Limestone

Right after the first minute of dry grinding, sample use to stick strongly 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. Moisture will lead to a faster sticking of sample too.

Still particles up to 3-4 mm can be found. For this, a longer dry grinding time is not recommended. A small amount of sample has been taken out for demonstration.

For a further grinding, we recommend grinding in suspension (see following result).

After 1 minute of dry grinding in result 1, we added 190 ml of water and ground the sample for 10 minutes.

To avoid over pressure, we ground the sample in steps of 3 minutes, followed by a programmed pausing time of 6 minutes. After several cycles, the outside temperature of the bowl should be checked (remain below 80°C); grinding time or programmed pausing time might be readjusted afterwards.

This is counting for all further grinding trials in suspension too.

After 10 minutes of grinding, a d50 < 5 µm and 100 % < 60 µm has been achieved. This is a typical result, achievable with 30 mm Ø grinding balls. For a fine grinding, balls should be changed to e.g. 5 mm Ø. For this, see result 3.

A small sample has been taken out after a total grinding time of 10 minutes.

After a total grinding time of 20 minutes, already 30 % of sample use to be < 1 µm as desired.

Because the slurry use to fill the bowl to the maximum and viscosity of sample has been increased immensely, we need to take out about 180 g of slurry (which correlates with ~ 93 g of dry sample.

For this, we recommend grinding with maximum 250 g of limestone per 500 ml grinding bowl.

As liquids, also solvents like benzine, hexane, ethanol or isopropyl alcohol can be taken instead of water too.

We proceeded grinding in result 4.

After we needed to take out some sample to proceed grinding and maintain a motor oil slurry again (see result 3), about 250 g of sample and ~310 ml of water use to be filled inside the grinding bowl.

We proceeded grinding to a total of 60 minutes. Afterwards, present fineness has been checked with our Laser Particle Sizer ANALYSETTE 22 NanoTec plus again.

For a proper detection of fine particles (< 2 µm), it is important to use Mie parameters for measurement. We used best fitting parameters from our list of possible refractive indices for this material and found a d50 < 350 nm. If the adequate Mie parameters for your special kind of sample can be provided, a recalculation of the result is possible.

Possibly the desired fineness has been already reached after a total grinding time of 30-40 minutes.