Abstract
The suspended buffer is the core equipment for solid backfilling coal mining. Its dynamic response feature during the collision process with gangue particles is of vital importance for its strength check and service life prediction. In this paper, the SolidWorks and Femap software were adopted to establish a numerical model for the dynamic response of the buffer. The influence of the gangue particle size and gangue feeding speed on the dynamic response of the suspended buffer, which were mainly revealed by the maximum Mises stress, was studied. Once the gangue particles impacted the suspended buffer, the maximum Mises stress of the suspended buffer rapidly increased to the peak value and then turn to the periodic vibration process. It can be obtained through function fitting that the maximum Mises stress peak, the maximum and minimum values of the maximum Mises stress within the periodic vibration of the conical protective cover and the buffer base were in power functional relation with the gangue particle size and in a linear relationship with the gangue feeding rate. In addition, the vibration frequency of the maximum Mises stress of suspended buffer had a power function relationship with the gangue particle size, while had nothing to do with the feeding rate. According to the aforementioned fitting functions and 15601 backfilling working face parameters in D.Ping Coal Mine, it was determined that the crushed size of the gangue particle was 50 mm. The field-measured buffer maximum Mises stress and vibration parameters agreed well with the numerically simulated ones, which verified the reliability of the simulation to some extent.
1. Introduction
The fully mechanized backfill mining method is an essential means for green coal mining (Yilmaz et al.2018; Sun et al.2018a) and is widely used to recover protective coal pillars, control the height of water-flow fractured zones and treat gangues produced from protective layer mining (Zhu et al.2017; Emad et al.2018; Madzivire et al.2018; Sun et al.2018b). Because gangues are not only cheap and easy to get, but also could reduce environmental pollution, most mines use gangues as a backfilling material (Vargas et al.2017; Zhang et al.2017). If the gangue particles are sent directly from the surface to the underground through the feeding pipe, the high-speed moving gangue particles will inevitably cause damage to the storage bin, so Chinese scholars have designed the suspended buffer, which could reduce the kinetic energy of the high-speed gangue particles by colliding with them, thereby lowering the impact on the storage bin and achieving safe and efficient transportation of gangue particles from the surface to the underground (Ju et al.2016).
As shown in figure 1a, the suspended buffer is mainly composed of a buffer mechanism and a limiting mechanism. The buffer mechanism includes a conical protective cover, a buffer base and a buffer spring; the limiting mechanism comprises an upper limiting nut, a lower limiting nut and a guide rail. The conical protective cover is fixed on the buffer base. The buffer base is connected with the guide rail. The upper end of the buffer spring is connected with the buffer base. The lower end is fixed to the lower limiting nut. The upper and lower limiting nuts are fixed to the buffer base and guide rail s, respectively, and the upper ends of the guide rails are welded. Guide rails are fixed to the outer wall of the feeding pipe. As shown in figure 1b, the gangue particles are put into the buffer device through the feeding pipe, buffered by the conical protective cover and scattered to the storage bin at a low speed. The buffering mechanism achieves repeated buffering and recovery under the action of the buffer spring.
Main structural components of the suspended buffer.
The basic requirements for the suspended buffer to ensure its buffering effect are that it works normally and for a long time. Whether the suspended buffer is functioning well or not is judged by the strength check, while its service life is determined by the fatigue prediction. In order to complete the processes of strength checking and fatigue prediction, the dynamic response of the suspended buffer when it is impacted by the gangue particles, which is determined by two main feeding parameters, the gangue particle size and gangue feeding rate, should first be obtained. Therefore, it is necessary to analyse the effect of gangue particle size and gangue feeding rate on the dynamic response of suspended buffer.
