Parametric study of electrostatic separation of South African fine coal Parametric study of electrostatic separation of South African fine coal

Parametric study of electrostatic separation of South African fine coal

  • 期刊名字:矿业科学技术(英文版)
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  • 论文作者:BADA Samson,TAO Daniel,HONAKER
  • 作者单位:School of Chemical and Metallurgical Engineering,Department of Mining Engineering
  • 更新时间:2020-06-12
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论文简介

leonlineatwwww.sciencedirect.comMINING氵到Science DirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20(2010)0535-054Parametric study of electrostatic separation ofSouth african fine coalBADA Samson, TAO Daniel", HONAKER Rick, FALCON Lionel, FALCON RosemarySchool of chemical and Metallurgical Engineering, Faculry of Engineering and the Built Environment,University of the witwatersrand, Johannesburg 2050, South Africa2Department of Mining Engineering, University of Kentucky, Lexington KY40506,USAAbstract: The fact that water requirements are a major problem for present and future developments in material beneficiation, andthe construction of a new power plant in South Africa, forms the basis for the utilization of a Rotary Triboelectrostatic Separator(RTS)for beneficiation of South African pulverized coal. The cleaning potential of Majuba and Koorfontein coal was first evalu-ated using kinetic froth flotation tests on the-l77 um coal fraction. The RTS tests were conducted under varied process parametersParameters such as applied separating voltage, air injection velocity, particle feed rate and splitter position were investigated. Twotion results show that the rTs reduced Majuba coal initially containing about 30% ash to a clean product of 14.30%,or19.46%, ash at a combustible recovery of 15. 10%, or 53.02%, respectively. Similar separation performance was also achieved withthe Koorfontein coal. The mineral and organic compositions in the feed, after single stage and after the second stage separationswere characterized using X-ray diffraction analysis. The results show a better separation for the second stage coal productsKeywords: coal cleaning: particle charging: RTS technique; separation stage1 Introductionparticle electrical resistivities, are responsible for theelectrostatic separation of particles". The amount ofAlthough many techniques have been reported for charge exchanged between two contacting surthe wet coal beneficiation of South african fine coadue to the difference in the work functions and con-little information is available on dry beneficiation ductivities of the particles determines the degree ofusing the tribo-electrostatic technique. Wet coal separation. The degree of separation can be improvedseparation techniques use water as the separation me- by adjusting the electric or other contacting, forcesdium. This necessitates a costly dewatering processuch as the gravitational and centrifugal forces. Theand increases the transportation costs of the cleaned transfer of electrons(charge) from the surface of onecoal product. The dry coal beneficiation method caparticle to another occurs through contact chargingserve as an alternative to the wet coal cleaning proc(tribo-electrification)and frictional charging mechaesses, which is very important for coal producing nisms, which can arise from sliding, particle-wallcountries such as South Africa(SA)where fresh wa- charging, vibration at contact-surfaces or deformation.ter is scarce. Dry beneficiation of fine coal can be This leads to charge distribution at the stresseconomically competitive because energy need not be pointThis phenomenon enables two chargedexpended in drying the fine coal, which is mainly phases to develop, and maintain, a charge and to beused as fuel In addition, dry beneficiation will also deflected towards one of two oppositely polarizedserve as a potential technique for recovering and util- electrodes. Hence, application of the various opera-izing coals that would have otherwise been classified tional parameters available on the separator can im-unrecoverable because of environmental or economiprove the separation efficiency of the coal particlescal constraintsAn innovative rotary tribo-electrostatic separatorThe differences in the electronic surface structure developed at the University of Kentucky, USA, wasbetween minerals and organic phases in coal, and the used for the separation of coal organics from mineralmatter. The differential charge imparted to the coalReceived 06 November 2009, accepted 25 February 2010中国煤化工 a rotary charger. Corresponding author. Tel: 27 785422016the mineral matterE-mail address: Samson. Bada@students. wits. aczawithCN Gvely in the charg-doi:0.106S1674-5264(09602398ber and then subjected to an attractive forceMining Science and Technologfrom the positive electrode in the separation chamber. 