Mitigating Risks in Coal Mining: Simulation-Based Strategy for Oxidation Zone Control Using Inorganic Paste Backfill at the Working Face Corners

Abstract

Insufficient stability of the top plate at the corner of an easily combustible coal seam comprehensive mining face may lead to a natural fire within the goaf. While corner sealing is crucial for minimizing air leakage, current sealing methods struggle to effectively prevent such leakage. Additionally, the distribution characteristics of the oxidation zone in the goaf after sealing are unclear, making it difficult to control the extent of the oxidation zone. To address these issues, a new type of inorganic paste filling material was developed, taking into account the conditions of the Cuncaota II Mine. Various corner-filling schemes were developed, and numerical simulations were used to study the effects of different corner-filling strategies and varying filling interval distances on the width of the oxidation zone in the goaf. Based on these findings, a working face corner-filling technology was proposed and applied to the 22,122 working face. The research results indicate that the mountain sand-based paste filling material, using mountain sand as the filling aggregate and cement and fly ash as the binding materials, not only meets the pumping requirements but also exhibits excellent self-supporting characteristics, thereby addressing the corner filling needs of the working face. The width variation in the oxidation zone in the goaf is influenced by the position and interval distance of the corner filling, showing a pattern of initially decreasing and then increasing with the rise in the filling interval distance, reaching a minimum at a filling interval of 50 m. Field observation data demonstrate that, following the application of the aforementioned filling technology, the width of the oxidation zone in the goaf of the 22,122 working face is reduced by 37.5%, and air leakage decreases by 66.7% compared to the unfilled condition. This technology effectively narrows the range of the oxidation zone in the goaf, ensuring the safety of working face production.

1. Introduction

China possesses substantial coal reserves and has traditionally relied on coal as its main energy source. The progress of the coal industry is crucial for the nation’s economic growth [1,2]. Nevertheless, the coal industry faces a range of obstacles, including water, fire, gas, and roof collapses during mining, limiting its development. Among these challenges, the spontaneous combustion of residual coal in the goaf, triggered by air leakage at the working face corners, is a major issue that impacts the safety of coal production in mines [3,4,5].

The occurrence of spontaneous combustion disasters in goaf areas is often attributed to the self-ignition of residual coal upon exposure to oxygen [6]. Through the collapse pattern of the goaf roof [7,8,9], a suspended structure forms at the corner of the working face due to the support provided by the coal wall and hydraulic supports. This region exhibits a high void ratio, designating the two corners as the primary pathways for air leakage within the goaf. Effectively addressing air leakage at the corners of the working face has consistently posed a significant challenge in mitigating spontaneous combustion and fires originating from residual coal in goaf areas [10,11]. Understanding the relationship between internal airflow and resistance within the goaf area indicates that increasing resistance can fundamentally diminish air leakage [12]. Filling and sealing the corners of the working face constitute reliable methods for enhancing resistance. Safely and efficiently sealing the pathways of air leakage at the corners of the working face to reduce air infiltration into the goaf has become a critical aspect of ensuring the secure extraction of coal in mining operations.

The currently prevalent methods for corner sealing mainly consist of building cement walls and applying high-molecular-weight materials. However, these sealing techniques face problems, such as an insufficient sealing efficacy, an extensive construction effort, and high-molecular-weight materials being prone to self-ignition. In this context, the potential use of inorganic paste materials with proper fluidity, strong self-supporting properties, and appropriate strength for filling corners in the working area and sealing the main air leak passages in the goaf should be evaluated.

Research into inorganic paste materials for use as coal mine filling materials was initiated by scholars in the 1950s [13,14]. Different types of inorganic filling materials, including fly ash paste, high-sand filling materials, foam cement, inorganic-cured foam materials, and ultra-high-water materials, were studied and provided a solid foundation for practical applications [15,16]. Huang Xingli and colleagues [17] utilized inorganic foaming filling materials to seal ventilation channels in the goaf, using mobile grouting technology. This refined inorganic filling material significantly reduced the initial setting time, effectively addressing the issues of spontaneous combustion and fires in coal seams. Feng Guangming [18] developed ultra-high-water filling materials and successfully applied them to fill mining working faces, tackling the challenge of high filling costs. Liu Yong [19] conducted experiments on high-sand filling materials, using water, cement, fly ash, and wind-blown sand as inorganic filling aggregates, and determined the optimal concentration ratio suitable for water-preserving mining, significantly impacting water-preserving mining practices in western mining areas. Zhao Xuefei [20] utilized various fine-tail grains of sand as filling aggregates, examining the filling performance of cemented paste materials under different aggregates and additives, and found that substituting cement with high-alumina clay resulted in minimal changes in the compressive strength of the filling body while gradually enhancing stability in later stages. Zheng Juanrong [21] and collaborators introduced different types of early-strength agents to tailings cemented paste filling materials and discovered that Na₂SO₄ and NaOH early-strength agents had superior effects. Zhang Xuebo [22] used advanced Fluent numerical simulation software to conduct research on air leakage in the goaf. The simulation results led to the application of a custom test scheme using tracer gas SF6 on the fully mechanized mining face, revealing the air leakage patterns of the working face.

