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Effects of characteristics of clapboard unit on separation of < 6 mm fine coal in a compound dry separator
Powder Technology 321 (2017) 232–241
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effects of characteristics of clapboard unit on separation of b 6 mm fine coal in a compound dry separator Zhenfu Luo a,⁎, Yuemin Zhao a, Xiaodong Yu b, Chenlong Duan a, Shulei Song a, Xuliang Yang a a b
School of Chemical Engineering & Technology, China University of Mining and Technology, Xuzhou 221116, China College of Mining Engineering, North China University of Science and Technology, Tangshan 063009, China
a r t i c l e
i n f o
Article history: Received 7 June 2017 Received in revised form 28 July 2017 Accepted 6 August 2017 Available online 09 August 2017 Keywords: Separator Fine coal Compound dry separation Ash distribution
a b s t r a c t With the increasing mechanization of coal mining, the proportion of fine coal being mined has been rising. To date, the problem of dry separation of fine coal has not been satisfactorily solved. Based on analysis of the surface structure characteristics of a compound dry separation machine, an experimental system with a compound dry separation unit for fine coal separation was established. We analyze the most important structural elements: the effect of the bed plate on fine coal separation and the distribution of the unit bed material layer of the ash separator unit under different structural partition conditions. The value of the comprehensive index, segregation degree of ash and separation efficiency were introduced for evaluation of results. The results indicate an optimum set of baffle parameters (θ = 45, d = 55–60 mm, and h = 60 mm) for which the segregation degree of ash in the bed material Sash is 0.98, the maximum value of the comprehensive index P is 3.43, and the separation efficiency is the highest. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With coal as the main source of energy in China, coal production in 2015 was 3.75 billion tons, accounting for approximately 60% of China's energy consumption [1]. This is consistent with the long-term development trend for China's energy production and consumption of coal [2]. With the popularization and application of a wide range of comprehensive mechanized coal mining methods, coal (− 6 mm) in fine coal content increased gradually, some areas of coal (− 6 mm) fine coal content reached about 70% [3]. Thus, clean and efficient use of fine coal is important in promoting energy savings and emission reduction to achieve sustainable development. At the same time, highefficiency fine coal separation technology can achieve full grain dry separation of coal, which is of considerable significance in improving the core competitiveness of coal dry separation. At present, the dry separation of fine coal mainly concentrates on air-dense medium fluidized beds, electric separation, and high-gradient magnetic separation. In the former, the separation medium layer is formed by the combination of a narrow grain size, heavy gas, and an external force field. However, owing to the small size of the feed material and the difference in the weight of the added plasmid, the removal efficiency is low and the separation cost is increased. The latter needs to have a smaller particle size, lower moisture, and less processing, and there ⁎ Corresponding author. E-mail address: zfl[email protected] (Z. Luo).
http://dx.doi.org/10.1016/j.powtec.2017.08.028 0032-5910/© 2017 Elsevier B.V. All rights reserved.
are still many problems with industrial amplification that cannot be resolved [4–5]. Therefore, it is important to study a simple and reliable method for the dry separation of fine coal. Fine coal (− 6 mm) compound dry separation technology, with its simple process, lowcost separation, operation, and maintenance, and processing capacity, has important research and application value for industrialization [6–8]. So far, the compound dry separator has realized commerce application for coarse raw coal in the world, and the efficient lower limit of separation is larger than 6 mm, which indicates the compound dry separator cannot separate efficiently fine coal (6 mm). The behavior and interaction among fine particles are different from that among coarse particles. Therefore, it is of considerable significance to develop a new type of dry separation technology suitable for −6 mm fine coal separation. 2. Materials and methods 2.1. Experimental equipment The process flow of a − 6 mm fine coal compound dry separation system is shown in Fig. 1, which is mainly composed of a raw coal preparation part, a separation part, and an air supply dust removal part [9]. The raw coal is divided into two types of products, which are produced by the buffer feeder and then separated in a compound dry separator. The air supply in the dust removal part mainly includes a blower, draught fan, and dust collector. The blower provides the wind
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Table 1 Proximate analysis of the 0–6 mm coal sample.
Feed
Proximate analysis
Value
Moisture content (%) Ash content (%) Fixed carbon (%) Volatile content (%) Calorific value (MJ/kg)
11.92 36.85 29.51 21.62 17.77
Concentrate Tailings Mineral flow line
Air flow line
1 classification screen, 2 crusher, 3 surge bin, 4 compound dry separator, 5 dust collector, 6 induced draft fan, 7 air bag, 8 air blower
analysis of raw coal is presented in Table 1: the ash content is 36.85%, moisture content is 11.92%, volatile content is 21.62%, fixed carbon content is approximately 29.51%, and the calorific value is 17.77 MJ/kg. The result for the sink-float experiment using raw coal is given in Table 2.
Fig. 1. Fine coal dry separation process flow chart.
2.3. Evaluation index power required by the separating machine, and the dust collector is mainly used for the purification and recovery of the dust generated in the separation process. The test mainly analyzes the separation process of fine coal using the bed baffle effect. Material forms the corresponding change on the partition mainly occurs between the parameters, from the bottom along the height direction of the partition bulkheads bed material space is divided into 5 segments, and stratified sampling for laboratory analysis of ash. The schematic of the sampling is shown in Fig. 2. The schematic of the angle adjustment is shown in Fig. 3.
(1) The separation of raw coal is mainly done to reduce the ash content of coal, reduce the inorganic mineral impurities in coal, and increase the content of combustible coal. Based on the concept of ash separation degree, the material in the separator between the surfaces of the FGX dry separator is divided into n sections from the top to the bottom, along the height of the separator. Then, sampling and determination of ash and yield in each section is done using the formula shown in Eq. (1).
