Why doesn't the ar fizz when urea is added?

[LWR]

Hi there, I've been trying to refine some gold and am having a bit of trouble getting the Aqua regia method to work.
I've used the nitric acid + hydrochloric acid. And when I add the urea to get rid if the nitrates it doesn't fizz or give off fumes. And when I try to precipitate the gold with SMB the liquid goes brown and then slowly returns to its original colour. So I added heaps of urea and tried again with the SMB and the same thing happened.
Every where I've read about it ,the urea is meant to fizz or give off fumes.
And I'm using laboratory grade urea.
Can anyone help?. Any info you can give would be much appreciated. Thanks.

 

[FrugalRefiner]

LWR, there are several better ways to eliminate excess nitric in an AR solution. The first is to avoid using too much nitric in the first place. Many members have written about adding nitric a little at a time until the solids are dissolved. Harold taught us that adding a button of refined gold will use up any excess. Many members use sulfamic acid to eliminate the excess nitric. Lou taught us that urea doesn't really destroy excess nitric. Hope that helps.

Dave

 

[jeneje]

LWR, at this point in your process you would be better off to use a copper bus bar to cement out your gold, filter and refine the black powders again. While you cement out your gold with copper, it would be a great time to download CM Hoke book on refining precious metals and do some reading. I think frugalrefiner has a link in his sig line.

Ken

 

[MarcoP]

Yeap, this time I remembered correctly but went to find the original sources just to be sure. Urea only eliminates NO2(g) dissolved in solution not nitric adic, so you still have free nitric in solution.

At this point of your journey you could go the sulfamic acid way (as FrugalRefiner suggests), via the classic evaporation-to-syrup cycles or cement with copper.

Edit: Strange you are worried more about the frizzing problem then the fact that "gold won't drop".


Sources:
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=1294&p=10853&hilit=urea+no2#p10853
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=5001&p=42977&hilit=urea+no2#p42977

 

[Palladium]

How much gold? What type of materials? Stannous?