| China's coal industry is currently in a critical transformation period,coal workers have clearly recognized the deep processing and fine machining are the only way to keep the coal industry regain vitality.Ultra-clean coal(UCC)is a valuable research field of clean coal utilization,the ash content can be controlled within 1%,or even lower,based on the production actual needs.The ultra-low ash content characteristic make the ultra-clean coal can be used not only as energy supply,but also can be used in the preparation of super capacitor,graphite substitute product,electrode and other industry fields.At present,taking environmental protection and energy conservation into consideration,the most commonly used method to product ultra-clean coal is the hydrophobic-flocculation flotation.Now,the knowledge of floccus hydrophobic flocculation theoretical research is systematic and comprehensive,but there is a lack of study on the floccus formation mechanism in the process of ultra-clean coal preparation.Therefore,this paper was divided into four parts which were around the theories of ultra-clean coal preparation process.The paper explored the influence of initial stage coal sample dissociation on the geometrical feature,physicochemical and mechanochemistry properties of coal samples,studied the theoretical basis of hydrophobic flocculation of fine coal particles during the formation of floccus,discussed the influence of energy input control in energy field and chemical agent system on floccus quality,and at last,analyzed the interaction motions like collision and attachment between floccus and bubbles in flotation environment.The first part of the study was based on the preparation of clean coal.In order to study the mechanism of ultra-fine comminution on the quality of ultra-clean coal and select the ultra-fine crushing equipment which is most suitable for production processing,took the ultra-fine grinding processing requirements and existing ultra-fine grinding equipment into consideration,selected four kinds of ultra-fine equipment(grinding mill,colloid mill,air mill and ball mill)to ultra-fine grind coal samples.And the degree of dissociation,surface morphology,functional group content,specific surface area,pore size distribution,surface potential,etc.were studied and analyzed.Through further research and analysis,it was concluded that the colloid mill was the most suitable equipment for ultra-clean coal processing.Analysis of the coal particles geometric characteristic showed that the coal samples grinded by colloid mill had a wide particle size distribution range,coarse and fine grain contents were uniform,the ditch shape uneven surface had no obvious fracture traces and gully morphology of varying depth,with maximum average roughness 2.27 nm,indicated that the colloid mill exposed a new generation surface,more functional groups and molecular clusters,it was advantageous to the formation subsequent floccus.Analysis of coal samples interfacial chemical properties showed that the colloid grinded coal sample had the maximum solid-liquid interfacial tension,62.22mJ/m2,and had the maximum contact angle,109.70 degrees.That meant the water molecules between coal particles were more likely to be excluded,the coal particles were more easily formed floccus.The colloid grinding coal samples had the maximum wetting heat with non-polar oil,-1.384J/kg,that meant,the effect between non-polar oil and coal samples was intensively,which was in favor of the non-polar oil adsorbed and spread on the coal particles surface and strengthened the hydrophobic particles and further tightened the oil-bridge connections between the particles.Study on coal chemical surface mechanical properties showed that the coal samples crystallite parameter coal had a certain regularity changes,D(002)from small to large was as follows,colloid mill,jet mill,stirring mill and ball mill.And jet mill had larger coal core stacking height and average diameter,showed the the approximate graphite lattice characteristics,which had a higher metamorphism degree and were advantageous to subsequent floccus formation.The samples processed by colloid mill had lower Zeta potential in different pH environment and the isoelectric point was slightly higher,caused bigger inter-particle electrostatic repulsion force.Therefore,the colloid milling coal samples need more machinery in the floccus formation process to some extent.The second part of the study was aimed at the theoretical analysis about coal particle hydrophobic flocculation process,the calculation of the coal sample flocculation-rate of coal sample indicated that the basic of hydrophobic-flocculation theory was the sample hydrophobicity.However,it is believed that the coal particles hydrophobic flocculation effect can be enhanced by controlling the mechanical energy input amount and adding non-polar oil during the floccus formation.The system potential energy in the process of forming hydrophobic-flocculation floccus was quantitative calculation,it was found that the traditional theory of colloid stability theory can't reasonably explained the system potential energy change,it must add the hydrophobic potential into the calculation of system potential calculation.In order to calculate the system energy,it need to use the coal particle Debye length(?