18 cm in diameter. A variable voltage DC power sup.At the same time organic matter charged positively ply was employed to study the effect of electric fieldand was subjected to an attraction from a negative strength within the separation zone. The charger rota-electrode. Clean coal product and refuse were then tion speed was fixed at 2500 r/min and the applicollected by air cyclones and analyzed to determine charger potential was 1.5 or 0 kv for Majuba andthe separation efficiencyKoorfontein coal, respectively. A schematic represen-Recent work on the tribo-electrostatic separation of tation of the octagonal triboelectrostatic separatorSouth African Coal(SAC) has focused on the two used in this study is presented in Fig.stage separation of two coals as a demonstration ofthe feasibility of achieving high grade clean coalconditions regardless of the recovery. This paper de-scribes the application of single-stage separation withmiddles recycling and two stage separation techniques. These trials were performed under optimumprocess conditions designed for maximum combustiParticle separationCo-nlowble recovery and improved calorific values and weredone using Majuba and Koorfontein coal samplesThe total sulfur of the feed and cleaned coal fromVariable voltageboth the single and two stage separations was analyzed along with the yield. Four main operationalparameters affecting process separation efficiencywere investigated. The results under different separating voltage, feed rate, injection air velocity andsplitter position, both for single and two stage separa-tion tests are presented. a discussion concerning theProduct binspotential application of the RTS technology to beneficiation of South African fine coal is included2 Experimental2.1 Materials and test procedurestriboelectrostatic separator with an octagonal rotorfrom the Majuba and KoorfonteinFor each RTS separation test the feed coal wasSouth Africa were used for the transported pneumatically into the charging chamberbo-elecseparation experiments. The sam- were it was imparted with positive and negativeples were first crushed in a hammer mill and then charge. The positively charged particles are deflectedpulverized to-177 Hm. A representative portion of towards the negative electrode(R plate)while theeach coal was prepared for tribo-electrostatic separa- negatively charged particles are attracted towards thetion and for kinetic flotation tests. Calorific meas- positive electrode (L plate). Uncharged particles fallurements were carried out on the feed coal and on the in between two electrodes and are collected as midproducts obtained from the one- and two-stage dlings. The middlings were then subjected to anothertribo-electrostatic separation tests. X-ray diffraction stage of separation to generate three more productsstudies of all samples helped identify the mineral Thus, a total of five products were obtained givingcomposition in the feed and cleaned coals. The kifive data points on the separation performance curvenetic froth flotation test was carried out with a-177 In some RTS tests all three products collected fromum coal sample in a 4-litre Denver mechanical flota- the first stage of separation were subjected to a sec-tion cell under the following conditions: impeller ond stage of separation for a total of nine productsspeed, 1200 r/min; SNF Flomin F-100C frother dos- The feed and products were each analyzed for ashage,15x10, and, #2 fuel oil collector dosage, 910 percentage, phase composition, total sulfur contentand calorific value. A combustible recovery vScuThe coal samples used for the tribo-electrostatic mulative ash performance curve was prepared forseparation experiment were kept in an oven at 53C evaluation of separation performance with both theovernight to remove surface moisture prior to separaMaition. For each test, 100 g of sample were fed through中国煤化工a vibration feeder and transported pneumatically at a 2.2CNMHGvelocity of 1.9 m/s for Majuba coal, or 2.5 m/s forle pauucic-sIzc dIstribution was de-Koorfontein coal, into a cylindrical charging