Effects of energy consumption on the separation performance of fine coal flotation

Fuel Processing Technology 115 (2013) 192–200

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Effects of energy consumption on the separation performance of fine coal flotation Xiahui Gui a,⁎, Gan Cheng b, Jiongtian Liu c,⁎, Yijun Cao c, Shulei Li a, Qiongqiong He a a b c

School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China Chinese National Engineering Research Center of Coal Preparation and Purification, Xuzhou 221116, Jiangsu, China

a r t i c l e

i n f o

Article history: Received 17 December 2012 Received in revised form 9 May 2013 Accepted 22 May 2013 Available online 2 July 2013 Keywords: Fine coal Flotation Energy consumption Agitation Energy input

a b s t r a c t To explore the laws of energy consumption in the fine coal flotation process, an energy consumption test system was established. In this investigation, it was discovered that fine coal with high ash content from Kailuan mine in China is difficult to float. The energy input in coal flotation process was changed by adjusting the shaft power and flotation time. A flotation rate test was conducted under different shaft speed conditions, and the characteristics of the flotation products with varying energy consumption were analyzed. The results showed that the floatability of the floating material decreased with the energy consumption increased. With the increase of energy consumption, the rate of increased yield of fractions is from higher to lower as following: 0.043–0.074 mm, 0.074–0.124 mm, −0.043 mm, 0.124–0.246 mm, 0.246–0.495 mm, +0.495 mm. Higher combustible matter recovery was achieved under high shaft speed conditions within the same flotation time. However, high energy input may increase the pollution of high-ash fine mud in the concentrate. The coarse intergrowth particles, such as coal and rocks were non-liberated and part of the micro-fine-grained coal particles, could be recovered by high energy input. For example, 18.74% concentrate yield was obtained for 1.5–1.6 g/cm3 intergrowth particles at 144 J energy input at the later stage of flotation process while the shaft speed is 1500 r/min. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Fine coal separation is an important method for improving the quality of coal, removing harmful and unusable impurities, and reducing sulfur content. In addition, coal separation is an important factor in reducing air pollution and in saving transport energy. The particle size of flotation feed has undergone miniaturization with further development, thus increasing the difficulty in conducting fine coal separation. However, energy saving and emission reduction efforts have promoted the quality of flotation equipment through large-scale flotation process refinement. These processes are mainly reflected in the reduction of energy consumption and the selective recovery of coarse and micro-fine particles during coal flotation. In fine coal flotation, coarse and micro-fine particles significantly affect the energy consumption of the processing unit. In addition, the oxidation of coal has also a great effect on the floatability. While the coal is oxidized, the coal will be difficult to float. It has been reflected in China [1–3]. Therefore, exploring the laws of energy consumption during coal flotation process is of great importance. ⁎ Corresponding author at: School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China. E-mail addresses: [email protected] (X. Gui), [email protected] (J. Liu). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.05.017

It is difficult to determine the effects of energy consumption on the flotation process. The flotation cells have complex hydrodynamic environments, in which the sub-processes of solid suspension, gas dispersion, and flotation are strongly interdependent. A number of studies have explored this issue by examining the flotation process in standard stirred tanks that are agitated by Rushton turbine impellers [4–10]. The concept of the capture probability of particles/bubbles has recently received considerable research attention. However, increasing the collision rate remains a difficult task while reducing the probability of detachment of particles/bubbles. Previous studies are in agreement that a higher probability of collision emerges as the value of flotation machine airflow and agitation Reynold's number increases [11–17]. Flotation rate constant was found to increase approximately linearly with increasing power intensity for all particle and bubble sizes [18]. This study investigates the relationship between energy consumption and fine coal flotation performance. The research object is a fat coal from Kailuan mine in north China. The examined flotation parameters include the dosages of frother and collector, scrubbing time, as well as the slurry concentration and air flow rate for the mechanical flotation cell. Energy consumption during the fine coal flotation process was measured by using a torque sensor. To demonstrate the effect of energy consumption on flotation products of different sizes and densities, size and density analyses of the flotation rate test products were conducted.

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2. Method and materials 2.1. Method 2.1.1. Flotation energy consumption test system The flotation energy consumption test system used in this study included flotation and energy consumption test equipment. An XFD 0.75 L laboratory-scale inflatable flotation machine was used for the flotation rate test. A torque sensor that has data acquisition, data conversion, and data processing functions was selected as the energy consumption measurement equipment. The torque sensors converted torque into flotation energy consumption by using the data conversion and data processing units after data collection. The professional measuring torque parameters of the TQ-662 sensor made by Beijing WorldCom Kechuang Limited were used in the test. The sensor principle was based on the dynamic resistance strain method. Strain gauges were attached to the shaft on the strain bridge. Shaft torque was obtained when power was switched on and was then converted into power by the voltage/frequency unit. A physical map of the torque sensor and the measuring principle are shown in Figs. 1 and 2, respectively. The technical parameters of the torque sensor are shown in Table 1. The torque sensor was connected between the power source and the load by using two sets of coupling (Fig. 3). Fig. 2. Measurement principle diagram.

2.1.2. Flotation energy consumption calculation method The primary function of the flotation process is concentrated in the agitator shaft of flotation cells. The power consumed by the coal flotation process is calculated as shown in Eq. (1), where P is the effective power obtained by the mechanical stirring shaft, M is the stirring shaft torque, and n is the speed of the agitator shaft (synchronized with the spindle motor). P¼

2πMn 60

W ¼ Pt:

ð1Þ ð2Þ

The stirring shaft torque and speed of the agitator shaft were measured by using the torque sensor and the inverter in the flotation cells, respectively. First, the torque value was measured when no pulp was present in the flotation tank, and the unloaded power was calculated by using Eq. (1). Second, coal samples for the test requirements were added to the flotation tank, after which the torque value was obtained to calculate the unloaded power. Third, the effective power was calculated against the load power by subtracting the idling power. Finally,

the flotation energy consumption was calculated by using Eq. (2), where W is the flotation energy consumption, and t is the flotation time. 2.1.3. Sample analysis A Rigaku D/Max-RA-type rotating anode X-ray diffractometer was used for X-ray analysis. The working conditions are as follows: Cu target operating voltage of 40 kV, current of 50 mA, graphite monochromator, scattering slit of 1°, receiving slit of 0.3 mm, and sampling interval of 0.02°/step. An SPB200 vibrating Taylor screen was used for size analysis to obtain the ash content at different grain sizes. The decreasing order of mesh apertures was 0.495, 0.246, 0.124, 0.074, and 0.043 mm. The sample was dried and dispersed by using a sample splitting device, and then a 100 g sample was extracted for size analysis. The Taylor screen was fixed in turn to reduce the mesh aperture from top to bottom. The vibrating Taylor screen was switched on after feeding. The machine was stopped every 5 min to check on the screening manually. The screen was removed from the tray sequentially from top to bottom to check on the screening manually for 1 min. When the weight of the hutch product did not exceed 1% of the weight of the retained product, the

Fig. 1. Actual product diagram of torque sensor.

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Table 1 Technical parameters of the torque sensor. Technical parameters

Index

Measuring range Output signal Supply voltage Response frequency Ambient temperature

0 N·m to ±1 N·m 4 mA to 20 mA 24 VDC 100 US −40 °C to 80 °C

Table 2 Ratio of organic solution at different densities (temperature: 15 °C). Technical parameters

Index

Maximum load Insulation resistance Zero drift Accuracy Relative humidity

0 N·m to ±1.5 N·m >200 M Ω b0.5% 0.1 b90% RH

screening process was completed. Then, the hutch product was mixed into the next grain size for manual checking. Manual checking was performed for all grain sizes. A GL-21 M high-speed centrifuge was used for density analysis, with a centrifuge speed of 3000 r/min. The specification of the centrifuge tube was 4 ∗ 250 ml. Four centrifuge tubes with volumes of 250 ml were arranged symmetrically in the centrifuge. The centrifugal liquid was prepared by mixing an organic solution comprising carbon tetrachloride, benzene, and tribromethane. The preparation method for different densities of organic solution is shown in Table 2. First, 60 g of the coal sample was divided into four average parts by using a sample splitting device. Thereafter, 15 g of coal sample was poured into four centrifuge tubes separately. Second, a centrifugal liquid with similar density (1.3 g/cm3) was poured into the centrifuge tubes (low density first), and the mixture was stirred evenly. The liquid filled 85% of the height of the centrifuge tube. Third, the centrifuge machine was switched on. Timing was started when the speed exceeded 2000 r/min. The tubes were abstracted, and the floating material was poured out after 12 min. Centrifugal liquid at a volume of 1.4 g/cm3 was added into the sinking material in the tubes for the next sink-and-float test by using the centrifugation process. Finally, each density product was washed, filtered, dried, weighed, and analyzed.

