October 2013

Savings from the bottom up

Large-diameter boreholes increase options for miners

By Ian Ewing

At the Young-Davidson mine, 60 kilometres west of Kirkland Lake, Ontario, Aurico Gold is betting on the rarely used method of borehole hoisting to extract up to 8,000 tonnes per day from as deep as 1,500 metres underground. At the brownfield site, home to historical mining operations from the 1930s to the 1950s, the existing underground shafts were unsuitable for the high production rates Aurico was anticipating, says Young-Davidson general manager Luc Guimond. “Borehole hoisting was chosen because we had infrastructure in place that allowed us to look at this project differently,” he points out.

Borehole hoisting has never been considered an ideal method of raising production material in mines. Historically, imperfect, narrow shafts have limited the volume of material it was possible to raise, by constraining the volume and speed of the skips used. In fact, until now, borehole hoisting had only ever been a temporary, short-term arrangement until other extraction routes were completed.

It is the confluence and expansion of two existing technologies that has given hard rock miners like Aurico new options. The first technology is a raise drill machine with the ability to create large-diameter boreholes in hard rock. The second is the ability to drill vertical pilot holes very accurately. Together, these technologies have made it possible for some mines, such as Young-Davidson, to create large enough shafts with tight enough tolerances for production shaft use – and to save time and money while they are at it.

New ways to use existing technology

Creating large-diameter boreholes in hard rock has been possible since around 2003, when Cementation Canada, which is the contractor behind the shaft at Young-Davidson, brought the technology to North America. The capability is mainly due to the ongoing improvement of raise drill machines. The South African and Australian units of Cementation have been doing large-diameter raise bore holes of up to six metres in diameter in the softer rock of those countries for years. By leveraging the overseas units’ experience with large-diameter holes and the Canadian unit’s experience with medium-diameter (three to four metres) raises in hard rock, the company was able to build raise drill machines like the Strata 950, capable of pulling large-diameter shafts through hard rock. Now Cementation, which designs and builds the machines itself, has two of only a handful of similar machines on the continent.

The other key to allowing high-volume borehole hoisting is the accuracy of the shaft. That comes not from the raise drill machine but from the pilot hole drilled for the raise drill string. Here, Cementation uses a device built by Micon, a German firm. The rotary vertical drilling system (RVDS) is placed on the drill string right at the drill bit. The tool uses an internal gyroscope to sense any deviation from the desired drilling path and uses jacks to push against the drillhole wall in the opposite direction. Constant real-time monitoring above ground ensures incredible accuracy – as little as 20 millimetres, or less than an inch, deviation over the length of a 400-metre shaft.

“Neither of these are new technologies,” says Cementation president Roy Slack. “But put them together and make them bigger, and all of a sudden you have a new application.”

The near-perfectly vertical pilot hole can then guide the large-diameter reamer to that same accuracy. The combination of tight tolerances and an enormous shaft allows viable production volumes to be hoisted by permitting larger skips to traverse the shaft faster.

That sinking feeling? Not here

“You have to have certain tolerances within a hoisting plant,” explains Guimond. “In conventional sinking, it’s constantly surveyed as you’re sinking. You can correct it as you’re advancing the sinking face. In this case, you don’t really see the final product until you’ve done the final reaming.”

“We’ve been quite successful in pulling these large-diameter holes,” notes Slack. “There are always challenges, whether it’s issues with the ground; sometimes we’ll have the borehole sloughing and we have to deal with that, sometimes there are water inflows, and that’s a particular challenge on its own. But there are different ways to deal with that. We have had some issues over the years, but no problems that have prevented us from finishing any of the holes we’ve started.”

Presented with the option, it turned out to be an easy decision for Aurico. “It was substantially cheaper, quicker and a lot safer,” says Guimond, than conventional shaft sinking. “The big driver was schedule, [but] the cost of the excavation itself was also cheaper than conventional methods. We will shave eight months off the total shaft schedule to completion and the cost difference will be about $30 million.”

Slack says these kinds of savings are typical. “In an example we looked at recently, the borehole option was about 60 per cent of the cost of a traditional blind sink and saved about 12 months off the schedule.”

Trying something new did require Aurico to put substantial trust in their contractor, though. “We were aware of the technology of raise boring,” says Guimond, “but we were always skeptical of being able to drill a fairly long pilot hole and then ream it and make sure it’s within tolerance to maintain its vertical requirement. But the technology has caught up to allow us confidence in going in this direction.”

Production unaffected

The method is not for every application. The main prerequisite is access, both from the top and bottom of the hole, so the reaming head of the raise drill machine can be put in place underground. “The ground has to be fairly competent,” adds Slack. Also, the size of the shaft is currently limited to around 5.5 to 6 metres. Depending how much ventilation is required, that may not be enough. “Where the conditions are right, though, it is worth considering,” Slack says.

In spite of the unorthodox delivery method, production rates at Young-Davidson are not limited by the use of borehole hoisting. Aurico’s mill is currently operating at about 7,000 tonnes per day, of which 1,500 tonnes is coming from the underground operations. As the open pit is phased out over the next several years, however, more and more material will come through the borehole. By 2016, Aurico expects to be milling 8,000 tonnes per day, all from underground. “In terms of production, there’s no real difference,” says Guimond. “At the end of the day, whatever your design capacity for your plant is, you can still achieve that. That’s not going to change.

“The advantage is all in the time and cost of raising the hole.”

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