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The borehole hydraulic coal mining system extracts coal seams through a hole drilled in overburdens. The mining device is lowered through the hole into the coal seam where it fragments the coal with high-pressure water jets. The fragmented coal is pumped to the surface as slurry by a jet pump located in the center of the mining device. The coal slurry is then injected into a slurry pipeline to transport to the preparation plant.
The hydraulic borehole mining system has several advantages over conventional mining systems. (1) The system can extract coal resources with the most challenging geology, which will be impossible using conventional mining system. (2) The system can ensure the safety of mining procedures, shorten the mining period and decrease the investment of infrastructure and operational cost without pollution. It is a promising environment-friendly and cost-effective mining method. In some countries, the system has been successfully employed in mining the deposits of phosphates, uranium, coal, apatite, quartz-sand and gravel and so on.
In China, the hydraulic borehole mining techniques develop very slowly. There's no integrative research and application of such system. This paper presents one kind of hydraulic borehole coal mining system. The three key elements that influence the success of the system are hydraulic lift of jet pump, air lift and water jet disintegration. These elements are discussed and analyzed in this paper using theoretical analysis and field experiments. The integrative research on the system achieved some useful results, which can be set as reference in the design of other kinds of hydraulic borehole mining systems.
The design of jet pump, air lift system and water jet has to be integrated organically in order to improve mining efficiency, coal recovery rate and decrease energy consumption. The design of hydraulic lift jet pump must meet the requirement of the minimum floating speed of coal particles. The optimization of non-dimensional parameters and prevention of cavitation has to be considered in the design. With regard to selecting the nozzle types of jet pump, center nozzle and annular nozzle can be picked up according to the size of extracted particles. Through air-lift and back pressure, the water head can be decreased to improve the lift capacity of jet pump and decrease the power loss. The air-lift has great limitation if it's used solely to extract coal, but if it's employed in conjunction with jet pump, the lift capacity of jet pump can be increased greatly. With water jets, the air-lift can improve the fragmentation radius and capacity. The main factors that influence the effect of water jet are the submergible status of jet, jet pressure and flow. The ideal jet of the monitor in the borehole hydraulic coal mining system is a non-submergible free jet. Through air-lift, the non-submergible free jet can be set up in the mining hole.
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