Mine pit ecological restoration method based on coal gangue

Abstract

The invention discloses a mine pit ecological restoration method based on coal gangue. The mine pit ecological restoration method comprises the following steps that an impermeable layer is laid at the bottom of a coal mine goaf; laying a coal gangue solidification layer on the impermeable layer; a flame-retardant layer is arranged on the coal gangue curing layer; laying a soil layer I on the last flame-retardant layer; plants are planted on the first soil layer, a second soil layer is laid in a non-plant-planting area of the first soil layer, and the second soil layer is a mixture of soil and a crust material. The coal gangue solidification layer can effectively solidify metal ions in coal gangue and prevent the metal ions in the coal gangue from escaping. The flame-retardant layer is additionally arranged on the coal gangue curing layer, so that the possibility of spontaneous combustion of the coal mine goaf can be further reduced. The crust material provided by the invention has good wind erosion resistance, and can more effectively protect surface soil. By means of the method, the ecological environment of the goaf is effectively improved, and ecological restoration of the coal mine goaf is achieved.

Description

Technical Field

[0001] This application belongs to the technical field of coal-based solid waste treatment and mine ecological restoration, and relates to a method for ecological restoration of coal mine goafs. Background Art

[0002] Abandoned coal mine goafs usually result from mining activities of mineral resources, including two main methods: open-pit mining and underground mining. With the depletion of resources and the end of mining activities, these coal mine goafs are gradually abandoned. The existence of abandoned coal mine goafs not only affects the beauty of the natural environment but also may bring a series of environmental problems, such as geological disasters like ground subsidence, groundwater pollution, landslides, and debris flows, as well as ecological damage and waste of land resources. Therefore, attention must be paid to the treatment and restoration of abandoned coal mine goafs.

[0003] Coal gangue, as a by-product with a high density and strong bearing capacity, can be used as backfill material to achieve land reclamation. This approach not only reduces the land area occupied by the stacking of coal gangue and realizes its nearby utilization but also helps to alleviate the problem of ground subsidence caused by underground coal mining activities, thus bringing significant economic and environmental benefits.

[0004] However, there are also some problems with directly using coal gangue for backfilling. For example, its porosity is relatively high, which easily allows air to mix in and increases the risk of spontaneous combustion. In addition, heavy metals in coal gangue may pollute the environment through leaching.

[0005] In the prior art, after backfilling with coal gangue in the abandoned coal mine goaf, a layer of soil is laid for vegetation planting. However, the soil in the mining area is greatly affected by wind erosion. Wind erosion causes the loss of the topsoil, resulting in a decrease in soil fertility and affecting the productivity of the land. Long-term wind erosion changes the soil structure, leading to land desertification and the formation of land that is difficult to utilize. Wind erosion will blow away the relatively fertile topsoil and affect plants. The problem of wind erosion also needs to be considered during the restoration of coal mine goafs.

[0006] Therefore, how to effectively restore coal mine goafs is a technical problem that needs to be solved in the prior art. Summary of the Invention

[0007] In view of the problems existing in the prior art, the present invention discloses a method for ecological restoration of mine pits based on coal gangue. The present invention pours materials that can solidify metal ions in coal gangue into the gaps of coal gangue, forming gels in-situ between coal gangue to solidify and adsorb metal ions in coal gangue, so as to prevent heavy metals from seeping out and polluting the environment. The present invention also adds a flame retardant layer on the coal gangue solidification layer to reduce the possibility of spontaneous combustion in the goaf of coal mines. The present invention also newly develops a crust material, which has good wind erosion resistance and can more effectively protect the surface soil. Therefore, the method of the present invention effectively improves the ecological environment of the goaf and realizes the ecological restoration of the coal mine goaf.

[0008] To achieve the above object, the present invention adopts the following solutions:

[0009] The first aspect of the present invention is to provide a method for ecological restoration of mine pits based on coal gangue, including the following steps:

[0010] (1) Level the bottom of the coal mine goaf, and lay an anti-seepage layer on the leveled bottom of the coal mine goaf;

[0011] (2) Lay a coal gangue solidification layer on the anti-seepage layer;

[0012] (3) Set a layer of flame retardant layer on the coal gangue solidification layer; lay a coal gangue solidification layer on the flame retardant layer, and then set a layer of flame retardant layer on the coal gangue solidification layer; optionally repeat step (3);

[0013] (4) Lay a first soil layer on the last layer of flame retardant layer; plant plants on the first soil layer, and lay a second soil layer on the non-planting area of the first soil layer, and the second soil layer is a mixture of soil and crust material.

[0014] As a preferred embodiment, the anti-seepage layer is successively a clay layer, a bentonite layer and a polyethylene LDPE film layer from bottom to top; preferably,

[0015] The thickness of the clay layer is 30-40 cm;

[0016] The thickness of the bentonite layer is 20-30 cm;

[0017] The thickness of the polyethylene LDPE film layer is 1.0-1.5 mm.

[0018] As a preferred embodiment, the crust material includes the following raw materials in parts by weight:

[0019] Sodium alginate is 0.5-0.52 parts;

[0020] Polyvinyl alcohol is 4-5.3 parts;

[0021] Glycerol is 5-6 parts;

[0022] Sodium tripolyphosphate is 0.12 - 0.13 parts;

[0023] Calcium chloride is 0.05 - 0.06 parts;

[0024] Sodium dodecylbenzenesulfonate is 0.5 - 0.8 parts.

[0025] In the crust material system prepared by the present invention, sodium alginate and polyvinyl alcohol are reaction substrates, providing cross - linking sites for the subsequent polymerization of glycerol, sodium tripolyphosphate and calcium chloride. Sodium tripolyphosphate acts as a cross - linker to complete the chemical cross - linking between polymers, and then forms a network structure through the ionic cross - linking of calcium ions and sodium alginate. Sodium dodecyl sulfate is an emulsifier, which can effectively improve the solubility and wetting performance of the material. Sodium alginate and polyvinyl alcohol are monomers that form a network matrix, enabling the material to have viscosity and adsorption, and enhancing its stability and mechanical properties.

[0026] As a preferred embodiment, the thickness of soil layer 1 is 90 - 140 cm; the thickness of soil layer 2 is 3 - 5 cm.

[0027] As a preferred embodiment, in soil layer 2, the addition amount of the crust material is 2 - 3% of the soil mass;

[0028] The preparation method of soil layer 2 is to spray the crust material onto the soil under stirring conditions.

[0029] Based on the existing conventional soil, the present invention sprays the crust material in the non - plant - growing area of the soil, which can effectively prevent soil wind erosion. Moreover, the non - plant - growing area where the crust material is sprayed has a higher terrain, while the plant - growing area where the crust material is not sprayed has a lower terrain, which is more conducive to water retention and more beneficial to the growth and survival of plants.

