The Rochester Mine employs a comprehensive heap leach processing circuit for the extraction of silver and gold, with several key components and operational parameters detailed below:
1. Crushing Circuit
– Historical Method (1986–2019):
– Utilized a three-stage crushing circuit with tertiary cone crushers to produce a nominal 3/8-inch product.
– From 2003–2019, two Nordberg MP800 cone crushers operated in open-circuit configuration (~9.5M tons/year).
– Current Method (2019–Present):
– High-Pressure Grinding Rolls (HPGR) replaced tertiary cone crushers to optimize gradation, permeability, and recovery.
– X-Pit Crusher:
– Primary crushing via jaw crusher, followed by screening and secondary crushing.
– Tertiary HPGR is being fine-tuned for optimal heap leach performance.
– Throughput: ~9.5M tons/year (historical), with pebble lime added for pH control.
– Limerick Crusher (Future, POA 11 Expansion):
– Designed for 78,000 tpd (~28.5M tons/year).
– Will produce a 3/8-inch product for placement on the Stage VI heap leach pad (HLP).
2. Heap Leaching
– Current HLPs:
– Four valley-fill HLPs (Stages I–IV) with capacities ranging from 23M to 180M tons.
– Stage IV (Active): Receives crushed and ROM ore, with ~139.9M tons placed (78% of design capacity).
– Stage VI (Future, 2022–2038):
– Permitted for 300M tons, with Merrill-Crowe plant capacity of 13,750 gpm.
– Leaching Process:
– Cyanide heap leaching with solution application at ~0.004 gpm/ft².
– Optimal conditions: pH ~10.0, cyanide concentration 1.5–3 lb/ton.
– Pregnant solution collected via gravity drainage and processed in the Merrill-Crowe plant.
3. Merrill-Crowe Recovery & Refining
– Process Flow:
1. Clarification: Leaf filters remove suspended solids.
2. De-aeration: Vacuum towers (Crowe towers) remove dissolved oxygen.
3. Precipitation: Zinc dust addition forms metal precipitates.
4. Filtration: Plate-and-frame presses separate precipitates.
5. Refining:
– Retorting removes moisture/mercury.
– Flux-smelting in a propane-fired furnace produces doré bars (~350 lbs each, 98% Ag, 1% Au).
– Recovery Rates:
– Gold: 89% (project-to-date, varies by pad).
– Silver: 57% (project-to-date, varies by pad).
– Merrill-Crowe efficiency: ~99% for both Au and Ag from solution.
4. ROM Ore Processing
– Blasted but uncrushed ore is directly placed on leach pads and capped with lime-enhanced crushed material for pH control.
– Historically used but not currently in the mine plan.
5. Key Technical Challenges & Solutions
– HPGR Optimization: Adjusting operational parameters for gradation and permeability to enhance leach recovery.
– Future Expansion (POA 11):
– Stage VI HLP and Limerick crushing facility will support extended mine life.
– New Merrill-Crowe plant (13,750 gpm) to handle increased solution flow.
6. Capacity & Throughput
– Current Crushing: ~9.5M tons/year (X-Pit).
– Future Crushing (Limerick): 78,000 tpd (~28.5M tons/year).
– Leach Pad Capacities:
– Stage IV: 180M tons (78% filled).
– Stage VI: 300M tons (future).
7. Notable Innovations
– HPGR Adoption (2019): Improved crushing efficiency and leach performance.
– Merrill-Crowe Upgrades (2011–Present): Increased solution flow from 5,400 gpm to >12,000 gpm with 99% recovery efficiency.
This summary provides a detailed technical overview of the Rochester Mine’s processing and recovery methods, emphasizing operational parameters, equipment, and future expansions.
