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Development of a pilot roller test machine for investigating the pulverizing performance of particle beds
Minerals Engineering 72 (2015) 65–72
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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Development of a pilot roller test machine for investigating the pulverizing performance of particle beds C.J. Chi a, Y.D. Zhou a,⇑, S.Q. Cao a, K. Arima b, Y. Chuman b, T. Okafuji b a b
State Key Laboratory of Hydroscience and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, PR China Nagasaki Research & Development Centre, Mitsubishi Heavy Industries, Ltd., Nagasaki, Japan
a r t i c l e
i n f o
Article history: Received 4 October 2014 Accepted 8 December 2014
Keywords: Roller test machine Particle bed Breakage Grinding Size reduction
a b s t r a c t Various types of pulverizers are commonly used in power plants for the purpose of breaking coal particles into fine powders to achieve optimum combustion for the boilers. To investigate the effects of factors that may influence the pulverizing efficiency, this study presents the development of a pilot roller test machine, which can significantly simplify the grinding conditions in actual pulverizers whilst the key variables involved in a rolling compression can be considered. The monitoring and data acquisition systems allow real-time monitoring of the pulverizing induced roller movements. Through parametric numerical analyses on an elastic feed bed of 5–30 mm in thickness and 500–1000 MPa in elastic modulus, it is found that the machine is capable of providing a maximum contact pressure stress in a range of 4.5– 17.5 MPa. A series of fundamental tests have been conducted by the developed machine using a type of bituminous coal and typical bound values of roller weight and speed. The size reduction results as well as the measurements of roller movement demonstrate the capability of the machine as a suitable tool for testing grinding performance. Some discussions of the potential extension of the machine are also given in the final part. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Fragmentation or size reduction of brittle material particles by various types of compression loading is of considerable scientific and industrial interest, particularly within the disciplines of mining, civil, and chemical engineering. Typical processes include mineral processing, thermal power generation, cement manufacturing, and pharmaceutical processing. In mining industry applications, comminution of mineral blocks or particles for a finer powder, such as grinding of ores and mineral particles in pulverizer machines, is widely accepted as an energy intensive process and many efforts have been devoted to better understanding of the particle breakage behaviour and improvement of energy efficiency of comminution (Tavares, 1999; Schaefer, 2001; Bourgeois, 1993). Single particle and particle bed breakage tests are commonly used for investigating the comminution mechanism in industrial operations (Liu and Schönert, 1996, Eksi et al., 2011; Barrios et al., 2011). Various compression testing methods have been proposed and applied to measure the breakage characteristics of single particle, which can be classified into single impact, double impact and slow compression according to the mode of application ⇑ Corresponding author. Tel.: +86 10 62794588; fax: +86 10 62782159. E-mail address: [email protected] (Y.D. Zhou). http://dx.doi.org/10.1016/j.mineng.2014.12.014 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved.
of stresses and the number of contact points (Gotoh et al., 1997). A comprehensive review of single-particle breakage tests was given by Sankara (1985). He compared the single impact, double impact and slow compression tests. The materials investigated varied from nearly defect-free materials, such as single crystals, to complex industrial granules. Similar to the basic mechanism of particulate attrition by Paramanathan and Bridgwater (1983), macroscopic effects of grinding induced size reduction may include particle impact on containing walls, particle to particle abrasion and fracture due to external stresses and internal stresses within the particles which form the bulk bed, whilst the microscopic effects include particle size, particle shape, particle structure and adhesion behaviour. A list of techniques commonly employed for testing the susceptibility of particles to attrition were summarized by Bemrose and Bridgwater (1987). Vertical roller mills have been well accepted for grinding of cement raw materials, clinker and slag, coal particles for cement kilns and power plants for several decades. Although much progress has been made in understanding the grinding process of various types of mill feeds (Schaefer, 2001), to the best knowledge of the authors, a systematic study of the grinding mechanism and comminution efficiency of particle beds by a pure roller compression cannot be found in literature. As one step toward such a goal, a pilot roller test machine has been developed in this study, which
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can significantly simplify the grinding conditions in actual pulverizers whilst the key variables involved in a rolling compression can be considered. Firstly, main design issues about the test machine, including the mechanical realization, measurement and data acquisition systems are introduced in the main portion of this paper. A rough estimate of the range of maximum contact pressure stress is also given based on some fundamental finite element analyses. Secondly, elementary pulverizing tests have been conducted using the machine considering variable roller weights and speeds, from which typical results of size reduction and roller movement are provided. Some discussions on the coming extension of the machine and concluding remarks are given in the final part. 2. Design and setup of a roller test machine The working principle of the roller test machine developed in this study is schematically shown in Fig. 1. The motions of the roller and the support table components are controlled by an AC servo motor, of which the rotation movement follows the order given from a control computer through the controller unit. The mobilized roller up-and-down displacements and its rotation speed, shall be monitored using photoelectric sensors and be recorded by a data acquisition system, which digitizes the analog sensor signals and transfers to the control computer for visualization and post-processing purpose. Main components for the mechanical realization of the test machine, the control and measurement system, as well as parametric numerical analyses for an estimate of the maximum contact pressure stress are described below. 2.1. Mechanical realization of rolling compression As mentioned above, three main parameters that were deemed to be mostly influential in the pulverizing process were focused in the design of the test machine, including the roller weight, the kinematics of the roller (especially its rotation speed), and the thickness of the feed bed. The test machine was purposely designed such that each of these three factors can be controlled individually for simulating various grinding conditions in actual practice. As shown schematically in Fig. 1 and in photo in Fig. 2, the machine consists of three main parts: the base frame, the support table and the loading roller. The features of these main parts are described as follows (from bottom to top): (1) A 5000 mm long steel-made base frame of sufficiently large stiffness was fixed onto the ground, which comprised a 150 mm thick concrete layer on a solid soil foundation and
was deemed strong enough for supporting the overall weight of the machine as well as vibration loads mobilized during the rolling tests. A straight and horizontal support table, which was also made of steel material and 2400 mm in length, was arranged on the base frame and its movement was constrained along the longitudinal direction through two liners installed on both lateral sides of the base frame (Fig. 3). A 2875 mm long ball screw was installed within the base frame and connected to the support table, which serves a purpose of driving the support table along the longitudinal direction. (2) A horizontal groove was purposely built into the support table for containing the particle bed during pulverizing tests (Figs. 2 and 3). A small width of 32 mm was chosen for the groove in order to obtain a higher pressure stress acting on the feed bed by the overburdened roller weight. The groove is 35 mm in depth, which poses a limit on the maximum layer thickness and the largest vertical motion of the roller part.
Controls on the table speed
Command
Order
Fig. 2. A front view of the roller test machine.
Roller AC servo motor
Controller unit
Support table Groove Liners
Motion control system
Motion monitoring system Computer
Test machine
Digital output
Base frame
Voltage output Photoelectric sensors
Counterweight blocks
Monitoring roller motion
Data acquisition hardware
Fig. 1. The working diagram of the developed roller test machine.
Servo motor Fig. 3. An iso-metric view of the test machine (3D design model).
