Experimental research on transport⁃resistance characteristics of gangue slurry and its flow trend in goaf

Slurry filling is deemed as a significant technique for harmless disposal of gangue under the condition of low interference.The safe transport of slurry in the pipe and its stable flow in the goaf area lays the foundation for the reliable operation of filling system.In order to deeply study the fundamental characteristics of gangue and slurry，and the piping resistance and flow dynamics of slurry in the goaf area，the basic parameters of gangue and gangue slurry were obtained through X-ray diffraction test，electron microscope scanning test，gangue particle size distribution test as well as slurry slump test.A long⁃distance loop piping system for transporting gangue slurry was designed and a series of loop piping tests were carried out under different flow velocities，thus the relationship between transport velocity and friction loss was established and the optimized transport velocity and friction loss of slurry could be determined.Then，grou⁃ ting tests were carried out in the adjacent goaf to analyze the flow and diffusion characteristics of slurry within the ca⁃ ving zone of the goaf area and measure the slurry flow induced by gravity gradient in the goaf，thereby revealing the particle sedimentation law in the process of slurry flow.According to the testing results，the engineering application of gangue slurry in grouting filling in the adjacent goaf was provided.The research shows that the gangue is predomi⁃ nantly composed of quartz and kaolinite.Also，some feldspar and mica，and a small amount of chlorite，calcite，siderite， pyrite，and other minerals could be found in the gangue.The particle size ranging from 1 250 μm to 3 000 μm，from 630 μm to 1 250 μm，from 315 μm to 630 μm，from 160 μm to 315 μm，from 75 μm to 160 μm as well as particle size smaller than 75 μm account for 18.18%，18.99%，19.35%，15.02%，11.73%，and 16.74% respectively.In addi⁃ tion，70% of slurry containing gangue has a slump of 28.1 cm，a slump flow of 50.1 cm as well as the flowing slope in⁃ duced by gravity of 3.85，which demonstrating a decent flowability.The resistance along with the slurry pipeline increa⁃ ses with flow velocity and an approximately proportional relationship could be found and expressed as y = 0.602 44x+0. 784 11.When the diameter of the pipeline is 60 mm，the optimal transport velocity and friction loss are determined to be 1.50 m/s and 1.68 kPa/m.Overall，the slurry flowing characteristics presents trends of first vertically expanding and then horizontally diffusing.A semi⁃circular covering layer is firstly formed under the discharging port，and then the slurry travels across the covering layer and spreads towards the bottom floor.Once it reaches the bottom boundary，it starts to diffuse laterally，with the average flowing slope induced by gravity reaching 7.1% after stabilization.In addi⁃ tion，particle separation phenomena could be observed when the solid particles flow in the goaf area，with the proportion of fine particles smaller than 200 mesh increasing slightly and coarse particles bigger than 200 mesh reduc⁃ ing in the flowing direction of slurry.The engineering application indicates that the gangue slurry could be reliably piped without sedimentation amid filling， and no aggregation and settlement occur within the permeable space of goaf，meaning that the slurry shows an excellent flowability.According to the visual results through the peepholes，the spreading distance of slurry could exceed 45 m，which means that the slurry piping velocity and gradient concentra⁃ tion have been reasonably designed to meet the system requirements.