Stannous indication became less strong

[user 65313]

Hi there, new member who has already learned a lot from this forum and yes I have read Hoke....

I am learning to process e-waste of which I have an abundance. I observed something unusual in my most recent efforts and am hoping someone can shed some light on what may have occurred before I go too much further and create a bigger problem.

First the observation... I had a positive stannous test in my aqua regia. I added a splash of sulphuric acid to the aqua regia to remove any lead which may have been present and immediately got a fine white precipitate which I assume to be Lead Sulphate. I filtered that solution via a paper coffee filter and some of the precipitate seemed to travel through with the solution. My first stannous test was very dark purple/black. After filtering the solution which I had added the sulphuric to, I now only get a fairly weak brown indication from the stannous. I have been evaporating/reducing the filtered solution and the latest stannous test does seem to have darkened but is certainly not the very dark result I had previously.

I added HCL to the filtered material and it went into solution but tested as no PM's present.

Is this typical or has something gone wrong? I have kept all materials from the process i.e. Aqua regia, sediment, filter and original material (crushed ic's ) which was also in the filter.

The gold was being extracted from pyrolised, and mechanically processed IC's (Crushed, sieved, removal of magnetic materials and HCL peroxide bath for base metals) as well as some clean gold foils which had also been extracted using the AP process.

Any suggestions to what has occurred would be appreciated. Do I just need to take a chill pill and finish the evaporation, rehydrate with HCl and drop the gold with SMB or should I be looking more closely at the residues and filtered materials?

 

[Lino1406]

Obviously you tested with stannous a sample, not the whole mass?

 

[user 65313]

Yes mate.tested on piece of paper

 

[nickvc]

You said you added some more HCl but did you add any more nitric to the material in your filter ?
It’s possible as your solution cooled your values cemented out back onto the metal in your filter.

 

[Martijn]

What did you process in AR?
How well were base metals separated from the PM's?
WIKI says: Lead(II) sulfate can be dissolved in concentrated HNO3, HCl,....
so probably lead sulphate?

 

[user 65313]

Many thanks for the feedback.

I will try adding some HNO3 to the HCl solution and test again. Good suggestion which I definitely hadn't considered.

The material was pretty well processed to remove base metals.The IC's were pyrolised, crushed in a ball mill. Run though a magnetic separation process, sieved to finer and finer levels and run through a Blue Bowl. The separated material from the blue bowl was then processed in AP solution to remove copper and any other base material affected by AP. Finally the material went into a warm aqua regia. The gold foils were separated from trimmed ram and sat in AP for some time to ensure removal of copper.

Over the weekend I'll process the evaporated Aqua regia, add some nitric to the HCl (containing dissolved lead sulphate(?) and test with stannous. Following that I will post an update.

Thank you all for your contributions.

 

[Geo]

When you evaporate a solution, you are only removing water. Anything in the solution becomes concentrated. To me, it sounds like there is an excess of nitric acid. Excess nitric acid will stop a positive stannous test even if precious metals are present. Test again and watch carefully. Does it make a dark color and then very quickly go clear? It can appear as a simple flash of color or it may not make a color at all.

 

[g_axelsson]

Whenever I try a new process and is left with solids after leaching out precious metals, I add a second leach with fresh acid. If I get a positive stannous test on the second run I know I left some precious metals in with the solids. If I get only a weak result or none at all then I know the first run was enough.

It gives me confidence in my refining and I don't have to worry about throwing away gold with the solid waste.

It usually doesn't take a lot of AR just to cover the leftover solids so it's a quick and cheap way to give ease of mind.

Hi there, new member who has already learned a lot from this forum and yes I have read Hoke....

Hi Marty and welcome to the forum. Just stating that shows that you both read the forum and Hoke's book. That's what we like! :lol:

Göran

 

[Shark]

Just checking, you did not boil the solution? If your copy of Hoke came from FrugalRefiners link then the word boil should have been corrected to mean evaporate and not boil. Boiling can cause losses.

 

[user 65313]

I think Geo nailed it. There was definitely an excess of Nitric. Took several evaporations (not boils) and addition of HCl to remove. Back to a strong stannous indication. Have checked the residues and no gold present plus adding SMB has bought down some decent precipitate so working on cleaning that up at the moment.
There is a fine white precipitate still but I will see what a wash with HCl does to that after I have finished rinsing off the AR from the sediment.
Once again thanks for sharing your knowledge with me everyone.