[0030] As a preferred embodiment, the preparation method of the crust material includes the following steps:

[0031] (1) Under heating conditions, add polyvinyl alcohol to water and mix evenly to obtain a polyvinyl alcohol solution;

[0032] (2) Under heating conditions, add sodium alginate to the polyvinyl alcohol solution and mix evenly, then add glycerol and mix evenly to obtain a mixture;

[0033] (3) Dropwise add an alkali solution to adjust the pH of the mixture, continue stirring for a period of time, first add sodium tripolyphosphate and mix evenly, add water, then add a calcium chloride solution and mix evenly; finally add sodium dodecylbenzenesulfonate and mix evenly to obtain the crust material.

[0034] As a preferred embodiment,

[0035] Step (1),

[0036] The heating temperature is 90 - 95 °C;

[0037] Step (2),

[0038] The heating temperature is 40 - 55 °C;

[0039] The mixing time of adding sodium alginate to the polyvinyl alcohol solution is 2.5 - 3.5 h;

[0040] The mixing time of adding glycerol until evenly mixed is 0.5 - 1.5 h.

[0041] As a preferred embodiment,

[0042] Step (3),

[0043] The lye is sodium hydroxide solution;

[0044] Adjust the pH to 9.5 - 10.5;

[0045] The continuous stirring time is 25 - 35 min;

[0046] The lye is sodium hydroxide solution with a concentration of 0.9 - 1.2 mol / L;

[0047] The mixing time of adding sodium tripolyphosphate until evenly mixed is 2.5 - 3.5 h;

[0048] Add water to make the volume of the solution 230 - 260 ml;

[0049] The volume of the calcium chloride solution added is 23 - 26 ml.

[0050] According to the preparation steps of the crust material, reactions occur among sodium alginate, polyvinyl alcohol, glycerol, sodium tripolyphosphate, and calcium chloride through the following steps. (1) Polyvinyl alcohol is completely dissolved in water to form an aqueous polyvinyl alcohol solution. The molecular chains of polyvinyl alcohol form crosslinking points through intermolecular hydrogen bonds between hydroxyl groups for physical crosslinking. After sodium alginate is dissolved in the aqueous polyvinyl alcohol solution, physical crosslinking occurs between the hydroxyl groups of polyvinyl alcohol and the hydroxyl groups of sodium alginate to form intermolecular hydrogen bonds, thereby initially forming a three-dimensional physical crosslinking network structure. (2) Glycerol itself contains a large number of hydroxyl groups, which will generate strong intramolecular and intermolecular hydrogen bonds with the hydroxyl groups of sodium alginate and polyvinyl alcohol, increasing the intermolecular force, facilitating the aggregation of molecular chains, having a certain adhesion, increasing the viscosity of the material. Due to the strong hydrogen bond action, a SA-PVA-Gly three-dimensional physical crosslinking network structure is formed. (3) Under alkaline conditions, the added sodium tripolyphosphate can undergo a chemical crosslinking reaction with the hydroxyl groups in the sodium alginate and polyvinyl alcohol polymers, promoting the formation of a network structure between high molecular weight polymers. The network structure formed by this chemical crosslinking has strong cohesion. It can effectively bond dust particles. When a water-soluble calcium salt is introduced into the high molecular polymer solution, the calcium ions in calcium chloride can form a coordination effect with the carboxyl groups (-COO-) of the G segments and adjacent G segments in the sodium alginate molecular chain, thereby causing physical crosslinking of the sodium alginate linear molecular chain to form a SA network through the "egg box" structure.

[0051] As a preferred embodiment, the preparation method of the coal gangue solidified layer is as follows:

[0052] After uniformly mixing the mixture A in the material with coal gangue, mixture B is obtained; mixture B is spread into a spreading layer with a thickness of 30 - 40 cm; the heated alkaline solution is added to mixture B for reaction to achieve in-situ gelation, thereby obtaining the coal gangue solidified layer;

[0053] The mixture A includes the following raw materials in parts by weight:

[0054] 15 - 40 parts of polyacrylamide, 10 - 30 parts of fly ash, 5 - 15 parts of crosslinking agent, 10 - 25 parts of calcium oxide, 1 - 10 parts of chitosan, 1 - 10 parts of montmorillonite, 4 - 15 parts of brucite fiber, 1 - 5 parts of rubber particles;

[0055] The addition amount of the alkaline solution is 200 - 400 parts.

[0056] Preferably, the mixture A includes the following raw materials in parts by weight:

[0057] 20 - 30 parts of polyacrylamide, 10 - 20 parts of fly ash, 5 - 10 parts of crosslinking agent, 10 - 20 parts of calcium oxide, 2 - 6 parts of chitosan, 5 - 10 parts of montmorillonite, 5 - 8 parts of brucite fiber, 1 - 3 parts of rubber particles;

[0058] The addition amount of the alkali solution is 200 - 300 parts.

[0059] As a preferred embodiment, in mixture A,

[0060] The number-average molecular weight of polyacrylamide is 3 million - 6 million;

[0061] The particle size of fly ash is 120 - 150 mesh;

[0062] The crosslinking agent includes one or more of N,N'-methylenebisacrylamide, phenolic resin, hexamethylenetetramine, and formaldehyde;

[0063] The particle size of calcium oxide is 100 - 150 mesh;

[0064] The number-average molecular weight of the chitosan is 300,000 - 500,000;

[0065] The particle size of the montmorillonite is 10 - 20 mesh;

[0066] The length of the brucite fiber is 3 - 6 mm;

[0067] The rubber particles are selected from nitrile rubber particles, and preferably the particle size of the nitrile rubber particles is 80 - 120 mesh.

[0068] As a preferred embodiment, the amount of the coal gangue is such that the mass ratio of polyacrylamide to coal gangue in mixture A is 1:4 - 8;

[0069] The alkali in the alkali solution is selected from sodium hydroxide and potassium hydroxide;

[0070] The weight part of the alkali in the alkali solution is 5 - 10 parts;

[0071] The heating temperature of the alkali solution is 30 - 80 °C.

[0072] As a preferred embodiment, the alkali solution is added to mixture B in n times, and the amount of the alkali solution each time is 1 / n of the volume of the alkali solution. The addition position of the first alkali solution is 1 / n from the bottom layer of the spreading layer of mixture B, and the addition position of the second alkali solution is 2 / n from the bottom layer of the spreading layer of mixture B. Repeat the above rule until the addition of the alkali solution is completed; the value range of n is 3 - 4.

[0073] As a preferred embodiment, the addition temperature of the first alkali solution is 60 - 80 °C; denote the addition temperature of the first alkali solution as T1, and the addition temperature of the nth alkali solution is T1 - 10(n - 1);

[0074] The time interval between each addition of the alkali solution is 15 - 30 minutes.

