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Preliminary study on foreign slime for the gravity separation of coarse coal particles in a teeter bed separator
International Journal of Mineral Processing 160 (2017) 76–80
Contents lists available at ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Review
Preliminary study on foreign slime for the gravity separation of coarse coal particles in a teeter bed separator Xiangning Bu a,b, Chao Ni a, Guangyuan Xie a,⁎, Yaoli Peng a,⁎, Linhan Ge b, Jie Sha a a b
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, PR China Department of Chemical Engineering, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 5 July 2016 Received in revised form 6 December 2016 Accepted 21 January 2017 Available online 23 January 2017
The teetered bed separator (TBS) is a gravity concentration device based on the principle of hindered settling velocity, which has been widely applied for the density separation of coarse coal particles. The influences of the teeter water velocity on the density separation of coarse coal particles were investigated based on the slip velocity model and the experimental results. The ash content of clean coal increased as the teeter water velocity decreased initially and increased afterwards according to the relationship between teeter water and minimum fluidization velocity. It was observed that the foreign slime had an effect on the separation of coarse particles, while the effect of the foreign slime was not as significant as that of the teeter water velocity. © 2017 Published by Elsevier B.V.
Keywords: Foreign slime Coarse particles Teetered bed separator Gravity separation Coal
Contents 1. 2.
Introduction . . . . . . . . . . . . . Materials and experimental procedure . 2.1. Coal samples . . . . . . . . . . 2.2. Experimental procedure. . . . . 3. Results and discussion . . . . . . . . . 3.1. Effect of teeter water flow velocity 3.2. Effect of foreign slime . . . . . . 4. Conclusions. . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . Appendix A. Slip velocity model . . . . . References. . . . . . . . . . . . . . . . .
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1. Introduction Teeter bed separators are hydraulic classifiers that have been recognized as low-cost and high capacity devices for both classification and density separation. Presently over 200 such units have been installed worldwide, in mineral processing applications including silica sand for construction grade, foundry and glass making purpose, mineral sands
⁎ Corresponding authors at: School of Chemical Engineering and Technology, China University of Mining and Technology, 1 Daxue Road, Xuzhou 221116, Jiangsu, PR China. E-mail addresses: [email protected] (G. Xie), [email protected] (Y. Peng).
http://dx.doi.org/10.1016/j.minpro.2017.01.009 0301-7516/© 2017 Published by Elsevier B.V.
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and hematite processing along with coal (Littler, 1986; Kumar et al., 2013; Tripathy et al., 2013). The conventional teeter bed separator (TBS) is developed from the concept of hydrosizer and the initial designs have been manufactured since 1934. There are various commercial units based on the conventional TBS such as floatex density separator (FDS), reflux classifier (RC), cross flow separator (CFS), allflux classifier and hydro float separator, which work based on the principles of fluidization and hindered settling with different types of equipment configuration, feeder and discharge systems (Tripathy et al., 2015). Drummond et al. (2002) demonstrated that the circuit efficiency could be improved by introducing the teeter bed separator between
X. Bu et al. / International Journal of Mineral Processing 160 (2017) 76–80
dense medium cyclones and froth flotation. However, it has been shown that these devices can be effectively applied to gravity separations provided that the size distribution of the feed is within acceptable limits, depending on the application (Bethell, 1988; Heiskanen, 1993; Honaker and Mondal, 1999; Luttrell et al., 2006; Kumar et al., 2013). This is due to the fact that when treating wider size distributions, coarse, low density material will be misplaced to the underflow due to its net greater sizing effect. In the same way, extremely fine, high density material will report to the overflow irrespective of its overall density. Several investigations carried out by using the conventional TBS are summarized in Table 1. Most of the published literature on TBS has focused on the prominence if operational variables on the gravity separation of coarse particles. Li et al. (2013) investigate the separation of coal particles inside the TBS by collecting the samples at different heights of the separation column. It was concluded that size based classification is predominant across the length of the column and the separation efficiency of − 0.25 mm particle is poor compared to the coarser fraction (+ 0.25 mm). It has been demonstrated that the performance of TBS units can be predicted reasonably well using a slip velocity model and steady-state mass balance equations, where the narrowly coarse particle fraction is considered to be mono-size for the theoretical calculation and its size distribution is not considered (Sarkar et al., 2008; Das et al., 2009). The computed data from four different slip velocity models have been compared and contrasted with the experimental observations by Sarkar and Das (2010). It has been shown that a slip velocity model based on the modified Richardson and Zaki equation, in which the dissipative pressure gradient is considered to be the primary driving force for separation, predicts the performance more accurately than the other three. In this study, the effect of the foreign slime of −0.25 mm on the gravity separation of coarse particles was investigated based on the experimental and computational results. 2. Materials and experimental procedure 2.1. Coal samples Two types of coal samples with different size distributions were prepared for the study. The narrowly sized, F1, of −1 + 0.25 mm nominal size and an ash content of 22.20% was designated as coarse fractions. Meanwhile, the widely sized, F2, having −1 mm nominal size with an ash content of 27.26% was selected as the coarse fractions with fine fractions. The size fraction of − 0.25 mm was considered as the foreign slime compared the coarse fraction. In fact, F1 was from F2 by removing coal smile of −0.25 mm. Sieve analysis of the feed F2 and ash content according to size distributions are shown in Table 2. It was observed that the ash content witnessed an increase trend with decreasing size. The ash content of − 1 + 0.25 mm size fractions was obviously lower than that of −0.25 mm. For all practical purposes, narrowly sized fractions showed in Table 2 were considered as a mono-size fraction in this study.
