| Lignite,long flame coal,non caking coal and weak claking coal belong to low rank coal,the metamorphism of which is relatively low.Low rank coal is abundant in our country,and its total reserve is about 50%.Generally,the surface of low rank coal is rough,porous and fissured,the oxygen content on coal surface is high,and the oxygen-containing group associates with water molecules to form hydrogen bond,which increase the hydrophilicity of coal sample surface.The floatation characteristics of low rank coal directly affect the high efficiency recovery and quality of low rank coal and increase the difficulty of coal preparation plant.However,traditional flotation reagents such as hydrocarbon oil collectors in China are difficult to spread on the surface of low rank coal and can not promote the adhesion behavior between low rank coal particles and bubbles.In addition,the conventional flotation separation method has weak recovery effect and adaptability to low rank coal recovery.Aiming at the above problems,two typical low rank coals,Shengli No.6 lignite and Zhuanlongwan long flame coal,were selected as experimental samples.The adhesion behavior of low rank coal and the enhancement of low rank coal-reactive oily bubble flotation process were systematically studied.The following results were obtained:The variable characteristics of low rank coal floatability were determined.The composition of coal sample was analyzed by sieve test,float and sink test,XRD test and coal rock composition test.The surface chemical structure and surface morphology were studied by XPS,FTIR,SEM and automatic surface and pore size analyzer.The surface wettability was studied by contact angle and Zeta potential measurement,and the free energy of coal surface was calculated based on Van Oss-Chaudhury-Good theory.The results show that the surface of low rank coal is very rough,and there are cracks and depressions and a large number of fine mineral particles attached to it.These hydrophilic minerals can improve the hydrophilicity of coal.The surface of low rank coal has a lot of oxygen-containing functional groups,which can easily form hydrogen bonds with water molecules and inhibit the adsorption of conventional oil collectors,and the surface of low rank coal is polar,and the high surface energy determines the easy adsorption of polar water molecules by low rank coal.The collision and adhesion process between bubbles and low rank coal surface before and after modification was analyzed.A bubble-particle collision and adhesion system was established.The microscopic behavior of bubble-particle collision and adhesion was observed and the velocity variation of bubble-particle collision process was analyzed.The results show that the hydrophobic modification of low rank coal surface by collector can reduce the number of bouncing of bubbles on the coal sample surface,thus contribute to the formation of gas-liquild-solid three-phase contact periphery,and enhance the low rank coal flotation from the viewpoint of micromineralization dynamics.The percolation behavior of particles and the effect of foam stability during foam decay were studied.The percolation behavior of long flame coal particles was investigated by particle percolation test,and the change characteristics of foam stability were analyzed using three-phase foam stability test system.The results show that the fine coal particles of long-flame coal mainly exist in the form of adhesion to the bubble surface.The better the hydrophobicity of the coal particles,the higher the adhesion probability and the degree of adhension to the bubble surface,and the gangue mineral particles and moisture coexist in the foam structure.No matter what kind of particle system,the foaming ability and foam stability of the system increase with the decrease of particle size,which indicates that small particles are more likely to play the role of foam stabilization.The mechanism of bubble-particle adhesion of low rank coal was analyzed.With the help of bubble-particle induction time,the mechanism of low rank coal bubbleparticle adhesion under different particle size,bubble compression deformation,bubble approaching velocity and so on has been studied.By changing the surface roughness of coal sample and soaking coal sample in acid or alkali solution,the bubble-particle adhesion of low rank coal was analyzed,and the change of surface wettability of coal sample after modification was characterized by contact angle test.Moreover,the floatability of the modified coal surface was verified by flotation test.The results show that when the particle size of coal sample is 0.125-0.074 mm,the bubble-particle adheres easily.Compared with the large bubble,the small bubble is more helpful to the bubble-particle adhesion.Additionally,bubble approaching velocity and bubble deformation have significant effects on bubble-particle collision and adhesion process.The rougher the surface and the worse the hydrophobicity,the longer the time required for bubble-particle attachment,which is not conducive to the low rank coal floatation.In addition,the floatability of coal samples can be improved to a certain extent by the treatment of appropriate concentration of acid solution or alkali solution,and the combustible material recovery can be increased.The characteristics of oil bubble flotation process were studied from the point view of oil bubble characteristics,kinetics and thermodynamics.The characteristics of bubble-low rank coal flotation and the interaction force between low rank coal and oil bubble was discussed.The maximum foam height and half-life of two-phase and threephase systems were compared and verified by flotation test.The greater the energy barrier between fine coal particle and oil bubble,the greater the difficulty of attachment and mineralization,and the greater the energy required by the attachment process between coal particle and oil bubble.Tthe foam stability is stronger after a layer of oil film coated on the foam surface,which is beneficial to concentrate enrichment.A method of low rank coal flotation with active oil bubble was proposed.The mechanism of low rank coal flotation strengthened by active oil bubble was studied,and the experiments of low rank oil bubble flotation were carried out.The results showed that the modified oil bubble has faster attachment speed and higher attachment force than the ordinary oil bubble,and the modifier molecules interact with the polar groups on the surface of low rank coal to further improve the collecting ability of the modified oil bubble.Through flotation machine and flotation column test,it can be found that conventional flotation process is difficult to adapt to low rank coal flotation.The experiments on oily-bubble column flotation of low rank coal samples showed that the introduction of oily-bubble can significantly reduce the amount of collector used in low rank coal flotation.The clean coal with low ash content of 11.16% and 78.73% of combustible matter recovery rate can be obtained by the flotation column.This paper provides technical basis and reference for large-scale flotation of low rank coal in China.The thesis contains 79 figures,35 forms and 271 pieces of references. |
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