Effect Of Surfactant Adsorption On Wettability Of Lignite And Its Regulatory Mechanism

Lignite is one of important fossil energies in our country,its proportion in coal consumption increased year by year with the growing demand for energy to meet economic development.The characteristics of lignite,such as high oxygen content and strong water-adsorption capacity due to low coalification degree,brings a large energy waste in the utilization,for example,high transportation cost and lower calorific value.However,traditional drying technologies for low rank coal are not perfect enough to solve the problem about moisture re-adsorption of dried coals,which can not satisfy the requirement of high efficiencies for development and utilization of lignite in present stage.Surfactant,as one chemical product to controlling the interface properties,has amphiphilic molecular structure.Surfactant could adsorb on the surface of lignite with formation of the directed compact adsorption layer which point its hydrophobic ends outward.The hydrophilicity of lignite is decreased,and the moisture re-adsorption is inhibited.In this study,Manglai lignite was used as experimental samples.Wettability and moisture re-adsorption of lignite was studied.The effect of molecular structure of surfactant,such as types of hydrophilic heads and species of hydrophobic ends（chain length,aromatic group,ring-like alkyl,Gemini alkyl）,on the efficiencies of the decrease in hydrophilicity of lignite were investigated.The effect of HLB values of nonionic surfactant was also considered.Adsorption characteristics and adsorption mechanism of three different surfactants(cetyltrimethyl ammonium bromide（CTAB）,Sodium dodecyl sulfate（SDS）and lauryl polyoxyethylene ether(C₁₂（EO）₁₅))on lignite surface were analysed by methods of instruments characterization and quantum simulation,for examples microcalorimeter,electrophoresis,surface tension,infrared spectroscopy,ultraviolet-visible spectrophotometer,X-ray photoelectron spectroscopy（XPS）and nitrogen adsorption.In addition,the effect of organic content,mineral matter and coalification degree on the wettability and the efficiencies of the decrease in hydrophilicity of lignite by surfactant adsorption were also studied.The main conclusions list as follows:Wetting heat of organic content in lignite was larger than that of mineral matter,indicating the dominant role of organic content in the heat release of wetting process.The wetting heat of kaolinite was higher than that of calcite and quartz.Moisture re-adsorption behavior of Manglai lignite could be described by the second-order kinetic model.The relationship between moisture content in lignite and the wetting heats was negative correlation,and the more moisture content in lignite,the smaller wetting heat.The moisture adsorbed on lignite surface as monolayer or clusters decrease the wetting heat significantly,and the moisture in pores or capillary condensation decrease wetting heat with a small scale.Oxygen-containing groups in lignite surface exert important influence on the wetting heat of lignite.With the increase of the coalification degree,the oxygen content in coal decreases,wetting heat is reduced.The exothermic amount of lignite wetted by polar wetting reagent and nonpolar wetting reagent was significant difference.The wetting heat of lignite wetted by water and anhydrous ethanol were larger than that by n-alkanes.After adsorption of surfactants on lignite,the wetting heat and moisture re-adsorption decreased significantly,which stems from the essence that the decrease in hydrophilicity of lignite.The decrease in hydrophilicity of lignite relies on the formation of surfactant layer with directed compact arrangement on lignite surface.This is closely related to the molecular structure of surfactant,and the specific performance as below: efficiencies of cationic surfactant and nonionic surfactant in decrease of the hydrophilicity were more effective than that of anionic surfactant;with the increase of alkyl chain length of surfactant,the efficiencies in decrease of lignite hydrophilicity increased;branched chain and benzene ring structure in surfactant were not benefit for decreasing the hydrophilicity of lignite surface;surfactant with Gemini structure has higher efficiencies in hydrophilicity decrease of lignite than surfactants with single or double hydrophobic group;effect of the Tween surfactant on the decrease in hydrophilicity was not obviously;alkyl glycoside increased the hydrophilicity of lignite;with the decrease of HLB values of nonionic surfactant C12（EO）n,the efficiencies in decrease of hydrophilicity present as increasing trend,and the maximum efficiency was reached at HLB value of around 13.6.The key of decreasing the hydrophilicity of lignite was the hydrophilicity restraint of organic content.As for mineral matter in lignite,the efficiency was variation depending on the species of mineral matter.In general,cationic surfactant was more effective for kaolinite,anionic surfactant was more effective for calcite and the efficiency for quartz was fluctuate obviously with adsorbed surfactant.Adsorption of CTAB on lignite is exothermic process.The adsorption behavior could be described by the Freundlich model,and the adsorption rate fitted well by the second-order kinetic model.Increasing temperature,alkaline solution and low ionic strength promoted the adsorption of CTAB on lignite surface.Adsorption amount of CTAB on lignite surface increased with the increasing oxygen content in coal.After demineralization of lignite by acid solution,the adsorption amount of CTAB and the efficiencies in decrease of hydrophilicity were decreased.In low concentration solution of surfactant,CTAB reached prior to organic oxygen-containing groups rather than mineral matter.The adsorption capacity of kaolinite was higher than that of calcite and quartz.Among those oxygen-containing groups,carboxyl group adsorbed more CTAB than other oxygen groups.Quantum simulation calculation results show the order on the CTAB adsorption ability for different oxygen-containing functional groups: phenolic hydroxyl> carboxyl> alcohol hydroxyl> carbonyl> ether group.The adsorption of CTAB on lignite occurred both on the surface and in pores.Increasing temperature,acid solution and low ionic strength promoted the adsorption of SDS on lignite surface.The absorption of SDS on mineral matter was higher than that on organic content of lignite,and the adsorption capacity of calcite for SDS adsorption was bigger than that of kaolinite and quartz.Mineral content in coal samples with different coalification degree has much effect on the adsorption of SDS.Although,the adsorption of SDS on oxygen-containing groups in lignite was small due to electrostatic repulsion,the carboxyl group adsorbed more CTAB than other oxygen groups.After adsorption of SDS,the special area,average pore dimeter and pore volume decreased suggesting that SDS was adsorbed on surface and in macropores of lignite.Adsorption of C₁₂（EO）₁₅ on lignite could be described by the Langmuir model,and the adsorption rate fitted well by the second-order kinetic model.Increasing temperature,alkaline solution and high ionic strength promoted the adsorption of C₁₂（EO）₁₅ on lignite surface.After demineralization of lignite by acid solution,the adsorption amount of CTAB was decreased slightly.With coalification degree decreased,the oxygen groups became more,the adsorption amount of C₁₂（EO）₁₅ on lignite surface increased.The adsorbed C₁₂（EO）₁₅ covered the oxygen-containing groups in lignite surface and blocked the micropores pore structure,the macropores also being thinned.This study has certain reference value on the application of surfactant to reduce coal hydrophilicity and decrease the moisture re-adsorption of dry lignite.This work also supplys some experimental data for the investigation of adsorption mechanism of surfactant on lignite surface and regulatory mechanism of hydrophilicity inhibiting.