Interfacial Interaction Mechanism Between Hydrocarbon Oil-carboxylic Acid Mixed Collector And Low-rank Coal

China’s low-rank coal resources are abundant and are an important part of coal production and supply.Flotation is an effective way to upgrade and utilize low-rank coal.However,the surface of low-rank coal is of many oxygen-containing functional groups and poor hydrophobicity,resulting in low flotation efficiency and high chemical consumption of traditional hydrocarbon oil collectors.Many scholars have found that a series of hydrocarbon oil-carboxylic acid mixed collectors can improve the flotation efficiency of low-rank coal to varying degrees,but the effects are different,and the structure-activity relationship of different carboxylic acids in the mixed collectors is the main reason for the difference.Moreover,the non-covalent bond interaction between reagent molecules and low-rank coal/water molecules affects their assembly structure at the coal/water interface and then relates to the interface adsorption behavior,which ultimately determines the efficiency of the low-rank coal flotation process.At present,the research on the microscopic action mechanism of hydrocarbon oil-carboxylic acid mixed collectors is gradually studied,but its assembly mechanism and structure at the interface of low-rank coal/water are still unclear.The multi-scale studies on the interactions between different kinds of hydrocarbon oil-carboxylic acid mixed collectors and low-rank coal under liquid phase conditions from quantum chemistry,all-atom molecules and coarse-grained molecules are helpful to clarify the structure-activity relationship of different kinds of carboxylic acids in the mixed collectors,to reveal the adsorption mechanism and assembly mechanism of hydrocarbon oil-carboxylic acid mixed collectors at the low-rank coal/water interface,and to construct the theory of multi-scale interface interaction between oil-carboxylic acid mixed collectors and low-rank coal.They provide a theoretical basis for the screening design of mixed collectors and the flotation enhancement of low-rank coal.In this thesis,Shendong low-rank coal was taken as the research object,dodecane(D)was used as hydrocarbon oil,and different carboxylic acids(valeric acid,decanoic acid,myristic acid,undecylenic acid,phenylpropionic acid,2-butyl octanoic acid,and sebacic acid)were respectively composed of hydrocarbon oil-carboxylic acid mixed collector(named D-carboxylic acid,for example,D-decanoic acid),the multi-scale interaction mechanism of hydrocarbon oil-carboxylic acid mixed collector at the low-rank coal/water interface was studied by using the method of simulation calculation and laboratory experiment.The effects of carboxylic acid carbon chain length,unsaturated bond type,carboxylic group position,and number of carboxylic groups were investigated.First,the interactions of carboxylic acids with low-rank coal/water were investigated by quantum chemical calculations.With the increase of carbon chain length,the interaction between carboxylic acid and low-rank coal/water also increased,but regardless of the carbon chain length,carboxylic acid molecules preferentially interacted with the sites with carboxyl groups on the low-rank coal surface.The carboxylic acid molecules had similar interactions with sites with hydroxyl groups on the low-rank coal surface or water molecules,and there may be competitive adsorption.The interaction between carboxylic acid with unsaturated carbon chain and low-rank coal/water was stronger than that of carboxylic acid without unsaturated carbon chain,and it strengthened the interaction with water molecules,even phenylpropionic acid preferentially interacted with water molecules,which was unfavorable for the mixed collector on the low-rank coal flotation.The position of the carboxyl group had little effect on the interaction strength of carboxylic acid and low-rank coal/water,and regardless of the position of the carboxyl group,the interaction between carboxylic acid and low-rank coal was stronger than their respective interactions with water molecules.The interaction between sebacic acid with two carboxyl groups and low-rank coal/water was very strong,which was bound to cause strong competition in the liquid phase environment,and may negatively affect the adsorption of D-sebacic acid mixed collector on the low-rank coal surface.Secondly,the adsorption behavior and strength of hydrocarbon oil-carboxylic acid mixed collector at the low-rank coal/water interface were investigated at the molecular scale through all-atomic molecular dynamics simulation,atomic force microscopy,and X-ray photoelectron spectroscopy.In the liquid phase environment,the carboxylic acid with moderate carbon chain length was helpful to improve the diffusivity of each molecular component of the mixed collector,and the corresponding mixed collector had a higher adsorption strength on the low-rank coal surface.Meanwhile,it was proved that the strong interaction between carboxylic acid with longer carbon chain length and water affected its interaction with low-rank coal.Carboxylic acids without unsaturated carbon chains helped to improve the diffusivity of each component molecular of the entire mixed collector and enhanced the adsorption capacity of the corresponding mixed collector on the low-rank coal surface.The carboxylic acid-containing carbon-carbon double bond was much stronger than the carboxylic acid-containing benzene ring.Due to its special topological molecular structure,2-butyl octanoic acid with the carboxyl position in the middle of the carbon chain made its corresponding mixed collector have a stronger three-dimensional space at the low-rank coal/water interface than D-decanoic acid with the carboxyl position at the end of the carbon chain.Most of the sebacic acids with two carboxyl groups had at least one carboxyl group facing the water phase,which was unfavorable to weaken the hydrogen bond grid between the low-rank coal/water interface and greatly reduced the adsorption of D-sebacic acid on the low-rank coal surface.On this basis,coarse-grained molecular dynamics simulation and sum-frequency vibration spectroscopy were used to reveal the molecular assembly mechanism and structure of hydrocarbon oil-carboxylic acid mixed collector at the low-rank coal/water interface.During the assembly process of the mixed collector molecules at the coal/water interface,the molecular axes of D and decanoic acid molecules with similar carbon chain lengths tended to be adsorbed in parallel on the chip surface,which was more conducive to the spreading of the reagent molecules.The strong interaction between the carboxylic acid with the unsaturated carbon chain and the D molecule affected the spreading and covering of the mixed collector on the low-rank coal surface.The first layer of adsorption of molecules in D-2-butyl octanoic acid with the carboxyl group in the middle of the carbon chain was more parallel and orderly,the carboxyl group in 2-butyl octanoic acid was more closely related to the low-rank coal surface,and the adsorption was more reliable to the surface of low-rank coal in the polar water phase.D and sebacic acid in D-sebacic acid with two carboxyl groups were more likely to form clusters or aggregates of the same kind of molecules,and more sebacic acid aggregated and tended to be adsorbed perpendicular to the surface of low-rank coal.In this way,the synergistic effect of each component in the mixed collector was not well achieved,so the overall coverage of D-sebacic acid on the hydrophilic and hydrophobic sites on the surface of low-rank coal was worse than that of D-decanoic acid.Finally,based on the multi-scale research results,the wetting behavior and flotation performance of low-rank coal with mixed collectors were studied.For hydrocarbon oil-carboxylic acid mixed collectors with different carbon chain lengths,the mixed collector with moderate carboxylic acid carbon chain length was more easily adsorbed on the surface of low-rank coal.For hydrocarbon oil-carboxylic acid mixed collectors with different unsaturated bond carbon chains,the use of unsaturated bond carbon chain mixed collector had the best flotation effect on low-rank coal,and the effect of the mixed collector with carbon-carbon double bond was better than that with a benzene ring.For hydrocarbon oil-carboxylic acid mixed collectors with different carboxylic acid positions,the mixed collector with carboxylic acid in the middle of the carbon chain had better spreading effect on the surface of low-rank coal.For hydrocarbon oil-carboxylic acid mixed collectors with different number of carboxyl groups,the mixed collector with only one carboxyl group was more favorable to remove the hydration film on the surface of low-rank coal.The thesis contains 114 figures,50 tables,and 180 pieces of references.