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Enrichment effect of coal and quartz particles in gas-solid fluidized bed with applied electrical field
Powder Technology 354 (2019) 743–749
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Enrichment effect of coal and quartz particles in gas-solid fluidized bed with applied electrical field Haifeng Wang a,b,⁎, Xuejie Bai b, Zhen Peng b, Xiaolu Zhao b, Jinshan Yang b, Shuai Wang c, Yaqun He a,b,c a b c
Key Laboratory of Coal Processing and Efficient Utilization (China University of Mining and Technology), Ministry of Education, Xuzhou, Jiangsu 221116, China School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China
a r t i c l e
i n f o
Article history: Received 16 September 2018 Received in revised form 14 June 2019 Accepted 24 June 2019 Available online 27 June 2019 Keywords: Fine coal Fluidization charging Triboelectrostatic separation Enrichment
a b s t r a c t Electrostatic effects are commonly found in gas-solid fluidized beds. With an electrical field applied in a gas-solid fluidized bed, the charged particles there would be separated according to the opposite charge polarity. A chargeto-mass ratio test showed that quartz can easily be given a negative charge when tribo-charging with coal. A binary particle mixture was employed to explore the enrichment effect in an electrical-field fluidized bed unit. The results showed that the mass fraction of quartz in the products decrease gradually from negative plate to positive plate. The increase of fluidization time, plate voltage and gas velocity, along with low bed height, were beneficial to the improvement of the enrichment effect. The mass fraction of quartz close to the positive plate was N40%, while that of negative was around 10%. This work demonstrates the possibility of separating fine coal using a fluidized bed with applied electrical field. © 2019 Elsevier B.V. All rights reserved.
1. Introduction As one of the most prevalent fossil energies around the world, coal is widely employed in many industries, such as thermal coal in power plants and metallurgical coal in coking plants [1]. The requirement for coal production varies from field to field, for example, thermal power plants need coal with a high heating value while steel manufacturers require coking coal with low ash content and high coking properties [2]. Meanwhile, in recent years, excessive coal consumption and associated pollution have focused attention on sustainability problems related to resources, environment and society [3,4]. Therefore, in order to meet the requirements and improve coal quality, coal needs to be processed before it can be used in other industries [2]. There are several beneficiation techniques employed in coal preparation, which include such as gravity-based separation, dense medium separation and froth flotation [1,5]. However, these techniques rely on sufficient water to maintain a high efficiency process. This causes a series of subsequent problems, such as dewatering of the coal slurry, reagent pollution and limited application in freezing cold and arid areas. Therefore, a dry coal beneficiation technique has been proposed as an
⁎ Corresponding author at: Key Laboratory of Coal Processing and Efficient Utilization (China University of Mining and Technology), Ministry of Education, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.powtec.2019.06.046 0032-5910/© 2019 Elsevier B.V. All rights reserved.
environmentally friendly and efficient way to cut operation cost and reduce the consumption of water. Electrostatic beneficiation is a process that makes use of the electrical field forces exerted on charged or polarised bodies [6]. Recent work using electrostatic methods for cleaning coal have focused on triboelectric charging of coal particles followed by separation of the dry charged materials in a static high voltage field [7]. Therefore, particles should be electrostatically charged by contact, friction or rubbing with other particles or walls prior to the separation stage [8]. There are many ways to charge particles for triboelectrostatic separation, for example, conveyer belt charging [9,10], cyclone charging [11], and drum charging [12], to separate materials such as plastics [9–11,13,14] and coal [7,12,15–17]. Moreover, tribocharging in gas-solid fluidized beds has also been employed for solid separation in recent years [18–20]. Tribocharging of particles in gas-solid fluidized beds has been exploited in a number of useful applications, including powder coating [21,22] for solid separation of such as coal [4,7,17], fly ash [18] and plastic particles [14,23]. Gas-solid fluidized beds, by their nature, are associated with continuous solid-solid contact and separation, as well as friction, as particles rub against each other and against the wall [8]. When two initially neutral material surfaces are brought into contact and then separated, charge is transferred so that one material is charged positively while the other is charged negatively, due to the different work functions of the two surfaces [24,25]. Therefore, we take advantage of the work function difference between coal and accompanying impurities, to charge them with opposite polarity in a gas-solid fluidized
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Fig. 1. Working principle of particle charging and separation in fluidized bed.
bed, and subsequently, to separate coal from gangue minerals in an electrical field generated between vertical electrode plates. For this study, we proposed a device that integrates triboelectrostatic separation and a gas-solid fluidized bed to explore preferable operating parameters for coal-quartz mixture separation. X-ray diffraction (XRD) and X-ray fluorescence (XRF) were performed to obtain substance and elemental composition of the coal samples. Subsequently, charge-to-mass ratio tests were performed to verify the charging property of quartz after tribocharging with coal. A set of single factor experiments were conducted to determine the effect of operating parameters on enrichment of the coal-quartz mixture. And raw coal samples were also employed to verify the possibility of using this method to separate coal from its gangue minerals.
Fig. 3. X-ray diffractogram of coal sample.
2.2. Equipment A bench-scale fluidized bed system, shown in Fig. 2, consisted of gas supply devices and gas-solids fluidized bed with mounted electrode plates. The injection of air was provided by a Roots blower and was monitored using a float flowmeter. The fluidized bed was made of Plexiglass, and was 200 mm high. The gas distributor, mounted at the bottom of the fluidized bed, mainly contained a 200 mm × 100 mm perforated-plate with a large number of nanopores. Both sides of the fluidized bed were equipped with electrode plates.
2.3. Materials 2. Experimental 2.1. Working principle Fig. 1 shows the basic principle of solid separation in fluidized beds with mounted electrode plates. Solid particles are transformed to a fluid-like state by upward-flowing gas [26], causing charge transfer among particles due to their intense and frequent collisions. Solid particles with lower surface work function were prone to be positively charged (blue in Fig. 1), while those with higher work function were inclined to gain negative charges (red in Fig. 1). Under the electrical field generated by electrode plates, positively charged particles would move toward the negative electrode plate, while the negatively charge ones would move to the positive.
