METHOD FOR RECOVERING RESIDUAL COAL PILLARS BY FREEZING WATER ACCUMULATED IN ROOM-AND-PILLAR MINING AREA

Abstract

A method for recovery of residual coal pillars by freezing accumulated water in room-and-pillar mining areas is provided. A feasibility of repeated mining of residual coal pillars in room-and-pillar goafs is distinguished based on production data and exploration data of a mine. An accumulated water in the room-and-pillar mining areas is frozen to replace a paste filling material, which envelopes collapse roofs and gangues in the room-and-pillar goaf as a whole. The room-and-pillar goaf is filled with a frozen ice body. Roadways and mining faces are arranged in the frozen ice body. Coal cutter cuts the residual coal pillars and the frozen ice body. With the advancing of the mining face, the residual coal pillars are gradually recovered and melted water after the cutting is pumped out.

Claims

1. A method for recovering residual coal pillars by freezing accumulated water in a room-and-pillar mining area, comprising: (S1) performing core drilling and peeping a room-and-pillar goaf; combined with original geologic data and technical data of a mine and distribution of a coal pillar group and a goaf group, plotting a distribution pattern diagram of an overlying stratum of the room-and-pillar goaf, residual coal pillars in the room-and-pillar goaf and the room-and-pillar goaf to guide safe production; and determining height, volume and water quality of accumulated water in the room-and-pillar goaf; (S2) measuring strength and Mohs hardness of a coal sample obtained by the core drilling, and measuring strength and Mohs hardness of an ice body formed from a water sample from the accumulated water in the room-and-pillar goaf; regarding the room-and-pillar goaf as an intact unmined coal seam, and in combination with mechanical parameters of the coal sample and the ice body, carrying out design of a mining face, equipment selection and roadway design of the mining face for recovery of the residual coal pillars in the room-and-pillar residual mining area; (S3) regarding an existing roadway on a same seam in parallel with a direction of the room-and-pillar goaf as a freezing auxiliary roadway; drilling a series of horizontal boreholes on a wall of the freezing auxiliary roadway towards an accumulated water area of the room-and-pillar goaf; arranging a plurality of first freezing pipelines respectively in the series of horizontal boreholes for communication with the accumulated water of the room-and-pillar goaf, and in a case that there is no suitable roadway to choose from the coal seam where the room-and-pillar goaf is located, choosing a roadway in the overlying stratum of the room-and-pillar goaf; drilling a series of vertical boreholes from top to bottom; and arranging the plurality of first freezing pipelines in the series of vertical boreholes in the overlying stratum; (S4) deploying a freezing workstation in the freezing auxiliary roadway arranged in step (S3); circulating a salt water through the plurality of first freezing pipelines to perform heat exchange with the accumulated water, so as to allow the accumulated water to enter an active freezing period, thereby converting the accumulated water into a frozen ice body with a certain carrying capacity, wherein a temperature range of the frozen ice body in the active freezing period is from 20 C. to 12 C.; based on characteristics that water changes with a shape of a contained body therein and conversion of water into ice results in volume expansion, freezing the accumulated water to abut against to a roof of the room-and-pillar goaf; regarding the frozen ice body from the accumulated water in the room-and-pillar goaf as a filling body, in which the residual coal pillars in the room-and-pillar goaf and broken rocks are contained,, wherein the frozen ice body in the room-and-pillar goaf, roof and floor of the room-and-pillar mining area, the residual coal pillars in the room-and-pillar goaf and boundary coal pillars of the room-and-pillar goaf together form a collaborative carrier to jointly bear a load transmitted from the overlying stratum; (S5) in a case that the accumulated water is insufficient such that the frozen ice body fails to fully abut against the roof of the room-and-pillar goaf, during a freezing process, additionally injecting water through the series of horizontal boreholes or the series of vertical boreholes drilled in step (S3) to ensure the frozen ice body to fully abut against the roof of the room-and-pillar goaf, and in a case that the room-and-pillar goaf is filled with the accumulated water, considering a volume expansion caused by freezing, partially removing the accumulated water through the series of horizontal boreholes or the series of vertical boreholes drilled in step (S3) to ensure the frozen ice body to just abut against the roof of the room-and-pillar goaf; (S6) after the accumulated water in the room-and-pillar residual mining area is completely frozen, reducing a cooling power of the freezing workstation to enter a negative freezing period to ensure that the frozen ice body will not defrost, wherein a temperature of the frozen ice body in the negative freezing period is kept at 10 C. to 5 C.; (S7) digging the frozen ice body to form a transporting roadway and a ventilating roadway therein; melting, by an explosion-proof electric heating bar, the frozen ice body along a central axis of the transporting roadway and the ventilating roadway, and pumping out water formed by melting the frozen ice body; after the transporting roadway and the ventilating roadway reaches a design size, arranging a second freezing pipeline on a surface of an inner wall of the transporting roadway and the ventilating roadway to maintain the surface of the transporting roadway and the ventilating roadway in a frozen state, so as to avoid melting of the surface of the transporting roadway and the ventilating roadway caused by heat radiation generated by air ventilation and equipment transportation and maintain the transporting roadway and the ventilating roadway in a design shape and the design size; transporting and deploying corresponding mining equipment through the transporting roadway and the ventilating roadway; arranging an open-off cut on a designated position of the mining face, and arranging the mining face; and (S8) in the negative freezing period, regarding the frozen ice body and the residual coal pillars as a whole, and stepwise cutting, by a coal cutter, the frozen ice body and the residual coal pillars from the open-off cut to recover residual coal in a whole coal seam; wherein ice debris formed by cutting of the frozen ice body is transferred, by a scraper conveyor, together with coal blocks formed by cutting; and arranging a water pump and a discharging groove on each of the mining face and the transporting roadway and the ventilating roadway, so as to pump out water formed by melting the ice debris during cutting and transporting.

