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Commercial hard-rock gold processing turns run-of-mine (ROM) rock into a gold-silver alloy called doré through a linked series of size-reduction, concentration, leaching, adsorption, recovery and waste-management steps. The usual route is to crush and grind the ore, recover any suitably coarse free gold by gravity, leach exposed gold with sodium cyanide, capture dissolved gold on activated carbon in CIP or CIL tanks, then elute, electrowin and smelt the recovered metal.
There is no universal gold-recovery percentage, residence time or doré composition. Mineralogy, gold liberation, hardness, sulphide content, plant design and operating controls determine the flowsheet. A mine may omit gravity or flotation, use a concentrate-leach circuit, or choose CIP instead of CIL after metallurgical testing.
1. Mining produces the plant feed
Gold processing begins with ROM ore, not with a uniform powder. Drill-and-blast open-pit mining or underground stoping breaks the ore, which is then hauled to a crusher or to a stockpile. ROM is the plant feed for the projects described here; the mining method, truck fleet, haul distances and stockpile arrangement are specific to each project.
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Before the plant is designed, samples are tested for head grade, gold grain size, association with sulphide minerals, clay content, hardness, preg-robbing behavior and elements that can consume cyanide or foul activated carbon. Those results determine how much crushing and grinding is needed and whether gravity, flotation, pre-aeration or a different leach treatment is justified.
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2. Crushing and coarse-ore stockpiling
Primary crushing
ROM rock is fed to a jaw or gyratory crusher. The crusher reduces large blasts of rock to a size that conveyors and the grinding circuit can handle. The product is commonly conveyed to a coarse-ore stockpile.
Stockpile and reclaim
The stockpile buffers interruptions in mining and evens out mill feed. Reclaim feeders draw ore from the stockpile at a controlled rate. Tocantinzinho’s published design specifically places primary crushing before a coarse-ore stockpile and reclaim system.
3. Grinding and classification liberate the gold
Crushed ore is mixed with water and reduced further in semi-autogenous (SAG) mills, ball mills or a combination of the two. The objective is liberation: gold particles must be exposed from the surrounding quartz, rock or sulphide minerals without spending excessive energy making an unnecessarily fine slurry.
Hydrocyclones classify the ground slurry. Coarser particles return to the mill, while particles fine enough for downstream treatment overflow to the gravity, flotation or leach circuit. A published Tocantinzinho design uses a final grinding P80 of 125 micrometres. P80 means 80 percent of the material is finer than that size; it is a design value for that project, not a global standard.
4. Optional concentration before leaching
Gravity recovery of coarse free gold
Some ores contain liberated, relatively coarse gold that can be recovered before cyanidation. Centrifugal concentrators or other gravity devices are installed on an appropriate part of the grinding circuit. The gravity concentrate may receive intensive cyanidation and separate electrowinning, reducing the amount of coarse gold that must pass through the main leach circuit.
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Gravity is not automatically beneficial. If gold is very fine, locked in sulphide minerals or distributed throughout the rock, a gravity circuit may recover little and add cost and operating complexity.
Flotation of sulphide-associated gold
When gold is hosted by sulphide minerals, flotation can produce a sulphide concentrate rather than treating all of the ore in the same way. The concentrate may be reground and then cyanide-leached. Tocantinzinho’s reported flowsheet includes a two-stage sulphide flotation circuit.
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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Flotation changes the mass and chemistry of the material entering leaching. It can concentrate gold and sulphides into a smaller stream, but it requires reagent control, additional equipment and a suitable response from the ore. A whole-ore leach is often simpler when flotation cannot make a valuable, readily leachable concentrate.
5. Conditioning and cyanide leaching
Pre-aeration and slurry preparation
Some plants condition the slurry with air or oxygen before cyanidation. This can help establish the chemical conditions needed for gold dissolution or reduce the effect of oxygen-consuming minerals. Whether pre-aeration is used, and for how long, is established by metallurgical testing rather than by a universal recipe.
