Arab J Geosci (2016) 9:465 DOI 10.1007/s12517-016-2493-8

ORIGINAL PAPER

Flow of top coal and roof rock and loss of top coal in fully mechanized top coal caving mining of extra thick coal seams Ningbo Zhang 1,2 & Changyou Liu 1,2 & Peiju Yang 1,2

Received: 15 July 2015 / Accepted: 25 April 2016 # Saudi Society for Geosciences 2016

Abstract This study investigates the flow and caving characteristics of top coal and roof rock, as well as top coal loss pattern in the fully mechanized top coal caving mining of extra thick coal seams. The two dimensional discrete element numerical simulation software program, particle flow code (PFC), is used for the simulation of top coal caving and the inversion analysis. The original locations, distribution, and migration pattern of caved top coal and lost coal were obtained. The analysis shows that in the initial site of caving, the caved bodies are in the form of arc shaped strips in front of the working face. During the caving, caved bodies of different heights move towards the lower rear of the face at different speeds. The lost coal and caved roof rock are originally located at the interface between coal seam and roof, the lost coal is mainly distributed in the goaf on the floor. Behind the support, the caved top coal bodies originally are arc shaped strips, with the highest points located at the midline of the caving opening. The strips are more curved near the goaf than those near the support. During top coal caving, the strips successively cave, with the adjacent outer strip replacing the caved one. The variations of top coal loss and waste rock ratio with time reflect the different phases of top coal caving. In order to improve coal recovery and limit the amount of caved roof rock, the waste rock ratio should be controlled below 10 %.

* Changyou Liu [email protected]

1

State Key Laboratory of Coal Resources and Mine Safety, China University of Mining and Technology, Xuzhou, Jiangsu 221116, Peoples’ Republic of China

2

School of Mines, Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, China University of Mining and Technology, Xuzhou, Jiangsu 221116, Peoples’ Republic of China

When the waste rock ratio reaches this value, the caving opening should be closed. This paper provides theoretical bases for the improvement of top coal recovery in the fully mechanized top coal caving mining of extra thick coal seams. Keywords Extra thick coal seams . Fully mechanized caving . Top coal flow pattern . Coal loss . Inversion For the theoretical study of fully mechanized top coal caving mining, a focus has been the flow and caving characteristics of top coal and roof rock during the caving process (Fu et al. 1999; Wang 2014; Khanal et al. 2014; Alehossein and Poulsen 2010; Huang and Liu 2006; Wang and Zhang 2015). The reason is that those two aspects are closely related to the determination of reasonable caving parameters and the improvement of top coal recovery. Intensive studies with considerable achievements have been done in the field. In the early stages of the application of top coal caving, Wu (1991) and Wu and Yu (1989) introduced the drawing ellipsoid theory of metal mining into coal mining and proposed the concept of coal drawing ellipsoid. They carried out simulation experiments and theoretical deductions, offering important bases for the theoretical and practical studies of top coal caving. Based on the discrete particle flow character of top coal, Wang and Fu (2002) proposed the loose medium flow field theory of top coal and revealed the flow patterns of roof rock and top coal. As large mining height fully mechanized top coal caving mining is applied to extra thick coal seams that are 15 to 20 m or thicker, top coal is getting thicker, the caving space for top coal and roof rock is larger, and the mining height is becoming larger. In such condition, the top coal flow pattern, top coal and roof rock mixing pattern, and top coal loss pattern are important bases for determining caving parameters reasonably, and for improving coal recovery. Therefore,

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the original locations of caved bodies which are composed of top coal and roof rock caved in one caving web, and movement and loss pattern of top coal during top coal caving should be thoroughly studied (Zhang et al. 2015). For extra thick coal seams that are more than 15-m thick, the coal seam condition, top coal and roof rock caving pattern, as well as top coal loss amount are significantly influenced by temporal/spatial factors. According to the fragment dimension theory of coal and refuse (Liu and Huang 2006), inversion analysis was used to research the caving process of top coal and roof rock. We performed inversion of top coal and roof rock caving to analyze the flow pattern and top coal loss. Inversion here means that, the simulation process can be repeated according to the numerical calculation, the numerical software was used to simulate the top coal caving process, and the IDs of top coal and roof rock caved in each cycle were recorded. They were marked with different colors (yellow and green alternately) in the initial model, and the analog computation for the top coal caving process was carried out again to analyze the top coal and roof rock flow and the top coal loss. This study aims to provide bases for the further improvement of top coal recovery and the formulation of corresponding technical measures.

