numerical studies of the effects of medium properties in dense medium cyclone operations

Minerals Engineering 22 (2009) 931–943

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Minerals Engineering

journal homepage: www.elsevier .com/locate /mineng

Numerical studies of the effects of medium properties in densemedium cyclone operations

B. Wang a, K.W. Chu a, A.B. Yu a,*, A. Vince b

a Lab for Simulation and Modelling of Particulate Systems, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australiab Elsa Consulting Group Pty Ltd., Mackay, Qld 4740, Australia

a r t i c l e i n f o

Article history:Received 16 December 2008Accepted 29 March 2009Available online 26 April 2009

Keywords:Dense medium cycloneMultiphase flowComputational fluid dynamicsSeparations

0892-6875/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.mineng.2009.03.019

* Corresponding author. Tel.: +61 2 93854429; fax:E-mail address: [email protected] (A.B. Yu).

a b s t r a c t

A mathematical approach is proposed to describe the multiphase flow in a 1000 mm industrial densemedium cyclone. A mixture multiphase model is employed to describe the flow of the dense medium(comprising finely ground magnetite contaminated with non-magnetic material in water) and the aircore, where the turbulence is described by the well established Reynolds stress model. The stochasticLagrangian particle tracking method is used to simulate the flow of coal particles. The proposed approachwas qualitatively validated using literature and industrial data and then used to study the effects of med-ium properties including medium density, magnetite type and non-magnetic content. It is found that asthe medium density increases, the pressure drop increases, resulting in a high pressure gradient force oncoal particles and reduced separating efficiencies. The segregation of magnetite particles becomes seriousas magnetite particle size increases, which leads to a high density differential and a high off-set. The vis-cosity of medium decreases and the segregation of magnetite particles become significant with thedecrease of non-magnetic content, resulting in a high density differential and off-set.

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1. Introduction

Dense medium cyclones (DMCs), also known as heavy mediumcyclones, are the work horses of the modern coal industry to up-grade run-of-mine coal in the 0.5–50 mm size range. Their workingprinciple has been well documented (for example, see King andJuckes, 1984; Svarovsky, 1984; Wills, 1992). The feed, which is amixture of dense medium slurry and raw coal, enters tangentiallynear the top of the cylindrical section under pressure, thus promot-ing a strong swirling flow. The refuse or high ash particles move to-wards the wall where the axial velocity vector points downwardand are discharged through the spigot. The lighter clean coalmoves towards the longitudinal axis of the cyclone, where thereis usually an axial air core present and the axial velocity vectorof slurry flow points upward; and passes through the vortex finder.Their invention about 70 years ago has been followed by continu-ous improvement and DMCs have proven to be effective in the coalindustry. Today DMCs process the vast majority of tonnes fed tocoal preparation plants.

The flow in a DMC is very complicated with the presence ofswirling turbulence, air core and particle segregation, and involvesmultiple phases: gas, liquid, coal and magnetic/non-magnetic par-ticles of different sizes and densities. Normally, the slurry includ-

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ing water, magnetite and non-magnetic content is termed‘‘medium”. Precise measurement of the velocity field in a DMC isvery difficult, mainly because of the presence of magnetite parti-cles in the medium. Medium properties play an important role inDMC operation. The factors that control the fluid mechanical med-ium properties in industry are medium density (solids content),magnetite type (largely expressed by particle size distribution)and non-magnetic content. He and Laskowski (1994) experimen-tally studied the effects of some medium properties on the separa-tion performance. They found that while the separation efficiencyand cut point shift (or off-set) for coarse particles were mainlydetermined by the medium stability, the separation performanceof fine particles was more sensitive to the change in medium rhe-ology. However, the mechanism behind this effect remains unclearand the effects of some key variables have not been systematicallystudied. In our recent work (Wang et al., 2009), a more comprehen-sive CFD model of DMC has been established. The resulting modelprovides a good tool to investigate the effects of medium proper-ties in dense medium cyclones.

