Design and Process Analysis of Gold Extraction from Refractory Gold Ores

With the depletion of easily processable gold ores, refractory gold ores (containing harmful impurities such as arsenic, sulfur, and carbon) have become a primary raw material source for the gold industry. Due to encapsulated gold particles or associations with sulfides, direct cyanidation recovery rates for these ores are typically below 80%. Thus, pretreatment processes are critical to enhance gold recovery. Current mainstream methods include bio-oxidation, pressure oxidation, and roasting oxidation. This article focuses on the design and workflow of circulating fluidized oxygen-enriched roasting, supported by experimental data and engineering practices from a concentrator case study.

Overview of Pretreatment Methods

1. Bio-Oxidation
   Utilizes bacteria (e.g., Acidithiobacillus ferrooxidans) to decompose sulfides and expose gold. While eco-friendly and low-energy, it is inefficient and limited to specific ores.

2. Pressure Oxidation
   Oxidizes sulfides under high temperature and pressure, requiring acidic/alkaline media. High equipment costs and operational complexity restrict its large-scale use.

3. Roasting Oxidation
   Removes sulfur and arsenic via high-temperature oxidation (450–800°C), releasing encapsulated gold. Circulating fluidized oxygen-enriched roasting combines fluidization and oxygen enrichment, significantly improving efficiency and sustainability.

Circulating Fluidized Oxygen-Enriched Roasting Process

1. Ore Pretreatment

• Ore Characteristics: Au 3.15 g/t, S 2.77%, As 0.06%, C 0.45% (Table 1).

• Crushing & Drying: Ore is crushed to 60% passing 200 mesh, with moisture reduced below 2%.

2. Optimized Roasting Parameters

• Temperature: 500°C (maximizes Au recovery at 81.11%).

• Time: 0.5 hours (prolonged roasting causes gold re-encapsulation).

• Oxygen Concentration: 30% (enhances oxidation, reduces gas volume).

• Cooling: Water quenching (simplifies workflow).

3. Roasting Workflow

1. Feeding & Fluidization: Ore is fed into a circulating fluidized bed via a screw feeder, with oxygen-enriched air (30% O₂) injected from the bottom.

2. Reaction & Impurity Removal: Sulfur and arsenic are converted to SO₂ and As₂O₃ gases; carbon oxidizes to CO₂. Alkaline gangue (e.g., CaO) fixes sulfur/arsenic, minimizing pollution.

3. 

4. Gas Treatment:

• Primary Dust Collection: Cyclones recover dust for recirculation.

• Secondary Combustion: Residual dust undergoes complete sulfur oxidation.

• Electrostatic Precipitation: High-temperature ESP (efficiency >99.5%) purifies emissions.

5. Calcine Handling: Overflow calcine and dust are water-quenched into slurry for cyanidation.

4. Key Equipment

• Fluidized Bed Reactor: Diameter 3.2 m, height 23.5 m, gas velocity 5 m/s.

• ESP: Field area 100 m², gas flow 123,326 m³/h, temperature 320–350°C.

• Fan Systems: Primary fan (320 m³/min) and natural gas supply fan (610 m³/min), both with variable frequency drives.

Results & Economic Benefits

• Gold Recovery: Cyanidation recovery reaches 86.25%, a 25% increase over direct leaching.

• Environmental Performance: Desulfurization >97.5%, arsenic fixation >95%, emissions meet national standards.

• Capacity: 2,500 t/day, 825 kt/year, operating 330 days/year.

Conclusion

The circulating fluidized oxygen-enriched roasting process effectively addresses the "encapsulation challenge" in refractory ores through optimized parameters and advanced equipment design, offering high recovery and environmental compliance. Future integration with intelligent controls and byproduct valorization (e.g., sulfur recovery) will further enhance its economic and ecological benefits, providing a cornerstone for sustainable gold extraction.