Establishment of model In order to investigate the top coal caving form and loss pattern in the fully mechanized caving of extra thick coal seams, the PFC2D3.10 program (Choi and Lee 2015; PFC2D 1999) was used to build a two dimensional model for the 8102 fully mechanized caving face of Tashan mine in Shanxi Province of China. At the studied caving face, the Permo Carboniferous 3~5# merged coal seam is being excavated. This relatively stable seam is 11.1 to 18.9-m thick, with an average thickness of 15 m. False roof of the seam is mudstone with a thickness of 0.01 to 0.05 m. The immediate roof is silt mudstone with a thickness of 0.51 to 30 m. The mining height is 5 m, while the thickness of top coal is 10 m (Wang 2013). A numerical calculation model was established based on the geological conditions described above. The model is 39.7 m in height. Laterally, the model’s horizontal movement is restricted. At the bottom, the model’s vertical movement is restricted. According to field measurements of top coal and roof rock size distribution, the top coal of the model is evenly divided into bottom, middle, and upper layers, with particles radii of 0.1, 0.11, and 0.12 m in the PFC code, respectively. The immediate roof, with its thickness set to 15.3 m, is divided into three layers of different sizes. According to the immediate roof’s impact range on top coal caving, the three layers are 5, 5, and 2 m in thickness, with equivalent radii of 0.14, 0.15, and 0.16 m, respectively (Liu et al. 2006). Table 1 (Zhang

et al. 2015) shows the mechanical parameters used by the calculation model. When PFC was used for the simulation of top coal caving, the process of caved top coal being hauled away from the working face by rear scraper conveyor was not simulated as the real situation. An open space was made instead of the rear scraper conveyor. The width of open space and the relative position of open space and supports are the same as the rear scraper conveyor’s. Numerical simulation To simulate the actual top coal caving, the simulation was started with the face start line and the first top coal caving cycle. Top coal caved boundaries were configured for the followed normal top coal caving cycles, which were then simulated with a cutting web of 0.8 m. Figure 1(b) illustrates this process. Coal loss is closely associated with the waste rock ratio which means the mass ratio of roof rock mixed in the caved material. Studies (Huang et al. 2007; Zhang et al. 2012; Chen et al. 2014; Yu et al. 2008) have shown that top coal recovery ratio increases with the waste rock ratio. However, when the waste rock ratio reaches a certain value, the contribution of its increase to the top coal recovery is negligible. Thus, in order to increase coal recovery ratio and limit the waste rock ratio in actual top coal caving process, a certain reasonable waste rock ratio should be predefined according to the coal seam’s conditions. At the predefined waste rock ratio, the caving should be stopped. To analyze the influence of waste rock ratio on the top coal and roof rock movement, original locations, top coal and roof rock loss process, the waste rock ratios of 0, 3, 6, 9, 12, 15, 18, and 21 % were set as criteria to close the caving opening. At different waste rock ratios, the original locations of caved bodies were inverted, in order to analyze the roof rock invasion characteristics during top coal caving, locate the original locations, and analyze the migration characteristics of lost top coal.

Simulation result and analysis Original locations and movements of caved bodies The body comprised by caved material in one caving cycle (web) is called caved body. Inversion analysis of the caved top coal’s original locations was performed, in order to analyze the migration of caved bodies and loss of top coal as the waste rock ratio increases. Different waste rock ratios were set as stopping criteria for caving. Numeric simulations of 30 top coal caving cycles were performed, during which the IDs of caved top coal and roof rock were recorded one cycle by one. After the calculation, the

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Physical mechanical parameters of coal and roof rock in different layers of the model

Table 1 No.

Color

Layers

Normal stiffness (GPa)

Shear stiffness (GPa)

Bulk density (N/m3)

Equivalent radius (m)

Porosity

Friction coefficient

1 2 3 4

Blue Yellow Red Black

Upper immediate roof Middle immediate roof Bottom immediate roof Upper top coal layer

