Cyanide leaching is a widely used method for extracting gold from ore, but it is highly toxic and must be handled with extreme care. Here’s an overview of the process:
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How Cyanide Extracts Gold
1. Crushing & Grinding – Gold-bearing ore is crushed and ground into fine particles to increase surface area.
2. Leaching – A dilute sodium cyanide (NaCN) solution (typically 0.01–0.05%) is applied to the ore in tanks or heaps.
– The cyanide dissolves gold by forming a soluble complex:
\[
4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH
\]
3. Recovery – The gold-cyanide solution is separated from the waste rock.
– Zinc Precipitation (Merrill-Crowe Process): Zinc dust is added to precipitate gold:
\[
2Na[Au(CN)₂] + Zn → 2Au + Na₂[Zn(CN)₄]
\]
– Activated Carbon (Carbon-in-Pulp/CIP): Gold adsorbs onto carbon, which is later stripped and smelted.
4. Refining – The recovered gold is further purified, often via electrolysis or chemical treatment.
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Dangers & Environmental Concerns
– Extreme Toxicity: Cyanide is lethal to humans and wildlife even in small doses.
– Environmental Risks: Spills can contaminate water supplies (e.g., the 2000 Baia Mare disaster in Romania).
– Regulations: Strict controls are required for storage, handling, and wastewater treatment.
Alternatives
– Thiosulfate Leaching: Less toxic but slower and more expensive.
– Chlorination (Miller Process): Used for refining but not large-scale extraction.
– Non-Chemical Methods: Gravity separation or flotation for high-grade ores.
Conclusion
While cyanidation is efficient for low-grade ores, its use demands rigorous safety protocols and environmental safeguards. Many countries enforce strict regulations, and alternatives are being explored for safer gold extraction.
Would you like details on specific aspects (e.g., heap leaching vs. tank leaching, detox methods)?