chamber termined using a laser particle size analyze CILASBADA Samson et alParametric study of electrostatic separation of South African fine coal(1092), having a measurement range of 0.04 to 500 ents was done from X-ray diffraction data. Qualita-um. The apparatus utilizes both Mie algorithms and tive evaluation of the common, predominant phasesFraunhofer diffraction of light from particles larger in within the coal powder samples was carried out withdiameter than the laser wavelength. A combination of a Bruker-AXS D8 Discover X-ray diffractometermultiple lasers and optical filters with a lens and with a rotating Cu-anode. The diffractometer wasphoto detector coupled to a computer loaded with operated at 40 kV and 40 mA over a 20 range fromSize Expert software allowed the computation of the -2 to-70 at a scan speed of 1 deg/min. mineralarticle size distribution. After calculation from theidentification was made using the DIFFRAcPLUSdiffraction data the size distribution was stored assoftware suitecumulative value percentage versus particle diameter3 Results and discussionProximate analysis was carried out with a LECOThermo-Gravimetric Analyser (TGA-701), Approxi3. 1 XRD analysis of Majuba and Koorforteinmately 1 g of coal was used for the analysis of inherent moisture, fixed carbon, ash content and volatilematter. The TGa was operated under a N2 atmosphereThe diffractograms(Fig. 2a for Majuba coal or Figfor moisture and volatile matter analysis to an end 2b for Koorfontein coal, where feed(bottom);temperature of 107 and 950C, respectively. Ash cleaned first stage product(middle); cleaned secondanalysis was carried out under a low oxygen atmos- stage product(top)) show the X-ray diffraction pat-phere to an end temperature of 750Cterns for particles smaller than 177 um. Note that theTotal sulfur in the feed and products was analyzed two coals consist mostly of crystalline minerals suchwith a LECO S632 analyzer, which has the capability as quartz, Kaolinite and pyrite and that there is anof operating to 1450oC with an analyzing time be- organic hump. The intensity of the lines from quartztween 60-120 secondsand Kaolinite was stronger in the feed coal than in theThe calorific value, the measure of the heat content cleaned coal after either the first- or second-stage ofof the coal, was determined using a LECO (AC-500) separation. Both feed coals show a much less intensebomb calorimeter with a Windows-based operating organic hump, and higher mineral matter, comparedsystem. This system uses an electronic thermometer to the cleaned second stage product. The organicmpernc, accuracy of 0.0001C that measures the hump corresponds to the quantity of organic matterTest results are ob-present in the coal since the organic material is antained withamorphous material.The iderneralogical constitu!AMm~一260°(a)Majuba coal(b) Koorfontein coalFig. 2 X-ray diffraction pattern3.2 Froth notation rate testsbe shown later. The difficulty of cleaning these coalsKinetic flotation tests were conducted on Majuba alternative cleaby froth flotation clearly indicates the necessity for anand Koorfontein coals to evaluate the potential forh as electrostaticcleaning with froth flotation. The results are shown inFig. 3. It can be seen that a clean coal product with19.61% ash and a combustible recovery of 24.60% 3.3 Effect of separation voltage on clean-coal ashwas obtained from the Majuba coal and that a cleancoal with 12.50% ash and a combustible recovery of山中国煤化工μ m Majuba coal7.80% was produced from Koorfontein coal. The re- showCNMHG.10% ash, 23.01%sults obtained from the electrostatic separation were volatile matter and 3.1y%o moisture. The KoorfonteinFar better than from this froth flotation testing, as will coal had 52.35% fixed carbon, 24.04% volatile matter,Vol20 No420.58% ash and 3.03% moisture. The ash content of the rts. a series of two-stage separation tests werethe cleaned coal collected at the negative electrode at conducted to investigate the effects of the applieddifferent separation potentials is presented in Table 1 separation voltage on separation efficiency during thewith recovered clean coal percent yield. The coal par- first stage of separation. Process parameters for teststicles were charged positively by the rotary charger with Majuba coal were otherwise fixed at 1.5 kvand were deflected towards the negative electrode. It charging voltage, 1.9 m/s air velocity and a splittercan also be seen in Table 1 that as the