Size Reduction

Preamble

The work reported here falls into three distinct areas. Part A provides a review of attrition and an appraisal of existing test methods. Part B presents the results of work on the behaviour of alumina particles in the annular attrition cell and in single particle crushing experiments and associated tests. Part C is concerned with developing insight into attrition kinetics by initial tests in the annular shear cell. The immediate goal was a clarification of the effect of molecular weight on the attrition of high density poly- ethylene.

"THD makes no warranty or representation with respect to the application of any result or technique described in this report, and THD shall not be responsible in any way for any claims or liabilities arising out of the application by any party or any result or technique described herein. Any application, authorized or unauthorized by THD or TFPRI, shall' be made at the users own risk."

This disclaimer is in accordance with the standard policy of the T-H. Delft.

"THD makes no warranty or representation with respect to the application of any result or technique described in this report, and TBD shall not be responsible in any way for any claims or liabilities arising out of the application by any party or any result or technique described herein. Any application, authorised or unauthorised by THD or IFPRI, shall be made at the users own risk."

This disclaimer is in accordance with the standard policy of the T.H. Delft.

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 tendency of materials to be subject to attrition. The work for IFPRI aimed to exploit this potential with 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 stress enabled 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 attrition of a material to particle properties and, although the predictions are not yet quantitative, to equipment performance.

A piece of equipment has been devised in which the gap promoting attrition can be varied on a controlled manner with a view to carrying out studies on the relation between attrition in process equipment and in the annular attrition cell.

A modified attrition cell was fitted with fins to simulate flow of particles through a gap and preliminary trials at constant pressure showed that a small gap clearance gave more attrition than a larger one. Constant volume testing yielded a finer product. ProvIsionally, the particle breakage mechanism is unaltered in this type of equipment.

With the cell in its normal format using extruded particles the Gwyn kinetic formulation has been found to be true whatever attrition production size, testing stress and initial particle shape is selected. A shift in breakage mechanism from bodily fracture to surface abrasion has been found at low stresses. In general, mechanisms in operation are flexical bending (for long particles), fracture across a principal dimension, chipping (which is particularly noticeable for angular particles), and surface abrasion.

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.

The attrition and breakage of particles has been examined in a cone cell which permits the clearance between a rotating blade and a wall to be varied between experiments. Breakage is considerable if the clearance between the blade and the wall is just under one particle diameter or about 1% - 2 particle diameters. This effect has been found for a number of absolute initial particle sizes and for two materials of entirely different structure namely a polycrystalline urea and catalyst beads. Breakage is not detectable if the clearance is less than 0.25 particle diameters or greater than 2.5 particle diameters. A pronounced minimum attrition rate is found at about 1.25 particle diameters. An explanation of such effects is offered in terms of the particle packing. The findings are of considerable significance for understanding how attrition occurs in processing equipment.

Studies on well characterised alumina extrudates have been conducted in the annular shear cell used in previous work for IFPRI. The cell has the advantage that the stress, strain and strain rate applied to deforming bodies of particulate solids are known. The extrudates have been made in the different strengths and have known and reproducible structures. A range of normal stresses varying by over three orders of magnitude could be imposed on the deforming bed of particles.

The less strong extrudate showed a progressive decrease in fine attrition product as the normal stress decreased which indicated a shift in dominant mode of breakage from fragmentation to abrasion, but each mechanism remained at every stress. Both photographic evidence and particle size distributions confirmed this behaviour. For the harder particles the photographic evidence was consistent with this behaviour but the particle size distributions were erratic; it is thought that there is some limitation to the use of the cell but this remains to be proved.

It was also found that re-use of broken material in the cell may affect behaviour and further is necessary to establish whether the unification of breakage data may be achieved for the full stress range as it was previously over a more restricted range. The annular shear cell is of most practical use for systems with high ambient stresses.

SUMMARY

This report is an extension of the paper given at the AMU Meeting held at Princeton and includes further analysis on the application of Fracture Mechanics to the chipping experiments reported at the Annual Meeting by the workers at the University of Birmingham. In addition it has been found that the initial pessimism of finding published work on fracture under applied compressive loads was not fully justified.

Some of the ideas of Fracture Mechanics are briefly reviewed and fracture criteria for both Linear Elastic Fracture Mechanics (LEFM), which is predominantly brittle, as well as for ductile failure in the presence of extensive plastic deformation are presented. The role of plastic deformation is stressed: it governs toughness and brittle/ductile failure, characteristics. The effect of size is considered and it is shown that the observed ductility of particles of materials which may be brittle in large scale components is a logical consequence of the theory. Kendall’s work on this topic is briefly described.

It is argued that fracture mechanics can be usefully applied to particle breakage, although LEFM will be of less use than for fracture of larger samples and plastic deformation must be incorporated in the treatment. Attrition is not seen to differ mechanistically from gross fracture and will depend on the same parameters, particle shape being a significant factor in determining whether small fragments are chipped off or not. Some ideas concerning the dependence of abrasion on yield stress are discussed and it is concluded that fracture toughness and yield stress are both important governing parameters.

Finally, some suggestions for future work are presented.