Fundamental Research On The Enhancing Effect Of Interfacial Nanobubbles In Flotation

With the intensification of micro-fine effect and low-grade of ore and coal,bubble mineralization in flotation system becomes difficult.The core of low-quality mineral resources and coal flotation upgrading is to solve the problem of increasingly fine particle size.Interfacial nanobubbles(abbreviated as “nanobubbles”)are an important way to solve the separation of fine particles,which has been proved to effectively improve the particle bubble capture efficiency.However,the mechanism of nanobubbles enhancing the interaction between particles and bubbles is still unclear,requires to be further researched.A comprehensive understanding of the nucleation and characteristics of nanobubbles from the perspective of basic science can provide a prerequisite for the application of nanobubbles in flotation.Clarifying the micro-action mechanism of nanobubbles on bubble mineralization can provide theoretical basis and guidance for nanobubbles to improve the flotation effect of low-quality ore and coal.Starting from flotation experiments,combined with the tests of induction time and particle size distribution,as well as atomic force microscopy(AFM)techniques,the influence mechanism of nanobubbles on particles flotation was explored.With the increase of air saturation in water,the flotation recovery and flotation rate of particles are gradually improved.The stronger hydrophobicity of particles,the more obvious increase in the flotation rate of microfine particles in nanobubbles water,and the more significant improvement in the flotation effect.The induction time and particle size distribution tests show that the adhesion time or induction time of particles is shortened in air oversaturated water,indicating that the floatability of particles is improved.In nanobubbles water,the stronger the hydrophobicity of particles,the more likely they are to agglomerate,resulting in an increase in the apparent particle size,which is beneficial for improving the probability of particle bubble collision.The AFM force curve measurement and surface scanning reveal that the presence of nanobubbles on particles surface causes the interparticle hydrophobic force and action range to be significantly higher than the corresponding values in normal water and degassed water,revealing the fundamental reason for the improvement of particle flotation effect by nanobubbles.The nucleation,mechanical properties and stability of nanobubbles were systematically investigated with AFM tip-probe technique in liquid-phase.In nanobubbles water,the size of nanobubbles gradually increases with time and reaches dynamic equilibrium.Meanwhile,the stronger hydrophobicity of substrate surface,the larger number and size of nanobubbles,indicating that the nucleation and growth of nanobubbles depend on the hydrophobicity of substrate surfaces.Nanobubbles are elastic soft matter,and their height can be obtained from the indentation depth on the approaching force curves between the hydrophilic probe and nanobubbles.The higher height of nanobubbles,the greater critical applied force required for the probe tip to be pressed in.However,the height of nanobubbles cannot be obtained from the force curves between hydrophobic probes and nanobubbles,especially for probes with strong hydrophobicity.Nanobubbles are sensitive to the external environment after formation,and when the system temperature increases/decreases,the height of nanobubbles increases/decreases,but the horizontal width is basically unchanged due to the pinning effect.The addition of Na Cl solution results in nanobubbles gradually losing their typical spherical cap shape and even merging along the HOPG surface steps.With the increase of Na Cl concentration,the solution air saturation increases and the size of nanobubbles increases significantly;besides,the surface tension of nanobubbles also increases gradually,but always significantly lower than the macroscopic surface tension of solutions.Based on the contact line pinning theory and hydrophobic surface potential theory,and combined with experiments,it is proposed that the low surface tension of nanobubbles is an important factor to maintain the stability of nanobubbles,which enriches the stability mechanism of nanobubbles.With the specially designed 5 μm colloidal particle probes to investigate the effect of individual nanobubble on the interaction of interparticle,the role of nanobubbles bridging in the particle-particle system was clarified,and the nanobubble capillary force model was refined.The interaction force between hydrophilic/hydrophobic particles and HOPG surface was measured after determining the location and morphology of nanobubbles,to investigate the bridging role of individual nanobubble in interparticle.It is found that nanobubbles increase the repulsion range between hydrophilic particles and HOPG surface,and the bridging range is close to the height of nanobubbles.Nanobubbles significantly enhance the long-range hydrophobic interaction between hydrophobic particles and HOPG surface,and the higher hydrophobicity of particle surface,the more obvious bridging enhancement is;the bridging range is proportional to the height of nanobubbles,but much larger than the height of nanobubbles.The contact angle and surface tension of air-bridge in the capillary force model were refined based on the preformed nanobubbles.It is proposed that the concave air-bridge model and the semi-convex air-bridge model are applicable to calculate the capillary force between hydrophobic particles and between hydrophilic particles,respectively.In the concave model,the capillary attraction increases when the contact angle of the air bridge increases on either the particle surface or the HOPG surface;when the distance between particles is very small,the capillary attraction increases sharply due to the Laplace pressure(attractive force)of the air bridge;when the distance between particles is large,the surface tension of the air bridge dominates the capillary force;the larger the volume of the air bridge,the larger the capillary attraction.In the semi-convex model,the attractive force is mainly generated by the surface tension of the air bridge,and the Laplace pressure is always greater than zero;the smaller the inter-particle distance or the larger the volume of the air bridge,the greater the capillary repulsion.Using bubble probe technique,the reinforcing effect of nanobubbles on the interaction between particle and bubble was investigated,and the mechanism of the effect of nanobubbles on the thinning and rupture of the liquid film between particle and bubble was revealed.It is always repulsive force between the hydrophilic surface and the bubble,and the liquid film between the two is always in a stable state.In nanobubbles water,the adhesion between hydrophobic surface and bubbles is more likely to occur,and the peak forces are significantly lower than the corresponding values in normal water,because the presence of nanobubbles increases the decay length of hydrophobic force and the critical rupture thickness of liquid film;The stronger the hydrophobicity of the substrate surface,the more significant the enhancement of adhesion between particle and bubble.The addition of Na Cl solution both compresses the double electric layer to reduce the electrostatic repulsion and may induce the formation of nanobubbles,thus reducing the difference in the adhesion effect between bubble particles in normal water and supersaturated water.This thesis contains 147 figures,12 tables,and 211 references.