Research On Mechanism Of Purifying Underground Reservoir Water Storage By Collapsed Coal Rock In Goaf Of Wanli No.1 Coal Mine

The underground reservoir of coal mine is an important technical way to protect the water resources in the western mining area.It can not only store the water body by using the free space of the broken rock mass in the goaf,but also purify the water quality by the water-rock interaction.Therefore,it is of great significance to study and reveal the mechanism and influence law of the water quality purification of the underground reservoir caused by the broken coal rock in the goaf,for the efficient utilization of the water quality and the rational selection of the location of the reservoir which is conducive to the water quality purification.Taking Wanli No.1 mine underground reservoir as the research object,the paper carried out a long-term water-rock interaction experiment with mine water,coal,sandy mudstone,aluminum mudstone and sandstone,and studied the influence law and purification mechanism of water-rock interaction on the water quality,and based on this,the site selection of underground reservoirs and the scheduling optimization of water storage reuse were carried out.The study found that the four lithological rocks all have obvious water quality improvement effects on mine sewage.Most of the water quality characteristic values show a declining trend of fluctuations,and there are still a few parameters（such as total hardness and E.coli）that show an increasing trend of fluctuations,and when the initial characteristic value of water quality is higher or the content of rock clay minerals is larger,the change range of water quality characteristics will be more obvious.Among the four kinds of lithological rocks,coal has the best purification effect on mine sewage;after 80 days of experiment,it was found that the reduction rate of coal on the turbidity,BOD₅,residual chlorine and petroleum parameters of mine water can reach more than 80%,and the reduction rate of alkalinity and total hardness is more than 30%.Affected by the time of water-rock interaction,some water quality characteristics（such as total dissolved solids,COD）will decrease first and then gradually increase,showing the deterioration of water quality caused by the long storage time of the reservoir.Based on the results of coal rock scanning electron microscopy and X-ray diffraction analysis,the influence mechanism of coal rock on the key water quality characteristics is analyzed.When the initial ion concentration of mine water is high,at the initial stage of water-rock interaction,due to the strong adsorption and sedimentation of clay mineral surface and soluble organic matter in rock,the total dissolved solid and total hardness of water body generally decrease,and then they will increase with the relative enhancement of dissolution of water on rocks;the active organic matter in coal and sandstone can ionize more H⁺to react with the alkaline matter in water,which makes the total alkalinity of water significantly reduced;the surface of aluminous mudstone is rich in clay minerals with dense morphology,which is conducive to the adhesion and reproduction of E.coli,and easy to lead to the increase of E.coli index in water.According to the difference of water purification effect of different lithology rocks,the site selection of underground reservoir is optimized.That is to say,the mined out area with more coal and less aluminum mudstone is preferred as the construction site of underground reservoir.The reservoir water storage time and its reuse operation are optimized by fully considering the deterioration effect of water and rock interaction time on water quality.That is to say,when the storage time of mine water in the reservoir reaches 5 weeks,it can be reused as water for some production purposes.In addition,the storage time shall not exceed 6 weeks,otherwise,some key water quality characteristic values of the water body will exceed the standard and the water quality will deteriorate.There are 45 figures,14 tables and 90 references in this paper.