Elemental gold has a
melting point of 1,063 deg C and a boiling point of 2,966 deg
C. In addition to its softness, it is both the most malleable
and most ductile of all elements. This means that it can be
hammered into extremely thin sheets (approaching a small
number of atoms) and can be drawn into extremely fine wire.
Gold in the form of very thin sheets, called gold leaf, has
many decorative uses. Elemental gold is an excellent
conductor of electricity and heat, surpassed only by the
other members of group 1B, copper and silver. Gold usually
forms compounds (and complexes) by giving up either one of
three of its valence electrons. It is commonly alloyed with
other metals, as in jewelry, in proportions that yield
desired hardnesses and colors.
An alloy of gold, silver, and
copper, in which the amounts of silver predominates, is
called "green gold". An alloy of
the same three elements in which copper predominates is
called "red gold"; An alloy of gold and nickel is
called "white gold". The purity of alloyed gold
is expressed by the karat system, where the percent of gold
by weight is given as a fraction of 24. Therefore, pure gold
is 24 karat, whereas 18-karat gold is 18/24, or 75%, gold by
weight. Gold dissolves in very few solvents, among them aqua
regia and the various solutions of cyanide that are used in
ore extraction. When gold does dissolve, it is generally by
forming complexes. Gold also forms amalgams with mercury and
as such is used in dentistry.
MINING
Gold is obtained by two principal mining
methods--placer and vein mining--and also as a by-product of
the mining of other metals. Placer mining is used when the
metal is found in unconsolidated deposits of sand and gravel
from which gold can be easily separated due to its high
density. The sand and gravel are suspended in moving water;
the much heavier metal sinks to the bottom and is separated
by hand. The simplest method, called panning, is to swirl the
mixture in a pan rapidly enough to carry the water and most
of the gravel and sand over the edge while the gold remains
on the bottom.
Panning is one of the
original methods used by the forty-niners and is imortalized
in story, art, and song . Then, a method that allowed for
processing larger quantities of ore, is a sluice box, a
U-shaped trough with a gentle slope and transverse bars
firmly attached to the trough bottom. The bars, which extend
from side to side, catch the heaviest particles and prevent
their being washed downslope. Sand and gravel are placed in
the high end, the gate to a water supply is opened, and the
lighter material is washed through the sluice box and out the
lower end. The materials caught behind the bars are gleaned
to recover the gold. A similar arrangement catches the metal
on wool, and may have been the origin of the legend of
Jason's search for the GOLDEN FLEECE. Another variation of
the placer method is called hydraulic mining. A very strong
stream of water is directed at natural sand and gravel banks,
causing them to be washed away. The suspended materials are
treated much as if they were in a giant sluice box. Today's
most important placer technique is dredging. In this method a
shovel of several cubic meters capacity lifts the
unconsolidated sand and gravel from its resting place and
starts the placer process.
Vein, or lode mining, is
the most important of all gold recovery methods. Although
each ounce of gold recovered requires the processing of about
100,000 ounces of ore, there is so much gold deposited in
rock veins that this method accounts for more than half of
the world's total gold production today. The gold in the
veins may be of microscopic particle size, in nuggets or
sheets, or in gold compounds. Regardless of how it is found,
the ore requires extensive extraction and refining. One-third
of all gold is produced as a by-product of copper, lead, and
zinc production. Copper, for example, must be
electrolytically refined to raise its purity from 99% to more
than 99.99% as required for many industrial purposes. In the
refining process an anode of impure copper is electrolyzed in
a bath in which the cathode is a very thin sheet of highly
refined copper. As the process continues, copper ions leave
the impure anode and are deposited as atoms on the cathode.
Because impurities are not transported through the bath, as
the anode is consumed, the impurities fall to the bottom as a
sludge. This anode sludge contains gold in quantities
sufficient to make recovery profitable. One-third of all gold
is obtained from such by-products. Silver and platinum are
also recovered from the copper anode sludge in quantities
large enough to more than pay for the total refining process.
EXTRACTION AND REFINING
In obtaining gold from vein ore, the ore is first
crushed in rod or ball mills. This process reduces the ore to
a powdery substance from which the gold can be extracted by
amalgamation with mercury or by placer procedures. About 70%
is recovered at this point. The remainder is dissolved in
dilute solutions of sodium cyanide or calcium cyanide. The
addition of metallic zinc to these solutions causes metallic
gold to precipitate. This precipitate is refined by smelting.
The purification is completed by electrolysis and the sludge
produced will contain commercial quantities of silver,
platinum, osmium, and other rare-earth metals.
USES OF GOLD
One of the principal uses of gold today is as a
currency reserve--usually stored in the form of gold bricks
weighing 13 kg (35 troy lb). Gold was for centuries used
directly as currency along with silver. During the 19th
century it assumed a role as the sole basis of the currencies
of most nations; paper MONEY was directly convertible into
gold. World War I disrupted this system. The original GOLD
STANDARD was gradually abandoned (the United States stopped
minting gold coinage in 1934), and the dollar eventually
emerged as the principal unit of international monetary
transactions. Since the 1970s gold has been bought and sold
on the market, with widely fluctuating prices, and gold
reserves maintain only a very indirect relationship with the
values of currencies. There is a large and rapidly growing
demand for gold in industrial processes. Its relatively high
electrical conductivity and extremely high resistance to
corrosion make the metal critically important in
micro-electrical circuits. Minute quantities dissolved in
glass or plastic sheets prevent the passage of infrared
radiation and make an efficient heat shield. Because of its
chemical stability, gold is in demand for bearings used in
corrosive atmospheres. It is also plated on surfaces exposed
to corrosive fluids or vapors. Its lack of toxicity and its
compatibility with living systems make it indispensable in
dentistry and medicine, and its beauty has made it
outstanding in the arts and crafts since ancient times.
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