# Nitric Acid recycling from AR



## IGutYa (Apr 19, 2010)

hi, im new to the forums but have spent many hours reading about the AR process. when it comes to neutralization of the nitric pre-drop or boiling off the nitric acid...

...instead of just boiling off the nitric into the air, could i use a distillation setup to recover the boiled off nitric acid from the AR ??

then proceed as normal with the drop with SMB, NaNO3, or similar?

Thanks


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## Harold_V (Apr 19, 2010)

IGutYa said:


> hi, im new to the forums but have spent many hours reading about the AR process. when it comes to neutralization of the nitric pre-drop or boiling off the nitric acid...
> 
> ...instead of just boiling off the nitric into the air, could i use a distillation setup to recover the boiled off nitric acid from the AR ??
> 
> ...


Unfortunately, not just nitric comes off. Not being a chemist, I am at a loss to detail what you'd get, but it may not be suited for use aside from, maybe, dissolving more values. 

I wonder if Lou might have an opinion?

Welcome to the forum!

Harold


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## IGutYa (Apr 19, 2010)

just considering things here...

of all the things that go into the mix, cannot each be removed via distillation at their respective boiling temp's?

and if it were to be done, could the resulting liquid be re-used for AR?

i have no plan for it other than to be reused in the AR process, just trying to think of an efficient method without the need of extra chemicals. especially considering how expensive & hard to get nitric acid is. knowing how it is produced, figured it could be captured & reused instead of being boiled into the air.


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## butcher (Apr 21, 2010)

Here's my thinking:
Aqua Regia
4HCl (H2O) + HNO3 (H2O)
Notice both have water, which will have a lower boiling point than the acids, and if no other metal was involved would be mostly the first gas to boil off, BUT with metals involved other gasses would evolve, as metals dissolve, lets say if we only had pure gold in solution, things in solutions change in solution as it progresses, here are some formula's to check out.

Au + 3NO3 + 6H --> Au3 + 3NO2 + H20
Au + 4Cl --> AuCl4

HNO3 + 3HCl --> NOCl + CL2 + 2H2O
After 15 min
4H + 3Cl + NO3 --> NOCl + CL2 + 2H2O

Au +3HNO3 + 4HCl -->HAuCl4 + 3NO2 + 3H2O

These are some of the ways I have seen it in formulas
Notice H20, NO2, Cl2 = water, nitrogen dioxide, and chlorine, these will be the gasses, and yes they do have different boiling points (boiling temperature and concentration in solution and the amount of water involved plays a big role here) and also different boiling Azeotropes so these Gases boil out in the order of H20, NO2, Cl2, but there concentration and separation would be hard to achieve in pure state of each gas, also most of them will not come over until there Azeotrope boiling point is reached (their concentration in solution as water boils off) also some of these will be forced from solution by the reaction on the metal being dissolved.

Well to make a long story short if you distilled the Aqua regia you can capture the gases, these gases would have to be bubbled back into water to work this will dilute the aqua regia for your next batch, and some portion of the gases may escape (not completely reconverting back to acid as they bubble into water in the receiver and escaping as gas), but this more dilute Aqua Regia will work again with heat (driving off some water), and you may need to make some small additions of new acid. HCl additions may be especially necessary as the gold takes the chlorine out of hydrochloric acid solution as gold chloride, and also the fact that chlorine gas and water would be making hypochlorous acid and hydrochloric acid
Cl2 + H20 --> HOCl + HCl

In the presence of sunlight, hypochlorous acid decomposes into hydrochloric acid and oxygen, so this reaction is sometimes seen as:

2Cl2 + 2H2O <---> 4HCl + O2 
(note above reaction can go both ways)
HClO is considered to be a stronger oxidant than chlorine.

HClO reacts with HCl to form chlorine gas:

HClO + HCl → H2O + Cl2



Now after a couple of times doing this you may be guessing as to how much of which acid was in solution, this is where it would be better to use this acid for recovery processes and not on the final stages of purifying your gold


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## golddie (Apr 22, 2010)

I need help in understanding the numbers in front of the chemical symbols
For example
3NO3 
I did a quick research and this is what I got
2Na 
the 2 means there are 2 sodium ions per sodium nitrate molecules
the word ion is also diffucult for me to understand

Thanks very much


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## machiavelli976 (Apr 22, 2010)

the numbers before the symbols mean numbers of atoms or molecules involved in a certain and possible reaction according with the correct formula of each compound involved and (or) resulted. the right formula of each substance is mathemathically conected to the valence of each type of atom inside a molecule or another. get some reading about atoms, ions and valence and you will see how symple is everything about that. pure algebra !


