Gold extraction or recovery from its ores may require a combination of comminution, mineral processing, hydrometallurgical, and pyrometallurgical processes to be performed on the gold ore.
Mining from alluvium ores can be acheived by the techniques of placer mining resulting in a concentrate. Hard rock ores are typically mined via open-pit or sub surface mining techniques.
Once the gold ore is mined it can be treated as is using a dump leaching or heap leaching process. In the case that the gold ore is not economically suited to dump leaching or heap leaching processes, either because it is of high grade or resistent to cyanide leaching at coarse particle sizes, comminution is used to crush and grind the gold ore into finer particles.
- 1 Orebody formation
- 2 Concentration
- 3 Leaching
- 4 Refractory Gold Processes
- 5 Gold Smelting
- 5.1 Mercury removal
- 5.2 Iron removal
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:
The cyanide process is the industry standard.
Thiosulfate leaching has been proven to be effective on ores with high soluble copper values or ores which experience pregrobbing by carbonaceous components.
Refractory Gold Processes:
Leaching might not work, if the gold is trapped within minerals which do not allow the leach solution access to the gold, and so the gold is not leached by the leach solution. If leaching doesn't work then this difficult to process (refractory) ore requires a refractory gold treatment step before cyanidation.
- These include:
- Concentrate or whole ore roasting
- Concentrate or whole ore bio-oxidation
- Concentrate or whole ore pressure oxidation in autoclaves
- Ultrafine grinding
Mercury is a health hazard, especially when in gas form. To remove this hazard, prior to 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 prior to smelting. Gravity concentrates can often contain high grinding steel contents, and so their removal using shaking tables or magnets is used prior to 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).
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