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Table 2 Size and ash distribution of the feed F2. Size range (mm)
Median size dmed (mm)
Wt (%)
Ash (%)
1–0.25 −0.25 Total
0.625 0.125
59.18 40.82 100.00
22.20 27.86 24.51
2.2. Experimental procedure The schematic of the TBS used in this work is shown in Fig. 1. The column was fabricated using 2400 mm diameter and 3200 mm long steel pipe. The experimental apparatus contained an actuator, PID controller, pressure probe, teeter plates and spigot valve. The feed slurry enters tangentially through a feed well and a fluidised or teetered bed is created based on the fluidization of the heavier particles in the suspension using an upward current of water. When a steady state is reached, the particles which are lighter than the density of the teetered bed will float and report to the overflow stream, while the higher density particles will subside and report to the downflow current. The column was allowed to attain the steady state and the steady state was confirmed by constant the effective density of the teetered bed. Steady state was also confirmed by mass balance of top and bottom solids flow rates with the feed rate of solids(Drummond et al., 2002). A simple PID controller and a capacitance pressure probe (4–20 mA) are utilized to keep the effective density of the teetered bed constant. The effective density is compared to the operating set point. If the effective density is too high, the spigot valve will be activated by the actuator and excessive solids are discharged as the underflow. Conversely, the control system acts to restrict the solids discharge if the effective density is too low (Drummond et al., 2002; Tao et al., 2012). Tests of F1 anf F2 were executed at dfferent teetered water velocities of 16.46, 22.05, 28.94 and 34.59 mm/s. Feed solid content and effective density of the teetered bed were constant at 45% and 1.18 g/cm3 respectively. These overflow and underflow products of F1 and F2 were screened into two size fractions (1–0.25 mm and −0.25 mm), filtered, dried, stored in sealed bottles. For simplicity, coarse fractions of 1– 0.25 mm of the feed F1 and F2 were named as C1 and C2, respectively. Those coarse coal fractions (1–0.25 mm) obtained from the overflow and underflow products were used as the feed in the float-sink analysis.
3. Results and discussion 3.1. Effect of teeter water flow velocity In order to investigate the influence of the foreign slime on the separation performance of coarse particles thoroughly, separation tests were carried out in a TBS with different teeter water velocities. The yield and ash content of clean coal (without slime) are summarized in
Table 1 Summary of the study on conventional TBS. Investigator
Material
Equipment type
Particle size (mm)
Variables
Bethell, 1988 Drummond et al., 1998 Newling et al., 1998 Galvin et al., 1999 Cho and Kim, 2004 Maharaj et al., 2007 Sha et al., 2012 Tao et al., 2012 Ni et al., 2015 Ergun et al., 2016
Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal
Plant Plant Plant Pilot Pilot Pilot Pilot Plant Pilot pilot
N0.074 1.2–0.15 1.2–0.35 2.0–0.375 1.7–0.15 2.0–0.038 1.0–0.25 1–0.25 1.0–0.25 0.5–0.038
NA NA NA Suspension density and teeter water velocity Set point and teeter water velocity Distributor configuration and teeter water velocity Column height NA Teeter water velocity Teeter water velocity and set point
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X. Bu et al. / International Journal of Mineral Processing 160 (2017) 76–80
Fig. 3. Recoveries of low, middle and high-density fractions of clean coal. Fig. 1. Schematic of the experimental apparatus.
Fig. 2. Recoveries of low, middle and high-density fractions of clean coal with different teeter water velocities are given in Fig. 3. As shown in Fig. 2, the yield of clean coal was visibly increasing with the increase in teeter water velocity. The higher the teeter water velocity was, the more particles there were in the upward stream. Hence, there was an increase in yield. As observed in Fig. 3, more and more particles having middle and high-density were recovered in the overflow product, which resulted in the increase of ash content. There was an interesting phenomenon that the ash content of clean coal decreased firstly, and then increased with the increment of teeter water velocity. As shown in Fig. 3, the recovery of materials having low density (−1.4 g/cm3) of clean coal increased, while the proportion of middle and high-density materials decreased with the increment of teeter water velocity from 16.46 to 22.05 mm/s. When teeter water velocity was very lower than the minimum fluidization velocity, the steady state of the teetered bed was not developed to achieve the efficiency density separation in a settling separator, and thus the separator was unable to provide clean coal with low ash content due to the loss of low-density materials. With the gap between the teeter water velocity and the minimum fluidization velocity narrowing, particles were fluidized completely and the teetered bed was established. More and more low-density materials were recovery with the establishment of teetered bed and thus the ash content of clean coal decreased. After that, it can be seen from Fig. 3 that more and more particles with middle and highdensity reported to clean coal with the increase of teeter water velocity from 22.05 to 34.59 mm/s. Therefore, there was an increase of ash content of clean coal. To sum up, the ash content of clean coal increased as the teeter water velocity decreased initially and increased afterwards
Fig. 2. Effect of teeter water velocity on the separation performance of coarse particles (without slime).