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## goldsilverpro (Apr 22, 2010)

golddie,

Very simply put, the ions are electrically charged atoms in solution. For example, when you dissolve table salt, NaCl, in water, it breaks up into Na+ and Cl- ions. The total positive charge in the solution will always* equal the total negative charge and the overall solution charge will be neutral. If you hung 2 electrodes in the solution and hooked up one to the positive (+) side of a DC power source (such as a battery or a battery charger) and the other electrode to the negative (-) side, the (+) ions would be attracted towards the (-) electrode and repelled by the (+) electrode. The (-) ions will be attracted towards the (+) electrode and repelled by the (-) electrode. Unlike charges attract and like charges repel. The (+) electrode is called the anode and the (-) electrode the cathode.

* An aside. With a series of 4 or 5 membrane chambers in a very specific arrangement, 2 electrodes in the end chambers, and a DC power supply, I once tried to create an ionic solution in one of the chambers that would have a greater total of positive charges than negative charges (and, visa versa, in one of the other chambers). I thought I might be able to create a really neat new power source (hopefully, without blowing myself up) by doing this. However, nature, for lack of a better word, wouldn't allow this to happen. I saw gassing at the (non-conductive!!) membrane surfaces. Water was splitting there and was providing either an exact amount of H+ or OH- ions in the 2 chambers to make up whichever ion deficit was required - this was quite unexpected! At that moment, I saw, firsthand, that everything in nature strives to stabilize and become neutral - it was a lightbulb moment. It was a very important lesson to learn, in that it constantly comes into play, no matter what you are doing.


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## Platinum (Apr 22, 2010)

goldsilverpro said:


> At that moment, I saw, firsthand, that everything in nature strives to stabilize and become neutral - it was a lightbulb moment. It was a very important lesson to learn, in that it constantly comes into play, no matter what you are doing.



Everything in nature follows these rules. Observation is the keyword. It's an enlighten moment.


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## butcher (Apr 24, 2010)

I will try to explain best I can,

2NaNO3 + H2SO4 --> 2HNO3 + Na2SO4
Here we have 2 moles of sodium (Na) Nitrate NO3 and we are adding or mixing 
one mole of sulfuric acid (H2SO4)

( 2 in front of NaNO3 means double the fertilizers molecular weight) 

The arrow is the change into the product

Here we have 2 moles of nitric acid (HNO3) and one mole of sodium sulfate (Na2SO4)

Now what the heck are mole's small brown critters in the garden?

Yes, but here they are weights, if we look at periodical table we see 

Sodium (Na) molecular weight is 22.98 g/mol, and nitrogen (N) is 14g/mol, and oxygen is 15.9g/mol

So for sodium nitrate we have a total molecular weight of 
22.98(Na) + 14(N) + {15.9X3} (3 Oxygen’s) = a total of 84.68grams per mole of sodium nitrate
Now in the equation above I had 2 NaNO3 so here we have 2 moles of sodium nitrate so 2 X 84.68 grams = 169.36grams of sodium nitrate
And this mixed with one mole of sulfuric acid
From periodic table
Hydrogen 1.00g/mol
Sulfur 32.064
Oxygen 15.999
So H2SO4
Here we have two hydrogen and one sulfur and four oxygen in sulfuric acid so 
H (1x2) + S 32 + O (15.999 X 4) = H2SO4 98g/mol

Now
2NaNO3 (169.36gram) + H2SO4 (98gram) --> will make nitric acid and sodium sulfate,
you can figure out how many grams of nitric and sodium sulfate you will get by looking at the periodic chart and plugging in the numbers

Notice that in the formula we had a 2 in front of the nitric acid
2HNO3 so we have made TWO moles of nitric acid

Not two little critters in the garden.

Hope this helps and I didn’t mess up to bad.


