aqua regia (without hydrochloric acid)

[Anders Hoveland]

It is commonly known that a mixture of concentrated nitric and hydrochloric acids can dissolve gold, where neither acid alone would work.

HNO3 + (3)HCl --> NOCl + (2)H2O + Cl2

Less commonly known, however, is that a mixture of concentrated nitric and sulfuric* acids can also dissolve gold. The reaction is slow at room temperature, but rapid with heating. The reaction equation is basically:

(2)Au + (3)NO3[-] + (18)H[+] --> (2)Au[+3] + (3)NO[+] + (6)H3O[+]

Interestingly, diluting the acid mixture with water causes the gold to solidify back out in metallic form! This can be prevented by using a solution of potassium permanganate instead of the plain water, or by boiling ammonium sulfate with the acids before diluting.

Mixtures of manganese dioxide and sulfuric acid can also dissolve gold.
Again, the reaction is slower at room temperature, but rapid with heating.
Potassium permanganate and sulfuric acid after a few minutes also dissolve gold.

*note that the sulfuric acid must be over 95% concentration, and the nitric acid should ideally be 70%, although 40% can also be used if a larger ratio is sulfuric acid is used. Theoretically, nitric acid alone would work, but 95% nitric acid is generally harder to prepare and not easily obtainable. Basically, the use of the sulfuric acid allows extremely acidic conditions necessary for the reaction.

for those chemists reading this that are wondering, the NO[+] ion is called the nitrosonium cation, and actually exists in solution as nitrosylsulfuric acid. A bottle of nitrosylsulfuric acid can be seen in the link below:
https://7007983270460794965-a-1802744773732722657-s-sites.googlegroups.com/site/ecpreparation/other/nitrosylsulfurik.GIF?attachauth=ANoY7co2YGoXyt3DPC9DxUX0-ZE3OtHpAb3SNjEUCJbnI310o8EpLKrOByu7Y_snB4vwpK0l8Uv0okd1GuqtWMQVYrTTzLxnBboUfdI4ngQlgGOz_Ma0vChKdLA61wiLUwPbKvIieVIvtUk2aA6aAvai1IhIbFKWNGVCnVnyRFzku_V_bRPOdqgqURyFe0vqAMZ93b7vas9qTnM9_aPGhJkjoOndg72FZQ%3D%3D&attredirects=1

 

[Arrowood]

Wow,. thats really interesting! If diluting the nitric and sulfuric acid can precipitate gold too,. why would you want to prevent this from happening? Just dissolve the gold, dilute and get it back as pure metal,.. seems like the perfect solution to rapid gold recovery to me.

I'm interested in the mang dioxide and sulfuric acid technique too. Whats the chemical equation look like for the mang dioxide and sulfuric reaction?

 

[4metals]

Note the concentrations of acid required, this is nasty stuff used hot. Not for the backyard processor. It would require a lot more benefits for me to consider this, be cautious and don't jump into anything. I'd like to hear from someone doing it, not someone posting it.

 

[kurt]

Hmmm – interesting – but I agree with 4metals – more info needed on procedure & precautions. I may be new to refining & still have a lot to learn – but I do know that when working with high concentrate sulfuric “great” care needs to be taken when mixing with other acids &/or when diluting.

Just out of curiosity – does it precip just the gold when diluted or gold & some other things or everything?

Kurt

 

[samuel-a]

That is quite interesting.

(5)Au + (3)NO3[-] + (18)H[+] --> (5)Au[+3] + (3)NO[+] + (6)H3O[+]

I'm no chemist, but this eqesion doesn't looks right to me,there's a missing of Sulfur somwhere.
Or maybe i need further explanation from you.

I sure want to experiment with that procedure in a test tube.

What is the resulting Au^-- compound?
What ratio of the acids?
how much gold will it dissolve?

It will be interesting to hear about it, if you have some hands on experience...
Thanks

 

[samuel-a]

I sure want to experiment with that procedure in a test tube.

Well... i did...

