The Mechanism Of The Stable Acoustic Aggregation Of Hydrophobic Particles In Ultrasonics Standing Waves Field

The slow rate and poor selectivity have always been a common problem for the flotation of fine or ultra-fine particles.As for this problem,flotation is often improved by selective flocculation or aggregation.Flocculation or aggregation is often achieved by means of hydrodynamic cavitation,such as stirring.However,there is few literature researched on flocculation or aggegation of particles by ultrasound.In this paper,the aggregation behavior of particles in the ultrasonic standing wave(USW)field was studied.The principle for the selective acoustic aggregation of hydrophobic particles was explored,and the formation of cavtation bubbles in USW field was revealed.This thesis has important theoretical significance and potential application value in the fields of ultrasound and flotation.The main results are as followed:Firstly,the behavior of particles in the USW field was explored through a USW system.The results show that the acoustic aggregation by USW was only achieved by the hydrophobic particles,and aggregates were very stable and strong.However,there is no aggregation effect on the hydrophilic particles.Particels required enough sound intensity to aggregate,and the aggregation effect was improved with the sound intensity within a certain range.When the sound intensity is high enough,the aggregation effect will not change significantly.Furthermore,the ultrasonic frequency is also crucial for the acoustic aggregation.Hydrophobic particles could hardly aggregate under 50 kHz USW,and the aggregation effect was obvious by 600 kHz USW.For the-45 ?m coal particle suspension,large quatities of aggregation were formed under 600 kHz USW instead of those at 50 kHz and 200 kHz USW.Secondly,the high-intensity focused ultrasound(HIFU)was seted up to measure the cavitation threshold of the particle suspension.Also,the size and distribution of gas nuclei in water or suspension was detected by the HIFU system.The cavitation threshold results indicate the difference in gas nuclei distribution between hydrophobic and hydrophilic particle suspensions,meaning that the gas nuclei only appeared in the hydrophobic particle suspension.Furthemore,the result indicates that the gas nuclei trapped on hydrophobic particle surface is the main reason for the selective acoustic aggregation.The number of gas nuclei increases with the surface roughness and concentration of the hydrophobic particles,which results in low cavitation threhoslds.The cavitation threshold is determined by the Blake threshold when the size of nuclei is much smaller than the resonance radius.According to the analysis on the Blake threshold,the cavitation threshold is reduced with the nuclei size,and the large-size nuclei is easy to be cavitated.Thirdly,the mechanism of the formation of cavitation bubbles during the acoustic aggregation process was researched.The results indicate that the acoustic aggregation belongs to different type under 50 kHz and 200 kHz USW.In 50 kHz USW field,the acoustic aggregation is named transient acoustic aggregation.Particles gathered into a ball at the point-shape antinodes under 50 kHz ultrasound.The aggregates were unstable and they will break immediately when the ultrasound was turned off.However,the acoustic aggregation formed at 200 kHz USW was called stable acoustic aggregation,which was totally different with that at 50 kHz.The formation of aggregate was independent with the shape of the sound field.The aggregates were stable and they could maintain their shape when the ultrasound was turned off.Additionally,the acoustic aggregations with different typies were determined by the rectified diffusion threshold,Blake threshold,and the transient cavitation threshold.Finally,the force and the movement behavior of particles and bubbles in USW field were analyzed.The results show that the acoustic radiation force on particles is very small,meaning that the acoustic effect on particle could be ignored.On the contrary,the acoustic radiation force on bubbles are strong.This force is improved when the bubble size is closed to the resonance radius of the sound field.During the acoustic aggregation process,stable cavitation occurs on the particles surface,and the bubbles and particles were gathered by the primary Bjerknes force.Then,the bubbleparticle aggregates were formed due to the attraction between cavitation bubbles,which called secondary Bjerknes force.According to the acoustic radiation force in the USW field,a novel dual bubbles flotation system in USW field was proposed.In this system,the efficiency of the particle-bubble interaction is significantly improved as compared to the conventional flotation in the gravity field.The stable cavitation bubbles were trapped on particles surface,and then the particles were attracted by the large flotation bubbles,which do not require the collision and adhesion process.The flotation efficiency was greatly improved by the dual bubble flotation system in USW field.In this study,the recovery by the USW flotation increased by a dozen or even dozens of points as compared to the conventional flotation.The USW is considered as a potential method to improve the flotation efficiency in the further.This paper contains 109 pictures,8 tables,and 156 pieces of references.