Thord Wennberg Lic2010

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LICENTIATE TH ES I S

Transporting Highly Concentrated Slurries

with Centrifugal PumpsThe Thickened Minerals Tailings Example

Thord Wennberg


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Transporting Highly Concentrated Slurries

with Centrifugal Pumps

The Thickened Minerals Tailings Example

Thord Wennberg

Lule University of Technology

Department of Chemical Engineering and Geosciences

Mineral Processing


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Printed by Universitetstryckeriet, Lule 2010

ISSN: 1402-1757ISBN 978-91-7439-169-5

Lule 2010

www.ltu.se


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Preface

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Published papers

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ContentsABSTRACT.............................................................................................................................. 5

PUBLISHED PAPERS ............................................................................................................ 7CONTENTS .............................................................................................................................. 9

INTRODUCTION .................................................................................................................. 11

&RQYHQWLRQDODQGWKLFNHQHGWDLOLQJVPDQDJHPHQW 2. OBJECTIVES AND SCOPE ............................................................................................. 15

3. FLOW OF THICKENED TAILINGS SLURRIES ........................................................ 17

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4. PERFORMANCE OF CENTRIFUGAL PUMPS HANDLING COMPLEX

SLURRIES .............................................................................................................................. 21

5. EXPERIMENTAL RESULTS .......................................................................................... 23

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6. DISCUSSION ..................................................................................................................... 27

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7. CONCLUSIONS ................................................................................................................. 358. FUTURE WORK ............................................................................................................... 37

9. REFERENCES...................................................................................................................39

10. NOTATIONS.................................................................................................................... 43


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1. Introduction7KHUHLVDQLQFUHDVHGLQWHUHVWWKURXJKRXWWKHZRUOGLQWKHWUHDWPHQWDQGSURFHVVLQJRIILQHJUDLQHGUHViGuDOSURGXFWVWDLOLQJVIURPPLQHUDOEHQHILFLDWLRQSODQWVOLPLWWKHYROXPHVWR

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1.1 Conventional and thickened tailings management

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PondTailings

Pipeline Dam

Figure 1. Conventional tailings disposal system.

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Figure 2. Schematic descriptions of a thickened tailings system with the thickener located

on a hill close to the disposal area.


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Solids concentration by mass

Tonneofsolids

Solids concentration by volume

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Figure 3. The relationship between water content per tonne solids and solids concentration

or the volume metric concentration, Ss = 2.9. Ms= mass flow rate of dry solids,

MO= mass flow rate of water. Cw is concentration by mass.

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Table 1. Flows of slurry and water and heat recovery potential in an example with the

capacity 90 tonnes /h of dry tailings in a sub-arctic region.

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2. Objectives and scope7KHREMHFWLYHRIWKLVVWXG\LVWRIRUPDEDFNJURXQGIRUIHDVLELOLW\FRQVLGHUDWLRQVEDVHGRQODERUDWRU\ DQG SLORWVFDOH H[SHULPHQWV LQ IOXPHV DQG SLSHOLQHV WRJHWKHU ZLWK YLVFRPHWULF

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3. Flow of thickened tailings slurries

3.1 Characterization

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ApparentviscosityPa

Shear rate JPf

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tressW

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plastic

Shear rate J

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W\

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(left). The apparent viscosity a versus shear rate for a Bingham plastic fluid is

shown to the right together with a constant viscosity; , at high shear rates

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120

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VT

Figure 5. Transition point, VT, where flow regime changes from laminar to turbulent in a

tailings slurry with Bingham like properties. The arrows indicate various

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Channel

B (m)

y (m)

Figure 6. Sketch of channel section with depth y and width B. The wetted perimeter, P, isB+2y for the area, A = By.

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4. Perf ormance of cen trifugal pumps ha ndling complex

slurries7\SLFDO SXPS SHUIRUPDQFH HIIHFWV ZKHQ SXPSLQJ YLVFRXV 1HZWRQLDQ OLTXLGV DUH VKRZQ

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Flow rate

H

K

Higher viscosity

Water viscosity

System curve

Pump head curve

Efficiency curve

Figure 7. Schematic sketch on pump characteristics depending on fluid Newtonian viscosity.

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7KH W\SH RI VOXUULHV FRQVLGHUHG KHUH EHKDYHV RIWHQ LQD QRQ1HZWRQLDQ ZD\ DQG H[KLELW D\LHOGVWUHVV:KLOHWKHGHUDWLQJIRUD1HZWRQLDQIOXLGFRQYHUJHVIRUGHFUHDVLQJIORZUDWHV)LJXUHDQRSSRVLWHWHQGHQF\PD\RFFXUIRUQRQ1HZWRQLDQVOXUULHVZLWKD\LHOGEHKDYLRXUKHUHGHPRQVWUDWHGLQDQH[WUHPHVLWXDWLRQVHH)LJXUH

Pump head curve, water

Flow rate

H

K

Slurry

System curve

Efficiency curve, water

Figure 8. Extreme derating effect on the pumping head for a non-Newtonian slurry with a

yield stress typical giving a large head requirements for low flow rates.


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)/2:5$7(

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Figure 9. Sketch defining the reduction in head and efficiency of a centrifugal pump

pumping a solids water mixture. H and H0 are heads in slurry and water service,

respectively. The corresponding efficiencies are and0.

