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
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[edit] Panning
Gold panning is a mostly 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 gravel and other material. As gold is much more dense than rock, it quickly settles to the bottom of the pan. The silt is usually removed from stream beds, often at a bend in the stream, or resting on the bedrock bed of the stream, where the weight of gold causes it to separate 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 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 viability.
[edit] Metal detecting
A new small scale technique using metal detectors for finding gold. Using a piece of electronic equipment, called a metal detector, a person may walk around area systematically scanning below the surface. If the meter gives a positive reading a quantity of gold may be present up to a meter below the surface being scanned at the time. This technique is very easy to operate, highly mobile, and very popular among gold diggers.
[edit] Sluicing
Using a sluice box to extract gold from placer deposits has been a common practice in prospecting and small-scale mining throughout history to the modern day. A sluice box is essentially a man-made channel with riffles set in the bottom. The riffles are designed to create dead zones in the current to allow gold to drop out of suspension. The box is placed in the stream to catch water-flow and gold bearing material is placed at the top of the box. The material is carried by water through the box where gold and other heavy material settles out behind the riffles. Lighter material flows out of the box as tailings.
Larger commercial placer mining operations employ screening plants or trommels to remove the larger alluvial materials such as boulders and gravel before concentrating in a sluice box or jig plant.
[edit] Dredging
Although mostly historical, 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. These machines are much more efficient at extracting smaller gold than the old "bucket line" ever was. This means there is a better chance of you finding gold than ever. There are some large suction dredges (100 hp+ 10 inch) used in commercial production throughout the world. Smaller ones with 2 to 4-inch (100 mm) suction tubes are used to sample the areas behind boulders and along the potential pay streaks, until color (gold) first appears.
[edit] Hard rock mining
Hard rock gold mining is done when the gold is encased in rock, rather than as particles in loose sediment. Sometimes open-pit mining is used, such as the Ft. Knox Mine in central Alaska. Barrick Gold Corporation has one of the largest open-pit gold mines in North America, located on its Goldstrike property in northeastern Nevada. Other gold mines use underground mining, where the ore is extracted through tunnels or shafts. Hard rock mining produces most of the world's gold.
[edit] Byproduct gold mining
Gold is also produced by mining in which it is not the principal product. Large copper mines, such as the Bingham Canyon mine in Utah, often recover considerable amounts of gold and other metals along with the copper. Some sand and gravel pits, such as those around Denver, Colorado, may recover small amounts of gold in their washing operations.
[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 sulfuric 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 1970s have seen activated carbon used in extracting gold from the leach solution. The gold is absorbed into the porous matrix of the carbon. Activated carbon has so much internal surface area[1], that fifteen grams (half an ounce) has the equivalent surface area of the Melbourne Cricket Ground (18,100 square meters). 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 100,000 m³ of wastewater contaminated with heavy metal sludge and up to 120 tons of cyanide into the rivers of Tisza.
[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 inheriting their money. One who does this is called a gold digger.
[edit] References
[edit] Footnotes
- ^ "Porous Carbon: Room for Exaggeration" http://ergobalance.blogspot.com/2006/11/porous-carbon-room-for-exaggeration.html
[edit] General references
- Ali, Saleem H. ["Gold Mining and the Golden Rule: A Challenge for Developed and Developing Countries."] Journal of Cleaner Production, Vol 14: 3, 2006. [1]
- Dennis Garrett ("Blue Ribbon Mine, Alaska Trommel/Sluice")
- Eathan Mertz ("Sluice Box Use & Set-Up")
[edit] External links
- Gold Prospecting Tools
- Gold Prospecting and Lost Treasure in British Columbia
- Gold Prospecting and Metallurgy
- Gold mining in the Pacific Islands (including a web-video)
[edit] See also