Anthracite coal was used as the sample for XRD. In order to determine the elemental composition of the anthracite coal, XRD was conducted using a Bruker D8 Advance XRD with a copper anode at 40 kV. The X-ray intensities were measured in the range of 3° ≤ 2θ ≤ 70° with step size of 0.019° 2θ. The resulting diffractograms were further smoothened and analysed using the phase quantification method. Fig. 3 shows X-ray diffractogram of the anthracite coal. It is clear from Fig. 3 that several main peaks in the diffraction curve correspond to quartz, calcite and kaolinite. Furthermore, an elemental assay test was conducted using XRF (Bruker S8 Tiger). Table 1 shows the elemental content of anthracite coal sample, which included minerals in the sample composed of Si, Al and Ca, etc. Specifically, “CO3” in Table 1 represents the total amounts
Fig. 2. Schematic of overall layout of fluidization triboelectrostatic separator.
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Table 1 Elemental contents of the coal sample. Elements
TiO2
S
K2O
MgO
Fe2O3
CaO
Al2O3
SiO2
CO3
Content(%)
0.219
0.243
0.303
0.34
0.847
1.43
4.35
9.02
82
of carbon existed in various forms. Combined with the XRD results, we were able to calculate that the contents of quartz, calcite and kaolinite in the anthracite coal was 3.84%, 2.38% and 9.53%, respectively. Given that the minerals in the anthracite coal represent the impurities found in most coal samples, an experimental sample was prepared by mixing low-ash clean coal with quartz. The binary particle samples used in the operating condition experiments were made of 75% lowash clean coal and 25% quartz by weight. The coal sample, provided by the Taixi coal preparation plant (Ningxia, China) was clean coal with 2.87% ash content. The quartz sample was 94.98% in purity, with 0.7% of loss on ignition. The sample was crushed and ground in an agate mortar, and then was wet screened to obtain a binary mixture with a size range of −0.2 + 0.045 mm. 2.4. Charge-to-mass ratio tests To verify the charging characteristics of the quartz after shaking and tribocharging with coal, charge-to-mass ratio tests were conducted with the device shown in Fig. 4. The charge-to-mass ratio test system consisted of a shaking table, a Faraday cup, a screen and an electrometer. The screen mesh size was 0.355 mm, which is smaller than the size of the coal samples (−0.6 + 0.5 mm) but larger than that of the wetscreened quartz samples, namely, 0.045 mm, −0.074 + 0.045 mm, −0.125 + 0.074 mm and −0.2 + 0.125 mm. This was done to ensure that the quartz particles could fall into the Faraday cup while the coal particles remained above the screen. A sufficient number of coal particles was spread on the surface of the screen and charged by shaking with a small quantity of quartz particles. Subsequently, the quartz particles dropped into the Faraday cup and were measured using a Keithley 6514 electrometer.
Fig. 5. Diagram of product collector.
(4) Experiment 4: Vary the fluidization time between 1 min and 45 min, at U = 60 kV, υ = 16.53 cm/s and H = 5 mm. To verify that this method is also feasible for raw coal enrichment, coal samples (ash content:15.61%) were employed to conduct anther set of expeiments. The operating conditions were: U = 60 Kv, υ = 16.53 cm/s, H = 5 cm. This experiment were repeated for three times. The ambient conditions were stable: temperature T = 20 ± 2 °C and relative humidity RH = 35 ± 5%. As shown in Fig. 5, the collector contained seven cubicles numbered sequentially from #1 to #7, with #1 cubicle being close to the positive plate while #7 was close to the negative one. The collector (length: 200 mm, width: 90 mm) was designed to perfectly match the fluidized bed (length 210 mm, width: 100 mm), so that the particles could be recovered after the separation process. That is, once enrichment process finished, the collector were inserted into the fluidized bed from the top, where the original bed cover was dismantled. The products in the bed were divided by clapboards in the collector. Subsequently, the fluidized bed was reversed to collect particles into the collector, which was then retracted from the bed. By weighing the output in each cubicle and testing its ash content, the corresponding mass fraction of quartz, which was employed as a significant factor for evaluating the enrichment effect, could be obtained. 3. Results and discussion
2.5. Operating condition experiments 3.1. Charge-to-mass ratio tests A set of single factor experiments were performed to explore the effect of operating conditions on the enrichment of the coal and quartz content in a gas-solid fluidized bed with applied electrical field. Specifically, we defined the domain of variation of the operating parameters: high voltage as U; gas velocity as υ; static bed height as H and fluidized time as t. All single factor experiments included the following operations.
The charge-to-mass ratios of quartz particles after contact with, and separation from, coal particles are shown in Fig. 6 (error bars: Mean ± SD) from three repeated experiments. With increasing particle size, the charge-to-mass ratio of the quartz particles increased slightly at
(1) Experiment 1: Vary the high voltage between 20 kV to 60 kV, at υ = 16.53 cm/s, t = 30 min and H = 5 mm. (2) Experiment 2: Vary the superficial gas velocity between 3.31 cm/s and 16.53 cm/s, at U = 60 kV, t = 30 min and H = 5 mm. (3) Experiment 3: Vary the static bed height between 2.5 mm and 20 mm, at U = 60 kV, t = 30 min and υ = 16.53 cm/s.
Fig. 4. Schematic diagram of charge-to-mass ratio test system. (Screen: 200 mm in diameter; Faraday cup: 180 mm in diameter, 150 mm in height)
Fig. 6. Charge-to-mass ratio of quartz particles in different size range charging with coal particles.
