Dissolving gold with HCl and TCCA

[Yggdrasil]

Hi!
My first test dissolving gold was done with HCl and TCCA.
The idea behind this, was to avoid neutralizing the HCl to quick.
TCCA came from Pool tablets.

The gold came from the end collecting from AP leaches.
Some of the gold was sticking quite persistent to the boards so they were partially scraped off,
which generated substantial amounts of solder mask and such.

Dissolving was done in a beaker on a magnetic stirrer. (No heat applied)
I processed it by, covering the material with HCl and then add small amounts of powdered pool tablets
now and then and let it stir for a few hours before letting it rest until it cleared up.
Then the liquid was siphoned off and a new run started, I kept this going until the liquid was clear.
The collected liquid was covered by some paper to keep insects and dust out of it.

When I was preparing to filter the solution,
I discovered that the paper had a brownish/yellow discoloration on the part that had been subjected to the
vapors from the liquid.

Do the gold chloride evaporate by itself in low temperatures (around 5 Celsius), or is it most likely acid fumes that reacts with the paper?

Next is to let the liquid clear completely and let it concentrate before dropping the gold.
How is gold best stored when accumulating? As a liquid or as wet gold powder?

Hmm I need some training to put images correctly in my posts. :wink:

PS! Do anyone have a proper description on the mechanism which makes SO₂ drop the gold?
Is it a displacement process, such as adding Copper/Zinc/Iron to a solution?


Edit to add comment and for clarity:

Since this is my first go at dissolving gold in any form I have no reference regarding smell looks and such.
There is a distinct faint smell from the gold chloride solution.
It was also a bit mesmerizing to look at the drops from the funnel,
during filtering to see the turbulent swirl in the liquid as the drops fell into it.
It seemed thick as a brine.
I'm afraid this may get addictive :roll:

 

[FrugalRefiner]

I discovered that the paper had a brownish/yellow discoloration on the part that had been subjected to the
vapors from the liquid.

Do the gold chloride evaporate by itself in low temperatures (around 5 Celsius), or is it most likely acid fumes that reacts with the paper?

The discoloration is likely due to acid / chlorine fumes.

How is gold best stored when accumulating? As a liquid or as wet gold powder?

If you break a container with gold in solution, you will lose some as it soaks into whatever surface it hits. At least with a powder,you can scoop most of it up.

PS! Do anyone have a proper description on the mechanism which makes SO₂ drop the gold?
Is it a displacement process, such as adding Copper/Zinc/Iron to a solution?

It's a reduction reaction, i.e., the SO₂ gives up an electron to the dissolved gold, allowing it to return to a metallic state.

I'm afraid this may get addictive :roll:

:lol:

Dave

 

[Yggdrasil]

Thanks Dave.

The discoloration is likely due to acid / chlorine fumes.

That was what I suspected, but just had to ask

For storage before melting, would one prefer dry powder or wet powder?
Or is it so heavy by itself that it will be stationary in the container even if dry?

I see now that filters, tissues, gloves and such will accumulate in substantial amounts.
Is it advisable to collect them in a container with a lid, before incinerating them?
Of course separate the gloves and other plastic/rubbery items from the papers ?

 

[g_axelsson]

If you suspect gold chloride on something just add some stannous to it and you will see the purple stain if there is gold chloride there. Do a test on your used filter and you will see the effect.

Göran

 

[Yggdrasil]

Hi Gøran!

I will do
My Stannous had gone bad I think, so I'm brewing a new batch.
Are there conditions that will create false negatives in the stannous tests?

 

[g_axelsson]

Excessive SO₂ creates a brown stain that can be mistaken for gold. Molybdenum seems to make blue stains. Apart from that I don't know of anything else than precious metals that gives a stain. That doesn't mean that there isn't something else that can give a false positive.

Göran

 

[Yggdrasil]

Hi!
My question was false negative
Are there conditions which will hide the gold in solution?

 

[g_axelsson]

Sorry about that... yes, excessive oxidizer (nitric or chlorine) will redissolve the precipitated gold. The purple is colloidal metallic gold so it can be dissolved again. Usually you see the purple color for a while at least but left to it's own it will disappear.
Spot plates, cotton tops or filter paper can have different reactions and timing on how the stain appears or disappears when you have excessive oxidizer.

Göran

 

[Yggdrasil]

Thanks.
I'll warm it for a while while reducing the volume.
I can retest it then

 

[Yggdrasil]

I have now put the rest of the gold foils into solution.


The foils was collected from mostly ram, clean cut fingers and some small boards that showed possible traces of gold on the non finger areas.
Rams and boards was depopulated with heat gun so they had quite some solder still on them.
The AP leach was started with this still on the boards, which may explain the slow start.
I added 30% HCl to the boards and some water and a splash of H₂O₂, started the bubbler and it took literally forever to start.
Almost a month went by before I added another splash of peroxide (a couple of milliliter), now it turned green within 15 minutes and kept going strong The temperature was hovering around freezing all this time.
This my first AP batch so I had nothing to start with.

When it was finished it had foils and quite a bit of black powder in it. (Some of the fingers had a cover of black powder clinging to them)
All was collected and cleaned in fresh HCl.

It should have been quite clean as it have been sitting in 30% HCl for more or less a couple of months.
The HCL was still clear and no bubbles have been noticed.

Still when it was dissolved, a significant amount of white precipitation appeared.

Is this Silver from the solder or some Tin salt?

