Gold mining
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- "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 earth and rock:
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[edit] Placer (Sediment) mining
[edit] Panning
Gold panning is a mostly historical, manual technique of sorting gold. Wide, shallow pans are filled with sand and gravel that may contain gold. Water is added and the pans are shaken, sorting the gold from the rock and other material. Gold being much denser than rock, quickly settles to the bottom of the pan. The gravel is usually removed from streambeds, often at a bend in the stream, or resting on the bedrock bed of the stream, where the weight of gold causes it to settle out of the water flow. This type of gold found in streams or dry streams are called placer deposits.
Gold panning is the easiest technique for searching for gold, but is not a commercially viable for extracting gold from large deposits, except where labor costs are very low and or gold traces are very substantial. It is often marketed as a tourist attraction on former goldfields. Before production methods can be used a new source must be identified and panning is a good way to identify placer gold deposits so that they may be evaluated for commercial viablility.
[edit] Dredging
A mostly historical, 'industrial' way of gold panning, this involves dredging up large amounts of gold-bearing sediments for later processing in various ways. Dredges were for example used strongly during various New Zealand gold rushes. Unlike the panning method, it required capital investment, and thus unviable for most individual prospectors, though under the correct circumstances, it offered rich yields. Very large dredges (often known as "bucket line dredges") were used in many places during the early gold rush periods in the U.S. and elsewhere. Their use was discontinued many years ago but a few of them still exist and are on display as tourist attractions.[citation needed]
Currently, some dredging is done by small scale miners using suction dredges. These are small machines floating on the water and are usually operated by one or two people. Unlike the old bucket line dredges, modern suction dredges have little to no detrimental impact on the area being mined.[citation needed]
[edit] Hydraulic mining
Hydraulic mining is a type of placer mining used in areas where large amounts of loose gravel and sand or soil are poorly packed and may be washed away with a heavy stream of water. Fire hoses (Water cannons) are sometimes used to strip away entire hills of loose gravel, which are then run through a sluice (a wooden trough with ripples). Gold, being heavier, does not move as easily as other material in the sluice. This technique can damage the environment, causing mud in streams below the mining site and erosion damage at the site itself.
[edit] Hardrock gold mining
Hard rock mining is removing rock from the ground, in which miners tunnel and blast into rock, seeking deposits of gold. Veins of gold ore are often found several inches or feet wide in certain rock formations in a volcanic deposit and in certain bed layers in a sedimentary deposit, hence the minerals may be removed, collected, and treated to process the gold and other valuable metals (such as silver) from them. This technique uses the most energy and is typically viable only where exceptional gold grades warrant the associated expense. Deposits where this is the case may have significantly higher grades than those needed to offset the costs and so underground mining can be quite profitable. Hard rock mining produces much of the world's gold.
Typical hard rock mining involves a cycle wherein holes are drilled in the rock to be broken, explosives are placed in the holes then detonated. Rock broken by the explosion is then removed from the area, typically with mechanized equipment. Where ground conditions warrant, rock bolts and other means of ground support may then be installed and the cycle can be begun again with drilling. By this means an advance of some few meters may be achieved. Hard rock mining is the most dangerous type of mining.
[edit] Gold Ore Processing
In placer mines, the gold is recovered by gravity separation. For hardrock mining, other methods are usually used.
[edit] Cyanide process
Cyanide extraction of gold may be used in areas where fine-gold bearing rocks are found. Sodium cyanide solution is mixed with finely-ground rock that is proven to contain gold and/or silver, and is then separated from the ground rock as gold cyanide and/or silver cyanide solution. Zinc is added to the solution, precipitating out residual zinc, as well as the desirable silver and gold metals. The zinc is removed with sulphuric acid, leaving a silver and/or gold sludge that is generally smelted into an ingot that is shipped to a metals refinery for final processing into 99.9999% pure metals.
Advancements in the 1970's have seen activated carbon used in extracting gold from the leach solution. The gold is absorbed into the porous matrix of the carbon. Carbon has so much internal surface area1, that fifteen grams (half an ounce) has the equivalent surface area of the Melbourne Cricket Ground (18,100 square metres). The gold can be removed from the carbon by using a strong solution of caustic soda and cyanide. This is known as elution. Gold is then plated out onto steel wool through electrowinning. Gold specific resins can also be used in place of activated carbon, or where selective separation of gold from copper or other dissolved metals is required.
The cyanide technique is very simple and straightforward to apply and a popular method for low-grade gold and silver ore processing. Like most industrial chemical processes, there are potential environmental hazards presented with this extraction method in addition to the high toxicity presented by the cyanide itself. This was seen in the environmental disaster in Central-Eastern Europe in year 2000, when during the night of 30 January, a dam at a goldmine reprocessing facility in Romania released approximately 100,000m³ of wastewater contaminated with heavy metal sludge and up to 120 tonnes of cyanide into the rivers of Tisza and Danube.
[edit] Idiomatic use
In colloquial English, the term gold mine is used to describe a very profitable economic venture. For example, a business or investment opportunity that provides (or could potentially provide) a considerable amount of money for the owners or shareholders as net earnings could be considered a gold mine. (A profitable physical gold mine could also be called a gold mine due to its profitable nature.)
An example sentence using the term gold mine in the idiomatic sense could be: "This oil deposit is a regular gold mine. We make more and more money every year!"
As a verb, to gold mine can mean to date or marry someone rich (normally much older) in the hopes of inhereting their money. One who does this is called a gold digger.
[edit] References
1("Porous Carbon: Room for Exaggeration" http://ergobalance.blogspot.com/2006/11/porous-carbon-room-for-exaggeration.html)
[edit] See also
