| Coal seam water injection is a common method to reduce working dust and prevent methane outburst.Most coal seams in China are difficult to be wetted,and traditional water injection measures are difficult to achieve ideal results.Electrochemical method can significantly improve the effect of water injection in coal seams.However,previous studies rarely focused on thin coal,and less on the influence of modified coal pore characteristics and capillary effects on water absorption laws.Electrochemical modification experiments were performed on lean coal samples taken from the No.11 coal seam of Gaoyang Coal Mine in Shanxi Fenxi Mining Bureau.The improvement of lean water absorption effect after modification was studied,and the mechanism of electrochemical influence on lean water absorption characteristics was analyzed.The main work and conclusions are as follows:(1)The electrochemical modification experiment equipment was set up and the electrochemical modification experiment was carried out on the thin coal sample.The modification scheme was as follows:the electrode was copper,aluminum and graphite,the electrolyte was Na2SO4 solution of 0.05 mol·L-1,0.10 mol·L-1 and 0.15 mol·L-1,the potential gradient was 2 V·cm-1,and the modification time was 120 h.The main phenomena found in the process of modification were as follows:the current firstly increased and then decreased with time,and gradually tended to be flat.The current peaks of copper,aluminum and graphite electrodes were 1.13 A,0.25 A and 0.46 A respectively.The higher the concentration of electrolyte,the greater the current;The temperature change in the experiment was similar to the current,but the temperature in the anode region was slightly higher than that in the middle region and the cathode region,the temperature of the graphite electrode was higher than that of the copper electrode and the aluminum electrode,and the temperature of the high concentration electrolyte was higher than that of the low concentration electrolyte.(2)The water absorption of lean coal samples before and after modification was tested.The results show that the water content of lean coal changed with water absorption time in three stages.The water content of coal sample increased rapidly in 0-12 h,and water absorption accounted for 64.77%of the total.The water absorption rate slowed down in 8-60 h,and water absorption accounted for 29.44%of the total.In 60-120 h,the water absorption reached saturation,accounting for 5.79%of the total.The natural water absorption of lean coal was about 1.61%.After modification,the natural water absorption of lean coal increased in different degrees.Under the condition of 2V·cm-1 potential gradient and 0.05 mol·L-1 Na2SO4 electrolyte,after modification by copper,aluminum and graphite electrodes,the natural water absorption of coal samples of anode area increased to 3.35%,2.17%and 3.48%,the middle area increased to 2.63%,2.42%and 2.77%,the cathode area increased to 2.04%,1.94%and 2.24%,respectively.Under the condition of 2V·cm-1 potential gradient and graphite electrode,after modification of 0.05mol·L-1,0.10 mol·L-1 and 0.15 mol·L-1 Na2SO4 electrolytes,the natural water absorption of coal samples in anode area increased to 3.48%,4.04%and 4.78%,the middle area increased to 2.77%,2.55%and 3.24%,and the cathode area increased to 2.24%,2.75%and 3.42%,respectively.The mechanism of electrochemical modification to increase the water absorption of thin coal sample was revealed as follows:first,the electrochemical action made the coal pore fissure more developed,and a large amount of water entered and stored in the pore;Second,the adsorption capacity of water on the surface of coal after modification was significantly improved.(3)The wave peak intensity of thin coal samples before and after electrochemical modification was measured and analyzed.The peak strength of-OH at wave number 3750?3550 cm-1,the peak strength of C=O at wave number 1610 cm-1,and the peak strength of C-O and C-O-C at wave number1030?1000 cm-1 were all increased,indicating that the hydrophilicity of coal was improved.At 2910 cm-1 and 2860 cm-1 of coal samples in the cathode area,the peak intensity of-CH2 and-CH3 increased,the relative content of oxygen increased,and the coal surface was more likely to combine with hydrogen atoms in water to form hydrogen bonds,indicating that the coal's water absorption capacity was enhanced.The mechanism of the change of group on coal surface caused by electrochemical modification was revealed as follows:the redox reaction in the electrochemical process caused the key position of some groups to break and recombine,which changed the original group distribution.