Mechanism Of Soluble Organic Matter Influencing On Coal Microscopic Surface And Wettability

Coal dust is the main factor affecting the safety of underground operation.Wet dust removal methods such as coal seam water injection,spray dust reduction are still widely used in China's underground dust removal technology,and the wetting characteristics of coal dust surface is the key factor that restricts the above-mentioned wet dust removal effect.The presence of soluble organic matter has an impact on the physical and chemical characteristics of coal,including the wetting characteristics of the microscopic surface of coal dust.In this paper,four different coal samples including long-flame coal,fat coal,coking coal and anthracite coal were selected to carry out Ultrasound-Microwave assisted extraction with tetrahydrofuran(THF)to prepare and extract residual coals.Capillary upward osmotic wetting experiments were carried out on raw coal and residual coal.Deionized water and 0.2% sodium dodecyl sulfate solution were selected as wetting solvent respectively.The moisture absorption of raw coal and residual coal was measured within 120 min,so as to determine the wetting characteristics of raw coal and residual coal.The experimental data show that the wettability of coal decreases with the increase of coal rank;the wetting characteristics of residual coal after extraction have been reduced,and it is more difficult to wet than raw coal.The microscopic surface adsorption characteristics and chemical oxygen-containing functional group content of raw coal and residual coal were measured by low temperature nitrogen adsorption experiment and X-ray photoelectron spectroscopy(XPS).The results show that soluble organic matter has different forms of occurrence in different coal samples,therefore,after THF extraction,the surface structure of different coal samples shows different changes.However,the overall performance is that the pore radius of coal samples with relatively low metamorphism(long-flame coal and fat coal)increases,and the pore number of coal samples with relatively high metamorphism(coking coal and anthracite)increases.The relative content of surface polar hydrophilic oxygen-containing functional groups(mainly hydroxyl groups)decreased,while the relative content of lower active oxygen-containing functional group(ether groups)increased.Based on the variation of surface pore structure and surface chemical structure(surface functional group composition)of raw coal and residual coal,the mechanism of soluble organic matter on the micro-surface wetting characteristics of coal dust was explored:(1)The presence of soluble organic matter fills a portion of the pores on the surface of the coal dust,change the pore morphology,reduce the pore size,and even block some pore,blocking the connected pore,thus affecting the distribution of pore on the micro surface of coal dust.(2)Organic oxygen atoms in polar oxygen-containing groups are easy to combine with hydrogen in water to form hydrogen bonds,which makes the surface of coal dust easier to wet.Oxygen-containing atom compounds in soluble organic matter change the chemical structure of coal dust micro-surface and provide more polar oxygen-containing groups for coal samples.(3)The existence of soluble organic matter can change the micro-surface structure of coal dust by reducing the pore size,complicating the pore morphology and increasing the number of polar hydrophilic oxygen-containing functional groups,which together ameliorate the wettability of the micro-surface of coal dust.