Sciencemadness Discussion Board - Aqua regia dangers

Aqua regia dangers

mnick12 - 5-1-2010 at 23:13

So I have a few questions regarding the dangers of aqua regia. The reason why I am asking is because a few hours ago I took apart one of my dad's old brief-case sized laptops from the early 90s to see if I could salvage any silver or gold. After an hour of work I able to extract around 50 or 60 small gold wires. After that I mixed 15ml of RFNA with around 35ml of conc. HCl and placed the solution outside. Next I added the gold wires and immediately the solution fizzed and a lot of HCl and various nitrogen oxides began to flow out of the beaker. After about 15 minutes the reaction finished. So here is my question what are the dangers of heating the solution to dryness? I know it will become more corrosive, and a lot of nasty fumes will evolve but is there any chance of something like an explosion? As an alternative I could dry the solution over a desiccator, but that would take a dog's age.
Thanks for any input!

woelen - 6-1-2010 at 00:41

If you carefully heat the solution then you do not have to fear an explosion. Just be sure not to have any organic contaminants in the solution.

But a much better way to recover the salt is adding excess sodium sulfite to the liquid. This reduces the gold and causes it to precipitate. Then you can decant the liquid, rinse the gold with water and redissolve in a much smaller quantity of aqua regia and then slowly evaporate this liquid. Why are you using red fuming nitric acid for making the aqua regia? Plain 65...70% acid, mixed with conc. HCl also does the job and this is much easier to handle than the red fuming acid and it also is much cheaper.

mnick12 - 6-1-2010 at 00:55

Hello and thanks for the reply.
The reason why I am using fuming nitric acid is because I distilled it from a H2SO4/NaNO3 mixture, and while buying 70% may be cheaper in the long run I really don't need 2.5 liters of the stuff using up space in my lab. Now if I get some more space I may consider buying some so I don't have to make it every time I need it. Also will sodium metabisulfite work, because I do not have any of the sulfite.
thanks!

woelen - 6-1-2010 at 10:13

Yes, the metabisulfite also works.

argyrium - 6-1-2010 at 12:22

Add small amounts of metabisulfite or sulfite a little bit at a time. You will be producing sulfur dioxide gas and you want to do this slowly. You will also be producing NOx so all of his should be done out of doors or in a hood (at least use a respirator).

The less the residual amount of HNO3, the faster the reduction. You might want to boil your pregnant AR down first to drive off much of the HNO3 (adding fresh HCl when it gets syrupy - until no more red/brown vapors evolve). The solution will need to be then diluted w/ DI water before the SO2 stage.

Remember that the fumes may get nasty - please be careful.

mnick12 - 6-1-2010 at 12:38

Thanks for the the help.
Anyway I have not boiled down the solution yet because my hot plate is being used for a esterification right now. Something I noticed though there is a small crop of crystals on the bottom of the beaker, I think these are silver nitrate but I am not sure. Should I filter the crystals before I reduce the solution?

argyrium - 6-1-2010 at 13:36

What colour are the crystals and what shape are they? You started off with 50mL of liquid - approx. what is it now? What was the starting mass of the wires?

Evilblaze - 6-1-2010 at 13:44

Quote: Originally posted by mnick12

I took apart one of my dad's old brief-case sized laptops from the early 90s to see if I could salvage any silver or gold.

Ohh, I had the same problem half year ago, the only difference was that I recovered gold from CPU-s.
The original idea was the same what you also did, the aqua regia. I made it from 60% HNO3 and 30% HCl. It was quite powerful...
But there are easier (and safer) methods for dissolving gold. The HCl + H2O2 mix works also really good. At second time (when I realized aqua regia has a terrible smell

) I used AP (acid-peroxide) mix. It dissolved the gold in 10 minutes. The used chemicals were: 30% HCl and 35% H2O2.

And for recovering the precious metals (just an idea), from pH balanced solution: maybe with pyrogallol? As I know it reduces Au, Ag from their salts into pure metal. Any suggestion?

mnick12 - 6-1-2010 at 13:55

Oh I am glad someone else is trying out this experiment.
I personally have no experience in recovering precious metals, so I am not really the one to ask.

And yes aqua regia is terrible stuff, when I was dissolving the solution I had all my safety gear on, and I was coughing through my gas mask.

Argyrium.
The solution now is about 25ml in volume, and the solution has the color of nickel nitrate. Now I am unsure of the original weight of the wires though id guess it was around 7grams. Now the crystals are clear-ish and they look like potassium nitrate that has been recrystallized, but they are much thinner and more voluminous.

densest - 6-1-2010 at 14:07

The big problem with reducing the metals is getting base metals as well: copper, nickel, lead, tin, and iron are the biggest culprits. You must remove nitrate ion for most of the precipitation methods to work. I use urea and boil until it stops fizzing as that doesn't generate noxious NO/NO2 gas. The peroxide/HCl method works but outgasses Cl2 something fierce. Iron as Fe+++ in solution is a pretty strong oxidizing agent so it can mess things up too.

