Underground pneumatic separation of coal and gangue with large size (≥ 50 mm) in green mining based on the machine vision system

Powder Technology 278 (2015) 223–233

Contents lists available at ScienceDirect

Powder Technology journal homepage: www.elsevier.com/locate/powtec

Underground pneumatic separation of coal and gangue with large size (≥ 50 mm) in green mining based on the machine vision system Kehong Zheng a,b,⁎, Changlong Du a, Jianping Li a, Bingjing Qiu a, Daolong Yang a a b

College of Mechanical and Electrical Engineering, China University of Mining and Technology, Xuzhou 221116, China Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, Salt Lake City, UT 84112-0114, USA

a r t i c l e

i n f o

Article history: Received 3 November 2014 Received in revised form 12 March 2015 Accepted 16 March 2015 Available online 25 March 2015 Keywords: Pneumatic separation High pressure airflow Gangue particles PFC3D

a b s t r a c t Coal gangue separation is of key important for green mining. Underground pneumatic separation of coal and gangue based on the machine vision system is proposed in this study. This method has the potential to be used in coal mining due to the real time control and simple work principle. Here, the theoretical pneumatic separation distance formula of coal and gangue is derived based on energy conservation principle. Then hard-sphere model is applied to simulate the particle motion pattern in a three-dimensional model with a pressure–velocity boundary, and key factors affecting coal gangue pneumatic separation for large diameter (≥50 mm) are analyzed. An air–solid multiphase flow simulation was conducted to clarify all the three factors (airflow velocity u, the conveyor velocity v0 height difference hp between conveyor belt and air nozzle) have great influence on separation distance. Especially, the optimal parameters for best separation distance are presented at u = 320 m/s,v0 = 0.5 m/s,hp = 0.4 m. Besides, the theoretical pneumatic separation distance formula is corrected based on experiment data, the error between experiment value and calculate value of the separation distance is less than 15%. The corrected formula has important guiding significance and practical value for coal gangue pneumatic separation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction With the widely use of mechanized coal mine production and the increasingly extraction of thin seam, large number of gangue mixed raw coal has greatly increased, which affects the efficiency of coal preparation and increases the cost of preparation. Meanwhile gangues are stacked on the ground after preparation, which become the hazard source for environment. In order to reduce the damage to environment of gangue, various separation methods have been proposed based on the physical property differences between coal and gangue. The separation of gangue from coal underground can not only improve the quality of raw coal, decrease the cost of preparation, but also provide materials to the gangue filling underground and realize the real green mining underground[1–2]. Underground pneumatic separation of coal and gangue is based on the machine vision system, that is an integrated technology using computer image processing technology and analysis technology to identify the target of various patterns of coal and gangue. Machine vision system of recognition coal and gangue can be divided into the following steps: (1): Read digital image gotten from the camera by computer. (2) Determine the identification characteristic of coal and gangue.(3) Find gangue materials based on the identification algorithm.(4) Determine the size ⁎ Corresponding author at: College of Mechanical and Electrical Engineering, China University of Mining and Technology, Xuzhou 221116, China. Tel.:+86 13815319204. E-mail address: [email protected] (K. Zheng).

http://dx.doi.org/10.1016/j.powtec.2015.03.027 0032-5910/© 2015 Elsevier B.V. All rights reserved.

and the location of gangue, then convert these information to the control single of high pressure air nozzle. With the development of computer technology, video-signal on-line transition and processing technology has been perfected and raised, machine vision system have got great success in many fields [3]. S. AlThyabat and N.J. Miles' proposed on-line particle size analysis was one of the key motivations for the introduction of image-based particle sizing systems to the minerals industry [4]. Jayson Tessier described a general machine vision approach for on-line estimation of rock mixture composition, and has illustrated on a very challenging nickel mineral system: very heterogeneous minerals, similar coloration, and rock fragments can be dry or wet [5]. Xu W. et al. verified the 3D quantitative analysis of grain boundary fracture in the breakage of single multiphase particles using X-ray micro-tomography [6]. Andersson T described an algorithm which is demonstrated with results of measurements of limestone particles on conveyor belts [7]. S. Al-Thyabat took into account some well known problems associated with imaging moving particles such as camera location, particle overlap, image blurring, conveyor speed, dust generation and treatment [8]. Kernel-based methods to estimate particle size ranges on a pilot-scale conveyor belt as well as edge detection algorithms were considered in reference [9]. Hamzeloo E. et al. introduced a method to estimate the particle size distribution on the industrial conveyor belt [10]. A Monte Carlo method was used to simulate the phenomenon of particle size percolation at the transfer point of a moving belt conveyor in reference [11]. C.L. Lin and J.D. Miller

