Second refining - how to get rid of residual AgCl

[FigiMon]

Hello,

I have redissolved 160g of gold powder that was previously precipitated with FeSO4. The solution has been diluted to 1000 mL and as can be seen in the picture below has some very fine AgCl suspended in it. The AgCl simply will not filter out, I must have filtered ten times. Vacuum filtration, gravity filtration through cotton plug. Nothing works .



Please help me get rid of this before I go to precipitatipn with SMB.

Thanks

 

[goldenchild]

If you're sure it's silver chloride then you can go ahead and precipitate the Ag. Then you would use ammonia washes to get rid of the chloride. I know some on here won't agree but truthfully if the amount of chloride is that miniscule you would get rid of it during the melt. A fast and hard torch would vaporize all the chloride. If you'd like to experiment you could always take a small sample of AgCl put it in a melting dish and torch it. You would only lose a few pennies for the test but the knowledge gained is worth it.

 

[FigiMon]

Thanks, tried your suggestion and it worked. I drew off a portion and precipitated the gold. After the usual washing steps I was left with 9.8 g of dried gold powder that I melted down again. This button is now almost dissolved and the solution looks crystal clear, no sediment whatsoever.

Still, I would really love to get rid of the AgCl in the remaining solution if I could. I do my melting in an oven and would like to keep that free of any silver chloride sticking to the walls etc.

Is there something I can do to make the silver chloride clump up a bit? Tried heating but that did nothing.

 

[UncleBenBen]

You can try ice to dilute and chill the solution. Lowering the temperature will reduce the solubility of AgCl. Sometimes you have to run the solution through the same filter a few times to get it clear, the pores in the filter material will eventually clog up enough to catch the smaller particles.

 

[FigiMon]

Thanks for your suggestion but I think most of the silver chloride has already precipitated out. The problem is it is so incredibly fine. Still hasn't settled one bit after several hours. I have tried repeated vacuum filtration through triple filters and it still gets through.

 

[denim]

Have you ever tried a charmin plug with vacuum filter? I have experienced your situation plenty of times and the charmin plug always works for me. Crystal clear after filtering.

 

[snail]

Time is your friend, if you can let it settle for 2 or three days It will clear tremendously. Then a fine filter paper like a Whatman number 5.
Decant the clear solution first, leaving the sediment for last. The sediment will clog the fine paper quickly.
Lately I don’t use the extremely fine filter.
After I precipitate I use several light boils in full strength HCl, then decant while still hot.
I believe it was recommended by Lou. the hot acid dissolves a considerable amount of Silver Chloride.
Really cleans up the gold.

 

[FigiMon]

Charmin plug? Would that be toilet paper in the büchner funnel?

EDIT. Looked it up online now. Yes I tried gravity filtration through cotton plug. Usually works but not this time.

Plan right now is to leave the solution undisturbed for a few days and see what happens.

 

[denim]

I don't know for sure but I am assuming a charmin plug is MUCH tighter than a cotton plug. Look on the third page of this thread, I posted some picts of my charmin plug vacuum filtration setup.

https://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=28845&start=60

Edited to include link

 

[FigiMon]

Yep, I see what you mean now. Thanks very much. I'll let it settle until tomorrow and then go with the toilet paper.

 

[goldenchild]

If you do the ammonia washes you don't have to precipitate then melt and then dissolve the gold again. Simply precipitate the solution you have now and use ammonia washes. That will remove all the silver chloride. You can speed up the washes with a little bit of heat.

[youtube]https://www.youtube.com/watch?v=4uZqAWwguXU[/youtube]

 

[FigiMon]

Thanks for your tip. I have done this reaction a few times and I know it works really well on freshly precipitated AgCl.

The hazy solution is from a second refining. The gold dropped in the first refinings was actually washed with both ammonia and HCl. Perhaps I wasn't careful enough in the washing steps though. Or somehow the silver chloride I am now seeing was trapped inside the little gold crumbs and not reached by the washes.

As a side note I believe the AgCl came from a bracelet that I thought was 18K white gold but turned out to be yellow gold underneath a thick Rh plating. This went straight into AR without inquartation together with other white gold.

I am grateful for all suggestions and plan to take on the now partially settled solution tomorrow with the following steps.

1. Decanting as much as possible of the clear liquid
2. Filtering whatever is left and hazy through a toilet paper plug.
3. Precipitate with SMB and being really careful in the washing this time.
4. Melt into a bar (hopefully of very high purity )

 

[nickvc]

Whenever I had a murky solution I simply put it on heat with a watchglass over and let it heat to near boiling and left at that temperature until the it clarified, it can take some time but it usually worked so long as you had no tin in the solution.
Good washes in boiling Hcl will normally remove much of the contaminates followed by several boiling water washes until no color shows in the water.
If none of these work try washing your precipitate in dilute sulphuric as Hcl and nitric seem incapable of removing whatever is causing this.

 

[FigiMon]

Thanks guys for all your suggestions!

Following the steps I outlined above I ended up with a crystal clear solution and this precipitated gold powder:

Look at the cool effect from the flash :lol:
This powder was first melted in a crucible and the re-melted directly in the mould and left to cool in the furnace. Pretty happy with the result

 

[FrugalRefiner]

Nice bar! Interesting texture on the top.

Dave

 

[FigiMon]

Thanks! I think it is because it cooled so slowly. After 90 minutes it was still way too hot to touch. I was a little disappointed at first because when it was molten it was like a perfect little loaf of bread but now I quite like it. Perhaps I will re-melt it though and cool it more quickly to preserve the shape and finish.

 

[butcher]

What was the source of gold, did it contain any platinum group metals?
Are the bumps or raised spots on the bar harder than the rest of the gold when you sand or file the bar?

The reason I ask is I have made rings and jewelry from some of my silver cement and the rings would develop bumps of extremely hard spots (PGM metals) that you could not file or sand.

The picture of your solution, the gold powder looks to be very pure, the bar looks great, my only concern on the melted bar is the small bumps that look like some kind of impurity making the small bumps.

 

[denim]

Well done! Looks like a real nice bar. Now, on to your next batch armed with a little more knowledge!

 

[FigiMon]

The source for this bar was 18K-23K yellow gold jewellery. There was one large bracelet that I thought was white gold but turned out to be yellow underneath thick Rh. I can't see there having been much PGMs in the source material. Nowhere in the process have I seen any sign of Pt or Pd with SnCl2 testing.

It is a bit hard to see in the images but the little bumps run in two ridges either side of the crater in the middle. Could it be aggregated gold crystals? Like when molten bismuth solidifies and crystals form just under the surface.

 

[butcher]

It very well could have been the gold cooling and the torch trying to remelt the crystalizing gold that made the bumps, if the bumps are not hard it is more likely how the gold cooled and crystalized while melting, than the possibility of PGMs that formed the bumps.

The only reason I mentioned it was because I had similar bumps in cast jewelry made from cemented silver that was high in the PGM's, in my case the small bumps were very hard compared to the silver alloy as a whole.


Everything in your pictures looks to be very pure, the solution powder and the gold.

That bar of gold well, it just looks good in your hands looks heavy in there too.