Dec '10/Jan '11

Engineering Exchange

Going for the gold: Optimized planning means high production for Detour Lake

By Heather Ednie


Detour Lake construction camp | Photo courtesy of Detour Gold Corporation

There are few mining operations the size of the Detour Lake open pit gold mine that are poised for development in Canada. And unlike most big mining projects these days, this one, in northeastern Ontario, carries minimal risk and has faced few major technical complications. And that makes it a bit unusual, says Paul Chawrun, director of technical services at Detour Gold.

“It’s not as technically challenging as other world-class operations,” explains Chawrun, noting Detour will employ conventional mining methods. “In fact, the greatest benefit is that it’s a low-risk, high recovery operation with a solid workforce base — being more of a ‘standard’ operation is what stands out today. It’s a matter of doing a lot of leg work because it’s a big deposit. The sheer size led to a lot of testing to prove it out.” Sound geology and an emphasis on utilizing currently available technology are allowing the company to seize an opportunity that is, well, golden.

The Detour Lake Mine was first operated by Placer Dome between 1983 and 1999, initially as a small open pit that was then converted into an underground mine. However, when the price of gold tumbled in 1998, the operation was shut down and reclaimed. The property became available in mid-2006 and Detour Gold Corporation acquired it in January 2007 with the vision of developing the deposit as a bulk mineable, low-grade operation. In just over three years, Detour Gold succeeded in proving that Detour Lake was both technically feasible and economically valid after delivering a positive feasibility study in May 2010.

The project recently received the green light from the provincial government to begin construction while detailed engineering and procurement are going full speed ahead.

Mapping the mine strategy

Detour Gold contracted BBA to carry out the feasibility study and detailed engineering of the project. Patrice Live, project manager – mining for BBA, says a combination of factors made the project feasible, namely, optimizing the grade and stripping ratios, limiting the dilution coefficient and maximizing the cash flows of the operation, particularly in the early years to accelerate project payback.

“We executed a vast series of studies and simulations, looking at dozens of scenarios to maximize the best option for the mining scheduling,” Live says. “We have developed an optimized project that is feasible and solid.”

The pit design completed by BBA resulted in a mine plan containing 347.5 million tonnes grading 1.02 g/t gold (including low-grade stockpiled ore). The mine plan maximizes head grade recovery in the early years for a faster payback of capital. Total gold production over a 16-year mine life is estimated to be 10.4 million ounces, averaging 649,000 ounces per year (plus 200,000 ounces of silver). The life-of-mine waste-to-ore ratio is estimated at 3.3 to 1.

“Pit development is staged to optimize ore grade to maximize our cash flow at US$850 per ounce,” adds Chawrun. “Our mineral resource continues to grow as the deposit is open at depth and to the west.” A future expansion plan would be considered at a gold price of greater than US$1,000 per ounce. The expansion means lowering the cut-off grade, leading to a lower waste to ore ratio.

Currently, BBA oversees the engineering process. A number of pieces of equipment have already been ordered, including 795F (diesel/electric) 320-tonne trucks from Caterpillar. “They are new machines, but have undergone very serious testing in several countries under different climates over the last five years,” Live says.

Because Detour Gold aims to make the mine a world-class operation, it wanted to equip it with the latest proven technology. “In the first year, we will require six trucks. Then, the fleet will be increased to 18 for full production and increased more as needed. After careful scrutiny and analysis, the BI RH-340B hydraulic shovels have been selected as the primary ore-loading tool to reduce the dilution coefficient and maximize recovery. Cable shovels are likely to be the next primary loading equipment to be selected.”

Optimizing the mill plan

Detour Lake
As the original mine complex was completely demolished and almost all infrastructures removed, Detour Gold has to rebuild the site from scratch, including erecting a new mill with two independent lines totalling 61,000 tonne per day capacity. Like the mine, the main challenge was to optimize the capital and operating costs, as well as productivity, through technology and innovative design.

“The mill design includes two SAG mills and two ball mills,” explains André Allaire, director – mining and metals for BBA, who was the company’s project manager for the feasibility study of the project. “Detour Gold purchased the largest twin pinion SAG mills available from FLSmidth. Having two lines gives Detour Gold additional flexibility during periods of maintenance.”

Size, however, was not the only deciding factor in the mill’s design. The Detour Lake material has a low grinding capacity for SAG mills and therefore results in an approximate 45,000 tonne per day capacity for the two mills. However, with the addition of a 1,000 kW gyratory crusher, followed by two 745 kW cone crushers before the SAG mills, the capacity increased significantly. “It helps the SAG mills by reducing the feed size from a P80 of 165 millimetres to a P80 of 50 millimetres,” Allaire explains. “The SAG mill throughput is largely increased because the Detour gold ore is considered hard to break.”

“Precrushing isn’t common in Canada, but we’ve proven its merit at both the Troilus operation and Agnico-Eagle’s Goldex operation,” Allaire adds. “We investigated other options, such as high-pressure grinding rolls (HPGR), but they could not be justified compared to conventional methods. In the end, adding the pre-crushing units gives the best solution in terms of price and profitability.”

After crushing, the material travels to a gravity recovery circuit, then on to an intensive leach reactor for the recovered gold, and the remainder to a CIP (carbon-in-pulp) carousel circuit, prior to stripping. The carrousel CIP is relatively new to North America, found here only at Agnico-Eagle’s Meadowbank and Osisko’s upcoming Canadian Malartic project.

In a conventional CIP system, the slurry and carbon move counter current to each other. In the carousel system, the carbon stays within each tank and the pulp is piped tank to tank. Once fully loaded, the tanks are emptied out. “The tanks will be emptied once every 14 days,” Allaire adds. “With two systems including seven tanks each, that translates into emptying a tank each day.”

Allaire said the carousel technology was adopted because the CIP tanks are three to six times smaller than a conventional CIP, making it much easier to fit them inside the building. As well, the gold loading on the carbon is higher, reducing the gold inventory in the plant and the stripping circuit size.

Let’s get it started

Currently, about 80 of BBA’s employees are working on the Detour Lake detailed engineering, overseeing the planning and construction of the powerline and executing procurement. “In Eastern Canada, there are not that many large operations like Detour Lake,” Allaire says. “BBA is a new player with much experience and it’s exciting to be building these large projects, and building our own company at the same time.”

AMEC is responsible for the construction management, with much of the same team that built the Victor Mine for De Beers.

Production is scheduled to begin in the first quarter of 2013. Detour Gold received provincial permits this past November to go ahead with construction while they continue with the regulatory process on a number of other areas. Detour Gold will be in full construction mode starting in January 2011.

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