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Small Home-based gold cyanidation. Appropriate CN concentration & pretreatment DIY (7 replies and 13 comments)
Stibnut,
I am hopeful that any replies you receive here are strongly, very strongly to discourage you from this attempt. Cyanidation is NOT something that any novice should experiment with, EVER.
Simply put, when you reach step 7, if not before, you will die.
The reason this will happen is that due to your inexperience you have not accurately assessed or mitigated the risks, and when neutralizing cyanides with hypochlorites (or bleach) you run the higher risk of creating a MORE TOXIC material than the cyanide. In this case you, by nature of describing strong bleach as your neutralizer, are in direct risk of creating cyanogen chloride. This will kill you.
There are many more risks and aspects that you have not become aware of, and for that reason you should cease all activity in performing cyanidation.
That is my only advice to you.
Do you have any idea of what minerals might be present in your "pay dirt"? Is the gold free or locked in sulfides or silica minerals? Knowing what you have and the mode of occurrence is a really good starting point for any leaching project.
There appears to be very little free gold, at least visible free gold. Unfortunately it's locked in a storage unit as I'm in the middle of a move and in the last stages of writing a paper for a journal, so I can't go dig it out now. I will make getting a little sample a priority though.
What sorts of simple tests might I perform to determine whether silicates or sulfides predominate? Geology is my one real blind spot in the sciences, so I can't tell minerals apart by appearance. If sulfides predominate, would they be amenable to (close your eyes, Craig) being treated with HCl to see if they give off H2S? In small quantities, outside, of course. And is there a test I can do to see if silica makes up a large proportion of this material?
You can test for carbonates with a drop of acid, HCl will do, on the ore. If carbonates are there it will fizz, CO2. If you have a microscope, see if you have yellowish shiny particles, likely cubic if pyrite, or brassy looking chalcopyrite.which is an iron copper sulfide and these are the most likely sulfides associated with a simple gold ore. There are a host of other possibilities which is a bit more complex to identify with simple suggestions. Where did you get the pay dirt? Do you know where it is from? Silver minerals are often associated with grey or brown gray minerals, galena or sphalarite, which will be shinnier than the quartz.. Quartz is likely to look somewhat irregular in shape without a noticeable form. River gravel will likely show rounded shapes from tumbling in the water. Having said all of that I would do your 50 gram test with the initial assumption that the gold is free and amenable to direct cyanidation. Refractory gold treatments open up a total new set of issues.
I would also use H2O2 to ultimately neutralize your cyanide solutions. I do not think that you will need to use Cu to catalyze the reaction. However, add VERY slowly as peroxide tend to be very sensitive to temperature and the reaction will dramatically accelerate when it starts to get warm and will foam dramatically out of a beaker or bucket. Add slowly, stir and wait. Add more again slowly, stir, and wait.
I would like to add that cyanide ties up the hemoglobin in blood causing it not carry oxygen, so cyanide can asphyxiate a person. The most important antidote, which is required is oxygen. Amyl nitrites are helpful in replacing the cyanide hemoglobin bond to be removed through the urine; however oxygen is essential. You do not need the poppers if you flood the person with oxygen. You need to have a small tank of breathing oxygen with a mask as the primary antidote as the person with cyanosis is dying from lack of oxygen. You can force oxygen into the blood stream with an oxygen source which will safe the life. You have 6 minutes maximum prior to the lack of oxygen causing permanent brain damage.
You need to do any chemical experiments in a fume hood, which is drawing the fumes away from you. A less satisfactory solution is to have a very highly ventilated space with the fumes directed away from you. There are many chemical suggestions for the safe use of cyanide; however the above conditions are the starting points. I agree with Craig that you don't want to do experiments if you have not taken appropriate physical precautions for doing any chemical testing.
Thanks for that tip. I was going to do this outside in the first place, which for small-scale things like the new scaled-down plan should be nearly as good as a fume hood, as long as I take care not to hover over it or allow prevailing winds to blow gas in my face.
