Method for recovering residual coal pillars by Freezing water accumulated in room-and-pillar mining area

A method for recovering residual coal pillars by freezing accumulated water in a room-and-pillar residual mining area, comprising:

• (S1) performing core drilling and peeping the room-and-pillar residual mining area;
• 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 residual mining area, residual coal pillars in the room-and-pillar residual mining area and the room-and-pillar residual mining area to guide safe production;
• and determining height, volume and water quality of accumulated water in the room-and-pillar residual mining area for subsequent design of freezing the accumulated water in the room-and-pillar residual mining area;
• (S2) measuring strength and Mohs hardness of a coal sample obtained by the core drilling from the residual coal pillars in the room-and-pillar residual mining area, and measuring strength and Mohs hardness of an ice body formed from a water sample from the accumulated water in the room-and-pillar residual mining area;
• regarding the room-and-pillar residual mining area 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, roadway design of the mining face and arrangement of an open-off cut 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 residual mining area as a freezing auxiliary roadway;
• deploying a freezing workstation;
• drilling a series of horizontal boreholes on a wall of the freezing auxiliary roadway towards an accumulated water area of the room-and-pillar residual mining area;
• 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 residual mining area;
• and
• in a case that there is no suitable roadway to choose from the coal seam where the room-and-pillar residual mining area is located, choosing a roadway in the overlying stratum of the room-and-pillar residual mining area;
• drilling a series of vertical boreholes from top to bottom;
• and placing and arranging the plurality of first freezing pipelines in the series of vertical boreholes in the overlying stratum for communication with the accumulated water of the room-and-pillar residual mining area;
• (S4) deploying the freezing workstation in the freezing auxiliary roadway arranged in step (S3);
• circulating a salt water through the plurality of first freezing pipelines arranged in the accumulated water of the room-and-pillar residual mining area to perform heat exchange with the accumulated water, so as to allow the accumulated water of the room-and-pillar residual mining area to enter an active freezing period, thereby converting the accumulated water in liquid phase 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 residual mining area;
• regarding the frozen ice body from the accumulated water in the room-and-pillar residual mining area as a filling body, in which the residual coal pillars in the room-and-pillar residual mining area and broken rocks are contained with the accumulated water to form a frozen body, and a whole room-and-pillar residual mining area is filled with the frozen ice body frozen from the accumulated water, wherein the frozen ice body in the room-and-pillar residual mining area, roof and floor of the room-and-pillar residual mining area, the residual coal pillars to be recovered in the room-and-pillar residual mining area and boundary coal pillars of the room-and-pillar residual mining area together form a collaborative carrier to jointly bear a load transmitted from the overlying stratum;
• (S5) in a case that the accumulated water frozen in step (S4) is insufficient such that the frozen ice body fails to fully abut against the roof of the room-and-pillar residual mining area, during a freezing process of the accumulated water in step (S4), 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 frozen from the accumulated water in the room-and-pillar residual mining area to fully abut against the roof of the room-and-pillar residual mining area;
• and
• in a case that the room-and-pillar residual mining area is filled with the accumulated water, considering a volume expansion caused by freezing the accumulated water in step (S4) and the accumulated water is much, 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 frozen from the accumulated water in the room-and-pillar residual mining area to just abut against the roof of the room-and-pillar residual mining area;
• (S6) after the accumulated water in the room-and-pillar residual mining area is completely frozen, reducing a cooling power of the freezing workstation in step (S4) 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 in the negative freezing period 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 the 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;
• the frozen ice body is melted during cutting, 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, so as to pump out water formed by melting the ice debris during cutting and transporting.