# Research on Key Parameters and Engineering Experiments of Coal Gangue Slurry Filling Technology
## 1 Introduction  
As China's primary energy source and an essential chemical raw material, coal plays a fundamental and primary role as a \"ballast\" and \"regulator\" in ensuring energy security [1, 2]. However, in the process of coal resource development, about 10-20% of coal gangue needs to be discharged, which inevitably causes environmental pollution and ecological damage in the mining area [3, 4]. The Chinese government and various provincial and municipal governments have introduced a series of policies and regulations to improve the comprehensive utilization rate of coal gangue. However, the total utilization rate of coal gangue is still not high, and coal gangue accumulated over the years has not been effectively treated. The green disposal of coal gangue has become one of the problems affecting coal resource extraction and ecological environmental protection in China [5, 6]. Therefore, how to achieve the efficient removal of coal gangue while safeguarding \"green mountains and clear waters\" is a significant development challenge faced by coal enterprises [7, 8]. Long-term theoretical research and practice in China have proven that underground filling in coal mines is an essential technical approach for the disposal and utilization of coal gangue. The underground filling technology of coal gangue has a development history of several decades in China [9, 10, 11, 12]. It has come in the form of three underground filling technologies for different needs in different periods, including roadway dumping filling, solid filling of working faces, paste filling of working faces, short-wall cementation filling, and grouting filling of overlying strata. These coal gangue filling technologies have beensuccessfully applied in dozens of mining areas and hundreds of pairs of mines in China. They have achieved various goals, such as the green discharge of coal gangue and the safe recovery of coal resources [13; 14; 15; 16; 17; 18]. However, the existing underground filling technology of coal gangue has the problem of spatiotemporal interference between coal mining and filling, which seriously restricts the efficient extraction of coal and the large-scale filling of coal gangue. In terms of the solid filling and paste filling technology of working faces, coal mining and filling are carried out in the same space successively. After mining 1-3 coal knives, filling is carried out once, and then upon continuing to mine 1-3 coal knives, filling is carried out again. This process is repeated until the entire working face is fully filled. In addition, it is not easy to arrange a filling working face specifically for handling coal gangue in a high-yield and efficient mine with a capacity of tens of millions of tons [19; 20; 21; 22]. For roadway dumping filling and short-wall cementation filling, the speed of coal excavation in the roadway limits the filling efficiency, making it difficult for the coal mining and filling capacity to exceed 500,000 tons/year [23]. For the grouting filling of overlying strata, it is necessary to drill grouting holes on the ground in advance before the mining face, and the timing of the grouting filling is directly related to the migration law of the overlying strata in the mining site [24; 25]. The mining of coal resources in China has gradually shifted westward, and the tens of millions of tons of coal obtained from mines in the western regions are also facing the problem of the large-scale green disposal of coal gangue. However, existing underground filling technology cannot meet the high production and efficient operation of mines while achieving underground disposal of coal gangue.