C.J. Chi et al. / Minerals Engineering 72 (2015) 65–72
(3) A cylinder-shaped steel roller, 200 mm in thickness and 400 mm in diameter, was arranged on the support table with the shaft in a horizontal plane and orthogonal to the advancing direction of the support table (Fig. 3). The roller was designed to be fixed horizontally, and can move up and down freely in the vertical direction by slider (guide) rails (Fig. 4(a)) that were installed on both lateral sides of the base frame. The desired rotation with reference to the central shaft can be achieved when a sufficient large friction action is triggered along the roller-table (or feed bed) interface, which reasonably comes from the advancing or receding of the support table component. Specifically, the original thickness of the roller (200 mm) was narrowed to a much smaller value of 30 mm near the outer surface, as given in Fig. 3, which fits the groove width value, and can reasonably favour the increase of the compression stress on the feed bed to a great extent. The total weight of the roller and the adjunct components, including the shaft, the bearing and the slider rail for controlling the roller motion, is 67 kg. (4) In order to simulate various rolling conditions of different pressure levels in practice, it was designed that counterweight blocks could be added on both sides of the roller shaft by using dual tension rods (Figs. 3 and 4). The counterweight blocks were made of lead material of a big density 11.3 g/mm3, and the maximum surcharge can be as large as 350 kg considering the requirements of a proper operation of the shaft and bearing components. It can be seen that an equivalent roller weight in a range of 67–417 Kg can be chosen for the grinding tests on this machine. According to the simplified analysis results given in the following Table 1, the lower and upper limit of contact pressure stress for a horizontal elastic layer of 5–30 mm in thickness and 500– 1000 MPa in elastic modulus are 4.5 MPa and 17.5 MPa respectively. Displacement sensor Print mark sensor Roller
Slider rails Tension rods Counterweight blocks
(a) A schematic view of the sensors arrangement
(b) The print mark poster on the lateral surface of the roller Fig. 4. The arrangement of photoelectric sensors for monitoring roller motion during pulverizing tests.
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2.2. Control, measurement and data acquisition In addition to the above main mechanical parts of the test machine, two systems, including the motion control system and the motion monitoring system (Fig. 1), were designed for the pulverizing test study. The motion control system can precisely control the horizontal movement of the support table, leading to a rolling compression action by the roller on the table as well as the feed layer on it. By the motion monitoring system it becomes possible to measure the real-time response of the roller’s rotation speed and its vertical displacement during the testing process. The motion control system consists of three main parts, an AC servo motor (Type SGDV-120A01A002000 by Yaskawa Electric (China) Co., Ltd), a system controller unit, and a motion control card (Type MPC08SP by Leetro automation corporation, China). The main supply is three-phase 380 V that is reduced to 220 V by a step-down transformer to power the AC servo motor. For a given motion of the support table, the inputs for the motion control card placed in the computer can be determined and the corresponding control commands are given to the servo motor through the controller unit. As mentioned above, a ball screw links the servo motor and the support table. Reasonably a proper control of the motion of the ball screw can drive the support table according to a prescribed way. The motion monitoring system consists of two types of photoelectric sensors, a data acquisition system, electrical accessories and other input–output devices. Through the introduction of two types of high-performance photoelectric sensors, non-contact measurements of the roller motion become feasible, which is important for investigating the interaction between the roller part and the feed bed. A high resolution (<8 lm) measurement of the vertical up-and-down movement of the roller was designed in our machine using a CP08MHT80 type displacement sensor by Wenglor sensoric GmbH (German), of which the measurement rate can be as high as 600 per second. The displacement sensor was installed onto the frame component of the machine with its transmitter diode normally pointing to the uppermost portion of the roller (Fig. 4(a)). Furthermore, a method was proposed to measure the angular speed of the roller using a print mark sensor of Type WM03PCT2, also by Wenglor sensoric GmbH. This type of sensor is originally designed to recognize contrast marks for sorting, positioning and quality control applications, such as in the printing machines and packaging industry applications in the food, beverage and pharmaceutical industries. Based on its capability for precise positioning of print marks and high frequency response (up to 5000 per second), a non-contact measurement method was proposed to monitor the rotation speed of the roller, which is described as follows: (1) A poster of the same outer diameter as the roller, with 90 black print marks of circular sector shape uniformly distributed along the circumferential direction, as given in Fig. 4(b), was stuck to one lateral surface of the roller. The angle at the centre for each black print mark is 2°. (2) A print mark sensor was installed onto the side frame with its transmitter diode normally pointing to the above poster, and its position was adjusted such that the sensor’s light spot lies within the range of print marks in the radial direction (Fig. 4(a)). By teaching the two grey-scale values of the black mark and the background to the sensor, it can recognize the switching between the black marks and the white background when the roller rotates during grinding tests. (3) The sensor output of voltage versus time would be recorded by a data acquisition system, which is to be described below, and the response of the roller rotation speed with time can then be determined. A typical case of constant roller rotation
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Table 1 Numerical results of the maximum contact pressure stress for a roller statically arranged on a horizontal coal layer. Roller mass (kg)
Maximum contact pressure stress in the contact region (MPa) h = 5 mm
67 210 417
h = 10 mm Ec = 1000 MPa
Ec = 500 MPa
Ec = 1000 MPa
Ec = 500 MPa
Ec = 1000 MPa
4.8 9.2 13.6
6.6 12.3 17.5
4.6 8.9 12.5
6.4 12.1 16.8
4.5 8.5 11.6
6.3 12.0 16.0
speed is schematically shown in Fig. 5, where a high voltage (10 V) indicates the recognition of black print marks, while a low voltage (0 V) means the light spot is on the white background. From the response curve monitored during pulverizing tests, the time variation of the roller angular speed can be simply determined as the speed that the sensor light spot passes the alternating print mark and underground section, that is, by the equation below:
xðti Þ ¼
h = 30 mm
Ec = 500 MPa
p
ð1Þ
90Dt i
in which x(ti) denotes the roller angular speed at the middle point of each zone of constant voltage response lasting for a time interval Dti (i = 1, 2, . . . N). From a series of data points of roller speed versus pulverizing time, the whole development history of roller rotation during grinding tests can be determined. The data acquisition system for the roller test machine includes the data acquisition module and the corresponding software. We chose DAM-3058 by DonghuaTest Ltd. (China) for our machine’s data acquisition module, which can record the analog voltage or current output signal, and the data acquisition sampling frequency can be as high as 12,000 readings per second. The corresponding software allows a real-time visualization of the time variation of sensor signal on a computer displayer.
stress level allowed for the test machine, a series of numerical analyses were conducted in this section using the finite element method. Fig. 6 shows the finite element model for the roller-feed bed-table system. A linear elastic assumption was made for all components, and the elastic parameters for the steel-made roller and table are Es = 210 103 MPa and vs = 0.3. The interaction along the interfaces between the roller and the feed bed as well as the feed bed and the table was modelled by a master–slave contact algorithm, in which a small friction coefficient of 0.2 was adopted. A static equilibrium solution of the roller compressing on the particle bed was investigated. The effects of different test conditions were investigated, including the roller weight, thickness of feed bed and the elastic modulus of the bed material. A range of the modulus of bed material (Ec) of 500–1000 MPa was chosen based on the compression test results of prismatic coal samples. Table 1 lists a summary of the maximum contact pressure stress from the numerical results. The results in Table 1 show that for a feed bed of smaller thickness, a higher contact pressure stress can be mobilized, and the increase of its overall stiffness is also beneficial for the build-up of pressure stress. A range of maximum contact pressure stress between 4.5 and 17.5 MPa can be expected for the grinding tests using the machine on a feed bed of 5–30 mm in thickness and 500–1000 MPa in elastic modulus. The peak contact pressure stress, 17.5 MPa, is deemed to be representative of the grinding conditions in actual pulverizers.
