Abstract
It is widely accepted that particles stabilise flotation froths and that stable froths result in improved flotation performance. Predicting the effect of particle addition on froth stability is, however, challenging. Dynamic surface tension measurement using maximum bubble pressure presents an attractive technique to investigate the effect of surfactant and particles at the air-water interface. The range of bubble lifetimes that can be studied (typically 0.1 to 60 s) is analogous to variations in air rate in flotation cells, and the corresponding changes in surface tension give an indication to the diffusion and adsorption rates of particles at the interface. In this paper, we use dynamic surface tension measurements to investigate the effect of particles on bubble surfaces at the microscale and link this to bulk froth stability measurements carried out using a froth column. Using the maximum bubble pressure method, the results show that the addition of particles results in lower surface tension, both at the dynamic (i.e. short) bubble lifetimes and towards equilibrium (i.e. 60 s bubble lifetime). This corresponds with the bulk froth stability measurements, where the three-phase system yielded more stable froths than the surfactant only system. Furthermore, increased particle loading at the air-water interface, whether through higher surfactant concentrations or lower air rates (longer bubble lifetimes), gave lower surface tension and higher froth stability. This demonstrates the link between bubble loading and froth stability. It is proposed that the maximum bubble pressure technique can be used to predict froth stability for two- and three-phase systems, enabling the effect of particle loading to be accounted for and quantified. Moreover, the technique has the potential to allow rapid determination of particle and surfactant diffusion at the air-water interface and prediction of the corresponding effect on bulk froth behaviour.
Highlights
Froth flotation is a separation technique that is based on differences in the surface properties of particles and which is used at a huge scale in the minerals industry
The bubble lifetime is automatically increased from the minimum to the maximum and the surface tension measured for each bubble
The effect and magnitude of the turbulence on the measured value of dynamic surface tension was determined through an investigation into stirring using deionised water only (Fig. 3)
Summary
Froth flotation is a separation technique that is based on differences in the surface properties of particles and which is used at a huge scale in the minerals industry. For particle sizes more typical of flotation systems, Brian and Chen [17] suspended iron and silicon oxide particles in three size range up to 106 μm at solids concentrations as high as 45% They employed the maximum bubble pressure method and agitated the solution to suspend the particles, which apparently increased the surface tension. The maximum bubble pressure method is attractive for studying foam and froth systems as it replicates the dynamic process of bubble formation and the diffusion of surfactant molecules and/or hydrophobic particles to the airsolution interface. The effect of surfactant concentration and particle addition on surface tension and froth stability is measured. It is shown that the maximum bubble pressure tensiometer results can be correlated to froth stability trends for different combinations and concentrations of particles and surfactant. The pHPZC and pHIEP for silica particles have been reported [25] as 4.7 and 2.7, respectively, the surfaces can be assumed to be negatively charged in deionised water
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