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MT

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R1OMT

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H0 = H/HR 7KHSXPSPXVWWKHQRSHUDWHIRUWKHURWDU\VSHHGFRUUHVSRQGLQJWR H07KHYDOXHRIZDWHUHIILFLHQF\

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5. Exp eri men tal res ults

5.1 Flow resistance in pipelines and flumes (Paper B and E)

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6XPPDUL]HGSLSHDQGIOXPHZDOOVKHDUVWUHVVUHVXOWVYHUVXVSVHXGRVKHDUUDWH9'LQ)LJXUH)RUIUHHVXUIDFHIORZ''+ $3 ZKHUH$ LV WKHDYHUDJHVHFWLRQ DUHD DQG3 LV WKHDYHUDJHZHWWHGSHULPHWHU

1

10

100

1000

P

P

P

Pipeline diameter

and flume

)OXPHP

8V/D (s-1

)

0.1 1 10 100 1000

Shears

tres

sw

(Pa)

Figure 10. Summary of experimentally found shear stresses versus the pseudo shear rate8V/D for various tests in pipeline loops and in a flume.

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5.2 Deposition slope evaluations (Paper E)

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0

5

10

15

20

25

30

38 45 50

C (%)

Slope(%)

)ORRU

%DWFK

7UHQFK

7UHQFKFLUFXODWHG

%DWFKFLUFXODWHG

Figure 11. Observed deposition angles from 0.025 m diameter discharge pipelines, batch

laboratory-scale flume tests and pilot-scale trench results. Unfilled and filled

marks correspond to tests at two companies with similar particle size

distributions of the tailings. Solids density 2900 kg/m3.

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5.3 Pump performance (Paper A and B)

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0

2

4

6

8

10

12

14

16

30 40 50 60 70 80

Flowrate (m3/hr)

Head(m)

0

10

20

30

40

50

60

70

80

Efficien

cy(%)

Head, Water Head, Slurry Efficiency, Water Efficiency, Slurry

Figure 12. Pump performance derating for a simulated tailings product characterized by ayield stress of about 100 Pa. From Addie & Sellgern (2007).

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5.4 Overall result summary

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6. Discussion

6.1 Centrifugal pump performance (Paper A and B)

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QBEP

Shock losses

Frictio

nlossesActual pump

H-Q curve

Theoreticalheadline

Hf

Hs

H

Q Figure 13. Theoretical head curve and hydraulic losses from the ideal head curve. Partly

from Wilson et al.(2006). Hs and Hfdefined in Eqs. 13 and 14, Sayers (1990).

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HSQBEPQ2

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Hf

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Notation Product CRotary

speed

Impeller

diameterHR ER

Pump type, comments and

reference

.DROLQ

E\PVHPLRSHQYDQHLPSHOOHU%RRWOH

6OXGJH E\FORVHGYDQH

LPSHOOHU6HOOJUHQHWDO

&OD\VDQG

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6HOOJUHQ

Figure 14. Apparent viscosity for four sets of tests with mainly non-settling slurries with

three centrifugal pumps with various impeller diameters and configurations.

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Figure 15. The effect of slurry on the pump head and efficiency at solids concentrations by

volume of 31 and 35 % of a scrubber sludge product. Impeller diameter 0.63 m.

y = 35 Pa and p = 0.03 Pas for 35 %. From Sellgren et al.(1999).


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1.5 N

N

B

A

Relative flow rate

Re

lat

ive

hea

d BEP- Line

1

21

2

Figure 16. An increase in head from A to B through an increase in rotary speed, N, for a

constant flow rate means operating further away from the BEP-region. From

Wennberg et al. (2008).

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Figure 17. The pressure on the suction side of the pump and the eye of the impeller when the

suction pressure is lowered bubbles starts forming in the eye of the impeller.

From Girdhar and Moniz (2004).

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Figure 19. Rheogram for C-values from 43.2-46.5 % for the backfilling paste measured with

a vane-type Bohlin-viscometer. Comparison with the pipeline-loop results inFigure 10.

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50 100 150 200 250 300

5

10

15

20

25

30

35

40

Q (m3/h)

H (m)

1800rpm

1600rpm

1400rpm

BEP-LineOperating

domain

Figure 20. Schematic working domain for the pump when handling high concentrations

shown in Fig.19. The pump was a 3-vane all-metal unit with a semi-open impeller

with diameter 0.35 m.


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Paper A

Predicting the performance of centrifugal pumps when handling complex slurries

Wennberg T, Sellgren A and Whitlock L

Proceedings, 14th International Conference on Transport & Sedimentation of Solids particles,

June 23-25 2008, Saint Petersburg, Russia


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PREDICTING THE PERFORMANCE OF CENTRIFUGAL PUMPS

WHEN HANDLING COMPLEX SLURRIES

Thord Wennberg1, Anders Sellgren

2, Lee Whitlock

3

1Luossavaara-Kiirunavaara AB, Sweden,[email protected] University of Technology ,Sweden ,[email protected] Industries Inc., U.S.A.,[email protected]

Abstract: The term complex here refers to the intermediate area between homogeneously andheterogeneously flowing slurries. Typical examples are residual products (tailings) from the

mining industry with average particle sizes of 20 to 100 microns. The performance derating are ofspecial interest when centrifugal slurry pumps are applied to these slurries at high solids

concentrations. The Hydraulic Institutes slurry (ANSI/HI 2005) and viscous Newtonian liquid(ANSI/HI 2004) standards are briefly covered with focus on applicable viscosities for non-Newtonian slurry behaviour. Various experimental results are discussed and found to givemaximum deratings in head and efficiency of about 10 and 15 %, respectively, provided stablehead curves can be maintained. Magnified shock losses, circulatory flows and blockage ofslurry and possibly vapour tendencies in the pump entrance region may be considered asmechanisms behind unstable head curves together with the flow behaviour of the slurry.

