Analysis
of powder samples in dry is often an
unavoidable alternative as well as being
a preference. In many instances, the
selection of solid sample dispersion
is totally dependent on the end use
of the product. Dry powders that are
free-flowing, anti-caking and anti-clogging
require no external dispersion aids,
a free fall by gravity is usually sufficient
for dispersion. For other dry powders,
dispersing particle agglomerates into
individual particles is needed.
Existing methods for the dispersal of
an agglomerated dry powder into an air
stream utilize pressurized air as the
means of dispersal. Powder is accelerated
in a straight line to produce high shear
forces; usually the flow of powder is
designed to impact against a surface
or series of surfaces to achieve complete
dispersion. The complete fractional
sample shall be dispersed and used for
the measurement. All particles should
have the same approximate velocity in
the measurement zone. Often, large sample
quantities are used for dry dispersion,
which can assist the representation
of coarse particles in a wide size distribution.
It is necessary to check that comminution
of the particles does not occur and
conversely that a good dispersion has
been achieved. This is usually often
done by direct comparison of dry dispersion
with a liquid one: ideally, the results
should be the same.
In using pressurized air, two processes
occur simultaneously under strong shear
force, i.e., dispersion of agglomerates
and breaking of particles. In an ideal
situation, upon increasing the dispersing
energy the amount of fines is increased
at first, due to improved dispersion;
then, sometimes a point is reached,
where the size distribution is nearly
constant with increasing energy. At
still higher energies the amount of
fines may rise again as a result of
comminution. If such a plateau is reached,
its centre defines the optimum dispersing
energy. However, in real situation hardly
exists such a plateau since dispersion
and breaking always occur at the same
time. It is especially true for highly
aggregated or fragile particles. Therefore,
to find an optimum pressure to disperse
particles without breaking any often
is impossible and operators normally
find a pressure arbitrarily in experiments
testing different levels of pressure.
A new method of producing a dry dispersion
of powders has now been invented. This
method, named Tornado, utilizes a novel
means to create high shear forces in
a suspension of powder in air by a rapid
change of air flow direction rather
than by acceleration in a straight line.
In the operation, the sample holder
is moved up to the suction probe and
the vacuum creates a flow of air through
the sample holder. The side walls of
the suction probe include a pair of
curved vertical grooves designed to
increase the shear forces by inducing
the air flowing into the sample holder
to enter with a twisting flow. A rapid
change in the direction of air flow
is produced at the tip of the suction
probe to form vortices like a “Tornado”.
These mechanisms produce high shear
forces which disperse the powder; the
powder is then fed from the suction
probe to the suction channel and then
to the measuring cell.
Another major advantage of this technology
compared to conventional dry powder
feeding system using a pre-set rate
of flow of powder to the measuring system
is that it provides direct control of
the feed rate by adjusting the distance
between the probe and the powder surface
via the feedback from the light obscuration
level to ensure optimal loading in the
measuring cell. It measures the entire
sample presented to the instrument with
no need of sample preparation and minimal
operator intervention. The sample is
placed in a sample holder and brought
to the cell in the optical bench by
a vacuum system which is fully controlled
by the computer. Figure 1 shows the
basic principle of the Tornado DPS.
Figure 1. The operation principle of the Tornado DPS.
