Aggressive expansion planextends Coeur’s Rochester Mine

Following more than a century of active mining at the site of Coeur Mining’s Rochester silver and gold mine in Pershing County, NV, the company reached a critical turning point in 2018 when it became evident that the highest-grade ore at the long-operating mine was mostly depleted from an asset that was already considered a low-grade deposit. In 2016, Coeur’s plan of operations amendment (POA) 8 commenced to extend the life of the mine to 2023, and in 2017 its POA 10 heap leach expansion plans began. However, it was evident that more production was needed if the mine’s operations were to continue over a longer period of time. “We looked at the fact that the costs were rising and the grade wasn’t getting any better,” Coeur Mining chief operating officer Mick Routledge told Mining Engineering. “We realized that we would have to improve performance and that we would have to make a significant investment in Rochester.” With that, the team began to plot POA 11, which would become the largest expansion in the mine’s history. The $730 million expansion was completed in 2023 with the commissioning of the new 300-Mt (330 million-st) stage VI heap leach pad and new Merrill-Crowe processing plant. With the completion of the expansion, the mine becomes the largest openpit heap leach operation in North America and the third largest in the world. It is also the largest silver reserve asset in the United States, the third largest in North America and the fourth largest in the world. POA 11 has transitioned Rochester into Coeur’s long-lived, transformational cornerstone asset and positioned it to generate significant free cash flow. This comes at an opportune time as silver price and demand are expected to increase. Silver is the most conductive metal of electricity and as such, it is being used in numerous green energy technologies, primarily in solar panels, electric vehicles (EVs) and 5G technology. By 2030, EV demand in the United States and Europe alone could require as much as 2.5 billion oz of silver. This growing demand is also leading to chronic structural deficits. Over the past four years, there has been a deficit in the silver balance; in the past year there has been a supply deficit of 142 million oz, which is projected to grow to above 200 million oz in 2024, Routledge said. Global silver demand is forecast to reach 1.2 billion oz in 2024 which would be the second-highest level ever. Industrial applications are forecast to post a 4 percent rise in 2024 to a record 690 million oz. “Inventories of silver are dropping hard and the fundamentals as an investment tool as well as an industrial metal will be the driver,” Routledge said. “Fundamentally, silver is usually a secondary product, which means it is very hard for others to ramp up production to meet the price peaks. As a primary silver producer we can and have been able to ramp up.” With the expansion, the Rochester Mine has become the primary silver source for Coeur in the United States. But the path from planning to completion was an interesting and challenging one. There were early victories such as the successful commissioning of a new crushing circuit and high-pressure grinding roll (HPGR) unit along with the substantial construction of the stage VI heap leach pad. There were challenges as well, not the least of which were a global pandemic, the historically high inflation that followed, and the fortunate find of higher-grade ore in a most unfortunate place, under old site infrastructure.Through it all, the expansion project continued. Plans changed along the way and eventually a massive three-stage crushing plant (the key to the expansion) was built and in July 2024, the company reported that it had achieved midyear target run rates of a daily throughput rate of 80 kt/d (88,000 stpd), which is over two and a half times more than what was previously achieved at the mine. “With the ramp-up to steady-state throughput now achieved, Rochester is expected to lead the company to a free-cash-flow inflection point during the second half of the year, which will enable the company to reduce debt levels,” said Mitchell J. Krebs, chairman, president and chief executive officer of Coeur. “The team has worked tirelessly to complete the ramp-up and will now turn its focus to a range of optimization efforts while establishing an operating rhythm at the higher mining, crushing and placement rates now being achieved.” In July, Mining Engineering visited the Rochester Mine to learn more about the expansion project that reshaped the northern Nevada mine. Rebirth of the mine “We are still mining the same resource that we have always mined, and we are discovering more about the resource as we go,” Tim Dimock, general manager, Coeur Rochester Mine, told Mining Engineering. The Rochester and Rochester East pits are currently being mined while exploration work continues on a