Limestone

As desired, half of the provided sample was used for a test with Planetary Micro Mill PULVERISETTE 7 classic line and a grinding bowl made of zirconium oxide.

After one minute of dry grinding, the sample got comminuted to a fine powder, lightly sticking to bowl and grinding balls.

This uses to happen after the majority of particles reached a fineness of < 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.

We added 1,5 ml of isopropyl alcohol after 1 minute of dry grinding and proceeded grinding. By grinding in suspension, sticking of particles will be prevented.

To avoid over pressure, we ground the sample in steps of 1 minute, followed by a programmed pausing time of 2 minutes. After several cycles, the outside temperature of the bowl should be checked (remain below 70°C); grinding time or programmed pausing time might be readjusted afterwards.

After 5 minutes of total grinding time, we checked the particle size by optical microscopy. Still a couple of particles > 20 µm can be found. We proceeded grinding for a total of 10 minutes. Afterwards, no particles > 20 µm have been spotted by microscopy.

The original sample which was left over was used for our second trial.

With agate, this start size of sample could be ground as well, even when the density of agate is much lighter compared to zirconium oxide. We expect that the lightweight agate needs a bit more time to achieve a similar result compared to result 1.

After 1 minute of dry grinding, we added about 2 ml of isopropyl alcohol and proceeded grinding for 10 minutes. Afterwards, fineness has been checked by optical microscopy again. One particle > 20 µm has been found inside the sample we took.

We kept on grinding for a total of 15 minutes. After 15 minutes, no further particles > 20 µm have been spotted.

To achieve better grinding results, it is still possible to keep on grinding, also a change to smaller grinding balls (e.g. 5 mm Ø) is possible to improve the grinding result.

Because no more sample has been provided to check results with other machines, we used sand, fractioned to 1,0-2,0 mm by sieving to demonstrate a grinding result wit hour Vibratory Micro Mill PULVERISETTE 0. We expect that sand uses to be way harder to grind compared to the limestone sample which was provided.

To flush all lightly sticking particles back into the center of the mortar, we added 4 ml of isopropyl alcohol.

The grinding process was interrupted after a total grinding time of 10 minutes. It has shown that still some fragments up to 70 µm could be found afterwards. We expect that even after a total grinding time of 60 minutes, still particles > 20 µm might be spotted.

We also like to mention that the lid made of plexiglas will suffer after longer contact with solvents like isopropyl alcohol; for this Vibratory Micro Mill PULVERISETTE 0 and Mortar Grinder PULVERISETTE 2 might not be recommendable tools for comminution of sample.

For the desired amount of sample and start size, also a Mini-Mill PULVERISETTE 23 might come into question.

After 5 minutes of dry grinding with 15 mm Ø grinding balls, still a few particles > 20 µm are present. A small portion has been packed for demonstration.

Because of sticking of particles, we added 2 ml of Isopropyl alcohol and proceeded grinding. We recommend to seal the 2 half of the bowl with a bit of scotch tape additionally – the zirconium oxide grinding bowls don’t use to have a special gasket like the steel bowls use to. This way, we can make sure the isopropyl alcohol will not rinse out of the bowl during operation.

After a total grinding time of 10 minutes, the present fineness got checked again by optical microscopy. No particles > 20 µm had been spotted.

Agate is not available for grinding PULVERISETTE 23 bowls anymore; it is still possible to use agate grinding balls in zirconium oxide bowls as well.