Inorganic paste filling materials have become the preferred choice for mining operations and the sealing of working faces. However, practical challenges arise when using these materials to seal corner leakage paths at working faces, mainly due to their high cost and low recovery rate. Therefore, there is a continuous need to develop new types of inorganic paste filling materials that consider the geological conditions of mines and the storage conditions of surrounding materials. The objective of this development is to provide an efficient and reliable sealing solution for corner leakage paths at working faces [23]. The sealing of corner leakage paths can significantly impact the pressure field within the goaf. The effects of filling at different corners and intervals can vary greatly, affecting ventilation patterns and gas distribution in the goaf, as well as directly influencing the cost of filling and normal mining operations at the working face [24,25,26]. Currently, the distribution characteristics of the oxidation zone in the goaf under different filled corners and intervals between two corners are not well understood, posing a challenge in establishing rational and effective corner-filling strategies for working faces.

In this paper, our focus is on the engineering background of the 22,122 working face of Cuncaota Ⅱ Mine. We analyze the surrounding storage materials and the actual situation of the working face, including performing a particle gradation analysis and an X-ray diffraction analysis of the filling aggregate. Moreover, we conduct fluidity tests, quick-setting tests, and expansion tests on mountain sand-based paste filling material. Through these tests, we successfully develop a new type of mountain sand-based paste filling material.

Additionally, we employ ANSYS FLUENT 2022 R1 numerical simulation software to analyze the impact of different corner fillings and different interval distance fillings on the distribution characteristics of the oxidation zone in the goaf. By applying inorganic paste filling technology at the corner of the working face, we achieve positive results on the 22,122 working face. The successful application of this method provides a valuable reference for other mines with similar conditions.

2. Engineering Background

The Cuncaota II Mine is located in the Yijinhuoluo Banner, Ordos City, within the Inner Mongolia Autonomous Region. The coal seams extracted from this mine are Type I, which makes them prone to spontaneous combustion. They have a significant mining height and long working faces, with relatively fewer collapses in the upper and lower triangular areas. However, there is a considerable amount of residual coal near the two roadways in the goaf, posing a significant risk of spontaneous combustion. Classified as a low-gas mine, the risk of surpassing gas pressure limits is relatively low. The main safety concern affecting mine production is the spontaneous combustion of residual coal in the goaf due to air leakage. Therefore, immediate measures are necessary to mitigate the likelihood of spontaneous combustion in the goaf and ensure the safety of mine production.

The 22,122 working face is located in the No. 1 mining area, spanning 835.5 m in length and 340.9 m in width, covering a total area of 284,821.95 m². The coal seam varies in thickness from 2.14 m to 4.81 m, with an average thickness of 2.93 m. The roof of the working face consists mainly of sandy mudstone and siltstone, followed by mudstone and medium-grained sandstone, with localized occurrences of fine-grained sandstone and coarse-grained sandstone. The floor primarily consists of siltstone and fine-grained sandstone. The stratigraphic column of the working face is shown in Figure 1. In the auxiliary haulage roadway of the 22,122 area, the roof consists mainly of continuous medium-grained sandstone, with good rock layer integrity and reduced susceptibility to collapse during mining operations. However, there is a significant issue of air leakage at the two corners of the working face. Currently, measures such as wind curtains and cement bag walls are primarily used to mitigate air leakage. The 22 coal seam is characterized by its overall black coloration with black-brown streaks. The primary constituents of the coal rock include dark coal, bright coal, and some occurrences of cannel coal and vitrinite. The coal exhibits an asphalt luster, featuring a uniform and banded structure, blocky texture, extensive crack development, semi-hardness, and a propensity for spontaneous combustion. The coal seam has a natural ignition period of 42 days, with flame lengths exceeding 400 mm, and the coal dust within the seam possesses explosive properties.

To fully grasp the impact of corner filling on changes in the gas concentration in the goaf, the oxygen concentration in the goaf is monitored using a 260 m long 2-inch seamless steel pipe. This pipe is positioned in the direction of the main withdrawal channel, starting from the end position of the inlet and return air lane of the 22,122 working face. By employing the oxygen concentration method, the approximate range of the oxidation zone in the goaf is determined. This method relies on measuring the oxygen concentration at various distances from the goaf to the working face. Areas with an oxygen concentration greater than 15% are classified as heat dissipation zones. Areas with an oxygen concentration between 15% and 5% are considered oxidation spontaneous combustion zones. Lastly, areas with an oxygen concentration below 5% are identified as asphyxiation zones. Sampling probes are strategically placed at 25 m intervals, starting from the corner of the working face. Each intake and return airway has three probes, labeled as “Inlet 1,” “Inlet 2,” “Inlet 2,” “Return 1,” “Return 2,” and “Return 3”. To protect the probes from damage, probe protection devices are utilized, and seamless steel pipes are connected using flange plates to ensure strong connections. Through sampling and an analysis of gas concentration data, it is observed that, on the intake side of the 22,122 working face, the oxygen concentration decreases to below 15% at a distance of 134 m from the working face, indicating entry into the oxidation zone. At a distance of 229 m, the oxygen concentration further decreases to below 5%, signifying entry into the asphyxiation zone. The oxidation zone is approximately 95 m wide. On the return side of the goaf, the oxygen concentration decreases to below 15% at a distance of 42 m from the working face, entering the oxidation zone. At a distance of 185 m, the oxygen concentration drops below 5%, indicating entry into the asphyxiation zone. The oxidation zone on the return side has a width of around 141 m. Figure 2 illustrates the situation of the 22,122 working face and the distribution diagram of the “three zones” of spontaneous combustion in the goaf.

