| The foam stabilization mechanism has important theoretical and practical significance for the intensification of flotation process.In this study,the drainage process and dynamics of foam were systematically studied with the two-phase foam of flotation regarded as the target subject.The effects of typical salts?Na Cl?and surfactants?Methyl Isobutyl Carbinol,MIBC;Sodium Dodecyl Sulfate,SDS;Dodecyl Trimethyl Ammonium Bromide,DTAB?on foam stability was investigated from the macroscopic foam properties,and the basic unit of"bubble pair"in the foam was selected to study the bubble coalescence behaviour.The dynamics of liquid film drainage and dynamic evolution of the interaction force between bubbles on micro-nano scale were studied to reveal the stability mechanism of two-phase foam for salt and surfactant systems,so as to provide a new theoretical perspective for understanding the foam stability in flotation.Foamscan was used to study the kinetics of macroscopic foam growth and decay in Na Cl,MIBC,SDS,and DTAB systems.The results showed that Na Cl,MIBC,SDS,and DTAB could increase the foam stability,which was shown by the increase in foam volume and half-life.The foam stabilizing ability of surfanctants was significantly greater than Na Cl and the order of stable foam ability was as follows:SDS>DTAB>MIBC.Hydration ability of the gas-liquid interface was enhanced,leading to increase in liquid content and macroscopic drainage rate in foam;the macroscopic drainage rate is positively correlated with the liquid content in foam.Addition of salt and surfactants could inhibit bubble coalescence and decrease the bubble size.The surface tension of Na Cl and surfactant solutions was measured,and it was found that the surface tension slightly increased in Na Cl solution,while it was significantly decreased in surfactant solutions.The ability of surfactants to reduce the surface tension decreases in the following order:SDS,DTAB,and MIBC.The foam stability mechanism of Na Cl might be attributed to the ion-specific effect of the sodium ions on the gas-liquid surface.The foam stabilising effect of the surfactant is mainly due to the surface tension reduction.Microscopic bubble coalescence was further studied through the visualization platform of dynamic bubble coalescence,which focused on the variation of bubble coalescence time.The results indicated that the bubbles coalesced quickly after having contact in DI water,and the bubble coalescence time decreased with an increase in the approach velocity of bubbles.With the addition of Na Cl,MIBC,SDS,and DTAB,the bubble coalescence time increased with concentration.When the concentration of SDS and DTAB increased to 10-5 and 10-3 mol/L,the bubbles no longer coalesced.Dynamic liquid film apparatus was used to analyse the dynamics of micro liquid film drainage.Firstly,the thin-film micro-interference technique was used to record the interference fringe sequence during the liquid film drainage process to complete the reconstruction of the spatiotemporal evolution of liquid film.It was found that the liquid film thinned rapidly and ruptured at 0.92 s,and the critical rupture thickness was 25.46nm in DI water.The addition of Na Cl and surfactants could slow down the liquid film drainage and improve the stability of liquid film.The results of bubble size and approach velocity on stable liquid film drainage showed that,the Laplace driving force between bubbles decreased as the bubble size increased,leading to the decrease in the liquid film drainage rate and increase in the equilibrium film thickness.With the increase in bubble approach velocity,the gas-liquid interface had"dimple"deformation owing to the hydrodynamic force,and the approach velocity had a slight effect on equilibrium film thickness.Stokes-Reynolds-Young-Laplace?SRYL?equation was used to integrate and calculate the curvature and disjoining pressures.In DI water,when the film thickness above 250 nm,the curvature pressure of the bubbles was the main driving force for film drainage.When the liquid film thickness reduced to 250 nm,an attractive separation pressure began to appear between bubbles,and the liquid film thinning and rupturing rapidly occurred owing to the attractive separation pressure effect.Combined with the extended DLVO theory,the attractive force was primarily owing to the hydrophobic force between bubbles;this force was the primary cause for thinning and rupture of liquid film between bubbles in DI water.Na Cl and surfactants showed foam stabilization effect in different degrees through inhibiting the hydrophobic attraction force between bubbles,and the inhibition of hydrophobic ability was in descending order:SDS>DTAB>MIBC>Na Cl.To further verify the common foam stabilization mechanism of Na Cl and surfactants based on the hydrophobic force suppression;the interaction force between bubbles in stable liquid film was directly measured using AFM bubble probe technology.The SRYL model was used to fit the experimental data and analyse the disjoining pressure components between bubbles and their dynamic evolution in the stable liquid film.The results showed that the interaction force between bubbles always had a repulsive effect in 10-5mol/L SDS solution.When the surface potential of the bubble was-105 m V and Debye length of the solution was 75 nm,the classic DLVO theory could fit experimental data well,which indicated that SDS can effectively shield the hydrophobic attraction between bubbles.In addition,the repulsive force between the bubbles increased with the approach velocity.The liquid film could not be ruptured with the increase in AFM setpoint?external pressure?.The liquid film between bubbles had a strong ability to resist external force interference.It can be seen from the thinning dynamics profile of liquid film that bubbles could resist the external force by deforming,resulting in the flat film area gradually increasing with the increased external force.The two-phase foam stability mechanism based on the inhibition of hydrophobic force provide a theoretical reference for foam stability in flotation.This paper contains 120 pictures,8 tables,and 201 pieces of references... |
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