KEY WORDS: complex slurries, pump performance derating, entrance losses

1. INTRODUCTION

The term complex here refers to the intermediate area between homogeneously andheterogeneously flowing slurries or non-settling and settling types of mixtures,respectively. Typical examples are residual products (tailings) from the miningindustry with normal average particle sizes of 20 to 100 microns, see for exampleWennberg and Sellgren (2007). Today there is an increased interest in using less water,which means handling of tailings slurries at high solids concentrations. Criteria forthickening and pumpability are often referred to in rheological terms, for example yield

stress, i.e. homogeneous flow. It is known that these slurries may not behave in acompletely non-settling way flowing homogeneously during pipeline transportationbecause larger particles may show settling tendencies.

The performance of centrifugal pumps is influenced by the solids in the slurrythrough deratings of the clear water head and efficiency. The Hydraulic Institute hasdeveloped a new standard (ANSI/HI 2005) for centrifugal slurry pumps were guidelinesare given to predict the performance when handling typical settling or non-settlingslurries.


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The performance of a centrifugal pump is affected when handling slurries. Therelative reduction of the clear water head and efficiency for a constant flow rate androtary speed may be defined by the ratios and factors shown in Fig. 1.

Water

Slurry

H

HoH

Head ratio: HR = H/HoHead reduction factor: RH = 1- HR

Efficiency ratio: ER = / oEfficiency reduction factor: R = 1-ER

o

Fig. 1. Sketch defining the reduction in head and efficiency of a centrifugal pump pumping a solid

water mixture. H and H0 are heads in slurry and water service, respectively. The correspondingefficiencies are and 0.

A generalized diagram for estimation of the performance derating in head for settlingslurries is given in Figure 2, see ANSI/HI (2005) or Addie et al.(2007)

HR

RH

(%)

Fig. 2 Generalised solids-effect diagram for pumps of various sizes (impeller diameters).For solidsconcentration by volume 15% with relative density of solids, 2.65, and a negligible amount of fine

particles smaller than 75 micron. From ANSI/HI (2005).

Abritrary HR or RH values from Fig. 2 are then obtained through concurrentcorrections for solids concentration, relative density and content of particles smaller than75 micron, for details see ANSI/HI (2005) or Addie et al. (2007).


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Typical pump performance effects when pumping viscous Newtonian liquids are

shown Fig. 3.

Flow rate

HHigher viscosityWater viscosity

Fig. 3. Schematic sketch on pump characteristics depending on fluid viscosity

ANSI/HI (2004) provides a generalized derating method of the water performancewhen pumping viscous Newtonian liquids (Fig. 3), i.e. when the property can be definedby a constant viscosity value. It can be seen from Fig. 3 how both the efficiency and thebest efficiency region is influenced, indicating that the pump behaves as a smaller unit.Correction factors for head, flow rate and efficiency are based on viscosity, head, flowrate and rotary speed.

Non-settling type of slurries means that the liquid contains solids in the form of verysmall particles, practically giving no segregation or settling tendencies during thehandling in pumps and pipelines. These mixtures behaves often in a non-Newtonian wayand exhibit a yield stress which is defined as the minimum stress required causing theslurry to flow. Highly concentrated industrial slurries can often be approximatelyrepresented by a yield stress and a straight line for larger shear strains in a rheogram. Theslope of the line is often denoted tangent viscosity. If the linear relationship prevails tozero shear strain then the slurry is a Bingham type of medium, for which the slope(tangent viscosity) is termed plastic viscosity or coefficient of rigidity.

Pump performance derating results for two non-Newtonian slurries fluids causingunstable head curves are shown in Figs. 4 and 5.


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Fig. 4. The effect of slurry on the pump head and efficiency at solids concentrations by volume of31 and 35 % of a scrubber sludge product. Impeller diameter 0.63m. y =35 Pa and =0.03 Pas

for 35%. From Sellgren et al.(1999).

30

25

20

15

0 25 50 75 100

Flow rate (l/s)

Hea

d(m)

Fig. 5.Kaolin, y =240 Pa,= 0.034 Pas. A 0.15 by 0.1m pump with a 5-vane metal impeller with

diameter 0.365m. BEP at about 50 l/s. From Kabamba (2007).

The flow rates for which the head cannot be maintained for the fine-particle slurriesin Figs. 4 and 5 are about 50% of the flow at the best efficiency flow rate (BEP).Theresults in the figures show how the head diverges for low flow rates, which is opposite towhat is shown in Figure 3 for viscous Newtonian liquids. The deratings in Figure 3 arecaused by increased flow resistance within the pump due to an increased liquid viscosity.The slurries in Figs 4 and 5 are composed of small solid particles which takes up 20 to35% of the volume. Most of the kaolin particles are smaller than about 2 microns and thesuspension can be considered to be fully homogeneous.


