The Rochester Mine is an open pit, heap leach, and Merrill-Crowe operation in Lovelock, Nevada, that has been processing gold and silver ore since 1986.
Report context
This technical report, dated December 16, 2020, covers the Rochester Mine located in Lovelock, Nevada, U.S.A. The report describes the processing facilities as of late 2020, including historical operations from 1986 through October 2020, and proposed future expansions under Plan of Operations Amendment (POA) 11.
Processing route
Crushing
Rochester historically utilized a three-stage crushing circuit with cone crushing in the tertiary position from 1986 through mid-2019 to produce a nominal 3/8-inch product. In 2019, the operation adopted HPGR crushing technology to replace the cone crushers in the tertiary position. The report states that HPGR operating parameters are being optimized for gradation, permeability, and recovery.
The X-pit crusher was installed in 1986. From 1986 to 2003, a closed-circuit tertiary crushing system achieved a 3/8-inch product. In 2003, the closed-circuit tertiary system was replaced with two Nordberg MP800 crushers in open-circuit configuration to achieve a nominal 3/8-inch product at approximately 9.5 million tons per year. In 2019, the open circuit tertiary cone crusher system was replaced with a single HPGR crusher. The crusher is permitted to operate 24 hours per day, 365 days per year.
Currently, material from the mining operations is end-dumped from Komatsu HD1500 150-ton haul trucks into a dump hopper. Material is then processed via three-stage crushing: jaw crushing, screening, secondary crushing, and tertiary crushing. Pebble lime is added to the final crushed product belt to control pH during heap leach processing. Overland conveyors deliver final crusher product to the load out area stockpile near the Stage IV leach pad, and Komatsu HD1500 haul trucks transfer material onto the active leach pad.
Historical placement of X-pit product was on Stage III HLP from 2011 through mid-year 2017. Since July 2017, the Stage IV leach pad has been receiving all X-pit crusher product.
ROM ore
ROM ore is classified as blasted but uncrushed ore transferred directly to the leach pads from mining operations via haul trucks. The ore is capped with lime-enhanced crushed product for pH control during leaching. No ROM is included in the current mine plan, though it has been used historically.
Heap leaching
There are currently four dedicated valley-fill heap leach pads (HLPs) at the Rochester Mine: Stage I, II, III, and IV. The leach pads are typically constructed in 30- to 60-foot lifts with ultimate heights ranging from 200 to 400 feet above liner.
Stage I has been filled to design capacity, recontoured, and reclaimed with topsoil and vegetation. Stage II has been filled to design capacity and is periodically leached. The Stage III leach pad has a final design capacity of approximately 90 million tons and achieved its design capacity by 2019. Stage IV was loaded with crushed and ROM product from the 1990s through 2013 with approximately 100 million tons, and was partially expanded under POA10 to add approximately 80 million tons of additional capacity, with the second phase completed in 2019.
Future leach pads include Stage V and Stage VI expansions. The Stage VI leach pad is permitted to contain 300 million tons and is proposed to be put into operation in 2022 as part of POA 11. An additional expansion of Stage IV and Stage VI is being contemplated for mine life beyond 2032.
Leaching and recovery
Leaching on the heap leach pads utilizes solution containing cyanide applied via drip tube at a rate of approximately 0.004 gallons per minute per square foot. Efficient silver extraction occurs at a pH near 10.0, with cyanide concentrations maintained at 1.5 to 3 pounds per ton of solution. Pregnant solution percolates to the pad liner and migrates via gravity drain lines to a collection point.
The Merrill-Crowe process plant clarifies pregnant solution using leaf filter clarifiers, removes dissolved oxygen using two vacuum de-aerator towers, and adds zinc dust to precipitate precious metals. The precipitates are separated using plate and frame filter presses.
Refining
Precious metal precipitates are removed from filter presses, placed into trays, and retorted to remove moisture and extract mercury. Batch flux-smelting using a propane-fired furnace follows. Slag impurities are skimmed, and the final product is poured into doré bars.
Since 2011, the Merrill-Crowe plant has improved solution flow from 5,400 gpm to greater than 12,000 gpm, with gold and silver recovery rates of nearly 99% from precious metal in solution.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Historical crushed ore placed (1986–October 2020) | 267,429,430 tons | Actual |
| Historical ROM ore placed (1986–October 2020) | 44,489,538 tons | Actual |
| Gold contained placed (all production) | 2,007,643 oz | Actual |
| Silver contained placed (all production) | 283,744,981 oz | Actual |
| Historic tertiary crusher rate | ~9.5 million tons/year | Design |
| HPGR tertiary operation | Single HPGR, 2019 to present | Operating |
| Limerick crusher design rate | ~78,000 tpd / ~28.5 million tons/year | Proposed design |
| Stage III leach pad capacity | ~90 million tons | Design |
| Stage IV leach pad approved capacity | 180,000,000 tons | Design |
| Stage VI leach pad permitted capacity | 300,000,000 tons | Proposed/Permitted |
| Merrill-Crowe solution flow | >12,000 gpm (improved from 5,400 gpm since 2011) | Actual operating |
| Limerick Merrill-Crowe design flow | 13,750 gpm | Proposed design |
| Gold recovery, project-to-date (all pads) | 89% | Project-to-date actual |
| Silver recovery, project-to-date (all pads) | 57% | Project-to-date actual |
Project website: https://www.coeur.com/operations-projects/rochester-nv/default.aspx
Technical qualifications
The report notes that the X-pit crushing facility will be removed and replaced with the Limerick crushing facility as part of POA 11. The Limerick Merrill-Crowe process plant is proposed to operate from 2022 through approximately 2038, sized for 13,750 gpm to process solution from the Stage VI leach pad.
The report states that the facility has sufficient capacity to process planned feed material with POA 11 approval, and sufficient energy, water, and process materials are available.
Source: 2020 Rochester Mine NI 43-101 Technical Report, December 16, 2020, Sections 17.1–17.6.