This paper reports our recent effort in modelling the gas–li-quid–solid flow in a 1000 mm DMC. It shows that the combinationof the Reynolds stress model (RSM), volume of fluid (VOF) free sur-face model, mixture model and stochastic Lagrangian model cansatisfactorily describe the flow and performance of a DMC. Aftervalidation, this model was used to study the effects of mediumproperties including the medium density, magnetite type and

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Nomenclature

CD drag coefficientd particle diameter, mDT,ij turbulent diffusion termg acceleration due to gravity, m s�2

k kinetic energy, m2 s�2

p static pressure, PaPij stress production termRe Reynolds numbert time, su instantaneous velocity, m s�1

v instantaneous velocity, m s�1

w instantaneous velocity, m s�1

u0 dispersion velocity, m s�1

�u time average velocity in axial direction, m s�1

x axis, m

a volume fractioneij dissipation termuij pressure strain terml fluid viscosity, kg m�1 s�1

q density, kg m�3

f normally distributed random number

Subscriptsc continuous phased solid phasedr drift velocityi, j, k 1,2,3m mixturep particleq qth phase

932 B. Wang et al. / Minerals Engineering 22 (2009) 931–943

non-magnetic content on the separation performance. The under-lying mechanism was explored by analysing the internal flow ina DMC.

2. Model description

Recognising that the flow in a DMC is quite complicated, wedivided our modelling into three steps, as shown as Fig. 1. InStep 1, only air and slurry with a certain density are considered.The two phases are treated as fluids of homogeneous viscosityand density. Turbulent flow is modelled using the RSM, andthe VOF free surface model is used to describe the interface be-tween the medium (defined as the mixture of water, magnetite

Fig. 1. Steps used in the

Table 1Mathematical models used in the present DMC modelling.

Model

Turbulence Reynolds stress model (RSM)

Multiphase Volume of fluid multiphase model (VOF)

Mixture multiphase model

Particle Lagrangian particle tracking model (LPT)

Viscosity Viscosity correction

where: ui ¼ �ui þ u0i; q ¼P

aqqq; um ¼Pn

q¼1aqqq uq

qm; qm ¼

Pnq¼1aqqq; lm ¼

Pnq¼1aqlq; FD

and non-magnetic content) and the air core (here defined asthe regions with air volume fraction larger than 90%). In thisstep, the primary position of the air core and the initial velocitydistributions are obtained. The method is similar to that used formodelling the multiphase flow in a hydrocyclone (Wang et al.,2007a).

In Step 2, six additional phases are introduced to describe thebehaviour of magnetite particles with different sizes. The multi-phase model is changed from the VOF to the so-called mixturemultiphase model. A correction is also necessary to estimate theviscosity effect of magnetite and non-magnetic particle size distri-bution. Detailed density and velocity distributions of differentphases are obtained at the end of this step.

present modelling.

Equations@q@t þ @

@xiðquiÞ ¼ 0

@@t ðquiÞ þ @

@xjðquiujÞ ¼ � @p

@xiþ @

@xjl @ui

@xjþ @uj

@xi

� �h iþ @

@xjð�qu0iu

0jÞ

@@t qu0iu

0j

� �þ @

@xkquku0iu

0j

� �¼ DT;ij þ Pij þ /ij þ eij

@aq

@t þ uj@aq

@xj¼ 0

@@t qui þ @

@xjquiuj ¼ � @p

@xiþ @

@xjl @ui

@xjþ @uj

@xi

� �þ qgi

@@t ðqmÞ þ @

@xiðqmumÞ ¼ 0

@@t qmumi þ @

@xjqmumiumj ¼ � @p

@xiþ @

@xjlm

@umi@xjþ @umj

@xi

� �þ qgi þ @

@xjðPn

q¼1aqqqudr;qiudr;qjÞd u!p

dt ¼ FDð u!� u!pÞ � rpqp

lm ¼ 3:8lcð1�ad

0:62 Þ�1:55

¼ 18ld2

pqpCD

Rep

24

Table 4Variables considered in this work.

