Help with Determining gold content in a pin sample

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Lobby

Well-known member
Joined
Jan 25, 2011
Messages
194
Location
San Antonio, TX
Hi folks! I hadn't been here in awhile. Heck, perhaps a few years. I hope you've all been well.

I'm a jewelry designer / fabricator, and a scrap gold buyer. I've been buying gold filled stuff for a while. It's piled up and I wanted to get rid of it.

I found a refiner that will process my gold filled scrap. They pay 90%. I melted it (oxy propane) (over 600 grams), but didn't have a mold to pour it into, so I just let it cool off in the crucible. I then broke the crucible off.

I "polished" the blob with a wire wheel grinder, then pickled it for a few minutes in a sodium bisulfate solution.

I analyzed the bar with my XRF, and got readings between 4% and 6%. Obviously, that's higher than what you'd expect out of gold filled, but perhaps there were some pieces with GF clasps that were actually karat gold.

I sent to the refiner. He remelted and pulled pin samples. He stated his analysis was 1.77%. Hmmm. When I analyzed the pins, I got between 3% and 5%. Granted, my XRF isn't calibrated at those low levels.

I didn't accept his offer, politely, so he sent them back. I have the pins and the blob now.

I'm sending the pins to a friend who has a different model xrf so she can analyze them.


What options do I have for determining the actual gold content in these pins? (I've refined Au and Ag several times, but only have about 1 liter of nitric acid left)

- I suppose I could weigh the pins (10.7 grams each) and dissolve into nitric. Then collect the Au particulates, weigh and go from there. But 2% of 10.7 grams isn't a lot. I'm worried about recovery affecting my yields.

- I suppose I could ask one of you to perform a fire assay.

- I have cupels and an assay oven. But I've never performed one and am worried about how accurate my first tries would be.


I would appreciate any advice...
 
Well the maths proves that if it was plum 1/10 10k the assay should be 4.1% Au, so if you had some 14k gold filled in the lot it could reach 6%.
Can your xrf determine the copper content? If so and you have just copper and gold you could set up a copper cell and recover the gold from the slimes, you only need sulphuric, some copper and a power source. Bear in mind cells work best with high grade feed stock so it needs to be 95% copper or very close, if it's below that you could add more copper to raise the percentage.
 
Adding more copper to a sample to run it in a copper cell is futile. The problem with low grade material is that the contaminants is fouling the electrolyte. Adding more copper only increases the time to run the sample, the contaminants is still the same amount.

There are only two times when it is useful to add copper.
- If there is something that passivates the anode, too much gold or silver for example.
- If you are running a continuous system where some of the electrolyte is removed and it can't handle too much contaminants.

As for the XRF of the blob, you mentioned that you pickled it before testing. That would have removed some of the base metals at the surface so it would read higher than a filed down surface. I don't know how big difference it would make in the end though.

Göran
 
Thanks. I sent the pins to a friend with an XRF. Am waiting to hear back.

What I suspect happened is that the refiner analyzed the pins only at the center, where the gold content is the lowest. Of course, that's unethical (if the pin is representative of the entire molten gold lot, then all the gold in the pin must be taken into account). Many refiners cheat you that way with pins.

Do y'all think that refining one of those pins with nitric, then filtering the gold from the solution is feasible?
 
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