Observations on impact attrition of granular solids

Powder Technology, 49 (1986) 53 - 57

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Observations on Impact Attrition of Granular Solids K. R . YUREGIR, M . GHADIRI and R . CLIFT Department of Chemical and Process Engineering, University of Surrey, Guildford (U.K.) (Received February 20, 1986)

SUMMARY Results of preliminary experiments investigating attrition of granular solids by impaction are reported . Solution-grown sodium chloride crystals, 1 .2 mm cubes, were accelerated in an air eductor and impacted on to a target plate at a velocity of 29 m s - ' . The initial stages of impact were recorded using an image converter camera, at a speed of 50 000 frames per second. Qualitative analysis of the results indicates that the macroscopic surface features of these particles created during the crystallization process play a dominant role in determining the characteristics of the resulting debris . The fracture morphology, however, is similar to that for a more perfect melt-grown crystal under quasi-static compression .

INTRODUCTION

Little work has been reported on the fundamental mechanisms of particle attrition, and there is a serious lack of understanding relating rate of attrition and the characteristics of the debris produced to the physical properties of the material . Most of the studies carried out so far have used testing devices which attempt to simulate the real process . Small-scale grinding mills [1], single-orifice fluidised beds [2], and shear cells [3] are the devices most frequently employed . These techniques are obviously useful in assessing the relative attritability of a particular powder . However, the mechanics of particle movement in these devices is very complicated and is not amenable to a detailed analysis . It is also known that interaction between the material properties and the rate of application of forces can strongly influence the pattern of crack formation and propagation [4], but again due to 0032-5910/86/$3 .50

the complexity of these systems it is not possible to assess the importance of this interaction . Alternatively, it is possible to test single particles for fracture and wear using techniques such as indentation and scratching of the surface by a micro-indenter [5], compression of a particle between two platens [6], drop-shatter tests [7] and impaction tests [8] . The single-particle approach addresses the problem in a fundamental way because these tests can be carried out under welldefined and controlled conditions, while the effect of applied forces can be analysed quantitatively by fracture mechanics . A complete analysis of attrition in an item of process equipment can then be carried out by coupling the findings of the single particle tests with models describing motion of fluid and particles within the system . This approach has been adopted in this work to study the mechanisms of attrition resulting from high-velocity particle impacts such as occur in pneumatic conveyors, cyclones, etc . Naturally, particle impact is chosen as the single particle test . Sodium chloride is used as a `model' granular material because it is a well-characterised ionic crystal whose structure and physical properties have been studied extensively . It is also the subject of parallel work [9, 10] investigating its fracture under quasi-static indentation and compression tests. In this paper, the results of preliminary experiments using solution-grown NaCl crystals are reported . EXPERIMENTAL

A schematic diagram of the experimental rig is given in Fig. 1 . The particles were accelerated to the required impact velocities in an air eductor where compressed air (0 .5 MPa) was allowed to expand after passing through a nozzle . The particles were introd© Elsevier Sequoia/Printed in The Netherlands

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1 AUTOMATIC PARTICLE SEPARATOR/ FEEDER 2 EOUCTOR 3 PHOTOELECTRIC TIME-OF-FLIGHT DETECTORS 4 TARGET/FORCE TRMGO XER ASSEMBLY

MICRO COPFUfER CHARGE AMPLIFIER

.,

DIGITAL STORAGE OSCILLOSCOPE

Fig . 1 . Schematic diagram of the experimental rig .

uced into the air stream in the nozzle section individually and were impacted on to the target in random orientation . The acceleration tube had an internal diameter of 3 .9 mm, and was 1 .0 m long . The particle velocity was determined by measuring its time of flight between a pair of photoelectric sensors located close to the target plate . The target assembly was placed within a Perspex collection chamber fitted with a filter on the exhaust to capture the debris produced by the impact . It incorporated a force transducer to record the peak load experienced by the particle during impact . This was a load washer (Kistler 9001), pre-stressed between

Fig. 2 . SEM micrograph showing the surface morphology of the solution-grown sodium chloride crystals .

a target plate (made of Dural) and the base plate . The granular salt crystals used were

lighting . The camera and the lighting units

grown from solution in an Oslo productionscale crystalliser . The size range was between

were triggered by a photoelectric sensor

1 .2 and 1 .4 mm, nominally . The crystals were

positioned very close to the surface of the target plate . The impact tests were carried

generally cubic, with rounded corners and curved edges, as shown on the SEM micro-

out under room temperature conditions .

graph, Fig . 2 . The impact process was photographed

velocity of 29 m C' .

The air flow rate was set to give a particle

using an image converter camera, Imacon 790* . The camera can provide eight to six-

PHOTOGRAPHIC SEQUENCES

teen exposures of an event at speeds up to 20 million frames per second . In this work,

The photographs record the initial stages

the framing speed was set to 50 000 fps, giv-

of impact . As produced, the first frame is in

ing an exposure time of 4 As and an inter-

the top right, with the next below it and

frame delay of 20 µs . A typical plate containing 8 frames is therefore a record of 160 µs .

subsequent frames moving to the left . An 'almost-face-first' impact, Fig . 3, breaks the

Standard electronic flash units were used for

crystal into large fragments, the cracks forming normal to the impacting surface, and

*Manufactured by Hadlands Photonics, UK .

parallel to the planes in the (100) zone . Analysis of the fragments produced, Fig . 4,

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Fig. 3 . Solution-grown crystal impacting in an almost face-first orientation, where cracks travelling in (100) direction have fragmented the crystal .

Fig. 5. Corner-first impact where {100} planes near the impact point are flaked away .