68% HNO3 / 98% H2SO4 at 1:1 ratio.
Room temp' = nothing
Boiling = nothing

I guess there's a reason why this method is less common....
It did clean the gold real nicely though.

p.s - how would one filter that kind of mixture (assuming it did dissolved some gold) without making nitrocellulose from the filter.

 

[Anders Hoveland]

Excess sulfuric acid needs to be used. This is an equilibrium reaction, and the gold is not going to dissolve easily. The mixture needs to be extremely acidic. Even a 1:1 ratio of 70% HNO3 to 95% H2SO4 is not going to be concentrated enough. For good results, use a 1:10 rato of 70% nitric acid to 98.5% concentrated sulfuric. Essentially, there can be no water in the reaction!

Even in the hot boiling mixed acids, the gold takes several minutes to dissolve.

The NO[+] ion hydrolyzes (reacts with) water to form nitrous acid.

NO[+] + (2)H2O --> HNO2 + H3O[+]

Nitrous acid is fairly reactive, and can act as either a reducing or an oxidizing agent. It will reduce the dissolved gold (Au+3) to elemental form (Au). This explains why the gold precipitates back out when the reaction is diluted with water.

(2)Au[+3] + (3)H2O (3)HNO2aq --> (2)Au + (6)H[+]aq + (3)HNO3aq

(note that "aq", which stands for "aqueous", means it is dissolved in water)

If fuming nitric acid is added to the reaction containing the dissolved gold, the gold will solidify out as a purple solid. The gold is probably still in its elemental form, but small particle sizes of gold are known to exhibit strong colorations, from red to purple.

Sorry for all the chemistry information, but some of you will undoubtedly be curious about the specifics of the reaction.
Nitrous acid is unstable, and only exists in the form of solutions which gradually degrade after several minutes. Solutions of nitrous acid exist in equilibrium with nitrogen dioxide and nitric oxide, the latter of which is an unstable radical which can either react with the oxygen in air to form more nitrogen dioxide, or if left on its own will disproportionate into nitrogen dioxide and nitrous oxide after several minutes.

(2)HNO2 <==> H2O + NO2 + NO

(3)NO --> N2O + NO2


In the reaction,
(2)Au + (3)NO3[-] + (18)H[+] --> (2)Au[+3] + (3)NO[+] + (6)H3O[+],
sulfate ions are not shown because they do not directly take place in the reaction. The literature even states that phosphoric acid can be used in place of the sulfuric acid.

The above reaction is in ionic form. Some of you may prefer to see it in the form:
(2)Au + (3)HNO3 + (15)H2SO4 --> (2)Au(SO4H)3 + (3)NOSO4H + (6)H2SO4*H2O

Note that the "Au(SO4H)3" only exists in the solution, it cannot be isolated. Gold trinitrate, if it even exists, would also be nearly impossible to obtain as a pure solid. Gold trinitrate only exists in highly concentrated solutions of nitric acid. When these solutions are diluted with water, auric oxide precipitates out. Similarly, auric oxide dissolves only in very concentrated acids, since it is only very weakly basic.

Au2O3 + (9)HNO3 <==> (2)Au(NO3)2 + HNO3*H2O


Edit: Note that the equation should have had (2) atoms of gold, not 5. The mistake has been corrected.

To answer some of your questions, when the reaction is diluted and the gold precipitates out, it is pure gold. Nothing else solidifies out. Diluting concentrated sulfuric acid is potentially very dangerous. Some of you that have taken chemistry have undoubtedly heard "Never add water to acid!". Adding water to concentrated sulfuric acid generates enough heat to instantly cause the water to boil, which can cause some of the corrosive acid to vigorously splash/spray out! If you simply poured a cup of water into the reaction to try to dilute it, you would undoubtedly end up being rushed to the hospital, and likely suffer permanent eye damage if you were not wearing goggles!

