Canadian Mining Journal

Feature

Antamina blows your mind

You may already have heard that Antamina in north-central Peru is the world's largest combined copper/zinc mine, owned by a consortium of major mining companies--Noranda Inc. (33.75%), BHP Billiton (3...


You may already have heard that Antamina in north-central Peru is the world’s largest combined copper/zinc mine, owned by a consortium of major mining companies–Noranda Inc. (33.75%), BHP Billiton (33.75%), Teck Cominco Ltd. (22.5%) and Mitsubishi Corp. (10%). You may know that, at 4,300 m altitude, it is one of the world’s highest mines and has already reached commercial production, four months ahead of schedule. With a capital cost of US$2.3 billion is has been the world’s most expensive mining development and its US$1.32-billion project financing is the largest financing ever for a greenfield mining project. You may even know that, at US$0.30 per pound of copper (net of byproduct credits), Antamina will have operating costs in the lowest quartile of global producers.

But did you know that the town of Huaraz, three hours drive from the mine, is a mecca for mountain climbers as it is surrounded by peaks including Huascarn, Peru’s tallest mountain, stretching above 6,700 m altitude? Huaraz is right by the ancient Inca Trail. “There are archeological sites around here that just blow your mind,” says Ron Simkus, president and CEO of Compaa Minera Antamina S.A. (CMA), the operating company of the mine.

Simkus is enthusiastic about more than the mountain treks. He has been on the site only since May 2001 when he arrived after 25 years of almost continuous service within the Cominco family, including a recent stint as president and CEO of the Highland Valley mine in Logan Lake, B.C.

Geology

The Antamina polymetallic deposit is skarn-hosted, unusual in its persistent mineralization and predictable zonation. It has a SW-NE strike length of more than 2,500 m, and is up to 1,000 m wide. Most of the resources are in the northern half of the deposit, around the south shore of Lago Antamina. The deposit is located mainly between elevation 4,350 and 3,790 m, but crops out as high as 4,650 m elevation and has been drilled as deep as 3,632 m elevation.

The skarn is generally zoned around a central monzonite porphyry intrusion. First is brown garnet skarn with chalcopyrite. Green garnet skarn with chalcopyrite and sphalerite is next. Wollastonite-diopside-green garnet skarn with bornite and sphalerite occurs on the south margin of the green garnet skarn. Silty limestone, variously hornfelsed and marbleized, occurs around the garnet mineralization. It shows veins, veinlets and lenses of galena with minor pyrite, chalcopyrite, sphalerite and fluorite.

Chalcopyrite accounts for over 90% and bornite for less than 10% of copper in the deposit. Pyrite and magnetite are common throughout. The central intrusives are mineralized with molybdenite. The stated reserves and resources increased 13% in December 2000, and are shown in the table.

Mining

Antamina is a conventional open pit mine. The mine will operate over a 26-year life at a rate of 70,000 tonnes of ore and 146,000 tonnes of waste per day, but this rate may be increased. Pre-production development included mining about 107 million tonnes of material.

The pit is using large equipment to reduce the unit production costs. The main production drills, Bucyrus International 49RIIIs,

drill 31.1-cm-diameter blastholes. The ore is blasted using ANFO and emulsion at a powder factor of 0.23 kg/tonne. The benches are 15 m deep, and the ultimate pit wall slope will be between 43* and 57* from horizontal.

The BI 495 rope shovels, with an 80-tonne bucket capacity, can fill the 232-tonne Caterpillar 793C trucks in just three passes. The heavy diesel equipment suffers in the thin air at high altitude. The engines have to be turbocharged, and a special blend of fuel is being used to reduce carbon build-up.

An in-pit 150-cm x 226-cm gyratory crusher reduces the ore to -15 cm. It is stored on the pit floor in stockpiles of up to 3 million tonnes, divided into six types: chalcopyrite copper ore with low or high bismuth (48%); chalcopyrite copper-zinc ore with low or high bismuth (42%); and bornite copper ore with low or high zinc (10%). Material between 0.7 and 1.0% Cu equivalent, is being stockpiled for processing at the end of mine life.

