problem of extracting gold from cyanide solution

[Yggdrasil]

Hello,
we recived a solution which is used in jewellery to dissolve small part of watch to get back the diamound.
It contain KCN, Au, Pd, ...pH more than 13. Solution is red/orange. It should contain around 300-400 gram of gold in 10-15 Liters.
First time we handle this material. We did on few mL some test.
So first we tryed electrolysis, with Pt anode and Ti cathode. It works and some material comes out.
However, the solution stay yellow/orange. It dosent change color even if we let the reaction run.
Addition of Zn dust, dosent give any reaction. There is no buble and no precipitate. It's maybe because of lack of free KCN.
Under fume hood, we add HCl to the solution which contain Zn. There is some buble and at some point the solution become colorless.
After letting it stand overnight, the pH go back to around 11-12, which we understand why and the color become light orange which we dont understand why.
We push the addition of HCl and Zn, until there is no more buble and pH is around 2. The solution dont become colorless again. It seems some copper fine powder or gold fine powder.

Off curse, we have some precipitate. So, we were thinking about this ways to hanlde this:
1. Take out most of metal with electrolysis and then wash the solution with activated carcoal to get everything. (we didnt try the carcoal yet)
2. Build a fume scrubber which contain NaOH or bleach and acidify the solution and heat it to get ride of the HCN. Then add Zn dust. But it seems we need to finish it with carcoal as well.

Any suggestion?

This sounds dangerous.
Have you measured the free CN?
There are meters for this.

 

[Swissgoldrefiner]

This sounds dangerous.
Have you measured the free CN?
There are meters for this.

No, i didnt, but as it dont react with Zinc and most of reference and even this topic sugest that the reaction to start need free CN, i supose there is no free CN.

 

[Yggdrasil]

No, i didnt, but as it dont react with Zinc and most of reference and even this topic sugest that the reaction to start need free CN, i supose there is no free CN.

That is strange since Zinc dissolves in both Sodium and Potassium Hydroxides
Are you sure you have Zinc metal?

 

[nickvc]

You could try adding some lye which should help the zinc to dissolve and if it does some of the gold should precipitate out, if that fails if you have access to cyanide add a little more and see if the zinc dissolves which again should precipitate soemof the gold.
Try a sample and see which one works.

 

[Swissgoldrefiner]

That is strange since Zinc dissolves in both Sodium and Potassium Hydroxides
Are you sure you have Zinc metal?

yep...zinc dust 63 mesh

 

[Swissgoldrefiner]

You could try adding some lye which should help the zinc to dissolve and if it does some of the gold should precipitate out, if that fails if you have access to cyanide add a little more and see if the zinc dissolves which again should precipitate soemof the gold.
Try a sample and see which one works.

In 911metallurgist they suggest as a precipitant activator, sodium bisulphite is of practical benefit where insufficient alkaline cyanide is present.
It s too long to read all the blog, but i am working on it. At some point they suggest that "Excess alkalinity must be neutralized to about pH 6.6."
Which seems strange, too close to acidic condition.

 

[Yggdrasil]

In 911metallurgist they suggest as a precipitant activator, sodium bisulphite is of practical benefit where insufficient alkaline cyanide is present.
It s too long to read all the blog, but i am working on it. At some point they suggest that "Excess alkalinity must be neutralized to about pH 6.6."
Which seems strange, too close to acidic condition.

No, it is acidic when it pass 7, in the range where Zinc is the least soluble.
Zinc is at its lowest solubility from 6.6 to 7.9 if I'm not mistaken.
So it is madness to push it so low.
Better lift it to 10-14 range and it should be safe and the Zinc will react well.
Nick has used Cyanide if I'm not mistaken, so take his advice.
Edit to fix spelling

 

[Swissgoldrefiner]

Next day coloration, color looks like copper dust. In the bottom, there is two kind of powder, one black and one "red".
It may be, the Au and Pd, as well as some Cu (it was 18K white gold with 12.5% of Pd).
For sure it's not Zn, as when i add more Zn, free HCl react and i have Hydrogen buble forming.

