Cement (high aluminia)

At maximum speed (1500rpm), sample starts sticking within 30s of grinding.

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 just become compressed by the used grinding balls. These clusters of particles also contain bigger particles which will not be ground any further too.

To determine the fineness afterwards, we used a Vibratory Sieve Shaker ANALYSETTE 3 Pro and sieve an aliquot of 5g for 20 minutes. Afterwards, we found the following distribution: 88,7% < 80µm; 10,1% 80-140 µm and only 1,2% > 140 µm.

For a grinding to 100% < 150 µm, we recommend grinding in suspension.

In this trail, we wanted to show that a reduction of speed will delay a sticking of sample, but fineness should not be improved significantly.

Therefore, we set speed to 600rpm. Light sticking of sample sets in after 150 seconds of grinding.

Again, an aliquot of 5g ground and mixed sample has been sieved out with our Vibratory Sieve Shaker ANALYSETTE 3 Pro. This time, following distribution of particles has been found: 79,4% < 80 µm; 18,2% 80-140 µm and 2,4% > 140 µm.

As shown, when sticking of sample occurs, fineness will not change significantly.

No bigger pieces of ~ 10mm length have been left. We are guessing that 10mm long pieces might become ground as well. Else, a 250ml bowl with 30mm balls might be required for comminution of sample.

By using 30g of sample, the 80ml bowl use to be filled almost to the maximum.

After 1 minute of grinding, only a light sticking of sample occurs. After 2 minutes of dry grinding, the sample became compressed mainly at the wall. Sample is still easy to detach.

Following fineness has been determined by sieving an aliquot with Vibratory Sieve Shaker ANALYSETTE 3 Pro: 77,5% < 80 µm; 16,5% 80-140 µm and 6% > 140 µm.

To improve the dry grinding result, it is possible sieving out desired fine fraction and grinding the coarse fraction again until the minimum useful bowl volume is reached. For this, maybe about 35 g of sample should be ground stepwise to achieve 30 g of desired fineness.

For grinding the complete sample in one step to < 150 µm, we recommend grinding in suspension (see result 4).

In our last trial, we want to show the grinding result in suspension. Sticking of particles can be hindered this way. Better endfineness can be achieved.

To avoid swelling of sample by using water, we added ~11ml of isopropyl alcohol (IPA) to achieve motor oil like viscosity (for best grinding results with low abrasion on bowl and balls). Also isopropyl alcohol will evaporate much faster if sample should be dried afterwards with e.g. vacuum rotary evaporator.

After only 5 minutes, grinding has been interrupted and particle size distribution has been determined by our Laser Particle Sizer ANALYSETTE 22 NanoTec plus.

Measurement no. 60861 on separate page shows, that a d50 of <16,2 µm and a d90 < 43,8 µm has been achieved. No particles > 90 µm could be found.

Even better endfineness can be achieved by longer grinding times. Also a change of ball size might be plausible for further size reduction.