Aqua regia process post sulfuric stripping

[gregory-d]

I have edited this a little but it is probably better to see the replies below.

After removing the gold from components using a sulphuric acid stripping cell I want to move to the next stage and refine this into gold powder.

I have come up with a process I think is correct to go to the next step.
Could someone just check it to see if I am doing something wrong.
I haven't done it before and would like to check before I do something stupid.

Dilute the gold laden sulphuric acid sediment left over after the stripping.

Filter the dilution.

Reusing the filtrate to wash any residue thus reducing the waste filtrate

1. Incinerate the Filter Paper containing gold residue to remove the organic matter

The organic matter could effect the aqua regia reaction with the gold.
Incinerate: Burn the filter paper to convert organic material into ash.

2. Treat Ash with Nitric Acid

Add Nitric Acid: Treat the ash with concentrated nitric acid (HNO₃) to
dissolve other metals and form metal nitrates.

3. Combine with Hydrochloric Acid

Add to HCl:
Transfer the nitric acid-treated solution (with dissolved metal nitrates)
directly into concentrated hydrochloric acid (HCl).
This will generate aqua regia as the nitric acid reacts with HCl:
HNO3+3HCl→Cl2+NOCl+2H2O
The generated aqua regia will start dissolving the gold.

4. Additional Nitric Acid

Add More Nitric Acid: Add additional nitric acid to ensure that all gold is dissolved.
Adjust the amount based on the observed reaction.
Heat the solution gently as the reaction deminishes and add some more.
When it looks like the reaction has finished add a small amount of gold and heat more to ensure all
nitric acid is spent.

5. Filter and Recover Gold

Filtration: Filter the solution to remove any remaining solid residues.
Ensure thorough rinsing of the filter paper to recover any gold that may be trapped.

Gold Precipitation: Precipitate the gold from the aqua regia solution using sodium metabisulfite (Na₂S₂O₅).
Test solution with Stanneous chloride and repeat additions until no more gold is in solution.


Thanks.
Gregory

 

[Yggdrasil]

I have come up with a process I think is correct to go to the next step.
Could someone just check it to see if I am doing something wrong.
I haven't done it before and would like to check before I do something stupid.

Dilute sulphuric acid Explain what your goal is with this?

Filter the dilution.

Reusing the filtrate to wash any residue thus reducing the waste collecting every thing.
This filtrate can be treated with the left over caustic soda mix used to remove the solder mask on the PCBs Why do you want to do this?
The reaction between caustic soda and sulphuric acid produces sodium sulfate and water:
NaOH + H2SO4→Na2SO4+2H2O2 NaOH + H2SO4→Na2SO4+2H2O
It's important to add the caustic soda slowly and carefully to avoid excessive heat generation and splashing.
Ensure proper mixing and monitor the pH to reach neutralisation. This belongs under your the waste treatment.

1. Incinerate the Filter Paper

Incinerate: Burn the filter paper to convert organic material into ash. What do you expect to find in these filters?

2. Treat Ash with Nitric Acid Better drop the Sulfuric step and go straight to HCl, this will remove Tin which will create problems with the Nitric.

Add Nitric Acid: Treat the ash with concentrated nitric acid (HNO₃) to
dissolve other metals and form metal nitrates.

3. Combine with Hydrochloric Acid
No and No, do not add the dirty Nitric to HCl, if you use Nitric first, filter it clean and set your dirty solution aside. Or use the previous step first.

Add to HCl:
Transfer the nitric acid-treated solution (with dissolved metal nitrates)
directly into concentrated hydrochloric acid (HCl).
This will generate aqua regia as the nitric acid reacts with HCl:
HNO3+3HCl→Cl2+NOCl+2H2O
The generated aqua regia will start dissolving the gold.

4. Additional Nitric Acid
If you have clean foils from the previous steps only very little Nitric is needed.
Add enough HCl to cover your material, add about half the ml as the suspected weight of Gold then add a few ml/drops now and then until it is dissolved.
Clean Gold needs from 0.5 to 1.5 ml Nitric per gram of Gold

Add More Nitric Acid: Add additional nitric acid to ensure that all gold is dissolved.
Adjust the amount based on the observed reaction.
Heat the solution gently as the reaction deminishes and add some more.
When it looks like the reaction has finished add a small amount of gold and heat more to ensure all
nitric acid is spent.

5. Filter and Recover Gold
Add ice and dilute the solution to remove AgCl and then filter til crystal clear (After denoxing of course) if necessary filter multiple times through same filter.

Filtration: Filter the solution to remove any remaining solid residues.
Ensure thorough rinsing of the filter paper to recover any gold that may be trapped.

Gold Precipitation: Precipitate the gold from the aqua regia solution using sodium metabisulfite (Na₂S₂O₅). There are other options.
Test solution with Stanneous chloride and repeat additions until no more gold is in solution.


Not sure yet how to treate the spent Aqua regia Set it aside in your stock pot awaiting cementation and waste treatment.

Thanks.
Gregory

What material are you treating?
Comments in Bold in the quote.

 

[gregory-d]

What material are you treating?
Comments in Bold in the quote.