Silver chloride will precipitate. It dissolves in NaOH and can be reduced with glucose, for instance. Lead chloride will precipitate in a chilled solution and will redissolve in hot water. Silver chloride won't redissolve.

Gold precipitates with SO2 or oxalic acid at 70-80C. SO2 gas is better than adding sulfite/bisulfite because the salts change the pH, but sulfites work. Oxalic acid is very clean. You may get a red coating on your glassware - that's gold!

Palladium and platinum are difficult: both are present in small amounts and don't precipitate from dilute solutions. Boiling the syrupy mess down is essential for those. Once you've removed gold with SO2, NH4Cl will drop platinum as a complex. Dimethylglyoxime drops nickel and palladium.

If your solution is mostly things like iron, tin, lead, nickel, etc. adding zinc dust or flakes or iron dust to precipitate out precious metals and copper, washing the ppt., and redissolving will make separating the precious metals -much- easier.

You can easily lose 3/4 of your precious metals by being sloppy. A gram of gold is less than 3mm on a side - very easy to lose!

Evilblaze - 6-1-2010 at 14:14

Hmmm, green color? I'm afraid that is the color of the copper-chloride and not nickel.
The salt on the bottom are soluble? Than they are not silver. The silver in aqua regia forms insoluble AgCl.

Let's see what could you do....
Maybe: Add NaOH or KOH to the solution, heat it as much you can. Than all the metals what you need will turn into insoluble form. The copper will also participate as a blue solid, after when you heat it will turn black (but I think its not a new information to you). Place the obtained solid into something and heat it over 200 Celsius. The Gold will brake down to metallic gold (what will be a black powder). You can remove the CuO by HNO3 or HCl, it will dissolve in it and the gold will left back. Good luck! This is the simplest method, it worked for me.

http://www.goldrefiningforum.com/
This forum is about recovery of precious metals, you will find here everything.

[Edited on 6-1-2010 by Evilblaze]

[Edited on 6-1-2010 by Evilblaze]

mnick12 - 6-1-2010 at 18:20

Thanks for the link evilblaze. And now that I think about it, it does make more sense that it would be copper rather than nickel. Also i'm fairly certain the crystalline material is the nitrate rather than the chloride since there was an excess of nitric acid in the solution, but I will verify this later by doing a chloride test. And if I get a chance I will react some of the solution with a hydroxide ( like you suggested) and after I will attempt to reduce it with some carbon in my grill.

densest:
Thanks for the advice. I like the idea of using SO2 for the reduction. If I get the chance tonight or tomorrow I will try a SO2 reduction and post pictures, but this is only if I have the time (otherwise I may have to wait a week) and if I can figure out how to upload pictures.

Swede - 7-1-2010 at 07:49

If pursuing it for the fun and knowledge, excellent, but I don't know how much Au you are going to recover from a single PC. The gold seen is mostly plating, not solid (AFAIK) and I cannot imagine the yield is going to be profitable on such a small scale. But perhaps it is... I just don't know.

And yes the huge problem is going to be the base metals that dissolve along with the Au. It sounds like there is some excellent advise here to separate the base metals from the gold.

[Edited on 7-1-2010 by Swede]

mnick12 - 7-1-2010 at 10:22

Thanks swede,
I also don't think that I am going to get any measurable amount of gold, but if there is any hope its going to be one of these older computers. I forget where maybe it was TV who knows, but a gold recovery plant claimed older computers contained a significantly higher amounts of gold compared to todays computers.
Even if I cant get any gold, I would be perfectly happy with some silver. Now I hope I will be able to reduce the solution with SO2 today, and if I do I will post pictures.

Swede - 7-1-2010 at 14:11

Quote: Originally posted by mnick12

Thanks swede,
I also don't think that I am going to get any measurable amount of gold, but if there is any hope its going to be one of these older computers. I forget where maybe it was TV who knows, but a gold recovery plant claimed older computers contained a significantly higher amounts of gold compared to todays computers.
Even if I cant get any gold, I would be perfectly happy with some silver. Now I hope I will be able to reduce the solution with SO2 today, and if I do I will post pictures.

I do know those commercial operations can gather significant amounts of gold. I think the whole process is cool and worthy of pursuit, if for nothing more than the excellent chemistry behind it. Good luck and I look forward to the pics!

No Gold!

mnick12 - 7-1-2010 at 16:01

So as promised I tried the SO2 reduction of liquid part of the solution, and the only thing I made was sulfuric acid.

before reduction with SO2.bmp - 291kB

this is the solution before the reduction

the setup.bmp - 357kB

here is the setup

near the end.bmp - 357kB

heres the solution near the end of the reaction, notice the NO2 fumes. I am pretty sure that means there was some leftover HNO3 in the solution, because the SO2 is getting oxidized to H2SO4 by the HNO3.

