Sag Moly Cop

From FAG … (fully)

to SAG … (semi)

to BAG ! (barely) The Theoretical Rationale behind CURRENT TRENDS IN OPERATING PRACTICE OF SEMIAUTOGENOUS GRINDING OPERATIONS Dr. Jaime E. Sepúlveda MolyMoly -Cop Grinding Systems

Basic Concepts SEMIAUTOGENOUS GRINDING

The concept of AUTOGENOUS GRINDING was born from the idea of avoiding the use and consumption of steel grinding balls, by replacing them with the same rocks contained in the fresh feed ore.

Feed the mill with large rocks (up to 10”12”), so avoiding the traditional crushing, classification and multiple storage stages of intermediate size particles. Use these rocks as a ‘zero-cost’ grinding media: Autogenous Grinding. Add large diameter steel balls (up to 6”): Semiautogenous Grinding. Considering that rocks are lighter than balls, it was assumed (wrongly?) that such rocks should fall from the highest possible position and therefore, SAG mills adopted their typical “pancake” shape: D>L.

Alternative Circuit Configurations SINGLESTAGE GRINDING (FAG or SAG) Product

Feed

Water

Alternative Circuit Configurations DOUBLESTAGE GRINDING (DSAG) Product

Feed

Water

The mid size rocks, denominated Critical Sizes or Pebbles do not act as grinding media and they do not allow themselves to be ground. They use up space in the charge affecting the productivity of the mill. As a corrective measure, it has been arranged for such Pebbles to leave the charge through the mill grate, classifying and crushing them by conventional methods.

Semiautogenous Grinding WHICH WOULD BE THE ACTUAL ROLE OF THE ‘ROCKS’? Do they Are they grind ground by themselves? media?

ROCKS

Do they Grind?

Large (> 4”)

Yes, less than Balls

No

Yes

Medium (2” to 4”)

Very little !

Little ! require large balls

Very little !

Small (< 2”)

No

Yes

No

Alternative Circuit Configurations DOUBLESTAGE GRINDING WITH PEBBLE CRUSHING (SABC(SABC-1) Pebbles

Feed

Product

Water

Alternative Circuit Configurations DOUBLESTAGE GRINDING WITH PEBBLE CRUSHING (SABC(SABC-2) Pebbles

Feed

Water

Product

Since Fully Autogenous Grinding (FAG) was first proposed, early last century, there has been a continuous evolution in operational practices with regard to:

With time, the fully AUTOGENOUS option has been gradually diverting from its original conception to become nowadays just a simple case of a poorly operated CONVENTIONAL BALL MILL …

The addition of increasing amounts of steel balls as ancillary grinding media, The sustained increment in diameter of such balls, The removal and crushing of the critical sizes (pebbles) that otherwise would accumulate in the load and … The pre-crushing (elimination) of either the larger rocks or the intermediate particle size fractions contained in the fresh feed ore.

Consequently, little is left today of the original intention of using the larger rocks as autogenous grinding media for the smaller particles. This presentation is aimed at illustrating the theoretical rationale behind the observed current trends in SAG operating practices, with the aid of Moly-Cop Tools 2.0.

Mineral Grinding Processes

Software for the Analysis of

2.0

My Grandpa made it!

Moly-Cop Tools Molyis available free of charge to all interested parties [email protected]

The model included in Moly-Cop Tools was first published at the SAG 2001 Conference by J. E. Sepúlveda, “A Phenomenological Model of SemiAutogenous Grinding Processes in a MolyCop Tools Environment”, Vol. 4, pp. 301-315, Vancouver, Canada. After that, the model has been providing quite satisfactory descriptions of actual SAG processes, in all cases where the proper plant and/or pilot scale data has been made available.