according to the relationship between teeter water and minimum fluidization velocity. Cut density (ρ50) is the relative density corresponding to 50% of feed material reporting to the overflow, which can be generated by partition curves for each test obtained from sink-float data of coarse coal particles in overflow and underflow products. Besides, the cut density can also calculated by the slip velocity model with a given teeter water velocity and particle size (given in the Appendix A). The experimental and calculated cut ρ50 values are summarized in Fig. 4. The determination coefficient (R2) of the predicted model was 0.9588, which indicated the slip velocity model predictions are in reasonably close agreement with the experimental results. The model under-predicted the cut density under different teeter water velocities, which can be explained by the underestimation of density distribution (Sarkar and Das, 2010). 3.2. Effect of foreign slime In this study, the effect of foreign slime on the separation performance of coarse partocles was summarized in Table 3. With the introduction, the yield and ash content of clean coal of coarse particles increased under different teeter water velocities. Table 4 shows the recoveries of low, middle and high-density fractions of clean coal, which indicates more and more materials having middle and high-density were recoveried in the presence of foreign slime. Foreign slime was predicted to mostly report to the overflow without density separation. Meanwhile, some fine particles will be misplaced to the reject due to entrainment of downward current. Though the rest participated in the
Fig. 4. Comparison between experimental and computed cut density values.
X. Bu et al. / International Journal of Mineral Processing 160 (2017) 76–80 Table 3 Effect of foreign slime on the separation performance of coarse particles. Teeter water velocity (mm/s)
26.80 35.90 47.10 56.30
Without slime
Foreign slime
Yield (%)
Ash (%)
Yield (%)
Ash (%)
49.53 59.10 66.34 73.37
8.11 6.83 7.31 8.01
49.28 60.79 67.91 74.20
8.32 7.01 7.65 8.34
Table 4 Effect of foreign slime on yields of low, middle and high-density fractions of clean coal. Density fraction (kg/m3)
16.46 mm/s
b1.4 1.4–1.6 N1.6
42.15 40.82 6.07 6.56 1.31 1.90
No slime
22.05 mm/s
Foreign No slime slime
28.94 mm/s
Foreign No slime slime
51.88 52.25 6.56 7.87 0.66 0.67
34.56 mm/s
Foreign No slime slime
58.02 59.01 7.59 8.15 0.74 0.75
Foreign slime
62.41 62.90 10.78 10.25 0.18 1.05
formation of the teetered bed, the concentration of the fines in the teeter bed will be very different to that in the feed (Li et al., 2013). As observed in Table 3, the teeter water rate had a significant effect on the separation based on the experimental results compared to the foreign slime. Though the teeter water velocity has a greater effect on segregation than foreign slime, the effect of foreign slime on the gravity separation of coarse particles cannot be negligible. The presence of the fines in the teeter bed can increase the voscosity of the teeter water and the drag force of coarse particles (Allen, 1990). The increase in the drag force decreases the hindered settling velocity of coarse partilces, which results in that more and more materials having middle and high-density report to the overflow product with the indroduction of foreign slime. In order to gain a comprehensive understanding mechanism of the effect of the foreign slime effect on the separation of coarse particles, an in-depth interfacial adsorption study should be ongoing in the future. 4. Conclusions (1) The slip velocity model was useful to predict the separation process of coarse particles, while the predictions were underestimated compared the experimental results. (2) The teeter water velocity has a more apparent influence on the separation performance of coarse particles compared to the foreign slime, but the effect of the foreign slime cannot be negligible. Acknowledgments This work was supported by National Nature Science Foundation of China (Grant No. 51474213 and 51374205). The authors are also grateful to the assistance of Program for Postgraduates Research Innovation in Universities of Jiangsu Province (Grant No. KYLX_1410). Finally, the authors are gratefully acknowledged to the financial support from China Scholarship Council (Grant No. 201506420030).