224

K. Zheng et al. / Powder Technology 278 (2015) 223–233

introduced on-line Particle Size Analyzer (OPSA) system which has been developed at the University of Utah to measure particle size distributions under plant condition [12]. Young-Don Ko provided an improved prediction model and can be used for real time measurement of particle size distribution in the industrial operation [13]. Scholars mainly probed into a general machine vision approach for on-line estimation of coal and gangue, pneumatic separation of coal and gangue has not been studied adequately. Pneumatic separation has widely adopted in agriculture filed and recycled for printed circuit boards (PCB), but it is not applied too much in mineral separation. Researchers have done lots of research on the pneumatic separation of small particles (particle size that is mainly less than 5 mm), and few research has been done on large particle diameter (particle size that is larger than 50 mm). Zhang Junxiong described a method that seeds are separated by high pressure airflow in the air [14]. Gamal Rashad Gamea carried out a series of experiments on pneumatic separation under different conditions [15]. M.Panasiewicz analyzed the process of pneumatic separation in an air stream of broken-up lupine seed [16].XU M. et al. proposed an effective mechanical process including impact crushing from printed circuit boards (PCB) and pneumatic separation for metal recovery scraps was investigated [17]. Guo C., Kumar V., Hayashi N., Oki T. et al. also proposed a method to realize the recycling of the PCB by pneumatic separation [18–20]. Besides, researchers have also focused on the pneumatic conveying. I. Lecreps et al. have discussed the factors which influenced the pneumatic conveying [21–23]. K. Li, and S.B. Kuang have presented a numerical study of the effects of friction and restitution coefficients of particles on horizontal pneumatic conveying by a combined approach of computational fluid dynamics for gas and discrete element method for particles [24]. In this work, the process of pneumatic separation is presented and a separation distance theory formula of coal and gangue with large

diameter is established. Firstly, the principle of underground pneumatic separation is introduced. Then, we use hard-sphere model to simulate the particle motion pattern in a three-dimensional model with a pressure–velocity boundary to find the key factors affecting coal gangue pneumatic separation for large diameter (≥50 mm). At last, theoretical pneumatic separation distance formula is corrected based on the experiment data. 2. Principle Underground pneumatic separation of coal and gangue mainly contain two parts: machine vision and pneumatic separation process with advantages of real-time processing, higher intelligence and lower cost. It is mainly based on coal and gangue physical feature differences and image recognition to realize separation, which is suitable for preliminary separation of coal and gangue. In order to decrease the difficulty of underground pneumatic separation, roll-type crusher is used to crush coal gangue to 100 mm. Then spiral size screen is applied to screen out materials below 50 mm, limiting the granularity of material within a range of 50 mm–100 mm.Coal and gangue, which size is between 50 mm and 100 mm, are sent to the pneumatic separation equipment to realize underground separation of coal and gangue. The process of pneumatic separation which is based on machine vision is shown in Fig. 1. Firstly, materials of coal and gangue will be sent into the roll-type crusher 3, which will be crushed under 100 mm. Coal particles, which diameter is less than 50 mm, will transport to coal bunker through conveyor belt. Coal and gangue particles, having a diameter between 50 mm and 100 mm, will transport by conveyor 18 to a fixed sieve 5 for further screening where multi-layer materials are transformed into a single layer. Then, coal and gangue through the secondary screen will transport to machine vision system 16 through

Fig. 1. Separation system of coal and gangue.

K. Zheng et al. / Powder Technology 278 (2015) 223–233

225

3.2. Gangue particle trajectory affected by the high pressure value hp tp

Gangue particles are recognized by the machine vision system when coal and gangue particles are thrown from the conveying belt, image sensor will send the information to computer to activate the high pressure value, then gangue particles will be affected by the airflow, the movement of coal and gangue pneumatic separation can be divided into three stages: 1) horizontal cast movement of particle before falling into the airflow domain; 2) movement of particle in air flow domain; 3) movement of particle after leaving the air flow domain. The following three sections will analyze the gangue particle movement process respectively.

airflow domain

h j tj Sj

coal particle trajectory 1

Si S0

tf hf

gangue particle trajectory 2 Sf

ΔS Fig. 2. Coal and gangue pneumatic separation model.

conveyor belt 7. Image information of coal and gangue after identification will be sent to computer 13 through image sensor 15, thus highpressure air value will be controlled by computer 13 to realize coal and gangue pneumatic separation. 3. Particle motion model Coal and gangue particles will do horizontal projectile motion when they are thrown from the conveying belt. The trajectory of coal particles without the affect of high pressure airflow is shown in Fig. 2 of curve 1. And the trajectory of gangue particles with the effect of high pressure airflow is shown in Fig. 2 of curve 2. Air flow directions of high pressure value and particle movement are of the opposite direction. Motion diagram is shown by Fig. 2. As shown in Fig. 2, ΔS represents the pneumatic separation distance between coal and gangue; hp represents the height difference between gangue mass center on the conveyor belt and the upper boundary of the airflow domain; tp represents the motion time of gangue before falling into the air flow domain; vp represents the velocity in direction y of gangue before falling into the air flow domain; hj represents the height of the airflow domain, which is also the diameter of the nozzle; Si represents the displacement in x direction of gangue before affecting by the airflow; tj represents the time of gangue moving in airflow domain, Sj represents the displacement in x direction of the gangue moving in airflow domain; vf represents the velocity in y direction of the gangue in airflow domain; ẋ(tj) represents the velocity in x direction of gangue in airflow domain; tf represents the time of gangue after the gangue has left the airflow domain; Sf represents the displacement in x direction of the gangue movement after leaving the airflow domain. 3.1. Coal particles trajectory without affected by high pressure value

3.2.1. Horizontal projectile motion of particles before falling into the airflow domain Particles will be thrown from the conveyor belt before falling into the airflow domain, and then gangue particles will do horizontal projectile motion. Particle's motion diagram is shown in Fig. 3. Then Si and vp can be expressed respectively as: sffiffiffiffiffiffiffiffi 2hp Si ¼ v0 g

ð3Þ

sffiffiffiffiffiffiffiffi 2hp : vp ¼ g g

ð4Þ

3.2.2. Movement of particle in air flow domain The separation medium of pneumatic separation is high pressure air. In order to calculate the effect of pneumatic separation quantitatively (separation distance ΔS (m) is regard as the only index to evaluate the effect of pneumatic separation), we need to calculate the force of the high pressure air act on gangue. If dynamic pressure head of the airflow domain is known in special location. The force of high pressure airflow act on gangue can be calculated by Eq. (5). F ¼ ∬ P d dA