Based on the above analysis, it is evident that there is a significant issue of air leakage at the corners of the 22,122 working face. The current sealing techniques are not effective, resulting in a considerable amount of airflow entering the goaf. Additionally, in the goaf of the 22,122 working face, particularly on the return side, the width of the oxidation zone is excessively large, leading to a heightened risk of spontaneous combustion in the goaf. It is essential to implement effective sealing methods at the corners of the working face to minimize air leakage into the goaf, decrease the width of the oxidation zone, and ensure safe mining operations.

3. Experimental Analysis of Mountain Sand Paste Filling Material

3.1. Selection and Physicochemical Property Testing of Mountain Sand Paste Filling Material

The area surrounding Cuncaota II Mine is abundant in four types of filling materials: slag, mountain sand, loess, and fly ash. However, the high viscosity of loess makes it unsuitable as a filling aggregate. Additionally, there is no coarse slag warehouse in the mining area. The concrete mixing station in the mining area mainly contains mountain sand and fly ash, creating a unique situation. Mountain sand and fly ash are chosen as the primary raw materials for developing a new inorganic paste filling material, and selective experiments are carried out to determine the optimum ratio for the mountain sand-based paste filling material.

The dry screening method was used to determine the gradation of mountain sand and fly ash in the concrete mixing station of the mining area. An X-ray diffraction analysis was then conducted using a diffractometer. The results show that the particle distribution of the mountain sand is relatively uniform, although its continuity is poor, making it suitable for use as a filling aggregate. In contrast, the particle distribution of fly ash is discontinuous, making it unsuitable as a filling aggregate, but it can serve as a substitute for cement in filling material. Adding fly ash to slurry can reduce the amount of cement used and improve the workability and durability of the paste [27].

Table 1. Particle size characteristics of filling materials.

Filling Material	Control Particle Size d60 (μm)	Median Particle Size		Effective Particle Size d10 (μm)	Average Particle Size dcp (μm)	Uniformity Coefficient Cu	Curvature Coefficient Cc
		d50 (μm)	d30 (μm)
Mountain sand	134.68	114.00	77.17	42.40	146.41	3.18	1.04
Fly ash	24.00	18.60	10.71	4.97	27.26	4.83	0.96

provides detailed information on the particle size characteristics and primary chemical compositions of the filling materials. In contrast, the X-ray diffraction analysis results for the filling materials are depicted in Figure 3.

3.2. Analysis of Performance Testing for Filling Materials

The filling materials used for sealing the corners in working faces should have reasonable flowability to ensure rapid accumulation in the corners and efficient blockage to prevent air leakage. They also need to exhibit good self-supporting characteristics, short-term solidification, and a certain degree of expansiveness to reach the roof and prevent airflow leakage into the goaf. Analyzing the flowability, setting time, and expansiveness of the mountain sand-based paste filling material through slump tests, viscosity tests, and quantitative tests for accelerators and expanders aims to determine the optimal ratio for the filling material applicable to the corners of the 22,122 working face [28,29,30].

The typical range of slump for a mine filling system paste is 15 to 25 cm. Pastes with slumps between 16 and 20 cm exhibit good self-supporting properties, but they require specific capabilities from pumping equipment. Considering that the mine uses an HBMD-30/9-45S mine filling pump, capable of handling paste within a slump range of 120 to 200 mm, it is recommended to maintain a slump range of 16 to 20 cm for the filling slurry to ensure the effective pipeline transport of high-concentration pastes.

Based on the physicochemical properties of the mountain sand-based paste filling material, various mix ratios (cement: fly ash: mountain sand) and mass concentrations of the mountain sand-based paste were prepared. Slump, slump spread, and viscosity tests were carried out on slurries with different ratios using a slump cone and an SC145 mortar viscometer. The test results, depicted in Figure 4, indicate a decrease in the slump, slump spread, and viscosity values of the mountain sand-based filling slurry with an increase in the mass concentration. Using a slump range of 16 to 20 cm as the standard, the optimal concentration for the 1:2:6 mountain sand ratio is 83%, with a slump of 17.8 cm; for the 1:2:10 ratio, the optimal concentration is 84.5%, with a slump of 18.2 cm; for the 1:4:15 ratio, the optimal concentration is 84%, with a slump of 19.1 cm; and for the 1:4:20 ratio, the optimal concentration is 84.5%, with a slump of 16.3 cm.

A series of quantitative tests were carried out on a J85 rapid-setting agent by adding 3%, 6%, and 9% to different proportioned slurries. The initial setting time of the sand-based grout was measured using a Vicat apparatus. The results, presented in

Table 2. Quantitative test results of J85 rapid-setting agent.

