Fascinating gold chemistry discovered by accident!

[Alondro]

I had a leftover waste solution of metabisulfite with dilute HCl (which also included sulfuric acid formed from the reaction) after dropping gold, and was curious to see if this could be used to strip base metals from pins.

I put some pins into in and closed the lid on the plastic jar, and placed it into our warm boiler room. The base metals partly dissolved over time, and eventually the solution became saturated, along with forming base metal sulfates and sulfites in various insoluble complexes.

After pouring out the saturated solution, which tested negative for gold, I added HCl and dissolved all the remaining base metal cores of the pins. Again, the solution tested negative for gold, but was forming black solids in addition to blue-green copper chloride and sulfate crystals. Once more, the solution was poured out and this time distilled water was added to dissolve the salts. I closed it up again, and that when interesting things began to happen! The solids began to change, first turning a greenish-grey and floating, then turning black over time and sinking. The solution began to test positive for gold and the foils began to disappear. I found this extremely interesting, as a dilute highly aqueous solution of only chlorides and sulfates/sulfites with no nitrate theoretically should not dissolve gold. And yet it was!

So, I dropped in some copper bits to cement the gold, and added more water out of a sense of suspicion of what was taking place, until the stannous test became negative, about 3 days after the cementation reaction began. A very dense black powder mixed with a significant amount of gold dust resulted (mixed in with the remaining gold foils) which proved utterly insoluble. I washed it several times in hot water.

I observed the acid waste solution as well in a beaker. It continuously forms an insoluble blue-green crust on the surface as it reacts with air, likely unstable sulfites or sulfides oxidizing and releasing SO2 at the surface, while forming copper hydroxide.

Then, I began to check what the black powder could be. After eliminating all the base metal salts (most from what is found on pins would be highly soluble salts, with only lead sulfate being insoluble... but that's white, and there could only be a tiny amount of it from the solder on the base of the pins.

The clue came from a faint rotten eggs smell I would detect every time I opened the jar during the various steps. Apparently, I was hitting on just the right combination of chemicals to produce hydrogen sulfide when the jar was closed tight, and in the aqueous acid mix, a small amount of gold was able to dissolve. It turns out that this very dilute chloroauric acid, when hydrogen sulfide is present, can precipitate out gold(III) sulfide! And with my set-up, more dilute chloroauric acid was made as the sulfide dropped out, freeing up the acid chloride to react again with more of the gold foils.

It is also likely that increasing the water percentage at the step where gold had been dissolving caused the sulfides to begin to decompose back to metallic gold, hence the very fine gold dust mixed in with the black powder. If it is indeed gold sulfide, then it should decompose entirely to gold and sulfuric acid with modest heating of the moist powder.

This is rather exciting from a chemistry point of view, as methods of producing gold(III) sulfide appear to give inconsistent results. If it proves to be gold(III) sulfide and I can repeat this result, I could make a standard protocol and get a chemistry paper out of it!

 

[Golddigger76]

I had a leftover waste solution of metabisulfite with dilute HCl (which also included sulfuric acid formed from the reaction) after dropping gold, and was curious to see if this could be used to strip base metals from pins.

I put some pins into in and closed the lid on the plastic jar, and placed it into our warm boiler room. The base metals partly dissolved over time, and eventually the solution became saturated, along with forming base metal sulfates and sulfites in various insoluble complexes.

After pouring out the saturated solution, which tested negative for gold, I added HCl and dissolved all the remaining base metal cores of the pins. Again, the solution tested negative for gold, but was forming black solids in addition to blue-green copper chloride and sulfate crystals. Once more, the solution was poured out and this time distilled water was added to dissolve the salts. I closed it up again, and that when interesting things began to happen! The solids began to change, first turning a greenish-grey and floating, then turning black over time and sinking. The solution began to test positive for gold and the foils began to disappear. I found this extremely interesting, as a dilute highly aqueous solution of only chlorides and sulfates/sulfites with no nitrate theoretically should not dissolve gold. And yet it was!

So, I dropped in some copper bits to cement the gold, and added more water out of a sense of suspicion of what was taking place, until the stannous test became negative, about 3 days after the cementation reaction began. A very dense black powder mixed with a significant amount of gold dust resulted (mixed in with the remaining gold foils) which proved utterly insoluble. I washed it several times in hot water.

I observed the acid waste solution as well in a beaker. It continuously forms an insoluble blue-green crust on the surface as it reacts with air, likely unstable sulfites or sulfides oxidizing and releasing SO2 at the surface, while forming copper hydroxide.

Then, I began to check what the black powder could be. After eliminating all the base metal salts (most from what is found on pins would be highly soluble salts, with only lead sulfate being insoluble... but that's white, and there could only be a tiny amount of it from the solder on the base of the pins.

