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

CHINA PARTICUOLOGY Vol. 1. No. 2. 52-56. 2003COAL DRY BENEFICIATION TECHNOLOGY IN CHINA:THE STATE-OF-THE-ARTQingru Chen*and Lubin Wei?school of Chemicaca! Engineering and Technology, China University of Mining andXuzhou 221008, PR Chinacal and Environmental Engineering, China University of Mining anBejing 100083, P R. ChinaAuthortowhomcorrespondenceshouldbeaddressed.E-mail:cheng@cumt.edu.cnAbstract In China, coal is the major source of energy and its leading role in energy consumption would not changein the next 50 years. Coal preparation is the essential component of Clean Coal Technology. In China more thantwo-thirdsof available coal reserves are in and areas which results in the unfeasibility with conventional wet processingfor 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 development of the new technology is given in this paperKeywords coal preparation, dry beneficiation, fluidized bed1 Introductionefficient dry beneficiation technology is of even greaterIn China, coal is the major source of energy and its focus of Chinese coal industry shifts to her western areaseading role in energy consumption would not change inne next 50 years (Shi, 1995).More than 80% of coal is 2. Dry Coal Beneficiation Technology indirectly burned. As a result, the emission of 70% smokedust and 85%SO, comes from coal combustion Environ-China: A Brief Historymental pollution problems caused by coal combustion and Dry coal beneficiation methods, including hand pickingutilization are being addressed by the coal industryfrictional separation, magnetic separation, electric separaThe development of the coal industry must be compati- tion microwave separation, pneumatic oscillating table, airble with national economy and devote major efforts to de- jig and air-dense medium fluidized bed beneficiation etcveloping Clean Coal Technology (CCT). Coal preparation are carried out according to differences in physical properis the essential component of CCT. In view of the energy ties between coal and refuse such as density, size, shapestatus in China, the only feasible way is to develop CCT to lustrousness, magnetic conductivity, electric conductiviresolve the conflict between energy utilization and envi- radioactivity, frictional coefficient and so on. Of these dryonmental protectionbeneficiation methods, pneumatic beneficiations(oscillatOnly 33% coal (18% steam coal)is being cleaned prior ing table and air jig) and air-dense medium fluidized bedto utilization in China, thereby resulting in significant en- have been commercializedergy waste and serious environmental pollution. Many Research and development of dry beneficiation tech-factors have led to the current situation, among which the nologies started in 1967 in China. The pneumatic separa-shortage of water resource is a major cause In China tor for removing gangue and its flowsheet were designedmore than two-thirds of available coal reserves are in arid by Beijing Institute of Mine Design, which was establishedareas, i.e. 60.3% of available reserves of one trillion tons in Mashan Mine of Jixi Coal Bureau, Heilongjiang Provincein Shanxi, Shaanxi and Inner Mongolia; 22. 3% in the eight and Tianshifu Mine of Benxi Coal Bureau, Liaoning Prov-provinces of Xinjiang, Gansu, Ningxia, Qinghai etc. and ince. The Pneumatic Separator(Type N)was operated to17. 4% in the remaining 19 provinces of East China In arid remove gangue in Subang Coal Mine of Longyan Coalareas there is no enough water resource required by con- Bureau, Fujian Provinceventional processing. For example, about 3-5 tons of wa- Now, these pneumatic separators are not employedter is needed for jigging one ton of coal, and a consider- because of the following disadvantages: strict requirementable amount of fresh water should be added continuously. for narrow size range of feed coal, low beneficiation effi-Second, the considerable reserves of low rank coal in ciency, high air- flow rate and serious dust pollution to atChina cannot be beneficiated by wet preparation due to its mospheredegradation in water. Third, high moisture content ofThec△ awateredeveloped by Tang-cleaned coal from wet separation (up to 12%)makes shan中国煤化工 ch Institute in1990storage and transportation very difficult due to freezing in CompCNMH Beneficiate 0-80 mmcold area, calling for shutting down operations of some size pfplants in winter. High capital and operation costs are The dry coal beneficiation technology with air-denseneeded for the conventional wet preparation. Developing medium fluidized bed has been under development byChen Wei: Coal Dry Beneficiation Technology in