| China is rich in low rank coal resources,with proven reserves accounting for about 55% of the coal resources and output accounting for about 40% of the total coal output,which is an important part of the production and supply of coal energy in our country.The surface of low rank coal is of many oxygen-containing functional groups and poor hydrophobicity,which hinder the adsorption,spreading and effective adhesion of traditional hydrocarbon oil collectors on the coal surface.Low flotation efficiency,large consumption of collector and high cost are the main problems that restrict the efficient utilization of low rank coal.In view of this,this paper puts forward using the fatty acid-hydrocarbon oil to strengthen the low rank coal flotation and takes Shendong long flame coal as study subject.The behavior characteristics of fatty acid-hydrocarbon oil in the gas-solid interface,liquid-liquid interface,liquid-solid interface were proved and theoretical system of fatty acid-hydrocarbon oil synergistic reinforcement of low rank coal flotation was built via studying on dispersion property of reagents,spreading and adsorption of reagents on the coal surface,and the influence of reagents on bubble-coal particle interaction,which have provided theoretical and technical support for efficient flotation and utilization of low rank coal.The surface characteristics of low rank coal were firstly studied.The results showed that the C content and O content of Da Liuta slack coal were 75.5% and 15.35% respectively,and the fine grain content was higher.The yield of-0.074 mm was 66.74% with 36.86% ash content.With high intermediate density grade content,organic combustible materials were closely bound with gangue,which made it difficulty to separation.Gangue minerals were mainly quartz,kaolinite and calcite.The coal surface contained a variety of oxygen-containing functional groups,which mainly existed in the form of C-O with few C=O and O=C-O.Coal surface was of a large number of cracks,fine particles embedded in it and large surface area,small pore size and more porosity.N-dodecane and butyric acid,hexanoic acid,octanoic acid,oleic acid and linoleic acid are typical representatives of hydrocarbon oil and fatty acid,respectively.The dispersion characteristics of fatty acid-hydrocarbon oil in solution and its interaction with coal particles were studied.The presence of fatty acid not only reduced the oil-water interfacial tension,but also shortened the time for the oil-water interfacial tension to reach equilibrium.The dispersion characteristics were analyzed by focusing beam reflectometer in situ.It was found that he average chord length of oleic acid-dedocane was minmum at 13.81 ?m,and the maximum number of microdroplet was 894.07.The presence of fatty acid can shorten the induction time of reagent droplet-coal particle,and the longer the chain length of fatty acid,the shorter the induction time.EDLVO theory was used to calculate the interaction energy between the droplets and coal particles.The results indicated that there was no barrier between microdroplets and coal particles.Within long range,the kinetic energy obtained in the pulp was the main energy source of the collision.Within short range,the hydrogen bonds between the fatty acid molecules and the surface of the coal particles made them stronger and tighter together.The spreading behavior and driving mechanism of fatty acid-hydrocarbon oil on coal particle surface were revealed.After the action of fatty acid-hydrocarbon oil,the hydrophobicity of the coal surface was increased,the hydration film was thinned,and the possibility of agglomeration between coal particles was increased.Fatty acid-hydrocarbon oil can reduce the free energy of droplet spreading at the soild-water interface,promote the spreading,increase the spreading driving force of microdroplet and decrease equilibrium contact angle,but at the same time it reduced the molecular fluxion in the three-phase contact line area with increasing the spreading driving resistance and reducing the spreading speed of microdroplet.Molecular dynamics(MK)model can well fit the relationship between the movement velocity of contact line and the contact angle in the process of microdroplet spreading.The microdroplet spreading speed,molecular substitution length and molecular exchange frequency of fatty acid-hydrocarbon oil were lower than dedocane.The adsorption thermodynamic and kinetic properties of fatty acid-hydrocarbon oil molecules were found out.The adsorption of dodecane and fatty acid-hydrocarbon oil on the coal surfac belonged to heterogeneous multilayer adsorption,and that of butyric acid,hexanoic acid and octanoic acid were categorized as monolayer adsorption.Neither of the above two partterns was applicable to oleic acid and linoleic acid.?G of reagent adsorption were-20~0kJ/mol,which belonged to the typical physical adsorption.The absolute values of wetting heat of fatty acid-hydrocarbon oil were higher than single reagent,and that of oleic acid was a maximum of 207.174 g/J.The adsorption process accorded with the pseudo-second order kinetic rate model.The kinetic process of fatty acid-hydrocarbon oil was slow,and the force between reagent molecules and coal surface was strong.A two-scale mechanism of interaction between fatty acid-hydrocarbon oil and low rank coal surface was constructed.In the mesoscopic scale,the fatty acid in the microdroplet interacted with polar sites of coal surface and sealed the water molecules in the pores,which promoted dodecane to spread and form a continuous oil film.In microscopic molecular scale,fatty acid and dedocane molecules respectively interacted with hydrophilic sites of coal and hydrophobic sites of coal,meanwhile hydrocarbon chain of two molecules formed a relatively stable molecular layer on coal surface by van der Waals force and hydrophobic force,with strong intermolecular interactions formed by the additivity effect and synergic effect of various parts,which led to the final assembly of the two on the low rank coal surface into a kind of “supramolecular structure”.The influence mechanism of fatty acid-hydrocarbon oil on the adhesion process of bubble-low rank coal was investigated.EDLVO theory and Stefan-Reynolds model were used to study the wetting film evolution mechanism between bubble and low rank coal.The results indicated that there was an exponential relationship between the hydrophobic constant and the induction time,the calculated hydrophobic constant of fatty acid-dodecane was larger than that of single reagent,and the hydrophobic constant of oleic acid-dodecane was the largest.The increase of hydrophobic constant can enhance the hydrophobic force of bubble-coal particle,reduce the potential energy of bubble-coal particle,increase the critical hydration film thickness,accelerate the wetting film thinning,shorten induction time of bubble-coal particle.After treated by reagents,the process of collision and adhesion of rising bubbles on the coal surface was accelerated,the length of contact line was increased,and the effect of fatty acid-dodecane was better than that of single reagent.The relationship between the spreading rate and the contact angle of three-phase wetting periphery met MK model,the three-phase wetting peripheral spreading rate of oleic acid-dedocane was the fastest,and the adhesion of bubble-particle was most likely to occur.The mechanical properties and energy characteristics of bubble detaching from coal surface were studied by microforce balance.After treated by reagents,the test detachment force of bubble-coal surface increased,the calculation model of detachment force was established,which can fit the test values well.The activation energy,detachment work and total detachment energy of fatty acid-dodecane increased significantly and were larger than those of fatty acid or dodecane.The synergistic effect between fatty acids and hydrocarbon oils increased the stability and strength of bubble-coal surface adhesion.Finally,based on the above theoretical results,the experimental study on flotation of low rank coal with fatty acid-hydrocarbon oil was carried out.The results showed that fatty acid-dodecane had a better collecting capability than fatty acid or dodecane.With the increase of the carbon chain of fatty acid,the collecting capability was enhanced and the selectivity was reduced.The flotation effect of adding mixed fatty acid and was better than that of adding fatty acid and dodecane one by one,and the optimal proportion of fatty acid in fatty acid-hydrocarbon oil was 20%.When the dasage was 3 kg/t,the flotation performance of oleic-dedocane was the best with 89.41% combustible recovery,which were 25.77% and 48.86% higher than those of oleic acid or dodecane.This paper contains 116 figures,31 tables and 232 pieces of references. |
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