1. Processing and Recovery Methods Overview
The Fort Knox Mine operates two distinct ore processing lines:
– Mill Circuit (CIP): Comprising crushing, grinding, agitated cyanide leaching, and carbon-in-pulp (CIP) recovery.
– Heap Leach (CIC): A run-of-mine (ROM) valley-fill heap leach operation with carbon-in-column (CIC) gold recovery.
Gold is recovered from both circuits via electrowinning and refined into doré bars at the on-site refinery.
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2. Mill Circuit (CIP) – Detailed Analysis
Feed & Capacity:
– Processes higher-grade ore at 36,287 t/d (40,000 stpd).
– Operates 24/7, achieving 81–83% recovery (dependent on feed grade).
Crushing & Grinding:
– Primary Crushing:
– Gyratory crusher (1.52×2.59 m, 700 hp) reduces ore to <20 cm (8 in).
– Capacity: 65,317 t/d (72,000 stpd).
– Conveyed 800 m to a 272,000 t (300,000 st) coarse ore stockpile.
– Grinding:
– SAG Mill: 10.4×4.6 m (34×15.25 ft), dual 7,000 hp motors.
– Discharge screened; oversize (“pebbles”) crushed by a 2.13 m (7 ft) cone crusher (500 hp).
– Ball Mills: Two parallel lines, each with a 6.1×9.1 m (20×30 ft) ball mill (7,000 hp) in closed-circuit with hydrocyclones.
Gravity & Leaching:
– Coarse Gold Recovery:
– Knelson concentrators recover gravity gold, processed in an Acacia reactor (high-temperature leaching).
– Gold electrowon and refined; tails returned to grinding.
– Leaching & CIP:
– Hydrocyclone overflow passes through trash screens, thickened, and pumped to seven-stage leach tanks (94,000 gallons each) with cyanide.
– CIP Circuit: Gold adsorbed onto activated carbon; loaded carbon stripped via hot caustic elution, electrowinning, and refining.
– Tails detoxified and stored in the Tailings Storage Facility (TSF).
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3. Heap Leach (CIC) – Detailed Analysis
Facilities:
– Walter Creek Heap Leach (WCHL):
– Valley-fill ROM heap leach with 15 m (50 ft) lifts.
– Impermeable synthetic liner; year-round irrigation via buried drippers.
– 416 ML solution reservoir; pumped to dual CIC circuits (61,000 L/m capacity).
– Barnes Creek Heap Leach (BCHL):
– Future expansion, similar to WCHL, with in-heap solution storage.
Operations:
– Leaching: Cyanide solution irrigates heaps; gold-laden solution processed via CIC.
– Gold Recovery:
– Activated carbon in CIC adsorbs gold; stripped and electrowon.
– Series Operation: WCHL feeds BCHL; combined pregnant solution processed in CIC.
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4. Key Technical Specifications & Innovations
– Equipment Highlights:
– SAG/Ball Mills: High-power (7,000 hp) for efficient grinding.
– Acacia Reactor: High-intensity leaching for gravity concentrate.
– CIP/CIC Circuits: Optimized carbon adsorption with elution/regeneration.
– Throughput:
– Mill: 36,287 t/d; Heap Leach: Variable (ROM-dependent).
– Recovery Rates:
– Mill: 81–83%; Heap Leach: Not explicitly stated but relies on CIC efficiency.
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5. Challenges & Solutions
– Mill Shutdown (2020): Due to TSF capacity limits; transitioned to full heap leaching (BCHL).
– Heap Leach Variability:
– Cell geometry and snow impacts addressed via flexible stacking/irrigation.
– Dual-pad system (WCHL + BCHL) optimizes solution flow.
– Power Demand:
– Mill consumes 200,000–300,000 MWh/year; heap leach less energy-intensive.
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6. Conclusion
Fort Knox employs a hybrid processing approach, combining high-capacity milling for high-grade ore and scalable heap leaching for ROM material. The integration of gravity recovery, CIP/CIC, and electrowinning ensures robust gold recovery, while future focus shifts to heap leaching post-mill closure. Operational flexibility and advanced equipment (e.g., Knelson concentrators, Acacia reactor) underscore the site’s technical sophistication.
