Positive stannous test after HCL rinse on Karat gold batch?

[Hartbar]

Good morning, I did three distilled water boils on my gold powder from karat gold, after dropping with SMB. I then did a HCL boil, as always I tested for gold in solution, got a slight positive result with stannous chloride. HCL solution took on a pale yellow/green color. I kept the solution. Powder seemed very nice and good yield, have not melted a bead yet. Any thoughts and input welcome please.

 

[Hartbar]

Process was: inquart to 6k gold with silver. Nitric diluted leach, rinse, nitric leach again. Rinse to remove silver chloride. Light incineration of flakes. AR. Sulfamic acid to negate nitric. Filter twice. Chill. Drop with SMB. Light boil to reduce fumes. Settle 12 hrs. Rinse with distilled water 3 times. HCL boil. Stannous test point. 3 more distilled water rinses. Dry powder, melt.
I assume there was still some nitric present even though I used sulfamic prior to drop?

 

[nickvc]

No the chances are that you dissolved a very small amount of your gold , the HCl absorbed a small amount of oxygen which allowed some gold to be dissolved , I always add rinse HCl to either my next dissolution or into the stock pot .

 

[Hartbar]

Thank you nickvc, so no major issue I guess?
Shouldn’t effect purity or yield that much?

 

[nickvc]

It should improve the purity as if it can dissolve gold it will dissolve any base metals as well and it will only dissolve a very small amount of gold which you can recover later.

 

[Shark]

The times I have had this happen the recovery was to small to weigh on scales that measured to the nearest .1 of a gram. Very minimal loss and as Nick mentioned it will remove more base metals as well.

 

[orvi]

To me, it happen practically on regular basis. But I process e-scrap, so there is often quite significant excess of nitric, solutions are overloaded with base metals... So it is maybe also due to this. I can imagine that oxidation potential of HCL/O2 near boiling point of HCL azeotrope is enough to start very slowly dissolving gold, but I do not know this for sure. Someone correct me if I am wrong.

Anyway, this happens. I always check first wash of gold with stannous. If it is significantly positive, I let the beaker cool a little bit, then add little spatula of SMB, let it boil for some time to pack the gold nicely with the rest of the ppt and then decant and proceed as usual
If it is just minor gold, I have one collection beaker, where I add all the "leftovers" (slightly positive acid boils, last bits of gold dust settled from the beakers, used positive stannous tests - i do them on watchglass, etc).

 

[Hartbar]

Not sure why, but it’s never happened to me on gold filled batches. Twice now on karat gold refines. I’m curious if it’s something in my procedure?