J. Cent. South Univ. Technol. (2011) 18: 374-380

DOI: 10.1007/s11771-011-0706-6

Modularized dry coal beneficiation technique based on gas-solid fluidized bed

ZHAO Yue-min(赵跃民)¹, LI Gong-min(李功民)², LUO Zhen-fu(骆振福)¹, LIANG Chun-cheng(梁春成)¹,

TANG Li-gang(唐利刚)³, CHEN Zeng-qiang(陈增强)¹, XING Hong-bo(邢洪波)¹

1. School of Chemical Engineering and Technology, China University of Mining and Technology,

Xuzhou 221116, China;

2. Tangshan Shenzhou Manufacturing Co., Ltd., Tangshan 063001, China;

3. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering,

Chinese Academy of Sciences, Beijing 100190, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: A 40-60 t/h modularized dry coal beneficiation process with a novel method to control the bed was designed around a gas-solid fluidized bed separator. Furthermore, the hydrodynamics of medium-solids consisting of wide-size-range magnetite powder (0.3-0.06 mm) and <1 mm fine coal were numerically studied. The simulation results show that the fluidization performance of the wide-size-range medium-solid bed is good. The separation performance of the modularized system was then investigated in detail using a mixture of <0.3 mm magnetite powder (mass fraction of 0.3-0.06 mm particles is 91.38 %) and <1 mm fine coal as solid media. The experimental results show that at separation densities of 1.33 g/cm³ or 1.61 g/cm³, 50-6 mm coal can be separated effectively with probable error, E, values of 0.05 g/cm³ and 0.06 g/cm³, respectively. This technique is beneficial for saving water resources and for the clean utilization of coal.

Key words: dry coal beneficiation; modularization; gas-solid fluidized bed; wide-size-range medium-solids

1 Introduction

Water based wet separation processes are the main methods utilized in the field of coal beneficiation today [1]. These techniques are, however, unsuitable for coals that tend to slime in a wet separation process or for those located in arid or cold region. Compared with wet processing, the air shaking table and air jig, which use air as the separation medium for dry beneficiation of coal, have high separation density and low separation efficiency. Fluidization technology was first applied to coal processing in the form of dry density-based beneficiation technologies using gas-solid fluidized beds by FRASER and YANCEY [2]. Since then, studies related to these methods have been performed in many countries around the world. Magnetite powder and sand were used as solid medium for dry beneficiation of 50-  3 mm coal by YOUROVSKY et al [3] in the former Union of Soviet Socialist Republics. 75-0.7 mm coal and other minerals were separated in a vibrated fluidized bed containing magnetite powder by DOUGLAS et al [4] in United Kingdom. The fluidized counter-current cascade principle was proposed and applied to the beneficiation of sand and carbon mixtures by BEECKMANS and his coworkers at Western Ontario University, Canada [5-6]. At the University of Alberta in Canada, the mercury association with components of Alberta sub–bituminous coal was studied, and the experiments of mercury and ash removal were performed using air dense medium fluidized beds [7-8]. An air dense medium fluidized bed separator was designed and applied to dry beneficiation of 25-6 mm coal using magnetite powder as solid medium in India [9]. The pilot-scale dry beneficiation of coal was carried out at Okayama University in Japan [10]. A study was supported by the European Coal and Steel Community, and performed by Delft University of Technology (Netherlands), Nottingham University (UK) and RWTH Aachen (Germany) to evaluate the dry beneficiation and to investigate the potential improvements [11-12]. An efficient dry beneficiation device, AKAFLOW, for fine coal (<3 mm) and other raw materials was proposed by WEITKAEMPER et al [13], and the pilot-scale experiments for various types of coal were carried out. A dry density beneficiation technique using an air-with- dense-medium fluidized bed as the separation medium has been presented by workers at China University of Mining and Technology [14-16]. The industry scale separation experiment for 50-6 mm coal using KZA2050 system was performed in China. However, there are some problems with the technique. The equipment was arranged in a conventional way, located in a building 27 m high. The feedstock was elevated to the top of the building from where it proceeded through each processing point. This arrangement leads to a large construction cost, a large area and volume, for the processing system. Moreover, it has not been easy to accurately control the density and height of the bed, or to maintain its uniformity and stability. A 40-60 t/h KZX-40 system, with a gas-solid fluidized bed separator, therefore, was developed to solve these problems. A new dry coal beneficiation technique using a gas-solid fluidized bed is proposed based on the modularization of the processing components.