Parameter Value

Feed density (kg m�3) 1400, 1450, 1500, 1550, 1600Magnetite type: size

(content in vol%)Ultra fine Super fine Fine Medium10 (35.1%), 10 (30.5%), 10 (23.0%), 10 (18.3%),20 (30.3%), 20 (25.6%), 20 (22.0%), 20 (20.0%),30 (17.3%), 30 (14.6%), 30 (14.9%), 30 (16.2%),40 (8.2%), 40 (11.5%), 40 (8.1%), 60 (19.8%),50 (5.6%)and

50 (9.9%)and

50 (15.7%)and

70 (10.5%)and

80 lm(3.5%)

80 lm(7.9%)

80 lm(16.3%)

100 lm(15.2%)

Non-magnetic contentin the dry medium(mass%)

0, 7.5, 15, 22.5, 30

B. Wang et al. / Minerals Engineering 22 (2009) 931–943 933

In Step 3, the results of the fluid flow are used in the simulationof the flow of coal particles described by the stochastic Lagrangianparticle tracking model (LPT). The characteristics of the DMC sep-arating performance, such as partition curve and medium split,are then estimated.Therefore, the whole process involves fourCFD models for different phases and one viscosity correction mod-el, with their governing equations listed in Table 1. Details and jus-tification of the use of these models can be found elsewhere (Wanget al., 2009).

3. Simulation conditions

A 1000 mm dense medium cyclone deployed in an industrialcoal preparation plant (Rong, 2007) is simulated in this work.

Fig. 2. Geometry (a) and mesh (b) representation of the simulated DMC (Dc =1000 mm).

Table 2Geometric parameters of the DMC considered.

Parameter Symbol Dimension (mm)

Diameter of the body Dc 1000Side length of inlet (involute) Li 266Diameter of vortex finder Do 450Diameter of spigot Du 337Length of cylindrical part Lc 1200Length of vortex finder Lv 700Length of conical part Lp 1880

Table 3Operational conditions and medium properties of the DMC considered.

Parameter Dimension

Medium feed flow rate 1120.8 m3/h (inlet velocity is 4.4 m s�1)Medium feed density 1550 kg m�3

Magnetite type: size(content in vol%)

Superfine: 10 (30.5%), 20 (25.6%), 30(14.6%), 40 (11.5%), 50(9.9%) and 80 lm(7.9%)

Non-magnetic contentin the dry medium (mass%)

15%

Non-magnetic size(content in mass%)

24 (55.5%) and 125 lm (45.5%)

Orientation angle 10� to horizontal

The geometry and mesh representation of the cyclone are shownin Fig. 2 and geometrical parameters are given in Table 2.

The whole computational domain is represented by hexahedrongrids. In the vicinity of the walls and vortex finder, a finer grid isused than for the remainder of the cyclone. In total, 80,318 cellswere used in the present work, with trial numerical results indicat-ing a greater number did not change the solution.

Table 3 lists the operational conditions and medium propertiesused in this work. The inlet slurry velocity and the coal particlevelocity were both 4.4 m s�1 (i.e., the feed flow rate was1121 m3/h). The feed medium density was 1550 kg m�3. The pres-sure at the two outlets (vortex finder and spigot) was atmospheric(assumed to be 101.325 kPa).

Six different sized magnetite particles, with their volume frac-tions approximated by a Rosin–Rammler distribution, are used todescribe the behaviour of magnetite phases (data in brackets rep-resent volume fraction): 10 (30.5%), 20 (25.6%), 30 (14.6%), 40(11.5%), 50 (9.9%) and 80 lm (7.9%), respectively. Coal particleswith a density range from 1200 to 1700 kg m�3 were injectedwith the dense medium at the inlet. Two different non-magneticphases were also considered in the work: 24 (55.5%) and 125 lm(45.5%).

