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AN OVERVIEW OF THE DESIGN, CONSTRUCTION, COMMISSIONING AND EARLY YEARS OF OPERATION OF THE SAG/BALL MILL GRINDING CIRCUIT AT PHU KHAM

COPPER, GOLD OPERATION IN LAOS.

*J.B. Hadaway1 and D.W. Bennett2

1FLSmidth Pty Ltd. Unit 3.0, Ground Floor, 63-85 Turner Street

Port Melbourne VIC 3027 Australia (*Corresponding author: john.hadaway@flsmidth.com)

2Principal Metallurgist PanAust Limited

Level 2, 99 Melbourne Street South Brisbane QLD 4140 Australia

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AN OVERVIEW OF THE DESIGN, CONSTRUCTION, COMMISSIONING AND EARLY YEARS OF OPERATION OF THE SAG/BALL MILL GRINDING CIRCUIT AT PHU KHAM

COPPER, GOLD OPERATION IN LAOS.

ABSTRACT

The operation at Phu Kham commenced design and construction in early 2006 and was commissioned in May 2008. This paper will examine and discuss the project from the inception of flow sheet development and equipment selection, equipment design and manufacture, construction, commissioning and ramp up to full production. The paper will touch on the strategies employed to achieve a short schedule and the efficient use of capital, but with the main emphasis on the grinding circuit. Various challenges during the early years of operation will be discussed including the effect of ore characteristics, including competency, on the performance of the grinding circuit. Actions to overcome these challenges and to enhance performance will also be discussed. Finally, current and future plans to improve the performance of the grinding circuit and the overall plant will be summarised.

KEYWORDS SAG Mill, Ball Mill, Phu Kham

INTRODUCTION

Phu Kham is a copper-gold operation located in the Republic of Laos. It is situated approximately 120km north of the capital, Vientiane (Figure 1).

Figure 1- Phu Kham Cu/Au Operation

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The Phu Kham operation is majority owned and operated by PanAust Limited, which is publically listed on the Australian Stock Exchange.

HISTORY AT PHU KHAM

PanAust Limited was founded in 1996. In 2001, after a number of years evaluating potential opportunities in SE Asia, it acquired an 80% interest in Phu Bia Mining (PBM) which held the rights to develop the Phu Kham deposit. In 2003 PanAust commenced the study phase for the Phu Kham heap leach project and the copper/gold operation. This work was completed and concluded that the development of the deposit was attractive and established a plan for a two stage development strategy. Stage 1 was to be a gold heap leach operation followed by the construction of the Cu/Au concentrator some two to three years later.

In 2005 the gold heap leach operation went into production. In this year PanAust also acquired the balance of the 20% interest in PBM.

In 2006 approval was given for the development of the $241m Phu Kham Project and during 2007 ore was mined and stockpiled in preparation for the commencement of operations in 2008. Also during 2006 and 2007 the design, procurement and construction of the concentrator was in progress. Commissioning of the plant was commenced in the first part of 2008 with operation commencing in earnest in March and first production of copper concentrate in April.

The mine was initially designed to treat 12 Mtpa of ore and this level of production was achieved early in the life of the operation. The Phu Kham project sets a high standard in terms of the efficient use of capital and represents a very low capital cost outcome when measured in capital invested per tonne of annual throughput. The staged development was also the key to achieving early cash flow ahead of the main investment; thus providing a significant improvement in the overall NPV of the project.

In October 2010 approval was given for the Phu Kham upgrade project which will take the annual throughput from 12 to 16 Mtpa, and maintain copper production with increasing ore hardness and lower grade throughout the life of the operation.. At the same time the mills will produce a finer grind to provide increased copper recovery. The upgrade project involves the installation of a second 13 MW secondary ball mill and flotation capacity, and is due to contribute to increased production from Phu Kham in the second half of 2012.

DISCUSSION

Mill Sizing and Selection

The Phu Kham operation provides significant processing challenges due to the complex nature of the deposit. The ore varies from highly weathered and altered material containing soft clays to hard and abrasive primary silicate rock. The intense faulting and folding of the deposit has caused intermingling of different zones, which required robustness in grinding plant design to cope with the significant ore hardness variation.

From the beginning of the feasibility study the grinding circuit for the Phu Kham Project was

conceived as a SAG/BALL circuit (Figure 2). A major advantage of this design for Phu Kham is the avoidance of fine crushing which is difficult for ores with high clay content, while retaining ability for treating competent rock. ROM ore is crushed by a Fuller-Taylor 54 x 77 NT Primary Gyratory Crusher, before transport to a coarse ore stockpile via an overland conveyor. Two reclaim apron feeders discharge ore from the stockpile to the SAG mill feed conveyor.. The discharge from the SAG mill is screened by a

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trommel screen mounted at the discharge end of the mill. The oversize material is recycled to the SAG mill feed via conveyors and the undersize material is combined in the cyclone feed sump with the ball mill discharge and feed to the ball mill classifying cyclones. Product overflow from the cyclones is gravity fed to subsequent flotation processes and the underflow is feed to the ball mill.

Figure 2 - Phu Kham Cu/Au Operation – Basic Grinding Flowsheet

Figure 3 - Phu Kham Cu/Au Operation – SAG and Ball near final stage of installation

During the study in early 2005 the circuit throughput was set at 9 Mtpa and the key grinding circuit design parameters were as shown in Table 1.

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Table 1 - Initial Grinding Circuit Key Design Parameters.

SAG Mill

Feed Size, F100 235mm

Feed Size, F80 115mm

Ore SG 2.9

Required Mill Discharge Size, P80 2,000 micron

Ball Charge 10 to 18%

Total Charge 26% Design Bond Ball Mill WI

(closing 106 microns) 15.5 kWh/tonne

Design Bond Rod Mill WI (closing 1180 micron) 17.4 kWh/tonne

Circuit Fresh Feed Rate 1,125 tph

Utilisation 91.3%

Ball Mill

Product Size (Cyclone O/F) P80 106 micron The initial selection at this stage was for a single 34’ dia. x 17’6” flange to flange long, twin

pinion, 10.5 MW SAG Mill and a single 24’ dia. x 36’ flange to flange long, twin pinion, 12 MW secondary ball mill.

By early 2006 better definition of the reserve and further test work indicated that the project NPV

would improved by increasing the throughput of the plant from the nominal9 Mtpa to 12 Mtpa. The grinding test work was conducted on the various ore types in the defined ore body. See Table 2 for a summary of the test work.

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Table 2 – 2006 Revised Grinding Test Work Results.

The mill selection parameters were also revised as indicated in the table 3.

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Table 3 – 2006 Revised Grinding Circuit Key Design Parameters.

SAG Mill

Feed Size, F100 300 mm

Feed Size, F80 125 mm

Ore SG 2.9

Required Mill Discharge Size, P80 Not defined

Ball Charge 10 to 18%

Total Charge 26% Design Bond Ball Mill WI

(closing 106 microns) Refer Table 2

Design Bond Rod Mill WI (closing 1180 micron) Refer Table 2

Circuit Fresh Feed Rate - Nominal 1,500 tph

Circuit Fresh Feed Rate - Maximum 1,750 tph

Availability 91.3%

Ball Mill

Product Size (Cyclone O/F) P80 106 micron Based on the revised test work and selection parameters a number of simulations were conducted

to find the optimum selection to ensure ample grinding capacity while at the same time maintaining objective of achieving the best use of capital. This led to the final selection of a 34’ dia x 20’ f/f long twin pinion, 13MW, variable speed SAG mill and 24’ dia x 40’ f/f long, twin pinion 13 MW, fixed speed ball mill. The mills selection summary is shown in Figure 4.

