Placer (sediment) mining
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 denser 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.
Metal detecting
With 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. This technique is very easy to operate, highly mobile, and very popular among gold diggers.
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. These operations typically include diesel powered earth moving equipment including excavators, dozers, wheel loaders and rock trucks.
Metal detecting
With 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. This technique is very easy to operate, highly mobile, and very popular among gold diggers.
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. These operations typically include diesel powered earth moving equipment including excavators, dozers, wheel loaders and rock trucks.
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. A suction dredge consists of a sluice box supported by pontoons, and attached to a suction hose which is controlled by the miner working beneath the water. Some believe that modern suction dredges have a significant environmental impact to fisheries by disturbing spawning gravels and fish migration. However, most small scale dredging operations affect the local fish only by stirring up additional food from the bottom of the river. 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 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.
Other larger scale dredging operations take place on exposed river gravel bars at seasonal low water. These operations typically use a land based excavator to feed a gravel screening plant and sluicebox floating in a temporary pond excavated in the gravel bar and filled from the natural water table. Pay gravel is excavated from the front face of the pond and processed through the floating plant, with the gold trapped in the onboard sluicebox and tailings stacked behind the plant, steadily filling in the back of the pond as the operation moves forward. This kind of gold mining is characterized by its low cost, as each rock is moved only once, as well as by its low environmental impact, as no stripping of vegetation or overburden is necessary, and all process water is fully recycled. These operations are typical on New Zealand's south island and in the Klondike region of Canada.
Hard rock mining
Other larger scale dredging operations take place on exposed river gravel bars at seasonal low water. These operations typically use a land based excavator to feed a gravel screening plant and sluicebox floating in a temporary pond excavated in the gravel bar and filled from the natural water table. Pay gravel is excavated from the front face of the pond and processed through the floating plant, with the gold trapped in the onboard sluicebox and tailings stacked behind the plant, steadily filling in the back of the pond as the operation moves forward. This kind of gold mining is characterized by its low cost, as each rock is moved only once, as well as by its low environmental impact, as no stripping of vegetation or overburden is necessary, and all process water is fully recycled. These operations are typical on New Zealand's south island and in the Klondike region of Canada.
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
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.
Gold ore processing
In placer mines, the gold is recovered by gravity separation. For hardrock mining, other methods are usually used.
Cyanide process
Main article: Gold cyanidation
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, 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.
Cradle
A cradle was rocked back and forth while water was poured over it. The sand and gold were washed through the screen of the cradle, leaving the gravel behind.
History of gold mining
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.
Gold ore processing
In placer mines, the gold is recovered by gravity separation. For hardrock mining, other methods are usually used.
Cyanide process
Main article: Gold cyanidation
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, 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.
Cradle
A cradle was rocked back and forth while water was poured over it. The sand and gold were washed through the screen of the cradle, leaving the gravel behind.
History of gold mining
Romans used hydraulic mining methods on a large scale to extract gold from extensive alluvial deposits, such as those at Las Medulas. Mining was under the control of the state but the mines may have been leased to civilian contractors some time later. The gold helped finance the growth of the empire, and was an important motive in the Roman invasion of Britain by Claudius in the first century AD, although there is only one known Roman gold mine at Dolaucothi in west Wales. Gold was a prime motivation for the campaign in Dacia when the Romans invaded Transylvania in what is now modern Romania in the second century AD. The legions were led by the emperor Trajan, and their exploits are shown on the grand column in City Hall.
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