# evaporating the Nitric acid



## Refiner232121 (Aug 25, 2011)

I was refining 400 grams of gold from cnc machine and it was in shavings.
I did the usual and during evaporation I had a hard time evaporating the Nitric acid.
I must have boiled it for one day and I couldnt get rid of it and in the end we bought urea and that did the job and the jobe came out good.
My question is regarding the evaporation of nitric 
I know Harold used pure gold but how long should the evaporation take and did I do something wrong
Also I would like to know if Sulphur dioxide gas can be used when the nitric is removed with urea
Thank You


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## Palladium (Aug 25, 2011)

Refiner232121 said:


> Also I would like to know if Sulphur dioxide gas can be used when the nitric is removed with urea
> Thank You





It always pays to calculate and use just the amount needed so there is no great excess. SO2 can be used after urea to drop the gold. From what I understand SO2 can also be used to burn off the excess nitric without using urea. But I think when it’s used this way it is just to burn off residual nitric, not nitric in great excess. I may be wrong. If you over gas with SO2 it will not only cause gold precipitation but also other unwanted metals like copper.


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## Refiner232121 (Aug 25, 2011)

Hi Palladium
Thanks for that help
Than it is a better thing to use SMB
I have tried SMB and it works and I dont think you have to worry about over using it
I was also wondering if it is possible to use SMB and Suplhur dioxide gas at the same time
First use the gas than smb


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## philddreamer (Aug 25, 2011)

You probably didn't have a hard time, its just that its tedious if you're not not familiar doing it.
You need to heat the solution close to, but NEVER to boil. It must be heated until you see a slight boil, then turn down the heat. Wath the level @ which you star & follow the evaporation until it starts to boil. (Lower heat a bit more.)This now means the you got to the point where you are getting rid of the nitric. 
At this point add a bit HCL. Do this 2 or 3 times. You'll notice that the solution thickens a bit like jelly. 
Then proceed to let cool & add 2 to 3 times water & add SMB. Some just add ice, a chilled solution "drop" the gold better.
Or, instead, add a button of pure gold, after weighing it, @ it will consume the nitric, when there is no more reaction, nitric is gone. This is way faster than evaporating. Or, the best thing to do is, do your math so you only add the nitric needed for the complete digestion. I use less nitric, & add what its needed at the end, if some material hasn't dissolve. This way you won't ever need urea. If you have a bit of nitric left, a little extra SMB will get rid of it, too. 
Harold used the sulphur dioxide gas with excellent results for precipitating his gold.

(And between you & me, don't ever say boil when it calls for evaporation, so Harold won't pull on ear! :lol: 
I hope this helps some.

Take care!

Phil


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## Geo (Aug 25, 2011)

i dont think you can effectively evaporate nitric acid from the solution. urea is the preferred method of removing nitric, just add a few pellets at a time until theres no reaction or you can put a piece of gold into the solution and when there is no more reaction the nitric has been used up. sulfur dioxide will precipitate the gold out of solution, the preferred way to do this is to introduce sodium metabisulfite to the solution. it can be added dry or dissolved in water. use equal weight SMB as gold in solution SMB weight = Au weight. this works as long as all nitric has been neutralized if not more SMB will be required to precipitate.


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## Refiner232121 (Aug 25, 2011)

Hi Folks 
I know that Harold has said not to boil the solution but the process was taking so much time that I said I must have done something wrong and I boiled it and also I noticed that I cracked the beaker.
I think I used way too much nitric because the solution was not getting siropy 

I am wondering how much gold would I loose in the steam because of the boiling
I had 400 grams of metal at the starting point

Anyway the job got done and I was happy about that 
Thanks


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## Palladium (Aug 25, 2011)

The beaker should be covered with a watch glass.


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## philddreamer (Aug 25, 2011)

Geo, evaporating nitric is effective, but one must know how to do it. 
Butcher tought me the steps, plus reading thru the posts I picked up more details on how to do it right. I don't evaporate anymore because I only add the nitric needed & I also add a bit extra SMB to get rid of any nitric, & I always get a good precipitate. 
Everyone ends up picking a way of going about it since, as we all pointed, there're several ways of getting rid of the nitric. 

