Report No. 8677 Rev C January 2011
AMAYAPAMPA GOLD PROJECT COMMINUTION CIRCUIT DESIGN REPUBLIC GOLD
Orway Mineral Consultants (WA) Pty Ltd, Level 4, 1 Adelaide Terrace, East Perth, Western Australia, 6004 Tel: +61 8 6210 5601 Fax: +61 8 6210 5555 Email:
[email protected] [email protected] Web: www.orway.com.au A.B.N: 40 093 277 126
CONTENTS EXECUTIVE SUMMARY ..................................................................................................................................... .....................................................................................................................................I 1.0
INTRODUCTION ................................................................... ...................................................................................................................................... ...................................................................1
2.0
GEOLOGY AND TESTWORK ....................................................................... ................................................................................................................. ..........................................2 2.1
Geology ....................................................................... .......................................................................................................................................... ...................................................................2
2.2
Testwork ...................................................................... ......................................................................................................................................... ...................................................................4
2.3
Ore Interpretation ...........................................................................................................................6
3.0
2.4 Viscosity Analysis ....................................................................................................................... ........................................................................................................................... .... 6 PROCESS DESIGN CRITERIA ............................................................................................................... ............................................................................................................... 9
4.0
POWER UTILISATION ......................................................................................................................... ........................................................................................................................... .. 10
5.0
CRUSHING CIRCUIT MODELLING ......................................................................................................11 5.1
6.0
Simulation & Crusher Specification ................................................................... .............................................................................................. ...........................11
GRINDING CIRCUIT MODELLING ................................................................ ........................................................................................................ ........................................14 6.1 6.2
Mill selection ................................................................................................................................. .................................................................................................................................14 Throughput estimates .......................................................................... .................................................................................................................. ........................................15
7.0
CIRCUIT SELECTION.................................................................................... SELECTION............................................................................................................................ ........................................17
8.0
MINE SCHEDULE ORE BLENDS .................................................................. .......................................................................................................... ........................................20
9.0
CONSUMABLES ................................................................... .................................................................................................................................... .................................................................23
10.0 CONCLUSION...................................................................................................................................... ........................................................................................................................................ ..24 11.0 DISCLAIMER ......................................................................... .......................................................................................................................................... .................................................................25 12.0 STANDARD WARRANTY .................................................................. ...................................................................................................................... ....................................................25 13.0 OWNERSHIP ........................................................................ ......................................................................................................................................... .................................................................25
TABLES Table 2-1
Comminution Testwork Summary ...............................................................................................5
Table 2-2
Comparison of Testwork Results with Database D atabase ..................................................................... ......................................................................... .... 6
Table 3-1
Process Design Criteria ........................................................................................................... ............................................................................................................... .... 9
Table 4-1
