You need to explain this a little more completely. Are you suggesting using the whirlwind inside a centrifuge? You wouldn't want the carrier (in this case air) swirling inside a centrifuge as this would remix the materials you are trying to separate. But maybe that isn't what you meant. What sort of material are you thinking of separating? I know you specified metals and plastics, but do you mean large pieces such as recyclables or small particles like powder? Please explain, I'm interested.

Since air is a fluid there is no more probability of remix of heavy and lighter particles than with any other fluid. The particles can be as large or as small as convenient but large particles and pieces would have aerodynamic effects that might have undesired remix results. Chopped recyclables would probably work OK. The main advantage would be in the lack of necessity of using a filtering process for a reusable transport fluid except to use a filter to prevent contamination of environmental air at the vent.

The reason they use liquids in a centrifuge is to suspend the particles. The particles settle out fairly slowly, allowing them to stratify as they settle. Air will not suspend particles effectively unless there is movement of the air (an updraft). This will tend to cause remix. The liquid used in a centrifuge is not moving at all from the perspective of the material inside the centrifuge. That is why it doesn't cause remix.

As air is merely a less dense fluid in which particles can be suspended equally as well as with a liquid and if the chamber within which the air is spun by some form of periheral air jets is sufficiently tall perhaps a mere passage through the chamber from top to bottom with no necessity for an updraft would accomplish the separation. It is only a speculative idea and perhaps not useful.

Yes, I realize this is all speculative and I don't mean to insult you. The site's name is "Why Not?", so I'm just saying why not. I really don't know if your idea has potential, maybe it does, I'm just sharing what I know on the topic.

I know that air blowers are used to separate certain materials, but only when the aerodynamic drag is dramatically different for the materials to be separated. Smaller particles actually have more aerodynamic drag than larger particles. Smaller particles have more surface area relative to their mass. This is why dust will stay aloft in a gently breeze that would never support pebbles. Size has a bigger affect on aerodynamic drag than density, unless of course the density difference is enormous and the size variation small. Shape can also have a bigger affect than density. Aluminum is denser than plastic, but a sheet of aluminum foil has more drag than a plastic toothbrush.

Most centrifuges run for at least several minutes if not hours. This is because it actually takes a long time for the particles to settle. The longer it takes, the more effective the separation. Bear in mind that the separation is never perfect. Obviously, once everything has settled, continuing to spin it is pointless. I suppose that for separating extremely small particles (dust sized) using air might work, being that they will settle slowly even in still air. However, they will tend to separate by size rather than chemical composition unless the particle size is extremely uniform.

The centrifugal procss is merely a method of purposefully directing the gravity gradient and if the collectors of susceptible material are strategically placed to capture the desired materials something useful might be accomplished. I am not insulted by a sensible criticism of an idea as the whole point of the site is to noodle with a concept and I certainly have no great expertise in the area.

Uranium enrichment is done with uranium hexafluoride, a gas. There are no complex aerodynamic factors to consider. Additionally, uranium hexafluoride goes through thousands of centrifuge stages (all of slightly differing design) before it's finally refined and pure.

A gas centrifuge does not use aerodynamics to separate. They work off of the weight of different fluid species. The plastic and metallic chips will remain mixed at the end, because they do not constitute a fluid phase. The speeds involved will also cause damage to the centrifuge if a solid is injected.

You would have to add a chemical to turn the aluminum and various plastics into a gas before sending them through a couple thousand stages of gas centrifuges.

However, that leaves the possibility of using a different aerodynamic device to seperate them out.