A safe and environment-friendly treatment method for coal and rock roof overhang at end of top coal caving face

Abstract

This invention relates to the field of top-coal caving mining, specifically a safe and environmentally friendly method for handling the overhang of coal and rock at the end of a top-coal caving face. It employs a high-voltage pulse device to generate shock waves that create continuous horizontal fracture interfaces within the coal and rock roof, reducing the thickness of the overhang and consequently shortening its length to meet on-site requirements. This invention replaces explosive detonation and proposes a reasonable method for determining operational parameters and a suitable construction process for addressing the overhang at the end of the top-coal caving face. The impact rock-breaking energy of this invention is controllable, highly safe, and has a uniform impact energy density distribution, preventing excessive rock fragmentation and flyrock hazards. It requires no large-scale warning and is environmentally friendly, producing no toxic or harmful gases. Other operations can proceed immediately after the initial impact, minimizing disruption to production.

Description

Technical Field

[0001] This invention relates to the field of top coal caving mining, specifically a safe and environmentally friendly method for handling coal and rock overhang at the end of a top coal caving working face. Background Technology

[0002] In some coal mining faces, the roof above the return roadway is hard, and there is often a situation where the overhang area at the end of the working face is too large (e.g.) Figure 1 (As shown). The roof slab at the end of the working face is suspended but does not collapse, resulting in an excessively large suspended area. This provides excellent space for gas accumulation at the end, leading to a large and long-term accumulation of gas at the working face end. This could potentially trigger gas exceedances and gas explosions, posing a significant safety hazard. Generally, hydraulic fracturing or blasting methods are used to weaken the rigid roof slab in advance to reduce the length of the suspended roof at the end.

[0003] For top-coal caving faces, the overhanging roof at the end is composed of coal and rock. Although the strength of coal and rock is lower than that of hard rock, the good integrity of coal and rock in some top-coal caving faces, coupled with the thickness of the top coal, results in a long overhanging roof at the end of the face, exceeding the mine's limitations. This phenomenon is particularly common in extra-thick coal seam mines in Shanxi and Inner Mongolia, my country. For hard, dense rock, hydraulic fracturing can produce good rock-breaking effects, but for coal and rock with significant anisotropy, the effect may be reduced. Blasting to treat the roof may cause underground gas explosions, posing a significant safety hazard. Excessive explosive energy, combined with the fact that the strength of coal and rock is far lower than that of hard rock, makes it a case of "using a large horse to pull a small cart." Shallow-hole blasting often produces flyrock, requiring long-distance and long-term vigilance for safety reasons, which to some extent reduces production time and space. Furthermore, explosive explosions produce large amounts of toxic and harmful gases, which is inconsistent with the concept of green mining. Therefore, it is necessary to invent a safe and environmentally friendly method for treating the overhanging coal and rock at the end of top-coal caving faces. Summary of the Invention

[0004] To address the shortcomings of the existing technologies, this invention proposes a safe and environmentally friendly method for handling coal and rock overhang at the end of a top-coal caving face, comprising the following steps:

[0005] S1, Determine the high-voltage pulse discharge operation layer: Determine the maximum cantilever length l of the working face end, simplifying the cantilever structure of the working face end to a cantilever beam structure, based on the measured ultimate tensile strength σ of the coal and rock roof. t Determine the appropriate thickness of the cantilever beam. The height or depth of the high-voltage pulse discharge operation layer from the coal and rock roof of the mining roadway is:

[0006] S2, Determine the excitation energy of high-voltage pulse discharge: Using numerical simulation or field test methods, adjust the voltage and capacitance, and determine the reasonable excitation energy of a single high-voltage pulse discharge and the corresponding capacitance and voltage operating parameters based on the fracturing effect of the coal and rock roof;

[0007] S3, Determine the spacing between high-voltage pulse discharge workstations: Using numerical simulation or field experiments, determine a reasonable spacing between high-voltage pulse discharge workstations. Based on the determined spacing, after completing the high-voltage pulse discharge operation, a continuous horizontal fracture interface can be generated within the coal and rock roof, reducing the thickness of the cantilever beam formed by the end-of-roof overhang.

[0008] S4. Construct advance boreholes in the mining roadway: Construct advance boreholes into the coal and rock roof in the advance support section of the mining roadway according to the spacing determined in step S3. The bottom of the advance borehole is constructed to the pulse discharge operation layer determined in step S1.

[0009] Preferably, the advanced support section is located 15m ahead of the working face, and the daily advance of the high-voltage pulse discharge operation is not less than the advance of the working face.