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The ANSI/HI (2005) slurry pump standard relates for non-settling slurries that

corrections can be based on a characteristic viscosity, for example the tangent viscositydefined above or other viscosity concepts. With industrial non-settling slurries typicalderatings in head and efficiency may not exceed 5 to 10% when operating close to the

best efficiency point of the pump. However, with respect to the complex behavioursometimes observed for lower flows, it is suggested in the standard that the pumpmanufacturer should be consulted for guidance regarding non-Newtonian effects onpump performance.

1.1 MODELLING

Apparent viscosity is the viscosity at a particular rate of shear expressed in terms of aNewtonian fluid. With a non-Newtonian fluid with a yield stress, the apparent viscosityincreases dramatically for small shear rates. Most modelling approaches of theprogressively derating with decreasing flow rates have been based on assumptions abouta representative shear rate within the pump. The corresponding apparent viscosity is thenused with the Hydraulic Institute Newtonian liquid method to calculate the deratingfactors. Sellgren and Addie (1994) simply used the bulk shear rate 8V/D (V is velocity)calculated for the pump suction pipeline diameter, D, to simulate the reduction in head inFig.4. The agreement was good for the conditions covered in Fig 4, however theapproach does not represent conditions in the pump entrance region and it could not bevalidated when compared to other data, for example Walker and Goulas (1984).

Pullum et al. (2007) related an apparent viscosity to a characteristic geometric sectionwithin the pump, thus representing a characteristic shear rate. The detailed section

geometry is so far mainly determined by experimental data, however results correspondto realistic main pump dimensions and the approach seems promising for generalisations.

A different modelling approach by Wilson and Sellgren (2006),directly linked to theyield behaviour simulated well the experimentally observed dramatic drop in pumpinghead at flow rates less than about 50 % of BEP in Fig.4. They expressed a dimensionlessyield stress parameter to the actual flow rate over the flow rate at BEP. Build-uptendencies of material within the pump have been documented in pictures with an acrylicpump, Pullum (2006). These attached lumps can occlude fractions of the passages withinthe pump which may seriously influence the head produced.

2. OBJECTIVES

The objective in this paper is to characterize the effect of solids on the performance ofcentrifugal slurry pumps for various industrial slurries with a complex behaviour asdefined above. The effect on the performance is often moderate to low when the headcurves are fully maintained. Experimental results will be presented and variousmechanisms causing the effect of solids will be discussed.


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3. CHARACTERISATION

Values of HR and ER in the range of 0.9 to 1 were presented by Cooke( 2007) fromresults obtained for various high-density tailings products with yield stresses of up to100 Pa and from 0.01 to 0.07. He related the derating parameter to a chart that goes

back to Walker and Goulas (1984) and a pump Reynolds number.

K

UZ miD2

Re 1

where is rotational speed, Di, impeller size ,m slurry density and and plastic ortangent viscosity Cooke (2007) recommended that installations are not designed for Re0.95.With Re in excess of 107 any derating can be neglected. One of the tailingsproduct investigated by Cooke had 70% of the particles larger than 75 micron with a d50

here estimated to about 0.1mm. Even if the settling slurry diagram in Fig.2 has not beenintended for this type of mixtures, it gave HR values of about 0.95, i.e. in accordancewith observations and Eq.1.

3.1 SIMULATED TAILINGS DATA

A simulated a tailings product was characterized by a fully sheared yield stress ofabout 100 Pa., Addie and Sellgren (2007). The rheology was evaluated from the pipelineloop results and expressed in terms of the yield stress and a tangent viscosity. Thecorresponding tangent viscosity was estimated to 0.055 Pas. Resulting pump deratingsare given in Fig. 7.The pump was a modified 3-vane metal unit with an open shroud 0.3

m diameter impeller having a simple auger-like inducer.

0

2

4

6

8

10

12

14

16

30 40 50 60 70 80

Flowrate (m3/hr)

Hea

d(m)

0

10

20

30

40

50

60

70

80

Efficiency

(%)

Head, Water Head, Slurry Efficiency, Water Efficiency, Slurry Fig.7 Pump performance derating for a simulated tailings product characterized by a yield stress of

about 100 Pa. From Addie and Sellgren (2007).

It follows from Fig. 7 that the estimated head curve was stable HR and ER werefound to be 0.95 and 0.86, respectively. These values agreed reasonably well with the


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Re-criterion in Eq.1 and the HI-method based on Newtonian fluids. Whitlock andSellgren (2002) reported tests with a paste-like slurry with solids concentration byvolume, C=38 % in the 0.3 m impeller pump described in connection with Figure 7,seeTable 1. It follows that values of HR and ER were 0.9 and 0.85.

Table 1Resulting HR and ER fory =240 Pa and=0.07 Pas. Whitlock and Sellgren (2002)

Product CRotary

speed

Impeller

diameterHR ER

Pump type,

comments and

reference

Clay-sand

38 1300-2000 0.3 0.9 0.850.1 by 0.075 m open

shrouded 3-vaneimpeller

3.2KAOLIN AND CMC DATA

Bootle (2006) investigated the derating effects for high and low yield stress kaolinslurries with C= 28 and 37% with a 4-vane semi open 0.244-m impeller pump. Achemical additive was used to reduce the yield stress from 200 Pa to 7.5 Pa, see results inTable 2.