The clue came from a faint rotten eggs smell I would detect every time I opened the jar during the various steps. Apparently, I was hitting on just the right combination of chemicals to produce hydrogen sulfide when the jar was closed tight, and in the aqueous acid mix, a small amount of gold was able to dissolve. It turns out that this very dilute chloroauric acid, when hydrogen sulfide is present, can precipitate out gold(III) sulfide! And with my set-up, more dilute chloroauric acid was made as the sulfide dropped out, freeing up the acid chloride to react again with more of the gold foils.

It is also likely that increasing the water percentage at the step where gold had been dissolving caused the sulfides to begin to decompose back to metallic gold, hence the very fine gold dust mixed in with the black powder. If it is indeed gold sulfide, then it should decompose entirely to gold and sulfuric acid with modest heating of the moist powder.

This is rather exciting from a chemistry point of view, as methods of producing gold(III) sulfide appear to give inconsistent results. If it proves to be gold(III) sulfide and I can repeat this result, I could make a standard protocol and get a chemistry paper out of it!

I would check the black powder for pgm's.
They can easily be masked by the color of your solution and go unnoticed until the gold is precipitated.

Otherwise that is very odd, let us know what it turns out to be .

 

[Yggdrasil]

I had a leftover waste solution of metabisulfite with dilute HCl (which also included sulfuric acid formed from the reaction) after dropping gold, and was curious to see if this could be used to strip base metals from pins.

I put some pins into in and closed the lid on the plastic jar, and placed it into our warm boiler room. The base metals partly dissolved over time, and eventually the solution became saturated, along with forming base metal sulfates and sulfites in various insoluble complexes.

After pouring out the saturated solution, which tested negative for gold, I added HCl and dissolved all the remaining base metal cores of the pins. Again, the solution tested negative for gold, but was forming black solids in addition to blue-green copper chloride and sulfate crystals. Once more, the solution was poured out and this time distilled water was added to dissolve the salts. I closed it up again, and that when interesting things began to happen! The solids began to change, first turning a greenish-grey and floating, then turning black over time and sinking. The solution began to test positive for gold and the foils began to disappear. I found this extremely interesting, as a dilute highly aqueous solution of only chlorides and sulfates/sulfites with no nitrate theoretically should not dissolve gold. And yet it was!

So, I dropped in some copper bits to cement the gold, and added more water out of a sense of suspicion of what was taking place, until the stannous test became negative, about 3 days after the cementation reaction began. A very dense black powder mixed with a significant amount of gold dust resulted (mixed in with the remaining gold foils) which proved utterly insoluble. I washed it several times in hot water.

I observed the acid waste solution as well in a beaker. It continuously forms an insoluble blue-green crust on the surface as it reacts with air, likely unstable sulfites or sulfides oxidizing and releasing SO2 at the surface, while forming copper hydroxide.

Then, I began to check what the black powder could be. After eliminating all the base metal salts (most from what is found on pins would be highly soluble salts, with only lead sulfate being insoluble... but that's white, and there could only be a tiny amount of it from the solder on the base of the pins.

The clue came from a faint rotten eggs smell I would detect every time I opened the jar during the various steps. Apparently, I was hitting on just the right combination of chemicals to produce hydrogen sulfide when the jar was closed tight, and in the aqueous acid mix, a small amount of gold was able to dissolve. It turns out that this very dilute chloroauric acid, when hydrogen sulfide is present, can precipitate out gold(III) sulfide! And with my set-up, more dilute chloroauric acid was made as the sulfide dropped out, freeing up the acid chloride to react again with more of the gold foils.

It is also likely that increasing the water percentage at the step where gold had been dissolving caused the sulfides to begin to decompose back to metallic gold, hence the very fine gold dust mixed in with the black powder. If it is indeed gold sulfide, then it should decompose entirely to gold and sulfuric acid with modest heating of the moist powder.

This is rather exciting from a chemistry point of view, as methods of producing gold(III) sulfide appear to give inconsistent results. If it proves to be gold(III) sulfide and I can repeat this result, I could make a standard protocol and get a chemistry paper out of it!

I'm puzzled.
Sulfites and HCl form SO₂ and NaCl and
Sulfates and HCl form Sulfuric acid and NaCl in the initial reaction, right?
Or are you counting on the transformation of SO₂ to Sulfuric which will be minute?

 

[Alondro]

I'm puzzled.
Sulfites and HCl form SO₂ and NaCl and
Sulfates and HCl form Sulfuric acid and NaCl in the initial reaction, right?
Or are you counting on the transformation of SO₂ to Sulfuric which will be minute?