ChinaMineral Processing Research Center of China University It is well known that the calculation of drag on the sepaof Mining and Technology(CUMT) since 1984. It utilizes rated materials is essential for processing with dense meair-solid suspension as beneficiatingwhose den. dia. However, this problem has not been well solved up tosity is consistent with beneficiating density, similar in prin- now. Daniels (1962)measured the terminal velocity ofciple to the wet dense medium beneficiation using liq- spheres falling through fluidized beds, thus obtaining anuid-solid suspension as separating mediumempirical correlation for the drag coefficient. The correla6 The air-dense medium fluidized bed used in dry coal tion was dimensionless but apparently somewhat arbitraryeneficiation is not only pseudofluid in nature, but also has Besides, this kind of pure empirical correlation has limiteda stable and uniform density. The heavy portion in feed- applicability. Then, Daniels(1965)tried to calculate thestock whose density is higher than the density of the fluid- viscosity of the fluidized beds, assuming that the fluidizedized bed will sink, whereas the lighter portion will float, particles behave as a Newtonian fluid. The viscositiesthus stratifying the feed materials according to their den- calculated by him for the same fluidized bed were consid-sity( Blagov, 1974; Beeckmans Goransson, 1982: Leo. erably scattered. The falling sphere method used to studynard,1979)rheological characteristics of gas-solid fluidized beds hashe first dry coal beneficiation plant in China with been criticized because the measurement device signifiair-dense medium fluidized bed was established by CUMT. cantly disturbed the state of fluidization ( Grace, 1970)The plant was inspected and accepted by the Chinese The rheological characteristics of fluidized beds wergovernment in June, 1994. Since then, new applications studied using once again the falling sphere method (Weihave been found and a 700, 000 t/a dry coal beneficiation Chen, 2001). The experimental results of Daniels andplant with air-dense medium fluidized bed has been put this study indicated that the fluidized bed behaves as ainto commercial testing(Chen et al., 1991; Chen et al., Bingham fluid. The plastic viscosity and yield stress canbe obtained by measurement of the terminal settling ve-locity of spheres and linear regression of the experimental3. Theory and Application for Coal Dry data. Both plastic viscosity and yield stress increase withBeneficiation with Air-dense Medium cient can be calculated by the following equationsFluidized BedCD=(1+0.15Rec0),In order to obtain efficient dry separation condition inair-dense medium fluidized bed stable dispersion fluidiza-Re=d P/ution and micro- bubbles must be achieved. Its required共=H+r0d/31,physical properties are that bed density is well distributedwhere Cp stands for the drag coefficient, Rem for thein three-dimensional space and does not change with the fied Reynolds number, do for the diameter of thetime; bed medium is of low viscosity and high fluidity. Itsfluidized bed density is identical to the beneficiation den- object, u, for the relative velocity between the objesity and may be expressed by the following equationfluidized particles, l for thety, u for th=(1-)+g(1) plastic viscosity and to for the yield stressThe calculated results show favorable agreement withwhere P, is the density of the solid particles; Pe is the experimental data. Fig. 1 compares the calculated dragdensity of the air; P, is the average density of the fluid- cozed bed; Pso is the separation density of the fiuidizedbed; e is the bed porosityThe homogeneity and stability theory of bed density withair-dense medium fluidized bed was established. so adispersion fluidized bed with a high-density dense phase,and a multitude of micro-bubbles was formed The purebuoyancy of beneficiation materials plays a main role influidized bed, and the displaced distribution effect shouldbe restrained. The displaced distribution effects includeviscosity displaced distribution effect and movement dis-placed distribution effect (Wei et al., 1996). The former iscaused by viscosity of the fluidized bed. It decreases withincreasing air flow velocity. Movement displaced distribu-tion effect will be large when air flow rate is too low or toohigh. If medium particle size distribution and air flow arewell controlled, both displaced distribution effects could be中国煤化工controlled effectively. A beneficiation displaced distributionCNMHGmodel may be used to optimize beneficiation of feedstockwith a wide particle size distrbution and multiple compo.nents in the fluidized bedFig 