separation spacing of 0.6, 1.5 or 2.7 cm For tests on Koorfon-oltage increased the clean coal yield and ash content tein coal the process was run at zero charging voltageoth increased. This can be attributed to more coal 2.5 m/s air velocity and a splitter spacing of 0.6, 1.5particles being diverted into the clean coal fraction at or 2.7 cm. These conditions were maintained for boththe higher separation potentialsthe first and second stages of separation. The rotarycharger was operated at a constant speed of 2500The results obtained from two stage separationtests are shown in Fig. 4 for Majuba and Koorfonteincoal, respectively. The results from the first separa-Majuba(-177μm)tion stage are in Table 2. Both figures show that betterseparation is obtained at higher separation voltage121620242832since the separation curve moved toward the upperleft comer with increasing voltage. With Majuba coalFig 3 Combustible recovery/cumulative ash curve fromthe product ash increased from 10.82% to 12.74%kinetic flotation tests: 910 g/t#2 fuel oil; 15x10- SNF Flomin and the combustible recovery increased from 3.61%F-100C; 1200 r/min impeller speedto 9. 19%o as the applied separation voltage increasedfrom 10 to 20 kV. a combustible recovery of 50%Table 1 Cleaned coal yield and ash content, negativewas associated with a product ash of about 18.5%atlectrode, Majuba and Koorfontein coals(o the separation voltage of 20 kV. Fig. 4b shows aMajubaKoorfonteinproduct ash of 9.98%, or 13. 12%, for combustibleClean coal Clean coal Clean coal Clean coalrecovery of 3.51%, or 11.30%, respectively, usingKoorfontein coal. A coal product of approximately1082103217% ash was obtained with a combustible recovery of50% using Koorfontein coal when the separation11.814381088voltage was set at 20 kV. Table 2 shows the calorific5.65values and the total sulfur content of the cleaned coalafter both the first and second stages of separation3.4 Effect of separation voltage on first and sec- The rTS clearly produced higher heating value coalond stage separationproducts. More importantly RTS was very effective atreducing the sulfur content, which dropped by moreThe electric field strength in the separation cham- than two thirds after two separation stagesber is an important parameter for the performance of10k·125kv40*·15kV20kv4Cumulative ash(%)Cumulative ash (%Fig. 4 Effect of separating voltage two stage separation: constant splitter distance, air velocity, feed rate and separating voltageTable 2 Triboelectrostatic separation of Majuba and Koorfontein coalsMajuba feedSingle pasSecond pa中国煤化工 Second passCalorific value(k/g) 20.15Total sulfur92CNMHGAsh(%)30.22BADA Samson et alParametric study of electrostatic separation of South African fine coal3.5 Effect of splitter spacing on single and two from the negative electrode. All data from two dif-ferent single stage separation tests fall on the samecurve suggesting that changing the splitter positionThe separations discussed above revealed that a did not affect the separation efficiency, although it didsecond stage of separation resulted in better separa- change the product ash content and recovery yieldtion in terms of product ash heating value and sulfurcontent. However, better quality products can also beThis is reasonable since different splitter positionsobtained by adjusting the product splitter spacing atwill split the product into different streams but it willthe bottom of the separation chamber. To investigatenot influence separation sharpness.the possibility of producing cleaner coal products byFig. 5b shows separation curves obtained from theadjusting the splitter position, in lieu of using the single and two-stage separation tests performed withecond stage of separation, two trials at splitter posiKoorfontein coal. The cleanest coal product hadtion R=o6 cm c=1.5 cm and L=2. 7 cm or at R=1. 09.98% ash and was obtained after two stages of sepacm, C=1.5 cm and L=2. 3 cm were performed. Other ration with a 3. 51% combustible recovery. The split-process conditions were: injection gas velocity 1.9 ter was 0.6 cm from the negative electrode duringm/s, applied charging potential 1.5 kV, separationthese trials. When the splitter was set 1.0 cm from thchamber voltage of 12.5 kv and rotation at 2500 negative electrode the cleanest coal obtained had anr/min.For the Majuba coal, Fig. 5a, single stage clean coal of 13. 