Science Raking in Gravity Thickeners

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86 (2008) 114–130

www.elsevier.com/locate/ijminpro

Int. J. Miner. Process.

Raking in gravity thickeners

M. Rudman a,⁎, K. Simic b, D.A. Paterson c, P. Strode b, A. Brent b, I.D. Šutalo c

a CSIRO Mathematical and Information Sciences, Private Bag 33, Clayton South, Victoria 3169 Australiab AJ Parker CRC for Integrated Hydrometallurgy Solutions, CSIRO Minerals, Bayview Rd Clayton, VIC 3169, Australia

c CSIRO Materials Science and Engineering, PO Box 56 Highett, Victoria 3190 Australia

Received 18 December 2006; accepted 25 December 2007Available online 10 January 2008

Abstract

Thickener rakes are essential in the transport of sediment bed material to the underflow in conventional thickeners, howeververy few studies of bed transport have been published. In this paper, results from pilot-scale thickener experiments with tailor-madeyield stress slurries are presented and compared to companion Computational Fluid Dynamics (CFD) simulations. Rake torque is akey issue in thickener operation and it was found that the yield stress of the suspension is the major factor in determining raketorque. Over a range of rake speeds, the measured torque was an almost linear function of yield stress. CFD simulations of theexperiments allowed torque to be estimated, and results are shown to be within 20% of the measured values in all cases except thelowest (zero) yield stress suspension. Residence time distributions of solids in the bed were also measured and unusual results werefound in which the relationship between residence time and distance from the underflow is not linear (or even monotonic). CFDresults clearly show that for uniform sized rake blades, the over-delivery of an outer blade (compared to the next inner blade) setsup recirculation in the bed, especially in the outer regions of the tank, and this can result in long material pathways and hence longresidence times. This picture is further complicated by the relative contributions of rake delivery and underflow rate, and indicatesthat a simple picture of plug flow in the bed is far from reality. The study illustrates the value that can be obtained from validatedCFD modelling of thickener rakes.Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

Keywords: Gravity thickening; Raking; Transport; Dewatering; Modelling

1. Introduction

A common way to separate solids from liquids for highvolume applications is to utilise settling under the influenceof gravity in large tanks, typically termed clarifiers,washers, or thickeners depending on the intended purposeof the separation step. In order to effect faster settling,flocculant is often added to the feed slurry and the sediment

⁎ Corresponding author. Tel.: +61 3 9545 8093; fax: +61 3 95458080.

E-mail address: [email protected] (M. Rudman).

0301-7516/$ - see front matter. Crown Copyright © 2008 Published by Elsdoi:10.1016/j.minpro.2007.12.002

that forms is often of a thick consistency with high vis-cosity and a yield stress. The term “conventional thick-ener” refers to those tanks with a shallow base slope and arelatively shallow depth of sediment, and is the type ofequipment that is the focus of this study. Material thatsettles in a conventional thickener moves towards thedischarge partly under the action of gravity and bymechanical transport using rakes.

Thickener rakes fulfil three main functions: 1) to movesediment to the underflow, 2) to assist in dewateringsediment that settles onto the thickener bed and 3) toscrape deposits away from the base, and sometimes the

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115M. Rudman et al. / Int. J. Miner. Process. 86 (2008) 114–130

walls, of the tank. It is the first of these functions that is thefocus of this paper. Although thickener rakes are essentialin the transport of bed material to the underflow in con-ventional thickener designs, and effective rake design andoperation can reduce the likelihood of rat-holing, little isknown about transport due to rake action in thickeners.The literature describing raking consists of just a handfulof previous studies.

Based onmeasurements made on full-scale thickeners,Günthert (1984) found that increasing either bladevelocity or blade height would improve raking andmovement of bed material to the underflow. Warden(1981) and Albertson and Okey (1992) developed singleequation mathematical models for spiral rake blades toinvestigate transport. Warden modelled the effect of a setof rake blades by approximating them as a continuousspiral. The validity of this approach is questionable, giventhat the pressure gradient along a rake blade between theupstream and downstream ends of individual blades isvery different to that along the length of a spiral rake. Theequations developed by Albertson and Okey (1992) hadempirical constants that had to be determined experimen-tally or estimated mathematically, thus making generalapplication difficult. Neither of these models takes thenon-Newtonian nature of the bed into account and neitheris able to provide details of the flow patterns that aregenerated by the rakes. Most of these studies have beenrelated to the wastewater treatment industry, whereas ourfocus here is on “conventional” thickeners that are usedwidely in minerals processing applications.

The only papers published to date that have sought tostudy the transport of bedmaterial to the underflow duringraking in thickeners have been the works of Frost et al.(1993), Szalai et al. (1994) and Šutalo et al. (2003). Frostet al. (1993) used a three-dimensional ComputationalFluid Dynamics (CFD) model to predict the flow througha circular flat-bottomed wastewater thickener, with anemphasis on showing how raking efficiency was affectedas blade angle and sizes were modified. They found thatthe efficiency increased as the blade height and lengthwere increased and that the optimal blade angle wasbetween 20 and 30°. They did not show global transportpatterns for their simulations. Szalai et al. (1994) used atwo-dimensional CFD model (with a third, swirlcomponent of velocity) to predict the liquid-only flowin a circular clarifier of conventional design and attemptedto model the effect of the rake mechanism by artificiallyinducing swirl at the bottom of the thickener. Althoughthey showed global transport patterns, the results did notagree with measured data. They stated that CFD model-ling should include the rakes and therefore a three-dimen-sional model would be required.