2 Design of industry scale modularization system using gas-solid fluidized bed

2.1 Separation principle of gas-solid fluidized bed

The modularized dry density beneficiation of coal is a dense-media separation method. The gas-solid fluidized bed has fluid-like characteristics, as illustrated in Fig.1.

Notable properties of a fluidized bed are:

1) The upward gas current fluidizes the solid medium to the same height in connected chambers (Fig.1(a)).

2) The surface of the bed maintains the same horizontal level when the container is inclined (Fig.1(b)).

3) The pressure drop between two points in the bed is approximately equal to the difference between the hydrostatic heads at the two points (Fig.1(c)) so that

Δp=p₁-p₂=ρg(h₁-h₂)                           (1)

where Δp is the pressure drop, p₁ is the pressure at point 1, p₂ is the pressure at point 2, ρ is the bed density, g is the gravitational acceleration constant, h₁ is the depth from point 1 to the bed surface and h₂ is the depth from point 2 to the bed surface.

If the solid medium is fluidized completely, then according to the principle of mechanical balance, Eq.(1) can be expressed as

Δp=(h₁-h₂)(1-ε)(ρ_p-ρ_g)g                       (2)

where ε is the bed voidage, ρ_p is the density of the solid and ρ_g is the density of the gas.

Eqs.(1) and (2) allow the bed density, ρ, to be expressed as

ρ=(1-ε)(ρ_p-ρ_g)                               (3)

Since ρ_g is small, far smaller than ρ_p, Eq.(3) may be rewritten as

ρ=(1-ε)ρ_p                                   (4)

4) Fluidized media will spray out of a hole in the chamber wall, which is similar to the liquid behavior (Fig.1(d)).

5) Coals with a density less than the bed density will float to the top surface of the bed, while those denser than the bed sink to the bottom of the container. This process is subjected to Archimedes law (Fig.1(e)). So,

ρ₁<ρ<ρ₂                                     (5)

where ρ₁ is the density of floating coal and ρ₂ is the density of sunk coal.

2.2 Design parameters

A 40-60 t/h gas-solid fluidized bed separator was designed. The structure of the separator is shown in Fig.2. The technical indexes and design parameters of the separator are presented in Table 1. A modularized KZX-40 system for dry coal beneficiation was then constructed around the separator. Table 2 shows a comparison of design parameters for the KZA2050 and the KZX-40 systems. The modularized system greatly reduces the construction and operating costs and decreases the area and volume required, compared with the KZA2050 system. Construction costs and the number of workers needed are reduced by 60% and 80%, respectively. The footprint and volume of the system are decreased by 75% and by 89.81%, respectively. These facts show the improved intensity, efficiency and economy of modularized coal beneficiation technique.

Fig.1 Pseudo-fluid properties of gas-solid fluidized beds

Fig.2 Schematic diagram of 40-60 t/h gas-solid fluidized bed separator

Table 1 Technical indexes and design parameters of separator

Table 2 Comparison of design parameters between KZA2050 system and KZX-40 system

2.3 Process flow

The modularized KZX-40 system consists of raw coal pre-treatment (drying and screening), coal separation, medium solid recovery, air supply and dust capture and removal. The engineering flow sheet for the system and the arrangement of the processing components are shown in Figs.3 and 4, respectively. Maintaining a uniform and stable bed density and height is very important for effective density-based separation of coals. However, during the separating process, fine coal (e.g. <1 mm) accumulates continuously in the system. This fine coal exists to some degree in the raw coal and also arises from the collision among coal lumps or between coal and the equipment. This accumulation inevitably results in the fluctuation of the bed density and height and eventually affects the separation efficiency. The fine coal content of the bed is too high for effective coal separation when the mean bed density becomes lower than some critical value. When this happens, the split-flow of the medium solids withdrawn from the system should be increased to remove the fine coal and to increase the bed density. Larger split-flows will restore the mean bed density more quickly. Once the mean bed density returns to the normal state, the split-flow continues at a certain level to maintain the bed uniformity and stability. Bed density and height control are key operations in the separating process. The sooner the medium solids split by distributor are recycled back into the fluidized bed, the steadier and more uniform the density and height of the bed will be. However, the KZA2050 system had one solid stream recycled back into medium-solid storage. The second stream was separated by the magnetic separator and entered into magnetite-powder storage. Then, the solids from these two storage bins moved back into the fluidized bed. In this case, obviously, the bulk densities of the solids in two storage tanks were needed to accurately adjust the density and height of bed. This resulted in an increased workload and created other problems difficult to solve in an industrial scale coal beneficiation environment. Density and height were, therefore, controlled by experience and frequently fluctuated widely.

