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After 5 minutes of grinding, sample starts sticking to the scraper, mortar and pestle. This uses to happen after the majority of particles reached a fineness of < 20-30µm. Interacting forces between fine ground particles will become bigger as their own g-force. Therefore, particles will stick to each other and just become compressed by the used grinding balls. These clusters of particles also contain bigger particles which will not be ground any further too.
For this, no longer dry grinding is possible.
Fineness has been determined by sieving the complete sample with a Vibratory Sieve Shaker ANALYSETTE 3 Pro and the proper test sieve of 50µm mesh. About 90,25% have been ground to < 50µm as desired.
It is possible to separate desired fine ground sample (by sieving of sifting) and keep on grinding the coarse fraction.
It is also possible grinding in suspension to achieve 100% < 50µm.
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