The range of surface subsidence and collapse is generally larger than the area of goaf. When the goaf is rectangular, the subsidence basin is roughly elliptical (Figure 5-4). The range of elliptical basin is related to the movement angle δ of coal seam strike direction and the movement angles β and γ of inclined direction.

The subsidence and caving of α coal seams with different dip angles are different. The physical and mechanical properties of overlying strata in coal seam have great influence on surface subsidence. Under the same dip angle of coal seam, the moving angle increases with the increase of rock strength. In general, the movement angle of bedrock is 50 ~ 90, and the movement angle of topsoil is 45 ~ 55. Therefore, the harder the overburden is, the greater the ground subsidence is. According to the characteristics of overlying strata, the movement angle values are shown in Table 5-3. Using the data in Table 5-3 and the actual related data of the studied mine, the surface subsidence range of the corresponding coal seam after mining can be predicted by conventional mapping method. According to the analysis of the measured data of Xiangshan Coal Mine in Hancheng, Shaanxi Province, when the goaf is 2ha (that is, when the1000-meter long working face advances 200m), the surface movement is active, and most of the collapse cracks occur at this time. Statistics show that the average ratio of ground subsidence area to underground coal mining area is 1.2, and the average ratio of subsidence amount to mining amount is 0.6 ~ 0.7. For gently inclined coal seams, the maximum surface subsidence depth is generally 70% of the total thickness of coal mining (Table 5-3).

Figure 5-4 Schematic Diagram of Ground Subsidence in Underground Coal Mining

A- inclined coal seam; B- steep seam

1- subsidence area; 2-fracture zone; 3- moving area; 4- goaf; 5— Horizontal projection plane of goaf; 6- Moving Boundary Line

Table 5-3 Movement Angle of Overlying Rock with Different Hardness

5.4.3.2 mining subsidence index

The area and depth of surface subsidence area caused by mining subsidence are related to coal seam occurrence, mining depth and goaf treatment methods. The area of collapse fracture zone is positively correlated with the mining scale. For example, in underground coal mining, people are used to using the land area (ha) of 10,000 tons of coal ground subsidence as an index to reflect the degree of surface morphological damage, which is usually called collapse index or collapse rate, reflecting the correlation between coal mining amount and collapsed land occupation to some extent. All mining areas and regions have their average values. For example, in 1996 and 2000, the coal mine collapse index in Shaanxi was 0.258 and 0.237, and that in Gansu was 0.236 and 0.5438+0. Ningxia is 0.378 and 0.368, Xinjiang is 0. 146 and 0. 160, and Qinghai is 0. 178 and 0. 188.

Experience shows that the boundary value of coal mine subsidence can be determined by extrapolating the mining boundary by 0.5 times the mining depth. When the mining depth H=500m, the influence boundary value of subsidence is about 250m, the subsidence area is about 1.2 times of the mining area of coal seam, and the maximum subsidence value is 70% ~ 80% of the mining thickness of coal seam.

1995 to 2003, Shaanxi 185 coalfield geological exploration team, Shenfu Coal Survey and Design Branch, Tangshan Branch of Coal Research Institute and other units were in Daliuta Coal Mine 1203 working face and Bulianta Coal Mine 22 165438 working face in Shendong Mining Area of Shaanxi and Inner Mongolia. Observation stations were set up at 45 10 1 working face of Yujialiang Mine and 881kloc-0/working face of Sunjiagou Mine, and the ground subsidence of underground coal mining was observed on the spot. The measured (calculated) values are shown in Table 5-4. Using the table of movement and deformation distribution function (Table 5-5), the predicted value of movement and deformation on the strike normal section (Table 5-6) is calculated, and the surface movement and deformation curve of the mining area is made according to the relevant parameter values in Table 5-6 (Figure 5-5).