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However, if the slip velocity of the particle is greater than the rising water, the particle settles downwards and reports to the underflow stream; otherwise, it is carried away to the overflow (Galvin et al., 1999). There are two categories of settling: free and hindered settling. In free settling, individual particles fall freely in a boundless medium without being hindered by other nearby particles. Gravity, drag and medium resistance created from dominated friction affects the free settling velocity. When the force is balanced, the acceleration of particles will be equal to zero and the settling velocity can reach the maximum. The maximum velocity is termed as the terminal free settling velocity and defined as (Gui et al., 2010): x
νt ¼ kd
ρp −ρ f ρf
!y
ρf μf
!z ð1Þ
where νt is the terminal free settling velocity (m/s), d is the spherical diameter of the particle (m), ρp and ρf are the densities of the particle and the carrying fluid (g/cm3), respectively, and μ f is the kinematic viscosity (Kg ⋅ m−1 ⋅ s−1). The parameters of constants (k, x, y and z) depend on Reynolds number (Re). The Reynolds number of the particle at its terminal settling velocity is given as: Re ¼
ρp ν t d μf
ð2Þ
Hartman et al. (1989) have pointed out that the terminal settling velocity calculated using Eq. (2) has substantial error. An explicit equation proposed by Zigrang and Sylvester (1981) can be expressed as : 2 32 ! 1:5 0:5 0:5 d Re ¼ 4 14:51 þ g ρp −ρ f ρ f 1:83 −3:815 μf
ð3Þ
Eq. (3) is derived from the simple balance of forces acting on a single, spherical particle failing in a uniform field for particle settling velocities in liquid-solid system, which provides a direct approach to calculate Reynolds number (Galvin et al., 1999). The expression of Zigrang and Sylvester has been used throughout the present work. Contrary to free settling, the particle hindered settling is influenced by the presence of particles in nearby vicinity. Hindered settling is a common phenomenon in TBS. Many models of hindered settling velocity have been developed to adequately model the density separation in a hindered bed. It is concluded that the model proposed by Galvin et al. (1999) in which the dissipative pressure gradient is considered as the main driving force for the separation, offers better performance predictions than other models (Happel, 1958; Sarka and Das, 2010). The Galvin et al.’s equation of hindered settling velocity is defined as (Asif, 1998; Galvin et al., 1999):
νh;i ¼ νt;i
ni ¼
ρi −ρs ρi −ρ f
!ni −1
5:1 þ 0:27Re0:9 1:0 þ 0:10Re0:9
ð4Þ
ð5Þ
Appendix A. Slip velocity model In a TBS, particles are separated by the segregation based on different hindered settling velocities. Therefore, the separation can be characterized by the relative velocity of each particle with respect to the velocity of an upward current of water (teeter water), which is the slip velocity. Particles with the same slip velocity as the teeter water have equal chances of settling or being transported upwards by water.
in which νh, i is the hindered settling velocity (m/s), ρi and ρs are the densities of the species i and the suspension (g/cm3), respectively. References Allen, H.S., 1990. The motion of a sphere in a viscous fluid. Philos. Mag. 50, 519–534. Asif, M., 1998. Segregation velocity model for fluidized suspension of binary mixtures of particles. Chem. Eng. Process. 37, 279–286.
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Bethell, P.J., 1988. Current and future processing flowsheets. In: Klimpel, R.R., Luckie, P.T. (Eds.), Industrial Practice of Fine Coal Processing. Society of Mining Engineers of AIME, Littleton, CO, pp. 317–329. Cho, H., Kim, J., 2004. Recovery of carbon from Korean anthracite by hindered-settling separation. Resour. Process. 51 (1), 52–55. Das, A., Sarkar, B., Mehrotra, S.P., 2009. Prediction of separation performance of floatex density separator for processing of fine coal particles. Int. J. Miner. Process. 91 (1– 2), 41–49. Drummond, R.B., Swanson, A.R., Nicol, S.K., Newling, P.G., 1998. Optimization studies on a 75 T/H teetered bed separator at Stratford Coal Preparation Plant. In: Partridge, A.C., Partridge, I.R. (Eds.), Proceedings of the 13th International Coal Preparation Congress. Australian Coal Preparation Society, Brisbane, Australia, pp. 215–224. Drummond, R.B., Swanson, A.R., Nicol, S.K., 2002. Teetered bed separators the Australian experience. J. South. Afr. Inst. Min. Metall. 102 (7), 385–391. Ergun, L., Bahrami, A., Ziypak, M., Özer, A., 2016. In-plant testing of teetered bed separator in Omerler washing plant. In: Litvinenko, V. (Ed.), Proceedings XVIII International Coal Preparation Congress. Springer International Publishing, Saint-Petersburg, Russia, pp. 849–854. Galvin, K.P., Pratten, S.J., Nicol, S.K., 1999. Dense medium separation using a teetered bed separator. Miner. Eng. 12 (9), 1059–1081. Gui, X., Liu, J., Li, Y., Cao, Y., Liu, C., Li, G., 2010. Study on settlement characteristic of the grain in fluidized bed. XXV Int. Min. Process. Congr. (IMPC) 2010 Proceedings, Brisbane, Aus. September, pp. 757–765. Happel, J., 1958. Viscous flow in multiparticle systems: slow motion of fluids relative to beds of spherical particles. AICHE J. 4 (2), 197–201. Hartman, M., Havlin, V., Trnka, O., Cˇárský, M., 1989. Predicting the free-fall velocities of spheres. Chem. Eng. Sci. 44 (8), 1743–1745. Heiskanen, K., 1993. Particle Classification. Chapman & Hall, London. Honaker, H.Q., Mondal, K., 1999. Dynamic development of fine coal separations in hindered-bed classifier. Coal Prep. 21, 211–232. Kumar, C.R., Bhoja, S.K., Tripathy, S.K., Mohanan, S., Venugopalan, T., Suresh, N., 2013. Classification performance evaluation of floatex density separator for coal fines. Fuel 108, 303–310.
