Some of the world’s largest open-pit copper mining operations are heading underground with ambitious block cave production plans

By Steve Fiscor, Editor-in-Chief

The natural progression for an open-pit mine in a rich orebody that extends at depth is to eventually convert to some form of underground mining to extract the ore. During the near term, two open-pit mining icons will do just that. The world’s largest copper and gold mine, Freeport McMoRan Copper & Gold’s Grasberg mine in Papua, Indonesia, will reach its open-pit limits and head underground in 2016. Similarly, Codelco’s Chuquicamata mine, one of the largest open-pit copper mines in Chile will do the same in 2018.

While this natural order of progression is nothing new to metal miners, the size and scope of what is being planned today is breaking new ground. South Africa’s Palabora Mining was probably the most recent large copper miner to transition from open-pit to block cave. It was and remains a learning experience for the mine engineers and today they are quite proud of what they have achieved: a block cave operation that produces at an average rate of 30,000 metric tons per day (mt/d). Freeport and Codelco are considering 160,000 mt/d and 140,000 mt/d respectively.

What further complicates the situation is that Grasberg and Chuqui are located in remote regions with limited access to resources. The existing workforce will have to be retrained to bring these operations online quickly. It also means that the engineers will have to get it right the first time. The advantage they have is that both Freeport and Codelco have experience with block cave mining.

While six to eight years seems like a long time, replenishing these large sources for copper ore has serious implications for the parent companies and world markets. Codelco is in the midst of preparing a feasibility study of the engineering design work to properly plan the conversion process. Freeport’s Grasberg block cave development is underway and detailed design work is ongoing. They will rely on this advanced level of mining engineering to successfully launch the new underground era for copper mining worldwide.

Palabora Mining Set the Precedent
During the early 1990s, Palabora Mining embarked on a similar series of feasibility studies to convert its open-pit copper mine in the Limpopo Province to an underground mine. In 1996, it announced it would proceed with the development of an underground block cave mine with a production rate of 30,000 mt/d.

Palabora Mining’s engineering design work set a precedent for converting from open-pit to underground design; no other block cave mine has been put into as competent an orebody, according to the mine owners. The block height of the cave is also testing previous limits at 450 m in the center increasing up to 700 m on the periphery.

The construction and development of the underground mine was completed in October 2004, but the Palabora Mining underground project could not be considered a success until it finally averaged 30,000 mt/d for a month. This occurred for the first time in May 2005, giving Palabora’s block cave one of the fastest ramp-ups to full production in history.

The production footprint measures 650-m long and 200-m wide and consists of 20 production cross-cuts and 320 drawpoints. With the orebody’s coarse fragmentation, a high degree of secondary breaking activities is required to free hang-ups and reduce oversize chunks to keep ore flowing through the drawpoints.

The mine’s fleet of LHDs tip 3,000 buckets per day into four jaw crushers on the northern side of the mine. Ore is reduced to less than 220 mm and fed onto a high capacity conveyor system up to the shaft complex for hoisting to surface.

During 2009, the mine averaged more than 31,600 mt/d. More than 11.5 million mt were hoisted last year, a slight decrease over 2008 tonnages. The decrease was due to a failure on a winder drum. The ore grade averaged 0.67% copper.

Today, the performance from the cave continues to improve. Increased mobile equipment availability and a general awareness on the possible impact of the open-pit waste have compelled the underground team to focus closely on proper draw control principles. Other improvements include the use of two LHDs in one crosscut and a continued use of oversize reduction philosophy where a rock breaker and a loader operate in the same production crosscut simultaneously to improve productivity.

Chuquicamata Subterránea
After more than 100 years of production, Codelco’s Chuquicamata, one of the greatest open-pit copper mines, which is located in northern Chile, will deplete its surface minable reserves and move to underground production by 2018. This is a significant milestone for one of the world’s largest copper producers. Yielding 450,000 mt of copper in concentrate annually, Chuqui represents more than one-fourth of Codelco’s total copper production. From that perspective, it’s easy to understand why Codelco has been working diligently to make the transformation to the next phase, Chuquicamata Subterránea (underground) as smooth as possible. After nine years of preliminary work, Codelco has reached the fourth stage of a six-stage (17-year) timeline to bring this mine into production in 2018 and to go on to achieve full production of 140,000 mt/d by 2025.

The Chuqui Underground project (Chuqui UG) will offer Codelco at least 50 years of copper production. More than 1.7 billion mt of reserves with an average grade of 0.7% copper will be extracted from the underground mine. Copper production in concentrate is expected to total 320,000 mt/y. To mine the ore, Codelco will retrain the Chuqui workforce to transition from open-pit mining to underground mining. Around $2 billion will be invested in the project to mine the first ore.

Codelco has decided to pursue a block cave approach. Among other objectives, Codelco also expects Chuquicamata underground production costs to come in between the lowest and second-lowest quartiles among world copper producers.

