Study On The Effect Mechanisms Of Froth Stability And Particle Motion At The Air-Water Interface On Froth Phase Flotation

In the process of conventional flotation,particles are easily detached from bubble surface due to the strong bubble coalescence at the interface of pulp-froth phase,resulting in a decrease of flotation efficiency.Related studies have shown that most of the mineral particles detachment occurs at the pulp-froth phase interface.In this view,the dissertation proposed a study of froth phase flotation based on two-phase foam and three-phase froth characteristics as well as the motion law of embedded particles in the particle monolayer at the air-water interface,aiming at through exploring the effect mechanisms of froth stability and particle motion at the air-water interface on froth phase flotation to provide theoretical guidance and technical support for in-depth understanding the process of bubble coalescence and particle detachment during flotation and efficient flotation of the two common diffcult-to-float coal slimes.Firstly,the dissertation takes the foam stabilization as the starting point,and studies the characteristics and stability intensification of two-phase foam and three-phase froth from the following three perspectives: frother,mixed frother and particle.Results indicated that the greater the surface activity of frother,the stronger the foaming ability,the better the static stability of foam,the longer the bubble coalescence time,and the lower the critical coalescence concentration.The applicable theoretical calculation methods of critical coalescence concentration and bubble sauter mean diameter of alcohol frother and polyethylene glycol frother were given,and its reliability were verified.It was found that the blending of methyl isobutyl carbinol(MIBC)and polypropylene glycol(PPG425)has a certain synergistic effect.The strong hydrophobic particle-stabilized froth has the highest half-life time,and the moderately hydrophobic particle-stabilized froth has the largest froth layer height,while the low hydrophobic particles have little effect on the froth stability.When the bubble surface was covered with particles,the coalescence time of the bubbles was significantly increased,meanwhile,the intensity of the interface oscillation induced by bubble coalescence was obviously weakened,and the oscillation time was shortened.Analysis suggested that this was mainly related to the damping effect of the particles that adsorbed on the bubble surface.Secondly,the improved Langmuir-Blodgett trough was used to investigate the structural evolution and rearrangement of the agglomerates during the compression and expansion of the particle monolayer at the air-water interface as well as the motion law of embedded particles under oscillation at the microscopic level.Results indicated that the particles at the air-water interface were all existed in the form of agglomerates.When the particle monolayer was compressed,the interfacial particles rearranged in a manner of agglomeration rotation and void collapse.At the same time,the structure of agglomerates was gradually transitioned from a horizontal intermediate chain and a multi-"block" structure to a typical hexagonal dense and a multi-layer agglomerate structure.When the tightly packed particle monolayer was expanded,the agglomerates dispersed again,but compared to before compression it was more uniform.The coalescence of bubble and air-water interface can induce an oscillation of particle monolayer,at this time,the horizontal drag force and inertial force as well as the interaction forces between the particles codetermined the rearrangement and motion behavior of the interfacial particles.In the absence of frother,the adjacent particles of different size at the air-water interface had similar intantaneous speed and mean square displacement(MSD).Meanwhile,with the increase of particle hydrophobicity and interficial particle coverage percentage as well as the decrease of bubble size and solution pH value,the instantaneous speed and MSD of the interfacial particles were all decreased.In the presence of frother,the motion of the interfacial particles was enhanced,but with the increase of 1-pentanol and PPG425 concentration,the instantaneous speed and MSD showed completely different change laws.It was speculated that the main reason for this phenomenon was that the PPG425 molecules migrated from solution to the solid particle surface,which through changing the surface tension of air-water interface and hydrophobicity of particle surface to further affect the motion behavior of interfacial particles.Then,based on the behavior and force analysis of particles in the froth layer,the process of froth phase flotation was revealed.Menawhile,guided by the classical flotation theory,the recovery model of froth phase flotation was proposed.Studies have shown that the process of froth phase flotation could be regarded as an effective combination of froth separating and conventional flotation,which resulted in a high flotation efficiency.The particles had the maximum supporting force at the top of froth layer,and the second maximum supporting force was achieved at the interface of pulp-froth phase.The change law of each force at the froth-air interface and pulp-froth interface as a function of particle size showed that the fine particles were easily form a cover on the bubble surface,while the coarse particles were more likely to recycle in the froth layer.Since the collision probability between bubbles and particles in the froth layer was close to 100 %,thus the capture probability of froth phase flotation was mainly related to the bubble-particle adhesion probability and the desorption probability of particles on the mineralized bubble surface.The recovery of froth phase flotation decreased with the increase of residence time of particles in the froth layer.Finally,based on the above research,the application research of froth phase flotation of coarse coal slimes and oxidized coal slimes were carried out.Results indicated that the froth phase flotation of coarse coal slimes could increase the combustible recovery by 11.6 % compared with conventional flotation under the same experimental conditions.At the same time,the clean coal ash increased by 1.48 %.When the PPG425/2-octanol mixed frother was used to strengthen the froth phase flotation effect of coarse coal slimes,the combustible recovery increased by 9.34 %,and the clean coal ash increased by 0.46 %.When the moderately hydrophobic fine coal particles were used to strengthen the froth phase flotation effect of coarse coal slimes,the combustible recovery increased by 6.27 %,but the clean coal ash decreased by 0.58 %.The froth phase flotation of oxidized coal slimes could increase the combustible recovery by 22.25 % compared with conventional flotation under the same experimental conditions.At the same time,the clean coal ash increased by 1.19 %.BBD response surface methodology was used to analyze the influence law of four common operating parameters on clean coal ash and combustible recovery.It was found that the wash water rate had the greastest influence on clean coal ash,while the frother concentration had the greastest influence on combustible recovery.The operating parameters of froth phase flotation of oxidized coal slimes were optimized,and the best experimental conditions were obtained as following: frother concentration 0.4 kg/t,superficial gas velocity 1.39 cm/s,feed height 200 mm,and wash water rate 277.83 ml/min.In this condition,the predicted clean coal ash and combustible recovery were 9.67 % and 45.63 %,respectively,both of which were in agreement with the verification results.This dissertation contains 86 figures,15 tables and 213 references.