Based on the aforementioned analysis, a novel method was proposed to control the density and height of the bed implemented by a master-control triple valve, a sub-control triple valve and a screw conveyor that transported dust from system onto the belt conveyor for the media. If the density and height of the bed were in a normal state, the two triple valves directly recycled the dust-containing split solid stream back to the separator. The two currents of solid media contribute to the stability and uniformity in bed density and in bed height. The recycled dust mainly contributes to the height stability. When the density and height of the bed fluctuate, the split-flow can be increased and the split solid medium controlled by the two triple valves can also enter the two storage bins according to the observed variations. This novel method, hence, can optimize the process flow and enhance the uniformity and stability of the bed density and height, which improves the fluidization quality. Moreover, it can be assumed that wide-size-range magnetite powder is suitable for this system. The acceptable size range in the modularized system can be broader in comparison with that in the KZA2050 system.

Fig.3 Engineering flow sheet of modularized system

Fig.4 Modularized dry coal beneficiation system: 1—Dryer; 2—Gas-solid fluidized bed separator; 3—Valve; 4—Media storage; 5— Master-control triple valve; 6—Distributor; 7—Magnetic separator; 8—Sub-control triple valve; 9—Exit for sinkers; 10—Storage of magnetite powder; 11—Valve; 12—Bag collector; 13—Screw conveyor; 14, 15—Screen; 16—Exit for floaters; 17—Belt conveyor for feedstock; 18—Gas distributor; 19—Entry for feedstock; 20—Belt conveyor for medium solids; 21—Bucket elevator; 22—Draft fan

3 Results and discussion

3.1 Hydrodynamics of bed containing wide-size-range medium-solids

0.3-0.15mm magnetite powder was utilized in the KZA2050 system. As pointed out by ZHAO et al [17], the difficult preparation of the large Geldart B magnetite powder resulted in a high cost and the activity of the large particle bed was relatively low. As an expanded work, the hydrodynamics of the bed, which contains 0.3-0.15 mm and 0.15-0.06 mm magnetite powders, and <1 mm fine coal, were therefore numerically investigated. The mass fractions of the three components were 46%, 46% and 8%, respectively. Computational fluid dynamics (CFD) software, Fluent 6.2.16, was employed to simulate the wide-size-range medium-solid bed. The parameters for the simulations were similar to those used in our previous work [17]. The variation of the bed void fraction with time is shown in Fig.5. It can be seen that the bed height is stable with only slight fluctuation. Because of the constant height of scraper conveyor in the separator, the stable bed height is necessary and beneficial for the separation. The fluctuations in the pressure drop and the density of the bed under a superficial gas velocity of 1.8U_mf (U_mf is the minimum fluidization velocity) are illustrated in Figs.6 and 7, respectively. When the fluidization state is steady, the pressure drops at fluidized bed height of H_f=55, 165 and 275 mm fluctuate around 4.69, 2.62 and 0.63 kPa, respectively. The standard deviations of the three pressure drops are 0.08, 0.11 and 0.10 kPa, respectively. This indicates a stable and uniform pressure drop of the bed. As seen in Fig.7, the bed densities at three bed heights tend to be uniform and fluctuate near 1.86 g/cm³. This indicates that the wide-size-range (0.3-0.06 mm) magnetite powder and <1 mm fine coal can be fully mixed with a stable and uniform bed density.

