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Neither gravity separation nor cyanide leaching is universally better. Gravity is a physical, reagent-free concentration step that works best when gold is coarse and liberated. Cyanide leaching chemically dissolves exposed gold, including much of the fine or locked gold that gravity misses, but it requires controlled chemistry, containment and long-term waste management. For many ores, the strongest starting hypothesis is a hybrid circuit: recover coarse gold by gravity, then cyanidate the gravity concentrate and/or the remaining ore. The correct choice depends on mineralogy, liberation, grind size, residence time, reagent consumption, recovery and the full environmental and economic balance.
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Gravity and cyanidation recover different populations of gold. A gravity concentrator separates particles because gold has a higher density than most gangue minerals. It can remove readily recoverable coarse gold early, often before a long leach residence time. Cyanidation uses an alkaline, oxygenated solution containing cyanide to dissolve exposed gold into a soluble gold-cyanide complex. It can treat fine particles and gold released by grinding that a gravity device cannot capture.
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A gravity-only plant may leave substantial value in its tails when the deposit contains fine, disseminated or locked gold. A cyanide-only plant may recover more total gold, but it carries reagent, power, oxygen, water-treatment, monitoring, permitting and closure obligations. The practical question is therefore not “Which method has the higher headline recovery?” It is “Which flowsheet delivers the required recovery at an acceptable cost and risk for this ore?”
The most defensible general design is often gravity recovery followed by cyanidation of the gravity concentrate and/or the gravity tails. That is a starting hypothesis, not a guarantee. Published figures are test-specific and cannot be transferred to an unnamed deposit without mineralogical and metallurgical testwork.
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How gravity separation recovers gold
The physical principle
Gravity equipment exploits the density contrast between gold and the surrounding rock. Water or another fluid carries the ground ore across a device that creates differences in settling, hindered settling or centrifugal force. Dense gold particles migrate into a concentrate stream while lighter gangue reports to tails.
The separation step itself uses no cyanide reagent. That can simplify reagent handling and reduce the volume of material that must enter a leach circuit. Gravity can also remove gold early, allowing the downstream mill or leach plant to process a smaller mass of high-value concentrate under more intensive conditions.
What gravity needs from the ore
- Coarse particle size: Gold grains must be large enough for the device to separate them reliably from the surrounding slurry.
- Liberation: Gold must be physically free, or nearly free, from the minerals that carry it. A gold grain locked inside sulfide or silicate will follow the host particle rather than behave as free gold.
- Limited flattening or smearing: Very malleable gold can flatten, attach to surfaces or report unpredictably unless the circuit is designed for it.
- Controlled feed density and water: Excessive slimes, unstable flow or poorly controlled classification can reduce the density contrast that the equipment needs.
Grinding can improve liberation, but overgrinding may create slimes that are difficult to recover by gravity. The optimum grind is therefore a trade-off between releasing gold and preserving particles that gravity can capture.
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A strong gravity concentrate does not demonstrate that gravity alone is economically sufficient. The unrecovered gold in the tails may be fine, locked or associated with minerals that require further grinding and leaching. Gravity tests should assay both concentrate and tails and report recovery on the same feed basis; a rich concentrate can still represent a modest share of the total gold.
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How cyanide leaching recovers gold
The chemical mechanism
In cyanidation, cyanide is added to an alkaline slurry while oxygen is available. Exposed gold dissolves and forms a soluble cyanide complex that can later be recovered from solution by adsorption, precipitation or another downstream process. The exact recovery circuit is separate from the leach chemistry, but the leach must expose enough gold surface for dissolution to proceed.
Variables that control extraction
- Grind size: Finer grinding can expose more gold, especially when it is locked in other minerals. It also increases grinding energy, can create slimes and may change settling and filtration behavior.
- Residence time: Gold that dissolves slowly may require longer leach tanks. Longer residence time increases equipment volume and operating cost.
- Cyanide concentration and consumption: More initial cyanide does not automatically produce a proportional recovery gain. Side reactions can consume cyanide without dissolving additional gold.
- pH and lime control: Alkalinity is maintained to control cyanide chemistry and reduce the chance of hydrogen cyanide formation. The operating set point and control strategy must be established in engineering testwork.
- Oxygen transfer: Dissolved oxygen supports the reaction. Poor mixing or inadequate oxygen transfer can limit extraction even when cyanide is present.
- Preg-robbing and mineralogy: Some carbonaceous or sulfide minerals can adsorb dissolved gold or consume reagents, reducing recovery unless the flowsheet addresses them.
Cyanidation can therefore recover gold that gravity misses, but “cyanide recovery” is not a fixed property of a deposit. It is the result of a specified grind, chemistry, residence time and test method.