[0010] S5, perform high-voltage pulse discharge operation

[0011] S51, Prepare a high-voltage pulse discharge system: Specifically, the high-voltage pulse discharge system includes a high-voltage rectifier circuit, a capacitor, and electrodes. The high-voltage rectifier circuit is connected to the capacitor through a first switch, and the capacitor is then connected to one end of the electrodes through a discharge cable and a second switch on the discharge cable. The electrodes include a positive electrode and a negative electrode.

[0012] S52, Install a pulse discharge device in the pre-drilled hole; specifically, insert the drill head at the other end of the electrode into the sealing water bag, so that the drill head and the sealing water bag are connected together; then send the drill head and the sealing water bag together into the bottom of the pre-drilled hole, and use a detachable sealing device to seal the pre-drilled hole.

[0013] S53, using the operating parameters determined in step S2, to generate shock waves in the water between the electrodes, thereby damaging the coal and rock roof; specifically, the first switch is turned on, the high-voltage rectifier circuit first charges the capacitor, then the first switch is turned off and the second switch is turned on, the electrodes suddenly receive several thousand volts of high voltage from the capacitor, causing the water between the electrodes to generate shock waves, effectively damaging the coal and rock roof; the second switch is turned off, the detachable sealing device is removed, and the electrodes and the blasting head are taken out.

[0014] S6. Establish a mechanism for verifying and providing feedback on the effectiveness of end-end overhang treatment: By observing the length of the end-end overhang on-site, verify whether the expected target has been achieved. If so, proceed to the next work cycle; if the expected target has not been achieved, correct the operation parameters in steps S1-S3, repeat the engineering practice, and verify the engineering effect until the engineering operation parameters that achieve the expected end-end overhang length are obtained.

[0015] Beneficial effects: 1. This invention uses a high-voltage pulse device to generate shock waves that create a continuous horizontal fracture interface in the coal and rock roof, thereby reducing the thickness of the end-end overhang and thus reducing the length of the end-end overhang to meet the requirements of the engineering site; this invention replaces explosive detonation and proposes a reasonable method for determining operating parameters and a process construction method for the end-end overhang of the top coal caving face.

[0016] 2. The impact rock-breaking energy of this invention is controllable, safe, and has a uniform impact energy density distribution, which will not lead to excessive rock fragmentation or flying rock hazards. It does not require large-scale warning and is environmentally friendly, producing no toxic or harmful gases. Other operations can be carried out immediately after the operation is completed, with minimal interference to production. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the overhang at the end of the top coal caving face;

[0018] Figure 2 This is a force diagram of a cantilever beam, simplified from the top-coal caving face end roof suspension.

[0019] Figure 3 This is a layout diagram of the row spacing for high-voltage pulse discharge work areas;

[0020] Figure 4 This is a schematic diagram of advanced drilling being carried out in a mining roadway.

[0021] Figure 5 This is a schematic diagram of the layout of the advanced drilling internal pulse discharge operation device;

[0022] In the diagram: 1. Working face; 2. End cantilever beam; 3. Advance support section; 4. Coal and rock roof; 5. Horizontal fracture interface; 6. Goaf; 7. Mining roadway; 8. High-voltage pulse discharge working layer; 9. Advance borehole; 10. Electrode; 11. Positive electrode; 11. Negative electrode; 11. Blasting head; 12. Sealing water bag; 13. Detachable sealing device; 14. Detailed Implementation

[0023] like Figure 2-5 As shown, this invention proposes a safe and environmentally friendly method for handling coal and rock overhang at the end of a top-coal caving face, comprising the following steps:

[0024] S1, Determine the high-voltage pulse discharge operation level

[0025] The end cantilever 2 of the working face 1 is simplified into a cantilever beam 3 structure. The stress of the cantilever beam 3 structure is as follows: Figure 2 As shown, q is the uniformly distributed load on the cantilever beam 3 (end-end cantilever 2) from the overlying strata, h is the thickness of the cantilever beam 3 (end-end cantilever 2), l is the maximum length of the cantilever beam limited by the engineering site, and M is the bending moment of the section where the observation point is located within the cantilever beam. o Let y be the bending moment at the section at the fixed support end, and y be the distance between the point of observation and the neutral axis of the section.

[0026] The normal stress σ at any point within the cantilever beam is calculated as follows:

[0027]

[0028] In formula (1), J z The cross-sectional distance of the cantilever beam.

[0029] According to the theory of mechanics of materials, the maximum bending moment occurs at the fixed end of the cantilever beam. The maximum bending moment M is calculated. o for The ultimate tensile strength of the coal and rock roof was obtained through mechanical testing as σ. t Then, the following equation can be obtained for the cantilever beam in its limit state:

[0030]

[0031] As can be seen from formula (2), h∝l, which means that the thickness of the cantilever beam is proportional to the length of the cantilever beam. To reduce the length of the cantilever beam, the thickness of the cantilever beam can be reduced. That is, the smaller the thickness of the cantilever beam, the easier it is to collapse, and the smaller the overhang length at the end.