SAFETY PRECAUTIONS: Concentrated sulfuric acid is extremely dangerous and corrosive. Be sure to wear a waterproof apron, long rubber gloves, eye goggles (preferably the splash proof type), and long sleeved shirt and pants. Have a big bucket full of water ready in case of emergency, so you can douse yourself with the water if any of the acid spill on you. Optional additional safety precautions are to have an eye wash bottle, filled with baking soda dissolved in water, that can be sprayed in your eyes/face if acid gets in. You may also wish to have someone else around, in the event you need assistance. Be sure to tell them before that you will be working with corrosive acid before the procedure so you will not have to do any explaining in the event you need immediate help. If any acid spills on you, throw water on the affected area of skin immediately. The acid burns skin very rapidly. A delay of only a few seconds can make a big difference. Immediately rinsing with water can greatly reduce the extent of chemical burns. Nitric acid should only be heated outside, or under a fume hood with good ventilation, as mildly poisonous fumes are given off.

The reaction should be extremely diluted, with at least 20 times its original volume with water, before being filtered. You can also add some baking soda (sodium bicarbonate) to the diluted mixture before filtering. The baking soda is going to cause much bubbling, so be sure to use a very big container so it does not overflow. Add the baking soda in small portions until the addition of more baking soda ceases to cause any more bubbling. Be patient. The neutralization is going to consume a large quantity of baking soda, and this will likely take ten minutes to do. Ideally, you do not want to use more baking soda than required, because too much baking soda may not dissolve, and it will then solidify out with your gold, but of course you could later wash the gold with vinegar to dissolve out the baking soda.

After all the acid has been neutralized, you can decant off most of the liquid from the top and discard it. Scoop out the gold solids at the bottom and wash it in water, then filter the pure gold out with coffee paper.

 

[Harold_V]

Excess sulfuric acid needs to be used. This is an equilibrium reaction, and the gold is not going to dissolve easily.

Then I question why you have taken the space on this forum to post what you did.

Try to understand that this isn't a place for you to display your prowess in chemistry. If you post methods that have no particular advantage, or are dangerous, you are misleading those that seek good, sound advice. That isn't going to be well accepted here, and you're going to find yourself at odds with me, something that isn't going to end up in your favor.

I took note that you were banned from another forum, just as I took note that GSP suggested that you tend to post things that go ***boom***. That's going to come to an immediate halt here---otherwise you will have worn out your welcome.

Harold

 

[goldsilverpro]

Well, Anders, it seems that everything you post is fraught with dangers. Also, everything I've seen from you so far is totally worthless to the refiner. I think you're one of those sick puppies that like to stir the danger pot and then stand back to see what happens. You already have 2 strikes against you. Any more dangerous posts and you'll be banned. Period!

It took a while for me to catch on and wade through your tripe, but I now see that this super hot 90H2SO4/10HNO3 is a "wet ashing" mix. Nothing new. I have used it many times and can guarantee it is extremely dangerous to use. The gold will definitely dissolve in it but will ppt out as a powder with continued heating - once the nitric and any traces of water present have been expelled and and a vast plume of very dangerous white fumes are coming off the solution. I only used it for specific analytical purposes and, due to the grave dangers in using it, I limited the quantity I used to about 200-500 ml. I once was offered quite a bit of money to set it up on a large scale for processing gold bearing ion exchange resins. I turned this down due to the inherent dangers in the process.

I would highly recommend that NO ONE even attempts to use this. It likes to bump and spit and it's hard to use it without getting burned. Like I said, I have quite a bit of experience with it and know that the burns take quite awhile to heal. There is NO APPLICATION for this in refining or, at least none that would override the dangers. There are much safer ways to do anything that this mix will do.

 

[Anders Hoveland]

The reaction is more interesting from a chemical perspective than a practical way to refine out gold. Nevertheless, the reaction may be useful to directly dissolve gold-silver alloys, without having to go to the trouble of inquartation.

Yes, it is extremely dangerous. The dangers of using concentrated mixed acids are commonly taken for granted among those that frequently perform nitrations. Obviously those unfamiliar with such procedures should think twice before handling such high concentrations of acid.

