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Historians believe that gold was the first metal known
to man. It was love at first sight; ever since, man has desired gold for
its sheer beauty and for the ease with which he can make it into beautiful
objects. Because they have survived in situations where objects made of
other materials have perished; gold artifacts are a source of much
information about ancient civilizations. In such civilizations gold was
not used only in jewelry. It gradually became a major symbol of wealth and
power. It even became a standard of value, hence the expression "as good
as gold." Over the centuries, gold has been used as money and in modern
times, it has come to be used in industry such as dentistry, computers,
electronic circuits, and even in the aerospace industry.
Egyptian wall relief's from 2300 BC show gold in
various stages of refining and mechanical working. During these ancient
times, gold was mined from alluvial placers-that is, particles of
elemental gold found in river sands. The gold was concentrated by washing
away the lighter river sands with water, leaving behind the dense gold
particles, which could then be further concentrated by melting. By 2000 BC
the process of purifying gold-silver alloys with salt to remove the silver
was developed. The mining of alluvial deposits and, later, lode or vein
deposits required crushing prior to gold extraction, and this consumed
immense amount of manpower. By AD 100, up to 40,000 slaves were employed
in gold mining in Spain. The advent of Christianity somewhat tempered the
demand for gold until about the 10th century. The technique of
amalgamation, alloying with mercury to improve the recovery of gold, was
discovered at about this time.
The colonization of South and Central America that
began during the 16th century resulted in the mining and refining of gold
in the New World before its transferal to Europe; however, the American
mines were a greater source of silver than gold. During the early to
mid-18th century, large gold deposits were discovered in Brazil and on the
eastern slopes of the Ural Mountains in Russia. Major alluvial deposits
were found in Siberia in 1840, and gold was discovered in California in
1848. The largest gold find in history is in the Witwatersrand of South
Africa. Discovered in 1886, it produced 25 % of the world's gold by 1899
and 40 % by 1985. The discovery of the Witwatersrand deposit coincided
with the discovery of the cyanidation process, which made it possible to
recover gold values that had escaped both gravity concentration and
amalgamation. With E.B. Miller's process of refining impure gold with
chlorine gas (patented in Britain in 1867) and Emil Wohlwill's
Electro-refining process (introduced in Hamburg, Ger., in 1878), it became
possible routinely to achieve higher purity than had been allowed by fire
refining.
Silver Facts:
Silver reflects!
The film coating on mirror backings is silver. Mirrors are used
in telescopes, microscopes, spacecraft and solar panels, as well
as bathrooms! Don't forget the silver transparent coating on
double-pane thermal windows.
Silver conducts
heat! Those silver ceramic lines fired into your car's
rear window keep the window clear of frost and ice.
Silver conducts electricity!
Silver is the metal of choice for switch contacts because it does not
corrode. Every time you start your microwave, dishwasher, television set,
car engine, etc., silver contacts complete the electrical circuit. The
same thing happens when you tap the keys of your computer keyboard, adjust
your car's power seats, or release the power trunk lock. Silver is there.
Silver kills bacteria!
Silver chemically affects the cell membranes of bacteria, causing them to
break down.
Bacteria do not develop resistance to silver, as they do
to many antibiotics. Silver solutions are used in burn treatments. Silver
gauze packs the wounds of patients during transport to medical facilities.
Silver nitrate drops are used to clean the eyes of newborns. Hikers use
portable silver-based water purification systems for drinking water.
Silver rings! Silver has
a pure acoustic resonance and is preferred by musicians for making high
quality silver bells and musical instruments.
Silver captures images!
Silver salts are the basic image capture and forming materials in
photography. Every picture of your sweet little darling contains silver,
as well as medical/dental x-rays and your favorite movie.
Silver is pretty! Don't
forget the silver in your jewelry or traditional tableware. It's a
favorite medium because it is soft and malleable, and can be shaped into
any form. Silver has been used in cherished heirlooms and gifts for
centuries.
Gold Facts:
Gold rocks! Compact discs
pressed on 24-karat gold provide the ultimate in sonic quality. Artists
available on ULTRADISC II, by Mobile Fidelity Sound, include Elton John,
Steve Winwood, B.B. King and Aretha Franklin.
Gold travels! The
Pathfinder "robotic geologist" relies on sophisticated electronics to
direct its Mars landing and movement. Its intricate gold circuitry enables
new computer technology to transmit the Pathfinder's information back to
Earth.
Gold communicates! Every
time you type an e-mail on your computer, gold circuits relay the keyboard
data to the microprocessor. Gold is essential in computer circuitry
because of its electrical conductivity and because it does not degrade
over time.
Gold protects!
Gold-plated reflectors used on Air Force One, the U.S. President's
airplane, confuse an incoming missile's heat-seeking signal so the
missile's guidance system cannot focus on its target. Gold provides the
highest reflectivity of heat radiation
Exploration
The gold mining process begins with geologists discovering a deposit.
Exploration Drilling
After finding indications of gold, drill rigs are used to obtain
samples from below the surface. These samples are analyzed for their gold
content. If there is enough gold in the deposit to be produced
economically, a mine is permitted and developed.
Open Pit Mining
The ore is mined in four steps: drilling, blasting, loading and
hauling.
Blasthole Drilling
In the case of a surface mine, a pattern of holes is drilled in the pit
and filled with explosives.
Blasting
The explosives are detonated in order to break up the ground so it can
be loaded by large shovels or front-end loaders into haul trucks.
Underground Mining
Some orebodies are more economically mined using underground methods.
In this case, a tunnel called an adit or a shaft, is dug into the earth.
Sort tunnels leading from the adit or shaft, called stopes, are dug to
access the ore. The surface containing the ore, called a face, is drilled
and loaded with explosives. Following blasting, the broken ore is loaded
into trucks and taken to the surface. Once mining is completed in a
particular stope, it is backfilled with a cement compound.
Ore and Waste Haulage
The haul trucks transport the ore to various areas for processing. The
grade and type of ore determine the processing method used. Higher-grade
ores are taken to a mill. Lower grade ores are taken to leach pads. Some
ores may be stockpiled for later processing.
Ore Processing
Heap leaching
The ore is crushed or placed directly on lined leach pads where a weak
cyanide solution is applied to the surface of the heap. The solution
percolates down through the ore, where it leaches the gold and flows to a
central collection location. All of the solution is recovered in this
closed system, preventing any infiltration into the ground below.
Milling
The ore is fed into a series of grinding mills where steel balls grind
the ore to a fine slurry or powder.
Oxidization
Some types of ore require further processing before gold is recovered.
In this case, the slurry is pressure-oxidized in an autoclave (shown on
right) before going to the leaching tanks or a dry powder is fed through a
roaster in which it is oxidized using heat before being sent to the
leaching tanks as a slurry.
Leaching
The slurry is thickened and run through a series of leaching tanks. The
gold in the slurry adheres to carbon in the tanks.
Stripping
The carbon is then moved into a stripping vessel where the gold is
removed from the carbon by pumping a hot caustic solution through the
carbon. The carbon is later recycled.
Electro-winning
The gold-bearing solution is pumped through electro-winning cells or
through a zinc precipitation circuit where the gold is recovered from the
solution.
Smelting
The gold is then melted in a furnace at about 2,100°F and poured into
molds, creating doré bars.
Gold Bullion
Doré bars are unrefined gold bullion bars containing anywhere from 60%
to 95% gold.
Refining
The bars are sent to a refinery for further processing into pure gold.
Reclamation
To restore the landscape for future uses such as ranching, recreation
or wildlife habitat protection, reclamation, including revegetation, is
implemented concurrently during mining.
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