[0075] The coal gangue solidified layer of the present invention includes the following reactions: under the action of an alkali solution, fly ash, calcium oxide, coal gangue, and a small amount of antigorite fibers undergo a geopolymer reaction, and the generated gel can effectively solidify heavy metal ions in the coal gangue; in addition, during the above geopolymer reaction, a large amount of heat is released, and at the high temperature of the exotherm, polyacrylamide and a cross-linking agent will also form a gel in-situ and fill between the coal gangue, forming a dense network structure with each other, coating and blocking the coal gangue, which can reduce the escape and mobility of metal ions in the coal gangue, achieving the effect of long-term solidification.

[0076] In the above solution of the present invention, chitosan and montmorillonite can effectively adsorb heavy metals; montmorillonite can also absorb water and swell, increasing the effectiveness of the blockage through the swelling filling and compaction effect generated by the swelling, and the rubber particles can undergo elastic deformation to further fill the space between the coal gangue through the deformation; the antigorite fibers are distributed in both the gel of the geopolymer reaction and the gel of polyacrylamide, which can increase the bonding effect of the above two gels and increase the tightness of the contact between the above gels.

[0077] The gel system of the present invention shows a slightly alkaline property. Under alkaline conditions, heavy metal ions are prone to precipitate, and thus are easily adsorbed and wrapped by the gel, which is also beneficial to reducing the leaching of heavy metal ions.

[0078] Therefore, the present invention uses multiple mechanisms to solidify and adsorb metal ions in coal gangue, and the materials of the present invention can form a gel in-situ between the coal gangue, so that the gel fills between the coal gangue, forming an effective blockage between the coal gangue, thereby preventing the escape of metal ions in the coal gangue caused by the scouring of liquids such as water, and effectively solving the problem of solidifying metal ions in coal gangue.

[0079] As a preferred embodiment, the flame retardant layer is obtained by solidifying a flame retardant slurry prepared by mixing fly ash, lime, cement, sodium silicate and water; the thickness of each flame retardant layer is 10 - 20 cm.

[0080] As a preferred embodiment, the mass ratio of fly ash, lime and cement is (45 - 70):(18 - 30):(15 - 20);

[0081] The addition amount of sodium silicate is 4 - 7% of the mass of the cement;

[0082] The liquid-solid ratio of the flame retardant slurry is 0.65 - 0.85 ml of water per g of solid mixture, and the solid mixture is a mixture of fly ash, lime, cement and sodium silicate.

[0083] As a preferred embodiment, the thickness of the first soil layer is 100 - 150 cm.

[0084] The present invention has the following advantages:

[0085] The crust material of the present invention is degradable and has good wind erosion resistance, which can more effectively protect the surface soil, thus being beneficial to the ecological restoration of the goaf in coal mines.

[0086] The coal gangue solidification layer of the present invention can effectively solidify metal ions in coal gangue and prevent the escape of metal ions in coal gangue. The coal gangue solidification layer of the present invention uses gel to adhere coal gangue blocks, improving the connection between coal gangue blocks, which can prevent the entry and circulation of oxygen and is also beneficial to reducing the combustibility of coal gangue.

[0087] The present invention also adds a flame retardant layer on the coal gangue solidification layer. The material of the flame retardant layer of the present invention is simple to prepare and can further reduce the possibility of spontaneous combustion in the coal mine goaf. Through the method of the present invention, not only the effective utilization of coal gangue is realized, but also the ecological environment of the goaf is effectively improved, realizing the ecological restoration of the coal mine goaf. Detailed implementation mode

[0088] Abbreviations:

[0089] PVA: Polyvinyl alcohol, polyvinyl alcohol;

[0090] SA: Sodium alginate, sodium alginate;

[0091] STMP: Sodium trimetaphosphate, Sodium trimetaphosphate;

[0092] Gly: Glycerol, Glycerol;

[0093] SDBS: Sodium dodecyl benzene sulfonate, sodium dodecyl benzene sulfonate.

[0094] Testing methods:

[0095] Viscosity test

[0096] Viscosity is one of the important performance indicators of soil crust materials and is used to measure the bonding effect of crust materials on soil particles. This test measures the viscosity of the sample according to the rotation method in GB / T10247-2008, and the equipment used is a rotational viscometer (RheolabQC).

[0097] Hardness test

[0098] The hardness of the material is used to reflect the compressive strength after the sample is applied to the surface soil to form a crust and is one of the important indicators for detecting the performance of the crust material. An LX-A type Shore hardness tester is used to test the hardness of the sample.

[0099] Water retention test

[0100] The water retention performance of the crust material is evaluated by the water retention rate. The higher the water retention rate, the stronger the water retention ability. The test steps are as follows:

[0101] (1) Prepare 18 samples of 600 g of soil fugitive dust, and dry the samples in a forced-air drying oven (temperature control range: 10 - 300 °C) at 75 °C.

[0102] (2) Put the treated soil sample (600 ± 0.05 g) into a petri dish with an inner diameter of 12.5 ± 0.1 cm and a height of 63 mm.

[0103] (3) Evenly spray the prepared compound solutions with different concentrations on the surface of the soil dust sample. After complete curing for 48 h, record the weight of the sample. The water retention rate is calculated according to the following formula.

[0104]

[0105] In the formula, MRR (%) is the water retention rate, m1 (g) is the weight of the petri dish after spraying the crust material;

[0106] m2 (g) is the weight of the petri dish measured after 48 h;

[0107] m3 (g) is the weight of the sprayed crust material.

[0108] Permeability test

[0109] The experiment is carried out according to the penetration rate method in GB / T16913 - 2008 "Test Methods for Physical Properties of Dust", and the wetting rate is used to reflect the wettability of the material to the soil.

[0110] Degradation rate test

[0111] The degradation rate test is carried out to evaluate the biodegradation performance of the crust material. The steps are as follows:

[0112] (1) Spray the crust material on a glass plate (thickness less than 2 mm), place the glass plate in a vacuum drying chamber, and dry it at 60 °C until the weight is constant.

[0113] (2) Thoroughly mix the completely dried crust material and soil and put them into a 500 mL jar, and then place the jar at room temperature (25 °C) for 14 cycles (1 cycle is 5 days).

[0114] (3) Take out the cured film in the jar and dry it, and record the degradation rate of the cured film over time. The degradation rate is calculated as follows:

[0115]

[0116] Where: M0 is the initial mass of the crust material, mg;

[0117] M1 is the mass of the crust material after a certain period of time, mg;

[0118] μ is the degradation rate of the crust material.

[0119] Wind erosion experiment

[0120] To verify the actual application effect of the crust material, the natural wind of an open-pit coal mine was simulated to design a wind erosion resistance test. An axial flow fan was used to generate wind in this experiment. The first measurement point, No. 1, was located 0.3 m away from the fan outlet. After this position, a measurement point was set every 0.5 m, with a total of 3 measurement points. The wind speed at each point was collected by an anemometer, and the average value of three repetitions was used as the final value. For each test, a soil sample to be tested was placed at each measurement point, and a crust material solution (the spraying amount of the crust material was 2.5% of the soil mass) was sprayed on the sample. After the sprayed solution dried naturally, the fan was started.