Sash 2.2. Material properties The 0–6 mm coal was separated by using the FGX (Compound Dry Separator) fine coal combined dry beneficiation process. The proximate
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u 1 X ¼t ðAi =A0 Þ2 ; n−1 i¼1
ð1Þ
where Ai is the ash content of coal in the ith sampling point, A0 is the initial ash content of the feed coal, and n is the total sampling number. In this article, n corresponds to 5 parts. The statistical
Table 2 Density composition of the 0–6 mm coal from float-sink experiment. Density (g/cm−3)
Fig. 2. Partial sampling.
Fig. 3. Partial longitudinal sampling coordinates.
b1.3 1.3–1.4 1.4–1.5 1.5–1.6 1.6–1.7 1.7–1.8 1.8–2.0 N2.0 合计 (Total)
Yield (%)
17.34 34.45 8.82 3.41 1.04 0.89 0.84 33.2 100
Ash content (%)
5.79 8.83 15.82 25.78 34.74 43.56 53.3 88.35 36.85
Cumulative floats
Cumulative sinks
Separating density
Yield (%)
Ash content (%)
Yield (%)
Ash content (%)
Density (g/cm3)
Yield (%)
17.34 51.79 60.61 64.02 65.06 65.95 66.79 100
5.79 7.81 8.98 9.87 10.27 10.72 11.25 36.85
100 82.65 48.20 39.38 35.97 34.93 34.04 33.20
36.85 43.37 68.06 79.76 84.87 86.37 87.49 88.35
1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
51.79 43.27 12.23 4.45 1.93 1.31 0.84 33.62
Fig. 4. Partition angle adjustment coordinate system.
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=35°
=40°
=45°
=50°
=55° Fig. 5. Influence of partition angle on bed material distribution.
index indicates that the quantity of ash from the coal bed of the FGX dry separator is different from that of the coal sample, and the higher the value of ash segregation, the better the separation efficiency [10].
(2) The main purpose of the FGX dry separator for − 6 mm coal separation is to reduce the ash to improve the calorific value of coal. Based on the ash content in the original coal, calorific value, the selected coal ash, and heat generation to establish
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the following comprehensive index P, the formula shown in Eq. (2) is devised. P¼
γc Q c =Q o ; Ac =Ao
ð2Þ
where Q c /Q o is heat transfer ratio and A c /A o is absolute drop ratio. The higher the heating ratio, indicating the high calorific value of coal, the lower is the absolute drop ratio. This indicates that the lower the ash, higher is the yield, and the higher the P value, and the better is the separation efficiency [11]. 3. Results and discussion 3.1. Effect of partition plate angle The partition plate is installed on the bed surface at a certain angle. The coordinate system from the material feeding end to the gangue section is the X-axis, the discharge side is in the direction of the Yaxis, and the angle between the partition plate and the X-axis is θ, as shown in Fig. 4. In the experiment, the height of the partition plate is 55 mm and the distance from the back plate edge is 60 mm. Fig. 5 shows the separation degree and its influence on the separation efficiency of the partition material, when angles of the partition are 35°, 40°, 45°, 50°, and 55°. As can be seen from Fig. 5, the material layer is not obvious along the height of the bed and the ash distribution clutter when the angle is small. The mixing is significant when the distance is 35–60 mm. At this time, the ash separation degree (Sash) of the bulkhead unit is small, at 0.41. With the increase of the angle, the ash distribution tends to be regular, and stratification is obvious along the bed height. At small angles, the particles experience less drag, and there is less formation of large voids at the backplane. Particles can quickly move through the void from the partition so that the material particles are not stratified before the next partition cycle. With the increase of the angle, the resistance of the partition plate to the particle drag force increases, and the gap between the partition and plate is relatively reduced. The particles in the partition during the residence time experience the synergistic efficiency of vibration and wind. The optimal density to complete the stratification is seen at θ = 45°. At this point, the comprehensive separating index, P, has a value of 3.21. With further increases of the angle, the horizontal particle resistance force is very large and the gap formation at the back of the plate is too small, resulting in the lower part of the partition between the high-density particles being unable to quickly move to the backplane area. Under the action of the pressure, the material moved to the gangue section; a serious backmix phenomenon, causing a deterioration of the separating efficiency, was seen when the angle increased from 50° to 55°. In the Y-axis direction, different degrees of backmix occurred between 40 and 60 mm and the layer efficiency deteriorated. The influence of angle change on ash separation and the comprehensive index P is shown in Fig. 6. It can be seen from the Fig. 6 that the ash separation degree and comprehensive index P were increased first and then decreased with the increasing of angle. When the angle is small, the ash separation degree and comprehensive index P are small. With the increasing of angle, both of them gradually increase. When the angle is 45° both of them reach the maximum, which indicates that the material distribution at this time achieved the best condition and separating efficiency. When the angle is further increased, the corresponding values are reduced and the separating efficiency is deteriorated.