Theoretical Background SPECIFIC SELECTION FUNCTION, ton/kWh 1.000

Balls on Particles Rocks on Particles Self-Breakage Overall

SiE

0.100

0.010 10

100

1000

10000

Particle Size, microns

100000

1000000

Complex Circuit Simulation ... SABC-1 Mesh # Opening By-Pass D50/Ds m

1000 131488 2.90

1 304800 0.000 1.00 100.00 Upper

∅

0

0.00

1 304800 Lower 0.000 1.00 100.00

⊕1

0 0.00 2.90

61.50

F80 % - 1.5"

3

Diameter, ft Lenght, ft Speed, % Critical Charge Level, % Balls Filling, % % Solids (slurry) App. Density, ton/m3 Gross kW kWh/ton

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

12" 8" 6" 4.15" 2.95" 2.1" 1.48" 1.05" 0.742" 0.525" 0.371" 3 4 6 8 10 14 20 28 35 48 65 100 150 200

2

ton/hr % of Feed % Moisture

% - 1/2"

Ore Density, ton/m

369 36.91 2.90


61.50

% - 1/2"

0.017 0.9 4

3

2.80

1000 40.00 76.72 167.0

ton/hr (all mills) % Solids % - 100# P80

243 0.315 0.331

d50c Bpf Bpw

76.10

% Solids

2

131488 58.97 Water, m /hr

Mesh

ton/hr

1

⊕

0

Remarks Base Case Example

1 Split

Mesh # Opening By-Pass D50/Ds m

Simulation N°

ton/hr, Fresh Feed F80 % Moisture

344

Grate 5 76200 0.070 0.70 3.00

35.30 15.00 78.00 26.00 10.00 76.00 3.331 10093 10.09

Size Distributions Opening Fresh Crushed Crushed Feed Pebbles 1 Pebbles 2 304800 100.00 100.00 100.00 203200 97.60 100.00 100.00 152400 83.93 100.00 100.00 101600 73.57 100.00 100.00 76200 67.87 100.00 100.00 50800 62.82 100.00 100.00 38100 58.97 100.00 100.00 26670 53.78 98.07 98.07 18850 49.78 90.24 90.24 13335 42.74 61.50 61.50 9423 38.32 48.04 48.04 6680 34.00 31.84 31.84 4699 29.28 23.55 23.55 3327 25.65 18.08 18.08 2362 22.57 14.32 14.32 1651 20.19 11.53 11.53 1168 18.16 9.20 9.20 833 16.79 7.80 7.80 589 15.65 6.65 6.65 417 14.66 5.74 5.74 295 13.79 5.06 5.06 208 12.84 4.43 4.43 147 12.01 3.96 3.96 104 11.12 3.50 3.50 74 10.28 3.10 3.10

% Solids % - 100# T80 3 m /hr

Screen 10 13335 0.017 0.90 4.00

Mesh # Opening By-Pass D50/D m

72.79 21.26 6112 731

# of Cyclones Diameter Height Inlet Vortex Apex

4.00 26.00 78.00 10.00 10.00 5.00

psi

10.19

% - 200# in Mill Discharge 29.66

(Guess) (Actual) (Delta)

89 3

m /hr, Water

Water, 3 m /hr

475

% Solids

Circ. Load, % 2.367 2.367 0.000

60.01

2.00 19.00 24.00 76.00 38.00 38.00 72.00 5.395 4631 9.26

# of Mills Diameter, ft Lenght, ft Speed, % Critical Charge Level, % Balls Filling, % % Solids (slurry) App. Density, ton/m3 Gross kW kWh/ton

In conjunction with other unit operationm /hr 1723 models, such as Conventional Ball Milling, Hydroclassification, Screening and Crushing, the referred SAG model can be applied, PROCESS RESTRICTIONS with Moly-Cop Tools, to represent fairly Current Min/Max SAG Power, kW 10093 11500 complex circuit arrangements. Pebbles, ton/hr 369 400 3

BM Power, kW Product Size, P80 Pump Capacity, P*Q 3 Total Water, m /hr

4631 167.0 17554 1470

3730 185.0 30000 2000

Remarks OK OK KO OK OK OK

Complex Circuit Simulation ... SABC-2 Mesh # Opening By-Pass D50/Ds m

1189 131488 2.90

1 304800 0.000 1.00 100.00 Upper

∅

Simulation N°

1 Split

Mesh # Opening By-Pass D50/Ds m

ton/hr, Fresh Feed F80 % Moisture

0

0.00

1 304800 Lower 0.000 1.00 100.00

⊕1

3

Diameter, ft Lenght, ft Speed, % Critical Charge Level, % Balls Filling, % % Solids (slurry) App. Density, ton/m3 Gross kW kWh/ton

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

12" 8" 6" 4.15" 2.95" 2.1" 1.48" 1.05" 0.742" 0.525" 0.371" 3 4 6 8 10 14 20 28 35 48 65 100 150 200

