Copyright CSC Publishing
As appeared in MARCH 2014 | PBEI
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EXPERT
Specific gravity and bulk
density in mixing
David S. Dickey MixTech
Q
Our product has three ingredients, each with a different
specific gravity (0.93, 1.25,
and 2.5). We need to control the mix
better than we’ve been doing. How can
we do it with these ingredients?
A
The answer to your question
may be in what you haven’t
told me. The issue of specific
gravity has at least two different meanings. By specific gravity, I assume that
you’re referring to the particle specific
gravity. In that case, the low specific
gravity is probably a polymer or natural
fiber, the intermediate specific gravity
could be anything from a denser polymer to a low-density mineral, and the
densest material could be ground glass,
sand, or any of several similar minerals.
The type of material or differences in
types may contribute to other mixing
problems.
A second and equally possible
meaning could be bulk density. For bulk
density, particle density and the void
fraction in the powdered material is represented in the value. Particle size and
shape will change the bulk density, even
for materials that are chemically the
same. Different forms of sugar may have
specific gravities ranging from 0.56 to
0.85. Sodium compounds may cover a
significant range of specific gravities,
from sodium silicate at 0.5 to sodium
sulfite at 1.63. Many different materials
from organic compounds or polymers to
minerals and even metal powders may
have specific gravities in the range you
mention for your components.
The simple answer to questions
about density difference is that it’s usually not the primary cause of mixing
problems. Particle size is usually a much
more important factor. The particle-toparticle interaction in a powder blend is
usually sufficient to prevent segregation
by density difference unless the particles
are very large or smooth and round. In a
few cases, such as fluidized mixing, particle density may affect product uniformity. Otherwise, particle size can cause
poor mixing or subsequent segregation,
usually with the larger particles nearer
the top surface of the bulk material. A
particle size distribution with some
large particles and many small ones may
not flow easily and cause caking or lump
formation.
One obvious and sometimes practical solution to particle size distribution
problems is to grind all of the particles to
the same size, or at least eliminate large
particles (those larger than 150 microns).
Small particles (less than 10 microns)
rarely separate due to sifting or segregation, but they may cause dusting.
One of the most common blending
errors with all types of powders is overfilling the blender. The more difficult
the blend, the more important a proper
fill level becomes. With tumble
blenders, fill level is extremely important, especially if the fill level is more
than half the open volume of the
blender. Because all of the actual blending occurs on the powder’s sliding surface, a tumble blender filled two-thirds
full may have a central core region in
which no blending occurs even after
very long blend times. Similarly, in a rotating blade blender such as a ribbon or
paddle blender, a fill level above the top
of the blades makes stagnant or poorly
mixed zones extremely likely. For best
blending results, horizontal blenders
with rotating blades should only be onehalf to two-thirds full.
Poor particle uniformity in an overfilled blender is a recipe for extended
blend time, another common problem
with powder mixing. Most blenders
will completely mix powder contents in
5 minutes or less. Some mixers may require only 30 seconds for complete uniformity. Extending the blend time then
results in particle breakage, increased
particle size distribution, or static
charge buildup. Some polymers and
other nonconducting materials are especially prone to static charging, which
will cause other types of segregation and
particle interactions, resulting in poorer
final blends. Other processes, such as
liquid addition, heating or cooling, and
any type of physical or chemical reaction
may extend required blend times, but
not for the sake of basic uniformity.
Minor ingredients that represent
less than 0.5 percent of the batch should
be preblended in a more concentrated
mixture. Other factors, such as addition
order, loading location, and poorly
mixed locations in the blender can also
cause uniformity problems. In some
ways, identifying the cause of a blending
problem may involve a long list of questions and answers.
A last and often critical cause of
poor powder blending is not in the
mixer itself, but in the handling following the blending operation. In extreme
cases, even material just flowing out of
the blender can cause segregation or a
degree of “unmixing.” Vibration in handling, packaging, and moving may
cause further separation.
I doubt that density difference is the
only explanation for your mixture problems. I’ve described some of the most
common problem areas, but there are
many more possibilities in specific cases.
David S. Dickey, consultant
MixTech Inc.
Coppell,TX USA
+1 937 431 1446