How cyanide dissolves gold
Sodium cyanide is added to the conditioned slurry. In the presence of oxygen, cyanide forms a soluble gold-cyanide complex from exposed gold surfaces. Agitation, cyanide concentration, pH, oxygen availability, temperature, particle size and residence time all affect the dissolution rate. Reagent conditions are selected for the particular ore because sulphides, copper minerals, organic carbon and other constituents can consume cyanide or interfere with recovery.
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Cyanidation is an industrial process requiring engineered tanks, monitoring, secondary containment, worker protection and controlled waste treatment. It is not a safe household or small-scale experiment. A design number illustrates the project-specific nature of the process: the Tocantinzinho Feasibility Study (2022) specifies 36 hours of CIL residence time. That figure should not be treated as a requirement for every ore or plant.
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After or during leaching, activated carbon contacts the slurry and adsorbs dissolved gold complexes. Screens keep carbon in the tanks while slurry moves onward. The two common arrangements differ in where adsorption occurs.
| Arrangement | Where adsorption occurs | What the name means operationally |
|---|---|---|
| Carbon-in-pulp (CIP) | In dedicated adsorption tanks after a separate leach section | Leaching and carbon adsorption are distinct stages in sequence |
| Carbon-in-leach (CIL) | In the leach train itself | Leaching and adsorption occur together in the same series of tanks |
Neither label guarantees a particular recovery or residence time. The choice depends on leach kinetics, soluble losses, slurry characteristics, carbon inventory, tank volume and the project’s capital and operating constraints. Technical reports for different projects document both configurations.
7. Washing, elution and carbon regeneration
Screening and acid washing
Loaded carbon is screened from the slurry and transferred to the carbon-treatment area. Acid washing removes scale and other foulants that block adsorption sites. The wash solution is handled as a controlled process stream.
Elution (stripping)
In the elution circuit, a hot, chemically controlled solution strips the adsorbed gold from the carbon. The result is a gold-rich pregnant eluate. The stripped carbon is retained for reuse rather than discarded.
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Thermal regeneration
Heating restores carbon capacity by removing organic contaminants and residual compounds that reduce adsorption performance. Regenerated carbon returns to the adsorption circuit. Screening, washing, elution and regeneration form a recirculating loop; poor carbon management can lower recovery even when leaching itself is performing well.
8. Electrowinning produces cathode sludge
Pregnant eluate flows through electrowinning cells. An electrical current plates gold onto cathodes. The deposit is not pure gold: it is a sludge containing gold, silver and other metals carried through the solution. Gravity concentrates, when treated separately, can also feed an intensive-cyanidation and electrowinning route.
Electrowinning is therefore a recovery step, not the final refining stage. Cell operation, solution chemistry, current density and sludge handling affect the quality and quantity of the deposit.
9. Filtering, drying and smelting make doré
Sludge preparation
Electrowinning sludge is filtered to remove liquid and then dried. The dry material is blended with selected fluxes. Fluxes help collect metal and separate unwanted components during melting; the exact recipe depends on the sludge chemistry.
Smelting
The fluxed charge is melted in a furnace. Dense molten metal settles as a gold-silver alloy, while slag captures much of the non-metallic fraction. The poured product is doré.
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Doré is an intermediate bar sent to an off-site refinery for further separation and purification. It is not normally 99.99 percent pure bullion. Gold-to-silver ratio, residual base metals and other impurities vary by ore and plant, so a reliable composition requires assay data rather than a generic fineness claim. Aura Minerals’ 2025 Era Dorada Gold Project technical report summarizes this part of the route as: “Gold refining – Gold electrowinning (sludge production), filtration, drying and smelting to produce gold doré.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.10. Cyanide destruction, tailings and water recovery
Cyanide detoxification
After soluble gold has been recovered, residual cyanide in process streams is detoxified or destroyed before discharge, recycling or deposition. The treatment selected depends on cyanide species, discharge criteria, water chemistry and the site’s permit.
