SUMMARY
This report is the result of a research programme on the subject of particle attrition. Fox the purpose of the report we have taken to this mean the process of mechanical damage to particles which is undesired, since that is the topic which stimulated the IFPRI committee to set up the project. By the end of the project, the word attrition is taken to denote one of the mechanical breakage processes which may occur when particles are stressed, regardless of whether the breakage is required or not. The thesis of the report is that particle breakage is a complex process which can only be understood and modelled if the various mechanisms are identified.
The first chapter of the report concerns, then, the identification and definition of four different mechanisms which can occur within the total process of particle breakage. The first mechanism is fracture, the splitting of the particle into several smaller fragments and dust. The resistance to fracture is dependent upon the previous stress history of the particle and this is the mechanism of fatigue, the gradual weakening of the particle due to continuous stress loading.
These two mechanisms occur throughout the whole volume of the particle whereas the other two occur at the surface of the particle. Attrition is the gradual wearing away of the surface of the particle due to the very high compressive stresses at the points of contact. A much larger portion of the surface may be involved if the particle is rolling or rotating and this results in abrasion, a polishing of the contacted surface due mainly to the shearing stresses at the surface.
A quantitative assessment of the susceptibility of a sample of particles to these four mechanisms depends upon experimental tests and four levels of testing are defined and discussed. At the most theoretical level, a knowledge of the basic elastic constants of the particle material, combined with the use of a large computer, can make predictions on the breakage behaviour of particles. The major limitation to such calculations is pre-knowledge of the fatigue factor, the incidence of faults and flaws which is present. At the other extreme, a measurement of the breakage behaviour can be made in an actual plant and under actual conditions. Here, the limitation is that such tests are completely specific and expensive, impossible in fact if the process of interest has not yet been constructed. Between these two extremes lie two possibilities. In the first we carry out tests on a sample of particles individually and build up, thus, a statistical description of their strength characteristics. These tests are classified as single particle tests. The alternative is to test the whole sample simultaneously under. conditions of stress and strain which are as closely controlled and uniform as possible. Such tests are bulk test. By classifying the sort of tests and the mechanisms which they must determine, the broad field of particle strength testing is defined. The relevence of the particular subject of this report, single particle impact testing, is thus clarified.
The second chapter gives some examples of the practical importance of particle breakage. The first example, lean phase pneumatic conveying is a case where particle impact is the predominant mechanism. Measurements of extreme fracture and attrition are reported. In the second example, fluidised beds, impact is not the only mechanism and may not be so violent but, nevertheless, it is still important. The third example is that of particles sliding against a boundary under compressive strength. Here, impact is not important at all. The three examples are thus chosen to illustrate the wide range of processes and conditions under which particle attrition may occur, dilute conditions with high velocity impact, packed beds with high compressive stresses or some intermediate condition. The theme of the chapter is to illustrate that, in all these cases, all the four mechanisms occur simultaneously and that they are interactive. Thus, particularly for fracture and attrition, the rate at which one occurs is affected by the rate of the other.
The third chapter is the heart of the report. In it we describe the single particle impact tests which are carried out and discuss their significance. The ostensible variables in the test are the velocity and angle of impact. In the chapter it is demonstrated, however, that the normal and tangential stresses which are generated by the impact are more truly the primary variables and that the link between the performance of particles in a test and in a process can be established by knowledge of the stresses in both cases. The measurement of stress during an impact, which has very short duration, still presents a challenging experimental problem. The chapter illustrates how the four breakage mechanisms can be assessed by the single particle tests, both by instantaneous and retrospective measurements. It is clearly demonstrated that a sample of particles may have a broad distribution of strength and that the parameter should be considered as a statistical distribution. It is also shown that the ranking of particles may not be the same fox the different mechanisms, that is that the sample which is most resistant to fracture is not necessarily that which is also resistant to attrition. It seems to be possible to define an "attrition constant" which relates the rate of attrition to the forces exerted. The most important implication of the chapter is the comparison of impact and compression tests. In this chapter we strongly imply that there is no fundamental difference between the two, rather do they represent extreme differences in the rate of loading. In future, this parameter should be considered to be an important variable in particle strength testing.
The concluding chapter is short, simply summarising the mechanisms, their interactive nature and the need for a unified impact and compression testing procedure.
This project was primarily an experimental project. Science follows a regular pattern of observation, hypothesis and confirmation. The experimental work in this thesis can be considered to have made some interesting observations but also to have lead to some positive if tentative hypotheses. They now require further confirmation.
The real conclusion is that the project is concluded but the work not.