- Gold extraction
Goldextraction or recovery from its ores may require a combination of comminution, mineral processing, hydrometallurgical, and pyrometallurgical processes to be performed on the ore. [cite web
title=Gold Extraction - Gold Mining - Washing, Amalgamation, Leaching, Smelting
Once the ore is mined it can be treated as a whole ore using a
dump leachingor heap leachingprocesses. 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 cyanideleaching 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.
Types of ore
Gold occurs principally as a
native metal, usually alloyed to a greater or lesser extent with silver(as electrum), or sometimes with mercury (as an amalgam). Native gold can occur as sizeable nuggets, as fine grains or flakes in alluvial deposits, or as grains or microscopic particles embedded in other rocks.
Ores in which gold occurs in chemical composition with other elements are comparatively rare. They include
calaverite, sylvanite, nagyagite, petziteand krennerite.
Gravity concentration has been historically the most important way of extracting the native metal using pans or washing tables. However,
froth flotationprocesses may also 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 treated with cyanidesalts, a process known as cyanidation leaching, followed by recovery from the leach solution. Recovery from solution typically involves 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 pyriteor 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 anodeslime during the electrorefining process.
If the gold can not be concentrated for smelting, then it is leached by an
cyanide processis the industry standard.
Thiosulfateleaching has been proven to be effective on ores with high soluble coppervalues or ores which experience pregrobbing by carbonaceous components.
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:
# Pressure oxidation
# 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
electrowinningor Merrill-Crowe processes are usually heated in a retortto 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 amalgamationprocess 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
copperor silverpresent, 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 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.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Gold mining — Gold mine redirects here. See Goldmine for other uses of the term. Gold mining consists of the processes and techniques employed in the removal of gold from the ground. There are several techniques by which gold may be extracted from the Earth.… … Wikipedia
Gold cyanidation — (also known as the cyanide process or the MacArthur Forrest process) is a metallurgical technique for extracting gold from low grade ore by converting the gold to a water soluble coordination complex. It is the most commonly used process for gold … Wikipedia
Gold — (Aurum), nächst Eisen und Aluminium das am weitesten verbreitete, meist aber in geringer Menge vorkommende Metall. 1) Vorkommen (mineralogisch und geologisch). G. findet sich meist gediegen und dann fast immer legiert mit Silber, auch mit Eisen,… … Meyers Großes Konversations-Lexikon
Gold (disambiguation) — Gold is a highly sought after precious metal that for many centuries has been used as money.Gold may also refer to:Business* Barrick Gold, the largest gold mining company in the world * Gold s Gym, international chain of fitness centers… … Wikipedia
Gold mining in China — has recently made that country the world s largest gold producer. For the year 2007, gold output rose 12% from 2006 to 276 tonnes (or 9.7 million ounces) to become the world s largest for the first time overtaking South Africa, which produced 272 … Wikipedia
Gold — This article is about the metal. For the color, see Gold (color). For other uses, see Gold (disambiguation). platinum ← gold → mercury … Wikipedia
gold processing — Introduction preparation of the ore for use in various products. For thousands of years the word gold has connoted something of beauty or value. These images are derived from two properties of gold, its colour and its chemical stability.… … Universalium
Gold prospecting — is the act of going equipped to find gold in rocks or in stream beds with a view to exploiting that discovery. This usually means individual prospectors, as opposed to those trained and educated individuals working for mining companies. Up until… … Wikipedia
Gold sink — is an economic process by which a video game s ingame currency ( gold ), or any item that can be valued against it, is removed. Most commonly the genres are role playing game or massively multiplayer online game. The term is comparable to… … Wikipedia
extraction — UK US /ɪkˈstrækʃən/ noun [U] ► PRODUCTION, NATURAL RESOURCES the process of removing a substance from the ground or from another substance: »The local economy is overwhelmingly dependent on oil and gas extraction. »Hard rock mining refers to the… … Financial and business terms