At present, the principal research on buffering includes: Liu et al. (2014) analysed the vibration response characteristics and structural stability of buffers and optimized the feeding process of gangue particles; Ju et al. (2016) used the theoretical analysis method to study the elastic collision and plastic collision characteristics of gangue particles and buffers in a collision cycle, based on which, the damping spring vibration equation and the vibration deflection equation of the supporting beam were established; Zhang et al. (2012) revealed the influence of the gangue particle size and gangue density on the motion state of gangue particles in the feeding pipe; Wang et al. (2016) designed a cellular buffer and studied the energy absorption efficiency of different sizes of cellular buffers through impact tests; Uddin et al. (2017) studied the deformation characteristics of hexagonal hollow tubes under lateral pressure, on this basis of which a buffer with high energy absorption efficiency was designed and applied to control high-speed drop devices; Wen et al. (2013) studied the expanded polystyrene (EPS) buffer layer of a motorcycle helmet and obtained the energy absorption characteristics of EPS buffer layers of different densities through their experiments; and Huang et al. (2014) studied the ultimate bearing capacity of foam buffer package materials under static and dynamic loads; by using highly micronized fiber, the compressive and tensile strengths of the foam buffer package were increased.
The aforementioned studies mainly focused on the vibration characteristics, the energy absorption efficiency of the buffer, and the movement state of gangue particles. However, there was no in-depth study on the influence of gangue particle size and gangue feeding rate on the dynamic response of the buffer, which are fundamental to the research of the strength verification and service-life prediction of the suspended buffer. In this paper, based on the analysis of the design criteria of a conical protective cover and buffer base, Femap software was used to establish the dynamic response numerical model for a suspended buffer. Subsequently, the effect of gangue particle size and gangue feeding rate on the dynamic response of the buffer was analysed. Then, the strength check and fatigue life prediction of the suspended buffer were conducted. Finally, based on the backfilling parameters of the D.Ping Coal Mine, the requirements of buffer material strength and the service life, the reasonable gangue particle size was determined, and an industrial test was carried out in D.Ping Coal Mine.
2. The design disciplines of the conical protective cover and buffer base
2.1. Conical protective cover
The conical protective cover is a component of the suspended buffer that is in direct contact with the gangue particles. Gangue particles collide with the conical protective cover and generate wear. Therefore, the conical protective cover should have strong wear resistance to ensure that it has a certain service life. When the material is selected, the wear resistance is related to the total amount of gangue particles put onto the suspended buffer. According to the calculation and wear test, the suitable wear-resistant material is determined. The commonly used wear-resistant material is high chromium nickel-molybdenum alloy with a thickness of 30 mm.
According to the installation process of the conical protective cover, the conical protective cover and the buffer base are connected by welding, so the selected material should have certain weld ability. At the same time, it also needs to meet certain strength requirements. Under normal condition, Q235 steel is usually chosen, and the corresponding welding rod model is J507 carbon steel electrode.
In summary, the material selection of the conical protective cover requires itself to be a contradictory unitary body. If the wear resistance is satisfied, the strength must be increased, while the increased strength in turn can affect the weld ability of the conical protective cover. According to the material selection requirements, combined with its manufacturing process, the conical protective cover is generally designed as a two-layer structure. The inner layer is Q235 steel, and the outer layer is a high-wear-resistant material. The high-wear-resistant material is cast on the outside of the conical buffer. 60° of cone angle can reduce the rebound of gangue materials, avoiding clogging from the field test. The geometric parameters of the conical protective cover are shown in figure 2.
Conical protective cover.
2.2. Buffer base
The buffer base is located between the conical protective cover and the buffer spring, and its function is to transmit the impact load uniformly from the conical protective cover to the buffer spring. The buffer base needs to bear a large load, so the material chosen must have a certain strength. The load on the buffer base is related to the gangue particle size and gangue feeding rate. After calculation, the geometric parameters of the buffer base are shown in figure 3.
Buffer base.
3. The impact numerical model for the suspended buffer
The suspended buffer reduces the kinetic energy of gangue particles by colliding with them to achieve safe and efficient transportation of the gangue particles from the ground to the downhole. In order to analyse the effect of the gangue particle size and gangue feeding rate on the dynamic response law of the suspended buffer, first a numerical model needs to be constructed. Secondly, the parameters of the gangue particle material are adjusted, and then the influencing factors of the dynamic response of the suspended buffer are determined. Finally, the numerical simulation schemes are determined.