10 ash was obtained at aP [1309%separation when the splitter was positioned at 0.6 cm combustible recovery for a splitter position 068from the negative electrode resulted in a clean coproduct with 14.70% ash from a feed coal of 30% ashfrom the negative electrode. In contrast, a cleanedcoal having 14. 20% ash was collected with about aat a combustible recovery of 16.30%. A second stage 20.83% combustible recovery when theof separation further reduced the clean coal ash to11.10% with a 5.90% combustible recovery. A 1.0 cm away from the negative electrode.cleaned coal of 16.24% ash. at 27. 20% combustiblefrom the single separation stage are far btribo-electrostatic separation results using Handlovarecovery, or 13. 26%ash at 11. 20% combustible re- coal as reported in referencecovery was obtained from either single-or two-stageseparation,respectively, when the splitter was 1.0 cmSingle(0.6: 1.5& 2.7)cmSingle(1.0;15&2.3)cmSecond(06;1.5&2.7)cm· Second(1.01.5&23)cmCumulative ash ()Fig 5 Effect of splitter position on single-and two-stage separations: charging voltage 1.5 kV:separating voltage 12.5 kV; air velocity 1.9 m/s; rotation 2500 r/min3.6 Effect of air injection velocityThis might result from a reduction in particle resi-dence time within the charging chamber. That wouldInjection air velocity is known to have an influence reduce the number of contacts between particles ofon turbulence, particle/particle interactions and the rotor, which affects the resulting particle chargetribo-charging time and, thus, particle charge density. densityTo understand the effect on the rts performanceThe ash content in Majuba coal was reduced fromtwo-stage separation tests were conducted using dif- a feed of 30% to 10.76%, for approximately 5.21%ferent injection air velocities. The velocity ranged combustible recovery, or to 18.54% for a combustiblefrom 1.3 to 2.5 m/s and the charger rotated at a con- recovery of 40.70% when 1.3 m/s was the injectionstant speed of 2500 r/min, the separation chamber air velocity. As the air injection velocity increased tovoltage was 12.5 kV and the applied charging poten中国煤化工t12.21% and thetial was 1.5 kV for tests using Majuba coal. Fig 6a comthan 2%, Similarshows that the injection air velocity should be main- trendCNMH Geparation of Koortained below 1.9 m/s: higher air injection flows of 2.5 fontein coal, see Fig. 6b, over injection velocitiesm/s were detrimental to particle separation efficiency. varying from 2.5 to 3. 7 m/s. Here the charger rotationMining Science and TechnolVol20 Nospeed was 2500 r/min, the charging voltage was o kv gave separation curves falling below those obtainedand the separation voltage was 12.5 kV. A low air at 2.5 m/s. Fig. 6b appears to indicate that an injecinjection velocity of 2.5 m/s yielded a product with tion air velocity of 2.5 m/s is better than one of 3. 1 or10.62% ash from an initial feed of 22% ash. These 3.7 m/s for producing lower ash products but it mayresults are in agreement with those described in ref- not be the best for producing higher ash productserence. Increasing the air velocity to 3. 1 or 3.7 m/s-3. I m/s1=2如立Cumulative ash(%)Cumulative ash(%)(b)Koorfontein coalFig. 6 Effect of air injection velocity on separation performance3.7 Effect of feed ratetribo-electrostatic separator used for fine coal clean-Fig 7 shows the cumulative ash versus combustigble recovery curves for different feed rates. All testsFig. 7b shows the separation data for Koorfonteinwith Majuba coal were conducted with a 1.5 kv coal as a function of particle feed rate with a 0 kVcharging voltage, a 12.5 kV separation voltage, a 2.5charging voltage, a 12. 5 kV separation voltage and a m/s injection velocity, a 2500 r/min rotation speed1.9 m/s injection velocity. The rotor was operated at2500 r/min and the splitters were positioned 0.6, 1.5 and with the splitters set at 0.6, 1.5 or 2.7 cm from theor 2.7 cm from the negative electrode. Fig. 7a showsnegative electrode. The cleanest coal, 10.62% ashthat slightly better separation was obtained at the was produced at a feed rate of 12 g/min starting withhigher feed rate over the range examined. This a 22% ash feed. As the feed rate increased to 17probably results from the increase in particle-particle os increase to 23 g/min showed slightly poorercollisions at the higher feed rates that gave rise to a furthehigher average charge density on the particle surface. performance, which may be caused by fewer parti-Other workers have reported similar results with their cle-charger contacts at the higher feed rate● Feed rate=12gminFeed rate12 g/minFeed rate=17 g/minFeed rate=23 g/min10121416182022Cumulative ash (%mulative ash(%(a)Majuba coal(b) Koorfontein coalFig. 