[0032] In practical engineering, if l is the maximum cantilever beam length limited by the engineering site, then the thickness of the cantilever beam must be reduced to at least [a certain value]. To ensure the cantilever beam collapses before reaching its maximum cantilever length, thus meeting site requirements, the thickness of the cantilever beam must be carefully considered. A thinner cantilever beam is more prone to collapse, resulting in a shorter cantilever length. However, excessively reducing the cantilever beam thickness can lead to a collapsed beam that is too thin to fully fill the goaf. Therefore, the thickness cannot be excessively reduced. To ensure the cantilever beam collapses before reaching its maximum cantilever length while maximizing its thickness to fill the goaf as much as possible, a reasonable cantilever beam thickness is determined. Correspondingly, the height or depth of the high-voltage pulse discharge working layer 9 from the coal and rock roof of the mining roadway 8 should be [missing information].

[0033] S2, Determine the excitation energy of the high-voltage pulse discharge.

[0034] The excitation energy of a high-voltage pulse discharge is calculated based on the stored energy. Under the rated discharge voltage, the excitation energy of a single discharge is proportional to the capacitance of the energy storage device, and the excitation energy is proportional to the square of the discharge voltage, as shown in the following formula:

[0035]

[0036] In the formula, Q E The unit for excitation energy is J; C is capacitance, unit is F; U is voltage, unit is V;

[0037] By using numerical simulation or field testing, the voltage and capacitance are adjusted, and based on the fracturing effect of the coal and rock roof, a reasonable single high-voltage pulse discharge excitation energy and corresponding capacitance and voltage operating parameters are determined.

[0038] S3, Determine the spacing between high-voltage pulse discharge workstations.

[0039] High-voltage pulse discharge is performed inside the coal and rock roof 5 above the advanced support section 4 of the top coal caving face 1. The shock wave generated by the high-voltage pulse discharge damages the coal and rock roof 5, creating a distinct horizontal fracture interface 6 within the roof. This limits the length of the overhang 2 at the end of the coal and rock roof 5 entering the goaf 7. As shown in step S1, the thickness of the coal and rock below the horizontal fracture interface 6 is...

[0040] Numerical simulation or field experiments were used to determine a reasonable spacing between high-voltage pulse discharge operations. Based on this determined spacing, after completing the high-voltage pulse discharge operation, a continuous horizontal fracture interface 6 will be generated within the coal and rock roof 5, reducing the thickness of the cantilever beam formed by the end-end overhang. like Figure 3 As shown, a represents the spacing between high-voltage pulse discharge operations, b represents the row spacing between high-voltage pulse discharge operations, and h represents the height of the coal and rock roof after the high-voltage pulse discharge operation is completed. It is also the high-voltage pulse discharge operation layer, where H is the height of the 8th mining roadway;

[0041] S4, advanced drilling was carried out in the mining roadway.

[0042] like Figure 4 As shown, according to the spacing determined in step S3, advance boreholes 10 are constructed in the advance support section 4 of the mining roadway 8 toward the coal and rock roof 5. The bottom of the advance borehole is constructed to the pulse discharge operation layer 9 determined in step S1. The advance support section is specifically within a 15m range of the advance working face. The daily high-voltage pulse discharge operation advance is not less than the working face advance.

[0043] S5, perform high-voltage pulse discharge operation

[0044] S51, Prepare a high-voltage pulse discharge system. The high-voltage pulse discharge system includes a high-voltage rectifier circuit, a capacitor, and an electrode 11. The high-voltage rectifier circuit is connected to the capacitor through a first switch. The capacitor is then connected to one end of the electrode 11 through a discharge cable and a second switch on the discharge cable. The electrode includes a positive electrode 111 and a negative electrode 112.

[0045] S52, such as Figure 5 As shown, the other end of the electrode 11, the drill head 12, is inserted into the sealed water bag 13 and effectively sealed, so that the drill head 12 and the sealed water bag 13 are connected together; then the drill head 12 and the sealed water bag 13 are sent together to the bottom of the advanced borehole 10, and the advanced borehole 10 is sealed with a detachable sealing device 14. The purpose of sealing the borehole is to improve the utilization rate of high voltage pulse discharge energy, so that most of the energy is used to break the rock, rather than being lost to the air.