Tailings handling
Tailings are the finely ground solids and process water left after recovery. A plant may thicken the tailings and pump them to a storage facility, or filter them to produce a drier stackable material. Water is often recovered and returned to the process, reducing fresh-water demand and limiting the volume sent to storage.
Tailings-storage design, seepage control, embankment stability, cyanide monitoring, emergency planning and closure requirements are part of the processing system, not an afterthought. The technically appropriate choice depends on climate, topography, water balance, seismic conditions and regulations.
11. Published design figures and what they do—and do not—tell you
| Project or study | Published figure | Qualification |
|---|---|---|
| Tocantinzinho Project technical report (2019) | 4.3 million tonnes per year nominal mill capacity | Planned plant capacity for that project design |
| Tocantinzinho Project technical report (2019) | 174,000 ounces of planned annual gold production | Planned annual output for that project, not a universal mine result |
| Tocantinzinho Project design | 125 micrometres final grinding P80 | Specified grind target for that ore and flowsheet |
| Tocantinzinho Feasibility Study (2022) | 36 hours CIL residence time | Project design parameter, not a general cyanidation requirement |
These figures illustrate scale and design practice, but they cannot be used to infer the recovery, operating cost or doré grade of another mine. No single global recovery percentage, cost benchmark or doré-fineness statistic is established for all mines.
12. Why two gold mines use different flowsheets
Engineers compare routes against the ore and the site’s constraints rather than copying a standard diagram. The decisive questions include:
Quick Recap
- Mineralogy and liberation: Is gold free after grinding, locked in sulphides, or associated with minerals that consume cyanide?
- Particle size and hardness: How much crushing and grinding energy is needed, and what grind size gives acceptable liberation?
- Coarse versus sulphide-associated gold: Does gravity recover a meaningful coarse fraction, or is flotation needed to collect sulphide-hosted gold?
- Whole-ore versus concentrate leach: Is it more effective to cyanide the entire ground stream or a smaller, reground concentrate?
- CIP versus CIL: Do leach kinetics, adsorption behavior and tank residence requirements favor separate adsorption or simultaneous leach-and-adsorb operation?
- Water and waste management: Can the site thicken or filter tailings, recover process water and meet cyanide-discharge controls?
- Capital and operating complexity: Will extra circuits, reagent systems and maintenance deliver enough additional recovery to justify their cost and regulatory burden?
13. Common misconceptions and practical limits
- “Every mine uses gravity.” Gravity is useful only when the ore contains recoverable coarse free gold.
- “Every mine floats sulphides.” Flotation is selected when sulphide concentration improves the overall route; many ores are treated by whole-ore leaching.
- “CIL always takes 36 hours.” The 36-hour value is a Tocantinzinho design parameter, not a universal rule.
- “Electrowinning makes bullion.” It makes cathode sludge that still contains silver and other metals.
- “Doré is pure gold.” Doré is a gold-silver intermediate product whose composition must be confirmed by assay and later refining.
- “A published recovery number applies everywhere.” Recovery changes with mineralogy, liberation, deleterious elements, grind size, reagent control and operating practice.
14. The complete mine-to-doré sequence
- Mine and haul ROM ore to the crusher or a stockpile.
- Primary-crush the rock and regulate feed through a coarse-ore stockpile and reclaim system.
- Grind in SAG and/or ball mills and classify with hydrocyclones.
- Recover coarse liberated gold by gravity when testing shows it is worthwhile.
- Float sulphide minerals, regrind the concentrate and leach it when the ore requires that route.
- Condition the slurry with air or oxygen where the flowsheet calls for pre-aeration.
- Leach exposed gold with sodium cyanide under controlled residence time and reagent conditions.
- Adsorb dissolved gold on activated carbon in CIP or CIL tanks.
- Screen, acid-wash, elute and thermally regenerate loaded carbon.
- Electrowin gold from the pregnant eluate to produce cathode sludge.
- Filter and dry the sludge, add fluxes, and smelt it into gold-silver doré.
- Detoxify residual cyanide and thicken, filter or pump tailings while recovering process water where practical.
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