Table 2For C=28%, y =200 Pa ,=0.14 Pas. For 37%, y =7.5 Pa ,=0.018 Pas after adding a chemical.

Product CRotary

speed

Impeller

diameterHR ER

Pump type, comments

and reference

Kaolin2837

1500 0.2440.990.99

0.830.82

0.186 by 0.1 m semi-open 4-vane impeller,

Bootle (2006)

It follows from Table 2 that the performance is not affected despite the big change inyield stress in this case and the values for HR 0.99 and ER 0.82/0.83. Any derating effectwas not observed on the head.Results by Kabamba with a CMC fluid is shown in Fig.8. CMC is a non-Newtonianliquid with pseudoplastic properties and no yield behaviour.

50

40

30

20

0 20 40 60 80 100 120

Flow rate (l/s)

Head(m)

Fig. 8. Head curve for a CMC fluid with pseudoplastic coefficients 3.69 and 0.598. Bingham

parametrar for large shear rates gave y= 50 Pa and =0.2Pas. Pump and pumping conditions as inFig.5. From Kabamba (2007).


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The tendency of a lowering head in Fig.8 indicates that a yield stress not necessarily

is a determining factor for the development of an unstable head curve.

4. DISCUSSION

With the exception of Bootles high-yield stress results in Table 2, it was found thathead could not be fully maintained at lower flow rates as shown here for results fromvarious non-settling fine particle slurries in Figs. 4 and 5 and a yield-free power lawliquid in Fig. 8. These media show various degrees of non-Newtonian behaviourexpressed by an apparent viscosity that increases for decreasing flow rates.

The observed set-in of destabilisation effects was related to how far away from theBEP-region the pump operates. The further to the left of BEP the more recirculation andshock losses which basically increase with the square of the flow rate at BEP minus the

actual flow rate. An increase in rotary speed in this domain for a constant flow rate maycontribute to the destabilisation because the distance away from BEP increases, seeschematic example in Fig.9.

1.5 N

N

B

A

Relative flow rate

Relative

head BEP- Line

1

21

2

Fig. 9. An increase in rotary speed from A to B through an increase in rotary speed, N, for aconstant flow rate means operation further away from the BEP-region.

The head curve was stable for the data in Table 1 at C=38%. However, with C = 41 %and a yof about 350 Pa the situation in Fig.9 occurred when a tendency of a drop inhead was met with an increase in rotary speed. Detailed evaluation of experimental datashowed that HR dropped from 0.84 to 0.70 in the BEP-region when N increased from


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1775 to 1925 rpm for practically constant values of flow rate, C, and pipeline frictionlosses during about 7 minutes, Fig. 10.

0.6

0.7

0.8

0.9

1

1750 1800 1850 1900 1950

N (rpm)

HRan

dER

ER

HR

Fig.10. Decrease in head and efficiency ratio for C= 41% ( y about 350 Pa) when N increases with

practically constant flow rate and pipeline friction losses during about 7 minutes.

This situation may be caused by progressive build-up of material within the pumpwith geometrical modifications of flow passages within the pump that promote losses andaffects the suction performance. With a constant flow rate the effect of the apparentviscosity is not caused by a decreased flow rate, only of the change in local flow patternassociated with the increase in rotary speed. Following Eq.1, an increase in N shouldincrease Re and thus contribute to a stabilisation. However, the situation is complex, thecause of the increase in N may also be an increase in . In addition, Sery et al. (2006)reported from kaolin tests a slightly smaller drop in head when the rotary speedincreased.

The 4-vane semi open 0.244-m impeller pump used by Bootle (2006) for kaolin wasmodified with an enlarged suction inlet from 0.1 to 0.186 m and integrated flow inducerscoops, which project into the enlarged inlet. The suction performance is stronglyinfluenced by the detailed configuration of the entry of the impeller inlet with the vanes.The rapid changes in direction along the flow path cause shock losses which togetherwith recirculation dominates the losses for low flow rates compared to the BEP-region.

Roudnev (2004) found that NPSHR can be several times larger than the water valuebased on experimental results from several Russian references from 1972 to 1986 withBingham-type of slurries, for example heavy media magnetite products. The greaterrequirement was related to blockage tendencies in the entrance region by the yield behaviour resulting in higher velocities and thus lower pressure. It is not known if thefindings reported by Roudnev was associated with any unstable head curve for low flowrates. However, the discussed mechanisms have some similarities to possible blockagefrom vapour in the entrance region sometimes observed for liquid pumping at low flow


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rates with strong local recirculation causing cavitational conditions, Girdhar and Moniz(2004).

An unstable head curve at low slurry flow rates may be the combined effects ofrecirculation with strong local eddy currents and vortexing together with a complex

solid-liquid mixture or fluid that magnifies the entrance shock losses to an extend that thepump head cannot be maintained.

5. CONCLUSION

Various experimental results are discussed and found to give maximum deratings inhead and efficiency of about 10 and 15 %, respectively, provided stable head curves canbe maintained. Magnified shock losses, circulatory flows and blockage of slurry andpossibly vapour tendencies in the pump entrance region may be considered asmechanisms behind unstable head curves together with the flow behaviour of the slurry.