2.3. Numerical analysis of the contact pressure stress by roller compression 3. Roller test study One may note the above test machine’s incapability of measuring the contact pressure stress distribution along the roller-bed interface mobilized by the rolling compression. The difficulty lies in the fact that the contact area shall change with the rolling process and an installation of membrane sensor(s) on the contact surface may disturb the interaction and in turn influence the pulverizing behaviour. In order to give a rough estimate of the contact pressure
3.1. Test cases To verify the effectiveness of the above developed test machine, a series of fundamental tests were conducted. The particle material
12
Δti 10 Roller
Voltage (V)
8
6
4
2
Table
0 0
2
4
6
8
10
feed bed
Time (s) Fig. 5. Time variation of voltage output from the print mark sensor (a particular case of constant roller rotation speed as an instance).
Fig. 6. Finite element modelling of the roller-feed bed-table interaction in grinding tests (coal layer thickness h = 20 mm as an instance).
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C.J. Chi et al. / Minerals Engineering 72 (2015) 65–72 Table 2 The basic properties of selected bituminous coal particle material.
1
Type
HGI
Particle density (kg/ m3)
Initial surface moisture content (%)
Bituminous
60
1388
7.3
Table 3 A summary of elementary test cases by the roller machine. Test case
Layer thickness (mm)
Roller weight (kg)
Table speed (m/s)
Case I Case II Case III
20 20 20
100 200 400
0.1, 1.0 0.1, 1.0 0.1, 1.0
Percentage of passing particle
0.8
W=100kg W=200kg 0.6
W=400kg Feed
0.4
0.2
0 0.05
ω
0.5
Particle size (mm)
(a) v =0.1 m/s
Particles in this range are chosen for size reduction assessment
W 1
θ=30º
Coal bed h
v
0.8
10 mm
>500 mm
Fig. 7. A schematic illustration of the arrangement of feed bed in pulverizing tests.
selected for the tests is a type of bituminous coal from Jiamusi, Heilongjiang Province, China. Table 2 lists the basic properties of the coal particle material. A type of jaw crusher was applied to prepare the feed particles and the output coal particles was scalped by 1.25 mm. The particle size distribution of the feed coal was determined by sieve analyses according to the Chinese standard GB/T 2565–1998. The particle size distribution in mass was found to be 4% up to 0.5 mm, 5% between 0.5 and 0.63 mm, 63% between 0.63 and 1.0 mm, and 28% between 1.0 and 1.25 mm. One can note that the feed samples for the tests contain particles mostly within a narrow range between 0.63 and 1.25 mm, which are similar to the sampling requirements for the common Hardgrove grindability index (HGI) tests (ISO 5074:1994). Typical factors which may influence the breakage efficiency of particle bed by rolling compression were chosen in this study and a summary of the test conditions is given in Table 3. Three different roller weights, 100 kg, 200 kg and 400 kg, were chosen, which respectively represent the minimum, intermediate and maximum bound values. Similarly, two velocities of the support table were chosen, including a lower one of 0.1 m/s and a higher of 1.0 m/s that is normally adopted in pulverizers. A feed bed of 20 mm thickness containing the coal particles was adopted in each test, of which the length of a constant thickness section is 330 mm, and an inclined slope of 30° was arranged at both ends (Fig. 7) for supporting the main particles into a horizontal bed. Only the central portion of the particle bed, as indicated by a red rectangle in Fig. 7, is to be collected for size reduction assessment by sieve tools. The purpose of such a treatment is to exclude the side effect near both ends caused by varying thickness, and the designed test
Percentage of passing particle
Table W=100kg W=200kg 0.6
W=400kg Feed
0.4
0.2
0 0.05
0.5
Particle size (mm)
(b) v =1.0 m/s Fig. 8. Effect of roller weight (W) on the particle size reduction by one-time rolling compression (a–b).
conditions can be representative of the circularly arranged long coal bed under continuous grinding in actual pulverizers. Three tests were conducted for each case as given in Table 3.
3.2. Typical test results 3.2.1. On the particle size reduction Firstly, the roller weight (W) was taken as an independent variable, and Fig. 8 presents the mean test curves of particle size distribution before and after one-time pulverizing process. It can be
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1
20
16
Roller uplift dispment (mm)
Percentage of passing particle
0.8
v=0.1m/s 0.6
v=1.0m/s Feed
0.4
W=100 kg W=200 kg 12
W=400 kg
8
4
0.2
0 0
1
2
3
4
5
Time (s)
(a) v=0.1 m/s
0 0.05
0.5 20
Particle size (mm)
(a) W=100kg 16
Roller uplift dispment (mm)
1
Percentage of passing particle
0.8
v=0.1m/s
0.6
v=1.0m/s Feed
W=100 kg W=200 kg
12
W=400 kg 8
4
0.4 0 0
0.1
0.2
0.3
0.4
0.5
Time (s)
(b) v=1.0 m/s
0.2
Fig. 10. Comparison of monitored roller vertical displacement during pulverizing tests (h = 20 mm cases were taken as instances).
0 0.05
0.5
Particle size (mm)
(b) W=400kg Fig. 9. Effect of table speed (v) on the particle size reduction by one-time rolling compression (a–b).
seen that reasonably an increase of the roller weight from 100 kg to 400 kg can significantly improve the disintegration intensity, for either a lower or a higher table speed allowable for the machine. For the W = 400 kg case, nearly 30% of the feed particles of 0.63–1.25 mm in size were disintegrated by one-time rolling compression, and the generated products mostly fell in a size range between 0.25 and 0.5 mm. Moreover, the figure shows that for the considered range of roller weight, the products for the finer particles (<0.25 mm) by one-time rolling compression increased by 3%,
which is not so great as expected and the effects of original particle size distribution of the feed need to be further investigated. Secondly, take the speed of the support table (v) as an independent variable, the particle size distribution curves before and after one-time rolling compression are compared in Fig. 9. It can be observed that generally for the chosen layer thicknesses and roller weights, a lower table speed may benefit the mobilization of fine products to a minor extent. The effect of adjusting the table speed within a range of 0–1.0 m/s seems to diminish with the increase of layer thickness from additional test studies. Similarly, from the case of a layer thickness of 20 mm, the results show that with the roller weight enlarged, the influence of the table speed is negligible. Even though only limited cases have been presented in this study, it may be appropriate to propose the adoption of a lower roller speed in actual pulverizers, since it require less energy input without cutting down the products of fine particle size.
C.J. Chi et al. / Minerals Engineering 72 (2015) 65–72
roller angular speed (rad/s)
0.6 0.5 0.4 0.3
W=100kg W=200kg
0.2
W=400kg
0.1 0 0
1
2
3
4
5
0.4
0.5
Time (s)
(a) v=0.1 m/s
Roller angular speed (rad/s)
8 7 6 5 4 W=100kg
3
W=200kg
2
W=400kg 1 0 0
0.1
0.2
0.3
Time (s)
(b) v=1.0 m/s Fig. 11. Comparison of monitored roller angular speed during pulverizing tests (h = 20 mm cases were taken as instances).
