Gold Cyanidation Process
Step by Step Explained

Cyanide cannot reach gold it cannot touch. A gold particle sitting inside a grain of pyrite or quartz, with no exposed surface, will pass through a leach circuit unreacted. The grinding circuit exists to break rock until gold surfaces are exposed, and the extent of this liberation sets a hard ceiling on recovery.

Crushing takes run-of-mine ore down to 10 to 15 mm through jaw and cone crushers. Grinding in SAG mills or ball mills reduces it further. The standard target is 60 to 80 percent passing 75 microns. This target made sense for the oxide ores and coarse free gold deposits that dominated early cyanidation practice. It is inherited rather than derived at many modern operations. Where gold occurs as fine inclusions within sulfide grains at 5 to 15 micron size, a 75 micron grind liberates a fraction of the gold present, and the rest rides through the leach tanks as inert passengers. Ultrafine grinding below 20 microns changes the economics for these ores, at significant energy and maintenance cost.

Slurry density after grinding runs 35 to 50 percent solids. Oxygen transfer degrades nonlinearly above about 43 to 45 percent solids because slurry viscosity interferes with bubble dispersion from spargers and impellers. This matters because oxygen transfer is the rate-limiting factor in most leach tanks.

Freshly ground sulfide minerals consume dissolved oxygen fast. Pyrrhotite is the worst. Marcasite and arsenopyrite follow. These minerals oxidize rapidly on fresh surfaces, stripping dissolved oxygen from the surrounding slurry. Adding cyanide to oxygen-depleted slurry accomplishes two things, neither of them useful: gold dissolution stalls because the cathodic half-reaction cannot proceed without oxygen, and cyanide reacts with sulfur species to form thiocyanate (SCN⁻), which is irreversible and produces no gold.

Pre-aeration for 4 to 12 hours allows sulfide surfaces to passivate with oxide films, dissolved oxygen to build to the 6 to 8 mg/L range, and acid from sulfide oxidation to be neutralized by lime already in the circuit. The step produces no gold and occupies tank volume. It gets compressed when throughput targets are tight. Cyanide consumption rises 20 to 40 percent when pre-aeration is inadequate, spread across months of data in a way that does not point clearly at any single cause.

Lime raises slurry pH to the 10 to 11 range. At pH below 9.4, the pKa of HCN, more than half the dissolved cyanide exists as hydrogen cyanide gas. HCN kills at low airborne concentration. At pH 10.5, about 92 percent of cyanide stays as CN⁻.

Quicklime or slaked lime is used. Each operation picks one based on availability, handling infrastructure, and delivered cost. The pH target of 10.5 is conservative. Operating at 9.5 to 10.0 in enclosed tanks with HCN monitoring is feasible and offers faster dissolution kinetics because HCN diffuses through the boundary layer at the gold surface faster than CN⁻. Few plants run this way. The safety controls cost money and attention, and the kinetic benefit is modest enough that it does not force the issue.

Excess lime puts calcium into solution. Calcium carbonate scale forms on carbon, pipes, and screens. Dolomitic lime adds magnesium to this scale, producing a deposit that resists acid washing. The lime purchasing specification at most operations is driven by delivered cost per tonne. Metallurgical impact per tonne of gold produced would be a better criterion. The difference between a clean calcitic lime and a high-magnesium dolomitic lime can cost several percent of carbon adsorption performance, compounding over months in a way that is difficult to trace back to the lime supply change that caused it.

Sodium cyanide enters the slurry at 200 to 600 ppm for most ores. Elsner’s equation gives the overall stoichiometry:

Gold dissolution is electrochemical. Anodic sites on the gold surface release gold atoms as Au⁺ ions that immediately complex with cyanide. Cathodic sites reduce dissolved oxygen by accepting the electrons that gold oxidation released. The two half-reactions are coupled through the metal. The rate is controlled by whichever half-reaction is slower. In most leach tanks, oxygen reduction at the cathode is the bottleneck.

Raising cyanide concentration when recovery drops is the default response at many operations. If the constraint is oxygen, this accomplishes nothing except increasing reagent cost and the cyanide destruction bill at the tail end of the circuit. Dissolved oxygen measurement in dense slurry is unreliable unless probes are maintained obsessively and positioned in representative locations within the tank. A probe mounted near the impeller reads well-aerated slurry. Two meters away, in the bulk of the tank, oxygen may be 30 to 50 percent lower. The probe position determines what the control room sees, and what the control room sees determines what actions are taken.

Total plant cyanide consumption is 0.5 to 1.5 kg NaCN per tonne for oxide ores, 5 to 10 kg/t for sulfidic or copper-bearing feeds. Dissolving a kilogram of gold requires only 0.51 kg NaCN stoichiometrically. Less than 15 percent of consumed cyanide does useful work. Copper minerals take the largest share. Iron forms ferrocyanide complexes. Sulfur species produce thiocyanate. HCN leaves through the slurry surface. A total cyanide titration says how much was consumed. A speciation analysis says what consumed it. These are different questions, and only the second one leads to actionable decisions about reducing consumption.

Six to ten agitated tanks in series. Eighteen to 36 hours total residence time. Air or oxygen sparging. Dual impeller agitation.

The extraction curve is steep early and flat late. Most recoverable gold dissolves in the first 4 to 6 hours: free gold and well-liberated grains reacting at rates limited only by surface kinetics and reagent transport. After that, the curve bends. Gold dissolving in hours 8 through 30 is coming out of partially liberated grains where cyanide diffuses through cracks and along grain boundaries. This fraction dissolves slowly regardless of what happens to bulk cyanide or oxygen concentration, because the rate limit is diffusion through narrow physical channels in the rock, not the chemistry at the gold surface.

Each additional hour of tank time captures less gold than the previous hour. At some gold price, the last tank in the train is not paying for itself. At a higher gold price, another tank would be profitable. The crossover is specific to each ore, each grind, and each price environment. Changes in gold price take two to three years to propagate through the capital approval process into physical plant modifications, so circuits tend to be sized for the gold price at the time of design and become economically suboptimal in either direction as the price moves.

In CIL circuits, activated carbon occupies the leach tanks alongside the dissolving slurry. Gold adsorbs onto carbon as fast as it dissolves, keeping dissolved gold concentration low. This matters for ores with natural carbonaceous material that would otherwise scavenge dissolved gold (preg-robbing). Carbon advance rate through the tank train must match gold dissolution rate. Too slow: dissolved gold accumulates and preg-robbing losses increase. Too fast: carbon leaves under-loaded and elution throughput is wasted.

Gold in solution as Au(CN)₂⁻ is captured by one of two methods.

Carbon adsorption uses coconut shell activated carbon at 6x12 mesh (1.7 to 3.4 mm). The aurocyanide ion adsorbs onto graphitic surfaces within carbon pores, probably as an ion pair with a co-adsorbed cation. Loaded carbon carries 3,000 to 15,000 grams of gold per tonne. Carbon activity degrades with each adsorption-elution-regeneration cycle, declining to 70 to 80 percent of initial capacity after roughly 20 cycles.

Interstage screens between tanks use 0.6 to 0.8 mm wedge-wire apertures to retain carbon while passing slurry. Carbon fines from attrition pass through these screens and exit with the tailings. These fines carry gold. The loss is real and ongoing, dispersed through millions of tonnes of tailings in a way that disappears into the unaccounted column of the metallurgical balance. Fine carbon recovery screens or flotation cells on the tailings line capture this gold. Operations that install them find that the recovery is economically significant. Operations that do not install them have no measurement of what they are losing, because the loss exists below the resolution of the sampling and assay system that tracks the rest of the circuit.

Zinc cementation (Merrill-Crowe) applies to heap leach and CCD circuits producing clarified pregnant solution. Zinc dust reduces aurocyanide to metallic gold:

De-aeration before zinc addition is non-negotiable. Dissolved oxygen above 0.5 mg/L reacts with zinc preferentially, wastes zinc, and produces zinc hydroxide that blinds the filter press. The vacuum tower must hold absolute pressure below 40 kPa. Worn seals and corroded shells leak air into the tower silently, and the resulting oxygen in the feed solution degrades gold precipitation for weeks before the cause is identified, because the tower is a passive piece of equipment that does not attract operational attention the way a pump or agitator does.

Lead nitrate at 0.05 to 0.2 g/L activates the zinc surface by creating micro-galvanic cells. Without it, zinc passivates with a zinc cyanide film and gold cementation grinds to a halt.

The Zadra process strips loaded carbon with 1% NaOH and 0.1% NaCN flowing at 95 to 100°C, atmospheric pressure, over 48 to 72 hours. The AARL process uses concentrated caustic cyanide pre-soak, then hot deionized water at 110 to 135°C under 300 to 500 kPa, finishing in 8 to 12 hours. AARL is standard at modern plants.

AARL elution works by concentration gradient. Deionized water entering the column creates maximum chemical potential difference between the gold-loaded carbon pore solution and the passing eluant. Ionic contamination in the water collapses this gradient. Conductivity above 50 µS/cm degrades stripping noticeably. Water treatment infrastructure (reverse osmosis, ion exchange) is a permanent operating cost at sites with poor raw water quality.