[0121] Before exposing the specimen to the induced wind, the mass of each specimen was measured and denoted as M i . Then the specimen was placed at the measurement point, and the fan frequency was adjusted through a speed control motor controller to control the induced wind speed. The total mass of the specimen was measured every 1 h after the fan was started and denoted as M f . This process was repeated. After the test, the soil loss rate Sl (kg / m 2 ) of each specimen was determined, which was defined as the mass of the soil eroded by the wind per unit surface area (m 2 ), and the calculation formula was as follows:

[0122]

[0123] In the formula, A is the surface area of the treated soil sample, m 2 .

[0124] Preparation example 1

[0125] The preparation method of the coal gangue solidified layer is as follows:

[0126] (1) Prepare the materials for solidifying metal ions in coal gangue. The materials include the following raw materials in parts by weight:

[0127] 15 parts of polyacrylamide, 30 parts of fly ash, 5 parts of crosslinking agent, 15 parts of calcium oxide, 6 parts of chitosan, 1 part of montmorillonite, 8 parts of brucite fiber, 4 parts of rubber particles, and 300 parts of alkali solution.

[0128] Among them, the crosslinking agent is hexamethylenetetramine; the number-average molecular weight of polyacrylamide is 5 million; the particle size of fly ash is 140 mesh; the particle size of calcium oxide is 120 mesh; the number-average molecular weight of chitosan is 400,000; the particle size of montmorillonite is 20 mesh; the length of brucite fiber is 6 mm; the rubber particles are selected from nitrile rubber particles, and the particle size of nitrile rubber particles is 100 mesh; the base in the alkali solution is selected from sodium hydroxide; the weight fraction of the base in the alkali solution is 7 parts.

[0129] When preparing the above materials for solidifying metal ions in coal gangue, the raw materials except the alkali solution are mixed evenly to obtain mixture A; the alkali solution and mixture A are stored separately.

[0130] (2) When applying the above materials to solidify metal ions in coal gangue to prepare a coal gangue solidified layer, first mix mixture A in the materials with coal gangue evenly (where the mass ratio of polyacrylamide to coal gangue is 1:5) to obtain mixture B; spread mixture B into a spreading layer with a thickness of 40 cm; add the heated alkali solution to mixture B for reaction. Among them, the alkali solution is added to mixture B in 4 times, and the amount of alkali solution each time is 1 / 4 of the volume of the alkali solution. The position of the first addition of the alkali solution is 1 / 4 from the bottom layer of the spreading layer of mixture B, and the position of the second addition of the alkali solution is 2 / 4 from the bottom layer of the spreading layer of mixture B. Repeat the above rule until the addition of the alkali solution is completed; the temperature of the first addition of the alkali solution is 70 °C; the addition temperatures of the 2nd - 4th times of the alkali solution are 60, 50, and 40 °C; the time interval between each addition of the alkali solution is 25 minutes; after the above reaction, in-situ gelation is achieved, thereby solidifying metal ions in coal gangue and obtaining a coal gangue solidified layer.

[0131] Preparation Example 2

[0132] The preparation method of the coal gangue solidified layer is as follows:

[0133] (1) Prepare materials for solidifying metal ions in coal gangue, and the materials include the following raw materials in weight fractions:

[0134] 30 parts of polyacrylamide, 10 parts of fly ash, 10 parts of crosslinking agent, 20 parts of calcium oxide, 2 parts of chitosan, 10 parts of montmorillonite, 8 parts of brucite fiber, 2 parts of rubber particles, 200 parts of alkali solution.

[0135] Among them, the crosslinking agent is hexamethylenetetramine; the number-average molecular weight of polyacrylamide is 3 million; the particle size of fly ash is 120 mesh; the particle size of calcium oxide is 150 mesh; the number-average molecular weight of chitosan is 500,000; the particle size of montmorillonite is 10 mesh; the length of brucite fiber is 5 mm. The rubber particles are selected from nitrile rubber particles, and the particle size of nitrile rubber particles is 120 mesh; the base in the alkali solution is selected from sodium hydroxide; the weight fraction of the base in the alkali solution is 5 parts.

[0136] When preparing the above-mentioned material for solidifying metal ions in coal gangue, the raw materials other than the alkali solution are mixed evenly to obtain mixture A; the alkali solution and mixture A are stored separately.

[0137] (2) When applying the above-mentioned material to solidify metal ions in coal gangue and preparing a coal gangue solidified layer, first, mixture A in the material is mixed evenly with coal gangue to obtain mixture B (where the mass ratio of polyacrylamide to coal gangue is 1:4); mixture B is spread into a spreading layer with a thickness of 30 cm; the heated alkali solution is added to mixture B for reaction. Among them, the alkali solution is added to mixture B in 3 times, and the amount of alkali solution each time is 1 / 3 of the volume of the alkali solution. The first addition position of the alkali solution is 1 / 3 from the bottom layer of the spreading layer of mixture B, and the second addition position of the alkali solution is 2 / 3 from the bottom layer of the spreading layer of mixture B. Repeat the above rule until the addition of the alkali solution is completed; the addition temperature of the first alkali solution is 80 °C; the addition temperatures of the 2nd - 3rd alkali solutions are 70 °C and 60 °C; the time interval between each addition of the alkali solution is 20 minutes; after the above reaction, in-situ gelation is achieved, thereby solidifying metal ions in coal gangue and obtaining a coal gangue solidified layer.

[0138] Preparation Example 3

[0139] The preparation method of the coal gangue solidified layer is as follows:

[0140] (1) Prepare a material for solidifying metal ions in coal gangue, and the material includes the following raw materials in parts by weight:

[0141] 20 parts of polyacrylamide, 20 parts of fly ash, 8 parts of crosslinking agent, 15 parts of calcium oxide, 4 parts of chitosan, 5 parts of montmorillonite, 4 parts of brucite fiber, 3 parts of rubber particles, and 300 parts of alkali solution.

[0142] Among them, the crosslinking agent is hexamethylenetetramine; the number-average molecular weight of polyacrylamide is 5 million; the particle size of fly ash is 150 mesh; the particle size of calcium oxide is 100 mesh; the number-average molecular weight of chitosan is 300,000; the particle size of montmorillonite is 20 mesh; the length of brucite fiber is 3 mm.

[0143] The rubber particles are selected from nitrile rubber particles, and the particle size of the nitrile rubber particles is 80 mesh; the alkali in the alkali solution is selected from sodium hydroxide; the weight fraction of the alkali in the alkali solution is 10 parts.

