Characterising Small Samples Of Powder
The tapped density is an useful feature, because this provides the driving force for gravity flow. The 'angle of repose' has no meaning with cohesive powders, because it can adopt almost any value from flat to overhanging, depending on how it is prepared.
Wall friction is a valuable design parameter. A series of measurements at different contact pressures can show both static and dynamic friction values and bring out any tendency for the material to cohere to contact surfaces. The results will depend on the surface chosen, but checking with different surfaces and finishes allows optimum constructional materials and finish to be specified.
The awkward feature to measure is shear strength in relevant conditions of compaction. One useful quality control method is a load/compaction graph. This indicates the compressibility of a powder and the resistance offered to compaction. Small amounts of product implies small scale equipment, therefore light compacting forces are of key interest. The Ajax tensile tester offers a useful method of detecting fundamental 'cohesive' forces within a mildly compacted test sample. This values reflects the potential flow difficulty that the material will exhibit in small scale equipment. Ajax has a publication on the subject of powder testing if you would care to send your address and further details of your specific interests to tech@ajax.co.uk.
Lyn Bates ■
Characterization Of Powders
Dear sir
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Best regards
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Dr. Ivan Peschl ■
Re: Characterizing A Powder
We do a lot of hopper design for poor flowing materials in small equipment.
You need to understand that the challenges involved in measurement and design for reliable flow get greater as the scale gets smaller - because you are trying to get the material through smaller holes with less force to do it.
So "simple" measurements are even more useless when designing small scale equipment than they are for large scale stuff!
The only real way to do this is with a shear cell. Or better yet, if you only have an occasional requirement, give the sample to an expert to do it for you - it will cost less than buying a shear cell and training someone to use it.
Have a look at our web page - www.bulksolids.com - for more information. ■
Characterizing Powder Flowability
I do see the opportunity for using some standardized powder flow measurements in a Quality Control situation. In an aritcle I wrote for Poder Handling and Processing magazine, 3/2002 issue on page 218, I illustrated how ANSI CEMA 550 Standard Flowability Ratings could be generated using alternative techniques. In my PhD thesis work, I also found correlations between powder flow ratings and a variety of other powder related factors. My personal favorite is the Hausner Ratio, which in my doctoral studies correlated well with at least 5 other parameters.
See: Characterization of Bulk Material Behavior for Granular Materials, PHD Thesis, University of Rochester, Rochester, NY USA 1996. James H. Cain ■
Characterizing a powder
I am aware of the difficulties of characterizing a powder by simple methods and the inadequacy of the traditional measurements.
Nonetheless for a small scale effort - where flows are a few gm/min( <1 kg/min) and dimensions are on the order of centimeters rather than meters- and the powder is both cohesive and electrostatic do the simple angle of repose, tapped density measurements have any meaning ?
Can they be used as a quality control measure ?
Thanks
barry ■