Melting gold flakes

[marveg]

Hello everybody!
I work in a PCB factory, and (unfortunately) we have sometimes defective pcbs with gold finishing. Also, because we conduct the ENIG process in panel, we have little pieces of scrap parts plated with gold.
We decided, for the gold recovery, to treat the scraps and defective boards with sulphuric/peroxide mixture, with the idea of dissolve all nickel and copper under the gold, and let gold flakes undissolved, so we then could easily filter it.
So at this moment, i have a lot of filtered gold flakes, with impurities comming from soldermask small pieces and also FR4 fibers. Next step is to purify it.
My question is: it is possible to directly smelt the impure gold in a small furnace or is it mandatory to dissolve gold, filter soldermask and fiberglass leftovers and then precipitate gold?
Im trying to avoid using AR...
This is an example of the kind of scraps i have.

These are the gold flakes, after washing several times and before filtering process.
And finally, part of the filterd product, after thoroughly drying it

Any comment or advice is welcome! and thanks in advance!

 

[FrugalRefiner]

For what you have, you don't need AR. You can use HCl and either bleach or hydrogen peroxide to dissolve the gold. Melting along with the impurities will leave you with impure gold.

CuCl₂ (aka AP) would be a better choice than the piranha (sulfuric and peroxide) solution to dissolve the base metals under the gold plating.

Dave

 

[Alondro]

Also, after dissolving with the HCl-bleach or peroxide methods, BEFORE you drop the gold, add a few drops of sulfuric acid to the solution, stir and allow to settle for an hour or two. That will grab any lead contamination and precipitate it as insoluble lead sulfate.

 

[marveg]

For what you have, you don't need AR. You can use HCl and either bleach or hydrogen peroxide to dissolve the gold. Melting along with the impurities will leave you with impure gold.

CuCl₂ (aka AP) would be a better choice than the piranha (sulfuric and peroxide) solution to dissolve the base metals under the gold plating.

Dave

Just to point out, all my impurities come from "organic" sources (dielectric and soldermask). My guess is, same as you can melt gold directly with paperfilter, is it possible to burn my impurities?

There is one reason why i use sulfur peroxide mix, it is because i can reuse wastes from production lines, and so, its a cost zero recovering. But yes, etching rate of copper is fast when using copper chloride, and i also have it, so i will try it, thanks for the advice!

 

[Alondro]

Just to point out, all my impurities come from "organic" sources (dielectric and soldermask). My guess is, same as you can melt gold directly with paperfilter, is it possible to burn my impurities?

There is one reason why i use sulfur peroxide mix, it is because i can reuse wastes from production lines, and so, its a cost zero recovering. But yes, etching rate of copper is fast when using copper chloride, and i also have it, so i will try it, thanks for the advice!

If it's carbon-based, it will burn off. If you think there are other salts, you can try lower-temp smelting in an anoxic environment first (a covered crucible). Any oxygen-containing salts will combine with the carbon and be released as CO2. Some anions will simply decompose. Then you'll have a cleaner mass to then direct-melt with flux. You can even smelt directly like that if you pick the correct flux recipe. The flux will gobble up most trace base metals and salts and leave behind the PMs.

mbmmllc (Mount Baker Mining and Materials) on Youtube has many videos about smelting and making the recipes based upon the material.