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Antofagasta's Chilean push.

The development of the Los Pelambres and El Tesoro projects will significantly boost Antofagasta's ranking amongst the world's leading copper producers.

The first copper concentrate shipments from the new $1.36 billion Los Pelambres copper mine in Chile began in January this year from the Punto Chungo port facility at Los Vilos. Antofagasta, the UK-listed mining group which holds a 60% equity interest in the project, anticipates that Los Pelambres will be amongst the world's lowest-cost copper producers, with cash operating costs forecast to average around $0.43/lb over a 30-year mine life.

Antofagasta originally owned 100% of the Los Pelambres project but in May 1997, in order to provide equity finance, it agreed to sell a 40% stake to two Japanese consortia for $275 million. It subsequently led negotiations on behalf of the group in arranging project finance of $950 million. Under the existing ownership of Minera Los Pelambres, apart from Antofagasta's 60% interest (held through its wholly-owned Chilean subsidiary Anaconda Chile, which is responsible for all of the group's mining operations), Nippon Mining and Metals holds 15%, Mitsubishi Materials 10%, Marubeni 8.75%, Mitsubishi Corp. 5% and Mitsui and Co. 1.25%. Nippon Mining and Mitsubishi Materials have agreed to purchase 400,000 t/y of concentrates on a 12-year basis.

Development of Los Pelambres, located 200 km north of Santiago in Chile's IV Region, began in November 1997, with Bechtel International appointed as contractor under an EPCM contract. During the development phase, almost 50 Mt of material was removed from the area of the open pit in order to access the ore, and some 13 km of avalanche-proof tunnels have been constructed to accommodate the conveyor system from the primary crusher to the concentrator. Crushing of the first ore began in August last year and commissioning of the concentrator began two months later. The concentrate is pumped via a 120 km pipeline to the new port facility at Los Vilos where it is filtered prior to shipment to smelters overseas.

A mineable open pit reserve of 934 Mt averaging 0.77% Cu and 0.023% Mo has been delineated at Los Pelambres within a measured and indicated reserve of 2,400 Mt at a grade of 0.67% Cu and 0.016% Mo (at a 0.4% Cu cut-off grade). The total resource amounts to more than 3,000 Mt at an estimated grade of 0.65% Cu and 0.014% Mo, and only 31% of the total geological resource is included in the 30year mine plan. Copper mineralisation includes chalcopyrite, chalcocite, bornite, coyellite and molybdenite.

At its ore-treatment design capacity of 85,000 t/d, Los Pelambres is expected to produce some 743,000 t/y of concentrate containing 246,000 t of payable copper. During the first five years of operation, however, annual payable copper production is expected to be nearer 271,000 t. Under a future projected Phase 2 expansion at the mine, daily ore throughput could be increased to more than 120,000 t.

Ore is fed by 240 t haul trucks into a fixed crusher station equipped with a 1,524 mm x 2,794 mm primary gyratory crusher located outside the pit limit at an elevation of around 3,000 m. The design capacity of the crusher station is 9,000 t/h at a nominal product discharge size of 180 mm. The crushed ore is discharged directly onto a 14,000 t/h capacity belt conveyor feeding a mine stockpile with a live capacity of 18,000 t and a total storage capacity of approximately 90,000 t.

Power recycle

The copper concentrates which began to be pumped from the mine late last year to the coast (MJ, November 12, 1999, p.385) contain molybdenum and silver by-products, which help reduce the net operating costs. However, it is not widely known that the mine has a third by-product: electricity.

Los Pelambres lies high in the Andes at 3,200 m, close to the border with Argentina. The closest suitable site for the concentrator was chosen at 1,600 m, necessitating the conveyor belt system to carry the ore to the concentrator. Four variable speed belt feeders have been installed in a concrete reclaim tunnel below the mine stockpile which discharge directly onto the first flight of the coarse ore conveying system.