Li, Y., Zhao, W., Xu, S., Xia, W., 2013. Changes of size, ash and density of coal particles on the column axis of a liquid–solid fluidized bed. Powder Technol. 245, 251–254. Littler, A., 1986. Automatic hindered-settling classifier for hydraulic sizing and mineral beneficiation. Trans. Inst. Min. Metall. 95, 133–138. Luttrell, G.H., Westerfield, T.C., Kohmuench, J.N., Mankosa, M.J., Mikkola, K.A., Oswald, G., 2006. Development of high-efficiency hydraulic separators. Miner. Metall. Process. 23 (1), 33–39. Maharaj, L., Pocock, J., Loveday, B.K., 2007. The effect of distributor configuration on the hydrodynamics of the teetered bed separator. Miner. Eng. 20 (11), 1089–1098. Newling, P., Weale, W., Crisafulli, P., Swanson, A., 1998. An innovative approach to a Boutique Mine and coal preparation plant at Stratford coal. In: Partridge, A.C., Partridge, I.R. (Eds.), Proceedings of the 13th International Coal Preparation Congress. Australian Coal Preparation Society, Brisbane, Australia, pp. 36–42. Ni, C., Xie, G., Liu, B., Bu, X., Peng, Y., Sha, J., 2015. Effect of bubbles addition on teetered bed separation. Int. J. Min. Sci. Technol. 25 (5), 835–841. Sarkar, B., Das, A., 2010. A comparative study of slip velocity models for the prediction of performance of floatex density separator. Int. J. Miner. Process. 94 (1), 20–27. Sarkar, B., Das, A., Mehrotra, S.P., 2008. Study of separation features in floatex density separator for cleaning fine coal. Int. J. Miner. Process. 86, 40–49. Sha, J., Xie, G., Wang, H., Liu, J., Tang, L., 2012. Effect of the column height on the performance of liquid–solid fluidized bed for the separation of coarse slime. Int. J. Min. Sci. Technol. 22 (4), 585–588. Zigrang, D.J., Sylvester, N.D., 1981. An explicit equation for particle settling velocities in solid-liquid systems. AICHE J. 27 (6), 1043–1044. Tao, Y., Wang, L., Li, Z., Sun, M., 2012. Study on coarse slime separation by teeter bed separator in Chenglao coal preparation plant. In: Young, C.A., Luttrell, G.H. (Eds.), Separation Technology. SME, New York, pp. 177–180. Tripathy, S.K., Mallick, M.K., Singh, V., Murthy, Y.R., 2013. Preliminary studies on teeter bed separator for separation of manganese fines. Powder Technol. 239, 284–289. Tripathy, S.K., Bhoja, S.K., Kumar, C.R., Suresh, N., 2015. A short review on hydraulic classification and its development in mineral industry. Powder Technol. 270, 205–220.
Contents lists available at ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Review
Preliminary study on foreign slime for the gravity separation of coarse coal particles in a teeter bed separator Xiangning Bu a,b, Chao Ni a, Guangyuan Xie a,⁎, Yaoli Peng a,⁎, Linhan Ge b, Jie Sha a a b
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, PR China Department of Chemical Engineering, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 5 July 2016 Received in revised form 6 December 2016 Accepted 21 January 2017 Available online 23 January 2017
The teetered bed separator (TBS) is a gravity concentration device based on the principle of hindered settling velocity, which has been widely applied for the density separation of coarse coal particles. The influences of the teeter water velocity on the density separation of coarse coal particles were investigated based on the slip velocity model and the experimental results. The ash content of clean coal increased as the teeter water velocity decreased initially and increased afterwards according to the relationship between teeter water and minimum fluidization velocity. It was observed that the foreign slime had an effect on the separation of coarse particles, while the effect of the foreign slime was not as significant as that of the teeter water velocity. © 2017 Published by Elsevier B.V.
Keywords: Foreign slime Coarse particles Teetered bed separator Gravity separation Coal
Contents 1. 2.
Introduction . . . . . . . . . . . . . Materials and experimental procedure . 2.1. Coal samples . . . . . . . . . . 2.2. Experimental procedure. . . . . 3. Results and discussion . . . . . . . . . 3.1. Effect of teeter water flow velocity 3.2. Effect of foreign slime . . . . . . 4. Conclusions. . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . Appendix A. Slip velocity model . . . . . References. . . . . . . . . . . . . . . . .
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1. Introduction Teeter bed separators are hydraulic classifiers that have been recognized as low-cost and high capacity devices for both classification and density separation. Presently over 200 such units have been installed worldwide, in mineral processing applications including silica sand for construction grade, foundry and glass making purpose, mineral sands
⁎ Corresponding authors at: School of Chemical Engineering and Technology, China University of Mining and Technology, 1 Daxue Road, Xuzhou 221116, Jiangsu, PR China. E-mail addresses: [email protected] (G. Xie), [email protected] (Y. Peng).
http://dx.doi.org/10.1016/j.minpro.2017.01.009 0301-7516/© 2017 Published by Elsevier B.V.