Mineral Processing

Because of its variability, the ore is run in seven- to 20-day campaigns. A 1.8-m-wide conveyor moves the crushed ore about 3 km from the pit to the Yanancancha concentrator on the opposite side of the mountain; 2.7 km of the route is through a tunnel. The employees’ modular camp accommodations, built by Atco, are also located at Yanancancha.

The grind size is relatively coarse at 100 m for zinc ore and 150 m for copper-only ore. Grinding consists of a single closed circuit SAG mill without pebble crushing, followed by three ball mills in parallel. The mills are from FFE Minerals Canada, and Technequip supplied the hydrocyclones.

The deposit is amenable to conventional flotation as the base metal sulphides are coarse grained and the iron sulphide content is low.

The ball mill discharge enters three rows of seven copper (bulk) flotation rougher cells. The overflow goes to the zinc flotation side. The underflow follows the copper side, passing through five cleaner scavenger tank cells and two stages of cleaners, plus regrind in two vertical grinding mills. Minnovex supplied the column flotation cells. The bulk concentrate product (30% Cu and up to 3% Zn) is thickened and then retreated in a reverse flotation circuit to recover a separate molybdenum concentrate and/or to remove a bismuth/lead concentrate. The copper concentrate slurry, at 42% water, is ready for shipment via pipeline.

The slurry entering the zinc side is conditioned and then passes through a similar route of roughers, cleaners and scavengers, with regrind. The zinc concentrate (54% Zn and up to 1.8% Cu) is thickened for transport via pipeline. Reject from the zinc roughers forms the final tails.

In its first 10 years, the plant will produce 910,000 tonnes of chalcopyrite copper concentrate and 490,000 tonnes of zinc concentrate each year, plus smaller amounts of bornite copper concentrate and molybdenum concentrate. The expected metal recoveries are 89-95% for copper, 80-86% for zinc, 65-90% for silver and 65-70% for molybdenum.

Says Ron Simkus: “The mill is running exceptionally well. Within the first 60 days of operation we achieved design tonnage and design metallurgy. We’ve been having the usual commissioning problems, and we’re taking them on one at a time.”

In late August and early September, the plant averaged about 88,000 tonnes throughput per day. However, the surface material that is being milled now is quite soft and goes through the mill quite easily, although it still carries 1.3% Cu. Harder millfeed is not expected until late next year.

“This gives us an opportunity to test the downstream parts of operation,” says Simkus. “The flotation, the pipeline and the filtering systems are all being pushed to the limit. Even with the high volumes, we’re averaging over 90% copper recovery, and the pipeline and filter plant are handling it quite well.”

Tailings Disposal

The tailings disposal facility is in the Ayash valley close to the Yanancancha plant, and is large enough to store all the tails during the expected mine life. Its 230-m-tall impermeable dam wall is the highest rock-filled dam in the world.

Because the tailings are rich in suphides, they could be acid-generating. Therefore, disposal will be subaqueous until beaches are established. Most of the disposal will eventually be subaerial from the beaches.

About 80-90% of the water used in the concentrator is recycled, including water reclaimed from the tailings facility. Diversion ditches constructed around the facility minimize natural water inflow. In average years, there will be little or no water discharge during the dry season. However, precipitation in the wet season is high, about 155 cm annually, so excess water is discharged through a decant structure. If required, the decant can be shut off and the water in the pond treated. Seepage from the tailings facility
can be collected in a downstream pond and pumped back to the tailings pond if needed to maintain downstream water quality.

According to Steven D. Botts, CMA’s vice-president of environment, health and safety, the site is currently reviewing two concepts for tailing reclamation–dry and wet–to determine which would carry the least long-term environmental risk.

Product Transportation

The original plan was to truck the concentrate from the Yanancancha plant to the coast, but CMA scrapped the plan, as the amount of traffic would pose unacceptable risks to the towns along the route. Now, only the molybdenum concentrate will travel by truck, and will go to the port of Callao.

Most of the concentrate is carried in a 25-cm-diameter pipeline built by Pipeline Systems Inc. using state-of-the-art technology and materials. The concentrate slurry travels 302 km from plant to the port site of Punta Lobitos along the coast of Huarmey, a drop of 4,300 m. It averages 277 m3/hour ( i.e., almost 7.2 km/hour), taking about 34 hours. Like the plant, the pipeline operates in batch mode, with the different concentrates separated by a 60-minute water buffer.