-1)constant which is used to describe the surface charge scope characterization,however,there is no definite value for the constant of the coal as a mixture.The Debye length was related to the theoretical research and data calculation about settlement of the coal slurry,coal samples selective flocculation,the influence of coal samples surface modification,the slurry stability properties and fine coal particles dispersion in the solution,etc.This paper used atomic force microscope(AFM)measurement the force distance and used backstepping method to obtain the coal sample Debye length in liquid environment according to EDLVO theory.The force distance curve reflected the change of micro force between coal samples with distance,and in order to improve accuracy of force distance curve accurately many measures were used in the experiments.Firstly,probe modification technology was used to modify the silicon nitride tip and added the coal particle on the tip.Next,the same kinds of coal sample were polished into flake as substrate.At last,the test was placed in the solution environment.In the experiment,coal samples with different metamorphic degree were selected as test samples,like Eerduosi candle coal(EE),Shenmu non caking coal(SM),Huainan bottle coal(HN)and Taixi anthracite coal(TX).The test environment was neutral,acidic and alkaline aqueous solution.Experiment results showed that the range of coal samples Debye length in different pH environment ranged from 1.12×10-9 to 9.78×10-9m,the Debye length increased with the increase of p H value.For different kind coal samples,the Debye length decreased with the increase of coal metamorphism.With the OH-ion content increased in solution,more H+ ions were attached to coal surface,which leading to increasing of formed double layer thickness.With the increasing of coal metamorphism degree,the negative charged and amount of oxygen groups and negatively charged groups on coal sample were reduced,that leading to diminishing of coal surface electric double layer thickness in the environment with same PH value.In the third part,this paper studied the mechanisms of the energy input and the effect of various additives on the hydrophobic-flocculation.In the study of mechanical energy input mechanism,the adding mechanical energy for particles was the flocculation essential condition which ensured the system potential energy cross the barrier and particles succeed formed floccus was theoretically verified by analyzed the system energy curves during hydrophobic flocculation process.Through analysis the force on coal particles in flow field,the paper theoretically confirmed that the appropriate mechanical energy input can promote the floccus formation,but the excessive mechanical energy would turn to destroy the formed floccus.The paper used mathematical dimensional analysis methods,with the stirring speed represented the amount of mechanical energy input,established the relationship between floccus morphological characteristics and mechanical energy input,analyzed the characteristics and rules of floccus morphology changes in the process of mechanical energy input according to the fractal dimension analysis method.It was found that the fractal dimension increased with the increasing of stirring speed and the peak value was 1.762 at 2000r/min,whereafter,with the stirring speed continued increasing the fractal dimension decreased rapidly and maintained at about 1.74 finally.Due to the reactivity of fractal dimension about the floccus density,even the floccus compactness,the conclusion indicated when the stirring speed is 2000r/min,the floccus had the maximum fractal dimension and suitable size with compact structure.The multi-fractal spectrum showed that between 500-2000r/min,f(?max)increased from 0.677 to 0.796,f(?min)decreased from 1.483 to 1.202,it showed that the number of small probability object(large particles)increased significantly and the number of large probability object(small particles)reduced significantly at this stage.After 2000r/min,f(?max)showed a change trend of first decreased significantly then kept stable,f(?min)showed a change trend of first increased significantly then kept steady,it indicated that during this period the large floccus disintegrated into small floccus until the formation and fragmentation of floccus reached a dynamic equilibrium.Based on the test results of mechanical energy input and ultra-clean coal separation effect,the conclusion was that,the floccus suspension system which had the larger floccus group average particle size,the more content of large floccus and the more homogeneous floccus morphology,was the most advantageous product condition to ultra-clean coal separation.Study on the effects of reagents on hydrophobic-flocculation found that,the potential energy curves between non-polar oil and coal particles had a small barrier and a large valley,non-polar oil can adsorb and spread on the coal particles surface spontaneously.At this point,the right amount of mechanical energy input can accelerate the non-polar oil and coal particles close to each other,so that the two agglutinated successfully.By comparing flocculation separation efficiency of the coal samples with different contact angle before and after added the non-polar oil,it was found that with the increasing of contact angle,the auxo-action of non-polar oil on the coal particle flocculation efficiency increased,but,after the coal surface contact angle greater than 80°,the promotion role became smaller.It indicated that,only when the mineral surface hydrophobic significant enough,non-polar oil promotion effect on mineral hydrophobic flocculation could effectively