How much gold can ar dissolve ?

[razvanflorin]

Hi guys ! i would need a fast answer. How much gold can Aqua Regia dissolve ? 3:1 ml ??

Kind Regards - Florin.

 

[goldsilverpro]

Assuming an open-top container, about 2 pounds/gallon = 1g/4.2ml @ about 4:1 - 4.5:1. It's always close but never the same exact amount twice. Most common base metals take about the same amount.

However, except for rare situations, pre-mixing the AR is a bad idea. Everything ends up better if you cover with HCl (about 150ml/ounce of metal), heat to about 140F (never boil). and add a little nitric (about 5ml/ounce of metal). When the reaction slows down, stir a little and then add a little more nitric. Repeat until an addition of nitric produces NO reaction. Stop adding.

With this addition method done properly you will only use the necessary amount of nitric to dissolve the metal and never add a excess. Since gold precipitation with SMB (or other chemical) is hindered by a excess of nitric, this will make it much simpler to drop your gold.

 

[kernels]

As above, the REAL question is, how much Nitric Acid do I need to dissolve a gram of gold in AR ? Because of the nature of the process, it doesn't really matter how much Hydrochloric Acid you use, as long as you use enough.

 

[MissRoxy]

heat to about 140F (never boil). and add a little nitric (about 5ml/ounce of metal)

laughs now i understand whyi get better results the hotter it gets in calif

 

[anachronism]

As above, the REAL question is, how much Nitric Acid do I need to dissolve a gram of gold in AR ? Because of the nature of the process, it doesn't really matter how much Hydrochloric Acid you use, as long as you use enough.

Assuming you are talking about a re-refine where the gold is almost pure then the answer is much less than a cc. You can dissolve an awful lot of pure gold with 10cc. If however you are talking about "dirty" product in AR with copper etc to dissolve before the gold goes into solution then the answer is still the same but you need to take into account how much it took to dissolve the base metals. At which point it all starts to get a little cosmic when you're trying to crunch the numbers.

Every not pure dissolve solution is going to be different Kernels, and you have to take that into account. Suffice to say that 1cc per gramme is not a hard and fast rule.

 

[kernels]

Yeah, actually, I wasn't asking a question really, more pointing out to the original poster that they were asking the wrong question. Thereby also suggesting that it is a silly idea to pre-mix their AR.

I did do the stochiometry early on when I started and the simple answer is, for every gram of gold, you need 1.93mL of HCl and 0.32mL of HNO3.

That obviously works for pure gold, and is also reasonably close for something like foils recovered from RAM.

 

[Rmwatson78]

I have been purchasing a clear premixed Aqua Regia solution on ebay and although it seems to work effectively, the reactions are definitely milder and it seems to take more of it than if I had just bought and added the hcl and nitric in 3:1 or 4:1 proportions myself.

 

[nickvc]

As a guide I'd suggest to work out what HCl you will need and add 10-20% more then slowly add nitric in small increments until all reaction stops, you need to heat your solution to use the acids economically and fully, when all reaction stops add some more HCl and watch for any reaction and if none you have dissolved your values but if you get some more reaction then add further HCl and small increments of nitric.
My advice is not to premix AR but make it as needed in your beaker too much HCl is never bad but too much nitric is costly and causes problems.

 

[Shark]

I wish I could answer with certainty, but the best I can say is, using GSP's method, I put 25.8 grams into 150ml HCl with 20ml of 67% nitric. I have just reached the point of increasing the heat to denox any remaining nitric, but I am guessing there isn't very much left. The last tiny piece took several hours with minimal bubbling. I added nitric 2ml at a time.

View attachment 33178

Edit for better picture size.

 

[g_axelsson]

That beaker looks huge!

I usually calculate 1 ml nitric and 4 ml HCl per gram Au and add half of it to start with and it usually dissolves 80% of the gold. Then I pour off the bulk of the solution and add a bit of fresh HCl and more nitric according to how much gold I judge is left. If I need to I might repeat the last stage.

This way I work with less volume and higher concentration of AR for the last few bits of gold and that speeds up the process.

Adding 2 ml nitric to 200 ml liquid only produces a 1% strength AR so there is no surprise that it takes forever to react with the last piece of gold.

Göran

 

[butcher]

That is pretty much the amounts, and ratio's I use to calculate how much I need, 3.8ml HCl acid and 0.95 ml HNO3 acid for each gram of fairly pure gold.
keep in mind that these ratio's (which come from Hoke's book and stoichiometric calculations) are for gold only without consideration of base metals. Copper or base metals involved can drastically change how much of both of these acids are needed. Base metals can consume much more of both of these acids, Excess HCl acid is not a problem, adding nitric in small additions, as needed to complete the reactions...