[0046] S53, set the warning distance and warning procedure; using the operating parameters determined in step S2, turn on the first switch, the high-voltage rectifier circuit first charges the capacitor to thousands of volts, then turn off the first switch and turn on the second switch, the electrode suddenly receives thousands of volts of high voltage from the capacitor, tens of thousands of amperes of discharge current are formed in the water medium between the electrodes, and tens of thousands or even hundreds of thousands of joules of heat energy are generated instantaneously, causing the water to vaporize and generate a huge impact force on the water, generating a shock wave, which can effectively destroy the coal and rock roof; turn off the second switch, remove the detachable sealing device 14, and take out the electrode 11 and the blasting head 12;

[0047] This invention uses high-voltage pulse discharge instead of explosive detonation as the rock-breaking power. The high-voltage pulse discharge process refers to the sudden application of a strong electric field (103-104V) between the positive and negative electrodes, which causes the water between the electrodes to be ionized and vaporized in a very short time (μs level), generating a high-temperature, high-pressure, and high-speed expanding shock wave to effectively break rocks. It is not only safe and effective, but also green and environmentally friendly.

[0048] S6. Establish a mechanism for verifying and providing feedback on the effectiveness of end-ceiling cantilever treatment.

[0049] By observing the overhang length at the end of the ceiling on-site, we can verify whether the expected target has been achieved. If it has, we proceed to the next work cycle. If the expected target has not been achieved, we correct the operation parameters in steps S1-S3, repeat the engineering practice, and verify the engineering effect until we obtain the engineering operation parameters that achieve the expected overhang length at the end of the ceiling.

Claims

1. A safe and environmentally friendly method for handling coal and rock overhang at the end of a top-coal caving face, characterized in that, Includes the following steps: S1, Determine the maximum overhang length of the end cap as limited by the construction site. The cantilever structure at the end of the working face is simplified to a cantilever beam structure, based on the measured ultimate tensile strength of the coal and rock roof. Determine the appropriate thickness of the cantilever beam. The height or depth of the high-voltage pulse discharge operation layer from the coal and rock roof of the mining roadway is: ; S2, determine the reasonable excitation energy of a single high-voltage pulse discharge and the corresponding capacitor and voltage operating parameters; S3, determine a reasonable spacing between high-voltage pulse discharge workstations to create a continuous horizontal fracture interface within the coal and rock roof, thereby reducing the thickness of the cantilever beam formed by the end-of-roof overhang. ; S4. According to the spacing determined in step S3, advance drilling is carried out in the advance support section of the mining roadway towards the coal and rock roof. The bottom of the advance drilling is constructed to the pulse discharge operation layer determined in step S1. S5, perform high-voltage pulse discharge operation S51, Prepare a high-voltage pulse discharge system; S52, Install a pulse discharge device in the advanced borehole; S53, Use the operating parameters determined in step S2 to generate shock waves in the water between the electrodes to damage the coal and rock roof. In step S51, the high-voltage pulse discharge system includes a high-voltage rectifier circuit, a capacitor, and electrodes. The high-voltage rectifier circuit is connected to the capacitor through a first switch, and the capacitor is then connected to one end of the electrodes through a discharge cable and a second switch on the discharge cable. The electrodes include a positive terminal and a negative terminal. In step S52, the drill bit at the other end of the electrode is inserted into the sealing water bag to connect the drill bit and the sealing water bag together; then the drill bit and the sealing water bag are sent together to the bottom of the pre-drilled hole, and the pre-drilled hole is sealed with a detachable sealing device. In step S53, the first switch is turned on, and the high-voltage rectifier circuit first charges the capacitor. Then the first switch is turned off and the second switch is turned on. The electrode suddenly receives a high voltage of several thousand volts from the capacitor, which causes the water between the electrodes to generate a shock wave, damaging the coal and rock roof. The second switch is turned off, the detachable sealing device is removed, and the electrode and the blasting head are taken out. S6. Establish a mechanism for verifying and providing feedback on the effectiveness of end-cantilever roof treatment.

2. The method for safe and environmentally friendly treatment of coal and rock overhang at the end of a top-coal caving face according to claim 1, characterized in that, In step S4, the advance support section is specifically 15m ahead of the working face, and the daily advance of the high-voltage pulse discharge operation is not less than the advance of the working face.

3. The method for safe and environmentally friendly treatment of coal and rock overhang at the end of a top-coal caving face according to claim 1, characterized in that, By observing the overhang length at the end of the ceiling on-site, we can verify whether the expected target has been achieved. If it has, we proceed to the next work cycle. If the expected target has not been achieved, we correct the operation parameters in steps S1-S3, repeat the engineering practice, and verify the engineering effect until we obtain the engineering operation parameters that achieve the expected overhang length at the end of the ceiling.

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