Precipitating Gold

[NobleMetalsRecovery]

I read today that you can use sulfuric acid to drop gold out of AR. I think you would add it during the last boiling down the solution to rid it of Noxx. The sulfuric acid decomposes and releases SO2 and SO3. SO2 will drop the gold, that's what's released from sodium metabisulfite.

Seems like it would work also with HCL/bleach or HCL/H2O2. just add it and boil it down.

It's too cold, and wet, and windy to do this in VA, maybe someone else can give it a try.

 

[qst42know]

Where did you read this?

I use a bit in my AR when there is a possibility of lead and as an aid for de-noxing, as Hoke recommends. I have not experienced gold dropping because of it.

 

[butcher]

from what I undertand its used to help rid nitric from aqua regia (also to precipitate lead as sulfite ) also helpful for keeping the solution from crystalizing out salts,
when ridding nitric acid from Aqua regia solutions evaporation techniques, not sure there would be any appreciable sulfur dioxide generated from it.
not positive. i also believe hokes book mentions this technique. I have not heard of it as precipitant unless when used to make ferrous sulfate, or in SO2 generation and so on.
also the man from mexico (Juan?)who is really good on silver answered a question of a problem where chocolate solution of gold having trouble precipitating from solution, adding sulfuric helps (probably from not eliminating nitric from solution?) but my thoughts were that it was the acid that helped here.

 

[NobleMetalsRecovery]

I'm not sure, my library of Gold related books is about 100 different titles. Once I find it again, I'll let you know.

The key point is that you'd have to boil it to the point where the sulfuric acid decomposes. So normally it wouldn't happen.

 

[butcher]

as the concentration increases so does the boiling point of the sulfuric acid, so before much sulfuric to boil off you would have to get over 600 degrees F or so, or boiling to 98% concentration before much of any SO2 released (maybe memory or a misunderstanding?).

 

[NobleMetalsRecovery]

I found it. The book is "Gold, Recovery, Properties, and Applications. Edmund M. Wise. 1964. page 174.

"Dissolve the gold residue in aqua regia, adding sulfuric acid, filtering, and boiling to S02 fumes. Finely-divided gold remains."

 

[NobleMetalsRecovery]

as the concentration increases so does the boiling point of the sulfuric acid, so before much sulfuric to boil off you would have to get over 600 degrees F or so, or boiling to 98% concentration before much of any SO2 released (maybe memory or a misunderstanding?).

I found some pictures today on the internet of someone making concentrated sulfuric acid out of battery acid. They boil it down to 25% of the original volume. But, they caution you to do it outdoors because it's releasing SO2 fumes the whole time while it is getting more concentrated.

So I don't think you would need to boil down all the way.

 

[NobleMetalsRecovery]

http://amazingrust.com/Experiments/how_to/Concentrating_H2SO4.html

 

[qst42know]

Having only worked with poor mans Ar and occasionally boiled to only the beginnings of dryness I suspect the gold remaining would be found in a quantity of salts.

When I have over cooked my solution and it had began to crystallize I re-hydrated the salts with HCL and dropped with SMB. I don't know what the end result would have been if I continued to heat to completely dry.

I would be interested in hearing what one of the chemists on the forum have to say of this technique.

 

[Platdigger]

Sounds similar to boiling a mix of metals in concentrated sulfuric, were nearly every metal will disolve except for gold.
Randy

 

[butcher]

SO2 will boil off after sulfuric in solution is 98%, as white fumes, and you could boil dry and have a gold salt solution,and salts of every thing else in solution, but would not be pure. to me that would kind of defeat my purpose,of using Aqua regia. was this a recovery process, or refining process? either way I would not use it.

 

[butcher]

when boiling sulfuric to concentrate mostly water bois off first (yes very little SO2 will) untill you get to concentrated azeotrope 98% then the SO2 boils out with very little water vapors, this azeotrope is why we can boil weak sulfuric to concentrate it, if SO2 boiled out early at low concentrations we could not concentrate it to 98%.
nitric Acid has an Azeotrope of 68% so if boiling from weak solution, mostly water boils off very little NOx untill we get to 68% concentration that is as concentrated as we can get it then the water and acid boil off at 68% till dry.
check out a chart of sufuric acid concentration and its boiling point temperatures.

