Study On Interaction Mechanism Between Lignite And Collector Based On Molecular Simulation

With the abundant reserves,lignite slime is basically not beneficiated,but is directly incorporated into the gravity cleaned product for power generation.This practice not only wastes the resources,but also brings about serious environmental pollution.Due to the poor floatability,conventional flotation reagent kerosene makes a difficulty in separating fine-grained lignite and gangue with high efficiency.Currently,surfactants are often used to improve the hydrophobicity of lignite surface.However,among a wide range of surfactants,it is significant to explore the adsorption mechanisum between the surfactants and lignite surface.This paper explored the interfacial interaction between Inner Mongolia lignite surface and collectors in multiscales.In microcosmic,the interaction was studied between polar groups of surfactant and the oxygen functional groups on the surface of the lignite.In mesoscopic,based on van Oss-Chaudhury-Good theory,the adsorption force between the lignite and nonionic surfactants was analyzed from the perspective of the interfacial free energy.In macroscopic,the adsorption configurations of the nonionic surfactants on the lignite were constructed by molecular mechanics and molecular dynamics.First,the physicochemical properties of the lignite components with different densities were explored and the surface properties that affected the adsorption of the reagents were analyzed.On the surface of the lower density lignite?-1.45 g/cm³?,the O element is mainly in the terms of organic oxygen functional groups,such as OH,O-C and O=C.The intermediate density lignite?1.45-1.80 g/cm³?is mainly consisted of Si-O,O=C and COOR and the high density?+1.80 g/cm³?is mainly inorganic O-H and Si-O.There is little difference in the surface potential with pH values of the different density lignites,so the organic oxygen functional groups have similar influence on the electrokinetic potential with the inorganic functional groups.The nonpolar component of surface free energy of the lower density lignite is significantly larger than that of the higher density products.However,the Lewis surface free energy,g_S^-,is obviously higher in the higher density lignite than that of the lower density ones.The hydrophilicity index,???+???,increases slightly in the higher density than in the lower density products.The molecular structure properties of the ultra low-ash lignite were analyzed by X-ray diffraction?XRD?,high resolution transmission electron microscopy?HRTEM?,X-ray photoelectron spectroscopy?XPS?,Fourier transform infrared spectroscopy?FTIR?and ¹³C nuclear magnetic resonance(¹³C NMR).The aromatic structures are mainly 2×2 and 3×3 small ring-shaped from HRTEM.The content of oxygen was37.39%on the surface of ultra low-ash lignite,with the contents of 14.23%,6.34%,10.07%and 6.75%for O-H,C-O,C=O and COOR,respectively.On the basis of synthesis of molecular structure parameters of the lignite,a molecular structure model of the lignite was developed with the chemical formula C₁₆₆H₁₃₀O₄₉.It is verified that the lignite molecular structure model is representative by comparing the infrared spectrum of the ultra-low ash lignite and the theoretically calculated group vibrational spectrum of the lignite molecular structure.It is the foundation for subsequent molecular simulation and calculation.XPS was used to analyze the adsorption performance of cationic dodecylamine?DDA?and non-ionic surfactant sorbitol oleate?span 80?on the lignite surface.The amine peak on the surface of different density lignite particles increased significantly after DDA adsorption which showed that the dodecylamine molecules were adsorbed on the negatively charged surfaces of the lignite samples.Span 80 has a selective adsorption on the lower density lignite surface.The surface oxygen functional groups O=C and OH can form hydrogen bonds with the polar groups-OH and-COOR in the span 80 molecules.But the epoxy ether bonds are difficult to form hydrogen bonds with the oxygen functional groups on the lignite surface.According to quantum chemical calculations,the order of the energy released from the hydrogen bonds by the oxygen functional groups on the lignite surface and water molecule is:-COOH?-OH?-O-?C=O.Furthermore,the interaction was calculated between the polar groups of the three nonionic surfactants with different non-polar groups and oxygen functional groups on the lignite surface.The hydrogen bonding energy between the polar group and the oxygen functional group hydroxyl is the strongest,followed by methoxyl,carbonyl and carboxyl.By comparing the calculation results of the interaction between the polar groups of span 20,span 60,and span 80 and the oxygen functional groups,the increase in carbon chain length enhances the interaction between the polar groups of span 60 and the oxygen functional groups on the lignite surface.In addition,the unsaturated double bonds in the carbon chain are beneficial for the interaction between the polar groups of the span 80 and hydroxyl groups on the lignite surface.The adsorption energy between miscible reagents of the different ratios of surfactant to kerosene,and lignite surface was calculated from the perspective of interface free energy.The interfacial free energy between the reagent and water is calculated according to van Oss and Good interfacial free energy equations.The polar interaction between kerosene and water is weak,which is consistent with the fact that kerosene is difficult to disperse in water.The span series surfactants have a larger interaction with water molecules because they contain polar groups and can form hydrogen bonds with water molecules.With the increase of the surfactant concentration in the surfactant-kerosene solution,the the interaction between the mixed solution and the lignite becomes weaker so the surfactant concentration should be controled.In the emulsion experiments,the contents of small size emulsion drop were higher in span 80-kerosene-water emulsion.Besides,it can adapt to the change of the proportion of water phase without emulsification effecicency changed.The interface interaction between the lignite and different molecules was studied based on the molecular mechanics and the molecular dynamics simulations.The strongest one was electrostatic interaction in the formation mechanism of hydration film on the lignite surface with the value-22.46 kcal/mol,which was far greater than van der Waals's and the long-range force.Dodecane adsorption simulation results show that the main energy is from van der Waals forces.The adsorption of span 80 on the lignite is the strongest,followed by span 60.The main interactions are van der Waals and electrostatic interaction,indicating that the surfactant has strong adsorption force on the lignite surface with polar and non-polar groups in the surfactant molecules.The increase length in a certain range,and containing unsaturated double bonds in the non-polar carbon chain is favorable for the adsorption of surfactants on the lignite surface.After the adsorption of span 80,the lignite surface has the weakest interaction with the water molecules,so span 80 acts as a barrier to water molecules near the lignite surface.The lignite flotation results also verified that the addition of non-ionic surfactant molecules can promote the flotation of the lignite with the highest efficiency of span 80.The molecular simulation method can be used to theoretically calculate the adsorption force between the reagents and the lignite so that it can predict the flotation efficiency.