magnesium carbonate

400g use to be the maximum amount of sample which can be fed into a 500 ml grinding bowl. With up to 4 bowl fasteners, maximum 1200 g of sample can be ground in one batch.

After 5 minutes of dry grinding, the sample starts sticking to 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.

A longer dry grinding time is not recommended.

To determine the achieved fineness, we sieved an aliquot of sample with our Vibratory Sieve Shaker ANALYSETTE 3 Pro and a test sieve with 200 µm mesh. All sample has been able to pass the test sieve. The desired fineness of < 200 µm has been achieved.

For a better fineness, it is possible separating fine ground sample by sieving or sifting (classifying) and proceed grinding the coarse fraction until sticking occurs again.

It is also possible adding anti-sticking agents like Aerosil to delay the sticking of sample.

For a grinding of the complete sample to < 20 µm, a grinding in suspension is required.

Ethylene glycol can be used as anti-sticking agent. It is necessary to add 0,1-0,5%w to delay a sticking of sample. Afterwards, sample can be ground way finer.

Because volume of sample rose during grinding, we used the half of ground sample from result 1 for this trial to prevent over filling. After adding the ethylene glycol, we ground for additional 15 minutes. Sample started to stick to bowl and balls again. A fineness of 100 % < 75 µm has been detected afterwards.

Longer dry grinding times are not recommended.