coal dry beneficiation technology in china: the state-of-the-art

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CHINA PARTICUOLOGY Vol. 1, No. 2, 52-56, 2003 COAL DRY BENEFICIATION TECHNOLOGY IN CHINA: THE STATE-OF-THE-ART Qingru Chen 1, * and Lubin Wei 2 1 School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221008, P. R. China 2 School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, P. R. China *Author to whom correspondence should be addressed. E-mail: [email protected] Abstract In China, coal is the major source of energy and its leading role in energy consumption would not change in the next 50 years. Coal preparation is the essential component of Clean Coal Technology. In China more than two-thirds of available coal reserves are in arid areas, which results in the unfeasibility with conventional wet processing for coal preparation. The uniqueness of dry coal beneficiation technology with air-dense medium fluidized bed is dis- cussed in this paper and a detailed survey of the current status of theoretical study, commercial application and devel- opment of the new technology is given in this paper. Keywords coal preparation, dry beneficiation, fluidized bed 1. Introduction In China, coal is the major source of energy and its leading role in energy consumption would not change in the next 50 years (Shi, 1995). More than 80% of coal is directly burned. As a result, the emission of 70% smoke/ dust and 85% SO 2 comes from coal combustion. Environ- mental pollution problems caused by coal combustion and utilization are being addressed by the coal industry. The development of the coal industry must be compati- ble with national economy and devote major efforts to de- veloping Clean Coal Technology (CCT). Coal preparation is the essential component of CCT. In view of the energy status in China, the only feasible way is to develop CCT to resolve the conflict between energy utilization and envi- ronmental protection. Only 33% coal (18% steam coal) is being cleaned prior to utilization in China, thereby resulting in significant en- ergy waste and serious environmental pollution. Many factors have led to the current situation, among which the shortage of water resource is a major cause. In China more than two-thirds of available coal reserves are in arid areas, i.e. 60.3% of available reserves of one trillion tons in Shanxi, Shaanxi and Inner Mongolia; 22.3% in the eight provinces of Xinjiang, Gansu, Ningxia, Qinghai etc.; and 17.4% in the remaining 19 provinces of East China. In arid areas there is no enough water resource required by con- ventional processing. For example, about 3-5 tons of wa- ter is needed for jigging one ton of coal, and a consider- able amount of fresh water should be added continuously. Second, the considerable reserves of low rank coal in China cannot be beneficiated by wet preparation due to its degradation in water. Third, high moisture content of cleaned coal from wet separation (up to 12%) makes storage and transportation very difficult due to freezing in cold area, calling for shutting down operations of some plants in winter. High capital and operation costs are needed for the conventional wet preparation. Developing efficient dry beneficiation technology is of even greater significance for coal cleaning and efficient utilization as the focus of Chinese coal industry shifts to her western areas. 2. Dry Coal Beneficiation Technology in China: A Brief History Dry coal beneficiation methods, including hand picking, frictional separation, magnetic separation, electric separa- tion, microwave separation, pneumatic oscillating table, air jig and air-dense medium fluidized bed beneficiation etc. are carried out according to differences in physical proper- ties between coal and refuse such as density, size, shape, lustrousness, magnetic conductivity, electric conductivity, radioactivity, frictional coefficient and so on. Of these dry beneficiation methods, pneumatic beneficiations (oscillat- ing table and air jig) and air-dense medium fluidized bed have been commercialized. Research and development of dry beneficiation tech- nologies started in 1967 in China. The pneumatic separa- tor for removing gangue and its flowsheet were designed by Beijing Institute of Mine Design, which was established in Mashan Mine of Jixi Coal Bureau, Heilongjiang Province and Tianshifu Mine of Benxi Coal Bureau, Liaoning Prov- ince. The Pneumatic Separator (Type ) was operated to remove gangue in Subang Coal Mine of Longyan Coal Bureau, Fujian Province. Now, these pneumatic separators are not employed because of the following disadvantages: strict requirement for narrow size range of feed coal, low beneficiation effi- ciency, high air-flow rate and serious dust pollution to at- mosphere. The Complex Dry Separators were developed by Tang- shan Branch of China Coal Research Institute in 1990. Complex Dry Separators are used to beneficiate 0-80 mm size of coal for removing gangue. The dry coal beneficiation technology with air-dense medium fluidized bed has been under development by

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Page 1: Coal dry beneficiation technology in china: the state-of-the-art

CHINA PARTICUOLOGY Vol. 1, No. 2, 52-56, 2003

COAL DRY BENEFICIATION TECHNOLOGY IN CHINA: THE STATE-OF-THE-ART

Qingru Chen1,* and Lubin Wei2

1School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221008, P. R. China 2School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, P. R. China