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first and then dropped rapidly. A plausible reason for this phenomenon is that the rising quartz particle size led to decline of the specific surface area, which consequently resulted in decrease of the collision probability. However, in the case of particles smaller than 0.045 mm, the agglomeration and adhesion of fine particles led to decrease of the valid collision number of quartz particles, and further decrease of the charge-to-mass ratio. However, coal-quartz collisions are just a part of charging process, and it should be noted that particle-wall collisions are also ubiquitous in fluidized bed. Particle-wall collision also has an impact on coalquartz charging process. Hence, the presence of walls in fluidized bed could finally interfere outcomes of fine coal in separation process. But it is hard to determine what is the proportion of particle-wall collision accounts for in charging process. In this work, we vaguely regard particle-particle collisions as the main source of charge. 3.2. Operating condition experiments Previous research has demonstrated that particle charging and separation processes are significantly influenced by plate voltage [27], superficial gas velocity [28] and fluidization time [29]. Meanwhile, as a critical parameter in a fluidized bed, the static bed height greatly influenced the enrichment effect of the binary particle mixture. In this work, we carried out a set of experiments to explore the effect of operating conditions, including plate voltage, superficial gas velocity, fluidization time and static bed height, on the enrichment of coal and quartz particles. 3.2.1. The effect of plate voltage The fluidized bed with applied electrical field could be regarded as a electrical condenser, and coal-quartz mixture can be seen as charged particles. In this model, electrical field force (F), particle charge (q) and plate voltage (V) follow these relationships: F ¼ qE
ð1Þ
E ¼ U=d
ð2Þ
force is proportional to its acceleration, that is, the charged particles could move further in an electrical field with a higher plate voltage. The motion of particles in this fluidized beds results from a balance between gravitational force, drag force, electric field force and interparticle forces such as electrostatic exerted on the particles [8]. On one hand, electrostatic charges can change the forces on particles, thereby inducing particle-wall adhesion, inter-particle cohesion, and agglomeration, which can further affect electrostatic phenomena in the bed. On the other hand, charged particles are separated in the electrical field generated between vertical electrode plates according to the magnitude and polarity of their charges by being deflected toward the positive or negative electrode. The results of the varying plate voltage experiments are shown in Fig. 7. Here, the horizontal axis refers to the cubicle numbers marked in Fig. 5, while the vertical axis represents the mass fraction of quartz output. As shown in Fig. 7, with greater voltage, quartz particles were more attracted by the positive plate, while coal particles were more likely to move to the negative one, given the polarity difference after contact charging of the two different minerals. Hence, in general, the mass fraction of quartz decreased gradually when close to the negative plate. With voltage of 20 kV and 30 kV, few evident changes were found in the distribution of coal and quartz, showing that the enrichment of coal and quartz was minimal with relatively low voltage. However, with increase of plate voltage, the mass fraction of quartz particles near the positive plate increased while that near the negative plate decreased. At 60 kV, the mass fraction of quartz in cubicle #1 was 42.76% while it was 11.21% in cubicle #7: a 13.79% reduction from the proportions of the binary mixture. These results furtherly demonstrated that the applied high voltage had an impressive enrichment effect on the coal and quartz particles in the mixture due to the electrical field force applied to the particles charged with opposite polarity. Within a consistent atmosphere, the increasing plate voltage might lead to an intense electrical field, which would also increase the electrical field force applied to the particles. Therefore, the more intense the plate voltage applied to the fluidization bed, the better the enrichment effect of coal and quartz particles would be.
where, F refers to electrical field force, q represents particle charge, E implies intensity of electrical field, U is short for plate voltage and d is the distance between two positive and negative plates. It can be concluded from Eqs. (1) to (2) that, higher plate voltage could lead to more intensified electrical field, which furthurly results in a greater electrical field force. With a given particle, electrical field
3.2.2. The effect of gas velocity Gas velocity can be an important factor in causing buildup of electrostatic charges in fluidized beds, and it may also play a significant role in determining carryover of particles from gas-fluidized beds. Moughrabiah [28] has found that, as the gas velocity increased, the degree of
Fig. 7. Mass fraction of quartz along the direction of the electric field at different voltage. (Superficial gas velocity: 16.53 cm/s; Fluidization time: 30 min; Static bed height: 5 mm.)
Fig. 8. Mass fraction of quartz along the direction of the electric field for different gas velocity. (Plate voltage: 60 kV; Fluidization time: 30 min; Static bed height: 5 mm.)
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mixture would also increase. However, higher gas velocity may also produce ultra-fine coal particles, due to the fact that coal is a brittle mineral. Ultra-fine coal particles are very light and difficult to drop into the collector with the present of airflow in fluidized bed, even though they have a larger charge-to-mass ratio.
Fig. 9. Mass fraction of quartz along the direction of the electric field for different static bed height. (Plate voltage: 60 kV; Superficial gas velocity: 16.53 cm/s; Fluidization time: 30 min.)
bed electrification also increased, probably due to stronger interactions among particles and between particles and the wall caused by an increase in bubble rise velocity, frequency, or gas flow rate. Mehrani [30] found that at higher gas velocities, the fines entrained from the bed carried opposite charges, leaving behind a net charge of opposite polarity inside the bed. The results of the experiments varying gas velocity are shown in Fig. 8. We determined that the mass fraction of quartz barely changed at relatively low gas velocity. However, with increase of the gas velocity, the mass fraction of quartz close to the positive plate increased while that near the negative plate decreased. At 16.53 cm/s, the mass fraction of quartz in cubicle #1 was 41.64%, but only 10.31% in cubicle #7. The gas velocity is a significant factor for evaluating particle charging. Increase of the superficial gas velocity results in more violent particle movement and higher probability of particle collision in the fluidized bed; hence, particles in the fluidized bed would be expected to charge more intensively. Meanwhile, another plausible explanation of this phenomenon may be that the increasing gas velocity leads to reduction of the interaction force between particles. Therefore, with increase of the gas velocity, the enrichment effect of coal and quartz particles in the
Fig. 10. Mass fraction of quartz along the direction of the electric field for different fluidization time. (Plate voltage: 60 kV; Superficial gas velocity: 16.53 cm/s; Static bed height: 5 mm.)
