How Gold Is Extracted From Ore: Step-by-Step Process
By the time a truck dumps its load at a gold processing plant, the hardest-looking work seems done. The rock is out of the ground. But the gold itself is still locked inside that rock, usually in particles far too small to see, and getting it out is a chemical and mechanical chain of its own. That chain is called extraction, and it is what happens between the mine and the shiny metal most people picture.
This guide follows how gold is extracted from ore, step by step, from the moment ore arrives at the plant to the moment it leaves as a rough gold-silver bar called doré. Getting the rock out of the ground is a separate story, covered in our guide to how gold is mined. If you would rather start with the short version of the whole journey, our gold ore to gold bar overview gives the quick summary, and this article gives the detail.
Step 1: Crushing the Ore
Ore arrives at the plant as rough rock, sometimes in pieces as large as a small car. Nothing can be extracted from rock that size, because the gold is sealed inside it. So the first job is simply to make the pieces smaller.
Large crushers, most commonly jaw crushers followed by cone crushers, bite the rock down in stages. At OceanaGold’s Waihi operation in New Zealand, for example, ore is stockpiled at the plant, fed by loader into a hopper, and passed through a jaw crusher before anything else happens. The crushed ore, now closer to gravel than boulders, is ready for the much finer work of grinding.
Step 2: Grinding and Making a Slurry
Crushing alone does not free the gold. The crushed ore is ground with water in large rotating mills, typically a SAG mill (a semi-autogenous grinding mill, where the rock helps grind itself alongside steel balls) followed by a ball mill filled with hardened steel balls. The turning drums and falling balls reduce the rock to a wet, pumpable mixture called a slurry, or pulp.
The point of all this grinding is a concept called liberation. Gold particles must be physically freed from, or at least exposed on, the surface of the rock around them before any later process can reach them. A leaching solution cannot dissolve gold it cannot touch.
That is why plants grind ore so fine. At Waihi, the finished slurry is typically ground until about 80% of its particles are smaller than 70 microns, roughly the consistency of fine beach sand. Classifying screens and hydrocyclones keep checking the size: fine particles move on, oversize material loops back into the mills for more grinding. Exact targets differ from plant to plant, because every ore body behaves differently, but the principle is the same everywhere. Fine, even grinding decides how well everything after it can work.
Step 3: Gravity Concentration — Catching the Free Gold First
Some of the gold in an ore body is already free: small flakes and grains that grinding has fully liberated. Plants recover this gold early, before any chemicals are used, with gravity concentration.
Gravity methods exploit one simple fact: gold is far denser than the rock and sand around it, so it settles faster and sinks where lighter material washes away. Shaking tables, jigs and modern centrifugal concentrators all use this weight difference, often helped by spinning or shaking, to pull the heavy gold particles into a small, rich concentrate.
It is the same basic principle a prospector uses when panning a river, scaled up and mechanised. Gravity recovery is cheap, chemical-free and worth doing first, because every grain caught here is a grain the chemical plant downstream no longer has to handle. Ores where most of the gold can be recovered this way or by simple leaching are called free-milling ores.
Step 4: Froth Flotation — For Gold Locked With Sulfides
Not all gold is free. In many ores the gold is finely spread through sulfide minerals such as pyrite. The particles are too small and too locked inside other minerals for gravity to catch, but flotation can concentrate them.
In a flotation cell, the slurry is mixed with water, air and small amounts of reagents that make the sulfide mineral surfaces repel water. Air bubbles are blown through the mixture, the treated sulfide particles stick to the bubbles, and they rise to the surface as a mineral-rich froth that is skimmed off. The result is a sulfide concentrate containing much of the gold in a far smaller volume of material, which then goes on to leaching or more intensive treatment.
Flotation does not extract the gold itself. It upgrades the ore, separating the valuable sulfide fraction from waste rock so the expensive chemical steps that follow only have to treat a fraction of the original material.
Step 5: Cyanide Leaching — Dissolving the Gold
The central step in most modern plants is cyanide leaching, also called cyanidation. A dilute solution of sodium cyanide, with oxygen from air, dissolves gold out of the ground ore and holds it in solution as a soluble gold-cyanide complex. Chemists write the reaction as the Elsner equation: 4Au + 8NaCN + O2 + 2H2O → 4Na[Au(CN)2] + 4NaOH.
Lime is added to the slurry before and during leaching to keep it alkaline. This matters for safety as much as for chemistry, because cyanide solutions must stay alkaline to prevent the formation of deadly hydrogen cyanide gas. Cyanide is highly toxic, and everything about this step, from lined tanks to monitoring and water treatment, is built around containing it. Cyanidation is nevertheless the most common industrial extraction method in the world, because it dissolves gold efficiently even from low-grade ores that no physical method could treat.
Tank leaching with CIP and CIL
In a tank plant, the slurry moves through a series of large, stirred leach tanks with enough retention time for the cyanide to dissolve the gold. Two layouts dominate:
- Carbon-in-pulp (CIP): leaching happens first, in its own tanks. The gold-bearing slurry then passes through separate adsorption tanks where activated carbon collects the dissolved gold (see Step 6).