*Author to whom correspondence should be addressed. E-mail: [email protected]

Abstract In China, coal is the major source of energy and its leading role in energy consumption would not change in the next 50 years. Coal preparation is the essential component of Clean Coal Technology. In China more than two-thirds of available coal reserves are in arid areas, which results in the unfeasibility with conventional wet processing for coal preparation. The uniqueness of dry coal beneficiation technology with air-dense medium fluidized bed is dis-cussed in this paper and a detailed survey of the current status of theoretical study, commercial application and devel-opment of the new technology is given in this paper. Keywords coal preparation, dry beneficiation, fluidized bed

1. Introduction In China, coal is the major source of energy and its

leading role in energy consumption would not change in the next 50 years (Shi, 1995). More than 80% of coal is directly burned. As a result, the emission of 70% smoke/ dust and 85% SO2 comes from coal combustion. Environ-mental pollution problems caused by coal combustion and utilization are being addressed by the coal industry.

The development of the coal industry must be compati-ble with national economy and devote major efforts to de-veloping Clean Coal Technology (CCT). Coal preparation is the essential component of CCT. In view of the energy status in China, the only feasible way is to develop CCT to resolve the conflict between energy utilization and envi-ronmental protection.

Only 33% coal (18% steam coal) is being cleaned prior to utilization in China, thereby resulting in significant en-ergy waste and serious environmental pollution. Many factors have led to the current situation, among which the shortage of water resource is a major cause. In China more than two-thirds of available coal reserves are in arid areas, i.e. 60.3% of available reserves of one trillion tons in Shanxi, Shaanxi and Inner Mongolia; 22.3% in the eight provinces of Xinjiang, Gansu, Ningxia, Qinghai etc.; and 17.4% in the remaining 19 provinces of East China. In arid areas there is no enough water resource required by con-ventional processing. For example, about 3-5 tons of wa-ter is needed for jigging one ton of coal, and a consider-able amount of fresh water should be added continuously. Second, the considerable reserves of low rank coal in China cannot be beneficiated by wet preparation due to its degradation in water. Third, high moisture content of cleaned coal from wet separation (up to 12%) makes storage and transportation very difficult due to freezing in cold area, calling for shutting down operations of some plants in winter. High capital and operation costs are needed for the conventional wet preparation. Developing

efficient dry beneficiation technology is of even greater significance for coal cleaning and efficient utilization as the focus of Chinese coal industry shifts to her western areas.

2. Dry Coal Beneficiation Technology in China: A Brief History

Dry coal beneficiation methods, including hand picking, frictional separation, magnetic separation, electric separa-tion, microwave separation, pneumatic oscillating table, air jig and air-dense medium fluidized bed beneficiation etc. are carried out according to differences in physical proper-ties between coal and refuse such as density, size, shape, lustrousness, magnetic conductivity, electric conductivity, radioactivity, frictional coefficient and so on. Of these dry beneficiation methods, pneumatic beneficiations (oscillat-ing table and air jig) and air-dense medium fluidized bed have been commercialized.

Research and development of dry beneficiation tech-nologies started in 1967 in China. The pneumatic separa-tor for removing gangue and its flowsheet were designed by Beijing Institute of Mine Design, which was established in Mashan Mine of Jixi Coal Bureau, Heilongjiang Province and Tianshifu Mine of Benxi Coal Bureau, Liaoning Prov-ince. The Pneumatic Separator (Type Ⅳ) was operated to remove gangue in Subang Coal Mine of Longyan Coal Bureau, Fujian Province.

Now, these pneumatic separators are not employed because of the following disadvantages: strict requirement for narrow size range of feed coal, low beneficiation effi-ciency, high air-flow rate and serious dust pollution to at-mosphere.

The Complex Dry Separators were developed by Tang-shan Branch of China Coal Research Institute in 1990. Complex Dry Separators are used to beneficiate 0-80 mm size of coal for removing gangue.

The dry coal beneficiation technology with air-dense medium fluidized bed has been under development by

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Chen & Wei: Coal Dry Beneficiation Technology in China

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Mineral Processing Research Center of China University of Mining and Technology (CUMT) since 1984. It utilizes air-solid suspension as beneficiating medium whose den-sity is consistent with beneficiating density, similar in prin-ciple to the wet dense medium beneficiation using liq-uid-solid suspension as separating medium.

The air-dense medium fluidized bed used in dry coal beneficiation is not only pseudofluid in nature, but also has a stable and uniform density. The heavy portion in feed-stock whose density is higher than the density of the fluid-ized bed will sink, whereas the lighter portion will float, thus stratifying the feed materials according to their den-sity (Blagov, 1974; Beeckmans & Goransson, 1982; Leo-nard, 1979).