Well I still have the crystalline product from the beaker, I am not sure what it is though I think its AgNO3. I hope you like the pictures

densest - 7-1-2010 at 17:13

The gold won't precipitate until the oxidizers are used up.

A reasonably sensitive test for gold is the purple of Cassius test - add a drop or two of SnCl2 solution and you should get a nice purple. The threshold is about the same as recoverable gold. It helps to get rid of the nitrate ion first.

Many chips used to have substantial amounts of precious metals in exotic solder alloys, leadframes, wires, etc. internal to the chip. One must grind them up or at least smash them well to get the values out. Older ceramic (especially monolithic ceramic "chip") capacitors used a palladium alloy on the ends (under the solder).

SnCl2 is air sensitive so keep your stock of solids well closed and make fresh test solution every week or so. Dissolve the SnCl2 in a little dilute HCl. Probably make only 10 ml at a time and keep it in a very small bottle well closed. SnCl2 used to be reasonably available - I don't know of any illicit uses for it.

Auric chloride is a beautiful golden yellow in solution.

[Edited on 8-1-2010 by densest]

[Edited on 8-1-2010 by densest]

Evilblaze - 8-1-2010 at 07:44

Ohh, this looks bad, really bad. When I made this, my solution was yellow colored. At first it was green, but when the gold started to dissolve the solution turned to yellow. It looked really nice:

After this first I tried electrolysis with platinum electrodes. This worked, my only problem was, that (in that time) I hadn't had any normal power supply and the electrolysis was really slow. I just filtered this solution and I added the KOH. The solution turned black and the filtered product was purified. The yield was really little, but it worked (I did this because I was interested in this and not for money...).

The electrolysis:

After adding the KOH:

[Edited on 8-1-2010 by Evilblaze]

[Edited on 8-1-2010 by Evilblaze]

Formatik - 8-1-2010 at 19:57

Besides also the other mixtures mentioned in the aqua regia analogues thread, there is also a mixture which will dissolve gold. The 'makeshift aqua regia', made from alkali nitrate with conc. HCl, or alkali chloride and conc. HNO3. I used KNO3 and conc. HCl with some heating some time ago to dissolve some gold. Though I didn't attempt to recover by precipitation.

mnick12 - 9-1-2010 at 11:59

Thanks for the help everyone, sadly I wont be able to do anything with the leftover salt for at least a week.
But I was able to test the copper contaminated compound with a NaCl solution, and I observed a TINY amount of what I presume to be silver chloride precipitate out of the solution. So it looks like I have a small amount of silver nitrate in my crystaline solid ( Yaaa!

) but I still dont know what the other stuff is.
Oh and good job evilblaze, looks like you had more luck thane me!

Evilblaze - 9-1-2010 at 12:21

Quote: Originally posted by mnick12

So it looks like I have a small amount of silver nitrate in my crystaline solid ( Yaaa!

)

When you add aqua regia to a metal it will turn this metal to a metal-chloride and not nitrate. If you add silver to aqua regia than nothing will happen. It won't dissolve.

Or if you add more nitric acid than what is in the "recipe" than the nitric acid will dissolve the silver, but the nitrosyl chloride/hydrogen chloride will turn it into silver chloride what is insoluble.

You added NaCl to your solution.... Maybe the solution was dilute and the NaCl didn't dissolved and this was the crystal what is on the bottom of the mix?

Fleaker - 11-1-2010 at 19:07

One can get appreciable amounts of silver to dissolve in aqua regia by saturating it with KCl (or if used at a low temperature, ammonium chloride). This is useful if one is analyzing the gold content of a karat gold ring and does not wish to inquart it.

Aqua regia is fairly safe in my experience, provided you keep things that don't belong in there out, namely anything organic. In the many, many, many liters of it I've used, I've never had an accident attributable to misuse of the acid.

Also, do not needlessly use aqua regia. For future reference: if you seek to remove the gold plating, you can easily do it with acid-peroxide (H2O2 and HCl), it dissolves the base metals first and the gold falls off as a foil. Many use it for electronics processing. You can even dispense with the dilute hydrogen peroxide and use an aquarium bubbler. It works by forming cupric chloride.

As a general rule when recovering gold or any other precious metal: NEVER use aqua regia unless you must. Your objective first should be to concentrate the gold in a form as pure as possible; only then do you dissolve it in aqua regia and precipitate with whatever way seems appropriate/needful.

I suggest you examine this forum: http://goldrefiningforum.com

mnick12 - 12-1-2010 at 14:12

Thanks fleaker,
the only reason why I used aqua regia was because I did not have any 35% hydrogen peroxide, and I did not think 3% would work. Also I was curious because up until that point I had never used aqua regia in amounts greater than 5ml. Thanks for the advice though.