Refiner: What's the source of your material? Did you calculate how much gold would be recovered? How much gold did you end up recovering?
You can loose plenty of gold, depending on how hot your solution & how much of it you evaporate. Like Palladium stated, You need a watch glass, so vapors condense & fall back into the solution. Heat should be enough for only the water to evaporate, & as the solution concentrates, the nitric starts to boils; this is when you cutback on the heat & add some HCl.
You need to know that is going to take some time.

I also learned in my early days thru this great forum, that you don't heat your solutions with PM's straight on top of a hotplate, use a Corning (& also the watch glass), of course... As you can see, I had gold all over the place. :roll: 

Write down what you've learn so you can follow the steps on the next batch & minimize any problems.

Take care!


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## Harold_V (Aug 25, 2011)

Geo said:


> i dont think you can effectively evaporate nitric acid from the solution. urea is the preferred method of removing nitric, just add a few pellets at a time until theres no reaction or you can put a piece of gold into the solution and when there is no more reaction the nitric has been used up. sulfur dioxide will precipitate the gold out of solution, the preferred way to do this is to introduce sodium metabisulfite to the solution. it can be added dry or dissolved in water. use equal weight SMB as gold in solution SMB weight = Au weight. this works as long as all nitric has been neutralized if not more SMB will be required to precipitate.


I strongly disagree with the notion that urea is the preferred method of dealing with free nitric, and that nitric can't be eliminated by evaporation. The addition of pure gold is very effective, but evaporation works perfectly well, assuming one uses proper procedures. 

It might interest you to know that I refined gold for more than 20 years and never used urea at any time. My logic is, and was, why add anything to a solution that can complicate recovery?

Harold


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## Refiner232121 (Aug 25, 2011)

Hi Folks
Thanks for your help
My beaker was covered but I did have minor spills and an accident because we poured in the SMB and I didn't know that you had to mix it and I kept on pouring and than he stirred and than all of a sudden it boiled over
But I did collect everything on tissue paper
The thing is the shavings were a mixture of 18,14 ,10 and maybe some silver because he is not being very clear with me.
and it was 400 grams and I got 180 refined gold and it could be 23K
I did this for a friend and he was with me 
my friend said that on average it was 14k
so I should have had a yield of 230 pure.
but I still have some in the water in the filter and we rushed and a bit with the silver siphoning and I figure about 10 15 grams of gold still in the water and filters and tissue paper
also in the scrubber
he is telling me it should have been 230 but I doubt that I lost 40 grams form boiling.
there is no way he could have known the karate of the shavings 
Anyway I have all the solution and I said he can take it and we will see what we are going to do
what do you folks think


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## Geo (Aug 25, 2011)

i apologize for speaking for everyone. if your a novice such as myself you take advise where you can get it sometimes. ive never been able to master evaporating nitric from solution without messing something up, im sure with the correct teaching i would be able to but almost all that ive spoken to one on one about it recommended urea to neutralize nitric acid as a simpler way to go about it. i know it adds another process to the equation but you dont lose any gold either. after reading the post here and talking to you fine members that share their knowledge i know now not to add more nitric than is needed to do the job. i only use AR when i have to and prefer to use hcl+cl myself.


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## goldenchild (Aug 25, 2011)

I just want to point out that when using more gold or a "timesaver" to get excess nitric out of solution, the reaction ceasing doesn't necessarily mean all nitric is out of solution. Additional HCL should be added to confirm that it isn't just the HCL to nitric ratio being out of wack. Remember HCL gets used up. One very good technique to combat this suggested by freechemist is to add water to your AR. This causes the HCL to be used up at a decreased rate.


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## Harold_V (Aug 26, 2011)

Refiner232121 said:


> he is telling me it should have been 230 but I doubt that I lost 40 grams form boiling.


Don't doubt it. It is VERY possible. If you evaporated for a prolonged time with the solution at a boil, especially if you had material left undigested that was slowly dissolving, you most certainly COULD have lost that much, and you probably did. That's exactly what I'd expect when you don't follow prescribed procedures, and lack patience to do the job properly. 

You can make a determination of losses by closely inspecting the area near the discharge of your vapors. If you evaporated outside, not in a hood, look for purple staining on objects near your work area. If your fumes escaped to the atmosphere, there is little you can do to determine the loss. If you evaporated by boiling, using a fume hood, you'd find some of the gold condensed in the hood, with any that came in contact with metal as cemented gold.