characterize the rule of change of material in the bed, separation efficiency is indirectly analyzed by testing samples from coordinates in the bed as shown in Fig. 3 and analyzing the spatial variation of material ash content based on the height of the clapboard, as shown in Figs. 7 and 8. As can be seen from Fig. 7, when the height of the bed is relatively low, the material ash content along the X-axis gradually increases. The ash content along the Y-axis gradually increases with increasing distance, but the value of ash content along the X-axis is higher, on average, which shows the material in the bed cannot be well layered at the moment, and the separation efficiency proves to be poor. The mark points in the Fig. 7 indicate the distribution of the ash in the three-dimensional space under different bed height. The x-axis direction indicates the bed width, and the Y-axis direction indicates the length of the bed. With the increase of the height of bed, the value of material ash content gradually decreases along the X-axis, and reaches its lowest when H = 60 mm. The numerical value has more variation along the Y-axis, showing that material in the test bed is remarkably layered in total density separation, and P (comprehensive index) is 3.41, which illustrates the best separation efficiency. As the height of the bed increases, the value of ash content along the X-axis gradually increases and the amplitude of variation gradually decreases along the Y-axis, so the back-mixing phenomenon appears in the material of bed, especially when the height of bed is 70 mm and the material ash content is unevenly distributed. This illustrates that because the height of bed is large and the shatter value of the bed decreases, airflow weakens and short circuits and channeling phenomenon appear in partial areas, the result being that material cannot be better layered and that particles obtain different energy in the transmission of vibrational energy (Fig. 9). At this point, material in the bed becomes dominant due to particle segregation, the comprehensive index of separation gradually decreases, and separation efficiency lessens. The comprehensive index of separation with the variation of bed height is shown in Fig. 8.
3.3. Influence of the length of clapboard on the separation process In this study, the longitudinal distanced from the feed end to the bed surface of the waste bed is gradually shortened due to the fine coal separation bed of the trapezoidal structure. In view of this, to examine the effect of the length of the baffle on the separation process, it is now defined as the vertical distance of the baffle from the backplane (d) as shown in Fig. 8. Fig. 10 shows the separation efficiency for a angles of the partition of 45°, backplane height of 60 mm, and backplane lengths of 50–55 mm, 55–60 mm, and 60–65 mm.
3.2. Influence of the height of clapboard on the separation process The clapboard angle adopted in the experiment is 45°, with distances from the backboard and blow down of 60 mm. The height of the backboard ranges from 50 to 70 mm at intervals of 5 mm. To
235
Fig. 6. The efficiency of angle on separating efficiency.
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H=50 mm
H=55 mm
H=60 mm
H=65 mm
H=70 mm Fig. 7. Baffle bed material ash with bed with high variation.
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Fig. 11. Influence of partition length on separating index of back plate material. Fig. 8. The relationship between the comprehensive index P and the bed height.
d
Fig. 9. Partition mark.
It can be seen from Fig. 10, when the length of the baffle (d) is smaller than 50–55 mm, the material along the longitudinal distribution of the baffle is more chaotic, where the mark points in the Fig. 10 indicate the longitudinal distribution of the material ash in the bed. The more dense mark points indicate that the material distribution is more closely. The distribution of ash points directly reflects the density distribution of the material. Fig. 10 also shows that the distribution of the high-ash and the relatively low-ash particle is relatively uniform, indicating that the material at this time cannot be effectively stratified by density, in
which the distribution of low-ash materials and high-ash materials is relatively small and the distribution of ash content between the two is relatively uniform. At this point, the ash separation degree (Sash) of the bed material is 0.51, indicating that the bed material failed to form an effective separation by density, and the comprehensive index P is 1.16. Thus, the separation efficiency is poor. The reason is that the length of the baffle is small, and the distance between the baffle and the backplane is shortened. The bottom of the particles is forced to the surface of the material by the backplane and the stratified particles cannot be transported to the next area, resulting in particles backmixing and the back-plan area having lower high-ash material content. As the length increases, the distance between the baffle and the backplane increases gradually. In this area, the high density particles can be stratified and transported to the next separating area, and when the d is 55–60 mm, the stratification efficiency is the best. From the ash distribution point in the figure, it can be seen that the highash materials are concentrated in the edge of the backplane, while the low-ash materials are concentrated in the discharge side, and the middle-ash material distribution is relatively small. At this time, the ash separation degree (Sash) of the bulkhead unit reaches a maximum of 0.98, and the comprehensive separating index P is 3.43. With the further increase in length, the length of the baffle unit reduced, and the effective separating area of the material decreased, resulting in poor separating efficiency. The distance between the baffle and the
Fig. 10. Influence of partition length on bed material change.
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Table 3 Basic operation parameters in the separation experiments. Code
Factors
A B C
Angle Length Height
Table 5 Variance analysis of quadratic model.
Unit
Minimum
Maximum
Level
mm mm
35 55 50
55 65 70
3 3 3
backplane is too large, resulting in the excessive concentration of the backplane material and the short circuit of the airflow at the bed, which seriously disturbs the material density segregation. Particle back to the mixed phenomenon is serious: it can be seen that the distribution of high-ash and low-ash material is relatively chaotic, and that the distribution of ash content over the whole material is relatively uniform. The ash separation degree and the comprehensive separating index of the partition unit are reduced. The relationship between the ash separation (Sash), the composite separating index (P), and the ash content of the backplane is shown in Fig. 11 as the length (d) of the separator changes. 3.4. Determination of optimum partition parameters The effects of single factors on the separating process are studied in this paper. In order to study the interaction between a partition parameter and other factors, the optimal partition parameters were determined. Therefore, the response surface method is used to model and analyze the corresponding problems of multi-factor influence (i.e., ash separation degree) [12]. In order to explore the relationship between ash separation degree and various factors, the significance of research factors and the interaction between factors, we determined the optimal parameters for the baffle. Table 3 shows the surface method response for the 0–6 mm fine coal. The results of the analysis and the first approximation model suitability test are shown in Table 4. It can be seen from the Table 4 that the approximation model fitness test results show that the second order polynomial model is a systematic recommendation model with Prob N F b 0.5, which indicates that the model has high goodness of fit for the response surface. The variance analysis of the second-order polynomial model recommended by the system is shown in Table 5. The results show that the F value of the second-order polynomial model is 29.75, indicating that the effect of the combination of test factors on the sulfur separation is significant. It is clear that the Prob N F values of the study items B, A2, B2, and C2 in this experiment are b0.05, indicating that the four external water contents have significant effect. It can be seen from Fig. 12(I) that the shape of the response surface is more prominent along the height change direction: the comprehensive index P caused by the height change is changed significantly, with increasing height, P first increases then decreases with P reaching its maximum value when the height is 60 mm. The relative value of the length has little effect on the index P value.