(full disclosure: I've done this sort of thing before. Not with cyanide, but with something involving generating lots of CO, far more than I would HCN, for another experiment. No negative effects given I was careful to do it outside)
I didn't know cyanide had a CO-like effect as well as its main method of causing asphyxia jamming up mito complex III or IV (can't remember which) by binding to the iron at the core of the complex. I suppose that makes sense, though: CN- and CO are iselectronic, so cyanohemoglobin would be a big problem. I'll add an oxygen tank to my list.
Cyanide ties up the iron in hemoglobin so it won't carry oxygen. Bodies need oxygen to survive a dose the rest is more academic.
Right, it definitely would do that. The reason HCN is more toxic per dose than CO is the inhibition of ATP synthesis in the mitochondria, but even if it weren't for that, it would still kill the same way CO does: by binding hemoglobin iron that way.
Ordered an oxygen tank+regulator+mask. Amazingly, it was only $70 altogether, plus something else reasonable for getting it filled. As long as I claim it's not for medical use, everything gets so much cheaper than if the medical bureaucracy gets its hands on it. And I'm not going to be too worried about any trivial industrial-grade contaminants if I get poisoned. 😉
Well, I'm going to do it. It might win me a Darwin, but chemistry's a passion I'm willing to risk my life for. Still, I'm committed to doing this in the manner least likely to kill me. That's why I am buying the HCN detector, amyl nitrites, thiosulfate, two pH meters for redundancy, and so forth. Not to mention the standard goggles and chemical-resistant gloves.
Some people go BASE jumping, some go rock climbing, some do amateur chemistry experiments. Only difference is that i'm more of a knowledge junkie than an adrenaline one. We all have our own thresholds for what constitutes an acceptable risk, and this falls on the "just barely safe enough, given many precautions" side for me, whereas it wouldn't for you or probably most other people.
I'll make two adjustments based on your advice: the first trial run will be on about 50 g of "paydirt" rather than 500, and I'll choose a different oxidizing agent for disposal. I did know there was a cyanogen risk, but it does seem to be larger, and cyanogen more toxic, than I had anticipated. That's why I'm reaching out to people who know more than I do - to minimize the risks of an inherently risky project.
So it sounds like an alternative oxidizing agent would be better. Perhaps potassium permanganate or hydrogen peroxide? I could use the Prussian blue test to determine when the solution has been detoxified enough to pour down the toilet. I have 35% H2O2 and 3%; not sure whether to go high or low here, as I don't want vigorous bubbling spreading aerosols everywhere or overflowing with still-toxic water present. I saw somewhere else that H2O2 needs a catalyst of some sort, such as a very small amount of copper sulfate.
Let's do a back-of-the-envelope calculation for how much NaCN I need on the first trial batch of 50 g. I'm going to assume that this really contains 0.5 g Au, but that I need a stoichiometric excess of about 5-fold to account for all the other things that CN reacts with. I'll start with the Elsner equation:
4 Au + 8 NaCN + O2 + 2H2O → 4 Na[Au(CN)2] + 4 NaOH
So I would need 2 CN's for every Au I anticipate, if CN only reacted with Au. But most of the stuff that would react with CN is not gold, sadly. So I'll go for a 10:1 molar ratio.
0.5 g Au / (196.97 g/mol) = 2.538 mmol Au.
So that means I want 25.38 mmol NaCN. (25.38)*(49.01 mg/mmol) = 1.24 g NaCN.
If I somehow managed to ingest or inhale 1/10 of that whole quantity, then I would probably die. That would be pretty difficult to do, though, short of eating it or vaporizing it all to HCN by throwing it in acid.
Does anybody have a feel for the concentration that would be appropriate? Do I want a relatively concentrated solution, just barely enough to cover the paydirt? Or are there benefits to a more dilute one?