In the most complete study of raking to date, Šutaloet al. (2003) combined CFD and small-scale experimentalmodelling in which an optically clear polymer gel wasused to simulate a thickener sediment bed. They used flowvisualisation and measured velocity fields using ParticleImage Velocimetry (PIV). The work showed that rakeblades suck material behind them as well as pushingmaterial in front of them toward the underflow. It was alsoclearly shown that the overall transport of material fromthe periphery of the thickener to the underflow occurred atthe level of the rake blades and appeared as a spiral patternthat took one or more revolutions to traverse the distancefrom the tank periphery to the underflow. This pattern wasgenerated by many rake passes. A comparison betweenthe experimental velocity measurements and CFD pre-dictions for individual rake components was good, sug-gesting that the CFDmodel developed there could be usedwith some confidence to predict transport in operatingthickeners. No CFD simulations of global transport werepresented in that study.

In conventional thickeners, it is common for rakeblades to be a uniform size along the rake arm. Warden(1981) first stated that this configuration will lead to amismatch in delivery between neighbouring blades and anover-raking of the bed at the periphery of the tank whensufficient material is being delivered to the underflow. Thesmall-scale experimental results of Šutalo et al. (2003)show that this over-raking at the periphery results in dyedmaterial being segmented by rake passage, with materialthat flows below the arm beingmoved inward andmaterialthat flows above the arm moved outward. Together withCFD predictions, they showed that there is significantrecirculation due to the mismatch in blade delivery.

A major uncertainty in the experimental results ofŠutalo et al. (2003) is the validity of using aqueouspolymer gels to model dense, flocculated fine particlesuspensions that are found in minerals processing applica-tions. Another is the ability of the CFD model developedthere to accurately predict global transport of the bed. In thepresent paper, experimental results from a 2 m diameterpilot-scale thickener in which flocculated, fine particlethickener underflows are used to approximate a thickenerbed. Specifically, torque measurements and residence timedistributions measured using an optical tracer are com-pared to results obtained from a CFD model based on thatreported in Šutalo et al. (2003). Because the bed in thepresent study is opaque and it is not possible to see what ishappening in the pilot unit, the CFD model allows anunderstanding of this complex flow to be gained thatwould be difficult to infer from the pilot-scale results alone.

The paper is organised as follows: In Section 2 adescription of the pilot-scale thickener process circuit

116 M. Rudman et al. / Int. J. Miner. Process. 86 (2008) 114–130

and measurement techniques is presented. A briefoutline of the CFD model is provided in Section 3.Rake torque measurements are presented in Section 4and compared to data extracted from CFD simulationsof the pilot-scale runs. Residence time results andcompanion CFD predictions that explain them are givenin Section 5. Finally, summary and conclusions arepresented in Section 6.

2. Experimental techniques

2.1. Pilot-scale thickener circuit

The pilot-scale thickener facility used in this study is aclosed loop system and is shown schematically in Fig. 1.A sample of slurry is mixed continuously in a 4.5 m3

mixing tank, and pumped out of the mixing tankunderflow into the pilot thickener. The thickener tank isa 2 m diameter by 2 m high side wall tank with a 14° floorand a small discharge hopper (45° slope). This “thickener”is not used as a sedimentation and dewatering tank in theexperiments reported here and instead plays the role of athrough-flow holding tank in which the raking experi-ments are undertaken. The slurry is distributed into thethickener uniformly across a radius via a manifoldcomprising of four equi-spaced 19 mm nozzles. Under-flow from the pilot thickener is fed back into the mixingtank. An on-board PID program in the variable frequencydrive uses a feedback signal from the ultrasonic levelsensor (Milltronics) to maintain a stable slurry height in

Fig. 1. Schematic of the pilot-scale thickener flow cir

the thickener. The slurry level in the mixing tank wasdictated by the requirement that an appropriate level ofslurry must cover the impellor to ensure good mixing.Underflow was regulated by a PID loop (ABB ModelACS400) and flow meter (Danfoss DN25) combination.Other variables monitored on the underflow includedensity and temperature.

Once steady slurry height and underflow conditionswere attained in the thickener tank, the rake was turnedon. The rake is mounted on a central shaft that is driv-en by a motor and gearbox at speeds between 0.2 and2 rpm. The tip speed of the outer blade at 1 rpm corre-sponds to the same tip speed as that in a 40 m thickeneroperating at a rake speed of 3 rev h−1 (20 min rev−1).AVFD (Sumitomo NTAC-2000) was used to control therake speed either by manual setting or by utilising theon-board programming capabilities.

2.2. Rake arm and rake blade design

The rake arms are made from slotted rectangularsection plate and the blades are bolted to the arm at thedesired positions and angles. This allows rapid changesto be made to the number of blades, the blade spacing,angle of attack and type/size of blades (see Fig. 2a). Forthe conventional rake studies here, five blades were usedand were mounted at 30° from the direction normal tothe arm (see Fig. 2b).

For most tests, the blades were a uniform size at290×75 mm and it is noted that the radial position of the

cuit used during the rake investigation studies.

Fig. 2. (a) Slotted rake arm system demonstrating easy blade positioning and blade angle setting and (b) schematic of rake configuration used in thetests.


trailing edge of one blade matches the radial position ofthe leading edge of the next (i.e. there was no blade“overlap”). For a few runs, blades that were 50% taller(290×112 mm) were used and a single 75 mm high“paddle” blade that ran parallel to the rake arm was alsoused in a few cases— the latter approximates the case ofa rake that has been completely scaled-up as it occursfrequently in minerals processing applications. The rakearms were attached to a common hub that was secured tothe rake shaft by a taper lock arrangement.

2.3. Torque measurement

The rake shaft is of hollow construction to allow directmeasurement of torque using a TorqueTrak 9000 Digitaltelemetry system (Binsfeld Engineering). A full Wheat-stone Bridge strain gauge mounted to measure torque on arake shaft was connected to a battery powered radiotransmitter. Strain data is transmitted to a radio receiver andthe receiver outputs this data as a ±10 V signal suitable fora standard data logging system. Prior to any experimentbeing conducted, themeasurement systemwas zeroedwiththe rake shaft stationary and the arms and thickener clear of

any fluid. For this work, the highest gain setting of 8000was selected to ensure the most sensitive measurements.

