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
ton/hr % of Feed % Moisture
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"
ton/hr % of Feed % Moisture
∅
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
ton/hr % of Feed % Moisture
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.
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
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.
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
14.0
1200
13.5
1000
13.0
800
12.5
600
12.0
400 22% Total Filling 26% Total Filling 30% Total Filling
11.5
200
11.0
Mill Throughput, tph
Simulated Conditions
kWh/ton
Effect of % BALLS IN THE CHARGE
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.
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
14.0
1200
13.5
1000
13.0
800
12.5
600
12.0
400 22% Total Filling 26% Total Filling 30% Total Filling
11.5
200
11.0
Mill Throughput, tph
Simulated Conditions
kWh/ton
Effect of % BALLS IN THE CHARGE
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
Simulated Conditions
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
Mill Throughput, ton/hr
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”.
Simulated Conditions
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
D = 36’φ φ L = 17’ Vel. = 76% Crit. % Solids = 78% % Filling = 23% % Balls = 15% Grate = 2” Ball Size = 5” Circuit Type = SABC-1
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
Simulated Conditions
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 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
Simulated Conditions
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.
Mill Throughput, ton/hr
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 !!!
