Engineering & Mining Journal

APR 2019

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LEACH PAD DESIGN 30 E&MJ • APRIL 2019 You need to then incorporate additional geotechnical design parameters to put it all together." For a valley fill pad, with sections steeper than the threshold, "you are go- ing to have to bench it," Koningen said. "Or you can do slopes and have horizon- tal sections between them. You can't just run the liner the whole way up to the top because you are going to get some com- paction and movement as the pad height increases." The current trend in liners is toward LLDPE, he said. "It is more flexible, and you can select the thickness depending on how high you want the heap to get." The pads at Santana will likely be un- derlined with local clay. "In our experi- ence in Mexico, there have been enough local clays available that you can have an underlining compacted clay liner," Kon- ingen said, "and then an HDPE or LLDPE liner on top so you get your two layers of containment, or what is referred to as a composite liner." To further protect the liner, Koningen said he plans to limit the amount of liq - uid allowed to build up on it. "I am not a fan of building in liquid containment into your pad designs where you start to get a meter or two meters of liquid above the liner because that is counterproductive," he said. Optimal is a "pretty minimal amount of liquid above the liner," Koningen said. "Then, even if there is some minor punc- tures and things like that, it will easily seal off with the underliner material." Some of the initial test work focuses on the geomechanics of the ore and the possible percolation rate. "We do early testwork, looking at permeability and the fact that, if you go up 50 or 60 meters, how much liquid can still get through," Koningen said. He advised it is crucial to complete studies on the geomechanics of the ore early on. "Figure out what crush sizes you really need and have a good handle on whether you've got mineral constituents in there, like clays and other things like that, that are going to cause problems with permeability," he said, "because most of the problems you have with your pad are related to that." Vanadium Op Plans Acid HLP Prophecy Development Corp. is planning to run an acid heap-leach system at its Gibellini vanadium project in Eureka County, Nevada, USA, using technology and processes similar to those employed by nearby gold miners, according to com- pany leadership. Gibellini is planned to be an open-pit operation with an initial capital cost of $116 million and producing 9.65 million pounds (lb) of vanadium pentoxide per year at an average all-in sustaining cost of $6.28/lb for up to 14 years. The to- tal inferred resource is 52.3 million tons. The mine is expected to move roughly 3.4 million tons of material per year. Ron Espell, vice president of environ- mental and sustainability, Prophecy De- velopment, told E&MJ the ore is exposed at the surface at a "very low" strip ratio. "It is just sitting there," he said. As part of the PEA from 2013, when the project was owned by American Va- nadium, samples from trenching totaling 18 tons were sent to a third party for pilot heap-leach testing. There the ore was crushed and dried and put through 12-in.-column and bottle roll tests. The average recovery rate was roughly 72% from a head grade averaging 0.29%. "The final pregnant-strip solution was 6.1% va- nadium, 250 g per liter sulfuric acid, with approximately 2% iron and aluminum," Prophecy Development reported. Espell said the company is in the process of redesigning the operation as it was originally planned by the previous owner. Included in the redesign is the heap leaching system. "Looking at the geotechnical work and the heap leach pad, I think the previous American Vana- dium plan for the heap was 200-ft tall," he said. "All that stuff is being evaluated right now." The primary challenges to be ad- dressed in the redesign arise from the differences between the plans for the heap leaching system for Gibellini and the ones in place by neighboring gold mines. "No. 1 is it will be the first heap leach for vanadium in the world," Espell said. "The biggest challenge is it is an acid heap-leach pad and an acid heap- leach process," he said. "The challenge there, from an environmental perspec- tive, is we are in Nevada, and Nevada is gold country." Gold miners in Nevada use cyanide for heap leaching. "As an industry, we have cyanide heap leaching down to child's play," Espell said. Cyanide is unstable and breaks down easily, helping with smooth, safe, regulatory problem-free closures of mine sites, he said. "Now with the acid heap-leach pad, collectively in Nevada, they don't have any experience closing an acid heap- leach pad," Espell said. "The regulators want a lot of detailed information. And rightly so, they ask, how are you going to neutralize the thing so that at the end of the day it doesn't produce a long-term source of acid drainage?" Loading of mineralized material on to a test heap-leach pad at the Santana gold project. (Photo: Minera Alamos) An 81-oz doré bar produced from heap leaching operations at the Santana gold project. (Photo: Minera Alamos)

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