ð5Þ

A

The approximate calculating schematic diagram of force act on gangue particles by nozzle is shown in Fig. 4. Due to the complexity geometry of the gangue surface, multiple rectangular polyline can be used to approximate its physical shape. The force of airflow act on gangue can be simplified as the sum of the product of the ring area's dynamic pressure head and the corresponding ring area. It can be calculated by Eq. (6): F¼

m X

ð6Þ

P di Ai

i¼1

When coal particles is thrown from the conveying belt, it will be recognized by the machine vision system, then image sensor will send the information to computer that will control the high pressure value. At last, coal particles will fall freely from the conveyor belt without the effect of high pressure value. In order to calculate the key parameters of the movement, assumptions are made as follows: velocity of conveyor belt is v0; the total motion height is hp + hj + hf; the motion time of coal particle is t0; and coal particle displacement in x direction is S0. Thus t0 and S0 can be obtained by Eqs. (1) and (2) respectively. vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u  u2 h þ h þ h p j f t t0 ¼ g

where, F represents the force of the high airflow that acts on gangue. Pdi represents the high pressure head of the ring area i. Ai represents the area of i. m represents the total number of the ring. In order to analyze the movement process of gangue particles in high pressure airflow domain, assumptions are made as follows: 1) high pressure airflow is parallel liner uniform flow; 2) the effect of gangue

o

x

ð1Þ

θ vp

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u  u2 h þ h þ h p j f t S0 ¼ v0 g

v0 h p tp

y ð2Þ Fig. 3. Gangue particles' horizontal cast movement before entering the airflow domain.

226

K. Zheng et al. / Powder Technology 278 (2015) 223–233

r

In Eq. (10), u is the speed of airflow. According to Newton's second law, Eq. (10) can be change into Eq. (11):

x3 x2

x1

 1  2 2 m€x ¼ − ρA u þ x : 2 

x

ð11Þ

Then general solution of the displacement in x direction of gangue moving in the airflow is obtained by taken Laplace transform of Eq. (11). The general solution is shown in Eq. (12).

Fig. 4. Schematic diagram of calculating approximate force applied to a gangue particle.

rotation in airflow domain is ignored; 3) the effect of the dynamic pressure is considered and the influence of other factors is ignored; 4) the density of high pressure airflow is constant. The schematic diagram of gangue's motion in airflow domain is shown in Fig. 5. As shown in Fig. 5, hj represents the height of airflow, d represents the diameter of gangue, u represents the velocity of the high pressure airflow, and g represents the acceleration of gravity. During the process of analysis, air resistance and the increment of horizontal momentum of airflow domain are ignored. The displacement in y direction can be obtained by Eq. (7).

   At j uρ−2muC 1 AρC þ 2m ln cos   2 2m x tj ¼ Aρ

ð12Þ

The velocity in x direction of gangue movement in airflow domain can be obtained by taking divertive of Eq. (12). The result is shown in Eq. (13).     At j ρu−2muC 1 0 x t j ¼ −u tan 2m

ð13Þ

ð7Þ

In order to calculate the constant term C1 and C2, the initial position of the gangue is x(0) = 0, and the initial velocity of gangue is x′(0) = v0. When ẋ and u are in the opposite direction, we will get the general solution of x(tj) and x′(tj) through Eqs. (12) and (13). C1 and C2 is shown in Eq. (14):

Then tj can be obtained through Eq. (7), which represents the time of gangue movement in airflow domain. The result is shown in Eq. (8) (the equation has two roots, one is the result, and the other is extraneous root).

ð14Þ

  t s ¼ ∫ 0j vp þ gt j dt ¼ h−d

tj ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi v2p þ 2g ðh−dÞ−vp

ð8Þ

g

When gangue particles fell into airflow domain, gangue will be affected by two forces, the one is gravity, and the other is the dynamic pressure. Airflow dynamic pressure is converting to static pressure on the condition that the velocity of airflow u is greater than the gangue's horizontal velocity v0, and airflow must keep dynamic pressure pd = ρẋ2/2. It can be converted to dynamic pressure difference Δpd = ρ(u2 − ẋ2)/2. According to Eq. (5), the force of airflow can be obtained as Eq. (9):  X 1 X 2 2 F ¼ ∬pdA ≈ p i Ai ¼ ρ ui −x Ai : 2 i i 

 1  2 2 ρ u −x A: 2 

ð10Þ

o

The general solution of x(tj) and x′(tj) are shown in Eqs. (15) and (16). 0

0

  x t j2 ¼

0

111

u C C BAt uρ−2m arccosB B 0 1 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B j2 CC B B B 2 2 CC v þ u B CC B 0 u B B CC−2m ln @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiC 2m ln B A CC B cosB 2m B CC B v20 þ u2 B CC B @ AA @ Aρ

ð9Þ

It can be seen approximately as a plane when curvature of gangue surface is not too large, so ∀ui = u, Eq. (9) can be changed to: F¼

0 1 8 > > > u B C > > arccos@qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA > > > 2 2 > v0 þ u > > > < C1 ¼ 0u 1: > > > u B C > > −2m ln @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA > > > 2 2 > v þ u > 0 > > : C2 ¼ Aρ

x u d 2 h j tj

y airflow direction Fig. 5. Schematic diagram of gangue's motion in nozzle.

ð15Þ 0

0

11

u C BAt ρu−2m arccosB @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B j2 C B 2 2 C   v þ u B C 0 0 C x t j2 ¼ −u tanB B C 2m B C B C @ A

ð16Þ

3.2.3. Particle movement after leaving the air flow domain When gangue particles left the lower boundary of airflow domain, gangue particles will continue do horizontal cast movement. The formula of velocity vf in y direction is shown in Eq. (17). tj

v f ¼ ∫ 0 ðvp þgtÞdt

ð17Þ

tf can be obtained by Eq. (18), and the result is shown in Eq. (19).   t h f ¼ ∫ 0f v f þ gt dt

  t t t 2 ¼ ∫ 0f gtdtþ∫ 0f ∫ 0j vp þ gt d t

ð18Þ

K. Zheng et al. / Powder Technology 278 (2015) 223–233

tf ¼

227

4. Experiment simulation of air–solid multiphase pneumatic separation

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 2 −gt j −2t j vp þ 8gh f þ gt 2j þ 2t j vp

ð19Þ

2g

The displacement Sf of gangue in x direction after leaving the airflow domain can be calculated from Eq. (20):   0 Sf ¼ x t j t f :

ð20Þ

3.3. The solution of separation distance To sum up, the separation distance ΔS can be calculated through Eq. (21), which reflects the basis motion law of gangue influenced by airflow field and coal without influence of airflow field.

In order to verify the feasibility of pneumatic separation for coal and gangue, a “fixed coarse-grid” fluid scheme is implemented in PFC3D for particle-fluid coupling simulations. This scheme solves the continuity and Navier–Stokes equations for incompressible fluid flow numerically in a Eulerian Cartesian coordinate system, and then derives the pressure and fluid velocity for each fixed cell by including the influence of particles, and the corresponding porosity, within each cell. Driving forces from the fluid flow are applied to the particles as body forces. These forces are also added to the fluid equations and cause change in momentum, as reflected by the change in the pressure gradient in the flow direction. The flowchart [25] of the entire PFC3D calculation with the fluid scheme is shown in Fig. 6.

4.1. DEM model setup △S ¼ S0 þ S j þ S f −Si vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u  u2 h þ h þ h p j f t ¼ v0 g 0 0

0

111

u C C BAt uρ−2m arccosB B 0 1 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B j CC B B B C v20 þ u2 C B CC B u B C B C B C ffiA 2m ln B cosB CC−2m ln @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2m 2 2 B CC B v þ u 0 B CC B @ AA @ þ 0

0

Aρ 11

u C BAt ρu−2m arccosB @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B j C sffiffiffiffiffiffiffiffi B 2 2 C þ u v B C 2uh f 2hp 0 C   −v þ tanB 0 B C g 2m B C t j gt j þ 2vp B C @ A

ð21Þ

With the limitation of assumption, big differences exist between the calculation result of theoretical model and practical model. In order to simplify the calculation and correct the difference between theoretical value and practical value, parameters kn (nonlinear correction factor) and kr (linear correction factor) are introduced. kn reflects the convergence rate of the fitting function, kr reflects the convergent gain and used to adjust the fitting effect of formula based on experimental value. Theoretical value will appropriate to experiment value by adjusting the value of kn and kr. The modified formula with correction factor is shown in Eq. (22). △S ¼ S0 þ S j þ S f −Si vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u  u2 h þ h þ h p j f t ¼ v0 g 0 0

The modeling of large system is still a big challenge, in spite of the rapid development of computer technology. Available model should be established to reduce the computation time without loss in accuracy. Fig. 7(a) shows a front view of the model at the initial stage. The gangue hopper is built at the top right of conveying belt 1 that is aiming at reducing the computation time. In Fig. 7(a), 300 balls, with diameters ranging from 50 mm to 100 mm, are created in the gangue hopper at the top right of conveying belt 1. The total length of conveying belt 1 is 6000 mm, and the diameter of belt roll is 800 mm. A pneumatic boundary is specified at x = 0 and 7000 mm, by which particles existing at the end of the conveying belt appear at the other end after crossing the boundary. In the fluid scheme, 550(22 × 5 × 5: x-, y- and z-directions) fluid cells are created in a rectangular space (x = [0, 7000 mm], y = [−400 mm, 400 mm] and z = [300 mm, 400 mm]), which covers the rectangular space. A pneumatic boundary should be set for the fluid grid. However, in this simulation, an approximation is made by specifying the velocity boundary at the right end of the model and a pressure boundary as 0.0 Pa at the left end, x = 0 mm. The slip boundary, in which the fluid velocity parallel to the wall surface is not zero at the wall surface, is specified at the surrounding four walls. The fluid cells can be seen in Fig. 7(b) and (c) and the material properties are shown in Table 1. As can be seen in Table 1: the average ball diameter ranges from 50 mm to 100 mm; viscous damping is introduced to dissipate energy at contacts, and the damping constant is 0.1, which is equivalent to a restitution coefficient of 0.8. Air is used as the fluid and is injected

Start tf =tm tm=tm+dtm

0

111

Law of motion

u C C BAk t uρ−2m arccosB B 0 1 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B n j CC B B B 2 2 CC v þ u B CC B 0 u B B CC−2m ln @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiC 2m ln B A CC B cosB 2m B CC B v20 þ u2 B CC B @ AA @ þ 0

tm:time of DEM calculation dtm:timestep of DEM calculation

0

Aρ 11

u C BAt ρu−2m arccosB @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B j C sffiffiffiffiffiffiffiffi B 2 2 C v þ u B C 2uh f 2hp 0 C   −v −kr ð22Þ þ tanB 0 B C g 2m B C t j gt j þ 2vp B C @ A

Yes

tm>tf +dtf

tf =tm

Fluid Scheme

Force-displacement law No

Cal.end?

tf:time of fluid calculation dtf:timestep of fluid calculation

Yes

End Fig. 6. Flowchart of entire PFC3D calculation with fluid scheme.

228

K. Zheng et al. / Powder Technology 278 (2015) 223–233

Gangue hopper

Fluid boundary

Conveying belt 1 v0 hp u

Conveying belt 2

Collecting box

(a). A front view of the model at an initial stage The motion of gangue particles Fluid cells

Fluid Domain `

(b). The general view of the model with the fluid Domain

(c). Enlarged view of fluid cell

Fig. 7. Air–solid multiphase pneumatic separation.

with a uniform velocity profile in x direction that is constant for all velocity boundaries. The simulation procedure is shown as follows:

fluid cell grid point accounting for balls within the fluid cell and applying driving forces to those balls.

1) Several flat walls and infinite walls are created to make an initial assembly, which are including conveying belt, hopper and collecting box. Extra flat walls are created at both ends in order to make cells (not fluid cells) over the entire model. 2) A ball assembly, which diameters are ranging from 50 mm to 100 mm, is created in the gangue hopper at the top right of the conveying belt 1. Balls are dropped under gravity until their movement diminishes and reach an equilibrium state. 3) Delete the two flat walls, air is injected at an apparent velocity in x direction, with a constant velocity profile from the right boundary at x = 7000 mm. The simulation continues for 7 s.

4.2. Trajectory analysis of different diameter particles

The time step of the fluid calculation is 1.0 × 10−3 s. At each fluid time step, PFC3D calculates the fluid velocity and pressure at each Table 1 Materials properties. Parameter

Value

Units

Ball Diameter Number Density Normal stiffness Shear stiffness Friction coefficient

50–100 300 2700 1 × 106 1 × 106 0.7

mm

Air Density Viscosity

1.205 1.8 × 10−6

kg/m3 Pa.s

Wall Normal stiffness Shear stiffness Friction coefficient

1 × 106 1 × 106 0.3

N/m N/m

kg/m3 N/m N/m

At the starting point, air is injected from the left side with an apparent velocity of 260 m/s in the negative direction of x, while the two side walls confining the assembly are removed simultaneously. After some transient response, a plug is formed at approximately 1 s. The front view from the initial stage to 1 s is shown in Fig. 6. It can be obtained from Fig. 8 that particles with different diameters have different movement trajectories. Gangue particles will do horizontal cast movement before falling into the fluid domain, that can be seen at t = 0.2 s and t = 0.4 s in Fig. 8. The gangue particles will change their trajectory when they are falling into the fluid domain, which can be seen at t = 0.6 s, t = 0.8 s and t = 1.0 s. As can be seen in Fig. 8, small particles are more apparently influenced by the fluid domain than large particles, thus can be blown far away than large particles. The movement trajectory of particle can be drawn by calculating the displacement of each particle in each time step. For example, the movement trajectory of particles with the diameter 60 mm and 80 mm particles are shown in Fig. 9. It can be seen from Fig. 9 that movement trajectory of particles with diameter of 60 mm and 80 mmare different from each other. The ground position of particles is also different from each other. As shown in Fig. 9, area A is the domain where the particle changed its trajectory. Particle has complex movement in area B, that is mainly due to the influence of other particles. 4.3. Speed analysis of particles with different diameters Gangue particles will step into the stage of horizontal cast movement and will be affected by the fluid domain after they are thrown from the conveying belt. Gangue particles are mainly affected by gravity and the contact force between particles, so the movement speed is changed continually.

K. Zheng et al. / Powder Technology 278 (2015) 223–233

t =0. 2s

t =0. 6s

t =0. 4s

t =0. 8s

229

t =1. 0s

Fig. 8. Front view (initial stage to 1 s; apparent velocity, v0 = 1 m/s, u = −260 m/s).

Velocity for 50 mm, 70 mm, 90 mm and 100 mm particles in x-, yand z-direction are shown in Fig. 10. As can be seen in Fig. 10(a), velocity has greatly changed from 4.5 s to 6.5 s. Velocity in x direction is absolutely different for different diameters, which decrease with the increase of the particles diameter. It can be concluded that fluid domain

has significant influence on the movement speed in x direction. As can be seen in Fig. 10(c), velocity in z-direction for the four grade particles has the same change trend and the peak value is almost the same. Thus it can be concluded that fluid domain has little influence on the movement speed in z-direction.

A

A

B

B

(a) Particle movement trajectory of 60mm

(b) Particle movement trajectory of 80mm

Fig. 9. General view of particle trajectory for different diameters.

230

K. Zheng et al. / Powder Technology 278 (2015) 223–233

(a) Velocity of particles in x direction

(b) Velocity of particles in y direction

(c). Velocity of particles in z direction Fig. 10. Particle velocity for different diameters.

3.2

5. Result and discussion

5.1. Different factors affect separation distance

2.8 2.6 2.4 2.2

(m)

2.0 1.8

ΔS

In Section 3.3, the theoretical pneumatic separation distance formula of coal and gangue is derived. The separation distance ΔS, which reflects the basis motion law of gangue influenced by airflow field while coal without influence of airflow field, is shown in Eq. (22). As can be concluded from previous analysis, the larger separation distance ΔS, the better separation effect.