- Carbon-in-leach (CIL): leaching and carbon adsorption happen in the same series of tanks. The carbon grabs the gold as fast as it dissolves, which is especially useful for ores with “preg-robbing” carbonaceous material that would otherwise steal the dissolved gold back out of solution.
At Waihi, the leach circuit uses a series of six tanks, with oxygen added to assist dissolution, before the slurry moves on to the carbon adsorption tanks.
CIP and CIL are the two tank routes compared side by side in our guide to CIP vs CIL differences explained.
Heap leaching for low-grade ore
Grinding and tank leaching cost money, so very low-grade ore often skips them. In heap leaching, crushed ore is stacked in a large pile, a heap, on an impermeable lined pad. Cyanide solution is sprayed or dripped over the top and slowly percolates down through the pile, dissolving gold as it goes. The gold-bearing liquid, called the pregnant solution, drains to a lined pond at the bottom, as the US Geological Survey describes for large operations such as Peru’s Yanacocha mine.
Heap leaching is slower and less tightly controlled than a tank circuit, and some gold is always left behind in the heap. Its advantage is that it can treat huge volumes of low-grade ore without the cost of fine grinding, which is exactly the ore that would otherwise be left as waste.
Step 6: Recovering Gold From the Solution
After leaching, the gold exists only as an invisible presence dissolved in liquid. Step 6 turns it back into a solid. Most plants use activated carbon to do this; the main alternative is zinc precipitation.
Carbon adsorption and elution (stripping)
Activated carbon has an enormous internal surface area, and dissolved gold-cyanide complex sticks (adsorbs) strongly to it. In the adsorption tanks, carbon granules are moved through the slurry in the opposite direction to its flow, so the freshest carbon meets the most depleted slurry and the most loaded carbon meets the richest slurry. By the last tank, the slurry is essentially barren of gold, and the loaded carbon is pumped away carrying the gold with it.
The gold is then stripped from the carbon in a process called elution. The loaded carbon is washed, including an acid wash, and a hot caustic cyanide solution is circulated through it, washing the gold back off into a small volume of rich liquid called pregnant eluate. The cleaned carbon is reactivated, typically by heating it in a kiln, and returned to the circuit to be used again. (A carbon safety screen is also used to catch any fine loaded carbon before the barren slurry leaves the plant.)
Electrowinning
The pregnant eluate passes through electrowinning cells, where an electric current is applied between metal plates. The dissolved gold (and silver, which follows gold through the whole process) migrates out of the liquid and coats the cathodes. At Waihi those cathodes are stainless steel plates; when they are rinsed, they yield a muddy, metal-rich sludge. That sludge, dried, is the raw material for the final step.
Merrill-Crowe zinc precipitation (the alternative)
The older alternative to carbon and electrowinning is the Merrill-Crowe process. The gold-bearing solution is first clarified and de-aerated, then zinc dust is added. Zinc is chemically more reactive than gold, so it takes gold’s place in the dissolved complex, and the gold falls out of solution as a fine solid precipitate that can be filtered off. Merrill-Crowe is still used in some operations, particularly where solutions carry high levels of silver, but carbon adsorption handles most modern tank plants.
Step 7: Smelting Into Doré Bars
The dried sludge or precipitate from recovery is mixed with fluxes, chemicals that help impurities separate during melting, and heated in a furnace. At Waihi the furnace reaches about 1,200 °C. The molten material is poured into a cascade of moulds, the glassy slag of impurities is removed from the top, and what remains is a doré bar: a semi-pure alloy of gold and silver. At Waihi a typical doré bar weighs around 20 kg and, before refining, actually contains more silver than gold; the mix varies with the ore.
Doré is the end of extraction, not the end of the story. A doré bar is not pure gold, and it cannot be sold as finished bullion. It is shipped to a refinery, where the gold and silver are separated and purified in the gold refining process. The melting stage itself, and how fluxes and slag work, is covered in more depth in our guide to the gold smelting process.
When Ore Fights Back: Refractory Ores and Pretreatment
Everything above assumes the cyanide can reach the gold. In refractory ores it cannot, for two main reasons. The gold may be locked inside sulfide minerals at a microscopic scale, so fine that even grinding does not liberate it. Or the ore may contain natural carbonaceous material that behaves like the plant’s own activated carbon, absorbing dissolved gold back out of solution, a problem called preg-robbing.
Refractory ores need pretreatment that breaks the sulfide cage open before leaching. The main options are:
- Ultrafine grinding: grinding far finer than normal, to expose more of the locked gold.
- Roasting: heating the ore or concentrate in air to burn off sulfides and carbon.
- Pressure oxidation: treating the slurry with oxygen under high pressure and temperature in an autoclave, which oxidises the sulfides.
- Biooxidation: using bacteria that naturally feed on sulfide minerals to break them down before leaching.
Pretreatment adds cost and complexity, which is why the geology of an ore body, not just its gold content, decides whether a deposit can be mined at a profit.
