Limestone

After the first minute of grinding, sample is sticking lightly to grinding bowl and 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.

Dry grinding has to be aborted after a total grinding time of 3 minutes. Only a grinding in suspension should be performed when longer grinding times (for finer grinding results) will be required.

After 3 minutes, we determined a fineness of ~ 69 % < 63 µm by sieving of an aliquot with our Vibratory Sieve Shaker ANALYSETTE 3 Pro.

It might be possible to improve the dry grinding result when sticky particles (< 63 µm) will be separated (e.g. by sieving out) and only proceed grinding of coarse fraction until sticking will start again.

Just for demonstration, we took a further sample and used 20 mm Ø grinding balls for comminution.

The sample uses to be brittle enough to get ground with those grinding balls as well.

It took about 5 minutes until an equal level of sticking was reached (compared to 3 minutes of grinding with 30 mm Ø grinding balls). With our Vibratory Sieve Shaker ANALYSETTE 3 Pro, we determined ~ 71,7 % < 63 µm.

Also smaller grinding balls don't achieve a significant better dry grinding result when sample will start sticking.

See result 3 for a grinding in suspension (with water).

To avoid a sticking of sample (as shown by dry grinding), we added ~120 ml of water to the sample. For a start size of 4 mm (previous grinding trials), we estimate that an equal result will be achieved with about one additional minute (max. 11 minutes).

To avoid over pressure, we ground the sample in steps of 5 minutes, followed by a programmed pausing time of 10 minutes.

After 10 minutes of grinding, the sample was tested with our Laser Particle Sizer ANALYSETTE 22 NanoTec plus. We determined 100 % < 40 µm and d50 < 2,44 µm.

For the desired fineness, less grinding uses to be required then 10 minutes.