"That poet, writer, geologist, appears again
as Novalis, the New...the New constellation. And in support of how
the warblings I make in the sentient soul and warblings I make in
the intellectual soul, and the news cycle and the eternity and depth
contemplated in Spiritual Science as Consciousness Soul, becomes
slowly, eternal Spirit Self, we can refer to how gold and cyanide
work together as something that dissolves the unessential so that
the gold is not harmed but the other elements are dissolved.
"The children's father was constantly coming in and going out, and
each time he would see how the children were going, and kindly greet
Ginnistan. He always had something to say to the scribe, who would
listen attentively, and after he had transcribed it would lean over
and hand the pages to a noble godly woman who was leaning against an
altar who had a dark bowl full of clear water. She dipped the pages
into the bowl, and looked into it with a cheerful smile. If when she
drew them out the writing remained and had become shining, she would
hand the leaf back to the scribe to be fastened into a large book.
Often, if his efforts had been in vain and were washed away, the
scribe's countenance would turn sullen.
"From time to time, the woman would turn to Ginnistan and the
children, dip her finger into the bowl, sprinkle some drops on them.
As soon as the drops touched the wet-nurse, the child or the cradle,
they would melt into a blue haze, displaying a thousand strange
sights, always changing."
"Since the 1890's, cyanide has been used to recover gold from gold
bearing ores. And today, over 115 years later, most of the worlds
gold is recovered with cyanide playing a large part in the
beneficiation of the yellow precious metal. Chemically, it is a
rather simple reaction:
4 Au + 8(NaCN) +O2 + 2 H2O = 4 NaAu(CN)2 + 4 NaOH
That presumes that the only elements are the gold, Sodium Cyanide
and water. However, as any geologist will tell you, no two ores are
the same, and their chemical composition will vary greatly
throughout the ore body. These "extra" elements in the mineral
compounds will often play havoc with a chemical reaction, as
Copper is definitely worth mentioning, since copper minerals will
dissolve in cyanide solutions, and cause a increased use of cyanide,
the copper-cyanide complexes formed by the dissolution will tend to
inhibit the dissolution of gold in the cyanide solution. Zinc, the
element used to precipitate gold from solution, if present in the
ore, will bond with the cyanide to form a zinc cyanide compound.
Another element that plays with the cyanide chemistry is nickel.
Nickel, however does not interfere with the gold going into
solution, but rather the precipitation of the gold from the cyanide
Arsenic and antimony do present a larger problem, by reacting with
the cyanide and using up all of the excess oxygen, leaving little or
no oxygen to effect the dissolution of gold. Carbonaceous gold ores
can have the carbon adsorb the gold onto its surface, and as a
result will not be recovered from the pregnant solution.
Leaching gold from sulfide ores is difficult, at best. Generally,
the recovery for cyanide leaching of sulfide or refractory ores is
no better than 30%, which is not a worthwhile venture.
The use of alkalies such as calcium oxide, will prevent the
decomposition of cyanide in solution to form hydrogen cyanide gas.
It reduces the volume of cyanide required to leach the gold or
silver. In addition, hydrogen cyanide is highly toxic to people. So,
the few dollars spent on adding a cheap calcium oxide to the ore or
solution, prior to leaching is worth the money spent. Most cyanide
leaching is carried out at a alkaline pH of between 10 and 11,
depending upon lab testing of individual ores and the optimum
leaching/chemical use rates.
The cyanide solution strength is also important in leaching gold,
with the typical range of solution being in the 0.02% -0.05% NaCN.
The gold particle size has a tremendous effect on the time required
for dissolution in a cyanide solution. Generally, the finer the
gold, the quicker it will dissolve. A 45 micron particle of gold
would dissolve in 10-13 hours, while a 150 micron particle might
take from 20 to 44 hours to dissolve in the same solution.
Oxygen plays an important role in the leaching of gold in a cyanide
solution, also. It has been proven that the rate of dissolution of
gold in cyanide solution is directly proportional to the amount of
oxygen present. Normal water will have 8-9 ppm dissolved oxygen
present in it. If this oxygen is used up by other reactions, it may
be necessary to aerate the solution, inducing oxygen into it, to
speed up the reaction. With cost being always the determining factor
(except in safety), the decision to aerate and speed up the reaction
will be made based upon economics and laboratory testing. It is not
used much anymore, because most leaching is heap leaching, carried
out in the outdoors, where drip emitters or sprays distribute the
cyanide solution to a large structure of gold ore, called a "heap".
And while the pile of ore is called a heap, it is not a haphazard
pile of rocks. Much thought and design goes into the making of a
heap leach, to derive the best, most economical solution for
recovering the gold from the ore.
Once the gold has been dissolved in the cyanide, and the ore body
has been reasonably depleted of its gold, there are two main
processes for recovering the gold from the pregnant cyanide
solution. One is the Merrill-Crowe zinc precipitation process and
the other is the adsorption of the gold onto activated carbon. The
oldest method, Merrill Crowe, involves first removing the oxygen
from the solution, then mixing a fine zinc powder with it (-200
mesh), and recovering the very fine gold precipitate on a precoat
filter, since the gold precipitate is very fine, ranging from a few
microns to 50 or so microns. The zinc reacts with the cyanide:
2Au(CN) + Zn = 2Au + Zn(CN)4-2..."