Best method of precipitating gold from aquaregia?

[decentdude21]

Depends on how diligently you do your work. And also it depends on ammount of gold. Working in 100 g batch, and losing 100 mg in the process mean that your efficiency will be 99,9%. And it is all possible to accomplish this number with proper dissolution, nitric killing operation, proper settling and decantation and also with recovery of traces of gold from gold powder acid washes.
But when you work with only say 0,5 g gold, handling loss will be much greater than 0,1% - it is well known, that working with small ammounts make losses higher.

It you get out 99%+ from 1 kg of clean material running say 3g/kg (eg pins), you´ve done a good job, I would say. But with dirty stuff and lots of junk, possibilities for gold to adsorb on vast solid matrix (like leaching ashes from incinerated IC´s)... You could run as low as 80%, and you will happily throw away the ashes and remains, because stannous was negative. But if you will do the smelt and "assay-like" cupelation of the thoroughly leached ashes, you will found not-negligible ammount left behind.

It is maybe more complicated than you want to hear but with deep knowledge of chemistry and what could possibly ruin your recovery, losses will shrink quite sharply

Hope that it will help you in some way. Good luck with refining.
Stay safe.

Thank you

 

[decentdude21]

It is a bit more complicated. Remains of nitric acid are still present, but it does have much with redox potential of liquid vs. redox potential required to dissolve the gold back to the solution.

Nitric acid undergo decomposition in acidic chloride enviroment, producing mainly nitrosyl chloride and chlorine. Chlorine is very quick, reacting immediately with anything around, but also undergo slow decomposition to oxygen and hydrochloric acid (but this is minor and slow reaction). It also evaporates from reaction, mainly while still hot - removing "oxidizing force" of the juice. And alongside with NOCl, giving the AR fumes the unmistakeable "yellow" hue.
Nitrosyl chloride isn´t that powerful oxidant, and undergo quick degradation to nitrogen dioxide in AR solutions.
Also with strong solution of AR, dissolution of gold isn´t particulary quick reaction. When there are remains of "weak" AR, or better said some "oxidizing species", gold will be dissolved anyway, but VERY SLOWLY. So it buy you time to let it settle and filter the gold in usual fashion and only minor losses.

So, overall if you keep the AR hot for long periods of time, most of active oxidizers, capable of dissolving gold (mainly chlorine) will simply evaporate or dissociate into compounds, which are very mild oxidizers in terms of dissolving gold. It does not say that they cannot dissolve the precipitated gold, but it is a very slow reaction.

I personally experienced this to happen, when I just didn´t de-nox the AR solution afterwards. Gold drop appeared to be as normal as usual. Gold dropped, started to settle. Then I came back in the morning, and all gold was redissolved so I decided to push it with more sodium sulfite. Again, gold dropped, I came next morning, and some precipitate was clearly on the bottom of the beaker, but stannous told gold presence in solution (after TWO subsequent drops). Third drop accomplished the thing, but it can illustrate how fine and long-settling are the reactions of residual nitric in AR.

Similar thing happen when you denox with urea. Urea is incapable of reacting with nitric acid in a way it will be destroyed. It just form a salt. But nitric undergo decomposition in chloride enviroment as I discussed above. So the formed N(III) species like NOCl or nitrous acid finally react with urea, creating nitrogen and carbon dioxide. But also, this reaction is only driven by decomposition of nitric acid. Taking into account, that for nitric in presence of urea, it is needed to break free from the urea.HNO3 salt, settling the equilibrium takes even more time.
So it is fairly common to encounter the situation, when you take hot AR gold solution from heat, and directly treat it with urea. It will hiss with two or three spoonfuls, but then stop. That does not mean that you neutralized ALL nitric, but instead that you killed all N(III) species lurking around in the juice. There could be plenty of undecomposed nitric acid, which could be still present after "sucessfull" de-noxing with urea.

That is why we always advise to use sulfamic acid, because sulfamic acid is capable of destroying not only N(III) compounds into nitrogen, but also nitric acid to nitrous oxide. But it need to be hot to work on nitric itself.

