You've been working hard with suction dredge or sluice and now you've got one bodacious pile of black sands with a tidy sum of gold somewhere inside it. Now you're faced with panning and panning until you wished you never HEARD of gold panning. Is there an easier way of recovering the gold? You bet!
Check out gold recovery per Allen Mershon and his mini-sluice. Start with a three- or four-foot piece of 3-inch diameter corrugated plastic drainpipe. Cut it lengthwise in half to form the chute of the sluice. The end of the sluice empties into a bucket. A small battery-powered electric motor (often a marine bilge pump) circulates the water.
You can spoon-feed the black sands into the sluice, or as Larry Weilnau demonstrated, you can rig up a funnel which will also provide a continuous feed. (One good thing about this club is that you get good input from many knowledgeable people.)
You should first dry and then classify (sieve) your black sands to -40 mesh. Here in Colorado, you won't get much gold in the +40 fraction, but you can sluice this later, anyway. It's just that if the size variation of the feed is too great, then the sluice won't work as well.
You should always dry the material out of doors and without the application of high heat. This is because there may be volatile and very toxic material in the black sand, such as mercury and tellurium and assorted man-made chemical substances, which, if roasted in a pan, will VAPORIZE AND KILL YOU! And your children, pets, etc. If you insist on drying balck sands over a fire, at least do it alone so that you will be the only victim.
After you have sluiced some of your material, stop and pan the rejects from the bucket. This will tell you what you have missed. It will be a guide to adjusting the angle of the sluice, the water flow, or the size distribution of the feedstock. You should expect to sluice ALL of your material more than once, which means drying all the rejects and sluicing them again, and maybe again. Mom Nature just doesn't give up her gold without a fight. But this is still easier, for most people, then panning through many pounds of black sands.
This is still a strictly trial-and-error process, so don't get frustrated. Anyone else would have to do the same thing and the mini-sluice is doing the "heavy" work anyway.
The mechanics of gravity separation are so complex that a thousand Einsteins with a thousand mainframe computers could not produce an exact mathematical formula. Don't worry about it. You're not striving for theoretical precision, only practical application. Good Luck!