For example, at Golden Reward in South Dakota, project owners are installing a track-mounted stacker engineered by Bateman Engineering of Lakewood, Colo., and built by Jervis B. Webb Co. The system incorporates two tracks along the length of the heap leach piles and a radial arm stacker. Another track system is used in removing the ore by bucket wheel.
The Golden Reward owners (Coin Lake Gold Mines, Minven and Moruya Gold N. L.) have learned that South Dakota’s environmental regulations are especially rigorous — as severe as, if not more severe than, those governing heap leaching in California. Just getting a passing grade in environmental approvals alone can be costly and time-consuming. To persuade a skeptical public of Golden Reward’s environmental commitment, the joint venturers spent $1.4 million to computerize snow-making equipment and build a lodge at a nearby ski hill.
“It was either that or forget the (environmental) permits,” said Chris von Hessert, a Coin Lake director. “We had to put to rest fears that we were going to destroy the ski hill.” One state-wide referendum and a second in the county finally gave the project a green light. The approval process was begun in 1985.
There is another way in which Golden Reward differs from other heap leach projects. Again, environmental concerns dictated the modification. A simple poly vinyl chloride (pvc) liner is not sufficient to prevent cyanide seepage, according to state environmental officials. So the bed upon which the heaps will rest consists of two 6-inch layers of clay separated by a 300-mil pvc liner, a 12-inch layer of drain rock above the clay/poly layer and, finally, a topping of asphalt. Installed with the lining is a leak detection system for close monitoring of the liner’s effectiveness.
The asphalt cap also allows for an on/off pad; that is, the pads are loaded, leached, detoxified and unloaded in an 8-to-10-week cycle. The spent ore is dumped into the mined-out pits, because South Dakota wants to eliminate the deep open pits that are left behind after mining.
At Florida Canyon, the material- handling system has attracted plenty of attention. The heap pile is pie- shaped — not a common sight at such operations — and the conveyor system redirects ore from the pit to a track- mounted, 1,500-ft-long radial conveyor that sweeps over the heap, much like the minute hand of a clock, distributing ore evenly in 15-ft lifts. The ore is tripped off the conveyor by a special machine, known as a tripper. It rides across the top of the mobile conveyor and triggers a cross-conveyor that puts ore on the piles.
While stacker technology is spreading through the industry, some operators are still reluctant to go the complete-conveyor route. Charles Stone, the project manager for R. A. Hanson on the Florida Canyon operation, said in a telephone conversation from Spokane that planning engineers can clearly see the operating cost savings. Mine operators, however, worry that breakdowns could be time- consuming and costly. “Most miners are used to trucks and shovels and front-end loaders. If one of these breaks down, the operati on can continue. But if one part of our conveyor stacking system breaks down, the whole system stops. That’s where we build in reliability. Pegasus has had its system running for three years now and it has been highly successful.”
At Viceroy Resource Corp.’s Castle Mountain project, on the verge of a go-ahead, the question of what technology to use has been settled. Now, the company is awaiting environmental approvals from the U.S. Bureau of Land Management and the county of San Bernardino in California. Once running, this operation will incorporate most of the recent heap leach innovations — drip-lines rather than sprinklers and conveyor/stackers. And, with their customary zeal for the environment, regulatory agencies will ensure that Viceroy innovates environmentally as well. For instance, the company is committed to fencing property roadways to protect the desert tortoise. Also, the upper benches of the ore pits will be treated with rock-staining solutions to reduce visual contrast with adjacent, undisturbed slopes, says the company. Canada’s Heap Leachers
Heap leaching in Canada is not as routine as it is in the southwestern U.S. The harsh winters see to that. But a strategy has been developed to make year-round leaching at least a possibility in the Great White North. It’s called vat leaching, an idea that has been kicking around for some years. It was tested last winter at Gordex Mineral’s Cape Spencer mine, in New Brunswick.
Each of Gordex’s two vats is 75 m wide, 100 m long and from 5 to 10 m deep. The walls are formed from crushed waste rock and lined with a liner made by Geoguard. “It looks like a big bathtub,” said Mine Manager Rick Brown. This past winter, Gordex loaded 70,000 tonnes of ore into one vat. Buried under a 3-to-4-ft waste rock cover to prevent freezing, driplines circulated the weak cyanide through the ore.
Brown told The Northern Miner Magazine he was satisfied with the winter recoveries, which registered between 60% and 65%. Warm-weather recoveries run 70%. However, he is convinced the vats are unnecessary. Making the heap piles larger and then burying the drip-lines under a waste- rock cover should do the trick.
Unfortunately, the Gordex operation was shut down earlier in the year, owing to lack of funds. The problem relates to debt charges at the corporate level, not operating costs. “We’ve got just about every bug worked out and now we don’t have the money to buy chemicals,” Brown said. He added that they had even decided on the optimum crush size (- 3/8 inch, not the – 5/8- and – 3/4-inch crushing that had been experimented with earlier). The Gordex reserve stands at about 400,000 tonnes grading 0.06 oz per tonne gold.
In central Canada, Pamour has been patiently learning the ropes at its Timmins, Ont., heap leach complex. Low-grade ore from the Pamour open pit is crushed to – 3/8 inch and loaded on to three pads. The pads are cycled so that while one is being loaded, another is being leached and the leached rock from a third is being hauled to the post-leach waste pile. Leaching begins in June and ends in November. Milder winters might allow for a protracted cycle, but winter leaching is out of the question.
“We leached one pad through the winter with mixed results,” said Ken Blower, vice-president operations. “The leaching was slower and it was difficult to get a uniform distribution of leach solution because ice blocked the flow,” he said. The sprinkler-fed cyanide solution formed an ice layer over parts of the heap, inhibiting percolation.
Pamour expects to leach between 150,000 and 200,000 tons of ore each year at a grade of 0.031 oz per ton gold. Recoveries should average 65% for an annual yield of between 3,022 and 4,030 oz. These are rough numbers bec ause Pamour’s heap leaching has yet to run a full cycle.
Heap leach production was initially scheduled to handle about 300,000 tons per year, but it has been scaled back. Because of this, the crushing circuit has been refined. The old system incorporated a 30×48-inch Hewitt Robbins primary crusher, supplied by Piedmont Industries, which crushed rock to — 6 inches. This material was fed to a 4×10-ft Ty-Rock screen. Oversize reported to a 5 1/2-ft
standard Symons cone crusher and the undersize went to a surge bin that fed two sides of a 6×20-ft screen. To achieve – 3/8-inch leachable ore, the final step in this system was a 4 1/4-ft shorthead Symons.
In the scaled-down version, the Ty- Rock screen and the 4 1/4 shorthead were eliminated so that the feed comes from the primary crusher to the 5 1/2 standard and to only one side of the 6×20 screen. The other side is scalped. “The whole idea is to get everything down to – 3/8 inch. So we’re taking off the fines as soon as possible,” said Denis Allarie, president of Leo Allarie & Sons of Timmins. His company and Pamour are involved in a joint venture, known as Lodestar Crushing and Construction, which operates the crushing plant. The new plant runs at a rate of 200 tons per hour.
Loading the pads is accomplished with a conveyor and stacking system supplied by Power Screen of Ajax, Ont. This method, rather than having heavy haulage equipment compact the heaps and thus inhibit percolation, has become the preferred stacking method on this continent. All told, something like 15 conveyors ranging in lengths from 50 ft to 280 ft do the stacking. “It’s an involved process.”
Back on the east coast, Hope Brook Gold’s Newfoundland mine is churning out the ounces. In 1988, heap leaching accounted for 49,464 oz. Recoveries are about 62%, but General Manager Bill Fotheringham said he hopes recoveries will rise to 70%. This summer, Hope Brook will wind up its heap leach operation to focus entirely on underground mining and conventional milling. “We had intended to mine and heap leach only in the pre- production period,” said Fotheringham. “And it has worked well.”
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