Years ago I purchased one of those little spring loaded push button magnets to remove the black sand when the material gets worked down. Works pretty good in a gold pan each time you pan down, but is very slow and tiring with concentrates from a sluice. My friend Alf came up with a more efficient and cost effective idea.

a) Find an old speaker (friend/yard sale/or the dump). Completely remove the magnet (you want a magnet approx. 3 inches in diameter) from all the speaker attachments and materials.

b) Completely clean out a PLASTIC peanut butter/jam/mayonnaise jar with a mouth wide enough for the magnet to fit through. Remove all labels/glue etc. from the outside. Place magnet inside and seal with the lid. The preferable jar will have a fairly flat bottom so that the magnet will be as close as possible to the black sand.

c) Size your concentrates with a small screen (window screen will work) and spread the undersize material (about 1/8 inch deep) in a plastic gold pan, large Tupperware or aluminum containers with a large flat bottom (glass can break and some metals will magnetize). With the magnet at the bottom of the jar, run the Mega Magnet over the sands. Don't worry about being gentle, push those sands around. Your Mega Mag will pick up the magnetic sands. To drop the sands simply give the jar a quick shake so that the magnet jumps to the top of the jar breaking the magnetic connection and the sands drop off.

You will find you can speed up your processing greatly with this unit and pick up more of the magnetic sands. Once the magnetics are removed it is much easier and more efficient to clean out your new cons for your rewards.

Some other pointers:
1) It will work BEST when the material has been dried out. It will work either way, however more small gold particles will be trapped between the black sand granules when used wet. With either method, "dropping" the material a couple of times will help to liberate the trapped gold particles.
2) After you pick up some sand, drop it in the same pan (off to the side) a couple of times and pick it up again. Each time you pick it up, go a little more gently and swirl the material a little. It even seems to help if you swirl the Mega Mag just above the black sand and let the sand come to the magnet. When the sand gets picked up initially, there may be small gold grains caught in the 'chains' of magnetic sand. Each time you drop it/pick it up again, there is less chance of the gold getting 'robbed' from your pan.
3) SAVE ALL MAGNETIC SANDS YOU REMOVE. Along with fine grains of gold that may yet be caught up with the sand: some platinum group metals are magnetic! There may be other rare earth minerals you won't know of unless the material is assayed (which you might do in the future). With this in mind drop the sands you remove into a separate container. MARK THE CONTAINER AS FINISHED MATERIAL (and location/date etc. where it came from) so you don't have to rework it again and again and again... A Ziploc baggie with a note in it is good and a length of twine stapled to the bag and label allows you to fish it out if it becomes buried. If possible, allow your saved material to dry in the container before putting the lid on so that you have a weight reduction factor for handling afterwards.