7 Effect of particle feed rate on separation4 Conclusions3)Better separation and lower ash coal productswere achieved using the two-stage separation rather1)RD analysis has revealed that both South Af- than the single-stage separation. Majuba feed coal,rican coals contained high concentrations of quartz, 30% ash, was cleaned to a 10.82% ash product at akaolinite and pyrite. The organic component was en- combustible recovery of 3. 61 after the second stageriched in the clean coal products obtained after sin- ofgle-and two-stage RTS separation testspr中国煤化工29%ater0me2)The RTS separation technique was more effec- stageCNMHGtive than froth flotation at upgrading both South4)Expenmental resuits snow that a lower ashAfrican coalsproduct was obtained at 10 kv for Majuba coal butBADA Samson et alParametric study of electrostatic separation of South African fine coalthat a higher combustible recovery was obtained at 20 [2] Kelly E G Spottiswood D J. The theory of electrostaticseparation voltage. a better separatlon curve wasseparations: a review part I: fundamentals. Miner Eng,observed at the higher separation voltage for bot1989,2(1):33-46[3] Manouchehri H R, Hanumantha R K, Forsberg K s5)The effect of feed rate on Majuba coal separaReview of electrical separation methods: part 1:fundamental aspects. Miner and Metall Process, 2000tion indicates a strong dependence of separation on17(1):23-36particle-particle collisions. More significant reduction [4] Gidaspow D, Wasan D T, Saxena S, Shih YT, Gupta R,in the ash content was observed as the feed rateMukherjee A. Electrostatic desulfurization of coal inincreased from 12 to 24 g/min. Feed coal containingfuidized beds and conveyers. AIChE Symposium, 1987.30% ash was reduced to 10.50% ash at a 24 g/mil255:74feed, as compared to 11. 10% ash at 12 g/r[5 Mazumder M K, Sims R A, Biris A s, Srirama PK, SainiminD, Yurteri C U, Trigwell S,lamar Twenty-first6)The Rts process is very effective at reducingsulfur content and increasing the calorific value of theto industry and mediciccoals tested in this stud2006,61:2192-2211[6] Hutton A C, Mandile A J. Quantitative XRD measure-Acknowledgementsment of mineral matter in Gondwana coals using theRietveld method. J African Earth Sci, 1996, 23(1)The authors gratefully acknowledge the financial [7] Soong Y, Link T A, Schoffstall M R, Turcanva Lsupport of the South African National Energy Re-Balaz P. Marchant S. Schehl RR. Triboelectrostaticsearch Institute (SANERI). Appreciation is also exseparation of mineral matter from Slovakian coals. ActaMontanistica Slovaca Rocnik, 1998(3): 393-400pressed to Mr Ed Thompson with the Department of [8 Tao D, Fan MM, Jiang X K Dry coal fly ash cleaningMining Engineering of the University of Kentuckyusing rotary triboelectrostatic separator. Mining Scienceand the Kentucky Geological Survey for grantingand Technology, 2009, 19(5): 642-647permission to perform XRD analyses.[9] Yoon R H, Yan E S. Luttrell G H, Adel G T. POC-scaletesting of a dry triboelectrostatic separator for fine coalReferencescleaning 6h Quarter Technical Progress Report, 199[10] Yoon R H, Yan E S. Luttrell G H, Adel G T. POc-scalesting of a dry triboelectrostatic separator for fine coal[1] Lockhart N C Dry beneficiation of coal-review papercleaning.7 Quarter Technical Progress Report, 1997Powder Technol. 1984. 40: 17-42( Continued from page 523)[12] Fan J C. Mei C L Data Analysis. Beijing: Science Press,Grey Relational Analysis [BSc. dissertation]. Xuzhou2002. (In Chinese)China University of Mining Technology. 2007.(In[13] Mei C L, Zhou J L. Practical Statistical Methods. BeiChinese)jing: Science Press, 2002.(In Chin[17] Tian Y L, Zhou L H. The study on the methods[14] Gao H X. Applying Multivariate Statistical Analysisdicting coal or gas outburst based on BP neural neBeijing: Peking University Press, 2005. (In Chinese)Systems Engineering-Theory Practice, 2005[15] He X Q. Wang E Y Nie B S, Liu M J, Zhang L. Elec-102-106.(In Chinese)romagnetic Dynamics of Coal or Rock Rheology. Bei- [118] Yu QX. Gas Protection and Control in Mine. Xuzhoujing: Science Press, 2003. (In ChineseChina University of Mining Technology Press, 1992[16] Ou X Y. Study and Application of Prediction of coal aGas Outburst by Neural Network Method Based on中国煤化工CNMHG

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