12 12 12 4

12 12 12 4

2550 2550 2550 1400

0.16 0.15 0.14 0.12

0.35 0.35 0.35 0.35

0.7 0.7 0.7 0.5

5 6

Black Black

Middle top coal layer Bottom top coal layer

4 4

4 4

1400 1400

0.11 0.1

0.35 0.35

0.5 0.5

original locations of caved coal and rock before the first top coal caving cycle were retrieved and labeled with different colors. Figure 2 shows the inversion result, where the top coal caved in odd number cycles is colored green, and the top coal drawn in even number cycles is colored yellow. As can be seen in Fig. 2, ① the original locations of caved bodies are in the form of continuous arc shaped strips. ② During top coal caving, the top coal located initially at the top of the coal seam near the immediate roof is the primary source of coal loss. The location is in the gap between the strips caved in two successive top coal caving cycles. ③ The waste rock (roof rock mixed in the caved material) located at the bottom of the immediate roof near the coal seam is the primary source of caved roof rock; the initial location is at the top of the caved strips. ④ When the waste rock ratio is 0 (Fig. 2a), i.e., the caving opening is closed as soon as roof rock caved, obvious and massive loss of top coal occurred. At lower waste rock ratios, the caved bodies at the original locations are in other irregular shapes (Fig. 2a, b). As the waste rock ratio increases, the caved bodies, caved in each cycle, at the arc shaped strips original locations gradually become consistent in shape and size with each other (Fig. 2c, d, e). The recovery of top coal increases when the waste rock increased resulting in higher waste rock ratio and less top coal loss. ⑤ With the mixed ratio of coal and gangue further increases, for example from 9 to 12 %, the top coal loss reduces little. Most of the lost coal cannot be caved when they move to the range of rear repose angle.

As the inversion analysis shows, the caving process has relatively large influence on the coal body which caves within the scope of six caving cycles in front of the working face. Therefore, the six cycles were further simulated, in order to analyze the migration of the 20th caved body as the working face moves forward. Figure 3 shows the inversion result. Figure 3 illustrates gradual change of the caved top coal’s original locations in front of the working face during the 15th to 20th top coal caving cycles. As can be seen, the caved top coal is in the form of arc shaped strips. As the working face moves forward, top coal body of the strips gradually inclines towards the caving opening, with the arc’s height gradually reduced. At last, the top coal body becomes a semi elliptic body and caves from the opening. In order to analyze the migration characteristics of the arc strips at the original location, the caved top coal and roof rock are marked during the 20 cycles of top coal caving. Five particles are marked as 5010, 14,516, 14,692, 20,164, and 22, 823, respectively. The marked particles are evenly distributed along the vertical direction, with a height difference of 2.5 m between each two points. Figure 4(a) shows the original locations of the points. During mining, the five points are monitored, with the migration tracks shown in Fig. 4(b). As the tracks in Fig. 4(b) show, with the coal strips’ bottom as base point, the points above it move towards the bottom rear at different speeds and with different total movements in an unsynced manner. At different layers, the top coal

a

b 1 2 3 4 5 6

Roof height 24.7m

Immediate roof (0,30) (0,20)

Top coal

Direction of mining (0,15) Rear scraper conveyor Start line

Initial state of the model

Goaf

caved repose angle

Caving height 10m Cutting height 5m

(80,15)

Caving opening

Cutting width (web) 0.8m

State of the model after initial top coal caving

Fig. 1 Illustration of the calculation model. a Initial state of the model. b State of the model after initial top coal caving. The number in the left of the model (a) and the layers’ color are in accordance with the ones in Table 1.

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Fig. 2 Inversion of caved bodies’ original locations at different waste rock ratios

a One of the caved bodies

Immediate roof Original location of lost top coal in thirty caving cycles

Top coal Caving opening

Cutting coal Original locations of caved top coal in thirty caving

Working face

b

c

d

e

Inversion result at waste rock ratio of 0

Inversion result at waste rock ratio of 3%

Inversion result at waste rock ratio of 6%

Inversion result at waste rock ratio of 9%

Original location of caved roof rock(waste rock) in thirty caving

Inversion result at waste rock ratio of 12%

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Immediate roof Caved top coal Top coal coal

15th cycle

16th cycle

17th cycle

18th cycle

19th cycle

20th cycle

Fig. 3 Inversion of location change of caved top coal during cycles 15–20

movements are different. Higher location of the coal means larger movement. However, before reaching the caving opening, the points’ movements do not intersect, as shown in Fig. 4(c).

Migration characteristics of lost top coal To analyze the coal loss process and the migration characteristics of lost top coal, after the last cycle of top coal caving, the top coal within 16 to 54-m range along the coal seam’s orientation above the goaf was analyzed (labeled in green). In different top coal caving processes, the locations of lost top coal were retrieved, and the migration tracks and variation characteristics were analyzed. Figure 5 shows the inversion result at the coal recovery ratio of 83.3% (and waste rock ratio of 3%). As Fig. 5 shows, the lost top coal is mainly located at the top of the seam near the roof. Such coal is distributed unevenly in comb shape. As the working face moves forward, the comb shaped top coal is stretched and moves towards the lower rear of the support in a certain curve. The top coal reaches the floor before the immediate roof and is in the range of caved repose angle and, thus, is lost.