The process of surface movement and strata movement is influenced by geology and coal mining conditions. Through the observation data and comprehensive data of each mine working face (Table 5-4), the main parameters of surface movement and deformation can be predicted. Budget steps:

Table 5-4 Measured (Calculated) Value of Mining Collapse in Shendong Mining Area

(1) Determine the estimation parameters:

Inflection point offset So=30m,

Main influence radius

(2) Estimated maximum movement and deformation value:

The maximum subsidence value wo = mqcos α = 3.7×1000× 0.6× cos2.5 = 2255 (mm);

Maximum tilt value;

Maximum curvature value;

Maximum horizontal displacement uo = bwo = 0.27× 2255 = 609 (mm);

Maximum horizontal deformation value

(3) Estimated movement and deformation value on the main section: the origin of the X axis is selected at the O point 30m away from the actual boundary of the working face (since So is positive, it should be measured in the direction of the goaf) and points to the goaf (Figure 5-5). Pre-calculation should adopt the table of distribution function values of movement deformation (Table 5-5), and the pre-calculation method and results are shown in Table 5-6. According to the data in Table 5-6, the prediction curve of surface movement and deformation in Shendong mining area is drawn, as shown in Figure 5-5.

Figure 5-5 Prediction Curve of Surface Movement and Deformation in Shendong Mining Area

1- subsidence curve; 2— Inclined curve; 3— Curvature curve; 4- horizontal motion curve; 5— Horizontal deformation curve

A -- the actual coal wall position; B—— imaginary coal wall position used in calculation; ABC-the original position before sinking; Ab1c1-the actual position of the roof after sinking; D- inflection point; So-offset

Table 5-5 Surface Subsidence Movement and Deformation Distribution Function Table

Note: When the value is "+",a () takes the number of the previous line, and a "() takes the symbol" "; When the value is "",a () takes the number of the next line, and a "() takes the+sign.

Table 5-6 Prediction Value of Movement and Deformation of Strike Normal Section

Ground subsidence and ground fissures in 5.4.3.3.

5.4.3.3.1Leading influence of working face

The dominant influence in the advancing process of the working face can be illustrated by the strike profile (Figure 5-6). After the working face advances from the open cut to point A for a certain distance, the strata movement begins to spread to the surface, which is called the initial distance. When the observed surface subsidence distance reaches 10 mm, the surface begins to sink, and the starting distance is mainly related to roof lithology and goaf. General working face advance (0.2 ~ 0.3) H0. If calculated by area, the starting distance of Daliuta Mine 1203 working face is between 10 ~ 13m, so the starting distance is determined to be 0.2H0.

Figure 5-6 Advance influence in the process of working face propulsion

When the working face advances to point B, the subsidence curve W 1 is obtained, and the 1 point in front of the working face begins to sink due to mining. When the advancing distance is about 1.4H0, that is, 85m (point C), the subsidence curve W2 is obtained, and the surface begins to sink due to mining. In the process of working face advancing, the front surface sinks due to the influence of mining, which is called lead influence. L 1, L2 and L3 are the main influencing distances. Ω1,Ω 2, Ω 3 are the leading influence angles, and the leading influence angle of Yujialiang Mine is 79, which is equal to the boundary angle δ 0 (64) when the mining face is finished and the surface movement is stable. The comprehensive leading influence distance of Shendong mining area is l = h0ctg δ o =130ctg1= 72m.

Surface moving basin

The actual observation shows that when the length D2 and width D 1 of the goaf reach or exceed (1.2 ~ 1.4) H0 (H0 is the average mining depth), the surface can be fully mined (a flat bottom appears in the surface moving basin). In Shendong mining area, D2/H0= 15.7, D1/H0 =1.51,and the surface can be fully exploited.

The surface moving basin at the end of mining is shown in Figure 5-7. The moving basin in Shendong mining area is divided into three boundaries: ① the outermost boundary delineated by the boundary angle of 6 1 (with the point of surface subsidence 10mm as the demarcation point), namely ACBD；; In the figure; (2) The boundary of dangerous movement is defined by whether the ground movement and deformation in the basin do harm to buildings, and the standard of whether it does harm to buildings is measured by the critical deformation value. At present, we have adopted a set of critical deformation values 1 1, with an inclined deformation i≤3mm, a horizontal deformation ε≤2mm and a curvature k≤0.2mm/m2. Based on this index, outside the delineated range, it is a zone where surface movement and deformation have not caused obvious damage, and within the delineated range, it is a zone where surface movement and deformation have harmful effects on buildings. Shendong mining area is delineated with a movement angle of 75, as shown in Figure A' C' B' D③, which moves the fault boundary of the basin, and Shendong mining area is delineated with a fracture angle of 79, as shown in Figure A "c" b "d, obviously ACBD area > A "c" b "d area > A" c "b" d area, so we call it A ". The long axis of moving basin should be the length of working face plus the leading influence distance l = h0ctgδ = 130ctg75 = 35m, then the surface movement distance above the side of open-cut coal pillar is 55-30=25m (Figure 5-7), that is, 2044+35+25=2 104m, that is, caving.