2. The method of claim 1, wherein a plurality of temperature sensors are placed together with the plurality of first freezing pipelines and evenly arranged in the accumulated water in the room-and-pillar residual mining area to establish a real-time dynamic monitoring network to monitor a temperature of the accumulated water or the frozen ice body in the room-and-pillar goaf in real time.

3. The method of claim 1, wherein in step (S1), width and height of each of the goaf group and the coal pillar group in the room-and-pillar goaf are obtained by searching the geologic and technical data of the mine; a three-dimensional laser scanner is adopted to determine a distribution orientation, size and volume of the goaf group and determine a depth, distribution range and volume of the accumulated water in the room-and-pillar residual mining area; and in step (S2), the room-and-pillar residual mining area is regarded as the intact unmined coal seam for mining design; wherein corresponding mechanical parameters are comprehensively considered based on mechanical parameters of the ice body and the mechanical parameters of the coal sample; and a uniaxial compressive strength of the ice body is 3-6 MPa and a Mohs hardness of the ice body is 2.8-4.

4. The method of claim 1, wherein in step (S3), the existing roadway on the same seam in parallel with the direction of the room-and-pillar goaf is subjected as the freezing auxiliary roadway; the plurality of first freezing pipelines are arranged in the accumulated water in the room-and-pillar goaf through the series of horizontal boreholes or the series of vertical boreholes, wherein the number of each of the series of horizontal boreholes and the series of vertical boreholes is determined by a required cooling capacity, the volume of the accumulated water and a radius of each of the plurality of first freezing pipelines; the plurality of first freezing pipelines are configured to form a closed line for salt water circulation to replace the heat of the accumulated water, so that the accumulated water is frozen into the frozen ice body.

5. The method of claim 4, wherein when a width of the mining face is larger than 50 m, and it is difficult to completely freeze the accumulated water in the room-and-pillar goaf by the plurality of first freezing pipelines arranged through the series of horizontal boreholes or the series of vertical boreholes, the frozen ice body with a thickness of 10-20 m can be frozen in the boundary coal pillars of the room-and-pillar goaf; the transporting roadway and the ventilating roadway and the mining face are arranged in the frozen ice body formed in an inner side of the boundary of the room-and-pillar goaf; the transporting roadway is widened and is provided with a digging auxiliary chamber, wherein the digging auxiliary chamber is configured to arrange the freezing workstation; and the transporting roadway has a function of the freezing auxiliary roadway.