So, dilution will help you to mitigate the re-dissolution of dropped gold, but it isn´t bulletproof and all reliable way how to do it. But if you master this way, good for you I wouldn´t do it this way, but it could certainly work in your favor.

I need more information about sulfamic acid reaction with AR gold solution. I want to know what are the conditions for sulfamic acid and how to do it?

I have refined both gold and silver, and I want to know more about them.

I have used sodium hydroxide and urea to neutralize AR gold solution. And used SMB and hydrogen peroxide to precipitate the gold.

Now I want to know more what other chemicals can be used to neutralize the gold dilution and precipitate the gold.

 

[orvi]

I need more information about sulfamic acid reaction with AR gold solution. I want to know what are the conditions for sulfamic acid and how to do it?

I have refined both gold and silver, and I want to know more about them.

I have used sodium hydroxide and urea to neutralize AR gold solution. And used SMB and hydrogen peroxide to precipitate the gold.

Now I want to know more what other chemicals can be used to neutralize the gold dilution and precipitate the gold.

Just put sulfamic acid into hot AR solution (70-80°C at least). Do it slowly, reaction could be quite vigorous if you dump in whole tablespoon. After stirring, add a bit more and observe. When effervescence of gases ceases, you´re done.

I am interested how sodium hydroxide is used for neutralization of AR gold solution and what is the purpose of this. Sounds very wild

 

[justinhcase]

Just put sulfamic acid into hot AR solution (70-80°C at least). Do it slowly, reaction could be quite vigorous if you dump in whole tablespoon. After stirring, add a bit more and observe. When effervescence of gases ceases, you´re done.

I am interested how sodium hydroxide is used for neutralization of AR gold solution and what is the purpose of this. Sounds very wild

Why would you add sulfamic to a hot solution?
That will be an extremely vigorous reaction, almost the exact opposite of what you want to achieve with a laden solution.
Froth and bubbles loos value.
Evaporation is the best way to remove a volatile compound like an acid.
I would recommend you use that as much as possible, Sulfamic acid or a button of gold is very useful to finish off a "De-Nox" when you already have evaporated to a viscous concentrate.
But I would not really use them as an alternative to evaporation.

 

[4metals]

Be careful when adding the sulfamic acid as the colorless gas it gives off is no laughing matter!

With HNO2, sulfamic acid reacts to give N2, while with HNO3, it gives N2O.
HNO2 + H3NSO3 → H2SO4 + N2 + H2O
HNO3 + H3NSO3 → H2SO4 + N2O + H2O

They don't call it laughing gas without reason.

If you are going into a scrubber it’s no big deal but if you are not, just imitate Bill Clinton and don’t inhale !

 

[orvi]

Why would you add sulfamic to a hot solution?
That will be an extremely vigorous reaction, almost the exact opposite of what you want to achieve with a laden solution.
Froth and bubbles loos value.
Evaporation is the best way to remove a volatile compound like an acid.
I would recommend you use that as much as possible, Sulfamic acid or a button of gold is very useful to finish off a "De-Nox" when you already have evaporated to a viscous concentrate.
But I would not really use them as an alternative to evaporation.

Reaction of nitric acid with sulfamic acid is very slow at low temperatures, and than exponentionally increase in reactivity. On the other hand, sulfamic acid react very well with N(III) species even at room temperature. So you experience slower, tame reaction and hissing. Very similar to the urea addition. But that does not mean nitric isn´t present afterwards. At low temperatures, nitric could survive in HCL for quite some time.
Just let the denoxed cold solution sit for say two hours and than add pinch of sulfamic.
Of course, these things are only relevant if quite a big excess of nitric is used. If you overdose it just a bit, that small ammount is relatively quickly destroyed by HCL to byproducts, that react with sulfamic immediately and also at r.t. .

If you add it slowly, pinch by pinch from the start, you will be absolutely OK. Just be prepared to use larger container from the start.
I agree that evaporation is superior method, do not introduce contamination etc... But I personally do not have time and means to evaporate HCL and slowly ruining my fumehood with additional corrosive fumes. Few spoons of sulfamic acid are also cheaper than volumes of HCL (for me). Additional cost of electricity for heating the solution for prolonged times... So I use sulfamic acid when solution is hot, to be quick and to be sure. Watchglass on the top of the beaker if I work with some rich concentrated solution. But otherwise that 0,1 mL that could possibly go off as aerosol from the container with first scoop (then the reaction slow down) do not bother me that much.
Maybe this isn´t most scientific approach to the subject, but I am in somewhat specific situation with refining, where I must consider time management as one of the priorities.

 

[orvi]

Be careful when adding the sulfamic acid as the colorless gas it gives off is no laughing matter!

With HNO2, sulfamic acid reacts to give N2, while with HNO3, it gives N2O.
HNO2 + H3NSO3 → H2SO4 + N2 + H2O
HNO3 + H3NSO3 → H2SO4 + N2O + H2O

They don't call it laughing gas without reason.

If you are going into a scrubber it’s no big deal but if you are not, just imitate Bill Clinton and don’t inhale !

I would assume that everybody working with AR or acids should use fume hood or at least powerful fan outside. If you do not have huge excess of nitric in juice, I would say the ammount of N2O isn´t that concerning.
You need to get a good whiff of practically pure N2O to feel light-headed or bit "drunk". Assuming that subject doesn´t stick the head into the bucket, it should be relatively low-risk operation.
Alongside with N2O, HCL vapors go mixed with it, so I think you won´t be able to breathe much of the gas, as it would be impossible without coughing and choking.

But it is good to know what are the hazards of denoxing with sulfamic

 

[4metals]

I come from the world of professional refiners and all, or most, use scrubbers and all use good exhaust hoods. It’s when I started posting here that I learned of those who often use neither, and even work upwind! So just making everyone aware of the potential hazards has become a habit.
I remember when I first posted plans to build a simple fume hood, so many members were thankful. That spoke volumes to me.

 

[decentdude21]

Just put sulfamic acid into hot AR solution (70-80°C at least). Do it slowly, reaction could be quite vigorous if you dump in whole tablespoon. After stirring, add a bit more and observe. When effervescence of gases ceases, you´re done.

I am interested how sodium hydroxide is used for neutralization of AR gold solution and what is the purpose of this. Sounds very wild

Sodium Hydroxide is used for maintaining the pH of AR gold solution between 3 to 5. The reason for the maintaining the pH is to reduce the amount of nitric in the solution. After maintaining the pH, use SMB to precipitate the gold. In the middle of the reaction, keep checking the pH of the solution, if it goes down, then again use sodium hydroxide.

 

[Yggdrasil]

So why do you want to keep it in the 3-5 range??

 

[orvi]

Never heard of this approach. It certainly won´t kill any nitric, as it just can convert it to the nitrate salt. Amen, if you have iron in solution, filtering that iron junk will take forever

 

[decentdude21]

Never heard of this approach. It certainly won´t kill any nitric, as it just can convert it to the nitrate salt. Amen, if you have iron in solution, filtering that iron junk will take forever

So what is the best alternative of sodium huydroxide?

 

[4metals]

This can become an interesting thread Decent, here in the US we do refining a bit different. We had a great thread a few years back from a large refinery in India and their ways were a bit different to me but we helped them improve their process and they benefitted greatly.

Similarly it would be interesting if you detailed the process you learned to refine and we got into a give and take conversation of your methods vs our methods.

Who knows, we may learn something, you may learn something, and maybe you can lean enough to gain employment in India again as a refiner.

 

[decentdude21]

This can become an interesting thread Decent, here in the US we do refining a bit different. We had a great thread a few years back from a large refinery in India and their ways were a bit different to me but we helped them improve their process and they benefitted greatly.

Similarly it would be interesting if you detailed the process you learned to refine and we got into a give and take conversation of your methods vs our methods.