reflected,namely,the essence of the non-polar oil to promote the hydrophobic flocculation was the coal particles hydrophobic nature of the coal particles to accelerate the process of hydrophobic flocculation.In additions,the non-polar oil amount and floccus morphology was closely,the appropriate non-polar oil amount played a bridging role between the floccus and the floccus volume growth rapidly,in addition,the non-oil filled in the floccus intervals and made the floccus shape nearly spherical.The study of the effect of other additives on coal sample showed as follows:(1)The coal surface positively charged and coal samples surface potential increases gradually with the increasing pH value in strong acid solution,the dispersion of coal particles became larger.The coal samples with low surface Zeta potential were easily to form flocculation,but at the same time,the mineral parts contained high ash were also trended to flocculate,may lead to coal products yield and ash content all increased.The sorting tests indicated the ultra-clean coal yield and ash showed a regular variation with change of pH value,namely,the lower pH value the lower ash content and yield.Through the above analysis,the appropriate increase of the slurry pH value of could achieve the effect of reducing ash to a certain extent,but also caused the wastage of coal yield.(2)The use of dispersant,to some extent,reduced the content of Al and Si in coal samples,which indicated that the addition of dispersant could effectively remove some clay minerals and improved the floccus selectivity.(3)The influence on weight percentage of Al,Si and S elements showed only when the consumption of consumption was at a higher level.The sorting texts results showed that there was no significant change in the yield and ash content of ultra clean coal.It was found that the addition of inhibitor had little effect on the improvement of the quality of coal preparation.Taken the amount of mechanical energy input and dosage as factors,taken ultra-clean coal yield and ash content as response values,the paper used three kinds of coal with different rank to design response surface analysis for ultra-clean coal yield and ash coal based on BBD experimental design method.The results showed that,the significantly arranges as follows: the dosage>stirring time>stirring speed.The interaction of mixing time and dosage played a dominant role on yield and the interaction of mixing speed and dosage played a dominant role on yield content.The fourth part researched the contact progress between bubble and floccus by establishing an image capture and analysis system and calculating the collision and attachment probability based on statistics theory.The results showed that when the floccus size diameter less than 200?m,the adsorption efficiency increased with the particle size increasing,while the floccus size diameter greater than 200?m,the adsorption efficiency decreased with the particle size increasing.There was a correlation between the floccus density and size,when the diameter ranged from 10?m to 170?m,the floccus density increased with the particle size increasing,growth also increased significantly,while the floccus diameter greater than 170?m,with the floccus diameter increasing the floccus density began to decline.Floccus density decreased with the particle size increasing around 170?m,and the adsorption efficiency between floccus and bubbles decreased with the particle size increasing around 200?m.The reasons of the distinguish of the turning point the promotion effect of floccus size diameter increasing on adsorption efficiency neutralized the inhibition effect of floccus density decreasing on adsorption efficiency in some extent.Therefore,the collision efficiency between floccus and bubbles was mainly determined by the floccus particle size,and the adsorption efficiency between floccus and bubbles was the result of the floccus size and density interaction effect,meanwhile,the floccus density played a leading role.The results of probability statistical data analysis showed that,the efficiency and the probability of collision between the bubble and floccus had the same trend,both increased with the floccus size increasing and the increase trend was gradually slow down,the adsorption efficiency and the probability of trend growth but there are less turning point start small.The efficiency and the probability of attachment between the bubble and floccus increased with the floccus size increasing,but both had a turning point where the probabilities became reduced.The reason for this phenomenon was that with the floccus diameter increasing,the collision area increased,showed as the increasing of collision times.For larger floccus,the inertial force were larger,were less susceptible to be influenced by the flow and prone to contact with bubbles.In addition,for the floccus fallen down,the main force that promote contact with the bubble was inertia,and the inertial force depended on the floccus size,when the particle size was same,the lager density the large inertia force,that lead to smaller hydraulic resistance.The research showed that the floccus density showed a trend of first increased and after decreased with the particle size increasing.Therefore,there was a certain floccus granularity acted as the turning point where the collision probability and collision efficiency increased slowly down,the attachment probability and attachment efficiency began turning to decrease. |
PDF Full Text Request