number of targets on site. During exploration, one of the discoveries that was made was that some of the highest-grade ore that remained at the site was also unfortunately below existing infrastructure. A number of facilities including the old crusher, water tank and offices were moved to give access to the ore. The initial plans were to increase throughput production from a range of 10 to 12 Mt/a (12 to 14 million stpy) to about 20 Mt (22 million stpy), but it was quickly decided that the throughout would have to be higher, and eventually that target was pushed to 30 Mt/a (32 million stpy), an increase in production of two and a half times. To reach that level of production the mine required a massive upgrade to its crushing circuit. Crushing circuit Rochester needed to increase the volume of ore being processed to compensate for the low-grade ore; thus, the largest part of the eventual $730 million investment went into the three-stage crushing circuit that incorporates the HPGR technology and pre-screens that are designed to help accelerate the rate of silver recoveries and lower unit costs at a higher throughput rate. Improved efficiency was the key to the expansion plans. The mine has very low strip ratios, about 0.2 to 1, but it also has some complex and difficult ground conditions with three types of ore — with varying types of ore hardness — making it difficult to blast for consistent size to be fed to the primary crusher. “The system needed to be able to cope with all those types of ore, especially from a fines generation perspective, because if you put too many fines on a heap leach pad it blinds the heap leach pad and disrupts the steady flow of solution down through the pad,” said Routledge. “We needed to have an even structure across the whole particle size distribution (PSD) and it all needed to work together as a system.” The three-stage circuit starts with a massive Metso 60 × 89 in. gyratory crusher that reduces the PSD from a maximum feed size of 36 in. to a minus 5 in. product. The crushed ore is then fed into one of two Nordberg MP 1000 secondary cone crushers by conveyor, and the MP 1000 product then feeds into one of two HPGR tertiary crushers. There are vibrating screens ahead of the cone crushers and the HPGR crushers to bypass smaller ore to the final product belt. The final stage includes two HPGRs that crush the ore to less than 5/8 in. The final material is then transported to the stage VI leach pad by truck. The goal was to find a way to control the fines to below the 10 percent mark, which was challenging with the various ore types. While looking over the Rochester pit at a blast pattern that would happen later in the day of Mining Engineering’s visit, production superintendent Aaron Evans explained that the new crushing system has allowed for a bit more wiggle room at the blast. He compared the old and new crushers to the Three Bears in the Goldilocks fable, with the old crusher much like the bears each looking for the perfect porridge. With the previous crushing circuit, the team was always in search of the perfect porridge to feed the crusher; with the new system in place, there is more room for different-size ore following the blast. “The screening plant ahead of the HPGR circuit was not in the original flow sheet, but there were just too many fines coming from the pit,” said Routledge. “The HPGR is a fantastic machine but it was already getting too much fine material so we had to have a way to screen that material and pass it around the HPGR. We built this screen system with an additional Kamengo bypass feeder and use an intelligent camera system to optimize size distribution, relieving potential bottlenecks for the future. We can now bypass about 40 percent of material already meeting sizing targets around the HPGRs.” By screening the material the team has the ability to control the product going to the final load-out area. This provides the flexibility to manage the ore and to create the correct PSD and gradation profile. It allows the team to maximize the recovery from the heap leach pad as well. Heap leach pad With a 300-Mt (330 million-st) capacity, the stage VI heap leach pad is by far the largest heap leach pad on site. To date, the mine has placed approximately 17 million tons on the stage VI heap leach pad, which is just about 5 percent of the overall capacity provided for by the mine expansion. Material from the crushing circuit is placed on the leach pads, and sodium cyanide is used to leach the metal from the mineralized ore. The mine expects full-year 2024 production to be between 4.8 million and 6.6 million oz of silver and between 37,000 and 50,000 oz of gold. The stage VI heap leach pad will feed the new Merrill-Crowe processing plant while the previous refinery and Merrill-Crowe plant remain in operation to take residual leach material from pads II, III and IV, said Dimock. Merrill-Crowe processing plant “It has