[0144] When preparing the above-mentioned material for solidifying metal ions in coal gangue, the raw materials other than the alkali solution are mixed evenly to obtain mixture A; the alkali solution and mixture A are stored separately.

[0145] (2) When applying the above materials to the metal ions in solidified coal gangue to prepare a coal gangue solidified layer, first mix the mixture A in the materials with coal gangue evenly to obtain mixture B (the mass ratio of polyacrylamide to coal gangue in the materials is 1:6); spread mixture B into a spread layer with a thickness of 40 cm; add the heated alkali solution to mixture B for reaction. Among them, the alkali solution is added to mixture B in 4 times, and the amount of alkali solution each time is 1 / 4 of the volume of the alkali solution. The position where the first alkali solution is added is 1 / 4 from the bottom layer of the spread layer of mixture B, and the position where the second alkali solution is added is 2 / 4 from the bottom layer of the spread layer of mixture B. Repeat the above rule until the alkali solution addition is completed; the temperature when the first alkali solution is added is 75 degrees; the addition temperatures of the 2nd - 4th alkali solutions are 65, 55, and 45 degrees; the time interval between each addition of the alkali solution is 18 minutes; after the above reaction, in-situ gelation is achieved, thereby solidifying the metal ions in the coal gangue and obtaining a coal gangue solidified layer.

[0146] Preparation Example 4

[0147] The preparation method of the coal gangue solidified layer is as follows:

[0148] (1) Prepare materials for solidifying metal ions in coal gangue. The materials include the following raw materials in parts by weight:

[0149] 40 parts of polyacrylamide, 10 parts of fly ash, 15 parts of crosslinking agent, 25 parts of calcium oxide, 1 part of chitosan, 10 parts of montmorillonite, 15 parts of brucite fiber, 1 part of rubber particles, and 400 parts of alkali solution.

[0150] Among them, the crosslinking agent is hexamethylenetetramine; the number-average molecular weight of polyacrylamide is 3 million; the particle size of fly ash is 130 mesh; the particle size of calcium oxide is 140 mesh; the number-average molecular weight of chitosan is 300,000 - 500,000; the particle size of montmorillonite is 10 - 20 mesh; the length of brucite fiber is 6 mm. The rubber particles are selected from nitrile rubber particles, and the particle size of nitrile rubber particles is 120 mesh; the alkali in the alkali solution is selected from sodium hydroxide; the weight fraction of the alkali in the alkali solution is 7 parts.

[0151] When preparing the above materials for solidifying metal ions in coal gangue, mix the raw materials except the alkali solution evenly to obtain mixture A; store the alkali solution and mixture A separately.

[0152] (2) When applying the above materials to the metal ions in solidified coal gangue to prepare a coal gangue solidified layer, first mix the mixture A in the materials with coal gangue evenly to obtain mixture B (the mass ratio of polyacrylamide to coal gangue in the materials is 1:8); spread mixture B into a spreading layer with a thickness of 40 cm; add the heated alkali solution to mixture B for reaction. Among them, the alkali solution is added to mixture B in 4 times, and the amount of alkali solution each time is 1 / 4 of the volume of the alkali solution. The position where the first alkali solution is added is 1 / 4 from the bottom layer of the spreading layer of mixture B, and the position where the second alkali solution is added is 2 / 4 from the bottom layer of the spreading layer of mixture B. Repeat the above rule until the addition of the alkali solution is completed; the temperature when the first alkali solution is added is 80 °C; the addition temperatures of the 2nd - 4th alkali solutions are 70, 60, and 50 °C; the time interval between each addition of the alkali solution is 15 minutes; after the above reaction, in-situ gelation is achieved, thereby solidifying the metal ions in the coal gangue and obtaining a coal gangue solidified layer.

[0153] Preparation Example 5

[0154] The preparation method of the coal gangue solidified layer is as follows:

[0155] (1) Prepare materials for solidifying metal ions in coal gangue. The materials include the following raw materials in parts by weight:

[0156] 25 parts of polyacrylamide, 18 parts of fly ash, 8 parts of crosslinking agent, 16 parts of calcium oxide, 3 parts of chitosan, 6 parts of montmorillonite, 6 parts of brucite fiber, 1 part of rubber particles, and 250 parts of alkali solution.

[0157] Among them, the crosslinking agent is hexamethylenetetramine; the number-average molecular weight of polyacrylamide is 5 million; the particle size of fly ash is 120 mesh; the particle size of calcium oxide is 140 mesh; the number-average molecular weight of chitosan is 400,000; the particle size of montmorillonite is 15 mesh; the length of brucite fiber is 4 mm. The rubber particles are selected from nitrile rubber particles, and the particle size of the nitrile rubber particles is 120 mesh; the alkali in the alkali solution is selected from sodium hydroxide; the weight part of the alkali in the alkali solution is 8 parts.

[0158] When preparing the above materials for solidifying metal ions in coal gangue, mix the raw materials except the alkali solution evenly to obtain mixture A; store the alkali solution and mixture A separately.

[0159] (2) When applying the above materials to the metal ions in solidified coal gangue to prepare a coal gangue solidified layer, first mix the mixture A in the materials evenly with the coal gangue to obtain mixture B (where the mass ratio of polyacrylamide to coal gangue is 1:5); spread mixture B into a spreading layer with a thickness of 30 cm; add the heated alkali solution to mixture B for reaction. Among them, the alkali solution is added to mixture B in 3 times, and the amount of alkali solution each time is 1 / 3 of the volume of the alkali solution. The position where the alkali solution is added for the first time is 1 / 3 from the bottom layer of the spreading layer of mixture B, and the position where the alkali solution is added for the second time is 2 / 3 from the bottom layer of the spreading layer of mixture B. Repeat the above rule until the addition of the alkali solution is completed; the temperature when the alkali solution is added for the first time is 75 degrees; the addition temperatures of the 2nd - 3rd times of alkali solution are 65 and 55 degrees; the time interval between each addition of the alkali solution is 22 minutes; after the above reaction, in-situ gelation is achieved, thereby solidifying the metal ions in the coal gangue and obtaining a coal gangue solidified layer.

[0160] Comparative Example 1

[0161] It uses basically the same method as Preparation Example 3, with the only difference being that brucite fiber, chitosan, rubber particles, and montmorillonite are not added to the materials for solidifying the metal ions in coal gangue.

[0162] Comparative Example 2

[0163] It uses basically the same method as Preparation Example 2, with the only difference being that the materials include the following raw materials in parts by weight:

[0164] 50 parts of polyacrylamide, 5 parts of fly ash, 25 parts of crosslinking agent, 30 parts of calcium oxide, 15 parts of chitosan, 15 parts of montmorillonite, 20 parts of brucite fiber, 8 parts of rubber particles, 400 parts of alkali solution.

