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After a total dry grinding time of 5 minutes, sample starts sticking lightly to bowl and grinding balls.
This uses to happen after the majority of particles reached a fineness of < 20-30 µm. Interacting forces between fine ground particles will become bigger as their own g-force. Therefore, particles will stick to each other and become compressed by the used grinding balls. These clusters of particles also contain bigger particles which will not be ground any further.
To reach a better fineness, it is possible to separate sticking fine ground particles (by sieving or sifting) and proceed grinding the coarse particles.
Also anti-sticking-agents (like e.g. Aerosil) can be used to delay a sticking of sample. A grinding in suspension will be capable grinding the complete sample way finer.
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