The Crusher Corridor for the Rochester mine’s POA11 descends over the course of a mile and provides
32 million tons per year of capacity. (Photo: Coeur)

Coeur essentially built a similar but much larger ore handling and mineral processing facility on the other side of the ridge from its existing pit or the X pit operations. At the top of the ridge, a dual truck dump feeds a massive Metso primary (gyratory) crusher. A conveyor system transports the coarse ore down the ridge to a stockpile that feeds a dual set of Metso MP 1000 secondary crushers. Conveyors transport the ore from the secondary to General Kinematics (GK) screening installation that sifts the ore and separates the material already meeting size targets prior to a set of two high-pressure grinding rolls (HPGRs), which create a stockpile of ore that is placed on the leach pad by haul trucks.

“This is the largest operation of its kind anywhere in the world,” said Mitch Krebs, chairman, president and CEO, Coeur Mining. “It will be the country’s largest domestic source of silver, produced and refined here in America. The project itself is massive. From the point where the trucks dump ore into the gyratory to where it is loaded out is more than a mile long. For the first time, Rochester has a relatively flat heap leach pad, which is also the largest heap leach pad ever built.”

The new Stage 6 heap leach pad is massive. Rochester has already placed 15 million tons on it, and it will grow to more than 300 and up to 400 feet high throughout the next 10 years of the mine’s life, ahead of the next phase of heap leach pad development. The pregnant leach solution is processed at a Merrill-Crowe facility with a 13,800 gallon per minute (gpm) capacity. It’s nearly twice the size of the plant used on the other side of the ridge.

Timing as they say is everything. Construction on POA 11 started in 2020, at the same time as the onset of COVID-19. The pandemic not only impacted activities at the mine as far as construction, but it also impacted Coeur’s ability to secure various components, which extended timelines. COVID-19 was only one of the obstacles the company had to overcome.

The capacity of the mine’s haulage system will more than double over a 2-year period. (Table: Coeur)

Construction Starts During the Pandemic
The baseline permitting for the project began in 2015. Coeur worked with its neighbors to secure the water rights for the project and in 2018 it secured a land package adjacent to Rochester. In 2020, construction of the Stage 6 leach pad began along with pre-construction activities in the X pit, where Rochester was commissioning a new crushing circuit using one of the MP1000s and an HPGR.

During its May 2020 meeting, the Coeur board approved the POA 11 Expansion. When they received the official thumbs up from the board, Krebs recalls the directors wondering if they should even meet in person with the way the world was evolving. “The pandemic was setting in, everything was starting to really grind to a halt, and that’s when we decided to push ‘Go’ on this project,” Krebs said.

At roughly the same time, Krebs decided to bring Mick Routledge, senior vice president and COO, Coeur, on board. He had been involved with several large-scale mining projects, and his career path included 20 years with Rio Tinto. Routledge recalled the initial X pit activities. “We did some interesting things in the old Rochester area,” he said. “It was very painful financially, but it worked out spectacularly. We had used a jaw crusher to replicate the gyratory. Down the lane, instead of having an MP 800, we installed an MP 1000 and we had previously implemented an HPGR as the tertiary crusher. We made a significant investment in the screen plant in between the secondary and tertiary to help us test and manage all the different ores in the pit.”

A pre-screen installation was placed between the secondary and tertiary crushing stages. (Photo: Coeur)

Coeur decided to change the design of the 3-stage crushing system for POA 11 and add a $100 million screening plant between the secondary and tertiary crushers. “The new gyratory crusher, which is a beast, feeds two parallel lines effectively, with each having an MP 1000 and an HPGR,” Routledge said. “We decided to build a giant screen tower now equipped with GK screens and a Kamengo bypass feeder. We just doubled down on this new setup with everything that we had learned in the X pit during those two years.”

This approach worked really well, Routledge explained. “We went from zero to commercial production levels above 70% within one month,” he said. “We hit a nameplate average of, 88,000 tons per day (t/d) by the end of June. All of this took place within four months. And really that was a testament to the team and the operational readiness program. We had hired all the people, and we had the team in place. This was a very efficient expansion — a 20% headcount increase for two and a half times the throughput.”

The construction phase had its share of challenges. “The project needed more FEL ahead of sign off,” Routledge said. “The FEL really was lighter than it should have been on construction, design, engineering, metallurgical work, understanding of orebody, etc. There were a lot of work fronts that could have benefitted from being more mature before sign off on this project. So, we learned a lot.”

Granite handled the earthworks which proved very challenging along the crusher corridor and Kiewit TIC managed the construction activities, and they did a great job, Routledge said. Newfields served as the engineer of record for the heap leach pad and some of the drainage systems. “The original estimate for the cut-andfill on the groundworks corridor for the crusher had to be revised,” Routledge said. “We found that it needed a significantly higher level of blasting and rock movement compared to just free dig. That added a lot of time to the schedule.” COVID-19 impacted the construction timeline as well. “Equipment lead times grew and turnover became challenging,” Routledge said. “At one point, with electricians alone we were turning over 30 a month of the 100 we had hired for the job. Finding high quality people was difficult, but Kiewit TIC worked hard to provide the right people for the job.”