6. The method of claim 1, wherein in step (S4), a type, power and number of the freezing workstation are determined based on the required cooling capacity calculated by the volume of the room-and-pillar goaf and the volume of the accumulated water in the room-and-pillar goaf in step (S2); a salt water circulation system selects a CaCl.sub.2 solution as a refrigerant, and a cooling water circulation system is cooled naturally by digging a pool; after determination of freezing parameters, a trial run of the freezing workstation and the plurality of first freezing pipelines is carried out in a designated position of the freezing auxiliary roadway in the mine; and a formal construction is carried out after a whole system consisting of the freezing workstation and the plurality of first freezing pipelines runs correctly.

7. The method of claim 1, wherein in step (S5), the frozen water has an expanding volume, and the volume of a frozen ice body is 1.1 times that of an original water; in a case that the accumulated water does not abut against the roof of the room-and-pillar goaf, and a distance between a water level of the accumulated water and the roof of the room-and-pillar goaf accounts for less than 10% of a total height of room-and-pillar goaf, the frozen ice body can fully abut against the roof of the room-and-pillar goaf; in a case that the accumulated water is less, the frozen ice body cannot abut against the roof of the room-and-pillar goaf, artificial water injection is used to increase an amount of the frozen ice body to fill a whole room-and-pillar goaf, which plays a supporting role for the overlying stratum.

8. The method of claim 1, wherein in step (S6), after the accumulated water in the room-and-pillar residual mining area is completely frozen, an uniaxial compressive strength of the frozen ice body in the negative freezing period is 3 MPa-6 MPa; an extension strength of the frozen ice body in the negative freezing period is about of its compressive strength; a compressive strength of the frozen ice body under a side limit of the boundary coal pillars is 5-10 MPa.

9. The method of claim 8, wherein the negative freezing period in step (S6) refers to a phase in which, after the freezing effect of the active freezing period on the accumulated water in the room-and-pillar mining area, the accumulated water in liquid phase is frozen completely, and a freezing process is basically completed; in the negative freezing period, the frozen ice body only needs to maintain a frozen state to ensure that it will not defrost; a temperature of a circulating salt water increases, and the temperature range of the frozen ice body increases from 10 C. to 5 C.; and the Mohs hardness of the frozen ice body is reduced.

10. The method of claim 1, wherein in the stepwise cutting of the frozen ice body and the residual coal pillars in step (S8), the frozen ice body and the residual coal pillars are regarded as a whole while maintaining the negative freezing period, and the coal cutter works under the cover of a hydraulic support, and a roof collapses with mining.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a sectional view of an overlying stratum of a room-and-pillar goaf according to an embodiment of the present disclosure.

[0036] FIG. 2 shows a distribution of a residual coal pillar in the room-and-pillar goaf and the room-and-pillar goaf according to an embodiment of the present disclosure.

[0037] FIG. 3 shows an arrangement of a freezing line and a distribution of a frozen ice body according to an embodiment of the present disclosure.

[0038] In Figures: 1, overlying stratum; 2, residual coal pillar; 3, accumulated water; 4, boundary coal pillar; 5, transporting roadway; 6, freezing auxiliary roadway; 7, freezing workstation; 8, first freezing pipeline; and 9, frozen ice body.

DETAILED DESCRIPTION OF EMBODIMENTS

[0039] This application is further described by embodiments below, but is not limited to the following embodiments.

[0040] To make the understanding of the objects, features and effects of this application more clearly, a method for recovering residual coal pillars by freezing accumulated water in a room-and-pillar mining area will be further described below with reference to the accompanying drawings.

