Neither method is universally “better.” Gravity separation exploits the density difference between gold and the surrounding minerals, so it is most effective on coarse, liberated particles. Cyanide leaching chemically dissolves exposed gold and can recover fine or locked gold that gravity misses after suitable grinding. For many hard-rock ores, the best starting hypothesis is a hybrid circuit: recover the readily recoverable coarse gold by gravity, then cyanide-leach the gravity concentrate and/or the remaining ore.
Published results illustrate why a single recovery percentage is misleading. A Rainy River assessment estimated gravity would recover about 20–30% of the gold in that ore, while cyanide treatment of gravity tails in separate Silvercorp/SRK tests recovered 70.9–95.2% under a 24-hour, 1.0 g/L NaCN condition. New Gold reported 89.8–93.2% combined gravity-plus-cyanide recovery across tested grind sizes. Those figures describe particular samples and test conditions, not a guarantee for an unnamed deposit.
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What gravity separation and cyanide leaching actually do
Gravity separation: a physical upgrade
Gravity circuits separate particles according to their effective settling behavior and density. Gold is much denser than common gangue minerals, so a particle that is both dense and sufficiently coarse can be concentrated without dissolving it. The separation step uses no cyanide reagent; it produces a smaller, higher-grade stream that can be treated separately or sold, depending on the operation.
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Cyanide leaching: chemical dissolution
Cyanidation converts exposed metallic gold into a soluble cyanide complex in an alkaline, oxygenated slurry. The process can attack gold particles too fine or too poorly shaped for gravity, provided the gold surface is accessible and the minerals do not consume cyanide or prevent contact. Grind size, slurry residence time, oxygen availability, pH control, and reagent consumption all affect extraction.
Leaching is not simply “adding cyanide.” The U.S. Geological Survey notes that process solutions contain the CN− anion, while ore leaching can generate hydrogen cyanide, metal-cyanide complexes, cyanate, and thiocyanate in addition to the desired gold complex. Their behavior in process water, tailings, seepage, and receiving waters is part of the design problem.
What published testwork shows
The following results are useful reference points, but each belongs to a named project, sample, or laboratory condition. They should be compared only with tests run on the same feed and under equivalent assumptions.
| Test or assessment | Process | Reported gold recovery | Conditions and qualification |
|---|---|---|---|
| Rainy River assessment | Gravity concentration | About 20–30% | Estimated share of gold hosted in the ore; project-specific assessment figure reported by the Canadian Environmental Assessment Agency in 2026. |
| Silvercorp/SRK tests | Cyanidation of gravity tails | 70.9–95.2% | 24-hour test with 1.0 g/L NaCN; results reported in 2026 for the tested material. |
| New Gold Rainy River tests | Gravity plus cyanidation | 89.8–93.2% | Range across the listed grind sizes; New Gold report, 2018. |
| U.S. Bureau of Mines test | Gravity/flotation followed by cyanidation | 94% overall | Recovery for one tested sample in a 1988 flowsheet study. |
The Rainy River assessment also states that intensive cyanide leaching of the gravity fraction was 98–99% efficient after gravity concentration. That number describes treatment of the concentrate, not recovery of every gold atom in the original ore. A high concentrate-leach efficiency can coexist with modest overall gravity recovery if most gold never enters the concentrate.
Which gold favors each method?
Choose gravity first when gold is coarse and liberated
- Assays and mineralogical work show a meaningful coarse or nugget-sized fraction.
- Gold is released from its host during a grind that still leaves particles large enough for gravity equipment.
- A rapid pre-concentration step can remove gold before long residence-time leaching.
- Reducing cyanide inventory, leach volume, or concentrate mass has economic value.
Gravity is particularly useful as a recovery step ahead of leaching, because it can remove a high-value fraction early. It is not automatically a complete flowsheet: the gravity tails must be assayed and tested rather than assumed barren.
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Use cyanidation when gold is fine, disseminated, or locked
- Gold occurs below the practical size range of the gravity circuit.
- Gold is disseminated through particles that require finer grinding for liberation.
- Gravity-tail assays remain high after a properly operated concentration test.
- Mineralogy indicates that exposed gold is chemically accessible and cyanide-consuming minerals are manageable.
Cyanide can recover gold left behind by gravity, but the result depends on ore behavior. Sulfides, preg-robbing carbonaceous material, copper and other reactive minerals, and incomplete liberation can raise reagent consumption or lower extraction. These effects must be measured on representative samples.