Who knows, we may learn something, you may learn something, and maybe you can lean enough to gain employment in India again as a refiner.

Nice, we will do.

 

[Geo]

Sulfamic acid is a great tool for removing excess nitric acid in solution. As with any process, precautions must be taken. The reaction will increase with each increase in degree of temperature. It is a very easy way to create a runaway reaction. I start to add sulfamic acid at 120°F in very small increments. I then increase the amount that I add after the initial violent reaction. A visual indicator is to watch the space between the liquid and the watch cover. If the solution is evolving NO2, the sulfamic acid is depleted. If the space is clear of color, N2O is still being generated. Leave the reaction on heat. When the space has a red tint, add a little more sulfamic acid. Repeat the additions of sulfamic acid in small increments until the space in the beaker is colorless thirty minutes after the last addition. You can add a heaping amount and there may be unreacted sulfamic crystals in the bottom of the beaker and still have free nitric acid if you don't heat and add in increments, depending on amount of free nitric acid available in the solution.
The best way to avoid this is by not adding too much nitric acid or solid nitrate to begin with. Add an excess of HCl and nitric acid or solid nitrate in small increments until all metal is dissolved and the reaction stops and will not start back with fresh addition of oxidizer. In this way, the amount of free nitric acid in solution will be minimal at best. Remember, with sulfamic acid, it is a 1 to 1 ratio by weight needed to destroy the free nitric acid. For each gram of excess nitric acid or solid nitrate, it takes a gram of sulfamic acid to destroy it.

 

[decentdude21]

Sulfamic acid is a great tool for removing excess nitric acid in solution. As with any process, precautions must be taken. The reaction will increase with each increase in degree of temperature. It is a very easy way to create a runaway reaction. I start to add sulfamic acid at 120°F in very small increments. I then increase the amount that I add after the initial violent reaction. A visual indicator is to watch the space between the liquid and the watch cover. If the solution is evolving NO2, the sulfamic acid is depleted. If the space is clear of color, N2O is still being generated. Leave the reaction on heat. When the space has a red tint, add a little more sulfamic acid. Repeat the additions of sulfamic acid in small increments until the space in the beaker is colorless thirty minutes after the last addition. You can add a heaping amount and there may be unreacted sulfamic crystals in the bottom of the beaker and still have free nitric acid if you don't heat and add in increments, depending on amount of free nitric acid available in the solution.
The best way to avoid this is by not adding too much nitric acid or solid nitrate to begin with. Add an excess of HCl and nitric acid or solid nitrate in small increments until all metal is dissolved and the reaction stops and will not start back with fresh addition of oxidizer. In this way, the amount of free nitric acid in solution will be minimal at best. Remember, with sulfamic acid, it is a 1 to 1 ratio by weight needed to destroy the free nitric acid. For each gram of excess nitric acid or solid nitrate, it takes a gram of sulfamic acid to destroy it.

Ok thanks for sharing your experience.

 

[decentdude21]

Best chemical for removing excess nitric acid in aquaregia? And how to use it?

 

[Yggdrasil]

Best chemical for removing excess nitric acid in aquaregia? And how to use it?

You had a very thorough answer from Geo, why do you ask again?

 

[M Usmani]

Sulfamic acid is a great tool for removing excess nitric acid in solution. As with any process, precautions must be taken. The reaction will increase with each increase in degree of temperature. It is a very easy way to create a runaway reaction. I start to add sulfamic acid at 120°F in very small increments. I then increase the amount that I add after the initial violent reaction. A visual indicator is to watch the space between the liquid and the watch cover. If the solution is evolving NO2, the sulfamic acid is depleted. If the space is clear of color, N2O is still being generated. Leave the reaction on heat. When the space has a red tint, add a little more sulfamic acid. Repeat the additions of sulfamic acid in small increments until the space in the beaker is colorless thirty minutes after the last addition. You can add a heaping amount and there may be unreacted sulfamic crystals in the bottom of the beaker and still have free nitric acid if you don't heat and add in increments, depending on amount of free nitric acid available in the solution.
The best way to avoid this is by not adding too much nitric acid or solid nitrate to begin with. Add an excess of HCl and nitric acid or solid nitrate in small increments until all metal is dissolved and the reaction stops and will not start back with fresh addition of oxidizer. In this way, the amount of free nitric acid in solution will be minimal at best. Remember, with sulfamic acid, it is a 1 to 1 ratio by weight needed to destroy the free nitric acid. For each gram of excess nitric acid or solid nitrate, it takes a gram of sulfamic acid to destroy it.

