01-08-2012, 12:41 PM
Size Reduction and Size Enlargement
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PARTICLE-SIZE DISTRIBUTION
Specification for Particulates Feed, recycle, and product from
size reduction operations are defined in terms of the sizes involved. It
is also important to have an understanding of the degree of aggregation
or agglomeration that exists in the measured distribution.
The fullest description of a powder is given by its particle-size distribution.
This can be presented in tabular or graphical form. The
simplest presentation is in linear form with equal size intervals (Table
20-1). The significance of the distribution is more easily grasped when
the data are presented pictorially, the simplest form of which is the
histogram. More usually the plot is of cumulative percentage oversize
or undersize against particle diameters, or percentage frequency
against particle diameters. It is common to use a weight basis for percentage
but surface or number may, in some cases, be more relevant.
The basis of percentage; weight, surface, or volume should be specified,
together with the basis of diameter; sieve, Stokes, or otherwise.
The measuring procedure should also be noted.
PARTICLE-SIZE MEASUREMENT
There are many techniques available for measuring the particle-size
distribution of powders. The wide size range covered, from nanometers
to millimeters, cannot be analyzed using a single measurement
principle. Added to this are the usual constraints of capital costs versus
running costs, speed of operation, degree of skill required, and,
most important, the end-use requirement.
If the particle-size distribution of a powder composed of hard,
smooth spheres is measured by any of the techniques, the measured
values should be identical. However, there are many different size distributions
that can be defined for any powder made up of nonspherical
particles. For example, if a rod-shaped particle is placed on a sieve,
its diameter, not its length, determines the size of aperture through
which it will pass. If, however, the particle is allowed to settle in a viscous
fluid, the calculated diameter of a sphere of the same substance
that would have the same falling speed in the same fluid (i.e., the
Stokes diameter) is taken as the appropriate size parameter of the
particle.
SIZE REDUCTION AND SIZE ENLARGEMENT
drying, cooling in a dessicator, and weighing. Stokes diameters are
determined from the predetermined times and the depths, with correction
for the changes in depth due to the extractions. The cumulative,
mass, undersize distribution comprises a plot of the normalized
concentration against the Stokes diameter. A reproducibility of 2
percent is possible using this apparatus. The technique is versatile in
that it is possible to analyze most powders which are dispersible in liquids;
its disadvantages are that it is a labor-intensive procedure and a
high level of skill is needed.
The hydrometer method is simpler in that the density of the suspension,
which is related to the concentration, is read directly from
the stem of the hydrometer while the depth is determined by the distance
of the hydrometer bulb from the surface (ASTM Spec. Pub.
234, 1959). The method has low resolution but is widely used in soil
science studies.
PRINCIPLES OF SIZE REDUCTION
Since a large part of the literature is in the German language, availability of
English translations is important. Translation numbers cited in this section refer
to translations available through the National Translation Center, Library of
Congress, Washington, DC. Also, volumes of selected papers in English translation
are available from the Institute for Mechanical Processing Technology,
Karlsruhe Technical University, Karlsruhe, Germany.