3.0

1.6 1.4

v0=0.5m/s

1.2

As can be seen in Eq. (22), the speed of airflow u, the conveyor velocity v0 and the height difference between conveyor belt and air nozzle hp are selected as three mainly factors. 5.1.1. The influence of different conveying velocities on separation distance In order to study the influence of different conveying velocities on separation, the conveying velocity 0.5 m/s, 1.0 m/s, 2.0 m/s and 3.0 m/s were selected respectively. The speed of airflow u and the height

v0=1.0m/s

1.0 0.8

v0=2.0m/s

0.6

v0=3.0m/s

0.4 45

50

55

60

65

70

75

80

85

90

95

100

105

d(mm) Fig. 11. The relationship between the conveying velocity and separation distance.

K. Zheng et al. / Powder Technology 278 (2015) 223–233

3.0

2.5

2.0

ΔS(m)

difference hp between conveyor belt and air nozzle are kept constant. Separation distance under different conveying speeds is shown in Fig. 11. As can be seen in Fig. 11, separation distance decreases with the increase of particle diameter and conveying velocity. When gangue particles fall into airflow domain, gangue will be affected by two forces, the one is gravity, and the other is the airflow pressure. Airflow dynamic pressure that converts to static pressure on the condition of the velocity of airflow u is greater than gangue's horizontal velocity v0. With the increase of the conveying belt velocity, the particles can get more kinetic energy in x direction. Airflow domain needs to overcome more kinetic energy to change particle's trajectory. It also can be seen from Eq. (9) that the dynamic pressure difference ΔPd will decrease with the increase of gangue's horizontal velocity v0. Thus, particles with large diameter and high horizontal velocity would have small separation distance ΔS.

231

1.5

hp=1.4m hp=1.1m

1.0

hp=0.8m hp=0.4m

0.5

0.0 45

50

55

60

65

70

75

80

85

90

95

100

105

d(mm)

5.1.2. The influence of different airflow velocities on separation distance In order to study the influence of different airflow velocities on separation, airflow velocities 260 m/s, 280 m/s, 300 m/s and 320 m/s are selected respectively for the research of separation effect. Conveying velocity v0 and the height difference hp between conveyor belt and air nozzle are kept constant. The relationship between the airflow velocity and particle diameter is shown in Fig. 12. As can be seen in Fig. 12, separation distance decreases with the increase of particle's diameter. However, the separation distance increases with the increase of airflow speed. When gangue particles fall into airflow domain, gangue will be affected by two forces, the one is gravity, and another is the airflow pressure. Airflow dynamic pressure that converts to static pressure on the condition of the speed of airflow u is greater than the gangue's horizontal velocity v0. With the increase of the airflow velocity, the particles can get more kinetic energy in x direction to overcome particles' kinetic energy in x direction to change particles trajectory. It can be seen from Eq. (9) that the dynamic pressure difference ΔPd will become larger with the increase of gangue's airflow velocity u. Thus, the particles with higher airflow velocity would have larger separation distance. 5.1.3. The influence of different height differences on separation distance In order to study the influence of different airflow velocities on separation effect, the height differences 0.4 m, 0.8 m, 1.1 m and 1.4 m are selected respectively. The speed of conveying velocity v0 and the height difference hp between conveyor belt and air nozzle are kept constant. The relationship between the airflow velocity and particle diameter is shown in Fig. 13. 4.5 4.0 3.5

Fig. 13. The relationship between the height difference and separation distance.

As can be seen in Fig. 13, separation distance decreases with the increase of particle's diameter. Separation distance also decreases with the increase of the height difference between conveyor belt and air nozzle. With the increase of the height difference between conveyor belt and air nozzle, the particles can get more kinetic energy in x direction. The airflow needs to overcome more kinetic energy to change particles trajectory. So the height difference has great influence on the separation distance. 5.2. Orthogonal experiment of pneumatic separation 5.2.1. Orthogonal experiment In this study, separation distance ΔS (m) is defined as the only index to evaluate the effect of pneumatic separation. From the analysis of the Section 5.1, the airflow velocity (u), conveyor velocity (v0) and height difference between conveyor belt and air nozzle (hp) are selected as three mainly factors. Factors and levels are listed in Table 2. According to the identified level of the factors, orthogonal table L9 (34) is applied in the test. Orthogonal test arrangement and results are shown in Table 3. Variance analysis and range analysis were used to generate variance analysis table and range analysis table to deal with the result of the orthogonal test. 5.2.2. Variance analysis and range analysis of the orthogonal test It can be seen from Table 2 that each factor at different levels is approximately linearity, so the method of regression analysis is to obtain the linear function relationship between them appropriately. Thus, the regression equation of coal gangue separation distance is obtained as follows: y ¼ −2:568 þ 0:023x1 −1:813x2 −0:4786x3

ð23Þ

3.0

where, y is the separation distance of coal and gangue, x1 represents the airflow velocity, x2 is the height difference, andx3 is the velocity of the conveyor belt. Then, variance analysis is carried out on the regression

ΔS(m)

2.5 2.0 1.5

u=260m/s u=280m/s u=300m/s u=320m/s

1.0 0.5

Table 2 Levels of factors. Factor

0.0 45

50

55

60

65

70

75

80

85

90

95

100

d(mm) Fig. 12. The relationship between the airflow speed and separation distance.

Level

A airflow velocity u(m/s)

B height difference hp(m)

C conveyor velocity v0(m/s)

1 2 3

260 280 320

0.40 0.80 1.40

0.50 1.00 2.00

105

232

K. Zheng et al. / Powder Technology 278 (2015) 223–233

Table 3 Experimental results of separation effect.