Power Modelling Results ........................................................................................................... ...........................................................................................................10
Table 5-1
Crusher Specification ........................................................................ ................................................................................................................ ........................................11
Table 6-1
Mill Specification ........................................................................................................................ ........................................................................................................................14
Table 6-2
Throughput estimate estim ate - Tertiary Crush Ball Milling .....................................................................15
Table 6-3
Throughput estimate - SS SAG ...................................................................... ................................................................................................. ...........................15
Table 6-4
Performance of Various Feed Types T ypes through SAB ................................................................... ...................................................................15
Table 9-1
Liner and Grinding Media Consumption ..................................................................... .................................................................................... ...............23
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FIGURES Figure 2-1
Comminution sample - Fresh rocks ................................................................ ............................................................................................. .............................3
Figure 2-2
Rheology Measurements Measur ements for the Fresh Ore Sample ...................................................................7
Figure 2-3
Rheology Measurements Measurem ents for the Transition Ore Sample ............................................................ ............................................................7
Figure 2-4
Rheology Measurements for the Oxide O xide Ore Sample...................................................................8
Figure 5-1
Primary Crushing Option ...........................................................................................................12
Figure 5-2
Tertiary Crushing Option ................................................................... ........................................................................................................... ........................................13
Figure 6-1
Throughput estimates ................................................................................................................ ................................................................................................................16
Figure 7-1
Simplified flowsheet: Tertiary crush c rush – ball milling option ...........................................................17
Figure 7-2
Simplified flowsheet: Single Stage SAG mill m ill ............................................................................. .............................................................................18
Figure 7-3
Simplified flowsheet: SAB circuit ............................................................................................... ...............................................................................................19
Figure 8-1
Ore type contribution to mill m ill feed f eed .................................................................... ............................................................................................... ...........................20
Figure 8-2
Tertiary crush ball milling option o ption – blend power requirements ..................................................20
Figure 8-3
SS SAG milling option – blend power requirements .................................................................21
Figure 8-4
SAB option - SAG mill – blend power requirements..................................................................21
Figure 8-5
SAB option - ball mill – blend power requirements....................................................................22
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EXECUTIVE SUMMARY OMC was requested to undertake a comminution circuit design for the Republic Gold Amayapampa Project in the South-West region of Bolivia between Oruro and Potosi Cities. The target throughput provided for this project is 2.74 Mtpa or 340 tph equivalent and a grinding product P 80 of 150 µm. The testwork was completed on samples that represent the three main ore types that will be treated at Amayapampa. The mill design was based on the Fresh ore only with throughput estimates made for the other two ore types. The following table summarises the ore characteristics. Parameter
Unit
Oxide Ore
Transition Ore
Fresh Ore
UCS
MPa
27
29
40
Crushing Work Index
kWh/t
-
-
5.5
0.249
0.1683
0.1093
Abrasion Index Bond Ball Mill Work Index
kWh/t
7.9
13.0
15.2
Bond Rod Mill Work Index
kWh/t
11.4
12.3
15.2
Ore SG
2.63
2.60
2.79
Breakage Characteristics (A x b)
61.6
93.4
52.4
Three circuit configurations were considered, namely three stage crushing followed by single stage ball milling, single stage SAG milling and SAG – ball milling. The major equipment selected for this project is as follows: Crushing equipment Parameter Primary Crusher Model Number of Crushers Installed Power Secondary Crusher Model Cavity Number of Crusher Installed Power Tertiary Crusher Model Cavity Number of Crusher Installed Power
Republic Gold
Unit
Primary
Tertiary
kW
Metso C125 or equivalent 1 160
Metso C125 or equivalent 1 160
kW
HP800 –or equivalent Std Medium 1 600
kW
HP800 –or equivalent Short Head Medium 1 600
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Grinding Equipment
Parameter
Tertiary Crush
Unit
SAG Mill Mill Diameter (Inside Shell) Effective Grind Length (EGL) Imperial Recommended Installed Power
m m ft x ft kW
Ball Mill Mill Diameter (Inside Shell) Effective Grind Length (EGL) Imperial Recommended Installed Power
m m ft x ft kW
SS SAG
SAB
7.92 6.40 26.0 x 21.0 7,300
6.71 5.25 22.0 x 17.1 4,000
5.49 9.75 18.0 x 32 5,000
4.88 7.62 16.0 x 25.0 2,850
The following graph summarises the throughput expected on the individual ore types.
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Discussion The tertiary crush – ball mill option generally results in the most stable milling operation with the least risk of throughput and grind excursions. If this option is considered and the oxide component of the resource is substantial, then it is recommended that the materials handling properties of the oxide ore be evaluated. This will ensure that there is minimal sticky clay associated with this ore that will be detrimental to the performance of the three stage crusher plant. The capital cost associated with a three stage crusher plant is often higher than for the other options The single stage SAG milling option provides a lot of flexibility when treating variable ore types. For soft ores it can be set up to operate at high ball charge and low speed, while for the more competent ores a lower ball charge and higher speed will be required. It is however important to realise that it is not possible to change from one scenario to the other on a daily basis and the mine schedule should be reviewed if this option is considered. SS SAG milling provides good expansion options, but also requires reasonably skilled operators. Republic Gold
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The SAB option is a good compromise between the ball milling and the SS SAG milling options. The ore variability showed that the circuit becomes SAG mill limited in some cases and ball mill limited in others. Again, reasonably skilled operators are required to run the circuit efficiently. Rheology testwork indicated that viscosity should not pose any difficulties in the comminution circuit; however more extreme viscosities were measured on fine samples at 70% solids. It is strongly recommended that this phenomenon be investigated and understood to avoid any potential downstream pumping and processing issues
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1.0
INTRODUCTION
Paul Pyke of Republic Gold requested OMC to undertake a comminution circuit design for the Republic Gold Amayapampa Project in the South-West region of Bolivia between Oruro and Potosi Cities. OMC have had previous involvement with the project (OMC Report number 8503 - March 2010). Additional comminution testwork has since been conducted to increase the confidence in the circuit design. The circuit selection is based on samples that were selected to represent the various oxidation states of the Amayapampa ore. OMC has been informed that additional variability samples will be sourced to further validate the design. The target throughput is 2.74 Mtpa or 340 tph at a grinding product of 80% passing 150 µm. This report covers: •
A review of testwork conducted
•
Ore interpretation based the testwork data and a regional geological description;
•
A comminution circuit design based on the process design criteria provided;
•
Major equipment specifications;
•
Process description and engineering design brief to allow costing;
•
Recommendations for any further work to reduce the design risk.
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2.0
GEOLOGY AND TESTWORK
2.1
Geology
Based on the information provided by the client, the three main ore types are classified as follows: Oxide Zone: Fractured – very fractured black/grey shale with bleached zones that include zones with abundant sand grains. Fractures in filled with clay and iron oxides (limonite). Some sandstones contain sideritic carbonate cement with abundant box works. The quartz veins show fracture filled with iron oxides and box works. The thickness of this zone varies up to 35 m. Transition Zone: Fractured – massive black/grey shales with bleached zones. Also, layers of fine grain silicified sandstones. The pyrite content is less 1% that occurs in fine grain or euhedral crystals. The fractures are in filled with clay and iron oxide. Quartz veins with oxide and less sulphide (pyrite). The thickness of this zone is irregular because there are different levels between the surfaces, up to 110 m. Fresh Zone: Broken, foliated and massive black /gray shales – sandstones shales. Pyritic black shales with singenetic sulphides. Some sandstones contain sideritic carbonate cement pyritized and less barite, pyrite in veins, veinlets and disseminated and occurrence of quartz-sideritic veins. Authigenic pyrite, coarse grained and euhedral arsenopyrite.
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Figure 2-1
Comminution sample - Fresh rocks
The following additional commentary was provided by the site geologist (e-mail dated 3/12/2010): The Pyrite crystals are common in black shales that are related to deep deposits in the sea. In this case most of the rocks that are located in the Paleozoic belt in Bolivia (that correspond exactly with the pollymetallic belt) are silicoclastic sequences constituted by shales, black shales and sandy shales, depending of the depth where they were deposited. For example in Amayapampa we have an Ordovician sedimentary sequence where one can see different members, some of them with more organic material (graphitic), other sectors with more sand and other sectors with only shales. There are thus two types of pyrite based on the source: the first one, related with the environment of deposition - in this case what the mineralogist used to call singenetic related to euxinic conditions. In general they are euhedrals (cubics) and used to leave prints or cubic open spaces that are called box works. This type of Pyrite, in general don´t have interest for mineralization. There is a second type of Pyrite that it is related with the mineralization called hydrotermal pyrite. This type of pyrite is associated with the base metals / Au / and as a gangue with the quartz. In general this type of pyrite it is not euhedral, but subhedral to anhedral. This pyrite fills fractures and also is disseminated in the sandy fractions and has more economic interest.
