# How I can denox with urea,correctly?



## Mida (Jan 19, 2009)

Hi People...
I have purchased a kilogram of 99% urea on ebay.I'm new with this chemical substance and I dont know how to use it for the denox.I added some prills to gold solution in AR,expecting to see some effervescence,but nothing.The prills goes in solution,leaving only an oily trail.Can someone explain me the correct process?Thank you.


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## butcher (Jan 20, 2009)

prills may have a waxy shell, heat your solution, crushing prills helps also , careful adding urea they will foam solution, and boilover is possible. just add till no fizz ( adding to much is also an easy thing to do) and Ph>1.

Nitric has a tendancy to get to a certain concentration and want to boil foam out all at once (no matter which method you use).

I find evaporation solution works better to deNOx, and just using a little urea to test works better for me. DeNOxing Aqua regia is such an important part,that for me Urea is not the way to do it, its too easy to get it wrong, you may have better luck than I do with it? proper evaporation Is better if you ask me.


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## Harold_V (Jan 20, 2009)

butcher said:


> proper evaporation Is better if you ask me.


I evaporated all of my solutions. It makes no sense to me to add materials to values, especially when further processing may be required. Indications are that urea complicates the recovery of the platinum group of metals. I never could see the value in using urea. 

Harold


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## Mida (Jan 21, 2009)

I noticed that if the solution of urea is hot and I add it to nitric,I got some effervescence.If its cold,I got oly a white foam on th top of the liquid. If I leave the mixed solution of nitric and urea, to cool,I got white crystals.Is it normal for urea?I ask this because Ebay seller makea lot of frauds.....


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## butcher (Jan 22, 2009)

sounds normal for urea, Urea is harder to work with than evaporation,it may take a little longer,but evaporation is more fool proof, Harold told us of a trick of using a little gold when evaporating to use up nitric in boiling solution this works real well.


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## Mida (Jan 22, 2009)

Using urea,what are the substance produced from the neutralization of the nitric acid in AR,chemically speaking?


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## butcher (Jan 23, 2009)

6HNO3 + 5CO(NH2)2 --->8N2 + 5CO2 + 13H20
Nitric acid + Urea ---> nitrogen + carbon dioxide + water
or 
HNO3 + (NH2)2CO --> HNCO + NH3 + HNO3
when heated looses ammonia
2HNO3 + CO N2 H4 ---> C2 H3 N2 O2 + NH3
nitrous acid and urea
2HNO2 + CON2H4---> CO2 + N2 + 2H2O
when heated looses ammonia and yields first biuret and then finally cyanuric acid
2CO(NH2)2 ---> C2 H3 O2 (biuret)+ NH3 (acetate)
3CO(Nh2)2 ---> C3H3O3N3 (cyanuric acid) ( White solid) + 3NH3
too much urea may precipitate urea or other salts and urea that can get complicated.
heres another
Au + HNO3 + 4HCL---> HAuCl4 +NO + H2O
or
HAuCl4 + (NH2)2CO + NO + 2H2O

I am only studying chemistry in my backyard on my own so my equations may not jive maybe ?

LOU Help needed can you explain this my head is spinning and I maybe barking up the wrong tree here??

like I said just evaporate, much more simple and works better, and you dont end up with all of these C's O's N's and XYZ's in your solutions :?


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## Lino1406 (Jan 25, 2009)

With NO2: N2 + CO2 (as mentionned)
with HNO3 : Urea nitrate adduct (the crystals you see
on solution top) (NH2)2CO.HNO3


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## EVO-AU (Jan 27, 2009)

Harold: 

I just saw a post of yours in regards to evaporation. Tell me, if you don't mind, how much solution did you do at a time and disragarding type of flame, distance of flame to dish, etc. how long a time did the evaporation take ? I know this is a wide question, but any information from a source such as your experience would give me a tangent with which to proceed. 

Appreciate, Phill


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## Harold_V (Jan 28, 2009)

Phill,
The volume of solution I evaporated varied, but as much as a couple liters was not unusual. From the perspective of volume, I limited the gold content to something less than 18 troy ounces in a four liter beaker, if that helps. I never handled volumes greater than that, choosing, instead, to handle several beakers, assuming a large lot was at hand. The greatest amount of gold I precipitated at any one time would be in the vicinity of 75 ounces, ±, and split in four units. I did that on a regular basis, when I'd re-refine my gold. 

In such a case, I'd evaporate until the volume was reduced to less than a liter, at which time it was filtered in preparation for precipitation. Please bear in mind, when I evaporated gold solutions, my common practice was to include a button of gold, with the purpose of consuming any free nitric instead of evaporating to eliminate the excess. I evaporated beyond the point of expulsion of the nitric to eliminate free HCl as well, so the solution wouldn't destroy the filter paper. I generally evaporated until the gold chloride had a deep red color, at which time I'd take it up with water, to achieve the desired liter of solution. That insured it would filter without destroying the paper. 

As far as distance from flame---it really makes little difference. If you had your solution a great distance from the flame, you might have the flame set higher. A ready answer isn't forthcoming due in part to the various types of vessels one might use. If a Coors evaporating dish is employed, the surface area is much greater than it might be if a small Griffin beaker was used. The critical thing is to keep the solution from evaporating too fast. If it achieves a temperature at which it can boil, you are guaranteed to lose gold in the vapors. That's a tough lesson to learn. (Don't ask!)