	Cement:Fly Ash:Mountain Sand	Mass Concentration/%	Addition Amount of J85/%	Initial Setting Time/min
Mountain sand-based filling material	1:2:6	83	3	145
	1:2:6	83	6	139
	1:2:6	83	9	150
	1:2:10	84.5	3	152
	1:2:10	84.5	6	143
	1:2:10	84.5	9	141
	1:4:15	84	3	145
	1:4:15	84	6	144
	1:4:15	84	9	135
	1:4:20	84.5	3	162
	1:4:20	84.5	6	148
	1:4:20	84.5	9	179

, show that the initial setting time of the slag-based paste filling material fluctuated with the amount of the J85 rapid-setting agent, initially decreasing and then increasing, across different mix ratios and concentrations. The shortest initial setting time occurred when the rapid-setting agent was added at 9%, with a mix ratio of 1:4:15 and a mass concentration of 84% for the mountain sand-based paste filling slurry.

Additionally, a quantitative selection test was conducted on the sand-based paste filling material using a UEA-type expansion agent at dosages of 0.25%, 0.5%, 0.75%, and 1%. The expansion rates before and after the test samples were calculated, and the results, presented in Figure 5, show that the maximum expansion rate initially increased and then decreased with the increase in the UEA expansion agent dosage. Notably, at a dosage of 0.5% of the UEA-type expansion agent, the sand-based paste filling material exhibited the fastest expansion rate and the most optimal expansion effect.

3.3. Analysis of Filling Material Strength Test

Based on the aforementioned test results, a strength test was performed on the sand-based paste filling material using a selected mixing ratio of 1:4:15, a mass concentration of 84%, a dosage of 9% of the J85 rapid-setting agent, and a dosage of 0.5% of the UEA-type expansion agent. A total of eight sets of cylindrical filling specimens, measuring 50 mm × 100 mm, were prepared, with three specimens in each set. The compressive strength and tensile strength of the specimens were measured at curing ages of 3 days, 7 days, 14 days, and 28 days to analyze the variation in the strength characteristics of the paste filling material [31,32]. The testing process is shown in Figure 6. The test results are presented in Figure 7.

The tensile strength of the sand-based filling specimens was notably lower than their compressive strength. The early strength fulfilled the criteria for filling the corners of the working face. Even after 28 days of curing, the filling materials demonstrated some residual strength after failure, ensuring that the sand-based paste filling material does not fully collapse under pressure and can effectively block the airflow movement in the goaf for an extended period.

4. Flow Field Analysis of Corner-Filled Goaf Area Using ANSYS FLUENT

4.1. Model Establishment

After the implementation of the fully mechanized coal mining face, the voids resulting from the roof collapse are intricately linked to the lithology of the roof, mining methods, and the fragmentation coefficient of the coal–rock mass. Researchers view the goaf as a vast porous medium area. Variations in airflow within the goaf are influenced by factors such as porosity, permeability, source terms, and ventilation methods within the goaf area. ANSYS Fluent 2022 R1, numerical simulation software that accommodates multiple variables, is widely used for simulating gas migration in the goaf area [33]. When simulating airflow in the goaf using Fluent, aside from the fundamental model establishment and grid division, it is crucial to optimize the formula for goaf porosity through the utilization of the User-Defined Function (UDF) based on the collapse and fragmentation coefficient of the goaf rock mass to accurately capture the simulation results.

According to the research of scholars [34], based on the definition of porosity and the fragmentation coefficient of fractured rock mass, Formula (1) can be derived:

$$\phi =1-\frac{1}{K_{\mathrm{p}}}$$

(1)

where K_p is the fragmentation coefficient of the collapsed rock mass, and φ is the porosity of the collapsed rock mass. The porosity of the caved rock mass considering the influence of gravity satisfies the following equation:

$${\varphi }_G={\beta }_1\sigma +{\varphi }_G$$

(2)

$${\varphi }_G(Y=0)={\beta }_1\frac{(1-{\varphi }_G)\gamma (\frac{l_y}{2}-y)\sin \alpha }{{\sigma }_0}+\left[1+e^{-0.15(\frac{l_y}{2}-|y|)}\right]\cdot \left\{1-\frac{h_{{}_d}}{h_d+H-\left[H-h_d(K_{P_b}-1)\right](1-e^{-\frac{x}{2l}})}\right\}$$

(3)

By solving the above equation, we obtain

$${\varphi }_G(x,y)=1+\frac{\left[1+e^{-0.15(\frac{l_y}{2}-|y|)}\right]\cdot \left\{1-\frac{h_d}{h_d+H-\left[H-h_d(K_{p_b}-1)\right](1-e^{-\frac{x}{2l}})}\right\}-1}{1+{\sigma }_0^{-1}{\beta }_1\gamma (\frac{l_y}{2}-y)\sin \alpha }$$

(4)

where Φ_G(x, y) is the variation curve of porosity in the goaf; β₁ is the regression coefficient; σ is the relative axial stress, Mpa; γ is the bulk density of the caved rock mass, N/m³; l_y is the width of the goaf in the strike direction, m; H is the mining height, m; h_d is the thickness of the immediate roof, m; K_Ph is the residual fragmentation coefficient of the immediate roof rock mass; l is the distance of roof collapse, m; and α is the inclination angle of the mining face.

