gold precipitating from wash

[Minimum]

Hello everyone. I'm not sure if this is the place to post this question. But here goes. I am in the process of recovering gold foils from clipped ram and other boards. I have 2 batches going at once right now. The first was gold foils cleaned with hcl then dissolved with hcl cl. Than I dropped with smb. I decanted that solution into my waste collector. Next I did my first hcl wash on batch 2 foils. That hcl was filtered into my waste collector. But when the two were mixed in my waste bucket I noticed brown sediment. Almost immediately. I'm sure there was excess smb in the solution. But my question is it possible this is gold? And how come there is so much. I only washed with hcl. Is it possible that the hcl dissolved that much of the foils? This is a new waste bin with only the two additions of waste so it's pretty clean looking. This picture is of my waste bin.

 

[4metals]

Thread has been moved to help needed section.

 

[Geo]

Most likely the first solution was still dropping gold. If the gold is dropping out fine, it may take a day or more for it to all settle.

 

[Minimum]

I let the first solution sit for about 2 days. I then decanted to waste. It wasn't until I added the wash mix to waste that it produced brown powder. And it appeared almost instantly when I added the wash. I was just surprised because I didn't think straight hcl would dissolve that much gold. Plus I tested the hcl cl mix with stannous and it came back negative. I'm just trying to get this straight because I am keeping track of yields. And now I don't know where this yield came from.

 

[Geo]

Just HCl will not dissolve gold. Two days is a long time for sulfur dioxide to stay in solution too. Only you can solve the mystery. Take a small sample of the precipitated powder and dissolve it. Then test it with stannous chloride. If it is gold, just be glad you found it before you chucked it. Always test solutions before and after dropping the gold. Test every time you do something different to it. That's the only way to follow the gold no matter where it goes.

 

[solar_plasma]

There is one more reason, why it is always valuable to test, even if you are sure, what you have: you get better in judging the results under the most different circumstances from time to time.

 

[4metals]

There is always the possibility that you dropped all of the gold out of the solution but the liquid in the waste had gold in it which was precipitated from the SO2 in the liquid you added to the existing waste.

 

[Anonymous]

If I had to pick a theory I think I'd go with the one that said that your waste still had gold in solution and that addition of your second waste with excess SMB precipitated the gold from the first waste.

 

[Justincase]

There will always be some Au in your liquid until it has sat for a number of week's in your piggy bank with copper at the bottom.
It may be in very fine particle's which give no reaction to stannous or as you have found it is quite easy to have some in solution.
The chlorine in your wash water will dissolve a surprising amount of Au when you move onto acid washing as well.
The more you drop and gain experience the less you will leave behind for the piggy bank.
Taking your time and making sure each stage has completed fully will save time, effort and money in the long run.
Refining naturally work's best at a glacial pace.I always leave my wash water and spent liquor sat over night before adding to the piggy bank just in case there is a gram or two that is taking it's time to settle down.Until it is relatively clean Au's natural tendency to clump start's to hold little lost sheep in place ,some very fine Au will always escape your decanting.

 

[g_axelsson]

Next I did my first hcl wash on batch 2 foils.

Was that a wash after precipitating gold or on foils before dissolving them?
If it was before dissolving it could contain a lot of other substances, for example lead chloride. Lead chloride would combine with the leftovers that SMB creates and form NaCl, HCl and PbSO₄. The lead sulfate would precipitate out as a white powder.
Tin chloride and SMB also forms an insoluble precipitate, but this one is brown. I would guess that together it could form a brown sediment quite rapidly.

How is the powder moving? I've never managed to get a nice pile of gold in the center of a beaker like that, the gold is too heavy to be moved by water or it's too fine and doesn't settle fast enough to form a nice pile, just a thin layer. From the picture it looks like the powder is light enough to easily be moved by the water, like tea leaves forms a pile when you stir your tea. That effect is from the liquid moving in an inwards and upwards direction, collecting the leaves or the light sediment as a small pile in the center.

My guess is that whatever you have it isn't gold.

Do you have any of the wash water left? If you didn't put everything into the waste bin then I suggest that you test it with tin chloride to see if it contains any gold. At least I wouldn't continue to pour if I got an unexpected reaction. I would stop and investigate before going forward.

To the record, SO₂ would leave the solution quite fast, in a day it should be gone. But SMB could stay behind if the pH was neutral and when mixed with HCl from the new wash water fresh SO₂ would be released. The SO₂ is only created in an acidic environment.

By the way, do you have any gold standard solution to test if your stannous is still working?

I have once tried to drop gold with SMB from a more or less neutral gold chloride solution and it looked like nothing happened. When I added a bit HCl a dark cloud of gold formed instantly.
Nowadays I try to drop the gold from at least 1/3 HCl and 2/3 water, to give the SMB someting to react with and that works great.

And all my left over from dropping the gold and washing afterwards goes to the stock pot where it dissolves some low level scrap and leaves the precious metals behind. Since I run a lot of small batches (below a gram and up to a couple of grams) and just collect the powder for later final refining I get quite a lot of the gold from the stock pot. I got one gram from it the first time when I had collected 50g totally. Then I got some PGM:s too. I haven't tried any real PGM refining yet, that is for a later date, just collecting the black powder for the moment.

Göran

 

[4metals]

In large scale aqua regia refining, dropping gold with metabisulfite, it is not uncommon to have 0.05% of the gold remaining in the aqua regia after you are done. This is recovered along with any Platinum group metals by cementation. If you keep running totals of your recovery you get an idea what to expect in your stock pot.

 

[jason_recliner]

How is the powder moving? I've never managed to get a nice pile of gold in the center of a beaker like that, the gold is too heavy to be moved by water or it's too fine and doesn't settle fast enough to form a nice pile, just a thin layer. From the picture it looks like the powder is light enough to easily be moved by the water, like tea leaves forms a pile when you stir your tea. That effect is from the liquid moving in an inwards and upwards direction, collecting the leaves or the light sediment as a small pile in the center.

That regularly happens with my powder too, after I vigorously swirl the beaker.
It's the opposite to what I expect. I would have thought centrifugal force would push the heavier gold to the edges, but it indeed collects in the middle starting with the biggest chunks. As the fluid motion slows, finer particles gradually spiral in, collecting in the centre.

If I were a physicist, I might know why. So my theory is it forms like new stars. I'm going to test that theory by swirling a beaker of gold powder so fast that I create a red dwarf in my kitchen. Wish me luck!

 

[Geo]

The vortex in the center is an area of low pressure within the liquid. The pressure increases the farther from the center you go. It would take something much heavier than precipitated gold to stay at the edge of the beaker if you stir the liquid until a vortex forms.