I tore down a tank and tried to recover the flourite (mixed with MGOCPM) a few months ago.
Did something pretty similar to what cjstl mentioned.
I'd let it dry as much as possible, put some in a large bin (or bucket, trash can, large pot, etc.), add a bunch of water to the bucket (I'd often use water from water changes to minimize waste), stir it up real good, and immediately pour off the water with suspended sediment. The heavier particles (like the flourite) will settle out almost as soon as you stop stirring them, so mostly just the organics and really fine particles will be in suspension. Pretty much like winnowing.
I've found letting it dry as much as possible twixt winnowings helps, as some of the organic particles will float when dried out.
Still had rocks/sand from the MGOCPM, and snail shells, but other then that the stuff is pretty clean.