3.2.2. On the roller motion measurement The monitoring system in the test machine allows the measurement of roller movement during pulverizing tests on the coal beds. Fig. 10 presents typical results of the roller vertical displacement with time, and the response during the compression on the feed layer was focused. For each case of variable roller weights and table speeds, a feed coal bed of the same geometry (Fig. 7) was carefully prepared before the test, ensuring all the coal beds were approximately of the same compaction degree. The results in Fig. 10 demonstrate that the development of the roller vertical displacement followed a similar variation pattern for all the cases. Each curve is roughly symmetric with respect to the midpoint, which can be attributed to the symmetry of the geometry of the feed layer. It is also shown that the slope of the descending portion is a bit smaller than that of the ascending portion, which can be reasonably explained by the difference in mechanical interaction between the roller and coal bed particles at the two stages. The ascending portion in each curve indicates the climbing of the roller onto the coal bed, and the compression from the roller has a trend of compacting the particle towards the central portion. For the descending portion, differently the roller passed most portion of the coal bed and its compression on the particles arranged on the sloping portion may tend to carry away particles and lengthen the bed to some extent. If an assumption is made that the roller keep in touch with the bed during pulverizing on it, the displacement curves in the above Fig. 10 also equivalently indicate the surface profile of the coal bed
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after one-time rolling compression along the longitudinal section. It can be expected that the rolling compression in each test would decrease the original thickness of the feed bed (h = 20 mm), and the adoption of different roller weights would mobilize different surface settlements of the particle layers. The monitoring results of the roller displacement in Fig. 10 approve the expectations well. From a series of tests using different roller speeds in a range of 0.1– 1.0 m/s, it was found that one-time rolling compression can induce the surface settlement of the feed bed by 3.1–3.9 mm (W = 100 kg), 4.8–5.6 mm (W = 200 kg), and 6.0–7.0 mm (W = 400 kg). Obviously, an increase of the roller weight from 100 kg to 400 kg, which in turn enlarged the contact pressure stress along the interface, can mobilize an extra surface settlement of the coal bed of 3.0–4.0 mm. It was also found that the variation of the roller speed can induce some slight difference in the settlement response. Besides the roller vertical displacement, its rotation speed response with time simultaneously monitored during pulverizing on the feed bed is also shown in Fig. 11. If an assumption is made that no slippage occurs along the two interfaces, namely between the roller and the feed bed, and between the support table and the layer, a linear relationship can be formulated between the given table speed v and the roller rotation speed w as w = v/r, where r indicates the roller radius (200 mm). The curves in Fig. 11 demonstrate that averagely the roller rotation speed follows the linear relationship, which are 0.5 rad/s and 5.0 rad/s for v = 0.1 m/s and v = 1.0 m/s case respectively. Moreover, notable perturbations of the roller speed were detected during pulverizing on the bed, which became more prominent for the case of a larger table speed of v = 1.0 m/s. From these perturbations it can be inferred that during the roller rolling on the feed bed, momentary slippages were triggered along the roller-bed interface for each test, which can be attributed to the complex interaction along the dual interfaces and the nonlinear behaviour of the coal particles forming the feed bed.
4. Discussions and conclusions The typical test results described above clearly demonstrate the capability of the developed machine for conducting pulverizing tests on particle beds under various conditions. In addition to the above elementary functions, further developments shall be made by the authors on the test machine to extend its capabilities, including but not limited to the following: (1) A photoelectric sensor is to be installed onto the side frame for a non-contact measurement of the surface profile of the feed bed. The aim is to determine the geometric feature of the feed bed before and after the rolling compression, which is essential for the evaluation of the energy spent on the internal deformation within the coal layer and the assessment of pulverizing efficiency. (2) A particular working condition of the roller, that is, a pure sliding on the feed bed with rotation around the shaft prohibited, is to be achieved by using caliper brakes for the roller. An initial vertical gap between the roller and the feed bed can also be obtained by lifting the roller with two jacks on both sides. Such a designed condition can somewhat simulate the actual operating condition in vertical-spindle pulverizers (https://www.mhps.com/en/products/detail/coal_pulverizer. html), as the discrepancy between the velocity at the surface of the conical roller and that at the table surface, particular due to the inconsistency near the inner and outer side surfaces, would cause slippage at both lateral sides. The breakage efficiency contributed by the roller sliding action can be investigated by such a test arrangement.
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(3) In actual vertical-spindle pulverizers, it is common to use spring frame hydraulic preload or a spring canister connecting to the journal assembly of each grinding element for extra pressure on the feed bed. The stiffness property of these springs shall influence the grinding performance. Similarly, a spring frame can be added to the above test machine for imposing extra pressure load on the roller shaft, which can provide an opportunity for better investigation into the influence of spring frames of various stiffness properties. Based on the work presented in the above sections, the following conclusions can be drawn in regard to the developed roller test machine: (1) The present roller test machine can consider key variables that may influence grinding efficiency by rolling compression, including the thickness of feed bed, roller weight (or grinding pressure), the velocity of the support table and in turn the roller rotation speed. A special sliding action by constraining the rotation of the roller can also be simulated. The capabilities of the developed machine allows a deeper investigation into the grinding performance of various actual pulverizers through laboratory test study. (2) The roller movements, including its vertical displacements and angular speed mobilized during pulverizing process, can be monitored by photoelectric sensors in a noncontact manner in the developed machine. These measurements can help determine the work for deforming the feed bed as well as that for particle disintegration, which is of great significance for the assessment of the grinding performance. (3) A series of 3-D finite element analyses have been conducted to study the maximum contact pressure stress on an elastic horizontal feed bed allowed by the test machine. For a feed bed of 5–30 mm in thickness and 500–1000 MPa in the modulus, it is found that a maximum contact pressure stress of about 17.5 MPa is available for the test machine.
(4) A series of fundamental rolling tests have been conducted on particle beds belonging to a type of bituminous coal material. The effects of roller weights and the speed of the support table have been studied and reasonable test results were provided. Besides the purposely-designed one-time pulverizing test, the test machine is also capable of doing rolling tests on single particle, and repeated pulverizing tests considering various combinations of controlling factors. Acknowledgements The authors are grateful for the kind support from Mitsubishi Heavy Industries Ltd., Japan. Additionally, the support from the National Natural Science Foundation of China (No. 51479096) and from State key Laboratory of Hydroscience and Engineering (Nos. 2012-KY-04, 2013-KY-02, and 2014-KY-1) as well as the support by Tsinghua University Initiative Scientific Research Program under Grant 20131089285 are gratefully acknowledged. References Barrios, G.K.P., de Carvalho, R.M., Tavares, L.M., 2011. Modeling breakage of monodispersed particles in unconfined beds. Miner. Eng. 24 (3), 308–318. Bemrose, C.R., Bridgwater, J., 1987. A review of attrition and attrition test methods. Powder Technol. 49 (2), 97–126. Bourgeois, F.S., 1993. Single-particle fracture as a basis for microscale modeling of comminution processes. Department of Metallurgical Engineering, University of Utah. Eksi, D., Hakan Benzer, A., Sargin, A., et al., 2011. A new method for determination of fine particle breakage. Miner. Eng. 24 (3), 216–220. Gotoh, K., Masuda, H., Higashitani, K., 1997. Powder Technology Handbook, 2nd ed. Marcel Dekker, New York. ISO 5074:1994 Hard coal – determination of Hardgrove grindability index. Liu, J., Schönert, K., 1996. Modelling of interparticle breakage. Int. J. Miner. Process. 44, 101–115. Paramanathan, B.K., Bridgwater, J., 1983. Attribution of solids—I: cell development. Chem. Eng. Sci. 38 (2), 197–206. Sankara NS. Australasian institute of mining and metallurgy bulletin and proceedings 1985; 291(4). Schaefer, H.U., 2001. Loesche vertical roller mills for the comminution of ores and minerals. Miner. Eng. 14 (10), 1155–1160. Tavares, L.M., 1999. Energy absorbed in breakage of single particles in drop weight testing. Miner. Eng. 12 (1), 43–50.
