Gold Processing5 min read
A free-cyanide reading of 300 ppm looks healthy — until the gold barely dissolves and HCN alarms start to trend. The number alone does not tell you which cyanide species the gold…
Author: Wayne Ncube
A free-cyanide reading of 300 ppm looks healthy — until the gold barely dissolves and HCN alarms start to trend. The number alone does not tell you which cyanide species the gold surface can use, whether oxygen is present at the same place and time, or which pH keeps molecular HCN off the solution. This article builds the chemistry that those readings only sample.
HCN / CN⁻ speciation · pKₐ ≈ 9.3
At pH = pKₐ half of total cyanide is molecular HCN; the plant window keeps most of it as CN⁻.
The overall dissolution reaction is usually written:
4Au + 8CN⁻ + O₂ + 2H₂O → 4Au(CN)₂⁻ + 4OH⁻
Four electrons leave gold; eight cyanide ions form two ligands per gold; oxygen accepts the electrons; hydroxide leaves as a product. The stoichiometry already tells an operator two things:
The dicyanoaurate(I) complex, Au(CN)₂⁻, is soluble and stable under alkaline cyanide conditions. Under- or over-complexing, base-metal competition, and surface films all act on the same equilibrium path.
Hydrogen cyanide is a weak acid:
HCN ⇌ H⁺ + CN⁻, pKₐ ≈ 9.3 at ambient temperature
Fraction of total cyanide present as HCN:
f(HCN) = 1 / (1 + 10^(pH − pKₐ))
At pH = pKₐ the split is 50/50. Raise pH by one unit and HCN falls by roughly an order of magnitude of the remaining small fraction. The interactive below moves the pH slider against this relationship.
| pH | Approx. HCN share of total cyanide | Practical reading |
|---|---|---|
| 9.3 | ~50% | Unacceptable: half the cyanide is volatile HCN |
| 10.0 | ~17% | Still a large volatile pathway |
| 10.3 | ~9% | Lower bound often used to protect against HCN |
| 10.5 | ~6% | Typical alkaline window upper end |
| 11.0 | ~2% | Safer for HCN, more calcium and viscosity load |
These are equilibrium fractions from pKₐ = 9.3; site temperature, ionic strength, and analytical method shift the exact curve. The direction does not.
Two costs appear:
The plant therefore balances HCN risk, cyanide availability, carbon condition, and lime cost — not a single pH maximised in isolation.
Dissolved oxygen at the gold surface controls the cathodic half-reaction. Common failure patterns:
Cyanate formation is irreversible for gold dissolution:
3CN⁻ + 2O₂ + H₂O → 3CNO⁻ + 2OH⁻
Once cyanide is oxidised to cyanate, it no longer complexes gold. Free-cyanide assays that ignore cyanate overstate active inventory.
Two practical lenses:
Copper, sulfide, and iron minerals intercept the same cyanide and oxygen molecules Elsner needs. A maintained cyanide-to-copper mass ratio near 3 is a common teaching rule; below about 2.5, gold extraction slows. Preg-robbing carbonaceous matter competes for Au(CN)₂⁻ after it forms — extra cyanide then feeds the thief.
See the companion cyanide-consumers article for the mineral-by-mineral detail.
| Symptom | Plausible chemistry gap | Evidence |
|---|---|---|
| High free CN, low recovery | Oxygen limited or surface film | DO profile, sulfide/copper assay, bottle-roll with air |
| Low free CN despite high dosing | Strong consumers or destruction | Reagent balance, CN:Cu ratio, CNO⁻ if measured |
| HCN risk indicators rising | pH below window or failed lime addition | pH trend at every dosing point, lime system |
| Carbon loads poorly after a lime campaign | Calcium blinding | Acid-wash performance, carbon pore/activity test |
| Recovery oscillates with DO | Sparger or air-lance problem | DO controller, tank-by-tank DO |
pKₐ ≈ 9.3
HCN / CN⁻ half-split pH (illustrative)
10.3–10.5
common alkaline plant window (illustrative teaching range)
4:8:1
Au : CN⁻ : O₂ stoichiometric sketch of Elsner
Cyanide is acutely toxic; HCN can form when chemistry or ventilation fails. This article is educational and does not authorise reagent handling. Site procedures, detection, PPE, and qualified supervision control real work.
Further reading in this series: