Most sources will tell you that Aqua Regia really isn't for black sands. But I have used that method successfully many times. Trick is in cleaning up all the other garbage in the sands, because you never know all what's in the black sands that will react. I roast the black sands first. Get rid of sulfides, etc. Then I use hydrogen peroxide and vinegar 50/50 to get rid of any lead in the sands if it's there. Then a good soak in HCL. Oh and don't forget to remove all the magnetic material such as magnetite. Reduces the quantity you need to process.
I think how successful you are and what your recovery will be using AR directly on black sands will really have a lot to do in the chemistry of the sands. As I said, processing black sands here, it works well for me. Where you are and with what minerals are in your black sands? I don't know.
I spoke of dangers of AR in another thread. So I am not going to give recipes and processes here. Suffice it to say that AR is dangerous on a few fronts. First being it's strength as an acid (acid safety handling precautions should be used.) Even the vapors from AR can instantly corrode the best of stainless steels! And the dangers of the vapors. They CAN kill. Even days later if you get too much. (As I recall it can destroy the liver.) So it's definitely something to do outside unless you have a lab that has a fume hood up to the task of venting the vapors absolutely and safely. If you live close to someone else, out on your back patio is not going to be a good place either I will bet. And then there are compounds that are explosive that can be inadvertently created. Small batches of AR processed probably won't yield a large quantity of these, but it could and that is a danger too. There are a lot of variables in this method, too many for a lot of people to be safe using.
This is an issue that needs to be solved though. My Gold Well sluice really collects a lot of fine gold. Because of that, more people are going to need a method that is safe and effective to get that fine gold out. Most miners only have to deal with such a small amount of fine gold that they really don't care much about it, it's not worth the effort to collect. But with the Gold Well, the amount of super fine gold can be significant. So I have been looking for a good solution. None of the standard gravity devices (bowl, wheel, table, etc.) seem to be efficient enough. They miss too much in my opinion. How do I know? Because I have taken a lot of material that's been processed through them and put it through the GW, and recovered a lot of fine gold that was missed by them.
I am however looking into the MineLab. One customer of mine who processes concentrates commercially in the northwest swears by it, so I will look into it and see. I will give an update when I find out how it does.
Basically it boils down to this:
Enhanced gravity methods (centrifuge type devices)
Standard gravity/mechanical devices (bowl, table, spiral)
Chemical methods (Aqua Regia, Cyanide, Mercury ... all have significant dangers of one kind or another)
Smelting (when finished you will have to use a chemical method to get pure gold if that's what you are after)
Send it out and have it done professionally for a fee: Problem has been a lot of people distrust refiners for one reason or another.
Well it's something I think about on a daily basis and I hope to come up with a good answer. So far I haven't.
Bottom line, chemical methods all have some inherent danger. But then so does life I guess, right? Just make sure if you choose a chemical method to make sure you know what you are doing. Might cost you your life if you don't or a trip to the ER. But if you are an intelligent, responsible adult and are willing to study before you leap, I think you might be ok using chemical means but make sure you really do have all that much fine gold to make it worth while, and see if there isn't a mechanical method that works for you well enough first!
