Problem precipitating gold out of KCN gold stripping solution

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ItayEdry

Member
Joined
Jul 23, 2018
Messages
8
A recent project that I had in my workshop required me to remove gold (probably 22k) out of a set of silverware (sterling silver). To do that I tried a few methods and the winning one was a potassium cyanide gold stripping solution sold by a plating supplier. The project was successfuly done and now I am left with 7L of gold bearing solution that I would like to recover the gold from. I have been reading the C. M hoke book and learning the art of refining but I am new to that and I ran into a problem of not being able to precipitate the gold out of the KCN solution. More precisely, working with 1L of solution, I was able to precipitate some gold. I used 325 mesh zinc powder for the precipitating process but nothing happened so I added 3% hydrogen peroxide (outside with a fan blowing on it away from civilization and with a fume grade mask). When adding the hydrogen peroxide I had some precipitation and I thought that all the gold in there precipitated. Now after getting the end result of the refining process I only have about 3g of fine gold and that leads me to suspect that not all the gold was precipitated since the 1L of sulotion can hold up to 60g of gold. Any advice on how to correctly precipitate the gold out of the solution will be welcomed.

Some more technical details :
-ph of solution is about 12
-I used 325 mesh zinc then added zinc bar to stand over night.
-I experimented with small amounts of 10ml solution and observed precipitation by adding hydrogen peroxide to the solution without adding any other metal (my thought is that the KCN turned into HCN those leaving free gold molecules, but no chemist so I'm not sure)
-reading about it I realize that KCN can also desolve silver so I don't really expect to get 60g/L of gold but 3g seems way too low considering the thinness of the gold plating.
 
A few questions.

How do you know how much gold you're likely to get?
What's you're overall expected return and what's it based upon?
Why did you add peroxide when trying to precipitate- what was your thought process there?

Jon
 
ItayEdry said:
A recent project that I had in my workshop required me to remove gold (probably 22k) out of a set of silverware (sterling silver). To do that I tried a few methods and the winning one was a potassium cyanide gold stripping solution sold by a plating supplier. The project was successfuly done and now I am left with 7L of gold bearing solution that I would like to recover the gold from. I have been reading the C. M hoke book and learning the art of refining but I am new to that and I ran into a problem of not being able to precipitate the gold out of the KCN solution. More precisely, working with 1L of solution, I was able to precipitate some gold. I used 325 mesh zinc powder for the precipitating process but nothing happened so I added 3% hydrogen peroxide (outside with a fan blowing on it away from civilization and with a fume grade mask). When adding the hydrogen peroxide I had some precipitation and I thought that all the gold in there precipitated. Now after getting the end result of the refining process I only have about 3g of fine gold and that leads me to suspect that not all the gold was precipitated since the 1L of sulotion can hold up to 60g of gold. Any advice on how to correctly precipitate the gold out of the solution will be welcomed.

Some more technical details :
-ph of solution is about 12
-I used 325 mesh zinc then added zinc bar to stand over night.
-I experimented with small amounts of 10ml solution and observed precipitation by adding hydrogen peroxide to the solution without adding any other metal (my thought is that the KCN turned into HCN those leaving free gold molecules, but no chemist so I'm not sure)
-reading about it I realize that KCN can also desolve silver so I don't really expect to get 60g/L of gold but 3g seems way too low considering the thinness of the gold plating.
The best proprietary cyanide gold strippers contain ingredients that prevent attack on mainly copper base materials. They might also work on silver, but I haven't tried that. These are designed for stripping faulty gold plating, without damaging the base metal, so the parts can be replated. Although they are occasionally used for gold recovery, that's not what they're designed for. These added ingredients bind the gold very tightly and make it extremely difficult to separate the gold. To make a long story short, those standard zincing methods won't work on many, or all, of these proprietary strippers.

The most difficult one I've ever used is Techni-Strip AU, made by Technic. Although I was in contact with Technic, they never gave me enough information to drop the gold. They sell a chemical, which is rumored to be sugar, which is supposedly added to allow the gold to be plated out, but it never worked for me. Also, a company named ACR (American Chemical and Refining, I think) developed an additive to enable the recovery of the gold. There may be a patent on it. These stripper companies want people to send the solutions to them for gold recovery, so maybe they're not trying too hard to help you do it yourself.

Contact the manufacturer and talk to someone in the lab, if possible. They might tell you how to do it.

Somewhere in the 30 gigs of 2000+ articles and books on chemistry and PMs I have accumulated, there is a long hands-on article on using sodium hydrosulfite (sodium dithionite) to drop the gold from these difficult cyanide strippers. Although I haven't used it, I've seen it used with success, although they kept the details of it's usage secret. Unfortunately, I can't find it. I do know that I copied it off the sciencemadness.org forum. I couldn't find it on there, either, but I know it's there, unless they removed the older posts. Maybe Lou has it. I do know that it's somewhat dangerous.

Here's some past threads on the subject
http://www.goldrefiningforum.com/phpBB3/search.php?keywords=techni-strip+au&terms=all&author=goldsilverpro&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

And a couple of patents using hydrosulfite
US3271135 and US3271136

And an article from Science Madness (see 1st page), but it's not the one I was looking for.
http://www.sciencemadness.org/talk/viewthread.php?tid=5700#pid80768

On the Techni-Strip AU MSDS, they list lead oxide as an ingredient. Maybe that's the culprit.

