If anyone is wondering why some folks are looking toward smelting to recover gold from black sands, here are some considerations which may be prompting those thoughts.
I became interested in the "black sands problem" a couple years ago, and read up on it as much as I could. The result is that there have been claims that there is much more to this than merely separating flour gold from black sand. People have reported that after they have removed all visable gold, even checking with microscopes to be sure that what's left is totally black sand, that gold remains in that sand.
The point being that it seems that they have found gold inside small grains of black sand. There has been discussion of how the gold came to be completely covered in black material. Similar discussions have been on the mysterious "black gold nuggets" supposedly found, on several occasions, around the Salton Sea area of California, and how they came to be there.
Many have tried the chemical approach, only to find that no acid, or combinations thereof, will remove the black material in every case. Yet fire assays have shown it to contain gold. Is it covered in black silica? But some do report that it seems to be iron, and can be removed with copious amounts of acid.
My casual thoughts have been that crushing to a find powder should expose the gold to acids. However, I also see that there may be limitations to crushing sands in high volume. Also, you would need to know which sand concentrates contained gold, and which were merely black sand throughout.
I have also thought that if fire assays can free up the gold, in small sample amounts, then smelting should be able to release it in larger amounts. Of course, not knowing hardly anything about either one, makes my opinion very iffy.
But, from time-to-time, I continue to see reports of gold inside black sands. And, although I've dredged a couple of times, and processed with various gravity methods to recover visable gold, I've never tried to get inside of any black sands, myself. And seeing all the exaggerations about "micron" gold in water, that come down the pike now and then, I'm cautious about the realities and occurance rates of the "black-gold sand" phenomenon.
Sending black sand to a refiner that you didn't know well, would be about the most unpredictable way of recovering gold from it. Unless you had a way of determining the amount of gold it contained before sending, it would be a total guessing game, and fertile ground for any less-than-honest refiners. And how would you know if you're getting a good sample for assay, if it all looks the same?
Even with multiple samples, the gold bearing sands could be anywhere in a lot. If there was a weight difference, then some kind of weight cut-off point would need to be established, to separate the "good' black sand from the rest. If the black coating over the gold was not magnetic, then some kind of magnet system might be possible.
So far, nobody has reported a proven way to separate the "good black" from the other black.
And the beat goes on....