While construction was taking place, Rochester ran a series of fullscale haulage tests to determine whether they could feed the primary. The trucks dumped into stockpiles next to the existing crusher. “We wanted to make sure that we could run that cycle and that we had enough trucks scheduled to deliver all of that material,” Routledge said. “We ran five full scale tests. During the first test, we learned a lot. We met the specifications during the second test and then we optimized with the next two tests to make sure that we had the right cycles before we did the fifth and final test. At that point, we were delivering material with a vengeance.”

Over the next 10 years, Rochester will place 300 million tons on its Stage 6 leach pad. (Photo: Coeur)

Managing Fines
Construction was already underway when the decision was made to add the screening facility into the 3-stage crushing circuit. Working with Shijie Wang, Coeur’s general manager of metallurgy, Routledge and a large team of Coeur employees and consultants looked at what was really needed to manage the fines on this system. “It wasn’t going to work without the screen,” Routledge said. “We had to move quickly to add a huge screen system to the flowsheet and shoehorn it into the design because we already had the footprint. Groundworks had already started.”

Today, the new screening installation works as it was planned, helping Rochester to effectively manage fines. From the secondary stockpile, the crushed material passes through a Kamengo feeder to the screening facility. The oversize material from the screening facility, anything greater than 0.25-in., is passed along to the HPGRs. The undersize material bypasses the HPGRs and ends up in the final stockpile. As much as 40% to 50% of the material can be redirected to the final stockpile. The fines are blended with the crushed material from the HPGR before being placed in the final stockpile.

Using a “hoteling” system, Rochester can track performace of the leach pad as it knows
what ore was placed in what room on each floor. (Photo: Coeur)

Rochester uses HPGRs to meet the size specifications. “HPGRs are fantastic at creating bug dust,” Routledge said. “A lot of companies used them ahead of the milling process where they want the ore to be as fine as possible for the flotation circuits. We’re not looking for that level of performance.

“HPGRs also offer really good control as far as size and capacity,” he said. “You can run an HPGR from 60 to 70 bar like we do or up to 150 to 160 bar and crush everything that goes in it. The big stuff gets smaller, but the small stuff gets even smaller. We have enough small stuff.

“We bypass that fine material and use the HPGR to produce a nice, normalized curve so that the size fraction and the matrix that we put onto the heap leach pad has the right flow characteristics,” Routledge said. “You do not want the material to be too fine, but you don’t want it too coarse. You want the cyanide solution to touch the right amount of ore to get the best recovery.

For about every 1/8-in. in top size reduction, Rochester sees approximately 1% increase in its silver recovery. “The use of HPGRs is super valuable,” Routledge said. “If, however, it’s pushed too far and the levels of fines reporting to the heap leach pad grows to 12% to 15% or higher, it will blind the heap leach pad and it will not flow properly.”

Inventory Management
The intermediate stockpiles created at each stage of crushing allows Rochester to manage inventory all the way down the line. If they need to shut down the primary, they can still run the crushing system for around five days because they can place nearly 500,000 tons in that coarse ore stockpile. They could perform maintenance on the gyratory without taking the whole system offline.

“We’ve been working on that strategy all the way down the line so that we can manage stockpiles both from a blending perspective and from a planned maintenance and reliability perspective,” Routledge said. “It gives us those windows of time where the pressure isn’t on to get those tons out the door.”

The Rochester mine achieved commercial production on March 31 and processing at a rate of 88,000 t/d by the end of June. “That is actually an [annual/overall] air permit constraint rather than an equipment capacity constraint,” Routledge said. “If you divide 32 million t/d by 365 days, you get 88,000. We’re operating at levels above a 100,000-120,000 t/d through that crusher and we’re only six months into the ramp up here. The good thing is that we will have headroom. When we have planned down days, we have the inventory capacity in the stockpiles to catch back up and hit the annual target. The expected run rate will need to be somewhere around that 2.5-3 million tons per month to get where we need to be.”

Modern Leach Pad Feeds Larger Merrill-Crowe Plant
Rochester’s No. 6 leach pad is massive. Over the next 10 years, it’s estimated that the mine will stack 300 million tons on it. With a new leach pad, Routledge and his team were able to use the latest tools at their disposal. “We installed flowmeters and instrumentation around all discharge points on this pad,” he said. “As we start building it, we are creating a hoteling system. Like a hotel, the leach pad has floors with rooms on each floor. In each room, we know exactly where the ore originated and the PSD. We can then match the metallurgical column testing on that cell with our resource and reserve model.

“We run the cyanide irrigation system for the first 60 or 90 days depending on the size of the cell, and we can see very quickly the performance in the overall pregnant solution coming from the heap leach pad,” Routledge said. “The system is very sensitive now because we’re close to the liner. It is allowing us to optimize the size fraction of our crushing system during the next six months to make sure we get the best performance for the energy that we must put into that rock. We want to see the best performance we can get out of the pad before we get to the point where it will be between 300- and 400- ft high. At that point, the lead times to understand the performance of each cell will be much greater.