Another thing I keep in mind is dilution and heat. Dilution may take longer but it does not waste my acids as gases, heating speeds reactions but can gas off more of the volatile components during the reactions.

Dilution at the first when acids are concentrated can keep gasses in solution longer to react with metal, instead of making deadly clouds of gas which waste the acid. keeping the temperature low as possible at first also helps. As the acids are consumed and convert the metal to dissolved salts in solution, some of the acids are converted to more water in the reaction, diluting the remaining free acid further. As the solution becomes more diluted and weaker in free acid, mild heating now can drive off water through evaporation, and concentrate the free acid in solution. At the end of the reaction raising the temperature more drives off the volatiles concentrating acid and making it more vigorous due to the heat of the reaction. This evaporation can proceed after the metals are completely dissolved and water has been fairly well driven out of solution, to evaporate off any free nitric you may have added in excess.

Note: during the evaporation process to de-Nox the solution of free nitric acid, base metals involved can form nitrate salts (i.e. copper nitrate), which can not be driven out of solution through evaporation, these nitrates can reform free nitric acid in solution when HCl or another acid is added. This is why it can take more than one evaporation process and we add a little HCl acid during each step...

Water is more volatile than nitric acid, Nitric acid more volatile than HCl acid, HCl acid much more volatile than H2SO4 (which can take high temperatures to decompose and gas off out of solution...
Boiling points for pure solutions change with dilution or concentrations...
Pure water boils at around 100 deg.C, (212 deg. F), add a little metal salt or a little nitric and we change that boiling point quite a bit, each concentration of an acid will have different boiling points...

This concept is very helpful when distilling or converting one metal salt into another metal salt (i.e. converting copper chloride into copper sulfate)...

 

[goldsilverpro]

Richard

Actually, I find that it takes about the same ball-park amount of AR to dissolve gold as it does to dissolve copper or iron - about one gallon of AR to dissolve 2 pounds of metal in an open-top container.

 

[butcher]

Chris,
Looking closer at the some of the reaction which may occur, I can see I am wrong with what I think, as usual, that is what happens when I think, instead of confirming my thoughts through experimentation or research. Before I open my big mouth.

looking at some reactions and calculating how much here are some of what I get (to prove myself wrong).
Oxidation of some metals in nitric alone:
copper in nitric
Cu (s) + 2 NO3- (aq) + 2 H+ (aq) → Cu2+ (aq) + 2 NO2 (g) + H2O (l)
silver in nitric acid:
Ag (s) + 2 NO3- (aq) + 2 H+ (aq) → Ag+ (aq) + 2 NO2 (g) + H2O (l)
gold by nitric acid:
Au (s) + 3 NO3- (aq) + 6 H+ (aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l)
(then dissolution of gold by forming chloraurate):
Au3+ (aq) + 4 Cl- (aq) → + AuCl4- (aq)

------------------------------------
Dissolving gold, the nitric acid ionizes neutral gold atoms to gold ions in a redox reaction, then hydrochloric acid provides chloride ions forming Au3+
The conversion of a neutral metal into a positively charged cation (oxidation) happens because of loss of the gold atoms electrons. Redox reaction where gold changes its oxidation state by transferring of electrons. Oxidation of Au to Au3+, the gold atom loses three electrons.

Oxidation of gold by nitric acid:
Au (s) + 3 NO3- (aq) + 6 H+ (aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l)
Dissolution of gold by formation of chloraurate:
Au3+ (aq) + 4 Cl- (aq) → + AuCl4- (aq)

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Calculations comparasion of dissolveing copper and gold in Aqua Regia
---------------------
Copper in Aqua Regia
1.5 Cu (s) + 3HCl (aq) + HNO3 (aq) --> 1.5CuCl2 (aq) + NO (g) + 2H2O (l)
Copper dissolved
1.5moles Cu @ 63.54g/mol = 95.31g Cu
3moles HCl used @ 36.46g/mol = 109g HCl
1 mole HNO3 @63g/mol = 63g HNO3
products ------->>
1.5 moles CuCl2 @ 134.45g/mol = 201g Copper II Chloride
1 mole NO @ 30g/mol = 30g Nitric oxide
2 moles H2O @ 18.01g/mol = 36.02g water
-----------------------------------
Gold in Aaqua Regia
Gold disolved
Au (s) + 3HCl (aq) + HNO3 (aq) --> AuCl3 (aq) + NO (g) + 2H2O (l)
1 mole of gold @ 196.96g/mol = 196.96g Gold
3moles HCl @ 36.46g/mol = 109g HCl
1 mole HNO3 @ 63.01g/mol = 63.01g HNO3
products ----->>
1 mole AuCl3 @ 303g/mol = 303g Auric Chloride
1 mole NO @ 30g/mol = 30g Nitric oxide
2 moles H2O @ 18.01g/mol = 36.02g water

Thanks Chris that why your called the Pro

 

[Shark]

Washed and ready for the second refine. Sorry for the thread hijacking, but it fit right in with the project I had been working on.

