Impact energy analysis of turbulent water sprays for continuous centrifugal concentration

Joumal of Harbin Instiute of Technology (Nev Series)，Vol. 16, No. 1.2009Impact energy analysis of turbulent water sprays for continuouscentrifugal concentrationREN Nan-qi' , CHEN Lu zheng'，XIONG Da-he2任南琪,陈禄政,熊大和.(1. State Key Lab of Urban Water Reoure and Fnvironment, Harbin Insiute of Technology ，Harbin 150090, China;2.Sson Magneic Separaor Lud. . Ganzhou 341000,Abstract: A SLon fll-scale continuous centrifugal concentrator was used to reconcentrate hematite from a highgradient magnetie separation concentrate to study the elet of impact angle , concentrate mass and drum rotationspeed on the impact energy of turbulent water sprays for continuous centrifugal concentration, under conditionsof feed volume flow rate around 9 m'/h, feed solid concentration of 25% - 35% and reciprocating velocity ofwater sprays at 0. 05 m/s. The results indicale that a minimal critical impact energy is required in the watersprays for achieving continuous concentration of the concentrator; an unfitted impact angle reduces the impacteficeiency, and the highest impact eficiency of 0.6416 is found at the mpact angle of 60° ; the increase in con-centrate mass leads to an increase in impact energy, and the highest impact eficiency is maintained when theconcentrate mass varies in the range of0. 44 -0. 59 kg/s; when the concentrate mass and the pressure of watersprays are kepl at around 0. 45 kg/s and in the range of0.4 -0.6 MPa respectively, the impact energy increa-ses proportionally with the increase of drum rotation speed.Key words: centrifugal concentration; turbulent impacl; hematite; reconcentrationCLC oumber: X751Document code; AArticle ID: 1005-91132009)01-0091-05High gradient magnetic separation (HGMS) is ef-China [4-6]. However, this kind of centrifugal concen-ficient for concentration of fine weakly magnetic miner-trator was gradually replaced mainly by flotation andals, but generally it can not be used to produce a quali-magnetic separators in the past 20 - 30 years, due to itsfied concentrate produet1.2]. When HGMS is used forincapability of working continuously and its very lowconcentrating hematite, for instance, the strong mag-processing capacity 17.8.netic force on magnetic particles results in high recover-A SLon continuous centrifugal concentrator wasy of the less magnetic particles, thereby deterioratingdeveloped by introdueing reciprocating high pressurethe quality of magnetic concentrate. Flotation is usually water sprays to remove the settled bed of heavy particlesa choice necessary for obtaining a high quality magnetic in centifugal field. The heart in the SLon continuousconcentrateweakly magnetic minerals processingconcentration is the impacting of reciprocating waterflowsheets. However, flotation always causes relatively sprays on the settled bed. An adequate impact energyhigh energy and reagent consumptions and environmen-is required in the water sprays for achieving continuousal pollutions. Moreover, high quality raw materials areconcentration of the concentrator. In this study, a SLonincreasingly required by the iron and steel making in- - 1600 x 900 full-scale continuous centrifugal concen-dustries to improve quality and to'reduce cost 5 ' .trator was used to reconcentrate hematite from a HCMSEnvironmental protection requirements and provi- concentrate to study the efeet of impact angle, concen-sion of high quality raw materials make it necessary to trate mass and drum rotation speed on the impact ener-find ways and means to eficiently reconcentrate HCMSgy of turbulent water sprays.magnetic concentrates to obtain high quality products.Centrifugal concentration was found to be useful in sat- 1 Experimentalisfying these requirements. It has been reported byChen et al. in 2006 that an intermittent centrifugal con- 1. 1 SLon Full-Scale Continuous Centrifugal Con-centrator was capable of obtaining a concentrate assa-centratorying 64. 39% Fe by reconcentrating a HCMS magneticAs showm in Fie. 1. the SLon - 1600 x 900 full-concentrate from iron tailings, which has been recently scale中国煤化工ftator consists (discarded in excess of 100 million tons each year in a coniTYHCNMHG'_ting mechanism inReceived 2007 -02 -26.Sponsored by the National Natunl Science Foundation of China (Crant No. 50638020)..91●Joumal of Harbin Institue of Technology (New Series)。