Gold Processing7 min read
Only a small fraction of the cyanide charged to a gold plant ever dissolves gold. The rest is consumed by other minerals, oxidised in the circuit, or destroyed during effluent…
Author: Wayne Ncube
Only a small fraction of the cyanide charged to a gold plant ever dissolves gold. The rest is consumed by other minerals, oxidised in the circuit, or destroyed during effluent treatment. When a plant responds to a high tailings grade by adding more cyanide, it is often feeding the very reactions that were stealing the leach in the first place. This article reviews the main consumers and how to recognise them.
Sodium cyanide dissociates readily:
NaCN → Na⁺ + CN⁻
In water, cyanide participates in an equilibrium with hydrogen cyanide. At about pH 9.3, half of the total cyanide exists as HCN. Raising pH shifts the balance toward free cyanide ions, which is one reason lime is dosed before the leach — both to protect against HCN loss and to keep cyanide available for dissolution.
But not all remaining cyanide is available for gold. Both free cyanide and HCN can be oxidised by oxygen to cyanate, which does not dissolve gold:
4HCN + 3O₂ → 4CNO⁻ + 2H₂O
3CN⁻ + 2O₂ + H₂O → 3CNO⁻ + 2OH⁻
A plant-wide mass balance may show that only a small fraction of the charged cyanide actually dissolves gold. The balance goes to other metals — the cyanide consumers — and to oxidation in the plant and in waste treatment. Knowing this reframes the dosing question: the plant is not only buying dissolution, it is also paying for every consumer in the ore.
Sulfide minerals are among the most consequential consumers. They decompose in the leach and follow two damaging paths:
How serious the effect is depends on the specific mineral and the available oxygen. The practical lesson is that a sulfide-rich ore can show adequate free-cyanide readings and adequate reagent addition while still leaching slowly. The barrier is at the surface, not in the bulk solution.
Copper minerals dissolve in cyanide to form a range of copper-cyanide complexes, and the speciation depends on cyanide concentration and pH. The damage is twofold: dissolving copper consumes both cyanide and oxygen that were intended for gold.
A useful teaching rule is to maintain a cyanide-to-copper mass ratio of approximately 3 for good gold extraction. If the ratio falls to around 2.5, extraction may slow. Even moderate levels of copper-cyanide in solution can exert a slight drag on gold dissolution; confirm the ratio with site-specific tests.
This is why copper ores are diagnosed on a ratio, not on a dose. Raising cyanide to cover copper consumption also raises reagent cost and the cyanide load in the effluent; the ratio tells you when you have added enough and when you are simply paying the consumer.
A free-cyanide measurement is a concentration. It does not tell you how much cyanide the ore consumed since the last addition. The useful operating record pairs free-cyanide readings with a reagent balance:
consumption = cyanide added − free cyanide remaining − cyanide destroyed or removed in the measured streams
The exact accounting depends on the plant boundary, but the principle is important. A target of 300 ppm free cyanide can mean a healthy leach in one ore and a severe overfeed in another. If addition is high, free cyanide is high, and gold recovery is still poor, the problem is unlikely to be solved by raising the setpoint.
Read the chemistry in this order:
That sequence prevents a plant from treating a concentration target as a diagnosis.
Iron behaviour is more varied, and the mineral form decides the outcome:
| Iron mineral form | Behaviour in cyanide leach |
|---|---|
| Oxides (hematite, magnetite), silicates | Generally do not dissolve; little direct cyanide demand |
| Carbonates (siderite) | Slightly decompose below about pH 10; mostly unreactive at high pH |
| Sulfides (pyrrhotite) | Readily break down; consume cyanide and oxygen; form iron-cyanide complexes and sulfur compounds |
Pyrrhotite is the aggressive case. It drives formation of the stable ferrocyanide ion and can deposit an iron-hydroxide layer on gold surfaces, hindering extraction. Interestingly, high ferrocyanide concentrations themselves can be tolerated — the problem is the consumption and the surface film, not the complex in isolation.
A distinct loss mechanism deserves its own category. Carbonaceous matter, clay minerals, and graphite in the ore can adsorb the gold-cyanide complex directly from solution. This is preg-robbing: the ore itself behaves like activated carbon and steals dissolved gold before the circuit can capture it.
Preg-robbing cannot be solved by cyanide dosage. It is addressed by circuit design — CIL places activated carbon in the tanks while gold is still dissolving, giving the engineered carbon an early advantage — and by ore-blending or treatment decisions upstream.
An illustrative ore-change case is useful. As feed shifts from an oxide-like feed to a sulfide-bearing feed, recovery can fall even when reagent consumption is adjusted:
| Illustrative period | Feed grade (g/t) | Tailings grade (g/t) | Recovery |
|---|---|---|---|
| Oxide-like feed | 2.80 | 0.34 | 88% |
| Sulfide-bearing feed | 3.09 | 0.65 | 79% |
| After grind and cyanide changes | 2.17 | 0.51 | 76.5% |
In the example, cyanide concentration was raised from 800 to 1200 ppm and the SAG feed size reduced, yet the tailings grade remained roughly double the illustrative 0.3 g/t target and cyanide consumption rose. The ore change altered the consumer population; more reagent mostly paid the consumer bill. These values are teaching examples, not a plant record.
The disciplined sequence is:
Tailings diagnostic tree
Start with a mass-balance check, then branch into physical, chemical, and sampling causes.
~0.5%
share of plant cyanide that typically dissolves gold
CN:Cu ≈ 3
illustrative cyanide-to-copper ratio for good extraction
pH ≥ 10.3
illustrative floor to limit HCN formation
Cyanide is acutely toxic and HCN can form under unsuitable chemistry. This article supports engineering study and test planning; any reagent work must follow the site chemical-management plan and qualified supervision.
Further reading in this series: