Gold Cyanidation: How Cyanide Leaching Recovers Gold

Cyanidation has been the backbone of gold recovery for well over a century, and despite decades of research into alternatives, it remains the only commercially proven method for dissolving gold from ore at scale. Its dominance comes down to a simple fact: cyanide forms a stable, soluble complex with gold, and no other cheap reagent does the same job as effectively.

This article explains how the process works and, more usefully, which plant conditions decide whether it works well.

The chemistry in plain terms

The overall reaction is well understood. In the presence of dissolved oxygen, cyanide dissolves metallic gold to form a soluble dicyanoaurate complex, while the oxygen is reduced to hydroxide. Two things follow from this equation that matter on the plant floor.

First, oxygen is not optional. Cyanidation needs both cyanide and dissolved oxygen at the same time, and in practice oxygen is very often the limiting factor rather than cyanide. Sparging air into the leach tanks is standard, and oxygen enrichment is used where kinetics are slow.

Second, the reaction consumes alkali. Hydroxide is produced and the pH tends to rise, but the surrounding chemistry — particularly the formation of hydrogen cyanide gas at low pH — means the leach is deliberately run alkaline to prevent cyanide loss and to protect workers.

The variables that decide recovery

Grinding: the ceiling on recovery

No leach can dissolve gold it never touches. The fineness of grinding therefore sets an upper limit on cyanidation recovery, and this is why gold plants spend so much effort on grinding circuits. The goal is not the finest possible grind, but the grind that achieves the required liberation degree without creating a flood of slimes that slows settling and complicates thickening. There is an important counterpoint, though: because the reaction takes place at the gold surface, leach kinetics depend on exposed surface area. Gold circuits can therefore justify grinding finer than a base-metal circuit would, and over-grinding is penalised less in gold than it is elsewhere.

Oxygen: the variable most often neglected

Many underperforming leach circuits are short of oxygen rather than cyanide. Air sparging introduces oxygen, but transfer efficiency depends on agitator design, tank geometry and pulp density. In high-grade or fast-leaching circuits, oxygen enrichment can be justified, and it usually shows up as a faster leach rather than a higher ultimate recovery.

Cyanide consumers

Ore minerals that react with cyanide raise reagent cost and can depress gold recovery. Copper sulphides are the most common problem; pyrrhotite and some iron sulphides also consume cyanide, and carbonaceous material adsorbs dissolved gold. Measuring cyanide consumption per tonne in a lab leach is essential — an ore that consumes heavily may be uneconomic no matter how much gold it contains.

Where cyanidation sits in the circuit

Cyanidation is rarely the whole plant. In a typical flow sheet, ore is crushed and ground, coarse free gold is removed by gravity or flotation, and the remaining pulp is leached. In a CIL circuit, activated carbon is added directly to the leach tanks so that dissolved gold is adsorbed as soon as it forms; in a CIP circuit, leaching and adsorption happen in separate stages.

After adsorption, loaded carbon is stripped in an elution circuit and the gold is recovered by electrowinning before smelting. Our overview of gold CIP processing covers that route in detail.

Environment and safety

Cyanide is acutely toxic, and its handling is regulated almost everywhere. Modern practice addresses this in three ways: keeping the circuit alkaline so hydrogen cyanide gas is not formed, containing and recycling process water in a closed circuit, and destroying residual cyanide before tailing is discharged. Detoxification using sulphur dioxide and air, peroxide, or Caro’s acid is standard and is usually a condition of the operating permit rather than an optional improvement.

Frequently asked questions