LOS BRONCES DEVELOPMENT PROJECT Nelson Sánchez

The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
LOS BRONCES DEVELOPMENT PROJECT
Nelson Sánchez; Rolf Fiebig; Pierre Perrier; Alejandro Vásquez
ABSTRACT
The Los Bronces Development Project (LBDP) was approved in October 2007 by the
Anglo American plc Board at a capital cost of some US$1.74B, and is scheduled to be
completed by the end of 2010. The project represents a big step forward for the Los
Bronces mine since it will increase the treatment rate by 140%, the mining rate by 64%
and almost double copper production during the first four years of the project. This
economy of scale will also position Los Bronces as a much bigger and efficient mining
operation, better prepared to face adverse market scenarios and allowing Anglo to
consolidate its position in base metals.
The project addressed numerous scope alternatives such as plant capacity vs mine
capacity, cut-off grade strategy to maximise value based on the existing flotation and
leaching process, cutback strategy, etc.
Main infrastructure aspects of the project are as follows:
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Dump area with 2400Mt capacity for waste and low grade material
Mining equipment: 1 x electric shovel, 12 x 300t trucks, 1 x drill
Primary crusher: 60” x 89”
Three new power lines (144MW capacity)
Grinding plant at 87kt/day nominal capacity (includes a 4.3km tunnel with a
regenerative conveyor belt)
Grinding plant: 1 SAG mill 40’ x 25’, 2 ball mills
Slurry pipeline (52 km from grinding to flotation plant) with five choke stations
Flotation plant (rougher, cleaning, regrinding circuits and molybdenum flotation).
The history of Los Bronces started in 1867 when it was discovered. In 1978 ENAMI
(Government company) sold Los Bronces to Exxon for US$112M. The same year a snow
avalanche destroyed San Francisco Flotation Plant. In 1979 the plant was rebuilt at
4.9kt/day and from 1979 to 2002 several plant expansions were made up to 54kt/day
capacity. In 2002 Anglo acquired Los Bronces for US$1.3B.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
The 2002 resource and reserve statement showed reserves at 459Mt at 1.02%CuT and
resources at 780Mt at 0.80%CuT. The 2007 statement shows reserves at 1,480Mt at
0.67%CuT and resources at 1,644Mt at 0.45%CuT.
Defining the size of a plant expansion is the key challenge that any expansionary project
faces. The scope of this paper is a general review of the project, describing all the mining
techniques that were used to define the best options that maximise value. In this context,
a range of plant capacities (from 58 to 230kt/day) combined with a range of mining
capacities (from 83 to 166Mt/a) were evaluated in detail, defining for each option a mine
plan, Capex and Opex, and cut-off grade strategy for flotation and leaching processes and
the associated Net Present Value (NPV) that was used as the selection criteria.
The Los Bronces mineral resources potential provides good growth opportunities for the
mine beyond LBDP implementation, offering an exciting invitation to continue studying
new initiatives that maximise the value of this deposit.
LOS BRONCES DEVELOPMENT PROJECT
1.
Background
•
Location
The Los Bronces operation is located in central Chile and is split between two sites
connected by an ore slurry pipeline.
The mine is located some 65km north-east of Santiago. Primary crushing, grinding, and
related infrastructure are sited adjacent to the open pit mining operation. Marginal grade
ore leaching is performed on dumps downstream of the mine site, where related solvent
extraction/electrowinning facilities are located.
The mine site is in the Andean Mountains and lies in an area of steep relief in the upper
reaches of the San Francisco Valley at an elevation of between 3400m above sea level
(masl) and 4100masl. Access is by public roads from Santiago.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
A 56km mostly buried pipeline runs from the Los Bronces grinding plant site to the
flotation facility at Las Tórtolas, traversing three tunnels while dropping in elevation
from 3450masl to 750masl.
A R G E N T I N A
CHAGRES SMELTER
Pacific
Ocean
LAS TORTOLAS
FLOTATION PLANT
Santiago
LOS BRONCES MINE
CHILE
Las Tórtolas is located in the Colina valley, some 40km north of Santiago. Access is via
the San Martin highway or the Pan-American highway from Santiago. The flotation plant
is situated in an open valley at an elevation of 750masl. Tailings impoundment is
adjacent to the plant site, using sand dams and taking advantage of favourable
topography.
The climate at Los Bronces is typical of the high Andes. Mean annual snowfall,
concentrated mostly in the winter period (April through September), is 764cm.
Temperatures range from -18ºC in winter to 19ºC in summer.
The climate at Las Tórtolas is temperate. Mean rainfall is 220mm/a and temperatures
vary from 0ºC in winter to 34ºC in summer.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
•
Surface Mining 2008
Strategic Considerations
The Anglo American plc Base Metals Division strategy is to achieve long term
sustainable maximisation of shareholder value through finding, acquiring, developing and
operating large, long life, low cost base metals businesses in a zero harm, socially and
environmentally responsible manner.
Within the framework of this strategy, it is the objective of the copper division to grow its
cost effective copper production from approximately 5% in 2005 to in excess of 7.5% of
world mine production over a 10 year time horizon. This implies more than doubling
existing copper production to a level of approximately 1.5Mt/a.
Los Bronces is one of Anglo Base’s existing two world class operations. Doubling
production at Los Bronces is consistent with and the first step in the achievement of the
copper division growth strategy (the Collahuasi expansions, Quellaveco and Michiquillay
are the other assets which are included in the current strategic growth plan).
The long term prospects for the copper industry are particularly positive. Although
current prices are presently far in excess of the long term equilibrium levels and
unsustainable, a period of above average demand growth is being experienced and
supply/demand fundamentals are, for the foreseeable future, more favourable than have
been seen for some considerable period of time.
2.
Project Description
An investment of US$1,744M (nominal) was proposed for an 87kt/day expansion to the
sulphide ore treatment capacity at the Los Bronces operation.
The expansion will increase the ore treatment rate from 61kt/day (base case) to
148kt/day. Average production of fine copper in concentrates (2011-2020) will be about
380kt/a, twice that forecast for the base case (2007 life of mine (LOM) forecast).
The proposed investment covers the following scope:
•
Mining Fleet: Addition of 1 x 73yd3 electric shovel, 12 x 300t capacity haul
trucks, 1 x 10⅝” drill rig and support equipment (bulldozer, wheel dozer, water
truck, etc.) to increase mine capacity from 84 to 128Mt/a (including the
incremental 87kt/day ore).
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
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Primary Crushing: A modified 60” x 89” (1000hp) primary crusher, to
complement an identical unit currently under construction for the existing
operation.
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Coarse Ore Transport: A downhill, regenerative conveyor system for coarse ore.
The conveyor will traverse a route that includes a new 4km tunnel connecting Los
Bronces with the new grinding plant site at Confluencia.
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Grinding Plant: Coarse ore stockpile, conventional semi-autogenous grinding
circuit, ore thickeners and ore slurry preparation tanks. The new plant will
operate concurrently with the existing grinding plant at Los Bronces.
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Ore Slurry Transport: A new, 28” diameter, 51km long pipeline, installed along
the existing ore slurry pipeline route to Las Tórtolas.
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Flotation: New copper and molybdenum flotation equipment at Las Tórtolas
(near the existing flotation plant).
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Tailings: Tailings thickener, transport systems (pumps, pipelines and flumes),
cyclone station expansion and modification, extensions to sand dam drainage and
new borrow dams consistent with a design to achieve ultimate tailings
impoundment capacity of 2,100Mt.
•
Water Supply: Expansion of pump stations and pipelines to increase capacity to
recirculate process water from Las Tórtolas to Confluencia. Expansion to fresh
water intake and pump system in the Riecillos valley.
•
Power Supply: Additional high-tension supply lines, two new substations and
modifications to existing power lines and substations
The project was approved in October 2007, with advanced commitment for critical path
grinding equipment and interim engineering. On this basis, project start-up is forecast in
January 2011.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
Figure 1: General layout of main expansion areas
CONFLUENCIA
LOS
BRONCES
(3450 masl)
EMERGENCY POND
CHOKE STATION
CS 0A
HOLDING TANK
ORTIGA TUNNEL
28” SLURRY PIPELINE
CS 1A
From Confluencia 51 km
Discharge
Box
CS 3A
LAS TORTOLAS
CS 1.5A
CS 2A
VIZCACHAS
Laguna Seca
TUNNEL
EL DURAZNO BRIDGE
By Pass Peldehue
COLINA BRIDGE
(750 msal)
3.
Mining Design
3.1
Terms of Reference
Mine design starts with the Terms of Reference, which include:
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Minimum life of mine required for the company for a feasible expansion.
Long term price forecasts of main products and consumables.
Preliminary estimates of operating costs, fixed and variable, for indexation to
associate with different plant treatment and mining capacities.
Block model with copper and molybdenum grades.
Mining and property restrictions.
Geotechnical design criteria.
Geometallurgical parameters: copper recovery, copper concentrate grade,
plant treatment rate (dependent on rock hardness), molybdenum recovery, etc.
Area for waste dumps and tailings dam.
Technology to be used, size and type of mining equipment, primary crusher,
grinding, flotation, etc.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
3.2
Surface Mining 2008
Mine Design
Based on the Terms of Reference and always considering the answer to the main question
“Which is the optimum expansion?”, various options of possible expansions in plant
treatment rate and associated mining capacity were designed, considering existing
technology.
The determination of the optimal expansion was developed during the pre-feasibility
study, allowing the analysis in detail of only the optimal case in the feasibility study, with
some sensitivities.
The pre-feasibility study considered that, based on the types of grinding system available
in the market, a new grinding plant could add between 15 and 120kt/day to the existing
plant capacity. Thus the Los Bronces plant could reach a total treatment rate in the range
75 to 180kt/day. The higher capacity was the maximum possible expansion due to
restrictions of minimum mine life associated with the existing ore reserves at the time of
the study.
For each plant capacity option, mine plan alternatives were analysed to determine the
mining capacity and the copper cut-off grade strategy that maximised the Net Present
Value NPV) in each case. Figure 2 depicts the sequence that was followed to obtain the
NPV for each mine-plant combination.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
Figure 2: Design & evaluation stages for each Mine-Plant combination
Optimal mining sequence definition
Terms of reference
Rutine
Lersch &
Grossman
Design parameters (Slope
Angle,...)
Block Model (CuT, MoT, CC, etc)
Initial Topography
Metallurgical parameters
Final Pit
Pushback selection
Economic model
Whittle
Mine-Sight
Datamine
Mining Plans, production level, Cost and investment
Economic evaluation
(NPV - RRI - ROCE)
Optimal alternative
selection
3.3
Accesses and pushbacks design
Mine and plant alternatives definition
Cut off grade strategy (by Opticut / COMET– K. Lane)
Mining plans (by Mine-sight / M821)
Fine Cu and Mo production
Equipment fleet calculation, labor, operation cost and
investment.
Final Pit Selection, Mine Extraction Sequence (Phases)
For each mine-plant alternative, the final pit and optimum reserve extraction sequence
were determined (volume of resources economically exploitable). The basis for the
optimisations were economic (given long term metal price forecasts, operating costs,
associated initial and replacement capital expenditures) and technical ( grade model,
metallurgy, geotechnical parameters, etc).
The Whittle software used in this task applies the Lerchs & Grossman optimisation
algorithm. For final pit limit selection, Whittle analyses the incremental NPV according
to the methodology shown in Figure 3.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
Figure 3: Final pit shell selection
VAN
Incremental
INCREMENTAL
NPV
Incremental NPV graphic:
•Best Case: Pit by pit extraction until
final pit.
ase
tC
Bes
e
Cas
ied
f
i
c
Sp e
Wors
t Cas
•Worst Case: Bench by bench until
final pit wall, extraction.
e
Zona de elección
Area of the optimal
del Pit Óptimo
pit criteria
#
Elegido
Rev F=1
PIT’s
•Specified Case: is defined, thinking in
a possible extraction and treatment
rate. This gives a more realistic case
then the best ore the worst case.
range
interest
Rango Pit
de pit’s
queofnos
interesa analizar
Taking into consideration the size and operational constraints of the selected equipment
and the desired production level, the design process was carried out in phases up to the
final pit design, including the required access ramps. The project used MineSight
software in this design stage, and the result of this work is presented in Figure 4.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
Figure 4: Design of operational phases
3.4 Definition / Determination of the Optimal Expansion
Opti-Cut software, based on Kenneth F. Lane theory, was used to optimise cash flows
depending on the mine and plant capacities, through adjusting cut-off grades for the ore
feed to the grinding plant. Considering the design phases, the maximum NPV was
calculated for different combinations of mining capacity and ore fed to the grinding plant.
