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Gold processing plants turn mined rock into gold by a linked sequence of size reduction, mineral separation, chemical extraction and high-temperature treatment. Ore is first crushed and ground so gold grains are exposed. Depending on the ore, the plant then uses gravity, flotation, cyanide leaching or a combination of these routes. Dissolved gold is recovered onto activated carbon in carbon-in-pulp or carbon-in-leach circuits, or precipitated with zinc in a Merrill–Crowe circuit. The recovered material is electrowon or filtered, smelted into doré bullion and sent to a refinery for final separation.
What happens to gold ore at a processing plant?
A mine delivers run-of-mine ore from an open pit or underground operation. The ore is a mixture of valuable minerals, waste rock and water; only a small fraction of its mass is gold. A plant therefore does not simply “melt the rock.” It progressively liberates gold, separates it from gangue, transfers it into a manageable intermediate product and then purifies that product.
- Feed preparation: primary and secondary crushers reduce large rocks, then mills grind the particles and classifiers control the pulp size.
- Physical concentration: gravity equipment may recover coarse native gold, while flotation can collect gold-bearing sulfide or other minerals in a concentrate.
- Leaching: when gold is fine or dispersed, an alkaline cyanide solution dissolves it as a gold-cyanide complex.
- Solution recovery: activated carbon captures dissolved gold in CIP or CIL plants; Merrill–Crowe plants remove oxygen and add zinc dust to precipitate gold and silver.
- Finishing: carbon eluate or zinc precipitate is recovered by electrowinning or filtration, dried and smelted to doré bullion.
- Refining: doré is shipped to a refinery, where gold is separated from silver and other metals to the required purity.
The exact flowsheet is selected from test work on the ore rather than from a universal recipe. Grain size, host minerals, sulfide and telluride content, clay, permeability, water supply, reagent consumption, recovery targets and tailings chemistry all affect the design.
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Crushing run-of-mine rock
Trucks feed a primary crusher that breaks blasted rock into smaller pieces. Secondary and tertiary crushers can reduce it further before milling. Screens separate particles that are already small enough from oversize material that must be crushed again. This stage creates a steady feed for the grinding circuit and removes the size variation that would make later separation unstable.
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Grinding and classification
Rod mills, ball mills or other grinding mills use rotating media to reduce the crushed ore to a fine pulp of rock and water. The objective is liberation: a gold grain must be physically freed from quartz, sulfide, clay or another host before a separator or leach solution can reach it. Overgrinding wastes energy and can create slimes that settle poorly or consume excess reagent, while undergrinding leaves gold locked in particles that pass to tailings.
Hydrocyclones or screens classify the mill discharge. Coarse particles return to the mill; correctly sized material moves to gravity, flotation or leaching. Milling is the front end of nearly every industrial gold route because chemical recovery cannot compensate for gold that remains enclosed in unbroken mineral grains.
2. Gravity separation recovers coarse, liberated gold
How density separation works
Native gold is far denser than most silicate gangue. Gravity circuits exploit that difference rather than changing the minerals chemically. Pans, sluices, jigs, shaking tables and centrifugal concentrators create flowing or pulsating conditions in which heavy particles settle while lighter material is carried away. The resulting concentrate contains a much higher gold grade than the original feed.
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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 matchIndustrial plants use engineered, high-throughput equipment and controlled water flows; a small prospecting sluice illustrates the same density principle but is not a substitute for a plant circuit. Gravity recovery is especially valuable when the ore contains visible or coarse gold that cyanide would dissolve slowly or that might be lost in a large tailings stream.
Where the gravity concentrate goes
A clean, high-grade concentrate may be smelted directly. More commonly, it is cleaned in a secondary gravity stage and sent to a leach reactor so residual gold can be recovered. The gravity tailing continues through flotation or cyanidation. Gravity is therefore often a parallel recovery step, not an alternative that eliminates every downstream process.
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3. Flotation makes a mineral concentrate when chemistry requires it
Attaching valuable minerals to bubbles
In flotation, ground pulp is conditioned with reagents that make selected mineral surfaces attach to air bubbles. Froth carrying the targeted gold-bearing sulfide or other mineral overflows into launders, while unwanted rock remains in the underflow. The rougher concentrate is normally cleaned in additional flotation cells to improve grade; the final product is thickened and filtered.