Vol 16, No. 1, 2009na. The feed is 48% -50% Fe grade and has a parti-cle size of 86% - 89% < 74 μm. During this investi-gation, feed volume flow rate and feed solid concentra-Ltion were controlled at around 9 m'/h and in the rangeof 25% -35% , respectively.1.3 MethodsAs shown in Fig. 2, feed volume flow rate wascontrolled by an adjustable valve located below the feedbox with a volume of 0.50 x0.45 x0.45 m'. Feedvolume flow rates were measured using a sample canand a chronometer. Reciprocating mechanism of theconcentrator was connected to a nearby tap water pipethrough a high pressure water tube. A pressure pumpWater inletwas installed in the pipe line to produce high pressurewater sprays. The required water sprays pressure wasachieved through another adjustable valve located be-tween the pressure pump and the reciprocating mecha-,FeedFlowing filmnism, and a pressure gauge was used to indicate theoperating pressure of water sprays. For all tests in thisstudy，reciprocation of water sprays was fixed at0.05 m/s. Drum rotation speed is adjustable from 160Tailings，to 260 r/ min through an AC transducer connected tothe drum driver.1. Reciprocating mechanism; 2. High pesure wsprays;3. Concentrate cllecting box; 4. Guard; 5. Drum6. Tailings discharging channelFig. 1 SLon - 1600 x 900 fll-scale continuous centrifugalTop wter日mconcentrator”Concentote Feedthe upper chamber of the drum. Below the reciproca-ting mechanism is installed a concentrate collectingbox.This continuous centrifugal concentrator operates由。Overlowon the prineiple of centrifugal acceleration in a flowingfilm of a few millimeters thick, wherein particles sepa-Tailing↓rate depending on the diference in density and the1. SLon - 1600 x 900 fll-eale contimuous crntrifugal concentaorcharacteristics of flowing film!。While the concen-2. Pesure pump; 3. Adjusable valve;trator is being operated for concentration, the feed ma-4. Pressure gauge; 5. Feed box; 6. Sample canterial enters through a feeding device into the rotatingFig.2 Experimental setup of SLon - 1600 x 900 full-scaledrum as a surry. The feed material enters through acontinuous centrifugal concentrationfeeding device into the rotating drum as a slurry. Theslurry is thrown by the drum onto the wall and rotatesAfter sufcient time is allowed for the concentratorwith the drum in the form of a flowing film. Heavy par-to realize its steady operation a typical sample was col-ticles in the flm settle down onto the drum surface un-lected from the concentrate stream for weighing and as-der the strong action of centrifugal force. Light parti-say. The feed volume flow rate and the feed solid con-cles continuously flow out with the flowing film to be-centration were measured and examined peridically.come tailings. W hile the reciprocaling mechanism is2 Theoreticalworking, the nozzles fixed on the mechanismect highpressure water sprays against the rotating drum and thesettled bed of heavy particles on the drum surface is2.1 Flow Patter of Water Sprayscontinuously flushed down into the concentrate collec-As shown in Fig.3, a centrifugal field is built upting box.intl”中国煤化工heary prticles1.2 Description of Feed Materialrotatedown by the highHematite material fed to the concentrator was frompressCNMHGthe magnetic produet stream of a SLon pulsating HGMSwuw palcruun waucr oprays 15 defined accordingseparator “at Hainan lron and Steel Company in Chi-to the Reynolds Number at the nozzle exit:●92●Joumal of Harbin Institue of Technology (New Serie)。Vol. 16, No. 1, 2009Re=dup(1)tinuity equation (13] :qm =其dupwhere d, u, p and μ are the nozle diameter, the flowvelocity of water sprays at the nozzle exit, the densityThe process of water sprays impacting and dis-and the viscosity of water, respectively.charging the sttled bed is in compliance with jet fluidThe impact veloeity of water sprays is estimated u-momentum balance :sing the Bermouli's lawP。= K .cosC. P.(5)u =φ、压2)where K, is the impact efciency coefficient, related tothe impact angle and the characteristics of feed, etc. ;where φ is the impact velocity cofficient, related to the a is the impact angle.orifice geometry of nozzles; in this study it was esti-By incorporating Eqs. (3) and (4) into (5):mated atφ = 1.0; p is the pressure of water sprays.u=K.2-c).ND.4. mecosawhere K is the dimensionless cofficient, K = π/K.Following the approximation between L and b:where n is the number of water sprays.A minimal critical impact energy P。is required inF.↓mwater sprays for completely impacting down the settled1. Drum;2. Stled bed; 3. Water spray; 4. Nozlebed, thereby achieving a continuous centrifugal con-Nis the drum rotation speed; me i the concentrate mas;qn is the water spray mass rate; u is the impact vloit;centration. When 号=一, P is given by:F. is the centrifugal field; a i the impact anglem。.(2-c) .NDFig.3 High pressure water sprays impacting down the set-Po = [n°qmu]mim =Ktled bed of heavy particles in centrifugal field(6)A water spray is turbulent when Re > 30. TheAnd the actual impact energy of water sprays isminimal critical pressure of water sprays for impactingP =n.qmi.down the settled bed is around 0. 