This model ignores the effects of coal particles on the slurryphase and the interaction between individual coal particles. Thistreatment has been widely accepted for dilute flows (Ma et al.,2000; Narasimha et al., 2006). For dense flows, the so-called com-bined approach of CFD and discrete element method (CFD–DEM)can be applied (Xu and Yu, 1997; Yu and Xu, 2003; Feng and Yu,2004; Chu and Yu, 2008), which is able to account for particle–par-ticle and particle–fluid interactions. Such a CFD–DEM approach isbeing undertaken in our other DMC modelling investigations(Wang et al., 2007b; Chu et al., 2008, in press).

The effect of changing the medium properties was examinedthrough a parametric study. Table 4 shows the variables consid-ered in this work. They are the medium density, magnetite typeand non-magnetic content. For each case, only one variable waschanged at a time with other variables adopting the same valuesas the base case dense medium cyclone (Tables 2 and 3).

Table 5Comparison of the flow in the DMC between the predicted and measured results.

Operatinghead (Dc)

Overflow(RD)

Underflow(RD)

Densitydifferential (RD)

Experimental 8.3 1.470 1.790 0.32Numerical 7.94 1.515 1.745 0.23


4. Results and discussion

4.1. Model validation

The proposed model was initially validated by comparing theexperimental and simulation results generated for a 350 mmDMC (Wang et al., 2009). In this work, the validation extends to

Fig. 3. Comparison of measured and predicted DMC partition curves for differentsize fractions: (a) 11 � 4 mm; (b) 4 � 1.4 mm; and (c) 1.4 � 0.25 mm.

a 1000 mm DMC which is indeed at an industrial scale. Quantita-tive comparison between the predicted and measured mediumflow characteristics is shown in Table 5, in which the RD is definedas the relative density with respect to water. It can be seen that thepredicted medium flow characteristics agree reasonably well with

Fig. 4. Operational performance as a function of medium feed density: (a)operational head; (b) density differential; and (c) medium split to overflow.

Fig. 5. The effect of medium feed density on partition curves of different sized coal particles: (a) 7 mm; (b) 4 mm; (c) 1.4 mm; and (d) 0.5 mm.


the experimental measurements, although the simulated opera-tional head and density differential are a little lower than thosemeasured. The low operating head is considered to be a conse-quence of ignoring the effect of solid (coal) flow on medium flow.

Fig. 3 shows a comparison of the measured and predicted DMCpartition curves. The general shapes of the partition curves for par-ticles of different sizes are similar to those measured experimen-tally. However, the simulated cut points are higher than thosemeasured. This could be caused by many factors, for example,the differing effect of viscous forces on different size particles,neglecting some liquid–solid forces in the model, the non-interact-ing particle assumption inherent to the Lagrangian approach andso on. How to improve the accuracy will be an aspect for futurework, in the mean time, the results in Table 5 and Fig. 3 suggestthat the proposed approach can at least be used to assess qualita-tively the performance of a DMC. Below we will use this model toexamine the effects of medium properties.

4.2. Effect of medium feed density

Fig. 4 shows that the operational head and the medium splitreporting to overflow remain constant as the medium feed densityincreases from 1400 to 1600 kg m�3. As the feed density increases,a constant head can only be achieved by a commensurate pressuredrop. Fig. 4 also indicates that the density differential (the differ-ence between the underflow and overflow medium densities) in-

creases to a maximum and then decreases. However, in absoluteterms the effect is small.

Fig. 5 shows the partition curves of different sized coal particleswith different medium feed densities. For all particle sizes, the par-tition curve shifts to the right as the feed density increases.

Fig. 6 presents the general performance for DMC separation, i.e.,off-set and Ep, under different feed densities. The so called cutpoint RD50 (the particle density at which 50% of particles reportto underflow) or off-set (the difference between the cut pointand feed medium density) and Ecart probable Ep (=(RD75 �RD25)/2) are the parameters commonly used to assess separationefficiency in industry and can be obtained from the partition curve.For all particle sizes, the CFD model predicts that the off-set in-creases with feed density, which agrees with the practical observa-tion (Gottfried and Jacobsen, 1977). The figure also indicates thatfeed density has an effect that is dependent on particle size. Forcoarser particles (>1.4 mm), Ep decreases a little with increasingfeed density. However, for finer particles (<1.4 mm), Ep increasessignificantly with increasing medium density.