3.2.3. The effect of static bed height Static bed height, to same extent, determines the total amounts of particles existing in the fluidized bed. Since the charging process and the separation process take place simultaneously, the total mass of coal-quartz mixture should not be too large nor too small. That is, with small amounts of coal and quartz particles, there would not be sufficient collisions among particles, leading to a poor outcome of coal separation. On the contrary, with excess particles, the charged particles could be obstructed by other particles when moving toward the plate. The results of experiments varying the static bed height are shown in Fig. 9. We can see from Fig. 9 that with the condition of 2.5 mm static bed height, the mass fraction of the quartz particles in cubicle #1 was 45%, while it was b10% in cubicle #7. This all shows that coal and quartz particles could be significantly enriched using a shallow bed height. When the static bed height was increased to 20 mm, the mass fraction of quartz particles in cubicle #1 was as low as 30%, and there were minimal changes near the negative plate. This set of experiments clearly reflects that this kind of fluidized bed is effective with shallow beds of material, and that the application of this enrichment approach to deeper beds of material remains to be explored. 3.2.4. The effect of fluidization time In this work, fluidization time is roughly equivalent to the working time of air supply system, and could also be known as solid residence time. It could be easily assumed that the longer the fluidization time is, the higher the probability of collision among particles would be. Therefore, particles collide with each other could obtain the opposite charges. Simultaneously, the present of external electrical field make it possible for charged particles to move toward plate. The results of the experiments with varying fluidization time are shown in Fig. 10. We can conclude from Fig. 10 that with short fluidization time, the mass fraction of quartz particles in positively charged areas was not relatively high; however, with increase in the fluidization time, the enrichment effect seemed more obvious. After 15 min, there were no obvious changes in positively charged areas. As the fluidization time increased to 45 min, the mass fraction of quartz in cubicle #1 reached 45%, while it was b8% in cubicle #7.
Fig. 11. Ash content and yield of products from each cubicle using raw coal sample. (Plate voltage: 60 kV; Superficial gas velocity: 16.53 cm/s; Static bed height: 5 mm; fluidization time: 30 min.)
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Computational simulations of fluidized beds with electrostatic effects have indicated that charged particles are subject to axial and lateral segregation because of the non-uniform potential distribution and particle charging in the bed [31]. Positive and negative electrical currents can be traced in different locations of the bed, with opposite polarities associated with different zones, demonstrating the interplay between hydrodynamic and electrification phenomena [8]. Due to the presence of adhesive forces between particles and the walls arising from electrostatic forces, particle motion is significantly hindered. However, the extend of fluidization time was likely to reduce the effect of this phenomenon and the continuous fluidization of particles could enhance the charging procedure and therefore achieve better enrichment effect of the charged particles in the mixture. 3.3. Enrichment of raw coal Given that remarkable results were obtained previously, we therefore extended our work to raw coal. As shown in Fig. 11, the ash content of each product decreased gradually from the positive plate to the negative one, except that an increase was obtained when closing to the negative plate. This is because after crushing, grinding and screening process, mineral particles could be almost fully dissociated from coal particles. With the presence of air flow, solid particles turned into fluidization state and were contacting and rubbing with each other, therefore charge transfer (or electron or material transfer) took place between particle surfaces due to the work function differences between coal and mineral surfaces. It was found that associated minerals, specifically quartz, were negatively charged after contact and frictional electrification with coal particles, therefore, coal particles were positively charged. Mineral particles with net negative charge were inclined to move to positive plate, leading to the ash content increase near the positive plate. While coal particles charged positively have a tendency to move to negative plate, resulting in low ash content near the negative plate. Therefore, ash content of product in each cubicle shows a modest decrease trend from positive plate to negative one. The reason of slight increase in cubicle #7 is that positively charged particles moving toward negative plate collided with the plate and furtherly bounced into the cubicle #6, making it the lowest ash content over other cubicles. As for yield of products from each cubicle, shown in Fig. 11, it is found that cubicle #1, which is close to the positive plate, yield more than a quarter of total amounts of product. While little difference were found among yields of other cubicles. Combined with ash content results, it could be concluded that most of associated minerals in coal samples were inclined to move toward positive plate. Therefore, it is feasible to remove minerals from coal using fluidized bed with applied electrical field. 4. Conclusion In this study, we proposed a novel device that applied high voltage to a fluidized bed to study the enrichment effect on a mixture of coal and quartz particles according to the difference of charging properties between two kinds of materials. The XRD test illustrates that the main impurities of the coal samples applied in this work were quartz, calcite and kaolinite (3.84%, 2.38% and 9.53%, respectively), as determined from the XRF results. The charge-to-mass ratio tests were conducted to verify the charging characteristics of quartz when contacting and then separating from coal particles. The results showed that the particle size of quartz plays an important role in tribocharging. Decreasing the particle size increases the specific surface area, leading to enhancement of the collision probability of coal and quartz particles. However, with particle size b0.045 mm, due to agglomeration and adhesion of extremely small particles, the valid collision number of quartz particles may decrease and actually lower the charge-to-mass ratio.
The operating conditions, namely gas velocity, static bed height and plate voltage, significantly influenced the enrichment of coal and quartz particles in the fluidized bed. The particle enrichment effect was evidently improved with increased plate voltage and gas velocity, and with less static bed height. The conditions of 60 kV plate voltage, 16.53 cm/s gas velocity and 5 mm static bed height were found to be feasible operating parameters for separating coal particles from those of gangue minerals. Under the feasible conditions mentioned above, with increase of the fluidization time, the enrichment effect of minerals first increased; then levelled off. With 15 min fluidization time, particles in the fluidization bed generally completed the enrichment process. The ash content of the materials in cubicle #7 was 10%, which is approximately 15% less than that of the binary particle mixture. This shows an obvious enrichment effect that reduced the ash content. Besides, experiments with raw coal are also consistent with results obtained above: ash content could be reduced from 15.61% to 7.29%, and more than a quarter of yields were high ash content products. The results from this research clearly indicate that it is feasible to separate fine coal particles from those of gangue minerals by applying high voltage to a fluidized bed. A couple of measures should be taken to converted this process into an pilot scale test, in order to separate solid particles continuously and steadily. Firstly, nitrogen should be introduced as gas supply, to reduce relative humidity in the bed. In addition, relative humidity in the atmosphere have large impact on charging process [32,33]. Therefore, the cost of reducing humidity could be great. And it is also the main constraint on converting our laboratory system to an industrialized one. Secondly, a device that integrates circulating fluidized bed should be introduced to our system, in order to reintroduce the middling products to fluidized bed and enhance the separation process. Meanwhile, improvements should be made on the product collecting approach to remove the products continuously, instead of halting the system to collect final product in this work.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Enrichment effect of coal and quartz particles in gas-solid fluidized bed with applied electrical field Haifeng Wang a,b,⁎, Xuejie Bai b, Zhen Peng b, Xiaolu Zhao b, Jinshan Yang b, Shuai Wang c, Yaqun He a,b,c a b c
Key Laboratory of Coal Processing and Efficient Utilization (China University of Mining and Technology), Ministry of Education, Xuzhou, Jiangsu 221116, China School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China
a r t i c l e
i n f o
Article history: Received 16 September 2018 Received in revised form 14 June 2019 Accepted 24 June 2019 Available online 27 June 2019 Keywords: Fine coal Fluidization charging Triboelectrostatic separation Enrichment
a b s t r a c t Electrostatic effects are commonly found in gas-solid fluidized beds. With an electrical field applied in a gas-solid fluidized bed, the charged particles there would be separated according to the opposite charge polarity. A chargeto-mass ratio test showed that quartz can easily be given a negative charge when tribo-charging with coal. A binary particle mixture was employed to explore the enrichment effect in an electrical-field fluidized bed unit. The results showed that the mass fraction of quartz in the products decrease gradually from negative plate to positive plate. The increase of fluidization time, plate voltage and gas velocity, along with low bed height, were beneficial to the improvement of the enrichment effect. The mass fraction of quartz close to the positive plate was N40%, while that of negative was around 10%. This work demonstrates the possibility of separating fine coal using a fluidized bed with applied electrical field. © 2019 Elsevier B.V. All rights reserved.