This latter system, supplied by Krupp Canada, incorporates three 1,800 mm wide, downhill regenerative flights, for a total length of 12,700 m and with a total vertical drop of 1,300 m. The conveyors run at 6.0 m/s and can handle minus 200mm coarse crushed ore at an initial design capacity of 5,800 t/h (corresponding to a process plant capacity of 85,500 t/d). The system's capacity can be upgraded to meet the future 127,500 t/ d plant throughput requirements by simply adding additional drive components.

Siemens of Germany supplied the electrical equipment for the regenerative conveyor system and secured an initial three-year maintenance contract, for a total of $30 million. The ten 2.5 MW motors that operate the system have an electronic control system which allows the speed and torque of the motors to be controlled by adjusting the AC supply. The frequency convertors allow a range of 0-120 rev/min. In any conveyor system, this offers a number of advantages, including much smoother start-up and stopping, and therefore lower maintenance costs and the ability to meet the same safety factors with lower-specification components; and the optimisation of the running speed and electrical load whatever the material flow onto the belt, saving up to about 20% in energy.

In the case of Los Pelambres, the gradient also means that as the frequency convertors control the downward movement of the ore, they act as generators, producing up to 25MW of electricity which is fed into the national grid. Minera Los Pelambres calculates that the recycled energy will total an average of around 56 GWh annually at the mine's initial full-production rate of 85,000 t/d of ore. The system only uses power to start, with both normal running and stopping being generative. The system does have disc brakes, but they are for use in emergency. To put the cost saving into perspective, Minera Los Pelambres calculates that the energy saved is roughly equivalent to that used by the project in its open pit (electric shovels and drills) and at its port facility.

The last flight of the coarse ore conveying system discharges directly, via a moveable discharge head, to the concentrator coarse ore stockpile. Coarse ore is reclaimed through two concrete reclaim tunnels, each of which includes four reclaim hoppers and four reclaim belt feeders. A third reclaim tunnel will be provided for possible future expansion. The belt feeders are rated at a maximum capacity of 1,220 t/h and each set of four feeders discharges directly onto a SAG mill feed conveyor. Each reclaim tunnel incorporates a grinding ball hopper and feeder which discharges grinding media directly onto the SAG mill feed conveyor.

The grinding facility includes two parallel grinding circuits, each incorporating a 10.98 m diameter by 5.18m EGL (equivalent grinding length) SAG mill with 12.68 MW gearless drive, and two 6.4m diameter by 10.2m EGL ball mills, each driven by a 7.1 MW direct coupled, synchronous drive motor. The combined design throughput of the two grinding circuits is 3,850 t/h. Pennsylvania-based FFE Mineral Corp. has secured a three year maintenance contract, worth $13 million, for the mine's concentrator.

Each SAG mill is equipped with a discharge trommel screen and is designed to operate in conjunction with two mills. The SAG mill discharge flows to a common, two compartment sump, and the slurry is pumped to a cyclone cluster in closed circuit with the ball mills. Cyclone overflow reports to the Cu-Mo bulk flotation circuit, whilst the underfiow returns to the ball mills.

Advanced control

The Siemens equipment for the regenerative conveyor system was provided by the company's industrial projects and technical services division (ATD). The division also supplied other equipment at Los Pelambres including the two gearless drives to power the processing plant's SAG mills. The motors are integral to the mills, with the armature built around the mill body, so there is no need for a mechanical transmission from motor to mill with inherent transmission losses. The clearance between armature (rotating mill) and stator is constantly monitored, and the mill is stopped automatically if it falls below 2 mm at any point. Robert Hardt, vice president - mining for Siemens' ATD division, explained to MM that the system also allows the speed of the mill to be varied in the range 0-9.55 rpm, according to ore characteristics and throughput rate, unlike a conventional fixed-speed unit in which the throughput rate must be varied according to ore characteristics.