The first dry coal beneficiation plant in China with air-dense medium fluidized bed was established by CUMT. The plant was inspected and accepted by the Chinese government in June, 1994. Since then, new applications have been found and a 700,000 t/a dry coal beneficiation plant with air-dense medium fluidized bed has been put into commercial testing (Chen et al., 1991; Chen et al., 1994).

3. Theory and Application for Coal Dry Beneficiation with Air-dense Medium Fluidized Bed

In order to obtain efficient dry separation condition in air-dense medium fluidized bed, stable dispersion fluidiza-tion and micro-bubbles must be achieved. Its required physical properties are that bed density is well distributed in three-dimensional space and does not change with the time; bed medium is of low viscosity and high fluidity. Its fluidized bed density is identical to the beneficiation den-sity and may be expressed by the following equation:

b p g 50(1 )ρ ε ρ ερ ρ= − + ≈ , (1) where pρ is the density of the solid particles; gρ is the density of the air; bρ is the average density of the fluid-ized bed; 50ρ is the separation density of the fluidized bed; ε is the bed porosity.

The homogeneity and stability theory of bed density with air-dense medium fluidized bed was established, so a dispersion fluidized bed with a high-density dense phase, and a multitude of micro-bubbles was formed. The pure buoyancy of beneficiation materials plays a main role in fluidized bed, and the displaced distribution effect should be restrained. The displaced distribution effects include viscosity displaced distribution effect and movement dis-placed distribution effect (Wei et al., 1996). The former is caused by viscosity of the fluidized bed. It decreases with increasing air flow velocity. Movement displaced distribu-tion effect will be large when air flow rate is too low or too high. If medium particle size distribution and air flow are well controlled, both displaced distribution effects could be controlled effectively. A beneficiation displaced distribution model may be used to optimize beneficiation of feedstock with a wide particle size distrbution and multiple compo-nents in the fluidized bed.

It is well known that the calculation of drag on the sepa-rated materials is essential for processing with dense me-dia. However, this problem has not been well solved up to now. Daniels (1962) measured the terminal velocity of spheres falling through fluidized beds, thus obtaining an empirical correlation for the drag coefficient. The correla-tion was dimensionless but apparently somewhat arbitrary. Besides, this kind of pure empirical correlation has limited applicability. Then, Daniels (1965) tried to calculate the viscosity of the fluidized beds, assuming that the fluidized particles behave as a Newtonian fluid. The viscosities calculated by him for the same fluidized bed were consid-erably scattered. The falling sphere method used to study rheological characteristics of gas-solid fluidized beds has been criticized because the measurement device signifi-cantly disturbed the state of fluidization (Grace, 1970).

The rheological characteristics of fluidized beds were studied using once again the falling sphere method (Wei & Chen, 2001). The experimental results of Daniels and this study indicated that the fluidized bed behaves as a Bingham fluid. The plastic viscosity and yield stress can be obtained by measurement of the terminal settling ve-locity of spheres and linear regression of the experimental data. Both plastic viscosity and yield stress increase with increasing size of the fluidized particles. The drag coeffi-cient can be calculated by the following equations:

0.687D m

m

24 (1 0.15 )C ReRe

= + ,

m o r b eRe d u ρ μ= , e 0 o r3d uμ μ τ= + ,

where CD stands for the drag coefficient, Rem for the modi-fied Reynolds number, do for the diameter of the falling object, ur for the relative velocity between the object and fluidized particles, μe for the effective viscosity, μ for the plastic viscosity and τ0 for the yield stress.

The calculated results show favorable agreement with experimental data. Fig. 1 compares the calculated drag coefficient with experimental results.

Fig. 1 Comparison of computation with experimental data.

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CHINA PARTICUOLOGY Vol. 1, No. 2, 2003

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The first dry coal beneficiation plant with air-dense me-dium fluidized bed has been established for beneficiation of 50-6 mm size fraction coal as shown in the flowsheet of Fig. 2. The separator is shown schematically in Fig. 3. The experimental plant has a capacity of 50 t/h.

Fig. 2 Flowsheet of dry coal beneficiation plant with air dense

medium fluidized bed.

Fig. 3 Schematic diagram of separator with air dense medium fluid-

ized bed. Advantages of this new dry coal beneficiation technol-

ogy include: High precision. It compares favorably with the best

existing wet heavy medium beneficiation for effective beneficiation of coal of 50-6 mm size with an Ep value of 0.05-0.07.