REFERENCES1. Addie G.R. and Sellgren A.2007.A first slurry pump standard and some

implications for paste applications. Proceedings of the Tenth InternationalSeminar on Past and Thickened Tailings 13-15 March 2007, Perth, Australia.

2. Addie G.R., Roudnev A.S. and Sellgren A.2007.The new ANSI/HI centrifugalslurry pump standard, Proceedings, The 17 th Int. Conference on the HydraulicTransport of Solids, Cape Town, South Africa,pp.205-218

3. ANSI/HI 12.1-12.6. 2005.standard for rotodynamic (centrifugal) slurry pumps.4. ANSI/HI 9.6.7. 2004. Effect of liquid viscosity on rotodynamic (Centrifugal and

vertical) pump performance 9.6.7,2004.

5. Bootle M.J., 2006. Practical aspects of Transporting Past with RotodynamicSlurry Pumps. Proceedings of the Ninth International Seminar on Past andThickened Tailings 3-7 April 2006, Limerick Ireland. pp. 413-427.

6. Cooke R. Thickened and paste tailings pipeline systems-Design procedure, part2. Proceedings of the Tenth International Seminar on Past and ThickenedTailings 13-15 March 2007, Perth, Australia.pp.129-140.

7. Girdhar P. and Moniz O. 2004: Practical centrifugal pumps. Design, operationand maintenance. Elsevier.

8. Kabamba B.M. 2007.Evaluation of derating procedures for centrifugal pumpperformance, Magister Technologiae thesis, Cape Peninsula University of

Tecnology, South Africa.9. Pullum, L., 2006. Personal communication: courtesy of the Thermal and FluidsEngineering Group, CSIRO, Australia, 2006.

10. Pullum, L., Graham.L, J, W., Rudman, M., 2007. A De-Rating Method forCentrifugal Pumps Pumping Paste: Proceedings of the Tenth International

Seminar on Past and Thickened Tailings 13-15 March 2007, Perth, Australia.pp. 175-184.

11. Roudnev,A.S.2004 Slurry pump suction performance considerations,Proc.16 thInt. Conference on Hydrotransport, BHR Group,Santiago,Chile,pp137-150


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12. Sellgren A. and Addie G.1994. Pump solids effect when handling non-settlingslurries, Proceedings, Hydro-mechanisation 9. Miskolc, Aug. 29-31,Hungary.

13. Sellgren, A., Addie, G. & Yu, W.C., 1999. Effects of non-Newtonian mineralsuspensions on the performance of centrifugal pumps, Journal of MineralProcessing and Extraction Metallurgy Review, 1999. Vol. 20, pp 239-249.

14. Sery G., Kabamba B., Slatter P., 2006. Paste pumping with centrifugal pumps:Evaluation of the Hydraulic Institute Chart De-Rating Procedures. Proceedingsof the Ninth International Seminar on Past and Thickened Tailings 3-7 April2006, Limerick Ireland. Australian Centre for Geomechanics, Perth, 2006. pp.403-412.

15. Walker, C.I. & Goulas, A.,1984. Performance characteristics of centrifugalpumps when handling non-Newtonian homogeneous slurries, Proc. Instn. Mech.Engrs. Vol. 198A, No. 1, 1984.pp. 41-49.

16. Whitlock L. and Sellgren A.,2002. Pumping a simulated paste-like sludge with amodified centrifugal pump, Proceedings, The 7th European Biosolids Organic

Residuals Conference, November 17-17,Wakefield, UK,2002.17. Wilson K.C., and Sellgren A.,2006. Effect of strongly non-Newtonian slurrieson the head and efficiency of centrifugal pumps, Proceedings, 11th Internationalconference on transport & sedimentation of solid particles., Georgia, September,2006.

18. Wennberg, T., Sellgren A. 2007. Pumping evaluations with paste tailingsthickened close to the surface disposal area. Proceedings,The 17th Int.Conference on the Hydraulic Transport of Solids, Cape Town, South Africa.


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Paper B

Pumping evaluations with paste tailings thickened close to the surface disposal area

Wennberg T and Sellgren A

The 17th international conference on the hydraulic transportation of solids in pipes,

Hydrotransport 17, 2007, Cape Town, South Africa


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Thord Wennberg, Anders Sellgren

The South African Institute of Mining and Metallurgy

The 17th

International Conference on the Hydraulic Transport of Solids Hydrotransport 17

Pumping evaluations with paste tailings thickened close to the surface disposal area 1

PUMPING EVALUATIONS WITH PASTE TAILINGS

THICKENED CLOSE TO THE SURFACE DISPOSAL AREA

Thord WennbergLuossavaara-Kiirunavaara AB, Sweden

Anders SellgrenLule University of Technology, Sweden

Abstract

Elevated location of a paste-thickener on a ridge close to the disposal area is considered at a Swedish ironore mine. About 0.7 Mtonnes of thickened tailings are planned to be layered as paste in the vicinity of thethickener during several years with distribution up to 900m after about 20 years. In order to clarify the

pipeline pumping characteristics of the tailings product for volumetric solids concentration from 40 to50%, experiments in loop systems with pipeline inner diameters from 0.063 to 0.152m have been carriedout with various types of pumps. The results and comparisons carried out here showed that the rheologycannot simply be related to the performance of centrifugal pumps, factors related to pump design and

built up tendencies of material within the pump and other particle properties may be of equal importance.During the first years of operation in the considered application only one thickener underflow pump isrequired. Therefore the reliability and performance for various pump configurations are planned to besystematically investigated in order to meet the most effective long-term solution for paste thickeneroperation, pipeline distribution and placement in the disposal area.