What Happens to the Rest: Tailings and Cyanide Safety
An extraction plant produces far more waste than gold. The barren slurry left after adsorption, now called tailings, is pumped to a dedicated tailings storage facility, a lined and engineered dam built to hold it permanently. Process water is recovered from the tailings pond and reused in the plant, as at Waihi, where water decanted from the tailings pond feeds back into grinding.
Because cyanide is acutely toxic to people, fish and wildlife, plants operate under strict containment, monitoring and water-treatment rules, and leftover cyanide in tailings is typically broken down (detoxified) to regulated levels before discharge or storage. Even so, tailings and cyanide management remain the most closely watched environmental aspects of gold extraction, and failures are treated as serious industrial accidents, not routine events.
One historical method deserves a mention only as a warning. Mercury amalgamation, mixing liquid mercury with gold-bearing material, was once common and still appears in some artisanal mining. Mercury is severely toxic to the people who use it and to the waterways it reaches, and it is not part of the modern plant process described in this article.
Extraction Methods Compared
| Method | Principle | Best for |
|---|---|---|
| Crushing & grinding | Breaking rock down and liberating gold particles into a slurry | Every ore; the preparation all other steps depend on |
| Gravity concentration | Gold’s high density makes it settle where lighter rock washes away | Free, visible gold in free-milling ores; used early, before chemicals |
| Froth flotation | Reagents make sulfide minerals stick to air bubbles and float as a froth | Gold locked with sulfide minerals; produces a concentrate for leaching |
| Cyanide leaching in tanks (CIP / CIL) | Dilute cyanide and oxygen dissolve gold; activated carbon collects it | Most free-milling ores in a controlled plant, low to high grade |
| Heap leaching | Cyanide solution trickles through a lined pile of crushed ore | Very low-grade ore in large volumes, where fine grinding is not economical |
| Refractory pretreatment (roasting, pressure oxidation, biooxidation) | Breaking open sulfide minerals or removing preg-robbing carbon before leaching | Refractory ores that direct cyanidation cannot treat effectively |
Key Takeaways
- Extraction is the plant stage between mining and refining: ore in at the plant gate, doré bars out.
- Grinding and liberation come first, because no later process can recover gold it cannot reach.
- Plants use physical methods before chemicals: gravity catches free gold, and flotation concentrates gold locked in sulfides.
- Cyanide leaching does the main dissolving work, in tanks (CIP or CIL) for most ores, or in heaps for very low-grade ore.
- Activated carbon, elution and electrowinning turn dissolved gold back into a solid, which is smelted into doré and sent for refining.
- How much gold is recovered depends on the ore and the method, which is why no single recovery figure applies to every plant.
FAQs
What is the difference between gold mining, extraction and refining?
Mining gets the ore out of the ground. Extraction, covered in this article, is the processing-plant work that separates gold from that ore and produces doré bars. Refining is the final stage that purifies doré into high-purity gold and separates it from silver.
Why is ore ground into such a fine powder before extraction?
Because the gold must be liberated, freed or exposed at the surface of the rock particles, before gravity, flotation or cyanide can reach it. Gold sealed inside a lump of rock cannot be dissolved or separated, so plants grind ore to a sand-like slurry and send oversize material back for more grinding.
What chemicals are used to extract gold from ore?
The main one is a dilute sodium cyanide solution, used with oxygen to dissolve gold during leaching. Lime is added to keep the slurry safely alkaline, and smaller amounts of flotation reagents are used where flotation is part of the flow sheet. The stripped gold is recovered from solution using activated carbon and electrowinning, or zinc dust in the Merrill-Crowe process.
Is cyanide the only way to extract gold?
No. Gravity concentration and flotation recover or concentrate gold without dissolving it, and they are used before and alongside leaching in most plants. Alternative lixiviants such as thiosulfate exist and are used in a few specific cases, but cyanidation remains the dominant industrial method because it works efficiently on a wide range of ores.
What is a doré bar?
A doré bar is the semi-finished product of a gold plant: an unrefined alloy of gold and silver, cast at the plant after smelting, which is then shipped to a refinery to be purified and separated into finished gold and silver.
How much gold is typically recovered from ore?
There is no single honest figure. Recovery depends on the ore’s mineralogy, how finely it is ground, and which methods are used. Well-run tank plants on free-milling ore recover most of the gold the ore contains, while heap leaching and refractory ores recover less, which is exactly why plants test each ore body before designing the flow sheet. Any source quoting one universal percentage for all ores should be read with caution.
To understand what gold recovery rates mean in practice, see our guide to how gold recovery works in modern processing plants.
Sources and Further Reading
- OceanaGold, Waihi Operation: Ore Processing (operating company’s own stage-by-stage description, used above for the Waihi examples)
- US Geological Survey (USGS EROS): The Mining Process (heap leaching at Yanacocha)
- HowStuffWorks: Extracting Gold (carbon-in-pulp chain explainer)
- 911Metallurgist: Testing Refractory Gold Ore (refractory ores and pretreatment)
- USGS: Sodium cyanide hazards to fish and other wildlife from gold mining operations