The chemistry going on can be more complex than straightforward reactions because they're not pure solutions. There are so many metals that unexpected redox reactions can take place.

Also, with mixed metal pins, the less reactive metals can steal electrons in the right conditions, while the more reactive metals eat up anions and can also shove additional electrons around as they are oxidized.

In an anoxic container with dilute acid solutions of sulfates and sulfites, this means there is the potential for the typical oxidants to be used up, and the sulfur at some point to drastically change its oxidation state.

Also, upon further investigation, much of that black powder may end up being copper(II) sulfide, which is known to form easily from hydrogen sulfide dissolved in any copper(II) salt solution.

I can test that by its solubility in HCl versus nitric acid. Cu(II)S will NOT dissolve in HCl, but it WILL dissolve in nitric.

 

[Alondro]

I would check the black powder for pgm's.
They can easily be masked by the color of your solution and go unnoticed until the gold is precipitated.

Otherwise that is very odd, let us know what it turns out to be .

There shouldn't be any PGMs. This was all from a small batch of typical gold-plated pins. I'm almost certain at this point it's a mixture of copper and gold sulfide.

I can force it to decompose into metal with a simple blowtorch flame. Copper sulfide decomposes at 932F, which is why it was one of the primary ancient copper ores. Very easily smelted.

 

[Alondro]

Update: Testing about a gram of the black powder showed it dissolved and reacted in nitric to produce a blue solution. So that proves SOME of it is Cu(II) sulfide.

However, even adding nitric to excess still left a good portion of the black powder untouched. This is either Cu(I) sulfide, which should not be able to form under acidic conditions, so that's unlikely... It cannot be gold(I) sulfide as that gradually decomposes into gold and elemental sulfur (no sign that this is happening after 3 days of the compound sitting in air)...

So it's either gold(III) sulfide, or the decomposition product of gold(III) sulfate, which would be hydrogen disulfoaurate(III), which I can't find a description of anywhere.

Next step will be to see how heating affects this mysterious black compound. All of the gold compounds in this class I've found any information on will decompose rapidly back into gold metal when heated merely in a typical blowtorch flame.

 

[JBo]

I just experienced something very similar!

I had just finished precipitating gold with SMB. The precipitate had the typical tan/brown color of pure gold power and stannous chloride test came back negative for gold in solution.

I then wanted to give the gold a quick DI water boil/rinse before melting so I poured off the clear HCl/SMB solution, added diH2O, and turned up the heat. Ten minutes later I came back to a bright yellow solution and almost all the gold precipitate was gone. I then tested with stannous chloride and it came back positive.

Is it possible for residual chloride ions or sulfides from the SMB to pull fine gold power back into solution? I have searched through HOKE and dozens of chemistry forums and I haven’t seen anything like this until this post.

Edit: I let the solution sit over night and it was still yellow and positive for gold in solution the next morning.

 

[Yggdrasil]

I just experienced something very similar!

I had just finished precipitating gold with SMB. The precipitate had the typical tan/brown color of pure gold power and stannous chloride test came back negative for gold in solution.

I then wanted to give the gold a quick DI water boil/rinse before melting so I poured off the clear HCl/SMB solution, added diH2O, and turned up the heat. Ten minutes later I came back to a bright yellow solution and almost all the gold precipitate was gone. I then tested with stannous chloride and it came back positive.

Is it possible for residual chloride ions or sulfides from the SMB to pull fine gold power back into solution? I have searched through HOKE and dozens of chemistry forums and I haven’t seen anything like this until this post.

Edit: I let the solution sit over night and it was still yellow and positive for gold in solution the next morning.

Chloride ions by itself can not do this under normal conditions.
What was used to dissolve the Gold in the first place?

 

[JBo]

Chloride ions by itself can not do this under normal conditions.
What was used to dissolve the Gold in the first place?

About 0.2g Au was dissolved in 25mL of 6M (~20%) HCl that was then incrementally dosed with 68% Nitric acid. (Unfortunately I don’t remember how much nitric I added in total! I think that was my first mistake.)

After all the gold was dissolved in aqua regia the solution was “de noxed” by gentle heating over the course of 24 hours to reduce the volume by about 50%. The solution was not boiled. I know this was not probably not necessary since I used incremental nitric dosing, but I figured better safe than sorry. The gold was then very easily precipitated out of solution with only 1 spoonful of SMB. I didn’t dilute the aqua regia before precipitating. After precipitation I let the gold settle over night and then the clear HCl/SMB solution was decanted. Then I added the DI water and boiled. Ten minutes later I came back to a yellow solution and the gold dissolved.

