Bubble Behavior And Its Effect On Separation Of Fine Coal In An Electric Field Fluidized Bed

As a dry sorting method of fine particles,triboelectric separation utilizes the difference in electrical properties between materials to achieve sorting.Fluidization triboelectric separation is a novel method of electric separation,which combines the fluidized frictional charge and electrostatic separation by an electric field fluidized bed to achieve efficient separation of minerals.In order to explore the role of bubbles in the fluidization triboelectric separation process,this paper uses coal and its associated minerals as research objects,and the fluidization characteristics of a self-made twodimensional electric field fluidized bed test system are studied;combined with highspeed camera and image processing technology,the bubble characteristics in electric field fluidized bed are analyzed,and the regulation of voltage on bubble behavior is clarified;through the separation experiment,the influence of operation factors on sorting effect is revealed,and the role of bubble in the separation process is discussed.The main conclusions of the paper are as follows:Firstly,the study on the fluidization characteristics of electric field fluidized bed shows that the applied voltage has a significant effect on its fluidization characteristics.Compared with the conventional fluidized bed,the bed pressure drop of the electric field fluidized bed is smaller,but the bed expansion ratio increases.The quadratic polynomial model of voltage and minimum fluidized gas velocity is fitted,and it is found that the minimum fluidized gas velocity is negatively correlated with voltage.For the bed density,it decreases with the increase of voltage.Specifically: in the bed height direction,the bed density shows the smallest at the top of the bed,and the middle part is slightly larger than the bottom of the bed;while bed density varies in the bed width direction.In general,the bed density changes regularly after the application of voltage,which shows high in the middle and low at both ends.The results show that the bed density is not only affected by the bed side wall,but also by the applied voltage,which is due to its influence on the movement of charged particles and bubbles.Based on the research of fluidization characteristics,the bubble characteristics in the electric field fluidized bed are analyzed by combining high speed camera and image analysis technology.The bubble is distributed in a single-peak curve along the bed height direction,and the area with the highest frequency of bubble occurrence(corresponding to the bed height)is the lower middle part of the bed.The position of the peak increases gradually from H=10 mm to H=60 mm with the increase of quartz mass fraction or the decrease of fluidization number.In the bed width direction,as the voltage increases,the distribution of bubble number gradually changed from more in the middle to more at both ends.The bubble diameter increases linearly from bottom to top along the bed height direction,then slowly increases to a maximum of about 10 mm,and finally decreases to about 7 mm due to bubble collapse.The tracking of the bubble rising trajectory shows that as the voltage increases,the slanting trajectory gradually increases during the bubble motion,while the bubble velocity gradually decreased from 0.37 m/s to 0.22 m/s.Comprehensive analysis of the bubble growth process,the bubble shape is approximately circular at the bottom of the bed,then flat at the middle of the bed,and finally reached the maximum flatness near the top of the bed.The increase of the fluidizing gas velocity will reduce the bubble aspect ratio,while the increase of applied voltage will increase the bubble aspect ratio and make the bubble flatter.The bubble coalescence and collapse behavior indicates that the bubble carries a large amount of particles and affects the movement of surrounding particles during the rising process,and the bubble will eventually reach the top of the bed and break away.At this time,the particles in the bubble are also fall on the surface of the bed,resulting redistribution of particles in the bed.The fluidization triboelectric separation experiment was carried out with actual coal samples,and the effects of fluidization number,sorting voltage and static bed height on product ash and combustible material recovery are investigated.The test results show that when the fluidization number is 1.6,the separation voltage is 40 kV,and the static bed height is 6 cm,the negative product ash content obtained in the test is 13.86%,the positive product it is 23.69%,and the combustible material recovery is up to 33.59%.When taking a higher level of fluidization number and sorting voltage and a lower level of static bed height,the difference in ash content between the positive and negative products is more obvious,and the separation effect is better.Combined with the influence of operating factors on bubble characteristics and separation effect,the role of bubbles in the fluidization triboelectric separation process is discussed.It is found that when increasing the voltage and the fluidization number,the bubble distribution and the trajectory graph indicate that more bubbles move toward both ends,and the sorting effect is also improved,which indicates that the changes of bubble motion and sorting effect are consistent.The image shows that the bubble contains a large amount of charged particles.Due to the lateral electric field force,the movement of bubbles and particles will shift to the electrode plate.In addition,it is noticed that the particles brought to the top by the bubble are easier to move towards the plate due to the reduction of surrounding restraint.The experimental results also show that the separation effect of upper materials is better,which proves that the bubble is beneficial to fluidization triboelectric separation.The paper has 57 pictures,11 tables,and 94 references.