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and hematite processing along with coal (Littler, 1986; Kumar et al., 2013; Tripathy et al., 2013). The conventional teeter bed separator (TBS) is developed from the concept of hydrosizer and the initial designs have been manufactured since 1934. There are various commercial units based on the conventional TBS such as floatex density separator (FDS), reflux classifier (RC), cross flow separator (CFS), allflux classifier and hydro float separator, which work based on the principles of fluidization and hindered settling with different types of equipment configuration, feeder and discharge systems (Tripathy et al., 2015). Drummond et al. (2002) demonstrated that the circuit efficiency could be improved by introducing the teeter bed separator between
X. Bu et al. / International Journal of Mineral Processing 160 (2017) 76–80
dense medium cyclones and froth flotation. However, it has been shown that these devices can be effectively applied to gravity separations provided that the size distribution of the feed is within acceptable limits, depending on the application (Bethell, 1988; Heiskanen, 1993; Honaker and Mondal, 1999; Luttrell et al., 2006; Kumar et al., 2013). This is due to the fact that when treating wider size distributions, coarse, low density material will be misplaced to the underflow due to its net greater sizing effect. In the same way, extremely fine, high density material will report to the overflow irrespective of its overall density. Several investigations carried out by using the conventional TBS are summarized in Table 1. Most of the published literature on TBS has focused on the prominence if operational variables on the gravity separation of coarse particles. Li et al. (2013) investigate the separation of coal particles inside the TBS by collecting the samples at different heights of the separation column. It was concluded that size based classification is predominant across the length of the column and the separation efficiency of − 0.25 mm particle is poor compared to the coarser fraction (+ 0.25 mm). It has been demonstrated that the performance of TBS units can be predicted reasonably well using a slip velocity model and steady-state mass balance equations, where the narrowly coarse particle fraction is considered to be mono-size for the theoretical calculation and its size distribution is not considered (Sarkar et al., 2008; Das et al., 2009). The computed data from four different slip velocity models have been compared and contrasted with the experimental observations by Sarkar and Das (2010). It has been shown that a slip velocity model based on the modified Richardson and Zaki equation, in which the dissipative pressure gradient is considered to be the primary driving force for separation, predicts the performance more accurately than the other three. In this study, the effect of the foreign slime of −0.25 mm on the gravity separation of coarse particles was investigated based on the experimental and computational results. 2. Materials and experimental procedure 2.1. Coal samples Two types of coal samples with different size distributions were prepared for the study. The narrowly sized, F1, of −1 + 0.25 mm nominal size and an ash content of 22.20% was designated as coarse fractions. Meanwhile, the widely sized, F2, having −1 mm nominal size with an ash content of 27.26% was selected as the coarse fractions with fine fractions. The size fraction of − 0.25 mm was considered as the foreign slime compared the coarse fraction. In fact, F1 was from F2 by removing coal smile of −0.25 mm. Sieve analysis of the feed F2 and ash content according to size distributions are shown in Table 2. It was observed that the ash content witnessed an increase trend with decreasing size. The ash content of − 1 + 0.25 mm size fractions was obviously lower than that of −0.25 mm. For all practical purposes, narrowly sized fractions showed in Table 2 were considered as a mono-size fraction in this study.
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Table 2 Size and ash distribution of the feed F2. Size range (mm)
Median size dmed (mm)
Wt (%)
Ash (%)
1–0.25 −0.25 Total
0.625 0.125
59.18 40.82 100.00
22.20 27.86 24.51
2.2. Experimental procedure The schematic of the TBS used in this work is shown in Fig. 1. The column was fabricated using 2400 mm diameter and 3200 mm long steel pipe. The experimental apparatus contained an actuator, PID controller, pressure probe, teeter plates and spigot valve. The feed slurry enters tangentially through a feed well and a fluidised or teetered bed is created based on the fluidization of the heavier particles in the suspension using an upward current of water. When a steady state is reached, the particles which are lighter than the density of the teetered bed will float and report to the overflow stream, while the higher density particles will subside and report to the downflow current. The column was allowed to attain the steady state and the steady state was confirmed by constant the effective density of the teetered bed. Steady state was also confirmed by mass balance of top and bottom solids flow rates with the feed rate of solids(Drummond et al., 2002). A simple PID controller and a capacitance pressure probe (4–20 mA) are utilized to keep the effective density of the teetered bed constant. The effective density is compared to the operating set point. If the effective density is too high, the spigot valve will be activated by the actuator and excessive solids are discharged as the underflow. Conversely, the control system acts to restrict the solids discharge if the effective density is too low (Drummond et al., 2002; Tao et al., 2012). Tests of F1 anf F2 were executed at dfferent teetered water velocities of 16.46, 22.05, 28.94 and 34.59 mm/s. Feed solid content and effective density of the teetered bed were constant at 45% and 1.18 g/cm3 respectively. These overflow and underflow products of F1 and F2 were screened into two size fractions (1–0.25 mm and −0.25 mm), filtered, dried, stored in sealed bottles. For simplicity, coarse fractions of 1– 0.25 mm of the feed F1 and F2 were named as C1 and C2, respectively. Those coarse coal fractions (1–0.25 mm) obtained from the overflow and underflow products were used as the feed in the float-sink analysis.
