Newcomer needs Experienced Insight on Unknown Crystals

[spentfire]

Greetings all!

First let me say that this forum is amazing for its open, collegial atmosphere and top-notch information. I am an analytical chemist, but so much of this gold refining technique is more art than science that I think I need some help from a more experienced hand on some reoccurring white crystals that don't act like the usual suspects. I've tried to search for an answer in the forums, but I haven't come across any pertinent information yet. Some back-story may help in figuring out what happened...

I made a 1.11g "gold" button a year or so ago out of unpopulated flexible circuit boards salvaged from the dumpster. My (uninformed) process was to etch away the backing copper/solder from gold traces with nitric. Once filtered and rinsed I melted the "gold". The end result was a cruddy, oxide-surfaced button that needed to be polished with crocus cloth before it really looked golden. Obvious to me now, the button was probably highly contaminated.

Flash forward about a year when I started reading this forum and Hoke, and realize that I had made some very rookie mistakes. So I restarted by dissolving the button in HCl-Cl. Right off, I notice a LOT of white crystals forming in the yellow solution as the button slowly decreased in size. "Right, it must be silver", I think to myself, as silver chloride is insoluble and can inhibit the attack of gold. However, upon filtering off the crystals and rinsing with water, all of the crystals redissolved. "Ok, it can't be silver chloride. It must be slightly soluble lead chloride, I'll catch the lead with sulfuric acid in a later step", I thought.

I used a fair amount of HCl-Cl (~500 mL all totalled), and I know I had excess Cl2 so I decided to heat the solution drive off chlorine gas and evaporate some volume. I dropped the volume by three quarters and let it cool over night. The next morning, the white crystals were back. I added more DI water to the solution, and, you guessed it, the crystals redissolved. I add concentrated sulfuric acid to remove what must be lead contamination, but no precipitation occurs. As long as the solution is dilute, the solution is clear as a bell, but if the concentration increases, saturation and crystallization of something happens.

Now to the question: What can this somewhat-soluble precipitate be? I know that gold(III) chloride has a solubility of ~65g/100 mL, so that can't be the suspect. Could it be gold(I) chloride? I thought that was soluble in HCl. Tin? I know I had problems with metastannic acid when I first started this process a year ago.

Any insights?

P.S. Sorry for the long post, but more information is probably better, right?

 

[martyn111]

copper(1) chloride?

 

[spentfire]

martyn111:

That is a possibility, I suppose, but my understanding is that copper (I) chloride is highly soluble in concentrated hydrochloric acid, so it shouldn't have precipitated in my HCl-Cl. I didn't see any green coloration in the resulting solution, but looks can be deceiving.

 

[samuel-a]

Sound like NaCl to me.

There's no need to evaporate HCl+Cl solution, just heat to drive off Cl. drop with SMB/coppers/SO2. wash and collect gold. melt.

 

[Lou]

Isolate and run a pXRD.

 

[dtectr]

Sound like NaCl to me.

There's no need to evaporate HCl+Cl solution, just heat to drive off Cl. drop with SMB/coppers/SO2. wash and collect gold. melt.

I agree with Sam on this - It is likely sodium salts - If I understand & I'm NO chemist - the HCl liberates the Cl2, leaving a solution that contains, among other things, I guess, water & sodium chloride. Since Clorox (a friend of the family died from COPD from working at a plant that manufactured it, according to him, by combining chlorine gas with sodium hydroxide) is so dilute, I imagine the excess of water keeps the salts in solution.

If you ever let Clorox evaporate, or develop a pinhole leak in your container & the leak evaporates, the resulting residue is crystalline & white, I can only assume its some form of sodium.

 

[spentfire]

samuel-a and dtectr:

Why is it that the most obvious possibilities are so easily overlooked? :shock:

It may very well be sodium chloride, although the crystals didn't appear to be cubic in structure (like NaCl), but much more complex.