The relationship between torsional strain, ε, andtorque, T (Nm), for a hollow rotating shaft is describedmathematically by:

e ¼ 16; 000DO 1þ mð Þp D4

O � D4I

� �E

T ð1Þ

where DO and DI are the outer and inner diameter of theshaft respectively (mm) and the material properties of theshaft are described with E, the modulus of elasticity (Nmm−2) and ν, the Poisson ratio (dimensionless). Based onpredicted torque loadings, a hollow stainless steel shaftwith 2.45 mm thick wall and outer diameter of 75.45 mmwas chosen and was suitable for all measurements madein this study. The rake shaft is 2 m longwith the lower endsealedwith awelded end cap and the other end flanged forbolting to the gearbox output shaft.

Measured torque results have a random local variationwith a standard deviation of about 1.3 Nm over one rakerotation and these are averaged out. In addition, a repeat ofan identical rake geometry and slurry showed a variation

Fig. 3. The K/S value (absorbance/scatter) as a function of the solidstracer concentration.


of about ±3.5 Nm (±12.5%) for a temperature variationof ±3.5 °C, possibly as a result of instrument drift. Thepilot thickener was out in the open and subject to ambientconditions that varied, so the measured torque resultspresented in this paper have been averaged and correctedfor instrument drift.

2.4. Solids tracer technique

X-ray diffraction analysis (XRD) revealed that thesolids component of the feed slurry contains a significantproportion of kaolinite that is known to strongly adsorb arange of commercially available dyes. Consequently, anoptical technique was chosen to measure solids residencetime distributions using a reflectance spectrometer (Gre-tagMacBeth, ColorEye XTH). After trials, the dye chosenwas Azure A because it required less dye to saturate thekaolinite and the colour response was more intense thanalternatives. Dyed material was rheologically indistin-guishable from un-dyed material and provided sufficientcontrast at low concentrations for good detection.

A saturated solids tracer slurry was prepared bymixing2 g of Azure A with 1 kg of feed slurry (solids content11.3 wt.%) for 10 min. By adding this dyed tracerincrementally to untreated slurry and measuring the re-sponse with a reflectance spectrometer, a calibration curvewas determined using the Kubelka–Munk theory of re-flectance (Kubelka and Munk, 1931). The most sensitivelight wavelength for this system was determined to be540 nm and this was the wavelength at which measure-ments were made. Denoting the reflectance at 540 nm as afunction of tracer concentration as R(c), define the K/Svalue as a function of c, using

K=S ¼ 1� R cð Þð Þ22� R cð Þ � 1� R 0ð Þð Þ2

2� R 0ð Þ ð2Þ

Plotting K/S as a function of dyed tracer concentration(Fig. 3) allows the solids concentration to be calculated as

Solids tracer wt:kð Þ ¼ K=S � 0:02970:2716

ð3Þ

For the pilot-scale tests, 20 kg dyed tracer batches weremade up by adding 2 g of Azure A per kilogram of slurryand then mixing thoroughly to ensure a uniform coverageof all particles in the sample. Residence time tests wereperformed by pumping a known mass of tracer through a12mm ID stainless steel tube into the slurry bed. Once thetracer had been delivered, the tracer delivery pump wasturned off, the addition lance withdrawn and the rake andthickener underflow pumps were started simultaneously.If the location of tracer addition was in the volume swept

by the rake, the mass of tracer added was generally in therange from 0.5–1 kg, whereas outside of this volume themass added was between 1–2 kg. The greater mass oftracer was added in anticipation that there would begreater dispersion of this material by the time it appearedin the underflow. Note that slurry is re-circulated in aclosed loop fashion in the pilot thickener system andconsequently any tracer that appeared in the underflowwas diluted to below detection levels in the mixing tankbefore being transferred back into the pilot thickener.

2.5. Slurry preparation and characterisation

All pilot-scale experiments were undertaken at a sandmining operation, with the clay-based feed to the pilot-scale thickener circuit being drawn from the underflow ofthe plant tailings thickener. The initial feed to this tailingsthickener was all sub 100 μm. To modify the rheology ofthis material, quantities of attapulgite clay were incre-mentally added to the slurry and mixed until homo-geneous. A Haake VT550 rheometer was used with acruciform vane to measure the static yield stress and a cupand bob (MV-DIN) was used to measure rheograms(shear stress as a function of shear rate). All measurementswere performed at ambient temperature conditions. Therheograms are shown in Fig. 4. The rheology is shear-thinning and there is little evidence of thixotropy.

A Herschel–Bulkley (H–B) rheology model wasfitted to the rheograms,

s ¼ sy þ j g:n ð4Þwhere τ is the total stress, τy is the yield stress, κ is theconsistency, g: is the strain rate and n is the flow index.Note that τy, κ and n are model fitting parameters anddo not have a physical significance of themselves, hencethe vane yield stress and fitted H–B yield stress usuallytake different values (although they are similar inmagnitude). Note that the model fit values are those that

Fig. 4. Rheograms of shear stress versus strain rate for the six slurries.


were subsequently used in the CFD modelling. Thesolids content of each slurry was calculated by dryingsamples. Table 1 summarises the material properties ofthe slurries used in the pilot thickener raking studies.

The dependence of static yield stress on solids contentis typical of mineral slurries, with little variation at lowsolids and, at higher solids concentration, quite largeincreases in yield stress with small variations in solidscontent. The range of static yield stress (0–132 Pa) isrepresentative of typical slurry yield stresses encoun-tered in the minerals processing industry.

3. CFD modelling

The CFDmodel solves the primitive variable forms ofthe equations for conservation of mass andmomentum inan Eulerian framework with values stored at the cornersof an unstructured tetrahedral mesh. The simulationswere carried out using the commercial code CFX 5.4®which uses a finite volume discretisation with the Rhie–Chow algorithm for pressure calculation. Simulation

Table 1Slurries used in the raking studies

Slurryno.

