The
AVR
Acidification
Volatilization
Reabsorption
CYANIDE RECOVERY PROCESS
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Air + HCN Gas
NaOH + HCN(g) → Na+ + CN- + H2O
Stripping
Column
Recirculated
Solution/Slurry
Sulfuric Acid
Ca(OH)2 + 2HCN(g) → Ca+2 + 2CN- + 2H2O
Clean Gas
HCN(aq) → HCN(g)
Absorption
Column
High‐Cyanide
Leach Solution
Low‐Pressure
Sodium Hydroxide or
Milk of Lime
Air
Acidification
Tank
(pH 2.0‐7.0)
Fan
Pump
Recovered Cyanide
2CN- + H2SO4 → 2HCN(aq) + SO4-2
Bleed
Stripped Solution
Stream
Pump
Milk of Lime
Low‐Cyanide
Leach Solution
Neutralization
Tank
(pH 10.5‐11.5)
Pump
Reagent
Sulfuric Acid (H2SO4)
Used For
Acidification
Milk of Lime (Ca(OH) 2)
Neutralization and HCN Absorption
Sodium Hydroxide (NaOH)
HCN Absorption
Ca(OH)2 + 2HCN(aq) → Ca+2 + 2CN- + 2H2O
Ca+2 + SO4-2 + 2H2O → CaSO4-2H2O(s)
Notes:
1. Tanks/columns containing acid solution are covered and ventilated to the absorption column.
2. Hydrogen cyanide dissolved in aqueous solution is designated as HCN (aq).
3. Pregnant or barren leach solution can be treated by AVR.
4. Sodium hydroxide or milk of lime can be used in the absorption column.
5. A small bleed fan & stack is used to maintain the entire system under slight vacuum.