Innovations VERTICAL RETREAT

Because bulk mining generally, and VRM in particular, represented such dramatic departures from previous practice, most current research projects are aimed simply at refinements. Nevertheless, these projects hold substantial promise.

Multi-deck blasting, for one, could be significant, but perfecting the technique has stymied researchers so far. Essentially, successful multi-deck blasting would entail loading explosives sufficient for several separate blasts (the blasts could be days or weeks apart), then detonating only a portion at a time. The productivity gain results from eliminating the need for an explosives crew to enter the stope several different times. Instead, they would load sufficient explosives in a single shift for several days blasting.

VRM is a well-documented and accepted method of exploiting orebodies wider than 20 ft. Essentially, VRM consists of blasting large blocks of ore from the bottom up. It avoids excessive dilution and is safer than previous methods by a factor of 10.

When The Northern Miner Magazine visited the 3,000-ft level at the Stobie mine, Peter Garrood, supervisor of technical services for Inco’s Frood- Stobie-Garson complex, explained how the system works.

A bottom sill is driven across the 45-ft-width of the orebody on the 3,000-ft level and a similar sill is driven 200 ft above it on the 2,800-ft level. From the top sill, an in-the-hole drill puts 6.5-inch-diameter holes down the entire 180 ft of the ore block to the bottom sill in order to blast one “panel” of ore. Thus, each panel measures about 180x45x50 ft.

Blasting technology is the key to VRM, which was first developed in the 1970s. A circular blast pattern and blasting only into the bottom sill void (which results in an inverted crater at the bottom of the panel) make the system work. It avoids having to ream a slot raise for the initial blast void. Using this blasting technique, each panel is mined by blasting a daily 10-ft slice from the panel bottom. That broken ore is then mucked out from the bottom sill by a load-haul-dump machine, usually operated by remote control.

Further 10-ft slices are blasted until the panel is mined to within 30 ft of the top sill. At that point, a final crown blast removes that large block of ore, or crown pillar. This final blast is a form of multi-deck blasting that is already well developed involving simultaneous blasting of what would be three decks.

Currently, other than that crown blast, each slice in the panel has to be loaded with explosives, so one shift a day is spent on loading each lift. After that lift is blasted, an 8-cu-yd load- haul-dump machine working a single shift of 150 round trips can muck out the 1,500 tons of ore created by a single blast. The stope must be mucked out to provide enough room for the next blast. Explosives are then loaded for the next blast.

While the drill holes for each panel, up to 180 ft deep, are driven in one go over a period of about a month, the blasting, 10 ft at a time, is a daily occurrence. For each blast, the explosives team must repeatedly load and prime the explosives in the same holes.

What has researchers stumped is a way to pack explosives into the entire panel and prime it in such a way that each lift can be blasted separately as production schedules dictate. Developing a multi-deck blasting technique for the crown lift of each panel was the first step. However, this crown blast is detonated in three parts of a single blast. The first is blasted to the bottom sill, the second to the top sill and the third both ways, all within microseconds.

Researchers working on the multi- deck technique can’t find a way to isolate those blasts so that detonating one blast won’t cross over and detonate or damage the remaining explosives already packed.

“It’s that crossover or shoot- through that’s the problem,” says Dennis Doucette at c-i-l, the explosives company working closely with Inco on the project for the past year or so. “We’ve tried to isolate the phenomenon that is causing it. We have some theories, but nothing conclusive I can let out yet,” Doucette added. David Joyce, manager-technical service group, said: “We do have ideas and theories we are working on. We are conducting joint research with Inco to solve this problem.”

Researchers at Inco Research say it is premature to talk about their work into the problem. “That (multi-deck blasting) is our ultimate goal,” said one Inco researcher about the elusive technique. “Frankly, we’ve been looking for it ever since we really absorbed VRM about eight years ago.

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