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Background of low-level grouting filling test
Dahaize Coal Mine is located west of Yuyang District, Yulin City, Shaanxi Province. The industrial site of the mine and the coal preparation plant are arranged separately and connected by a 3.0 km trestle. The designed production capacity of the mine is 15.0Mt/a, which adopts full vertical shaft development and zonal ventilation, and is equipped with two significant mining height fully mechanized working faces. Dahaize Coal Mine is a new mine. Currently, working face 20101 in panel 201 is being mined, and working face 20201 in panel 201 is being arranged. The sand, gravel, and other materials used for concrete production underground shall be vertically fed through the No. 1 blanking hole and No. 2 blanking hole.
According to the current situation of mine construction, underground mining, and coal preparation plant, the gangue of Dahaizi Coal Mine mainly comes from the tunneling gangue during the capital construction period and the washing gangue of the coal preparation plant, with the amount of washed gangue about 0.7 Mt/a. According to the preliminary design of the mine, all the coal gangue during the construction of the Dahaize Coal Mine is used to lay the special railway subgrade for the mine. Furthermore, the washed gangue is treated with the technology of filling goaf with coal gangue slurry.
Simulation test of low-level grouting filling
To explore the feasibility of low-level grouting filling and its slurry intervention law, a simulation experiment of low-level grouting filling was conducted in the gangue hill of Dahaize Coal Mine, analyzing the flow and diffusion law of gangue slurry in the simulated goaf during low-level grouting.
Simulation model for gangue accumulation in local caving zone
Based on the stepped rock block collapse accumulation characteristics in the lower part of the arc-shaped triangular area after the extraction of coal seam 2 in Dahaize Coal Mine and low-level grouting filling area, the rock block collapse morphology of the accumulation caving zone is simulated on the ground. The overall model is stepped, with a size of 30.0 m × 18.5 m × (0.8 –2.5) m, the low-level grouting pipe is laid at the bottom of the model, and the boundary of the model is constrained by sand, as shown in Fig. 5.
Simulation test model for low-level grouting filling.
Simulation test process for low-level grouting filling
Firstly, the gangue from the coal washing plant is crushed into powder with a certain particle size and added with water and additives to produce a 70.0% concentration of gangue slurry at the concrete mixing station, the ratio of gangue, water, and additives is 35:14:1. Secondly, the slurry is continuously transported by concrete tank trucks to the vicinity of the local collapse zone accumulation model of the gangue yard. Finally, the slurry inside the tank trucks is pumped through pipelines to the simulated goaf for grouting and filling.
Simulation test analysis of low-level grouting filling
From on-site observation and monitoring data, it can be seen that the flow and diffusion of gangue slurry in the gaps between rock blocks in the simulated caving zone have the following characteristics:
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(1)
The flow of gangue slurry in the gaps between rock blocks is selective. When the size of the gap between rock blocks is greater than 3–4 times the maximum particle size of the accumulated gangue slurry, the gangue slurry is distributed in the gaps between surrounding rock blocks. When the size of the rock block gap is less than 3–4 times the maximum particle size of the accumulated gangue slurry, there is no or a small amount of gangue slurry in the rock block gap near the rock block gap.
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(2)
Starting from the outlet of the grouting pipeline, the overall vertical height of the gangue slurry far from the outlet point shows a downward trend, with the downward trend in the direction of inclination being significantly greater than the downward trend in the direction of strike. The maximum vertical height of the slurry is 2.0 m, and the horizontal distance from the pipeline outlet is 0.4 m. The minimum vertical height of the slurry is 0.75 m, and the horizontal distance from the pipeline outlet is 25.4 m. The gangue slurry is prone to form slurry accumulation points at the dip and strike boundaries, and the maximum accumulation amount is at the intersection of the strike and strike boundaries, as shown in Fig. 6.
Figure 6 
Flow and diffusion law of low-evel grouting filling.
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(3)
Along the flow direction of the gangue slurry, the proportion of coarse particles in the gangue slurry shows a decreasing trend, while the proportion of fine particles shows an increasing trend, the mass fraction of the gangue slurry also shows a decreasing trend. And as the goaf deepens, the proportion of coarse particles in the gangue slurry decreases, and the mass fraction of the gangue slurry also decreases.
