Friability Indices

These difficulties are avoided in Gwyn s (1969) design (Fig. 3). Here, the attrition products are not kept inside the system but it is rather assumed that they are elutriated. In the enlarged diameter top section, gravity separation defines the limiting diameter of the elutriable particles. The attrition rate is assumed to be given by the elutriation rate. The steady-state elutriation rate can, therefore, be used as a friability index. 

Other Tests. The so-called tumbler tests are usually used for testing material like coke, coal, iron ore pellets or tablets. They can be divided into drum tests and ball mill type tests. The latter type is used to derive both the Hardgrove Index and the Bond s Work Index, which are often used to classify the material friability as described in Sec. 3. They are generally more suited to coarse material. The Hardgrove Grindability test requires an initial size range form 595 to 1190 microns. 

The percent of fines passing the 0.0117-in. screen is considered breakage due to attrition or abrasion rather than shattering, and is reported as dust index. In the above example this.is 20.1 percent. Note that the higher the friability value, the more easily the material under test degradates. 

Fig. 4. Relationship between the calculated spreading coefficient WS(B/A) and granule friability ( ), tablet crushing force ( ) and tablet capping index (A) [71]...

The objective was to obtain tablets of low friability and satisfactory hardness. Good compression properties were also needed, that is a high transmission ratio and also a high cohesion index (see below). A granulate with at least 20% fine particles was also considered desirable, for the powder to flow correctly. 

Coeff. % fines 3 2 friability (%) 3 hardness (kP) > 4 transmission ratio ys cohesion index... 

Figure 6.6 Response surfaces (a) % fine particles (b) friability (c)-(d) hardness (e) transmission ratio (f) cohesion index. 

Hardness/abrasiveness Friability GrindabiUty Dustiness index Thermal properties Calorific value Heat capacity Thermal conductivity Plastic/agglutinating Agglomerating index Free swelling index... 

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