“We are now starting the basic engineering stage,” said Sergio Olavarria, engineering director, Chuquicmata Subterránea mine project, Codelco. “We plan to have the infrastructure design work completed by the end of the year. In 2011, construction will begin on what will be one of the largest, safest and most efficient underground mines in the world.”

At 140,000 mt/d, the annual extraction rate will be 65 million mt. Chuqui UG will require two shafts between 11- and 12-m in diameter for development extraction and ventilation. The orebody will be mined in four 215-m lifts during a 42-year mine life. The first lift, Level 1, will begin at an elevation of 1,840 m. The cave will consist of production blocks as large as 36,000 m2. Four years before the first level is depleted, they will begin to prepare the next level. Final production is in approximately 2060. At that point the mine will reach a total depth of nearly 790 m.

During the caving process, they plan to pre-condition the current area of mining using a hydro-fracturing technique. “We will break the rock mass before the caving process begins,” Olavarria said. “We will use water for hydro fracturing and other holes for explosives. We think this approach will work well for the geology, which is similar to that of Palabora.” In addition to guiding the cave and managing dilution, Olavarria explained, it might also reduce the potential for air blasts.

The mine would use 30 6.9-m3 LHDs to move ore from as many as 12 production crosscuts to an orepass feeding a crusher installation. The engineers are not committing to remote operation yet. They are looking at their experience at El Teniente, before they commit to any type of automation.

Crushed ore would travel by belt haulage to an underground storage silo, which meters ore onto a main conveyor belt. The ramp for the main belt is 7-km long, which conveys ore to the surface. “The current plan for the main belt is three flights with two transfer stations, but we are also studying the possibility of using one transfer stage,” Olavarria said. “Because the conveyor would move material more than 1,000 m vertically up a 15% slope, it would consume 60 mega-
watts—making it one of the highest power-consuming conveyors.”

Chuqui UG will employ 3,000 to 3,500 miners. The training approach for miners will consist of 80% field training and 20% classroom. Codelco expects the entire training process for both new workers and converting existing workers from the open-pit to take about three years to complete.

Block Caving at Freeport Indonesia
When the Grasberg open-pit mine reaches final production, Freeport Indonesia plans to have the skill and infrastructure in place to launch the Grasberg block cave mine and ramp production up to 160,000 mt/d. The mine is under development and scheduled for production in 2016.

Underground mining is nothing new to Freeport Indonesia. For more than 30 years, it has mined ore from the East Ertsberg Skarn System (EESS) by block caving methods through a systematic set of lifts. The total remaining EESS reserves are more than 775 million mt, which will yield 11 billion lb of payable copper and 13 million oz of payable gold.

The Gunung Bijih Timor (GBT) mines first worked the deposit from 1980-1994 and during that period the mines, which were initially designed for 5,000 mt/d, peaked at 30,000 mt/d. From1994-2003, the Intermediate Ore Zone (IOZ) mine worked the next lift and similarly its design capacity of 10,000 mt/d  exceeded initial plan and peaked at 19,000 mt/d. The IOZ mine encountered and had to learn how to cope with wet muck, a combination of water and fines that resulted in occasional large flows of mud in the production areas. Wet muck is a safety hazard, is difficult to handle and results in less productive mining. To overcome this challenge, the IOZ mine introduced tele-remote mining with LHDs.

Today, the Deep Ore Zone (DOZ) mine works the current lift. Considered one of the largest caving mines in the world, it was originally designed at 25,000 mt/d. As additional ore was discovered at the DOZ the planned production rate underwent a series of expansions from 25,000 mt/d to a final sustained production of 80,000 mt/d, which was achieved in February 2010. Freeport plans to sustain a production level of more than 80,000 mt/d through at least 2015. The DOZ mine has already produced 112 million mt since it opened in 2003 and it will produce an additional 282 million mt through 2020.

The DOZ mine, currently working at the 3,125 m level, inherited the wet muck conditions from the IOZ mine. In fact, of the 490 active drawpoints, 20% are classified as wet. The DOZ mine has also improved underground construction standards. One initiative in the DOZ is the use of an engineered, boltable linter set in the drawpoints, which requires less concrete.

Mining the DMLZ Ore Body
The DMLZ will be the deepest and highest stress block cave mine in the Ertsberg District to date. The reserve is located below the DOZ mine, between the 3,125- and 2,590-m elevations. Development operations commenced in November 2008, with the start of a rail spur leading from the AB Adits, and in January 2009 with a conveyor and service decline access (Figure 2).

Production plans revolve around a block cave mining approach, similar in many respects to current operations at the DOZ mine.

A departure from the DOZ operation is the main inclined conveyors (~15%) planned to transport production ore up to the mill area. The DOZ mine, being located at the mill elevation, required relatively short, flat conveyors to transfer ore from the crushers to surface stockpiles. The DMLZ will use twin gyratory crushers fed by trucks operating above on the haulage level. Ore will be transported to the surface over a series of 1,829-mm conveyor belts totaling approximately 4 km in length. This conveyor system raises the ore 550 m vertically from the crusher discharge up to an existing conveyor near the surface.

