| The spontaneous combustion fire in goaf often occurs in the deep underground.The location of the fire point is hidden.It is difficult to determine the scope of the fire area,so the concealment is very strong.In addition,it is difficult for the backward personnel to enter the fire area to carry out work,which makes the accurate positioning of the spontaneous combustion fire in goaf become a worldwide problem.In the 1990s,Taiyuan University of technology proposed to apply the surface isotope radon measurement method to the detection of underground fire sources in coal mines,which has a history of more than 30 years.However,the mechanism of radon exhalation in the overburden of goaf is not clear,and the law of radon exhalation with temperature is not clear,which causes some trouble to realize the accurate location of underground fire source.In order to explore the relationship between radon emission and environmental temperature in overburden and reveal the mechanism of radon emission affected by the internal change of overburden under high temperature environment,based on radon emission theory of overburden,this paper studies the migration and emission mechanism of radon in sandstone,shale,mudstone and limestone samples by means of theoretical analysis,laboratory simulation experiment and grey correlation analysis.The radon emission characteristics of four kinds of rocks at different temperatures are studied through experiments.The changes of water content,specific surface area,pore volume and fracture structure of overburden were analyzed by means of specific surface and pore size analyzer,thermogravimetric and differential thermal analyzer,SEM scanning electron microscope.The main research contents and results are as follows:1.The high temperature produced by the spontaneous combustion of the coal in the goaf destroys the lattice structure of the rock,promotes the process of radon emission,strengthens the diffusion and convection of radon in the rock fracture network to a certain extent,accelerates the process of radon migration from the depth of the rock to the shallow layer,and finally leads to the abnormal precipitation of radon in the overburden when the coal in the goaf is spontaneous combustion.2.The radon measurement experiment is carried out,and the radon exhalation law in different overburden under different temperature is analyzed and summarized according to the experimental results.The results show that the radon exhalation shows a trend of fluctuating growth as a whole with the increase of temperature,and the maximum increase is related to lithology,and the maximum increase varies with different lithology.Before 450?,the maximum radon exhalation of sandstone is117.0bq/m~3,which is 1.64 times of the background radon concentration of sandstone.The maximum radon emission from shale,mudstone and limestone is 6.6,2.9 and 2.8 times of the background radon concentration,respectively,which has a certain guiding significance for the detection of fire area and the judgment of fire point temperature by isotopic radon measurement.3.In addition to temperature,water content,specific surface area,pore volume and fracture structure are all important factors affecting radon emission in overburden.Before 200?,the decrease of moisture content is the main reason for the increase of radon emission.Since then,the pore and fracture structure have become the main factors affecting radon exhalation.However,when it comes to a certain overburden,due to different lithology,the temperature range that has become the main factor may be different.When the temperature is about 350?,the new cracks of?m level appear and extend to the deep inside of the rock.The holes and fracture structure gradually merge into the fracture network,which provides a channel for free radon in the deep part of the rock to move to the shallow layer and greatly promotes the rapid release of radon in the deep part of the rock.4.Four The grey correlation analysis of radon exhalation quantity with temperature and pore structure is carried out.The grey correlation degree of radon exhalation quantity with temperature,specific surface area and pore volume of four kinds of rocks is more than 0.7,which shows that radon exhalation quantity has the same change trend with temperature,specific surface area and pore volume.It also proves the high temperature produced by spontaneous combustion of residual coal and its effect on high temperature from another angle The change of pore structure of the overlying rock is the reason for the abnormal increase of radon emission. |
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