STUDY ON IMPROVING THE CLASSIFICATION EFFICIENCY OF A LARGE-SCALE HYDROCYCLONE BASED ON CFD SIMULATION

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1 Eleventh Internatonal Conference on CFD n the Mnerals and Process Industres CSIRO, Melbourne, Australa 7-9 December 2015 STUDY ON IMPROVING THE CLASSIFICATION EFFICIENCY OF A LARGE-SCALE HYDROCYCLONE BASED ON CFD SIMULATION Baoyu CUI 1, Dezhou WEI 1 *, Cae ZHANG 1, Tanshu LI 1, Kaka LIU 1, and Yuqng FENG 2 1 College of Resources and Cvl Engneerng, Northeastern Unversty, Shenyang , PR CHINA 2 CSIRO Mneral Resources, Prvate Bag 10, Clayton South, VIC 3169, AUSTRALIA *Correspondng author, E-mal address: dzwe@mal.neu.edu.cn ABSTRACT As the most mportant classfcaton equpment n mneral processng ndustry, hydrocyclone has been studed wdely. Dversfed and large-scale hydrocyclones are necessary to meet the needs of present ndustral producton, but the followng queston s that the physcal expermental studes on hydrocyclones become more dffcult. Numercal smulaton has been proved to be an effectve method for the nvestgaton of hydrocyclones because of ts veracty, hgh effcency and low cost. In ths study, numercal smulaton method based on Computatonal Flud Dynamcs (CFD) was used to mprove the classfcaton effcency of a Φ660 mm hydrocyclone, whch was used n the classfcaton of a low-grade hematte ore n Chna. As the frst step of model development, CFD model was set up and run usng a scaled-down hydrocyclone (Φ50mm). In parallel, physcal experments facltated wth PIV flow feld measurement and real mneral partcles were conducted n the same szed hydrocyclone n order to valdate the developed CFD model. Followng model valdaton, the CFD model was extended to nvestgate the Φ660mm hydrocyclone. The classfcaton effcency was ncreased obvously by optmzng the operatonal and structural parameters of the hydrocyclone. INTRODUCTION Hydrocyclone has been wdely used n many areas of separaton due to the advantages of ts low operatng cost, hgh throughput, low space requrement, etc, and t s used almost n all of the mneral processng plants n the world. Multple nvestgatons on hydrocyclones have been carred out snce the 1950s (Kelsall, 1952). Now a commonly accepted wsdom s that the separaton characterstcs of the hydrocyclone depend on the flow feld, and turbulence ntensty of whch has a maor mpact on hydrocyclone s classfcaton effcency (Tea et al, 2014). Classfcaton effcency s one of the most mportant parameters to evaluate the performance of a hydrocyclone. In mneral processng, consumptons of energy and steel balls can be decreased; capacty and separaton can be ncreased va mprovng hydrocyclone s classfcaton effcency. Before the tme when the mportance of the nternal flow feld was formally recognsed, desgn of a hydrocyclone was regarded as a black box. Nowadays, the study of flow feld nsde hydrocyclone s a hot research area. The flow feld n a hydrocyclone, whch s characterzed by a complex three dmensonal swrlng flow wth an ar core, s decded by varous operatonal and structural parameters. Large-scale and strong turbulence s the man character of flow feld nsde hydrocyclone, local hgh turbulence ntensty reduces the radal regular dstrbuton of partcles. The detaled velocty profles n a hydrocyclone were measured frst by Kelsall usng a stroboscope (Kelsall, 1952). Wth the development of laser and hgh-speed camera technologes, laser Doppler velocmetry (LDV) and partcle mage velocmetry (PIV) are utlzed to nvestgate the flow feld nsde hydrocyclones (Marns, 2010, Jonas and Hannes, 2004). Dabr and Petty (1984) measured the tangental and axal veloctes n hydrocyclones usng LDV as early as CFD smulaton becomes an mportant numercal method for the detaled study of the flow feld of hydrocyclones due to the rapd development of computer technology and mathematcal models (Jose and Raaman, 2005). The frst successful CFD predctve work on hydrocyclones was reported by