Dry Sieving at Near-Ultrafine Particle Sizes

 

If you’re trying to size very fine powders by sieving you may be left feeling a bit frustrated. Typically gravity does not help much when trying to sieve very fine powders. Forces such as electrostatic repulsion become dominant and particles tend to remain suspended over vibrating surfaces rather than pass down through openings in mesh – rendering dry sieving below ~45 μm almost impossible.

The introduction of water into the sample allows gravity to pull the suspended particle down through the mesh openings. While this opens up new opportunities with the ability to sieve down to lower sizes there are downsides – like the need to remove water once the sample is sized and the problem of screen blockage and overflowing (and subsequent sample loss). 

The development of the Cyclosizer helped the sub-sieve sizing process by extracting 5 fractions below 50 micron – without using sieves. A “dry” equivalent to a Cyclosizer would be an Air Classifier that uses the Coandâ Effect to separate a dry sample into typically three size ranges. However these are typically 4x the cost of a Cyclosizer. 

There is also a dry sieving technique that can be used. This requires an Air Jet Sieve and their are limitations: Only small samples and single sieves can be used, and there is a potential to lose the undersized portion of the sample to the filters of a vacuum cleaner (although the use of a cyclone before the vacuum cleaner can mitigate that). 

 

 

 

If you pre-screen your dry sample on a standard-type sieve shaker (like an EML 200) you may have your smallest fraction at below 40 to 45 μm. This can be placed in an Air Jet Sieve where the air-assisted feature of the machine means finer aperture sieves can be utilised significantly quicker. The benefit to this method is your samples are dry and do not require dewatering, but you’re still limited, realistically, to a 20 micron sieve aperture.  Anything finer is horrendously expensive and fragile – a terrible combination. 

The other downsides are the cost of the equipment, the sample size limit and the fact that you can only use one sieve at a time. 

Any sieve below 20 μm would probably not have woven wire mesh as the sieving media but would instead have electroformed nickel sheets. At those apertures electroformed nickel sheets may have fewer openings for the same surface area when compared to woven wire mesh and so the sieving process may need to be run for longer. 

At such fine apertures the electroformed mesh becomes week and expensive and so they are typically only available in small sieve frames. 

The equipment needed to achieve dry sieving below 25 μm would (as an example) be:

 

Nexopart Air Jet Sieve + Vacuum Cleaner (usually supplied with machine)

 

 

Microsieving Adaptor

 

 

Micro Precision Sieve (40/38/36/32/25/20/15/10 μm)