Understanding particle-bubble attachment: experiments to improve flotation modelling

CSIRO has developed a model of flotation using computational fluid dynamics (CFD) software that incorporates fluid motion in realistic vessel geometries. The model also predicts particle capture by bubbles in the system using equations based on the induction time concept. This work looks to experimental studies at the microscale to provide an improved basis for the attachment predictions in the overall model of flotation. A model cell has been set up to study the behaviour of 150 micron glass particles in the vicinity of a captive 1.3 mm air bubble. The interaction is recorded on high-speed video, permitting direct estimation of relevant parameters such as the approach velocity, and the duration of particle sliding over the bubble surface. A new experimental configuration has allowed the particle path toward, around, and away from the bubble to be totally unimpeded. Particle trajectories show a significant deviation at surprisingly large separations, due to the hydrodynamics. However, surface properties are more important in determining eventual attachment. Sliding durations have been measured, and these can be related back to a threshold duration to achieve attachment, known as the 'induction time'. Events where a sliding particle 'jumps in' toward the bubble may indicate the precise moment of attachment, providing a more accurate estimate of induction time. Flotation modellers must consider processes ranging from bulk hydrodynamics to molecular chemistry. However, the micromechanics that forms a bridge between those two processes turns out to be critical. Direct observation of particle-bubble interaction through high-speed videography provides insight into flotation modelling, to allow prediction of attachment based on a realistic combination of hydrodynamic and chemical factors.