I used a three burner gas hot plate for all of my heating. Think of the common gas stove and you'd have a pretty good idea of the distance between the flame and the beakers. The big difference is I ALWAYS used an asbestos pad between the burner and the beaker. Beakers can deal with heat, but they do not do well when heated unevenly, so a thin layer of asbestos prevents cracking the beaker. Needless to say, today, asbestos isn't readily available, so a different product would be required, but I advise always having something between the beaker and flame. 

Time, for me, was a few hours, again, depending on the volume involved. I generally evaporated while I had other batches in various stages of progress, parting the silver and base metal, or even dissolving the resulting gold. I never timed any of my operations, so I am unable to provide any meaningful comments. 

The best way to get familiar with evaporation is to pursue the activity. You won't lose any values, even if you carry the process too far-----not as long as you don't toss anything, and you don't allow your solution to boil. Keep close watch on the column of vapors that come off. If you see the slightest indication of color, you are evaporating too fast. Cut the heat. When a solution is evaporating at an acceptable rate, you often think nothing is happening. Place a cold object (not metal) above the solution to see if anything condenses on the surface. If it does, you are likely running at an acceptable temperature. Do remember, as you eliminate all the water, the boiling point will rise---as will the temperature of the solution, so be careful when introducing any solutions. You risk cracking your vessel, or having a violent eruption of steam. Adding sulfuric acid (to expel lead) when the solution is highly concentrated and hot can be an interesting, and dangerous, experience. 

Harold


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## Mida (Jan 28, 2009)

Harold,
What its the best temperature I can set to evaporate the nitric acid?


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## Harold_V (Jan 28, 2009)

Mida said:


> Harold,
> What its the best temperature I can set to evaporate the nitric acid?


It would be reckless to base the operation on temperature. When you start evaporating, you'll come to realize that as the solution becomes more and more concentrated, it will tolerate more heat. There is no specific heat at which you should evaporate, aside from keeping the temperature below the boiling point of the solution. 

Key to successful evaporation is to not allow the solution to boil----nor to evaporate too quickly. Start out with a low flame and observe. If you see little or no action, kick up the heat slightly-----and keep close watch on the vapors that come off. They _*must *_be colorless. 

Sorry I'm not more help-------I never tied temperature with results----I just followed the instructions provided in Hoke's book, then made the change of adding a button of gold to assist in eliminating free nitric. That truly sets you free, and cuts short the amount of time spent evaporating. In the end, I evaporated to condense my solutions, not to eliminate nitric. 

Harold


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## butcher (Jan 29, 2009)

My thinking is as you are just wanting to boil off the nitric acid,and if it is at first concentrated in your solution,you should not go much above 80 degrees centigade(that is boiling point for pure nitric), now as the nitric is boiled down the concentration will lower in solution till it reaches Azeotrope of 68% in your solution 
( this may be the point you see it want to foam out all at once, watch solution carefully, I lower temperature when I see alot of small bubbles form, and having a lot of space in your vessel is helpful, also a water spay bottle handy too)
after concentration is 68% the boiling point of nitric is 120 degrees,
(it will evaporate at lower than these temperatures, and best kept below them)

the trick Harold suggests of using a litlle gold to react nitric has helped me tremendously,you will not have the trouble with boilovers.

I do mine on an electric hotplate.


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## zachy (Jun 16, 2016)

It can be used to remove the urea nitrogen dioxide in a platinum solution was treated with AR, but I want desahacerme of nitrous dioxides fast and efficiently, I have found that puedee do with urea, anyone know?


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## kernels (Jun 16, 2016)

On the subject of evaporating your solutions . . . Large surface area and flat bottom ! I used to evaporate from a 1000mL beaker, took forever. Moved to using a large kitchen-type pyrex 'jug' and it has cut the time to about 30% of what it used to be. I keep the temperature around the 70 to 80 degree C and am usually evaporating 400mL of solution on the first evaporation, I then add about 200mL HCl when the solution gets syrupy and evaporate that down again. Then I dilute, cool and filter.


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## zachy (Jun 17, 2016)

I have to evaporate about 1.5 liters, resulting from diluting 200 grams of alluvial platinum, then it is very slow all, I already use the pot pyrex wide and large background, but want to get the much faster nitrous dioxides, I use 80 degrees for this evaporation operation and get syrup, but as I say very slow, can I add urea ?, but I'm afraid because it can be a trigger is formed. what do you think?


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## FrugalRefiner (Jun 17, 2016)

You do not want to use urea if there is platinum in solution. It will make it more difficult to recover the platinum because it will form complexes.

Dave


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## zachy (Jun 23, 2016)

FrugalRefiner said:


> You do not want to use urea if there is platinum in solution. It will make it more difficult to recover the platinum because it will form complexes.
> 
> Dave



I use better formic acid, is added so slowly, because the reaction can SER4 violent, if it adds much, I think it gave me good result because it looked out much red smoke. someone uses formic acid as well?


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## Kiranos (Sep 11, 2020)

Hi people 
I have tried to denox my aqua regia solution by urea I added too much then I tried to precipitate the gold with sodium metabusilfate but suddenly the solution took a very high temperature also it released a very irritating smoke and the big problem is that there is a foam at the top and bottom of the solution crystalazied. really it is spleen so I can collect my solution again..? and try the precipitation..? with sulfiric acid and heat it..?


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## Lino1406 (Sep 11, 2020)

Add water, mix, let settle, skim the upper froth and see if something settling below


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