%solids

Density(kg m−3)

Composition Vaneyieldstress(Pa)

Herschel–Bulkleyparameters

τy κ n

1 11.9 1080 Diluted UF 0 0 1.2 0.272 18.2 1122 UF 7.5 4 3.2 0.253 30.6 1240 UF+12.4 wt.% 16 16 0.7 0.54 34.4 1272 UF+16.2 wt.% 33 28 0.56 0.65 37.8 1310 UF+19.6 wt.% 57 48 0.6 0.66 41.4 1345 UF+23.2 wt.% 132 95 10 0.6

“UF” refers to undiluted underflow from the tailings thickener that wasused as the base slurry, and the percentage in the composition columnis the wt.% of added attapulgite.

meshes contain 1.2 to 1.7 million nodes except for thepaddle blades which only needed 0.75 million. TheCFX® proprietary ‘higher order upwind’ differencingscheme is used for the mass and momentum equations.User CEL expression language was used for program-ming the Herschel–Bulkley rheology model, the inflowconditions and certain aspects of the rotating flow. Otherkey aspects of the model are:

1. The simulation is performed in a rotating coordinateframe attached to the rake. In this frame, the geometryis stationary (although the tank walls and conerotates), making the simulation simpler to undertake.

2. The bed material is assumed homogeneous (as in thepilot-scale experiments) and no dewatering is includedin the model.

3. The Herschel–Bulkley rheology parameters mea-sured during the pilot-scale experiments are used inthe simulation.

4. The inflow is based on unreported CFD simulations ofthe exit flow from a full-scale thickener feedwell. It isset to be a smooth function of radius with a flow ratethat is distributed in a hyperbolic fashion around thecircumference. It also approximates the pilot-scaleslurry feed system.

Several Eulerian techniques for calculating residencetime distributions were trialled, although the results weregenerally unsatisfactory due to numerical diffusion. ALagrangian approached was then developed in whichstreamlines were calculated from the steady velocity fieldin a post-processing step. It is not possible to calculatestreamlines by simply subtracting the rotation from thevelocity field first (to move into a stationary coordinateframe) because in the stationary frame the rake (and hencecomputational geometry) is changing with time. To

Fig. 5. The arrangement of the pilot-scale thickener CFD model showing the rake layouts for (a) paddle blade, (b) 5-blade rake and (c) detail of themesh for the 5-blade rake. The taller rake blades have the similar layout as the geometry in the middle.


perform the streamline calculations in the stationary framewould thus turn a steady problem into an unsteady oneand would require vast amounts of unnecessary inter-polation and additional error. Hence the followingprocedure was developed:

1. Velocity output files were read into the Tecplot 10®software and for each RTD, 125 streamlines were

calculated in the rotating coordinate frame (inwhich thegeometry is steady). The streamline starting positionswere uniformly distributed in a (5×5×5) cm3 box.

2. There is no timing information available from step 1,hence an integration programwaswritten that integratedalong a streamline from the start position, estimating thedistance travelled and the mean speed betweensubsequent points on the streamline. This was then


used to estimate a time increment between points on thestreamline.

3. For each streamline, the time increment was added tothe total elapsed time along that streamline, and thestreamline position was rotated through an anglecalculated as the product of the total elapsed time andthe rotational speed of the coordinate frame. Theresulting streamline is then the streamline as it wouldappear in the stationary frame (i.e. the one in which therake is rotating).

The raking geometries modelled are shown in Fig. 5a,b. The top of the bed is taken to be 0.6 m above the topedge of the cone, which is an average figure for the heightin the pilot-scale studies (which varied between runs).This is well above the top of the rakes and any smalldiscrepancies in the height will not affect the rakingbehaviour. Fig. 5c shows details of the unstructuredsurface mesh used around the inner portion of the rake.

Mesh refinement studies are an important part of allCFD modelling. Due to the complexity of the geometriesused here, a mesh refinement study of the complete pilot-scale thickener geometry was not undertaken. Insteadsimulations were performed for the flow around indivi-dual rake blade/arm elements and an assessment made ofthe mesh resolution required to obtain converged resultsin terms of predicted forces on the blade/arm surfaces.Results showed that provided a blade contained approxi-mately 12×40 elements, and that there was a concentra-tion of elements at the edges of the blades, predicted liftand drag forces changed by less than 2% for increasedresolution. This resolution was used or exceeded in thecalculations undertaken here and special care was takenwith mesh generation to provide high resolution aroundthe raking structure without jeopardising the resolutionin the bulk of the fluid. The meshes are consequently

Fig. 6. Pilot-scale measurements of the effect of rake speed and slurry rhComputational results discussed in Section 4.2 are shown as the dashed line

considered to be fine enough to reliably model the flowpattern in the tank and to estimate rake torques.

4. Rake torque results

During commissioning of the circuit, it was estab-lished that a minimum slurry height of 200 mm abovethe cone was required before slurry depth did notinfluence torque measurements (this corresponds to aslurry depth above the rakes at the tank periphery ofapproximately 100 mm). Consequently during theexperiments, the depth of slurry in the thickener waskept in the range 0.4–0.9 m above the cone. For most ofthese runs the underflow rate was 3.0 m3 h−1 except forthe highest yield stress slurry, where the underflow wasreduced to 2.5 m3 h−1 due to pumping difficulties. Forcomparison, the volume swept by the two (75 mm high)inner rake blades of this system is equal to approxi-mately 2 m3 h−1 at a rake speed of 1 rpm. Depending onhow much of this volume is actually moved inwardby the rake (as opposed to being swept circumferen-tially) the tank is probably over-raked at a rake speed of2 rpm, and under-raked at a rake speed of 0.2 rpm,with the possibility of rat-holing in the latter case. Inunreported experiments the underflow withdrawal ratewas not seen to affect torque.

