Crystallized Nitric Acid

[pokermandown]

I have been processing pins, sweeps, fingers and cpu's for over a year. I have made plenty of mistakes and have found this forum to be outstanding. Particularly so when things don't go perfectly, which is really lacking in Hoke's book. I have searched and searched and can't find any discussion about what went wrong with my latest batch. I had 600 grams of heavy duty slightly magnetic pins from some test equipment from the 1980's. They are close to 2 inches long and AP will not penetrate the gold plating.


Here is my process:

1. Burn off the oils with a propane torch - glowing slightly red - not even close to melting temp
2. Break into to two 300 gram batches place each in a 2L beaker
3. Cover in distilled water 225 ml
4. Place on hot plate that is turned off
5. Cover with watch glass
6. Add 67% nitric 25 mi at a time while reaction is vigorous

7. turn on hot plate
8. Add nitric 3 ml at a time when nearing completion. I typically wait 90min to 2hrs between additions

When it appears there is no reaction, I let the hot plate run for 8 hrs just to make sure I didn't get over anxious. Then I turn it off and let it cool. I have learned recovering gold is not for the impatient. I turned it off at 8 am yesterday and when I got home from work it was raining and 50 degrees F. I went to look at the solutions and one looked different than the other.

One of the two crystallized!

There is perhaps 150ml of liquid still in the beaker and the rest is a paste.
The other beaker looks perfectly normal.
Is there a discussion of this on the forum? Any idea what I did or should have done? Thank you for your help!

 

[Topher_osAUrus]

Yep.

Thats why Hoke states to dilute the acid after reaction, before decanting. To prevent crystallization of the metal salts in solution.

Add some water, maybe warm it up a bit, let it settle and go from there.

 

[g_axelsson]

If dilution doesn't work then I would look into metastannic acid, it can really gum up a reaction. But try to dilute first.

Nice pictures and good information, you surely does know how to ask a question.

Göran

 

[Topher_osAUrus]

I was thinking the same thing!

If only *everyone* posed their problems as well as he did..

And, I thought it looked like tin paste at the bottom as well, so he will probably have to deal with that after dilution. Although, the pins in the picture looked pretty clean. I guess it doesn't really take a lot of tin to cause a lot of heartache.

 

[g_axelsson]

The tin is usually in the alloy, for example in phosphor bronze.
https://en.wikipedia.org/wiki/Phosphor_bronze

Göran

 

[aga]

If the two beakers were pretty much the same, might be a good idea to add a little water to the 'good' one just in case it also decides to go gummy overnight.

I have learned recovering gold is not for the impatient.

Amen to that !

Impatience definitely makes it take longer. Best thing to do is wait as fast as possible.

 

[pokermandown]

I will dilute both and heat. Hopefully I can reverse the process. Thanks!

 

[pokermandown]

Solved! I doubled the volume and heated it for 60 minutes. All the crystals went back into solution. Thank you all for the input!

 

[Topher_osAUrus]

Thats good to hear! Good luck with the rest of the process.