At operations leaching flotation concentrate by CIL, carbon accumulates organic compounds from xanthates, dithiophosphates, and their degradation products. These block access to gold-bearing pore sites during elution. Alcohol pre-treatment, soaking loaded carbon in 5 to 20 percent ethanol or methanol before the caustic soak, displaces these organics and opens up the pore structure. The yield improvement is 3 to 5 percentage points of gold from the carbon. The technique is simple enough to trial in a single elution batch.

Copper co-adsorbed on carbon strips alongside gold. At copper-gold operations, copper in eluate can exceed gold by a factor of 5 to 20, flowing through into electrowinning and reducing doré purity.

Gold-bearing eluate passes through electrowinning cells at 2.5 to 3.5 V, depositing gold on steel wool cathodes. Optimal voltage sits around 2.8 to 3.0 V for most eluate compositions. Above 3.0 to 3.2 V, hydrogen evolution begins at the cathode, wasting current and dislodging deposited gold from the wool.

Cathodes are pulled periodically, acid-washed, dried, and smelted at roughly 1100°C with borax, silica, and soda ash. The doré bar runs 70 to 92 percent gold, balance mostly silver, and ships to a refinery for purification to 99.99 percent.

Acid washing in 3 to 5 percent HCl removes carbonate scale and metal precipitates. Thermal regeneration at 650 to 750°C in a rotary kiln under steam pyrolyzes organic fouling and restores micropore structure. The window is narrow. Below 650°C, organics persist and activity recovery is incomplete. Above 750°C, the carbon structure densifies, shrinks, and loses micropore volume permanently. Target: 700°C, 10 to 15 minutes. Mass loss per cycle: 3 to 7 percent, replaced by fresh makeup carbon.

Small quantities of metallic gold nucleate inside carbon micropores during elution and accumulate over many cycles. This gold is not stripped by normal elution. It grows invisibly within the circulating carbon inventory and is recovered only when carbon is retired and incinerated. The amount can be significant over years of operation.

Barren slurry contains 10 to 100 ppm WAD cyanide. The SO₂/Air process oxidizes it to cyanate using sodium metabisulfite or SO₂ gas with copper sulfate catalyst. Hydrogen peroxide is an alternative that avoids sulfate loading at higher reagent cost. Regulatory targets for WAD cyanide are 10 to 50 ppm depending on jurisdiction. The International Cyanide Management Code recommends 50 ppm maximum at the discharge point.

Iron cyanide complexes (ferrocyanide, ferricyanide) are not touched by the SO₂/Air process. They are stable under alkaline conditions in the dark. Ultraviolet light decomposes them. Shallow tailings beaches in direct sun release free cyanide from these complexes at rates sufficient to kill birds and other wildlife that land on or drink from tailings ponds. The hazard concentrates in arid climates with strong solar radiation and large areas of exposed, shallow tailings. Water cover attenuates UV penetration. Maintaining that cover in a water-scarce environment is a water balance problem layered on top of a chemistry problem.

Gold liberation is the ceiling. All downstream optimization operates below this ceiling. Mineralogical characterization (QEMSCAN, MLA) performed as mining progresses through different ore zones shows whether the leach circuit is running near its liberation limit or whether grind size changes could unlock inaccessible gold. This data is more valuable than reagent trials.

Dissolved oxygen controls the cathodic half-reaction rate. In most leach tanks, this is the constraint. Switching from air to oxygen sparging has delivered 2 to 5 percentage point recovery increases and 15 to 30 percent cyanide savings at multiple operations. The barrier is capital and logistics for oxygen supply. The unrealized gold production from suboptimal oxygen does not appear as a cost in any ledger.

Cyanide speciation, specifically free CN⁻ after metal complexation, governs dissolution kinetics. Total cyanide is a poor proxy. A plant reporting 400 ppm total cyanide while processing copper-gold ore may have 50 ppm or less of free cyanide doing useful work. The SART process recovers copper from cyanide solution, regenerates the cyanide, and has made certain copper-gold deposits economically treatable that previously were not.

Ore blend consistency matters because cyanidation chemistry equilibrates to a given feed. When feed composition swings between ore types, every reagent set point is temporarily wrong. The circuit spends hours to days re-equilibrating, and during the transition, recovery drops. Consistent blending from well-managed stockpiles holds feed composition within a narrow band and allows the leach circuit to operate near its optimum continuously. The people responsible for this, the mine planner and stockpile coordinator, often have no visibility into how their blending decisions propagate through the process chemistry downstream.