4.1. Measured torque

Torque measurements showed evidence of randomnoise, cyclic behaviour related to the rake orientation, anddrift with time, the latter showing some correlation withtemperature even though the strain gauges are fully tem-perature compensated. Because of the random and cyclicvariations the mean torque was averaged over multiplerotations, from 10 revolutions for the lowest speed

eology on rake torque for the standard 30° blade rake (solid lines).s.

Fig. 7. Rake torque as a function of vane yield stress for all slurry types tested, with the line of best fit.


(0.2 rpm) up to 170 revolutions for the highest speed(2 rpm). The mean drift over time was corrected byexamining the drift in the single-revolution averages andsubtracting the trend.

Two key parameters of interest in this study are theeffect of yield stress on rake torque and the effect ofrake speed. Fig. 6 shows the effect of rake speed ontorque for the 5-blade rake in the different slurries(experimental results are the solid lines). The largestincrease in torque generally occurred when the speedwas increased from 0.2 to 0.5 rpm (especially in SlurryNo. 5 and 6), although this is not clear from the figurebecause of the log scale. At rake speeds greater than0.5 rpm, torque increased only slightly with speed for allslurry rheologies. Anecdotal evidence from operatingthickeners agrees with this lack of sensitivity of torque torake speed, and indicates that the major component ofrake torque is generated overcoming the slurry yieldstress. This is most readily explained by referring to therheograms shown in Fig. 4. As the rotation rate increases,the strain rate increases, and this changes the shear stressin line with changes in Fig. 4. When integrated out overthe surfaces of the raking structure the yield stress con-

Fig. 8. Effect of blade height on rake torque for a 5-blade r

tributes strongly to the torque. Thus the torque versusrotation rate curves resemble the rheology curves, withlarger increases at low rake speeds and smaller increasesas the rake speed increases. Of course this behaviourdepends strongly on the slurry properties and would notbe seen in a Newtonian slurry. If all the rake torque valuesshown in Fig. 6 are plotted as a function of yield stress, analmost straight line results (Fig. 7). This figure shows thatrake torque increases linearly with yield stress almostregardless of rake speed. This dependence suggests thatyield stress measurements may be used to obtain areasonable prediction of the likely torque for a thickenerrake, provided a suitable relationship can be determinedfor the given configuration. The results also suggest thatthe measured rake torque can be used to back-calculate adirect estimate of the bed yield stress, again provided thislinear relationship is known. As will be seen later, CFDoffers a practical way of determining the relationship.

In Frost et al. (1993) and Šutalo et al. (2003), taller rakeblades were claimed to provide better rake efficiency, andtorque measurements here for the 290×112 mm rakeblades are compared to the 290×75 mm blades in Fig. 8(for slurry #4, 33 Pa yield stress). Over the whole range of

ake in Slurry #4 (see Table 1 for material properties).

Fig. 9. Comparison of straight paddle and 5-bladed rake for slurry #4 (33 Pa vane yield stress).

Fig. 10. Locations at which the solids tracer was added to the slurrybed.


rotation rates the larger blade generated approximately60% higher torque. The additional torque for tall blades isslightly more than the increase in blade size (50%), hencethe additional efficiency of the tall blades comes at adisproportionate cost in required torque. One potentialbenefit with higher blades is that for a given rake speed,they deliver more material than smaller blades and willdecrease the likelihood of rat-holing. Alternatively, tallerblades could be used in a “staggered” blade arrangementin which alternate blades on opposite arms are removed,reducing the total blade area and torquewhile maintainingsatisfactory rake delivery to the underflow. Although nottrialled here, staggered blades were used to good effect insmall-scale modelling presented in Šutalo et al. (2003).

Fig. 9 shows the variation of torquewith rake speed fora paddle blade compared to the standard 5-blade rake forslurry #4 (33 Pa yield stress). The projected area of thepaddle blade is marginally greater (2%) than that of thestandard 5-blade arm. The level of drift correctionrequired for the paddle blade design are of the order of15% or higher, and consequently the results for the paddleblade are not as reliable as the other rakes considered herethat required only 2–5%corrections. For the paddle blade,the torque remains at an almost constant level that isapproximately 10–15% higher than for the 5-blade rake(possibly within experimental error in this case). Howeverthere is very little transport of material to the underflowwith the paddle blade (see discussion in Section 5). Theonly benefit in practice of using a paddle blade (or keepinga scaled rake in operation) is a reduction in the likelihoodof scale build-up on the thickener floor due to the repeatedshearing of the sediment. The generally similar level oftorque between the two raking systems suggests thatprojected area is also a key determinant in the level of raketorque (a conclusion also supported by the results shownin Fig. 8). Note that the variation in torque in Fig. 9 bothbetween, and within, rake designs is within the scatterseen in Fig. 7.

4.2. Computed torque

Rake torque can be estimated from the CFD simula-tions of the tank by numerically integrating the viscousand pressure forces on the surfaces of the rake blades andarms. The computed rake torques are compared to theexperimentally measured values in Fig. 6 as a function ofslurry rheology and rake speed for most of the slurries.The computed torques are represented as the dashed linesand are predicted to increase very slightly with rotationrate for all slurries, in good agreement with the pilot-scaleexperiments. Computed torque is consistently approxi-mately 10–20% higher than measured torque for thehigher yield stress slurries, slurry #3 to slurry #6. This isnot a large discrepancy and the difference between actualsediment rheology and the H–B estimates could easily bethe source of the difference. The computed torque for the

Fig. 11. Effect of rake speed on delivery of tracer material to the underflow for the 5-blade rake (290×75 mm blades) and slurry #4 (33 Pa vane yieldstress) from (a) location 1 and (b) location 2.


thinnest slurry (slurry #1) is about 20% lower than themeasured value at a rotation rate of 2 rpm but is only onethird of the measured value at 0.2 rpm, suggesting that

Fig. 12. Flow visualisation in a model thickener showing the break-up of a sreleased below the level of the rake arm at the 6 o'clock position in the ima

frictional losses in the experiment may be playing a rolehere, although this discrepancy is not properly under-stood. Note that the absolute level of torque is very low in

ingle compact dye blob resulting from rake motion. The dye blob wasge.

Fig. 13. Tracer sequence illustrating movement of slurry for the 5-blade rake (290×75 mm blades) operating at 1 rpm. Experimental measurements(plus offset) top, and CFD simulation results bottom. Results are normalised by respective peak values of RTD for location 1.

Table 2Comparison between measured and predicted mean residence times (inseconds) for material released at different locations and differentraking geometries

Location1

Location2

Location3

Location4

Standard5-blade(1 rpm)

Measurement 80 180 N/D 1850CFD 62 205 652 1506

Tall5-blade(0.67 rpm)

Measurement 180 230 600 1650CFD 76 218 626 1319

Paddleblade(1 rpm)

Measurement N/D N/D N/D N/DCFD 86 1341 1119 371


the case of slurry #1, being significantly less than thescatter in the results for the higher viscosity slurries, andsmall errors in the rheology parameter fitting could also bethe source of the discrepancy.

Overall, the results seen in Fig. 6 suggest that the CFDmodel provides quite good torque estimates, and that theCFD approach may be extended to predicting torque onfull-scale raking mechanisms with some confidence. Oneof the difficulties in practice is that the rheology of bedmaterial is rarely well characterised, but as seen in Fig. 6 itis a key factor in determining torque. However, thissituation is not necessarily as bad as it might seem becausethe rake could potentially be used as a rheometer to inferaverage rheology information, in particular yield stress, ofthe sediment bed under different process conditions.Determining a relationship such as that seen inFig. 7 couldbe undertaken to a reasonable approximation using CFD.

5. Sediment residence time results

Torque predictions are a crude means of validating therake CFD model, and in the absence of transport patternsor velocity measurements (both of which are difficult toobtain in opaque fine particle slurries), a suitablevalidation can be performed by a comparison of calculatedand measured residence time distributions (RTDs) fortracer released at different locations in the pilot thickener.For RTD measurements, slurry #4 (33 Pa) was used andthe dyed tracer was made with a sub-sample of thismaterial as described in Section 2.4. Four separateexperiments were run with tracer addition at the differentlocations as shown in Fig. 10. Locations 1 and 2 were adistance approximately 90 mm above the floor of the tankand were inside the volume of material swept directly bythe rake. Locations 3 and 4 were approximately 225 mmabove the edge of the floor cone and outside the swept

volume. The radial positions of locations 1 and 4 were 1/3R and of locations 2 and 3 were 2/3R. Once the solidstracer had been added at a specified location in thesediment bed, underflow pumping and rake rotation werestarted simultaneously and data measurement using thereflectance spectrometer was commenced, with measure-ments made every 2 s.

5.1. Effect of rake speed on solids residence time

Fig. 11a, b shows the appearance of tracer pulsesreleased at locations 1 and 2 respectively for threedifferent rake speeds with the 5-blade rake (recall theunderflow rate is 3m3 h−1). As the rake speed is increasedthe dyed material released in the raked zone appearssooner in the underflow, indicating that the rakes aremoving the material more rapidly at higher speeds asexpected. The tracer released at location 1 appears in asingle pulse for the three rake speeds indicating that it haseither remained as a coherent entity, or the part that did notremain coherent has not appeared at the underflow or has

Fig. 14. Tracer sequence for the 5-blade rake (290×112 mm blades) rotating at 0.67 rpm. Response (+1.5) from experiment (top) and from simulation(bottom). Responses normalised by the magnitude of the respective peak values for location 1.


been diluted below the level at which detection ispossible. The peak residence time for 1 rpm is not twicethat of 2 rpm (and the 0.2 rpm peak is not 5 times that of1 rpm) because underflow pumping also affects residencetime. Thus the rake is not the only factor that determinestransport and residence time, and the basic flow in the tankinteracts with the flow induced by the rake to produce thetracer RTDs. There is a distinctly different feature in thetracer curves for the dye released at location 2, where thetracer appears in the underflow asmultiple peaks. This canbe explained by results shown in Šutalo et al. (2003) inwhich a single dye blob at the periphery of the tank in amodel thickener is stretched and divided by the rakeaction (see Fig. 12 for an example).

5.2. Effect of tracer release location

Residence time distributions for dye released atlocations 1, 2 and 4 for the 5-blade rake with 75 mmhigh blades operating at 1 rpm are shown in Fig. 13 (top isexperimental measurement, bottom is CFD prediction).Unfortunately, problems with data corruption meant thatthe RTD measurement for location 3 were lost for this setof experiments. Also note that only 20% of the computedstreamlines reached the exit in the CFD simulations forlocation 4, further limiting the comparison. The CFDsimulation results for RTDs are in good agreement fortracer released at locations 1 and 2, and in reasonableagreement for location 4 (Average residence times for the4 release positions from experiment and CFD are listed inTable 2.). Fig. 13 shows that tracer released at location 1exits before that released at location 2, and tracer fromlocation 4 takes a very much longer time to reach theunderflow than tracer released in the raking zone. Oncematerial is in the raking zone under these operatingconditions it usually moves fairly rapidly to the under-flow. When it does reach the underflow, if it has travelled

more than a small distance, it appears as multiple peakswith a frequency approximately equal to the rake rotationfrequency, indicating the importance of rake passage ontransport. The residence time distribution is very broadindicating that there has been a significant degree ofstretching and possibly break-up of the original tracervolume (as seen in Fig. 12).