Figure 4 - Final Grinding Mill Selection Summary

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The final selection of the SAG mill volume, operating ball charge level and installed power was made with a view to ensuring that there was ample room for expansion in the future without the need for a second SAG mill. The volumes of the mills were made deliberately generous to ensure that full installed power could be achieved. It can be seen from figure 4 that the full installed power of both mills will be required at 12 Mtpa throughput when treating the primary ore, however in the first three years of operation up to 14 Mtpa design capacity is available on the softer transition ores. The final upgraded circuit configuration is discussed later in this paper. The installation of the 50%/50% SAG/Ball power necessitated the need to achieve a relatively fine transfer size of approximately 80% passing 500 micron at the SAG mill discharge and therefore a relatively high design pebble recirculation in the SAG mill. Space was made available for future pebble crushing capacity when hard primary skarn ore becomes a significant proportion of the feed blend in later years of operation. The circuit was designed to provide maximum operating flexibility over the range of known ore types that were expected to be encountered during the life of the operation.

The selection of the mills was influenced, as mentioned above, by the need to achieve efficient use of capital. As a result of this the decision was made to use matched drives and bearings on the SAG and ball mills. The drive design was optimised to provide maximum interchangeability. The following items are interchangeable between the SAG and Ball mill.

Main drive motors High speed couplings Gear reducers Low speed couplings Pinion gears Pinion bearings Inching drives Liquid resistance, secondary starters. Main bearings and lubrication systems Trommel screen frames

This achieved a significant reduction in the required insurance spare parts holding. The value of this saving is estimated at up to approximately AUD3,000,000.

In line with the philosophy of achieving low initial cost particular attention was given to the plant layout and the final “side by side” arrangement was chosen to provide a compact foot print and a layout that is sympathetic to the steep topography of the site. The site topography was also exploited to reduce civil works and maximise gravity flows in the process and hence reduce capital and operating costs.

As mentioned the site is located in mountainous terrain and the access roads limited the allowable size and weight of mill components. The sectioning of the shells was studied extensively and the final configuration of shells and heads was as follows.

Table 4 – Major Component Sectioning

Component Sectioning

SAG Shell 3 x 120̊ Segments

SAG Heads 4 x 90̊ Segments

Ball Shell 6 x 180̊ Segments

Ball Heads 2 x 180̊ Segments

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The sectioning summarised in Table 4 represents the optimum point in the trade-off between manufacturing cost and the cost and dimensional restrictions of transport to a maximum of around 7 metres in width. Consideration in this study was also given to the manufacturing capability of the component manufacturers under consideration.

Variable speed was required for the SAG mill. Various options were considered to meet the

specified speed range of 65 to 85% of mill critical speed, and the final selection was the IES Hyper-Synchronous drive Slip Energy Recovery (SER) in conjunction with Wound Rotor Induction motors (WRIM). This was deemed to be the most cost effective solution for the required speed range of 70 to 110% of the synchronous speed of the motors and provides high efficiency and low harmonics. To date this installation remains the largest of its type in operation.

Mill Supply and Project Execution

Critical to the success of the project was a fast construction schedule. In order to achieve the planned schedule, the delivery of the mills was required progressively up to 68 weeks from award. At the time, the standard delivery for mills of this size was approximately 80 to 90 weeks. In order to achieve the required improvement a number of important steps were introduced into the execution plan.

Engineering was distributed between various technology centres within the FLSmidth group.

Basic design of the SAG mill was undertaken in North America, ball mill basic engineering in South Africa and general layout and plant interface engineering in Australia. In this way the major activities were performed in parallel and the production of critical manufacturing data and vendor data was accelerated; allowing an early commencement of manufacture and plant design and construction.

Manufacturers were selected on the basis of proven track record, available capacity and location

in order to minimise shipping transit durations. The primary locations for manufacture were

o Shells, South Africa o Heads and trunnions, Australia o Gears and pinions, Australia.

The planned deliveries were achieved due to intensive expediting and planning and the mill

components arrived at the construction site in the time and sequence required to achieve the overall project schedule.

Figure 5 shows the overall mill supply and construction time line.

Figure 5 - Engineering, supply and installation schedule.

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Figure 6 – Overview of the Phu Kham Grinding Circuit Near Completion of Construction

Commissioning, Ramp Up and Early Operation

Pre commissioning was undertaken in February and early March 2008. This comprised;

• Thorough checking of interlocks and control systems • Final QA on the mechanical installation • Low load water testing and running of the mills.