The DMLZ ore body measures roughly 1,300-m long (oriented southeast to northwest) and is between 350- and 500-m wide. There are 43 production panel drifts planned. Panel length between the first and last drawpoints varies from 72 m at the southeast end of the level to 474 m, with an average of 372 m.

Orepasses are installed along the panel drifts at a maximum spacing of 170 m. Aside from the 12 panels short enough to require only a single orepass and the eastern two panels that share an orepass, the remaining 30 panels require two orepasses, for a total of 72 orepasses in the current design.

The DMLZ ore body will be mined using an advance undercut method similar to the DOZ mine. Drawbells are drilled and blasted from the drawpoint drifts into caved material previously blasted above on the undercut level. Drawbells are blasted in one shot using programmable detonators. The undercut blast forms the major pillar apex; the drawbell blast forms the minor pillar apex (Figure 3).

Undercutting will lead drawbelling by a minimum of 15 m horizontally. It will typically advance beyond this minimum point, but as a general rule should not lead drawbelling by more than three months. Exceeding this maximum, according to Freeport Indonesia, creates an elevated risk of blasted undercut material re-compacting and taking weight.

Caving operations will commence in the higher-grade east section of the mineable footprint, moving in a series of three blocks to the west. Draw columns along the north side of the mine tend to be shorter in height, averaging approximately 245-m height of draw (HoD). The majority of the production activity will occur in the southern half of the mine where HoDs reach 526 m.

Manpower for the DMLZ is forecast to peak at 2,550 during the 2009–2021 preproduction and production ramp-up years, with a range of 1,950 to 2,400 for the full production period. The first drawbell ton is schedule for January 2015 with peak production achieved during 2020.

The Grasberg Transition
Freeport Indonesia is using what it has learned from planning and operating the DOZ and planning the DMLZ to design and build the Grasberg block cave (GBC) mine that is planned to produce 160,000 mt/d. The GBC mine has reserves of 1 billion mt with an average grade of 1.03% copper and 0.81 g/mt gold. At a cutoff grade of 0.60% copper equivalent, the deposit would yield 23 billion lb of copper and 26 million oz of gold.

The GBC mine design comprises a total of 2,400 drawpoints covering an area of 700,000 m2. The drawbells will be 30-x 20-m.

The mine will use an extensive rail haulage network with 40-mt electric trolley locomotives to haul ore. Five to six trains will have 20 to 24 20-m3-cars. The trains will haul ore to three pockets feeding individual gyratory crushing stations. Inclined conveyors will transport ore to the surface.

Similar to the DOZ mine, caving would consist of an advanced undercutting system. The average column height would be greater than 450 m. Caving would be initiated in 2016. The maximum drawbell opening rate would be eight drawbells per month.

Workers will pass through a security checkpoint at the Ridge Camp rail yard, the area just outside the AB Adits. The area would also have a materials handling facility, waste muck handling facility, rolling stock maintenance depot, a mine rescue station and concrete batch plants.

Currently, mine engineers are developing geotechnical and drifting designs. The underground batch plant will be designed, procured and constructed during 2010 and 2011. Basic engineering for the ore flow system will be completed along with the underground electrical distribution system design. Detailed design engineering for the service shaft is also in progress.

The GBC mine plans to initiate service shaft excavation by early 2011. The mine also plans to complete an inter-level ramp and the first portion of the mine ventilation system in 2010.

During 2011 and 2012, the GBC AB terminal will be constructed with the rail installed to it, along with underground support infrastructure. By 2012, rail installation should be completed with train service to the GBC mine from surface. The mine will also commission the service shaft in 2013 and begin development on the undercut, extraction and rail haulage levels.

The crushing plant will be operational by 2014, along with the initial feed conveyor and main conveying system. The rail haulage system and main shop come online in 2015. By the end of 2015, the required development on the extraction and undercut levels will be completed to initiate caving.

Caving will begin in 2016 and the GBC mine will reach full production in 2022. The GBC mine will be the flagship for the district during the new “underground era.” What the company is learning in the DOZ today at 80,000 mt/d is critical to achieving the goals for tomorrow’s GBC mine.

1. Casten, T., Brannon, C., and Thomas, L., “A Review of the Caving Mines at PT Freeport Indonesia’s East Ertsberg Skarn System,” SME Annual Meeting, Feb. 28 March 3, 2010, Phoenix, Arizona, USA.
2. Duckworth, I., Casten T., and Rakidjan, “An Overview of the Proposed DMLZ Mine,” SME Annual Meeting, Feb. 28 March 3, 2010, Phoenix, Arizona, USA.
3. Brannon, C., Vergara, P., and Baker, R., “Grasberg Block Cave Mine: Design Considerations to Achieve 160,000 tpb,” SME Annual Meeting, Feb. 28-March 3, 2010, Phoenix, Arizona, USA.