Visualized Study On The Interaction Between Single Bubbles And Curved Solid Surface In Flotation Separation Process

Froth flotation is a surfactant-based separation process widely used in numerous industrial applications such as water purification, de-inking of wastepaper and soil remediation. In the flotation separation, gas is generally introduced into a flotation column in the form of small bubbles which act as carriers transporting suspended particles to liquid surface where particles are removed. Obviously, the collision and attachment between gas bubbles and particles is critical to the formation of aggregates of them, which is a fundamental step required for obtaining higher efficiency in flotation. However, most of the studies were carried out on the interactions of bubble-flat solid surface. The purpose of this work is to investigate the interaction of single air bubble with a curved surface experimentally. Because the bubble will slide away from the curved surface instead of staying below the surface if the attachment cannot occur, the curved surface arrangement could reveal easily whether or not the TPC (three-phase contact) is formed compared to the horizontal surfaces. For this purpose, we use a laboratory scale flotation column to investigate two particular stages of the collision and attachment visually using a high-speed video camera. The bubble impact and rebound velocity and maximal distance after the rebound have been determined to quantify the first stage. The attachment stage has been characterized by the TPC time of a gas bubble. The effects of the surface material、surfactant types and surfactant concentration on these quantities have been also studied. In order to analyse the experimental data of bubble-curved surface collision and attachment, a simple expression is derived for TPC time. The considered materials of the solid are cleaned glass as a hydrophilic surface and Teflon as hydrophobic surface. Sodium dodecyl sulfate(SDS)、Tea Saponin and cetyl trimethyl ammonium bromide(CTMAB) are three different surfactants.The results of collision stage show that the presence of surfactant strikingly affects the rebound process of a gas bubble, while no obvious effect of the surface material and surfactant concentrations on the impact velocity. An increase in surfactant concentration has been observed to suppress the rebound number and maximal distance of the bubble from the surface. Compared to pure water, the maximal distance of bubble reduced by about13%in SDS solution of5×10-5mol/L. The maximal distance of bubble is1.05～1.1in the5x10-5mol/L solution of SDS, the maximal distance of bubble is0.9～1.0in the5×10-5mol/L solution of CTMAB. The bubble restitution coefficient is reduced with the increased surfactant concentration. The bubble restitution coefficient of CTMAB is smaller than that in SDS solution. The estimated loss of kinetic energy during the impact of bubbles is increased with the increasing surfactant concentration at various situations. The estimated loss of kinetic energy is smallest in SDS solution.The results of attachment stage show that the bubble attachment is only observed at the hydrophobic Teflon surface below the surfactant CMC (critical micelle concentration). The TPC time of bubble is a strong function of the surfactant concentration as well as of the bubble diameter. The time of TPC is reduced with the increasing concentration of SDS until the concentration is1×10-3mol/L. There is no obvious effect of CTMAB concentration on the TPC time. When the concentration of Tea Saponin is increased from1×10-5mol/L to1×10-4mol/L, the time of TPC is changed from30ms to100ms. The time of TPC is larger in Tea Saponin solution than others. Based on the simple model of TPC time/we can find TPC time is proportional to the bubble radius、surfactant and critical thickness of liquid film. We can use the model of TPC time to analyse the effects of the surfactant concentration and types on TPC time.Results of this study are relevant for deeply understanding of collision and attachment mechanism and to determine the best conditions for selective flotation process.