Figs. 7 and 8 show the fluid and solid flow in the DMC when themedium feed density = 1400 and 1600 kg m�3, respectively. Figs. 7aand 8a indicate that the pressure drop increases as the medium feeddensity increases. A high feed density condition is associated with ahigh pressure near the wall. At the centre, the gauge pressureequals zero for both conditions. In other words, the inward pressuregradient force in the radial direction for the higher feed density case

Fig. 6. Separation performance versus medium feed density for different sized particles: (a) off-set and (b) Ep.

Fig. 7. Fluid and solid flow patterns in the DMC when medium feed density = 1400 kg m�3: (a) gauge pressure distribution; (b) medium density distribution; and (c) particledensity distribution.

Fig. 8. Fluid and solid flow patterns in the DMC when medium feed density = 1600 kg m�3: (a) gauge pressure distribution; (b) medium density distribution; and (c) particledensity distribution.



is higher. The effect of the pressure gradient force on solids isapproximately same for all particle sizes, and its presence helpsparticles to overcome the local centrifugal forces and report to over-flow. As a result, the partition curve for all particle sizes consideredshifts to the right when feed density is increased. Figs. 7b and 8bshow the medium density distributions for the two DMC conditions

Fig. 9. Simulated results of normalised average radial forces in the bulk downwardflow as a function of particle size at different locations from the top wall: (a)500 mm; (b) 1000 mm; and (c) 2500 mm.

examined. The medium flow is more uniform when the feed densityis low. The magnetite in the DMC with a high feed density has moreextensive segregation near the spigot and a high density ring is alsoformed. As the medium density increases, the medium viscosityalso increases. The effect of drag force increases commensurately,which increases the off-set, particularly for fine particles. Figs. 7c

Fig. 10. Operational performance versus magnetite type: (a) operational head; (b)density differential; and (c) medium split to overflow.

Fig. 11. The effect of magnetite type on partition curves of different sized coal particles: (a) 7 mm; (b) 4 mm; (c) 1.4 mm; and (d) 0.5 mm.

Fig. 12. Separation performance versus magnetite type for different sized particles: (a) off-set and (b) Ep.


and 8c show the spatial distribution of particles of different sizesand densities in the DMC, in which different colours represent dif-ferent densities. As the medium feed density increases, the densityof particles around the air core increases. The reason could be thatthe pressure gradient force is higher in the DMC with a higher med-

ium density. This helps particles to overcome the centrifugal forceand move inward. As a result, the partition curve shifts to the right.

Fig. 9 shows the magnitude of the three main forces acting onparticles in the radial direction in the main downward flow at 3axial locations. The forces considered are the pressure gradient


force per unit mass, centrifugal force per unit mass and drag forceper unit mass. It is noted that the values shown in the figure rep-resent the magnitudes of these forces. In all cases, the pressure

Fig. 13. Distribution of medium density in the DMCs with different magnetitetypes.

Fig. 14. Simulated results of normalised average radial forces in the downwardbulk flow as a function of particle size at different locations from the top wall: (a)500 mm; (b) 1000 mm; and (c) 2500 mm.

gradient force always points inwards while the centrifugal forcealways points outwards. Only the drag force shows a varyingdirectional characteristic with the direction dependent on the rel-ative velocity between particle and fluid. At 500 mm from the topwall, where the particles move to the area between the outsidewall of vortex finder and inner wall of cyclone body, the dragforce does not change significantly as the feed density increases.In the DMC with a feed density of 1400 kg m�3, the pressure gra-dient force is almost the same as the centrifugal force for thecoarse particle but slightly smaller for 0.5 mm coal particles. Thiswould tend to entrap particles in the bulk downward flow. As thefeed density increases to 1600 kg m�3, both the pressure gradientforce and the centrifugal force increase significantly. However, therate of increase of the pressure gradient force is higher. The dif-