Source
Sum of squares
df
Mean square
F value
Prob N F
Model A-Angle B-High C-Length AB AC BC A2 B2 C2 Residual Lack of fit Pure error
26.07 3.200E−003 0.57 0.14 4.000E−004 0.10 3.600E−003 18.16 3.97 1.26 0.68 0.67 0.012
9 1 1 1 1 1 1 1 1 1 7 3 4
2.90 3.200E−003 0.57 0.14 4.000E−004 0.10 3.600E−003 18.16 3.97 1.26 0.097 0.22 3.070E−003
29.75 0.033 5.88 1.39 4.107E-003 1.05 0.037 186.42 40.80 12.91
b0.0001 0.8613 0.0458 0.2772 0.9507 0.3393 0.8530 b0.0001 0.0004 0.0088
Significant
72.69
0.0006
Significant
The shape of the response surface is relatively gentle along the length direction, and there is no obvious salient feature. Fig. 12(II) shows the response surface and the corresponding contour plot shows that P is more sensitive to changes in length than to the angle, and that the response surface along the length direction is more prominent. We can see that the change in length caused by changes in P value is more significant. The mathematical model of the separating index P is established by regression analysis, and the significant relationship between the influence of each factor on the external water content is obtained as follows: Height N Length N Angle. The mathematical model is expressed as follows. Mathematical model in terms of factors of code: P ¼ 4:05 þ 0:02A−0:07B þ 0:13BC þ 0:01AB þ 0:16AC−0:03BC −2:08A2 −0:97B2 −0:55C 2
ð3Þ
Mathematical model in terms of actual operational factors: P ¼ −144:97 þ 1:7θ þ 1:17h þ 2:54d þ e−4 θh þ 3:2e−3 θd−6e−4 hd 2 2 −0:02θ2 −9:72e−3 h −0:02d
ð4Þ
4. Optimal parameters of bed material distribution characteristics In this paper, the parameters of the partition system are obtained as follows: θ = 45°, d = 55–60 mm, and h = 60 mm. Under the optimum partition parameters, the results show that the ash within the material in the separation area is evenly distributed along the discharge side, that the ash content in the coal is fluctuating between 8 and 10%, and the gangue area ash fluctuates between 52.8 and 59.6%. High ash materials are mainly concentrated in the back plate area, with an ash fluctuation range at 50.5–58.6%. At this point, the separation degree is 0.98, and the comprehensive index P has a value of 3.43. Along the feed section to the gangue section of the
Table 4 Models comparison and selection. Source
Sum of squares
df
Mean square
Mean vs total Linear vs mean 2FI vs linear Quadratic vs 2FI Cubic vs quad Residual Total
94.40 0.71 0.11 25.56 0.67 0.012 121.16
1 3 3 3 3 4 17
94.40 0.24 0.035 8.42 0.22 3.070E−003 7.13
F value
P-value prob N F
0.12 0.014 86.45 72.69
0.9478 0.9977 b0.0001 0.0006
The underlined data means that the quadratic model is the optimal model for fitting test data, and to distinguish between the data and others, the underline is set.
Suggested Aliased
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(I)
(II) Fig. 12. Partition parameter response surface diagram.
bed the material ash gradually increased. To the gangue section, the largest material ash appeared, the high ash materials are mainly concentrated in this section. The ash content of the material gradually decreases along the back plate to the discharge side. Most of the low ash materials are mainly concentrated at the discharge end, and the high ash material is concentrated in the back plate area. The ash distribution of the bed material and its corresponding contours are shown in Fig. 13. 5. Conclusions In this paper, the FGX separating machine bed separator is used to study the separating efficiency of fine coal, and the influence of the height and length of the partition and the angle of the bed on the separating process is systematically analyzed. The distribution
of the material is analyzed, and the phenomenon of backmixing of the bed particles is studied. At the same time, the best separating parameters are obtained, and the following conclusions are drawn: (1) The comprehensive index P was used to verify the separating efficiency of fine coal and the ash separation degree Sash as the indicator of the degree of density of the partition material. The greater the value is, the better the density segregation and separating efficiency. (2) The ash separation degree and P of the material of the separator increase and decrease with the increase of the height, length, and the angle of the bed, and the maximum value is obtained when (θ = 45°, d = 55–60 mm, h = 60 mm). At this time, the distribution of ash in the direction of the bed discharge is uniform, and the average ash content is 8.9%. The average ash
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Fig. 13. Optimum baffle parameters under bed material ash distribution.