2.2. Materials The coal samples were obtained from the flotation feed of Qianjiaying Coal Preparation Plant of Tangshan City, Hebei Province,

Centrifugal liquid density/g·cm−3

1.3

1.4

1.5

1.6

1.8

2.0

Carbon tetrachloride CCl4/% Benzene C6H6/% Tribromethane CHBr3/%

60 40

74 26

81 19

98

79

59

2

21

41

China. The fine coal was fat coal and was thus difficult to separate. Kerosene and octanol were used as collector and frother, respectively. Distilled water was used as solvent for the dilution of the coal sample. X-ray diffraction (XRD) was utilized for mineral analysis. As shown in Fig. 4, kaolinite is the primary rocky mineral in the coal samples, followed by illite, quartz, pyrite, and so on. Kaolinite, which is a kind of ore with fine-grained particles, usually measured in microns, shaped in clods, and is easily broken into powder, is the main constituent part of clay and argillaceous rocks. Kaolinite is difficult to separate if not dissociated thoroughly when embedded as fine or micro-fine particles. Table 3 shows that the lowest level content of the coal sample was − 0.495 mm. The majority of the material exists in the relative size range of − 0.246 ± 0.124 mm with 48.32% yields. The fine particles and the ash content of the fine-grained samples were both high. The − 0.074 mm samples accounted for 38.45% of yield, whereas the − 0.043 mm samples accounted for 16.73% of yield at an ash content of 27.08%, which is higher than those of the other fractions. Therefore, the fine particles with high ash content floated into clean coal easily through mechanical entrainment, thus covering the surface of coarse-grained coal. The density fraction results of the fine coal are shown in Table 4. It indicates that the major yield of the coal exists in the relative density range of −1.5 g/cm3. When the theoretical separation density was 1.4 g/cm3 and 1.5 g/cm3, the content of δ ± 0.1 was 54.19% and 30.07%, respectively. The washability of such coal samples indicates extremely difficult separation. The ash content of the coal sample with a density range of 1.6 g/cm3 to 1.8 g/cm3 was relatively low. Numerous intergrowth particles which were non-liberated particles formed by gangue minerals and coals were identified. Therefore, obtaining high ash tailings with low ash concentrate is difficult.

Fig. 3. Connection chart of the flotation energy consumption test system.

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products and one tailing product were obtained sequentially: Concentrate 1 (C1), Concentrate 2 (C2), Concentrate 3 (C3), Concentrate 4 (C4), Concentrate 5 (C5), as well as Tailings (T). 4. Results and discussion 4.1. Flotation condition test

Fig. 4. XRD patterns of coal sample.

3. Experimental 3.1. Flotation conditions test Flotation tests were conducted in an XFDIII 0.75 L laboratory flotation cell inflated by an external air pump. The procedure for the test is as follows: Firstly, distilled water was added to the flotation cell at a volume that is 1/3 of the tank, followed by the addition of the coal sample. Secondly, the sample was stirred until the coal particles were completely wetted, and then distilled water was added until the second tick mark on the tank was reached. Thirdly, the collector (diesel fuel) was added after pre-agitation for 3 min. The frother (octanol) was then added and mixed into the sample for 1 min. Flotation commenced 10 s later. The flotation time was 3 min. Flotation condition tests were conducted at varying reagent dosage, feed concentration, and air flow rate through one rough flowsheet. 3.2. Flotation energy consumption test To explore the relationship between energy consumption and flotation index, flotation rate tests were conducted in the flotation cell at different shaft speeds of 1500, 1800, and 2100 r/min. The best optimal conditions of reagent dosage, feed concentration, and air flow rate obtained in the flotation condition test were selected by using the efficiency index E = Yc ∗ At/Ac [19], where Yc is the product yield, and At and Ac are the tailing and product ash content, respectively. Fig. 5 shows the flowchart of the flotation rate test. The process employed for the flotation rate test is as follows: First, the coal sample was added for wetting and dispersion agitation for 2 min. Second, the collector was added for 1 min of agitation, after which the frother was added for 10 s of agitation. Third, flotation was commenced for 5 min. The flotation process was divided into five phases. The first two phases of the flotation time lasted for 0.5 min, the middle two phases lasted for 1 min, and the final phase lasted for 2 min. Five concentrate