Fig. 4 . Fragment of impacted crystal showing a {100} face produced upon breakage.

Fig . 6 . Solution-grown NaCl crystal impacting on a corner, where the impact axis is in line with a cube diagonal, and shedding the growth features to leave the central core to rebound with little damage.

indicates that fracture occurring on {100} is frequent, but there is also evidence of fracture on other planes. Overall distribution of the angles of new surfaces formed with respect to the cube faces still remains to be determined . Similar breakage is observed when a near-corner impact takes place, where layers parallel to the cube face, i .e., {100} planes, are removed near to the impact site, Fig. 5 . In a truly corner impact, where the impact axis is in line with the cube diagonal, however, most of the kinetic energy appears to be taken up by removal of the growth features surrounding the central core (see below), permitting the latter to rebound with very little damage, Fig . 6 . These highlight the importance of the impact orientation in initiating breakage. There is also evidence of angular rotation upon impact, but it is not quantifiable at this level of spatial resolution . The fragments produced are generally in two

distinct size ranges : a small number of large, cubical particles, approximately 1/3 to 1/2 the size of the original particles, and a large number of fine, dust-like particles smaller than 300 µm .

DISCUSSION

The surface morphology of the solutiongrown granular salt crystals is dominated by the growth features, e .g., plateaus separated by crevices, voids, etc . These features are formed during the crystallization process and are believed to result from attrition taking place in the industrial crystallizer [11] . They extend into the crystal, to a depth of approximately 300 µm, and can be observed by optical microscopy under transmitted

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Fig. 7 . Solution-grown NaCl crystal cleaved in half and immersed in dibutyl phthalate, showing the extent of growth features on the crystal .

light : Fig. 7 shows a crystal cleaved in half and made translucent by immersion in an organic oil of similar refractive index (dibutyl phthalate) . The central core can be regarded as a `single' crystal, but still includes occlusions where brine was trapped during crystal growth. The occlusions line up parallel to the planar growth surface, i.e., {100} plane . The spatial and time resolutions of the impact photographs did not allow detailed examination of crack initiation and propagation. Nevertheless, it appears that the pattern of fracture follows closely that of a meltgrown NaCl crystal under quasi-static compression . In a parallel study [10], formation and propagation of cracks in large melt-grown crystals, 1 cm cubes, by compression between two platens have been observed by time-lapse photography using polarised light . The majority of the cracks were found to form during the unloading stage . These cracks lay predominantly on (100} and their directions were parallel to the compression axis, i.e., (100) . Examination of the damage caused to the particles on impact, and of the debris produced, shows evidence of similar behaviour for the solution-grown crystals under the impact conditions, although the strain rate is six orders of magnitude higher . The fracture behaviour of melt-grown NaCl crystals is strongly dependent on the technique used . Badrick and Puttick [9] found that it was impossible to initiate cracks by micro-indentation using a standard Vickers pyramid indenter (total included angle 136 °) even under the maximum recommend-

ed load . A sharper indenter in the form of a cone of semi-angle 20 ° could initiate cracks after extensive local plastic deformation . Considerable lattice bending about (110) axes was observed, and it was found that indentations did not normally initiate along either (100) or (110), but lay close to (110) and might eventually propagate along (100). The crack morphology was similar to `angel wing' cracks observed in the indentation fracture of poly(methyl methacrylates) [121 . This type of fracture is qualitatively different from the median and lateral vent pattern cracking observed under conditions where material response is more brittle (e .g., at higher strain rates or lower temperatures [13, 14] ) . Cracks of `angel wing' type were also observed in the impact fracture of the solution-grown crystals. Figure 8 shows one of these cracks emanating from the boundary of a plastically deformed region . As this type of crack occurs in less brittle materials, it may be concluded that the strain rates experienced by the particles in impaction tests, i.e., at impact velocities of 29 m s 7l, were not sufficiently high to make the transition from a 'semi-brittle' to a `brittle' material, as observed in impact velocities of about 100 m s7l [14] .

Fig. 8 . Fragment of an impacted solution-grown NaCl crystal, showing extensive plastic deformation near the impact site and the `angle wing' crack formed in (110).

It should be noted that the outer layer of the solution-grown crystal is shed more easily during impact, forming the main bulk of the debris . This is caused by the apparently porous structure of the outer region which does not allow stresses to be transmitted

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effectively to the central core : fragmentation of the surface features inhibits propagation of strain, and therefore of cracks into the central crystal. It may therefore be concluded that, in impact attrition of solutiongrown NaCl crystals, the macroscopic growth features play a dominant role in determining the characteristics of the attrition debris, whereas the basic mechanisms of fracture are essentially governed by the material properties.

ported by a Cooperative Research Grant from SERC and IC Plc . We wish to express our thanks to our colleagues in that Department and in the New Science Group, ICI Plc . for valuable discussions and also to ICI for providing the salt crystals . Our thanks are also extended to Hadlands Photonics Ltd ., U.K. for making the Imacon Camera available for the study presented here .

REFERENCES CONCLUSIONS

Large solution-grown NaCl crystals (about 1 mm) produced in industrial crystallizers possess an outer layer, approximately 300 µm thick, which contains many crevices and voids and whose external surface has a stepped profile . These macroscopic growth features strongly influence the attrition behaviour of the solution-grown salt crystals and form the bulk of the debris produced . However, the basic mechanisms involved in fracture are qualitatively similar to those observed in quasi-static compression and indentation tests of more perfect melt-growth NaCl crystals .

ACKNOWLEDGEMENTS

This work is part of a programme investigating the mechanisms of attrition of particulate solids in collaboration with the Physics Department, University of Surrey . It is sup-

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