 

[Yggdrasil]

About 0.2g Au was dissolved in 25mL of 6M (~20%) HCl that was then incrementally dosed with 68% Nitric acid. (Unfortunately I don’t remember how much nitric I added in total! I think that was my first mistake.)

After all the gold was dissolved in aqua regia the solution was “de noxed” by gentle heating over the course of 24 hours to reduce the volume by about 50%. The solution was not boiled. I know this was not probably not necessary since I used incremental nitric dosing, but I figured better safe than sorry. The gold was then very easily precipitated out of solution with only 1 spoonful of SMB. I didn’t dilute the aqua regia before precipitating. After precipitation I let the gold settle over night and then the clear HCl/SMB solution was decanted. Then I added the DI water and boiled. Ten minutes later I came back to a yellow solution and the gold dissolved.

Boiling or heating will not deNOx the solution.
For that you need to evaporate to sirup, use Sulfamic acid or add Copper/Gold to consume the extra Nitric.
0.2gram of Gold need 0.2 ml +- Nitric in the AR to dissolve.

 

[JBo]

Boiling or heating will not deNOx the solution.
For that you need to evaporate to sirup, use Sulfamic acid or add Copper/Gold to consume the extra Nitric.
0.2gram of Gold need 0.2 ml +- Nitric in the AR to dissolve.

Boiling or heating will not deNOx the solution.
For that you need to evaporate to sirup, use Sulfamic acid or add Copper/Gold to consume the extra Nitric.
0.2gram of Gold need 0.2 ml +- Nitric in the AR to dissolve.

By “evaporate” do you mean without heat? What about low heat to help speed up the process? How do I know when the sirup is ready to be diluted and precipitated?

 

[Yggdrasil]

By “evaporate” do you mean without heat? What about low heat to help speed up the process? How do I know when the sirup is ready to be diluted and precipitated?

Have you studied the links I give new people and in there Hokes book?

Evaporate with heat and then add HCl when it reaches syrup consistency.

Much easier to use either Sulfamic or add a bit of Copper to deplete the Nitric.

 

[Alondro]

I just experienced something very similar!

I had just finished precipitating gold with SMB. The precipitate had the typical tan/brown color of pure gold power and stannous chloride test came back negative for gold in solution.

I then wanted to give the gold a quick DI water boil/rinse before melting so I poured off the clear HCl/SMB solution, added diH2O, and turned up the heat. Ten minutes later I came back to a bright yellow solution and almost all the gold precipitate was gone. I then tested with stannous chloride and it came back positive.

Is it possible for residual chloride ions or sulfides from the SMB to pull fine gold power back into solution? I have searched through HOKE and dozens of chemistry forums and I haven’t seen anything like this until this post.

Edit: I let the solution sit over night and it was still yellow and positive for gold in solution the next morning.

In my case, there was no nitrate anywhere in the mix. It was HCl, perhaps a trace of lingering chloritites (but likely not after having sat for months slowly eating away the next pins), and sulfates-sulfites from SMB.

I just happened to hit upon the right mix for the reactive metals to eat up all the oxygen and turn the reactions towards reducing sulfur.

As an update, the copper sulfide has all been transformed into copper nitrate now and removed. Now I have about 25 mls of gold foil flecks and fine brown mud (likely gold reduced from the unstable gold sulfide). A little bit of grey-white powder too that settles on top after washing, and that's probably lead sulfate from the pins' solder.

 

[JBo]

Have you studied the links I give new people and in there Hokes book?

Evaporate with heat and then add HCl when it reaches syrup consistency.

Much easier to use either Sulfamic or add a bit of Copper to deplete the Nitric.

Yes, I’ve read Hoke from cover to cover but I haven’t seen the links. Where can I find them?

 

[Yggdrasil]

Yes, I’ve read Hoke from cover to cover but I haven’t seen the links. Where can I find them?

Here:

1. Read C.M. Hokes book on refining jewelers scrap, it gives an easy introduction to the most important chemistry regarding refining.
It is free here on the forum: https://goldrefiningforum.com/phpBB3/viewtopic.php?f=54&t=19798
2. Then read the safety section of the forum: https://goldrefiningforum.com/forums/safety.47/
3. And then read about "Dealing with waste" in the forum: https://goldrefiningforum.com/threads/dealing-with-waste.10539/

Suggested reading:
https://goldrefiningforum.com/forums/the-library.101/
https://goldrefiningforum.com/threads/when-in-doubt-cement-it-out.30236/
https://goldrefiningforum.com/threa...le-read-this-before-you-post-about-ore.33333/


Forum rules is here.
https://goldrefiningforum.com/threads/gold-refining-forum-rules.31182/