content of the material at the backplane is 58.9%. The different layers of the material are obviously stratified. At this time, the ash separation degree (Sash) of the bulkhead unit is 0.98, the maximum value of the comprehensive index (P) is 3.43, and the separating efficiency is the highest. (3) Through the analysis of the influence of the factors on the separating process, the significance degree of the partition elements was found to be as follows: Height N Length N Angle; that is,
the influence of the height of the partition on the separating efficiency is obvious.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effects of characteristics of clapboard unit on separation of b 6 mm fine coal in a compound dry separator Zhenfu Luo a,⁎, Yuemin Zhao a, Xiaodong Yu b, Chenlong Duan a, Shulei Song a, Xuliang Yang a a b
School of Chemical Engineering & Technology, China University of Mining and Technology, Xuzhou 221116, China College of Mining Engineering, North China University of Science and Technology, Tangshan 063009, China
a r t i c l e
i n f o
Article history: Received 7 June 2017 Received in revised form 28 July 2017 Accepted 6 August 2017 Available online 09 August 2017 Keywords: Separator Fine coal Compound dry separation Ash distribution
a b s t r a c t With the increasing mechanization of coal mining, the proportion of fine coal being mined has been rising. To date, the problem of dry separation of fine coal has not been satisfactorily solved. Based on analysis of the surface structure characteristics of a compound dry separation machine, an experimental system with a compound dry separation unit for fine coal separation was established. We analyze the most important structural elements: the effect of the bed plate on fine coal separation and the distribution of the unit bed material layer of the ash separator unit under different structural partition conditions. The value of the comprehensive index, segregation degree of ash and separation efficiency were introduced for evaluation of results. The results indicate an optimum set of baffle parameters (θ = 45, d = 55–60 mm, and h = 60 mm) for which the segregation degree of ash in the bed material Sash is 0.98, the maximum value of the comprehensive index P is 3.43, and the separation efficiency is the highest. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With coal as the main source of energy in China, coal production in 2015 was 3.75 billion tons, accounting for approximately 60% of China's energy consumption [1]. This is consistent with the long-term development trend for China's energy production and consumption of coal [2]. With the popularization and application of a wide range of comprehensive mechanized coal mining methods, coal (− 6 mm) in fine coal content increased gradually, some areas of coal (− 6 mm) fine coal content reached about 70% [3]. Thus, clean and efficient use of fine coal is important in promoting energy savings and emission reduction to achieve sustainable development. At the same time, highefficiency fine coal separation technology can achieve full grain dry separation of coal, which is of considerable significance in improving the core competitiveness of coal dry separation. At present, the dry separation of fine coal mainly concentrates on air-dense medium fluidized beds, electric separation, and high-gradient magnetic separation. In the former, the separation medium layer is formed by the combination of a narrow grain size, heavy gas, and an external force field. However, owing to the small size of the feed material and the difference in the weight of the added plasmid, the removal efficiency is low and the separation cost is increased. The latter needs to have a smaller particle size, lower moisture, and less processing, and there ⁎ Corresponding author. E-mail address: zfl[email protected] (Z. Luo).
http://dx.doi.org/10.1016/j.powtec.2017.08.028 0032-5910/© 2017 Elsevier B.V. All rights reserved.
are still many problems with industrial amplification that cannot be resolved [4–5]. Therefore, it is important to study a simple and reliable method for the dry separation of fine coal. Fine coal (− 6 mm) compound dry separation technology, with its simple process, lowcost separation, operation, and maintenance, and processing capacity, has important research and application value for industrialization [6–8]. So far, the compound dry separator has realized commerce application for coarse raw coal in the world, and the efficient lower limit of separation is larger than 6 mm, which indicates the compound dry separator cannot separate efficiently fine coal (6 mm). The behavior and interaction among fine particles are different from that among coarse particles. Therefore, it is of considerable significance to develop a new type of dry separation technology suitable for −6 mm fine coal separation. 2. Materials and methods 2.1. Experimental equipment The process flow of a − 6 mm fine coal compound dry separation system is shown in Fig. 1, which is mainly composed of a raw coal preparation part, a separation part, and an air supply dust removal part [9]. The raw coal is divided into two types of products, which are produced by the buffer feeder and then separated in a compound dry separator. The air supply in the dust removal part mainly includes a blower, draught fan, and dust collector. The blower provides the wind
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Table 1 Proximate analysis of the 0–6 mm coal sample.
Feed
Proximate analysis
Value
Moisture content (%) Ash content (%) Fixed carbon (%) Volatile content (%) Calorific value (MJ/kg)
11.92 36.85 29.51 21.62 17.77
Concentrate Tailings Mineral flow line
Air flow line
1 classification screen, 2 crusher, 3 surge bin, 4 compound dry separator, 5 dust collector, 6 induced draft fan, 7 air bag, 8 air blower
analysis of raw coal is presented in Table 1: the ash content is 36.85%, moisture content is 11.92%, volatile content is 21.62%, fixed carbon content is approximately 29.51%, and the calorific value is 17.77 MJ/kg. The result for the sink-float experiment using raw coal is given in Table 2.
Fig. 1. Fine coal dry separation process flow chart.
2.3. Evaluation index power required by the separating machine, and the dust collector is mainly used for the purification and recovery of the dust generated in the separation process. The test mainly analyzes the separation process of fine coal using the bed baffle effect. Material forms the corresponding change on the partition mainly occurs between the parameters, from the bottom along the height direction of the partition bulkheads bed material space is divided into 5 segments, and stratified sampling for laboratory analysis of ash. The schematic of the sampling is shown in Fig. 2. The schematic of the angle adjustment is shown in Fig. 3.
(1) The separation of raw coal is mainly done to reduce the ash content of coal, reduce the inorganic mineral impurities in coal, and increase the content of combustible coal. Based on the concept of ash separation degree, the material in the separator between the surfaces of the FGX dry separator is divided into n sections from the top to the bottom, along the height of the separator. Then, sampling and determination of ash and yield in each section is done using the formula shown in Eq. (1).