3.1. Model construction
Finite element model of suspended buffer.
In that formula, C is the damping matrix; M is the mass matrix; K is the stiffness matrix; and α and β are the mass and stiffness damping ratio coefficients, respectively. After theoretical analysis and trial calculation, the model took α = 0.23 and β = 0 (Barry et al.2014). Thus damping matrix C can be expressed as C = 0.23M. The collision recovery coefficient between the gangue particles and the conical protective cover, and that between the gangue particles, were 0.4 and 0.2, respectively (Kartushinskii et al.2004). The dynamic friction coefficient between the gangue particles and the conical protective cover, and that between gangue particles, were 0.2 and 0.3, respectively (Kartushinskii et al.2004).
In the numerical calculation, the mass of individual gangue particles is calculated according to the gangue particle size. According to the feeding speed of gangue particles, the quantity of gangue produced per unit time is given in the numerical calculation software.
3.2. Influencing factors for impact dynamics of suspended buffer
The gangue particle size is directly related to the crushing cost, and the gangue feeding rate is affected by the backfilling parameters of the working face. Both of them are the main factors in the dynamic response of the suspended buffer.
3.2.1. Gangue particle size
The sizes of the gangue particles produced from roadway driving, protective layer mining, etc. are usually relatively large, so to crush them to a certain degree is generally required. The final gangue particle size is directly determined by the crushing process and crushing cost. The smaller the crushing particle size, the more complicated the crushing process and the higher the cost will be. However, the gangue particle size is also an important parameter that influences the dynamic response feature of the buffer. The smaller the gangue particle size, the lower the gangue feeding rate, the smaller the impact load of the gangue particles on the buffer, the weaker the dynamic response of the conical protective cover and the buffer base, and the less the suspended buffer is damaged. Therefore, there is a contradiction between the cost of gangue crushing and the dynamic response of the suspended buffer. The existence of a certain critical gangue particle size could not only ensure the dynamic response of the suspended buffer within a reasonable range, but also minimize the cost of gangue particle crushing.
3.2.2. Gangue feeding rate
The underground backfilling speed is constrained by the ground gangue feeding rate directly as well as the underground backfilling parameters: the length of the backfilling face, the backfilling height and the advancing speed of the working face. The change of such underground parameters put forward corresponding requirements on the ground gangue feeding rate. The higher the feeding rate, the larger the amount of gangue particles delivered per unit time, and the higher the delivery efficiency will be. However, the larger the particle feeding rate, the greater the impact on the suspended buffer, the stronger the dynamic response of the suspended buffer, and the more serious the damage. Therefore, there also exists a contradiction between the feeding efficiency and the dynamic response of the suspended buffer. There must be a critical feeding rate range, which could not only ensure the dynamic response of the suspended buffer within a reasonable range, but also maximize the feeding efficiency.
3.3. Simulation schemes
The movement of gangue particles in the feeding pipe is a typical gas-solid dual-phase flow. The main forces during the falling process include gravity, buoyancy, additional mass force, air bond resistance, gradient pressure and Basset force. Zhang et al. (2012) gave the movement law of the gangue particles in the feeding pipe under the above various loads. It was found that the gangue particles underwent acceleration in the feeding pipe until reaching the peak value. At this time, the impact load of the gangue particles on the suspended buffer was the largest, and the dynamic response of the suspended buffer was the strongest. The gangue particle size after treatment was in the range from 0 mm to 100 mm. When the gangue particle sizes were 20, 40, 60, 80 and 100 mm, the corresponding peak velocity was 33.0, 47.6, 58.7, 68.1 and 76.4 m s-1 (Zhang et al.2012).
To sum up, two feeding schemes are determined. Scheme I, at a gangue feeding rate of 500 t h-1, the diameter of gangue particles was increased in steps of 20 mm from 20 mm to 100 mm. The influence of the gangue particle size on the dynamic response of the suspended buffer was studied. In Scheme II, with a gangue particle size of 60 mm, the feeding rate was increased by 200 t h-1 from 100 t h-1 to 700 t h-1, and the influence of feeding rate on the dynamic response of suspended buffer was investigated, as shown in Table 1.