3

2.80

% - 1/2"


∅

2

Split 0.00

0 0.00

ton/hr % of Feed

% - 1/2"

1189 40.00 62.52 270.0

ton/hr (all mills) % Solids % - 100# P80

357 0.260 0.273

d50c Bpf Bpw

82.66

% Solids

2

131488 58.97 Water, m /hr

Mesh

0

Remarks Base Case Example


Ore Density, ton/m 61.50

0 0.00 2.90

61.50

F80 % - 1.5"

2

ton/hr

1

⊕

371 31.19 2.90

271

Grate 5 76200 0.070 0.70 3.00

35.30 15.00 78.00 26.00 10.00 76.00 3.331 10093 8.49

Size Distributions Opening Fresh Crushed Crushed Feed Pebbles 1 Pebbles 2 304800 100.00 100.00 100.00 203200 97.60 100.00 100.00 152400 83.93 100.00 100.00 101600 73.57 100.00 100.00 76200 67.87 100.00 100.00 50800 62.82 100.00 100.00 38100 58.97 100.00 100.00 26670 53.78 98.07 98.07 18850 49.78 90.24 90.24 13335 42.74 61.50 61.50 9423 38.32 48.04 48.04 6680 34.00 31.84 31.84 4699 29.28 23.55 23.55 3327 25.65 18.08 18.08 2362 22.57 14.32 14.32 1651 20.19 11.53 11.53 1168 18.16 9.20 9.20 833 16.79 7.80 7.80 589 15.65 6.65 6.65 417 14.66 5.74 5.74 295 13.79 5.06 5.06 208 12.84 4.43 4.43 147 12.01 3.96 3.96 104 11.12 3.50 3.50 74 10.28 3.10 3.10

% Solids % - 100# T80 3 m /hr

Screen 10 13335 0.017 0.90 4.00

Mesh # Opening By-Pass D50/D m

73.45 25.85 5052 588

# of Cyclones Diameter Height Inlet Vortex Apex

4.00 26.00 78.00 10.00 10.00 5.00

psi

13.51

% - 200# in Mill Discharge 16.08

(Guess) (Actual) (Delta)

353 3

m /hr, Water

Circ. Load, % 2.688 2.688 0.000

Water, 3 m /hr

391

% Solids

64.12

2.00 19.00 24.00 76.00 38.00 38.00 72.00 5.395 4631 7.79

# of Mills Diameter, ft Lenght, ft Speed, % Critical Charge Level, % Balls Filling, % % Solids (slurry) App. Density, ton/m3 Gross kW kWh/ton

m /hr 2009 In conjunction with other unit operation models, such as Conventional Ball Milling, Hydroclassification, Screening and Crushing, the referred SAG model can be applied, PROCESS RESTRICTIONS with Moly-Cop Tools, to represent fairly Current Min/Max SAG Power, kW 10093 11500 complex circuit arrangements. Pebbles, ton/hr 371 400 3

BM Power, kW Product Size, P80 Pump Capacity, P*Q 3 Total Water, m /hr

4631 270.0 27155 1759

3730 185 30000 2000

Remarks OK OK KO KO OK OK

Current Operational Trends in SEMIAUTOGENOUS GRINDING

Effect of % BALLS IN THE CHARGE

D = 36’φ φ L = 15’ Vel. = 78% Crit. % Solids = 76% F80 = 131448 microns Grate = 0.5” Screen = 0.5” Ball Size = 5” Circuit Type = SABC-1

30000 22% Total Filling 26% Total Filling 30% Total Filling

1000

25000

800

20000

600

15000

Max. Power

400

10000

200

5000

0

Mill Power Draw, kW

Simulated Conditions

Mill Throughput, ton/hr

1200

0 0

5

10

15

20

% Balls

One of the first “diversions” from Fully Autogenous Grinding was the addition of large diameter balls with the purpose of increasing mill power draw and so providing extra grinding capacity, giving rise to the so-called Semi Autogenous option. Under any circumstances, Operators must be alert not to exceed the design Maximum Power of the mill motor and drive mechanism.




1000

25000

800

20000

600

15000

Max. Power

400

10000

200

5000

0

Mill Power Draw, kW



1200

0 0

5

10

15

20

% Balls

Even at the same mill power draw, balls would be more effective than rocks to convert the available power into actual grinding, thanks to their higher density and spherical shape.