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## 4metals (Apr 24, 2010)

If you are using aqua regia the fumes you recover will be dilute aqua regia, heavier in nitric than HCl but the HCl is there. If you do this with a sealed reactor and a large condenser you will collect a significant fraction of the outgassing acids by using a diverter trap on the condenser. I have used this technique when boiling the acid for a day to drive off the excess nitric. Usually I just don't use a diverter and allow the condensed acid to return to the reaction and be consumed. This allows me to use aqua regia made up as 5:1 and get good dissolution.

You can recover the aqua regia in a complicated setup involving a closed reaction with an atmosphere of chlorine gas where the scrubber will collect re-usable aqua regia. A nightmare to operate.

If you are only dissolving silver, or inquarting your metal in silver and digesting in 50% nitric, you can use a similar scrubber, use a pure oxygen atmosphere and recover re-usable nitric. This is much easier to control. I suppose if one were to experiment with peroxide you could eliminate the oxygen and scrub with a peroxide solution and recover nitric as well. This has to be done in an area where no HCl is used as any chlorine in the air will bugger things up.


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## golddie (Apr 25, 2010)

Hi GSP,buther
Thanks for your posts 
Hi 4metals


> Usually I just don't use a diverter and allow the condensed acid to return to the reaction and be consumed. This allows me to use aqua regia made up as 5:1 and get good dissolution.


Can you explain this a bit
What do you do with the outgoing gas
You still have gas to deal with



> If you do this with a sealed reactor and a large condenser you will collect a significant fraction of the outgassing acids by using a diverter trap on the condenser.


If I wanted to divert the gas into a bucket of water
then boil it to make it concentrated 
wouldnt this be the same thing
Thanks


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## 4metals (Apr 25, 2010)

Goldie,

Using a condenser the cold surface of the condenser collects the gas as condensation which drips back into the reaction, it is not a method of preventing all of the gas from escaping so scrubbing is still needed. It will save on nitric though. Obviously the longer the condensing surface is and the more surface area in the piece of glassware for condensation, the more efficient it is. I guess if you had a big enough condenser you may get to the point where no gas comes out, it all condenses. But it would be one big condenser. 

Bubbling the gas through water will only catch a surprisingly small quantity of the out-gassing fumes, a diverter catches the acid which is dripping back down towards the reaction and funnels the condensed acid into a collection vessel so you can re-use it. It is not pure nitric at this point, it has some HCl in there as well.


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## butcher (Apr 25, 2010)

I suspect that 
NOx the NO2 gas most likely will need water in condenser to revert back to nitric acid, the chlorine or chloride gas would also need moisture from water.
when aqua regia is dilute their will be water steam in condenser, but as solution gets concentrated it will be hard to capture as much NOx and Cl2 gases even with a very tall and cold condenser.


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## IGutYa (Apr 27, 2010)

4metals said:


> Goldie,
> 
> Using a condenser the cold surface of the condenser collects the gas as condensation which drips back into the reaction, it is not a method of preventing all of the gas from escaping so scrubbing is still needed. It will save on nitric though. Obviously the longer the condensing surface is and the more surface area in the piece of glassware for condensation, the more efficient it is. I guess if you had a big enough condenser you may get to the point where no gas comes out, it all condenses. But it would be one big condenser.
> 
> Bubbling the gas through water will only catch a surprisingly small quantity of the out-gassing fumes, a diverter catches the acid which is dripping back down towards the reaction and funnels the condensed acid into a collection vessel so you can re-use it. It is not pure nitric at this point, it has some HCl in there as well.




so in a scrubber, i have seen activated carbon being used as the sole filter for acid, is there anything else to add in?? ive made a fume hood & activated carbon filter for the end, its just, im not sure if that will be enough.

the scrubber is for nitric distillation & AR process, not considering recycling much anymore, maybe but not too likely.


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## butcher (Apr 30, 2010)

the carbon can be a collector of gold. but 
my concern would be with stong nitric fumes you may be asking for trouble.
a scrubber for acids use bases to neutralize the acids.

read the posts dealing with waste, 
I believe this is where 4 metals wrote some very good information on how to build and manage a scrubber, as well as dealing with waste's.


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