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2.2
Testwork
Metallurgical test work was completed by Amdel Laboratories in Adelaide and the University of South Australia on oxide, transition and fresh ore samples. The results of this work are reported in Appendix 2. OMC have had no input into the metallurgical sample selection and are unable to comment on the representivity of these samples. OMC did however specify which comminution tests were required for design. The selected samples were subjected to the following testwork: •
Unconfined compressive strength, UCS
•
Impact crushing work index, CWi
•
Abrasion index, Ai
•
Bond rod mill work index, RWi
•
Bond ball mill work index, BWi
•
JK Drop Weight Test, JK / SMC Test
•
Rheology testwork
No variability testwork has yet been carried out. Table 2-1 summarises the comminution testwork results.
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. Table 2-1 Parameter
Comminution Testwork Summary Unit
Oxide Ore
Transition Ore
Fresh Ore
Average
MPa
27
29
40
Range
MPa
0 - 45
0 - 42
13 - 66
kWh/t
-
-
5.5
0.249
0.1683
0.1093
UCS
Crushing Work Index Abrasion Index Bond Ball Mill Work Index F80
µm
2144
1937
2462
P80
µm
77
79
80
Grindability
g / rev
2.75
1.56
1.25
Wi
kWh/t
7.9
13.0
15.2
µm
106
106
106
P80
µm
886
915
864
Wi
kWh/t
11.4
12.3
15.2
RWI:BWI
1.44
0.95
1.00
Ore SG
2.63
2.60
2.79
Axb
61.6
93.4
52.4
A
56.5
63.1
57.6
b
1.09
1.48
0.91
4.27
2.78
-
0.61
0.93
0.73
Closing screen Bond Rod Mill Work Index
Breakage Characteristics
DWi
kWh/m
ta
The breakage characteristics of the Oxide and Transition material are not what would typically be expected. Typically the oxidised ore is less competent than the transition and Fresh ore, but for the Amayapampa samples tested the oxide samples are more competent than the Transition ore. The BWi values follow a more typical sequence. It is recommended that this competency be verified once the variability samples have been sourced.
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2.3
Ore Interpretation
Table 2-2 compares the testwork results for the oxide, transition and fresh ore with OMC’s database. Table 2-2
Comparison of Testwork Results with Database
Parameter
Percentile Rank of Data Units
Oxide Ore
Transition Ore
Fresh Ore
Rod Mill Work Index
% Rank
10
13
34
Ball Mill Work Index
% Rank
3
23
43
Abrasion Index – Ai
% Rank
48
34
23
UCS
% Rank
9
10
15
A x b NOTE
% Rank
71
89
63
ta
% Rank
66
82
75
JK Appearance Function
NOTE: For the A x b values a higher rank implies a softer ore.
The ore types are considered below average in terms of competency (A x b) as well as grinding requirements (RWi and BWi). Abrasion indices are below average and liner and media consumptions are not expected to be excessive. 2.4
Viscosity Analysis
A laboratory was supplied with fresh, transition and oxide ore samples ground to a P 80 of 80 m, 79 m and 77 m respectively for rheology testwork. Viscosity measurements were taken for each of the samples at slurry densities varying between 50% solids and 70% solids. Standard plots of apparent -1 viscosity (Cp) against shear rate (s ) for each sample are presented in Figure 2-2 to Figure 2-4.
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Agitators
$
Pumps
Cyclones
Approximate Limit of Centrifugal Pumps
% %
High Viscosity
%
$
$
Figure 2-2
Rheology Measurements for the Fresh Ore Sample
Agitators
$
Pumps
Cyclones
Approximate Limi t of Centrifugal Pumps
% %
High Viscosity
%
$
$
Figure 2-3
Republic Gold
Rheology Measurements for the Transition Ore Sample
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Agitators
$
Pumps
Cyclones
Approximate Limit of Centrifugal Pumps
% %
High Viscosity
%
$
$
Figure 2-4
Rheology Measurements for the Oxide Ore Sample
The rheology plots for each ore sample consistently suggests that while the viscosity at 60% solids is considered high, it should still pose no difficulty in the comminution circuit where a coarser particle size distribution is likely to relieve potential viscosity issues. All of the ore samples tested exhibit more extreme viscosity at 70% solids suggesting pumping and processing difficulties downstream of the comminution circuit at this density. This severe increase in viscosity is surprising, and it is strongly recommended that the cause of the increased viscosity and influence of density be investigated in more detail.