View attachment 33208

 

[Wingedcloud]

Richard

Actually, I find that it takes about the same ball-park amount of AR to dissolve gold as it does to dissolve copper or iron - about one gallon of AR to dissolve 2 pounds of metal in an open-top container.

Hello.

Been reading this topic carefully, as I am currently toll refining ceramic cpu's and wanted to know how much AR to use per kg of CPU to optimize both acid consumption and leach time.
Saw this 1 gallon AR/2 pounds of metal from GSP and would like to ask something: you meant 1 gallon of a 3:1 ratio AR ?

Because, like Kernels wrote in this post also, the real question is how much nitric I need to fully dissolve both the base metals and the gold from the pins.
I am currently trying to make 4kg batches of CPU, covering them on HCl and adding nitric as needed. The thing is I never know how much nitric is too much. Don't want to add in excess and have trouble dropping the gold later.

If anyone can build up on this, would be much appreciated.

Winged.

 

[Geo]

That would be based on knowledge of how much gold is present not counting base metal. Leach the CPU's in 50/50 nitric acid or poor man's nitric acid until there is no more reaction to the acid. The nitric acid is cleaner and the PMN leaves more solids to deal with in the remaining solids but it is still an improvement before dissolving the gold. The dissolution of the gold should not be based on a premixed aqua regia amount. After the nitric leach, decant the solution leaving any solids behind and rinse any filtered foils back into the other solids. Add HCl until the acid covers all of the solids well. Have your beaker in the fume hood or other suitable fume control because there should be an immediate reaction with left over nitrates from the nitric leach. Place the beaker on low heat and gradually increase heat until it is hot enough to be uncomfortable to the touch. The reaction should continue. You may not need to add more nitrate depending on how well you dissolved and removed the base metal. If the reaction stops before ALL metal is dissolved, add a small amount of nitric acid or nitrate and cover. Allow the reaction to continue until it stops completely. Examine the solids. Any sign of metal of any kind means to add more nitrate. Continue until ALL metal is dissolved. This insures that all of the gold has dissolved and will remain in solution until it is precipitated.

 

[Wingedcloud]

Hello Geo, thank you very much for your reply.

I perfectly understood your recommended approach.
Have just one question: can the initial nitric leach be done efectively at room temperature?
Since I am using plastic buckets to leach, I have no proper way of heating them with damaging the bucket.

I like the method. Will consume more nitric than initially planned but it seems to be way more straightforward and keeps the gold out of "foul solutions.

Winged.

 

[Geo]

It can be done at ambient temperatures but it will be much slower. When you prepare the nitric acid leach, add equal amounts of tech grade nitric acid and distilled water. Use this same combination until all reaction stops. You should be able to cement any silver from this solution using solid copper metal. There should be a fair amount of silver recovered from ceramic CPU's. Also, use stannous chloride to test for the presence of palladium salts. With prices as they are, you may consider precipitating the silver using table salt or HCl as silver chloride if palladium is detected with a stannous test. If you are wanting to recover the palladium, it is much easier to separate the two salts at this stage.

 

[Wingedcloud]

Will follow your advice. Will store the nitric leach for later recovery and focus on the gold for now

Thank you very much for the help !

Winged.

PS: after re-reading your post, some questions ocorred:

- I am using commercial grade 60% nitric. Why have you refered tech grade? Or is it the same?
. Why not use tap water? Because it may contain chlorides and get the silver out of solution ?

 

[philddreamer]

Winged wrote:

Since I am using plastic buckets to leach, I have no proper way of heating them with damaging the bucket.

I used to have the same problem... until I bought from Duda Diesel a couple of their pail heaters. They work great! One word of caution, though... make sure you start heating at a low temp. If you start too high, you could end up melting a hole in your pail!!!
Yep... been there and done that! :mrgreen:
https://www.dudadiesel.com/search.php?query=%2Bdrum+%2Bheater

Take care and be safe!
Phil