Figure 5-7 Schematic Diagram of Surface Movement Basin Boundary in Shendong Coal Mine Area

ground fissure

The overlying strata of coal seam in Shendong mining area are mainly calcareous cemented sandstone. The compressive strength of sandstone is 22 ~ 48 MPa (the thickness-weighted average compressive strength is 42MPa). The compressive strength of coal seam is low, and it is easy to be slimed, softened and weathered when it meets water. Caving zone, fracture zone and bending zone are formed in overlying strata, but the surface is slowly and continuously deformed. However, if the mining depth is small, the caving zone and the fracture zone can reach the surface directly, and the surface is discontinuous. For example, the actual mining height m=3.5m, mining depth H0=6 1m and H0/m= 17 in Daliuta Mine. According to relevant data, in general, the height of caving fracture zone formed by weak strata is 9 ~ 12 times of mining height, and that of medium hard strata is 12 ~ 18 times of mining height. The roof of 1203 working face belongs to medium hard strata. If the height of the caving fracture zone is 18 times of the mining height, it is 18 times. There is a bending zone above caving zone, and the strata in the bending zone are in a state of bidirectional compression in the horizontal direction, which has good compactness and waterproofness. The height of bending zone is mainly affected by mining depth. When the mining depth is large, the height of the bending zone can greatly exceed the height of caving zone. At this time, the cracks formed by mining will not reach the surface, and the surface movement and deformation are relatively slow. Sometimes, there may be some cracks on the surface (caused by tensile deformation of the surface), but these cracks are large and small. When they reach a certain depth (< 5 m), they usually close and disappear by themselves, and usually do not communicate with underground cracks. This is the case with the surface cracks in the other three mines. However, due to the collapsibility of loose sand layer, this fracture zone will collapse again after being eroded by rain. Compacting this crack can prevent it from collapsing again.

Table 5-7 Mining Face and Surface Subsidence Area and Surface Subsidence Area of Ten Thousand Tons of Coal

Note: ① The coal density is 1.35t/m3, and the recovery rate of working face is 64.4% (the value of Daliuta 12032 working face), but the recovery rate of coal mining face is 65% ~ 94%, with an average of 88%.

(2) The surface subsidence of ten thousand tons of coal hectares.

(3) According to the survey data of geological disasters in Shenmu County, by the end of 200 1, the coal in Daliuta Mine was 3,676×104t, and the ground subsidence was 7700872m2, that is, the collapse index of 10,000 tons of coal was 0.2 1ha, which was the same as the national average of 0.2ha.

④ According to the survey data of geological disasters in Shenmu County, at the end of 200 1, the coal in Yujialiang Mine was 500× 104t, and the ground subsidence was 552000m2, that is, the collapse index of ten thousand tons of coal was 0. 1 1ha, which was 45% lower than the national average of 0.2ha.

To sum up, it can be seen that:

(1) As can be seen from Table 5-6 and Figure 5-8, the maximum surface inclination (x=0) has an inclination value i(0) of 4 1mm/m and a settlement value W(0)= 1 128mm, that is, the maximum settlement. When x < 0 and k (x) > 0, the subsidence curve is convex; When x > 0 and k (x) < 0, the subsidence curve is concave, and the surface point with the largest inclination angle (that is, the surface point with x=0) is the turning point of the subsidence curve from convex to concave, and the curvature value of this point is 0, which is called the inflection point (point D) of the subsidence curve.

(2) The main surface movement and deformation values all occur in the range of x =-γ ~+γ, which is called the main influence radius, and it is related to the tangent tgβ and the main influence angle β of mining depth H0. The average main influence radius of the surface in the mining area is 55m.

(3) Regardless of the cantilever effect of the roof, the inflection point of the subsidence curve is directly above the actual coal wall A, but the cantilever effect of the roof exists, and the inflection point D is directly above the imaginary coal wall B, so it is actually the inflection point offset caused by the cantilever effect, which is called inflection point offset. The average inflection point offset of surface movement in mining area is 30m, and the imaginary coal wall is the calculation boundary of mined-out area.