best answer in detail

 

[orvi]

Sulfamic acid is a great tool for removing excess nitric acid in solution. As with any process, precautions must be taken. The reaction will increase with each increase in degree of temperature. It is a very easy way to create a runaway reaction. I start to add sulfamic acid at 120°F in very small increments. I then increase the amount that I add after the initial violent reaction. A visual indicator is to watch the space between the liquid and the watch cover. If the solution is evolving NO2, the sulfamic acid is depleted. If the space is clear of color, N2O is still being generated. Leave the reaction on heat. When the space has a red tint, add a little more sulfamic acid. Repeat the additions of sulfamic acid in small increments until the space in the beaker is colorless thirty minutes after the last addition. You can add a heaping amount and there may be unreacted sulfamic crystals in the bottom of the beaker and still have free nitric acid if you don't heat and add in increments, depending on amount of free nitric acid available in the solution.
The best way to avoid this is by not adding too much nitric acid or solid nitrate to begin with. Add an excess of HCl and nitric acid or solid nitrate in small increments until all metal is dissolved and the reaction stops and will not start back with fresh addition of oxidizer. In this way, the amount of free nitric acid in solution will be minimal at best. Remember, with sulfamic acid, it is a 1 to 1 ratio by weight needed to destroy the free nitric acid. For each gram of excess nitric acid or solid nitrate, it takes a gram of sulfamic acid to destroy it.

That is very nicely written. True.
As it is with almost everything in refining, it takes some time. I tested the reaction of sulfamic acid on plain nitric (diluted), and rolling/significant reaction started only past 70-75°C. Then with every 5°C more, it become more violent, at 90°C I would call it "explosive with 0,5s delay"

Thing is, we aren´t dealing with plain nitric practically in any occasion. We have AR solutions, we need to de-nox. And chlorides in the solution help to decompose nitric acid to Cl2 and NOCl, which then decompose even further.
It roughly correlate with how quickly solution discolor, when you add HNO3 to HCl. If you mix the two at room temperature, not much is happening in first minute. But heat to 40-50-60...°C and speed of "yellowing" of the solution is much much faster. Yellow color is due to soluble NO2, Cl2 and NOCl in solution.

HCl in case of AR help to speed up the process of de-noxing, because reactions of N(III) species like NOCl or nitrous acid, which arise from nitric decomposition, are much faster than reaction of N(V). It has a lot with thermodynamics - simplified: When N(V) reacts with sulfamic acid, N2O is generated, and thus energy gain for the reaction isn´t that high, because N2O is relatively high-energetic molecule. But when N(III) react, N2 is evolved. Nitrogen´s triple bond has one of the highest energies in whole chemistry, so any reaction, where N2 could be evolved is usualy favored and proceeds very smoothly due to vast energy gain.

That is what is happening "behind the scenes" Warmer the solution, quicker the decomposition of nitric in AR is = quicker de-noxing. Past 75°C, undecomposed nitric will start to react too - but at this temperature, decomposition is very quick, and N(III) species react even at r.t. with sulfamic.

I personally heat to 60-65°C (or let cool to less than 70°C when I take it off the hotplate), adding sulfamic slowly until obvious rolling reaction stops. Then leave for 15 minutes at 60°C, add one spoon and observe. If I see any bubbles, I wait another 15 minutes and repeat. If no bubbles evolve, i left it cool down and proceed further