Due to the high risk of seismic activity, the pipeline is equipped with pressure-monitoring, automatic high pressure valves that will open if the pipeline is over-pressure, and emergency ponds to contain slurry spills.

The facilites at Punta Lobitos were constructed under an EPC contract with Sandwell International Inc. Concentrates are dewatered to 8% moisture using clarifiers and filter presses, and stored in stockpiles within a covered storage shed. The removed water is treated before release to a plantation, where mainly eucalyptus trees are being grown as a method of exfiltrating the water.

“When we shift [from one concentrate to another] at the mill we also have to watch our inventory at the port,” says Simkus. “The shed holds about 160,000 tonnes and we’re really amazed at how fast we can fill that thing up. So the marketing people are always dancing on coals, making sure that the ships are scheduled in a timely fashion in the right order. The other thing we always have to watch is the size of the ships, because certain ships don’t fit in certain harbours.”

About 1.5 million tonnes of concentrate will pass through this port each year, to be loaded onto ships as large as 50,000 tonnes, at the rate of about two per week.

Antamina reserves and resources

Tonnage (million tonnes) Cu (%) Zn (%) Ag (g/tonne) Mo (%) Equiv. Cu (%)
Measured plus indicated reserves 561 1.23 1.03 13.71 0.029 1.7
Total resource 665 1.1 1.0 13 0.03 1.5
Cutoff grade is 0.7% Cu equivalent

CMJ caught up with Ron Simkus, president and CEO of Compaa Minera Antamina, by telephone at his home in Huaraz in early September. He describes the mine that constitutes his latest “experience and challenge”.

CMJ: How is Antamina protected from seismic activity?

Simkus: The engineering design criteria throughout construction contemplated potential seismic events. The largest single asset, the mill, has a design capable of resisting a Class 4 earthquake and is located to avoid possible earth movements.

CMJ: What are the accommodations for the high altitude at Antamina?

Simkus: We keep close medical records of everybody. Visitors are tested prior to coming up and then they’re evaluated upon arrival. We check for blood oxygen and blood pressure just to make sure that there’s no risk.

We have an acute awareness, like most high-altitude mines, to the presence of lightening. If we have storms coming we can get very high potentials and so, if we sense that the potential’s high, we ask people to stand down and wait until the storm passes–actually come down from the mine. It’s kind of rare, but we have methods for detecting it if it’s coming.

CMJ: Is road safety a problem at Antamina?

Simkus: One of the things that we’re trying to focus on is fatigue. Most of our road accidents are caused by people falling asleep at the wheel, usually in pickup trucks on the highway or heading home. We’re asking people to take a more disciplined regimen for sleep and diet. We have a policy that people don’t drive at night. At this elevation, you’re a little oxygen-deprived already so sometimes you don’t feel it sneaking up on you but you’re pretty tired.

CMJ: What are the main challenges in the processing?

Simkus: The orebody is a skarn. A skarn is an interesting cat. The minerals themselves are very coarse-grained. Normally if you test the different ore types individually, they perform beautifully in the laboratory. The complexity is in the mining, because a skarn is extremely variable. It has simple components, but the components are scattered all over the pit. When you’re mining, you can be cutting across two or three drastically different ore types in the same face within a few hundred metres of each other.

The mine and the mill have a very co-ordinated effort to identify where the different ore types are. The mine actually builds stockpiles to try to create discretely different ore types. We treat them in campaigns. We actually mill one type of ore for a period of time and then switch to another. The changes are really quite dramatic. Some of the ore is almost exclusively copper, some of it is a mixture of copper and zinc, and some of it is very high in zinc. So your balance in the mill has always got to be shifted around to make it work.

We’ve done a number of transitions already and we were really quite surprised that we can do it cleanly, and actually make the transition without totally disrupting the plant. We’re going to be going heavy into transitions in the next six months, essentially shifting between copper and zinc ores to keep the pit in balance. We don’t want to be favouring one ore type over another and have the mine essentially paint itself into a corner. When they build their stockpiles they’ve got to get them out of the way, otherwise they’re kind of blocked in.