4.1.1. Flotation reagent dosage Considering the actual production of Qianjiaying coal preparation plant, the collector dosages were 120, 270, and 320 g/t while the frother dosages were 75, 110, and 145 g/t. The flotation feed concentration was 100 g/L, the shaft speed was 1800 r/min, and the air flow rate was 0.77 cm/s. Table 5 shows the separation index obtained by varying the collector and frother dosages. The ash contents of the concentrate products were approximately 12% to 13%, with no significant difference. The concentrate ash was 12.8%, and the tailing ash was the lowest when the reagent dosage was the smallest. Based on size and density analyses (Tables 3 and 4), the fine particles with high ash content were found to be highly floatable. The ash content of concentrates and tailings increased through the addition of higher dosages of collector and frother. However, the increase in concentrate ash was less than 1% while the combustible matter recovery could increase by more than 30%. As shown in Table 5, the maximum value of E is the ninth group of 179.46. However, taking into account that the qualified clean coal ash should be less than 13%. The value of E in item 8 is 156.69 with clean coal ash content of 12.51%. Therefore, the optimal reagent dosage was 320 g/t of collector and 110 g/t of frother. 4.1.2. Flotation feed concentration The conditions of flotation feed concentration were fixed. The shaft speed was 1800 r/min, air flow rate was 0.77 cm/s, collector dosage was 320 g/t, and frother dosage was 110 g/t. With the increase in flotation concentration, the ash content of concentrate and the combustible matter recovery gradually increased. This result is attributed to the increase in the fine mud content of the float coal due to the entrainment and a more stable slurry fluid environment. The XRD patterns and size analysis results of the coal sample are shown in Fig. 4 and Table 2, respectively. The results indicate that the coarse mineral intergrowth facilitated mudding of fine particles at high ash content. By contrast, the entrainment was minimal in this size range. Therefore, a lower feed concentration should be provided for flotation. When the concentration was 90 g/L, the combustible matter recovery was the highest, and a downward trend was identified in Fig. 6. Thus, 90 g/L may be the optimal concentration, along with 12.45% concentrate ash content and 64.78% combustible matter recovery. The air flow was approximately 0 cm/s to 1.85 cm/s, shaft speed was 1800 r/min, collector dosage was 320 g/t, and frother dosage was 110 g/t. Combustible matter recovery increased to the maximum and then gradually decreased with increasing air flow rate. An increasing trend of concentrate ash content was identified, but the value fluctuated between 12.5% and 13.5%. The role of combustible matter recovery is presented in Fig. 7. It shows that the air flow rate of 0.3 cm/s was

Table 3 Size analysis of coal sample. Size, mm

+0.495 −0.495 −0.246 −0.124 −0.074 −0.043 Total

+ + + +

Mass fraction, %

0.246 0.124 0.074 0.043

3.19 10.04 30.12 18.20 21.72 16.73 100.00

Ash, %

16.25 16.67 17.79 19.12 21.85 27.08 20.31

Positive cumulative

Negative cumulative

Mass fraction, %

Ash, %

Mass fraction, %

Ash, %

3.19 13.23 43.35 61.55 83.27 100.00 –

16.25 16.57 17.42 17.92 18.95 20.31 –

100.00 96.81 86.77 56.65 38.45 16.73 –

20.31 20.44 20.88 22.52 24.13 27.08 –

196

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Table 4 Density analysis of coal sample. Density fractions

Mass fraction

Ash content

Cumulative of floating objects

Cumulative of sediments

Content of δ ± 0.1

Mass fraction

Ash content

Mass fraction

Ash content

Density

Mass fraction

g/cm3

%

%

%

%

%

%

g/cm3

%

−1.3 +1.3–1.4 +1.4–1.5 +1.5–1.6 +1.6–1.8 +1.8 Total

20.12 31.92 22.27 7.80 5.49 12.40 100.00

5.90 9.79 16.02 24.50 40.42 65.99 20.12

20.12 52.04 74.31 82.11 87.60 100.00 –

5.90 8.29 10.60 11.92 13.71 20.19 –

100.00 79.88 47.96 25.69 17.89 12.40 –

20.19 23.79 33.11 47.93 58.14 65.99 –

1.30 1.40 1.50 1.60 1.70 1.80 –

52.04 54.19 30.07 10.55 5.49 15.15 –

adequate for coal particles to float. The effects of air flow rate on combustible recovery were consistent with the work of Xuan Qu et al., who found that the flotation recovery reached the maximum by increasing the aeration rate and then slightly decreased in coal flotation. The decrease in recovery at higher aeration rates is probably due to increased amount of relatively coarse particles detached from bubbles in the pulp under more turbulent condition [20]. Considering the concentrate ash content and combustible matter recovery, the optimal air flow rate of the sample was found to be 0.62 cm/s. In summary, the optimal flotation operating conditions are as follows: collector dosage 320 g/t, frother dosage 110 g/t, feed concentration 90 g/L, and air flow rate 0.62 cm/s. 4.2. Flotation energy consumption To evaluate the energy demand of different particles during fine coal flotation, three flotation experiments were performed at varying flotation cell shaft speeds of 1500, 1800, and 2400 r/min. The instantaneous torque that was measured in the flotation rate test was converted into

instantaneous power, as shown in Fig. 1. The cumulative energy consumption of each flotation product at different flotation times was then calculated. Fig. 8 presents the relationship between the ash content and combustible matter recovery of concentrate at different flotation times. The results show that the concentrate ash content increases with the flotation process. Coals with low ash content exhibit priority float, whereas those with high ash content float in the latter part of the flotation process. The combustible matter recovery curve at three shafts speed was distributed evenly. The complete flotation process can be achieved by increasing concentration combustible matter recovery through the increase in flotation cell shaft speed. The additional coals recovered under a high shaft speed of 2100 r/min were coarse particles with low ash, fine particles with high ash, and several non-liberated particles with surfaces formed by gangue minerals. This finding was verified by the subsequent product size and density analyses. The process characteristic of flotation energy consumption is presented in Fig. 9. It shows the traditional relationship between energy consumption and flotation index at different shaft speeds (Table 6).

Fig. 5. Flotation rate test flowchart.

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Table 5 Effects of reagent dosage on coal flotation performance. Reagent dosage, g/t

1 2 3 4 5 6 7 8 9

Concentrates

Tailings

Collector

Frother

Yield, %

Ash, %

Yield, %

Ash, %

180 180 180 270 270 270 320 320 320

75 110 145 75 110 145 75 110 145

38.3 49.87 55.98 40.15 51.42 59.86 47.51 60.22 66.28

12.82 13.14 13.61 12.44 12.58 12.91 12.62 12.51 13.27

61.7 50.13 44.02 59.85 48.58 40.14 52.49 39.78 33.72

25.11 27.87 30.08 25.12 27.97 31.21 27.19 32.55 35.93

Energy consumption increases with the flotation process at three different shaft speed conditions. Meanwhile, the cumulative combustible matter recovery and cumulative ash content also increased. A faster shaft speed requires a higher energy input into the flotation system and facilitates a faster flotation rate, but results in a smaller amount of concentrate floated for each unit of energy consumption. That is, the floatability of materials in the flotation cell decreases with the flotation process, and the energy consumption for each floating unit mass of clean coal gradually increases. Therefore, the relationship between flotation energy consumption and the floatability of fine coal was determined. That is, coals floated easily consume less energy while those floated hardly consume more energy. Fig. 9 shows a significant difference in energy consumption but similar combustible matter recovery. An ash content of 13.27% and combustible matter recovery of 73.35% were obtained at energy consumption of 688 J and shaft speed of 1500 r/min. With the similar combustible matter recovery, energies of 1260 J with shaft speed of 1800 r/min and 2232 J with shaft speed of 2100 r/min were consumed with 13.27% and 12.7% concentrate ash content, respectively. At a certain flotation time, a more complete flotation process could be achieved with high energy consumption. That is, cleaner coal could be recovered with a higher energy input at similar ash content. For example, when the flotation time was 5 min, the combustible matter recovery of 85.91% was achieved with consumed energy of 3636 J and shaft speed of 2100 r/min. However, the combustible matter recovery of 81.06% was achieved at consumed energy of 1656 J and shaft speed of 1800 r/min. The combustible matter recovery increased by 4.85 percentage points with an additional 1980 J of energy. The above analysis indicates that high energy input could intensify the flotation process of particles with poor floatability. The characteristics of energy consumption during coal flotation favorably affected the flotation process design and the optimization of flotation equipment

E

20.41 20.53 20.86 20.65 20.06 20.26 20.27 20.48 20.91

75.02 105.77 123.72 81.07 114.33 144.71 102.36 156.69 179.46

structure. Energy is the product of shaft power and flotation time. Thus, varying energy consumption for different floatability coal particles could be obtained by adjusting the shaft power input at different flotation times. Thus, the rule on the flotation process design concept based on energy consumption is proposed. Constructing increasingly stronger energy conditions to adapt to the physical property changes of the minerals in flotation is important in realizing the optimal combination of process design and physical property characteristics and in achieving the best mineralization reaction conditions. 4.3. Characteristics of the flotation product To demonstrate the effects of energy consumption on the flotation recovery of poor floatability particles of different sizes, the characteristics of the flotation product at different flotation times, shaft speeds, and energy consumptions were analyzed. The flotation products are shown in Fig. 5. 4.3.1. Size analysis With the increase of energy input, concentrate yield of each size fraction increases, and the increased yield of coarser fraction is less than that of finer fraction. Figs. 10, 11 and 12 show yield vs energy consumption curves of different narrow size fractions at the shaft speed of 1500, 1800, 2100 r/min, respectively. With the increase of energy consumption, the rate of increased yield of fractions is from higher to lower as following: 0.043–0.074 mm, 0.074–0.124 mm, −0.043 mm, 0.124– 0.246 mm, 0.246–0.495 mm and +0.495 mm. The coarser the particle is, the higher energy consumes in the flotation process when the size is bigger than 0.124 mm. The energy consumption of finest size fraction −0.043 mm is not the lowest and at a moderate level. However, the energy consumption of 0.043–0.124 mm size fraction is the lowest. 70

15.0

15.0 14.5

14.5

50 14.0

Combustible Matter Recovery

40

Cleans Ash

13.5

30 13.0 20 12.5

10 0 60

70

80

90

100

110

12.0 120

Combustible Recovery, %

60

60

Cleans Ash, %

Combustible Matter Recovery, %

70

Calculate raw coal Ash, %

14.0 50

13.5 13.0

40

12.5 30 20

Combustible Recovery

12.0

Cleans Ash

11.5 11.0

10

10.5

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

10.0 2.0

Feed Concentration, g/l

Air Flow, cm/s

Fig. 6. Effect of feed concentration on coal flotation performance.

Fig. 7. Effects of air flow rate on coal flotation performance.

Cleans Ash, %

Number

X. Gui et al. / Fuel Processing Technology 115 (2013) 192–200

20

100

80

19

90

70

18

80

60

17

50

16

40

15

30

14

20

13

10

12

20

11

10

0 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Cumulative Yield, %

90

Cleans Ash, %

Combustible Matter Recovery, %

198

5

70

50 40 30

0 0

14.5

70

14.0 13.5

60

13.0

50

12.5 40

12.0

30

11.5

20

11.0

10

10.5 0

500

Cleans Ash, %

Combustible Matter Recovery, %

80

200

300

400

Fig. 10. Cumulative yield of concentrate in different size fraction as function of the flotation energy consumption with the shaft speed of 1500 r/min.

Fig. 8. Ash content and combustible matter recovery of concentrate as function of the flotation time (collector dosage 320 g/t, frother dosage 110 g/t, feed concentration 90 g/L, and air flow rate 0.37 m3/m2·min).

