HCl and Bleach Process for Gold Foils

[eprigge]

I apologize, I'm sure the information I want is here somewhere I just can't seem to find it searching or in the library.

What is the process for dissolving gold foils (from AP-digested fingers) using HCl and bleach? I am thinking of submerging the foils in full-strength HCl and then adding small amounts of bleach, step-wise until they are gone. I assume room temperature/cold is better for keeping the Cl in solution? Don't add too much bleach at once so Cl doesn't blow off? I saw someone say that the bleach should be injected at the bottom of the mixture, though I'm not sure why.

Once dissolved and filtered, I have SMB for reduction. Is any post-processing of the resulting mixture needed before SMB?

Any specific advice would be appreciated, thank you!

 

[Yggdrasil]

I apologize, I'm sure the information I want is here somewhere I just can't seem to find it searching or in the library.

What is the process for dissolving gold foils (from AP-digested fingers) using HCl and bleach? I am thinking of submerging the foils in full-strength HCl and then adding small amounts of bleach, step-wise until they are gone. I assume room temperature/cold is better for keeping the Cl in solution? Don't add too much bleach at once so Cl doesn't blow off? I saw someone say that the bleach should be injected at the bottom of the mixture, though I'm not sure why.

Once dissolved and filtered, I have SMB for reduction. Is any post-processing of the resulting mixture needed before SMB?

Any specific advice would be appreciated, thank you!

Your plan will work.
The not add too much bleach goes equally much into not neutralizing the HCl.
Nothing special to do after dissolving, just let it sit a day or so or give it a good heating, not boil.

 

[haveagojoe]

@eprigge Just to add that the solution needs to be stirred a lot while adding the bleach, as the chlorine gas produced needs to make actual contact with each of the gold foils in order to dissolve them. I'm guessing that's the reasoning for the suggestion of injecting it at the bottom of the solution but stirring should work fine.
Yes room temperature is best as heat will drive off the chlorine too quickly and you'll just need to use more bleach than necessary.
Also consider how chlorine gas can be removed from the workspace, if you don't have a fume hood do it outside and stand upwind- or better if you can rig up a fan with some ducting it will make the job less unpleasant.

 

[eprigge]

Thanks guys. Yes, I see what you mean regarding solubility vs temperature. Diluting the HCl with water (having more solution) would help to keep the Cl from outgassing. Unless the pH has to be at a certain level. I have 20 deg Baume HCl so I could dilute it by over 8x and it would have a pH of zero. I know the bleach will consume acid. I'll cut the acid to 25% strength and use a larger volume of it. And use a stirrer at 40-50 F. At this temp, the solution should hold 10g Cl/L.

 

[Yggdrasil]

Thanks guys. Yes, I see what you mean regarding solubility vs temperature. Diluting the HCl with water (having more solution) would help to keep the Cl from outgassing. Unless the pH has to be at a certain level. I have 20 deg Baume HCl so I could dilute it by over 8x and it would have a pH of zero. I know the bleach will consume acid. I'll cut the acid to 25% strength and use a larger volume of it. And use a stirrer at 40-50 F. At this temp, the solution should hold 10g Cl/L.

You are overthinking it.
Just add HCl so it is covered.
Then add bleach little by little giving it time to react.
If it stops reacting add more HCl and so on.

 

[Alondro]

Having enough HCl is the most important part. Concentrated is fine. I just use an eyedropper of bleach at a time while stirring vigorously for a couple minutes between additions. Once the foils are almost gone, I stir longer before adding anymore bleach. Usually one more shot does it after that. It doesn't take much bleach per gram of gold in foil form, roughly 5 mls from my experience. And I add that 1 ml at a time into the foils that are in about 50ml of HCl per gram.

 

[GunBallistic]

Having enough HCl is the most important part. Concentrated is fine. I just use an eyedropper of bleach at a time while stirring vigorously for a couple minutes between additions. Once the foils are almost gone, I stir longer before adding anymore bleach. Usually one more shot does it after that. It doesn't take much bleach per gram of gold in foil form, roughly 5 mls from my experience. And I add that 1 ml at a time into the foils that are in about 50ml of HCl per gram.

How do you precipitate afterwards? Still sodium metabisulfite?

 

[Yggdrasil]

How do you precipitate afterwards? Still sodium metabisulfite?

Welcome to us.
Yes or Aacorbic acid and and...
Just as usual.

 

[Alondro]

How do you precipitate afterwards? Still sodium metabisulfite?

Yes, but remember that it's critically important to heat the solution for several hours, BEFORE precipitating, at a slight simmer to drive off all remaining chlorine in solution, otherwise the gold will begin to re-dissolve. If you can heat it in direct sunlight, that's even better, as UV rays drive the dissociation of chlorine from solution.

 

[GunBallistic]

Yes, but remember that it's critically important to heat the solution for several hours, BEFORE precipitating, at a slight simmer to drive off all remaining chlorine in solution, otherwise the gold will begin to re-dissolve. If you can heat it in direct sunlight, that's even better, as UV rays drive the dissociation of chlorine from solution.

Thank for that info. Would a UV light work just the same? I live in Seattle so we get sunlight 4 months out of the year

 

[eprigge]

Once you reach 100C, the solubility of chlorine in water drops to about zero. Even if small amounts of chlorine gas are left in the solution after heating, SMB will react with it and reduce it to chloride. I don't see what UV would do to remove it.

 

[eprigge]

Thank you for all of the advice. I finally have my gold dried and weighed from this batch. I was hoping for maybe 1.5g but I got 5.53g, an almost suspiciously large amount from 598g of fingers. They are mostly from 80s/90s Nortel test equipment so I suspected they would be good but... not this good. Perhaps it's impure? Anyway, I'm happy, thanks again for all your help. Hopefully, I will know enough to help others at some point.

 

[Yggdrasil]

Thank you for all of the advice. I finally have my gold dried and weighed from this batch. I was hoping for maybe 1.5g but I got 5.53g, an almost suspiciously large amount from 598g of fingers. They are mostly from 80s/90s Nortel test equipment so I suspected they would be good but... not this good. Perhaps it's impure? Anyway, I'm happy, thanks again for all your help. Hopefully, I will know enough to help others at some point.

Color look good.
But if you really dont like to have that much lying around I can always help you carry it to the trash can

Good job mate.

 

[haveagojoe]

Perhaps it's impure?

It will have some impurities if it has only been dissolved and precipitated once. The next step is to wash the powder thoroughly before redissolving it, filtering the solution and dropping the gold out a second time.

The recommended procedure for washing the powder, known around here as "Harold's washes", can be found here.

Hopefully the impurities won't weigh much so your finished product will be close to the weight you have currently. Nice work!

 

[FrugalRefiner]

Harold's washing process is well known and works well, but I personally like Lou's process as described in Gold has been refined and melted, now has orange hue.

Dave

 

[haveagojoe]

Harold's washing process is well known and works well, but I personally like Lou's process as described in Gold has been refined and melted, now has orange hue.

Dave

Thanks, that's very useful, I hadn't seen that thread before. Gold sponge is not usually available for us small-timers, but Lou's comments explaining that boiling the washes is not necessary- counterproductive even- are very helpful.
In @eprigge 's case I'd say the powder looks pretty clean and since it has come from solder-free fingers it probably won't have any silver in it, so I think the ammonia wash is actually probably not required. A few washes with water, HCl, then water again would most likely be perfectly adequate, would you agree? I know Harold would say that corners shouldn't be cut but that's what I'd do.