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The coal seam water injection is divided into deep hole water injection and shallow hole water injection according to the drilling depth. The deep hole water injection is in the wind inlet lane or the return air lane in front of the working face, and the hole is inclined parallel to the working surface along the coal seam. The hole depth is generally 2/3 of the working face oblique length, and the hole diameter is 75-100 mm. After sealing with a cement slurry or a rubber sealer, water injection can be started. Compared with shallow hole water injection, deep hole water injection cost is higher, drilling is more difficult, only suitable for medium and thick coal seams. The advantage is that the pre-wet range is large, it can be fully wetted, and it does not affect the coal mining work. However, in some mining areas, water injection is difficult and the amount of water injection is small because the coal seam is not damaged.
2 Mechanism and effect of coal seam water injection in coal mine safety
2.1 Dustproof
Coal seam water injection is the most effective dust prevention measure for mining face. The dust removal mechanism of water includes the following three aspects:
(1) The primary coal dust in the wet coal body, so that it loses its ability to fly;
(2) effectively wrapping each small part of the coal body to avoid the flying of fine coal dust when the coal body is broken during mining;
(3) The wetting effect of water makes the coal body plastically enhanced and the brittleness is weakened. When the coal body is subjected to an external force, many brittle fractures become plastic deformation, thereby greatly reducing the possibility that the coal body is broken into dust particles and reducing the amount of coal dust generated.
After the coal injection was carried out in the Xinwen Suncun Mine and the Benxi Caiyu Mine, the dust removal rates reached 73% and 79% respectively. Table 1 shows the dust reduction rate of the “three soft†fully mechanized caving face through the implementation of coal seam water injection and other dust prevention measures.
2.2 Prevention of coal and gas outburst
Research and experimental investigations show that the wetted coal body can significantly change the mechanical properties of coal, the elasticity and strength of coal decrease, and the plasticity increases, so that the pressure distribution in front of the roadway changes, that is, the high pressure to the deep part of the coal body. Transfer, the pressure concentration factor is reduced. After the coal body is wet, its gas permeability will also be reduced by hundreds of times. The water has a significant hindrance effect on the movement of gas. The gas emission and the gas discharge rate in the coal are greatly reduced. The above various changes indicate that the water injection wet coal can eliminate or reduce the prominent danger of the coal seam. After the six fully mechanized mining faces with prominent dangers in the two pairs of prominent mines in No. 3 coal seam of Yangquan mining area were filled with shallow-hole coal wall, the number of gas outbursts and discharges was reduced by 72.2% compared with that before use, and the average protruding strength was reduced. Also reduced by 36.5%.
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