PM pregnant NaCN solution - Zinc VS Thermal decomposition

[alexxx]

Hi to all,

I am slowly trying to learn how to safely and economically recover precious metals with Cyanide solutions.
Thanks to this forum and all the generous members who are sharing their priceless knowledge, you guys are awesome.

My main focus is the recovery of precious metals from electronic scrap (mainly AU plated parts).
But as you know, other precious metals of interest such as Ag, Pd and Pt could be recovered with cyanide NaCN (not only from electronic scrap).

I was reading several posts on this forum where it is stated that the addition of Zinc is the preferred method to drop Au & Ag out of NaCN pregnant solutions.
After reading a document on the High temperature leaching of PGM in catalysts - located here ;
https://www.google.ca/search?q=nacn+dissolving+pd&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-USfficial&client=firefox-a&channel=sb&gfe_rd=cr&ei=oGHVU8mNLozM8gf3_IDgBA

I came across the concept of Thermal decomposition of cyanide in order to drop / recover PGMs.
It is stated in this report (not sure if it is truly reliable), that the best PGMs recoveries were made with thermal decomposition VS zinc addition.
Not only that, but thermal decomposition of cyanide complexes looks like an interesting shortcut for waste treatment.

A few questions now to the members interested in sharing their knowledge and experience;

- Do you guys think that Thermal Decomposition of cyanide complexes could achieve better recoveries VS Zinc addition when recovering PGMs from a NaCN solution ?
- Do you guys think that Thermal Decomposition of cyanide complexes could achieve better recoveries VS Zinc addition when recovering Au & Ag from a NaCN solution ?
- Do you guys think that one could lose any values by overheating a NaCN solution containing dissolved PMs when trying the Thermal Decomposition process ?
- In the above paper, the Thermal Decomposition of cyanide complexes was performed in a pressure vessel. Do you guys believe one could simply heat a round bottom flask full of pregnant solution at 250C in order to achieve the complete thermal decomposition and obtain decent yields without losing any values ?
- Anything I might be missing here ?

thanks to all in advance,
cheers,

Alex

 

[4metals]

Alexxx

We do not like to discuss cyanide on the forum, and for good reason. The reactions you are referring to are carried out with high heat and pressure to effect the dissolution.

You want to cut a little corner because you are short one pressure reactor and boil the solution in a round bottom flask. It's questions like these that make us shy away from discussions about cyanide to begin with.

You are venturing down a risky road, I for one do not want to be part of that journey.

 

[alexxx]

I totally understand the reasons why the cyanide subject is avoided on this forum. And maybe, just maybe... It's not the very best approach...
And I must also say that I have enormous respect for members like you that are contributing to this forum. And making people more aware of the dangers related to the use of various chemicals.

In my personal opinion, I believe that the best way to prevent sad situations, in this case and in life in general, is to openly discuss sensitive subjects.
I would like to draw a parallel with another situation or subject that is often avoided in the day to day life.

Sexual education for instance. For those who are parents, at some point, discussing your children sex life is a sensitive and important subject.
Some people prefer to avoid discussing it, others are perfectly open...
Why, as parents, are we talking with our kids about their sex life. Simply because we want to drag their attention to the importance of being safe and we don't want them to make terrible mistakes.
It's about the same thing here.

I prefer my kids to come to me, ask me questions, ask for advices and openly discuss an important subject.
You know why, because at some point they will do it anyway, maybe with the wrong tools, or without the proper knowledge.

As for cyanide, do we still want to avoid this subject forever and push people away, experimenting on their own. We can't act like it doesn't exist... It's out there, available to anyone who is searching at the right place... We don't want our forum members or anyone trying dangerous things from a garbage ebook sold on ebay, buying a turnkey "easy to use" cyanide solution or learning from an unreliable website somewhere in the Internet.

If we are able to discuss this subject, and constantly point out the dangers related to it, I believe it's a step in the right direction.

I was reading somewhere something like, "cyanide is a powerful tool, you have to respect it... both for its amazing properties and for its great danger..."

And I'm just a kid asking questions, trying to learn from people I trust...

