# UREA and the nitrate



## Golder (Mar 10, 2009)

I have a problem to understand the effect of urea-using to neutralize the after-AR-goldchloride-solution to prepare it for precipitation of gold. I know, that the urea catches the nitrate ions. Urea is not necessary for neutralization of the HCL in excess, right?
Instead of using HCL in excess (the classic way) to drive the nitrate away from the solution under heat in the way of using urea I have the nitrate furthermore in my solution, right?
If it is so, then my question is: If I reheat my urea-neutralized solution for whatever, I get perhaps some free nitrate ions, which can destroy the precipitated gold togehter with the the chloride ions?
Or if I use the HCL-wash premature, will the chloride ions separate some nitrate ions which is dangerous for the gold?

Thank you for the support!
Golder


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## ps_jb (Mar 10, 2009)

Golder said:


> I have a problem to understand the effect of urea-using to neutralize the after-AR-goldchloride-solution to prepare it for precipitation of gold. Golder



I think that urea is used as a reduction agent for nitric acid. So it's just oxidation-reduction reaction of urea with nitric acid.
Most probably the products of such reaction are gases from decomposition of urea and HNO3. So there is no NO3- in solution.


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## Golder (Mar 10, 2009)

Hello ps_jb,

yes, HNO3 will be reduced to ammonia and some other gases (CO2) by the neutralization with urea. But:
1. What will happen with the new nitrates which already in existence after the AR-process? The classic way destroys these all with HCL in excess. The urea will not destroy this. And some new HCL for wahsing the gold powder can reactivate the AR-process by the rest nitrates, known as poor-mans AR, but here with maybe iron-nitrate or copper-nitrate.
2. HNO3 will be made from ammonia and oxygen. Maybe there is a danger for new HNO3 in traces?

Should I have this scruples?

What about a mix-way: Some urea AND a little bit HCL in excess to be sure, get all the gold from the solution?

Golder


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## Harold_V (Mar 10, 2009)

Golder said:


> What about a mix-way: Some urea AND a little bit HCL in excess to be sure, get all the gold from the solution?


I never used urea, so I may not have the full picture, but what I can recommend is to use a small piece of added gold when evaporating. By providing an excess of HCl, any unused nitric will be consumed by dissolving the added gold. I can think of no other method that works so well and is pain free. I used that method for years. 

The only other viable option is to add nitric sparingly, until all of the gold has been dissolved. That is the method that is advised by GSP. The problem with that system is that if you are dealing with fairly large volumes, as the amount of un-dissolved gold diminishes, the solution reacts slower and slower with the remaining gold, due to the increased volume of solution diluting the small amount of nitric that may remain, and the decreased amount of surface area of gold remaining. It would be very easy to add too much nitric and not realize you have done so. I would always recommend the addition of a solid piece of gold in evaporation, even if his method is incorporated. It insures you do not leave nitric behind. 

Harold


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## ps_jb (Mar 10, 2009)

Golder said:


> Hello ps_jb,
> 
> yes, HNO3 will be reduced to ammonia and some other gases (CO2) by the neutralization with urea. But:
> 1. What will happen with the new nitrates which already in existence after the AR-process? The classic way destroys these all with HCL in excess. The urea will not destroy this. And some new HCL for wahsing the gold powder can reactivate the AR-process by the rest nitrates, known as poor-mans AR, but here with maybe iron-nitrate or copper-nitrate.
> ...



1) Cu(NO3)2 and Fe(NO3)3 are very good water soluble salts. So just wash your Au powder well - and then use HCl. In general, you are right - HCl and HNO3 are practically equavalent acids by ability to produce each other from the salt. So if you will have some amount of nitrates - HCl will definitly produce HNO3.
2) No way to make HNO3 from ammonia and O2 and reaction conditions we are talking about. For that purposes you need huge pressure, T about 700-800C and catalysts.


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## Golder (Mar 17, 2009)

Harold,
this could be the "Rich Man's Trick" (RMT) . But current I only have 1 gram of newbie-gold, which rather will precipitate gold instead of
reducing the NO3-Ions 
But seriosly: It is also a smart way, to consume NO3 in excess with pure gold, I am sure. Thank you!
I am working with a variation of your second option: I use nitric very sparingly, then if it shows no more reactions I decant the whole used solution and add completely new AR in a smaller volume then before to dissolve the rest of the scrap. Theoretically the mostly gold will be in the last decanted solution. But I collect all the solutions for preparing the precipitation.

ps_jb,
thank you for your comments! It is also my opinion. Never wash the precipitated gold with HCL before washing extensive with water. If I am afraid that I have redissolved some gold, I am adding some SMB after the HCL-wash. Nitrates near HCL are really the poison for every precipitation of gold. I have learned, that this turning point is very important on which I want that the solution after eating all scrap, gold and other metalls very fast is no more eating a small bite of metalls. From one minute to another. To convert the aggressive solution that eats all the scrap I put in it into a peaceful solution that gives me back my gold I will work very patiently and extensive.

Golder


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## goldsilverpro (Mar 17, 2009)

Harold_V said:


> The only other viable option is to add nitric sparingly, until all of the gold has been dissolved. That is the method that is advised by GSP. The problem with that system is that if you are dealing with fairly large volumes, as the amount of un-dissolved gold diminishes, the solution reacts slower and slower with the remaining gold, due to the increased volume of solution diluting the small amount of nitric that may remain, and the decreased amount of surface area of gold remaining. It would be very easy to add too much nitric and not realize you have done so. I would always recommend the addition of a solid piece of gold in evaporation, even if his method is incorporated. It insures you do not leave nitric behind.