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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Development of a pilot roller test machine for investigating the pulverizing performance of particle beds C.J. Chi a, Y.D. Zhou a,⇑, S.Q. Cao a, K. Arima b, Y. Chuman b, T. Okafuji b a b
State Key Laboratory of Hydroscience and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, PR China Nagasaki Research & Development Centre, Mitsubishi Heavy Industries, Ltd., Nagasaki, Japan
a r t i c l e
i n f o
Article history: Received 4 October 2014 Accepted 8 December 2014
Keywords: Roller test machine Particle bed Breakage Grinding Size reduction
a b s t r a c t Various types of pulverizers are commonly used in power plants for the purpose of breaking coal particles into fine powders to achieve optimum combustion for the boilers. To investigate the effects of factors that may influence the pulverizing efficiency, this study presents the development of a pilot roller test machine, which can significantly simplify the grinding conditions in actual pulverizers whilst the key variables involved in a rolling compression can be considered. The monitoring and data acquisition systems allow real-time monitoring of the pulverizing induced roller movements. Through parametric numerical analyses on an elastic feed bed of 5–30 mm in thickness and 500–1000 MPa in elastic modulus, it is found that the machine is capable of providing a maximum contact pressure stress in a range of 4.5– 17.5 MPa. A series of fundamental tests have been conducted by the developed machine using a type of bituminous coal and typical bound values of roller weight and speed. The size reduction results as well as the measurements of roller movement demonstrate the capability of the machine as a suitable tool for testing grinding performance. Some discussions of the potential extension of the machine are also given in the final part. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Fragmentation or size reduction of brittle material particles by various types of compression loading is of considerable scientific and industrial interest, particularly within the disciplines of mining, civil, and chemical engineering. Typical processes include mineral processing, thermal power generation, cement manufacturing, and pharmaceutical processing. In mining industry applications, comminution of mineral blocks or particles for a finer powder, such as grinding of ores and mineral particles in pulverizer machines, is widely accepted as an energy intensive process and many efforts have been devoted to better understanding of the particle breakage behaviour and improvement of energy efficiency of comminution (Tavares, 1999; Schaefer, 2001; Bourgeois, 1993). Single particle and particle bed breakage tests are commonly used for investigating the comminution mechanism in industrial operations (Liu and Schönert, 1996, Eksi et al., 2011; Barrios et al., 2011). Various compression testing methods have been proposed and applied to measure the breakage characteristics of single particle, which can be classified into single impact, double impact and slow compression according to the mode of application ⇑ Corresponding author. Tel.: +86 10 62794588; fax: +86 10 62782159. E-mail address: [email protected] (Y.D. Zhou). http://dx.doi.org/10.1016/j.mineng.2014.12.014 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved.
of stresses and the number of contact points (Gotoh et al., 1997). A comprehensive review of single-particle breakage tests was given by Sankara (1985). He compared the single impact, double impact and slow compression tests. The materials investigated varied from nearly defect-free materials, such as single crystals, to complex industrial granules. Similar to the basic mechanism of particulate attrition by Paramanathan and Bridgwater (1983), macroscopic effects of grinding induced size reduction may include particle impact on containing walls, particle to particle abrasion and fracture due to external stresses and internal stresses within the particles which form the bulk bed, whilst the microscopic effects include particle size, particle shape, particle structure and adhesion behaviour. A list of techniques commonly employed for testing the susceptibility of particles to attrition were summarized by Bemrose and Bridgwater (1987). Vertical roller mills have been well accepted for grinding of cement raw materials, clinker and slag, coal particles for cement kilns and power plants for several decades. Although much progress has been made in understanding the grinding process of various types of mill feeds (Schaefer, 2001), to the best knowledge of the authors, a systematic study of the grinding mechanism and comminution efficiency of particle beds by a pure roller compression cannot be found in literature. As one step toward such a goal, a pilot roller test machine has been developed in this study, which
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can significantly simplify the grinding conditions in actual pulverizers whilst the key variables involved in a rolling compression can be considered. Firstly, main design issues about the test machine, including the mechanical realization, measurement and data acquisition systems are introduced in the main portion of this paper. A rough estimate of the range of maximum contact pressure stress is also given based on some fundamental finite element analyses. Secondly, elementary pulverizing tests have been conducted using the machine considering variable roller weights and speeds, from which typical results of size reduction and roller movement are provided. Some discussions on the coming extension of the machine and concluding remarks are given in the final part. 2. Design and setup of a roller test machine The working principle of the roller test machine developed in this study is schematically shown in Fig. 1. The motions of the roller and the support table components are controlled by an AC servo motor, of which the rotation movement follows the order given from a control computer through the controller unit. The mobilized roller up-and-down displacements and its rotation speed, shall be monitored using photoelectric sensors and be recorded by a data acquisition system, which digitizes the analog sensor signals and transfers to the control computer for visualization and post-processing purpose. Main components for the mechanical realization of the test machine, the control and measurement system, as well as parametric numerical analyses for an estimate of the maximum contact pressure stress are described below. 2.1. Mechanical realization of rolling compression As mentioned above, three main parameters that were deemed to be mostly influential in the pulverizing process were focused in the design of the test machine, including the roller weight, the kinematics of the roller (especially its rotation speed), and the thickness of the feed bed. The test machine was purposely designed such that each of these three factors can be controlled individually for simulating various grinding conditions in actual practice. As shown schematically in Fig. 1 and in photo in Fig. 2, the machine consists of three main parts: the base frame, the support table and the loading roller. The features of these main parts are described as follows (from bottom to top): (1) A 5000 mm long steel-made base frame of sufficiently large stiffness was fixed onto the ground, which comprised a 150 mm thick concrete layer on a solid soil foundation and
was deemed strong enough for supporting the overall weight of the machine as well as vibration loads mobilized during the rolling tests. A straight and horizontal support table, which was also made of steel material and 2400 mm in length, was arranged on the base frame and its movement was constrained along the longitudinal direction through two liners installed on both lateral sides of the base frame (Fig. 3). A 2875 mm long ball screw was installed within the base frame and connected to the support table, which serves a purpose of driving the support table along the longitudinal direction. (2) A horizontal groove was purposely built into the support table for containing the particle bed during pulverizing tests (Figs. 2 and 3). A small width of 32 mm was chosen for the groove in order to obtain a higher pressure stress acting on the feed bed by the overburdened roller weight. The groove is 35 mm in depth, which poses a limit on the maximum layer thickness and the largest vertical motion of the roller part.
Controls on the table speed
Command
Order
Fig. 2. A front view of the roller test machine.
Roller AC servo motor
Controller unit
Support table Groove Liners
Motion control system
Motion monitoring system Computer
Test machine
Digital output
Base frame
Voltage output Photoelectric sensors
Counterweight blocks
Monitoring roller motion
Data acquisition hardware
Fig. 1. The working diagram of the developed roller test machine.
Servo motor Fig. 3. An iso-metric view of the test machine (3D design model).
