first chemical leaching of gold occurred in the late 1800s
with the introduction on the market of "DuPont Mining Salts".
It did extract gold as gold chloride. It was slow and cumbersome
to utilize. Just a little later, around the turn of the century,
it was discovered that gold could be dissolved in solutions of
sodium cyanide. This method was superior because of its
simplicity, availability of chemicals, and low cost. After all,
who knew what "Mining Salts" were composed of? The
success of cyanide leaching is attested to by the fact that, for
most purposes, it is still the preferred method of gold extraction
and by the fact that about 80% of the world production of gold is
obtained by this method.
has, I think, received a very bad rap at the hands of "do-gooders",
zealots, the well meaning but mis-informed, and some who are just
plain stupid enough to believe the latter without checking out the
facts. Im afraid that many of our public officials fall in
to the latter category.
success of cyanide is due to several characteristics that we
should mention. It is an extremely versatile method. It can be
successfully used as a means to assay in 1 quart fruit jars, in
small-batch production such as extraction of a few tons of good
ore or concentrates in a small cement mixer, or in mega-ton
It is a
relatively simple process. A person can be trained in a day or two
to oversee the extraction and recovery in a safe and efficient
manner. For those of you who work in areas where there is good
gold but it is so fine that it is near impossible to catch it by
gravity, you should read on, open your mind, and think of the
|Some Facts about Cyanide
- Sodium cyanide is a very simple chemical composed of one atom
of sodium, one atom of carbon, and one atom of nitrogen. Its
chemical formula is NaCN.
- Sodium cyanide is a white solid that looks like confectioners
- It will be converted to hydrocyanic acid, HCN, if it contacts
- HCN is a gas, poisonous and is the stuff that was used to
execute condemned prisoners that is probably where it got most
of its bad reputation. There are dozens of toxic gases, which
would have done the job faster, but cyanide is cheap.
- It is toxic. It is classified, I believe, as a class two
poison. The "rotten egg" odor of Hydrogen sulfide
often emitted from industrial plants or from the catalytic
converter on your car is a class one poison. Much more toxic.
- Cyanide occurs naturally in many plants. It is found combined
with the sugar glucose to form a chemical known as amygdalin.
When amygdalin contacts the enzymes in your intestinal tract the
cyanide is split away from the sugar and is released as HCN.
- Cyanide reacts with the hemoglobin in your blood. It takes
the place of oxygen. Oxygen combines with hemoglobin in the
lungs and is transported to the muscles etc where it releases
its oxygen to be utilized in energy producing metabolism. The
hemoglobin is now returned to the lungs to pick up another load
of oxygen. Cyanide, as opposed to oxygen, can not be released
from the hemoglobin, thus, the hemoglobin cannot carry anymore
oxygen. Hemoglobin combined with cyanide is essentially dead. If
enough cyanide reacts with your hemoglobin the blood will be
unable to transport enough oxygen to maintain your body and you
will die of asphyxiation. As in the "gas chamber".
- When I was a kid growing up in south Georgia, we used to go
fishing with nothing more than a sugar sack, a few black
walnuts, and a couple of rocks. You simply put the walnuts (with
the husks) in the sack, find an area where the water is not
moving too fast, smash up the walnuts with the rocks, and throw
it in the water. In a couple of minutes you scoop up the fish.
The fish are perfectly safe to eat, since the HCN has already
bonded with the fish's blood. You can use peach pits, wild
cherry leaves, bitter almonds, and numerous other plant
materials. Its the amygdalin that is doing the job.
- Remember the "miracle" cancer cure derived from
apricot pits, "Laetril", that made millions for
Mexican scamsters a few years back. Amygdalin! Nothing else!
- Cyanide is destroyed by many naturally occurring chemicals as
well as by oxidizing agents such as clorox.
- Cyanide is destroyed by sodium thiosulfate or sodium nitrite.
- An average fatal dose of HCN is about 50-60 milligrams.
- Cyanide dissolves both gold and silver from alkaline
- Cyanide is a very "clean" extraction system, as it
does not extract many of the base metals such as iron. This is
in contrast to halide extraction, which dissolves large
quantities of iron, aluminum, and other metals.
|A Little About pH
we should take a little excursion into the world of acids and
bases, pH, and titratable acid/base. So, what is an acid? To put
it in its simplest form it is any substance which when dissolved
in water will have a pH of less than 7.0. What is a base? Just the
opposite. Any substance which when dissolved in water will have a
pH of more than 7.0. So what if the pH is exactly 7.0? Then the
solution is neither acid or basic, it is neutral.
scale that is used to measure pH goes from 0-14. Any pH below 7.0
is acid and any above is basic (or alkaline). If we are going to
work with pH there is one point which I must make and sometimes
people have a little difficulty in understanding it. The pH scale
of 0-14 is logarithmic not linear. In other words, most folks
would assume that a pH of 1 has twice as much acid as a pH of 2.
