Chemical Engineering November 2021 - 22

Cover Story
Understanding Attrition
in Solids Processing
Better qualitative and quantitative understanding of the mechanisms of particle breakage at the
level of the particles, the bulk material and the equipment helps minimize the risk, uncertainties
and impact of particle attrition in solids processing operations
Shrikant Dhodapkar and
Jörg Theuerkauf
The Dow Chemical Company
Ray Cocco
Particulate Solid Research, Inc. (PSRI)
U
nwanted breakage, degradation
or attrition of
solid particles during processing
and handling is a
ubiquitous issue in the chemical,
polymer,
pharmaceutical,
food,
mineral, cement, coal, mining and
agricultural sectors. While the fundamental
mechanisms of attrition
phenomena in solids-handling
processes are similar to those well
researched in comminution (grinding),
the varied mechanisms and
unquantified state of stresses and
strains in solids handling processes
and equipment generate additional
complexity and uncertainty.
A certain degree of attrition is inevitable
during the handling and processing
of bulk solids. By applying
a fundamental understanding of the
physics underlying particle attrition
and conducting a systematic analysis
of the attrition processes, the negative
consequences of attrition can be minimized
and mitigated. A two-pronged
approach of particle engineering and
process engineering must be taken
to find the optimal solution. Using the
systematic method discussed in this
article, the sources of attrition can be
identified and mitigated.
Consequences of attrition
In
almost
all
processes
involving
granules or granular fluid flow, product
attrition is a challenge. Among the
industry sectors that continue to address
attrition are the following: coal,
chemicals, petrochemical catalysts,
22
agriculture,
detergents,
cement,
plastics,
food, pharmaceuticals
and others.
Potential negative
impacts resulting from
the attrition of solid
materials in processes
include the following:
* Loss of product due
to fines generation -
especially when fines
cannot be recycled, or the cost of
recycling fines is high
* Reduced value due to inferior finish
(for example, in food or polymers)
* Loss of activity or functionality (for
example, in sorbents, catalysts or
seeds and grains)
* Processing problems caused by
increased fines (such as segregation,
bridging, clogging, feeding, dusting)
* Alterations in end-use applications
(for example, fines in polymers, active
pharmaceutical ingredient effectiveness
in pharmaceuticals, dissolution
rates for chemicals)
* Environmental issues (for example,
neighborhood impact on process
emissions due to fugitive dust)
* Safety hazards of fines, including
higher combustibility, explosivity and
reactivity, as well as industrial health
issues (inhalable dust emissions during
processing and end-use)
In most cases, attrition is unintentional
and undesirable. Engineers
strive to minimize it and its consequences
during process design and
operation. Attritability (or friability),
which refers to the tendency of the
particles to break during processing
and handling, is not a fundamental
property of a material. It is a cumulative
(and iterative) consequence of
FIGURE 1. The graphic conceptualizes the assessment of material and
processing conditions for attrition potential
material properties, as well as specific
unit-operation and processing
conditions (Figure 1). Broad generalizations
should not be made about
equipment, operating conditions or
product families without systematic
studies or an attrition audit. Such
studies on large-scale equipment
and processes are expensive to conduct,
and often are not feasible at the
design stage.
As with all engineering problems,
approaches based on the scientific
method (hypothesis generation, testing
and validation) are the most efficient
and robust. Various tools, such
as fishbone plots, affinity plots and
" five whys, " along with software for
statistical analysis, can be used.
To minimize the uncertainties,
risks, and impact of attrition on process
design, it is imperative to understand
(qualitatively and quantitively)
the fundamental mechanisms
of breakage at the particle, bulk and
equipment levels. The selection of a
suitable test method at the particle
or bulk level requires an understanding
of the mechanisms governing
attrition in any unit operation. The
challenging part is the quantification
of mechanical stresses and corresponding
frequencies during proCHEMICAL
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Chemical Engineering November 2021

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