Sash 2.2. Material properties The 0–6 mm coal was separated by using the FGX (Compound Dry Separator) fine coal combined dry beneficiation process. The proximate
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u 1 X ¼t ðAi =A0 Þ2 ; n−1 i¼1
ð1Þ
where Ai is the ash content of coal in the ith sampling point, A0 is the initial ash content of the feed coal, and n is the total sampling number. In this article, n corresponds to 5 parts. The statistical
Table 2 Density composition of the 0–6 mm coal from float-sink experiment. Density (g/cm−3)
Fig. 2. Partial sampling.
Fig. 3. Partial longitudinal sampling coordinates.
b1.3 1.3–1.4 1.4–1.5 1.5–1.6 1.6–1.7 1.7–1.8 1.8–2.0 N2.0 合计 (Total)
Yield (%)
17.34 34.45 8.82 3.41 1.04 0.89 0.84 33.2 100
Ash content (%)
5.79 8.83 15.82 25.78 34.74 43.56 53.3 88.35 36.85
Cumulative floats
Cumulative sinks
Separating density
Yield (%)
Ash content (%)
Yield (%)
Ash content (%)
Density (g/cm3)
Yield (%)
17.34 51.79 60.61 64.02 65.06 65.95 66.79 100
5.79 7.81 8.98 9.87 10.27 10.72 11.25 36.85
100 82.65 48.20 39.38 35.97 34.93 34.04 33.20
36.85 43.37 68.06 79.76 84.87 86.37 87.49 88.35
1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
51.79 43.27 12.23 4.45 1.93 1.31 0.84 33.62
Fig. 4. Partition angle adjustment coordinate system.
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=35°
=40°
=45°
=50°
=55° Fig. 5. Influence of partition angle on bed material distribution.
index indicates that the quantity of ash from the coal bed of the FGX dry separator is different from that of the coal sample, and the higher the value of ash segregation, the better the separation efficiency [10].
(2) The main purpose of the FGX dry separator for − 6 mm coal separation is to reduce the ash to improve the calorific value of coal. Based on the ash content in the original coal, calorific value, the selected coal ash, and heat generation to establish
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the following comprehensive index P, the formula shown in Eq. (2) is devised. P¼
γc Q c =Q o ; Ac =Ao
ð2Þ
where Q c /Q o is heat transfer ratio and A c /A o is absolute drop ratio. The higher the heating ratio, indicating the high calorific value of coal, the lower is the absolute drop ratio. This indicates that the lower the ash, higher is the yield, and the higher the P value, and the better is the separation efficiency [11]. 3. Results and discussion 3.1. Effect of partition plate angle The partition plate is installed on the bed surface at a certain angle. The coordinate system from the material feeding end to the gangue section is the X-axis, the discharge side is in the direction of the Yaxis, and the angle between the partition plate and the X-axis is θ, as shown in Fig. 4. In the experiment, the height of the partition plate is 55 mm and the distance from the back plate edge is 60 mm. Fig. 5 shows the separation degree and its influence on the separation efficiency of the partition material, when angles of the partition are 35°, 40°, 45°, 50°, and 55°. As can be seen from Fig. 5, the material layer is not obvious along the height of the bed and the ash distribution clutter when the angle is small. The mixing is significant when the distance is 35–60 mm. At this time, the ash separation degree (Sash) of the bulkhead unit is small, at 0.41. With the increase of the angle, the ash distribution tends to be regular, and stratification is obvious along the bed height. At small angles, the particles experience less drag, and there is less formation of large voids at the backplane. Particles can quickly move through the void from the partition so that the material particles are not stratified before the next partition cycle. With the increase of the angle, the resistance of the partition plate to the particle drag force increases, and the gap between the partition and plate is relatively reduced. The particles in the partition during the residence time experience the synergistic efficiency of vibration and wind. The optimal density to complete the stratification is seen at θ = 45°. At this point, the comprehensive separating index, P, has a value of 3.21. With further increases of the angle, the horizontal particle resistance force is very large and the gap formation at the back of the plate is too small, resulting in the lower part of the partition between the high-density particles being unable to quickly move to the backplane area. Under the action of the pressure, the material moved to the gangue section; a serious backmix phenomenon, causing a deterioration of the separating efficiency, was seen when the angle increased from 50° to 55°. In the Y-axis direction, different degrees of backmix occurred between 40 and 60 mm and the layer efficiency deteriorated. The influence of angle change on ash separation and the comprehensive index P is shown in Fig. 6. It can be seen from the Fig. 6 that the ash separation degree and comprehensive index P were increased first and then decreased with the increasing of angle. When the angle is small, the ash separation degree and comprehensive index P are small. With the increasing of angle, both of them gradually increase. When the angle is 45° both of them reach the maximum, which indicates that the material distribution at this time achieved the best condition and separating efficiency. When the angle is further increased, the corresponding values are reduced and the separating efficiency is deteriorated.