This is a classic example of what I keep telling readers not to do. Don't evaporate at a boil, and *NEVER* speak of evaporation by _discussing boiling_. You do more harm to others than can be undone by my constant harping on the subject, because they take your words at face value, stupid as they may be. Who can blame them? They are far more interested in their first recital than they are in learning to play the piano. Reading Hoke would eliminate that problem, but, then, who has time for that nonsense?



> what do you folks think


That's exactly what I think. I think you, and others, should spend more time trying to learn what to do before doing any of it. You'd stay out of trouble that way, and, with luck, not be out several thousand dollars---which, very likely, have been lost. 

Harold


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## Refiner232121 (Aug 26, 2011)

Hi Harold
Thanks for your help 
If you refined for 20 years without urea than you did something right.
It was my first time refined a batch of this amount so I was confused because I only refined amount of 20 grams or so.
The solution would not become sirohpy and I think its because I used too much nitric.
Hoke says when the brown fume stop than it means that there is no more nitric.
I also made a mistake with the measurements and I used 50/50 Nitric HCL and later I realized that so I added the HCL later
At one point I was getting brown fumes and I wasn't sure if that was nitric or gold going up in smoke.
I am saying that if I can use urea what would be wrong with that other than the fact that you have increased volume in the solution


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## Harold_V (Aug 27, 2011)

Refiner232121 said:


> I am saying that if I can use urea what would be wrong with that other than the fact that you have increased volume in the solution


My only real objection to the use of urea is much the same as a home "machinist" using insert carbide tooling to avoid learning to grind proper cutting tools from high speed steel. 

Does urea work? 

Of course it does. 

Does it do any harm?

Probably not, although there's a faction of people that firmly believe it interferes with the recovery of platinum group metals. Dunno. Can't speak from experience because I never used urea. 

Use if it you find it necessary, but don't avoid learning the evaporation process, and using it to conclusion. You will never be sorry for having a good understanding. 

For me, it was a matter of pride. I didn't like the idea of using any dodges that would prevent me from learning the basics. Once you understand how easy it is to evaporate, I saw no need to use urea, especially considering my refining technique included precipitating from concentrated solutions. I had to evaporate to accomplish that end. 

Harold


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## philddreamer (Aug 27, 2011)

I just added 20g of SMB powder & as soon as it hit bottom, a brown cloud. 
I took the pic about 40 seconds after SMB drop. That precipitate is before even starting to stir. 
It means math. was correct; the amount of nitric added was correct; all reaction stopped; material dissolved. I let it cool; filtered; added enough water before adding SMB powder. No urea. I didn't need the gold button, (this time).
In less than 45 min. most of the precipitate has settled. 

Thank you GRF!


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## Frankk12 (Aug 28, 2011)

Hi philddreamer


> I let it cool; filtered; added enough water before adding SMB powder. No urea. I didn't need the gold button, (this time).


You add SMB powder
so the gold falls and than you filter the solution 
I am confused 

Can you explain this a bit
Thanks very much


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## Palladium (Aug 28, 2011)

He added smb after filtering to remove any contaminates before precipitation. You want your solution to be free of any sediment before the addition of smb. Once your gold has been precipitated you do not filter the gold or it will get trapped in your filter. You dry the gold in the same container you precipitated it in. That's why it's important to remove any solids by filtering.


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## Frankk12 (Aug 28, 2011)

Hi Palladium
I thought as the final steps you would filter the refined gold powder and whatever is in the filter you burn.
Also while on the subject of burning how do I burn the filter that I used for refining that has nitric acid or AR.
should I use alcohol or gasoline


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## Palladium (Aug 28, 2011)

rewalston wrote:
when force drying gold, I'm assuming that you heat it while it is in the beaker that it was precipitated in? Also is there a clear indicator as to when the gold powder has been dried sufficiently?

That's correct. Washing and drying take place without removing the gold, reducing the chance of loss or contamination. 

As for drying, it will be perfectly obvious to you when the gold is dry, and even more obvious why you are admonished to incinerate substances when switching between acids, and you don't wish to dissolve values. 

As the water evaporates, the color of the precipitated gold changes, getting lighter. Before that happens, you should be swirling the gold regularly, to prevent sticking to the beaker. This will make sense once you've been there, done that. 