[0165] Comparative Example 3

[0166] It uses basically the same method as Preparation Example 2, with the only difference being that the materials include the following raw materials in parts by weight:

[0167] 10 parts of polyacrylamide, 40 parts of fly ash, 1 part of crosslinking agent, 5 parts of calcium oxide, 0.5 part of chitosan, 0.5 part of montmorillonite, 2 parts of brucite fiber, 0.5 part of rubber particles, 100 parts of alkali solution.

[0168] Comparative Example 4

[0169] It uses basically the same method as Preparation Example 1, with the only difference being that the alkali solution at 25 degrees is added to mixture B at one time.

[0170] Comparative Example 5

[0171] It uses basically the same method as Preparation Example 3, with the only difference being that calcium oxide is not added to the materials for solidifying the metal ions in coal gangue.

[0172] Testing method:

[0173] The solidified coal gangue in the above preparation examples and comparative examples, and the equal amount of coal gangue without any solidifying material were washed with equal amount of water, and the solidification rate of metal ions in the coal gangue in the above preparation examples and comparative examples was measured. The specific data are shown in Table 1 below:

[0174] The solidification rate is calculated from the leaching toxicity values before and after heavy metal solidification:

[0175] Q = (C0 - C1) / C0

[0176] In the formula: Q is the solidification rate (%); C0 is the initial leaching concentration of heavy metals in the coal gangue without any solidifying material (mg / L); C1 is the leaching concentration of heavy metals in the coal gangue after adding the solidifying material (mg / L).

[0177] Among them, the contents of Cr, Cd, Cu, and Pb ions in the heavy metal release solution of the coal gangue were measured by LA-ICP-MS.

[0178] Table 1

[0179]

[0180]

[0181] Through reasonable raw material ratios, the present invention can effectively solidify metal ions in coal gangue by using materials that solidify metal ions in coal gangue, and obtain a coal gangue solidified layer with very little metal ion overflow.

[0182] Preparation Examples 9 - 11

[0183] A preparation method of a skin-forming material with a crosslinked network structure comprises the following steps:

[0184] (1) Preparation of PVA solution: Place a 500 ml conical flask in a water bath, slowly and evenly add polyvinyl alcohol (PVA), and adjust the temperature to 95°C and stir magnetically for 3 h to prepare an aqueous PVA solution.

[0185] (2) Synthesis of the main material: Adjust the temperature of the water bath to 50°C. After the temperature is stable at 50°C, slowly add sodium alginate (SA) powder to the PVA solution in portions and stir magnetically for 3 h, and then add glycerol (Gly) and stir magnetically for 1 h until dissolved.

[0186] (3) Crosslinking of polymer chains: Adjust the pH of the solution to 10 by dropwise addition of 1.0 M NaOH; after 30 minutes, add sodium trimetaphosphate (STMP) to the solution and stir magnetically for 3 h until completely dissolved; under alkaline conditions, the phosphate groups of STMP react with the hydroxyl groups of PVA and SA to form new bonds through chemical crosslinking, add water to 250 ml; then add 25 ml of calcium chloride solution and stir magnetically until dissolved. Finally, add sodium dodecylbenzenesulfonate and stir magnetically until dissolved.

[0187] The synthesis mechanism of the crust material of the present invention includes: physical crosslinking between the hydroxyl groups of polyvinyl alcohol, the hydroxyl groups of sodium alginate and glycerol will form hydrogen bonds; sodium trimetaphosphate undergoes a chemical crosslinking reaction with the hydroxyl groups in the sodium alginate and polyvinyl alcohol polymers; the linear molecular chains of sodium alginate undergo ionic crosslinking with calcium chloride to form an egg-box structure.

[0188] The addition amounts of sodium alginate, polyvinyl alcohol, glycerol, sodium trimetaphosphate, calcium chloride, and sodium dodecylbenzenesulfonate in the above preparation examples are shown in Table 2.

[0189] Table 2

[0190]

[0191] After the crust material of the present invention binds a large number of soil particles, the gaps between the particles become tighter, and the soil surface becomes flatter and denser. The cross-section of the crust material film of the present invention at a magnification of 500 times can clearly show the formed network structure, which indicates that the monomers form a three-dimensional structure under the influence of the crosslinking agent.

[0192] The performance of the crust material prepared in the above preparation examples is shown in Table 3.

[0193] Table 3

[0194] Preparation Example Viscosity (cP) Hardness (HA) Osmotic rate (mm / min) Water retention rate (%) 9 80.15 79 0.48 97.1 10 76.50 73.40 0.63 96.60 11 78.50 79.50 0.49 96.70

[0195] The viscosity of the crust material prepared above in the present invention is 70 - 85 cP, which not only ensures the adhesion of the crust material to soil particles, but also avoids clogging problems during spraying construction. The precipitation in the mining area is less, and the crust material prepared above in the present invention has high anti-evaporation performance of the crust material and good water retention effect. The crust material prepared above in the present invention has good hardness, which can improve the mechanical properties and anti-interference ability of the soil solidification layer. The penetration rate of the crust material prepared above in the present invention is 0.4 - 0.6 mm / min, and this penetration speed is moderate. If the penetration speed is too slow, it will affect the effective wetting depth of the crust material, thus affecting the application effect; while if the penetration speed is too fast, it will affect the hardness of the film formation, resulting in the inability to form a complete and stable solidification layer, thus reducing the consolidation ability of the material. In summary, the comprehensive performance of the crust material prepared above in the present invention is excellent.

[0196] In addition, the degradation rate of the film of the crust material of the present invention gradually increases over time. After 10 days of the degradation process, there is an obvious mass reduction in the crust material film, and the degradation rate significantly accelerates. This mass attenuation is likely caused by the combined action of air, water, and microorganisms in the soil. After 60 days, the mass of the film is relatively stable, and the mass retention rate remains at about 89%, indicating that the material is in a relatively stable state at this stage. During the degradation process, SA is first invaded by microorganisms (such as fungi and bacteria) in the soil. PVA and Gly can react with metal salts in the soil to form peroxides. The peroxides can break the molecular chains and result in a lower molecular weight, further forming low-molecular-weight polymers. The polymers are degraded into carbon monoxide and water, indicating that the crust material is biodegradable in the soil, conforms to the concept of green development, and will not cause secondary pollution. Thus, it can be seen that the prepared crust material not only has good degradation performance and conforms to the environmental protection concept, but also can maintain the relative stability of the material performance for a long time.

[0197] Test the soil loss rate of Preparation Example 9 at a wind speed of 12 m / s. After 8 hours of wind erosion, the soil loss rates under the treatment of Preparation Example 9 are 9.87 g / m 2 , compared with the CG (untreated original soil sample) treatment after 8 hours of wind erosion, the reduction rates of the soil loss rate under the treatment of Preparation Example 9 are 99.48% in turn. It can be seen that the anti-wind erosion effect of the soil treated by Preparation Example 9 is prominent.