Some 10,480 [m.sup.3]/h of cyclone overflow reports to the flotation circuit which comprises four rows of nine 128 [m.sup.3] capacity rougher cells and two rows of six 128 [m.sup.3] cleaner scavenger cells. Flotation rougher and cleaner scavenger concentrates are gravity fed to a common regrind cyclone feed sump and pumped to a regrind cyclone cluster. The cyclone overflow feeds eight, 4.0 m diameter by 14 m high rougher cleaner column cells, whilst the cyclone underflow reports to two 750 kW regrind tower mills in closed circuit. Rougher cleaner concentrate from the column flotation cells is reground in a 370kW tower mill in closed circuit before being fed to two 4 m diameter by 14 m high recleaner column cells for final cleaning of the Cu-Mo concentrates.

The Cu-Mo concentrate from the rougher cleaner cells is thickened in a 42.7 m diameter concrete thickener and the thickened underflow pumped to two agitated storage tanks ahead of the moly plant which is designed to treat 2,386 t/d of concentrates. The conditioned bulk concentrate is fed to two rows of nine 8 [m.sup.3] nitrogen fed, rougher cells.

The moly rougher concentrate, meanwhile, is cleaned in a row of seven 9 [m.sup.3] nitrogensparged primary cleaner cells. The primary cleaner concentrate is thickened in 22.8 m diameter thickener, reground in a 1,000 kW vertical mill and pumped to the final cleaning stages. Here the concentrate is cleaned and recleaned in three additional column cell cleaning stages, the fourth cleaner concentrate being discharged to a 15.2 m diameter thickener for dewatering. This concentrate is filtered and leached so that the copper content of the concentrate is reduced to the final product specifications. Filter cake is batch leached in pressure reactors, filtered and conveyed to three holding bins. A system of screw conveyors is used to load the moly concentrate into drums or bags.

The rougher tailings from the and the final copper concentrate is pumped to a 42.7 m diameter concrete copper concentrate thickener from where the underflow is pumped to the concentrate pipeline pump house. The 120 km long, 178 mm nominal diameter, carbon steel slurry pipeline lined with HDPE transports the copper concentrate slurry from the concentrator to a filter plant located at the port site near Los Vilos. Pipeline Systems Inc. (PSI), working with local partner JRI, commissioned the concentrate slurry pipeline system on November 19, last year. The commissioning programme, which included operator training, was completed on schedule in mid-January this year.

The concentrate slurry is pumped from two, 11 in diameter by 11 m high agitated storage tanks located at the concentrator to the main line pump house, via a slurry test loop. The main line pump station incorporates a centrifugal pump for pumping slurry through the pipeline, and a positive displacement pump which is brought on line if pipeline blockage occurs or when then the pipeline is required to transport more than 110 t/h of concentrates after future expansion. The concentrate discharges into four 11 m diameter by 11 m high agitated storage tanks at the filter plant.

The total tailings feed of just over 11,000 [m.sup.3]h flows to two 128 m diameter tailings thickeners, the overflow being pumped to the process water tanks for recycle to the process circuits. The underflow, meanwhile, is routed by 8,500 m long launder to the Quillayes impoundment area at the Cuncumen River valley.

The tailings impoundment is designed to hold some 912 Mt of residues in three dams - Quillayes, Quebrada Seca and Las Lajas. The first dam was constructed in the Cuncumen River valley and the second two dams will be constructed in the Manque River valley. The diluted tailings are fed to two clusters of cyclones to produce a coarse (sand) fraction suitable for downstream dam construction which will reach an ultimate height of 175 m. The fine (slimes) fraction reporting to the cyclone overflow is stored behind the dam walls. Any seepage from the impoundment is intercepted by a cut-off trench and pumped back into the impoundment.