To more clearly show the complexity in the interactionbetween raking and underflow, results from a 5-blade rakewith taller blades (290×112 mm) are next considered.The rake speed in this case was 0.67 rpm which gave anidentical swept volume to the previous case and hence theflow patterns might be expected to be very similar. Theresidence time traces are shown in Fig. 14 (top). Tracerfrom locations 1 and 2 is predicted to appear later for thetall blades than for the shorter blades and tracer fromlocations 3 and 4 it is predicted to appear sooner. Thesedifferences in timing in the CFD analyses are small and nodoubt depend on details of the rake geometry. Thedifferences in the measured durations probably depend onthe exact azimuthal and vertical location at which thetracer was released.

The experimental results for locations 3 and 4 for thiscase are known to be reliable because replicate runsproduced almost identical tracer responses. Mostinteresting in Fig. 14 is that the sequence of tracerdetection (from first to last appearance) is locations 1, 2,3 and finally 4. Location 4 is much closer to the outletthan locations 2 and 3 but material released there exitslast. A discussion of the reasons for this is given inSection 5.3. The CFD RTD predictions for this case areshown in Fig. 14 (bottom), where there is extremelygood agreement for RTDs for locations 2 and 3, andreasonable agreement for location 4. CFD predictionsfor location 1 are significantly less than the measuredvalue and are closer to the standard blade result [seeTable 2 and Fig. 13 (top)]. In the absence of replicate

Fig. 16. Streamlines for material released at locations 1–4 for standard5-blade rake at 1 rpm (a, b) and tall blade rake at 0.67 rpm (c, d). Planview is shown in the left column (a, c) and side view in the rightcolumn (b,d). The arrow shows the direction of rake motion and thenumbers indicate the start positions of the streamlines.

Fig. 15. (a) Velocity vectors and contours of radial velocity on a planethrough the centre of the rake arm. Red represents outward velocityand blue inward velocity. (b) Velocity vectors and contours ofazimuthal velocity on the same plane. Red represents flow in theopposite direction to the rake and blue in the same direction as the rake.This data is plotted in the stationary coordinate frame.

Fig. 17. Streamlines for material released at locations 1–4 for tall 5-blade rake at 0.2 rpm (a, b) and 2 rpm (c, d). Compare to results for0.67 rpm in Fig. 16c, d. Top view is shown in the left column (a, c) andside view in the right column (b, d). The arrow shows the direction ofrake motion.


results for location 1, it is believed that the experimentalresults are in error in this case.

5.3. Explanation for RTD behaviour

The reasons for the sequence of tracer exit behaviourcan be explained by considering the velocity field ob-tained from a CFD simulation of the pilot-scale ex-periment (for tall blade rake, 0.67 rpm, slurry #4), asshown in Fig. 15a. The red contours represent flow that ismoving radially outwards and blue contours flow that ismoving radially inwards. The generally inward flow ofmaterial at the level of the rake blades transports much ofthe tracer placed at locations 1 and 2 to the underflowfairly rapidly. The outward flow of material above the

rake arm in the outer part of the tank transports tracerfrom locations 3 and 4 outwards a significant part of thedistance to the periphery of the tank before moving itdownwards into the raked region and eventually to theunderflow, thus explaining the long residence time.

Table 3Computed mean residence times in seconds for material released atdifferent locations in the tall blade rake at different rake speeds

Location 1 Location 2 Location 3 Location 4

0.2 rpm 55 419 901 1450.67 rpm 76 218 626 13192 rpm 31 141 219 449


Fig. 15b shows contours of swirl velocity, with bluecontours representing flow that is moving with the rake(out of the page) and red contours representing flow that isin the opposite direction to rake motion (into the page).The rake blades pushmaterial in front of them (and “suck”material behind them), but the induced flow above therake arm has a net motion in the opposite direction to therake. This general behaviour is even more noticeable withthe paddle blade and a discussion is left to Section 5.4.

Streamlines for material released at the 4 differentlocations are shown in Fig. 16 for the standard 5-blade

Fig. 18. Predicted residence time distributio

Fig. 19. Streamlines for material released at locations 1–4 for the paddle rakedirection of paddle motion.

rake (Fig. 16a, b) and the tall 5-blade rake (Fig. 16c, d).The effect of the reverse motion is seen for both geo-metries in the right-hand images (b, d). For both 5-bladegeometries, material released at locations 3 and 4 swirlsbackwards as it moves outwards above the rake armtoward the periphery of the tank before being moveddownwards into the raked zone. (Note that in the case ofthe taller blades, the dye released at location 4 makes thisinward and outward traverse twice because excessiveraking ejects it the first time from the raked zone). Oncethe streamlines originating at locations 3 and 4 reach alevel below the rake arm, they travel to the underflow in aspiral pattern with the same sense as the rake rotation (andsimilar to that seen in Fig. 12).

The volume swept by the inner two rake blades isapproximately 2/3 of the total underflow in both 5-bladecases shown in Fig. 16. Even if these blades moved 100%of the swept volume inwards (which unreported worksuggests they do not), any tracer released much closer to

ns for the paddle blade rake at 1 rpm.

at 1 rpm. (a) Top view and (b) side view (right). The arrow shows the


the rake shaft than location 4 would probably havemovedmore directly to the underflow, although neither thisexperiment or simulation was performed for this case.

The outward motion seen in Fig. 15 is the result ofexcessive raking in the outer parts of the tank resultingfrom uniform rake blade size (common to conventionalthickeners) as discussed by Warden (1981). This over-rakedmaterial cannot flow inwards because the next innerblade ismoving still lessmaterial, hence the only place forthis material to go is upwards and back outwards. Thismechanism also contributes to segmentation of the dyestreak seen in Fig. 12.