[0041] A certain mine is faced with a problem of low recovery rate of resources, so it is urgent to seek new methods to increase recovery rate and extend mine service life. It is a good method to recover residual coal pillars from a No. 2 coal seam which was mined by room-and-pillar mining in an early stage. The No. 2 coal seam has a thickness of 4.2 m, restricted by early backward mining technology, was mined by room-and-pillar mining method. During coal mining, coal pillars with different shapes were left between coal rooms to support top plates, and a recovery rate of coal resources in a whole coal seam was less than 40%. After mining, most areas of a goaf were gradually filled with mine water, which were typical room-and-pillar mining areas with a large amount of high-quality coal. Referring to a traditional paste filling mining method, it is necessary to pump out a large amount of accumulated water in the room-and-pillar goaf at one time, and then to set filling lines for filling, which will increase the cost, therefore, it is urgent to fine a scientific, efficient and economic coal pillar recovery method. In view of the above situation, the embodiments of this application will be further described below with reference to the accompanying drawings, and specific embodiments are described as follows.

[0042] (S1) Combined with original geology data and technical data of the mine and distribution of a coal pillar group and a goaf group, a distribution pattern diagram of an overlying stratum 1 of a room-and-pillar goaf, residual coal pillars 2 in the room-and-pillar goaf and the room-and-pillar goaf were plotted to guide safe production. A height of an accumulated water 3 in the room-and-pillar goaf was 4 m, and a volume of the accumulated water 3 accounted for 80% of the room-and-pillar goaf.

[0043] (S2) Before a mining design, the residual coal pillars 2 in the room-and-pillar goaf had a uniaxial compressive strength of 10.6 MPa and a Mohs hardness of 1.8-2. The accumulated water 3 in the room-and-pillar goaf was sampled, and a frozen ice body had a uniaxial compressive strength (under a freezing environment from 10 C. to 5 C.) of 3 MPa-6 MPa and a Mohs hardness of 2.8-4. Combining mechanical parameters, such as a compressive strength and Mohs hardness of the coal pillars and mechanical parameters, such as a compressive strength and Mohs hardness of the frozen ice body, a room-and-pillar mining area was regarded as an intact unmined coal seam to carry out design of a mining face, equipment selection and roadway design of a mining face for recovery of the residual coal pillars in the room-and-pillar mining area.

[0044] (S3) A mining face of the room-and-pillar goaf was 180 m. Drilling was subjected through an existing roadway on a same seam in parallel with a direction of the room-and-pillar goaf. A plurality of freezing pipelines 8 was arranged to freeze all the accumulated water 3 in the room-and-pillar goaf. Boundary coal pillars of the residual mining area was drilled towards the room-and-pillar goaf, and a part of the accumulated water in the room-and-pillar goaf was frozen in the boundary coal pillars of the residual mining area to form the frozen ice body with a width of 20 m. The accumulated water within a freezing range was frozen, and then a temperature of a circulating salt water increased in a small range, and a temperature of the frozen ice body increases from a range from 12 C. to 20 C. in an active freezing period to a range 5 C. to 10 C.

[0045] (S4) The frozen ice body was kept in a negative freezing period, and was dug, and then a transporting roadway 5 was arranged in the frozen ice body. The frozen ice body was stepwise molten by an explosion-proof electric heating bar along a central axis of the transporting roadway, and water formed by melting the frozen ice body was pumped out. The transporting roadway 5 with a designed size was formed by melting the frozen ice body. A second freezing pipeline was arranged on a surface of an inner wall of the transporting roadway 5, so as to maintain the surface of the transporting roadway 5 and avoid melt of the surface of the transporting roadway 5 caused by heat radiation generated by air ventilation and equipment transportation and maintain the transporting roadway in a design shape and the design size.

[0046] (S5) The transporting roadway was widened, and a side of the transporting road was provided with a digging auxiliary chamber. The digging auxiliary chamber was configured to arrange a freezing workstation 7. A side of the transporting roadway 5 was configured as a freezing auxiliary roadway 6.

[0047] A selection and arrangement of freezing equipment were as follows.

[0048] A refrigerating machine adopted a SKD136.1.H type screw chiller unit, where working conditions of individual unit were a cooling capacity of 116960 Kcal/h and a motor power of 114 KW.

[0049] Each refrigerating machine was equipped with a salt water circulating pump, where the salt water circulating pump adopted IS150-125-315 type with a single flow rate of 200 m.sup.3/h and a motor power of 37 KW.