Based on the geological conditions of the 22,122 working face and considering the distribution law of the fragmentation coefficient, a fitting process was conducted to obtain a porosity distribution map of the goaf with Z = 1.5 for the 22,122 working face using Python-3.9.6. The porosity, permeability, gas flow, and dissipation coefficient of the goaf were then imported into Fluent for solving using UDF functions. The resulting goaf model is illustrated in Figure 8.

4.2. Model Accuracy Verification

Before engaging in simulation studies involving different corner fillings, it is essential to validate the accuracy of the initial model. In the case where both corners of the 22,122 working face remain unfilled (C0), we conducted a simulation analysis of the oxygen concentration field in the goaf area. The accuracy of the model and parameters was evaluated by comparing the distribution ranges of the measured and simulated goaf oxidation zones. Figure 9 presents a visual comparison between the measured goaf oxidation zone width data and the results obtained from the numerical simulations. As shown in Figure 9, the measured oxidation zone on the intake side of the goaf spans from 134 m to 229 m from the working face, while on the return side, it ranges from 42 m to 185 m. The simulated oxidation zone on the intake side of the goaf covers the range of 147 m to 238 m from the working face, and on the return side, it extends from 28 m to 188 m. Despite potential sampling errors, the comparative analysis between the measured structure and simulation results suggests a rough similarity in the ranges of the measured and simulated oxidation zones. This indicates that the model and parameters established in the previous section effectively capture the distribution pattern of oxygen in the goaf area of the 22,122 working face, laying a solid foundation for subsequent simulation studies.

5. Analysis of the Distribution Characteristics of the Oxidation Zone in the Goaf under Different Corner-Filling Conditions

5.1. Simulation Analysis of Upper and Lower Corner Fillings in the Working Face

To evaluate the impact of different corner fillings on goaf air leakage and the extent of the oxidation zone, we introduced pre-set filling bodies behind the working face, expanding on the model established in the previous section. We examined three distinct scenarios: C1—filling only the upper corner of the working face, C2—filling only the lower corner of the working face, and C3—filling both corners. The pre-set filling bodies were 100 m long, 3 m wide, and 3 m high, with an approximate zero porosity. The ventilation parameters remained consistent with the conditions outlined in the previous study. The specific layout is shown in Figure 10.

Figure 11 depicts the pressure distribution in the goaf at a height of z = 1.5 m, showing continuous changes resulting from various backfilling strategies. The exit pressure on the return air side of the working face corner is set as the reference point at 0 Pa. The highest pressure occurs near the lower corner, driving gas flow within the goaf due to pressure differentials. A comparison between the unfilled condition and filling only in the upper corner reveals significant alterations in the pressure distribution within the goaf. After filling in the upper corner, the green pressure zone in the deep part of the goaf substantially expands, while the light green range of 4~5 Pa experiences a sharp reduction. This indicates a notable increase in pressure in the deep part of the goaf and a considerable decrease in the pressure difference between the lower corner and the deep part, ultimately leading to a reduction in overall air leakage from the goaf.

After comparing the unfilled and only partially filled lower corner areas, it becomes evident that filling the lower corner leads to the dominance of the deep goaf by pressure isopleths of 3–4 Pa in the blue area. Furthermore, the high-pressure range significantly decreases, resulting in a decline in pressure within the deep goaf. Consequently, an increased pressure difference between the lower corner and the deep goaf occurs. Fortunately, the filling material in the lower corner effectively obstructs the majority of airflow from infiltrating the goaf. Simultaneously, the reduced pressure difference between the upper corner and the deep goaf hampers the backflow of harmful gases into the working face, thereby reducing the accumulation of gas in the upper corner of the goaf.

Upon comparison of the unfilled area with the region filled with both the upper and lower corners, it is apparent that filling both the upper and lower corners of the working face leads to an increase in pressure within the deep goaf. However, the extent of this pressure increase is slightly lower than when only the upper corner is filled. Furthermore, the range of the low-pressure area is slightly larger than when only the upper corner is filled, but it is significantly smaller than when only the lower corner is filled. This indicates that, when both the upper and lower corners of the working face are simultaneously filled, the filling material in the upper corner has a greater impact on the overall pressure changes in the goaf. It effectively reduces the pressure difference between the deep goaf and the lower corner, thereby minimizing air leakage from its source. Additionally, the majority of airflow within the goaf is hindered by the filling material in the upper corner, making it difficult for gases like methane to flow back toward the working face.

Figure 12 depicts a contour map showing the distribution of the oxygen concentration in the goaf at a height of z = 1.5 m, with both the upper and lower corners filled. The graph indicates that the oxygen concentration distribution in the goaf roughly follows an “S” shape. A comparison of the three different filling conditions reveals a significant reduction in the width of the oxidation zone in the goaf compared to the unfilled condition. Notably, when both the upper and lower corners are filled, the width of the oxidation zone experiences the greatest reduction at various positions in the goaf, indicating the most effective control of the oxidation zone. Specifically, it reduces the width of the oxidation zone by 14.84% on the intake side, 21.07% in the middle, and 6.23% on the return side of the goaf. The widths of the oxidation zone in the goaf under different corner-filling conditions are summarized in

Table 3. Distribution of the oxidation zone in the goaf under different corner-filling conditions.