Fig.5 Void fraction of bed versus time: (a) 6 s; (b) 10 s; (c) 14 s

Fig.6 Fluctuation in pressure drop of bed containing wide-size- range solid media

Fig.7 Fluctuation in density of bed containing wide-size-range solid media

A mixture of <0.3 mm magnetite powder and    <1 mm fine coal was used as the solid media in the modularized system. The magnetite powder contains 91.38% (mass fraction) 0.3-0.06 mm Geldart B particles and thus has a wider size range compared with that (0.3-0.15 mm) used in the KZA2050 system.

3.2 Separation performance of modularized system

The fluidized bed can be used to separate 50-6 mm coal over a separation density range of 1.3-2.2 g/cm³ depending upon the variation in <1 mm fine coal content. An investigation of system performance was performed under two different conditions, one at a low separation density and the other at a high separation density. A well fluidized bed containing wide-size-range medium-solids was obtained using the novel method to control the bed density and height. This provides a means for separating 50-6 mm coal based on bed density. The float–sink results of the products at low and high separation densities are listed in Tables 3 and 4, respectively. The partition curves of the products are shown in Fig.8. The partition coefficient represents the mass fraction of coal particles reporting to the sink (the tailing). The coal particles with densities of ρ₂₅, ρ₅₀ (i.e. the separation density) and ρ₇₅ have the mass fractions of 25%, 50% and 75%, respectively. The probable error or Ecart probable error (E) value is equal to half of the difference between ρ₇₅ and ρ₂₅. The lower E value indicates the more effective separation. Good separation performance is demonstrated in Fig.8(a) with an E value of 0.05 g/cm³ at the low separation density of 1.33 g/cm³. As seen in Table 4 and Fig.8(b), using 50-6 mm coal with an ash content of 55.35% as feedstock, at a high separation density of 1.61g/cm³, the clean coal with an ash content of 14.67% and the tailing with an ash content of 71.26% are produced with an E value of 0.06 g/cm³. It is clear that the wide-size-range medium-solids are suitable for the novel method to control the bed, and the bed can effectively separate 50-6 mm coal.

Table 3 Float–sink results of products at low separating density of 1.33 g/cm³

Table 4 Float–sink results of products at high separating density of 1.61 g/cm³

Fig.8 Partition curve of product: (a) At low separation density of 1.33 g/cm³; (b) At high separation density of 1.61 g/cm³

During the dry beneficiation process, the dust is removed by the bag collector and recycled back into the separator. So, the system has the ability to work without environmental pollution and water. Additionally, this technique has high separation efficiency, and low construction investment and operating costs. These advantages make it applicable to the coals located in the regions where water resources are in short supply, or where the weather is cold, and also to the coals that tend to slime during wet separation.

4 Conclusions

1) Compared with the KZA2050 system for dry coal beneficiation, the modularized 40-60 t/h system has reduced the construction cost and the number of workers by 60% and 80%, respectively. The area and volume required for the equipment have been decreased by 75% and 89.81%, respectively.

2) A novel method is proposed for effectively maintaining the stability and uniformity of density and height of the gas-solid fluidized bed. The wide-size- range medium-solids, which consist of 0.3-0.06 mm magnetite powder and <1 mm fine coal, have good fluidization performance and are suitable for the dry coal beneficiation.

3) The modularized system using a wide-size-range medium-solids bed can effectively separate 50-6 mm coal, at a high or low separation density, with an E value in the range of 0.05-0.07 g/cm³.

4) The modularized dry coal beneficiation technique has advantages of no water consumed, no environmental pollution, high separation efficiency and low construction investment and operating costs.

References

[1] DWARI R K, RAO K H. Dry beneficiation of coal—A review [J]. Mineral Processing and Extractive Metallurgy Review, 2007, 28(3): 177-234.

[2] FRASER T, YANCEY H F. Process of separating loosely-mixed materials. US 1534846 [P]. 1925-04-21.

[3] YOUROVSKY A Z, GOROSHKO V D, KORSHUNOV V I, REMESNIKOV I D. New dry processes for coal preparation—magnetic, aero-suspension, radiometric [C]// Proceedings of 4th International Coal Preparation Congress. Harrogate, Yorkshire, 1962: 403-411.

[4] DOUGLAS E, JOY A S, WALSH T, WHITEHEAD A. Development of equipment for the dry concentration of minerals [J]. Filtration and Separation, 1972, 9: 532-538, 544.