1. Introduction As one of the most prevalent fossil energies around the world, coal is widely employed in many industries, such as thermal coal in power plants and metallurgical coal in coking plants [1]. The requirement for coal production varies from field to field, for example, thermal power plants need coal with a high heating value while steel manufacturers require coking coal with low ash content and high coking properties [2]. Meanwhile, in recent years, excessive coal consumption and associated pollution have focused attention on sustainability problems related to resources, environment and society [3,4]. Therefore, in order to meet the requirements and improve coal quality, coal needs to be processed before it can be used in other industries [2]. There are several beneficiation techniques employed in coal preparation, which include such as gravity-based separation, dense medium separation and froth flotation [1,5]. However, these techniques rely on sufficient water to maintain a high efficiency process. This causes a series of subsequent problems, such as dewatering of the coal slurry, reagent pollution and limited application in freezing cold and arid areas. Therefore, a dry coal beneficiation technique has been proposed as an
⁎ Corresponding author at: Key Laboratory of Coal Processing and Efficient Utilization (China University of Mining and Technology), Ministry of Education, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.powtec.2019.06.046 0032-5910/© 2019 Elsevier B.V. All rights reserved.
environmentally friendly and efficient way to cut operation cost and reduce the consumption of water. Electrostatic beneficiation is a process that makes use of the electrical field forces exerted on charged or polarised bodies [6]. Recent work using electrostatic methods for cleaning coal have focused on triboelectric charging of coal particles followed by separation of the dry charged materials in a static high voltage field [7]. Therefore, particles should be electrostatically charged by contact, friction or rubbing with other particles or walls prior to the separation stage [8]. There are many ways to charge particles for triboelectrostatic separation, for example, conveyer belt charging [9,10], cyclone charging [11], and drum charging [12], to separate materials such as plastics [9–11,13,14] and coal [7,12,15–17]. Moreover, tribocharging in gas-solid fluidized beds has also been employed for solid separation in recent years [18–20]. Tribocharging of particles in gas-solid fluidized beds has been exploited in a number of useful applications, including powder coating [21,22] for solid separation of such as coal [4,7,17], fly ash [18] and plastic particles [14,23]. Gas-solid fluidized beds, by their nature, are associated with continuous solid-solid contact and separation, as well as friction, as particles rub against each other and against the wall [8]. When two initially neutral material surfaces are brought into contact and then separated, charge is transferred so that one material is charged positively while the other is charged negatively, due to the different work functions of the two surfaces [24,25]. Therefore, we take advantage of the work function difference between coal and accompanying impurities, to charge them with opposite polarity in a gas-solid fluidized
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Fig. 1. Working principle of particle charging and separation in fluidized bed.
bed, and subsequently, to separate coal from gangue minerals in an electrical field generated between vertical electrode plates. For this study, we proposed a device that integrates triboelectrostatic separation and a gas-solid fluidized bed to explore preferable operating parameters for coal-quartz mixture separation. X-ray diffraction (XRD) and X-ray fluorescence (XRF) were performed to obtain substance and elemental composition of the coal samples. Subsequently, charge-to-mass ratio tests were performed to verify the charging property of quartz after tribocharging with coal. A set of single factor experiments were conducted to determine the effect of operating parameters on enrichment of the coal-quartz mixture. And raw coal samples were also employed to verify the possibility of using this method to separate coal from its gangue minerals.
Fig. 3. X-ray diffractogram of coal sample.
2.2. Equipment A bench-scale fluidized bed system, shown in Fig. 2, consisted of gas supply devices and gas-solids fluidized bed with mounted electrode plates. The injection of air was provided by a Roots blower and was monitored using a float flowmeter. The fluidized bed was made of Plexiglass, and was 200 mm high. The gas distributor, mounted at the bottom of the fluidized bed, mainly contained a 200 mm × 100 mm perforated-plate with a large number of nanopores. Both sides of the fluidized bed were equipped with electrode plates.
2.3. Materials 2. Experimental 2.1. Working principle Fig. 1 shows the basic principle of solid separation in fluidized beds with mounted electrode plates. Solid particles are transformed to a fluid-like state by upward-flowing gas [26], causing charge transfer among particles due to their intense and frequent collisions. Solid particles with lower surface work function were prone to be positively charged (blue in Fig. 1), while those with higher work function were inclined to gain negative charges (red in Fig. 1). Under the electrical field generated by electrode plates, positively charged particles would move toward the negative electrode plate, while the negatively charge ones would move to the positive.