(4) The ground subsidence area of 10,000-ton output of three measured working faces in Shendong mining area is 0.35~0.42ha, and the average value of the mining area is 0.387ha, which is almost 0.2ha 1 times of the national average value of 10,000-ton coal output, mainly due to the small mining depth of the working face and the large mining thickness of the coal seam.

(5) According to the mining parameters of Shendong mining area and the movement angle values of loose bed and bedrock, two surface protection zones, 5× 100m2 and 5×320m2, are set, and corresponding protective coal pillars are left. It is estimated that the coal pressure of the former is 16× 104t, while that of the latter is 24.8× 104t, resulting in a great waste of coal resources. Therefore, it is necessary to conduct a comprehensive technical and economic analysis on whether to leave coal pillars for protection in general village settlements and cultivated land, or resettle part of the population and requisition related land.

Reclamation of Abandoned Land in 5.4.3.4 Mining Area

A large amount of waste residue and waste rock are produced in the mining process, and the discharge pressure occupies a large amount of land. Waste water discharge and waste residue dust pollute the land, and also seriously damage the economic value of the surrounding land, which will lose the function of cultivated land in serious cases. Therefore, in order to protect the environment and land value, it is necessary to restore and control the destroyed and occupied land as much as possible in the production process to eliminate pollution hazards. After the mine is closed, the abandoned land will be fully restored, and its use value will be restored, and it will be reused as agricultural, forestry, animal husbandry, fishery, tourism or industrial, urban and rural construction land.

Reclamation of damaged land refers to the activity of restoring all kinds of abandoned land damaged by excavation, occupation and collapse in the process of mine construction and mining to usable state by taking engineering measures or biological measures. Broadly speaking, mine land reclamation is a continuation and component of mining engineering. The best reclamation method is to closely cooperate with mining technology, make unified planning and coordination, so as to meet the needs of both production and reclamation, thus achieving the win-win goal of mineral resources development and environmental protection. Land reclamation has become an important part of land development and utilization activities, and it is an important measure for sustainable utilization of land resources, alleviating the contradiction between man and land and improving the ecological environment.

According to the mining method, topography and climatic conditions of the mining area, the land reclamation mode suitable for this mining area is selected according to local conditions. According to the different objects of mine land reclamation, there are mainly subsidence area reclamation, waste residue yard reclamation, open pit reclamation, tailings pond reclamation and so on. According to the main uses of reclamation, there are agricultural reclamation, forestry reclamation, construction land reclamation, leisure reclamation and so on.

5.4.3.4.1Reclamation of subsidence area

Land subsidence and ground fissures in the mining area have destroyed a large number of fertile fields, villages and buildings on the ground, resulting in the deterioration of the ecological environment in the mining area. The contradiction between mining enterprises and affected residents is becoming more and more serious, which has become one of the social instability factors. It is an important task to control the subsidence and ground fissure area in mining area reasonably. Due to the unreasonable allocation of land resources, unclear ownership of coal mining subsidence land, imperfect reclamation policy and management mechanism, unfulfilled funds channels for reclamation, and the fact that reclamation theory lags far behind practice, it is difficult to reclaim coal mining subsidence land. Because of the different topography, hydrometeorology and other conditions in the subsidence area caused by underground mining, the degree of land destruction and reclamation methods are also different. For mountainous and hilly areas, as long as the local collapse funnel or collapse pit and cracks are filled up, the original topography can be restored. For the plain area, if the groundwater level is low and the rainfall is low, there will be no water accumulation in the subsidence area all the year round, so it can be planted or used for other purposes only by backfilling the bedding topsoil during reclamation. If the groundwater level is high or the rainfall is heavy, and the subsidence area accumulates water all the year round, it is necessary to remove the accumulated water or renovate the water surface and surrounding environment for breeding and sightseeing.

Through the mine filling system, the mined-out area is backfilled by wind filling and hydraulic filling with the gangue discharged from the ground gangue hill and coal washing plant, which can not only reduce the movement of rock strata and the surface, but also reduce the surface subsidence, consume a lot of gangue, reduce the ground pollution and prevent the spontaneous combustion of coal seam. This method is widely used in Germany, the Soviet Union, the Czech Republic and other countries, and Jiaozuo Mining Bureau also uses this method to backfill mined-out areas, which has achieved good results.

