Chemical Engineering April 2020 - 33

may perform well in the pelletizing
process, but not produce an adequate
crush strength.
The binding agent is a highly customizable
part of the process, and
often requires testing to determine
the best fit. Hundreds of binders are
available, and their selection is based
on effectiveness, intended product
use, cost and availability. The type
of binder and the concentration of
binder are critical parameters affecting
how well the material agglomerates,
as well as the amount of dust
generated by the pellets upon drying.
Moisture
Controlling the moisture content of
the material throughout processing
is the foundation of achieving desired
product characteristics and ensuring
that the process operates smoothly.
All materials have a unique range of
moisture within which they will successfully
agglomerate (assuming they
will agglomerate) via tumble-growth
techniques. Some organic materials,
like compost, animal feeds and sawUltrapen
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dust, have a wide range of acceptable
moisture content in which they
will agglomerate, while others, like
iron oxide, silica dust, many clays and
fertilizers, require a very narrow window
of moisture content. A delicate
balance must be achieved: too much
moisture and the material may form
clumps or become a slurry; too little
moisture and agglomerates will not
form properly, or may not form at all.
Since moisture will be added in the
form of a liquid binder, the starting
material feedstock must fall below
the identified range of moisture at
which the material will agglomerate.
The addition of binder will then bring
the material into the ideal moisture
range for agglomeration to occur.
Consequently, materials containing
a moisture content higher than what
has been identified as acceptable
will require some form of moisture
reduction prior to pelletizing.
Preconditioning
Preconditioning is not a requirement
in all pelletizing production lines.
Some operations simply feed raw
feedstock onto the disc pelletizer,
omitting the preconditioning step in
the pin mixer. However, preconditioning
does offer significant value, and
may be required depending on the
desired end-product characteristics.
The inclusion of a preconditioning
step provides several key benefits to
the pelletizing process:
Improved product uniformity.
While disc pelletizers do promote
blending of fines and binder, they are
not intended to thoroughly mix the
different feed materials. As a result,
pelletizing operations not utilizing a
preconditioning step may experience
an uneven distribution of binder
in pellets, resulting in non-uniform
product
formulation
crush strength.
The addition of a preconditioning
step ensures that the binder and
fines are thoroughly combined into
a homogeneous mixture before processing
on the disc pelletizer, promoting
a highly uniform product.
Increased production. The addiand
variable
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Chemical Engineering April 2020

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