Introduction

Luossavaara-Kiirunavaara AB (LKAB), an iron ore company with mines in the northern part of Swedenis continuously considering new technologies for the handling, transportation and disposal of waste rockand tailings. In the LKAB concentrator in Svappavaara about 0.7 Mtonnes of dry tailings per year istransported nearly 1 km as a slurry in parallel pipelines connected to a flume discharging into a tailings

pond. In order to limit the cost of tailings management in the existing area surface disposal of a highlythickened slurry at slopes of 5% has been considered. The thickening may take place adjacent to the plantor close to the disposal area about 1.5 km away, Fig.1, Wennberg and Sellgren (2005).

The location of the thickener is mainly a balance of the cost for high pressure pumping and thedisadvantage of having the thickening facility remote from the rest of the processing taking place in the

concentrator. Today, it seems that a location of the thickening at 2 in Fig.1 is the most attractivealternative.

Various design features related to the underflow pump, the downstream distribution system and thegeotechnical behaviour when the paste is in place are in the literature coupled to characteristic paste yieldstresses. Values related to unsheared and fully sheared conditions goes back to the effect of appliedshear and exposure time, i.e. the shear history. As will be discussed later there are some uncertainty aboutshear history effects.

Objective and Scope

The objective here is to present experimental results from various loops with inner diameters of 63, 75,101, and 152mm with centrifugal and positive displacement pumps, the latter of screw or piston type.These diameters are expected to be representative for various parts of the distribution system. Due to the

thickener location close to the disposal area, the discussion will be focused on the performance ofcentrifugal pumps when handling high concentration slurries.


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The 17th



D

S

W

+ 365

+ 342

LegendC: ConcentratorD: Disposal area

S: Settling pond

W: Clarification pond

N

+ 436

+ 369

C

1

+ 359

2

+ 411

0 0.5 1 km

Emergency

tailings line

WaterTailings

Thickened tailings

Figure 1. Schematic sketch of the two alternative locations of the high-density thickener.

It was not possible to do on-site large-scale pipeline pumping testing and direct shear history observationsat the time of pilot-scale paste generation because of low production rates. Therefore, the pipeline looptests were carried out with the actual thickened product at suitable locations with stored, transported and

re-mixed pastes. During the first years of operation in the considered application only one thickenerunderflow pump together with a short distribution pipeline is required. Therefore questions raised in thisstudy and shear history effects are planned to be systematically investigated on-site in order to meet themost effective long-term solution for paste thickener operation, pipeline distribution and placement in thedisposal area.

Experimental Study

The average particle size distribution was 25 % less than 10 microns with average particle size, d50 ,about27 microns and with about 80% of particles finer than 90 microns. Figure 2 shows schematically the testloop procedure, most often with segments with two pipeline diameters where simple gauges wereinstalled for pressure drop readings. The gauges were at times mounted close to the pump and sometimesa soft pipeline bend was included in the pressure readings. The flow rate was simply measured with a

barrel and a stop watch and the slurry density was obtained from a volumetrically calibrated tank anddried samples.

4 m 4 m50 m

Barrel for

flow control

Figure 2. Schematic description of one of the used loop systems. Truck feeding of a displacementpump in a 120m long loop with 0.15- and 0.10m- diameter pipelines.


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The measured pressure gradient dp/dx along a pipe is related to the wall shear stress w and the frictionloss gradient j (m slurry/m) through the following relationship.

44

)/( gjDDdxdp

w

g (1)

where and D is the slurry density and pipeline diameter, respectively.

The performance of a centrifugal pump is affected when handling slurries. The relative reduction of theclear water head and efficiency for a constant flow rate and rotary speed may be defined by the ratios andfactors shown in Figure 3.

Head ratio: HR = H/H0Head reduction factor: RH = 1- HR

Efficiency ratio: ER = 0Efficiency reduction factor: R = 1-ER

Figure 3. Sketch defining the reduction in head and efficiency of a centrifugal pump pumping a

solid water mixture. H and H0 are heads in slurry and water service, respectively. The

corresponding efficiencies are and 0.

In order to compare pump performance results with various rotary speeds, n, and literature data,dimensionless head and flow rate parameters were used, see for example Wilson et al. (2006).

Head:22

iDn

gH Flow rate:

3

inD

Q(2)

whereH is the head and Q is the flow rate and Di is the impeller diameter.

Results

Pipe wall stress results (Eq.1) versus the viscous scaling parameter 8V/D (V is velocity) for various

diameters and volumetric solids concentration ,C= 40.4%, are shown in Fig. 4 and compared to largerdiameter data from Saurmann (1982) for a tailings product with similar particle size distribution as usedhere.

Results for various diameters up to 0.152 m for C-values of about 48 and 50% are shown in Fig. 5 wheredirect yield measurements also were available for 48 %. This is the value considered for pipeline pumpingdesign which here has been estimated to be on the conservative side with respect to paste qualityrequirements.