El Tesoro financing secured

Smaller than Los Pelambres, but nonetheless still an attractive opportunity for Antofagasta, the El Tesoro development has stalled over the past eighteen months or so largely due to the prevailing weak copper market which several times postponed the finalisation of the debt funding for the project. Financing was originally scheduled for April 1998, but was delayed to a target of June 1999, pushing the planned commissioning date for the project back from May 2000 to January 2001. However, its was not until November last year, that Antofagasta and Equatorial Mining announced that their joint venture company, Compania Contractual Minera El Tesoro (CCMET) had signed definitive agreements for $300 million worth of funding for El Tesoro, of which, $205 million will be in the form of project debt finance. The balance will come in the form of equity from Equatorial, a subsidiary of the Australian financial services group AMP Ltd (Australia's largest fund management group), and Antofagasta. Antofagasta now plans the first cathode production from the project in the March quarter of 2001, following an 18-month construction period.

The Anglo-Norwegian construction group Kvaerner has been awarded a US$170 million turn-key contract for the bulk of the project's construction. Project work will also include the provision of power and water supplies, and access roads. With a predicted copper recovery approaching 70%, El Tesoro is expected to produce 75,000 t/y of high-grade cathode copper over an 18-year life, based on a mineable reserve amounting to 153 Mt, at a grade of 0.96% Cu. Cash operating costs are forecast at US$0.39-0.41/lb over the first five years of operation. Kvaerner completed the feasibility study and basic engineering studies for the project in October 1997. Detailed engineering studies continued into 1998 and provided the basis for the lump sum turn-key price.

The proposed El Tesoro operation comprises an open-pit mine, crushing plant, heap leach and solvent extraction-electrowinning facilities, along with associated infrastructure. According to Kvaerner, during the early stages of the operation the hydrometallurgical plant is designed for a constant production rate of 75,000 t/y of copper to cope with varying ore grades and mine production rates. The crushing and agglomeration plant is designed to process a variable amount of ore when required.

The leach process is designed to treat crushed and agglomerated oxide ore by sulphuric acid leaching in 'dynamic' on-off heaps followed by solvent extraction and copper recovery. The lixiviant is recycled raffinate from the 2,100 [m.sup.3]/h, two-train solvent extraction facility. According to Kvaerner, potential leaching problems caused by the high amounts of clay and friability of the ore will be mitigated by designing low-lift heaps and using spreader-stack systems for heap construction.

Among the copper giants

The El Tesoro copper oxide deposit occurs on a regional geological trend that hosts some of the world's largest copper mines and prospects, including Codelco's Chuquicamata and Radomiro Tomic operations, Cyprus/Codelco's El Abra mine and BHP's Escondida mine. The first mineral exploration of the El Tesoro project area dates back to 1942-1948 when the Sorpresa and Leonor mining claims were filed. In 1950, a majority interest of Sorpresa was acquired by Orengo Mines (part of the Luksic Group) and, six years later, it established the larger Santa Carmen property which included the original Sorpresa claim and covered a wider zone around the Leonor property. During the 1960s, underground mining commenced at Santa Carmen, with the ore sold on to Chuquicamata to be processed. Then, in the 1980s, Leonor was purchased by Minera Cobrenorte Ltda and underground mine development began, together with the construction of a small plant producing cement copper. In 1991, Equatorial Mining executed an option to purchase the L eonor property, establishing Compania Contractual Minera Leonor (CCML).

Between 1990 and 1995, both Anaconda Chile and CCML conducted numerous exploration programmes on their respective properties before a joint venture between Equatorial and the Luksic Group was established in December 1995 allowing joint development of the Leonor and adjacent Sorpresa/Santa Carmen properties. This joint venture became collectively referred to as the El Tesoro project.

The El Tesoro deposit is located in an Upper Tertiary (Miocene) sedimentary basin containing several layers of partially lithified polymictic gravels which cover an extensive area west of the mineral deposit. Towards the east, meanwhile, the sedimentary basin is bounded by a ridge of andesite and tuff containing dacitic to andesitic domes of Lower Tertiary (Eocene) age.

The volcanics and sub-volcanics are intruded south of El Tesoro by Tertiary quartz porphyries and granodiorites. Further east, Upper Jurassic calcareous rocks outcrop and overlie grey Paleozoic andesite. Hydrothermal alteration zones have been recognised in the area, possibly associated with Upper Tertiary intrusive activity.

