Analysis and Engineering Practice of Factors Affecting Top-Coal Recovery in a Large Dip Coal Seam

The recovery of top coal in the caving face directly impacts the efficiency of mining coal resources. The geological conditions and mining parameters are well known to be significant influences on the recovery of top coal. This study focused on the 9-301 working face, which is located in a thick coal seam with a large dip angle. The influences of the coal seam’s dip angle, mining direction, and coal caving mode and interval on the recovery were analyzed using PFC 2D simulation. Field trials were also carried out. The results of the numerical calculations show that the recovery of top coal is clearly affected by the dip angle, with recovery decreasing as the dip angle is increased. Mining from the top to bottom along the dip of the coal seam is beneficial to improve recovery. The top-coal recovery using the multicycle-sequence coal caving method is higher than when using single-sequence coal caving and single-interval coal caving modes. The top-coal recovery using “one cutting and one caving” (coal caving interval of 0.8 m) was higher than that under two cuttings and one caving (coal caving interval of 1.6m). During the field trials, the recovery of top coal under different caving intervals and modes was measured. The results show that the recovery of top coal is optimal when using one cutting and one caving with multicycle-sequence coal caving modes. The field measurements are consistent with the simulation results. The results of this study can help guide additional research for optimizing the recovery of top coal from thick coal seams with large dip angles.


Introduction
As part of China's 14th ve-year plan, the role of coal will change from being the country's primary energy source to that of a supporting role to guarantee energy security. e key developmental areas to improve coal processing include improved safety, e cient mining, and clean utilization. As coal is a nonrenewable resource, appropriate mining methods and parameters must be selected during mining to minimize the formation of waste material [1]. Coal seams with steep (>45°) dip angles account for approximately onefth of China's total coal reserves. Consequently, it is important to improve the e ciency of mining these steep coal seams. During coal mining, the application of fully mechanized top-coal caving technology can optimize the mining process, improve e ciency and output, and signi cantly reduce or eliminate site accidents [2][3][4]. Top-coal caving technology is the primary and preferred method to mine thick coal seams in China, with the method being exible and widely used [5,6]. e caving properties of top coal are the key factors in the selection of fully mechanized cave-mining methods and also determine the resource recovery. e recovery of coal is directly related to the improvement of the caving properties of the top coal present. Various studies have been conducted to investigate the relationship of top-coal caving at the working face [7][8][9] with various factors affecting the recovery. ese can be summarized as being either geological or technical factors [10]. Geological factors include the coal strength, mining depth, and fracture development. Technical factors include the caving interval, mode, and sequence, mining/caving ratio, roof-control distance, support type, and end-head loss. To date, numerous studies have been carried out to qualify the influence of these factors on the recovery of top coal. Zhu and Wang [11], Ma [12], and Liang [13] summarized the geological and technical factors that affect the caving properties of top coal. ese studies included the simulation of relevant technological parameters and selected mining schemes with improved recovery of top coal. Han and Bao [14], Wang [15], and Zu [16] all studied the mining/caving ratio. ese studies considered the mining/caving ratio, along with the caving interval and mode, analyzing the recovery and gangue content under different conditions to select optimal process parameters to improve recovery. Some studies have individually focused on one of these factors. Liu et al. [17] conducted in-depth investigations into the top-coal caving properties under the conditions of different mining/caving ratios and caving intervals. is study established a numerical model that was used to analyze the impact of the mining/caving ratio and caving intervals on the top-coal recovery, obtaining the relationship between the mining/caving ratio and the recovery. Zhang et al. [18] conducted a discrete element simulation of the top-coal caving process under different dip angles, analyzing the caving volume and the recovery of top coal. e results demonstrated that the caving volume of top coal initially increased and then decreased as the dip angle of the working face was increased. Zhao [19] analyzed the influence of the coal caving sequence on the recovery of top coal. By comparing the recovery under "single-sequence," "single-interval," and "single-segmental interval" schemes, it was suggested that "single-segmental interval" caving modes could greatly improve the recovery at the working face. Li [20] reduced the strength and lumpiness of raw coal by injecting water to weaken a top-coal seam with a steep dip angle. is resulted in an increased recovery of top coal, improving from 13% to 22%. Chen et al. [21] undertook field-and laboratory-based investigations and conducted a theoretical analysis of the influence of gangue on the caving properties of top coal. e authors proposed measures to improve the caving properties of top coal based on these investigations. Various researchers have established systems for evaluating the caving properties of top coal. Jiang [22] undertook a theoretical analysis of the various factors that affect the caving properties, establishing a multifactor evaluation system.
is provided a reference for other studies evaluating these properties. Du et al. [23] and Li [24] established functional models for simulating/evaluating topcoal caving using an analytic hierarchical process that provided guidance for improving the recovery of top coal.
In summary, various studies have investigated the factors that affect the recovery of top coal when utilizing a fully mechanized caving face.
ese studies have established various models to evaluate the top-coal caving properties, providing a basis for determining the parameters to optimize the process. e geological conditions during mechanized caving are complicated, especially when mining thick coal seams with steep dip angles. Further in-depth research on improving the recovery of top coal under these conditions is required. is study used the theory of ellipsoid coal caving [25][26][27][28], and particle flow code (PFC) [29][30][31][32][33][34] was used to simulate and analyze the influence of factors including the dip angle, caving mode, caving interval, mining direction, and other factors on the recovery of top coal. Field trials were conducted to verify the accuracy of the model and determine the process parameters for the 9-301 working face of a steep, thick coal seam. is study provides a reference for improving the recovery of top coal under similar conditions.

Overview of the Project
e 9# coal seam is currently being mined in the 9-301 working face of a mining area located in Shanxi Province. e average depth of the coal seam from the surface is 460 m, with an average thickness of 11.8 m. e average dip angle of the coal seam is 35°. e inclined length of the working face is 200 m, and the strike length is 1490 m. Top-coal caving is currently used for a mining height of 3.2 m. e caving thickness is 8.6 m, and the coal cutting interval is 0.8 m. e immediate roof of the coal seam is argillaceous limestone, with a thickness of 5-7 m. e main roof is sandy mudstone, with a thickness of 6-9 m. e immediate floor is mudstone, with a thickness of 1-2 m. e north side of the working face is the western concealed inclined shaft system, and the south side is the boundary of the mine field. e west side is the 9-101 working face, and the east side is unmined coal. e layout of the working face is presented in Figure 1, while the roof and floor of working face are reported in Table 1.

Simulations of the Factors Influencing the Top-Coal Recovery
3.1. Establishment of the Model. Particle flow code (PFC 2D ) and the discrete element method were utilized to simulate the movement and interaction of particle aggregates. e trend and strike models were established according to the conditions measured at the 9-301 working face. e strike model of section I-I and the trend model of section II-II in Figure 1 were established. e length and width of the model were 50 m × 50 m. Field investigations and laboratory testing determined the mechanical parameters of the coal and rock mass used in the model (Table 2). e size of the coal seam block was set as 100-200 mm. Considering the Gaussian random distribution, large or small blocks were discarded to reduce the calculation time and speed up the model convergence. e stress state of the coal seam was displayed via particle cluster mode. In the case of non-mining, the coal body was in an integral state without granular flow medium [35]. e trend model was used to study the influence of the dip angle, mining direction, and caving mode on the recovery of top coal. e strike model was used to analyze the influence of the caving interval on the recovery of top coal. e trend and strike models are reported in Figures 2 and 3, respectively.
Both sides and the top boundary of the model were fixed horizontally, and the vertical direction was free. e bottom of the model was fixed horizontally and vertically. According to the field stress testing, a horizontal force of 20.5 MPa was applied to both sides of the strike model, and 26.3 MPa was applied to both sides of the trend model, while 10.3 MPa was applied to the top of the model to replace the weight of the upper overburden.

Influence of the Coal Seam's Dip Angle on the Top-Coal
Recovery.
e dip angle of the coal seam significantly influences the mining efficiency of top-coal caving. When the dip angle is large, the gravitational influence of the coal body is increased, potentially leading to the collapse of the coal body. However, with continuous increase in the dip angle, the recovery decreases [36]. During this study, top-coal caving simulation tests were carried out with the dip angle of coal seam set to 15°, 25°, and 35° (Figures 4-6, respectively). e coal caving direction during the simulations was from    When the dip angle of the coal seam was 15°, the recovery was calculated to be 79.2%. When the dip angle was increased to 25°, the recovery was 76.3%, and when the dip angle was 35°, the recovery was 74.2%. e simulation results clearly demonstrate that the recovery rate decreases as the dip angle of the coal seam rises.