Anthracite coal was used as the sample for XRD. In order to determine the elemental composition of the anthracite coal, XRD was conducted using a Bruker D8 Advance XRD with a copper anode at 40 kV. The X-ray intensities were measured in the range of 3° ≤ 2θ ≤ 70° with step size of 0.019° 2θ. The resulting diffractograms were further smoothened and analysed using the phase quantification method. Fig. 3 shows X-ray diffractogram of the anthracite coal. It is clear from Fig. 3 that several main peaks in the diffraction curve correspond to quartz, calcite and kaolinite. Furthermore, an elemental assay test was conducted using XRF (Bruker S8 Tiger). Table 1 shows the elemental content of anthracite coal sample, which included minerals in the sample composed of Si, Al and Ca, etc. Specifically, “CO3” in Table 1 represents the total amounts
Fig. 2. Schematic of overall layout of fluidization triboelectrostatic separator.
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Table 1 Elemental contents of the coal sample. Elements
TiO2
S
K2O
MgO
Fe2O3
CaO
Al2O3
SiO2
CO3
Content(%)
0.219
0.243
0.303
0.34
0.847
1.43
4.35
9.02
82
of carbon existed in various forms. Combined with the XRD results, we were able to calculate that the contents of quartz, calcite and kaolinite in the anthracite coal was 3.84%, 2.38% and 9.53%, respectively. Given that the minerals in the anthracite coal represent the impurities found in most coal samples, an experimental sample was prepared by mixing low-ash clean coal with quartz. The binary particle samples used in the operating condition experiments were made of 75% lowash clean coal and 25% quartz by weight. The coal sample, provided by the Taixi coal preparation plant (Ningxia, China) was clean coal with 2.87% ash content. The quartz sample was 94.98% in purity, with 0.7% of loss on ignition. The sample was crushed and ground in an agate mortar, and then was wet screened to obtain a binary mixture with a size range of −0.2 + 0.045 mm. 2.4. Charge-to-mass ratio tests To verify the charging characteristics of the quartz after shaking and tribocharging with coal, charge-to-mass ratio tests were conducted with the device shown in Fig. 4. The charge-to-mass ratio test system consisted of a shaking table, a Faraday cup, a screen and an electrometer. The screen mesh size was 0.355 mm, which is smaller than the size of the coal samples (−0.6 + 0.5 mm) but larger than that of the wetscreened quartz samples, namely, 0.045 mm, −0.074 + 0.045 mm, −0.125 + 0.074 mm and −0.2 + 0.125 mm. This was done to ensure that the quartz particles could fall into the Faraday cup while the coal particles remained above the screen. A sufficient number of coal particles was spread on the surface of the screen and charged by shaking with a small quantity of quartz particles. Subsequently, the quartz particles dropped into the Faraday cup and were measured using a Keithley 6514 electrometer.
Fig. 5. Diagram of product collector.
(4) Experiment 4: Vary the fluidization time between 1 min and 45 min, at U = 60 kV, υ = 16.53 cm/s and H = 5 mm. To verify that this method is also feasible for raw coal enrichment, coal samples (ash content:15.61%) were employed to conduct anther set of expeiments. The operating conditions were: U = 60 Kv, υ = 16.53 cm/s, H = 5 cm. This experiment were repeated for three times. The ambient conditions were stable: temperature T = 20 ± 2 °C and relative humidity RH = 35 ± 5%. As shown in Fig. 5, the collector contained seven cubicles numbered sequentially from #1 to #7, with #1 cubicle being close to the positive plate while #7 was close to the negative one. The collector (length: 200 mm, width: 90 mm) was designed to perfectly match the fluidized bed (length 210 mm, width: 100 mm), so that the particles could be recovered after the separation process. That is, once enrichment process finished, the collector were inserted into the fluidized bed from the top, where the original bed cover was dismantled. The products in the bed were divided by clapboards in the collector. Subsequently, the fluidized bed was reversed to collect particles into the collector, which was then retracted from the bed. By weighing the output in each cubicle and testing its ash content, the corresponding mass fraction of quartz, which was employed as a significant factor for evaluating the enrichment effect, could be obtained. 3. Results and discussion
2.5. Operating condition experiments 3.1. Charge-to-mass ratio tests A set of single factor experiments were performed to explore the effect of operating conditions on the enrichment of the coal and quartz content in a gas-solid fluidized bed with applied electrical field. Specifically, we defined the domain of variation of the operating parameters: high voltage as U; gas velocity as υ; static bed height as H and fluidized time as t. All single factor experiments included the following operations.
The charge-to-mass ratios of quartz particles after contact with, and separation from, coal particles are shown in Fig. 6 (error bars: Mean ± SD) from three repeated experiments. With increasing particle size, the charge-to-mass ratio of the quartz particles increased slightly at
(1) Experiment 1: Vary the high voltage between 20 kV to 60 kV, at υ = 16.53 cm/s, t = 30 min and H = 5 mm. (2) Experiment 2: Vary the superficial gas velocity between 3.31 cm/s and 16.53 cm/s, at U = 60 kV, t = 30 min and H = 5 mm. (3) Experiment 3: Vary the static bed height between 2.5 mm and 20 mm, at U = 60 kV, t = 30 min and υ = 16.53 cm/s.
Fig. 4. Schematic diagram of charge-to-mass ratio test system. (Screen: 200 mm in diameter; Faraday cup: 180 mm in diameter, 150 mm in height)
Fig. 6. Charge-to-mass ratio of quartz particles in different size range charging with coal particles.
