convenience in this scheme is apparent, as one need not to follow the swirling trajectory of a rising bubble but merely keep the surface of the stagnant bubble in focus. A fast camera (8000 frames/sec) is used to capture the details of the particle/bubble approach and impact.

Preliminary work on the deformation of the bubble shape, under the flow field alone (without the presence of particles) at different Reynolds numbers, is used to assess the role of the bubble surface mobility on the impact. Under certain conditions, an interesting oscillatory motion of the bubble surface is noticed. Bubbles and particles of different sizes are examined. The velocity of the approaching particles is also varied in a range of values encountered in flotation processes.

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Figure 1: (a) Trajectory of a particle moving past a bubble in streamline flow. (b) Critical angle of incidence

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Figure 2: Experimental set-up

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Figure 3: Small bubble sliding across the bubble surface

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