These results raise some interesting points. Theyclearly show that the flow in a thickener bed when rakedwith uniform blades is very far from plug flow, and thatsignificant parts of the outer region of the bed is movedaway from the underflow and also mixed as aconsequence of the over-raking. Because the bladesthere in most cases will contribute significantly to therake torque, the presence of over-raking suggests that areduction in blade size may reduce total rake torque(which depends on total projected rake area) but willhave no adverse effect on solids transport. Thecomplexity of the flow patterns and interaction betweenrake speed, delivery and underflow rate is highlighted inFig. 17 that shows tracer lines for the tall blade rake at0.2 and 2 rpm respectively. Mean residence times for thethree different rake speeds in this case are presented inTable 3, where the exit sequence for 0.2 rpm is locations1, 4, 2 and 3 and for 2 rpm is locations 1, 2, 3 and 4 (thesame sequence as 0.67 rpm).

The pilot-scale experiment at 0.67 rpm potentiallydelivered too much material to the underflow (this is thereason that none of the tracer from location 4 appeared atthe underflow before location 3). When applied to a full-scale thickener, the outermost blades can be radiallytransporting 20–50 times as much bed material as theinnermost blades and the excess bed material will beconvected up and transported back out many timesbefore reaching the underflow. The issue of whatconstitutes appropriate rake delivery at the underflowis one that needs further investigation.

5.4. Paddle blade rake

The predicted RTDs for the paddle blade are shown inFig. 18, where a very different order of exit is found.Again, dye released at location 1 is the first to exit, but theorder thereafter is locations 4, 3 and finally 2. Because thepaddle blade does not pushmaterial toward the underflow,dye at location 4 is sucked into the underflow with littleinfluence from the paddle blade at all. Dye at location 2

slowly moves inwards due to underflow suction whilebeing pushed in front of the paddle. As in the case of the 5-blade rake, flow above the rake arm (location 3) movescounter to the rake direction although it alsomoves slowlyinwards (see streamlines in Fig. 19).

This reverse flow is counter-intuitive, and deserves abrief discussion. Because the material in the bed has ayield stress in these simulations, it will not slip at thetank walls or cone unless the total stress on the rakes isgreater than the total stress on walls and cone. As therake moves forward, bed material is moved in front of it,but because there must be conservation of mass (andbecause the entire bed is constrained to be stationaryaway from the rake), there must be reverse flow. Thesituation is similar to that of a particle falling in acontainer of fluid — there must be a net upward fluidflow to balance the net downward flow of solids. Thepresence of reverse flow was also noticed on the surfaceof the pilot-scale tank during raking experiments.

In the case of the paddle blade, the flow patterns seenin Fig. 19 show that the rake does not enhance transportto the underflow and the torque results shown in Fig. 9show that rake torque is higher than for a conventionalrake. Hence, operation of such a system (or continuingto use a rake that has been scaled-up by solids) is likelyto be ineffective as well as requiring more power than aclean system.

6. Discussion and conclusion

The difficulty in conducting pilot-scale experimentson an operating site (and the large volumes of materialsneeded to conduct them off site) limited the experimentaldata that could be obtained in this study to that presentedabove. Although the data is not comprehensive, whenconsidered in conjunction with CFD simulation results, anumber of features of rake operation and sedimenttransport in thickener beds have been elucidated.

It is perhaps unsurprising that the two main factors thatdetermine rake torque are the projected area of the rake inthe direction of travel and the yield stress of the materialthrough which it is moving. The pilot-scale results showthat for a given rake geometry, the rake torque follows analmost linear trend with bed yield stress and is a weakfunction of rake speed. This dependence suggests thatyield stress measurements of thickener bed material maybe used to obtain a reasonable prediction of the likelytorque provided a suitable relationship can be determinedfor a given rake configuration. Alternatively, if the yieldstress measurement cannot be reliably undertaken, anestimate of yield stress can be obtained from themeasuredrake torque provided the linear relationship is known. The


CFD model was seen to give good torque estimates, andCFDmodelling of the rakemoving through different yieldstress materials should provide sufficiently reliableinformation to determine the relationship between torqueand yield stress for any given rake geometry.

The key result from the residence time measurementsand predictions is that flow in a thickener bed when rakedby a rake with uniform blades is very far from plug flow.The outer part of the tank is significantly over-raked inmost cases and material here is needlessly recycled as aconsequence of this. Because the outer rake blades inmostcases will contribute significantly to rake torque, a re-duction in blade size (or even removal of some blades)should have no adverse affect on sediment transport andshould provide an opportunity to reduce overall raketorque. The results also show the importance of attainingsome balance between rake delivery and underflowwithdrawal. Toomuch rake deliverywill result in needlessrecirculation and anecdotal evidence suggests that insuffi-cient raking will increase the likelihood of rat-holing.Exactly where this balance lies cannot be determined fromthe results here and requires further study.

Acknowledgments

A significant part of this workwas conducted as part ofthe AMIRA P266D ‘Improving Thickener Technology’

project. The authors wish to thank the followingcompanies for their support: Albian Sands Energy,Alcoa World Alumina, Anglo Gold, Anglo Platinum,BHP Billiton, Cable Sands, Ciba Specialty Chemicals,Cytec Australia Holdings, De Beers Consolidated Mines,EIMCO Process Equipment, GL&V/Dorr Oliver, Glen-core AG, Iluka Resources, Kumba Resources, MetsoMinerals, Mt Isa Mines, Nabalco, ONDEO Nalco,Pasminco, Pechiney Aluminium, Queensland Alumina,Queensland Nickel, Rio Tinto, Tiwest, True NorthEnergy, WMC Resources, Worsley Alumina.

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Günthert, F.W., 1984. Thickening zone and sludge removal in circularfinal settling tanks. Water Sci. Technol. 16, 303–316.

Kubelka, P., Munk, F., 1931. Zeit. Fur Tetn. Physik 12, 593 (1931).Šutalo, I.D., Paterson, D.A., Rudman, M., 2003. Flow visualisation

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