14.0

1200

13.5

1000

13.0

800

12.5

600

12.0


11.5

200

11.0

Mill Throughput, tph


kWh/ton


0 2.0

3.0

4.0

Apparent Charge Density, ton/m

5.0 3

In some cases, it is possible to identify an Apparent Charge Density (determined by the balls/rocks ratio) that minimizes the overall Specific Energy requirement. If the feed contains large rocks – that essentially must grind themselves – we must assure that these large rocks get to absorb the necessary proportion of the total available energy, so the overall process can achieve optimal performance.


14.0

1200

13.5

1000

13.0

800

12.5

600

12.0


11.5

200

11.0

Mill Throughput, tph


kWh/ton


0 2.0

3.0

4.0

Apparent Charge Density, ton/m

5.0 3

However, regardless of this ideal Apparent Charge Density that would optimize the energy efficiency (kWh/ton) of the process, the overall effectiveness (mill throughput) of the operation is always achieved at higher balls/rocks ratios, up to the limit imposed by the available motor and drive power.

Effect of DISCHARGE GRATE OPENING


D = 36’φ φ L = 15’ Vel. = 78% Crit. % Solids = 76% F80 = 131448 microns % Filling = 28% % Balls = 16% Ball Size = 5” Circuit Type = SABC-1


1250 Screen Opening = 1/2 inch Screen Opening = 3/4 inch

1200 1150 1100 1050 1000 950 0.0

1.0

2.0

3.0

Grate Opening, inches

Another source of “diversion” of SAG milling technology has been the empirical confirmation that removing and crushing larger and larger pebbles (by opening the discharge grate slots) invariably translates into substantially improved mill grinding capacity. In plain words … it is like “the SAG mill is asking help from the Crushers”.


D = 36’φ φ L = 17’ Vel. = 76% Crit. % Solids = 78% % Filling = 28% % Balls = 12% Grate = 2” Ball Size = 5” Circuit Type = SABC-1

ton/hr

Effect of FRESH FEED SIZE DISTRIBUTION 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600

SABC-1 SABC-1 plus +6 inch Crushing SABC-1 plus 6x2 inch Crushing

21%

20

30

40

50

60

70

80

90

100

% - 2" in SAG Mill Feed

It has been repeatedly demonstrated in actual operational practice that “getting rid of the rocks” ahead of the SAG mill brings substantial throughput benefits, raising questions about the effective contribution of such rocks to the overall grinding process.

Taken from: J. E. Sepúlveda, “A SIMULATION ANALYSIS OF THE NET EFFECT OF FEED PARTICLE SIZE DISTRIBUTION ON SAG MILL PERFORMANCE”, Jan D. Miller Symposium, SME-AIME Annual Meeting, 2005.

Effect of Feed Size THE PELAMBRES CASE 3000 SAG 1 SAG 2

2900

Operating Conditions


ton/hr

2800 2700

21%

2600 2500 2400 2300 2200 40

45

50

55

60

% - 1.25" in SAG Mill Feed

Actual data in support of the previous statement was provided by the PELAMBRES (Chile) operation, back in 2001, in the context of their “mine-to-mill” approach.

Taken from: R. Palomo, Moly-Cop 2001: IX Mineral Processing Symposium.

65

Effect of Feed Size

ton/hr

THE COPPERTON CASE 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800

Lines 1 - 3 Line 4

30

35

40

45

50

55

% - Fines in SAG Mill Feed (*)

(*) D. King (2005), SME-AIME Annual Meeting

60



ton/hr

Effect of CIRCUIT CONFIGURATION 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600

DSAG SABC-1 SABC-1 plus +6 inch Crushing SABC-1 plus 6x2 inch Crushing SABC-2

20

30

40

50

60

70

80

90

100

% - 2" in SAG Mill Feed

The grinding capacity of any given circuit improves as its configuration evolves from DSAG to SABC-1 to SABC-2; that is, as the SAG mill contributes less and less to the overall grinding task! Also, as the larger feed rocks get to be pre-crushed, the Ideal Apparent Charge Density quickly approaches values close to the limiting maximum value corresponding to just ‘balls plus slurry’ (~5 ton/m3); that is, Conventional Grinding.

