Granite

The sample has been ground with grinding bowl and balls made of hardmetal tungsten carbide. In tungsten carbide, bowls use to be available up to a volume of 250 ml. By using other materials for bowl and grinding balls like zirconium oxide or steel, bowls up to 500 ml of volume use to be available.

After a dry grinding time of 3 minutes, sample starts sticking to grinding 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 delay a sticking of sample, it is possible adding anti-sticking agents like e.g. Aerosil. It is also possible to separate desired fine fraction by sieving or sifting (classifying) and proceed grinding the coarse fraction.

To achieve 100 % < 71 µm, also a grinding in suspension is possible.

To determine the present fineness after a dry grinding time of 5 minutes, an aliquot has been sieved with our Vibratory Sieve Shaker ANALYSETTE 3 Pro and a 71 µm mesh test sieve.

Also the second sample use to start sticking to bowl and grinding balls after a total of 3 minutes. Longer dry grinding times are not recommended.

The present fineness has been checked with our Vibratory Sieve Shaker ANALYSETTE 3 Pro again. Already 96 % of sample use to pass a 71 µm test sieve.

Probably the moisture of both samples use to vary lightly (higher moisture leads to a faster sticking of sample).