15.0

100

Energy, J

1500r/min Cumulative Combustible Matter Recovery 1800r/min Cumulative Combustible Matter Recovery 2100r/min Cumulative Combustible Matter Recovery 1500r/min Cumulative Cleans Ash 1800r/min Cumulative Cleans Ash 2100r/min Cumulative Cleans Ash

90

1500r/min

60

Flotation time, mins

0

+0.495mm 0.246-0.495mm 0124-0.246mm 0.074-0.124mm 0.043-0.074mm -0.043mm

10.0 1000 1500 2000 2500 3000 3500 4000

This is consistent with the mechanism of collision, adhesion and desorption of the bubbles and particles. The lowest collision probability of the finest particles(−0.043 mm) and bubbles is due to the smallest particle diameter. Because of the bigger particle diameter, there is a higher collision probability of the coarser size fraction(+0.124 mm) with the bubble, but also a higher desorption probability. So there is a greater probability for middle size fraction particles captured by bubbles, and a higher concentrate yield is obtained by a lower energy consumption. The energy consumption vs concentrate yield curves are closer while the size is less than 0.246 mm as shown in Figs. 10, 11 and 12. However, the energy consumption vs concentrate yield curves of the coarser size fractions (+0.246 mm) are below that of −0.246 mm size fractions. The energy consumption vs concentrate yield curves indicate a higher flotation separation efficiency of −0.246 mm size fractions which is suitable for flotation at different energy input conditions. Comparing Figs. 10, 11 and 12, a higher concentrate yield would be obtained at a high power input (2100 r/min). However, the increasing rate of concentrate yield is limited, especially while the coal size is coarser, i.e. 0.246–0.496 mm size fraction. While the shaft speed is 1500 r/min, the highest concentrate yield is 40.67% at the energy

Energy , J 100

1500r/min Cumulative Combustible Matter Recovery 1800r/min Cumulative Combustible Matter Recovery 2100r/min Cumulative Combustible Matter Recovery 1500r/min Cumulative Cleans Ash 1800r/min Cumulative Cleans Ash 2100r/min Cumulative Cleans Ash

+0.495mm 0.246-0.495mm 0124-0.246mm 0.074-0.124mm 0.043-0.074mm -0.043mm

90

Fig. 9. Combustible matter recovery and ash content of concentrate as function of energy consumption.

Table 6 Energy consumption of flotation products with different shaft speeds.

Cumulative Yield, %

80 70

1800r/min

60 50 40 30 20

Time/s

Agitation mixing

Flotation

190

30

30

60

60

120

C1

C2

C3

C4

C5

Energy consumption/J

0

Shaft speed r/min 1500 1800 2100

10

252 648 1404

36 108 216

36 108 216

72 216 432

72 216 432

144 432 864

0

200

400

600

800

1000

1200

Energy, J Fig. 11. Cumulative yield of concentrate in different size fraction as function of the flotation energy consumption with the shaft speed of 1800 r/min.

X. Gui et al. / Fuel Processing Technology 115 (2013) 192–200

21

100 90

2100r/min

Cumulative Ash Content, %

70 60 50 40

+0.495mm 0.246-0.495mm 0124-0.246mm 0.074-0.124mm 0.043-0.074mm -0.043mm

30 20 10

500

1000

1500

17 15 13 11 9

2000

5

2500

0

500

Energy, J

+0.495mm 0.246-0.495mm 0124-0.246mm 0.074-0.124mm 0.043-0.074mm -0.043mm

Cumulative Ash Content, %

19 17

1500r/min

15 13 11 9 7 5 100

0

1000

1500

2000

2500

Energy, J

Fig. 12. Cumulative yield of concentrate in different size fraction as function of the flotation energy consumption with the shaft speed of 2100 r/min.

21

2100r/min

7

0 0

+0.495mm 0.246-0.495mm 0124-0.246mm 0.074-0.124mm 0.043-0.074mm -0.043mm

19

80

Cumulative Yield, %

199

200

300

400

Energy, J Fig. 13. Cumulative ash content of concentrate in different size fraction as function of the flotation energy consumption with the shaft speed of 1500 r/min.

Fig. 15. Cumulative ash content of concentrate in different size fraction as function of the flotation energy consumption with the shaft speed of 2100 r/min.

consumption of 360 J. While the shaft speed is 1800 r/min, the highest concentrate yield is 48.37% at the energy consumption of 1080 J. While the shaft speed is 2100 r/min, the highest concentrate yield is 49.54% at the energy consumption of 2160 J (Fig. 12). The flotation energy consumption increased several times, but the increasing of concentrate yield only increased less than 10%. Therefore, comprehensive analysis of energy consumption vs concentrate yield curves would be useful for guiding the design of coal flotation process. Appropriate power input conditions should be selected for obtaining the highest yield of clean coal at the lowest energy consumption. Concentrate ash content of each narrow size fraction at different energy consumptions at the shaft speed of 1500 r/min, 1800 r/min and 2100 r/min is shown in Figs. 13, 14 and 15, respectively. The finer the particles, the higher the ash content is. The ash content of each size fraction increases with the increasing of energy consumption. The ash content vs energy consumption curve also indicates that the recovery of lower ash content particles only needs lower energy consumption (Figs. 13, 14 and 15).