CMJ: To what extent is Antamina Canadian? Are there many Canadian employees or contractors on site?

Simkus: Antamina is a Peruvian company formed by shareholders who are important international operators with main offices in Canada, Australia and Japan. During the construction phase, which lasted until May 2001, only 5% of the work force was foreign, including Canadian contractors and technical personnel.

Our organization is basically Peruvian. Key positions in the administration and operations areas are managed efficiently by senior Peruvian staff, and more than 31% of the current workforce is made up of people from the surrounding communities.

CMJ: How much of the ongoing operation will be handled by contractors?

Simkus: Almost none of it. We have a fixed workforce that does virtually all of the work The biggest contracts that we have are the catering contractors that run the food service, security, and the transportation of our employees from Huaraz and Lima up to the mine.

We have a fairly small complement of expatriates here. We only have about 60 out of a workforce of about 1,200. About half of them come from the shareholder companies. About half of them come from places like Batu Hijau in Indonesia and Alumbrera in Argentina–the usual group of experts that move from project to project.

One thing that’s really gratifying is that the Peruvian workers that we have are excellent. There’s a lot of mining in Peru, so we’ve been able to acquire quite a few experienced people, particularly on the milling side. We have a mill manager, Augusto Chung, who came to us from Southern Peru Copper. He’s an
excellent metallurgist and mill manager. Within the mill itself, a lot of our operators are graduate engineers, so from a technical basis these guys are really quite sharp as well as enthusiastic and really quite motivated.

The Peruvian trades people have been a remarkable success. Everybody, including the Bechtel Engineering people, were really complimentary about the quality of workmanship you get from the Peruvian trades people.

CMJ: What would be an average wage for a miner at Antamina?

Workers at Antamina receive wages that are competitive with the rest of the Peruvian mining industry. The wage of a typical shovel operator is approximately US$15,000 per year. They also receive health and dental programs, life insurance as well as education subsidies, and will soon participate in profit-sharing.

CMJ: How many families were resettled away from the Antamina site?

Simkus: Fifty-six nuclear families and individuals were resettled. I’ve met some of the relocated families near the town of San Marcos where most of them moved to.

Our resettlement plan was based on World Bank guidelines. It was designed to minimize the disruption in the lives of the people being resettled and replicate their way of life, while improving their living conditions as well as health and education facilities.

When families are relocated and compensated, some are really disciplined and know how to invest their money and look after themselves, and others can be duped by people who want to try to get their money from them. Generally the majority of them have done well. We continue to monitor these people to ensure that they are integrating into their new communities.

CMJ: What will be the impact of Antamina on the Ancash region and the country of Peru?

Simkus: For the Ancash region, Antamina will represent an additional increase in the GDP [gross domestic product] of approximately 40%. It is important to note that, according to present legislation, a percentage of all taxes and contributions to be paid by the company will be delivered by the National Treasury to local governments of the region. Thus, these governments will have more resources for development projects.

Additionally, it is important to take into consideration that Antamina is located in a remote region, very far from the main Peruvian infrastructure. People from this area will also benefit thanks to the construction of Antamina’s facilities, such as the access road, transmission lines, telecommunications, education and health facilities, etc.

For Peru, Antamina represents an increase of 30% in the export of minerals, which constitute 50% of the Peruvian exports, and an increase of 1.8% in Peruvian GDP. Naturally, foreign currency revenue will be conditioned to the prices of the metals.

CMJ: Is there much local or NGO [non-governmental organization] opposition to the building or operation of Antamina or the port site? Is there strong local or government support?

Simkus: There are significant challenges with the development of a project the size and complexity of Antamina, not the least of which are the expectations of the local residents. Although some groups have lodged opposition to the development of the mine, many others have participated in a constructive, consultative process with Antamina and now stand in support of the project.

The company responded positively to the concerns raised on trucking and passage through the national park, and chose a more costly alternative–to build a pipeline around the park to the port.

Both the local and national governments are strongly supportive of the Antamina mine.

We recognize that we may not meet everyone’s expectations, but we are actively consulting with and engaging local communities in the development of social and economic programs that we hope will address their concerns and improve their communities.


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