Load commissioning was conducted progressively in early March, gradually building to near full

load and throughput into early April. Commercial operation was achieved in April and the first shipments of concentrate were achieved in that month; ahead of schedule.

In general terms it could be said that mill commissioning was very successful and achieved without major problems. This was in no small part due to thorough preparation, planning, extensive pre-commissioning and personnel training.

PBM adopted a conservative ramp up schedule. Between May and August 2008 the plant

throughput increased steadily from around 1100 tph to the design capacity of 1500 tph.

During the first 12 months of operation a number of teething problems were encountered.

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The trommel panels suffered from premature problems related to tearing and overall capacity in

the first few months. This led to an intensive review and redesign of the panels, particularly in the first section of the screen which takes the bulk of the impact as the material discharges from the mill. Subsequent developments in the design and configuration of the spirals, dams and process water sprays have achieved significant improvements in the capacity, efficiency and reliability of the trommel screen. The screen now achieves a life that is consistent with the rhythm of the maintenance schedule and can handle significantly higher throughputs than anticipated in the original design.

Phu Kham is located in an extremely high rainfall area. As such problems were encountered with

the stockpile and reclaim area due to the high moisture content and presence of more clay content than expected in the initial, partially oxidised ore and at times restricted the feed available to the grinding circuit. This problem has been largely reduced by improvements in the operation of the mine, including a change to 10 metre benches that improved ore drainage, expansion of the ROM pad capacity for improved blending and reduced truck queuing, and improvements in chute design. clay content in ore is expected to further reduce as the mine moves in to the transition and primary zones.

Lightning strikes also affected production in the early stages and voltage fluctuations were

encountered on a regular basis. This in part led to problems with the main mill drive motors. Deterioration of the slip-rings and brushes led to an unplanned shut down while the spare motor was installed and repairs made to the damaged motors. This problem was overcome by a combination of the installation of surge arrestors on the HV supply to the motors and close attention to the maintenance of the slip rings, particularly the filters on the slip-ring cooling air supply.

The early operation of the grinding circuit has been characterised by less than full power draw on

the SAG mill. This is largely due to the treatment of low competency ores and the high clay content mentioned earlier. During this same period the ball mill operated at full power draw and has done so since the start of the operation. This basic feature of the operation has been present for the full period of operation, though the SAG mill power draw has increased as the competency of the ore has increased. Short-term variability in ore hardness has created challenges with SAG mill operation, with ball charge often lower than optimal due to sudden changes from hard ore to soft ore. It could be said however that the operation at Phu Kham has been “ball mill limited” since its inception. This is not uncommon with 50/50% power split SAG/Ball mill circuits. The need to achieve a fine transition size between the SAG and Ball mills means that the SAG mill charge will contain a higher proportion of fine particles which leads to increased slip in the charge and therefore reduced power draw. PBM have undertaken a program to improve SAG mill and grinding circuit performance and have conducted detailed plant surveys which has allowed development of a power model of the circuit. The circuit model in combination with the Phu Kham ore characterisation program will be used for improving prediction of throughput and product size throughout life of mine. The Upgrade Project

In October 2010 the the upgrade project for the Phu Kham Cu/Au operation was approved. This project is designed to ensure copper in concentrate production is maintained after 2013 when plant feed grades will decrease with increasing primary ore. In order to achieve the copper production target throughout the life of mine, plant throughput will increase for the current nominal 12 Mtpa to 16 Mtpa.

As the operation has been characterised as being “ball mill limited”, the upgrade study included a

review of ore hardness data and required ball mill power in order to achieve 16 Mtpa throughput at a nominal product size of 80% passing 106 microns over the life of mine. Two options were considered. The first option was the addition of a 6.5MW single pinion ball mill to provide a total of 19.5 MW of installed ball mill power, and the second option the addition of a another 13MW mill to provide a total of 26 MW of installed ball mill power. The throughput at 106 and 75 microns for the two options is shown in Figure 7.