[0050] A cooling water circulating pump of the freezing workstation adopted IS150-125-315B type with a single flow rate of 173 m.sup.3/h and a motor power of 18.5 KW.

[0051] Each freezing workstation needed to be equipped with additional salt water circulating pump and cooling water circulating pump as alternatives.

[0052] Main parameters for freezing construction were as follows: [0053] Refrigerant: freon R-22; [0054] Refrigerating machine oil: Hanzhong HBR-B03 refrigerating machine oil; [0055] Temperature of freezing salt water: active freezing period: from 25 C. to 20 C.; and negative freezing period: from 20 C. to 15 C.; [0056] Average temperature of accumulated water after freezing: from 12 C. to 8 C.; and [0057] Delivery pipe of freezing salt water: flexible low-temperature resistant metal pipe.

[0058] Specific freezing parameters and details were as follows.

[0059] A salt water circulation system selected a CaCl.sub.2) solution as a refrigerant. A cooling water circulation system was cooled naturally by digging a pool. After determination of freezing parameters, a trial run of the freezing equipment was carried out, and a formal construction was carried out after a whole system consisting of the freezing workstation and the plurality of first freezing pipelines run correctly.

[0060] (S6) A plurality of freezing workstations 7 were arranged in the freezing auxiliary roadway 6 set in step (S5) along a direction of the mining face according to schemes of the freezing construction. The circulating salt water was configured to replace heat of the accumulated water through the freezing line 8 in the accumulated water 3 in the room-and-pillar goaf, so that the accumulated water 3 in the room-and-pillar goaf enter the active freezing period, and water in a liquid phase become ice with a certain carrying capacity, and a temperature range of the frozen ice body was from 12 C. to 20 C. Characteristics that water changes with a shape of a containing body and frozen water has expanding volume were utilized, so that water was frozen and subjected to fully abut against a roof of the room-and-pillar goaf. The frozen ice body 9 frozen from the accumulated water in the room-and-pillar goaf was regarded as filling body, and the residual coal pillars in the room-and-pillar goaf and a broken stone were frozen into the frozen ice body. A whole room-and-pillar goaf was filled with a huge ice frozen by the accumulated water. The frozen ice body 9 in the room-and-pillar goaf, a top and bottom plate of the residual mining area, the residual coal pillars 2 in the room-and-pillar goaf and a boundary coal pillar 4 in the room-and-pillar goaf formed a whole with a certain bearing capacity.

[0061] (S7) The volume of the accumulated water 3 accounted for 80% of the room-and-pillar goaf, and a volume of the frozen ice body was 1.1 times that of an original water. When the accumulated water is less, additional water was injected through boreholes formed in step (S3), so as to ensure the frozen ice body 9 in the room-and-pillar goaf to just abut against the roof of the room-and-pillar goaf.

[0062] (S8) After the accumulated water in the room-and-pillar residual mining area was completely frozen, a cooling capacity was reduced by the plurality of freezing workstations 7 arranged along the mining face in step (S5) to enter the negative freezing period to ensure the frozen ice body will not defrost, where a temperature of the frozen ice body in the negative freezing period was kept from 10 C. to 5 C.

[0063] (S9) In the negative freezing period, the transporting roadway and the ventilating roadway arranged in step (S4) was examined and completed. Corresponding mining equipment were transported and arranged through the transporting roadway and the ventilating roadway, and an open-off cut arrangement was carried out on a designated position of the mining face, and the mining face was arranged.

[0064] (S10) In the negative freezing period, the frozen ice body 9 and the residual coal pillars 2 were regarded as a whole, which was gradually cut by a coal cutter from the initial cutting opening to dig and recover the coal in a whole coal seam. With the advance of the mining face, the roof will collapse with mining.

[0065] (S11) A part of the frozen ice body was melted during cutting, and ice debris formed by cutting of the frozen ice body was transferred, by a scraper conveyor, together with coal blocks formed by cutting. A water pump and a discharging groove were arranged on each of the mining face, the transporting roadway and the ventilating roadway, so as to pump out water formed by melting the ice debris during cutting and transporting.