What is attrition and comminution?

Attrition is the unintentional breakage of particles during particle processing and handling. Comminution is the willfully induced breakage, intended for size reduction of particles.

Our work on impact breakage of inorganic materials has shown that impact of the particles leads to localised loading which causes plastic deformation of the impact site. Radial, median and lateral cracks can emanate from the plastically deformed site and propagate into the particle, if the local stresses exceed the yield stress of the material. This behaviour demonstrates the semi-brittle failure mode, where limited plastic deformation can act as a crack precursor. The radial and median cracks are driven by circumferencial tensile stresses, developed mainly during loading, and they propagate through the body of the particle. The intersection of the radial or median cracks with the free surface of the particle leads to fragmentation, i.e. the production of a small number of large fragments which are comparable in size with the original particle. Lateral cracks, on the other hand, propagate approximately parallel to and underneath the free surface of the particle, and are for

med during the unloading stage as a result of residual tensile stresses, which are imposed by the relaxation of the elastically deformed material around the plastically deformed impact site. The intersection of lateral cracks with the free surface of the particle leads to chipping, i.e. the formation and detachment of thin platelets from the faces adjacent to the impact site. Chipping is generally associated with limited particle damage, while fragmentation can change drastically the particle morphology.

The impact breakage of particulate solids needs to be addressed on a more fundamental basis if predictions on a wide range of material properties and impact conditions are to be made. In this case, the different types of breakage need to be distinguished mechanistically and related to particle material properties employing the concepts of fracture mechanics. The objective of our work is to determine the fundamental mechanisms of impact breakage of several materials and identify the transition conditions from one mechanism to another. In addition, our work proposes a quantitative description of one of the breakage mechanisms, i.e. chipping, as a function of material properties and impact conditions with reference to indentation fracture mechanics. Some indications for describing quantitatively other mechanisms of impact breakage such as the fragmentation are also addressed in our work.