October 2015

Hard lessons

The team at Palabora Copper, wiser for its Lift I block cave project, is drawing on that experience to go deeper

By Michael Yang

When Palabora Mining Company (PMC) decided to transition its giant open-pit mine underground nearly 15 years ago, the South African copper producer took on a formidable challenge: to build a massive block cave in one of Africa’s hardest rock formations. The resulting Lift I operation set industry benchmarks in both secondary breaking and integrated design, but it was not without its growing pains. Now, as PMC develops its bigger and deeper Lift II expansion, it is drawing on lessons from the past and new technologies to ensure the $1-billion project is a success.

On paper, block caving is simple enough. Crosscuts beneath an ore body force the rock to progressively collapse on itself, forming an artificial cavern from which the rubble flows downwards into a system of pre-constructed funnels and tunnels.

“Think of it as an inverted open-pit mine capable of mining the same types of massive ore bodies,” said Nick Fouche, PMC’s general manager of growth. For a company with a half-century of experience operating its former 82,000-tonne-per-day open-pit mine – the largest in South Africa – the transition into an underground equivalent was expected to be easy enough.

Experience soon taught them that block caving is anything but.

“Although we had some of the best block cave expertise in the industry we were in new territory, and there were many unknowns as the company began developing the Lift I operation 15 years ago,” said Fouche. “We didn’t always have the right equipment, processes and experience, and it wasn’t until we addressed those that the operation became a success.”

Currently, PMC is the only copper producer in South Africa, supplying nearly 80 per cent of the refined metal in the local market. After its open-pit operation in the Limpopo province ended in the late-90s, the company became one of the first in the world to directly transition from an open-pit to a block cave, setting industry precedents with an eventual volume of 30,000 tonnes per day (t/d) and a central cave height of 450 metres. However, while Lift I began drawing ore in 2001, it would take four to five years for the operation to ramp up to full production due to problems dealing with the fragmentation of the rock. A lack of extensive cave monitoring has also meant that the prediction of the ore grade could not always be forecasted accurately. This, along with the North Wall subsidence, meant that the mine would reduce its life much faster than originally expected, shortening the mine life to the end of 2015 rather than 2023.

“Given the situation, we began looking at the possibility of a second block caving expansion, 450 metres directly below Lift I, in November 2011,” said Fouche. A study based on 72,000 m of core drilling data concluded that the deeper ore body is very similar in size, shape and grade to Lift I, at 0.64 per cent copper, so it can be economically mined using the same method. With a planned volume of 33,500 t/d and a production level 1,650 m under the surface, Lift II will be one of the largest and deepest block cave mines in the world when the first ore is drawn in late-2017. “We’ve done everything we could to ensure that the expansion will be a success from the get-go,” said Fouche.

Beneath the tall shrubs and grassy plains of Limpopo’s famous Bushveld lies some of the most mineral-rich deposits in Africa, in some of the continent’s hardest rock complexes. PMC’s mining operations sit on an apatite-rich pyroxenite formation in the northeastern region of the province – only the Bushveld igneous complex has more competent rock in the region.

“The issues associated with ramping up Lift I was very well-known in the industry, and it had to do with the fragmentation sizing of the rocks coming through the drawpoints,” said Hans-Dieter Paetzold, Palabora’s chief geologist. “We were expecting much greater fragmentation than what actually occurred because the rock is so strong.”

Production maxed out at 20,000 t/d due to the bottlenecks in the drawbells and the crushers from oversized boulders. Eventually, the business deployed an internal team of mining and processing experts to design a secondary breaking process with the appropriate equipment to manage the ramp up in a much more orderly and predictable manner. It took nearly two additional years for the operation to reach full production but when it did, Lift I had “one of the best secondary breaking systems in the world,” according to Fouche.

For Lift II, instead of staying in the same four-crusher configuration, each with only one tipping point, PMC is planning on using two larger 2,000 t/h ThyssenKrupp BK 63-75 gyratory crushers. The crushers have larger throat sizes that allow them to handle larger ore and four tipping points per crusher, enabling the load-haul-dumps (LHDs) to tip from a total of eight different tipping points.

The two-crusher configuration also enables the potential use of electric LHDs, due to the shorter distances between the crosscuts and the centrally located tipping points. “Using electric LHDs is one of the ways we can control the high heat down there,” explained Paetzold. Virgin rock temperatures at Lift II’s depth are expected to reach 58 degrees Celsius and designing an electric vehicle footprint for the cave is crucial in significantly reducing the overall heat load. The project team is still weighing the advantages of the cooler electric LHD against the greater flexibility of untethered diesel machines, or battery-powered units that require charging stations.