[5] BEECKMANS J M, MINH T. Separation of mixed granular solids using the fluidized counter current cascade principle [J]. The Canadian Journal of Chemical Engineering, 1977, 55(5): 493-496.

[6] DONG X, BEECKMANS J M. Separation of particulate solids in a pneumatically driven counter-current fluidized cascade [J]. Powder Technology, 1990, 62(3): 261-267.

[7] MAK C, CHOUNG J, BEAUCHAMP R, KELLY D J A, XU Z. Potential of air dense medium fluidized bed separation of mineral matter for mercury rejection from Alberta sub-bituminous coal [J]. International Journal of Coal Preparation and Utilization, 2008, 28(2): 115-132.

[8] ZHANG C, CHEN G, YANG T, LU G, MAK C, KELLY D, XU Z. An investigation on mercury association in an Alberta sub- bituminous coal [J]. Energy & Fuels, 2007, 21(2): 485-490.

[9] SAHU A K, BISWAL S K, PARIDA A. Development of air dense medium fluidized bed technology for dry beneficiation of coal—A review [J]. International Journal of Coal Preparation and Utilization, 2009, 29(4): 216-241.

[10] TANAKA Z, OSHITANI J, KUBO Y, ZUSHI T. Dry coal cleaning in Drewboy bath type by dry heavy medium [C]// Proceedings of First International Symposium on Dry Coal Cleaning, Clean Coal Technology. Xuzhou: China University of Mining and Technology Press, 2002: 73-78.

[11] de JONG T P R, MESINA M B, KUILMAN W. Electromagnetic de-shaling of coal [J]. Physical Separation in Science and Engineering, 2003, 12(4): 223-236.

[12] van HOUWELINGEN J A, de JONG T P R. Dry cleaning of coal: review, fundamentals and opportunities [J]. Geologica Belgica, 2004, 7(3/4): 335-343.

[13] WEITKAEMPER L, WOTRUBA H, WEITKAEMPER L, WOTRUBA H. Effective dry density beneficiation of fine coal using a new developed fluidized bed separator [C]// Proceedings of 16th International Coal Preparation Congress. Littleton: Society for Mining, Metallurgy, and Exploration, 2010: 587-595.

[14] ZHAO Yue-min, LUO Zhen-fu, CHEN Zeng-qiang, TANG Li-gang, WANG Hai-feng, XING Hong-bo. The effect of feed-coal particle size on the separating characteristics of a gas-solid fluidized bed [J]. Journal of the South Africa Institute of Mining and Metallurgy, 2010, 110(5): 219-224.

[15] LUO Zhen-fu, ZUO Wei, TANG Li-gang, ZHAO Yue-min, FAN Mao-ming. Preparation of solid medium for use in separation with gas-solid fluidized beds [J]. Mining Science and Technology, 2010, 20(5): 743-746.

[16] TANG Li-gang, ZHAO Yue-min, LUO Zhen-fu, LIANG Chun-cheng, CHEN Zeng-qiang, XING Hong-bo. The Effect of fine coal particles on the performance of gas–solid fluidized beds [J]. International Journal of Coal Preparation and Utilization, 2009, 29(5): 265-278.

[17] ZHAO Yue-min, TANG Li-gang, LUO Zhen-fu, LIANG Chun-cheng, XING Hong-bo, WU Wan-chang, DUAN Chen-long. Experimental and numerical simulation studies of the fluidization characteristics of a separating gas–solid fluidized bed [J]. Fuel Processing Technology, 2010, 91(12): 1819-1825.

(Edited by YANG Bing) 

Foundation item: Projects(50921002, 50774084) supported by the National Natural Science Foundation of China; Project(2007AA05Z318) supported by the National High-tech Research and Development Program of China; Project(BK2010002) supported by the Natural Science Foundation of Jiangsu Province of China; Project(20100480473) supported by the China Postdoctoral Science Foundation

Received date: 2010-11-30; Accepted date: 2010-12-10

Corresponding authors: ZHAO Yue-min, Professor, PhD; Tel: +86-516-83590092; E-mail: ymzhao@cumt.edu.cn; TANG Li-gang, PhD; Tel: +86-10- 82544820; E-mail: lgtang1983@yahoo.com.cn