I did some very rough back-of-the-envelope calculations assuming 6% NaClO bleach (~1.08 g/mL density), NaCl solubility of ~360 g/L, and ~125 mL final volume. I would have needed to add over 850 mL of bleach to the reaction to reach saturation, which is way more than I actually added. However, this might be within a factor of 4-5 of how much I probably used, and my calculations don't account for any other possible sodium compounds (typically <1%) added to the bleach to act as stabilizers,etc, so I won't discount NaCl as the culprit.

I will filter off the gold and try to reproduce the crystals by evaporating the mother liquor. If possible, I'll try to run some more tests on the crystals directly.

Thanks for the nudge in the (hopefully) right direction, everyone!

 

[samuel-a]

I will filter off the gold and try to reproduce the crystals by evaporating the mother liquor. If possible, I'll try to run some more tests on the crystals directly.

If for experiment sake, i say go for it.

For practical use, you'd be far better off using few ml of hot AR to dissolve buttons/foils.

 

[spentfire]

I would have used AR in the first place, but I had such a B***** of a time with metastannic acid the first time around with nitric, I didn't want an encore performance.

Thanks again for the insight!

 

[dtectr]

I would have used AR in the first place, but I had such a B***** of a time with metastannic acid the first time around with nitric, I didn't an encore performance.

Thanks again for the insight!

I did, as well (re:metastannic acid) & found that PATIENT Poor Man's AR (see "lazersteve" & "mic") minimizes it's formation. The incremental addition of nitrates seems to be the ticket.
The difference? I have to stand by & monitor.
The result? Didn't I just say that? :lol: :lol:

Further - PATIENT hot, concentrated HCl washes will get rid of most of it - the bright yellow tint is not from "gold", as I originally assumed (you know what 'assuming' leads to - :wink: :wink: ) but from "the TIN". It seems, according to this paper, that hydrogen sulfide is somehow formed??? http://www.public.asu.edu/~jpbirk/qual/qualanal/tin.html
and the heated washes seem to do the trick, eventually.

Forgive me, as I do not have the training to describe the formulae involved. Practice has seemed to bear this out.

Any analytical science would help me, if I can understand it.

 

[spentfire]

I would have used AR in the first place, but I had such a B***** of a time with metastannic acid the first time around with nitric, I didn't an encore performance.

Thanks again for the insight!

I did, as well (re:metastannic acid) & found that PATIENT Poor Man's AR (see "lazersteve" & "mic") minimizes it's formation. The incremental addition of nitrates seems to be the ticket.
The difference? I have to stand by & monitor.
The result? Didn't I just say that? :lol: :lol:

Further - PATIENT hot, concentrated HCl washes will get rid of most of it - the bright yellow tint is not from "gold", as I originally assumed (you know what 'assuming' leads to - :wink: :wink: ) but from "the TIN". It seems, according to this paper, that hydrogen sulfide is somehow formed??? http://www.public.asu.edu/~jpbirk/qual/qualanal/tin.html
and the heated washes seem to do the trick, eventually.

Forgive me, as I do not have the training to describe the formulae involved. Practice has seemed to bear this out.

Any analytical science would help me, if I can understand it.

That makes a fair bit of sense. I'll have to try Poor Man's or incrementally added nitric AR to keep the kinetics in check.

I never had any problems with yellow tinted metastannic, mine was always pure white jell-o (particle emulsion)! If you had any trace sulfur in the mix, the extreme oxidizing conditions of AR should make it nearly impossible for sulfide ion to exist (all sulfur should turn to sulfur oxides) so I am not sure that your yellow tinting was tin (II) or (IV) sulfide. However, I don't have any better explanation than yours, and, as you can get rid of it with hot conc HCl washes, its all a bit academic! I, as always, will defer to experience and practice over "book learning".

You've got my curiosity piqued; I'll have to dig some more to see if I can find a better explanation. But for now, I think I'll proceed as directed.

Thanks!