Glass

We started grinding the sample in dry form.

After the first minute of grinding, we checked the grinding bowl and found, that all sample was sticking to bowl and grinding balls. This is caused by interacting forces which use to get bigger as the g-force of particles. The particles start to stick to each other, as well as grinding balls and bowl too.

Such fine sample will only became compressed to clusters by further dry grinding. Coarse particles inside these clusters will get ground any further.

For achieving a better fineness, a grinding in suspension is necessary. For this, we added 30 ml of ethanol and proceeded grinding. The sample volume collapsed by adding solvent, so amounts of 100 g and more will fit into a 250 ml grinding bowl. For amounts of 250 g of sample, a 500 ml grinding bowl should be suitable.

To avoid over pressure, we ground the sample in steps of 5 minutes, followed by a programmed pausing time of 10 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. This is counting for all other grinding trials too.

Grinding has been started with d50 < 8,7 µm, we achieved d50 < 4,84 µm after 5 minutes of grinding, a d50 < 3,19 µm after 15 minutes and a slight improvement to d50 < 3,09 µm after a total grinding time of 30 minutes. For this, we aborted the grinding process after a total of 30 minutes.

You will find the measurements of particle size distribution on separate pages, measured with a Laser Particle Sizer ANALYSETTE 22 NanoTec plus.

We like to mention that sintered corundum use to have a coarse surface. Because of this, aluminum oxide mainly will be used for short time grindings of fibrous materials. In our trial. We found a wear off of > 2 g from grinding bowl and balls.

We would recommend grinding with zirconium oxide with much less abrasion and a pale color as well (for this, see the following trials).

For the second trial, we used 80 g of sample and zirconium oxide for grinding bowl and balls. Grinding balls made of zirconium oxide use to be available in smaller size as 10 mm Ø (min. ball size of Al2O3 grinding balls). With 5 mm Ø grinding balls, you will have 24 times higher amount of grinding balls which should be much more effective for grinding of such fine particles. Even smaller balls (e.g. 2 mm Ø) will be usable.

We interrupted the grinding process each 5 minutes and took a small sample for demonstration.

The d50 sunk from < 8,7 µm (start size) to < 3,18 µm after 5 minutes. After 10 minutes, a d50 < 2,71 µm has been detected. We interrupted the grinding process after the d50 < 2,35 µm has been achieved.

Find the particle distributions of the Laser Particle Sizer ANALYSETTE 22 NanoTec plus on separate pages.

We like to mention that sintered corundum uses to have a coarse surface which is beneficial for a grinding of biological samples. Compared to other materials, you will face much more abrasion (Al2O3) after longer grinding times.