What published testwork shows
The following figures illustrate the range of outcomes without serving as universal design values.
| Project or study | Reported result | Conditions or qualification |
|---|---|---|
| Rainy River assessment, Canadian Environmental Assessment Agency (2026) | Gravity concentration recovered about 20–30% of the gold hosted in the ore | Project assessment figure; not a universal benchmark |
| Rainy River 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 |
| Silvercorp Metals and SRK Consulting (2026) | Cyanide recovery from gravity tails was 70.9–95.2% | 24-hour test with 1.0 g/L NaCN; ore-dependent result |
| New Gold, Rainy River tests (2018) | Combined gravity-plus-cyanide recovery was 89.8–93.2% | Range across the listed grind sizes |
| U.S. Bureau of Mines (1988) | 94% overall gold recovery | One tested sample using gravity/flotation followed by cyanidation |
| Séguéla Gold Mine technical report (2026) | Extraction increased as grind became finer | Increasing initial cyanide from 0.05% to 0.10% produced only marginal residue-grade improvement while increasing cyanide consumption |
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. That economic conclusion belongs to the assessed project and its assumptions; commodity prices, permitting requirements, energy costs and ore characteristics can produce a different answer elsewhere.
Why gold texture and liberation decide the outcome
Coarse, free-milling gold
Visible or coarse gold that is liberated during crushing and grinding is the clearest case for gravity. Removing it early can reduce gold circulating in the mill and provide a high-grade stream for intensive treatment. Gravity is particularly attractive when the coarse fraction represents a material share of contained gold and when tails can be treated economically by another method.
Fine and disseminated gold
Fine gold has less settling advantage and is more easily carried with gangue or slimes. If it is exposed at the chosen grind, cyanide can dissolve it. If it remains locked, finer grinding or a different pretreatment may be necessary before leaching can work.
Gold associated with sulfides or carbon
Gold tied to sulfide minerals may require flotation, oxidation or finer liberation before cyanidation. Carbonaceous material can adsorb dissolved gold, a phenomenon known as preg-robbing. These associations are why a gravity test alone cannot define the final flowsheet.
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How to compare gravity, cyanide and hybrid options fairly
Use the same sample preparation and accounting basis for every option. A comparison that gives gravity a coarse feed and cyanide a finely ground feed is not a like-for-like recovery test.
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- Define the feed: Document head grade, moisture, size distribution and sampling method. Confirm that the sample represents the ore domain the plant will treat.
- Characterize the gold: Measure grain-size distribution, liberation, mineral associations, sulfide and oxide content, carbonaceous material and any evidence of preg-robbing.
- Run a gravity-recoverable-gold test: Record concentrate mass, concentrate grade, tail grade and calculated gold recovery. Assay the entire balance rather than relying on visual gold.
- Test direct cyanidation: Specify grind size, cyanide concentration, pH, oxygen condition, residence time and test temperature. Measure reagent consumption as well as extraction.
- Test gravity followed by cyanidation: Leach the gravity concentrate and cyanidate the gravity tails under defined conditions. Report recovery for each stream and the combined feed.
- Match grind and residence time: Compare circuits at equivalent product sizes and leach times, or clearly show why a different condition is required.
- Build a mass and water balance: Include concentrate, solution, tails, recycle streams and water-quality requirements. Recovery numbers without a balance can hide losses or unrealistic recycle assumptions.
- Evaluate variability: Repeat tests across ore types, oxidation states and grade ranges. A single composite can conceal the conditions that will control plant performance.
Economic trade-offs that matter to a mine
Gravity equipment can be a comparatively low-cost addition to a grinding circuit, but the relevant comparison is the complete flowsheet. Capital for concentrators, pumps, screens, intensive-leach units and concentrate handling must be weighed against the value of early gold recovery and any reduction in downstream leach volume.
Cyanidation adds tanks, agitators, reagent storage and dosing, lime and pH control, oxygen management, detoxification or destruction facilities, water treatment, monitoring and engineered containment. Grinding finer to improve extraction increases power demand and may require larger mills or classification equipment. Longer residence time increases tank volume and the footprint of the plant.
Revenue should be calculated from payable gold, not simply contained gold. Concentrate treatment, transport, smelting or refining terms can reduce the value of gravity concentrate. Conversely, a high overall recovery may not create value if reagent consumption, energy use or tailings management costs rise faster than recovered ounces.
- Gravity-only case: Consider it when gold is predominantly coarse and liberated, tails contain little payable gold, and the concentrate can be treated or sold on acceptable terms.
- Cyanide-only case: Consider it when gold is mostly fine or disseminated, gravity tests show limited recovery, and the site can meet cyanide-management and permitting requirements.