Table 5 Range analysis.

Experiment number

A

B

C

Separation distance ΔS(m)

1 2 3 4 5 6 7 8 9

260 260 260 280 280 280 320 320 320

0.40 0.80 1.10 0.40 0.80 1.10 0.40 0.80 1.10

0.5 1.0 2.0 1.0 2.0 0.5 2.0 0.5 1.0

2.45 1.53 0.3 2.25 1.31 1.58 3.15 2.95 2.15

Kj1 Kj2 Kj3 Rj

equation of coal and gangue separation distance to make significance experiment and the result is shown in Table 4. According to Table 4, all the three factors have significant influence on the separation distance of coal and gangue. In order to determine the optimal pneumatic separation parameters, we carry out comparative analysis of range between various levels of each factor. The range analysis is showed in Table 5. In Table 5, Kjm (m = 1,2,…n) is the sum of index values corresponding to factors in column j at level m. The value of Kjm determines the optimal level and combination of factors in column j. Rj reflects the ranges of the index with the variation of factors in column j, and the influence of the factor will be more significant if the value Rjis greater. The optimal parameters for pneumatic separation distance are presented at u = 320 m/s,v0 = 0.5 m/s,hp = 0.4 m. 5.3. Correction of the theoretical formula Based on the least square method, the formula in Eq. (22) in Section 3.3 will transform into a function of kn and kr parameters through variable substitution, then set up equations based on the experiments data. The result of kn and kr can be calculated finally. When ẋ and u are in the opposite direction, we can obtain nonlinear correction term kn = 1.55 × 105, linear correction term kr = 4.5. The final formula can be obtained as follows: △S ¼ S0 þ S j2 þ S f 2 −Si vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u  u2 h þ h þ h p j f t ¼ v0 g 0 0

0

111

u C C B1:55  105 At uρ−2m arccosB B 0 1 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B CC B j2 B B C v20 þ u2 C B CC B u C B CC−2m ln B q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2m ln B cos @ A B CC B 2m B CC B v20 þ u2 B CC B @ AA @ þ

B

C

7.85 5.79 4.03 1.273

6.98 5.93 4.76 0.74

the influence on separation distance. The speed of airflow u and the height difference hp between conveyor belt and air nozzle are kept constant. Fig. 14 shows the relationship of the experiment value and the theoretical value for the separation distance. Theoretical value and experiment value of coal gangue pneumatic separation distance have high consistent degrees, most of the error of experiment value and fitting result of separation distance is less than 10%. It can be concluded that Eq. (24) has important guiding significance and practical value for coal gangue pneumatic separation.

6. Conclusion In order to realize the underground pneumatic separation of coal and gangue, key factors that have influence on coal gangue pneumatic separation for large diameter (≥50 mm) were analyzed based on energy conservation principle. On the basis of the presentation, we may state as follows: 1) The theoretical formula of coal gangue pneumatic separation distance is derived. The formula reflects the basis of motion law of gangue influenced by airflow field and coal without influenced of airflow field. 2) An air–solid multiphase flow simulation was conducted to clarify its effect. Based on the above analysis, all the three factors (the airflow velocity u, the conveyor velocity v0 and the height difference hp between conveyor belt and air nozzle) have great influence on the separation distance. 3) The distance theoretical formulas of coal gangue pneumatic separation are corrected based on the analysis of experiment data, the error of experiment value and calculate value of the gangue broken rate is less than 15%. The corrected formula has important guiding significance and practical value for coal gangue pneumatic separation.

3.2

Aρ 11

0

0

A 4.28 5.14 8.25 1.323

u C BAt ρu−2m arccosB @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAC B j2 C sffiffiffiffiffiffiffiffi B v20 þ u2 C B C 2uh f 2hp B C   −v0 −4:5: þ tanB C g 2m B C t j2 gt j2 þ 2vp B C @ A

3.0

v=0.5m/s experiment value v=0.5m/s theoretical value v=2.0m/s experiment value v=2.0m/s theoretical value

2.8

ð24Þ

2.6 2.4

In order to verify the correctness of the theoretical formula, the conveying velocity 0.5 m/s and 2.0 m/s are selected respectively to study

ΔS(mm)

2.2 2.0 1.8 1.6 1.4

Table 4 Experimental results of variance analysis.

1.2 1.0

Factor

Squariance

DOF

Mean square

F value

0.8

A B C Regression Error Sum

2.9087 2.4371 0.0785 6.0974 0.1664 6.2638

2 2 2 3 5 8

286.667 0.767 1.1667 244,873.8

Distinctively Very distinctively Distinctively

0.6 0.4 45

50

55

60

65

70

75

80

85

90

95

100

d(mm) Fig. 14. The relationship between the experiment value and theoretical value.