Loading scheme of impact load.
| Scheme | Material particle diameter (mm) | Feeding rate (th-1) | Out-of-pipe speed (m s-1) |
|---|---|---|---|
| 20 | 33.0 | ||
| 40 | 47.6 | ||
| I | 60 | 500 | 58.7 |
| 80 | 68.1 | ||
| 100 | 76.4 | ||
| 100 | 58.7 | ||
| 300 | |||
| II | 60 | 500 | |
| 700 |
| Scheme | Material particle diameter (mm) | Feeding rate (th-1) | Out-of-pipe speed (m s-1) |
|---|---|---|---|
| 20 | 33.0 | ||
| 40 | 47.6 | ||
| I | 60 | 500 | 58.7 |
| 80 | 68.1 | ||
| 100 | 76.4 | ||
| 100 | 58.7 | ||
| 300 | |||
| II | 60 | 500 | |
| 700 |
Loading scheme of impact load.
| Scheme | Material particle diameter (mm) | Feeding rate (th-1) | Out-of-pipe speed (m s-1) |
|---|---|---|---|
| 20 | 33.0 | ||
| 40 | 47.6 | ||
| I | 60 | 500 | 58.7 |
| 80 | 68.1 | ||
| 100 | 76.4 | ||
| 100 | 58.7 | ||
| 300 | |||
| II | 60 | 500 | |
| 700 |
| Scheme | Material particle diameter (mm) | Feeding rate (th-1) | Out-of-pipe speed (m s-1) |
|---|---|---|---|
| 20 | 33.0 | ||
| 40 | 47.6 | ||
| I | 60 | 500 | 58.7 |
| 80 | 68.1 | ||
| 100 | 76.4 | ||
| 100 | 58.7 | ||
| 300 | |||
| II | 60 | 500 | |
| 700 |
3.4. Material parameter calibration for gangue particle
The parameters of the metal materials used in the simulation of the conical protective cover, the buffer base and the guide rail can be obtained experimentally. The shape of the gangue particles is simplified after the gangue particles are equalized to solid spheres; however, there would exist a certain error if the lab-measured parameters of the simplified gangue particles are adopted. Therefore, the material parameters of the gangue particles need to be calibrated. The vertical displacement of the buffer spring is relatively easy to monitor on-site, so it was compared with that of the simulation result. By continuously adjusting the input parameters of the gangue particles, the vertical displacement of field measurement and numerical simulation were consistent both in tendency and value.
3.4.1. Obtaining gangue particles
In order to check the material parameters of the gangue particles, a multi-stage vibrating screen was used to obtain a certain size of gangue particles. As shown in figure 5, gangues with a particle diameter of about 20 mm were obtained by this screening and then were placed underground through the feeding pipe. The gangue particles would collide with the suspended buffer.
Gangue particle screening process.
3.4.2. Experimental monitoring systems
A YWD-80 displacement sensor was installed to monitor the vertical displacement of the spring in real time. The maximum frequency of equipment acquisition can reach 100 KHz, and the sensor was fixed near the spacing hole. A monitoring system with a monitor interval of 5 s, shown in figure 6, was used to extract and store data.
Monitoring system for the vertical displacement of buffer spring.
The vertical displacement of the buffer springs for D = 20 mm was obtained by the displacement sensor and the data processing system. The material properties of the gangue particles were continuously adjusted to determine their input parameters: elastic modulus E2 = 16.1 GPa, Poisson's ratio μ2 = 0.28, cohesion force c2 of 5.6 MPa and internal friction angle φ2 of 22°. The in-situ measured and numerically simulated vertical displacements of the buffer spring are shown in figure 7.
Vertical displacement of the spring in numerical simulation and field measurement.