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first and then dropped rapidly. A plausible reason for this phenomenon is that the rising quartz particle size led to decline of the specific surface area, which consequently resulted in decrease of the collision probability. However, in the case of particles smaller than 0.045 mm, the agglomeration and adhesion of fine particles led to decrease of the valid collision number of quartz particles, and further decrease of the charge-to-mass ratio. However, coal-quartz collisions are just a part of charging process, and it should be noted that particle-wall collisions are also ubiquitous in fluidized bed. Particle-wall collision also has an impact on coalquartz charging process. Hence, the presence of walls in fluidized bed could finally interfere outcomes of fine coal in separation process. But it is hard to determine what is the proportion of particle-wall collision accounts for in charging process. In this work, we vaguely regard particle-particle collisions as the main source of charge. 3.2. Operating condition experiments Previous research has demonstrated that particle charging and separation processes are significantly influenced by plate voltage [27], superficial gas velocity [28] and fluidization time [29]. Meanwhile, as a critical parameter in a fluidized bed, the static bed height greatly influenced the enrichment effect of the binary particle mixture. In this work, we carried out a set of experiments to explore the effect of operating conditions, including plate voltage, superficial gas velocity, fluidization time and static bed height, on the enrichment of coal and quartz particles. 3.2.1. The effect of plate voltage The fluidized bed with applied electrical field could be regarded as a electrical condenser, and coal-quartz mixture can be seen as charged particles. In this model, electrical field force (F), particle charge (q) and plate voltage (V) follow these relationships: F ¼ qE
ð1Þ
E ¼ U=d
ð2Þ
force is proportional to its acceleration, that is, the charged particles could move further in an electrical field with a higher plate voltage. The motion of particles in this fluidized beds results from a balance between gravitational force, drag force, electric field force and interparticle forces such as electrostatic exerted on the particles [8]. On one hand, electrostatic charges can change the forces on particles, thereby inducing particle-wall adhesion, inter-particle cohesion, and agglomeration, which can further affect electrostatic phenomena in the bed. On the other hand, charged particles are separated in the electrical field generated between vertical electrode plates according to the magnitude and polarity of their charges by being deflected toward the positive or negative electrode. The results of the varying plate voltage experiments are shown in Fig. 7. Here, the horizontal axis refers to the cubicle numbers marked in Fig. 5, while the vertical axis represents the mass fraction of quartz output. As shown in Fig. 7, with greater voltage, quartz particles were more attracted by the positive plate, while coal particles were more likely to move to the negative one, given the polarity difference after contact charging of the two different minerals. Hence, in general, the mass fraction of quartz decreased gradually when close to the negative plate. With voltage of 20 kV and 30 kV, few evident changes were found in the distribution of coal and quartz, showing that the enrichment of coal and quartz was minimal with relatively low voltage. However, with increase of plate voltage, the mass fraction of quartz particles near the positive plate increased while that near the negative plate decreased. At 60 kV, the mass fraction of quartz in cubicle #1 was 42.76% while it was 11.21% in cubicle #7: a 13.79% reduction from the proportions of the binary mixture. These results furtherly demonstrated that the applied high voltage had an impressive enrichment effect on the coal and quartz particles in the mixture due to the electrical field force applied to the particles charged with opposite polarity. Within a consistent atmosphere, the increasing plate voltage might lead to an intense electrical field, which would also increase the electrical field force applied to the particles. Therefore, the more intense the plate voltage applied to the fluidization bed, the better the enrichment effect of coal and quartz particles would be.
where, F refers to electrical field force, q represents particle charge, E implies intensity of electrical field, U is short for plate voltage and d is the distance between two positive and negative plates. It can be concluded from Eqs. (1) to (2) that, higher plate voltage could lead to more intensified electrical field, which furthurly results in a greater electrical field force. With a given particle, electrical field
3.2.2. The effect of gas velocity Gas velocity can be an important factor in causing buildup of electrostatic charges in fluidized beds, and it may also play a significant role in determining carryover of particles from gas-fluidized beds. Moughrabiah [28] has found that, as the gas velocity increased, the degree of
Fig. 7. Mass fraction of quartz along the direction of the electric field at different voltage. (Superficial gas velocity: 16.53 cm/s; Fluidization time: 30 min; Static bed height: 5 mm.)
Fig. 8. Mass fraction of quartz along the direction of the electric field for different gas velocity. (Plate voltage: 60 kV; Fluidization time: 30 min; Static bed height: 5 mm.)
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mixture would also increase. However, higher gas velocity may also produce ultra-fine coal particles, due to the fact that coal is a brittle mineral. Ultra-fine coal particles are very light and difficult to drop into the collector with the present of airflow in fluidized bed, even though they have a larger charge-to-mass ratio.
Fig. 9. Mass fraction of quartz along the direction of the electric field for different static bed height. (Plate voltage: 60 kV; Superficial gas velocity: 16.53 cm/s; Fluidization time: 30 min.)
bed electrification also increased, probably due to stronger interactions among particles and between particles and the wall caused by an increase in bubble rise velocity, frequency, or gas flow rate. Mehrani [30] found that at higher gas velocities, the fines entrained from the bed carried opposite charges, leaving behind a net charge of opposite polarity inside the bed. The results of the experiments varying gas velocity are shown in Fig. 8. We determined that the mass fraction of quartz barely changed at relatively low gas velocity. However, with increase of the gas velocity, the mass fraction of quartz close to the positive plate increased while that near the negative plate decreased. At 16.53 cm/s, the mass fraction of quartz in cubicle #1 was 41.64%, but only 10.31% in cubicle #7. The gas velocity is a significant factor for evaluating particle charging. Increase of the superficial gas velocity results in more violent particle movement and higher probability of particle collision in the fluidized bed; hence, particles in the fluidized bed would be expected to charge more intensively. Meanwhile, another plausible explanation of this phenomenon may be that the increasing gas velocity leads to reduction of the interaction force between particles. Therefore, with increase of the gas velocity, the enrichment effect of coal and quartz particles in the
Fig. 10. Mass fraction of quartz along the direction of the electric field for different fluidization time. (Plate voltage: 60 kV; Superficial gas velocity: 16.53 cm/s; Static bed height: 5 mm.)