A 200-ton Cat 789 haul truck dumps its load into the gyratory crusher.
(Photo: Coeur)

“It’s very similar to the previous Merrill- Crowe plant from a technical perspective,” Routledge said. “It’s just bigger, and it has dual redundancy. It has more autonomy, particularly with the filter presses. The old Rochester Merrill- Crowe had manual filter presses.” NV Energy constructed a new substation near the Merrill-Crowe plant and new electrical infrastructure for the mine. Rochester’s power consumption has grown from 9 megawatts (MW) to 14 to 16 MW depending on the load at the site.

“Even though it was a brownfield project, it really was a greenfield project in the Limerick Canyon,” Routledge said. “We broke virgin ground, built a crushing system and leach pad from scratch. The Merrill-Crowe facility, high-voltage substation, all the ponds, all the water management, and all the storm management are all brand new.”

Risk and Reward
Rochester has flexibility and flexibility really can help mitigate risks, Krebs explained. “Our team has done a good job of designing a lot of flexibility into the project,” he said. “The fact that it’s behind us now and that everyone can see this really is a big deal.”

The original cost estimate for the POA 11 expansion was $425 million and the original timeline to completion was early 2023. Over time, the price tag grew to $730 million, and the timeline was pushed out one year. The project was affected by external forces beyond the company’s control. It also evolved over time as Coeur and the Rochester team learned, and they pushed the project through to completion. Now they will reap the rewards. “Had we not decided to make the changes when we did, the lessons would be much more painful today,” Krebs said. “While it was ugly in that 2019-2021 period, what we learned allowed us to make the right long-term decisions.

Even though mining was deemed essential, operating during the pandemic was difficult. Due to the mine’s location in northern Nevada, they also had to endure the elements, which included winter seasons with record snowfall and spring seasons with record rain. “The schedule started to really feel some pressure when you have so much snow that you can’t get anybody to the site,” Krebs said. “The weather was definitely another challenge that the project had to face.”

Financing the project was another hurdle the Coeur team had to overcome. With the activities in the X pit, they could no longer count on cash flow from Rochester. They also had to find a way to cover the growing costs. The new screening facility was a big part of that, but there was also the inflationary pressure that every company has had to deal with.

“Once you push Go on a project like this, you can’t push Pause and you certainly can’t push Stop,” Krebs said. “You need to just deal with what’s coming at you and put your head down and grind it through to the end. We had to live with a fair amount of inflation that certainly impacted the final cost of the project. You can try as you might with competitive bids and other tactics, but ultimately if prices go up, you’ve got to pay the prices to keep the project on track. Inflation was very much of a challenge.” Coeur also had to find a way to allocate $100 million of unanticipated additional capital for the screening facility.

“We had to get pretty creative with financing the project,” Krebs said. “We took on more debt. We had to issue some equity along the way to fill the gap. We sold a very exciting exploration project down in southern Nevada to AngloGold for $150 million in cash. We sold some other smaller non-core pieces to help fill in some of those gaps.” Prices were volatile throughout the build as well. “For a project of this size for a company of our size, there was a fair amount of balance sheet and liquidity uncertainty that we had to navigate our way through over the three years or so that we were constructing this expansion,” Krebs said.

“And now to see Rochester flipping into positive cashflow, which will push the company into positive cashflow, where we can start paying down the debt that we’ve incurred, that’s exciting,” Krebs said. While Rochester isn’t feeling pressure to add years to its mine life right now, it has been focused on drilling some of the higher-grade opportunities around the existing operation, Krebs explained. “If we bring some of that higher grade material into the early years of the operation, it will further accelerate that return on investment,” he said.

“Rochester will be our cornerstone asset for a long time as a consistent generator of free cash flow,” Krebs said. “We will continue to reinvest in the business, both in sustaining capital and into exploration at each of our mines. “Our anchor is now located in northern Nevada and, as an American company, that’s the closest thing we have to a backyard,” Krebs said. “It’s a great place to be.” When asked about the next project, Krebs smiled and said, “I think we have more than 100 little projects going on all over the place. And each one of those needs to have the proper FEL and the right team and the right schedule and resources allocated to it.”

As far as the next major project, that will likely be Silvertip in northern British Columbia, Canada. “It’s a huge system,” he said. “Once we understand the size, we will start the permitting process. It’s several years down the road.” Today as Rochester POA 11 continues to ramp up, gold prices have climbed to nearly $2,500/oz and silver is approaching $30/oz. “We could not have drawn it up any better really,” Krebs said. “Sure, we had some cost and schedule pressures along the way, but to be bringing online a huge new source of silver and gold production at a time that prices have increased dramatically. it’s sort of a perfect storm for us in terms of our ability now to generate some strong free cash flow and start to generate a return on this investment we have made.”