a

Inversion analysis of top coal caving tracks To analyze the caving characteristics of the top coal and roof rock caved in a single top coal caving cycle, the locations of uncaved coal and roof rock are marked green every 5 s until the roof rock reaches the opening and the top coal caving stops, as shown in Fig. 6. Figure 6 illustrates the variation of the top coal body’s state in the caving zone behind the support during the 13th top coal caving cycle. The top coal and roof rock caved in a single cycle form a semi elliptic shape behind the support. The long axis of the ellipse is perpendicular to the floor. As the caving continues, the long and short axes of the ellipse decrease at the same time and eventually become zero. During the 13th top coal caving cycle, reference points in the caved bodies were marked and tracked. Symmetrically about the midline of the opening, particles caved along y = 20 m (i.e., at the same level as the support’s top beam), located at (28.7, 20), (29.2, 20), (29.7, 20), (30.2, 20), (30.7, 20), (31.2, 20), (31.7, 20), (32.2, 20), (32.7, 20), (33.2, 20), and (33.7, 20), were chosen as reference points, and the relative IDs of them are 24,350, 23,876, 24,557, 24,279, 13,031, 10,475, 12,275, 11,403, 3972, 4401, and 5302. The

b

c

y/m

35 22823 20164

30

14516 14692

20 5010

15 0

Original locations of marked points

Fig. 4 Top coal migration tracking

40

x/m Migration tracks of marked points

lines composed of marked points’ location in different cycle

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y/m Lost top coal 30

15 0

16

Initial state

54

x/m

9th cycle

3th cycle

6th cycle

12th cycle

15th cycle

Fig. 5 Migration characteristics of lost top coal

coordinates of those points were recorded every 5 s, and then plotted, as seen in Fig. 7. As Fig. 7(b) shows, coal particles at different locations all move towards the caving opening in paths similar to straight lines, without any intersecting of the tracks. Particle 10,476 on the midline of the opening, and the particles 13,031 and 12,275 that are nearest to the midline, all move vertically. Particles 13,031 and 12,275 move symmetrically about the midline. The particles on the right side of the midline generally move slower than those on the left side. This is attributable to the upper rear thrust from the shielding support beam affecting the right side particles. This thrust offsets a part of gravity. Thus, the downward resultant on the right side particles is smaller than that on the left side particles, resulting in slower movement of the former. During the 13th top coal caving cycle, the IDs of caved top coal and roof rock were recorded every 5 s until roof rock was caved. After the top coal caving, the recorded IDs are retrieved at initial state. Figure 8 shows the inversion result, where the

top coal caved in odd number cycles are marked yellow, the top coal caved in even number cycles are marked green, and the caved roof rock is marked blue. As Fig. 8 shows, the amount of top coal and waste rock caved within every 5 s is generally consistent with each other. Owing to the influence of the support, the caved top coal is originally in the form of arc shaped strips. As the caving continues, the top coal and roof rock closest to the opening are caved, and the top coal and roof rock in the surrounding strip move downwards to fill in the cavity, until the top coal and roof rock in the outermost arc strip is caved and the cycle of top coal caving ends. If the opening is closed as soon as roof rock is caved, the outer coal located around the caved body will be left in the goaf. However, if the top coal caving continues after roof rock is caved, the formed arc strips consist of both top coal and waste rock of the immediate roof. As the top coal caving continues, the caved body contains more and more waste rock. Therefore, when the waste rock ratio reaches a certain point, further caving will contribute less and less to coal recovery.

Immediate roof Top coal Goaf Support

0s

15s

30s

Fig. 6 Caving characteristics of top coal and waste rock in the 13th top coal caving cycle

45s

60s

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b

y/m

y/m

a Immediate roof

21 24350

Centerline

20

23876 24667

30

19 24279 18

Caved body Top coal Goaf

13031 Shieldingsupport beam

17 12275

(33.7,20)

11403

16

20 (28.7,20)

0.5m

upper rear (33.7,20)

Caving opening

ramp thrust

Caving opening 15 31.2

3972

15 28

21

10476

29

30

31

32

33

34

x/m

4401 5302

x/m

Fig. 7 Top coal migration tracks in top coal caving zone

The dividing lines among the caved bodies caved every 5 s, as well as the dividing lines of caved bodies before and after top coal caving, can be described as Fig. 9. As Fig. 9 shows, the back ends of the dividing lines between original locations of caved bodies from successive caving cycles are located at the back of the opening. The front ends are located at the arc strips of the shielding support beam. As the top coal caving continues, the front ends gradually move towards to the upper part of the beam until reaching the intersection of the shielding beam and the top beam. The highest points of all dividing lines are distributed along the midline of the opening. Therefore, we define the original state of caved bodies as arc-shaped strips, the arc crown are at the midline of the caving opening. They are the highest points of the caved bodies’ original locations. These arc shaped strips are not regular. Near the goaf, the strips are more curved than those near the support, which is attributable to the effect of the shield support beam. In addition, the top coal near the shield beam has less mobility than the coal near the goaf.