Low investment. Since this technology greatly sim-plifies the coal beneficiation process and eliminates complicated and costly slurry treatment system, its capital and operating costs can be reduced to only

about half of those of a wet beneficiation plant with the same capacity.

No environmental pollution. This technology re-quires a small quantity of low pressure compressed air. Pollution is greatly reduced by the dust removal system. The dust emission in the exhausted air is much lower than that required by environmental pro-tection laws. The separator operates smoothly and steadily with little noise.

Wide ranges of beneficiating density. Stable fluid-ized bed can be obtained by using mixtures of mag-netite powder and fine coal as dense medium to produce a beneficiation density from 1.3 to 2.2 g.cm-3. Therefore, this technology can meet the needs of beneficiating different coals for different products. It can either be used to remove gangues at high-density or to produce clean coal at low-density.

4. Current Development of Dry Benefici-ation of Coal

To realize coal dry beneficiation of full size range of 300-0 mm, further research efforts on dry coal benefici-ation of different size fractions are under way and consid-erable progress has been made at the lab of Mineral Processing Research Center of CUMT.

4.1 4.1 Dry beneficiation technology with a vi-brated air-dense medium fluidized bed for fine coal of size fraction 6-0 mm

In air-dense medium fluidized bed, coarse coal behaves only according to its density with little dependence on the action of bubbles and can thus be beneficiated efficiently. Fine coal behavior is, however, highly dependent on the action of bubbles, tending to follow medium solids to backmix due to their small size (Luo & Chen, 2001). It is very difficult for fine coal to be separated in the available air-dense medium fluidized bed that is currently used for 50-6 mm coarse coal in China. However, the 6-0 mm por-tion in raw coal has been increasing as a result of in-creased mechanical mining to as much as 70%. In addi-tion, pyrite is mainly embedded in fine coal. It is therefore of immediate importance to develop a new fluidized bed separator suitable for fine coal. From the standpoint of fluidization principle, two approaches can be adopted to form a weakly bubbling or bubble-free fluidized bed. One is further reduction of the size of the medium solids. The other is to supply the bed with external energy to suppress bubbling of the bed. A comprehensive investigation has been conducted on the vibrated air-dense medium fluid-ized bed (Luo et al., 2000).

The results showed that as the size of medium solids was reduced and the bed was supplied with mechanical vibration energy, the gas-solid interaction was enhanced and a better dispersed fluidized bed was formed. Further investigation was also made on the mechanism of fluidiza-tion and separation in the vibrated air-dense medium flu-

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Chen & Wei: Coal Dry Beneficiation Technology in China

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idized bed and on the effects of the operating parameters including vibration parameters, airflow parameters, etc. The coal separation results obtained from a laboratory apparatus showed that, for 6-0.5 mm fine coal with ash content 16.57%, a desirable beneficiation with clean coal ash down to 8.35%, yield up to 80.20% and Ep value up to 0.065, was achieved.

4.2 Coal dry beneficiation technology with a deep air-dense medium fluidized bed for >50 mm coal

The available air-dense medium fluidized bed used for beneficiation of 50-6 mm lump coal is about 400 mm in bed height with sufficient space for effective beneficiation. Bed density is readily kept stable and uniform due to the effective suppression of bubble formation and growth. However, this bed height does not provide enough space for effective beneficiation of >50 mm coal. Further investi-gation was performed on a deeper laboratory air-dense medium fluidized bed with a cross-sectional area of 1 m2 that took into account effects of air distribution and proper-ties of medium solids, large coal beneficiation dynamics, etc. The results showed that the bed height required for beneficiation of >50 mm coal should be about 1200 mm in order to form a stable fluidized bed with small bubbles and a uniform bed density. Effective beneficiation of >50 mm coal with an Ep value up to 0.02 was achieved. The large coal dry beneficiation technology is of great value for waste removal from 300-50 mm large feedstock, espe-cially for big surface mines in China.

4.3 Coal triboelectric cleaning technology for <1 mm pulverized Coal

In triboelectric separation, a powder material is charged either positive or negative, depending on its surface elec-tric properties, as a result of wall friction or particle-particle collision in air flowing at a high velocity. When entering a high voltage electrostatic field, particles with opposite charges move to the opposite electrodes, resulting in the desired separation. Laboratory tests in a model triboelec-tric cleaner at CUMT showed that when the feed coal was comminuted down to 320 mesh (0.043 mm), at which the minerals embedded in coal were fully liberated, ultra-low ash coal with less than 2% ash was obtained. Currently a pilot system with triboelectric cleaning has successfully passed technical appraisal.

