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Gravity separation is best for coarse, liberated gold; cyanide leaching is needed to dissolve much of the fine, disseminated or locked gold that gravity misses. Neither method wins universally. Deposit mineralogy, grain size, liberation, grind, leach time, reagent consumption, water management and permitting determine the result. In many operations, the most defensible starting point is a hybrid circuit: recover gravity-recoverable gold first, then cyanide-leach the gravity concentrate and/or the remaining ore after metallurgical and economic testwork.
What the two processes actually do
Gravity separation exploits density
Gravity concentration separates particles by their response to fluid motion and gravity. Gold is much denser than most host-rock minerals, so a suitably liberated gold particle can report to a concentrate while lighter gangue moves to tails. The separation step does not require cyanide reagent.
That selectivity is also its limitation. A gold grain has to be sufficiently coarse and liberated, and the circuit must physically capture it. Very fine particles, gold attached to sulfides or silicates, and gold hidden inside composite grains can follow the lighter material. Grinding can expose more gold, but excessive grinding may create particles that are difficult for a gravity device to recover.
Cyanide leaching dissolves exposed gold
Cyanidation uses an alkaline process solution containing cyanide to chemically dissolve gold that is accessible at the particle surface. The dissolved gold forms a cyanide complex that can be recovered from solution downstream. Because the mechanism is chemical rather than purely density-based, leaching can treat fine gold and gold that becomes exposed only after suitable grinding.
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Leaching is not automatically complete. Extraction depends on mineralogy, liberation, grind size, cyanide concentration, oxygen, pH control, residence time and competing reactions. Preg-robbing behavior, sulfide minerals and other constituents can consume reagents or interfere with gold dissolution.
Why the methods target different gold populations
Think of an ore body as containing several gold populations rather than one uniform material. Coarse, free gold is a natural gravity target. Fine or disseminated gold may require grinding and leaching. Gold locked in another mineral may need additional liberation or a different pretreatment before cyanide can reach it. A fair comparison therefore measures gravity recovery, leach recovery of gravity tails and total recovery on the same feed, rather than comparing an isolated gravity percentage with a whole-ore cyanide result.
What published testwork shows about recovery
The figures below come from different projects, samples and test conditions. They demonstrate the range of outcomes, not a transferable guarantee for an unnamed deposit.
| Test or assessment | Reported result | How to interpret it |
|---|---|---|
| Rainy River project assessment, Canadian Environmental Assessment Agency, 2026 | Gravity concentration recovered about 20–30% of the gold hosted in the ore. | Project-specific gravity-only result; it is not a universal benchmark. |
| Rainy River project assessment, Canadian Environmental Assessment Agency, 2026 | Intensive cyanide leaching of the gravity gold fraction was 98–99% efficient. | Applies to the gravity fraction in that assessment, not to all ore. |
| Silvercorp Metals and SRK Consulting, 2026 | Cyanide recovery from gravity tails ranged from 70.9% to 95.2%. | Results were obtained with 24-hour leaching at 1.0 g/L NaCN; the range reflects ore or test differences. |
| New Gold Rainy River tests, 2018 | Combined gravity-plus-cyanide recoveries ranged from 89.8% to 93.2% across the listed grind sizes. | A combined-circuit result; grind size affects the outcome. |
| U.S. Bureau of Mines flowsheet, 1988 | Gravity/flotation followed by cyanidation achieved 94% overall gold recovery for one tested sample. | A sample-specific flowsheet result, not a design value for every ore. |
The Rainy River assessment also concluded that, among the alternatives it evaluated, gravity concentration coupled with whole-ore cyanidation offered the greatest return on investment and represented industry-standard practice for that project. That economic conclusion must be re-tested when the deposit, gold price, energy cost, permitting regime or flowsheet changes.
Gravity versus cyanide: the practical trade-offs
| Decision factor | Gravity separation | Cyanide leaching |
|---|---|---|
| Best gold target | Coarse, liberated and high-density gold | Fine, exposed, disseminated or previously locked gold after suitable grinding |
| Separation mechanism | Physical density response | Chemical dissolution into a cyanide-bearing solution |
| Cyanide in the separation step | None | Required, with controlled addition and monitoring |
| Typical role in a flowsheet | Early recovery of a high-grade fraction before long residence-time treatment | Whole-ore treatment, gravity-tail treatment or intensive treatment of a gravity concentrate |
| Main performance controls | Gold grain size, liberation, particle size distribution, water flow and capture efficiency | Grind, cyanide and lime/pH control, oxygen, residence time, mineralogy and reagent consumption |
| Environmental management | Water and tailings management; no cyanide is introduced by the gravity step itself | Containment, detoxification, water treatment, monitoring and long-term closure controls for cyanide-related species |
| Economic risk | Equipment can be a comparatively low-cost mill component, but gravity-only recovery may leave too much gold in tails | Can increase recovery, but reagent, oxygen, power, water, residence-time and closure costs can determine whether the extra gold is economic |
How a hybrid gravity-plus-cyanide circuit works
A hybrid circuit is not a compromise chosen by default; it is a way to assign each gold population to the process most likely to recover it. A typical evaluation follows this sequence:
- Crush and grind the ore to the test target. The grind must be recorded because liberation and leach extraction change with particle size.
- Recover gravity-recoverable gold as early as practical. Removing coarse gold can produce a concentrated product and reduce the amount of that gold exposed to a long leach residence time.
- Assay the gravity concentrate and gravity tails. Without separate assays, an apparent total recovery can hide gold that bypassed the gravity stage or remained in the tails.
- Intensively leach the gravity concentrate when justified. The Rainy River assessment reported 98–99% efficiency for intensive cyanide leaching of its gravity gold fraction, but that figure belongs to that project and fraction.
- Cyanide-leach the gravity tails or whole ore. This step targets fine and non-gravity gold. Silvercorp/SRK tail tests show why results must be measured: recovery ranged from 70.9% to 95.2% under a stated 24-hour, 1.0 g/L NaCN condition.
- Recover dissolved gold and manage the process solution. Cyanide-bearing streams require engineered containment, monitoring and treatment rather than discharge by assumption.
- Compare the complete flowsheet. Include capital, operating costs, reagent and water use, power, residence time, tailings treatment, detoxification and closure obligations.
Gravity can therefore replace some cyanide duty, but it cannot be assumed to replace cyanidation for an ore whose gold is predominantly fine or locked. Conversely, adding cyanide to every tonne without first removing readily recoverable coarse gold may increase residence time and reagent demand without delivering the best economics.
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Why grind size and chemistry change the answer
Finer grinding can expose more gold
The Séguéla technical report found that gold extraction increased as the grind became finer. Finer material can expose surfaces that were previously enclosed in gangue, allowing cyanide and oxygen to contact more gold. The benefit has to be balanced against grinding energy, equipment capacity, water use and the possibility that very fine particles behave poorly in downstream separation.
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In the Séguéla work, increasing initial cyanide concentration from 0.05% to 0.10% produced only a marginal improvement in residue grade while increasing cyanide consumption. That is a warning against selecting reagent strength by rule of thumb. The useful question is how much additional gold is extracted per additional unit of cyanide, after accounting for the cost and treatment of the resulting solution.
Residence time must be matched to the test
The 70.9–95.2% gravity-tail recoveries reported by Silvercorp/SRK were measured after 24 hours at 1.0 g/L NaCN. A shorter or longer residence time, a different cyanide concentration, a different grind or a different ore can produce a different result. A recovery number without its time, concentration and feed description is not a valid design comparison.
Oxygen, pH and mineralogy matter together
Cyanidation requires more than cyanide addition. Lime or another alkalinity-control strategy, oxygen availability and solution management affect the reaction environment. Sulfide and oxide mineralogy, preg-robbing behavior and other consumers can change both extraction and reagent consumption. These variables should be measured in the same test program rather than optimized one at a time from unrelated data.
Environmental and safety implications of cyanide
Gravity’s separation step is reagent-free with respect to cyanide, but a gravity circuit does not make an entire operation cyanide-free if its concentrate or tails are later leached. The environmental question must be asked for the complete flowsheet.
The U.S. Geological Survey explained in 2015 that cyanide is added to process solutions as the CN− anion, while ore leaching produces additional cyanide-containing or cyanide-related species. These include hydrogen cyanide, metal-cyanide complexes, cyanate and thiocyanate. Their behavior in tailings, process water and the surrounding environment determines ecological risk.
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A cyanide circuit therefore needs controls designed before equipment selection:
- Containment: ponds, tanks, pipelines and tailings facilities must prevent uncontrolled releases.
- pH and process control: operating conditions must limit formation and escape of volatile hydrogen cyanide while maintaining leach performance.
- Detoxification: residual cyanide and related species require an engineered treatment step appropriate to the selected process.
- Water management: recycle, treatment, storm-water separation and discharge criteria must be defined for normal operation and upset conditions.
- Monitoring: sampling should cover process water, tailings and relevant receiving environments, with action levels and response procedures.