top coal and waste rock at the point’s abscissa time. The average waste rock ratio (same as waste rock ratio) means the ratio of the quantity of caved waste rock to the total quantity of caved top coal and waste rock until the point’s abscissa time. As seen in Fig. 10, as the top coal caving continues, the top coal loss gradually decreases, and the average waste rock ratio drastically increases after a certain time. Such trends indicate the different characteristics of top coal loss and roof rock caving during different phases of top coal caving. In the phase when only coal is caved, the average top coal caving ratio is 80 kg/s, which lasted for about 65 s. In the coal rock caving phase, as the caving continues, the roof rock caved in unit time rapidly increases; when the average waste rock ratio reaches 10 %, the instantaneous waste rock ratio reaches 50 %. At this time, the top coal recovery ratio has reached around 90 %. When the instantaneous waste rock ratio reaches 80 %, there is little coal caving; the phase can be considered roof rock caving phase. In consideration of improving top coal recovery and limiting the amount of average waste rock, the average

Variations of coal loss and waste rock ratio with top coal caving

Immediate roof The 13th top coal caving cycle was simulated, with the weight of caved top coal and roof rock recorded every second, as well as the instantaneous waste rock ratio recorded. Figure 10 illustrates the recorded data. In Fig. 10, the abscissa is the caving time, the left ordinate is top coal quantity, and the right ordinate is waste rock ratio. The ordinate value of each point on the top coal quantity loss curve is equal to a difference, the difference is equal to the top coal quantity theoretically caved in each cycle of coal caving process minus the top coal quality that has been caved from the beginning of coal caving to the corresponding time of the point’s abscissa. Instantaneous waste rock ratio here means the ratio of the quantity of caved waste rock to the total quantity of caved

top coal Caved roof rock Caved top coal

Fig. 8 Inversion of single cycle top coal caving

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Fig. 9 Dividing lines between original locations of caved bodies Midline of caving opening

Coal-refuse dividing line before top coal caving

Coal-refuse dividing line after top coal caving Connecting line of highest points on boundary lines of caved bodies' original locations

Dividing lines between original locations of caved bodies of successive caving cycles Intersection of the shielding beam and the top beam Shielding support beam

Support Caving opening

lost during top coal caving is originally located at the top of the coal seam, whereas the caved roof rock is originally located at the bottom of the immediate roof. The lost top coal is originally distributed unevenly in comb shape. During top coal caving, the lost top coal reaches the floor before the immediate roof, thus becoming a part of the goaf rest corner on the floor. (2) Behind the support, the caved top coal is originally in the form of arc-shaped strips, with the vaults located at the midline of the caving opening. The vaults of the arcs are the highest points of the caved bodies’ original locations. Those strips are not regularly shaped. Near the goaf, the

Conclusions (1) The original position in the initial state of caved bodies is located in front of the working face in the form of arc shaped strips. During top coal caving, the caved bodies at different heights move towards the lower rear of the face at different speeds. As the waste rock ratio increases, the original arc-shaped strips are more uniform. The top coal

Loss

Fig. 10 Top coal loss and waste rock ratio with top coal caving

Average refuse rate

Instantaneous refuse rate

9000

100%

8000

90%

7000

80% 70%

Quantity/kg

6000

Phase two

60%

5000 50% 4000 40% 3000

Phase three

Phase one

30%

2000

20%

1000

10%

0 0

20

40

60

80

Times/s

100

120

140

0% 160

Waste rock ratio

waste rock ratio should be controlled below 10 %. This value can be used as a criterion for stopping top coal caving.

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arcs are more curved than those near the support. During the top coal caving, the arc strips successively cave. (3) The variations of coal loss and waste rock ratio with time reflect the different phases of caving. In order to improve top coal recovery while limiting the amount of caved roof rock, the waste rock ratio should be controlled below 10 %. This value can be used as a criterion for stopping top coal caving. Acknowledgments Financial support for this work, provided by the Independent Research Subject (No.SKLCRSM12X03) of State Key Laboratory of Coal Resources and Mine Safety of China University of Mining and Technology, the Scientific Research and Innovation Project (No. CXZZ13_0947) for College Graduates in Jiangsu, Top-notch Academic Programs Project of Jiangsu Higher Education Institutions, the Fundamental Research Funds for the Central Universities (China University of Mining and Technology)(2014ZDPY21). and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, are gratefully acknowledged.

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