Chemical Engineering August 2023 - 32

FIGURE 5. Cohesive bridging (a) and ratholing (b)
are two common no-flow conditions that can occur
downstream of screening equipment
Caking. This is a condition in which
stagnant material may agglomerate
into lumps, potentially causing
flow stoppages. The risk of this is increased
when handling hygroscopic
materials (Figure 9).
Dust generation. This is a mechanism
by which additional release of
fine particles can lead to overloading
of dust-collection equipment.
The effects of comminution and
FIGURE 6. Fine material can be a problem if it
builds up on the inside of bins and chutes
tropic materials to interlock and form
strong physical bridges.
Ratholing. This is a " no-flow " condition
where a stable, open channel
forms above the outlet and stagnant
material outside of the channel does
not empty (Figure 5b).
Buildup. This is a condition where
material is allowed to build up within
bins and chutes (Figure 6).
Limited discharge rate. This is a
" not-enough-flow " condition when interactions
between fine material and
air restrict the solids discharge rate.
Flooding. This is a condition whereinteractions
between fine material
and air can cause the solid material
to behave like a liquid, resulting in the
material overwhelming downstream
equipment (Figure 7).
Segregation. This is a condition in
which the material can segregate itself
based on a number of different
mechanisms. In comminution and
screening applications, this can lead
to reduced efficiency (Figure 8).
32
screening are often determined by
the quantity and distribution of the
particles' size following the sizing
step. If a crushing operation results
in a wider particle-size distribution,
segregation of particles can occur,
which can influence product quality
and flowability. On the flip side, if a
crushing operation leads to a narrow
particle-size distribution, mechanical
interlocking of similar sized
particles can occur, which can result
in flow obstructions, as well as
screen blinding.
An example from the cement industry
can help illustrate the concepts.
Limestone is a critical ingredient in
cement manufacturing and typically
makes up approximately 80% of cement
chemical composition. Blasting
is conducted in the limestone quarry
to extract limestone. Following blasting,
large limestone rock is fed into
a primary crushing operation, often
a gyratory crusher. Following primary
crushing, limestone is often transported
via belt conveyor to a primary
stockpile. Because the limestone has
undergone significant size reduction,
a large portion of the limestone
is now fine. Fine limestone can be
more cohesive than the original limestone
rock, and thus, more prone
to bridging, which can limit flowrate
out of the stockpile. Ratholing can
also occur within these
stockpiles, and this can
effectively reduce the live
capacity of the stockpile
significantly (often by up
to 90%).
Bridging and ratholing
are not the only negative
side effects that can
occur in this example.
Segregation of fine and
coarse limestone can
occur, which will not only
intensify cohesive bridging
and ratholing, but can lead to
separate " slugs " of fine and coarse
limestone downstream. If a slug of
fine material is discharged from the
stockpile, dusting can occur, leading
to material spillage, overloading
of dust collection equipment, and
inefficient use of crushing equipment,
which can reduce equipment
life. Likewise, if a slug of coarse
limestone is sent downstream, it
can mechanically interlock at hopper
outlets, creating " no-flow " conditions,
reducing crushing efficiency
and blinding screening equipment.
Process effects. Negative flowability
side effects are not the only
concern of improper particle-size reduction.
Improper crushing can have
a direct impact on product quality,
and can lead to either reprocessing
or increased scrap product. Crushing
can also reduce material integrity,
leading to " weaker " material, resulting
in less stable particles. Processes
that include chemical reactions can
be negatively impacted, because
particle size can have a direct impact
on reaction time and efficiency. The
same can be said for drying processes,
in which unsuccessful crushing
can lead to either over- or underdried
material. Inefficient screening
has a direct impact on product quality
and could allow foreign material to
enter the material stream.
Safety effects. As anyone in involved
in manufacturing industries will tell
you, safety is the number priority.
Crushing and screening can have
massive safety impacts. Obviously,
machine entanglement is a major
concern in the design of crushing
and screening applications. But the
potential safety hazards do not stop
there. For materials that are exploFIGURE
7. Solids flooding occur when fine material interacts with air
in such a way that the solids behave as a liquid
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