Figure 5-8 Fishbone Diagram of Land Reclamation in Coal Mining Area

The treatment of ground subsidence cracks in China is mainly achieved through the development and utilization of subsidence areas and comprehensive treatment. Since the early 1980s, the subsidence and crack areas have been reclaimed in a planned way, and some achievements and experiences have been made in Huaibei, Huainan, Xuzhou, Datun and Pingdingshan coal mining areas, and many comprehensive management models have been put forward (Figure 5-8). 627 rural houses on the first and second floors of Pangzhuang Coal Mine in Xuzhou are ***6.9× 104m2, which are built on the gangue foundation in the subsidence area, filled in layers, vibrated and compressed in layers, and affected by the mining of two underground floors and four working faces, with no loss of houses. Comprehensive prevention measures include: ① filling coal gangue and fly ash; (2) soil improvement; ③ Stripping reclamation; ④ Comprehensive utilization of collapsed land; (5) Ecological farming can control subsidence, as shown in Figure 5-9. As a filling material, solid waste has dual benefits of landfill waste and reclamation of subsidence area. For example, Daihe Coal Mine in Huaibei has built a four-story building 1650m2, such as a trade union, a kindergarten and a mining middle school, after the foundation filled with gangue in the subsidence area with a subsidence depth of 5m has been treated by dynamic compaction. However, coal gangue and fly ash generally only account for 20% of the total area of backfilled subsidence area, so it is necessary to use the method of not backfilling to reclaim some subsidence areas. Reclamation without filler mainly adopts the method of deep digging and shallow cushion to comprehensively control the subsidence area, and digs the bottom of the subsidence basin into a deep-water pond that can store water and raise fish, so that it has the functions of flood storage and irrigation at the same time, and the surrounding sloping fields can be transformed into horizontal terraces. Most areas in northwest China belong to loess plateau and hilly areas, and the terrain has not changed obviously after collapse. If the destroyed land needs to be cultivated, just fill up the local funnel-shaped collapse pit and ground fissure.

In Shendong Mining Area, which borders Inner Mongolia in Shaanxi Province, five garbage treatment plants have been established to treat coal gangue in a centralized way. Layered discharge, ditch filling and land reclamation are adopted to cover loess. After leveling, plant trees and grass, and turn the gangue hill into green space. Through data collection, investigation and interview, ground geophysical exploration, drilling, etc., Ningxia coal mining area has found out the mined-out area from history to now, conducted stability analysis and follow-up observation, mastered the ground collapse and its occurrence law, carefully studied and analyzed its occurrence area, scope, depth, time and speed, and put forward a forecast. For areas that require to reduce ground subsidence as much as possible, mining methods such as strip mining and room-and-pillar mining can also be used to fill mined-out areas, such as borehole separation grouting and gangue backfilling, to avoid or reduce subsidence of overlying strata. For the collapsed areas, coal gangue, power plant fly ash and a small amount of domestic garbage can be used for backfilling, ecological environment restoration and agricultural land reclamation, fish farming in local waters, and scenic spots can be built for treatment. For example, Shizuishan in Ningxia has created a number of green and construction land by using the existing coal gangue and filling the subsidence area through flat turning.

5.4.3.4.2 Reclamation of slag discharge yard

The discharge of mine waste soil, slag and waste rock occupies a large amount of land, which is itself an important pollution source in the mining area, polluting the atmosphere and water body, and also causing geological disasters such as landslides and mudslides. Reclamation of dump refers to the transformation of dump, the restoration of land and vegetation, and the control or elimination of the pollution of dump to the surrounding environment. The design of the dump should consider the future reclamation work. When the rock is stripped, the topsoil and waste rock should be collected and piled separately. According to the location, shape, waste rock properties and hydrometeorological conditions of the dump, the reclamation scheme is determined according to local conditions.

Reclamation of open-pit mining area

In the early stage of open-pit mining, the future reclamation of the mining area should be considered, and the fertile topsoil above the deposit should be stripped and piled separately to keep the original soil structure as much as possible. When backfilling the mined-out area, large pieces of waste rock or harmful rock and soil are placed at the bottom of the mine, the original topsoil is laid on the surface, or new soil suitable for cultivation is covered, and suitable plants are selected for planting or other purposes after leveling.

