coal

Light sticking of sample occurs after 3 minutes of dry grinding. With 95% < 250 µm (determined by sieving), the target of d90 < 250 µm is already achieved.

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.

Probably a total dry grinding time of 5 minutes is plausible for a further reduction of particle size. After 5 minutes, we are guessing that the sticking of sample will hinder a further size reduction by dry grinding.