SUMMARY
The term "attrition" is taken to embrace any unwanted particle damage that can arise during processing. This damage may be evident as particle fragmentation due to bodily fracture or as the creation of fines due to the removal of surface layers. Attrition due to mechanical means may arise from the bulk strain of particulate solids and powders or due to particle impact on a wall.
An annular shear cell has been exploited in order to study in a controlled, systematic manner the influence of normal stress and strain on the bulk attrition of a number of bulk solids. No such work had been performed previously, reliance being placed upon arbitrary tests in which the key parameters were not known or controlled. The cell offers the possibility of comparing the tendencyof materials to be subject to attrition. The work for IFPRI aimed to exploit this potentialwith a view to relating bulk attrition to particle characteristics with a view to improving process operation and to understanding particle design. To this end studies were performed on high density polyethylene granules and on alumina particles.
Various grades of high density polyethylene enabled the influence of polymer molecular weight and degree of branching to be examined. In this particular polymer system, bulk attrition was found to be reduced by increase in molecular weight and by the existence of branching.
Alumina extrudates were manufactured in accordance with a precise recipe. This enabled cylindrical particles with identical sizes but of significantly different strengths to be formed, these being assessed by a measurement of single particle crushing strength. Attrition experiments conducted with any of these extrudates at the same total breakage yielded the same product size distribution. Further experiments on one material at one stressenabled the behaviour of any other of the extrudates at any other stress to be predicted making use of the particle tensile failure stress, as deduced from the crushing tests, as a normalising parameter. It is logical to argue that a more fundamental property of the extrudates, the fracture toughness, should be employed to normalise the attrition results and therefore this was measured by casting the extrudates in the form of bars. However, this does not produce an improved means of correlating results, probably because it specifically excludes the flaw structure in the extrudates.
The attrition cell also offers the potential of providing guidance on the performance of operating equipment. Tests in a laboratory stirred vessel showed that the attrition rates were in the same order as those found in the cell and in the crushing tests but the relationship was not a linear one.
The studies have established that it is possible to relate simply and scientifically the bulk attritionof a material to particle properties and, although the predictions are not yet quantitative, to equipment performance.