Engineering & Mining Journal

APR 2019

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Page 32 of 83

LEACH PAD DESIGN APRIL 2019 • E&MJ 31 To answer that question, Prophecy Development hired "one of the best geo- chemists," Espell said. Another question for the geochemist is how to maximize use of the acid. "The idea is that the acid is going to be our most expensive reagent cost for the project," Espell said. "We want to save that acid." One plan being considered is to reuse it. "As we finish a cell, we actually rinse the acidity out of it and capture that acid to be able to use for the next cell's leach- ing," Espell said. "By doing that, we are removing that acidity so that at closure, we don't have an acid drain-down solu- tion to have to deal with. It makes closure much simpler." In routine operations, the miner will add a step to the end of the leaching pro- cess for each section of the heap. Nor- mally, after the barren solution leaves the mill, acid is added to it in the plant before it is routed back to the stacks. Instead, at Gibellini, the barren freshwater will be pumped "back to the top of the finished heap to flush the remaining acid water out to enable recovery of that acid" for use elsewhere on the heap, Espell said. The mine is planning to use a geomembrane system that is standard for mines in the area. "In Nevada, the liner systems are really well-defined in the mining regulations," Espell said. "It is called a water pollution control per- mit, which is a zero-discharge permit, and you have to obtain it from the state in order to run any heap leach pad. It is going to be virtually identical to a gold heap-leach pad." When it comes to challenges posed by climate, Prophecy Development will ap- ply lessons learned from history. Espell, who has worked for a couple of mines in the region, including McEwen Mining's Gold Bar project and another operated by Barrick Gold, said that winter leaching there now has been mastered. "In the early days of gold heap leach- ing, we had problems until we all figured it out," Espell said. "We used rain bird sprinklers. In the winter time, we'd create these tremendous ice castles. Then we got smart as an industry." The answer was to go to subsurface drip emitters. "That did two things for us," Espell said. "It allowed us to leach through the wintertime with no effect be- cause the solution is underground. The other thing it did from an environmen- tal perspective is it didn't allow for any pooling or ponding of those leach solu- tions on top of the heap leach pad that birds could get into and have wildlife mortalities." Espell described the rock as friable. "It is not super competent," he said. The mine will likely use a strategy he saw de- ployed at Gold Bar. "I recall that one had a clay issue," he said. "The ore had to be agglomerated and then leached." To maintain percolation at Gibellini, "we'll have to do the same thing," Espell said. "It will have to be an agglomerated particle. That is a classic thing out of the gold industry where if you need to stabi- lize the ore, you agglomerate it." Potential concerns about possible needs for special overliner or underliner materials are de-prioritized currently due to the expected limited size of the mine and stacks. "We have gold heap leach- es out there that are in excess of 600-ft tall," Espell said. "The industry standard liners are fine with stuff like that. This is going to be a baby compared to a gold heap-leach pad." Further, the geology and terrain of the area, described as "very mild and gen- tle," ensures "you couldn't pick a better place to have a heap leach pad," Espell said. "We're sitting on top of 400-plus ft of alluvial gravel," he said. "There is limestone hills above us and basically nothing but neutralizing-capacity lime- stone underneath the leach pad." The culture of the county also en- sures the site is primed for success, Es- pell said. "Eureka County is the mining county," he said. "The regulators here are very strategic in their approach to problem solving." The mine will benefit from a new fed- eral program designed to streamline the state environmental permitting process- es, Espell said. "The new secretarial or- der, No. 3355, demands that an EIS can be now no more than 150 pages, and they can't take more than a year to complete," he said. "That is a huge opportunity to get this thing permitted quickly." Gold Mine Closes Trouble-free HLP Cerro San Pedro, New Gold's open-pit gold and silver heap-leach operation near San Luis Potosí, in Central Mexico, is in the process of closing shop. The mine, formerly one of a handful owned by the company in North America and Australia, is operated by the subsid- iary, Minera San Xavier S.A. de C.V. Op- erations began in 2007. In 2015, it pro- Above, the mine map for Gibellini. The big square in the middle (9) is the main heap-leach pad. The mine road runs from the pit, on the left, to two smaller supplemental leach pads, in the bottom right. (Image: Prophecy Development)

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