Gold extraction

Gold revcovery from its ores may require a combination of comminution, mineral processing, hydrometallurgical and pyrometallurgical processes to be performed on the ore. Once the ore is mined, it can be treated as is using a dump leaching or heap leaching process.

In the case that the ore is not economically suited to dump leaching or heap leaching processes, either because it is of high grade or refractory to cyanide leaching at coarse particle sizes, commuinution is used to crush and grind the ore into finer particles. 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 consituent in a base metal concentrate, it can be recovered in the anode slime during the electrorefining process.

Cyanidation

If the gold is not concentrated, then it is leached by cyanide solution.

Refractory Gold Processes

If the gold is trapped inside minerals which are not pourous, and so the gold is not liberated to the leach solution, then a refractory gold treatment step may be required before cyanidation. These include:

 i) Concentrate or whole ore roasting
 ii) Concentrate or whole ore bio-oxidation
 iii) Concentrate or whole or pressure oxidation in autoclaves
 iv) Ultrafine grinding

Gold Smelting

Prior to smelting, gold precipitates from electrowinning or Merrill-Crowe processes are usually heated in a retort to recovery any mercury present, that would otherwise cause health and environmental problems due to 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 precipitated using nitric or sulfuric acids may be required. Iron removal 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 maintenance costs and increasing risk of run-aways, or holes in the pot through which the molten charge is lost.