Granule Consolidation, Attrition and Breakage

The approach taken here is to employ standard materials characterization tests to measure the materials properties of the granulated product. With this information, the mechanism of attrition, i.e., breakage versus erosion, is determined. The rate of attrition can then be related, semi-empirically, to material properties of the formulation and the operating variables of the process, such as bed depth and fluidizing velocity. 

It is inherently difficult to measure the strength of a material since this is strongly influenced by the microstructure of the material, i.e., the distribution of flaws which strongly influence the propagation of cracks. This concept is illustrated in Fig. 31, where the elastic stress distribution in an ideally elastic, brittle material is seen to become infinite as the crack tip is approached. The key properties which characterize the strength of a material are  

In order to ascertain these properties in a reproducible manner, very specific test geometry must be used since it is necessary to know the stress distribution at predefined, induced cracks of known length. For example, the measured tensile or compressive strength of a series of glass bar 

Using the picture shown in Fig. 31, the fracture toughness defines the stress distribution in the body just before fracture and is given by  

The above results are derived from linear elastic fracture mechanics and are strictly valid for ideally brittle materials with the limit of the process zone size going to zero. In order to apply this simple framework of results, Irwin (1957) proposed that the process zone, r be treated as an effective increase in crack length, Sc. With this modification, the fracture toughness becomes 

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