Fig. 15. Operational performance versus non-magnetic content: (a) operationalhead; (b) density differential; and (c) medium split to overflow.


ference between these two forces increases, hence promoting par-ticle movement towards the centre of the DMC. At 1000 mm fromthe top, where particles flow towards the cylindrical section, thedrag force remains relatively constant. The pressure gradientforce in the DMC with feed density 1400 kg m�3 is higher thanthe centrifugal force for all particle sizes except for the 0.5 mmcoal particles. This means most of coarse particles would migrateto the bulk upward flow and report to the overflow. However, ahigh proportion of fine particles would remain in the bulk down-ward flow. With a feed density of 1600 kg m�3, the difference be-tween these two forces becomes larger and the separationbecomes easier for all particle sizes, and the partition curve shiftsto the right. When particles move to the zone near the spigot, theabsolute value and difference between the pressure gradient forceand the centrifugal force become very high, so the particles havea strong driving force to separate. However, the force differencefor fine particles in the DMC with the feed density of 1600 kg m�3

is small and results in a high Ep.

4.3. Effect of magnetite type

Fig. 10 shows that the operating head and the medium split de-crease, and the density differential increases as the magnetite par-ticles become coarser (from ultrafine to medium grade). Thesegregation of magnetite would be a very important factor inDMC performance.

Fig. 16. The effect of non-magnetic content on partition curves of different

Fig. 11 shows the partition curves to underflow of differentsized coal particles with different magnetite types. For all sizes,the partition curve shifts to the right as the magnetite becomescoarser. This finding indicates that for a given particle density inthe range 1500–1700 kg m�3, fine magnetite particles can providean increased opportunity for coal particles to report to and be col-lected in the underflow.

Fig. 12 presents the off-set and Ep for different magnetite typesin the DMC. For all particle sizes, the off-set increases as magnetiteparticles become coarser (from ultrafine to medium). Ep increasesslightly for coarse particles and almost remains constant for fineparticles as magnetite particles become coarser.

Fig. 13 shows the medium density distributions in the DMCs ofdifferent magnetite types. The extent of segregation near the spigotwith medium grade magnetite is much more pronounced than forultrafine. At the same time, the orbit of the high density ring in thecylindrical section becomes wider. The mechanism of the effect ofmagnetite type on the separation performance is not straight for-ward. The local density and viscosity of medium both vary withthe change of magnetite type. Normally, since the magnetite segre-gates near the spigot, the local pressure gradient, medium densityand viscosity increase as magnetite particles become coarser.

As shown in Fig. 14, when particles are near the vortex finder,the pressure gradient force is larger than the centrifugal force withall magnetite types. However, for fine particles, the difference isvery small. At the location in the cylinder, the difference between

sized coal particles: (a) 7 mm; (b) 4 mm; (c) 1.4 mm; and (d) 0.5 mm.

Fig. 18. Distribution of medium density in the DMCs with different non-magnetic content.

Fig. 17. Separation performance versus non-magnetic content for different sized particles: (a) off-set and (b) Ep.


the pressure gradient force and the centrifugal force becomes largeand is the same for all magnetite types. At this stage, the separationbehaviours of all magnetite types are similar. In the cone near thespigot where particles usually have a high density, the centrifugalforce becomes larger than the pressure gradient force when usingultrafine magnetite. It causes particles to stay in the bulk down-ward flow and report to the underflow, resulting in a low Ep. Whenusing medium grade magnetite, the pressure gradient force ismuch higher than the centrifugal force. So the off-set and the Epare high.

4.4. Effect of non-magnetic content

Fig. 15 indicates that, as the non-magnetic content in the med-ium increases, both the operating head and differential decrease.The effect on differential is marked. The medium split increasesto a maximum and then decreases, however, the difference is sosmall that it can be ignored.

