Processing and Recovery Methods
The Marigold Mine employs a Run-of-Mine (ROM) heap leaching process combined with Carbon Adsorption/Desorption/Recovery (ADR) and electrowinning to produce a final doré product. The oxide ore is processed entirely via heap leaching, a cost-effective method for gold recovery.
1. Ore Stacking on Leach Pad
– Ore Handling: ROM ore is hauled by trucks and stacked in 6.1 m to 12.2 m lifts on the leach pad.
– pH Control: Pebble lime is added to haulage trucks from a silo to maintain optimal pH before dumping.
– Leach Pad Preparation: Fresh and spent ore are ripped and cross-ripped to enhance permeability.
– Solution Application:
– Drip tubing and impact sprinklers distribute barren solution at 0.122 to 0.143 L/min/m².
– Header and sub-header lines ensure uniform solution distribution.
2. Leaching Solution Distribution
– Pond System:
– Two barren ponds, four pregnant ponds, and one stormwater/overflow pond.
– Ancillary Systems: Sodium cyanide (NaCN) addition (two locations), sodium hydroxide (NaOH) for pH adjustment, and antiscalant injection.
– Leach Pad Expansion:
– Originally constructed in 1990, expanded to 20 cells (with permitting for an additional cell).
– Active Leaching Area: ~0.5 million m² at any given time.
– Solution Pumping:
– Two barren solution distribution systems (total capacity: 43.5 m³/min).
– One lean solution system (max flow: 5.7 m³/min), recycled from low-grade pregnant solution.
– Total flow to pad: 49.2 m³/min.
3. Pregnant Solution Processing
– Carbon Adsorption:
– Pregnant solution is pumped to five parallel carbon column trains (each with five columns).
– Total flow rate: 37.9 m³/min (handled by six vertical turbine pumps).
– Gold Recovery Efficiency (2017): 95.4% from carbon columns.
– Solution Recycling:
– Discharge solution reports to barren ponds for reconditioning (NaCN/NaOH addition) before recirculation.
4. Carbon Processing (Strip Circuit)
– Carbon Elution:
– Loaded carbon is stripped in two vessels (2-ton and 3-ton capacity).
– Stripping Conditions:
– pH 13 (adjusted with NaOH).
– Temperature 141°C (heated via boiler and heat exchangers).
– Solution flow rate: 114–151 L/min.
– Gold Recovery Efficiency (2017): 95.6% from strip circuit.
– Electrowinning:
– Pregnant eluant passes through two parallel 2.1 m³ electrowinning cells.
– A third cell is used for cathode replating.
– Barren solution is recycled until stripping is complete (target: <17.1 g/t gold).
5. Refining
– Retorting & Smelting:
– Electrowinning sludge is retorted (mercury removal) and smelted in a propane-fired furnace.
– Flux is added to produce final doré bars.
– Carbon Reactivation:
– Stripped carbon is screened, acid-washed, and thermally reactivated before reuse.
6. Ventilation & Emissions Control
– Mercury Scrubbing:
– Exhaust gases from stripping, electrowinning, retort, and smelting pass through:
– Deep bed scrubber (sulfur-impregnated carbon) for mercury capture.
– Wet scrubber (water mist) for additional mercury condensation.
Key Equipment & Capacities
– Carbon Columns:
– Existing (2017): 20 columns (five trains).
– Post-2018 Expansion: 30 columns (additional two trains).
– Pumping Systems:
– Barren solution pumps upgraded to 597 kW motors (increasing flow from 43.5 to 53.0 m³/min).
– Pregnant solution pumps expanded to match flow rates.
– Pond Volumes:
– Pregnant ponds: 137,059 m³ (post-expansion).
– Barren ponds: 40,648 m³.
Recovery Rates & Performance
– Heap Leach Recovery (1990–2017): 70.3% (90–120-day leach cycles).
– Carbon Column Recovery (2017): 95.4%.
– Strip Circuit Recovery (2017): 95.6%.
Innovations & Challenges
– 2018 Expansion Projects:
– Upgraded pumping capacity to 53.0 m³/min (eliminating lean solution system).
– Added two carbon column trains to increase throughput.
– Challenges:
– Managing gold-in-solution inventory in the pad.
– Optimizing reagent consumption (NaCN: 0.17 kg/t ore; lime: 0.68 kg/t ore).
Throughput & Capacity
– Solution Flow Rates:
– Pre-2018: 37.9 m³/min (pregnant), 49.2 m³/min (total to pad).
– Post-2018: 53.0 m³/min (matched pregnant/barren flows).
– Power Consumption: 2.3 million kWh/month (2017 average).
Reagent Consumption (2017 Averages)
| Reagent | Consumption (kg/t ore) |
|——————|———————–|
| Sodium Cyanide | 0.17 |
| Lime | 0.68 |
| Caustic Soda | 0.01 |
| Activated Carbon | 0.01 |
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This summary provides a comprehensive technical breakdown of the Marigold Mine’s processing circuit, emphasizing operational parameters, recovery efficiencies, and planned optimizations. Let me know if you need further details on any section.
