Nathaniel R Warner

At these sizes, wet screening is usually recommended but there are few screens in this size range and they are easily blocked. Dry methods especially will be prone to blockage due to agglomeration and lack of dispersed particles in this range. Typically other methods would be used for separation in this small size range. A hydrocyclone would be the most common (e.g. the Warman Cyclosizer) route but a standard air cyclone as used in the cement industry (I see you're in Civil Engineering) may allow the < 10 micron fraction to be separated as 'fines'. But less likely for a 2.5 - 10 micron fraction.

Even just shaking in water (if compatible) and removing the supernatant after a minute or so (the large stuff has sedimented out) may be a simple solution (bad pun). A Stokes' Law calculation for gravitational settling (a centrifuge would not be necessary) may allow you to home in on a crude guess at settling time (from a few seconds to a few minutes).

Seek out Professor Jim Adair at Penn State - he's a renowned expert in this area - and pass on my regards. You may already have done this...

Agree with Alan F Rawle. If not compatible with water - you are asking for a dry method - use cyclohexane. Dry the cyclohexane with a dessicant that you filter out before use, then use promptly.

@Nathaniel R Warner

Now a lab air classifier plus air jet sieve might just work. This usually would generate a < 20 mike-rons (named after Mike and Ron) fraction. See

https://www.youtube.com/watch?v=9tFSd5idKiQ

I like their lab cyclones as shown. Don't forget the vacuum hose. Now you could ask someone to do this for you (for a fee, of course).

Less than 500 people have seen the video, so it's a secret just between us....