What is Gold
n.
1.a. (Symbol Au)
A soft, yellow, corrosion-resistant element, the most malleable and ductile
metal, occurring in veins and alluvial deposits and recovered by mining or by
panning or sluicing. A good thermal and electrical conductor, gold is generally
alloyed to increase its strength, and it is used as an international monetary
standard, in jewelry, for decoration, and as a plated coating on a wide variety
of electrical and mechanical components. Atomic number 79; atomic weight
196.967; melting point 1,063.0°C; boiling point 2,966.0°C; specific gravity
19.32; valence 1, 3.
b. Coinage made
of this element.
c. A gold
standard.
2. Money;
riches.
3. A light
olive-brown to dark yellow, or a moderate, strong to vivid yellow.
4. Something
regarded as having great value or goodness: a heart of gold.
5.a. A medal made
of gold awarded to one placing first in a competition, as in the Olympics: won
9 golds in 13 events.
b. A gold
record.
Background
Gold, recognizable by its yellowish cast, is one of the
oldest metals used by humans. As far back as the Neolithic period, humans have
collected gold from stream beds, and the actual mining of gold can be traced as
far back as 3500 B.C., when early Egyptians (the Sumerian culture of Mesopotamia)
used mined gold to craft elaborate jewelry, religious artifacts, and utensils
such as goblets.
Gold's aesthetic properties combined with its physical
properties have long made it a valuable metal. Throughout history, gold has
often been the cause of both conflict and adventure: the destruction of both
the Aztec and Inca civilizations, for instance, and the early American gold
rushes to Georgia,
California, and Alaska.
The largest deposit of gold can be found in South
Africa in the Precambrian Witwatersrand
Conglomerate. This deposit of gold ore is hundreds of miles across and more
than two miles deep. It is estimated that two-thirds of the gold mined comes
from South Africa.
Other major producers of gold include Australia,
the former Soviet Union, and the United
States (Arizona,
Colorado, California,
Montana, Nevada,
South Dakota, and Washington).
About 65 percent of processed gold is used in the arts
industry, mainly to make jewelry. Besides jewelry, gold is also used in the
electrical, electronic, and ceramics industries. These industrial applications
have grown in recent years and now occupy an estimated 25 percent of the gold
market. The remaining percentage of mined gold is used to make a type of ruby
colored glass called purple of Cassius, which is applied to office building
windows to reduce the heat in the summer, and to mirrors used in space and in
electroscopy so that they reflect the infrared spectrum.
Physical Characteristics
Gold, whose chemical symbol is Au, is malleable, ductile,
and sectile, and its high thermal and electrical conductivity as well as its
resistance to oxidation make its uses innumerable. Malleability is the ability
of gold and other metals to be pressed or hammered into thin sheets, 10 times
as thin as a sheet of paper. These sheets are sometimes evaporated onto glass
for infrared reflectivity, molded as fillings for teeth, or used as a coating
or plating for parts. Gold's ability to be drawn into thin wire (ductility)
enables it to be deposited onto circuits such as transistors and to be used as
an industrial solder and brazing alloy. For example, gold wire is often used
for integrated circuit electrical connections, for orthodontic and prosthetic
appliances, and in jet engine fabrication.
Gold's one drawback for use in industry is that it is a
relatively soft metal (sectile). To combat this weakness, gold is usually
alloyed with another member of the metal family such as silver, copper,
platinum, or nickel. Gold alloys are measured by karats (carats). A karat is a
unit equal to 1/24 part of pure gold in an alloy. Thus, 24 karat (24K) gold is
pure gold, while 18 karat gold is 18 parts pure gold to 6 parts other metal.
Extraction and Refining
Gold is usually found in a pure state; however, it can also
be extracted from silver, copper, lead and zinc. Seawater can also contain
gold, but in insufficient quantities to be profitably extracted—up to
one-fortieth (1/40) of a grain of gold per ton of water. Gold is generally
found in two types of deposits: lode (vein) or placer deposits; the mining
technique used to extract the gold depends upon the type of deposit. Once
extracted, the gold is refined with one of four main processes: floatation,
amalgamation, cyanidation, or carbon-in-pulp. Each process relies on the
initial grinding of the gold ore, and more than one process may be used on the
same batch of gold ore.