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3.0
PROCESS DESIGN CRITERIA
The process design criteria for this project is summarised in Table 3-1. Table 3-1
Process Design Criteria
Parameter Crushing Circuit Throughput Availability
Grinding Circuit Throughput Availability
Grinding Product P80 Ore Parameters Crushing Work Index, CWi Bond Ball Mill Work Index, BWi Bond Rod Mill Work Index, RWi Abrasion Index Ore SG Breakage Characteristics Axb A b DWi Ta
Unit
Value
Source
tpa tph % h/a
2,737,500 521 60.0 5,256
Client Client Client Client
tpa tph % h/a
2,737,500 340 92.0 8,059
Client Client Client Client
µm
150
Client
kWh/t kWh/t kWh/t
5.5 15.2 15.2 0.1093 2.79
Testwork Testwork Testwork Testwork Testwork
52.4 57.6 0.91 5.99 0.73
Testwork Testwork Testwork Testwork Testwork
kWh/m
The following circuit options were considered: •
Tertiary Crushed - Ball Mill (Tertiary Crush)
•
Primary Crushed SS SAG (SS SAG)
•
Primary Crushed SAB (SAB)
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4.0
POWER UTILISATION
Power modelling was carried out to determine the grinding efficiency and power consumption expected for each configuration. Results are summarised in Table 4-1. Table 4-1
Power Modelling Results
Parameter Feed Rate Feed Size, F80 Product Size, P80 Specific Crush Energy from 150mm to F80 SAG Milling Specific Energy Ball Milling Specific Energy Total Circuit Specific Energy fSAG
Unit
Tertiary Crush
SS SAG
SAB
tph mm µm
340 10 150
340 125 150
340 125 150
kWh/t
0.41
0.01
0.01
kWh/t kWh/t kWh/t
12.7 13.1
15.9 15.9 1.23
9.1 6.8 15.9 1.23
kW kW kW
4,305 4,305
5,406 5,406
3,094 2,311 5,405
Grinding Power Required - SAG Mill - Ball Mill - Total
Power modelling suggests that Tertiary crushing will be the most energy efficient option which is not uncommon. Historically, this is counterbalanced by the likelihood that tertiary crushing is more costly to build than the other options and incurs higher maintenance costs. None of the primary crushed options are clearly superior from a power efficiency point of view; therefore circuit selection will be driven by differences in: •
Capital and operating cost
•
Company preference
•
Relative complexity and ease of operation
•
Likelihood of expansion
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5.0
CRUSHING CIRCUIT MODELLING
5.1
Simulation & Crusher Specification
Two crushing circuits were designed to match the comminution circuit options. Specifically, they were: •
Primary Crushing Circuit – designed as the front end crushing for the SS SAG and SAB options
•
Tertiary Crushing Circuit – designed as the front end crushing for the tertiary crush – ball milling option
The configuration of the Primary and Tertiary crushing circuits simulated are presented in Figure 5-1 and Figure 5-2. The crusher specifications simulated are presented in Table 5-1. Table 5-1 Parameter Primary Crusher Model Number of Crushers Feed Opening Closed Size Setting Installed Power Secondary Crusher Model Cavity Number of Crusher Feed Opening Closed Size Setting Installed Power Tertiary Crusher Model Cavity Number of Crusher Feed Opening Closed Size Setting Installed Power
Crusher Specification
Unit
Primary
Tertiary
mm mm kW
Metso C125 or equivalent 1 1,250 x 950 130 160
Metso C125or equivalent 1 1,250 x 950 125 160
mm mm kW
HP800 –or equivalent Std Medium 1 267 33 600
mm mm kW
HP800 –or equivalent Short Head Medium 1 92 16 600
It should be noted that the design is based on the assumption (client advice) that the material will flow freely without any sticky clays. If this is not the case, the crusher design must be revised.
