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Gold extraction or recovery from its ores may require a combination of comminution, mineral processing, hydrometallurgical, and pyrometallurgical processes to be performed on the ore.

Gold mining from alluvium ores was once achieved by techniques associated with placer mining such as simple gold panning and sluicing, resulting in direct recovery of small gold nuggets and flakes. Placer mining techniques in the present day are generally the practice of artisan miners. Hard rock ores form the basis of the majority of commercial gold recovery operations, and are mined using open pit or sub-surface mining techniques.

Once the ore is mined it can be treated as a whole ore using a dump leaching or heap leaching processes. This is typical of low-grade, oxide deposits. Normally, the ore is crushed and agglomerated prior to heap leaching. High grade ores and ores resistant to cyanide leaching at coarse particle sizes, require further processing in order to recover the gold values. The processing techniques can include grinding, concentration, roasting, and pressure oxidation prior to cyanidation.

Contents 1 Concentration 2 Leaching 3 Refractory Gold Processes 4 Gold Smelting 4.1 Mercury removal 4.2 Iron removal 5 See also

Gravity concentration or froth flotation processes may be used to concentrate the gold. In some cases, a gravity concentrate can be direct smelted to form gold bars. In other cases the concentrates are subjected to intensive cyanidation leaching, followed by recovery from gold in solution by carbon in pulp or electrowinning. If the gold is present as a minor constituent in a base metal concentrate, it can be recovered in the anode slime during the electrorefining process.

If the gold can not be concentrated for smelting, then it is leached by an aqueous solution:

1. The cyanide process is the industry standard.
2. Thiosulfate leaching has been proven to be effective on ores with high soluble copper values or ores which experience pregrobbing by carbonaceous components.

A "refractory" gold ore is an ore that is naturally resistant to recovery by standard cyanidation and carbon adsorption processes. These refractory ores require pre-treatment in order for cyanidation to be effective in recovery of the gold. A refractory ore generally contains sulfide minerals, organic carbon, or both. Sulfide minerals often trap or occlude gold particles, making it difficult for the leach solution to complex with the gold. Organic carbon present in gold ore may adsorb dissolved gold-cyanide complexes in much the same way as activated carbon. This so-called "preg-robbing" carbon is washed away because it is significantly finer than the carbon recovery screens typically used to recover activated carbon.

Pre-treatment options for refractory ores include:

1. Roasting
2. Bio-oxidation
3. Pressure oxidation
4. Ultrafine grinding

The refractory ore treatment processes may be preceded by concentration (usually sulfide flotation). Roasting is used to oxidize both the sulfur and organic carbon at high temperatures using air and/or oxygen. Bio-oxidation involves the use of bacteria that promote oxidation reactions in an aqueous environment. Pressure oxidation an aqueous process for sulfur removal carried out in a continuous autoclave, operating at high pressures and somewhat elevated temperatures. Ultrafine grinding may be used when liberation of gold particles from the surrounding mineral matrix is the primary refractory characteristic of the ore.

Mercury is a health hazard, especially when in gas form. To remove this hazard, before smelting, gold precipitates from electrowinning or Merrill-Crowe processes are usually heated in a retort to recover any mercury present, that would otherwise cause health and environmental problems due to its release (volatilization) during smelting. The mercury present is not usually from the mercury amalgamation process that is no longer used by formal gold mining companies, but from mercury in the ore that has followed gold through the leaching and precipitation processes.

In the event that there are high levels of copper or silver present, leaching of the precipitate using nitric or sulfuric acids may be required.

Nitric acid or forced air oven oxidation can also be used to dissolve iron from the electrowinning cathodes before smelting. Gravity concentrates can often contain high grinding steel contents, and so their removal using shaking tables or magnets is used before smelting. During smelting iron can be oxidized using nitre. Excessive use of nitre will corrode the smelting pot, increasing both maintenance costs and the risk of catastrophic leaks (known as run-aways, or holes in the pot through which the molten charge is lost).

• Gold mining
• Ore genesis