1 Comparison of computation with experimental dataCHINA PARTICUOLOGY Vol. 1, No 2, 2003The first dry coal beneficiation plant with air-dense me-about half of those of a wet beneficiation plant withdium fluidized bed has been established for beneficiationthe same capacity.of 50-6 mm size fraction coal as shown in the flowsheet of No environmental pollution. this technology re-Fig. 2. The separator is shown schematically in Fig 3. The quires a small quantity of low pressure compressedexperimental plant has a capacity of 50 t/hair. Pollution is greatly reduced by the dust removalsystem. The dust emission in the exhausted air ismuch lower than that required by environmental pro-tection laws. The separator operates smoothly andand . 50mmsteadily with little noiseWide ranges of beneficiating density Stable fluid-pickingized bed can be obtained by using mixtures of mag-netite powder and fine coal as dense medium toproduce a beneficiation density from 1.3 to 2.2 g-cmscreenTherefore, this technology can meet the needsbeneficiating different coals for different pro6mmused to remosuction fan4. Current Development of Dry Benefi-out to aciation of coalfloatsreceiverTo realize coal dry beneficiation of full size rangescreenn screer blower 300-0 mm, further research efforts on dry coal beneficia-tion of different size fractions are under way and consid-erable progress has been made at the lab of MineralProcessing Research Center of CUMT.-s possible coal circuit 4.1 Dry beneficiation technology with a vi-brated air-dense medium fluidized bed forK-extract air circuitfine coal of size fraction 6-0 mmfine coalair circuitIn air-dense medium fluidized bed. coarse coal behavesonly according to its density with little dependence on theFig. 2 Flowsheet of dry coal beneficiation plant with air dense action of bubbles and can thus be beneficiated efficientlyFine coal behavior is, however, highly dependent on theexhaustahausaction of bubbles, tending to follow medium solidsbackmix due to their small size(Luo& Chen, 2001). It isvery difficult for fine coal to be separated in the availableair-dense medium fluidized bed that is currently used for50-6 mm coarse coal in China. However, the 6-0 mm por.tion in raw coal has been increasing as a result of in-tion, pyrite is mainly embedded in fine coal. It is thereforecleaningsb bar creased mechanical mining to as much as 70% In addi-of immediate importance to develop a new fluidized bedcompressedseparator suitable for fine coal. From the standpoint ofluidization principle, two approaches can be adopted toFig 3 Schematic diagram of separator with air dense medium fluid- form a weakly bubbling or bubble-free fluidized bed. Oneis further reduction of the size of the medium solids. theAdvantages of this new dry coal beneficiation techno- bubbling of the bed. A comprehensive investigation hasHigh precision. It compares favorably with the best been conducted on the vibrated air-dense medium fluid-existing wet heavy medium beneficiation for effective izedbeneficiation of coal of 50-6 mm size with an Ep中国煤化工 size of medium solidsalue of 0.05-0.07CNMH Plied with mechanicalLow investment. Since this technology greatly sim- vIbrFraction was enhancedplifies the coal beneficiation process and eliminates and a better dispersed fluidized bed was formed. Furthercomplicated and costly slurry treatment system, its investigation was also made on the mechanism of fluidizacapital and operating costs can be reduced to only tion and separation in the vibrated air-dense medium fluChen Wei: Coal Dry Beneficiation Technology in Chinaidized bed and on the effects of the operating parameters cal properties of medium solids, particle size composition,including vibration parameters, airflow parameters, etc. bed structure parameters, operation parameters, etcThe coal separation results obtained from a laboratory Experimental results showed that using proper bedapparatus showed that, for 6-0.5 mm fine coal with ash structure and operation parameters, two relatively stablecontent 16.57%, a desirable beneficiation with clean coal beneficiation layers with different densities in the axiaash down to 8.35%, yield up to 80. 