The Mount Milligan Mine employs a comprehensive mineral processing circuit designed to recover copper, gold, and silver from run-of-mine (ROM) ore. The plant was originally designed for a nominal throughput of 60,000 t/d, later increased to 62,500 t/d following the installation of a secondary crushing circuit in 2016 and process optimizations.
Processing and Recovery Methods
1. Crushing Circuit
– Primary Crushing:
– Equipment: 1.525 m x 2.794 m (60″ x 110″) gyratory crusher
– Output: 150 mm top size
– Capacity: 3,500–4,000 t/h
– Secondary Crushing (added in 2016):
– Two 1,000 kW cone crushers
– Target product size: 60 mm or finer
– Live ore stockpile capacity: 60,000 t
2. Grinding Circuit (SABC Configuration)
– SAG Mill:
– Dimensions: 12.20 m (40’) diameter x 6.71 m (22’) EGL
– Drive: 23.5 MW gearless motor
– Discharge screen oversize recycled to pebble crushers (target: 12 mm product)
– Ball Mills (2 units):
– Dimensions: 7.32 m (24’) diameter x 12.50 m (41’) EGL
– Power: 26 MW total (two 6.5 MW motors per mill)
– Operated in closed circuit with twenty 840 mm (33”) cyclones
– Pebble Crushers (2 units):
– Each powered by 750 kW motors
– Final Grind Size:
– Design: 80% passing 212 µm
– 2019 average: 175 µm
3. Flotation Circuit
– Rougher/Scavenger Flotation:
– Two parallel trains of five 200 m³ tank cells (two rougher, three scavenger)
– Natural pH, 35–40% solids slurry density
– Reagents: PAX (potassium amyl xanthate), A3409 (gold collector), Polyfroth H28C (frother)
– Mass pull target: 12%
– Regrinding:
– Rougher concentrate: Reground to 80% passing 40–50 µm in a 1,125 kW tower mill (six 500 mm + two 375 mm cyclones).
– Rougher-scavenger concentrate + cleaner tails: Reground to 80% passing 18–25 µm in two 3,000 kW IsaMills™ (twenty-six 250 mm cyclones).
– Gravity Gold Recovery:
– Centrifugal concentrator recovers coarse gold from 20% of rougher concentrate regrind cyclone underflow.
– Gold concentrate is blended into final flotation concentrate.
– Cleaner Flotation (3 stages):
– First cleaner: Two 100 m³ tank cells
– Second cleaner: Four 30 m³ tank cells
– Third cleaner: Two 30 m³ tank cells + one 4 m x 12 m flotation column
– Final concentrate grade: ~21.5% Cu, 30–40 gpt Au
– pH control: ~11.3 for pyrite rejection
4. Tailings Management
– Rougher-scavenger tailings: Non-sulphide gangue, stored in TSF.
– Cleaner-scavenger tailings: Sulphide gangue, stored underwater in a lined pond to prevent acid generation.
5. Concentrate Handling & Dewatering
– Thickening: 12 m diameter high-rate thickener (60–65% solids).
– Filtration: 96 m² pressure filter (target 8.5% moisture).
– Transport: Trucked to Mackenzie rail loadout, then shipped from North Vancouver.
6. Power Consumption (2019 Data)
– Total: 535,708 MWh (32.76 kWh/t ore)
– Major consumers:
– SAG mill: 33.4%
– Ball mills: 29.1%
– Regrinding: 7.3%
7. Innovations & Challenges
– Secondary crushing circuit (2016) increased throughput by 4.2%.
– IsaMills™ for ultrafine regrinding improve liberation.
– Gravity gold recovery enhances overall gold recovery.
– Tailings segregation mitigates environmental risks (acid drainage prevention).
Conclusion
The Mount Milligan processing circuit integrates crushing, SABC grinding, flotation, regrinding, and gravity concentration to produce a high-grade copper-gold concentrate. Operational optimizations, including secondary crushing and advanced regrinding, have enhanced efficiency, with a nominal throughput of 62,500 t/d and a final concentrate grade of ~21.5% Cu and 30–40 gpt Au. Power-intensive grinding dominates energy use, while tailings management ensures environmental compliance.