Influence of the Mining Direction on the Top-Coal
Recovery.
e influence of downward and upward mining on the top-coal recovery was examined using a dip angle of 35°. Upward mining refers to the shearer cutting coal and hydraulic support caving coal from bottom to top along the inclination of the coal seam. Downward mining refers to the shearer cutting coal and hydraulic support caving coal from top to bottom along the inclination of the coal seam. According to the simulations (Figures 7 and 8     during downward mining. Downward mining and caving had a greater impact on the roof, but the area impacted was relatively weak. Consequently, downward mining along the dip of the coal seam is recommended for improved economic benefits.

Influence of the Caving Mode on the Top-Coal Recovery.
ere are three caving modes utilizing a fully mechanized caving face. ese include single-sequence coal caving, single-interval coal caving, and multicycle-sequence coal caving [37]. Eight groups of supports were arranged in the model. e simulations were carried out according to the three modes, with the effects compared: (1) single-sequence coal caving refers to coal caving in the order of 1#, 2#, 3#. . . supports, with the caving opening closed after contacting the gangue. (2) Single-interval coal caving refers to coal caving in the order of 1#, 3#, 5#. . . supports initially and closing after finding gangue. After a certain period, the coal outlets of the 2#, 4#, 6#. . . supports are opened to discharge the ridge coal. (3) Multicycle-sequence coal caving refers to opening the outlets of the 1# and 2# supports at the same       time, and simultaneously opening the outlets of the 3# and 4# supports after closing the outlets of the 1# and 2# supports. e mode described is followed until all supports have completed caving. e coal caving direction in the above model is from top to bottom along the dip of coal seam. e simulation process is presented in Figures 9-11. e recovery of top coal was 85.7% when multicycle-sequence coal caving was used, 82.7% in the case of single-interval coal caving, and 83.1% when single-sequence coal caving was utilized. Multicycle-sequence coal caving is the most effective method to improve the top-coal recovery. However, due to the simultaneous caving of multiple supports, the roof is impacted and the top coal is thrust from the top to the bottom.

Advances in Civil Engineering
is leads to the roof sinking towards the coal caving space, making it more difficult to control.

Influence of the Caving Interval on the Recovery of Top
Coal.
e coal caving interval refers to the advancing distance of the working face between coal caving. A reasonable coal caving interval is crucial to improve the recovery and maintain roof stability [38,39]. e caving interval is an important factor that affects the top-coal recovery and the gangue content of the working face. If the caving interval is too long or too short, the recovery will decline and the mining quality will be reduced [40][41][42]. At present, there are two commonly used technologies for coal caving, namely, "one cutting and one caving" (coal caving interval of 0.8 m) and "two cuttings and one caving" (coal caving interval of 1.6 m). e numerical simulation of these two coal caving processes is presented in Figures 12 and 13, respectively.
When the coal caving interval is larger than the short axis of the caving ellipsoid, the gangue above the support reaches the caving opening before the top coal within the interval range. is results in "backbone" coal loss and reduces the recovery of top coal. Conversely, if the coal caving interval is smaller than the short axis of the caving ellipsoid, then gangue in the goaf is discharged earlier than the top coal, resulting in the retention of the coal, which affects the quality of the coal mined [43][44][45][46]. e results of the simulations (Figures 11 and 12) show that the "one cutting and one caving" method (coal caving interval of 0.8 m) is more economical than the "two cuttings and one caving" method (coal caving interval of 1.6 m). e top coal and gangue can reach the coal discharge port at the same time, assisting in recovering the top coal and improving control of the support to the roof.

