Gold processing transforms raw ore or scrap material into pure gold through a series of physical and chemical steps. The exact method depends on the scale of the operation and the form of the starting material, but the core sequence is the same: crush, separate, dissolve, and refine. Industrial mines process thousands of tons of rock to extract tiny amounts of gold, while smaller operations can refine scrap jewelry or gold flakes using simpler chemistry. Here’s how each stage works.
Crushing and Grinding the Ore
Gold ore straight from a mine is mostly rock. A typical gold deposit might contain just a few grams of gold per ton of material, so the first job is breaking that rock down into fine particles to free the gold trapped inside. Industrial operations use jaw crushers to reduce large chunks to gravel-sized pieces, then ball mills or rod mills to grind them into a fine powder, sometimes as small as grains of sand. This step is called comminution, and it’s one of the most energy-intensive parts of the entire process. Without it, the gold stays locked inside mineral grains and can’t be recovered.
Gravity Separation
Gold is extremely dense, about 19 times heavier than water. That density difference makes it possible to physically separate gold particles from lighter surrounding rock using gravity. This is the oldest gold recovery method and still the first step in most modern operations.
Several types of equipment handle this job. Shaking tables use a sloped, ridged surface with flowing water to sort particles by weight, producing high-grade concentrate but processing relatively little material at a time. Jig concentrators pulse water through a bed of material, letting heavy gold sink while lighter minerals wash away. Spiral concentrators channel slurry down a helical track, where centrifugal force and gravity sort particles along the way. Centrifugal concentrators spin material at high speed to capture even very fine gold particles, and they’re portable enough for small-scale miners to use in the field.
Gravity separation is effective for “free gold,” meaning gold particles that have been fully liberated from the surrounding rock during grinding. But a significant portion of gold in most ores is too finely distributed to capture this way, which is where chemical methods take over.
Flotation
After gravity separation recovers the coarse gold, the remaining material often goes through froth flotation. This technique exploits differences in how minerals interact with water. The ground ore is mixed into a slurry with water and chemical reagents that make sulfide minerals (which commonly host microscopic gold) repel water. Air is blown through the mixture, creating bubbles. The water-repelling sulfide particles attach to the bubbles and float to the surface as froth, which is skimmed off. The result is a concentrated sulfide product containing gold that was too fine for gravity methods to catch.
Cyanide Leaching
Cyanide leaching, also called cyanidation, is the dominant method for dissolving gold out of ore or concentrate. The material is mixed with a dilute cyanide solution, which reacts with gold to form a soluble compound. The gold literally dissolves into the liquid, leaving the worthless rock behind as solid residue.
Some ores are “refractory,” meaning the gold is locked inside sulfide minerals so tightly that cyanide can’t reach it. These ores need an oxidation step first, using heat (roasting) or pressure (autoclave processing) or bacteria (bio-oxidation) to break down the sulfide minerals and expose the gold before cyanide leaching can work.
Once the gold is in solution, it needs to be pulled back out. Two main methods handle this. In carbon adsorption, the gold-bearing solution passes through activated carbon, which attracts and holds the dissolved gold. The gold is later stripped from the carbon using a strong cyanide and sodium hydroxide solution. In the Merrill-Crowe process, the solution is deoxygenated and filtered through zinc metal powder, which chemically displaces the gold from solution and deposits it as a solid precipitate. Either way, the end product at this stage is an impure gold material ready for smelting.
Smelting
Smelting melts the gold precipitate or concentrate in a furnace, typically with chemical fluxes that bind to impurities and float to the surface as slag. The slag is poured off, and the remaining molten gold is cast into bars called doré. Doré bars typically contain 60% to 95% gold, with silver and other metals making up the rest. These bars then move to a refinery for final purification.
Refining to High Purity
Refining is where gold reaches the purity levels required for bullion, electronics, and jewelry. Two main processes handle this, and large refineries typically use both in sequence.
The Miller process bubbles chlorine gas through molten gold. The chlorine reacts with impurities like silver, copper, and lead, forming chloride compounds that float to the surface as a scum and are skimmed off. This method is fast and works well at large scale, but it tops out at around 99.5% purity. That’s good enough for some applications but not for investment-grade bullion or electronics.
For higher purity, the Wohlwill electrolytic process takes over. Impure gold bars are used as anodes (the positive electrode) in an electrolytic cell filled with a gold chloride solution. When electricity flows through the cell, pure gold dissolves from the anode and deposits onto the cathode (the negative electrode) as 99.99% pure gold. The process requires anodes with at least 95% gold content, which is why the Miller process usually comes first. This two-step approach, Miller followed by Wohlwill, is the standard pathway for producing four-nines (99.99%) gold at commercial refineries worldwide.
Small-Scale Refining With Aqua Regia
For smaller operations, scrap gold, or jewelry refining, aqua regia is the go-to chemical method. Aqua regia is a mixture of concentrated hydrochloric acid and nitric acid in a 3:1 ratio. It’s one of the few substances that can dissolve gold, which resists almost every other acid on its own.
The gold item is placed in aqua regia, which dissolves it into solution. Any silver present won’t dissolve and instead forms a white solid (silver chloride) that settles to the bottom and can be filtered out. Once the gold is fully dissolved, a reducing agent like ferrous sulfate or sulfur dioxide gas is added, which causes pure gold to precipitate out of the solution as a fine powder. This powder is washed, dried, and melted into a button or bar. Skilled refiners can achieve 99.9% purity or better with careful technique.
Aqua regia refining requires proper ventilation and acid-resistant containers. The process produces toxic nitrogen dioxide fumes, and the acids themselves are highly corrosive. It’s practical for small batches but too slow and chemical-intensive for processing large volumes of ore.
Why Mercury Amalgamation Is Dangerous
Mercury has been used to process gold for centuries, particularly by small-scale and artisanal miners. The method is simple: liquid mercury is mixed with gold-bearing sediment, and it bonds with gold particles to form a heavy amalgam. The amalgam is then heated to boil off the mercury, leaving gold behind. It works, and it’s cheap, which is why an estimated 10 to 15 million artisanal miners worldwide still use it.
The health consequences are severe. Mercury vapor produced during the heating step is readily inhaled and attacks the nervous system, digestive system, immune system, lungs, and kidneys. Chronic exposure causes tremors, memory problems, vision disorders, poor coordination, and fatigue. Workers in amalgam-burning areas are routinely exposed to mercury vapor levels far exceeding the World Health Organization’s public exposure limit of 1.0 microgram per cubic meter. The damage extends beyond miners: communities living downstream of mining operations show elevated mercury levels from contaminated fish, and children exposed to mercury can develop seizures, hearing and vision loss, and delayed development. Pregnant women with high mercury exposure show dose-dependent effects on their children’s coordination and cognitive development.
Gravity concentration and cyanide leaching have largely replaced mercury in professional mining operations. For small-scale miners, centrifugal concentrators offer a mercury-free alternative that captures fine gold effectively and costs relatively little to operate.