Study On Flocculation And Filtration Mechanism Of Fine Low-Rank Coal Enhanced By Ultrasonic Pretreatment

Low-rank coal（LRC）has an important role in China’s coal industry,and its clean and efficient use can not only improve the quality development of the coal industry but also ensure the national energy supply.At present,washing is an effective way to improve the quality of LRC in China,of which filtration and dewatering is an essential part of the washing process.However,LRC contain a large number of hydrophilic functional groups and poor hydrophobicity on the surface,which makes their filtration difficult,especially for fine LRC.In this thesis,we investigate the mechanism of ultrasonic pretreatment to enhance flocculation and filtration of fine LRC by using low-field nuclear magnetic resonance（L-F NMR）,computational fluid dynamics simulation（CFD）,EDLVO theory and Darcy’s theorem calculations.The LRC samples used for the tests were obtained from the National Energy Group in Yulin,Shaanxi Province.The coal samples were ground and sieved to produce a D90of 38.15μm fine grain LRC and used for filtration studies.The properties of the coal samples were tested by SEM,XPS,contact angle and XRD.The results showed that the surface of the coal sample contained a large number of cracks and pores,with a small contact angle and poor hydrophobicity.Also,the surface of the coal particles contains more hydrophilic oxygen-containing functional groups,such as C-O and O=C.In addition,the sample contains some hydrophilic clay minerals,which are not conducive to sample dewatering.Firstly,the filtration effect of anionic polyacrylamide（APAM）,non-ionic polyacrylamide（NPAM）and cationic polyacrylamide（CPAM）on LRC was compared by flocculation and filtration tests.The results showed that the best filtration effect was achieved with 50 g/t CPAM.Ultrasonic pretreatment flocculation and filtration tests show that ultrasonic pretreatment can further promote the filtration of LRC based on the action of flocculants.The optimum ultrasonic pretreatment conditions were 30 KHz and 3 min of ultrasonic action time.At this time,the water content of the filter cake decreased by 2.36%and the filtration rate of LRC increased by 27.26 m L·（m²s）^-1.Meanwhile,the results of the filtrate viscosity and surface tension tests showed that the viscosity of the filtrate decreased by 0.11 m Pa·s and the surface tension of the filtrate decreased by 5.6 m N/m after the ultrasonic pretreatment,which showed that the ultrasonic pretreatment enhanced the filtration speed,reduced the water content of the filter cake,improved the filtrate properties and thus promoted filtration.Then,the interfacial mechanism of action of ultrasonic pretreatment enhanced flocculation and filtration of LRC was investigated using adsorption tests,XPS,zeta potential,FBRM,PVM and polarised light microscopy tests,and by calculations of EDLVO theory and fractal dimension of flocs.The results showed that the ultrasonic pretreatment enhanced the adsorption of flocculants on the coal surface.Under the optimum ultrasonic pretreatment conditions,the equilibrium adsorption of CPAM on the coal surface increased by 0.022 mg/g;the content of hydrophobic functional groups C-C/C-H on LRC surface increased by 0.52%;and the coal-water contact angle on the LRC surface increased by 4.5°.Interparticle interaction analysis showed that the ultrasonic pretreatment compressed the bilayer structure of LRC surface and reduced the absolute value of the surface zeta potential,thus weakening the electrostatic repulsion between the particles,resulting in a reduction in the total repulsion between the particles and promoting the formation of flocs within the action distance.At the same time,ultrasonic pretreatment increases the size and strength of LRC flocs.The maximum chord length and fractal dimension of the floc increased by 28.9892μm and0.04426,respectively,under the optimum ultrasonic pretreatment conditions,and the microstructure of the flocs combined with PVM and polarised light microscopy confirmed that LRC flocs were more stable and less susceptible to damage after ultrasonic pretreatment.Finally,the effects of ultrasonic pretreatment on the structural properties and filtrate flowability of the filter cake were analysed using L-F NMR,Darcy’s theorem and CFD simulations.The results showed that the ultrasonic pretreatment increased the porosity of the filter cake.Under the optimum sonication pretreatment conditions,the total porosity of the filter cake increased by 2.0264%and the pore porosity of the filter cake increased by 1.2055%,which reduced the filtration resistance of LRC flocculation and improved the permeability of the filter cake.Meanwhile,L-F NMR water fugacity tests showed that the free water content of the filter cake increased by 0.813%under optimal ultrasonic pretreatment conditions.This indicates that the ultrasonic pretreatment promotes more conversion of non-free water to free water in the filter cake,thus weakening the binding of water molecules in the filter cake and allowing more water to escape from the filter cake pore channels to reduce the cake water content.In addition,CFD simulations showed that the pressure difference between the lower part of the filter cake and the outlet of the Br?nsted funnel was 54,000 Pa and the fastest flow rate of the filtrate was 10.3 m/s without ultrasound treatment,whereas under optimal ultrasound pretreatment conditions this pressure difference increased by 11,000Pa and the fastest flow rate of the filtrate increased by 1 m/s.Therefore,ultrasound pretreatment enhanced the flocculation and filtration performance of the fine LRC performance.There are 63 pictures,8 charts,and 126 references in this thesis.