Chemical Engineering August 2023 - 30

Cover Story
Impact of Particle-Size
Control on Bulk-Solids
Flow Behavior
Crushing and screening of solids are essential to many solids-handling processes, but
engineers must be mindful about how those actions will affect flow behavior of the bulk material
McKinnon
Ray and Eric
Maynard
Jenike & Johanson, Inc.
IN BRIEF
SIZE REDUCTION AND
SCREENING
UNITENDED EFFECTS OF
SIZING
EVALUATING RISKS
CONCLUDING REMARKS
W
hen it comes to processing bulk
solids, (particle) size matters. Almost
all industries rely on some
sort of particle-size control within
their processes, whether it is comminution (reduction
of particle size by crushing, grinding
or milling) or screening (separating materials
based on particle size). For example, without
crushing mined material, it would be impossible
to extract valuable minerals and ores from
the surrounding rocks. And effective screening
is commonly used in the food industry to
separate foreign materials and contaminants
during food processing. However, crushing
and screening can have unintended consequences,
including causing process upsets in
downstream equipment and creating potential
safety issues. The good news is there are
scientific approaches to evaluate these risks,
and there are approaches for preventing poor
process flow as a result of upstream particlesize
control. This article touches on the background
of comminution and screening, how
both can impact processes, and how to evaluate
flowability risks.
Size reduction and screening
Broadly speaking, comminution is defined as
the act of reducing the average particle size
of material by mechanical means. It serves
as a critical process step in many industries,
including mining, pharmaceutical and food
industries. In general, comminution can be
broken down into five methods: blasting,
crushing, shredding, grinding and milling.
Blasting is commonly done to reduce the size
of large boulders, and of all the comminution
methods, is typically used only for the largest
particles. Blasting is also different from
the other four methods, in that it typically occurs
in the open, rather than within processing
equipment. Shredding typically involves
taking large, stringy materials (such as biomass,
recycle waste and others) and shears
the materials into smaller shreds. Grinding
involves the chipping or " grinding " of a single
material to create smaller particles.
Crushing can be done via three mechanisms:
compression, shear and impact [1, 2].
Compression is typically used for hard, brittle
particles and typically does not result in significant
generation of fines. Shear crushing
is similar to shredding, except that it is designed
to handle sticky, cohesive bulk materials
and powders. This method can lead to
anisotropic shapes. Size reduction by impact
is effective for hard, tough particles, and can
lead to size reduction of 100 times smaller.
FIGURE 1: Several different particle-size reduction technologies are available for bulk solid materials
Compression
Shear
30
Impact
Attrition
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