These proprietary gold stripper work wonderfully well. They make stripping a very simple chore, compared to the NaCN/H2O2 combination that I use. However, the gold in the NaCN/H2O2 combo is very easy to zinc out.
The moral is, never use a proprietary gold stripper, if you want to be able to get all the gold easily.
 
Thank you for your help goldsilverpro. It does look like what I am dealing with along the lines of the Techni-Strip AU, especially because I bought it from a plating supplier. It also seems like the best solution for extracting the gold out of the solution would be to incinerate the material after evaporation yet I couldn't find any info about what temperature will need to be used and if HCN is expected to be part of the emission. If you have additional knowledge about those missing parts I would love to know before I attempt incineration.

Jon, those are good questions. here are my answers:
Looking at the items that I had to remove the gold from I estimated (by looking at it only, no scientific process) that each one of them is plated with 1.0 - 2.0 grams of gold. The gold plating was very heavy well above the 5 micron that is considered heavy plating today. going by that very rough calculation it took 1 liter of solution to remove the plating from 15 - 20 pieces. Therefore the minimum expected will be 15g and the maximum will be 40g of what ever carat gold used for plating. I was told that the plating is 22k, if that is true then the minimum gold expected after refining would be 13.7g max 36g. Those calculations are backed by the fact (or claimed fact) that 1 litter of solution can hold up to 60g of gold. In conclusion 3g of gold out of 1L solution seems a bit low.
The reason I added hydrogen peroxide was because it was mentioned in CM Hoke's book as a method of accelerating gold drop out of alkali cyanide base solutions (though I can't find the page right now but it is in the early chapters).
 
Thanks very much for the answers that's really good of you to be so open.

A few more points if I may.

What product have you got where you think you may have over a gramme per item? I'd love a pic. If it's some kind of enterprise Telecoms then I'm with you on this but for the majority of kit it just doesn't happen.

FYI gold plating on IT connectors is in the main fine gold.

Yes if a solution can hold a particular amount then you need to contrast that with the fact that it tends to be an amount of metals rather than gold. Also the stronger the cyanide leach, the more aggressive it is, and the more metals it will attack.

I'm probably going to be pilloried here but after 7 years I think I can get away with it but I've never actually read Hoke. It's out of date and irrelevant (apart from some great testing) with regards to e-waste and modern products because most of this stuff, and the relevant interactions between elements weren't around when it was written. It had it's place and that was with a particular product set. Suffice to say I use peroxide as an oxidant- which dissolves gold in the presence of free CN- ions so that was why I asked. If you have free oxygen in a solution with CN- and gold present then it's gonna dissolve.
 
I had considered that it may be something along the lines of needing a small amount of free cyanide to actually initiate the precipitation.

Apart from the hcn that is produced, why can't you just add sulfuric acid?


Sent from my iPhone using Tapatalk
 
Some notifications (besides bothering others): Where is the stripped silver?
GSP, SHS works well, but it does the same as zinc, not better and it gives a
smell during CN reduction (probably some CN + S + O compound)
The zinc bar was not colored? Then there is no more gold. To ascertain it
I would add some caustic soda and put an aluminum sheet inside (more active than zinc)
And yes, Hoke is as actual as ever
 
I attached a couple of pictures. I wasn't dealing with e-wast but with an antique Tiffany's silverware set. The product I used to strip the gold is Davis-k gold stripper. I find that the CM Hook book is very useful for me since I am a jeweler and I could use most of those techniques to refine my own metals and reuse them. I should add that the silverware is made from sterling silver and that the gold stripper did an amazing job removing the gold from all the surface including the deep crevices with no damage to the silver. Now, I am also certain the some silver was dissolved as I have seem evidents to it so for sure the amount of gold / liter will be less then 60g if 60g is the max that the solution can hold. Also my estimation of the gold weight for the plating comes from experience in plating and buffing out gold plating (which buffing out the plating did not work for this project since the gold plating was so heady)
 

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Lino,
I'm not sure that I understand your method of recovering the gold in my case. I did you zinc to try and drop the gold but that did not work. The zinc bar that I left inside the solution turned black.
How will caustic soda help recover the gold in that case?
Wouldn't adding sulfuric acid be super dangerous since it will release huge amounts of HCN?
If possible I would like to recover the gold and live to see another day, otherwise I'll just forget about the gold and live to see another day
 
Yep Hoke was written for doing things like jewellery that's for sure, so it's a heck of a lot more relevant for what you're doing than a guy doing ewaste. 8) 8)

Can you get the MSDS sheet for the Davis K gold stripper?
 
According to description, the black color on Zn bar is silver, hence scrape it and return bar to solution and maybe a yellow hue will appear. Anyhow not much gold is expected, 3 g look fine to me from the forks described
 
Yes, sulfuric acid will release hcn, so if you aren't appropriately ventilated then don't. But a small aliquot will give you recovery information.

And I wondered the same lino, about the silver. This is concentrated kcy yes? So it should be leaching silver as well?


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Lino, I am not sure how much of the silver was dropped as I have not refined it yet and it is sitting inside a nitrite acid solution.
I am not prepared to deal with HCN therefore I wouldn't like to add acid into the solution. The question that I am trying to find an answer to is, will HCN be release if I evaporate the solution into a crust and incinerate it? also what will be the best method of doing so, torch or induction furnace?
I also added the tech sheet for the David-K mighty stripper
 

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This plan is unaccepted to me. Gold should go out by other means - easily (if present at all). Silver dropped? How
 
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