Black film on gold plated e-scrap in HNO3

[devin]

Hello friends. I am learning to recover PCM's, primarly gold right now. I tried a few experiments using nitric acid to seperate the gold flake from the base metals on misc. gold fingers and gold plated socket wires from misc. computer parts. I have come across some pins that are very old and look to be solid gold and quite thick. When these pins are put in the 70% nitric acid there is no change in size, no foils, no base metals being eaten. I black film coats these gold pins. What is the black film? If I put the blackened pins in fresh acid the pins will become golden again, then go back to black. Ive searched and have not found anything remotely close to what I am experiencing. Please come to my rescue!

 

[butcher]

devin,
"I am learning to recover PCM's"
Are you referring to Platinum group metals (PGM).

Gold will not dissolve in nitric acid, if the pins are heavily plated, the acid may not be able to penetrate the gold easily.

If there was a trace of chloride in your nitric acid (even from chlorine in water) you could have dissolved some of the gold, I suspect that is what the black powder you see is.

You could do a test, cut a few pins and test them in a test tube, heat test tube to saturate the small amount of solution with metals, using same acid, then take a few drops of solution, in a spot plate add a couple of crystals of ferrous sulfate and test solution for gold, I suspect you have some chlorides involved here.

 

[Lino1406]

When you submerge electronic waste
in nitric acid, at very short time the
exposed copper will dissolve violently.
Have you reached that stage?

 

[gurudatta]

devin,
mr butcher had given you correct answers.there is chlorine in your nitric acid which form silverchloride grey black coat or masking on your e scrap.

 

[Geo]

did you check the pins with a magnet?