How to prevent and extinguish fire by grouting method

Coal mine fire is one of the five disasters that seriously threaten coal mine safety production. It not only poses a great threat to the life and property safety of underground workers, but also may burn a lot of valuable resources. Therefore, how to effectively prevent and control mine fires has become a difficult problem for researchers and field technicians. There are many ways and means to prevent and control mine fires, such as grouting fire prevention, inhibitor fire prevention, gel fire prevention, nitrogen injection fire prevention, pressure equalization fire prevention, air plugging and curing foam fire prevention technology. These technologies have their own characteristics, their own conditions or scope of use, and have played an important role in ensuring safe production in mines. Based on the author's shallow understanding of mine fire prevention, this paper mainly talks about the method of grouting to prevent and extinguish goaf.

2 characteristics of grouting fire prevention technology

Grouting fire prevention is a direct technology to prevent coal spontaneous combustion. Grouting is to mix and stir solid materials such as clay, crushed shale and power plant fly ash with water to make slurry with a certain concentration, which is injected or sprayed in the goaf by means of slurry pipeline to achieve the purpose of fire prevention and extinguishing. Its main feature is that the coal body is wrapped by slurry with a certain concentration. On the one hand, it can isolate the coal body from oxygen, making it lack of combustion improver and achieving the function of fire extinguishing; On the other hand, it can reduce the temperature of coal through the evaporation of water and other liquids in the slurry, so as to achieve the effect of fire extinguishing; In addition, it can also play the role of bonding the bottom plate, thus increasing the air leakage resistance and ultimately reducing air leakage and oxygen supply. The essence of grouting fire prevention is to inhibit the oxidation rate of coal at low temperature and prolong the spontaneous combustion period. The advantage of grouting fire prevention technology is that the whole process is relatively simple and the cost is relatively low, which is the most widely used and the best effect technology in the mine. The disadvantage of grouting fire prevention and extinguishing technology is that due to the influence of gravity, slurry only flows to low-lying places and cannot be piled up to high places, so its effect on high-altitude fires is limited; Because the slurry may not cover the floating coal evenly, it is easy to form the phenomenon of "ditch pulling"; In addition, due to the fluidity of coal slurry, it may cause slurry leakage and slurry collapse, thus deteriorating the working environment and affecting coal quality.

3 goaf grouting fire prevention method

3. 1 grouting system

The grouting system consists of pulping equipment, slurry conveying pipeline and grouting borehole. At present, there are two main methods to prepare grouting slurry: hydraulic preparation and mechanical preparation. Hydraulic preparation is a simple pulping method, which uses high-pressure water gun to scour loose clay layer to mix water and soil into slurry, but the slurry concentration is difficult to ensure and the fire prevention effect is poor. Mechanical pulping is to send pulping raw materials and water into a mixing tank according to a certain proportion, stir with a blender, and send them underground through a grouting pipeline. However, the current grouting system generally has some shortcomings, such as easy pipe plugging, low slurry conveying strength, high requirements for slurry and large investment. At present, the existing grouting system for coal mine fire prevention and extinguishing has the characteristics of mobile light equipment, cooperative grouting of multiple groups of slurry pools, long-distance slurry transportation and so on. After filtration, there are multiple slurry delivery ports, and various grouting materials such as loess and fly ash can be used, which has the advantages of simple equipment, less investment, fast construction, strong slurry delivery and antifreeze.

3.2 Grouting method

There are many preventive grouting methods, which can be divided into pre-grouting before mining, irrigation with mining and post-mining grouting according to the relationship between coal mining and grouting.

Pre-grouting before mining is to inject coal seam before mining, which is suitable for mines with too many goaf and serious spontaneous combustion. Grouting while mining refers to grouting into the goaf while the coal mining face is advancing, mainly including drilling grouting, buried pipe grouting and grouting, which can timely grout the goaf with backward roof caving; Post-mining grouting refers to grouting after the whole goaf is closed. In order to ensure the timely and simple treatment of spontaneous combustion hazards, the buried pipe grouting method is adopted in the design.

Grouting pipe was laid in the goaf of return air roadway before caving by buried pipe grouting method. One end of the embedded pipe is connected with the goaf, and the other end is connected with the rubber hose. The length of rubber hose is generally 20 ~ 30m. With the advance of the working face, grouting pipe is gradually pulled by a prop-pulling winch, and grouting is carried out at regular intervals, so that the goaf of the working face can be filled with enough mud.

