Improvement of dynamic foam stability with low-frequency acoustic sound

Abstract

Exposing the froth phase of a flotation system to low-frequency acoustic sound was reported to improve the process efficiency, which was thought to be due to the improvement in the froth stability. However, no direct evidence of the stabilization effect of sound on froths or foams is available to date. The present work aims to resolve this issue by experimentally studying the effect of low-frequency acoustic sound on the stability of a foam, the two-phase counterpart of flotation froth, across a wide range of experimental conditions. Three different setups consisting of a flotation column or a bottom-driven mechanical flotation cell, with the foam being exposed to sound from above or below, were tested. The equilibrium height of the foam with a given flow rate of air supply was measured at steady state using the Bikerman’s method. The equilibrium foam height was found to increase within certain ranges of sound frequencies and amplitudes, and at various surfactant concentrations (3–50 ppm) and superficial gas velocities (0.3–2.0 cm·s−1). An increase in sound amplitude or superficial gas velocities was also found to broaden the range of sound frequency that led to the stabilization effect. These findings were largely common for the three different setups tested, suggesting that this is a versatile method for improving the stability of foams. A strong correlation was also found between the stabilization of foam demonstrated in the present work and the stabilization of froth reported in a previous study, implying that the stabilization mechanisms could be similar.