Cyanide destruction is the process of breaking down residual cyanide in mill tailings before disposal, lowering CN concentrations to regulatory limits. The dominant industrial methods are INCO SO₂/air (sulfur dioxide and air with copper catalyst) and hydrogen peroxide. Heap-leach operations typically rinse and rely on natural attenuation; agitated-leach operations require active destruction.
INCO SO₂/Air Process
The most common active cyanide destruction. SO₂ (as sodium metabisulfite or liquid SO₂) and air are sparged into the tailings slurry in the presence of a copper catalyst. CN⁻ is oxidized to OCN⁻ (cyanate, ~1,000x less toxic) and ultimately to CO₂ + N₂. Destruction efficiency: 95–99%. Reagent cost moderate.
Hydrogen Peroxide
H₂O₂ oxidizes CN⁻ to OCN⁻ directly, with copper catalysis. Simpler than INCO (no SO₂ handling) but higher reagent cost per tonne. Often preferred in remote sites where SO₂ logistics are difficult.
Natural Degradation
Free cyanide degrades naturally in sunlight and oxygen. Heap-leach operations typically rinse the heap with water at end-of-life, then rely on natural attenuation to lower residual CN below permit limits over time.
Regulatory Context
The International Cyanide Management Code is the industry-standard safe-handling and environmental framework. Most modern operations are signatories. Regulatory CN limits in tailings vary by jurisdiction — typically 50 mg/L (free CN) at the discharge point.
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