Sieve testing, as I have stated many times, is the Cinderella of particle size analysis because it delivers more value than expected from something that’s so easy to use and relatively inexpensive. However, the problem with standard sieving techniques using wire mesh sieves is that they begin to exhibit accuracy problems in the lower micron sizes.
Standard sieves are advertised with openings as low as 30m (microns). At these small sizes, tests usually end with poor and inconsistent results. Inherent variability in wire mesh causes wide relative difference in aperture sizes at these small sizes.
This is the area where alternate techniques such as laser diffraction become the accepted way to get accurate particle size analysis. These techniques, however, are significantly more costly and are technically more difficult to use.
There is another culprit contributing to the difficulty of getting good results for particle size analysis in the lower micron ranges.
However, Cinderella and Prince Charming have a couple of tricks up their sleeves.
The process of electroforming can produce sieves with apertures in the 2 to 3 micron range with acceptable variation between opening sizes – a far cry from woven wire mesh.
Their second trick is the development of the vacuum siever. This design places a rotating vacuum below a sieve that moves the sample over the sieve surface while pulling the particle through the openings. These vacuum sievers can be effective down to the 20m levels. One drawback of these vacuum-sieving instruments is that they will only process one sieve at a time.
Cinderella and the Prince have one more trick, which is the Sonic Sifter. This instrument can process 3 to 5 sieves at a time. The Sonic Sifter applies a vertical, oscillating column of air to the sieves; the air column oscillates 60 times per second to present
Costs for the different techniques of particle size analysis begin at about $1,000 for a basic shaker and 5 non-certified sieves. The Sonic Sifter prices are at the level of $7,000 without sieves. Vacuum sievers cost approximately $9,000. Finally, the Laser Diffraction equipment begins in the $25,000 to $30,000 level and frequently costs in excess of $50,000.
If it seems that the design of reliable particle size analysis in the low micron range is esoteric you are thinking correctly. The determination of the best method is a combination of the nature of the material, the accuracy and repeatability of the results needed, the budget available, and the technical ability of the operators.
The range of equipment and techniques available should provide an alternative that will work in most situations.
As usual, even analyses that should be easy turn out to disconcert and perplex me.
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By Art Gatenby