Chemical Engineering August 2023 - 31

FIGURE 2. Vibratory screeners, such as the one shown here, are effective
at separating over- and unders-sized material from a solid
sample
Material feed
Dust cover
Screen deck
Middle-frame
discharge
Screen deck
Spring
assembly
Base
Eccentric
bottom weight
FIGURE 3. A gyratory screener, like the one shown in the schematic
diagram, uses a rotational motion to separate solid materials of differing
particle sizes
However, impact crushing could
result in shattering of the material.
Milling typically handles size reduction
via a fourth mechanism: attrition.
Attrition mills are commonly used as
a tertiary size-reduction step, and
typically take longer than the previous
three mechanisms described.
Examples of each of these mechanisms
are illustrated in Figure 1.
Screening is another key processing
step found in almost all industries.
Screening is a mechanical
process used in material handling to
separate or classify particles into different
sizes. Proper screening is critical
to meet material specifications,
remove foreign material, and in certain
applications, can even improve
flowability of the material. Similar to
crushing, there are several types of
screening methods, and each has its
own set of pros and cons. For example,
vibrating screeners (Figure 2) are
great for separating oversized and
undersized materials (sometimes referred
to as " scalping " ) and generally
Unintended effects
of sizing
Comminution
and
screening are two crucial
stages in the processing
of materials across various industries.
However, improper execution can
have significant adverse effects on
the plant operations. These effects
may include reduced throughput,
increased downtime, compromised
product quality, and safety concerns.
Eccentric
top weight
Motor
Bottom frame
Bottom-frame
discharge
Top-frame
discharge
have a higher capacity
than other screeners. A
single vibratory screener
can have multiple decks
to allow for additional
material separation. Gyratory
screeners (Figure
3) are like vibrating
screeners, except they
utilize a rotational motion
to separate materials,
and can be utilized
for
material
classification.
Centrifugal screening
is a more aggressive
method of screening
that utilizes rotating paddles
to allow for a high
efficiency
of
screening
in a small footprint.
Tumbling screening is a
gentler form of screening
that combines vertical
and gyratory motion
in a large footprint and is
common in the biomass
industries (Figure 4).
To assess the impact of these effects,
we can categorize them into
three distinct areas: flowability, process
and safety.
Flowability effects. In the design
and selection of comminution and
screening equipment, solids flowability
considerations often take a
back seat to other parameters, such
as reduction efficiency, throughput,
screening efficiency, capital and operating
costs, as well as the equipment
footprint. However, this can
be a critical mistake, because comminution
and screening can have
drastic impacts on the flowability of
the bulk solids and powders, due to
increased fines generation, separation
of material, changes in particle
shape, and more. These changes
can lead to low-flow or no-flow conditions
and material quality issues.
Often, these issues are found in
equipment immediately downstream
of the crushing and screening operations
- commonly in bins, hoppers
and transfer chutes. Some of the
common solids-flow problems include
the following:
Cohesive bridging. This is a " noflow "
condition where material forms
a stable arch over bin outlets, due
to the material adhering onto itself.
A similar sensation, plugging, can
result in complete flow stoppages in
chutes (Figure 5a).
Mechanical interlocking. This is a
" no-flow " condition where material
forms a stable arch over bin outlets,
due to the ability for coarse, anisoFIGURE
4. This photo shows a tumbler screener, which is common in the biomass industries
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Chemical Engineering August 2023

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