One of the effective measures to control land destruction in open-pit mining is to take land reclamation as a link in the whole open-pit mining technology. For example, the mining sequence of building materials 60 1 emery mine in China is: topsoil stripping-topsoil storage-mining-mineral processing-tailings returning and filling mined-out areas-topsoil paving-leveling and building channels, and then giving them to farmers for fertilization and planting. Mining, stripping and reclamation methods are shown in Figure 5-9.

Figure 5-9 Schematic Diagram of Mining, Stripping and Recovery Methods

1-overburden pile; 2— Ore-bearing bed; 3- Farmland has been restored; 4- backfill waste rock; 5— Red sandstone

Reclamation operation is to send waste rocks and covering soil into the inner dump (mined-out area), and try to be consistent with the ground elevation before mining, especially the surface planting soil should maintain the original soil structure and be able to grow crops. Originally, the mine was only mined and not reclaimed. After mining, the mining area has become an area with three mountains (surface earth mountain, gangue hill and tailings pond) and two ponds (ponds and tailings pond) without fertile land, which has caused tension between workers and peasants. If it is not reclaimed, the mine will not be able to continue production. At the initial stage of reclamation, only heaping mountains, filling pits and moving soil are carried out, which has poor benefits and high cost (30,000 yuan/hectare). After the reclamation method of combining mining and filling is adopted, the cost is reduced to 4500 yuan/hectare, and the benefit is improved.

Zhungeer coalfield bordering Shanxi, Shaanxi and Mongolia is located in the arid loess plateau. The mining area has serious soil erosion, broken surface and sparse vegetation. The comprehensive treatment engineering test was carried out in the acid ditch of mining area. Garlic ditch was originally a natural scouring ditch, and the initial stripping was filled in this ditch by open-pit coal mining, forming a trapezoidal step area. The total dumping volume of Daosuangou dump is 75.28× 104m3, the total area of dumping platform is 2× 104m2, and the total area of four slopes is 1× 104m2.

The new idea of reclamation construction of Majiata open-pit coal mine in Shendong mining area is that governance and management promote each other and develop harmoniously. Reclamation adopts stripping and backfilling coal mining method, with layered backfilling, with waste rock and raw soil at the bottom and topsoil at the top. After backfilling, the reclamation area is113.33×104m2. 1999 autumn, 0.2m red mud was planted on 20ha land in reclamation area, and vegetables, corn, potato, sunflower, buckwheat and high-quality pasture were planted respectively, which grew well and achieved remarkable results. At the same time, the sewage treated by oxidation pond is used as water source. In view of the poor water-holding capacity of soil in reclamation area, irrigation pipe network is fully arranged in reclamation area, and fixed or mobile sprinkler irrigation is adopted to continuously and effectively improve the humidity of soil and near-surface air and solve the drought constraint problem in reclamation and greening. At present, Majiata reclamation area has formed a pattern of mutual promotion and coordinated development of governance and management. The green coverage rate reached 80%, which was 15.8 times higher than that before mining. * * * Planting 46.7 hectares of pasture, more than 654.38 million shrubs and 20,000 trees. It was rated as a national demonstration base for ecological construction by the Ministry of Water Resources and an AA-level tourist area by the Tourism Bureau of Inner Mongolia Autonomous Region. A new modern artificial ecological park has basically taken shape.

Reclamation of tailings pond

After the tailings pond stopped using, due to the evaporation and discharge of water, the surface dried up and was exposed to the air, forming a closed shell. The whole tailings pond is similar to swamp, with low bearing capacity. In windy season, the tailings pond area in arid area is shrouded in gray dust. After the wind stopped, dust fell on crops and buildings, affecting the health of residents. This is also a source of increasingly strong sandstorms in northwest China. Therefore, the reclamation of tailings pond should first deal with and improve its surface structure and improve its wind erosion resistance. The general reclamation steps are: digging the hard shell of the surface, filling it with gravel after digging the surface, neutralizing the acidity of acid tailings with limestone, neutralizing the alkalinity of alkaline tailings with dolomite, leveling the surface of tailings pile, spreading topsoil and mixing neutralizer and fertilizer for planting or other purposes. When tailings and residual chemicals contain toxic substances, it is necessary to study the harm of these harmful substances and its prevention and control measures. This not only relieves the shortage of urban land, but also effectively solves the problem of air pollution caused by tailings dust.