40 MPa, at which ReAccording to Eq. (6), the critical impact energyis about 8.3 x 10* according to Eqs. (1) and (2).is proportional to the concentrale mass and the drum'Thus, flow patterm of water sprays in this study is tur-rotation speed. However, a conclusion can not bebulent.drawn that this critical impact energy is proportional to2.2 lmpact Energy of Water Sprays for Continu-the cefficient K or reversely proportional to cosa, be-ous Centrifugal Concentrationcause the impact efficiency closely relates with otherAn adequate impact energy is required to counter-factors such as impact angle as illustrated later in thisact the rotating energy of the stted bed in centrifugalstudy.field when the water sprays impact it down. As shownin Fig. 3, the centrifugally settled bed has a rotating3 Results and Discussionmomentum, which can be expressed as:P。=m。+m.cn =π●(2-c).∞●DN (3)3.1 Elfrect of Impact Angle on Impact EficiencyThe effect of impact angle of water sprays on thewhere P。, m。, men and c are the rotating momentum,impact efciency was first studied with the concentratorthe solid mass, the fluid mass and the solid concentra-at a drum rotation speed of 200 r/ min and an impacttion of the sttled bed, respectively; b, D and N areenergy of42 N●m. The tests were caried out on thethe effective separating width, the average diameterconcentrator under discontinuous concentration condi-and the rotation speed of drum, respectively.tion so that Eq. (6) can be used for calculation of theAnd the impact momentum of water sprays againstimpact eficiency cofficient K. Fig. 4 ilustrates thethe sttled bed can be expressed as:test results of the concentrator applied to the hematiteP。=一.qmu(4)materials as a function of impact angle. As is noted, theconcer-rith the increase ofwhere M_ is the impact imomentum of water sprays; Lisimpac中国煤化工625/9)athe spacing between two neighboring water sprays; qnis:YHCN M H Gsed sharply 88 thethe mass rate of a water spray.impact angle further increased.The mass rate of a water spray is given by the con-:.93●Joumal of Harbin Intinue of Technology (Nen Series), VoL 16, No. 1, 20090.8ceeded 0.59 kg/s. The compronise between the above一Ptwo opposite aspects leads to the reduction of impact ef-0上十一m.l 0.6ficiency after 0. 59 kg/s concentrate mass On the oth-er hand, the impact energy can not be accordingly de-creased when the concentrate mass was below 0.44 kg/一弋+04, because a minimal cnitical water sprays pressure of0.4 MPa is also demanded to impact down the sttled20 t0.2bed. In such case, an impact energy waste occurredwhen the concentrate mass was lower than 0. 44 kg/s,thus generates a reduction of impact eficiency. There-60fore, the highest impact eficiency may be maintaineda/(°证the pressure of water sprays is kept in the range ofFlg.4 Etect of impact angle阳impact fficiency0.4 -0.6 MPa by using changeable nozzles in diame-Obviously, an unitted impact angle reduces theter for different concentrate masses.impact fficiency; this is similar to that of a jet cutting8Ca material .5. It can be seen from Fig. 4 that theK=0.6416water sprays achieved the highest impact efficiency a旺the impact angle of 60°. Test results in this studyshowed that the average solid concentration of concen-trate at the drum rotation speed of 200 r/min is82. 5%. Therefore, the highest impact eficiencyK; =0, 6416 is calculated by incorporatingP。 =42 N .m,20m. =0.62 kg/s,c = 82.5%,D = 1.767 m,N =200 r/min and a = 60° into Eq. (6).0.45 0.550.650.753.2 Effect of Concentrate Mass on Impact Energym/ (kgrs")Fig. 5 shows the efeet of the concentrate mass onFig.s Ertect of concentrate mass on impact energythe impact energy in the concentrator (N = 200 t/min) under continuous concentration condition, com-3.3 Effect of Drum Rotation Speed 瞪lmpactpared with that calculated in the light of Eq. (6) onEnergythe assumption that the water sprays removed the set-The efect of drum rotation speed on the impacttled bed at the highest impact fficiency. As it is theo-energy was studied on the concentrator under continu*retically expected，the inerease in concentrale m8ss ledous concentration condition. During this test, the con-to an increase in the impact energy. It was found that 8centrate mass and the pressure of water sprays weretoo small or too big concentrate mass resulted in akept at around 0. 