3.3 Selection of grouting parameters

The water-solid ratio of mud is an index reflecting the mud concentration, which refers to the volume ratio of water to solid mud in mud. Water-solid ratio affects grouting effect and mud transportation. The smaller the water-solid ratio of coal slurry, the greater the concentration of coal slurry, the greater its viscosity, stability and compactness, and the better the effect of wrapping residual coal to isolate oxygen, but at the same time, the smaller the dispersion range, the easier it is to block the coal slurry transportation pipeline; When the water-solid ratio is large, the amount of water used to transport the same volume of soil is large, the package isolation effect is not good, the mine water inflow increases, and it is easy to flow out when grouting in the goaf behind the working face, which deteriorates the working face environment. The water-solid ratio of slurry should be determined by experiments according to the distance of slurry, coal seam dip angle, grouting method, grouting material and season, and it is generally 4: 1 and 5: 1 in winter. The amount of grout required for daily grouting and the daily grouting amount can be calculated according to the following formulas respectively, where q is the amount of grout required for daily grouting, m3/d; M represents the mining height of coal seam, m; L represents the daily advancing degree of the working face, m; H represents the inclined length of grouting area, m; C stands for recovery rate,%; K represents the grouting coefficient, which is the ratio of the solid volume of grouting material to the volume of goaf to be grouted, and generally takes 0.05 ~ 0.15; Q water 2 represents the daily water consumption for grouting, m3/d; Q grout 1 represents the daily grouting quantity, m3/d; M stands for mud production, which is 0.88. It should be noted that the parameters of grouting system, such as grouting coefficient and water-soil ratio, must be determined by experiments in actual production according to factors such as coal seam ignition, transportation distance, coal seam dip angle, grouting method, grouting material and season, so as to ensure grouting effect and production safety. In addition, grouting works closely with mining works.

3.4 Selection of grouting materials

The selection of grouting materials for mine fire prevention and extinguishing has a certain emphasis, and the selected materials require particles less than 2mm, with fine particles accounting for the majority; Its main physical properties are as follows: the specific gravity is 2.4 ~ 2.8t/m3；; The plasticity index is 9 ~ 1 1 (loam); Colloidal mixture (calculated by MgO content) is 25-30%; The sand content is 25-30%; It is easy to dehydrate and has certain stability; Does not contain flammable materials.

At present, the commonly used grouting materials are loess, fly ash and so on. Compared with loess, the particle size of fly ash is larger, but its bulk density is smaller. As far as the fire extinguishing by grouting is concerned, the fly ash is light, the particle surface has a certain smoothness, and it is easy to be stirred into slurry, which is convenient for pipeline transportation. After being injected into the fire zone, the fluidity and stability are good; Fly ash has certain volcanic activity and good sealing performance; Fly ash has poor hydrophilicity and larger particle size than loess. After grouting, the slurry will be dehydrated quickly when it reaches a static state, and some heat will be taken away with the discharge of water. Therefore, fly ash can be used for fire extinguishing by grouting, which has the functions of isolation, wrapping and cooling.

3.5 grouting fire safety measures

After selecting grouting system, grouting method, suitable grouting parameters and grouting materials, corresponding measures should be taken to prevent Explosicum and water seepage accidents after grouting. The main safety measures are as follows.

(1) Grouting materials shall meet the requirements of relevant specifications, and the slurry-water ratio shall be strictly controlled, and the grouting quantity shall not be too large.

(2) There are ditches and sump pits in the gateway of the working face, and a small water pump is equipped, which can meet the discharge requirements of gushing water and mud-water separation water when the working face is inclined to mine.

(3) The underground grouting in this mine adopts the method of increasing mining and irrigation, and part of the grouting water will flow into the working face conveyor or gateway from the goaf. At this time, it is best to build a slurry filter seal in the roadway to keep the slurry in the goaf and release water.

(4) Strengthen water regime observation, observe and record the grouting quantity and displacement of goaf. If the displacement is too small, mud may accumulate in the grouting area; There is a large amount of mud in the drainage, and a mud channel may be formed in the goaf.

(5) Leave 20m security pillar between the fully-mechanized mining face and the replacement face, close the communication lane in time as required, and place a drainage pipe on the closed upper part to ensure that the water in the goaf will not collapse into the gateway of the production face when mining the working face. If it is found that there is more water in the closed room, measures must be taken or drainage must be carried out.

(6) Strengthen pipeline maintenance.

4 conclusion

Because the whole process of grouting fire prevention and extinguishing technology is simple and the cost is relatively low, it plays a very important role in mine fire prevention and extinguishing. Based on the author's shallow understanding of mine fire prevention and extinguishing, this paper briefly introduces the characteristics of grouting fire prevention and extinguishing technology, grouting system, grouting method, grouting parameter selection and grouting material selection, hoping to play a role in attracting jade.