Processing chips, black solution, need help!

Hey guys!

I'm processing some computer chips with relatively high gold content (2+ g/kg).
First, I dissolved the whole thing in nitric acid and heated it until NO production stopped. Got a dark blue/green solution that I filtered. Then put some AR on the remaining sludge to dissolve gold. Then I added some water before dropping with SMB, but I'm sure I added too much unfortunately.
No problem, I added some more water to dissolve SMB crystals, heated the system then poured off the top part. I got a brown precipitate that is a mixture of pure gold + metastannous in my opinion. So to dissolve the later, I poured HCl on the precipitate. It did in fact dissolve it but now the whole solution is black, any ideas why that might be? I'm guessing it may be some remains of the plastic carrier, some organic compounds reacting with HCl? Any idea how to get the gold? I hope it sinks to the bottom, but I can't see sh*t.
Maybe add some peroxide?

Thanks a lot for your precious time and help!

You didn't tell us what kind of chips they were.

Did you not incinerate them first?

Looks like you did everything wrong from the start.

Dr.xyz said:

You didn't tell us what kind of chips they were.

Did you not incinerate them first?

Looks like you did everything wrong from the start.

Click to expand...

No, I couldn't incinerate them, I only have access to hydrometallurgical methods.

By the way saying I did "everything wrong from the start" isn't helpful at all. I described the problem and the process, I need some ideas. Thanks.

Hi.
You Finnish people seems to be very blunt and direct ;-)
But of course what he says are very close to the truth, although not very helpful.

First of all, you said nothing of the amount of chips or kind of chips, which are not very important until you want to get to all of your gold. Initial leach of the base metals are often done with AP and/or HCl to avoid metastannic issues, especially when you suspect tin to be involved.

Testing with stannous are mandatory, in order to avoid throwing away your gold or the see if it as gone into solution.

Then put some AR on the remaining sludge to dissolve gold.

Click to expand...

Already here one may suspect metastannic issues.

No problem, I added some more water to dissolve SMB crystals, heated the system then poured off the top part.

Click to expand...

I sincerely hope you kept the solution, because this is where some of yor gold may have been. Stannous would have proved it either way.
The black color of your solution may well be dissolved plastic.
This is one of the things incineration could have prevented, and on the other hand most of the gold in chips are almost always in the bonding wirs that are inside the epoxy.

So basically you got off a bit on the wrong side this, but if you haven't thrown away the solution you poured off, all the gold is still there, and most likely most of it is still stuckinside the epoxy of the chip.
Read the threads from Tzoax he has posted very thorough processes for chips of many kinds, and his method for incineration are quite accessible for most.

My job is to determine the amount of gold in the chips. Unfortunately I don't have any information about their origin. I can't incinerate as I said, no instruments for that. I can only work with wet chemical methods.

Thank you for the reply. So any means of removing the black colour?
I indeed kept the solution and already tested with purple of Cassius, no reaction.


UPDATE: I waited a few hours for the gold to settle and decanted the pot, got a black powder that I dried, it turned into brown, so quite sure it is gold. The black solution wasn't that big of a deal after all.

I'm leaving for a week or so, but there are many better suited to answer your question.
Regarding the black color I have no good solution for you there.
The only thing I can think of, is to redissolve whatever fine gold powder in there with HCl and peroxide for instance and the filter it thoroughly, precipitate gold with SMB and decant the black solution.
Then you can continue washing your gold powder as described many places on this forum.
As always test with stannous chloride, before and after.

Still read Tzoax posts regarding chips, they are very informative, you may even find the number you are seeking is already in there.