El Tesoro, like the Mina Sur deposit at Chuquicamata, the Cascada deposit east of Iquique and the Huinquintipa deposit at Collahuasi, was formed by the dissolution of a detached porphyry copper deposit. These types of deposits are characterised by the occurrence of water and acid soluble minerals in what is an otherwise barren host rock.

The deposit consists of secondary copper mineralisation, principally silicates (chrysocollas), chlorides (atacamite) and, to a lesser degree, compounds of copper manganese known as copper wad (which forms the so-called insoluble copper). The mineralisation occurs as cement and fracture coatings within a 300 m thick series of poorly sorted beds and cross layered units that have a high clay content and are interbedded with well sorted gravels and conglomerates. There are practically no copper sulphides present.

Extensive exploration

The 150 m thick, lenticular shaped El Tesoro orebody has been extensively explored. Since 1981, there have been three major drilling programmes and additional detailed studies of the underground workings, shafts and pits, as well as numerous geological, geophysical and other reconnaissance programmes. This work, coupled with the extensive sampling and assaying of over 33,000 samples from 368 vertical reverse air circulation drill holes, has provided a comprehensive understanding of the mineralisation, and has established sufficient geological resources and mineable reserves for a mine operating in excess of 18 years at an annual rate approaching 9 Mt of ore. The mine plan, prepared by CCMET and Kvaerner, is based upon total mineable reserves of almost 153 Mt grading 0.96% Cu, of which, 98.6% are classified as proven and probable reserves.

The feasibility study considers the mining of more than 482 Mt of combined waste and ore over the mine life, preceded by a four month pre-production development period that will remove 3.5 Mt of waste and stockpile 1.8 Mt of relatively high grade (1.196% Cu) ore to supplement the production programme of 5.6 Mt of ore grading 1.37% Cu during the first year of operation. The average strip ratio over the mine life is 2.03:1 (waste:ore). Given the shape and depth of the deposit, the mine plan is also based on eight different operating phases, resulting in a production programme oriented towards exploiting the highest grades at the start of the operation and delaying the mining of the lower grades as long as possible. Total production over the life of the mine is expected to be 1.02 Mt of copper.

The design criteria for the processing plant that will treat the oxide ore at El Tesoro has been based upon considerable metallurgical studies, many of which have been undertaken at the pilot plant constructed on site. Over 115 simulated heap leaching tests on some 4,400 t of material have been carried out, producing 30 t of high quality copper cathodes.

The process developed for El Tesoro involves crushing, agglomeration and dynamic (on-off) heap leaching, involving irrigation with the raffinate obtained from solvent extraction. After the solvent extraction, which will take place in conventional mixing-settling tanks, the rich electrolyte will be filtered, warmed and then undergo and electro-deposition process to recover LME grade A copper cathodes.

The deposit will be developed as a conventional open pit operation, with a fleet of 177 t haul trucks, loaded by 17.6 [m.sup.3] front end loaders, transporting the ore to the crushing plant. Most of the copper mineralisation occurs in the finer fractions of the ore which will be crushed in a 25,000 t/d, three-stage open circuit. Discharge from a 1,270 mm x 1,651 mm primary gyratory crusher, supplied by Svedala, will be stored in a 6,800 t capacity reserve stockpile, before being further reduced by two secondary crushers and four tertiary units. The products of the scalping screens, tertiary screens and tertiary crusher make up the final product of 95% minus 12.7 mm material. This final product will be conveyed to a hopper feeding two inclined and rotating agglomeration drums where an agglomeration agent (Nalco 9670), concentrated sulphuric acid (20 kg/t) and process water are added to adequately bind the crushed ore.

A belt conveyor and mobile stacker will deliver the agglomerated ore to the leach pad, which is made up of 92 irrigation units for an area of 6,667 m2, set in two parallel rows. Ore will be stacked in 2.5 m high heaps on liners and raffinate irrigation will be undertaken with sprinklers installed on a grid placed over the heaps. Pregnant leach solution (PLS) will be channelled into collection ditches before gravitating to the pregnant solution pond where it will remain for a maximum of 18 h.

