Engineering & Mining Journal

APR 2019

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LEACH PAD DESIGN 28 E&MJ • APRIL 2019 www.e-mj.com The common thread in the advice on heap-leach pad (HLP) system design given to E&MJ by high-level people at a handful of junior mining companies cen- ters largely on preparing for the worst. Specifically, it covers the basic and early steps required to minimize the chances of the worst ever occurring. Whether the miner is operating in lush, subtropical Mexico or the barrens of Nevada, when planning a pad, the long-term goal of closing it in the distant future, without incident, should be the foremost operating motive, they say. That can mean planning for a 500-year storm. It can mean planning on establishing the best possible relations with the surround- ing community. Most of the time, howev- er, it means repetitive drilling and layers of lab testing on the ore to get a clear picture of what will happen to it when it is stacked sky high and irrigated. Sky high, in the Andes, is a good start- ing point. Polymetallic Op Plans 2 HLPs Filo Mining is set to build two heap leach pads at its 100% owned polymetallic Filo Del Sol Project, in the Atacama Region on the border of Chile and Argentina. The orebody is described as "a high- sulphidation epithermal copper-gold-sil- ver deposit associated with a large por- phyry copper-gold system." Jamie Beck, vice president, corporate development and projects, Filo Mining, described the deposit as unique. "Often, deposits have either an endowment of copper or an en- dowment of precious metals, gold and sil- ver," he said. "At Filo, what is unique is that we have all three metals in economic grades, so the challenge here is trying to find a way to extract the copper, gold and silver through a leach process." In a prefeasibility study, the mine reported goals of eventually producing gold at a rate of 160,000 ounces per year (oz/y), silver at a rate of 8.7 million oz/y, and copper at a rate of 67,000 met- ric tons (mt)/y. "What is also unique about Filo is we are planning a sequential leaching pro- cess," Beck said. "During the first stage, the plan is to acid leach for the copper on an on-and-off pad," he said. "After that, the ore is washed, neutralized and trans- ferred to a valley fill facility for cyanide leaching to recover the gold and silver." The project is situated high in the Andes, at an elevation averaging 4,800 meters (m) above sea level and subject to high winds much of the year. The fore- most issues considered when planning the pads were related to elevation, terrain and climate, Beck said. Temperatures at the site range from -20°C to 5°C. The average year-round temperature is -3° or -4°C, Beck said. "The difference between the summer and winter highs are not that much," he said. "We plan on it being a cold, windy envi- ronment all the time." The miner is also planning for perma- frost. "That can thaw, especially at the base of any pad and generate deforma- tions," Beck said. "As a result of that, we have considered a LLDPE (linear low-density polyethylene) geomembrane, that can tolerate the higher deformations. As an additional measure, a stability toe built into the valley fill pad has been de- signed." Due to elevation, high levels of ul- traviolet radiation can damage exposed patches of geomembrane, which some- times occurs when ponds and channels evaporate or drain. "Where that is go- ing to happen, we are going to switch to a HDPE (high density polyethylene) geomembrane," he added. A "key issue" in designing the valley fill pad, which will use the topography of the area, is "protecting the geomem- brane from any puncture due to rock exposure or friction over top of exposed rocks," Beck said. "There are some steep slopes and some exposed rock. Designing Leach Pads for the Longview Experts say studies conducted early should give insight that can be used throughout the life of the system By Jesse Morton, Technical Writer Two heap leach pads are planned for Filo Mining's Filo Del Sol polymetallic op situated high in the Andes on the Chilean border. (Photo: Filo Mining)

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