Can gravity concentration replace cyanide leaching?
Only when testwork demonstrates that gravity-only recovery meets the project’s recovery and economic requirements. The Rainy River estimate of 20–30% gravity recovery shows why gravity alone is often insufficient in hard-rock ore, but it is not a universal benchmark. An alluvial or exceptionally coarse-gold deposit could behave very differently from a disseminated underground ore.
The correct comparison is not “gravity percentage versus cyanide percentage” from unrelated studies. Compare gravity-only recovery, cyanide recovery of gravity tails, and total recovery from a combined circuit on the same composite feed. Include the gold remaining in every stream, including scavenger tails and any concentrate-handling losses.
Gravity-only, cyanide-only, or hybrid: a decision framework
1. Characterize the ore before selecting equipment
Measure gold grain-size distribution, liberation, mineral associations, sulfide and oxide mineralogy, and preg-robbing behavior. A head assay alone cannot tell you whether gold is recoverable by gravity or accessible to cyanide. Use multiple spatially representative samples so a nugget effect does not dominate the conclusion.
2. Run gravity-recoverable-gold tests
Test the planned grind and a realistic range of operating conditions. Weigh and assay the concentrate, middlings, and tails. Record mass pull as well as gold recovery: a circuit that recovers a small amount of gold into an extremely large concentrate may create downstream costs rather than savings.
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3. Compare matched cyanide tests
Run direct whole-ore cyanidation and gravity followed by cyanidation of the tails. Keep feed preparation, grind size, slurry density, residence time, pH, oxygen strategy, and sampling protocol comparable. Otherwise, an apparent process advantage may simply reflect a different test condition.
4. Record consumptions, not just recovery
Measure cyanide, lime, oxygen, water, power, and grinding requirements. In the 2026 Séguéla report, making the grind finer increased extraction, while raising initial cyanide from 0.05% to 0.10% produced only marginal residue-grade improvement and increased cyanide consumption. A small recovery gain may not pay for additional reagent, detoxification, residence time, or equipment.
5. Design environmental controls before committing to cyanide
Define containment, seepage collection, water treatment, cyanide monitoring, detoxification, emergency response, and closure measures at the flowsheet stage. Tailings and process-water chemistry must be considered over the operating life and after closure, not only during the initial metallurgical test.
6. Select the circuit from the full balance
Choose gravity-only, cyanide-only, or a hybrid arrangement by balancing recovered ounces, capital, operating cost, water and power demand, residence time, permitting obligations, worker safety, and long-term liability. Re-run the economic comparison when metal prices, mine plan, or permitting conditions change.
How a hybrid circuit captures different gold populations
A common concept is to grind enough for liberation, pass the slurry through gravity equipment, and split the resulting streams. The gravity concentrate can receive intensive leaching in a small, controlled vessel, while the gravity tails proceed to conventional cyanidation if tests show that fine or locked gold remains. This arrangement uses physical concentration for the fraction that responds quickly and chemical extraction for the fraction that does not.
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The sequence is not fixed. Some ores benefit from gravity before the final grind so coarse gold is removed early; others need additional grinding before gravity to liberate particles. The correct location is determined by size-by-size recovery data and the cost of circulating or regrinding material. A flowsheet should also account for concentrate handling, security, sampling, and the possibility that a high-grade gravity stream may contain minerals that consume cyanide rapidly.
For one U.S. Bureau of Mines sample, a gravity/flotation step followed by cyanidation achieved 94% overall recovery. The result supports the principle of combining mechanisms, not a promise that every hybrid circuit will reach 94%.
Why grind size and residence time change the answer
Grinding exposes gold surfaces but consumes energy and can alter the particle-size distribution that gravity equipment needs. A coarse grind may leave locked gold unavailable to cyanide; an unnecessarily fine grind can increase power demand, slime generation, and cyanide consumption without a proportional recovery gain.
Cyanide extraction also requires time for dissolution and for solution chemistry to remain favorable. Gravity can remove coarse gold before that residence-time requirement, reducing the amount of gold that must be dissolved in large tanks. When comparing circuits, use the same nominal grind and document actual size-by-size results, because two tests labeled with the same target grind can have different fines content and liberation.
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What must be controlled in the plant
- Alkalinity and pH: Lime or another approved alkali is used to maintain conditions that limit hydrogen-cyanide volatilization and support leaching.