3. Results and discussion 3.1. Effect of teeter water flow velocity In order to investigate the influence of the foreign slime on the separation performance of coarse particles thoroughly, separation tests were carried out in a TBS with different teeter water velocities. The yield and ash content of clean coal (without slime) are summarized in
Table 1 Summary of the study on conventional TBS. Investigator
Material
Equipment type
Particle size (mm)
Variables
Bethell, 1988 Drummond et al., 1998 Newling et al., 1998 Galvin et al., 1999 Cho and Kim, 2004 Maharaj et al., 2007 Sha et al., 2012 Tao et al., 2012 Ni et al., 2015 Ergun et al., 2016
Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal
Plant Plant Plant Pilot Pilot Pilot Pilot Plant Pilot pilot
N0.074 1.2–0.15 1.2–0.35 2.0–0.375 1.7–0.15 2.0–0.038 1.0–0.25 1–0.25 1.0–0.25 0.5–0.038
NA NA NA Suspension density and teeter water velocity Set point and teeter water velocity Distributor configuration and teeter water velocity Column height NA Teeter water velocity Teeter water velocity and set point
78
X. Bu et al. / International Journal of Mineral Processing 160 (2017) 76–80
Fig. 3. Recoveries of low, middle and high-density fractions of clean coal. Fig. 1. Schematic of the experimental apparatus.
Fig. 2. Recoveries of low, middle and high-density fractions of clean coal with different teeter water velocities are given in Fig. 3. As shown in Fig. 2, the yield of clean coal was visibly increasing with the increase in teeter water velocity. The higher the teeter water velocity was, the more particles there were in the upward stream. Hence, there was an increase in yield. As observed in Fig. 3, more and more particles having middle and high-density were recovered in the overflow product, which resulted in the increase of ash content. There was an interesting phenomenon that the ash content of clean coal decreased firstly, and then increased with the increment of teeter water velocity. As shown in Fig. 3, the recovery of materials having low density (−1.4 g/cm3) of clean coal increased, while the proportion of middle and high-density materials decreased with the increment of teeter water velocity from 16.46 to 22.05 mm/s. When teeter water velocity was very lower than the minimum fluidization velocity, the steady state of the teetered bed was not developed to achieve the efficiency density separation in a settling separator, and thus the separator was unable to provide clean coal with low ash content due to the loss of low-density materials. With the gap between the teeter water velocity and the minimum fluidization velocity narrowing, particles were fluidized completely and the teetered bed was established. More and more low-density materials were recovery with the establishment of teetered bed and thus the ash content of clean coal decreased. After that, it can be seen from Fig. 3 that more and more particles with middle and highdensity reported to clean coal with the increase of teeter water velocity from 22.05 to 34.59 mm/s. Therefore, there was an increase of ash content of clean coal. To sum up, the ash content of clean coal increased as the teeter water velocity decreased initially and increased afterwards
Fig. 2. Effect of teeter water velocity on the separation performance of coarse particles (without slime).
according to the relationship between teeter water and minimum fluidization velocity. Cut density (ρ50) is the relative density corresponding to 50% of feed material reporting to the overflow, which can be generated by partition curves for each test obtained from sink-float data of coarse coal particles in overflow and underflow products. Besides, the cut density can also calculated by the slip velocity model with a given teeter water velocity and particle size (given in the Appendix A). The experimental and calculated cut ρ50 values are summarized in Fig. 4. The determination coefficient (R2) of the predicted model was 0.9588, which indicated the slip velocity model predictions are in reasonably close agreement with the experimental results. The model under-predicted the cut density under different teeter water velocities, which can be explained by the underestimation of density distribution (Sarkar and Das, 2010). 3.2. Effect of foreign slime In this study, the effect of foreign slime on the separation performance of coarse partocles was summarized in Table 3. With the introduction, the yield and ash content of clean coal of coarse particles increased under different teeter water velocities. Table 4 shows the recoveries of low, middle and high-density fractions of clean coal, which indicates more and more materials having middle and high-density were recoveried in the presence of foreign slime. Foreign slime was predicted to mostly report to the overflow without density separation. Meanwhile, some fine particles will be misplaced to the reject due to entrainment of downward current. Though the rest participated in the
Fig. 4. Comparison between experimental and computed cut density values.
