Rising dynamics of particle-laden bubbles in presence of surfactant and turbulence

Abstract

In the minerals industry, froth flotation is a commonly used method for separating valuable minerals from gangue. The design and efficient operation of flotation cells require a knowledge of the interplay between the system hydrodynamics and its chemistry. These parameters however are often examined in isolation using bare bubbles due to the system complexity. This study aims at investigating the coupled effect of surfactant and turbulence on the rising dynamics of particle-laden bubbles of different bubble surface loadings. Specifically, the rise velocity and trajectories of particle-laden bubbles with bubble surface loadings of 0–87 % were examined in presence of methyl isobutyl carbinol surfactant in a near isotropic turbulent flow generated by oscillating grids. Particle-bubble aggregates were formed using a microfluidics T-Junction-based approach to attach coarse glass bead particles of 316 µm in size to bubbles with a diameter of 2.15 mm. The rising dynamics of the particle-laden bubbles were captured using a high-speed camera. The collected data on rise velocities and trajectories were then benchmarked against the rising behaviour of particle-laden bubbles in a quiescent water with no surfactant.

The experimental results showed that, in a quiescent water, addition of methyl isobutyl carbinol surfactant reduces the mean bubble rise velocity at low bubble surface loadings of less than 13%. At bubble surface loadings greater than 13%, the effect of methyl isobutyl carbinol surfactant on the bubble rise velocity was found to be negligible. The increased bubble surface immobility due to an increase in the bubble surface loading is believed to be responsible for the observed behaviour. Turbulence in the absence of surfactant on the other hand, consistently led to an increase in bubble rise velocity over the examined conditions.

Under the coupled effect of surfactant and turbulence conditions, the particle laden bubbles with bubble surface loadings greater than 13% exhibited an overall increase in the rise velocity indicating that turbulence was the governing factor at higher bubble surface loadings. At lower bubble surface loadings however, surfactant proved to counteract the effect of turbulence on the rise velocity, leading to an overall reduction of the velocity at loading below 13%.

In the absence of turbulence, the particle laden bubbles follow a path that is closer to rectilinear as the bubble surface loading increased or surfactant was added. This is the artefact of a reduction in bubble oscillation due to an enhanced bubble surface immobility. However, in presence of turbulence the trajectory of the particle-laden bubble was found to be generally non-rectilinear, due to stochastic interactions with the turbulent flow. The effect of surfactant on particle laden bubble trajectory was found to be negligible in turbulent flow conditions.