Rodes n 1987 (Rodes, et al, 1987). Nowadays, the CFD modelng software packages, ncludng ANSYS/Fluent, have shown suffcent accuracy n modelng the flow feld n a hydrocyclone, and smulaton results have been compared wth the LDV or PIV expermental measurement whch demonstrated good agreement (Tea et al, 2014, Cu, et al, 2014). In ths paper, numercal smulaton method based on CFD was used to mprove the classfcaton effcency of a Φ660mm hydrocyclone, whch was used n the classfcaton of a low-grade hematte ore n Chna. As the frst step of model development, CFD model was set up and run usng a scaled-down Φ50mm hydrocyclone, usng ANSYS/Fluent wth a Reynolds stress model (RSM) for turbulence calculaton, the volume of flud (VOF) model for capturng the ar-lqud nterface, and a mxture model for predctng partcle movement. In parallel, physcal experments facltated wth PIV flow feld measurement that uses real mneral partcles were conducted n the same szed hydrocyclone n order to valdate the developed CFD model. Followng model valdaton, the CFD model was extended to nvestgate the Φ660mm hydrocyclone. Turbulence ntensty of the flow feld was nvestgated to optmze the operatonal and structural parameters, the classfcaton effcency of the Φ660mm hydrocyclone was ncreased obvously. MODEL DESCRIPTION Hydrocyclone Geometry Table 1 lsts the parameters nvestgated n ths study. The model as nvestgated n the experment s based on a Φ50mm hydrocyclone. Copyrght 2015 CSIRO Australa 1

2 Item Φ50mm Φ660mm Feed pressure /MPa Cone angle / Underflow outlet dameter /mm Overflow outlet dameter /mm Table 1: Parameters nvestgated n the study Numercal model Smulaton of hydrocyclones requres accountng for the rregular nterface between ar core and the surroundng lqud, the turbulence nduced by the strong swrlng flow, as well as the movement behavor of partcles. Ths has been a challengng task n the numercal modelng of multphase flow. In ths paper, detals of the modellng approach are not lsted completely, only the key governng equatons are brefly provded. For ncompressble flud, the equatons for mass (or contnuty) and momentum n a general form are as follows: 0 (1) t x p ( u ) [ ( ) u ] g (2) t x x x x x The RSM model uses the followng transport equaton for the Reynolds stresses: u ( u ) ( t k ) P R S D t xk xk k xk (3) For VOF model, the governng equaton for the phase can be wrtten as 0 (4) t x For the mxture model, the nvestgated mxed phase vscosty s closely related to the volume fracton of -10 μm as found n the physcal separaton test of a Φ50 mm hydrocyclone, the mxed phase vscosty μm s defned as follows (Chu, et al, 2012, Q, et al, 2015): v 10 m m 3.8 w(1 ) (5) 0.62 where μm s the water vscosty, and v-10μm s the volume fracton of -10μm partcles. Pressure nlet was set and assumed to be constant. Pressure outlets were set for overflow and underflow and assumed to be at atmospherc or zero gauge pressure. No-slp boundary condtons were appled on all walls. Intally, the whole flud doman was assumed to be flled wth ar representng a startup condton of an actual operaton. Grd dependency studes were carred out, t was observed that 300,000 cells for Φ50mm hydrocyclone and 800,000 cells for Φ660mm hydrocyclone were optmal for compromsed consderatons of smulaton accuracy and computng tme. The meshes for the numercal smulaton are shown n Fgure 1. Fgure 1: Meshes for numercal smulaton (a) Front vew of the Φ50mm hydrocyclone; (b) Top vew of the Φ660mm hydrocyclone; (c) 3D vew of the Φ660mm hydrocyclone Expermental model To avod complextes arsng from testng of the full-scale hydrocyclone (.e. Φ660mm),, physcal experments were performed for a scaled-down verson of the unt (.e. Φ50mm). Fgure 2 shows photos of the expermental model. In order to reduce errors caused by laser scatterng, a couple of orthogonal planes were placed n the outsde of the transparent model. (a) (b) Fgure 2: Expermental model (a) hydrocyclone for PIV measurement; (b) hydrocyclone for real mneral partcles Polystyrene powder (-10μm, 1050kg/m 3 ) was used as tracer partcle n the PIV measurement, whch was carred out to obtan the flow veloctes over a vertcal plane passng the hydrocyclone centre axs. A total of 200 transent velocty frames were recorded to obtan a tme averaged value. Quartz partcles (2673kg/m 3 ) were fed nto the Φ50mm hydrocyclone, the sze dstrbuton s shown n Table 2. Partcle sze /μm Yeld Table 2: Sze dstrbuton of mneral partcles used n ths paper Copyrght 2015 CSIRO Australa 2