- Oxygen: Dissolved oxygen affects the reaction rate and must be managed through aeration, mixing, or another site-approved strategy.
- Reagent inventory: Cyanide addition should follow measured demand; excess addition can increase cost and the mass of cyanide-related species requiring treatment.
- Containment: Tanks, pipelines, ponds, and tailings facilities need engineered liners, leak detection, secondary containment, and inspection programs appropriate to the site.
- Detoxification and water treatment: The selected process must address residual cyanide and related compounds before discharge or recycle.
- Monitoring and emergency response: Operators need continuous or scheduled checks, worker protection, alarms, spill procedures, and documented response authority.
USGS identifies hydrogen cyanide, metal-cyanide complexes, cyanate, and thiocyanate as species that can arise during ore leaching. Their toxicity, persistence, transport, and transformation differ, so a permit and closure plan must evaluate more than free cyanide alone.
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Why gravity reduces—but does not eliminate—risk
Gravity concentration has no cyanide reagent in its separation step and can reduce the mass of material entering a leach circuit. It does not remove the need for cyanide controls if the concentrate or tails are subsequently leached, and it does not eliminate risks from other process chemicals, tailings, dust, or water management.
Economic comparison: avoid the cheapest-step trap
Gravity equipment can be a comparatively low-cost component of a mill, and removing coarse gold early may reduce leach volume or residence-time requirements. However, gravity alone may leave too much gold in tails to support mine economics. Cyanidation can deliver higher recovery, yet it adds tanks, reagent systems, oxygen management, detoxification, monitoring, water treatment, and long-term closure obligations.
The Rainy River assessment concluded that gravity concentration coupled with whole-ore cyanidation offered the greatest return on investment among the alternatives it evaluated and described that arrangement as industry-standard practice for that project. The conclusion cannot be transferred automatically: metal price, ore variability, energy cost, permitting regime, mine schedule, and available water can reverse the ranking at another operation.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchBuild the comparison on recovered payable ounces and total life-cycle cost. Include grinding, pumping, concentrate security, cyanide and lime consumption, oxygen, labor, maintenance, tailings capacity, detoxification, water treatment, closure, and contingency for ore variability.
Common failure modes and how to diagnose them
Gravity recovery is lower than expected
- Check whether the feed contains less coarse gold than the geological model assumed.
- Verify liberation at the actual cyclone overflow size; locked gold cannot be recovered efficiently by density alone.
- Review feed-rate fluctuations, pulp density, water balance, and equipment settings.
- Inspect concentrate and tailings sampling for nugget-related bias; repeat tests with larger, representative samples.
Cyanide consumption rises without a recovery gain
- Test for reactive sulfides, copper, carbonaceous preg-robbing material, and other cyanide consumers.
- Compare finer grinding with a controlled cyanide-dose study rather than increasing both variables at once.
- Check pH, dissolved oxygen, mixing, and residence-time profiles across the tanks.
- Assay residues and intermediate streams to determine whether gold is locked, adsorbed, or simply under-leached.
Laboratory recovery does not appear at plant scale
- Confirm that the laboratory composite represents the mine’s ore domains and weathering states.
- Compare laboratory and plant grind distributions, not only their nominal P80 values.
- Track solution losses, carbon or concentrate handling, and sampling bias between unit operations.
- Use variability testing and pilot or demonstration campaigns before making a permanent flowsheet change.
Questions to answer before approving a flowsheet
- What proportion of gold is coarse, liberated, and demonstrably gravity-recoverable?
- What recovery remains in gravity tails at the planned grind?
- Does cyanidation of those tails improve total recovery enough to cover its capital and operating cost?
- How do cyanide, lime, oxygen, water, power, and residence time change between alternatives?
- Which cyanide-related species will be present in tailings and water, and how will they be contained, treated, monitored, and managed at closure?
- How sensitive is the preferred circuit to ore variability, finer or coarser grind, and changes in metal price?
Verdict
Gravity separation is the cleanest and fastest way to capture the coarse, liberated portion of gold, but it is usually incomplete. Cyanide leaching reaches fine and locked gold that gravity leaves behind, at the cost of chemical handling, water treatment, monitoring, and long-term liability. The most defensible general approach is to test a hybrid circuit—gravity followed by cyanidation of the concentrate and/or tails—then select the final arrangement from deposit-specific recovery, cost, safety, and environmental results. No published percentage should be treated as a guarantee for a different orebody.
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