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After making the selection
on size, some selection must made on density.
Sand has a density of approximately 2.65
g/cm3, Sand, which was the most common
media in the paint and ink industry in sand
mills for some time, has sufficient mass
to disperse most pastes using mills with
peripheral velocities of 2,200 FPM (feet
per minute) Using sand or a synthetic
media with a density in this basic low range,
where possible, does have advantages.
The cost of such medias is low.
The media next to sand in density is the
high-strength glass bead, usually a straight
soda-lime type, with specific gravity of
2.7 to 2.75 g/cm3. The dispersing
ability of this bead can be considered the
same as sand, assuming the size to be the
same.
Finally, steel iron shots in the over-7.1
g/cm3 specific gravity range are quite common,
and fortunately, can be used in almost any
commercial mill. They are especially
useful in inks, very viscous pastes, and
for fast processing of primers or other
products in large volumes. They cannot
be used for clears or white, or any product
where discoloration or iron contamination
would be objectionable.
Very generally, then, the choice
of media density is based on type of material
to be dispersed.
Many plants keep one mill on shot or
heavy bead for blacks, primers, etc., or
one on large glass for whites and yellows,
and find it much more efficient than trying
to do everything with one mill and one media.
The general-purpose glass media are a high
effective and long-proven media for production
of a very wide range of dispersions.
The third factor controlling choice of media
is, of course, the material of which it
is made. Certain points pertain to
all, one of the most important being crush
strength.
There is NO force in a normally operating
mill, which is great enough to break up
even the weakest, sand. A foreign
object or a broken disc in a mill will break
up any media, and pass thru a feed pump
will do the same, as well as destroy the
pump.
Besides those just mentioned, two other
sources of media “failure” have
found by experience. One of these is that
when the mill has been freshly charged,
with whatever media, the new charge has
a tendency to “scour” the old
media from disc hubs, stabilizers, corners,
etc., generating a lot of fine media particles,
which take some time to rinse from the bed.
The other is the puzzling phenomenon of
the scalloped disc or impeller. The
normal flow around a sand mill disc is smooth
and laminar with little axial component.
When the disc wears to a point where there
are definite valleys in the flat surfaces,
and scallops in the smooth peripheral surfaces,
this laminar flow is interrupted.
When this occurs on the bottom discs of
a vertical mill where the beads are most
tightly packed, it seems to generate enough
force to actually break any kind of bead.
In the batch-type mill the same pattern
forms and the same wear or break-up factor
enters as well as a readable drop-off in
dispersing ability.
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