3 RESULTS AND DISCUSSION Model Valdaton The ar core as predcted and measured nsde the Φ50mm hydrocyclone s compared n Fgure 3, and axal velocty dstrbutons along a horzontal symmetry axs at the lower end of the cylndrcal secton (or upper end of the concal secton) of the hydrocyclone are also compared n Fgure 4. Results obtaned by both numercal and expermental methods are n very good agreement. (a) (b) Fgure 3: Comparson of ar core (a) CFD smulaton; (b) physcal measurement Axal velocty / m/s ar core LZVV boundary radal poston / mm CFD PIV Fgure 4: Comparson of axal velocty dstrbutons When solds volume fracton of the feedng partcles was 10%, Φ50mm hydrocyclone classfcaton tests of quartz partcles were carred out by both numercal and physcal methods. Sze dstrbutons of underflow products were compared n Fgure 5. For the physcal experment, results were averages based on fve tests. It can be seen that the mxture model as gven n equaton (5) has suffcent accuracy. Investgaton of Φ660mm Hydrocyclone The Φ660mm hydrocyclone s beng used n the classfcaton of a low-grade hematte ore n Chna, the sze dstrbutons of feedng and products are shown n Table 3. Partcle sze /μm Feedng Overflow Underflow Table 3: Sze dstrbuton of feedng and products of Φ660mm hydrocyclone Classfcaton effcency of the hydrocyclone E can be obtaned by: ( )( ) E 100 ( )(100 ) (6) where α, β and θ are proportons of partcles for a partcular sze group n feedng, overflow and underflow respectvely. Table 3 shows that the classfcaton effcency of -71μm partcles s as low as 39.32%, and the man reason can be obtaned from Table 3 s that there are too many -71μm partcles enterng the underflow. It s necessary to reduce the yeld of -71μm partcles n the underflow to ncrease the classfcaton effcency. Effectve methods nclude ncreasng the cone angle and overflow outlet dameter, and decreasng feed pressure and underflow outlet dameter. Fgure 6 shows turbulence ntensty of the flow feld wth dfferent cone angles. Wth the use of a 25 conc secton, the turbulence ntensty reduces generally, whch s useful for mprovng the classfcaton effcency. The turbulence ntensty ncreases around underflow outlet. Ths s helpful for removng the fne partcles before they enter the underflow. The underflow outlet dameter of 80mm and overflow outlet dameter of 210mm are determned based on the same method CFD smulaton Physcal experment Yeld / % Partcle sze / m Fgure 5: Comparson of sze dstrbutons of underflow products Fgure 6: Turbulence ntensty of the flow feld wth dfferent cone angles In ths study, the reducton of feed pressure can not only ncrease the classfcaton effcency but also reduce the energy consumpton. However, there s a mnmum feed pressure to mantan the stablty of the hydrocyclone. It can Copyrght 2015 CSIRO Australa 3