Alex

 

[nickvc]

Alexxx I can fully understand 4metals reluctance to openly discuss the use of cyanide, he like me has worked with it, and as I point out each time it's mentioned it doesn't allow for a second mistake when your using it. The availability legally of cyanide is thankfully controlled so most members will not be able to source it which unless you have a good laboratory with excellent extraction and all the proper safeguards is for the best, I'd hate to think of someone using it in their home or garden with kids, spouses and animals around, it looks innocuous but will kill very quickly.
If your recovering large amounts of values from e scrap I can see why you want to use it but with small volumes the workarounds such as AP and the sulphuric cell do the same job with less risk. Unless you have a dedicated safe lab area to work in I'd advise you and all others to find another route, yes it's a tool but one that is best left unused.

 

[Harold_V]

It is my opinion that we should always be careful to not endorse a substance that is a ready killer, and that describes cyanide nicely. Beyond that, depending on one's location, it's entirely possible that it is illegal to possess and use without proper permits, which are most likely very difficult to obtain.

Depending on one's location in the world, cyanide may be readily available, but it isn't in the US, although it used to be. So was dynamite, for which, the local hardware store was a source, but humans have a way of making a good thing bad (Timothy McVeigh did a wonderful job of that with ammonium nitrate, which used to be available at any hardware or garden store).

I fully agree that readers should be well versed on handling such dangerous reagents, but we also walk a fine line on this board, as we don't want to find ourselves liable for the insanity of others. To suggest methods to use cyanide is akin to endorsing the use of cyanide. All of the cautionary notes at our disposal should be provided, with the decision for its use to be made by the individual in question.

I am not an attorney, nor do I play one on TV. However, good sense should be the order of the day when this topic is discussed.

Harold

 

[Lino1406]

To begin with do you intend to have a "thermal decomposer" of cyanide with absorption/control over the emissions?

 

[alexxx]

To begin with do you intend to have a "thermal decomposer" of cyanide with absorption/control over the emissions?

Thermal decomposition looks a bit more complex than I tought, and unfortunately I have not found tons of references on how to perform correctly such operation.
I would be really interested in learning how this procedure is normally conducted, even if it involves expensive lab equipment.

I am not going to "experiment" with this one, but still very curious about that specific procedure.

 

[Lino1406]

There is a better alternative - thiosulphate converts cyanide to thiocyanate from which
the way is straight forward.
"30 and more recovery procedures"

 

[Gratilla]

Does anyone have direct experience of http://www.google.com/patents/US5178665 Recovery of Dissolved Gold by Sodium Borohydride?

The abstract states, "A process is disclosed for the direct recovery of gold from cyanide leach liquor ... The barren solution is in a condition such that it can be recycled to the upstream process."

Sounds ideal, but the patent needs to be read carefully. Example 1 states, "Over a period of 65 minutes, 100 ml of stabilized sodium borohydride (4.4M in 14M NaOH) was added dropwise ... Since the precipitation of metals was slow, the pH of the leach liquor was lowered to 3.0 - 3.5 by the addition of H2SO4."

If anyone reading this doesn't understand what this means, you're not ready for cyanidation.

 

[Lou]

EDIT: I do have direct experience with that (and have used borohydride on a variety of other difficult bastard complex salts, like thiourea, thiocyanates (another great way to get killed from H2S), thiosulfates, iodides, etc.). That patent sucks FYI.

It's also a great way to get killed.

 

[4metals]

Which is exactly why we try to avoid discussions involving cyanide. Perfect example, a patented process that can kill you.

Cyanide gives you a very limited number of screw ups, that is why it should only be handled by those who know how to do it and have the proper equipment to handle it.

 

[nickvc]

Which is exactly why we try to avoid discussions involving cyanide. Perfect example, a patented process that can kill you.

Cyanide gives you a very limited number of screw ups, that is why it should only be handled by those who know how to do it and have the proper equipment to handle it.

We definitely need a like button on the forum.
It sounds as if those that have and will use cyanide don't want to share their knowledge but I have to say that's not the reason.
This stuff kills, it gives very little leeway to make mistakes and you only make one unless your equipped to deal with it.
Leave this to professionals it's not a home refiners process!

