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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.
Additional recommended knowledge
Gravity concentration or froth flotation processes may be used to concentrate the gold. In some cases, particularly when the gold is present in the ore as discrete coarse particles, a gravity concentrate can be directly smelted to form gold bars. In other cases, particularly when the gold is present in the ore as fine particles or is not sufficiently liberated from the host rock, the concentrates are subjected to cyanidation leaching, followed by recovery from the leach solution. Recovery from solution typically involves by adsorption on activated carbon followed by solution concentration or stripping and or electrowinning.
Froth flotation is usually applied when the gold present in an ore is closely associated with sulfide minerals such as pyrite or arsenopyrite, and when such sulfides are present in large quantities in the ore. In this case, concentration of the sulfides results in concentration of gold values. Generally, recovery of the gold from the sulfide concentrates requires further processing, usually by roasting or wet pressure oxidation. These pyrometallurgical or hydrometallurgical treatments are themselves usually followed by cyanidation and carbon adsorption techniques for final recovery of the gold.
Sometimes gold is present as a minor constituent in a base metal (e.g. copper) concentrate, and is recovered as a by-product during production of the base metal. For example, 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:
Refractory gold processes
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:
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.
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 from electronics and other scrap
Gold can be extracted from electronics scrap, scrap jewelry, and dental scrap. The gold in these items is still valuable, but must first be extracted from the scrap in order to be returned into circulation in its purified form.
Recovering gold from electronics scrap begins with the enectration of large amounts of outdated or defective electronics and computer equipment. Additional sources include manufacturers scrap and surplus discrete electronics components. The greatest concentrations of gold in scrap electronics are in the CPUs, card edge connectors, and component interconnects. The gold on these items is typically plated in very thin layers.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gold_extraction". A list of authors is available in Wikipedia.|