evaporating the Nitric acid

[Refiner232121]

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

 

[Palladium]

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.

 

[Refiner232121]

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

 

[philddreamer]

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

 

[Geo]

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.

 

[Refiner232121]

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

 

[Palladium]

The beaker should be covered with a watch glass.

 

[philddreamer]

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!

 

[Harold_V]

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

 

[Refiner232121]

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

 

[Geo]

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.

 

[goldenchild]

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.

 

[Harold_V]

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

 

[Refiner232121]

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

 

[Harold_V]

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

 

[philddreamer]

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!

 

[Frankk12]

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

 

[Palladium]

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.

 

[Frankk12]

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

 

[Palladium]

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