It goes into the filter burn pile anyway

 

[facesfromparadise]

Hello,

I would say your white precipitate is more than likely tin paste from not removing solder prior to your process. Generally a good way to remove this and thus avoid tin paste is by prolonged treatment in hot HCl, once no solder is left or no reaction is seen upon addition of fresh acid you can decant this and then start your procedure with new acid, this is what I do.. there may be other ways though, the search bar is your friend!

Another note, if your white powder was/is silver chloride, it would discolor pretty quickly upon light exposure once filtered out of the chloride solution. Although if your boards were depopulated like you said the likelihood of there being silver in your feedstock would be minimal. Really the only components with a considerable amount of silver within the e-scrap realm (folks, correct me if I'm wrong I'm no expert!!) are keyboard mylars and MLCCs, which would have come off when heating with the heat gun.

Also as far as your black powder clinging to your boards after treatment with AP, it is likely that you used too much oxidizer (h2o2) in this instance, it really doesn't take too much to start dissolving gold as well as copper.. The gold will however cement back out on base metals if they are still present. Problem with this is that sometimes the gold will cement out INSIDE of the PCB itself (where copper tracings are present below the green solder mask), in which case it is extremely difficult to remove and get back all of your gold. This is one reason I think people generally try to avoid A/P'ing more PCB than they have to. Fingers themselves are fine, I usually just cut or crack them off of the ram sticks. Even if the components are heated off of the full sticks, the copper below the solder mask is still present and if you have any gold in solution by accident, that is where it will wind up. Also in regards to the tiny gold traces on boards... I chased these at one point as well.. it is good learning experience but it is really such a minimal amount of gold that in all likelihood you will be left wanting haha. It seems like you understand more or less what is supposed to happen with these steps but I would always advise studying more on this forum especially on the tried and true methods and techniques for things like solder removal issues with cementation etc. there is a lot to learn here!

Also I am not quite sure as to what would happen to the cyanuric acid that would be released with the chlorine when adding TCCA to HCl and using it in situ. Whether this would cause issues with precipitation or in other processes I'm not sure, let us know how you make out. I think a good general rule of thumb is to try not to over crowd your solutions with molecules and ions that don't need to be there. "crap in crap out"! Really, just get some bleach or KNO3 if you don't have access to HNO3. being conservative with the bleach and keeping track of pH, it works just fine. No need to reinvent the wheel as they say!

 

[facesfromparadise]

oh my goodness didn't realize how old this was! my apologies! hope everything worked out for you hahaha :lol:

 

[Yggdrasil]

Oops, I had forgotten the whole thread
I used appr 3ml 10 percent H₂O₂ in around 3L HCl.
So not much gold should have dissolved, it might been stuck from the initial application though.
The choice of TCCA was that it is an acid and would not ruin the Ph of the solution.
Not creating metastannic acid if tin was present was also a benefit.
I used way too much of the TCCA and also SMB.
One learns in time though
I have full access of HNO₃ so I'm using that on non Tin bearing material

BR Per-Ove

 

[im1badpup1]

Hi cyanuric acid precipitates out like that when tòo much tcca used. Im not stating for a fact thats what your precipitate is in this case.
But i know from a similar reaction ive performed countless times thats what happens.

 

[zachy]

How should it be done to dissolve the gold, using TCCA and HCL? Does this mixture dissolve the platinum? Or is it better to use HCL and H202?

 

[Geo]

Trichlor should be added in very small amounts due to the fact that it is a strong base and neutralizes HCl when added to it. Also, trichlor generates a huge amount of chlorine gas for a very small amount of dry solid. Trichlor contains around 90% available chlorine. That is a huge difference than chlorine bleach at 8% sodium hypochlorite. All metal is subject to attack by chlorine including heavy metals such as plutonium and uranium. It is the preferred ligand in most heap leach mining operations. It's easy to remove the free chlorine by simply allowing the gas to leave the solution on it's on or you can heat the solution to near boiling for a few hours to speed the process up.
Be very careful. Chlorine gas is both toxic and noxious. It is deadly to all forms of life.

 

[Yggdrasil]

Hi Geo!
Are you entirely sure TCCA are a base?
Its name is TriChlorCyanuricAcid.
I have used it with success on gold foils, not much needed,
I have never tried chlorine bleach though, which is a strong base
For PGMs I can not say.

 

[Geo]

Hi Geo!
Are you entirely sure TCCA are a base?
Its name is TriChlorCyanuricAcid.
I have used it with success on gold foils, not much needed,
I have never tried chlorine bleach though, which is a strong base
For PGMs I can not say.

"Trichloroisocyanuric acid is prepared from cyanuric acid via a reaction with chlorine gas and sodium hydroxide."
It would take a chemist to give the particulars but I can tell you that adding trichlor to HCl will raise the PH value.

 

[anachronism]

Hi Geo!
Are you entirely sure TCCA are a base?
Its name is TriChlorCyanuricAcid.
I have used it with success on gold foils, not much needed,
I have never tried chlorine bleach though, which is a strong base
For PGMs I can not say.

"Trichloroisocyanuric acid is prepared from cyanuric acid via a reaction with chlorine gas and sodium hydroxide."
It would take a chemist to give the particulars but I can tell you that adding trichlor to HCl will raise the PH value.

It has a pH of 3.

 

[zachy]

According to you, you can also dissolve platinum sponge with hydrochloric acid and sodium hypochlorite, it might work, to prevent evaporation of nitric acid.