[0198] Example 1

[0199] A method for ecological restoration of mine pits based on coal gangue, comprising the following steps:

[0200] (1) Level the bottom of the coal mine goaf, and lay an anti-seepage layer on the leveled bottom of the coal mine goaf; wherein, the anti-seepage layer is, from bottom to top, a 35-cm clay layer, a 30-cm bentonite layer, and a 1.0-mm polyethylene LDPE film layer in sequence.

[0201] (2) Lay a coal gangue solidification layer on the anti-seepage layer; the specific method is the same as that of the coal gangue solidification layer in Preparation Example 1;

[0202] (3) Set a flame retardant layer on the coal gangue solidification layer. The flame retardant layer is obtained by curing a flame retardant slurry prepared by mixing fly ash, lime, cement, water glass, and water; the mass ratio of fly ash, lime, and cement is 60:26:16; the addition amount of water glass is 6% of the mass of cement; the liquid-solid ratio of the flame retardant slurry is 0.8; the thickness of each layer of the flame retardant layer is 20 cm; lay a coal gangue solidification layer on the flame retardant layer and then set a flame retardant layer on the coal gangue solidification layer; repeat step (3) 5 times;

[0203] (4) Lay a first soil layer on the last fire-retardant layer; the thickness of the first soil layer is 100 cm; plant plants on the first soil layer, and lay a second soil layer on the non-planting area of the first soil layer. The second soil layer is a mixture of soil and crust material; among them, the preparation method of the second soil layer is to spray the crust material into the soil under stirring conditions. In the second soil layer, the addition amount of the crust material (prepared in Preparation Example 9) is 3% of the soil mass, and the thickness of the second soil layer is controlled to be 3 cm.

[0204] Example 2

[0205] A coal gangue-based mine ecological restoration method includes the following steps:

[0206] (1) Level the bottom of the coal mine goaf, and lay an anti-seepage layer on the leveled bottom of the coal mine goaf; among them, the anti-seepage layer from bottom to top is a 40-cm clay layer, a 25-cm bentonite layer, and a 1.5-mm polyethylene LDPE film layer in sequence.

[0207] (2) Lay a coal gangue solidification layer on the anti-seepage layer; the specific method is the same as that of the coal gangue solidification layer in Preparation Example 2;

[0208] (3) Set a fire-retardant layer on the coal gangue solidification layer. The fire-retardant layer is obtained by curing a fire-retardant slurry prepared by mixing fly ash, lime, cement, water glass and water; the mass ratio of fly ash, lime and cement is 50:24:15; the addition amount of water glass is 7% of the cement mass; the liquid-solid ratio of the fire-retardant slurry is 0.85; the thickness of each fire-retardant layer is 18 cm; lay a coal gangue solidification layer on the fire-retardant layer and then set a fire-retardant layer on the coal gangue solidification layer; repeat step (3) 5 times;

[0209] (4) Lay a first soil layer on the last fire-retardant layer; the thickness of the first soil layer is 140 cm; plant plants on the first soil layer, and lay a second soil layer on the non-planting area of the first soil layer. The second soil layer is a mixture of soil and crust material; among them, the preparation method of the second soil layer is to spray the crust material into the soil under stirring conditions. In the second soil layer, the addition amount of the crust material (prepared in Preparation Example 9) is 2% of the soil mass, and the thickness of the second soil layer is controlled to be 5 cm.

[0210] Example 3

[0211] A coal gangue-based mine ecological restoration method includes the following steps:

[0212] (1) Level the bottom of the coal mine goaf, and lay an anti-seepage layer on the leveled bottom of the coal mine goaf; among them, the anti-seepage layer from bottom to top is a 30-cm clay layer, a 20-cm bentonite layer, and a 1.5-mm polyethylene LDPE film layer in sequence.

[0213] (2) Lay a coal gangue solidified layer on the anti-seepage layer; the specific method is the same as that of the coal gangue solidified layer in Preparation Example 3;

[0214] (3) Set a fire-retardant layer on the coal gangue solidified layer. The fire-retardant layer is obtained by curing a fire-retardant slurry prepared by mixing fly ash, lime, cement, water glass and water; the mass ratio of fly ash, lime and cement is 45:30:15; the addition amount of water glass is 4% of the mass of cement; the liquid-solid ratio of the fire-retardant slurry is 0.65; the thickness of each fire-retardant layer is 10 cm; lay a coal gangue solidified layer on the fire-retardant layer and then set a fire-retardant layer on the coal gangue solidified layer; repeat step (3) 5 times;

[0215] (4) Lay a first soil layer on the last fire-retardant layer; the thickness of the first soil layer is 120 cm; plant plants on the first soil layer, and lay a second soil layer on the non-planting area of the first soil layer. The second soil layer is a mixture of soil and crust material; wherein, the preparation method of the second soil layer is to spray the crust material onto the soil under stirring conditions. In the second soil layer, the addition amount of the crust material (prepared in Preparation Example 9) is 2.5% of the mass of the soil, and the thickness of the second soil layer is controlled to be 4 cm.

[0216] Example 4

[0217] A method for ecological restoration of mine pits based on coal gangue, comprising the following steps:

[0218] (1) Level the bottom of the coal mine goaf, and lay an anti-seepage layer on the leveled bottom of the coal mine goaf; wherein, the anti-seepage layer is successively a 35-cm clay layer, a 25-cm bentonite layer and a 1.5-mm polyethylene LDPE film layer from bottom to top.

[0219] (2) Lay a coal gangue solidified layer on the anti-seepage layer; the specific method is the same as that of the coal gangue solidified layer in Preparation Example 4;

[0220] (3) Set a fire-retardant layer on the coal gangue solidified layer. The fire-retardant layer is obtained by curing a fire-retardant slurry prepared by mixing fly ash, lime, cement, water glass and water; the mass ratio of fly ash, lime and cement is 60:22:20; the addition amount of water glass is 6% of the mass of cement; the liquid-solid ratio of the fire-retardant slurry is 0.75; the thickness of each fire-retardant layer is 14 cm; lay a coal gangue solidified layer on the fire-retardant layer and then set a fire-retardant layer on the coal gangue solidified layer; repeat step (3) 5 times;

[0221] (4) Lay a first soil layer on the last fire-retardant layer; the thickness of the first soil layer is 100 cm; plant plants on the first soil layer, and lay a second soil layer on the non-planting area of the first soil layer. The second soil layer is a mixture of soil and crust material; wherein, the preparation method of the second soil layer is to spray the crust material into the soil under stirring conditions. In the second soil layer, the addition amount of the crust material (prepared in Preparation Example 10) is 2.5% of the soil mass, and the thickness of the second soil layer is controlled to be 4 cm.