The NPVs from these analyses were used as a guideline for more detailed study of the
most economically attractive options. These cases required the development of detailed
mine plans and related economic analysis using the company’s financial model.
Table 1 and Graph 1 show the combinations of plant treatment rate and mining capacity
studied.
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
Table 1: Matrix of Mine-Plant combinations
Capacidades
Mina(Mton/year)
(Mton/año)
Mine Capacities
Mill Capacities
Capacidades
Plantas
Sulphide
Sulfuros
(MTPA)
83
(227 KTPD)
Cathode
Cátodos
22
(58 KTPD)
XX
(Kton
Cu Fino año)
(Kton Fine CU year)
33
(90 KTPD)
XX
44
(120 KTPD)
XX
49
(135 KTPD)
XX
66
(180 KTPD)
XX
84
(230 KTPD)
XX
91
(250 KTPD)
99
(270 KTPD)
110
Preliminary
estimation
•EstimaciónNPV
Preliminar
del for
VAN
every
combination
mine-mill
de cada combinación
(300 KTPD)
128
(350 KTPD)
146
(400 KTPD)
of K. Lane
Uso Use
de Teoría
de K.Theory
Lane
(Determinar
el mejor VAN
las máximas
(Best
NPV determination
baseddadas
on maximun
mill and
capacidadesmine
Minacapacities
y Planta definidas
OPTICUT)
– OPTICUT)
166
(450 KTPD)
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
Graph 1: Final NPV for the different combinations
NPV Results
Mayor de
interest
Casos
Mayor
cases
interés 2 1
Resultados Opti Cut
(MUS$)
(VAN
MUS$)
NPV
(MUS$)
VAN (MUS$)
1500
1300
Mayor
Casos de
interest
Mayor
cases
interés
1 2
1100
900
700
58
90
120
135
180
230
Tratamiento
Planta (KTPD)
(KTPD)
Plant
Treatment
Movimiento
Mine
Movement
Mina
227
M in
Mine Movement
M o v im ie n to
270
300
350
400
Plant Movement
a
T r a ta m ie n to
450
P la n ta
M T P A
2 1
3 3
4 4
4 9
6 6
8 4
K T P D
5 8
9 0
1 2 0
1 3 5
1 8 0
2 3 0
M T P A
K T P D
8 3
2 2 7
1 0 3 5
1 1 9 2
1 0 8 3
1 0 1 0
8 2 9
9 9
2 7 0
9 6 2
1 2 0 2
1 2 8 1
1 2 7 9
1 0 5 5
1 1 0
3 0 0
9 6 9
1 1 4 5
1 3 2 1
1 2 7 3
1 2 0 4
9 2 6
1 2 8
3 5 0
9 6 9
1 2 1 1
1 3 4 9
1 3 6 0
1 4 0 5
1 1 3 4
1 4 6
4 0 0
9 7 3
1 1 5 8
1 3 3 9
1 3 6 5
1 5 0 5
1 3 0 4
1 6 6
4 5 0
9 0 9
1 1 7 8
1 3 1 5
1 4 0 4
1 5 3 0
1 4 7 4
7 7 6
3.5 Plant Capacity Definition
After this extensive analysis it was determined that, under the defined Terms of
Reference, the best mine-plant combination for Los Bronces was 132Mt/a total mining
capacity and 54Mt/a treatment at the sulphide plant, resulting in an increase in fine
copper production from an average of 220kt/a to 400kt/a.
Graph 2 compares the mining and plant treatment rates, copper grades and copper cut-off
grades for the Los Bronces Base Case and the Optimum plan, the Los Bronces
Development Project plan.
________________________________________________________________
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
Graph 2: Mining rate, plant treatment, copper grade, cut off grade: Base case vs.
4.
Future of the Los Bronces Mine District
As discussed earlier, the Los Bronces Development Project defined the best expansion
option according to the Terms of Reference and reserves available at the time.
During the course of this study, the mine has continued with a core drilling campaign in
the boundaries of the operation, increasing the available resources around the pit. This
could eventually support further expansions, giving Los Bronces a very bright future.
________________________________________________________________
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The Southern African Institute of Mining and Metallurgy
N Sánchez, R Fiebig, P Perrier, A Vásquez
Surface Mining 2008
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