The line in Fig. 5 for 48% is considered to represent an average representative design shear stress. Forexample, with D=0.15 m and 92 tonnes/h (0.7 Mtonner/y) then the flow rate is 68 m3/h with V about 1.1m/s and 8V/D = 57 s-1 giving w about 200 Pa in Fig.5. The corresponding pressure requirement fromEq.1 will be 530 kPa/100m or head losses of about 30m slurry per 100m horizontal pipeline.


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0

100

200

300

400

500

600

0 100 200 300

8V/D (sec

-1

)

Shearstress

w(

Pa) 0.063 m

0.076 m

0.101 m

0.2 m, Saurmann (1982)

Pipeline diameter

Figur 4. Shear stresses versus the rheological scaling parameter 8V/D from pipeline test whith aconcentration of 40.4 per cent by volume.

0

100

200

300

400

500

600

0 100 200 300

8V/D (sec-1

)

Shearstress

w(

Pa)

0.063 m

0.076

0.101 m

0.152 m

Vane-type viscometer

Pipeline diameter

47.7 %

50 %

Figur 5. Shear stresses versus the rheological scaling parameter 8V/D from pipeline test with

solids concentrations by volume of about 48 and 50 %.

The pipeline results in Figs. 4 and 5 were based on rough measurements and represent re-mixed andmildly to strongly circulated slurry or a product that had been stored for one to several weeks and then

brought into a loop system. It was not possible to evaluate any obvious rheological trends related pipelinediameter, age or exposure time in the loop system.

Operation in a 40 m long 0.065 m diameter hose was also used for indicative performance measurementsand observations for a centrifugal pump with a 0.065 m-suction and a 0.05 m-discharge diameter with a4-vane semi-open impeller with diameter 0.23 m. A few pump efficiency values ( ) were estimated fromthe variable speed unit. The flow rate at the start corresponded to a velocity of 1.55 m/s at 1480 rpm forC=45.5% with the intention to successively increase the concentration at approximately the same flowrate.


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With C about 48%, it was not possible to maintain the flow through increased rotary speeds, however thesystem operated for a long time in a semi-stable way at extremely low velocities of 0.1 to 0.3 m/s forrotary speeds of 1840 to 1940 rpm.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 0.002 0.004 0.006 0.008 0.01

Q/nDi3

gH/n

2D

i2

0

50

100

(%)

H Slurry

h SlurryHead water

Efficiency water

H Slurry

Slurry

Figur 6. Dimensionless representation of high concentration slurries derating result.

Discussion

Indicative measurements at an early stage with gravity flow in a 11 m long 25mm-diameter hose from apilot scale thickener at C=44% showed a yield stress slightly above 100 Pa, which make sense incomparison to the results in Figures 4 an 5 for about 40 and 48%,respectively, and the scatter related tothe various testing conditions.

Parts of the underflow in paste thickeners are today often circulated back into the base through pumpingin order to agitate the paste. Pilot scale thickening and trench deposition tests for a similar tailings productas investigated here with and without circulation with a centrifugal pump showed that the agitated pasteflowed more easily down the slope resulting in a lower deposition angle, Engman et al. (2004)

The thickened and flocculated slurry flows by gravity in the base of the thickener to the pump.Centrifugal pumps are known to impart high shear intensities resulting in a reducedslurry yield stress.Furthermore, the slurry is exposed to aging and various degrees of shear in the downstream pipelinedistribution system. Published results are difficult to evaluate and sometimes contradictory. For example,Schaan et al. (2005) reported for a flocculent-dosed coarse sand-silt/clay tailings paste that the dominatingrelative yield stress reduction took place in the 300 m long 0.075 m-diameter pipeline after twocentrifugal pumps where the yield stress decreased about 50%. An 80% decrease in yield stress through

pump passages for a red mud tailings slurry with no pipeline effect was reported by Sofra (2006).Arbuthnot and Triglavcanin (2005) found that a centrifugal pump in the underflow approximately halvedthe yield stress with a gold tailings paste. They found a similar effect for an underflow circulatingarrangement in the thickener. High pressure systems with positive displacement pumps are normally alsofed with centrifugal pumps. Therefore, independent of the system configuration, slope formingmechanisms can be considered to be related to shear stresses that are much lower than those observed inthe thickener underflow.

The reported shear history experiences and the observation of a slope dimishing effect related to pastecirculation in the thickener, raise questions related to system effectiveness and flexibility. With a long-term fixed overall slope requirement in the disposal area, flexibility can be promoted with a split of the

pipeline flow into several streams just before the discharge end, giving higher slopes following anindicative criterion, Engman et al. (2004). In order to plan for investigational procedures related to system

effectiveness during the first years of operation with simple underflow pumping arrangements, the initial


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choice of equipment will be important. Therefore the rest of the discussion will be focused on centrifugalpump performance derating when handling high concentration slurries.

The pump performance results in Fig.6 addresses the problem of maintaining steady state operation whenpumping a paste with various solids concentrations. Walker and Goulas (1984) presented results with an

unstable head curve for coal and kaolin clay slurries with yield stresses of about 8 Pa. Results for a four-vane closed impeller with diameter 0.352 m and a constantrotary speed of 1200 rpm are shown in Figure7.

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Thord Wennberg Lic2010