This is wrong. It is 10 times more acidic. Following this
logarithmic scale, a pH of 1 is 100 times more acidic than a
solution of pH of 3 etc. A pH of 9 is 10 times more alkaline than
a pH of 8.
thats O.K., If we have a solution of alkali that contains 1
gram/liter of lime and has a pH of say 12 and we want to adjust it
to 13 we simply add 10 more grams of lime, right? Well, I set you
up again. I wish I could say this was the case but it just wont
work out that way. You will find that it will take more lime than
you thought. This is the difference between pH and "titratable
acidity or alkalinity". Without going into a monologue just
let me say that this is due a thing called "buffering".
If anyone wants to pursue this subject get in touch and we will
climb that tree as high as you want to go. Just be aware that
frequently the amount of acid or base that you must add to your
reaction will not be what you expect.
|Cyanide Extraction of Gold and Silver
thing to consider is what sort of chemicals, facilities, and
equipment that you will need. You will need a supply of Sodium
cyanide. You will need a much larger supply of slaked lime
(calcium hydroxide). This is not limestone, or quicklime.
Frequently these terms are used interchangeably. You will hear
terms like "agricultural limestone" etc. Just be sure
you have Calcium hydroxide. The cheapest grade that you can buy.
Do not underestimate the amount that you need. Its cheap and
you do not want to start an extraction and run out of lime. Your
material could start to go acidic and you would be caught with no
alkali to counteract the acid. This could be bad because if the
solution should become acid enough the cyanide would be released
into the air as HCN and your values would fall out of solution.
would be prudent to have a supply of sodium thiosulfate for just
such an emergency. If things start to get out of hand just dump in
an excess. Your values will still fall out of solution but you
will have destroyed the cyanide in the extraction vessel and
precluded complaints etc. Its just good procedure. You will
probably never have occasion to use it.
will need some way to determine the approximate pH
(acidity/alkalinity) of the reaction. On a small laboratory or
pilot-plant scale you should be able to get by with "pH paper".
This is paper in strips or rolls which when dipped into the
solution is then compared to a color chart on the paper container.
You match the color of the strip with a color on the chart and
read off the pH. Many of these products will only work over a
certain range of pH. You would prefer a wide range paper going all
the way from 1-14. You will be operating in the range of 9-12.
will need something to keep the solution stirred, agitated, mixed
and aerated. On a very small scale such as a quart to a gallon
size jug you can probably do it by hand. Just shake or stir the
material every few minutes. In somewhat larger scale extractions I
recommend a cement mixer of a size appropriate to the amount of
material to be treated. On a cement mixer scale you might also
have to adjust the cyanide concentration to maintain its
concentration at about 0.4%. For this you will have to determine
the amount of cyanide present in solution. This is usually done by
a simple titration of the extracting liquid and addition of
cyanide as it is used up in the reaction. If any of you should
need a procedure to accomplish this please let me know. I will try
to modify this page to include a section on determination of
cyanide in solution. Because this site is primarily for the small
operator, we are not going to consider large scale or
heap-leaching at this time.
will need some sort of filter to separate the solids from the
pregnant (gold-bearing) solution. For a small batch, you can
utilize coffee filters and a funnel. There are things called "filter
funnels" which are much more efficient than a standard smooth
walled funnel. The filter funnel simply has a series of grooves
cut from the rim of the funnel down to the spout. These grooves
serve as channels for the liquid to escape through. Without these
the folded paper will sort of seal itself to the filter surface
which acts like a dam so that the only surface area available for
liquid to exit is through the very tip end of the paper. This can
be frustratingly slow. You can make an acceptable substitute by
taking a few thin strips of wood or plastic of swizzle stick or
soda straw size. Tie them all together at one end. Carefully
spread them to make sort of a "coolie hat" or cone. Put
this cone into your funnel and put your folded filter paper in on
top of it. Now you have a "Q&D" (quick & dirty)
filter funnel. A complete discussion of filters is well beyond the
scope of this page. If you have specific questions let me know and
well go in to more detail.
highly recommend that anyone who has no experience with cyanide
extraction start with about a quart or half gallon size jar with a
tight fitting top that does not leak. If you should make some sort
of mistake it is much easier to deal with a small mistake rather
than a large one. After all, if everything starts to go south you
can just throw the whole thing into a bucket of thiosulfate
solution. Remember, anytime that you are doing an extraction in a
closed vessel you should loosen the top occasionally to relieve
internal pressure and allow oxygen to enter. There must be oxygen
present or no gold/silver will be dissolved.
now that you have digested that wealth of information maybe there
are a few out there who are truly desperate and got no choice but
to continue, lets dissolve some gold/silver.
going to run a sample to see if there is enough really fine gold
present to go after it on a larger scale. Start by dissolving
about 10 grams of lime in one liter of water. If it doesnt
all dissolve, not to worry. Now weigh out 4 grams of sodium
cyanide and add it to the lime solution and allow it to dissolve.