In order to reflect the coincidence between field measurement and numerical simulation objectively and accurately, hypothesis testing was carried out. In this paper, the pair-samples t test in Origin software was used. The results show that the P value = 0.723 > 0.05. Therefore, there is no obvious difference between the two groups of data, which proves the reliability and accuracy of the numerical simulation. The data of 5 seconds was collected in this paper, but the periodicity can be seen from the curve of the vertical position of the spring; the results of the numerical simulation are consistent. Thus, to a certain extent, the rationality of the selection of the parameters of the gangue particles is verified.
4. Influence of gangue particle size and gangue feeding rate on the dynamic response of the buffer
The maximum Mises stress of the suspended buffer is the concentrated reflection of its dynamic response. Therefore the maximum Mises stress of the suspended buffer is selected as the monitoring index. The maximum Mises stress peak of the suspended buffer during the collision determines the strength check of the conical protective cover and the buffer base, and the maximum and minimum values of the maximum Mises stress in the vibration period determine the fatigue prediction of the conical protective cover and the buffer base.
4.1. Mises stress distribution law of conical protective cover and buffer base
In order to study the influence of the gangue particle size and gangue feeding rate on the maximum Mises stress of the conical protective cover and the buffer base, it is first necessary to obtain the position of the maximum Mises stress. Therefore, it is necessary to analyse the Mises stress distribution law of the conical protective cover and the buffer base. Figure 8 shows the Mises stress of the conical protective cover and buffer base at 0.085 s under the condition with a gangue feeding rate of 500 t h-1 and gangue particle size of 100 mm.
Conical protective cover and buffer base stress state.
The following can be seen from figure 8. (1) The Mises stress distribution laws of the conical protective cover and the buffer base are roughly the same. Both of their Mises stress concentration regions are located near the limiting hole, corresponding to a maximum value of 303 and 144 MPa, respectively. Moreover, their stress reduction regions are located at the geometric center. (2) The Mises stress distribution corresponds to the geometry of the conical protective cover and the buffer base, i.e. the Mises stress is symmetrical about the two geometric symmetry axes of the conical protective cover and the buffer base.
4.2. The influence factors on maximum Mises stress of suspended buffer
Through the Mises stress distribution law of the conical protective cover and the buffer base, the maximum Mises stress position of the conical protective cover and the buffer base can be accurately obtained. The maximum Mises stress at the corresponding position is extracted to study the effect of the gangue particle size and gangue feeding rate on the maximum Mises stress of the conical protective cover and the buffer base.
4.2.1. Gangue particle size
Analysis of the influence of the particle size of gangue on the maximum Mises stress of the buffer can provide a basis for determining the critical size of the gangue particle. As shown in figure 9, the maximum Mises stress of the conical protective cover and the buffer base in Scheme I is extracted.
The maximum Mises stress of suspended buffer under different gangue particle size.
The following can be seen from figure 9.
After the gangue particles impact the suspended buffer, the maximum Mises stress of the conical protective cover and the buffer base rapidly increases to a peak. When the particle size of the gangue is 20, 40, 60, 80 and 100 mm, the maximum Mises stress of conical protective cover and buffer base reaches a peak at 0.151, 0.122, 0.102, 0.090, and 0.085 s. The maximum Mises stress peak values S1 of the conical protective cover are 34.0, 85.0, 119.0, 136.0 and 144.0 MPa, respectively, while the maximum Mises stress peaks S2 of the buffer base are 71.2, 178.0, 249.0, 285.0, 303.0 MPa, respectively.
Once the peak value is reached, the maximum Mises stress of the conical protective cover and the buffer base turns to the periodic vibration. When the size of gangue particles is 20, 40, 60, 80, and 100 mm, the maximum Mises stress vibration period of the conical protective cover and the buffer base is the same, which are, respectively, 0.258, 0.173, 0.149, 0.138, and 0.130 s. The corresponding frequency f is 3.870, 5.780, 6.721, 7.252 and 7.692 Hz.
