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VIII Simposio Internacional de Mineralurgia Pag 1

Millsoft online simulation of SAG, AG and Ball Mills Metallurgical Raj Rajamani / Engineering Department Mike Dennis

Millsoft™ Online Simulation of SAG, AG and Ball Mills

Raj K. Rajamani

Metallurgical Engineering Department, The University of Utah, Salt Lake City,

Utah, USA

and

Michael Dennis

Process Engineering Resources Inc.

Salt Lake City, Utah, USA

www.processeng.com

OUTLINE

1. DEM concepts

2. 3D code and verification

3. 2D code and Online Interface

4. Comparison of 3D code and 2D code results

5. Cortez Gold Mines Study with Millsoft Online

6. Conclusions



1. DEM Concepts

What is DEM ?

(Discrete Element Method)

• Treat every ball or rock as individual particle.

• Calculate the collision of each ball with other balls and rock particles.

• Compute the force of impact of each ball against a lifter.

• Combine the calculations of all the collisions.



How many calculations ?

• Millions and millions, but we can do a 2D calculation in one or two hours. That is 6000 spheres in a 7.8 m diameter SAG mill.

• We can do 3D calculations in two to three days on a 3.0 GHz computer.

How do you calculate the ball impact with the lifter?

• Construct the lifter with three plates: left side, top face and right side.

• Calculate the impact force of balls with each plate and some times with the edge.



How is the mill rotation handled in DEM ?

• Choose time increment = 0.0001 sec

• At each increment move the mill shell plates according to mill speed * 0.0001.

How do you construct the SAG mill for 3D code?

• Using graphics and design software.

• See next slide for an example.



•The area of discharge is created

Feed end Dischargeend

Mill body not shown

Typical 3D result



2. 3D code verification

3D Verification with a lab Scale Mill

• 0.9 X 0.14 m mill

• Front face fitted with a Plexiglas plate.

• 50 mm grinding balls

• 220-260 balls

• Power measured with a torque sensor

• Video images compared with 3D code graphics.



20 % ball load, 30 % critical speed

Measured Power - 301 Watts

Predicted Power - 294 Watts

Lab mill 3D code




Lab mill3D code






Lab mill 3D code

3. 2D Code and Online Interface



PERI’s Millsoft™ Online Interface

Typical Millsoft™ Online Results



Top hat (7 degrees)

25 degree release angle



4. Comparison of 3D code and 2D code results

•32 x 32 ft. SAG Mill.

•54 lifter rows

•Feed size 3-4 inches

•Ball size 100 mm

•70% critical speed

•Installed Power 16.5 MW

3D Simulation of a 1 ft section of the mill



Cross section of the mill with velocity vectors

Isometric view showing mill length



View of the mill from the side. Velocity directions shown.

Ball and rock charge motion



2D snapshot 3D snapshot

Comparison of Millsoft™ Online with 3D simulation

12.2 MW computed power 12.63 MW computed power

2D snapshot 3D snapshot

Comparison of Millsoft Online with 3D simulation



5. Cortez Gold Mines Study with Millsoft™ Online

Mill #2 Operations

• Mill #2 commissioned 1997

• 10,000 tpd with a conventional SABC

• 26’ diameter x 12.5’ variable speed SAG with 3350 kW installed power

• Shell lifter design was rubber shell plate w/ hi-low cast 17°face angle lifters

• Pulp discharger design was rubber w/ Polymet radial pulp lifters



Design Change Steps

• Several DEM simulations on MillSoft to analyze charge motion:

– Eliminate every other lifter row to minimize packing and maximize lift as well as mill volume.

– Changed face angle to 28°

– This type of liner design has been well documented in other literature.

MillSoft Simulation: Old Hi-Low Design

17° face angle - media hits unprotected shell



MillSoft Simulation – 26 rows with 28o and 32o face angle

28° face angle 32° face angle

New Design Summary

Ref: Norcast 3/26/04

•Retained OEM shell plate thickness



Energy Utilization

SAG Mill Circuit Power Data

2000

2100

2200

2300

2400

2500

2600

2700

2800

2900

3000

Jan-

04

Feb-04

Mar-0

4

Apr-04

May-

04

Jun-

04

Jul-0

4

Aug-04

Sep-04

Sep-04

Oct

-04

Nov-

04

Dec-

04

Jan-

05

Feb-05

Mar-0

5

Apr-05

May-

05

Jun-

05

Jul-0

5

Aug-05

Aug-05

Sep-05

Oct

-05

Nov-

05

Dec-

05

Jan-

06

Feb-06

Mar-0

6

Apr-06

May-

06

Jun-

06

Jun-

06

Jul-0

6

Aug-06

Sep-06

Oct

-06

Nov-

06

Dec-

06

Time

SA

G k

W

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

% P

ow

er

to S

AG

/ k

Wh

/to

n

SAG Mill kW SAG Mill kWh/ton Plant Operating Wi, kWh/t % Power to SAG

TPLTM

New Shell Lifter design

Throughput

SAG Mill Circuit Operating Data

300

350

400

450

500

550

Time

Mill

Fee

d (

stp

h)

/ M

ill B

P (

psi

)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

Spp

ed

, rp

m

SAG Mill tons / hr SAG Mill feed psi speed

TPLTM

New Shell Lifter design



Shell Lifter Results Summary

• A decrease of 1.8 kWh/ton operating work index for the entire grinding circuit at same P80

• SAG mill power decreased from 2673 kW to 2410 kW.

• Mill availability increased by 1%.

• Mill throughput no measurable change

6. CONCLUSIONS

2D simulations agree with 3D simulations.Due to particle consolidation in 3D the charge fill lookslower than in 2D.

The verification of 3D results shown by comparison withvideo pictures of a 90 cm diameter mill.

At Cortez Gold Mines the shell lifters were designed withMillsoft™ which resulted in considerable energy savings.

A number of other mines including Alumbrera, Andina,Candelaria, Collahuasi, El Teniente, Los Bronces, andPelambres have used Millsoft™ Online simulations withmuch success.

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