 

[alexxx]

Which is exactly why we try to avoid discussions involving cyanide. Perfect example, a patented process that can kill you.

Cyanide gives you a very limited number of screw ups, that is why it should only be handled by those who know how to do it and have the proper equipment to handle it.

We definitely need a like button on the forum.
It sounds as if those that have and will use cyanide don't want to share their knowledge but I have to say that's not the reason.
This stuff kills, it gives very little leeway to make mistakes and you only make one unless your equipped to deal with it.
Leave this to professionals it's not a home refiners process!

ok... bump for the like button...

But still, I feel so sad in my heart that this knowledge is kept away from me... Especially when an "interesting and very emotional" thread is polluted by useless posts that is achieving the exact opposite of what was first intended.

I consider myself a professional in what I do, but I was not long ago a pure beginner.
I have learned lots of things during these 3 past years on this forum and in my current business as a recycler, and still learning new tricks everyday.
And all of us, at some point, were beginners in our respective fields...

For those of us who want to learn the inside outs of cyanide use, there might be a way ?
Any readings that you guys have in mind ? books ? Resources ? Do you really believe only a classic extensive chemistry formation is needed ?

Alex

 

[Gratilla]

Alex,

Here's a link that is informative about cyanide and its use:

http://webpages.charter.net/kwilliams00/bcftp/docs/cyanide.htm

It includes the statement:

"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."

Back to the patent. Sodium borohydride (familiar to the forum as a useful reductant for PMs) appears to be a viable alternative to activated carbon and zinc cementation for precipitating metals. Although I prefer to keep my posts short(ish), perhaps I should've included a bit more from the patent in my last msg.

"Fourteen liters of this gold-cyanide leach liquor was added to an appropriate sized beaker. Over a period of 65 minutes, 100 ml of stabilized sodium borohydride (4.4M in 14M NaOH) was added dropwise at room temperature to the stirred contents of the beaker. The precipitation of metals commenced almost immediately with evolution of hydrogen gas. Since the precipitation of metals was very slow, the pH of the liquor was lowered to 3.0-3.5 by the addition of H2SO4. the rate of precipitation of metal increased immediately. Upon analysis, the raffinate was found to have the following composition:"

The subsequent table showed 99% of Au & Ag and lower amounts of base metals as having been dropped.

It's not clear whether these excellent results required the low pH or that they could also have been achieved at the higher original pH (ie safe for cyanide use), albeit more slowly.

 

[g_axelsson]

A word of caution!

A patent is no guarantee that a process is working. The patent office never demands a working process, only that it would be new and different enough compared to earlier patent applications.

Nothing in the patent application needs to address safety.

I could probably apply and get a patent for smelting copper ore by blowing it up with a nuclear bomb and fractionally distill the vapor cloud. But that doesn't mean it is a working process or a safe way to do it.
A real example, Richard Feynman was awarded a patent for nuclear powered rocket airplanes driven by enriched uranium.

Göran

 

[Lou]

Thermal decomp is messy and crappy yielding. Some of the gold goes up in vapor and you get a purple mist everywhere.

Best in my opinion is a semi-batch/continuous setup: primary plating in a stainless tank with steel wool cathodes until you start producing too much gas at the cathode, then the largely stripped liquor (and all the dragout wash/rinses, if one is immersing parts) is drained and re-circulated through an ion exchange resin to pull Au/Ag, watching it on AA. The strip liquor is then freshened with more NaCN, sent to do the parts cleaning and the column stripped and that eluate containing gold is sent to either another electrowinning cell or hydrolytically destroyed (if thiourea) and the gold/silver collected as sulfides. The cathodes can then be melted as is if silver and no gold (insoluble iron can be fluxed out), or else treated in nitric acid to remove the silver, then sulfuric to remove the iron, then dissolved in AR or smelted to give 99% Au.

It will inventory metal, but that's the game.


Alexx, you may be better off using thiourea as a stripper than cyanide. Both are annoying, toxic, and sure to get people on your case.