Chemical Engineering April 2020 - 32

nents. In essence, the drying step
" cures " the pellets into their final
form. Without sufficient drying, pellets
may not hold their shape, could
clump together, or may even allow
mold or bacterial growth to occur.
The drying process also helps the
end-product to reach the desired
crush strength.
When carried out in a rotary dryer
(Figure 6), drying has an added advantage:
the rotational tumbling that
occurs in the drum may further round
and polish pellets.
Controlling particle characteristics.
Achieving the desired matrix
of end-product characteristics requires
a careful balance of process
and feedstock parameters. Material
and binder feed rates, disc speed
and angle, retention time and other
variables, are all used to control the
properties and overall quality of the
end product.
The flexibility of agglomerate pelletizing
allows operators to control
a wide range of particle characteristics,
and to produce an end product
that meets a range of specifications
for appearance and performance parameters,
including the following:
* Green or wet strength
* Dry-pellet crush strength
* Amount of degradation and attrition
* Bulk density
* Solubility
* Flowability
* Particle size distribution (PSD)
* Moisture content
* Surface quality
* Temperature
Pelletizing process add-ons
Various other production elements
can be integrated into the pelletizing
process to further tailor the process
to meet the unique application. Typical
integrations are listed below.
Cooling. Depending on the product,
a cooling step may be implemented
after drying. Cooling helps to prevent
product from caking during storage
and allows material to be bagged immediately
after production.
Coating. Coatings on the agglomerates
are sometimes used to further
improve granule performance, appearance,
or handling characteristics
in end products. A number of coating
devices are available, with the
coating drum being the equipment
of choice for superior results.
Recycle. Disc pelletizers generally
yield little recycle material, but a recycle
circuit can still be beneficial in
minimizing off-specification product. A
recycle circuit consists of a screening
system and mill for reducing oversized
granules, as shown in Figure 3. The
screen separates oversize and undersize
product from on-size product. The
oversize product is ground down, typically
in a hammer mill, and combined
with the undersize material to go back
into the process as recycle. Depending
on the material, a recycle process
may or may not be required to achieve
efficient and stable processing. When
employed, a recycle circuit can also
serve as a buffer in the process, should
an upset occur.
PELLETIZING
CONSIDERATIONS
There are many aspects to consider
in the pelletizing process, but three
important considerations stand out
as the most influential:
Binder
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32
The binding agent plays a critical role
in both the pelletizing process and in
the end product quality by influencing
pellet formation, green strength
and crush strength.
Pellet formation. Binder is used in
the pelletizing process to create the
tackiness that allows fines to adhere
to each other, promoting particle
growth. Without this tackiness, pellets
and fines would simply tumble with
inadequate or non-uniform growth.
Green strength. In addition to aiding
in pellet formation, the binder
also gives pellets the strength they
need to move through the process
without breaking down, a property
referred to as " green strength " or
" wet strength. " Pellets require an adequate
green strength to withstand
the various drop and transfer points
encountered during processing.
Crush strength. The binder also impacts
end-product crush strength,
solidifying the bond between components
upon drying. Some binders
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Chemical Engineering April 2020

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