[0222] Example 5

[0223] A method for ecological restoration of mine pits based on coal gangue, comprising the following steps:

[0224] (1) Level the bottom of the coal mine goaf, and lay an anti-seepage layer on the leveled bottom of the coal mine goaf; wherein, the anti-seepage layer is successively composed of a 36-cm clay layer, a 24-cm bentonite layer and a 1.3-mm polyethylene LDPE film layer from bottom to top.

[0225] (2) Lay a coal gangue solidified layer on the anti-seepage layer; the specific method is the same as that of the coal gangue solidified layer in Preparation Example 5;

[0226] (3) Set a fire-retardant layer on the coal gangue solidified layer. The fire-retardant layer is obtained by curing a fire-retardant slurry prepared by mixing fly ash, lime, cement, water glass and water; the mass ratio of fly ash, lime and cement is 70:25:15; the addition amount of water glass is 5% of the cement mass; the liquid-solid ratio of the fire-retardant slurry is 0.7; the thickness of each fire-retardant layer is 16 cm; lay a coal gangue solidified layer on the fire-retardant layer and then set a fire-retardant layer on the coal gangue solidified layer; repeat step (3) 5 times;

[0227] (4) Lay a first soil layer on the last fire-retardant layer; the thickness of the first soil layer is 130 cm; plant plants on the first soil layer, and lay a second soil layer on the non-planting area of the first soil layer. The second soil layer is a mixture of soil and crust material; wherein, the preparation method of the second soil layer is to spray the crust material into the soil under stirring conditions. In the second soil layer, the addition amount of the crust material (prepared in Preparation Example 11) is 3% of the soil mass, and the thickness of the second soil layer is controlled to be 3 cm.

[0228] Based on the existing conventional soil, in the non-planting area of the soil, the crust material is sprayed in the present invention, which can effectively prevent soil wind erosion, and the non-planting area sprayed with the crust material has a higher terrain, while the terrain of the plant planting area without spraying the crust material is lower, which is more conducive to water retention and more beneficial to the growth and survival of plants.

[0229] The present invention injects a material that can solidify metal ions in coal gangue into the gaps of coal gangue, forming a gel in situ between the coal gangue to solidify and adsorb the metal ions in the coal gangue, so as to achieve the purpose of preventing heavy metals from leaching out and polluting the environment. The present invention also adds a flame retardant layer on the coal gangue solidified layer to reduce the possibility of spontaneous combustion in the goaf of coal mines. Through the method of the present invention, not only the effective utilization of coal gangue is realized, but also the ecological restoration of the goaf of coal mines is realized, which has very important environmental and economic benefits.

[0230] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art should understand that based on the technical solutions of the present invention, various modifications or deformations that can be made by those skilled in the art without creative efforts are still within the protection scope of the present invention.

Claims

1. A method for ecological restoration of mine pits based on coal gangue, characterized in that, It includes the following steps: (1) Level the bottom of the coal mine goaf, and lay an anti-seepage layer on the leveled bottom of the coal mine goaf; (2) Lay a coal gangue solidification layer on the anti-seepage layer; (3) Set a layer of flame retardant layer on the coal gangue solidification layer; lay a coal gangue solidification layer on the flame retardant layer, and then set a layer of flame retardant layer on the coal gangue solidification layer; Optionally repeat step (3); (4) Lay a first soil layer on the last layer of flame retardant layer; plant plants on the first soil layer, and lay a second soil layer on the non-plant planting area of the first soil layer. The second soil layer is a mixture of soil and crust material.

2. The method for ecological restoration of mine pits based on coal gangue according to claim 1, characterized in that, The crust material includes the following raw materials in parts by weight: Sodium alginate is 0.5 - 0.52 parts; Polyvinyl alcohol is 4 - 5.3 parts; Glycerol is 5 - 6 parts; Sodium tripolyphosphate is 0.12 - 0.13 parts; Calcium chloride is 0.05 - 0.06 parts; Sodium dodecylbenzenesulfonate is 0.5 - 0.8 parts.

3. The coal gangue-based mine ecological restoration method according to claim 1, wherein The thickness of the first soil layer is 90 - 140 cm; the thickness of the second soil layer is 3 - 5 cm.

4. The coal gangue-based mine ecological restoration method according to claim 1, wherein In the second soil layer, the addition amount of the crust material is 2 - 3% of the soil quality; The preparation method of the second soil layer is to spray the crust material onto the soil under stirring conditions.

5. The method for ecological restoration of mine pits based on coal gangue according to claim 4, characterized in that, The preparation method of the crust material includes the following steps: (1) Under heating conditions, add polyvinyl alcohol to water and mix evenly to obtain a polyvinyl alcohol solution; (2) Under heating conditions, add sodium alginate to the polyvinyl alcohol solution and mix evenly, then add glycerol and mix evenly; obtain a mixed solution; (3) Dropwise add an alkali solution to adjust the pH of the mixed solution, continue to stir for a period of time, first add sodium tripolyphosphate and mix evenly, add water, then add a calcium chloride solution and mix evenly; finally add sodium dodecylbenzenesulfonate and mix evenly to obtain the crust material.

6. The method for ecological restoration of mine pits based on coal gangue according to claim 5, characterized in that, Step (1), The heating temperature is 90 - 95 °C; Step (2), The heating temperature is 40 - 55 °C; The time for adding sodium alginate to the polyvinyl alcohol solution and mixing is 2.5 - 3.5 h; The time for adding glycerol and mixing evenly is 0.5 - 1.5 h.

7. The coal gangue-based mine ecological restoration method according to claim 5, wherein Step (3), Adjust the pH to 9.5 - 10.5; The time for continuing to stir is 25 - 35 min; The time for adding sodium tripolyphosphate and mixing evenly is 2.5 - 3.5 h; Add water to make the volume of the solution 230 - 260 ml; The volume of the calcium chloride solution added is 23 - 26 ml.

8. The coal gangue-based mine ecological restoration method according to claim 1, wherein The preparation method of the coal gangue solidification layer is: Mix the mixture A in the material with coal gangue evenly to obtain a mixture B; spread the mixture B into a spread layer with a thickness of 30 - 40 cm; add the heated alkali solution to the mixture B for reaction to achieve in-situ gelation, thereby obtaining the coal gangue solidification layer; The mixture A includes the following raw materials in parts by weight: 15 - 40 parts of polyacrylamide, 10 - 30 parts of fly ash, 5 - 15 parts of crosslinking agent, 10 - 25 parts of calcium oxide, 1 - 10 parts of chitosan, 1 - 10 parts of montmorillonite, 4 - 15 parts of antigorite fiber, 1 - 5 parts of rubber particles; The addition amount of the lye is 200 - 400 parts.

9. The method for ecological restoration of mine pits based on coal gangue according to claim 1, characterized in that The flame retardant layer is obtained by curing a flame retardant slurry prepared by mixing fly ash, lime, cement, water glass and water; the thickness of each layer of the flame retardant layer is 10 - 20 cm.

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