Fig. 16 shows the separation efficiencies of different sized coalparticles with different non-magnetic content from 0 to 30% (drymass basis). For the relatively coarse particles, e.g. 7, 4 and1.4 mm, the partition curve shifts to the left as the non-magneticcontent increases, but the change is very small. For fine particles,there appears to be a pivot point at the medium density

(1650 kg m�3) with the effect on low density particles being tomove the curve up and, for high density particles, to move thecurve down. The net effect is to increase the Ep.

Fig. 17 presents the general separation performance with differ-ent non-magnetic content levels. The off-set decreases for largeparticles and remains constant for small particles as the non-mag-netic content increases. Ep increases slightly for coarse particlesand significantly for fine particles as the non-magnetic contentincreases.

Fig. 18 shows the medium density distributions in the DMCwith different non-magnetic contents. With a low non-magneticcontent, more extensive segregation occurs near the spigot thanwith a high non-magnetic content, resulting in a high density dif-ferential. The width of the high density ring becomes smaller as thenon-magnetic content increases.

As shown in Fig. 19, in the area near the vortex finder, the pres-sure gradient force is slightly larger than the centrifugal force forcoarse particles for all non-magnetic levels, which means the sep-aration occurs in this area. However, for fine particles, the differ-ence between the magnitudes of the two main forces is verysmall, allowing the drag force to play an important role and thusfine particles tend to have more random or stochastic movements.In the cylindrical section, the difference between the pressure gra-dient force and the centrifugal force becomes larger than that in

Fig. 19. Simulated results of normalised average radial forces in the downward bulk flow as a function of particle size at different locations from the top wall: (a) 500 mm; (b)1000 mm; and (c) 2500 mm.


the area near the vortex finder and the particles there thus have anincreasing propensity to separate. The difference in the DMC withlow non-magnetic content is slightly larger than that with highnon-magnetic content. When particles travel to the cone near thespigot, the force difference becomes very high in the DMC withlow non-magnetic content, mainly because of the segregation ofmagnetite. Under this condition, many high density particles arepushed to the upward bulk flow and discharged from the overflow,which results in a high off-set and a high Ep.

5. Conclusions

In order to understand the effects of medium properties such asthe medium density, magnetite type and non-magnetic content onDMCs performance, a CFD model has been developed and used toquantify the flow and particle fields in a 1000 mm industrial densemedium cyclone with different medium properties. The findingsare summarised as follows:

1. As the medium feed density increases, the operational head andthe medium split remain constant. The off-set increases but theEp is relatively insensitive to medium feed density. The pres-sures drop increases significantly with increasing medium feeddensity, resulting in a high inward pressure gradient force onparticles and reduced separating efficiencies.

2. The operating head and the medium split decrease, and thedensity differential increases, as the magnetite particles becomecoarser (from ultrafine to medium grade). The off-set increasesand Ep increases slightly for coarse particles and remainsalmost constant for fine particles as the magnetite particlesbecome coarser. The separation performance in the cylindricalsection is insensitive to magnetite particle size. However, sincethe magnetite segregates near the spigot, the local pressure gra-dient, medium density and viscosity all increase, which leads toa higher density differential. The difference between the pres-sure gradient force and the centrifugal force becomes large inthis area and the particles there have more opportunities tomove into the upward bulk flow, resulting in a higher off-setand Ep.

3. As the non-magnetic content in the medium increases, both theoperating head and density differential decrease. The mediumsplit increases to a maximum and then decreases. The off-setdecreases for large particles and remains constant for small par-ticles as the non-magnetic content increases. Ep increasesslightly for coarse particles and significantly for fine particles.Since the magnetite segregation near the spigot is very signifi-cant when operating with low non-magnetic content, the localpressure force tends to be high, resulting in a high density dif-ferential and off-set compared to the high non-magneticcontent.


Acknowledgments

The authors are grateful to the Australian Coal Association Re-search Program (ACARP) and Australia Research Council (ARC) forthe financial support of this work, and the industrial monitors, Har-vey Crowden, Dion Lucke, Ian Brake and Peter Newling, for helpfuldiscussion and suggestions.

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