This topic have drifted a bit, the gold isn't going to appear as flakes. The gold sits on different parts, as plating on fingers still attached to the board, as pieces of shredded pins, as bond wires embedded in shredded IC:s. So the density of the material containing the gold will be all over the scale from light to dense. The form of the particles will also affect how they move in the sorter. Larger particles will move as lighter but more dense particles.
My prediction is that it will be an utter failure. Gold will be with both fractions, the form of the particles will clog the channel unless a really high air speed is used and then the separation will fail.
Why not use magnetic separation if you want to remove ferrous material? Just remember that CPU pins are made of covar and are magnetic. Whatever method used, take samples after separation to see that the separation actually works. Test, not believe!
Göran