There's a little art in this. After a few times, you know when it's nearly finished and when it's finished. After a few times, the method feels normal. It fools you very rarely, but usually because you screw up your calculations or measurements. Just remember, it takes both HCl and HNO3 to dissolve gold. They are used up at individual rates as they dissolve the gold. When either is spent, the reaction stops.

The trick is to start with the scrap covered an inch or so with straight HCl. This is heated and a small amount of nitric is added. The reaction will fizz and foam a bit. When this dies down, give it a stir and add a little more nitric. As time goes by, make your nitric additions smaller. At some point, an addition of nitric will produce no reaction. At this point, you could be out of nitric or, you could be out of hydrochloric (muriatic). You don't know which.

You add a splash of HCl, heat, and observe. If it reacts, there's nitric left. When it slows, add a little nitric. Repeat all this until neither HNO3 or HCl produce a reaction. You are finished.

The back and forth thing can be avoided by using an excess of HCl to start with. That way, when an addition of nitric produces no reaction, you know the gold is dissolved.

At first, I always calculated the gold content and measured out the nitric before starting. It never was 100% right on, but it was close. The HCl was just estimated. The guy that taught me this just poured the additions from a plastic jug. He'd been doing it for 20 years and he never overshot.

I also poured off the acid, when it got saturated, a couple of times during the process and put on fresh HCl- then HNO3. The amount of HCl you use depends on how much gold you have left. I never let the acid level be more than an inch or two above whatever I was dissolving.

Keep it hot but don't boil it. The boiling action masks the dissolving reaction and makes it very hard to see. After any addition, give it a minute, or so, to heat up again before you observe the reaction. Only when the acid is hot can you trust the absence of a reaction.

When dropping the gold from AR solutions using this method (or, any other method), you can stand a little free nitric. However, there is a limit to this, since a little HNO3 kills a lot of sodium sulfite or SMB. I used sodium sulfite to drop my gold. It first reacted with any excess nitric - didn't affect the purity at all. When the nitric was gone, the gold dropped. I assume it will work the same SMB.

Mainly, I'm talking about karat gold. However, I use the method for most all forms of gold. Many times, you don't know how much gold you have and have to play the additions by ear. To keep nitric additions small, I may use a 10ml beaker or an eye dropper, at the end of the dissolution. The bigger the nitric container, the easier it is to add too much. When dissolving powder, start with very small nitric additions until you get the feel for how much it will take at a time.

The way I've described this, it sounds slow. It reality, it goes fast if you keep after it. Ideally, keep it going as fast as possible without adding too much total nitric or producing a foam-over. Keep the heat right under simmering.


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## Oz (Mar 18, 2009)

Just thought I would give some kudos here. That is a very detailed explanation step by step that is hard to put in words. 

Top notch and helpful to many. Hopefully the people that read it understand how much labor is avoided by following it.


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## goldsilverpro (Mar 18, 2009)

I've tried to explain this several times and have never done a very good job of it. It is hard to put into words.


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## Harold_V (Mar 18, 2009)

Oz said:


> Top notch and helpful to many. Hopefully the people that read it understand how much labor is avoided by following it.


Absolutely true----it can save a considerable amount of labor, in particular, for those that don't evaporate. I didn't rely on the process, if for no other reason, I evaporated my solutions to insure they were highly concentrated. I relied on the added button of gold to remove traces of nitric. It was my method of keeping vessels small enough to permit my limited work area and hot plate open for other batches that were in process. If a person doesn't evaporate, his method is the best possible way to keep volumes to a minimum. Most importantly, it doesn't waste nitric, which has become the expensive and most difficult component to acquire for those that use AR made from commercial acids. 

Well done, Chris. 

Harold


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## 61 silverman (Feb 17, 2010)

GSP , Now that's some talkin !! Exceptionally well stated Even I can get all of that, Combining Harold's pefered method with your method , Should work GREAT.. Thank You Mark


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## salman1122 (Mar 18, 2017)

is it necessary to put urea in aqua regia ? before adding smb . and how much should i put smb?


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## g_axelsson (Mar 18, 2017)

salman1122 said:


> is it necessary to put urea in aqua regia ? before adding smb . and how much should i put smb?


No, it is not, there are several alternatives. Urea isn't that commonly used today as there are better ways to handle excess nitric acid.
Look up denoxing in the library section and check the links at the bottom of this page.
http://goldrefiningwiki.com/mediawiki/index.php/Denoxing

I've never used urea so I don't have any experience with it. A few grains of sulfamic acid shows if you have an excess of nitric acid, if it is a small amount then I use sulfamic acid, else I use evaporation first.

Göran


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## Zoli55 (Mar 6, 2019)

You should take care if you react nitric acid with urea. They make urea-nitrate which is a highly explosive material.


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## Lino1406 (Mar 6, 2019)

Under which conditions will it explode?


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## zachy (Apr 19, 2019)

I have the same question, how can it explode?


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## butcher (Apr 20, 2019)

Normally in the dryer states, there are instances where it can be reactive in the liquid state of a solution.

There is no reason to use Urea, it is basically useless, when dealing with aqua regia, it will not destroy excess nitric acid.


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