# Where did my gold go?



## g_axelsson (Mar 19, 2013)

... oh, I think I found it ... let me tell you what happened.

It was a couple of years ago when I was quite new on the forum. I wanted to test process some ceramic CPU:s with HCl and then aqua regia. I put the CPU:s in a glass jar, added HCL and let it stand for a while, heating sometimes. After a while a lot of base metals were dissolved. I replaced the HCl and continued the slow dissolving.
Now I got two fiber CPU:s and I thought "What harm can it do?" and threw them in. So far no problem...
Finally I had so little base metals left I felt it was time for the aqua regia, my mistake was to not remove the fiber processors first. I made it quite weak as I wanted a slow process that could run unsupervised. If I only had known how long it was going to sit...

... real life interrups, lot of work, other interests, winter ... and before I knew it was two years later and I decided to start cleaning up some half way projects. This was easy, I saw that there were no metal left in the jar and the liquid tested strongly of gold. 8)
The fiber CPU:s didn't have a trace of pins left, you could look straight through the holes where the pins had been sitting.
Well, I washed off the auric chloride from the leftovers, filtered and dropped with SMB, got a beautiful powder in the bottom. Dried and weighed it... a dismal 0.3g from a bunch of pentium and other CPU:s. Where did my gold go?  
Back to the scrap, the ceramic bodies didn't have a trace on them, but on one of the fiber CPU:s there were some brown deposit together with a whitish hard crust on the die that had been facing the heat spreader. I put it under the microscope and behold, there were my gold. It had cemented onto the bottom of the fiber CPU. :lol: 


I then took the second cpu, it was almost totally covered by that white crust. I crushed it carefully and found more gold, not as much but all around the edge of the die.


Apparently some base metals had been left inside the fiber board and while it dissolved the gold it cemented onto the die. I also suspect that the white crust is silver chloride from silver brazing used to connect the die onto the heat spreader.

I suspect that there could be some gold inside the fiber boards, cemented onto the thin copper traces buried in the board. They will be incinerated in the summer to get any gold left inside.

I just wanted to share the experience of what happens when you have left over base metals after aqua regia process. Metals will cement back from the solution and it doesn't have to look as it did when it went into solution.
Another thing to learn from this is that don't rush, the gold will wait for you until you are ready to deal with it.

Göran


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## srlaulis (Mar 19, 2013)

Thanks for the very helpful real world scenario.


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## Geo (Mar 19, 2013)

that brown powder resembles cemented gold. if it were mine, i would dissolve it and test with stannous.


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## g_axelsson (Mar 19, 2013)

I thought it was clear, that brown powder is gold and massive too, couldn't knock it off with a glass rod with a melted end. It only got polished (I think it called burnished).

Another "real world" example is a jar with what I think is palladium in copper chloride. Strangely I couldn't cement it onto a copper plate, just made more copper chloride and a real mess. I finally tired on it and decided to kill the copper chloride with zinc. That batch is waiting for me at home, so in a couple of days I will know if there were any values or just a strange false positive.
... but that will be another post. :mrgreen: 

Göran


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