C.J. Chi et al. / Minerals Engineering 72 (2015) 65–72
(3) A cylinder-shaped steel roller, 200 mm in thickness and 400 mm in diameter, was arranged on the support table with the shaft in a horizontal plane and orthogonal to the advancing direction of the support table (Fig. 3). The roller was designed to be fixed horizontally, and can move up and down freely in the vertical direction by slider (guide) rails (Fig. 4(a)) that were installed on both lateral sides of the base frame. The desired rotation with reference to the central shaft can be achieved when a sufficient large friction action is triggered along the roller-table (or feed bed) interface, which reasonably comes from the advancing or receding of the support table component. Specifically, the original thickness of the roller (200 mm) was narrowed to a much smaller value of 30 mm near the outer surface, as given in Fig. 3, which fits the groove width value, and can reasonably favour the increase of the compression stress on the feed bed to a great extent. The total weight of the roller and the adjunct components, including the shaft, the bearing and the slider rail for controlling the roller motion, is 67 kg. (4) In order to simulate various rolling conditions of different pressure levels in practice, it was designed that counterweight blocks could be added on both sides of the roller shaft by using dual tension rods (Figs. 3 and 4). The counterweight blocks were made of lead material of a big density 11.3 g/mm3, and the maximum surcharge can be as large as 350 kg considering the requirements of a proper operation of the shaft and bearing components. It can be seen that an equivalent roller weight in a range of 67–417 Kg can be chosen for the grinding tests on this machine. According to the simplified analysis results given in the following Table 1, the lower and upper limit of contact pressure stress for a horizontal elastic layer of 5–30 mm in thickness and 500– 1000 MPa in elastic modulus are 4.5 MPa and 17.5 MPa respectively. Displacement sensor Print mark sensor Roller
Slider rails Tension rods Counterweight blocks
(a) A schematic view of the sensors arrangement
(b) The print mark poster on the lateral surface of the roller Fig. 4. The arrangement of photoelectric sensors for monitoring roller motion during pulverizing tests.
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2.2. Control, measurement and data acquisition In addition to the above main mechanical parts of the test machine, two systems, including the motion control system and the motion monitoring system (Fig. 1), were designed for the pulverizing test study. The motion control system can precisely control the horizontal movement of the support table, leading to a rolling compression action by the roller on the table as well as the feed layer on it. By the motion monitoring system it becomes possible to measure the real-time response of the roller’s rotation speed and its vertical displacement during the testing process. The motion control system consists of three main parts, an AC servo motor (Type SGDV-120A01A002000 by Yaskawa Electric (China) Co., Ltd), a system controller unit, and a motion control card (Type MPC08SP by Leetro automation corporation, China). The main supply is three-phase 380 V that is reduced to 220 V by a step-down transformer to power the AC servo motor. For a given motion of the support table, the inputs for the motion control card placed in the computer can be determined and the corresponding control commands are given to the servo motor through the controller unit. As mentioned above, a ball screw links the servo motor and the support table. Reasonably a proper control of the motion of the ball screw can drive the support table according to a prescribed way. The motion monitoring system consists of two types of photoelectric sensors, a data acquisition system, electrical accessories and other input–output devices. Through the introduction of two types of high-performance photoelectric sensors, non-contact measurements of the roller motion become feasible, which is important for investigating the interaction between the roller part and the feed bed. A high resolution (<8 lm) measurement of the vertical up-and-down movement of the roller was designed in our machine using a CP08MHT80 type displacement sensor by Wenglor sensoric GmbH (German), of which the measurement rate can be as high as 600 per second. The displacement sensor was installed onto the frame component of the machine with its transmitter diode normally pointing to the uppermost portion of the roller (Fig. 4(a)). Furthermore, a method was proposed to measure the angular speed of the roller using a print mark sensor of Type WM03PCT2, also by Wenglor sensoric GmbH. This type of sensor is originally designed to recognize contrast marks for sorting, positioning and quality control applications, such as in the printing machines and packaging industry applications in the food, beverage and pharmaceutical industries. Based on its capability for precise positioning of print marks and high frequency response (up to 5000 per second), a non-contact measurement method was proposed to monitor the rotation speed of the roller, which is described as follows: (1) A poster of the same outer diameter as the roller, with 90 black print marks of circular sector shape uniformly distributed along the circumferential direction, as given in Fig. 4(b), was stuck to one lateral surface of the roller. The angle at the centre for each black print mark is 2°. (2) A print mark sensor was installed onto the side frame with its transmitter diode normally pointing to the above poster, and its position was adjusted such that the sensor’s light spot lies within the range of print marks in the radial direction (Fig. 4(a)). By teaching the two grey-scale values of the black mark and the background to the sensor, it can recognize the switching between the black marks and the white background when the roller rotates during grinding tests. (3) The sensor output of voltage versus time would be recorded by a data acquisition system, which is to be described below, and the response of the roller rotation speed with time can then be determined. A typical case of constant roller rotation
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Table 1 Numerical results of the maximum contact pressure stress for a roller statically arranged on a horizontal coal layer. Roller mass (kg)
Maximum contact pressure stress in the contact region (MPa) h = 5 mm
67 210 417
h = 10 mm Ec = 1000 MPa
Ec = 500 MPa
Ec = 1000 MPa
Ec = 500 MPa
Ec = 1000 MPa
4.8 9.2 13.6
6.6 12.3 17.5
4.6 8.9 12.5
6.4 12.1 16.8
4.5 8.5 11.6
6.3 12.0 16.0
speed is schematically shown in Fig. 5, where a high voltage (10 V) indicates the recognition of black print marks, while a low voltage (0 V) means the light spot is on the white background. From the response curve monitored during pulverizing tests, the time variation of the roller angular speed can be simply determined as the speed that the sensor light spot passes the alternating print mark and underground section, that is, by the equation below:
xðti Þ ¼
h = 30 mm
Ec = 500 MPa
p
ð1Þ
90Dt i
in which x(ti) denotes the roller angular speed at the middle point of each zone of constant voltage response lasting for a time interval Dti (i = 1, 2, . . . N). From a series of data points of roller speed versus pulverizing time, the whole development history of roller rotation during grinding tests can be determined. The data acquisition system for the roller test machine includes the data acquisition module and the corresponding software. We chose DAM-3058 by DonghuaTest Ltd. (China) for our machine’s data acquisition module, which can record the analog voltage or current output signal, and the data acquisition sampling frequency can be as high as 12,000 readings per second. The corresponding software allows a real-time visualization of the time variation of sensor signal on a computer displayer.
stress level allowed for the test machine, a series of numerical analyses were conducted in this section using the finite element method. Fig. 6 shows the finite element model for the roller-feed bed-table system. A linear elastic assumption was made for all components, and the elastic parameters for the steel-made roller and table are Es = 210 103 MPa and vs = 0.3. The interaction along the interfaces between the roller and the feed bed as well as the feed bed and the table was modelled by a master–slave contact algorithm, in which a small friction coefficient of 0.2 was adopted. A static equilibrium solution of the roller compressing on the particle bed was investigated. The effects of different test conditions were investigated, including the roller weight, thickness of feed bed and the elastic modulus of the bed material. A range of the modulus of bed material (Ec) of 500–1000 MPa was chosen based on the compression test results of prismatic coal samples. Table 1 lists a summary of the maximum contact pressure stress from the numerical results. The results in Table 1 show that for a feed bed of smaller thickness, a higher contact pressure stress can be mobilized, and the increase of its overall stiffness is also beneficial for the build-up of pressure stress. A range of maximum contact pressure stress between 4.5 and 17.5 MPa can be expected for the grinding tests using the machine on a feed bed of 5–30 mm in thickness and 500–1000 MPa in elastic modulus. The peak contact pressure stress, 17.5 MPa, is deemed to be representative of the grinding conditions in actual pulverizers.