20% and Ep value up to direction in a fluidized bed were formed(Wei, 1998). In0.065 was achievedthis so-called dual density air-dense medium fluidized bed4.2 Coal dry beneficiation technology with athree products, i.e. clean coal, middling and tailings can beobtained simultaneously. The results of coal beneficiationdeep air-dense medium fluidized bed for were also acceptable, e.g., an Ep value of 0.06-0. for50 mm coalthe upper layer with a density of 1.5-1.54 g cm and an EpThe available air-dense medium fluidized bed used for value of 0.09-0.11 for the lower layer with densitybeneficiation of 50-6 mm lump coal is about 400 mm in 1.84-1.9 g-cmbed height with sufficient space for effective beneficiationBed density is readily kept stable and uniform due to the 5. Conclusionseffective suppression of bubble formation and growthHowever, this bed height does not provide enough space Dry coal beneficiation with air dense medium fluidizedfor effective beneficiation of >50 mm coal. further investi-bed is a highly efficient dry process worth an effort atgation was performed on a deeper laboratory air-densedevelopment. This new technology applied to benefinedium fluidized bed with a cross-sectional area of 1 mciating 50-6 mm size coal has claimed widespreadthat took into account effects of air distribution and properadoption in Chinaties of medium solids, large coal beneficiation dynamics, Dry beneficiation technology with vibrated air-denseetc. The results showed that the bed height required for medium fluidized bed for fine coal of size fraction 6-0beneficiation of >50 mm coal should be about 1200 mm inmm has given good experimental results. Further in-order to form a stable fluidized bed with small bubbles andvestigation is in progress on the mechanism of fluidizea uniform bed density. Effective beneficiation of >50 mmtion and separation in the vibrated air-dense mediumcoal with an Ep value up to 0.02 was achieved. The largefluidized bed and on the effects of the operating pa-coal dry beneficiation technology is of great value forrameters including vibration parameters, airflow pa-waste removal from 300-50 mm large feedstock, espe-Ultra-low ash coal with less than 2% ash was obtainein a model cleaner with triboelectric cleaning technol4.3 Coal triboelectric cleaning technology for ogy for <1mm pulverized coal. A pilot system has been<1 mm pulverized CoalinstalledIn triboelectric separation, a powder material is charged Dry technology to beneficiate raw coal of 300-0 mmeither positive or negative, depending on its surface elec-size is expected to be realizable in the near futuretric properties, as a result of wall friction or particle-particle Efficient dry beneficiation technology is unfolding acollision in air flowing at a high velocity. When entering anew path for coalssing in arid and cold areashigh voltage electrostatic field, particles with oppositecharges move to the opposite electrodes resulting in the Nomenclaturedesired separation. Laboratory tests in a model triboelec- Cp drag coefficienttric cleaner at CUMT showed that when the feed coal was do diameter of object,comminuted down to 320 mesh(0.043 mm), at which the Ep probable errorminerals embedded in coal were fully liberated, ultra-lowrelative velocity between object and fluidizedash coal with less than 2%ash was obtained currently aparticles, mspilot system with triboelectric cleaning has successfully P bulk density of fluidized bed, kg m 3voidage of bed4.4 Three-product beneficiation technology pr gas density kg- m 'articles, kgmPpwith dual-density fluidized bedIn order to simplify the coal beneficiation process and toield stress, Paoptimize product structure, a three-product dry coal ben- Rem中国煤化4eficiation technology with a dual density air-dense medium efluidized bed has been studied in cUmt. the mechanisCNMHGand characteristics of dual-density air-dense medium flu- Acknowledgementidized bed have been investigated by adjusting the physi- Financial support provided by National Natural Science Foun-CHINA PARTICUOLOGY Vol. 1. No. 2. 2003dation of China(Project 50025411, Project 59974030)for this Leonard, J W (1979). Coal Preparation(4th Edition), Chapter 11work is gratefully acknowledgedew York: AIMEReferencesCoal Techno, 1(1). 16-18 (n Chinese of China energy. CleanShi, D. H.( 1995). Clean coal is the futBeeckmans, J. M.& Goransson, M.(1982). Coal cleaning by Wei, L B (1998). Forming mechanisms of a dual-density fluidizedcounter-current fluidized cascade. CIM Bull, 75, 191-194bed,J. Central South Univ. Technol., 29(4), 330-333.(in Chi-Blagov, E. S.(1974). Hand Book of Coal Preparation nese(pp, 331-351. Moscow Petroleum Press. (in Russian)Wei, L. B.& Chen, Q. R.(2001). Calculation of drag force on anChen, Q.R., Yang. Y, Tao, X.X., Liang, C. C.& Chen, Z. Q. object settling in gas-solid fluidized beds. Part. Sci. Technol., 19(1994). 50 th coal dry cleaning demonstration plant with air 229-238dense medium fluidized bed. Fluidization 94 Science and Wei, L. B, Chen, Q. R.& Liang, C. 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ChemEng,48,30-33Manuscript received March 4, 2003 and accepted March 18, 2003中国煤化工CNMHG