- Closure: long-term containment, water treatment and financial liability continue after active gold production ends.
These requirements do not prove that cyanidation is unacceptable, nor do they make gravity automatically preferable. They determine whether the additional recovery can be achieved responsibly and at a cost the project can sustain.
How to choose between gravity-only, cyanide-only and hybrid treatment
Use a deposit-specific decision sequence rather than a generic recovery percentage.
1. Characterize the gold before choosing equipment
Measure grain-size distribution, liberation, mineral associations and the proportions of free, disseminated and locked gold. Include sulfide and oxide mineralogy and test for preg-robbing behavior. A gravity device cannot recover gold that is not physically available to it, and cyanide cannot dissolve gold that remains inaccessible without suitable liberation.
2. Run gravity-recoverable-gold tests
Process representative samples through a gravity test and assay both concentrate and tails. Report the feed grade, mass pull, concentrate grade and gold recovery. The Rainy River figure of 20–30% illustrates why this step is informative: gravity can capture a meaningful fraction while still leaving most of the gold for another treatment stage.
3. Compare matched cyanide tests
At the same feed and grind size, compare direct cyanidation with gravity followed by cyanidation. Keep residence time, cyanide concentration, alkalinity, oxygen conditions and sampling method explicit. Measure residue grade and solution extraction, not just a single headline recovery.
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4. Track consumption and utilities with recovery
Record cyanide, lime, oxygen, water, power and grinding requirements. A higher recovery can reduce project value if it requires disproportionate reagent use, longer residence time, additional grinding or expensive water treatment. The Séguéla result shows why a reagent increase should be judged by incremental extraction rather than by concentration alone.
5. Design risk controls before selecting the cyanide option
Define detoxification, containment, monitoring, emergency response and closure measures in the same feasibility comparison. Treat these as operating and life-of-mine requirements, not as add-ons after the recovery flowsheet has been chosen.
6. Select the circuit on full recovery-cost-risk performance
Choose gravity-only when testwork shows that the remaining gold is too little or too costly to recover, cyanide-only when gravity adds little value, or a hybrid circuit when the two methods recover clearly different gold populations. The selection should be revisited as ore zones, grades, grind requirements, reagent prices and permitting conditions change.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common mistakes and how to avoid them
Using a gravity percentage as a universal promise
The 20–30% Rainy River gravity result is often more useful as a warning than as a target. It came from a named project assessment and cannot be transferred to an unnamed deposit. Gold texture and liberation can make gravity recovery far lower or higher elsewhere.
Comparing unlike recovery denominators
A gravity-only percentage, a cyanide recovery from gravity tails and a combined whole-circuit recovery answer different questions. Label whether each figure is based on the original feed, the gravity tails or a concentrate, and keep assay methods and sample preparation consistent.
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Ignoring the grind-cost penalty
Finer grinding may increase extraction, as reported for Séguéla, but grinding consumes energy and can alter downstream handling. Compare the value of the extra gold with the cost of achieving the finer size.
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Increasing cyanide without checking marginal benefit
The Séguéla tests found only marginal residue-grade improvement when initial cyanide rose from 0.05% to 0.10%, while consumption increased. Establish a response curve for the ore instead of assuming that a stronger solution is always better.
Calling a circuit low-risk because gravity comes first
Gravity reduces cyanide duty only where it actually removes gold from the material that would otherwise be leached. If the concentrate or tails enter cyanidation, cyanide-related species and closure obligations remain part of the operation.
Treating one laboratory result as a plant guarantee
The U.S. Bureau of Mines result of 94% overall recovery came from one tested sample and one flowsheet. Pilot work, variability samples and operating controls are needed before converting such a result into a plant design value.
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Bottom line for operators and project reviewers
Gravity separation is selective, fast at removing coarse free gold and free of cyanide reagent at its separation stage, but it is incomplete when gold is fine, disseminated or locked. Cyanide leaching reaches gold that gravity misses, yet its performance and cost depend on grind, chemistry, residence time and mineralogy, and it brings obligations for containment, detoxification, monitoring and closure.
The strongest general conclusion is not “gravity” or “cyanide” in isolation. It is to test gravity recovery first, measure what remains, and compare matched cyanide and hybrid circuits on recovery, consumption, cost and environmental risk. Published examples range from 20–30% gravity recovery at Rainy River to 89.8–93.2% combined recovery in New Gold tests and 94% overall recovery in a U.S. Bureau of Mines sample, but none of those numbers substitutes for representative testwork on the ore being developed.
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