Scheme	Location within the Goaf	Spontaneous Combustion Area	Width of the Oxidation Zone
Unfilled	Inlet air side	146.68~237.8 m	91.12 m
	Middle	97.62~209.26 m	111.64 m
	Return air side	27.53~188.24 m	160.71 m
Filling of the upper corner	Inlet air side	159.7~238.3 m	78.6 m
	Middle	116.65~210.76 m	94.11 m
	Return air side	29.54~186.73 m	157.19 m
Filling of the lower corner	Inlet air side	158.2~238.8 m	80.6 m
	Middle	116.65~211.76 m	95.11 m
	Return air side	37.05~188.73 m	151.68 m
Filled	Inlet air side	155.69~233.29 m	77.6 m
	Middle	114.14~202.26 m	88.12 m
	Return air side	29.54~180.23 m	150.69 m

.

Figure 13 presents a comparative schematic diagram illustrating the width of the oxidation zone on the intake side, in the middle, and on the return side of the goaf under four different filling conditions. The diagram emphasizes that corner filling, as an effective measure to reduce the void ratio in the corner areas and seal the airflow leakage from the working face, significantly alters the internal flow field within the goaf, thereby exerting a notable impact on the distribution of the oxidation zone. Filling only the lower corner exhibits a limited inhibitory effect on the width of the oxidation zone on the return side of the goaf. Similarly, filling only the upper corner demonstrates poor inhibitory effects on the width of the oxidation zone on the intake side and middle of the goaf. However, the simultaneous filling of both the upper and lower corners proves to be an effective approach to shorten the width of the oxidation zone in the goaf.

5.2. Simulation Analysis of Different Filling Interval Distances at the Corners of the Working Face

A simulation study was carried out to investigate the optimal filling interval distance, building upon the analysis results from the preceding section. The study specifically targeted various filling interval distances for both the upper and lower corners of the working face. The simulated interval distances encompassed C4—10 m, C5—30 m, C6—50 m, and C7—70 m. The aim was to evaluate the impact of these diverse filling interval distances on the extent of the oxidation zone in the goaf.

Figure 14 displays the pressure distribution in the goaf on the z = 1.5 m plane with varying intervals between the filling bodies. It is evident from Figure 14 that an increase in the interval distance between the filling bodies results in a reduction in pressure in the deeper parts of the goaf. Specifically, with interval distances of 10 m, 30 m, and 50 m, the green range of the pressure contour lines (5–6 Pa) gradually diminishes, while the blue range (3–4 Pa) progressively expands. Additionally, when the interval distance reaches 50 m, the two pressure ranges become nearly identical, indicating a relatively balanced pressure field in the goaf. However, with an interval distance of 70 m, the expanded interval distance significantly increases the blue pressure zone of 2–3 Pa, amplifying the pressure difference between the return air side and the lower corner angle of the goaf and resulting in a greater influx of air leakage into the goaf. Furthermore, the presence of the two filling bodies on the return air side obstructs the movement of air leakage, causing its accumulation between the two filling bodies and adversely impacting the reduction in the width of the oxidation zone on the return air side.

In conclusion, by increasing the spacing between the filling bodies, the pressure difference between the corner of the working face and the deep part of the goaf is reduced. This effectively prevents the backflow of internal airflow to the upper corner angle, mitigating the risk of exceeding the gas limit in that area. Additionally, the secondary filling process makes it more challenging for air leakage from the lower corner angle to infiltrate the goaf, leading to a decrease in air leakage and facilitating the management of the spontaneous combustion zone in the goaf.

Figure 15 presents the oxygen concentration distribution in the goaf at z = 1.5 m with various interval distances between filling bodies. After the secondary interval filling of the two corners of the working face, a noticeable reduction in the width of the oxidation zone within the goaf is observed. This reduction is attributed to the increased resistance encountered by the airflow from the intake side, hindering its entry into the goaf and resulting in weakened airflow within the goaf. Consequently, this decrease in airflow within a certain range contributes to a narrower oxidation zone within the goaf, leading to improved control over air leakage from the working face and the oxidation zone. More detailed data on the width of the oxidation zone in the goaf for different interval distances of filling bodies can be found in

Table 4. Distribution of the oxidation zone in the goaf with different interval distances of filling bodies at different corners of the working face.

Scheme	Location within the Goaf	Spontaneous Combustion Area	Width of the Oxidation Zone
Spacing distance of 10 m	Inlet air side	154.19~218.77 m	64.58 m
	Middle	113.14~187.73 m	74.59 m
	Return air side	25.03~143.68 m	118.65 m
Spacing distance of 30 m	Inlet air side	153.19~216.77 m	63.58 m
	Middle	112.64~185.73 m	73.09 m
	Return air side	30.04~140.68 m	110.64 m
Spacing distance of 50 m	Inlet air side	153.69~217.27 m	63.58 m
	Middle	113.14~186.73 m	73.59 m
	Return air side	34.54~142.18 m	107.64 m
Spacing distance of 70 m	Inlet air side	153.19~229.79 m	76.6 m
	Middle	113.14~203.25 m	90.11 m
	Return air side	37.55~179.72 m	142.17 m

.

Figure 16 presents a schematic diagram demonstrating the extent of the oxidation zone in the goaf at various interval distances of filling bodies. It is evident from the figure that an increase in the interval distance, from primary filling to intervals of 10 m, 30 m, and 50 m, leads to a decrease in the average width of the oxidation zone within the goaf. Interestingly, at an interval distance of 50 m, the oxidation zone width in different parts of the goaf is the smallest. However, as the interval distance is further increased to 70 m, the measured width of the oxidation zone in the goaf exceeds that observed at 10 m, 30 m, and 50 m distances. In fact, in the central section of the goaf, the width of the oxidation zone even surpasses that of the primary filling condition. This indicates that excessively large interval distances between filling bodies have limited effects on reducing the width of the oxidation zone in the goaf. Therefore, it is important to establish a reasonable interval distance for filling to effectively control the width of the oxidation zone in the goaf.

5.3. Application of Inorganic Paste Filling Technology at Working Face Corner

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Process Flow for Filling Workforce Corners

According to the analysis presented earlier, a filling scheme is chosen for the corner-filling operation of the 22,122 comprehensive mining face. This scheme involves filling both corners at intervals of 50 m. The key steps in the inorganic paste filling technology process for the 22,122 working face corner include the preparation and transportation of filling materials, the installation of mobile filling pumps, the laying of pipelines, the pumping of inorganic paste filling slurry, and other critical steps. Taking into account the actual mine conditions, the filling operation for the working face corners is carried out using the surface slurry preparation and tanker transportation method. The specific implementation process is depicted in Figure 17.

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Results and Analysis of Corner Filling in Working Face 22,122

Through the processes of preparing ground slurry, transporting it via a mixer truck, and pumping it through pipelines, the inorganic paste slurry is efficiently pumped to specified locations at the two corners of the working face for filling. Once the filling process is concluded, overflow of the filling slurry at the top of the caved rock layer is observed, and it is depicted in Figure 18. Following two filling sessions, each at a 50 m interval for the two corners of the working face, air leakage before and after the filling is assessed using SF6 gas, employing a system of marking and tracking. SF6 tracing gas is released in the intake airway, and in the presence of air leakage in the working face, the SF6 concentration at measurement points along the path exhibits a decreasing trend. The leakage amount ΔQ between two sampling points can be computed using Equation (5) [35]. The distribution of release points and sampling points is visually represented in Figure 19. Under the measured ventilation rate condition of 1279 m³·min⁻¹ in the working face, the analysis of SF6 concentrations at sampling points 1 and 3 indicates that the air leakage amount ΔQ before filling is 147 m³·min⁻¹. Post-filling, the leakage amount ΔQ decreases to 49 m³·min⁻¹, marking a notable reduction of 66.7%. The majority of post-filling air leakage is directed into the goaf behind the hydraulic support, significantly diminishing air leakage at the two corners of the working face compared to the unfilled state.

$$\Delta Q=\frac{q(C_1-C_2)}{C_1{C}_2}$$

(5)

where q—amount of released SF6 gas, mL/min; C₁—SF6 concentration after analysis in the intake airway, ppm; and C₂—SF6 concentration after analysis in the return airway, ppm.

After conducting two filling and sealing operations at the corners of the working face, gas samples were systematically collected from the intake and return airway ducts, with close monitoring of their compositions. The obtained gas concentrations were then carefully compared to those measured before the filling process, as detailed in Figure 20. As shown in Figure 20a, after the filling and sealing procedures at the corners of the working face, the oxygen concentration on the intake side of the goaf decreased to 15% at a distance of 145 m from the working face, indicating entry into the oxidation zone. Furthermore, it decreased to 5% at a distance of 200 m, entering the asphyxiation zone. In comparison to the pre-filling conditions, the width of the oxidation zone in the goaf significantly reduced from 95 m to 55 m, reflecting a noteworthy decrease of 42%. Shifting attention to the return side of the goaf, as depicted in Figure 20b, post-filling, the oxygen concentration within the goaf declined to 15%, entering the oxidation zone at a distance of 39 m from the working face, and it further dropped to 5% at a distance of 135 m, entering the asphyxiation zone. Compared to the unfilled state, the width of the oxidation zone in the goaf was reduced from 143 m to 96 m, representing a reduction of 33%. The noticeable reduction in the widths of the oxidation zones on both sides of the goaf, when compared to pre-filling conditions, highlights the effectiveness of the filling material at the corners of the working face in preventing the leakage of oxygen into the goaf. This plays a crucial and positive role in averting the spontaneous combustion of residual coal in the goaf of the 22,122 working face, thereby further ensuring the safety of mining operations.

6. Discussion

This article intricately presents the geological conditions of the 22,122 working face in the Cuncaota II Mine, elucidates the dominant factors contributing to spontaneous combustion in goaf areas, and emphasizes the significance of sealing the corners of the working face. Through a comprehensive analysis of the physical and chemical properties of large-volume mountain sand and fly ash in the mining area, a revolutionary mountain sand-based paste filling material is successfully developed and applied to seal the corners of the 22,122 working face. The distribution characteristics of the oxidation zone in the goaf, under various corner-filling and interval-filling scenarios, are simulated and analyzed using ANSYS FLUENT. This study holds immense significance in mitigating air leakage in the working face and ensuring safe mining operations. While the existing literature has extensively explored inorganic paste filling materials [15,16,17,18,19,20,21,22], this article introduces an innovative approach by using mountain sand as the filling aggregate and fly ash as the filling auxiliary material, resulting in a novel mountain sand-based paste filling material. This novel material demonstrates superior self-supporting properties, a quick setting time, and the ability to fully expand and reach the ceiling, as compared to other inorganic paste filling materials. These attributes improve corner-filling efficiency, decrease filling costs, and contribute to the advancement of inorganic paste filling materials.

Numerical simulation studies that examine the division of oxidation zones in goaf areas offer crucial insights for ensuring the safe production of coal mines [6,24,25,34]. The variation in the width of the oxidation zone in goaf areas directly affects the advancing speed of the working face, and an excessively large oxidation zone poses a direct threat to the safety of coal mine production. While previous research has mainly focused on the impact of changes in airflow and goaf porosity on the range of oxidation zones, this study primarily focuses on the influence of filling materials at the corners of the working face on the distribution characteristics of oxidation zones in goaf areas, aimed at reducing air leakage into the goaf from the source. The analysis of the pressure field in goaf areas filled with different corner materials shows that changing corner fillings impacts the pressure difference between the interior of the goaf and the lower corners of the working face, thereby influencing airflow movement. The pressure field distribution in the goaf area becomes more uniform when both corners are filled, leading to a smaller width of the oxidation zone. Simulated experiments with various interval distances for filling in both corners indicate that the width of the oxidation zone in the goaf area tends to decrease initially and then increase with the increase in the filling interval distance. The range of the oxidation zone within the goaf area is minimized when the interval distance is 50 m.

This research paper proposes the utilization of inorganic paste filling as a solution for mitigating air leakage at the corners of operational areas within coal mines. This approach is successfully applied in the 22,122 working face and demonstrates a substantial decrease in air leakage when compared to unfilled conditions. Traditional methods such as cement bag walls or wind-blocking curtains are ineffective in sealing off the corners and have led to increased labor intensity for workers. In contrast, the use of inorganic paste filling not only offers superior sealing effects but also contributes to preventing spontaneous combustion in goaf areas. This technology introduces novel materials, techniques, and perspectives for addressing air leakage at the corners of operational areas, providing valuable insights for other mines encountering similar challenges.

7. Conclusions

This paper successfully developed a novel paste filling material based on mountain sand by conducting a compositional analysis of mountain sand and fly ash, along with a numerical simulation analysis of filling in the corners of the working face. Additionally, this study investigated the distribution characteristics of the oxidation zone in the goaf during the process. As a result, the use of inorganic paste filling at the corners of the working face was proposed and effectively implemented in underground field operations. The following conclusions were drawn:

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Through experiments involving the determination of particle size distribution and X-ray diffraction on commonly used filling aggregates, such as mountain sand and fly ash, it has been observed that mountain sand is an effective filling aggregate, while fly ash can be used as a substitute for cement as a filling auxiliary material. Based on this discovery, a new mountain sand-based paste filling material was developed. Through rigorous flowability tests, quick-setting tests, and expansibility tests, the optimal mass ratio for this new mountain sand-based paste filling material was determined to be 1:4:15 (cement: fly ash: mountain sand), with a solid concentration of 84%. Additionally, it is recommended to use a 9% dosage of a J85 quick-setting agent and a 0.5% dosage of a UEA expansion agent for optimal results.

(2)

Various corner-filling scenarios were simulated and analyzed for the 22,122 working faces using FLUENT 2022 R1 numerical simulation software. It was observed that filling significantly impacted the pressure field within the goaf area. The presence of filling material in the upper corner reduced the pressure difference between the deep goaf area and the lower corner, effectively mitigating air leakage at its source. Similarly, the presence of filling material in the lower corner decreased the pressure difference between the deep goaf area and the upper corner, impeding the flow of harmful gases back to the working face and reducing the likelihood of gas exceeding safety limits in the upper corner. Overall, these findings contribute to the prevention of gas accidents by minimizing the risk of gas accumulation in the working face.

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By employing FLUENT numerical simulation software, different interval distances for filling in the two corners of the 22,122 working face were examined. The findings indicate that the width of the oxidation zone within the goaf area was impacted by the interval distance, displaying a trend of an initial reduction followed by an increase. The minimum extent of the oxidation zone within the goaf area was observed at an interval distance of 50 m. It was determined that implementing a secondary filling in the corners of the working face was more effective in controlling the width of the oxidation zone within the goaf area. However, it is essential to avoid excessively large interval distances.

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The technology of inorganic paste filling was successfully implemented at the two corners of the 22,122 working face. After conducting two fillings with a 50 m interval between the upper and lower corners, the air leakage in the goaf area decreased by 66.7% compared to unfilled conditions. Moreover, the width of the oxidation zone on the intake side of the goaf area was reduced by 42%, while on the return side, it decreased by 33%. The filling material effectively acted as a barrier, preventing oxygen from entering the goaf area and reducing the risk of spontaneous combustion of residual coal. This advancement significantly contributes to the safe extraction of the working face.