- "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.
Placer 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. The pan is submerged in water and
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 panning material is usually removed from stream beds,
often at the inside turn in the stream, or resting on the
bedrock bed of the stream, where the density of gold
allows it to concentrate. This type of gold found in streams or dry
streams are called
placer
deposits.
Gold in goldpan, Alaska
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. 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 an area and systematically scan below the surface. The
sensor can give a positive reading for a quantity of gold to a
depth of as much as a meter below the surface. As the device is
easy to operate and highly mobile, this method of prospecting is
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 channel water flow. Gold-bearing material is placed at
the top of the box. The material is carried by the current through
the box where gold and other dense material settles out behind the
riffles. Less dense 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 the remainder in a sluice box or jig plant. These
operations typically include diesel-powered, earth-moving
equipment, including
excavators,
bulldozers,
wheel loaders and
rock trucks.
Alaskan Trommel at the Potato Patch,
Blue Ribbon Mine
Dredging
Although this method has largely been replaced by modern methods,
some dredging is done by small-scale miners using suction dredges.
These are small machines that float on the water and are usually
operated by one or two people. A suction dredge consists of a
sluice box supported by pontoons, attached to a suction hose which
is controlled by the miner working beneath the water.
State dredging permits in many of the United States gold-dredging
areas specify a seasonal time period and area closures to avoid
conflicts between dredgers and the spawning time of fish
populations.
Some states, such as Montana
, require an
extensive permitting procedure, including permits from the U.S. Corps of Engineers, the Montana
Department of Environmental Quality, and the local county
water-quality boards.
Some large suction dredges (100 hp+ 10 inch) are used in commercial
production throughout the world. Small suction dredges are much
more efficient at extracting smaller gold than the old "bucket
line" was. This has improved the chances of finding gold. Smaller
dredges with 2 to suction tubes are used to sample areas behind
boulders and along potential pay streaks, until "color" (gold)
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. The pond is 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 type of gold mining
is characterized by its low cost, as each rock is moved only once.
It also has low environmental impact, as no stripping of vegetation
or overburden is necessary, and all process water is fully
recycled.
Such 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 found as particles in
loose sediment. Hard rock mining produces most of the world's gold.
Sometimes
open-pit mining is used,
such as at 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.
South Africa
has the world's deepest hard-rock mine, which mines gold from as
deep as 3900 meters under the ground.
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.
The
largest-producing gold mine in the world, the Grasberg mine
in Papua, Indonesia
, is primarily a copper mine.
Gold ore processing
In placer mines, the gold is recovered by gravity separation. For
hardrock mining, other methods are usually used.
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 then shipped to a
metals refinery for final processing into 99.9999% pure metals.
Medgold was one of the first companies to use this method.
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 gravel was washed through the screen of the cradle,
leaving the gold 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.
Large gold mining companies
Barrick Gold,
Goldcorp and
Newmont Mining Corporation are
the world's three largest gold mining companies.
Small mining operations
While most of the gold is produced by major corporations, tens of
thousands of people work independently in smaller, artisan
operations, in some cases illegal.
Among them are the galamseys, the name for independent mine workers in
Ghana
, who are estimated to number 20,000 to
50,000. In neighboring
francophone countries, such workers are called
orpailleurs.
The high risk of such ventures was seen in
the collapse of an illegal mine at Dompoase, Ashanti Region
, Ghana on 12 November 2009, when 18 workers were
killed, including 13 women. Many women work at such mines as porters
. It was the worst mining disaster in
Ghanaian history.
Gold mining in popular culture
See also
United States:
British Commonwealth:
References
Further reading
External links
(1860s)
Top topics
Wikis
Quiz
Facts
Map
Search