Is there anything wrong with my plan? Will it actually work, in your experience, and are there better ways to do some of these things? I certainly would listen to any safety tips as well, besides "don't do it period". I want to do the safest amateur first-time gold cyanidation possible.
I suggest that you use a 1% NaCN solution strength and about a 20% pulp density. So, 200ml H2O + 2 g NaCN. I would add NaOH to 10.5 pH with the water in the slurry prior to adding the NaCN and check with pH meter before adding the cyanide. The pH will rise with the addition of the cyanide to about 11. A pH of 11 is quite satisfactory for leaching; I would not go to 12 as less effective and it will slow the leach. Do not let pH drop below 10 by adding a small amount of NaOH. Personally I would precipitate the gold with fine zinc or borohydride and then filter. Adding an electrowinning cell is another complete art and would make recovery more difficult with plating out over your cathodes and is totally unnecessary for the scale you are contemplating.
Okay, that makes sense. How rapidly does the pH tend to drop as leaching occurs?
Would it make any sense to use some sort of buffer solution rather than just NaOH, or would that just screw things up? If I could find a good pH 11 buffer, maybe the risk of going too low and getting HCN would be smaller. But nothing really comes to mind as a good buffer whose range centers near (or just above) 11.
Good to see that simple reducing agents are all I need to get the gold out. I take it bisulfite or metabisulfite would work as well? I still have some of those lying around from an earlier phase with aqua regia. I have zinc dust too, and at some point borohydride although it's gone missing recently. I'm disorganized at the best of times and made three moves in as many months...
The pH should not drop very rapidly particularly if you have carefully stabilized the pH at 10.5 before adding the cyanide. If carbonates are present, you might not get much if any pH drop. Cyanide is a bit peculiar, high concentrations will react with a lot more minerals than lower concentrations which are more selective for precious metals. A 1% solution is alright for lab extractions but a bit high for commercial leaching as you don't want to use or waste excess cyanide and you have more stability with larger volumes of solution. I would use the zinc for a precipitate not the metabisulfite. Zinc is a good reductant as gold is precipitated and zinc goes into solutiion as a zinc cyanide complex. It is pretty selective for precious metals in reasonable quantities, say 3 to 5 time stoiciometric.Ven Met (stabilized borohydride) or hydride is also pretty selective for precious metals.
I would not use any buffers for pH, for the lab NaOH is fine. CaO slurry for larger leaches in less expensive but a little more difficult to mix and control for a lab experiment. You don't want to add a lot of other chemistry into the mix with buffers.
Stibnut, in any case, you get the BEST FORMULATED QUESTION AWARD. How refreshing to see a complete question with lots of details. I hope to see you around here other.
Those watching and wondering why they get no answer or little feedback: Look at the initial post by Stibnut and emulate.
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Stibnut you need to go back to a few basics before going down the route of cyanidation. One of the first questions that need to be answered is how much gold is in the sample in the first place. While it is described as paydirt. 1 Oz/t equates to only $US38. At 15 Oz/t there is still only $US650 of contained gold. These are very high assays. Typically anything higher would have to have free gold in it. Before cyanidation check if there is any gravity recoverable free gold by panning. A small portion washed in a gold pan (collect the washings) should give a gold tail in the bottom of the pan if there is free gold. Using this method it is a lot easier to recover the gold than using cyanide. Grinding it finer would help in this step and if you want to cyanide it.
If your heart is set on using cyanide despite the dangers to those who are unsure what they are doing. Then a simple bottle roll set-up could be used to leach the gold. Provided the gold is cyanide leachable and not locked up in sulphides for example then roughly 500-1000g could be treated by this method. Running a pH around 10.5 and cyanide strengths of 200-250ppm over say 24hr should leach any gold. To recover the gold separate/filter the solids from the solution and precipitate the gold using excess zinc shavings. Note there needs to be some cyande present in the solution. The recovered zinc shavings were usually roasted outside or dissolved in sulphuric acid to get rid of the excess zinc with the remaining sludge smelted into a dore bar.