Once the 75 day leaching cycle has elapsed (40 days of intermittent watering and 35 days of rest), the leached ore will be removed in a continuous cycle and carried by conveyor to the tailings dump where it will be distributed by a mobile stacker.

Performance flexibility

There may be some flexibility for enhanced leaching performance, according to Antofagasta. For example, if the mining operations experience difficulty in consistently controlling the ore grade there are still several options to smooth potential grade variations of the PLS or the recovery grade, including:

* Varying acid additions to the agglomeration process to control the leaching kinetics. Both laboratory and pilot plant tests demonstrated that total copper recoveries and leaching kinetics improved with higher acid to copper ratios;

* Increase the number of days that the cells are under active irrigation. The practice of alternating active irrigation days has not been optimised and the actual active leaching cycle can probably be reduced;

* The leaching process will continue in the spent ore and, since the spent ore storage area is also lined, additional pregnant leach solution could also be recovered if CCMET chose to irrigate portions of this area.

The solvent extraction plant will incorporate two lines, comprising two extraction units, one washing unit and two stripping (re-extraction) units, and will upgrade the PLS from around 5 g/l to a strong electrolyte grading around 50 g/l. The projected recovery for the SX-EW plant is estimated at 90% with a treatment capacity of 1,047 [m.sup.3]/h of solution per line.

The rich electrolyte produced at the stripping stage of the SX process will initially undergo a filtration stage through garnet-anthracite dual filters, and will then be heated over heat exchangers before finally undergoing a two-stage electrowinning process. Two circuits, comprising 284 cells, will be utilised for this process which, at a current density of 250 A/[m.sup.2], permits a production in excess of 75,000 t/y of LME A grade copper cathodes. Daily harvesting of the final product will be undertaken using two bridge cranes and one automatic stripping machine able to produce more than 500 cathodes per hour. It is believed that by increasing the current density to 270 A/[m.sup.2] (the tankhouse electrical system can deliver up to 300 A/[m.sup.2]), the electrowinning plant could be operated up to around 85,000 t/y of cathodes.

Located in one of the most intensively developed copper mining regions in the world, El Tesoro benefits from a well established infrastructure base, including established contractors, services and con sumables suppliers. Furthermore, the project will benefit greatly from the experience Minera Michilla (a 74%-owned subsidiary of Antofagasta) has gained in operating several open heap leaching operations throughout the Antofagasta province of Chile, and the information available from previous underground room and pillar mining, and small scale surface excavations at the shallow eastern sub-outcrop of El Tesoro.

IT linkages for optimisatian

The optimisation of operating performance is central to the approach of Siemens' control systems, which draw heavily on the company's strength in information technology. The latter offers the potential to collect and analyse data throughout a production process, and thus to optimise operating conditions through a linked control system. Mr Hardt said that, in contrast, around ten years ago, monitoring and control systems for the various plant in a process were independent. The advantages of linking plant can be applied to the planning and implementation of maintenance. Modern communications technology also allows remote diagnostics, although the need for engineering support is backed in Siemens case by 2,600 plant-maintenance staff, out of Siemens ATD's total of about 29,000 in some 300 locations.

Gunter Menden, vice president - plant maintenance arm for Siemens ATD, believes that maintenance is undergoing three major changes in basic approach. First, preventative maintenance is moving away from the 'mean time before failure' approach to being driven by the critical requirements of the process. Second, the costing of maintenance is becoming more sophisticated, moving from a simple cost, for example per month, to costing in terms of units of production. This inherently takes into account the productivity and downtime benefits from maintenance that may appear more costly at first sight. And, thirdly, there is a growing acceptance of extending maintenance arrangements to cover equipment made by other manufacturers.
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Author:Clifford, Des
Publication:Mining Magazine
Article Type:Brief Article
Geographic Code:3CHIL
Date:Apr 1, 2000
Words:4470
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