During the periodic vibration, when the size of gangue particles is 20, 40, 60, 80 and 100 mm, the maximum value of the maximum Mises stress P1 of the conical protective cover is 27.6, 67.3, 96.7, 114.3, and 116.2 MPa; the corresponding minimum values of Q1 are 21.3, 55.0, 79.1, 87.2, and 92.8 MPa, respectively. The maximum values of the maximum Mises stress P2 of the buffer base are 56.7, 140.6, 203.4, 234.0, and 244.5 MPa, and the minimum values Q2 are 44.7, 115.1, 163.2, 190.5, and 198.2 MPa, respectively.
To obtain the relationship between the size of the gangue particle and the maximum Mises peak values S1 and S2 of the conical protective cover and the buffer base, the maximum values of the maximum Mises stress P1 and P2 within the vibration period, the minimum values of the maximum Mises stress Q1 and Q2 within the vibration period, and the vibration frequency f, the curves between particle size and the above parameters are plotted in figures 10 and fitted using the Origin software.
Effect of gangue particle size on the maximum Mises stress of suspended buffer.
The functional relationship between the gangue particle size and the above parameters obtained by fitting is |${S_1} = 7.689{D^{0.648}}$|, |${S_2} = 15.040{D^{0.664}}$|, |${P_1} = 4.45{D^{0.727}}$|, |${P_2} = 9.14{D^{0.730}}$|, |${Q_1} = 3.765{D^{0.711}}$|, |${Q_2} = 7.252{D^{0.735}}$| and |$f = 1.286{D^{0.395}}$|.
4.2.2. Gangue feeding rate
The gangue feeding rate is one of the key factors affecting the maximum Mises stress of the protective cover and the buffer base. The relationship between the maximum Mises stress and the impact time in the extraction Scheme II is shown in figure 11.
The Max Mises stress of suspended buffer under different gangue feeding rate.
The following can be seen from figure 11.
After the gangue particles impact the suspended buffer, the maximum Mises stress of the conical protective cover and the buffer base rapidly increases. When the gangue feeding rates are 100, 300, 500 and 700 t h-1, the maximum Mises stress of the conical protective cover and the buffer base all reach peak values at 0.90 s. The maximum Mises stress peaks P1 of the conical protective cover are 26.0, 64.1, 119.0 and 136.0 MPa, respectively, while the maximum Mises stress peaks P2 of the buffer base are 56.0, 130.0, 249.0 and 285.0 MPa, respectively.
The maximum Mises stress of the conical protective cover and buffer base first reaches the peak value and then enters periodic vibration. When the gangue feeding rate is 100, 300, 500 and 700 t h-1, the maximum Mises stress and vibration period of the conical protective cover and the buffer base are the same (both are 0.149 s) and the corresponding frequency is 6.721 Hz. Therefore, the vibration frequency of the maximum Mises stress of the conical protective cover and buffer base is independent of the feeding rate.
During the periodic vibration of the maximum Mises stress, when the gangue feeding rates are 100, 300, 500 and 700 t h-1, the maximum values of the maximum Mises stress of the conical protective cover P1 are 21.4, 52.4, 97.4 and 111.3 MPa. The corresponding minimum values Q1 are 16.9, 41.3, 76.0 and 87.0 MPa, respectively. The maximum values of maximum Mises stress P2 of the buffer base are 44.7, 103.8, 203.5 and 229.9 MPa, respectively. The corresponding minimum values Q2 are 36.3, 81.7, 156.2 and 179.5 MPa, respectively.
To obtain the functional relationship between the gangue feeding rate and the maximum Mises peaks S1 and S2 of the conical protective cover and the buffer base, the maximum values of maximum Mises stress P1 and P2 during the vibration period, the minimums of maximum Mises stress Q1 and Q2, and the vibration frequency f, the relationship curves between the gangue feeding rate and the above parameters are plotted, as shown in figure 12. By using Origin software to perform fitting, the functional relationship between the gangue feeding rate and the above parameters is |${S_1} = 0.4388v$|, |${S_2} = 0.2101v$|, |${P_1} = 0.355v$|, |${P_2} = 0.172v$|, |${Q_1} = 0.276v$|, |${Q_2} = 0.135v$| and |$f = 6.721 $|.