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2010 2011 2012 2013 2014 2015 2016 2017 2018

Thro

ughp

ut (M

tpa)

Maximum Design Nominal Design 26MW 106µm 26MW 75µm 19.5MW 106µm 19.5MW 75µm

Figure 7 - Phu Kham Throughput at 106µm and 75µm for 26 MW and 19.5 MW Installed Power It was concluded that the smaller 6.5 MW mill would not add sufficient capacity to meet the 16

Mtpa throughput at a maximum product size of 80% passing 106 microns after 2013, and therefore an additional 13 MW ball mill was required. The extra 13MW installed power also provides capacity upside above 16 Mtpa throughout life of mine, with a maximum product size of below 106 microns providing a benefit of increased copper recovery in rougher flotation.

The recommendation for the larger 13 MW mill was approved and a second identical FLSmidth

ball mill was ordered in late 2010. The identical mill and auxiliary equipment provides significant advantages in minimising spares inventory held at site, while ensuring compatibility of existing reline equipment and systems between the SAG and ball mills.

Once again the project schedule is tight, and the mill is the longest lead item in terms of delivery.

Delivery of the second mill is due in the last quarter of 2011 with start up due in the second quarter of 2012. As was the case with the first stage of the project, capital cost is very important. At all times the strategy has been to minimise capital expenditure while at the same time avoiding undue risk to the schedule and quality assurance of the equipment. For this reason the sourcing of the mill components was reviewed.

A review of all sourcing options was undertaken and various combinations of western and low

cost sources were analysed. Critical components were assessed in terms of the difficulty of manufacture and the potential risk to the project schedule.

The components in question were the shell, heads and trunnions. The risk ranking was assessed as

shown in Table 5.

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Table 5 – Major Component Sourcing

Component Risk Country of Manufacture Trunnions Low P.R. China

Heads High Europe Shell Medium P.R. China

This combination yielded the optimum balance between cost and acceptable risk. To date the

choices have proven to be right and all components are either on or ahead of schedule. It should be noted that a close and cooperative relationship with all suppliers is critical to the success. FLSmidth has employed its resources around the world to ensure this outcome.

Apart from the grinding capacity other mill related items were assessed to ensure that they had

sufficient volumetric capacity for the increased plant throughput. In particular the SAG mill trommel screen was assessed. Options were studied to establish the viability of fitting a larger trommel. It was concluded that the effective surface are of the trommel could be increased by approximately 50% with the installation of a new transition cone, pulp diverters and new trommel. The trommel was designed to be fitted with the same panels used in the existing operation, thus continuing the theme of commonality of spare parts. Figure 6 illustrates the proposed upgrade to the trommel.

Figure 6 – Proposed SAG Mill Trommel Upgrade As mentioned earlier the performance of the existing trommel screen has been improved

significantly over the first few years of operation. Subsequent to proposing the larger trommel it has been established (based on current site performance) that the existing trommel has sufficient capacity to meet the 16 Mtpa throughput. It has therefore been decided to defer to decision to upgrade the trommel. This has a very beneficial affect on the project in terms of reduced interruption and downtime during the start up of the second ball mill.

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CONCLUSIONS

It can be concluded that the development of the copper/gold deposit at Phu Kham has been a

success on many levels. The project is a first class example of the efficient use of capital. The rapid on-time development is a testament to the thorough testing, planning and innovative execution model employed during the planning, engineering, procurement, construction and commissioning phases of the project. The plant has performed beyond design expectations and overcome a number of challenging problems in the first few years of operation. Close working relationships between the staff of PBM and its key suppliers has ensured that high plant availability has been achieved.

ACKNOWLEDGEMENTS

The authors would like to thank the management and staff of PanAust Limited and Phu Bia Mining for their support and assistance with the preparation of this paper.

REFERENCES

Crnkoviz, I. & Georgiev, T. & Harbort, G. & Phillips, M. (2009). Commissioning and optimisation of the Phu Kham Copper – Gold Concentrator, Tenth Mill Operators’ Conference, Adelaide:AusIMM Lane, G. & Dickie, M, (2009). What is Required for a Low-Cost Project Outcome?, Project Evaluation Conference, Melbourne:AusIMM Meka, Z. & Lane, G. (2010). Recent Metallurgical Developments and Their Impact on Minerals Project Execution. XXV International Mineral Processing Congress (IMPC) , Brisbane