Flotation is useful when gold is associated with sulfides, tellurides or another mineral that can be selectively floated. It can reduce the volume that must undergo intensive leaching and may produce a concentrate that is treated on site or transported to another recovery facility. The remaining fine-rock tailings require containment and water management just as leach residues do.
Why flotation and leaching may be combined
A plant can float a sulfide concentrate and cyanide-leach the flotation tailing, or treat the concentrate separately after an oxidation step. The choice depends on where the gold sits and how much reagent each stream consumes. A gravity–flotation–leach flowsheet can recover coarse free gold first, concentrate mineral-bound gold next and leach the remaining accessible gold last.
4. Cyanide leaching dissolves gold from finely divided ore
Tank, vat and heap leaching
For many low-grade or finely disseminated ores, a weak cyanide solution contacts prepared ore in agitated tanks, vats or heaps. The solution is kept alkaline so the process can be controlled and to limit formation of volatile hydrogen cyanide. In simplified form, the reaction is:
4Au + 8NaCN + 2H2O + O2 → 4NaAu(CN)2 + 4NaOH
Oxygen, cyanide concentration, particle size, residence time, temperature, mixing and pH all influence dissolution. The gold enters solution as a soluble gold-cyanide complex; the rock left behind becomes leach residue or tailings.
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Heap leaching
In a heap operation, crushed ore is placed on an impermeable, lined pad. Irrigation distributes cyanide solution through the heap, and drains collect the gold-bearing pregnant solution at the bottom. Heap leaching has lower equipment intensity than finely ground tank leaching but requires suitable permeability and careful control of solution inventory, pad integrity and channeling. Clay-rich or poorly permeable ore may need agglomeration or a different route.
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Agitated leaching
Finely ground pulp is mixed in a series of tanks when faster kinetics and tighter control are needed. Air or oxygen can be added, and the tank sequence provides enough residence time for the targeted gold dissolution. The resulting pregnant solution is separated from solids before gold recovery, while carbon-in-leach can combine leaching and adsorption in the same train.
5. Carbon-in-pulp and carbon-in-leach recover dissolved gold
Adsorption onto activated carbon
Activated carbon has a high internal surface area that adsorbs the dissolved gold-cyanide complex. In carbon-in-pulp (CIP), leaching and adsorption are separate sections: clarified leach pulp moves through adsorption tanks containing carbon. In carbon-in-leach (CIL), carbon is present in the leach tanks so gold is captured as it dissolves. Screens retain carbon while slurry flows onward.
Carbon moves counter-currently toward the higher-grade solution, allowing the final adsorption tank to receive the leanest solution and the freshest carbon. Loaded carbon from the first adsorption position is screened out, washed and sent to elution.
Elution and electrowinning
In the elution circuit, hot caustic cyanide solution strips gold from the loaded carbon. The gold-rich eluate flows to electrowinning cells. An electric current plates gold and silver onto cathodes, producing a sludge or “cathode mud” that contains the precious metals along with some impurities. The barren carbon is reactivated and returned to the adsorption circuit.
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6. Merrill–Crowe uses zinc precipitation instead of carbon
Merrill–Crowe is suited to a clear pregnant solution, often where silver is significant or carbon adsorption is less advantageous. The solution is filtered and de-aerated to remove dissolved oxygen. Zinc dust is then added; it displaces gold and silver from the cyanide complex, forming a solid precipitate. Filters collect the precipitate, and the clarified solution is returned to the process after appropriate treatment.
The critical controls are clarification, oxygen removal, zinc dosage and filtration. Suspended solids can foul filters, while excess oxygen reduces precipitation efficiency. Unlike CIP or CIL, this route does not produce loaded carbon; its immediate gold-bearing intermediate is the zinc precipitate.
7. Smelting turns recovered solids into doré
Electrowinning sludge or Merrill–Crowe precipitate is dried and mixed with fluxes selected to combine unwanted oxides into a separable slag. A furnace melts the charge. Dense molten metal settles below the slag and is poured into molds as doré bullion, an intermediate alloy of gold and silver with residual copper, iron, lead or other metals.
Doré is not usually the final saleable purity. It is shipped to a refinery, where chemical or electrochemical refining separates gold from silver and other constituents. The refinery’s product specification, rather than the mine’s initial head grade, determines the final gold purity.
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Why some gold ores are refractory
Gold locked in sulfide or telluride minerals
In a refractory ore, gold is so finely enclosed in sulfide, telluride or another resistant mineral that ordinary grinding and cyanidation leave much of it inaccessible. A leach test may show low extraction even when the assay confirms substantial gold in the feed. Simply adding more cyanide can increase cost without opening the mineral structure.