4.4 Three-product beneficiation technology with dual-density fluidized bed

In order to simplify the coal beneficiation process and to optimize product structure, a three-product dry coal ben-eficiation technology with a dual density air-dense medium fluidized bed has been studied in CUMT. The mechanism and characteristics of dual-density air-dense medium flu-idized bed have been investigated by adjusting the physi-

cal properties of medium solids, particle size composition, bed structure parameters, operation parameters, etc.

Experimental results showed that using proper bed structure and operation parameters, two relatively stable beneficiation layers with different densities in the axial direction in a fluidized bed were formed (Wei, 1998). In this so-called dual density air-dense medium fluidized bed three products, i.e. clean coal, middling and tailings can be obtained simultaneously. The results of coal beneficiation were also acceptable, e.g., an Ep value of 0.06-0.09 for the upper layer with a density of 1.5-1.54 g.cm-3 and an Ep value of 0.09-0.11 for the lower layer with density of 1.84-1.9 g.cm-3.

5. Conclusions Dry coal beneficiation with air dense medium fluidized

bed is a highly efficient dry process worth an effort at development. This new technology applied to benefi-ciating 50-6 mm size coal has claimed widespread adoption in China.

Dry beneficiation technology with vibrated air-dense medium fluidized bed for fine coal of size fraction 6-0 mm has given good experimental results. Further in-vestigation is in progress on the mechanism of fluidiza-tion and separation in the vibrated air-dense medium fluidized bed and on the effects of the operating pa-rameters including vibration parameters, airflow pa-rameters, etc.

Ultra-low ash coal with less than 2% ash was obtained in a model cleaner with triboelectric cleaning technol-ogy for <1mm pulverized coal. A pilot system has been installed.

Dry technology to beneficiate raw coal of 300-0 mm size is expected to be realizable in the near future.

Efficient dry beneficiation technology is unfolding a new path for coal processing in arid and cold areas.

Nomenclature CD drag coefficient do diameter of object, m Ep probable error ur relative velocity between object and fluidized

particles, m.s-1 ρb bulk density of fluidized bed, kg.m-3

ε voidage of bed ρp density of fluidized particles, kg.m-3 ρg gas density, kg.m-3 μ plastic viscosity, Pa.s τ0 yield stress, Pa Rem modified Reynolds number, m o r b eRe d u ρ μ=

μe the effective viscosity

Acknowledgement Financial support provided by National Natural Science Foun-

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CHINA PARTICUOLOGY Vol. 1, No. 2, 2003

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dation of China (Project 50025411, Project 59974030) for this work is gratefully acknowledged.

References Beeckmans, J. M. & Goransson, M. (1982). Coal cleaning by

counter-current fluidized cascade. CIM Bull., 75, 191-194. Blagov, E. S. (1974). Hand Book of Coal Preparation

(pp.331-351). Moscow: Petroleum Press. (in Russian) Chen, Q. R., Yang, Y., Tao, X. X., Liang, C. C. & Chen, Z. Q.

(1994). 50 t/h coal dry cleaning demonstration plant with air dense medium fluidized bed. Fluidization '94 Science and Technology Conference Proceedings, Fifth China — Japan Symposium (pp.381-389). Beijing: Chemical Industry Press.

Chen, Q. R., Yang, Y., Yu, Z. M. & Wang, T. J. (1991). Dry clean-ing of coarse coal with air dense medium fluidized bed at 10 tons per hour. Eighth Annual International Pittsburgh Coal Conference Proceedings (pp.266-271). USA.

Daniels, T. C. (1962). Measurement of the drag on spheres mov-ing through gaseous fluidized beds. J. Mech. Eng. Sci., 4, 103-110.

Daniels, T. C. (1965). Measurement of the drag on immersed bodies in fluidized beds. Rheol. Acta, 42, 192-197.

Grace, J. R. (1970). The viscosity of fluidized beds. Can. J. Chem. Eng., 48, 30-33.

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Shi, D. H. (1995). Clean coal is the future of China energy. Clean Coal Technol., 1(1), 16-18. (in Chinese)

Wei, L. B. (1998). Forming mechanisms of a dual-density fluidized bed. J. Central South Univ. Technol., 29(4), 330-333. (in Chi-nese)

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Wei, L. B., Chen, Q. R. & Liang, C. C. (1996). Study on the mechanism of coarse material separation in the air-dense me-dium fluidized bed. J. China Univ. Min. Technol., 25(1), 12-17. (in Chinese)

Luo, Z. F. & Chen, Q. R. (2001). Effect of fine coal accumulation on dense phase fluidized bed performance. Int. J. Miner. Proc-ess., 63(4), 217-224.

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Manuscript received March 4, 2003 and accepted March 18, 2003.