Processing old transistors.

[Makcc]

Hello,
I processed about 400 grams of transistors, as attached, in diluted aqua regia.
Everything went into solution as expected and solution was brown. I added about 100 ml extra aqua regia.. Just in case.
I removed the nitric with Urea until solution was quiet and free of nitric. Stannous was positive for gold.
I added SMB and waited for the solution to settle. I decanted and there was nothing in the bottom of the beaker. Test on Stannous was negative.
I added some steel nails (I didn't find copper) and this resulted in black precipitates. I filtered and dried and added dried precipitate to aqua regia. There was immediate reaction but quite slow.
I added more aqua regia but the reaction stopped. I heated..
The solution tested positive for gold but there's a significant amount of black residue undissolved at the bottom of the beaker.
Any suggestions as to what this residue might be?

** I processed the aqua regia for the second digestion, showed black on Stannous .. De-noxed and added SMB... Nothing happened... No ppt.

 

[Geo]

"Everything went into solution as expected and solution was brown."
This was your first clue that something was not right, If all the metal was dissolved, it should have been closer to green, not brown. Can you take a picture of your positive stannous test and post it here?

 

[Arthur Brown]

What did you do the the transistors before they met the acid? How much precious metal are you hoping to find? Gold on internal connections is unavailable unless you start to break the transistors open. Gold on external parts will be negligible. If these are plastic (black epoxy usually) then they need machining open in (probably) a ball mill, til they are dust.
Overall your yield will max out at about 0.05% very likely you will get a lot less than this for general purpose transistors.
If you could get a tonne of transistors then treat it as ore, crush it and density separate it (shaker table?), then use chemicals.

Minimise chemical use and cost by maximising the mechanical separation.

 

[vgecas]

Hi,
I do not see what type of transistors you have dealt with, but because you went to AR straight, i assume they were fully metal. I would process such like this:
Burn off any paint, lacquer and anything that burns. Remove base metals as much as i can using nitric acid with an addition of little sulfuric acid, then use aqua regia for the remaining.