Effect of Balls/Rocks Ratio IDEAL APPARENT CHARGE DENSITY 12000

Total


D = 36’φ φ L = 17’ Vel. = 70% Crit. % Solids = 78% % Balls = 12% Grate = 0.5” Ball Size = 5” Circuit Type = DSAG

kW (Net)

10000 8000 6000

Balls

4000

Rocks

2000

Slurry 0 14

16

18

20

22

24

26

28

30

32

34

36

38

Total Mill Filling, %

As Total Mill Filling is increased (by the addition of large or mid size rocks), at constant Ball Filling, the Total Mill Power Draw increases, but the Net Power absorbed by the Balls actually decreases. If one is to accept that rocks are less effective than balls as grinding media (not to say, totally ineffective), then Mill Throughput will be higher at lower Total Filling levels. This empirical finding has led operators to run at fairly low Total Filling (below 24%) and relatively high (up to 20%) Ball Filling levels.

Meanwhile ... Has the IDEAL MAKE-UP BALL SIZE also been evolving? F80, mm

With the advent of the new century, SAG mill operators have been consistently realizing the clear advantages of using larger and larger balls, regardless of the ore feed particle size.


2400 2200

27

2000 1800 1600

56

1400 1200 120

1000

131

800 3.5

4

4.5

5

5.5

6

6.5

7

7.5

Make-up Ball Size, inches

For every ‘grinding task’, there is an Ideal Make-up Ball Size that maximizes mill throughput. Quite often, this Ideal Make-up Ball Size turns out to be larger than the largest commercially available ball size and increases consistently for coarser and coarser feeds.

Meanwhile ... Has the IDEAL MAKE-UP BALL SIZE also been evolving?

It should be noted that this trend of increasing make-up ball sizes has not yet been offset by the concurrent trend of feeding the mills with finer and finer particles.

Ave. SAG Ball Size, inches

5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 '90

'92

'94

'96

'98

'00

'02

'04

'06

Based on Historical Sales Records of Moly-Cop Chile S. A.

'08

So ... HOW ARE THEY RUNNING TODAY? Facility

Chuquicamata Andina Teniente SAG 1 Teniente SAG 2 Collahuasi MEL Laguna Seca MEL Los Colorados SAG 1 MEL Los Colorados SAG 2 MEL Los Colorados SAG 3 Candelaria Mantos de Oro Pelambres El Soldado Los Bronces SAG 1 Los Bronces SAG 2

Mill Diameter, ft

Mill Length, ft

32 36 36 38 32 38 28 28 36 36 28 36 34 28 34

15 15 15 22 15 20 14 14 19 15 14 17 17 14 17

Ball Filling, % 15.0 14.0 14.0 15.0 12.0 19.0 13.0 13.0 15.0 17.5 14.0 19.5 14.0 17.0 17.0

Total Filling, % 28.0 30.0 33.0 31.0 25.0 26.0 23.0 23.0 23.0 31.0 30.0 30.0 25.0 30.0 30.0

Ball Size, in 5.0 5.0 5.0 5.0 5.0 5.5 5.0 5.0 5.0 5.5 6.0 5.5 5.0 5.0 5.0

F80 Size, mm 120 76 170 100 152 80 80 80 80 128 64 90 117 60 60

Charge Density, ton/m 3 3.75 3.54 3.46 3.64 3.56 4.37 3.88 3.88 4.14 3.95 3.52 4.10 3.83 3.88 3.88

Circuit Type

SABC-1 SABC-2 SABC-2 SABC-2 SABC-1 SABC-1 SABC-1 SABC-1 SABC-1 SABC-2 Precrushing Precrushing SAC Precrushing Precrushing

Data obtained from direct interviews to the listed operations.

So ... HOW ARE THEY RUNNING TODAY? 200

FAG

F80 Size, mm

180 Too many balls!

160 140 120 100 Not enough balls!

80 60

BAG

Chuquicamata Andina Teniente Collahuasi Escondida Candelaria MDO Pelambres Anglo

40 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 Charge Density, ton/m3

Data obtained from direct interviews to the listed operations.

CONCLUDING REMARK It is very likely that many of the members of this audience would not share with me the ‘rightfulness’ of all of my todays statements. For now, in my defense, I just wish to express that, in real life ...

nobody is free of making mistakes !!!

Sag Moly Cop

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