Processing and Recovery Methods
The Fosterville Gold Mine employs a comprehensive and multi-stage processing circuit designed to maximize gold recovery from sulfide-bearing ores. The plant, commissioned in 2004, integrates crushing, grinding, gravity recovery, flotation, bio-oxidation (BIOX®), leaching, and specialized heated leach circuits.
1. Crushing and Milling
– Primary Crushing: ROM ore is crushed via a 760mm x 1,372mm single-toggle jaw crusher (90mm product size) with dust suppression systems.
– SAG Milling: A 3,500kW SAG mill (6.1m diameter x 6.1m length) grinds ore to P80 of 75µm in closed circuit with hydrocyclones. Design throughput: 120 dry tph (24/7 operation).
– Gravity Circuit: Recovers coarse gold from the grinding circuit recirculating load using Knelson concentrators and Gemini shaking tables.
2. Flotation
– Circuit Design: Rougher-scavenger-cleaner configuration to produce a gold-bearing sulfide concentrate (~8–10% of feed mass, grading ~20% S₂).
– Reagents: Copper sulfate (activator), potassium amyl xanthate (PAX, collector), and frother.
– Regrind: Concentrate is reground to 80% passing 20µm before thickening and feeding to BIOX®.
3. Gravity Recoverable Gold (GRG) Circuit
– Added in 2016 to target visible gold, with upgrades in 2018.
– Primary: Knelson concentrators process 100% of regrind recirculating load.
– Secondary/Tertiary: Gemini GT1000 and GT250 tables refine concentrates, with final product calcined and smelted.
4. Bio-Oxidation (BIOX®)
– Process: Oxidizes sulfide minerals to liberate gold via bacterial action (pH 1.0–1.6, 43°C).
– Key Features:
– Six reactors with cooling coils and medium-pressure aeration.
– Nutrient control: Nitrogen (N), potassium (K), phosphorous (P).
– Material Handling: SAF 2205 stainless steel used due to corrosive slurry.
– CCD Circuit: Three-stage counter-current decantation (wash ratio 4.0) removes solubilized arsenic/acid.
5. Leaching (CIL & Heated Leach)
– CIL Circuit: Six 190m³ tanks (48-hour residence, 30% pulp density) with 20–30g/L activated carbon.
– Heated Leach (HL): Unique to Fosterville, uses steam/caustic to combat preg-robbing (six 75m³ tanks, 8–12 hours).
– Recovery Rates:
– CIL recovery: 86–97% (2018 peak).
– Overall plant recovery: 85–97.3% (2018).
6. Elution and Smelting
– Pressure Zadra Elution: Strips gold from carbon; electrowinning and smelting produce doré bars.
Technical Specifications & Equipment
– SAG Mill: 3,500kW, 6.1m x 6.1m.
– Flotation Regrind Mill: 8ft x 13ft ball mill.
– BIOX® Reactors: 48-hour surge capacity, chloride tolerance <1,000ppm.
– Neutralization: Ground limestone/lime slurry for gypsum/ferric arsenate precipitation.
Throughput & Capacity
– Nominal Capacity: 830,000 tpa (2018: 456,909 tonnes milled).
– Expansion Potential: Space for doubling throughput via duplicate facilities.
Innovations & Challenges
– Heated Leach: Addresses preg-robbing by native carbon (in-house technology).
– Gravity Circuit Upgrades: Improved recovery of coarse gold (2016–2018).
– BIOX® Optimization: Strict chloride/cyanide management to protect bacterial culture.
Key Challenges & Solutions
– Preg-Robbing: Mitigated via HL circuit.
– BIOX® Sensitivity: Controlled water quality and separate reagent streams.
– Tailings Management: Dual residue streams (flotation/CIL) with in-pit storage for cost efficiency.
Recovery Performance (2018 Highlights)
– Overall Plant Recovery: 97.3% (up from 85% in 2009).
– Gold Produced: 356,230 oz (2018, vs. 102,336 oz in 2009).
The Fosterville circuit combines conventional and innovative methods to achieve industry-leading recoveries, supported by scalable infrastructure and rigorous process controls.