Once dry, continue to heat the gold. You're not finished, yet. As the temperature rises, you'll notice vapors, or light smoke, comes from the gold. There are substances that stick with the gold doggedly. What they are I have no idea, but it will be quite obvious that there is a presence of acid, assuming you (lightly) sniff the fumes. Heat until you no longer see the vapors, then allow the beaker to cool. Do not remove and place on a cold surface, for that will cause thermal fracturing of the beaker. 

I heated my beakers on a gas hotplate. I had a pad of asbestos between the beaker(s) and the flame, to limit uneven heating. The pads of which I speak used to be readily available from chemical supply houses. That is no longer the case, due to asbestos no longer being available. They likely have a replacement material now that performs the same function. 

Harold

and this http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=5197&p=44526&hilit=+processing+filters#p44526


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## philddreamer (Aug 28, 2011)

Thanks Palladium!
I've been cought up on 3 projects & forgot to follow thru.


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## Frankk12 (Aug 28, 2011)

Hi Palladium
Thanks for your help
I guess the way to burn the filter that have nitric and AR would be to use alcohol or gasoline


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## butcher (Aug 28, 2011)

Whenever I have to incinerate (powder's filters and so on) I will use a hydroxide solution to rinse them, and then several washes to remove salts formed.
my thinking here is that roasting gold with chlorides gold can be volatile, even when roasting with salts the gold can form chlorides and some will evaporate. silver chloride is also volitile, the hydroxide can help to make it silver oxide.

There is no need for alcohol (I have to admit I have used alcohol to wet fine powder to keep it from blowing away), and gasoline could be very dangerous.

if much powders they can blow away if torch too strong and brought too close too fast, so heating with just the tip of the flame until they start fusing and paper or volatile carbon that blows around easily has been incinerated, then torch can be turned up and heated hotter, (some may use torch on bottom of pan), I usually use a propane or electric hot plate or coal fire to heat from bottom and torch to heat top, crushing and stirring well to expose all powders to oxygen and heat, heating to red glowing hot and keeping on heat as close to hour as I feel needed, the gases that come off depend on chemicals you used, and what is formed chemically during the roasting procedure, they come off in somewhat of an order like water then NOx then chlorides with sulfates being very hard to get oxidized and to fume off (sometimes I may add some fine iron powder if I have sulfides in these fusions.
some powders (I call them metal salts {metal + acid is a metal salt of that acid}) some powders will dry then change to a syrup, (fuse), continued heating will drive off more acidic gases then these powders will then dry again and as all acids are fumed off will stop smoking and glow red hot.

Caution these fumes are deadly, or can give you breathing troubles later in life. They are also very corrosive to anything close to where you do this work.


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## philddreamer (Aug 28, 2011)

Thank you Butcher! 8)


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## butcher (Aug 28, 2011)

Well Phildreamer, I cannot say that I do it right but that is how I do it untill I know better.

I am just amazed at how fast you have been learning all of this, and always like reading your posts and answers, you are a valuable asset to the forum in my mind. And I just love seeing those gold pictures you post.


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## philddreamer (Aug 28, 2011)

I thank you for leading me in my first steps & explaining the "why" of things; I owe a lot to you for been so patiente.
This is a great forum!


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## Frankk12 (Aug 29, 2011)

Hi butcher
Thanks for your help and the help of others.
I was wondering about boiling nitric acid and the material to be refined.
Here the silver mixes in with the nitric and I was wondering if you boil the solution does the silver go up in vapour 
Thanks again


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## philddreamer (Aug 29, 2011)

Hi Frank!
If I may, it seems you're processing silver. The silver doesn't mix with the acid, it dissolves. If you boil the nitric & you don't have a cover, like a watch glass, you will loose silver.


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## Frankk12 (Aug 29, 2011)

Hi philddreamer
We know that gold should not be boiled because we will loose the metal in the steam and I wanted to know if the same will happen to silver
Thanks for your help


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## PreciousMexpert (Aug 29, 2011)

Hoke says to boil.
I guess not to long.
As soon as the silver dissolves than thats all you need


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## butcher (Sep 1, 2011)

I do not believe Hoke meant boil even though that is a term used to describe heating a solution by many of us before we realized the problems of using this word incorrectly.

Notice Hoke talks of using steam table for evaporating nitric acid, it would not boil the acid as the temperature would not get hot enough (unless steam was under High pressure and that is not the case here), the words we choose can have different meanings to different people, or we may use a word to describe a process and use a term that really does not describe the process as it should, and would cause problems for others if they took our word literally, that has been something that stands out in this forum in my mind, everywhere I have read the aqua regia process they spoke of boiling to rid nitric acid, here on the forum I learned the proper method slow evaporation.

Boil your aqua regia and you will loose gold, and you may just loose it all at once in one big boil over as nitric concentrates in solution.


Edit to add:
When dissolving silver your nitric is diluted to about 30 -40%, there is plenty of water and metal to dissolve, fume would contain mostly water and some NOx gases, (silver nitrate is not volatile that I know of like gold chloride is), a watch glass cover should always be used, they are good indicators of what your fumes may contain, and in my case keeps out the oak tree leaves and bugs, boiling the nitric when dissolving silver is not necessary, and I just heat mine just under the boiling point, to speed thing up, as when boiling the popping of gas bubbles will carry values and you can loose silver with them, even cold the silver will dissolve it just takes forever, heat speeds reactions of acids on metals.

Editted again: So I would not annoy Harold, by using their when I meant there.


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## philddreamer (Sep 1, 2011)

Thank you Butcher!
Butcher wrote:
"Boil your aqua regia and you will loose gold,..."
The novice can learn to tell when the solution is too hot by, watching the color of the drop on the watch glass. If the drop is the same color of the solution, you're evaporating values! The drop should be clear. Train your eye to recognize the difference between boiling bubbles & reaction bubbles.
Reaction bubbles are small & you'll have red fumes in your vessel. Boiling bubbles are much bigger & the fumes in your vessel will be more the color of the solution.

"...and you may just loose it all at once in one big boil over as nitric concentrates in solution."

The novice also needs to learn that the reaction itself will produce heat. 
If you are processing silver & you add ALL the nitric required @ once, but see no immidiate reaction for the next 5 minutes, the tendency is to crank up the hotplate. Well, when the reaction itself "kicks in" & produces its own sudden heat, plus your hotplate cranked up, chances are its going to boil over! Learn to control the heat of your solution. Once you've gained experience, & have learned how your solution will behave, then you can speed up the heating process.

Butcher, please, could you add more details.


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## butcher (Sep 2, 2011)

Sorry Phil, my mind is blank; I do like your description of these reactions.

Do not fill your pots too full give them room for expansion, incase things start foaming up, a water squirt bottle is handy to spray the foaming and may help to keep them in the pot, (also a suction bulb can remove liquids before the overflow from the pot), catch basins are always handy for spills or if some glass breaks, I use fiberglass trays like they have in cafeterias.
Remember do not change temperature of your glass too fast thermal shock breaks them and always at the wrong time, also metal powder’s in the bottom of a pot on heat can over heat the bottom portion of the reaction vessel being heated and crack the bottom of the pot, if you must heat them keeping them stirred will keep them off the bottom. 

If you do get a pot cracked at bottom on a stove, do not lift the pot, the bottom will most likely fall out and all the acids and gold with it, siphoning can remove the liquid. 

One of my favorite tools is a suction bulb that looks similar to a turkey baster suction bulb, the hole is smaller they sell them where you buy lab supplies, it is very handy for transferring liquids, another tool are called pipettes similar to eye droppers they can pick up drops of solution from above powders, these tools work well for decanting liquids without disturbing settled powders, I even cut off top bulb of an old pipette and stick it onto my suction bulb to be able to pick up more liquid through the pipette orifice.


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## philddreamer (Sep 2, 2011)

Excellent Butcher, I knew there were more details. 

Thanks!


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## saadat68 (Sep 22, 2017)

Hi
I didn't understand this section


> to speed thing up, as when boiling the popping of gas bubbles will carry values and you can loose silver with them, even cold the silver will dissolve it just takes forever, heat speeds reactions of acids on metals.


If I boil silver nitrate in for example 110 °C for one hour, I lose silver ? :shock:

What temperature is good for dissolving silver in nitrice acid without lose silver ?

Thanks


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