Chemical Engineering October 2016 - 40

Facts At Your Fingertips
Agglomeration Processes
Department Editor: Scott Jenkins
gglomeration is a natural phenomenon
in which solid particles
stick to each other or to
surfaces. When unwanted, agglomeration
can cause problems, such
as caking, buildup or lumping, but it
is also an important particle-size enlargement
process widely used in the
chemical process industries (CPI) to
help overcome challenges, including
segregation, difficult flow, low bulk
density and others. Agglomeration
can also help control particle-size
distribution and reduce potentially
hazardous aspects of the solids, such
as dust. This one-page reference provides
information on agitation, compression
and sintering methods of
agglomeration and their mechanisms.
Agglomeration methods to enlarge
particulate solids can be broadly
broken down into three general categories:
agglomeration by agitation
(sometimes referred to as wet agglomeration
or tumble/growth agglomeration;
pressure (compaction)
agglomeration, and agglomeration
using heat (sintering).
Wet agglomeration
Wet agglomeration processes combine
powder, liquid (usually water)
and, if necessary, a binder to impart
shear to form agglomerates. Processing
equipment may include rotating
drums, disc or pan agglomerators,
pin or ribbon mixers or fluidized
beds. Agglomeration can be induced
by a solvent or slurry atomized onto
the bed of particles, or by the controlled
sintering or partial melting of a
binder component of the feed (Figure
1). Next, moist particles join together
to form so-called green agglomerates.
Drying or curing takes place in
a final stage. The wet agglomerates
are created by first forming nuclei that
then grow into larger aggregates by
layering or coalescence. In some cases,
nucleation and aggregate growth
take place in two separate pieces of
equipment that are operated in series.
Nucleation gives rise to seed particles,
which are formed when several
individual particles adhere to each
other. The nucleation stage can be
40
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time-consuming because the seed
agglomerates are weakly bonded
and readily disintegrate back into individual
particles. Eventually, larger
aggregates are formed when small
agglomerates coalesce or individual
particles adhere to larger agglomerates.
Once larger agglomerates are
created, growth becomes accelerated
as the increased mass and higher
kinetic energy of agglomerates
cause them to pick up individual particles
more rapidly and incorporate
them onto their surfaces. The relative
rates of size enlargement (nucleation,
coalescence and layering) and
size reduction (attrition and consolidation)
establish the final particle size
along with the material's tendency to
wick moisture from its core to outer
layers. The optimal amount of liquid
added to a powder - the amount
that gives the resultant agglomerates
their greatest integrity and resistance
to breakage - is typically
40-90% of its liquid saturation. The
liquid saturation is the fraction of total
void space that can be filled with
the liquid. When water (or another
liquid) is added to a dry bulk solid,
liquid bridges will begin to form at
contact points between particles.
This is known as the pendular stage
of saturation. All free moisture is attracted
to the interfaces between the
solid particles by capillary effects,
and surface tension draws the particles
together. As saturation levels
are increased, the funicular stage is
eventually reached where all internal
solid surfaces become surrounded
by liquid. At this point, the mixture
becomes more fluid-like, tensional
forces disappear, and the agglomerates
become weaker. When the
powder becomes fully saturated,
it reaches its capillary state, and at
higher moisture levels, the system
begins to behave as a slurry.
Compaction agglomeration
Pressure agglomeration works by applying
external forces to dry particulate
solids to form enlarged particles.
Continuous sheets of solid material are
produced, as in roll pressing, or some
Growth
Wetting
Granule properties affected
include size, bulk density,
attrition, dispersion, flowability
Breakage
Consolidation
FIGURE 1. In wet agglomeration, a wetting step
leads to nucleation and particle growth
solid form is made, such as a briquette
or tablet. Continuous sheets or
strands may either break down in subsequent
handling to form a granulated
material, or the material may be further
processed through a variety of chopping,
spheronizing or forced screening
methods. A key factor in compression
agglomeration is the level of force applied.
Compaction processes range
from confined compression devices,
such as tabletting, briquetting machines
and ram extrusion to unconfined
devices, such as roll presses and
a variety of pellet mills.
Heat sintering
In agglomeration using heat (sintering),
atoms and molecules begin to migrate
across the interface where particles
touch each other. This happens at a
certain elevated temperature, which
is different for various materials. While
still in solid state, diffused matter forms
bridge-like structures between the surfaces,
which solidify upon cooling. In
the post-treatment of agglomerates,
this phenomenon produces strong
permanent bonds or
specific
final
properties in parts that may have been
manufactured by virtually any of the
other agglomeration techniques. n
Editor's note: Portions of this column have been adapted
from the following articles: Mehos, G. and Kozicki, C., Consider
Wet Agglomeration to Improve Powder Flow, Chem.
Eng., January 2011, pp. 46-49; and Ennis, B.J., Agglomeration
Technology: Mechanisms, Chem. Eng., March
2010, pp. 34-39. Additional references include the following:
Sochon, R.P.J and Salman, A.D., Particle Growth and
Agglomeration Processes, chapter in Chemical Engineering
and Chemical Process Technology, Vol. II, " Encyclopedia of
Life Support Systems; " and Pietsch, W., What is Agglomeration?,
Power and Bulk Solids, Feb. 27, 2008.
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