Engineering test of low-level grouting filling
Determination of slurry filling model in Dahaize Coal Mine
According to the existing conditions of the mine and the selection process of the slurry filling technology mode, the crushing system, pulping system, and pumping system are respectively arranged on the ground and underground near the No. 1 blanking hole. The low-level grouting filling mode is formed in the goaf behind the 20101 working face in panel 201. Then the ground and underground coordinated pulping and low-level grouting filling are established to explore the reasonable parameters of slurry filling technology of Dahaize Coal Mine and provide data support for the subsequent system expansion and reconstruction.
Engineering test process for low-level grouting filling
Because this test needs to use the forklift to transfer gangue, and the surge buffer has not been set up, its front-end crushing and back-end pulping systems cannot work continuously, so this test process is described by subsystem. Its test process is shown in Fig. 7.
The engineering test process of low-level grouting filling.
Process of a crushing system: the truck is used to transport the gangue of −50 mm in the gangue bin of the coal washing plant to the gangue boron storage near the No.1 blanking hole of the industrial site. Then the forklift is used to transfer the gangue to the crusher for crushing. The crushed gangue is then transferred to the screening machine by the forklift for screening. The finished particle aggregate under the screen is directly put into the underground buffer through the No.1 blanking hole. The large particle aggregate on the screen is unloaded to the ground. It is transferred to the crusher by forklift for further crushing.
Process of pulping and pumping system: during the filling operation, the forklift transfers the finished particle aggregate buffered at the bottom of the No. 1 blanking hole to the receiving hopper. Then, the quantitative feeder under the receiving hopper and the quantitative feed pump of the clean water silo transport the finished particle aggregate and mine water to the primary mixer at a ratio of 7:3 for pulping, and the qualified, prepared slurry flows automatically to the secondary mixer for buffer mixing. Then it flows automatically to the slurry receiving hopper in the filling pump. Finally, the filling pump will fill the gangue slurry in the slurry receiving hopper to the caving zone of the goaf behind the 20101 working face by low-level grouting through the south wing return air main roadway pipeline and 20101 return air roadway pipeline.
Design of engineering test parameters for low-level grouting filling
The filling material of coal gangue slurry is composed of coal gangue and mine water, and the coal gangue is the main aggregate of the filling material. Based on low-level grouting filling simulation test and multiple on-site tests and debugging, this engineering test removed the gangue with a particle size greater than 3 mm, resulting in an average proportion of 26.32% of the 1.25–3 mm particle aggregate, 53.03% of the 0.075–1.25 mm particle aggregate, and 16.74% of the 0–0.075 mm particle aggregate. The ratio of gangue slurry is coal gangue to mine water = 7:3, and the mass fraction of gangue slurry is 70%.
The grouting pump used in this experiment is a plunger pump, with a maximum theoretical delivery capacity of 200m3/h at the outlet and a maximum theoretical delivery pressure of 14.0 MPa at the outlet. The underground pipeline model is φ133 × 12 mm, arranged in sequence at the bottom of the north and south wing return air connecting roadway, No.1 return air shaft, south wing return air main roadway, and 20101 belt conveyor roadway, with a total length of approximately 4000 m, as shown in Fig. 8.
The system layout of low-level grouting filling.
Based on the experimental conditions and supporting equipment of this test, the critical flow rate for low level grouting filling in the return air roadway of Dahaize in 20101 was calculated according to the formula of Changsha Research Institute of Mining and Metallurgy:
$$V_{c} = 2.809\left( {\frac{{\rho_{s} – \rho_{h} }}{{\rho_{h} }}} \right)^{ – 0.308} C_{V}^{ – 0.308} D^{0.31}$$
(1)
In the formula, VC is the critical flow rate for slurry transportation, m/s. ρS is the density of gangue, kg/m3. ρH is the density of water, kg/m3; CV is the volume fraction of the gangue slurry. D is the inner diameter of the pipeline, m.
After calculation, the critical flow rate for low level grouting filling is 1.71 m/s. Based on 1.1 times the critical flow rate, the design flow rate for slurry transportation should not be less than 1.88 m/s, and the design flow rate should not be less than 63.12m3/h.
Analysis of engineering test effect of low-level grouting filling
The Dahaize Coal Mine adopted a coordinated arrangement system above and below the shaft and a low level grouting filling method to conduct a coal gangue slurry filling test in the goaf. Without affecting the normal production of the working face, the washed coal gangue was successfully filled into the collapse zone of the underground goaf. In the low-level grouting filling test, the pump pressure of the grouting pump is around 8–10 MPa, and the transportation state of the gangue slurry pipeline is stable. The average transportation flow rate is about 1.94 m/s, and the low-level grouting filling is carried out at an average interval of 30 m. The single filling amount of coal gangue is between 150 and 600 t.
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