Gold button refining issue (black Layer).

[AceHaseeb]

Hello,
So I process 1Kg of DDR2/3 RAM sticks (ICs + trimmed fingers) using AR & I melted the acquired gold dust into this small 1.25g button. I had done AR twice, once to drop gold and once to refine/purify it.

However this button is not pure gold since its color makes it obvious (very dull and not golden like pure gold);

I wanted to purify this, so I did AR on this button (not premix, added nitric drop by drop) with small applied heat in a beaker, to my surprise, the button dissolves just a bit and then a black layer envelopes it, preventing further reaction. This black layer can be peeled off easily, it is very flaky and weighs nothing. here is an image of that layer;

Now I am unable to dissolve it fully without stopping reaction every 10 minutes and taking it out, peeling off layer and placing back in.
I need some help here to purify it, whats going on? TIA

 

[Martijn]

Did you wash the powder after AR?
It sounds like you have a substantial amount of silver in the alloy making a silver chloride crust.

 

[AceHaseeb]

Did you wash the powder after AR?
It sounds like you have a substantial amount of silver in the alloy making a silver chloride crust.

Well, I did AR twice and to knockout silver I did nitric reaction even before AR.
Assuming theres somehow still some silver in there, what can be done now?

 

[Martijn]

Silver is slighty soluble in HCL. But after AR there should not be that much left.

You can hammer it flat or roll it out in a rolling mill to increase surface for the acid to work on. This will allow the acid to eat through the piece lots faster. If it is silver.

Otherwise inquarting is needed. Did you treat the loose wires and foils in nitric or did you melt them first together?

 

[Martijn]

How did you melt your gold? Fuel, crucible, furnace, etc.

 

[AceHaseeb]

Silver is slighty soluble in HCL. But after AR there should not be that much left.

You can hammer it flat or roll it out in a rolling mill to increase surface for the acid to work on. This will allow the acid to eat through the piece lots faster. If it is silver.

Otherwise inquarting is needed. Did you treat the loose wires and foils in nitric or did you melt them first together?

i dissolved all gold bonding wires (from ICs) and foils (from trims) with AR. but before this i had done nitric process on both till completion.
Then i precipitated it and did AR again for purification.
I melted the 'purified' dust using 1 LPG and 1 butane torch combined in a small crucible with just a pinch of borax and sodium carbonate.

 

[Shark]

During your ar processing, at what point did you filter the ar? And what filter did you use?

 

[AceHaseeb]

During your ar processing, at what point did you filter the ar? And what filter did you use?

I did AR with sequential adding of nitric acid, let it sit for 2-3 hours, added few more drops of nitric after wards just to see completion; it was complete. Then i added few drops of sulphuric acid to knockout some lead. I waited for 1 hour, then i filtered it using a funnel and cotton. Denoxed it and added hot SMB solution, waited overnight for dropped gold. My AR solution was also yellow-golden (the usual color).

 

[Shark]

Sounds like some silver may still be in the button. Follow Martins advice in redoing the button and add better steps for removing silver.

 

[orvi]

Black colour is strange, since I know no common metal, which will be black when finely divided and does not dissolve in AR. Silver can in fact form crust, as AgCl isn´t soluble in AR, but AgCl is white and silver is the "whitest" metal known

There is one possibility - that you melted with pieces of silicon and alloyed gold with Si. Just tiny percentage of silicon in golod cause discolouration. And as Si isn´t soluble in acids (except HF), precipitate could be it.
Every chip has silicon die, so it could be the case.

 

[AceHaseeb]

Sounds like some silver may still be in the button. Follow Martins advice in redoing the button and add better steps for removing silver.

Alright, thank you very much. I will hammer the button flat and try nitric, if that fails ill just repeat AR.
Assuming nitric isn't sufficient for taking care of silver, what would you suggest i should do to avoid this problem in future, given the fact I did the nitric step till completion.

Black colour is strange, since I know no common metal, which will be black when finely divided and does not dissolve in AR. Silver can in fact form crust, as AgCl isn´t soluble in AR, but AgCl is white and silver is the "whitest" metal known

There is one possibility - that you melted with pieces of silicon and alloyed gold with Si. Just tiny percentage of silicon in golod cause discolouration. And as Si isn´t soluble in acids (except HF), precipitate could be it.
Every chip has silicon die, so it could be the case.

Thank you for responding. After dissolving the gold pins and foils in AR, I filtered the process with thick cotton/funnel method. The filtrate was clear (as in there was no solid matter inside it visually). Are you suggesting that in my filtration process, somehow some silicon slipped through a jungle of cotton? Because i only melted the dropped gold powder after 2nd AR with borax and sodium carbonate.

 

[Martijn]

No. Put it in AR after flattening. There will not be enough silver in the piece to go for HNO3.

I think the black could be from workimg in daylight?

 

[AceHaseeb]

No. Put it in AR after flattening. There will not be enough silver in the piece to go for HNO3.

I think the black could be from workimg in daylight?

View attachment WhatsApp Video 2023-05-30 at 9.42.44 PM.mp4
I think it is dissolving now after flattening, will update later.

 

[Martijn]

Great!

Before you filter, let the solution cool down, add a drop of sulfuric to push out any lead and then chill with some ice cubes.
This will push out any soluble silver chloride by diluting the HCl and cooling, lowering the solubility even further.
Filter a couple times through the same filter until crystal clear.

Then precipitate.

Let the powder settle or gently boil with a couple ml of sulfuric and decant when clear.
Test the clear solution with stannous.

Then gently boil with some HCl, only covered a bit, until no more yellow is visible.

Wash clean with boiling water and melt in a clean glazed crucible without any flux.

Enjoy!

 

[Shark]

Alright, thank you very much. I will hammer the button flat and try nitric, if that fails ill just repeat AR.
Assuming nitric isn't sufficient for taking care of silver, what would you suggest i should do to avoid this problem in future, given the fact I did the nitric step till completion.

Dissolve your cleaned material in AR.
Add Sulfuric or sulfamic, I use sulfamic as it does two steps, lead removal and nitric removal at the same time.
Dilute with water, or chill with ice.
Filter while cold. Cotton packed tight in a funnel will work. (remember that nitric and cotton and water, can produce gun cotton if allowed to dry out)
Drop your gold.
Clean you powders and melt.

How ever you choose to do it, remember to test with stannous regularly through out the process.

 

[Shark]

Also, show us a picture or short clip of the AR once your button dissolves fully.

 

[AceHaseeb]

The button refuses to dissolve.
Ive scratched its surface and you can see the gold content, it weighs ~ 0.98g now.

Just peeling off the weird layer and re-AR.

 

[FigiMon]

I have had the same happen with 18K scrap. In my case it was a layer of AgCl. Try boiling in ammonia, if the layer vanishes it is probably AgCl. In my case rolling the pieces super flat made them dissolve quickly.

 

[FigiMon]

Actually, looking at the pictures again, your button looks a little too light to be pure gold. Hard to say definitely but I suspect you have a bit of silver in there.

 

[Martijn]

The button refuses to dissolve.
Ive scratched its surface and you can see the gold content, it weighs ~ 0.98g now.
View attachment 57329View attachment 57330
Just peeling off the weird layer and re-AR.

Flatten it again and put it back in the acid.