Sorry I thought I made it clear in the title.
Treating the gold residue from the sulphuric acid stripping cell.
I have modified the original post to reflect that now.
I am going through your suggestions and modifying the original post to make it clearer thank you.

 

[4metals]

Not sure yet how to treate the spent Aqua regia

dealing with waste

 

[nickvc]

With fine powders you can use HCl and peroxide and or HCl and bleach.

 

[Yggdrasil]

Sorry I thought I made it clear in the title.
Treating the gold residue from the sulphuric acid stripping cell.
I have modified the original post to reflect that now.
I am going through your suggestions thank you.

Then you will be dealing with pure Gold powder, no NaOH or other are needed.
Just let it settle (2-3 days) siphon off the Sulfuric and set it aside in a closed container until next use.
The sludge in the bottom will be diluted with water until very dilute.
Filter it well and burn filter well, or put straight into hot AR and simmer until paper is gone.
DeNOx, dilute and chill before filter well.
When crystal clear drop the Gold with your preferred method.

 

[gregory-d]

Then you will be dealing with pure Gold powder, no NaOH or other are needed.
Just let it settle (2-3 days) siphon off the Sulfuric and set it aside in a closed container until next use.
The sludge in the bottom will be diluted with water until very dilute.
Filter it well and burn filter well, or put straight into hot AR and simmer until paper is gone.
DeNOx, dilute and chill before filter well.
When crystal clear drop the Gold with your preferred method.

Thanks I thought as I cooked the basket there may be copper in there as well that was why I was doing the nitric first.

 

[gregory-d]

dealing with waste

Yes I shouldn't have put that line in it is all there is the forum.
Thanks

 

[Yggdrasil]

Thanks I thought as I cooked the basket there may be copper in there as well that was why I was doing the nitric first.

Since it is pure metal powder form the Sulfuric cell you can easily use Nitric first although not necessary.

 

[4metals]

A lot of what you may need is in the library. One of the reasons we needed a library was because the actual name of the thread is rarely indicative of the content. If you go to the Library section> Introduction> Library index thread, a thread opens that describes the actual topic of selected threads, in some semblance of order, to cover a lot of your questions.

 

[gregory-d]

A lot of what you may need is in the library. One of the reasons we needed a library was because the actual name of the thread is rarely indicative of the content. If you go to the Library section> Introduction> Library index thread, a thread opens that describes the actual topic of selected threads, in some semblance of order, to cover a lot of your questions.

Yeah sorry there is a lot of information there it is a little overwhelming at first. I thought I had covered but clearly not.

 

[gregory-d]

Since it is pure metal powder form the Sulfuric cell you can easily use Nitric first although not necessary.

Ok thanks. I can sort of see that now.
If I add extra gold and heat the solution near the end of the aqua regia process that should denox the solution shouldn't it?

 

[anachronism]

If you have nitric then you may as well go straight to AR and do it in one step. No point over complicating it.

 

[gregory-d]

If you have nitric then you may as well go straight to AR and do it in one step. No point over complicating it.

Yep thanks Yggdrasil did mention that.

 

[gregory-d]

I did come up with another one of my crazy ideas and I will just put it here for giggles. (doesn't deserve any more attention than that unless it really works well)
I am going to get an old record player turntable and put the sulphuric bath beaker on it after it's done to see if centrifugal force can accelerate the settling process.
The idea is that solids will slowly be spun out and the gravity will cause it to slide down the outside of the circular pyrex container I use for my sulphuric bath.
Oh yes and I will put the lid on it first of course.
No fancy dangerous stuff this time. Well not much it is sulphuric acid after all.

 

[Shark]

Filtering sulfuric with a paper filter isn't likely to work as it will break down the paper quick.

 

[gregory-d]

Filtering sulfuric with a paper filter isn't likely to work as it will break down the paper quick.

Yeah haha I found that out the hard way.
But if you dilute the sulphuric it works ok. (I poured the tailings in sulphuric into about 5 times the volume of water and three coffee filters thick held up)

 

[gregory-d]

With fine powders you can use HCl and peroxide and or HCl and bleach.

instead of aqua regia ?
I'll look it up thanks.

 

[gregory-d]

Ok now I am getting confused again HCl and Peroxide is AP?
What I understand is that it doesn't dissolve the gold it just dissolves everything else leaving the gold as a solid.
Is that right?
But what about silver chloride I read here that it precipitates out in acid. So that will also be in the sludge as well won't it.

 

[Shark]

Ok now I am getting confused again HCl and Peroxide is AP is that right?
Does it dissolve the gold or just dissolve everything else leaving the gold as a solid.

Peroxide is an oxidizer. HCl and an oxidizer dissolves gold. HCl and a strong peroxide can be used in place of AR, or HCl and nitric acid. For AP a 3% peroxide is the usual, but if you go to 12% or even 30% it will put very fine gold powders into solution.

AP is HCl with a tiny bit of, usually, 3% peroxide. Just enough to start the copper oxidizing without putting the gold into solution. Or at least as little gold as possible. This can also be done using an air supply, such as an aquarium air pump. This takes a little longer, but reduces the risk of the gold going into solution.

This is another way for many people to still work with gold in countries where nitric just isn't available. It also is another method in the tool kit for those interested in the science and wanting to know as many ways as possible to get to the gold.