3.2.3. The effect of static bed height Static bed height, to same extent, determines the total amounts of particles existing in the fluidized bed. Since the charging process and the separation process take place simultaneously, the total mass of coal-quartz mixture should not be too large nor too small. That is, with small amounts of coal and quartz particles, there would not be sufficient collisions among particles, leading to a poor outcome of coal separation. On the contrary, with excess particles, the charged particles could be obstructed by other particles when moving toward the plate. The results of experiments varying the static bed height are shown in Fig. 9. We can see from Fig. 9 that with the condition of 2.5 mm static bed height, the mass fraction of the quartz particles in cubicle #1 was 45%, while it was b10% in cubicle #7. This all shows that coal and quartz particles could be significantly enriched using a shallow bed height. When the static bed height was increased to 20 mm, the mass fraction of quartz particles in cubicle #1 was as low as 30%, and there were minimal changes near the negative plate. This set of experiments clearly reflects that this kind of fluidized bed is effective with shallow beds of material, and that the application of this enrichment approach to deeper beds of material remains to be explored. 3.2.4. The effect of fluidization time In this work, fluidization time is roughly equivalent to the working time of air supply system, and could also be known as solid residence time. It could be easily assumed that the longer the fluidization time is, the higher the probability of collision among particles would be. Therefore, particles collide with each other could obtain the opposite charges. Simultaneously, the present of external electrical field make it possible for charged particles to move toward plate. The results of the experiments with varying fluidization time are shown in Fig. 10. We can conclude from Fig. 10 that with short fluidization time, the mass fraction of quartz particles in positively charged areas was not relatively high; however, with increase in the fluidization time, the enrichment effect seemed more obvious. After 15 min, there were no obvious changes in positively charged areas. As the fluidization time increased to 45 min, the mass fraction of quartz in cubicle #1 reached 45%, while it was b8% in cubicle #7.
Fig. 11. Ash content and yield of products from each cubicle using raw coal sample. (Plate voltage: 60 kV; Superficial gas velocity: 16.53 cm/s; Static bed height: 5 mm; fluidization time: 30 min.)
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Computational simulations of fluidized beds with electrostatic effects have indicated that charged particles are subject to axial and lateral segregation because of the non-uniform potential distribution and particle charging in the bed [31]. Positive and negative electrical currents can be traced in different locations of the bed, with opposite polarities associated with different zones, demonstrating the interplay between hydrodynamic and electrification phenomena [8]. Due to the presence of adhesive forces between particles and the walls arising from electrostatic forces, particle motion is significantly hindered. However, the extend of fluidization time was likely to reduce the effect of this phenomenon and the continuous fluidization of particles could enhance the charging procedure and therefore achieve better enrichment effect of the charged particles in the mixture. 3.3. Enrichment of raw coal Given that remarkable results were obtained previously, we therefore extended our work to raw coal. As shown in Fig. 11, the ash content of each product decreased gradually from the positive plate to the negative one, except that an increase was obtained when closing to the negative plate. This is because after crushing, grinding and screening process, mineral particles could be almost fully dissociated from coal particles. With the presence of air flow, solid particles turned into fluidization state and were contacting and rubbing with each other, therefore charge transfer (or electron or material transfer) took place between particle surfaces due to the work function differences between coal and mineral surfaces. It was found that associated minerals, specifically quartz, were negatively charged after contact and frictional electrification with coal particles, therefore, coal particles were positively charged. Mineral particles with net negative charge were inclined to move to positive plate, leading to the ash content increase near the positive plate. While coal particles charged positively have a tendency to move to negative plate, resulting in low ash content near the negative plate. Therefore, ash content of product in each cubicle shows a modest decrease trend from positive plate to negative one. The reason of slight increase in cubicle #7 is that positively charged particles moving toward negative plate collided with the plate and furtherly bounced into the cubicle #6, making it the lowest ash content over other cubicles. As for yield of products from each cubicle, shown in Fig. 11, it is found that cubicle #1, which is close to the positive plate, yield more than a quarter of total amounts of product. While little difference were found among yields of other cubicles. Combined with ash content results, it could be concluded that most of associated minerals in coal samples were inclined to move toward positive plate. Therefore, it is feasible to remove minerals from coal using fluidized bed with applied electrical field. 4. Conclusion In this study, we proposed a novel device that applied high voltage to a fluidized bed to study the enrichment effect on a mixture of coal and quartz particles according to the difference of charging properties between two kinds of materials. The XRD test illustrates that the main impurities of the coal samples applied in this work were quartz, calcite and kaolinite (3.84%, 2.38% and 9.53%, respectively), as determined from the XRF results. The charge-to-mass ratio tests were conducted to verify the charging characteristics of quartz when contacting and then separating from coal particles. The results showed that the particle size of quartz plays an important role in tribocharging. Decreasing the particle size increases the specific surface area, leading to enhancement of the collision probability of coal and quartz particles. However, with particle size b0.045 mm, due to agglomeration and adhesion of extremely small particles, the valid collision number of quartz particles may decrease and actually lower the charge-to-mass ratio.
The operating conditions, namely gas velocity, static bed height and plate voltage, significantly influenced the enrichment of coal and quartz particles in the fluidized bed. The particle enrichment effect was evidently improved with increased plate voltage and gas velocity, and with less static bed height. The conditions of 60 kV plate voltage, 16.53 cm/s gas velocity and 5 mm static bed height were found to be feasible operating parameters for separating coal particles from those of gangue minerals. Under the feasible conditions mentioned above, with increase of the fluidization time, the enrichment effect of minerals first increased; then levelled off. With 15 min fluidization time, particles in the fluidization bed generally completed the enrichment process. The ash content of the materials in cubicle #7 was 10%, which is approximately 15% less than that of the binary particle mixture. This shows an obvious enrichment effect that reduced the ash content. Besides, experiments with raw coal are also consistent with results obtained above: ash content could be reduced from 15.61% to 7.29%, and more than a quarter of yields were high ash content products. The results from this research clearly indicate that it is feasible to separate fine coal particles from those of gangue minerals by applying high voltage to a fluidized bed. A couple of measures should be taken to converted this process into an pilot scale test, in order to separate solid particles continuously and steadily. Firstly, nitrogen should be introduced as gas supply, to reduce relative humidity in the bed. In addition, relative humidity in the atmosphere have large impact on charging process [32,33]. Therefore, the cost of reducing humidity could be great. And it is also the main constraint on converting our laboratory system to an industrialized one. Secondly, a device that integrates circulating fluidized bed should be introduced to our system, in order to reintroduce the middling products to fluidized bed and enhance the separation process. Meanwhile, improvements should be made on the product collecting approach to remove the products continuously, instead of halting the system to collect final product in this work.
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