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Cyanide Leaching

The first chemical leaching of gold occurred in the late 1800’s 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.

Cyanide 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. I’m afraid that many of our public officials fall in to the latter category.

The 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 heap-leaching operations.

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 possibilities.
 

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 confectioner’s sugar.

    · It will be converted to hydrocyanic acid, HCN, if it contacts acid.

    · 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. It’s 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 solution.

    · 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
I think 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.

The 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.

Well, that’s 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 won’t 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
First 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. It’s 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.

It 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. It’s just good procedure. You will probably never have occasion to use it.

You 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.

You 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 it’s 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.

You 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 we’ll go in to more detail.

I 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.

O.K., 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.

We are 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 doesn’t 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 won’t 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 don’t panic.

It’s 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.

Another 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.
 

Recovering values
There 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 don’t 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 doesn’t 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, it’s usually de-ionized. The one you want to catch gold/silver is a weak base, anionic resin. I’m 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 it’s best to simply burn it. Remember, with the resin you don’t 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.