Effect of feeding rate on maximum Mises stress of suspended buffer.
5. Effect of feeding parameters on buffer strength and fatigue
5.1. Strength judgment of suspended buffer
5.2. Fatigue prediction of suspended buffer
The maximum Mises stress amplitude S in the equation is half the difference between the maximum and minimum values of the maximum Mises stress in the vibration period.
6. Engineering applications
6.1. Engineering background
As shown in figure 13a, D.Ping Coal Mine is located in Yangquan City, Shanxi Province, China. The recoverable reserve of the mine is 4.08 million tons, with an annual production of 1.2 million tons, and the mining period is only three to five years. However, there is a large amount of coal that is under the buildings and farmlands within the coal mine area with estimated reserves of 73.54 million tons, 63.24 million tons of which are recoverable. As a result, mining under buildings and farmlands has become the major bottleneck for the sustainable development of D.Ping Coal Mine, so the mine has decided to use gangue backfilling coal mining methods for this part. The first gangue backfilling face is the 15 601 face. The production system and backfilling system of this face are shown in figure 13b.
Location of D.Ping Coal Mine and its working face layout.
The basic underground backfilling parameters for 15 601 gangue backfilling face are detailed in Table 2.
Backfilling process parameters.
| Working face length L (m) | Backfilling height H (m) | Mining speed V (m d-1) | Gangue surplus coefficient S | Gangue density γ (t m-3) | Effective feeding time T (s) |
|---|---|---|---|---|---|
| 80 | 3.0 | 2.4 | 1.4 | 2.5 | 4 |
| Working face length L (m) | Backfilling height H (m) | Mining speed V (m d-1) | Gangue surplus coefficient S | Gangue density γ (t m-3) | Effective feeding time T (s) |
|---|---|---|---|---|---|
| 80 | 3.0 | 2.4 | 1.4 | 2.5 | 4 |
Backfilling process parameters.
| Working face length L (m) | Backfilling height H (m) | Mining speed V (m d-1) | Gangue surplus coefficient S | Gangue density γ (t m-3) | Effective feeding time T (s) |
|---|---|---|---|---|---|
| 80 | 3.0 | 2.4 | 1.4 | 2.5 | 4 |
| Working face length L (m) | Backfilling height H (m) | Mining speed V (m d-1) | Gangue surplus coefficient S | Gangue density γ (t m-3) | Effective feeding time T (s) |
|---|---|---|---|---|---|
| 80 | 3.0 | 2.4 | 1.4 | 2.5 | 4 |
Substituting the parameters in that table into Equation (12) results in a feeding rate of 504 t h-1 for the gangue particles. For the convenience of equipment selection, the gangue feeding rate is set at 500 t h-1.
6.2. Reasonable particle size of gangue
The conical protective cover and the buffer base have a material yield strength of 235 MPa and a safety factor of 1.2, and the allowable stress is 196 MPa. At the same time, it can be seen that the gangue feeding rate in D. Ping Coal Mine is 500 t h-1.
From Equations (13) and (14), the gangue particle size D can be determined to be less than 48 mm by the strength of the conical protective cover and the buffer base.
From Equations (16) and (17), it can be concluded that according to the fatigue analysis, the size of gangue particles is D < 108 mm.
According to the strength check and fatigue analysis of the conical protective cover and the buffer base, the size of the gangue particle can be obtained: D < 48 mm. For the convenience of equipment selection, D = 50 mm is selected.
6.3. Engineering measurement
According to the study in Section 6.2, the critical size of the gangue particles is 50 mm. The specific processing and transportation procedures are as follows: the gangue particles were loaded into a charging hopper by a loader and sent to a screening machine through a belt conveyor. After screening, gangue particles with a particle size smaller than 50 mm were directly delivered into the belt, while gangue particles larger than 50 mm were sent into the crusher to be broken into pieces smaller than 50 mm before they entered the belt. The broken gangue particles were delivered to the well through the feeding pipe, and the velocity of the gangue particles was significantly reduced under the impact of the suspended buffer. Since it was put into production in December 2014, the conical protective cover and buffer base have been running smoothly without damage.