characterize the rule of change of material in the bed, separation efficiency is indirectly analyzed by testing samples from coordinates in the bed as shown in Fig. 3 and analyzing the spatial variation of material ash content based on the height of the clapboard, as shown in Figs. 7 and 8. As can be seen from Fig. 7, when the height of the bed is relatively low, the material ash content along the X-axis gradually increases. The ash content along the Y-axis gradually increases with increasing distance, but the value of ash content along the X-axis is higher, on average, which shows the material in the bed cannot be well layered at the moment, and the separation efficiency proves to be poor. The mark points in the Fig. 7 indicate the distribution of the ash in the three-dimensional space under different bed height. The x-axis direction indicates the bed width, and the Y-axis direction indicates the length of the bed. With the increase of the height of bed, the value of material ash content gradually decreases along the X-axis, and reaches its lowest when H = 60 mm. The numerical value has more variation along the Y-axis, showing that material in the test bed is remarkably layered in total density separation, and P (comprehensive index) is 3.41, which illustrates the best separation efficiency. As the height of the bed increases, the value of ash content along the X-axis gradually increases and the amplitude of variation gradually decreases along the Y-axis, so the back-mixing phenomenon appears in the material of bed, especially when the height of bed is 70 mm and the material ash content is unevenly distributed. This illustrates that because the height of bed is large and the shatter value of the bed decreases, airflow weakens and short circuits and channeling phenomenon appear in partial areas, the result being that material cannot be better layered and that particles obtain different energy in the transmission of vibrational energy (Fig. 9). At this point, material in the bed becomes dominant due to particle segregation, the comprehensive index of separation gradually decreases, and separation efficiency lessens. The comprehensive index of separation with the variation of bed height is shown in Fig. 8.
3.3. Influence of the length of clapboard on the separation process In this study, the longitudinal distanced from the feed end to the bed surface of the waste bed is gradually shortened due to the fine coal separation bed of the trapezoidal structure. In view of this, to examine the effect of the length of the baffle on the separation process, it is now defined as the vertical distance of the baffle from the backplane (d) as shown in Fig. 8. Fig. 10 shows the separation efficiency for a angles of the partition of 45°, backplane height of 60 mm, and backplane lengths of 50–55 mm, 55–60 mm, and 60–65 mm.
3.2. Influence of the height of clapboard on the separation process The clapboard angle adopted in the experiment is 45°, with distances from the backboard and blow down of 60 mm. The height of the backboard ranges from 50 to 70 mm at intervals of 5 mm. To
235
Fig. 6. The efficiency of angle on separating efficiency.
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H=50 mm
H=55 mm
H=60 mm
H=65 mm
H=70 mm Fig. 7. Baffle bed material ash with bed with high variation.
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Fig. 11. Influence of partition length on separating index of back plate material. Fig. 8. The relationship between the comprehensive index P and the bed height.
d
Fig. 9. Partition mark.
It can be seen from Fig. 10, when the length of the baffle (d) is smaller than 50–55 mm, the material along the longitudinal distribution of the baffle is more chaotic, where the mark points in the Fig. 10 indicate the longitudinal distribution of the material ash in the bed. The more dense mark points indicate that the material distribution is more closely. The distribution of ash points directly reflects the density distribution of the material. Fig. 10 also shows that the distribution of the high-ash and the relatively low-ash particle is relatively uniform, indicating that the material at this time cannot be effectively stratified by density, in
which the distribution of low-ash materials and high-ash materials is relatively small and the distribution of ash content between the two is relatively uniform. At this point, the ash separation degree (Sash) of the bed material is 0.51, indicating that the bed material failed to form an effective separation by density, and the comprehensive index P is 1.16. Thus, the separation efficiency is poor. The reason is that the length of the baffle is small, and the distance between the baffle and the backplane is shortened. The bottom of the particles is forced to the surface of the material by the backplane and the stratified particles cannot be transported to the next area, resulting in particles backmixing and the back-plan area having lower high-ash material content. As the length increases, the distance between the baffle and the backplane increases gradually. In this area, the high density particles can be stratified and transported to the next separating area, and when the d is 55–60 mm, the stratification efficiency is the best. From the ash distribution point in the figure, it can be seen that the highash materials are concentrated in the edge of the backplane, while the low-ash materials are concentrated in the discharge side, and the middle-ash material distribution is relatively small. At this time, the ash separation degree (Sash) of the bulkhead unit reaches a maximum of 0.98, and the comprehensive separating index P is 3.43. With the further increase in length, the length of the baffle unit reduced, and the effective separating area of the material decreased, resulting in poor separating efficiency. The distance between the baffle and the
Fig. 10. Influence of partition length on bed material change.
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Table 3 Basic operation parameters in the separation experiments. Code
Factors
A B C
Angle Length Height
Table 5 Variance analysis of quadratic model.
Unit
Minimum
Maximum
Level
mm mm
35 55 50
55 65 70
3 3 3
backplane is too large, resulting in the excessive concentration of the backplane material and the short circuit of the airflow at the bed, which seriously disturbs the material density segregation. Particle back to the mixed phenomenon is serious: it can be seen that the distribution of high-ash and low-ash material is relatively chaotic, and that the distribution of ash content over the whole material is relatively uniform. The ash separation degree and the comprehensive separating index of the partition unit are reduced. The relationship between the ash separation (Sash), the composite separating index (P), and the ash content of the backplane is shown in Fig. 11 as the length (d) of the separator changes. 3.4. Determination of optimum partition parameters The effects of single factors on the separating process are studied in this paper. In order to study the interaction between a partition parameter and other factors, the optimal partition parameters were determined. Therefore, the response surface method is used to model and analyze the corresponding problems of multi-factor influence (i.e., ash separation degree) [12]. In order to explore the relationship between ash separation degree and various factors, the significance of research factors and the interaction between factors, we determined the optimal parameters for the baffle. Table 3 shows the surface method response for the 0–6 mm fine coal. The results of the analysis and the first approximation model suitability test are shown in Table 4. It can be seen from the Table 4 that the approximation model fitness test results show that the second order polynomial model is a systematic recommendation model with Prob N F b 0.5, which indicates that the model has high goodness of fit for the response surface. The variance analysis of the second-order polynomial model recommended by the system is shown in Table 5. The results show that the F value of the second-order polynomial model is 29.75, indicating that the effect of the combination of test factors on the sulfur separation is significant. It is clear that the Prob N F values of the study items B, A2, B2, and C2 in this experiment are b0.05, indicating that the four external water contents have significant effect. It can be seen from Fig. 12(I) that the shape of the response surface is more prominent along the height change direction: the comprehensive index P caused by the height change is changed significantly, with increasing height, P first increases then decreases with P reaching its maximum value when the height is 60 mm. The relative value of the length has little effect on the index P value.
