Processing gold ore means turning rock containing tiny amounts of gold into a high-purity metal, and it happens through a sequence of physical and chemical steps: crushing, grinding, concentrating, leaching, recovering, and refining. The exact path depends on the type of ore, but most operations follow the same general flow. The end product is typically a doré bar containing 60% to 90% gold, which is then sent to a refinery for final purification.
Crushing and Grinding
Gold ore comes out of the ground in large chunks, sometimes the size of boulders. The first job is to break it down into pieces small enough for the gold to be separated from the surrounding rock. Jaw crushers reduce the ore to fist-sized fragments, and cone crushers take it down further. The material then moves to ball mills or rod mills, which are rotating drums filled with steel balls that grind the ore into a fine powder, often as small as grains of sand or finer.
This grinding stage is critical because gold particles are frequently locked inside the rock matrix. Until the ore is ground fine enough to physically free those particles, no chemical or physical method can reach them. The finer the grind, the better the recovery, but grinding is also one of the most energy-intensive steps in the entire process.
Gravity Concentration
Gold is extremely dense, roughly 19 times heavier than water, and gravity concentration exploits that property. As ground ore passes through the grinding circuit, batch centrifugal concentrators spin the slurry at high speed to separate heavy gold particles from lighter waste material. These devices have become the standard tool for recovering what the industry calls gravity-recoverable gold.
The concentrate from a centrifugal unit is often upgraded further using a shaking table, a vibrating surface that separates particles by both weight and size as water washes across them. Gravity methods work best on coarse, free gold and can pull a significant portion of gold out of the ore before any chemicals are needed. That makes them cheaper and cleaner than chemical methods, though they typically can’t capture very fine gold particles on their own.
Cyanide Leaching
The dominant chemical method for dissolving gold from ore is cyanide leaching. A very dilute sodium cyanide solution, typically between 0.01% and 0.05% cyanide, is mixed with the finely ground ore. The cyanide reacts with gold in the presence of oxygen to form a soluble gold-cyanide complex, effectively pulling the gold into the liquid phase and leaving the rock behind.
This process requires careful control. The solution is kept alkaline, usually with added lime, to prevent cyanide from converting into toxic hydrogen cyanide gas. Leaching can take anywhere from several hours to several days depending on the ore type, the fineness of the grind, and how the gold is distributed through the rock. For oxide ores that respond well to direct leaching, gold recovery rates typically fall between 75% and 90%.
Heap Leaching
For lower-grade ores, heap leaching is a simpler alternative. Crushed ore is stacked on a lined pad and the cyanide solution is dripped over the top, percolating down through the heap over weeks or months. The gold-bearing solution collects at the base and is piped to a processing plant. This method is less efficient than tank leaching but far cheaper, making it viable for ores that would otherwise not be worth processing.
Handling Refractory Ores
Some gold ores are called “refractory” because the gold is locked inside sulfide minerals like pyrite or arsenopyrite, and cyanide simply cannot reach it. These ores require a pretreatment step to break open those mineral structures before leaching can work. Without pretreatment, recovery rates can be disappointingly low.
Three main pretreatment methods are used. Roasting heats the ore in air to burn off the sulfides, though it only achieves around 75% gold recovery in some operations and releases sulfur dioxide, an air pollutant. Pressure oxidation (known as POX) uses high temperatures (180 to 225°C) and high-pressure oxygen to chemically decompose the sulfide minerals. It avoids the air pollution issues of roasting and generally delivers better recovery. Bio-oxidation is the most environmentally friendly option: specially adapted bacteria break down the sulfide minerals at low temperatures (15 to 25°C) over a period of days. It is now widely used for low-grade refractory concentrates, and studies have shown it can boost gold extraction from as low as 73% up to 99% in some cases.
After any of these pretreatments, the ore goes through standard cyanide leaching, typically recovering 80% to 90% of the gold.
Recovering Gold From Solution
Once gold is dissolved in a cyanide solution, it needs to be pulled back out as a solid. The two most common methods are carbon adsorption and electrowinning.
In the carbon-in-pulp (CIP) process, granular activated carbon, usually made from coconut shells, is added to tanks containing the gold-bearing slurry. The dissolved gold-cyanide complex sticks to the surface of the carbon granules. The system works in a counter-current flow: the slurry moves forward through a series of tanks while the carbon moves backward, from the last tank toward the first. By the time carbon reaches the first tank, it is heavily loaded with gold. Screens at each tank’s outlet prevent the carbon from washing forward with the slurry. The loaded carbon is then stripped of its gold using a hot chemical wash in a process called elution.
The gold-rich solution from elution goes to an electrowinning cell, where an electric current passes through the liquid. Gold ions are attracted to steel wool cathodes, where they deposit as a dark metal sludge. Cells typically run at 2 to 5 volts and are serviced about once a week to collect the accumulated gold. The sludge is dried and then smelted in a furnace, often with fluxes like borax, to produce a doré bar ready for shipment to a refinery.
Small-Scale and Artisanal Processing
Not all gold processing happens at industrial scale. Millions of small-scale miners worldwide process gold using much simpler methods. Traditionally, many have used mercury amalgamation, mixing mercury with gold-bearing sand to form an amalgam, then burning off the mercury to leave gold behind. This releases toxic mercury vapor and is a major environmental and health concern.
A safer alternative developed by small-scale miners in the Philippines uses borax as a smelting flux instead of mercury. The process starts with sluicing, using flowing water over riffled channels to concentrate heavy minerals. The resulting concentrate is mixed with borax (at least 30% of the estimated gold weight), placed in a clay bowl with charcoal, and heated. Borax lowers the melting point of gold, allowing it to pool together and separate from waste minerals at achievable temperatures. In controlled comparisons, this mercury-free method recovered up to 78% more gold than the mercury amalgamation approach, while costing less and avoiding toxic exposure. The main limitation is that borax can be difficult to find in remote areas, though it is common in towns where it is used in welding and jewelry work.
Cyanide Detoxification and Waste Management
Because cyanide is highly toxic, every gold processing operation must treat its waste before disposal. The most widely used method is the INCO SO₂/air process, developed by the International Nickel Company. It works by mixing the cyanide-containing waste with sulfur dioxide and air in the presence of a small amount of copper sulfate as a catalyst. This converts cyanide into cyanate, a compound that is 3,000 to 5,000 times less toxic.
The reaction requires roughly 3 to 7 pounds of sulfur dioxide per pound of cyanide, depending on whether the waste is a clear solution or a thick slurry with solids. Lime is added to keep the pH between 8 and 10. The equipment is relatively straightforward: a mixing vessel, an air sparging system, and chemical dosing lines. Large operations may treat hundreds of kilograms of cyanide per hour through these systems. The treated waste is then stored in lined tailings facilities designed to prevent any remaining contaminants from reaching groundwater or surface water.
Recovery Rates by Ore Type
How much gold you actually recover depends heavily on the ore’s mineralogy. Simple oxide ores, where gold sits freely in weathered rock, respond well to direct cyanide leaching and typically yield 75% to 90% recovery. Sulfide ores, where gold is trapped inside sulfide minerals, require pretreatment but can reach 80% to 90% recovery afterward. The most stubborn refractory ores, like the Carlin-type deposits common in Nevada, may need aggressive pressure oxidation to achieve acceptable yields.
Gravity methods alone rarely capture all the gold in an ore body, but they are excellent at recovering coarse particles cheaply. Most modern operations combine gravity recovery within the grinding circuit with cyanide leaching of the remaining ore, capturing gold across the full size range. This combined approach consistently delivers the highest overall recovery rates while reducing chemical consumption.