To verify the accuracy of the dynamic response of the suspended buffer to the size of the gangue particles and the feeding rate described in Section 4 further, the dynamic response parameters of the suspended buffer obtained via actual measurement and that found through numerical simulation were compared. Since the maximum Mises stress of the buffer base was greater than that of the conical protective cover, the maximum Mises stress of the buffer base should be monitored during field measurements. The maximum Mises stress of the buffer base was located near the limiting hole, as shown in figure 14, where the strain gauge was pasted. The strain at this location was obtained by a dynamic strain meter, and the maximum Mises stress was calculated by computer processing.
Buffer base stress monitoring.
As shown in figure 15, the dynamic change law of the maximum Mises stress of the buffer base is given by Origin software and compared with the numerical simulation.
Buffer base vibration parameters measured.
From figure 15, the maximum Mises stress peak S2 of the buffer base is 189.0 MPa, the maximum value of maximum Mises stress P2 is 157.0 MPa, the minimum value of maximum Mises stress Q2 is 112.9 MPa during the vibration period and the vibration frequency f is 7.1 Hz.
In the numerical simulation, v = 500 t h-1 and D = 50 mm were substituted into Equations (3), (7), (9) and (10). The maximum Mises stress peak value S2 of the buffer base is 202.0 MPa. The maximum value of maximum Mises stress P2 is 158.9 MPa, the minimum value of maximum Mises stress Q2 is 128.6 MPa and the vibration frequency f is 6.0 Hz.
In the numerical calculation and the field measurement, the maximum Mises stress peak S2 error of the buffer base is 6.9%, the error of maximum value of the maximum Mises stress P2 in the vibration period is 1.2%, the error of minimum value of maximum Mises stress Q2 is 13.3%, and the vibration frequency f error is 15.5%. The vibration parameters of the maximum Mises stress in the numerical simulation are consistent with that in the field measurement, and the reliability of the numerical simulation is verified to some extent.
7. Conclusions
Based on the analysis of the design principles of the buffer conical protection cover and buffer base, this study combined SolidWorks and Femap software to establish a suspended buffer vibration numerical model. The influence of gangue particle size and the gangue feeding rate on the dynamic response of the buffer was studied, and the following conclusions were drawn.
Mises stress distribution in the conical protective cover and buffer base is related to their Geometric characteristics. The maximum Mises stress of both is located near the limiting hole.
After the gangue particles impact the suspended buffer, the maximum Mises stress of the conical protective cover and the buffer base rapidly increase to a peak and then enter the periodic vibration.
The maximum Mises stress peak, and the maximum and minimum values of the maximum Mises stress of the periodic vibration of the conical protective cover and the buffer base have a power-function relationship with the gangue particle size and a linear relationship with the gangue feeding rate. The vibration frequency of the maximum Mises stress of the conical protective cover and the buffer base has a power function relationship with the gangue particle size and is independent of the gangue feeding rate.
Based on the backfilling parameters of the 15 601 backfilling face in D.Ping Coal Mine, it was determined that the feeding rate of the gangue was 500 t h-1. On this basis, according to the material strength and service life of the conical protective cover and the buffer base, the crushed size of gangue particle was 50 mm. Finally, an industrial test was conducted at the D.Ping Coal Mine 15 601 backfilling face. The on-site buffer operated in good condition. At the same time, the vibration parameters found in the numerical simulation are consistent with that in field measurement of the buffer base, and the reliability of the simulation analysis is verified to some extent.
Acknowledgements
This work was supported by the Special Funding Projects of Funded by the Research Fund of the State Key Laboratory of Coal Resources, Safe Mining, CUMT (SKLCRSM18KF08) and Talents Project of Liaoning Revitalization (XLYC201807219); the Special Funding Projects of Funded by The National Natural Science Foundation of China (51574055, 51504127). The authors gratefully acknowledge the financial support from the organization mentioned above.