Source
Sum of squares
df
Mean square
F value
Prob N F
Model A-Angle B-High C-Length AB AC BC A2 B2 C2 Residual Lack of fit Pure error
26.07 3.200E−003 0.57 0.14 4.000E−004 0.10 3.600E−003 18.16 3.97 1.26 0.68 0.67 0.012
9 1 1 1 1 1 1 1 1 1 7 3 4
2.90 3.200E−003 0.57 0.14 4.000E−004 0.10 3.600E−003 18.16 3.97 1.26 0.097 0.22 3.070E−003
29.75 0.033 5.88 1.39 4.107E-003 1.05 0.037 186.42 40.80 12.91
b0.0001 0.8613 0.0458 0.2772 0.9507 0.3393 0.8530 b0.0001 0.0004 0.0088
Significant
72.69
0.0006
Significant
The shape of the response surface is relatively gentle along the length direction, and there is no obvious salient feature. Fig. 12(II) shows the response surface and the corresponding contour plot shows that P is more sensitive to changes in length than to the angle, and that the response surface along the length direction is more prominent. We can see that the change in length caused by changes in P value is more significant. The mathematical model of the separating index P is established by regression analysis, and the significant relationship between the influence of each factor on the external water content is obtained as follows: Height N Length N Angle. The mathematical model is expressed as follows. Mathematical model in terms of factors of code: P ¼ 4:05 þ 0:02A−0:07B þ 0:13BC þ 0:01AB þ 0:16AC−0:03BC −2:08A2 −0:97B2 −0:55C 2
ð3Þ
Mathematical model in terms of actual operational factors: P ¼ −144:97 þ 1:7θ þ 1:17h þ 2:54d þ e−4 θh þ 3:2e−3 θd−6e−4 hd 2 2 −0:02θ2 −9:72e−3 h −0:02d
ð4Þ
4. Optimal parameters of bed material distribution characteristics In this paper, the parameters of the partition system are obtained as follows: θ = 45°, d = 55–60 mm, and h = 60 mm. Under the optimum partition parameters, the results show that the ash within the material in the separation area is evenly distributed along the discharge side, that the ash content in the coal is fluctuating between 8 and 10%, and the gangue area ash fluctuates between 52.8 and 59.6%. High ash materials are mainly concentrated in the back plate area, with an ash fluctuation range at 50.5–58.6%. At this point, the separation degree is 0.98, and the comprehensive index P has a value of 3.43. Along the feed section to the gangue section of the
Table 4 Models comparison and selection. Source
Sum of squares
df
Mean square
Mean vs total Linear vs mean 2FI vs linear Quadratic vs 2FI Cubic vs quad Residual Total
94.40 0.71 0.11 25.56 0.67 0.012 121.16
1 3 3 3 3 4 17
94.40 0.24 0.035 8.42 0.22 3.070E−003 7.13
F value
P-value prob N F
0.12 0.014 86.45 72.69
0.9478 0.9977 b0.0001 0.0006
The underlined data means that the quadratic model is the optimal model for fitting test data, and to distinguish between the data and others, the underline is set.
Suggested Aliased
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(I)
(II) Fig. 12. Partition parameter response surface diagram.
bed the material ash gradually increased. To the gangue section, the largest material ash appeared, the high ash materials are mainly concentrated in this section. The ash content of the material gradually decreases along the back plate to the discharge side. Most of the low ash materials are mainly concentrated at the discharge end, and the high ash material is concentrated in the back plate area. The ash distribution of the bed material and its corresponding contours are shown in Fig. 13. 5. Conclusions In this paper, the FGX separating machine bed separator is used to study the separating efficiency of fine coal, and the influence of the height and length of the partition and the angle of the bed on the separating process is systematically analyzed. The distribution
of the material is analyzed, and the phenomenon of backmixing of the bed particles is studied. At the same time, the best separating parameters are obtained, and the following conclusions are drawn: (1) The comprehensive index P was used to verify the separating efficiency of fine coal and the ash separation degree Sash as the indicator of the degree of density of the partition material. The greater the value is, the better the density segregation and separating efficiency. (2) The ash separation degree and P of the material of the separator increase and decrease with the increase of the height, length, and the angle of the bed, and the maximum value is obtained when (θ = 45°, d = 55–60 mm, h = 60 mm). At this time, the distribution of ash in the direction of the bed discharge is uniform, and the average ash content is 8.9%. The average ash
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Fig. 13. Optimum baffle parameters under bed material ash distribution.
content of the material at the backplane is 58.9%. The different layers of the material are obviously stratified. At this time, the ash separation degree (Sash) of the bulkhead unit is 0.98, the maximum value of the comprehensive index (P) is 3.43, and the separating efficiency is the highest. (3) Through the analysis of the influence of the factors on the separating process, the significance degree of the partition elements was found to be as follows: Height N Length N Angle; that is,
the influence of the height of the partition on the separating efficiency is obvious.
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