45 kg/s and in the range of 0.4-bigher impact energy consumption in comparison with0.6 MPa, respectively. It can be seen from Fig. 6 thatthat achievable at the highest impact eficiency ofK =the drum rotation speed has a very significant efect on0.6416. This higher impact energy consumption rethe impact energy. The impact energy proportionallyduces the impact fficiency according to Eq. (6). Asincreased with the increase of drum rotation speed.can be seen from Fig5, the m。- Pcurve is almost 8-perposed with the dashed line when the concentrate8[mass varied from 0.44 to 0.59 kg/s, that is, the high-est impact efficiency of 0.6416 was maintained in this0.45range; beyond the range, the impact efficiency was rduced.It was observed during the tests that parts of the40.30.concentrated particles were impacted back into thedrum rotating at the drum rolation speed of 200 r/ min一P 40.15when the pressure of water sprays was higher than 0. 6MPa. The returned concentrate particles results in把-duction of the concentrate mass, thus reduces the in-pact efficiency. However， a water sprays pressure中国煤化工268higher than 0.6 MPa was practically needed for achie-Fg6.YHC N M H Gimpact energyving continuous concentration of the concentrator withfixed nozzles in diameter when the concentrate mass ex-Two main factors determine this observation..94.Joumal of Harbin Institute of Technology (New Series)。Vol. 16, No. 1.2009Firstly, aggregating force between heavy particles inMinerls Proessing, 1998, 54(2): 11-127.the settled bed increases with the increases of drum ro-[2] Zeng w, Xiong D H. The latest application of SLon vericalring and pulsating high-gradient magnetic separator. Miner-tation speed, 8s a result of their exposure to a strongerals Engineering, 2003, 16(6): 563 - 565.centrifugal force. Secondly, the settled bed obtains a[3] Arol A I, Aydogan A. Recovery enhancement of magnetichigher rotating momentum when it rotates with the drumfines in magnetic separation. Colloids and Surfaces A,at a higher rotation speed. A combination of these two2004, 232(2-3): 151- 154.aspects demands a more adequate impact energy of wa-[4] ChenLz, Ren N Q, Xiong D H. Test research on recon-ter sprays onto the settled bed. It was also found duringcentration of Hainan Stee1' S tailings by high intensity mag-the tests that an increase in the pressure of water spraysnetic separation-centrifugal separator. Metal Mine, 2006 ,is required with the increase of drum rotation speed for10: 75 -77. (in Chinese)[5] Cai X. Advance in applying iron tailing as building materi-impacting down the settled bed, which may also be real. Metal Mine, 2000, 10: 45 -48. (in Chinese)sponsible for the above observation. In view of this, it[6] YuanS L Ways and tasks of comprehensive utilization andis advisable to increase the impact energy by using adisposal of metal mine solid wastes. I Express Information ofhigher water sprays pressure when the drum rotates at aMining Industry, 2004, 9: 1 -5. (in Chinese)higher rotation speed.[7] Chen Lz, Ren N Q, Xiong D H. Industrial lest on recove-ring microfine iron minerals by SLon continuous centrifugal4 Conclusionsseparators. Metal Mine, 2007, 1: 63-66. (in Chinese)[8幻] Wei Jintao, Yang Bo. A study of gravity concentrationtechnology on superfine particles. Joumal of Kunming Uni-A minimal eritical impact energy is required in theversity of Science and Technology, 2001 , 26(1):46 -47,water sprays for achieving continuous concentration of63. (in Chinese)the SLon concentrator. The impact angle of water[9] Ergun L, Ersayin S. Studies on pinched sluice concentra-sprays has a significant influence on the impact ffi-tion. Par I: The ffects of operating variables and sluiceciency. An ufited impact angle reduces the impactgeometry on the performance of pinched sluices. 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Synergeic ees of secondary liquid dropwhich parts of concentrate mass was impacted back intoimpact and solid particle impact during hydro-abrasive eno-sion of britle materials. Wear, 2004, 256(I1 - 12):the drum. The impact energy increases with the in-1190- 1195. .crease of drum rotation speed. This increase is mainly[ 13 ]Momber A W. The kinetie energy of wear particles genera-due to the fact that a higher drum rotation speed leadsted by abrasive waler-jet erosion. Joumal of Materials Pro-to an increase in the aggregating force between heavycessing Technology, 1998, 83(1 -3): 121- 126.particles in the settled bed and in the rotation momen-[14]Hu X G, Momber A w, Yin Y G. Hydro abrasive erosiontum of the bed.of stel-ibre reinforced hydraulic .oncrete. Wear, 2002,253(i -8): 848 -854.References:[15]Hu X G, MomberA w, Yin Y, et al. High-speed hydro-dynamic wear of steel-fibre reinforced hydraulic concrete.[1] Xiong D H, Liu s Y, Chen J. New technology of pulsatingWear, 2004, 257(5 -6): 441 -450.high geradient magnetic eparation. Intemational Joumal of中国煤化工MYHCNMHG●95.