1. Processing and Recovery Methods
The Fosterville Gold Mine employs a sophisticated processing circuit combining gravity recovery, flotation, bio-oxidation (BIOX®), carbon-in-leach (CIL), and heated leach (HL) to maximize gold recovery from refractory sulfide ores.
2. Key Unit Operations & Technical Specifications
– Crushing & Milling:
– Primary Jaw Crusher: 760mm x 1,372mm, producing 90mm crushed ore.
– SAG Mill: 6.1m diameter x 6.1m length (3,500 kW), grinding to P80 of 75µm in closed circuit with hydrocyclones.
– Throughput: Up to 120 dry tph, operating 24/7.
– Gravity Recovery:
– Knelson Concentrators: Installed in 2016 to recover coarse gold from SAG mill recirculating load.
– Shaking Tables (Holman 8000, Gemini GT1000/GT250): Upgrade in 2018 improved gravity recovery.
– Recovery Rates: Gravity recovery ranged from 12.9% (2017) to 99.2% (2018) depending on ore type.
– Flotation Circuit:
– Rougher-Scavenger-Cleaner setup producing a gold-bearing sulfide concentrate (8–10% of feed mass, ~20% S₂ grade).
– Regrind Mill: 8ft x 13ft, regrinding concentrate to 80% passing 20µm.
– Reagents: Copper sulfate (activator), potassium amyl xanthate (PAX, collector), and frother.
– Flotation Recovery: Consistently 95–97% (2014–2018).
– Bio-Oxidation (BIOX®):
– Oxidation of sulfide concentrate to liberate gold.
– 6 BIOX® reactors (pH 1.0–1.6, temp ~43°C, SAF 2205 stainless steel construction).
– Oxygen & Cooling: Medium-pressure air injection and cooling coils maintain optimal conditions.
– Sulfide Oxidation Efficiency: 93.9–98.8% (2017–2018).
– CCD & Neutralization:
– 3-stage Counter-Current Decantation (CCD) with wash ratio of 4.0 to remove solubilized arsenic/acid.
– Neutralization Circuit: Uses limestone/lime to precipitate gypsum and ferric arsenate.
– Leaching (CIL & Heated Leach):
– CIL Circuit: Six 190m³ tanks (48h residence time, 30% pulp density, 20–30g/L carbon).
– Heated Leach (HL): Six 75m³ tanks (8–12h residence, steam injection) to combat preg-robbing.
– Overall Leach Recovery: 95–97.3% (2017–2018).
– Elution & Smelting:
– Pressure Zadra Elution: 3.5t capacity, acid wash + stainless steel columns.
– Electrowinning & Smelting: Produces doré bars (refined at ABC Refinery, NSW).
3. Recovery Rates & Efficiencies (2009–2018)
– Overall Plant Recovery: Improved from 85% (2009) to 97.3% (2018) due to circuit optimizations.
– BIOX® Recovery: Consistently >97% post-2014.
– Heated Leach Contribution: Added 2–7.1% recovery (critical for preg-robbing ores).
4. Notable Innovations
– Heated Leach Circuit (2009): Proprietary in-house technology to address carbonaceous ore challenges.
– Gravity Circuit Upgrades (2016–2018): Enhanced coarse gold recovery via Knelson concentrators and shaking tables.
– In-Pit Tailings Storage: Reduced environmental footprint and costs.
5. Capacity & Throughput
– Nominal Capacity: 830,000 tpa (plant designed for expansion to double throughput).
– Actual Milling (2018): 456,909 tonnes at 24.93 g/t Au feed grade.
6. Key Challenges & Solutions
– Preg-Robbing Ore: Mitigated via heated leach circuit.
– BIOX® Sensitivity: Strict control of chlorides (<1,000ppm) and cyanide to protect bacteria.
– Tailings Management: Segregation of flotation/CIL residues and use of lined storage facilities.
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Conclusion
The Fosterville processing circuit is a high-efficiency, multi-stage system integrating crushing, grinding, flotation, bio-oxidation, and advanced leaching to achieve >97% gold recovery. Innovations like the heated leach and gravity upgrades have significantly enhanced performance, while modular plant design allows future expansion.