More details about the reaction. The concentrated acid mix that contains the dissolved gold should be gradually transfered into the larger bowl of water using a 10mL glass transfer pipette. You will also need a rubber pipette suction bulb. For those of you unfamiliar with this tool, it is basically like a turkey baster that is used to suck up a small quantity of liquid, then move it to another container. The pipette can be bought here:
http://www.pelletlab.com/pipette

Using the pipette to slowly add the acid mixture to the water is important for two reasons. First, safety. Water should never be added to concentrated acid, since this can result in the acid spraying up. Neither should the acid be poured into the water, because of the possibility of an accidental spill or splashing, and because it can be hard to control the rate that the liquid is poured in. Adding the acid in too fast can lead to overheating, which could result in boiling/splashing in the water. Second, it is important that each small portion of the acid quickly be diluted with as much excess water as possible. This will help prevent the gaseous nitrogen oxides (NO and NO2) from escaping. Although nitrosylsulfuric acid reacts with excess water to form a solution of nitrous acid, if not enough water is used nitrogen oxides will bubble out instead.

There will inevitibly be some loses of nitrogen oxides, in the form of some bubbling and some brown gas being given off. Unfortunately, when some of the nitrogen oxides escape, there will not be enough nitrous acid to completely reduce the gold. After neutralizing, all the gold will still precipitate out, but a small portion of it will be in the form of hydrated gold oxide, Au2O3. If the gold is going to later be melted, the gold oxide should not pose any problems, as the compound decomposes to the pure metal at 160°C, giving off oxygen gas.

One other note of warning, unless the gold oxide has been completely reduced, it should not be reacted with ammonia, as this will form the dangerous sensitive explosive known as "fulminating gold". In the event that the acid solution was previously boiled with ammonium sulfate to prevent precipitation of the gold, fulminating gold can result upon neutralization if too much ammonium sulfate was added.


More safety information:

Only use small quantities of mixed acids at a time. Be aware that with concentrated acids, even tiny drops can splash out and result in painful burns on exposed skin. To get some understanding of these dangers, try pouring cranberry into a glass, wearing a clean white long-sleaved shirt. Even with cautious pouring, you are likely to find one or two tiny little red stains on the sleeves afterwards, even though you were not aware of any splashing while the juice was being poured. If this was concentrated acid, painful burns would have been felt.

You may desire to cover your shoes with a plastic bags and a rubber band, so that if any of the acid spills onto the floor, it will not seap into your shoes. Protective shoe coverings can also be purchased:
http://www.labsafety.com/search/shoe%2Bcovers/
If you choose to wear rubber boots instead, it is advised that the top of the rubber be tied tight around your legs, so that if any of the acid is spilled on you, it will not drip down into the boots and collect in a puddle. If the acid is in contact with your skin for more than a few seconds, the burns will be much more severe. http://www.amazon.com/b?ie=UTF8&node=393294011

A boiling mixture of concentrated nitric and sulfuric acids is extremely dangerous, much more so than 70% concentrated sulfuric acid, for example. The chemistry of this mixture presents several unique hazards. Extremely concentrated sulfuric is a strong dehydrating agent, that will turn anything organic, such as a strip of paper or your skin, into black char immediately on contact. A note about treating concentrated nitric acid burns, after you immediately rinse the affected area with plenty of water, and neutralize with sodium bicarbonate solution, there is special recommendation for concentrated nitric acid burns. Use a swab dipped in chlorine bleach to gently scrub the affected area. Some of the yellow color from the burn should be absorbed onto the cotton swab. Continue to scrubbing with fresh swabs until no more yellow can be absorbed onto the cotton. Then rinse well in soapy water. Doing this will help remove some of the nitro compounds which have formed. These compounds act as allergens and greatly slow the healing process. In fact nitric acid burns take much longer to heal than sulfuric acid of the same concentration. The unique effects of concentrated nitric acid are due to the formation of nitronium ions, NO2[+], in equilibrium in the solution. The addition of highly concentrated sulfuric acid greatly enhances this equilibrium, and so the special burn effect of nitric acid will be greatly exaggerated by the acid mixture. In other words, it would be very important to treat the burns in the way described above, and the healing time is likely to be much longer.