Mining
• In lode or
vein deposits, the gold is mixed with another mineral, often quartz, in a vein
that has filled a split in the surrounding rocks. Gold is obtained from lode
deposits by drilling, blasting, or shoveling the surrounding rock.
Lode deposits often run deep underground. To mine
underground, miners dig shafts into the ground along the vein. Using picks and
small explosives, they then remove the gold ore from the surrounding rock. The
gold ore is then gathered up and taken to a mill for refinement.
• Placer
deposits contain large pieces of gold ore (nuggets) and grains of gold that
have been washed downstream from a lode deposit and that are usually mixed with
sand or gravel. The three main methods used to mine placer deposits are
hydraulic mining, dredging, and power shoveling. All methods of placer deposit
mining use gravity as the basic sorting force.
In the first method, a machine called a "hydraulic giant"
uses a high pressure stream of water to knock the gold ore off of banks
containing the ore. The gold ore is then washed down into sluices or troughs
that have grooves to catch the gold.
Dredging and power shoveling involve the same techniques but
work with different size buckets or shovels. In dredging, buckets on a conveyor
line scoop sand, gravel, and gold ore from the bottom of streams. In power
shoveling, huge machines act like shovels and scoop up large quantities of
gold-bearing sand and gravel from stream beds.
Hydraulic mining and dredging are outlawed in many countries
because they are environmentally destructive to both land and streams.
Grinding
• Once the
gold ore has been mined, it usually is washed and filtered at the mine as a
preliminary refinement technique. It is then shipped to mills, where it is
first combined with water and ground into smaller chunks. The resulting mixture
is then further ground in a ball mill—a rotating cylindrical vessel that uses
steel balls to pulverize the ore.
Separating the gold from the ore
• The gold is
then separated from the ore using one of several methods. Floatation involves
the separation of gold from its ore by using certain chemicals and air. The
finely ground ore is dumped into a solution that contains a frothing agent
(which causes the water to foam), a collecting agent (which bonds onto the
gold, forming an oily film that sticks to air bubbles), and a mixture of
organic chemicals (which keep the other contaminants from also bonding to the
air bubbles). The solution is then aerated—air bubbles are blown in—and the
gold attaches to the air bubbles. The bubbles float to the top, and the gold is
skimmed off.
Cyanidation also involves using chemicals to separate the
gold from its contaminants. In this process, the ground ore is placed in a tank
containing a weak solution of cyanide. Next, zinc is added to the tank, causing
a chemical reaction in which the end result is the precipitation (separation)
of the gold from its ore. The gold precipitate is then separated from the
cyanide solution in a filter press. A similar method is amalgamation, which
uses the same process with different chemicals. First, a solution carries the
ground ore over plates covered with mercury. The mercury attracts the gold,
forming an alloy called an amalgam. The amalgam is then heated, causing the
mercury to boil off as a gas and leaving behind the gold. The mercury is
collected, recycled and used again in the same process.
The carbon-in-pulp method also uses cyanide, but utilizes
carbon instead of zinc to precipitate the gold. The first step is to mix the
ground ore with water to form a pulp. Next, cyanide is added to dissolve the
gold, and then carbon is added to bond with the gold. After the carbon
particles are removed from the pulp, they are placed in a hot caustic
(corrosive) carbon solution, which separates the gold from the carbon.
• If the gold
is still not pure enough, it can be smelted. Smelting involves heating the gold
with a chemical substance called flux. The flux bonds with the contaminants and
floats on top of the melted gold. The gold is then cooled and allowed to harden
in molds, and the flux-contaminant mixture (slag) is hauled away as a solid
waste.
The Future
Because gold is a finite resource, its long-term future is
limited. In the short term, however, it will continue to find widespread use in
jewelry and in industrial applications, especially in the electronics field.
In the last few years, several companies have focused on
extracting gold from sulphide ore rather than oxide ore. Previous techniques
made such extraction difficult and expensive, but a newer technique called
bioleaching has made extraction more feasible. The process involves combining
the sulphide ore with special bacteria that "eat" the ore or break it
down into a more manageable form.