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Figure 5-1
Republic Gold
Primary Crushing Option
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Figure 5-2
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Tertiary Crushing Option
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6.0
GRINDING CIRCUIT MODELLING
6.1
Mill selection
Grinding mill equipment sizing were conducted to estimate mill specifications that would deliver the required throughput. Table 6-1 details the mill specification for the three circuit configurations. Table 6-1 Parameter SAG Mill Mill Diameter (Inside Shell) Effective Grind Length (EGL) Imperial L : D Ratio Discharge Arrangement Liner Type New Liner Thickness Backing Rubber Operating Mill Speed Speed Range Ball Charge - Operating - Maximum - Ball Size Recommendation Total Load - Operating - Maximum Pinion Power - Operating - Maximum Recommended Installed Power Ball Mill Mill Diameter (Inside Shell) Effective Grind Length (EGL) Imperial L : D Ratio Discharge Arrangement Liner Type New Liner Thickness Backing Rubber Operating Mill Speed Ball Charge - Operating - Maximum - Ball Size Recommendation Pinion Power - Operating - Maximum Recommended Installed Power
Republic Gold
Mill Specification Tertiary Crush
Unit
SS SAG
SAB
7.92 6.40 26.0 x 21.0 0.81
6.71 5.25 22.0 x 17.1 0.77
mm mm %Nc %Nc
Grate Steel 100 6 75 60 - 80
Grate Steel 100 6 75 60 - 80
% Vol % Vol mm
11 18 Up to 125
12 15 Up to 125
% Vol % Vol
25 35
25 35
kW kW kW
5,400 6,900 7,300
3,100 3,800 4,000
m m ft x ft
m m ft x ft
5.49 9.75 18.0 x 32 1.78
4.88 7.62 16.0 x 25.0 1.56
mm mm %Nc
Overflow Rubber 100 6 75
Overflow Rubber 80 6 75
%Vol %Vol mm
30 35 80
30 35 50
kW kW kW
4,300 4,730 5,000
2,400 2,680 2,850
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6.2
Throughput estimates
In order to make a more complete comparison of the options, each comminution configuration was resimulated for the oxide and transition ore. These results are presented in Table 6-2 to Table 6-4. Table 6-2
Throughput estimate - Tertiary Crush Ball Milling
Parameter Feed Rate BWi RWi Feed Size, F80 Product Size, P80 SG Product of Efficiency Factors Corrected Ball Milling Specific Energy Ball Mill Power Required
Unit
Fresh
Oxide
Transition
tph kWh/t kWh/t mm µm
340 15.2 15.2 10 150 2.79
700 7.9 11.4 10 150 2.63
415 13.0 12.3 10 150 2.60
kWh/t
1.163 12.7
1.004 6.1
1.101 10.3
kW
4,300
4,265
4,260
NOTE: The oxide throughput will probably be restricted to less than the value indicated by factors other than the mil. Other probable bottlenecks include: crush circuit limitations, discharge pumps and cyclones and other downstream circuit constraints.
Table 6-3 Parameter Feed Rate Feed Size, F80 Product Size, P80 SAG Milling Specific Energy SAG Mill Power Required
Throughput estimate - SS SAG Unit
Fresh
Oxide
Transition
tph mm µm
340 125 150
435 90 150
405 60 150
kWh/t
15.9
10.8
11.6
kW
5,400
4,700
NOTE
4,700 NOTE
NOTE: The SS SAG mill may be required operate at 60%Nc and high ball charge when treating oxide and transition ore.
Table 6-4
Performance of Various Feed Types through SAB
Parameter Feed Rate Feed Size, F80 Product Size, P80 SAG Milling Specific Energy Corrected Ball Milling Specific Energy Total Circuit Specific Energy SAG Mill Power Required Ball Mill Power Required Total Power Required
Unit
Fresh
Oxide
Transition
tph mm µm
340 125 150
407 90 150
376 60 150
kWh/t kWh/t kWh/t
9.1 7.1 16.2
7.6 3.2 10.8
5.2 6.4 11.6
kW kW kW
3,100 2,400 5,500
3,100 1,300 4,400
1,955 2,400 4,482
Throughput in the SAB option is limited by the SAG mill for the oxide (A x b = 61 and BWi = 7.9 kWh/t) while the Transition ore is limited by the ball mill (A x b = 93 and BWi = 13 kWh/t).
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Figure 6-1 graphically represents the throughput estimates for each configuration and ore type.
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Figure 6-1
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Throughput estimates
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7.0
CIRCUIT SELECTION
Tertiary Crushed – Ball Miling For this option, the ROM ore will be tertiary crushed to 80% passing 10mm prior to being fed to a single stage ball mill for grinding. The ball mill will be operated in closed circuit with hydro-cyclones. The cyclone overflow at the target grind size will report to downstream processing and the coarse cyclone underflow will recycle back to the ball mill for further grinding. The major concern for a tertiary crushing plant would be the clay content when processing Oxide ore as it could cause blockages in the crusher, screens, chutes and fine ore storage. However, the client confirmed that from visual inspection, the clay content is unlikely to be an issue for the tertiary crushing circuit. Ball milling is considered a low risk option, but the three stage crushing plant and fine ore storage often results in the highest capital cost. A basic circuit flowsheet is shown in Figure 7-1.
Figure 7-1
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Simplified flowsheet: Tertiary crush – ball milling option
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Primary Crushed – SS SAG Milling For this option, the ROM ore will be primary crushed by a jaw crusher to provide a coarse crushing product for single stage SAG milling. The SAG mill will be in closed circuit with hydro-cyclones. Due to the average competency (A x b = 52.4) of this ore, recycle crushing should not be required. It is however recommended to allow for future installation of recycle conveyors and/or recycle crushing if required. The single stage SAG mill is the most flexible of the grinding configurations for the particular ore types tested. It should however be set up to allow for high ball charge operation when the soft ores are treated. The required speed range (60 – 80% critical speed) is also extremely important to ensure that the mill can be operated at low speed and high ball charge when treating soft ores (typically ball mill operation), and high speed with a lower ball charge when treating the more competent ores (typical SAG mill operation). If future expansion is a consideration, the single stage SAG mill configuration provides the simplest expansion with future addition of a ball mill to increase throughput if the primary crusher is also adequately sized. A basic circuit flowsheet is shown in Figure 7-2.
Figure 7-2
Simplified flowsheet: Single Stage SAG mill
Depending upon environmental conditions, a fine ore bin is sometimes preferable to a stockpile to prevent feeding issues from frozen ore stocks.
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SAB Configuration The primary crushed product will be fed into a SAG mill, which operates in open circuit. The SAG mill product will combine with the ball mill product prior to classification by hydro-cyclones. The cyclone overflow at target grind size will report to the downstream process, and the coarse cyclone underflow will report to the ball mill for further grinding. SAB circuits are generally considered easier to operate than single stage SAG mill. In the case of the three ore types tested, this circuit becomes ball mill limited when treating Transition ores and SAG mill limited for oxides. These circuits are typically operated at very low ball charge in the SAG mill when treating soft ores (Transition ore in this case), with most of the comminution being done in the ball mill. As the competency increases, the SAG ball charge and speed is increased, allowing for more work being done in the SAG. As with the single stage option recycle crushing should not be required. It is however recommended to allow for the future installation of recycle conveyors and/or recycle crushing if required. The basic circuit is shown in Figure 7-3.
Figure 7-3
Simplified flowsheet: SAB circuit
As with the SS SAG option, the stockpile can be replaced with a fine ore bin in environmental conditions that can often result in frozen ore stocks.
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8.0
MINE SCHEDULE ORE BLENDS
An indicative first pass mine schedule has been provided to OMC to evaluate the affect of the ore blend on each circuit option. Figure 8-1 depicts the mill feed blend. 100% 90% 80% d e e f l l i m o t n o i t u b i r t n o C %
70% 60% 50% 40% 30% 20% 10% 0% 1
6
11
16
21
26
31
36
41
46
51
56
61
66
71
76
81
Month Oxide %
Figure 8-1
Transitio n %
Fresh %
Ore type contribution to mill feed
The pinion power requirement for each monthly blend was calculated for each option. The realistically sustainable minimum and maximum pinion power for the selected equipment was also calculated to establish whether the required power is achieved within the typical operating range. Tertiary crush – ball milling option Figure 8-2 shows that the ball mill will be required to operate at very low ball charges (<20%) during the first 10 months. Operating at such low ball charges results slurry pooling and thus in poor power efficiency. Over-grinding may occur during this period. The ball charge will be increased gradually to compensate for the increase in competency.
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Figure 8-2
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Tertiary crush ball milling option – blend power requirements
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SS SAG The power requirements for the ore blend for a single stage SAG circuit is shown in Figure 8-3. It is expected that the mill be operated at slow speed (60% Nc) initially. The speed and subsequently the ball charge will be increased as the ore competency increases.
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Figure 8-3
SS SAG milling option – blend power requirements
SAB For the SAB circuit, the SAG mill and ball mill power requirements were evaluated separately. The SAG mill will initially operate at low ball charge and low speed, which will be increased as the competency increases.
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Figure 8-4
SAB option - SAG mill – blend power requirements
The ball mill is expected to operate at low ball charge during the first few months. In the SAB circuit this could however be managed by reducing the ball charge in the SAG mill even more (operating the SAG as a pulper), thus increasing the power required from the ball mill.
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Figure 8-5
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SAB option - ball mill – blend power requirements
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9.0
CONSUMABLES
Table 9-1 presents the calculated mill liner and media consumption rates for three of the options. Table 9-1
Liner and Grinding Media Consumption Unit
Tertiary Crush
SS SAG
SAB
Liner Consumption – Fixed Jaw
hours
2,400
2,200
2,200
Liner Consumption – Moving Jaw
hours
3,400
3,100
3,100
Power Consumption
kWh/t
0.07
0.08
0.08
Liner Consumption
hours
5,700
Power Consumption
kWh/t
0.30
Liner Consumption
hours
4,600
Power Consumption
kWh/t
0.32
Ball Consumption
0.54
0.31
Steel Liner Consumption
0.09
0.05
Gross Power (kWh/t)
17.2
9.8
Primary Crusher
Secondary Crusher
Tertiary Crusher
SAG Mill
BALL MILL Ball Consumption
kg/t
0.54
0.29
Steel Liner Consumption
kg/t
0.07
0.04
kWh/t
13.7
7.4
Gross Power (kWh/t)
Steel liner consumption does not include waste factor, which could be as high as 30% for mill liners.
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10.0
CONCLUSION
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The testwork was completed on samples that represent the three main ore types that will be treated at Amayapampa.
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The mill design was based on the Fresh ore only with throughput estimates made for the other two ore types.
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Three circuits were evaluated, tertiary crush – ball milling, single stage SAG milling and SAG – ball milling.
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The tertiary crush – ball mill option generally results in the most stable milling operation with the least risk of throughput and grind excursions. If this option is considered, then it is recommended that the nature of the oxide ore be evaluated carefully to ensure that there is minimal clay associated with this ore. Sticky ore will be detrimental to the three stage crusher plant. The capital cost associated with the three stage crusher plant is often higher than for the other options
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The single stage SAG milling option provides a lot of flexibility when treating variable ore types. For soft ores it can be set up to operate at high ball charge and low speed, while for the more competent ores a lower ball charge and higher speed will be required. It is however important to realise that it is not possible to change from one scenario to the other on a daily basis and the mine schedule should be reviewed if this option is considered. SS SAG milling provides good expansion potential, but also requires reasonably skilled operators.
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The SAB option is a good compromise between the ball milling and the SS SAG milling options. The ore variability showed that the circuit becomes SAG mill limited in some cases and ball mill limited in others. Again, reasonably skilled operators are required to run the circuit efficiently.
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Rheology testwork indicated that viscosity should not pose any difficulties in the comminution circuit, however more extreme viscosities were measured on fine samples at 70% solids. It is strongly recommended that this phenomenon be investigated and understood to avoid any potential downstream pumping and processing issues.
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