105

K. Zheng et al. / Powder Technology 278 (2015) 223–233

Nomenclature ΔS coal and gangue pneumatic separation distance (m) hp the height difference between gangue mass center on the conveyor belt and the upper boundary of the airflow domain (m) tp the time of gangue before coming into the air flow domain (s) vp the velocity in direction y of the gangue before falling into the air flow domain (m/s) hj the height of the airflow domain (m) Si the displacement of gangue in x direction before affecting by the airflow (m) tj the time of gangue moving in airflow domain (s) Sj the displacement in x direction of the gangue moving in airflow domain (m) vf the velocity in y direction of the gangue moving in airflow domain (m/s) ẋ(tj) the velocity in x direction of the gangue moving in airflow domain (m/s) tf the time of gangue after the gangue leaving the airflow domain (s) Sf the displacement in x direction of the gangue movement after leaving the airflow domain (m) u airflow velocity (m/s) v0 conveying belt velocity (m/s) C1 constant confidence C2 constant confidence kn nonlinear correction factor kr linear correction factor Acknowledgment Financial support for this work, provided by the National High-Tech Research and Development Program of China (863 Program) (No. 2012AA062102), Innovation Training Project of Graduate Student in Jiangsu Province(CXLX13_936) and the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD), is gratefully acknowledged. References [1] M.G. Qian, J.L. Xu, X.X. Miao, Technique of cleaning mining in coal mine, J. China Univ. Min. Technol. 32 (2003) 343–348. [2] J.X. Zhang, X.X. Miao, Underground disposal of waste in coal mine, J. China Univ. Min. Technol. 35 (2006) 197–200.

233

[3] T.J. Gorden, A. Chris, Estimating size fraction categories of coal particles on conveyor belts using image texture modeling methods, Expert Syst. Appl. 39 (2012) 7947–7960. [4] S. Al-Thyabat, N.J. Miles, An improved estimation of size distribution from particle profile measurements, Powder Technol. 166 (2006) 152–160. [5] J. Tessier, C. Duchesne, G. Bartolacci, A machine vision approach to on-line estimation of run-of-mine ore composition on conveyor belts, Miner. Eng. 20 (2007) 1129–1144. [6] W. Xu, N. Dhawan, C.L. Lin, J.D. Miller, Further study of grain boundary fracture in the breakage of single multiphase particles using X-ray microtomography procedures, Miner. Eng. 46 (2013) 89–94. [7] T. Andersson, M.J. Thurley, J.E. Carlson, A machine vision system for estimation of size distributions by weight of limestone particles, Miner. Eng. 25 (2012) 38–46. [8] S. Al-Thyabat, N.J. Miles, T.S. Koh, Estimation of the size distribution of particles moving on a conveyor bel, Miner. Eng. 20 (2007) 72–83. [9] C. Aldrich, G.T. Jemwa, J.C. van Dyk, M.J. Keyser, J.H.P. van Heerden, Online analysis of coal on a conveyor belt by use of machine vision and kernel methods, Int. J. Coal Prep. Util. 30 (2010) 331–348. [10] E. Hamzeloo, M. Massinaei, N. Mehrshad, Estimation of particle size distribution on an industrial conveyor belt using image analysis and neural networks, Powder Technol. 261 (2014) 185–190. [11] Y.K. Yen, C.L. Lin, J.D. Miller, Particle overlap and segregation problems in on-line coarse particle size measurement, Powder Technol. 98 (1998) 1–12. [12] C.L. Lin, Y.K. Yen, J.D. Miller, Plant-site evaluations of the OPSA system for on-line particle size measurement from moving belt conveyors, Miner. Eng. 13 (2000) 897–909. [13] Y.D. Ko, H. Shang, A neural network-based soft sensor for particle size distribution using image analysis, Powder Technol. 212 (2011) 359–366. [14] J.X. Zhang, T. Chen, Z.D. Yu, Xinjiang cotton seed color separation system based on computer vision, Trans. Chin. Soc. Agric. Mach. 40 (2009) 161–164. [15] G.R. Gamea, Physical properties of sunflower seeds components related to kernel pneumatic separation, Int. J. Eng. Technol. 13 (1) (2013). [16] M. Panasiewicz, Analysis of the pneumatic separation process of agricultural materials, Int. Agrophys. 13 (1999) 233–239. [17] M. Xu, G.M. Li, J. Yin, W.Z. He, Crushing and pneumatic separation of printed circuit board scraps, Environ. Sci. Technol. 30 (2007) 72–74. [18] C. Guo, H. Wang, W. Liang, J.G. Fu, X. Yi, Liberation characteristic and physical separation of printed circuit board (PCB), Waste Manag. 31 (2011) 2161–2166. [19] V. Kumar, J. Lee, J. Jeong, M.K. Jha, B.S. Kim, R. Singh, Novel physical separation process for eco-friendly recycling of rare and valuable metals from end-of-life DVD-PCBs, Sep. Purif. Technol. 111 (2013) 145–154. [20] N. Hayashi, T. Oki, Effect of orifice introduction on the pneumatic separation of spherical particles, Mater. Trans. 55 (2014) 700–707. [21] I. Lecreps, O. Orozovic, T. Erden, M.G. Jones, K. Sommer, Physical mechanisms involved in slug transport and pipe blockage during horizontal pneumatic conveying, Powder Technol. 262 (2014) 82–95. [22] I. Lecreps, O. Orozovic, M. Eisenmenger, M.G. Jones, K. Sommer, Methods for in-situ porosity determination of moving porous columns and application to horizontal slug flow pneumatic conveying, Powder Technol. 253 (2014) 710–721. [23] I. Lecreps, O. Orozovic, M.G. Jones, K. Sommer, Application of the principles of gas permeability and stochastic particle agitation to predict the pressure loss in slug flow pneumatic conveying systems, Powder Technol. 254 (2014) 508–516. [24] K. Li, S.B. Kuang, R.H. Pan, A.B. Yu, Numerical study of horizontal pneumatic conveying: effect of material properties, Powder Technol. 251 (2014) 15–24. [25] PFC3D, Particle Flow Code in 3 Dimension, Ver 3.0 User's Manual, Minneapolis, 1999.