Flotation involves the interaction of particles with bubbles and consists of three sub-processes: collision, attachment and detachment. The collision of fine, hydrophobic particles (diameter < 50 µm) with bubbles is inefficient as they follow the fluid streamlines around the bubbles due to their low mass. However, the collision efficiency of ultrafine particles (diameter < 1 µm) with bubbles is enhanced by Brownian diffusion. Particle diffusion is a function of particle diameter, solution viscosity and temperature. Theoretical and experimental studies have shown that ultrafine particle-bubble collision efficiency increases with decreasing particle size (Nguyen et al. 2006). The objective of this research is to investigate the diffusion of ultrafine particles and their selective attachment to an air-water interface. Ultrafine fluorescent core-plain shell silica particles, synthesised using the modified Stöber method, are being used to quantify the interactions of particles with bubbles. The particle surfaces were modified by hydrophobisation. The Brownian diffusion of particles in aqueous dispersions was studied by Dynamic Light Scattering while the particle-bubble interaction was investigated in a Hallimond flotation tube. The results showed that particle diffusion coefficient decreases with increasing particle size under constant continuous phase conditions. For a given particle size, the diffusion coefficient increases with increasing temperature and decreasing viscosity. Preliminary flotation results showed that the recovery of particles increases with increasing hydrophobicity for a given particle size.

References
Nguyen, AV, George, P & Jameson,GJ 2006, ‘Demonstration of a minimum in the recovery of nanoparticles by flotation: Theory and experiment’, Chemical Engineering Science, vol. 61, pp. 2494-2509.