Investigation Of An Inhibiting And Sealing Material For Coal Spontaneous Combustion Under Small Pillar Mining Condition

Small coal pillar mining technology,which is effective in promoting the recovery rate of coal resources,has gradually become the major method for mining medium and thick coal seams.During roadway excavation and working face recovery,the small coal pillar is subjected to the superposition of concentrated stress and mining stress.Resultantly,the structural strength drops dramatically;the permeability and air leakage intensity increase significantly,which may induce disasters such as gas overrun explosion and coal spontaneous combustion（CSC）.The effective sealing of fracture zones in the small coal pillar and the adjacent old goaf is the key to preventing and controlling the compound disaster of gas and CSC in small coal pillars.However,the commonly used sealing materials,such as cement and yellow mud,are apt to shrink and crack after solidification and air drying.Besides,they do not own the chemical inhibition capacity for CSC,failing to prevent and control the compound disaster of gas and CSC.In view of the above problems,this dissertation presents a research from the following aspects:the evolution law of stress and plastic damage range within the small coal pillar,the influence mechanism of small coal pillar fracture development on the compound disaster of gas and CSC,the selection and preparation of anti-oxidation synergistic inhibiting and sealing（SIS）material,the inhibiting and sealing performance of the anti-oxidation SIS material,and the on-site engineering application test.It reveals the mechanism of the compound disaster of gas and CSC induced by small coal pillar fracture evolution,and develops the anti-oxidation SIS material with both an inhibition function and a sealing performance.The main results are as follows.（1）With the aid of the modern computer numerical simulation technology,the dynamic evolution law of peak stress during roadway excavation on both sides of the small coal pillar and working face recovery is analyzed through FLAC^3D numerical simulation software from both horizontal and vertical perspectives,and the plastic damage range of the small coal pillar is determined.On this basis,the dynamic development process and key areas of internal cracks in the small coal pillar are deduced.Furthermore,gas migration within the small coal pillar,the working face goaf and the adjacent old goaf during small coal pillar fracture development is simulated through ANSYS Fluent numerical simulation software.In addition,the influence mechanism of small coal pillar fracture development on CSC and compound disaster is revealed.Moreover,the area with CSC and compound disaster hazards before and after grouting in the small coal pillar and the adjacent goaf is comparatively analyzed,and the necessity of small coal pillar sealing by grouting for disaster prevention and control is clarified.（2）Regarding the advantages and disadvantages of a single physical or chemical inhibitor,it is proposed to combine the two organically to prepare a SIS material with dual characteristics.In material selection,a high-water-content material with both an excellent physical inhibiting function and a satisfactory sealing performance is selected as the physical inhibitor,and its basic performance is tested.Meanwhile,the synergistic antioxidant prepared by using multiple hindered phenolic antioxidant as the main antioxidant and phosphite antioxidant as the auxiliary antioxidant is selected as the chemical inhibiting component.The optimal molar ratio of the main antioxidant to the auxiliary antioxidant is determined as 5:2 by experimental test,and its chemical inhibitory performance is tested.Finally,the high-water-content material and the synergistic antioxidant are compounded.Through the CSC simulation experiment and the uniaxial compressive strength test,the optimal mass ratio of the synergistic antioxidant to the high-water-content material is determined as 1:8.Under this ratio,the material boasts an excellent CSC inhibitory performance as well as a satisfactory structural strength.（3）Through a combination of the CSC simulation test system with instruments such as electron spin resonance spectroscopy（ESR）,Fourier transform infrared spectroscopy（FTIR）and gas chromatography（GC）,the mechanism of the newly developed SIS material in inhibiting CSC is disclosed through experiments from three aspects,namely free radicals,functional groups and symbolic gases.Moreover,the SIS material is comparatively tested and analyzed with traditional inhibition materials such as yellow mud and Mg Cl₂.The results demonstrate that the SIS material can dynamically eliminate the new free radicals generated during CSC,thus notably reducing the concentration of free radicals in coal.In addition,it can reduce the rate of coal-oxygen compound reaction and the rate of functional group self-chemical reaction,thus inhibiting or delaying the natural oxidation of coal.Meanwhile,it boasts both physical and chemical inhibition functions which enable it to maintain excellent inhibiting performance throughout the entire temperature range.The SIS material is remarkably superior to cement and yellow mud which only have the physical inhibition function.（4）The characteristics of fracture development of the SIS material are explored macroscopically and microscopically,and the advantages and disadvantages of the SIS material in fracture development are compared with cement and yellow mud.Through experiments and theoretical analysis,the sealing mechanism of the synergistic material is revealed,and its permeability and air leakage sealing performance are tested.The results reveal that the relatively compacted internal structure of the SIS material makes it capable of retaining water during natural solidification and air drying.Consequently,the size and number of cracks on its surface are smaller than those on cement and yellow mud surfaces during the same incubation time.Moreover,the SIS material features good fluidity and high permeability,so they are able to seal the cracks and voids deep inside the broken coal,reducing the gas exchange frequency and air leakage intensity at both ends of the broken coal.Meanwhile,the SIS material has better sealing performance and a longer effective service life compared with cement and yellow mud,according to the air leakage test.（5）Taking the 81303 small coal pillar working face in Huayang No.1 Mine as the experimental working face,the prevention and control effect of the SIS material on the compound disaster of gas and CSC in the small coal pillar working face of the coal mine is investigated.The investigation results disclose that when the slurry of the SIS material is injected into cracks inside the small coal pillar and the adjacent old goaf coal,it crystallizes and solidifies in a short time,effectively sealing cracks to reduce air leakage.In doing so,the pressure difference increases dramatically while the O₂concentration decreases significantly.Meanwhile,the SIS material boasts both physical and chemical CSC inhibition functions which endow it with a long service life in inhibiting the coal-oxygen compound reaction of broken coal.Such dual effects,together with the low oxygen environment resulting from its excellent sealing performance,ultimately succeed in keeping the spontaneous combustion process of broken coal in the adjacent old goaf in the initial stage for a long time.Hence,the broken coal there is basically free from the risk of CSC.This study is of great theoretical and practical significance for ensuring the safe mining in small coal pillar working faces and improving the synergistic ability to prevent and control the compound disaster of gas and CSC.The paper has 89 pictures,41 tables,and 204 references.