Dissolving Gold with Manganese Dioxide
"Arrowood" asked about the reaction with manganese dioxide and sulfuric acid. The reaction is probably:
(2)Au + (3)MnO2 + (3)H2SO4 --> Au2O3 + Mn(SO4)2 + (2)H2O

where Mn(SO4)2 is manganese sulfate, and the gold oxide dissolves in the sulfuric acid. concentrated sulfuric acid still needs to be used, but it probably does not need to be quite so concentrated as required for the other reaction; a 70% concentration should be suitable.

 

[Harold_V]

More details about the reaction.

All details snipped.
That's what I like---someone that doesn't take NO for an answer.

You were properly warned.

Harold

 

[Geo]

every thing we touch from glass to fire to chemicals is all dangerous enough handled in the most careful way,i don't understand why someone would post a process that would endanger people needlessly.

 

[Harold_V]

every thing we touch from glass to fire to chemicals is all dangerous enough handled in the most careful way,i don't understand why someone would post a process that would endanger people needlessly.

Perhaps he's just a moron. What ever the reason, he won't be doing it again on this forum, and if he returns and doesn't alter his behavior he'll be gone once again. What we don't need (or want) is readers posting processes that can be interpreted as bordering on terrorism.

Harold

 

[Arrowood]

Does anybody know anything about the Manganese dioxide and sulfuric reaction, or the Potassium Permanganate and sulfuric reaction Anders mentioned? I'm curious.

 

[jimdoc]

Google search probably does. If Harold flagged him because of posting hazardous and non refining related posts, I think we should leave it at that here on this forum.

Jim

 

[Arrowood]

I understand why Harold booted him. Safety is important, and I want nothing to do with extremely hazardous chemical reactions. It seems to me that the mag dioxide and sulfuric reaction and/or the Permanganate and sulfuric reaction could be a lot less hazardous however, and might have some benefits. If they are dangerous reactions, then it would also be wise to know about them.

 

[Arrowood]

It appears that Manganese dioxide and sulfuric acid is used to process Molybdenite rather than gold from what I found about the reaction. That reaction appears to pretty safe. Mixing concentrated sulfuric acid with Potassium Permanganate however gives Mn2O7, which can be explosive! Not safe!

 

[Harold_V]

It appears that Manganese dioxide and sulfuric acid is used to process Molybdenite rather than gold from what I found about the reaction. That reaction appears to pretty safe. Mixing concentrated sulfuric acid with Potassium Permanganate however gives Mn2O7, which can be explosive! Not safe!

If you'll refer to a Merck Index, you'll come to understand that there are compounds of gold that are so unstable that they are known to exist in theory, only. Couple that with encouraging a reader to experiment with substances that are already unstable isn't in keeping with the purpose of this forum.

Explosives are made quite easily from gold, along with other heavy metals, so it is ALWAYS a good idea to stick with known processes. We don't want to be even remotely responsible for a process that risks being troublesome, let alone dangerous.

When we, as moderators, encounter anyone that displays a willing disregard for the safety of others in suggesting a process that offers risk, they have already outlived their welcome.

Harold

 

[Lou]

AndersHoveland is a well known know-it-all troll extraordinaire from Science Madness (and many pyrotechnica forums) who likes to sling equations that he got from the Re-dox balancer web calculator. He's been banned several times at that forum.


In any case, all of the stuff he wrote is utter garbage having no practical value whatsoever. He posted the same bull on SMDB and did not accept my correction there; I would have surely banned him here. He probably found this forum on a web search for a justifying reference to his inane/insane chemopsychobabble that was first posted there. Y'all were much more polite with him than I would have been.


"In theory, there is no difference between practice and theory," his moniker on SMDB; who can guess why?


Please all remember that there is ZERO TOLERANCE towards any illegal talk here. People like Anders are why you get so many questions when you buy nitric acid.

 

[Arrowood]

"In theory, there is no difference between theory and practice",. is a partial quote attributed to Jan L. A. van de Snepscheut. The whole quote is,.

"In theory, there is no difference between practice and theory, but in practice there is." He left out the most important part in my opinion. Why, I don't know, but I suspect it was to justify his own dangerous experiments.