Next, fill your container to about 1/3 full with ore. This should
be crushed so that there are no large rocks etc. Add enough of the
cyanide solution to make thin slurry. The container should not be
more than 2/3 full. Now you start mixing. You should now start to
use your pH paper immediately. The first reading should be rather
high 11 or 12. Within a very short time, no more than a minute,
test it again. The pH will probably be falling. If so, add lime
with stirring and continue to monitor the pH. You want the pH to
stay close to11. After a few minutes the pH will begin to
stabilize and you wont have to stay on top of it as in the
beginning. DO NOT LET THE pH DROP BELOW 8!!! At this point I
cannot give you a cut and dried recipe, it depends too much on the
type of ore you have, how fine it has been crushed etc. Just stay
on your toes for awhile and dont panic.
like everything else; it gets a lot easier with practice. After
some length of time the gold will be dissolved. The only way you
can tell if all the values are dissolved is to set up several
reactions and filter them over a period of time such as 1 hr, 2
hrs, 4 hrs, 8 hrs, etc. This will only have to be done once for
each type of ore that you test. After that you will know that
samples from area X requires 2 hrs for extraction, etc. For our
first try lets just extract for 4 hrs and filter it.
very important thing is that you must wash the solids (pulp)
several times to remove all the values. After the first pregnant
solution has been recovered, wash the pulp with an equal volume of
your cyanide/lime solution that has been diluted 1/10 with water.
You should wash the pulp this way three times. Combine all the
liquid. This pregnant solution should have the values in solution.
are numerous ways to recover the values from pregnant solutions.
We will not attempt to cover them all, just the ones that seem
practicable for our purposes.
- Activated carbon. You must use activated carbon.
Ground up charcoal briquettes will not work. Yes, you can make
your own carbon but its a grunt and not worth the trouble. Buy
or borrow it. You can adsorb the values two ways. First and most
efficient is "column adsorption". In this method the
carbon is slurried into a column of plastic or other material
and the pregnant solution allowed to flow through it. The values
should remain stuck to the column. There are ways to "strip"
the column with chemicals but it is frequently unreliable and
you usually dont know if you recovered all the values or
not. In most cases it is probably better to simply burn the
carbon to ash and deal with that.
- Ion Exchange resin. This is usually my preferred
method. This stuff will adsorb like a thousand times more
values/unit weight than carbon. It is much more reliable and
doesnt vary from batch to batch as carbon does. This is
the stuff that they use in water softeners and de-ionizers to
produce pure water. Did you know that most "distilled"
water is not distilled at all, its usually de-ionized. The
one you want to catch gold/silver is a weak base, anionic resin.
Im sure you could get it from a local water processor.
Treat it just like it was carbon. Like carbon it can be stripped
and re-used. If you have really loaded a lot of gold on it, like
carbon, I think its best to simply burn it. Remember, with
the resin you dont need much, a very small column will
suffice. You can test to know when the column is saturated and
will adsorb no more values. This will work for carbon or resin.
Catch a little of the column effluent in a test tube or some
such and add a little table salt dissolved in weak (1-5 %)
nitric acid. If you get a white cloud of silver chloride, your
column is saturated and is about to start losing gold. Silver
always "breaks through" a column before the gold does.
- Cementation. This is a tried and true method for
recovering values dissolved in almost anything except aqua
regia. The term "cementation" refers to the process of
replacing the dissolved gold with a base metal resulting in the
precipitation of the values. Historically, this process utilized
zinc. The reason was simple, it works, and at that time zinc was
the cheapest metal in the world. There were two basic systems
used. The "zinc box" was a wooden box filled with "mossy
zinc" a sort of shredded zinc, through which the value
bearing solution was allowed to flow. This was a very efficient
method because it presented a very large surface area of zinc so
that the reaction time was very fast. As the zinc dissolved and
replaced the gold/silver in solution the values precipitated to
the bottom of the box where they could be periodically recovered
as a black-brown paste. The other way was to set up a box is
with zinc plates arranged so that the flowing liquid ran over
one plate and under the next. This works perfectly well but was
a bit slower than the previous method. The third method, which
was used when you had the pregnant solution in a batch rather
than a stream, was to simply add small chunks of zinc or
powdered zinc to the solution and allow the values to settle
out. You can substitute aluminum for the zinc. These "cementation
slimes" will contain large quantities, sometimes as much as
75% of the cementing metal but that is easily removed by other
means or in the smelting process. It is usually better to remove
these contaminating metals. If you should smelt that much zinc
all at once it could create a large cloud of zinc vapor that is
unnecessary. The usual method of cleaning up these cementing
slimes is to treat the slime with dilute sulfuric acid until
most of the metals have dissolved in the acid, leaving your gold
behind in the remaining insoluble slime. This can now be smelted
with less danger of overexposure to metal vapors.