Study On The Enhancement Of Low-rank Coal Floatability By Surfactant Based On Molecular Simulation

As a valuable resource,it is of consideration important to realize the efficient and low-pollution utilization of fine low rank coal?LRC?particles.However,duo to the presence of lager number of polar oxygen-containing groups in LRC and the poor hydrophobicity of LRC,it is difficult to successful separate clean coal from gangue based on the wettability difference between organic and mineral matter.Using molecular simulation as a main method,the present study investigated the properties of interfacial water on LRC surface and it was found that the water molecule layer at the LRC surface is incompleteness.Meanwhile,for low-rank coal with a high degree of metamorphism,it is proposed to strengthen the spreading of nonpolar oil collector on the hydrophobic area of LRC to promote the flotation performance,and the microscopic mechanism of the effect of surfactant structure in the formation of oil-in-water emulsion droplet was explored.Besides,for low-rank coal with a high degree of metamorphism,it is proposed to modify the surface wettability of LRC surface by surfactant adsorption to promote the flotation performance,and the effects of the polar group type of surfactant,the number of EO[abbreviation for-?CH₂CH₂O?-group]units and the benzene ring structure in hydrophobic chain on the LRC flotation were investigated.In addition,kaolinite,the most common clay mineral in LRC,is taken as the research object.The agglomeration behavior between kaolinite and LRC fine particles in pulp as well as the influence of kaolinite on LRC flotation were studied.At last,based on the difference in adsorption strength of surfactants on the organic matter and minerals in LRC,it is proposed that the surfactant adsorbed on the surface of minerals in low rank coal can be selectively desorbed by ultrasonic pretreatment while the surfactant adsorbed on the organic could be kept as much as possible,thus further increasing the wettability difference of organic and mineral surface to improve the flotation performance of LRC.The main conclusions of the paper are as follows:The radial distribution function?RDF?results demonstrate that the structure of interfacial water at the LRC surface is substantially similar to that in bulk water and the order degree of the water molecules decreased with increasing distance from the LRC surface.The molecular configurations of water near the carboxyl,hydroxyl,ether and carbon chain are oriented to each group with hydrogen.It was found that the water molecule has strong adsorption positioning ability and it is preferentially fixed around the polar functional groups at LRC surface.Water layer at LRC surface is not complete but gathers around the polar oxygen-containing groups.The adsorption of surfactant could reduce the difference between interfacial water and bulk water significantly.Nonionic surfactant tetraethylene glycol monododecyl ether(C₁₂EO₄)is better than anionic sodium dodecyl sulfonate?SDS?in promoting the dispersion of dodecane in water.Compared with the dodecane-SDS-water system,emulsion droplet formed in dodecane-C₁₂EO₄-water system could stable for a longer time.The result from DFT?density functional theory?showed a larger interaction between SDS and the water molecules than that between C₁₂EO₄ and water molecules.The MD?molecular dynamics?simulation results suggested that the SDS headgroup exhibited a loose arrangement and a relatively large gap size at the dodecane/water interface.In contrast,the headgroups of C₁₂EO₄ were bent and interwoven with others to form a tight reticulation at the interface.Both the interfacial thickness and absolute value of interfacial formation energy in dodecane-C₁₂EO₄-water system are larger than these in dodecane-SDS-water system.Therefore,it could be concluded that the ability of the surfactant to form dodecane-in-water emulsion droplets depends on the arrangement of the surfactants at the oil–water interface rather than on the interactionThough the adsorption amount of n-dodecyl?-D-maltoside?DDM?on LRC surface is bigger than that of dodecyl ethoxyl ethers(C₁₂EO₇)due to the electrostatic action and hydrogen bonding originating from the polyhydroxyl groups of DDM,DDM is not suitable for LRC flotation improvement.The results of flotation showed that all C₁₂EO_n can obtain the same maximum flotation recovery.Increasing the number of EO in C₁₂EO_n lowers the necessary amount of reagent.Greater selectivity can be achieved with increasing EO number in C₁₂EO_n.The benzene ring in nonionic surfactant could increase its adsorption strength on LRC with a high degree of metamorphism by?-?stacking interaction but this doesn't work for LRC with a low degree of metamorphism.The adsorption state of oxygen atom in polar group could not be influenced by benzene ring in nonionic surfactant.Both benzene and oxygen-containing groups of LRC model molecules could interact with kaolinite surface.In the presence of water,LRC model molecules are more inclined to adsorb on the?0 0 1?surface in comparison with the?0 0?1?surface.The van der Waals attraction between the LRC model molecules and the kaolinite?0 0?1?surface is responsible for the kaolinite coating on LRC flotation.Due to the strong adsorption between the water molecule and the kaolinite?0 0 1?surface,it should be difficult to replace the adsorbed water molecules on the?0 0 1?surface for LRC model molecules.The increase in the polarity of nonionic surfactant could increase the adsorption strength and adsorption layer compactness on kaolinite?0 0?1?surface,thereby altering the kaolinite surface wettability from hydrophilic to hydrophobic and deteriorating the LRC flotation.The microscopic adsorption mechanisms of aliphatic alcohol polyoxyethylene ether on the organic and mineral?kaolinite?were investigated through DFT calculation.The results demonstrate that the adsorption force of C₁₂EO on kaolinite is mainly weak electrostatic interaction and Van der Waals originating from C₁₂EO and kaolinite?0 0 1?surface while C₁₂EO could interact with LRC model molecules by hydrogen bond.The interaction strength between C₁₂EO and LRC model molecules is stronger than that of kaolinite.Flotation experimental results confirm that C₁₂EO could be selectivity desorbed from mineral by ultrasonic treatment and retained in organic matter,which undoubtedly further enlarges the wettability difference between organic matter and minerals after adsorption of C₁₂EO,and promotes flotation of LRC.