Engineering & Mining Journal

APR 2019

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LEACH PAD DESIGN APRIL 2019 • E&MJ 29 We are planning to use some geotextile as a cushion were needed to protect the geomembrane from puncture." The on/off pad will be built using cut and fill and backfill "to develop a flat foundation for the geomembrane," Beck said. "No protective underliner is going to be required." The overliner material for both pads will be a permeable gravel, possibly gran- ite, Beck said. Stacks will max out at 7.5 m on the on/off pad. "The total area there is about 670,000 m 2 ," Beck said. "The load is pretty small and didn't drive the design of the liner system." "The valley fill pad we are planning on building is in two phases," he said. The first phase will accumulate 1.6 mil- lion m 2 of ore, followed by an expansion upward of an additional 630,000 m 2 of lined area. "The plan is to reach heights of about 100 m to get a total volume of 160 million m 3 ," Beck added. The mine is expected to move some 60,000 tons per day. "Process recovery is expected to be 80% for the copper, 70% for the gold, and 82% for the silver," Beck said. The project could span 14 years. Headlining milestones for 2019 cen- ter on drilling and testing. "We've got deep drill holes going in this season," Beck said. "We've identified a good por- tion of the oxide deposit, which this heap leach plan is addressing." The orebody sits atop a "big sulfide deposit," he said. "Before we get too far down the line on a construction decision, we want to understand more about what is lying underneath." At the core of the advice Beck has for miners in a similar situation is thor- ough testing to arrive at a comprehen- sive understanding of the orebody. "You can learn so much about things through your metallurgical test work," he said. "You want to make sure you are doing a significant and substantial number of tests so that you've got confidence that ultimately when you get up there and build this thing and put it into produc- tion, you are going to achieve the pro- duction targets." Beck said that while testing it is crit- ical to cross-reference results to further backstop key findings. "Don't be shy in running lots of bottle-roll tests and trying to determine correlations between similar samples: how they performed in a column leach and how they performed in a bottle roll," he said. "If that correlation proves to be strong, it can help you understand variability over the entire deposit." Such tests so far have enabled the company to conclude that the planned sequential leach process will be low cost up front and will entail relatively low oper- ating costs, Beck said. "The heap leach- ing at Filo allows us to have what looks to be a robust project," he said. "That is going to drive stakeholder returns." Gold Mine to Build HLP in Stages Minera Alamos Inc. told E&MJ it will like- ly build a gold heap-leach pad system in stages at its Santana gold project in So- nora, Mexico. Exploration started at the 8,500-hect- are project in 2008. Drilling revealed "the mineralization was open along the strike and to depth." The promising zones were described as having low-sulfi - dation hydrothermal breccias with quartz, calcite and some iron minerals. Drilling highlights included sizeable intersections near or at the surface with grades of about a gram per mt (g/mt), and including some grading at almost 2 g/mt. Bulk-test mining wrapped up last year, with roughly 50,000 tons of mate- rial moved, crushed and tested. "Approx- imately 1,100 oz of gold were recovered as a result of these bulk tests with addi- tional residual gold still being recovered from the leach pad," the miner reported. Minera Alamos reported it will publish the first technical report and resource es- timate on the project around mid-2019. Santana is one of three projects the com- pany operates in Mexico, all of which are in the permitting and planning stages. The pad at Santana will likely be built in stages for two obvious reasons, Darren Koningen, CEO, Minera Alamos, said. "You typically want to build them for the first year or two years and then expand them later," he said. "The reason we build these things in stages is not only capital related but is also done in order to assure we have adequate capacity in our collection pond system." The ponds have to be capable of han- dling whatever runoff is possible. Santa- na is subject to what Koningen called a three-months rainy season. "The worst thing you can do in that situation is build a huge leach pad with nothing on it," he said. "And then the rainy season comes, and you are collecting the water over a very large lined area." A better scenario is to have multiple levels of material in place at the start of the rainy season, Koningen said. "The ore in the heap acts as a sponge to absorb rainfall during large rain events, and by not having large unused areas of the heap helps us with our solution management by minimizing surges into the ponds," he said. "We typically evaluate these sce- narios as part of our engineering design using a water balance model." The biggest pad-design issue at the company's mines often centers on slopes, he said. "If you look at most of the work that has been done over the years, the weak point, as far as friction angels are concerned, is between the liner and the clay below the liner," Koningen said. The goal is to build the pad at an angle that does not exceed a certain threshold, which, he said, was typical- ly less than 9% to 10%. "Stability can often be further improved through an additional more-shallow buttress on the order of 2% to 3%, typically near the toe of the leach pad," Koningen said. "Otherwise, you have to stage it and bench it so you can have steeper areas. Above, a heap leach pad is prepped at Minera Alamos Inc.'s Santana gold project in Sonora, Mexico. (Photo: Minera Alamos)

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