(4)The coal-water contact Angle of thin coal samples before and after electrochemical modification was measured and analyzed.The coal-water contact angle of the natural lean coal was 93.0°,and the contact angle of the lean coal samples after electrochemical modification was reduced to varying degrees.Under the condition of 2 V·cm-1 potential gradient and 0.05 mol·L-1 Na2SO4electrolyte,after modification with copper,aluminum,and graphite electrodes,the contact angle of the coal samples in the anode region were reduced to 82.0°,83.0°and 81.5°.The contact angles of the coal samples in the middle region were reduced to 84.5°,89.0°and 84.5°.The contact angles of the coal samples in the cathode region were reduced to 90.5°,90.9°and 89.5°.Under the condition of 2 V·cm-1 potential gradient and graphite electrode,after modification of 0.05 mol·L-1,0.10 mol·L-1 and 0.15 mol·L-1 Na2SO4 electrolytes,the contact angle of the coal samples in the anode area was reduced to 81.5°,76.5°and 71.0°,the contact angles of the coal samples in the middle area were reduced to 84.5°,83.5°and 77.0°,and the contact angles of the coal samples in the cathode area were reduced to 89.5°,88.0°,and 87.5°.The electrochemical action drived the transformation of micropores in lean coal to macropores,and the proportion of macropores in the modified lean coal increased,which was conducive to the entry and storage of water.The pore size distribution curve of the modified thin coal sample showed that the position of the wave peak had changed,and the crests were concentrated on large apertures,which showed that the most probable pore size of the modified coal showed an increasing trend and the water flow ability was stronger.The contact angles of coal samples in the cathode area decreased to 89.5°,88.0°and 87.5°respectively.The mechanism of electrochemical modification to improve coal water contact angle was as follows:the hydrophilic groups on the surface of modified coal increased,the wettability of the surface increased,and the contact angle decreased.(5)The pore structure of lean coal samples before and after electrochemical modification was tested and analyzed.Electrochemical action changed the pore structure of lean coal samples.After modification,the proportion of large pores in lean coal samples increased,and the proportion of micro pores decreased,which was beneficial to the entry and storage of water.The pore size distribution curve of the modified lean coal sample showed that the position of the wave peak has changed,and the wave peaks appeared at the larger pore size,indicating that the most probable pore size of the lean coal after electrochemical modification shows an increasing trend,which was beneficial to water infiltration and storage.tial gradient was 2 V·cm-1,the pore size distribution of coal samples in the cathode area modified by 0.15 mol·L-1Na2SO4 electrolyte was the best.The percentage of micropores is 16.73%,which was 53.25%,43.04%and 65.62%respectively.The mechanism of electrochemical modification to change the pore structure of coal was revealed as follows:electrochemistry would dissolve some minerals in the pore of coal,through the action of electrophoresis,impurities in the pore migrated,and finally got out of the coal body,making the coal pore fissures more developed after modification,and the most probable pore diameter increased.(6)The capillary strength of thin coal samples before and after electrochemical modification was measured and analyzed.The average capillary strength of natural thin coal sample was-0.20 MPa,the negative value indicated that the capillary effect inhibited the water absorption in fine pores.After electrochemical modification,the average capillary strength of thin coal samples moved forward.Under the condition of 2 V·cm-1 potential gradient and 0.05mol·L-1 Na2SO4 electrolyte,after modification by copper,aluminum and graphite electrodes,the average capillary strength of the coal samples in anode area were 0.37 MPa,0.33 MPa and 0.48 Mpa.The average capillary strength of the coal samples in middle area were 0.26 MPa,0.06 MPa and 0.28 Mpa.The average capillary strength of the coal samples in cathode are were 0.02 MPa,0.05 MPa and 0.02 MPa.Under the condition of 2 V·cm-1 potential gradient and graphite electrode,after modification of 0.05 mol·L-1,0.10 mol·L-1 and 0.15mol·L-1 Na2SO4 electrolytes,the average capillary strength of the coal samples in anode region were 0.48 MPa,0.65 MPa and 1.65 MPa.The average capillary strength of the coal samples in middle area were 0.28 MPa,0.34 MPa and 0.62MPa.The average capillary strength of the coal samples in cathode area were0.02 MPa,0.09 MPa and 0.11 MPa.It indicated that the capillary action after modification further promoted water absorption of fine pores in lean coal.The mechanism of electrochemistry improving the capillary strength of the thin coal sample was as follows:the hydrophilicity of the modified coal sample was enhanced,and the capillary effect promoted the water content of the fine pores. |
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