2.3. Numerical analysis of the contact pressure stress by roller compression 3. Roller test study One may note the above test machine’s incapability of measuring the contact pressure stress distribution along the roller-bed interface mobilized by the rolling compression. The difficulty lies in the fact that the contact area shall change with the rolling process and an installation of membrane sensor(s) on the contact surface may disturb the interaction and in turn influence the pulverizing behaviour. In order to give a rough estimate of the contact pressure
3.1. Test cases To verify the effectiveness of the above developed test machine, a series of fundamental tests were conducted. The particle material
12
Δti 10 Roller
Voltage (V)
8
6
4
2
Table
0 0
2
4
6
8
10
feed bed
Time (s) Fig. 5. Time variation of voltage output from the print mark sensor (a particular case of constant roller rotation speed as an instance).
Fig. 6. Finite element modelling of the roller-feed bed-table interaction in grinding tests (coal layer thickness h = 20 mm as an instance).
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C.J. Chi et al. / Minerals Engineering 72 (2015) 65–72 Table 2 The basic properties of selected bituminous coal particle material.
1
Type
HGI
Particle density (kg/ m3)
Initial surface moisture content (%)
Bituminous
60
1388
7.3
Table 3 A summary of elementary test cases by the roller machine. Test case
Layer thickness (mm)
Roller weight (kg)
Table speed (m/s)
Case I Case II Case III
20 20 20
100 200 400
0.1, 1.0 0.1, 1.0 0.1, 1.0
Percentage of passing particle
0.8
W=100kg W=200kg 0.6
W=400kg Feed
0.4
0.2
0 0.05
ω
0.5
Particle size (mm)
(a) v =0.1 m/s
Particles in this range are chosen for size reduction assessment
W 1
θ=30º
Coal bed h
v
0.8
10 mm
>500 mm
Fig. 7. A schematic illustration of the arrangement of feed bed in pulverizing tests.
selected for the tests is a type of bituminous coal from Jiamusi, Heilongjiang Province, China. Table 2 lists the basic properties of the coal particle material. A type of jaw crusher was applied to prepare the feed particles and the output coal particles was scalped by 1.25 mm. The particle size distribution of the feed coal was determined by sieve analyses according to the Chinese standard GB/T 2565–1998. The particle size distribution in mass was found to be 4% up to 0.5 mm, 5% between 0.5 and 0.63 mm, 63% between 0.63 and 1.0 mm, and 28% between 1.0 and 1.25 mm. One can note that the feed samples for the tests contain particles mostly within a narrow range between 0.63 and 1.25 mm, which are similar to the sampling requirements for the common Hardgrove grindability index (HGI) tests (ISO 5074:1994). Typical factors which may influence the breakage efficiency of particle bed by rolling compression were chosen in this study and a summary of the test conditions is given in Table 3. Three different roller weights, 100 kg, 200 kg and 400 kg, were chosen, which respectively represent the minimum, intermediate and maximum bound values. Similarly, two velocities of the support table were chosen, including a lower one of 0.1 m/s and a higher of 1.0 m/s that is normally adopted in pulverizers. A feed bed of 20 mm thickness containing the coal particles was adopted in each test, of which the length of a constant thickness section is 330 mm, and an inclined slope of 30° was arranged at both ends (Fig. 7) for supporting the main particles into a horizontal bed. Only the central portion of the particle bed, as indicated by a red rectangle in Fig. 7, is to be collected for size reduction assessment by sieve tools. The purpose of such a treatment is to exclude the side effect near both ends caused by varying thickness, and the designed test
Percentage of passing particle
Table W=100kg W=200kg 0.6
W=400kg Feed
0.4
0.2
0 0.05
0.5
Particle size (mm)
(b) v =1.0 m/s Fig. 8. Effect of roller weight (W) on the particle size reduction by one-time rolling compression (a–b).
conditions can be representative of the circularly arranged long coal bed under continuous grinding in actual pulverizers. Three tests were conducted for each case as given in Table 3.
3.2. Typical test results 3.2.1. On the particle size reduction Firstly, the roller weight (W) was taken as an independent variable, and Fig. 8 presents the mean test curves of particle size distribution before and after one-time pulverizing process. It can be
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C.J. Chi et al. / Minerals Engineering 72 (2015) 65–72
1
20
16
Roller uplift dispment (mm)
Percentage of passing particle
0.8
v=0.1m/s 0.6
v=1.0m/s Feed
0.4
W=100 kg W=200 kg 12
W=400 kg
8
4
0.2
0 0
1
2
3
4
5
Time (s)
(a) v=0.1 m/s
0 0.05
0.5 20
Particle size (mm)
(a) W=100kg 16
Roller uplift dispment (mm)
1
Percentage of passing particle
0.8
v=0.1m/s
0.6
v=1.0m/s Feed
W=100 kg W=200 kg
12
W=400 kg 8
4
0.4 0 0
0.1
0.2
0.3
0.4
0.5
Time (s)
(b) v=1.0 m/s
0.2
Fig. 10. Comparison of monitored roller vertical displacement during pulverizing tests (h = 20 mm cases were taken as instances).
0 0.05
0.5
Particle size (mm)
(b) W=400kg Fig. 9. Effect of table speed (v) on the particle size reduction by one-time rolling compression (a–b).
seen that reasonably an increase of the roller weight from 100 kg to 400 kg can significantly improve the disintegration intensity, for either a lower or a higher table speed allowable for the machine. For the W = 400 kg case, nearly 30% of the feed particles of 0.63–1.25 mm in size were disintegrated by one-time rolling compression, and the generated products mostly fell in a size range between 0.25 and 0.5 mm. Moreover, the figure shows that for the considered range of roller weight, the products for the finer particles (<0.25 mm) by one-time rolling compression increased by 3%,
which is not so great as expected and the effects of original particle size distribution of the feed need to be further investigated. Secondly, take the speed of the support table (v) as an independent variable, the particle size distribution curves before and after one-time rolling compression are compared in Fig. 9. It can be observed that generally for the chosen layer thicknesses and roller weights, a lower table speed may benefit the mobilization of fine products to a minor extent. The effect of adjusting the table speed within a range of 0–1.0 m/s seems to diminish with the increase of layer thickness from additional test studies. Similarly, from the case of a layer thickness of 20 mm, the results show that with the roller weight enlarged, the influence of the table speed is negligible. Even though only limited cases have been presented in this study, it may be appropriate to propose the adoption of a lower roller speed in actual pulverizers, since it require less energy input without cutting down the products of fine particle size.
C.J. Chi et al. / Minerals Engineering 72 (2015) 65–72
roller angular speed (rad/s)
0.6 0.5 0.4 0.3
W=100kg W=200kg
0.2
W=400kg
0.1 0 0
1
2
3
4
5
0.4
0.5
Time (s)
(a) v=0.1 m/s
Roller angular speed (rad/s)
8 7 6 5 4 W=100kg
3
W=200kg
2
W=400kg 1 0 0
0.1
0.2
0.3
Time (s)
(b) v=1.0 m/s Fig. 11. Comparison of monitored roller angular speed during pulverizing tests (h = 20 mm cases were taken as instances).