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Pretreatment options
Plants may use flotation to make a smaller, higher-grade concentrate, then apply oxidation before cyanidation. Options include roasting, pressure oxidation, bio-oxidation and other licensed oxidation processes. These treatments break down the host mineral, expose the gold and alter sulfur, arsenic or carbon chemistry so the subsequent leach can work.
Carbonaceous, clay-rich and preg-robbing ores
Carbonaceous ore can adsorb dissolved gold before the recovery circuit captures it, a behavior known as preg-robbing. Plants may use passivation, competing carbon, flotation or other controls to prevent this loss. Clay can impair grinding, settling and heap permeability, requiring desliming, agglomeration or a different flowsheet. These mineralogical effects explain why two deposits with the same head grade can need very different plants.
How plants manage tailings, cyanide, mercury and water
Containing cyanide-bearing solutions
Leach tanks, pipelines and heap pads are engineered to contain process solution. Lined pads, secondary containment, leak detection, solution ponds and monitored tailings storage reduce the chance that cyanide-bearing water reaches soil or surface water. Detoxification can convert residual cyanide to less reactive forms before discharge or recycle, but the exact method and limits depend on the jurisdiction and process.
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Plants thicken tailings to recover water for reuse, reducing fresh-water demand and the volume sent to storage. Tailings facilities must account for settling, seepage, flood protection and long-term stability. Process water can contain cyanide species, dissolved metals and reagents, so sampling and treatment continue after the solid gold has been removed.
Mercury and air emissions
Mercury controls are required where ore or process equipment contains mercury. Dust collection, enclosed transfer points, furnace off-gas treatment and monitoring address particulate and gaseous emissions. A 2011 U.S. Environmental Protection Agency rule summary estimated that controls would reduce mercury emissions to about 1,200 pounds per year, a reduction of approximately 77% from 2007 levels; that figure is a historical U.S. estimate, not a universal current plant value.
Why there is no single percentage of ore that becomes gold
Most feed mass leaves as waste rock, flotation tailings, leach residue, slag or refinery by-products. The plant tracks a mass balance: tonnes entering each stream, gold grade in that stream and the gold recovered. Recovery is reported for a defined ore type and flowsheet, not as a universal percentage applicable to every mine. Coarse free gold, sulfide-bound gold, carbonaceous ore and heap-leachable oxide ore can produce very different results, and published sources do not establish one global average.
| Stage | What enters | What the plant produces | Main reason it is used |
|---|---|---|---|
| Crushing and grinding | Run-of-mine ore | Classified pulp | Liberates gold from host rock |
| Gravity | Ground pulp | Heavy-mineral concentrate and gravity tailing | Recovers coarse, liberated gold by density |
| Flotation | Conditioned pulp | Gold-bearing mineral concentrate and tailings | Collects sulfide or other floatable host minerals |
| Cyanide leaching | Prepared ore or pulp | Pregnant solution and leached solids | Dissolves fine or disseminated gold |
| CIP/CIL | Pregnant pulp and activated carbon | Loaded carbon and barren pulp | Adsorbs dissolved gold |
| Merrill–Crowe | Clarified pregnant solution | Zinc precipitate and barren solution | Precipitates gold and silver from solution |
| Smelting and refining | Electrowinning sludge or zinc precipitate | Doré, then refined gold and separated silver | Converts recovered solids into a marketable metal product |
How engineers choose a flowsheet
- Liberation size: coarse gold may justify gravity; fine locking may require fine grinding or oxidation.
- Host mineral: quartz, sulfide, telluride and carbonaceous hosts respond differently to flotation and leaching.
- Gold and silver content: high silver or a clear solution can favor Merrill–Crowe, while many operations use carbon adsorption.
- Clay and permeability: poor heap drainage may rule out heap leaching or require agglomeration.
- Water and reagents: availability, recycling potential and consumption affect operating cost and environmental design.
- Tailings chemistry: cyanide, sulfide oxidation, arsenic, mercury and dissolved metals determine containment and treatment requirements.
Metallurgical test work—commonly including comminution, gravity, flotation, leach and oxidation tests—maps these variables before construction. The result can be a simple gravity-and-leach plant or a multi-stage circuit with concentrate treatment, oxidation and separate recovery lines.
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