 

[LittleJohnnyH2SO4]

The white fumes from boiling concentrated H2SO4 are SO3, not SO2. (small amounts will decompose to SO2). This is sulfuric anhydride, sulfur trioxide. Horrible awful stuff and it looks white because it's sucking moisture out of the air forming tiny droplets of H2SO4. Also horrible, awful stuff. I don't think I need to explain what a 550F splash of water-hungry concentrated acid would do to any skin, eyes, or for that matter, anything it touches.

It is much more likely that the AuCl3 is being decomposed by the high temperatures and the formed chlorine gas is driven off by the heat. It looks like gold chloride decomposes 40C under the boiling point of sulfuric acid, and the higher temperature would make it go even faster.

 

[arthur kierski]

i bought a book (from a brazillian professor who purify pm for more then 20years) who says ---if you make a fine powder from your alloys and them heat in ar for 10minutes, all the gold goes into solution --add twice the volume of water to the initial ar, filtrate and to the filtrate add more 8 times the volume of the original ar solution---example=100 cc of ar,dilute to 1liter---300ccar to 3liters----then you add to this dilute ar smb,mix until the solution becomes brown and wait till the gold goes down----------------one does not need to add urea or heat the ar 3times to eliminate the hno3---------------
i did this experiment yesterday and tested the solution after filtrating the gold out with stanous chloride ,and it did not indicate any gold--in other words---it worked
any member of this forum can make this experiment and reach its own conclusions-----for me it worked

 

[NobleMetalsRecovery]

i bought a book (from a brazillian professor who purify pm for more then 20years) who says ---if you make a fine powder from your alloys and them heat in ar for 10minutes, all the gold goes into solution

It makes good sense that this would work. You do not mention what amount of AR is used compared to the amount of alloy. If you have just enough AR all the nitric will be used up. Actually its hard to get the exact amount of AR to dissolve all the alloy so you try to use just a tiny bit more than is needed. That way you will dissolve all the alloy and have very little nitric left in the AR solution.

By diluting the solution to such a large degree, it greatly weakens the remaining nitric in the AR solution. So the SMB can drop the gold. You should probably filter it as soon as it all drops. If you still have some nitric left in solution, it would be able to slowy begin to redissolve the precipitated gold.

 

[arthur kierski]

he gives an example100grams ---containing;55grams of gold,20grams of silver,15grams of palladium and10grams of copper----he uses 3times the ar,that is225ml of hcl and 75m ml of hno3----all the metals dissolve in 10minutes---he adds 1litre of water,filtrate and to the filtrate add another 1700ml of water to complete 3liters(ten times)-------them ads smb and stir the solution----- with incinerated ash containing lets say 50 grams of gold he does a bit diferent---first hcl wash,(1kilo of ash) them 500ml of ar 4to1 at 50centigrades for 1hour --then a quick boil of the ar,add1.5 liters of water and filtrate---- in the filter he adds 4 times 500ml of hot water to complete 4liters of all and then smb---note:i am passing his way---if one is interested,i sugest that he does the experiment-----for me it is important because i have dificulty with urea(never know whem the addition is sufficient) and also i do not like to evaporate many liters of ar to get rid of hno3----- this is specially writen to nobel metals recovery who asked for quantities of ar in the process---

 

[goldsilverpro]

How do you "powder" the alloy? With 20% silver in his alloy, I can't see it dissolving quickly (or, if at all) in AR unless it were truly powder.

 

[arthur kierski]

here we have a shreder(a machine that triturates the alloys ) which makes a powder less then 0,1mm---this machine is made by a firm called ----hidrometal--------and in the book there is a plan of this machine--pity i cant show you --i do not knoww how to make a picture and pass to the forum

 

[qst42know]

I think I would rather remove the nitric than deal with 10 times the volume of waste.

By ignoring any remaining nitric and diluting a surplus of SMB could consume the remaining nitric by forcing it to redissolve the first precipitated gold however the partially dissolved gold crystals that dropped first may be very fine and lost through the filter.

If you used Sulfuric acid and boiled until the gold dropped from your AR your starting material would need to be 100% free of lead

While these methods seem plausible, to me they have drawbacks that can be avoided.

 

[Platdigger]

Why not forget AR entirely (for this aplication of finely devided metals), by just using nitric to take up the silver, palladium, and copper, and then HCL/CL for the gold?
Randy