3.2.2. On the roller motion measurement The monitoring system in the test machine allows the measurement of roller movement during pulverizing tests on the coal beds. Fig. 10 presents typical results of the roller vertical displacement with time, and the response during the compression on the feed layer was focused. For each case of variable roller weights and table speeds, a feed coal bed of the same geometry (Fig. 7) was carefully prepared before the test, ensuring all the coal beds were approximately of the same compaction degree. The results in Fig. 10 demonstrate that the development of the roller vertical displacement followed a similar variation pattern for all the cases. Each curve is roughly symmetric with respect to the midpoint, which can be attributed to the symmetry of the geometry of the feed layer. It is also shown that the slope of the descending portion is a bit smaller than that of the ascending portion, which can be reasonably explained by the difference in mechanical interaction between the roller and coal bed particles at the two stages. The ascending portion in each curve indicates the climbing of the roller onto the coal bed, and the compression from the roller has a trend of compacting the particle towards the central portion. For the descending portion, differently the roller passed most portion of the coal bed and its compression on the particles arranged on the sloping portion may tend to carry away particles and lengthen the bed to some extent. If an assumption is made that the roller keep in touch with the bed during pulverizing on it, the displacement curves in the above Fig. 10 also equivalently indicate the surface profile of the coal bed
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after one-time rolling compression along the longitudinal section. It can be expected that the rolling compression in each test would decrease the original thickness of the feed bed (h = 20 mm), and the adoption of different roller weights would mobilize different surface settlements of the particle layers. The monitoring results of the roller displacement in Fig. 10 approve the expectations well. From a series of tests using different roller speeds in a range of 0.1– 1.0 m/s, it was found that one-time rolling compression can induce the surface settlement of the feed bed by 3.1–3.9 mm (W = 100 kg), 4.8–5.6 mm (W = 200 kg), and 6.0–7.0 mm (W = 400 kg). Obviously, an increase of the roller weight from 100 kg to 400 kg, which in turn enlarged the contact pressure stress along the interface, can mobilize an extra surface settlement of the coal bed of 3.0–4.0 mm. It was also found that the variation of the roller speed can induce some slight difference in the settlement response. Besides the roller vertical displacement, its rotation speed response with time simultaneously monitored during pulverizing on the feed bed is also shown in Fig. 11. If an assumption is made that no slippage occurs along the two interfaces, namely between the roller and the feed bed, and between the support table and the layer, a linear relationship can be formulated between the given table speed v and the roller rotation speed w as w = v/r, where r indicates the roller radius (200 mm). The curves in Fig. 11 demonstrate that averagely the roller rotation speed follows the linear relationship, which are 0.5 rad/s and 5.0 rad/s for v = 0.1 m/s and v = 1.0 m/s case respectively. Moreover, notable perturbations of the roller speed were detected during pulverizing on the bed, which became more prominent for the case of a larger table speed of v = 1.0 m/s. From these perturbations it can be inferred that during the roller rolling on the feed bed, momentary slippages were triggered along the roller-bed interface for each test, which can be attributed to the complex interaction along the dual interfaces and the nonlinear behaviour of the coal particles forming the feed bed.
4. Discussions and conclusions The typical test results described above clearly demonstrate the capability of the developed machine for conducting pulverizing tests on particle beds under various conditions. In addition to the above elementary functions, further developments shall be made by the authors on the test machine to extend its capabilities, including but not limited to the following: (1) A photoelectric sensor is to be installed onto the side frame for a non-contact measurement of the surface profile of the feed bed. The aim is to determine the geometric feature of the feed bed before and after the rolling compression, which is essential for the evaluation of the energy spent on the internal deformation within the coal layer and the assessment of pulverizing efficiency. (2) A particular working condition of the roller, that is, a pure sliding on the feed bed with rotation around the shaft prohibited, is to be achieved by using caliper brakes for the roller. An initial vertical gap between the roller and the feed bed can also be obtained by lifting the roller with two jacks on both sides. Such a designed condition can somewhat simulate the actual operating condition in vertical-spindle pulverizers (https://www.mhps.com/en/products/detail/coal_pulverizer. html), as the discrepancy between the velocity at the surface of the conical roller and that at the table surface, particular due to the inconsistency near the inner and outer side surfaces, would cause slippage at both lateral sides. The breakage efficiency contributed by the roller sliding action can be investigated by such a test arrangement.
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(3) In actual vertical-spindle pulverizers, it is common to use spring frame hydraulic preload or a spring canister connecting to the journal assembly of each grinding element for extra pressure on the feed bed. The stiffness property of these springs shall influence the grinding performance. Similarly, a spring frame can be added to the above test machine for imposing extra pressure load on the roller shaft, which can provide an opportunity for better investigation into the influence of spring frames of various stiffness properties. Based on the work presented in the above sections, the following conclusions can be drawn in regard to the developed roller test machine: (1) The present roller test machine can consider key variables that may influence grinding efficiency by rolling compression, including the thickness of feed bed, roller weight (or grinding pressure), the velocity of the support table and in turn the roller rotation speed. A special sliding action by constraining the rotation of the roller can also be simulated. The capabilities of the developed machine allows a deeper investigation into the grinding performance of various actual pulverizers through laboratory test study. (2) The roller movements, including its vertical displacements and angular speed mobilized during pulverizing process, can be monitored by photoelectric sensors in a noncontact manner in the developed machine. These measurements can help determine the work for deforming the feed bed as well as that for particle disintegration, which is of great significance for the assessment of the grinding performance. (3) A series of 3-D finite element analyses have been conducted to study the maximum contact pressure stress on an elastic horizontal feed bed allowed by the test machine. For a feed bed of 5–30 mm in thickness and 500–1000 MPa in the modulus, it is found that a maximum contact pressure stress of about 17.5 MPa is available for the test machine.
(4) A series of fundamental rolling tests have been conducted on particle beds belonging to a type of bituminous coal material. The effects of roller weights and the speed of the support table have been studied and reasonable test results were provided. Besides the purposely-designed one-time pulverizing test, the test machine is also capable of doing rolling tests on single particle, and repeated pulverizing tests considering various combinations of controlling factors. Acknowledgements The authors are grateful for the kind support from Mitsubishi Heavy Industries Ltd., Japan. Additionally, the support from the National Natural Science Foundation of China (No. 51479096) and from State key Laboratory of Hydroscience and Engineering (Nos. 2012-KY-04, 2013-KY-02, and 2014-KY-1) as well as the support by Tsinghua University Initiative Scientific Research Program under Grant 20131089285 are gratefully acknowledged. References Barrios, G.K.P., de Carvalho, R.M., Tavares, L.M., 2011. Modeling breakage of monodispersed particles in unconfined beds. Miner. Eng. 24 (3), 308–318. Bemrose, C.R., Bridgwater, J., 1987. A review of attrition and attrition test methods. Powder Technol. 49 (2), 97–126. Bourgeois, F.S., 1993. Single-particle fracture as a basis for microscale modeling of comminution processes. Department of Metallurgical Engineering, University of Utah. Eksi, D., Hakan Benzer, A., Sargin, A., et al., 2011. A new method for determination of fine particle breakage. Miner. Eng. 24 (3), 216–220. Gotoh, K., Masuda, H., Higashitani, K., 1997. Powder Technology Handbook, 2nd ed. Marcel Dekker, New York. ISO 5074:1994 Hard coal – determination of Hardgrove grindability index. Liu, J., Schönert, K., 1996. Modelling of interparticle breakage. Int. J. Miner. Process. 44, 101–115. Paramanathan, B.K., Bridgwater, J., 1983. Attribution of solids—I: cell development. Chem. Eng. Sci. 38 (2), 197–206. Sankara NS. Australasian institute of mining and metallurgy bulletin and proceedings 1985; 291(4). Schaefer, H.U., 2001. Loesche vertical roller mills for the comminution of ores and minerals. Miner. Eng. 14 (10), 1155–1160. Tavares, L.M., 1999. Energy absorbed in breakage of single particles in drop weight testing. Miner. Eng. 12 (1), 43–50.