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The Effect of the Grinding ChargeTrajectory on the Grinding Efficiency

Levi Guzman / Dante GarciaMoly-Cop

2011

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Agenda

Introduction

Former Simulation Methodology (Powell

Model)

DEM Methodology

Industrial Case Studies

2

Ball Mill

Simulations of the Mill Power Effect on the

Throughput

Conclusions

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Introduction

At the moment, the conventional grinding technologies of SAG and BallMills are power inefficient, they use from 3-5% of the total of the energy

consumed.

Some researchers have indicated

that the maximum grinding

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about 20% using intra particle

fracture.

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Introduction

Several researchers agree that within the main variables for optimising thegrinding process, are:

The selection of the optimal and grinding media (ball size,type, density, shape)

The optimisation of the grinding charge trajectory (the effectof the lifter, speed, grinding media size)

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On the other hand, it can be argued that the grinding efficiency ismuch more important than the grinding media cost, since the benefits ofobtaining a greater throughput or better product quality are several timesgreater than the magnitude of the grinding media cost.

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Normal Size Range and Approximate Energy Efficiencies

for Various Devices

Device Normal Size Range,mm

ApproximateEfficiency, %

Explosive

Gyratory Crusher

Cone Crusher

- 1000

1000 200

200 20

70

80

60

Rod Mill

Ball Mill

Stirred Mills

HPGR

20 5

5 0.2

0.2 0.001

20 1

7

5

1.5

20 30

Source: Fuerstenau, M., 2003. Principles of mineral processing

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Former Simulation Methodology (Powell Model)

- In a standard way wedevelop single media chargetrajectory simulations,without an analysis ofcollision impact-energydistribution, and without

consider the total chargemotion inside the mill.

- But they are useful in order

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zone from ball charge in themill.

- Also they allow to determinethe single energy at impact,

but they does not alloy tooptimise the power draw,neither the total chargemotion.

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Critical Trajectory SimulationsThe Effect of the % Critical Speed and Lifter Height

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Mill Diameter : 36Ball Size : 5Lifter Face Angle : 10Lifter Height : 6

Mill Diameter : 36Ball Size : 5Lifter Face Angle : 10% Critical Speed : 76%

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Critical Trajectory SimulationsThe Effect of the Lifter Face Angle and Ball Size

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Mill Diameter : 36Lifter Height : 10% Critical Speed : 76%Ball Size : 5

Mill Diameter : 36Lifter Height : 10% Critical Speed : 76%Lifter Face Angle : 10

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DEM Discrete (Distinct) Element Method Methodology

DEM is a numerical model capableof describing the mechanicalbehaviour of assemblies of balls orrocks inside the mill.

Is based on the use of an explicit

numerical scheme in which theinteraction of the balls and rocks ismonitored, calculated and combined,contact by contact (collisions) and

particle by particle. It calculates the impact-energy ofeach ball on the charge.

It can do a 2D simulation in one or

two hours. That is with < 10 000particles in a SAG mill.

It can do 3D simulations in two tothree days on a 3.0 GHz computer

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How We Can Improve the Grinding Efficiency?

Goal

Maximise Mill

Power drawLifter Face

Angle

Optimising the charge trajectoryinside the mill:

Modifying the lifter face angle

Changing the mills % criticalspeed

Mill charge level, optimal ballsize, etc.

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Industrial Case StudyIndustrial Case Study SAG Mill: StatusSAG Mill: Status

Mill Diameter : 9.7 m

Mill Speed : 69% C. S.Power draw Installed : 16.6 MW

Power draw Operating : 15.3 MW

Liner Material : Steel

Number of Lifter Rows : 54

Lifter Profile : Hi-Hi

Rock F80 Size : 75 mm

Rock Specific Gravity : 2.7

Charge Filling : 32%

Balls Filling : >15%

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The Effect of the Number of Lifter Rows on the ChargeTrajectory

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Comparison of the charge motion with 36 and 54 lifter rows, both with alifter face angle of 22. With 36 lifter rows the advantage would be agreater charge volume (capacity) than with 54 lifter rows.

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The Effect of the Lifter Face Angle on the Charge Trajectory

Comparison of the charge motion with lifter face angle of 22 and 30,both with 36 lifter rows. A lifter design with a lifter face angle of 22would be more aggressive.

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Dimensions

Diameter 7.3 m

Length 10.8 m

Installed Power 11.2 MW

Mill speed 10.0 rpm

Feed size- F80

- 3 mm

Product size-P80 - 1000 micron

Ball Size 3 inch or

2.5:3.0 mix

Industrial Case StudyIndustrial Case Study Ball Mill: StatusBall Mill: Status

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The Effect of the Lifter Profile on the Charge Trajectory

Compared to original lifter profile the double wave lifter is slightlymore aggressive.

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The Effect of the Mill Speed on the Charge Trajectory

Changing mill speed by 0.8 rpm to 10.8 the motion becomes more

aggressive preserving cascade.

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The Effect of the Lifter Face Angle on the Charge Trajectory

8250 kW 8746 kW

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The mill power draw increases gradually as the lifter face angledecreases from 30 to 10.

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The Effect of the Mill Power on the Throughput

LIFTER FACE ANGLE MILL POWER ( kW)

30 degrees 8250

25 degrees 8310

15 degrees 8730

10 de ree 8746

E =kWh

mt

kW

mt/h=

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For a grinding task (F80 -> P80), the required specific energy is thecontrolling parameter for the mill throughput.

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The Effect of the Mill Power on the Throughput

8200

8300

8400

8500

8600

8700

8800

MillPowe

r,kW

The Effect of the Mill Power on the Throughput

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1320 1340 1360 1380 1400 1420 1440

Throughput, mt/h

Lifter FaceAngle ()

Mill Power(kW)

Throughput(mt/h)

ThroughputDifference (%)

30 8250 1340 0.0025 8310 1349 0.6715 8730 1418 5.8210 8746 1421 6.02

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Conclusions

The present work shows that using DEM 2D simulation techniques it is possible

to determine potential benefits that are related with throughput, product quality

and optimal grinding charge trajectory for the SAG, Ball and Rod Mills.

Identified potential benefits could be obtained with an ideal grinding media

charge, mill speed as well as optimisation of lifter profile (specialty lifter face

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angle) which is directly related with mill power draw, internal motion of mill charge,optimal liner wear and throughput.

We suggest complementing the analyses of DEM simulations with Moly-Cop

Tools, software for the assessment and optimisation of grinding circuit

performance.