 

[anachronism]

You could try adding some lye which should help the zinc to dissolve and if it does some of the gold should precipitate out, if that fails if you have access to cyanide add a little more and see if the zinc dissolves which again should precipitate soemof the gold.
Try a sample and see which one works.

Bang on the money. Try some NaOH to get things moving.

 

[Swissgoldrefiner]

Bang on the money. Try some NaOH to get things moving.

What about electrolysis and then activated carcoal?

 

[anachronism]

What about electrolysis and then activated carcoal?

There are loads of ways to to recover the gold including those you mentioned. If you use the electrolysis correctly you'll get 99% of the gold out. Personally I use bunched up strips of Aluminium foil to recover the rest. Nick suggested I give it a bash a while ago and it's become my "go to" scavenge method.

 

[Swissgoldrefiner]

There are loads of ways to to recover the gold including those you mentioned. If you use the electrolysis correctly you'll get 99% of the gold out. Personally I use bunched up strips of Aluminium foil to recover the rest. Nick suggested I give it a bash a while ago and it's become my "go to" scavenge method.

But, after electrolysis, solution color goes from red/orange to yellow orange, is it normale to have that color?
In my opinion, if there is no metal, no color. I mean, usually when i go it with copper nitrate and push the electrolysis, i get clear solution.

 

[nickvc]

As a fast check to see if you still have gold in the solution simply suspend a strip of aluminum foil in it, if it comes out still silver colored chances are you have most of if not all of the gold out if it comes out with a brown tinge then you still have gold in the solution.

 

[anachronism]

But, after electrolysis, solution color goes from red/orange to yellow orange, is it normale to have that color?
In my opinion, if there is no metal, no color. I mean, usually when i go it with copper nitrate and push the electrolysis, i get clear solution.

We may be at cross purposes I'm talking about an alkaline Cyanide solution not AR. Oh and colour is no guide.

 

[Swissgoldrefiner]

As a fast check to see if you still have gold in the solution simply suspend a strip of aluminum foil in it, if it comes out still silver colored chances are you have most of if not all of the gold out if it comes out with a brown tinge then you still have gold in the solution.

I will try that. However, the test way from Hook, didnt give any results, with SnCl2 and with coperas.
And yes, i add HCl, boil it, let it cool...
But after addition of HCl, i have some precipitate, which looks like AgCl, but not exactly.

 

[Swissgoldrefiner]

We may be at cross purposes I'm talking about an alkaline Cyanide solution not AR. Oh and colour is no guide.

I am talking about alkaline cyanide solution as well. So, where the color comes from? It's sure some thing metalic, as at some point when i was playing with another sample the color desapear.

 

[anachronism]

I am talking about alkaline cyanide solution as well. So, where the color comes from? It's sure some thing metalic, as at some point when i was playing with another sample the color desapear.

Ahh thanks for clarifying. Strangely the colour doesn't come from the gold it actually comes from other metals in the solution. Pure Gold cyanide solution is colourless. It's almost clearer than water.

 

[Swissgoldrefiner]

Ahh thanks for clarifying. Strangely the colour doesn't come from the gold it actually comes from other metals in the solution. Pure Gold cyanide solution is colourless. It's almost clearer than water.

Great news, it help me to know that it should be colorless. I am sure there is Pd as well there, do you think the color comes from Pd?
I am preparing some DMG solution, i let it cool overnight and will try to the sample. Do you think DMG works in cyanide solution?

 

[nickvc]

I doubt you have much if any palladium in your solution as cyanide needs to be really hot to dissolve PGMs but you stated it was used on 18carat white gold so chances are there are base metals in your solution , I’m not sure why they used cyanide , AR would have been much quicker .

 

[Swissgoldrefiner]

I doubt you have much if any palladium in your solution as cyanide needs to be really hot to dissolve PGMs but you stated it was used on 18carat white gold so chances are there are base metals in your solution , I’m not sure why they used cyanide , AR would have been much quicker .

I agree AR would be much better, but they dont have acces to AR. It's famous swiss watch company. They probably work like this since years.
The stripping company claim that the solution can dissolve: "gold and it's alloy, Silver, copper, brass, nickel, palladium-nickel and rhodium by glitter (dont understand what that mean) on iron and/or inox, without attanking the iron and inox"