100 21

17 15

80

Cumulative Yield, %

Cumulative Ash Content, %

19

13 11 9

1500r/min

-1.3 g/cm3 1.3-1.4 g/cm3 1.4-1.5 g/cm3 1.5-1.6 g/cm3 1.6-1.8 g/cm3 +1.8 g/cm3

90

+0.495mm 0.246-0.495mm 0124-0.246mm 0.074-0.124mm 0.043-0.074mm -0.043mm

70 60 50

18.74% 40

144J

30 20

7

1800r/min

10 0

5 0

200

400

600

800

1000

1200

Energy, J Fig. 14. Cumulative ash content of concentrate in different size fraction as function of the flotation energy consumption with the shaft speed of 1800 r/min.

0

100

200

300

400

Energy, J Fig. 16. Cumulative yield of concentrate in different density fraction as function of the flotation energy consumption with the shaft speed of 1500 r/min.

200

X. Gui et al. / Fuel Processing Technology 115 (2013) 192–200

100

100 -1.3 g/cm3 1.3-1.4 g/cm3 1.4-1.5 g/cm3 1.5-1.6 g/cm3 1.6-1.8 g/cm3 +1.8 g/cm3

Cumulative Yield, %

80 70

90 80

Cumulative Yield, %

90

60 50 40 30 20

70 60 50 40

-1.3 g/cm3 1.3-1.4 g/cm3 1.4-1.5 g/cm3 1.5-1.6 g/cm3 1.6-1.8 g/cm3 +1.8 g/cm3

30 20

1800r/min

10

10

0

0 0

200

400

600

800

1000

1200

0

500

1000

1500

2000

2500

Energy, J

Energy, J Fig. 17. Cumulative yield of concentrate in different density fraction as function of the flotation energy consumption with the shaft speed of 1800 r/min.

Fig. 18. Cumulative yield of concentrate in different density fraction as function of the flotation energy consumption with the shaft speed of 2100 r/min.

4.3.2. Density analysis Energy consumption vs yield curves of different narrow density fractions at the shaft speed of 1500 r/min, 1800 r/min and 2100 r/min are shown in the Figs. 16, 17 and 18, respectively. The higher the density fractions, the higher energy consumes are needed in the flotation process. The recovery of middle density fractions can be improved by increasing the energy input at the later stage of flotation process. 1.5– 1.6 g/cm3, for example, 18.74% concentrate yield was obtained at 144 J energy input at the later stage of flotation process while the shaft speed is 1500 r/min (Fig. 16).

References

5. Conclusion 1) An increase in energy input during the flotation process ensures good concentrate quality and combustible matter recovery. When the combustible matter recovery exceeds 70%, the combustible matter recovery at higher energy input conditions is higher than at lower energy input conditions with similar fine ash content. 2) With a prolonged flotation process and increased energy input, the concentrate yield gradually decreased for each unit of energy input, the floatability of particles retained in the pulp worsened, and energy consumption increased per recovering unit of combustible matter recovery. The primarily clean coal loss at low energy input was coarse particles. The coarse intergrowth particles at middle ash content and the micrograined low ash content coal particles were recovered at high energy input conditions, at the cost of concentrate pollution by heterogeneous fine mud. 3) During the flotation process, the particles that were hard to float required high energy input. However, higher energy input during the flotation process did not necessarily yield better outcomes. A rational flotation process should be designed according to the specific requirements of the flotation indicators. Function is the product of power and time. The influence of the flotation process through increased flotation time under low power conditions or through changes in stirring power at the different flotation periods requires further study. Acknowledgments This research was supported by the National Key Basic Research Program of China (Grant no. 2012CB214905) and National Nature Science Foundation of China (Grant no. 51074157) for which the authors express their appreciation.

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