been pretty terrific in terms of the engineering and how fast we were able to get this up and in production,” said Zack Erickson, process plant superintendent. “It has been really intuitive and great to run.” There was a steep learning curve for the new Merrill-Crowe processing plant, which includes more instrumentation and controls, but it is simplified compared to the old Rochester Merrill-Crowe plant. “It has taken a few months to get up to speed and to get it up and running, but now we are in full operation,” Erickson said. One of the most complex aspects of the processing plant has been fine-tuning the de-aeration tanks. “They are the lungs of the operation,” Erickson said. “If we have too much oxygen in the solution we are not able to efficiently recover the metal, and this operation is all about efficiency.” Once the cyanide solution is recovered from the heap leach pad, it is passed through the initial filters to remove any debris. After that, it runs through the plant to the clarified pregnant tank, which feeds the Vieola de-aeration tanks where oxygen is pulled out of the solution. After passing through the de-aeration tanks, super fine zinc is added to the stream, at which point the silver and gold are precipitated out of the solution to create a low-percent-solid slurry.“The solid particles are our concentrate,” said Erickson. “To catch those particles we send them through the four 48-in. FLSmidth filter presses. They use every single plate at the same time, it is a unique design. They are fed solution at all times, and that solution then passes through a membrane through our barren solution to the barren tank where it is pumped back up the pad and starts the process all over again. It is a closed circuit with zero discharge.” Erickson said the Merrill-Crowe plant is operating at an average of 9,000 gpm and has operated as high as 11,000 gpm with headroom capacity up to 13,750 gpm with recovery rates of more than 97.5 percent for gold and more than 99 percent for silver. The Rochester Mine beneficially uses its water, recycles it and does not discharge any water, and thus it is a zero-discharge facility. Since 1991, Rochester employees have won 15 awards, recognitions and proclamations for excellence in environmental protection. These include awards for reclamation, wildlife habitat enhancement, conservation measures, cultural resources and operational efficiencies. Other equipment As part of the expansion, the mine expects to add 19 Cat 789 181-t (200-st) haul trucks to replace its existing fleet of haul trucks. The Caterpillar trucks, which will begin to be phased into operations later this year, are equipped with autonomous-ready solutions built in. The larger trucks will support an increase in the tons of ore delivered to the primary crusher and could blend in to a more autonomous future at the mine. “We are implementing a new truck dispatch system,” said Routledge. “Once we have that in place we will look into putting in a remote monitoring center, then a remote operating center and possibly an autonomous control center. Those are the long-term thoughts.” The mine plans to continue operating its Komatsu PC4000 shovel and two Cat 994K loaders.Drilling equipment includes three Atlas Copco DML drilling rigs and one Cat 650. The proposed production plan is expected to increase the mining rate from 65 kt/d (71,600 stpd) to 155 kt/d (170,000 stpd) by the end of 2024. Future of Rochester The POA 11 expansion gives Rochester a current mine life of 16 years, considering only proven and probable reserves, but Coeur’s large and prospective district-scale landholding gives lots of opportunity to extend mine life by exploration. Currently, exploration is focusing on three priority near-mine targets: East Rochester, Nevada Packard and Lincoln Hill. At East Rochester, re-logging of historic drill holes and geological mapping of the open pit have delivered a better understanding of the structural controls to mineralization within the deposit with implications for the wider district potential. This new interpretation has led to the identification of several high-angle faults with significant potential for additional higher-grade mineralization. Re-logging and mapping at Lincoln Hill is being incorporated into a new geologic model with the new information expected to be applied to further developing this resource. The past-producing Nevada Packard deposit hosts similar geology and structure to the Rochester deposit with additional oxide potential bounded by the Blackridge Fault to the east. The fault corridor between the Rochester and Nevada Packard pits is a high-priority target in 2024-2025.

Aggressive expansion planextends Coeur’s Rochester Mine

Authored by: William Gleason

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To view the past issues of the UCA magazine you must be a member of SME/UCA.

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