“We now know the importance of
predictive monitoring,
and will be putting in an extensive system for Lift II.”

 – Nick Fouche, PMC’s general manager of growth

 

Second guess

While Lift I’s secondary breaking configuration eventually became an industry benchmark for breaking extremely competent ore, the operation’s cave monitoring systems never achieved similar success. “We didn’t have an extensive cave propagation monitoring program in place from the start,” said Paetzold. “We had a few monitoring tools and draw bell sampling, but essentially after eight years it became difficult to predict how the cave will behave.”

With only a few time domain reflectometers (TDRs) in the Lift I ore body, the broad seismic system did not allow for sufficient information to paint a full picture of the cave at any given time. As a result, the north pit wall collapsed in 2004 after the crown pillar of the cave from Lift I broke through at the foot of the open pit north wall. Over 18 months, a failure measuring nearly 800 m by 300 m developed along the wall from the pit bottom. Up to 100 million tonnes of waste rock have diluted the ore in the deposit since then, significantly reducing the grade and shortening the mine life by five years.

“We now know the importance of predictive monitoring and are in the process of implementing an extensive system for Lift II,” said Fouche. TDRs and smart markers placed in open holes will be installed to cover the caving area between Lift I and Lift II. The collected data will then be stored and analyzed using GEMS software. In addition, the existing seismic system is being upgraded to establish exactly when the deeper cave will begin to affect operations and infrastructure above.

Early works

Though the mine life of the current cave is slated to end in December 2015, production out of the cave will still continue until late-2018 to reduce reserve losses. Because the operating block cave mine is located on top of the Lift II block cave development, the company needed to create a construction strategy to build Lift II without halting the existing operation.

That brought about the $220-million early works program in 2011, which involved completing twin decline ramps as well as electrical and ventilation upgrades to power both Lift I and Lift II simultaneously. “We didn’t want any kind of lengthy discontinuity between the two operations because we’ve developed the unique skills and experience necessary for block caving and aim to keep the block cave competency in place for the Lift II operation,” Fouche said.

This mindset is evident in the breakneck speed at which construction is progressing and the new technologies and processes Palabora is using to get there. Building twin decline ramps in the constrained underground environment usually means progress maxes out at around seven metres per day (m/d). However, the company worked with contractor Byrnecut to develop a process capable of reaching 10.5 m/d. “Instead of taking two cuts, one on each heading, we actually managed to rotate the headings and get three cuts across the two headings every day,” explained Fouche. The advance rates were also supported by the use of a Sandvik 621 loader, the largest ever to be sent down a 1,200 m shaft and the biggest loader operating underground in Africa.

Another piece of technology driving the expansion forward is contractor Master Drilling’s RD8 raise borer. Modelling indicates that the current vent shafts are in danger of collapsing with the propagation of the cave, “and it’s crucial that we get the first new vent shafts in by December 2016, because that’s when the construction level will surpass the current ventilation capabilities,” said Paetzold.

The company opted to use the raise boring method over the more conventional blind sinking method to drill the two, 6.1-m diameter vent shafts reaching 1,200 m from the surface down to the Lift I level. The RD8 raise borer is the largest and most advanced drill used in South Africa; with the capacity to bore up to 8.0-m diameter holes 1,500 m deep. The machine uses a combination of electrical and hydraulic power to drive the pistons and engine, and can be operated and monitored remotely by only four people.

Data indicate the shafts will intersect geological structures that could present problems during the construction phase when those structures are exposed. However, the RD8 also comes with the contractor’s new Remote Operated Shaft Support (ROSS) unit, developed especially for this project. Instead of waiting for the shaft to be completed and reamed out, the ROSS allows the team to remotely install shaft support up to 1,200 m from the boring unit during the actual drilling process. “If we ever suspect a fallout or failure, we can pause the reaming, hook up the unit, scan the area and then seal it off right away,” said Paetzold.

He also noted that building the foundation was a big factor in the decision. A normal blind sinking process usually requires a box cut and the area to be completely dug out until suitable rock to place a foundation on is found. However, the option of piling to support a foundation exists with raiseboring, which significantly reduced the overall construction time.

Paetzold said he expects the first shaft, which PMC began work on in March, to be completed by the end of 2016. “Both shafts will be in much faster than if we had decided to sink a single, eight-metre diameter blind sink shaft, and will be three to four times cheaper as well,” he said.

The Lift II expansion is expected to extend the mine life until 2033, and this time, the Palabora team is confident that it will get it right from the start. “Block caving truly is a unique skill that must be learned through time and experience,” said Fouche. “And we have been fortunate enough to be given a second opportunity to do it all over again.”


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