Again cyanide is toxic and should not be played with by those unsure what they are doing.
As I said at the beginning, I'm not expecting to make money out of this. I just want to extract a little gold and learn more hands-on chemistry. The more gold the better, but seeing as I've spent hundreds of dollars on equipment, and I did indeed buy the paydirt for about $1-2/lb, I'm not really after gold so much as I'm after chemistry, and CN's is fascinating, however dangerous.
Looks like zinc is the way to go - I'll try to keep track of the bag I have on my last move here. Also that reminds me to clean out some lead and antimony gloop out of a melting pot among other things, and wipe up all the galinstan residue I find, because I was totally doing that too and then using it with aluminum as a reducing agent. I made during my brief stay at my parents' place between leases. Managed to cause an impressive amount of mess while I was here...
I'm not a total novice at this stuff, even though I am indeed operating without proper lab equipment. I did a wide variety of reactions last year using sciencemadness.org and Google Scholar as resources. I made bromine and used it to brominate unsaturated compounds, successfully initiated my own Grignard reactions (albeit with terrible yields and bizarre side products), did a project where I bought a couple of polonium static eliminators and covered them with beryllium foil secured with duct tape and then activated some elements by very weak neutron bombardment (Be-9+He-4 --> C-12+n), made sodium amalgam, messed around with dissolving impure gold and other precious metals in aqua regia to recover the real thing, and on and on like that. I've never had any fires or injuries worse than minor burns and glass cuts. Maybe I was just lucky, but I'm pretty sure I was being careful in there too.
Cyanide's toxicity is on the high end of what I've dealt with, but I've built enough experience and collected enough safety equipment that I think it's worth a go. At a lethal dose in the ~100 mg range, it's enough that I respect it greatly but not enough that it's something I feel I can't touch with a ten-foot pole. There are chemicals like that: dimethyl mercury, HF and F2, high explosives, etc. But cyanide is a case of relatively informed consent.
How is it that 250 ppm could leach all gold, given that the gold gets dissolved into cyanide complexes at a 1:2 molar ratio with cyanide? If it's facilitating the gold being uptaken by something else then I'd believe that, but it looks like the way it stays soluble is by staying with the cyanide. Now maybe I got ripped off entirely and only have a few mg of gold, but barring that it seems I should use much more. Maybe I shouldn't use as much of an excess and 0.5% (5000 ppm) is better than 1.0%, though.
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Hi all,
I am a student in atmospheric chemistry who has developed a side interest in chemistry overall. Then I ended up doing metallurgy and found it quite interesting. It's time for something ambitious. And dangerous, I know, but I hope to persuade you that I'm aware of the risks and have taken enough countermeasures.
I am purchasing 200 g of NaCN from a vendor overseas, and I intend to use it on 60 lbs of "paydirt" that I bought a while ago. I'm under no illusions that I'll actually make money off the gold, especially after all the gear I'm buying, including an HCN detector plus some cheaper stuff (antidotes including thiosulfate and a bottle of amyl nitrites aka 'poppers', two pH meters, a bunch of CaO and NaOH, etc.), beakers, and a stir plate with bar along with an aquarium aerator to keep a flow of oxygen going. This is entirely a self-directed chemistry project where I'll sell whatever I get at the end to offset the cost a little bit, done as carefully as possible by someone with a bit of real chemistry training but still mostly an amateur.
The "paydirt" appears to contain pebbles of size 0.5-1 mm or so. I don't know my geology so I'm uncertain as to their composition. I want to start at a small scale of ~500 g of "paydirt" in a 2L beaker with large stirbar, and then use what I learned there to work through the rest of it. I have a plan and some questions - feel free to answer or critique:
Any feedback at all would be appreciated. Thanks! 🙂