X. Bu et al. / International Journal of Mineral Processing 160 (2017) 76–80 Table 3 Effect of foreign slime on the separation performance of coarse particles. Teeter water velocity (mm/s)
26.80 35.90 47.10 56.30
Without slime
Foreign slime
Yield (%)
Ash (%)
Yield (%)
Ash (%)
49.53 59.10 66.34 73.37
8.11 6.83 7.31 8.01
49.28 60.79 67.91 74.20
8.32 7.01 7.65 8.34
Table 4 Effect of foreign slime on yields of low, middle and high-density fractions of clean coal. Density fraction (kg/m3)
16.46 mm/s
b1.4 1.4–1.6 N1.6
42.15 40.82 6.07 6.56 1.31 1.90
No slime
22.05 mm/s
Foreign No slime slime
28.94 mm/s
Foreign No slime slime
51.88 52.25 6.56 7.87 0.66 0.67
34.56 mm/s
Foreign No slime slime
58.02 59.01 7.59 8.15 0.74 0.75
Foreign slime
62.41 62.90 10.78 10.25 0.18 1.05
formation of the teetered bed, the concentration of the fines in the teeter bed will be very different to that in the feed (Li et al., 2013). As observed in Table 3, the teeter water rate had a significant effect on the separation based on the experimental results compared to the foreign slime. Though the teeter water velocity has a greater effect on segregation than foreign slime, the effect of foreign slime on the gravity separation of coarse particles cannot be negligible. The presence of the fines in the teeter bed can increase the voscosity of the teeter water and the drag force of coarse particles (Allen, 1990). The increase in the drag force decreases the hindered settling velocity of coarse partilces, which results in that more and more materials having middle and high-density report to the overflow product with the indroduction of foreign slime. In order to gain a comprehensive understanding mechanism of the effect of the foreign slime effect on the separation of coarse particles, an in-depth interfacial adsorption study should be ongoing in the future. 4. Conclusions (1) The slip velocity model was useful to predict the separation process of coarse particles, while the predictions were underestimated compared the experimental results. (2) The teeter water velocity has a more apparent influence on the separation performance of coarse particles compared to the foreign slime, but the effect of the foreign slime cannot be negligible. Acknowledgments This work was supported by National Nature Science Foundation of China (Grant No. 51474213 and 51374205). The authors are also grateful to the assistance of Program for Postgraduates Research Innovation in Universities of Jiangsu Province (Grant No. KYLX_1410). Finally, the authors are gratefully acknowledged to the financial support from China Scholarship Council (Grant No. 201506420030).
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However, if the slip velocity of the particle is greater than the rising water, the particle settles downwards and reports to the underflow stream; otherwise, it is carried away to the overflow (Galvin et al., 1999). There are two categories of settling: free and hindered settling. In free settling, individual particles fall freely in a boundless medium without being hindered by other nearby particles. Gravity, drag and medium resistance created from dominated friction affects the free settling velocity. When the force is balanced, the acceleration of particles will be equal to zero and the settling velocity can reach the maximum. The maximum velocity is termed as the terminal free settling velocity and defined as (Gui et al., 2010): x
νt ¼ kd
ρp −ρ f ρf
!y
ρf μf
!z ð1Þ
where νt is the terminal free settling velocity (m/s), d is the spherical diameter of the particle (m), ρp and ρf are the densities of the particle and the carrying fluid (g/cm3), respectively, and μ f is the kinematic viscosity (Kg ⋅ m−1 ⋅ s−1). The parameters of constants (k, x, y and z) depend on Reynolds number (Re). The Reynolds number of the particle at its terminal settling velocity is given as: Re ¼
ρp ν t d μf
ð2Þ
Hartman et al. (1989) have pointed out that the terminal settling velocity calculated using Eq. (2) has substantial error. An explicit equation proposed by Zigrang and Sylvester (1981) can be expressed as : 2 32 ! 1:5 0:5 0:5 d Re ¼ 4 14:51 þ g ρp −ρ f ρ f 1:83 −3:815 μf
ð3Þ
Eq. (3) is derived from the simple balance of forces acting on a single, spherical particle failing in a uniform field for particle settling velocities in liquid-solid system, which provides a direct approach to calculate Reynolds number (Galvin et al., 1999). The expression of Zigrang and Sylvester has been used throughout the present work. Contrary to free settling, the particle hindered settling is influenced by the presence of particles in nearby vicinity. Hindered settling is a common phenomenon in TBS. Many models of hindered settling velocity have been developed to adequately model the density separation in a hindered bed. It is concluded that the model proposed by Galvin et al. (1999) in which the dissipative pressure gradient is considered as the main driving force for the separation, offers better performance predictions than other models (Happel, 1958; Sarka and Das, 2010). The Galvin et al.’s equation of hindered settling velocity is defined as (Asif, 1998; Galvin et al., 1999):
νh;i ¼ νt;i
ni ¼
ρi −ρs ρi −ρ f
!ni −1
5:1 þ 0:27Re0:9 1:0 þ 0:10Re0:9
ð4Þ
ð5Þ
Appendix A. Slip velocity model In a TBS, particles are separated by the segregation based on different hindered settling velocities. Therefore, the separation can be characterized by the relative velocity of each particle with respect to the velocity of an upward current of water (teeter water), which is the slip velocity. Particles with the same slip velocity as the teeter water have equal chances of settling or being transported upwards by water.
in which νh, i is the hindered settling velocity (m/s), ρi and ρs are the densities of the species i and the suspension (g/cm3), respectively. References Allen, H.S., 1990. The motion of a sphere in a viscous fluid. Philos. Mag. 50, 519–534. Asif, M., 1998. Segregation velocity model for fluidized suspension of binary mixtures of particles. Chem. Eng. Process. 37, 279–286.
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