Monitoring the Top-Coal Recovery.
e 9-301 working face was selected as the site for undertaking the industrial field trials. e migration monitoring points of the top coal were arranged, with boreholes installed in different areas of the working face. A total of 10 sections were arranged for monitoring. e 10# support was used as the starting point, with each monitoring section arranged at an interval of 10 supports. Six measuring points were arranged in the top coal for each section, at depths of 3, 5, 6, 7, 8, and 9 m, respectively. e measuring point at 9 m was located in the roof (Figure 14). e measuring point in the hole was equipped with a fixed barb device ( Figure 15). During field construction, boreholes were drilled between the adjacent supports, with a diameter of 42 mm and a depth of 9 m. e drill rod was used to push the monitoring device to a predetermined depth. e arrangement of measuring points for each section is presented in Table 3. After the top coal was discharged, an iron suction device at the head of the scraper conveyor and transfer machine was used to recover the measuring point device (Figure 16), which was used to count the maximum coal caving height (i.e., the actual coal caving height). e ratio of the actual coal caving height to the theoretical coal caving height was used as a comparative 9-3# 9-5# 9-6# 9-7# 9-8# 9-9# 90# 10# 10-3# 10-5# 10-6# 10-7# 10-8# 10-9# 100# Figure 16: Measuring point device after recovery.         Table 6: Monitoring data of top coal recovery under the condition of one cutting and one caving with single-interval coal caving ("√" means the monitoring point has been recovered, "×" means the monitoring point has not been recovered).       A comparison was carried out between the processes of "one cutting and one caving" (caving interval of 0.8 m) and "two cuttings and one caving" (caving interval of 1.6 m). e original monitoring data are attached at the end of the article (Tables 4-7). e statistical data of the recovery under these conditions are reported in Table 8. e caving method used was single-sequence coal caving. ree measurements were made under each condition. Using the process of "one cutting and one caving," the recovery of top coal was 0.93, 0.93, and 0.94, with an average of 0.93. Using the conditions of "two cuttings and one caving," the recovery of top coal was 0.90, 0.91, and 0.89, with an average of 0.90. e recovery of top coal with the process of "one cutting and one caving" was significantly higher than that of "two cuttings and one caving." e industrial test results were consistent with the simulation data.

Recovery of Top Coal with Different Coal Caving Modes.
Different coal caving modes were compared under the condition of "one cutting and one caving." e coal caving modes included single-sequence coal caving, single-interval coal caving, and multicycle-sequence coal caving. e results from the monitoring data are reported in Table 9. It is evident that the multicycle-sequence coal caving was the best, with the recovery of top coal reaching 98%. e recovery of top coal using the single-sequence coal caving was 93%, with single-interval coal caving having 87% recovery. erefore, the recovery of top coal can be improved using multicycle-sequence coal caving.
To verify the experimental results, field industrial tests lasting two months were carried out. Assuming that all of the coal within the cutting height of the coal shearer was recovered, the theoretical coal output, the actual output, and the recovery of top coal were determined. ese data are reported in Table 10, with the effects of different caving modes on the recovery of top coal compared. e results demonstrate that the recovery of top coal was the highest under the condition of "one cutting and one caving" with a multicycle-sequence coal caving, reaching 96%. e recovery of top coal was 90.5% under the condition of "one cutting and one caving" with single-sequence coal caving. e optimal choice of methodology resulted in an increase of 5.5% in the recovery.

Discussion
(1) ere are many factors affecting the recovery of top coal, and the interaction of multiple factors makes the movements of top coal more complicated. is study only analyzes factors such as the dip angle, mining direction, coal caving intervals, and coal caving modes. us, further analysis is required. (2) At present, there is still no effective method to observe the recovery of top coal. e method used in this study is based on the stability of top-coal's occurrence with consistent thickness, so there is still an error with respect to the actual situation.

Conclusion
(1) A numerical model was established to investigate the dip of the 9-301 working face. e effects of the coal seam's dip angle, the mining direction, and the   e simulation results showed that the dip angle of the coal seam has a strong influence on the recovery. e recovery decreased as the dip angle increased. Downward mining from the top to the bottom along the dip of the coal seam is conducive to improving the recovery of top coal. e recovery of top coal using the multicycle-sequence caving method is higher than that using single-sequence or singleinterval caving.
(2) e influence of the coal caving interval on the recovery of top coal was analyzed, with simulation results demonstrating that the recovery under "one cutting and one caving" (coal caving interval of 0.8 m) was higher than that under "two cuttings and one caving" (coal caving interval of 1.6 m). (3) e recovery of top coal under different caving intervals and caving modes was measured during field trials. Site monitoring results showed that the recovery of top coal was greatest under the condition of "one cutting and one caving" with the multicyclesequence caving. e field measurements were consistent with the simulation results.

Data Availability
Some or all data, models, or codes generated or used during the study are available from the corresponding author by request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.