- Hybrid case: Consider it when a meaningful coarse fraction is present but the tails still contain leachable gold. Early gravity recovery and downstream cyanidation can address different gold populations.
Environmental, safety and closure implications of cyanide
Cyanide management is more than preventing a spill at the reagent plant. The U.S. Geological Survey explains that ore leaching produces hydrogen cyanide, metal-cyanide complexes, cyanate and thiocyanate in addition to the desired gold cyanide complex. The behavior and persistence of these species in process water, tailings and seepage determine ecological risk.
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A cyanide circuit therefore needs engineered storage, secondary containment, controlled dosing, leak detection, emergency response, water-quality monitoring and a documented plan for detoxification or destruction of residual cyanide. Tailings facilities must be designed for seepage control, storm events, water balance and post-closure performance. Closure planning should begin before selecting the process, because residual cyanide species and affected water may require treatment and monitoring after production ends.
Gravity does not eliminate environmental obligations. Concentrate handling, tailings storage, acid-generating minerals, metals in drainage and water consumption still require assessment. Its principal advantage is that the separation step does not introduce cyanide, which can reduce one category of chemical risk and management complexity.
Common failure modes and how to investigate them
Gravity recovery is lower than expected
- Check whether the feed was classified correctly and whether excessive slimes bypassed the concentrator.
- Verify that gold is actually liberated at the operating grind; a dense composite particle may not behave as free gold.
- Inspect water flow, feed density, equipment settings and concentrate laundering for instability.
- Reconcile concentrate and tail assays with a full mass balance before changing equipment.
Cyanide extraction stalls
- Confirm grind size and liberation rather than increasing cyanide immediately.
- Measure pH, dissolved oxygen and free cyanide through the leach profile, not only at the feed point.
- Look for preg-robbing carbon, sulfide consumption or other minerals that consume cyanide or oxygen.
- Check residence-time distribution and mixing; nominal tank volume is not the same as effective leach time.
More cyanide gives little additional recovery
The Séguéla technical report is a specific example: raising initial cyanide from 0.05% to 0.10% produced only a marginal improvement in residue grade while increasing cyanide consumption. When extra reagent produces little benefit, investigate liberation, oxygen, preg-robbing and leach kinetics before increasing the dose.
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A hybrid flowsheet usually has three accounting stages: gravity concentration from a suitable mill stream, intensive treatment of the gravity concentrate, and cyanidation of the remaining ore or gravity tails. The exact arrangement depends on where gold is liberated and how the plant handles concentrate.
- Place gravity where liberation is sufficient: Installing it too early can send locked gold to tails; installing it too late may increase circulating loads and sliming.
- Keep concentrate mass small and controlled: Intensive treatment is practical when the concentrate is substantially smaller than the whole-ore stream.
- Assay each product: Measure gravity concentrate, intensive-leach residue, cyanide tails and final solution losses separately.
- Protect downstream chemistry: Remove problematic coarse material or reagents that could interfere with cyanidation, and maintain the water balance required by both circuits.
- Evaluate bypass and upset conditions: The plant should have a safe route for periods when the concentrator, intensive-leach unit or cyanide circuit is offline.
Combined circuits can capture coarse gold quickly while allowing cyanidation to treat the fine population. The U.S. Bureau of Mines reported 94% overall recovery for one sample treated by gravity/flotation followed by cyanidation, demonstrating the potential of staged recovery without implying that every ore will match it.
A practical selection checklist
- Do mineralogical data show coarse, liberated gold that gravity can capture?
- What fraction of total gold is gravity recoverable at the proposed grind and feed rate?
- How much gold remains in gravity tails, and is it exposed or locked?
- What grind size and residence time are required for acceptable cyanide extraction?
- What are cyanide, lime, oxygen, water and power consumptions at that recovery?
- Can the site provide containment, detoxification, monitoring and closure funding for cyanide-bearing streams?
- Are concentrate treatment charges, refining terms and payable-metal assumptions included in the economic model?
- Have tests covered all major ore types and expected variability?
- Does the final design retain a safe operating mode during equipment outages or abnormal water conditions?
Bottom line for a project decision
Choose gravity when the ore contains a material coarse, liberated gold population and the value of early concentration outweighs the gold left in tails. Choose cyanidation when fine or locked gold dominates and the project can responsibly manage cyanide chemistry, water and closure. In many deposits, use both: gravity captures the dense coarse fraction, while cyanidation recovers exposed fine gold from the concentrate and remaining ore.
Make the decision from matched testwork and a complete recovery-cost-risk model. The published 20–30% gravity figure, 70.9–95.2% gravity-tail cyanide range, 89.8–93.2% combined Rainy River range and 94% single-sample Bureau of Mines result show why no single percentage can answer the question for every mine.
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