Surface Physical And Chemical Properties Controlments Of Lignite And Its Influence Mechanism On Slurry Making Performance

With the rapid development of China’s economy and society,the demand for energy is also increasing rapidly.Therefore,it is of great significance to achieve efficient and clean utilization of coal resources to ensure national energy security and sustainable development of society and economy.As the essential technology of coal chemical industry and clean utilization of coal,coal water slurry(CWS)has high demands on coal properties due to the limitation of chemical additives.At present,high rank coal(HRC)with a high coalification degree is usually desired for CWS preparation.However,due to its high price and large consumption in various industrial scenes such as smelting,coking and ammonia synthesis,the application of high rank coal in CWS preparation is limited.Therefore,preparation of high concentration CWS by using lignite,which has advantages like abundant reserves,shallow burial and low price,is an urgent problem for CWS industry.It is also of great significance for rational,efficient and clean utilization of coal resources.Compared to high rank coal,lignite presents different physicochemical properties due to its low coalification degree.For example,lignite has high moisture content,abundant oxygen functional groups,developed pore structure and high ash content.These properties lead to the poor slurry making performance of lignite,such as poor compatibility with dispersant,low concentration,poor fluidity and stability.Therefore,it is necessary to upgrade lignite or synthesize specified dispersant for lignite to improve its slurry making performance.According to the surface properties of lignite surface,such as high oxygen content,developed pore structure and high ash content,this paper applies Gemini quaternary ammonium salt,emulsified hydrophobic organic material and amphoteric dispersant containing phosphate ester group to modify the surface physicochemical properties of lignite respectively.The aim of surface modifications is to reduce moisture content of lignite,improve its surface properties and optimize the compatibility with dispersant.Finally,the slurry making performance of lignite was improved.Besides,the mechanism of surface modification and its influence on dispersant adsorption and slurry making performance were discussed.The detailed research contents of this paper are listed as follows:(1)A variety of Gemini quaternary ammonium salts using 4,4’-bipyridine or 4,4’-bis(chloromethyl)biphenyl as spacer were synthesized and then applied to modify the surface hydrophobicity of lignite.The Influences of surface modification on the surface properties of lignite,the adsorption of dispersant and slurry making performance were investigated.The changes of surface physicochemical properties caused by using Gemini quaternary ammonium salts were studied.The influences of modification on surface properties of coal were analyzed by using SEM,XPS,chemical titration,contact angle,water reabsorption ratio and Zeta potential.The adsorption amount of Gemini quaternary ammonium salt on coal surface was measured by TOC analyzer.Then the influence of modification on the adsorption of NSF dispersant was investigated by UV spectrum.Finally,the slurry making performance of modified coal was tested.The test results show that the assembly of Gemini quaternary ammonium salt on coal surface causes changes in element composition of coal surface.The relative content of C increases significantly,while the content of O,Si and Al decreases.The Gemini quaternary ammonium salts cover hydroxyl and carboxyl groups on the coal surface,resulting in a decrease of their content.In addition,the introduction of hydrophobic spacer further enhances the hydrophobic modification performance,so that the contact angle of coal to water increases from 44.8°to 81.1°,and the water re-adsorption ratio decreases.The Gemini quaternary ammonium salts partially neutralize the negative charges caused by the ionization of oxygen functional groups,which reduce the negative potential of coal surface.Compared with common cationic surfactant cetyltrimethyl ammonium bromide(CTAB),the Gemini quaternary ammonium salt has two electrostatic attraction sites,resulting in a higher adsorption amount on lignite surface.With similar hydrophobic groups,the adsorption amount of CTAB on coal surface is 1.97 mg/g,while that of Gemini quaternary ammonium salt increases to 3.36 mg/g.The hydrophobic modification enhanced the adsorption amount of NSF dispersant on coal surface from 2.15 mg/g to 3.91 mg/g.The maximum concentration of CWS prepared by using modified lignite increases from 55.3%to 58.7%,while the slurry instability coefficient TSI decreases from 0.48 to 0.24,and the water separation ratio reduces from 4.7%to 3.1%.(2)The developed pore structure is another factor affects the surface properties of coal.However,the modification on coal surface using Gemini quaternary ammonium salt does not affect the pore structure significantly.Therefore,asphalt is introduced as the hydrophobic organic film-forming material to block the pores,aiming to further improve hydrophobic modification performance and reduce modification cost.The emulsified asphalt was prepared with different surfactants and sprayed to form a hydrophobic layer on the surface of lignite.The hydrophobic modification of lignite by using emulsified asphalt could enhance the coal rank and surface hydrophobicity and improve the adsorption of dispersant.The test results show that the emulsified asphalt forms a relatively smooth and complete hydrophobic layer on the surface of coal.It enhances the C/O ratio on coal surface and covers Si and Al element.In addition,the formation of hydrophobic layer leads to the decrease of hydroxyl and carboxyl group contents on coal surface.The modification also reduces the pore parameters such as specific surface area,pore volume and average pore diameter.It could effectively improve the surface hydrophobicity of Yili lignite,as the contact angle increases obviously from 44.8°to 123.7°,while the adhesion work reduces significantly.Meanwhile,the re-adsorption of moisture is inhibited effectively as the re-adsorption ratio reduces 9.09%.Besides,the adaptability of modified lignite to dispersant is improved,as the adsorption amount of NSF dispersant on coal surface increased from 2.15 mg/g to 4.53 mg/g after surface modification.The maximum concentration of CWS prepared by using modified lignite increases from 55.3%to 61.2%.The static stability of CWS prepared by modified coal is also improved obviously.The instability coefficient TSI decreases from 0.48 to 0.22,while the water separation ratio reduces from 4.7%to 1.8%.(3)Based on the concept of environmental protection and reuse,waste cooking oil is recycled and used in conjunction with Gemini quaternary ammonium salt to modify the surface properties of lignite.The test results show that the recycled oil and Gemini quaternary ammonium salt synergistically form hydrophobic layer on coal surface.It significantly enhances the C/O ratio and reduces the content of oxygen functional groups and blocks the pore structure.The contact angle of modified coal to water increases to 123.5°,while the water re-adsorption ratio reduces.In addition,the formation of hydrophobic layer is conducive to the directional adsorption of NSF dispersant on the surface of modified lignite through hydrophobic attraction,which enhances the adsorption density.It also increases the negative potential on the surface of composite coal particles,which is conducive to the dispersion of coal particles in CWS.The slurry making performance of lignite modified by using emulsified recycled oil is improved,as the maximum concentration increases from 55.3%to 61.5%.Besides,the stability of CWS prepared by using modified coal is also improved.The instability coefficient TSI reduces from 0.48 to 0.26,and the water separation ratio reduces from 4.7%to 2.1%.(4)As the metal cations exist in the ash of lignite,the phosphate ester monomer HEMAP,which could complex with metal cations,was introduced into the amphoteric dispersant structure.An amphoteric dispersant P(APEG-co-SS-co-HEMAP-co-DMC)containing multiple functional groups was designed and synthesized.It is applied to prepare high concentration CWS by using lignite.Compared to NSF and amphoteric dispersant without phosphate ester group,amphoteric dispersant with phosphate ester group showed better adaptability to Yili lignite.The complexation between phosphate ester and metal cations,as well as the electrostatic attraction between cationic monomer and negative oxygen groups,are conducive to the directional multi-sites adsorption of dispersant on lignite surface.Its adsorption amount on coal surface reaches 3.77 mg/g,higher than that of 3.53 mg/g for P(APEG-co-SS-co-DMC)and 2.15 mg/g for NSF.By using P(APEG-co-SS-co-HEMAP-co-DMC)as the dispersant,the maximum concentration of CWS prepared using Yili lignite increases to 61.3%,which is higher than that of 60.8%by using P(APEG-co-SS-co-DMC)and 55.3%by using NSF as the dispersant.The instability coefficient TSI decreases from 0.48 to 0.19,and the water separation ratio decreased from 4.7%to 2.0%.It indicates that the electrostatic repulsion and steric hindrance caused by sulfonate and polyether monomer effectively improve the dispersion of coal particles in CWS.Compared to the amphoteric dispersant without phosphate ester group and anionic dispersant NSF,the amphoteric dispersant containing phosphate ester group has better slurry making performance when using Yili lignite to prepare CWS.