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After a grinding time of 3 minutes, sample already starts to become compressed by the pestle because of sticking of particles. This use to happen when a fineness of d50 < 30 µm might be present; some hygroscopic samples tend to stick faster.
We determined a fineness of 87 % < 180 µm as desired. Because it is not allowed by the customer to proceed grinding in suspension, the grinding result can only be improved by separation of sticking, fine ground particles by sieving or sifting and proceed grinding the coarse particles.
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