4 be seen from Fgure 7, the ar core for feed pressure 0.04MPa shows sgnfcantly dfferent pattern from others, feed pressure 0.06MPa s recommended n ths case. 85.5μm 58μm 0.1MPa 0.08MPA 0.06MPa 0.04MPa Fgure 7: Ar core pattern for dfferent feed pressure For a solds volume fracton of 10% (feedng), feed pressure 0.06 MPa, cone angle 1 st 20 and 2 nd 25, underflow outlet dameter of 80 mm and overflow outlet dameter of 210 mm, solds volume fracton dstrbutons for dfferent partcle szes are shown n Fgure 8. For convenence of calculaton, the denstes of dfferent partcles were set accordng to ther respectve ron grade. Densty calculaton equaton s as follows: 100 (7) Where β s the ron grade of a partcular sze partcles n feedng, detals of whch are shown n Table 4. Partcle sze /μm Iron grade Table 4: Iron grade dstrbutons 40.5μm 30.5μm Accordng to the CFD smulaton results, classfcaton effcency for -71μm partcles s about 47.54%, ndustral test wll be performed to valdate the numercal result. 12.5μm Fgure 8: Solds volume fracton dstrbutons for dfferent partcle szes n the new Φ660mm hydrocyclone 202μm 127μm CONCLUSIONS Flow dynamcs and partcle separaton n hydrocylones of Φ50mm and Φ660mm have been numercally studed usng CFD models wth ANSYS/Fluent beng a numercal platform. The man fndngs from the present work are: 1) Usng a Φ50mm hydrocyclone as a testng bed, CFD smulaton results agreed well wth physcal expermental results n terms of ar core sze, lqud axal velocty dstrbuton and partcle separaton. The good agreement demonstrates valdty of the present modellng approach for hydrocylone studes, e.g. RSM and VOF models for predctng the ar core formaton and the corrected Mxture model for the descrpton of partcles movement behavour. 2) Large-scale and strong turbulence s the man characterstc of flow feld nsde hydrocyclone. The classfcaton effcency of hydrocyclone can be mproved through reducng the turbulence ntensty of flow feld nsde hydrocyclone by optmzng the operatonal and structural parameters. Copyrght 2015 CSIRO Australa 4

5 3) For the Φ660mm hydrocyclone, the classfcaton effcency for -71 μm partcles n the orgnal desgn s as low as 39.32%. Accordng to numercal results, when feed pressure 0.06MPa, cone angle 1 st 20 and 2 nd 25, underflow outlet dameter 80 mm and overflow outlet dameter 210 mm, the classfcaton effcency of -71 μm partcles can be up to about 47.54%. The smulaton results demonstrated that CFD model s a valuable research tool n mprovng research effcency of hydrocyclone. The usefulness of CFD model wll be fully demonstrated n the future for a detaled evaluaton of the desgn and/or operatons of ndustral scale hydrocyclones of varous types. REFERENCES KELSALL, D.F., (1952), A study of the moton of sold partcles n a hydrocyclone, Transactons of the Insttuton of Chemcal Engneerng, 30, TEJA, R.V., KEDAR, S.K., RAVI, G., NARASIMHA, M., (2014), Computatonal and expermental study of the effect of nclnaton on hydrocyclone performance, Separaton and Purfcaton Technology, 138, MARINS, L.P.M., DUARTE, D.G., LOUREIRO, J.B.R., MORAES, C.A.C., FREIRE, A.P.S., (2010), LDA and PIV characterzaton of the flow n a hydrocyclone wthout an ar-ore, Journal of Petroleum Scence and Engneerng, 70, JONAS, B., and HANNES, V., (2004) Velocty measurements n a cylndrcal hydrocyclone operated wth an opaque fber suspenson, Mnerals Engneerng, 17, DABIR, B., and PETTY, C.A., (1984), Laser Doppler anemometry measurements of tangental and axal veloctes n a hydrocyclone operaton wthout an ar core, Second Internatonal Conference on Hydrocyclone, Cranfeld, England, September. JOSE, A.D., and RAJAMANI, R.K., (2005), A comparatve study of three turbulence-closure models for the hydrocyclone problem, Internatonal Journal of Mneral Processng, 77, RODES, N., PERICLEOUS, K.A., DRAKE, SN., (1987), The predcton of hydrocyclone performance wth a mathematcal model, Process Internatonal Conference on Hydrocyclone, CUI, B.Y., WEI, D.Z., GAO, S.L., LIU, W.G., FENG, Y.Q., (2014), Numercal and expermental studes of flow feld n hydrocyclone wth ar core, Transactons of Nonferrous Metals Socety of Chna, 24, CHU, K.W., WANG, B., YU, A.B., VINCE, A., (2012), Partcle scale modellng of the multphase flow n a dense medum cyclone: effect of vortex fnder outlet pressure, Mnerals Engneerng, 31, QI, Z., KUANG, S.B., YU, A.B., (2015), Numercal nvestgaton of the separaton behavours of fne partcles n large dense medum cyclones, Internatonal Journal of Mneral Processng, n press. Copyrght 2015 CSIRO Australa 5