Chemical Engineering November 2021 - 23

cess residence time. Therefore, for
each unit operation in a process, one
must delineate the dominant " bulk
mechanisms " for particle breakage,
their corresponding impact on the
type of particle breakage (fragmentation,
surface abrasion, chipping,
crumbling), and the consequent impact
on bulk properties (bulk density,
flowability, fines content), which can
affect the process step or end use
of the product. A holistic view of the
process and product engineering is
the key to minimizing the deleterious
effects of breakage and attrition.
Attrition mechanisms
Attrition occurs when particles in
motion come into contact with other
particles, or the rigid wall of a vessel
or pipe, and the dissipated energy
triggers mechanisms for particle
breakage. The underlying mechanisms
of crack formation,
growth
and propagation are similar to those
observed during comminution or
grinding. Much like comminution
(grinding or size reduction), attrition
is statistical in nature. Not all particles
are identical, and neither are
the contact or collisions events. One
must rely on the measurement of average
values or use appropriate statistical
functions (such as the Weibull
distribution) to quantify it.
As with any solid, failure will occur
when stresses exceed the strength
of the section to which they are applied.
Sharp corners and local aspirates
of the small cross-section are
vulnerable to modest impact and
shear forces that cause an abrasive
form of attrition. This vulnerability results
in the particles attaining a more
rounded form without a significant
change of overall particle size, with
a degree of fines being created from
the broken fragments. More radical
fracture of the body of a particle
tends to generate a broader range
of reduced-size, sharp-edged particles.
Attrition is minimized by reducing
applied stresses.
Mechanisms at the particle level.
The failure modes for a particle can
be categorized as brittle, semi-brittle
and ductile. The propensity of a particle
to fail in a certain mode depends
on material properties, strain rate,
elastic/plastic state of stress, particle
size and contact geometry.
When a particle is subject to
external stress (normal or tangential),
and the peak stress
does not exceed the plastic
yield stress (onset of yield), the
reversible deformation is considered
elastic. Otherwise, the
deformation is not reversible,
resulting in viscous flow (that is,
a dent) or damage to the particle.
Any damage to the particle will
first be due to brittle failure, resulting
from defects, imperfections, microcracks
and surface flaws. Brittle
failures at high strain rates usually
result in cracks that split the particle
into fragments. Particles do not fail
in a predictable or consistent way
because the size and position of the
flaws within the particle are varied.
The Weibull distribution is commonly
used for interpreting fracture data
due to brittle failure.
When the impact stress exceeds
the onset of yield, plastic flow occurs
with viscous energy dissipation.
Yet, plastic flow can also generate
cracks and microcracks along
with compressive radial, normal and
tensile stresses during loading and
unloading cycles. Radial and normal
stress cracks could result in fragmentation,
while the lateral stress
cracks form chips.
This type of ductile failure can be
observed
in
soft
materials where
plastic flow ends with a rupture.
Plastic flow serves as the precursor
to ductile failure. Shear yield stress of
the material at the contact via shear
chains will determine the extent of
damage (ploughing, smearing or cutMechanism
Bulk
shear
FIGURE 2. The primary mechanisms of particle attrition
are fragmentation and surface abrasion
ting). It is known that an oblique angle
of attack and shear stress at contact
cause damage in ductile materials.
It should be noted that the mode
of failure or breakage is not wholly
inherent to the property of the material.
The strain rate, particle size and
shape can change the failure mode.
For instance, large particles can fail
in brittle mode due to the presence
of internal cracks and flaws, along
with
contact geometry. The fines
fraction of the same material can fail
in ductile mode due to the absence
of flaws and contact geometry. Similarly,
high-strain-rate impacts can
cause brittle failures, whereas lowstrain-rate
crushing could result in
semi-brittle or ductile failures.
Particle attrition during handling
and processing is not a singular phenomenon.
The mechanism of particle
breakage is broadly categorized as
follows: fragmentation/fracture, leading
to large sub-fractions; and surface
abrasion (chipping), leading to
fines (Figure 2). While the breakage
manifests itself either as a shift in the
peak (the parent peak) of the particlesize
distribution or the formation of a
new peak in the small particle-size
range (daughter peak), a closer exTABLE
1. PRIMARY MECHANISMS OF ATTRITION AT BULK-SOLIDS LEVEL
Description
Continuous stress during shearing where the load path varies depending on contact
renewals; Confinement during shear affects contact stresses
Contact sliding/
friction
Dilated flow
Impact
Crushing
Trapping, pinching,
chopping
Vibrations
Viscous shearing
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When material moves against a boundary, such as a vessel wall or surface of internals,
causing sliding, bouncing or rolling motion
Shearing motion of material with unconfined boundary and/or dilated state allowing
sufficient bed expansion during shearing
Due to rapid change in momentum brought by collision with other particles or walls
(such as moving blades)
Stressing particles to failure in confinement, either between two rigid surfaces or
within a confined bed
Cleavage or breakage of particles by a moving edge against trapped or confined
particle
Oscillatory motion of bed, resulting in frequent low-intensity impacts in dilated bed
Shearing action by the fluid medium surrounding the particles
NOVEMBER 2021
23
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Chemical Engineering November 2021

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