Particle size reduction has been around
almost since the beginning of time and has undergone many changes. We’ve all
seen the picture of the ox turning the millstone to grind grain, a method that
is still used in many parts of the world today. In 1922, Dr. Szegvari hit upon
the idea of using pebbles, a gallon can, a drill press, and a special agitator
he designed for the drill bit to achieve a fine dispersion. In less than an
hour, he invented an apparatus that would produce exactly what he needed. The
device was his first attritor, which is defined as a grinding mill with
internally agitated media.
A useful and simple equation that describes the grinding momentum
is M x V (mass x velocity), which enables us to see how the attritor fits into
the family of mills. For example, ball mills use large media, normally 1/2 in. or larger, and run at a low rpm (10-50).
Other mills, such as sand, bead and horizontal, use smaller media, from 0.3 to
2 mm, but run at a very high rpm (roughly 800-1200). Table 1 offers a
comparison of grinding mills.
1. The most efficient fine grinding takes place when both impact action and
shearing force are present.
The main feature that makes the attritor more
efficient than other mills is that its power input is used directly for
agitating the media to achieve grinding, not for rotating or vibrating a large,
heavy tank in addition to the media. The most efficient fine grinding takes
place when both impact action and shearing force are present, as illustrated in
When wet grinding, impact action is created
by the constant impinging of the grinding media due to its irregular movement.
Shearing action is present as the balls (media) in their random movement are
spinning in different rotation and, therefore, exerting shearing forces on the
adjacent slurry. As a result, both liquid shearing force and media impact force
are present. Such combined shearing and impact results in size reduction as
well as good dispersion.
The operation of the batch attritor
very simple. All the material can be loaded directly into the grinding tank; no
premixing or pre-dispersing is needed. Since the top-open grinding tank is
stationary, the process can be visually observed and corrections and additional
ingredients can be introduced at any time. The maximum feed material size can
be up to 10 mm, provided the material is friable; otherwise, any 10 mesh down
material is feasible.
Batch attritors are used to process
hard-to-grind materials, such as tungsten carbide, silicon carbide, ceramics,
glass frits, ink, chocolate, metal oxides and various metals. High-viscosity
slurries up to 30,000 cps can also be processed easily in batch attritors.
A circulation attritor
system is a combination of an attritor and a holding tank that is generally 10
times the size of the attritor. One of the essential requirements of the system
is the high circulation (or pumping) rate. The entire contents of the holding
tank are passed through the attritor at least once every 7-8 minutes.
At this rapid speed, the premixed slurry is
pumped through a confined media bed. The media act as a dynamic sieve, allowing
the fines to pass through quickly, while the coarser particles follow a more
tortuous path and are ground finer. The slurry can be continuously monitored,
additional ingredients can be added to the premix tank at any time during the
grinding, and the processing can be terminated precisely.
One advantage of the circulation system is that large quantities
of material can be handled with a smaller investment in grinding media and
attritor equipment. Another advantage is better temperature control, which is
achievable because the holding tank is jacketed for cooling or heating and acts
as a heat sink. In addition, the slurry passes through the grinding chamber
very quickly (20-30 seconds per pass), so it has less time to heat up. These
advantages are very important when the grinding chamber is lined with plastic
or rubber for metal-contamination-free processing.
best suited for the continuous production of large quantities of material.
Well-premixed slurry is needed to be able to use this type of process. The
slurry is pumped up through the bottom of a tall, narrow grinding tank and
discharged out the top of the tank. The residence time required for certain
fineness is controlled by the pumping rate. Continuous attritors can be set up
in a series, using larger media and grid openings for the coarser feed, then
subsequent units with smaller media to achieve a finer grind.
An expanded moving bed of media achieves the
principle of attritor dry grind processing (a condition described as kinematic
porosity). The dry particles are subjected to various forces such as impact,
rotational, tumbling and shear; therefore, micron-range fine powders can be
easily achieved. In addition, combinations of these forces create a more
spherical particle than other impact-type milling equipment.
Dry milling can provide reduced transportation
costs compared to wet grinding, since 50% of the gross weight is liquid in many
wet slurry processes. Because the removal of the liquid from a wet grinding
process involves not only another process step but also requires large amounts
of energy, dry grinding can provide reduced energy costs. Another advantage is
the elimination of waste liquid disposal. Following stricter environmental
regulations, the disposal of any waste liquid (whether water or solvent) can be
Dry grind attritors use grinding balls from 5
to 13 mm, and the shaft RPM generally runs from 75 to 500. These types of
attritors are suitable for harder-to-grind materials such as metal powder,
metal carbides and glass chunks. The feed material size for these machines can
be quite coarse, but smaller than the grinding media chosen.
Figure 2. This mill was developed to combine
advantages of circulation grinding with small media.
Small Mill Media
About 20 years ago, the industry trend began
to move in the direction of smaller and smaller particles. Most people involved
with milling agree that small media mills of one type or another are the
preferred method to produce these particles. Small media is generally defined
as 3 down to .25 mm, but mills are now available that utilize media as small as
0.1 mm. Small media milling provides several advantages over larger sizes, including
putting more energy into the process and allowing more media per volume, and it
results in a large surface area-to-weight ratio as well.
Small media mills have undergone a transition
over the years as manufacturers continually look for increased efficiencies.
The first generation of small media mills utilized a cantilevered chamber with
a wedge wire screening mechanism. At that time, customers were looking for
media as small as .25 mm. Mill manufacturers had trouble with mills packing
out, however, and the industry moved back to media of about .75 mm.
Second-generation mills were designed with a shorter chamber and a
rotor to agitate the media and material. The screening mechanism was changed
from the wedge wire screen to rotating caps, rotating screens or a series of
rings to provide more reliable operation.
One mill in particular was developed to
combine the advantages of circulation grinding with small media (see Figure 2).* The
mill is designed to accommodate media from 0.3 to 1.0 mm, and a screening
mechanism was developed that allows for high product flow without media
plugging. This is accomplished by having a screen of rings separated by
appropriately sized spacers between them. Since the rings are the full diameter
of the mill, the slurry can pass through a maximum open area, thus eliminating
any plugging of the screen.
3. The mill parts that will be in contact with the material being milled
(chamber, rings and discs) can be made out of special harder metals or
By using different-sized spacers, users can
maximize the open area for the size of media being used. The mill utilizes
specially designed delta discs that are indexed to provide directed and uniform
media distribution throughout the mill chamber while simultaneously providing
greater random media motion and improved milling. The mill parts that come in
contact with the material being milled (chamber, rings and discs) can be made
out of harder metals for longer life or out of ceramics, such as MgO-stabilized
zirconium oxide, for applications where contamination is an issue (see Figure 3).
*DMQ Small Media Mill
developed by Union Process.
The grinding media is another important factor to consider for
optimizing the milling process, since media cost can be a significant part of
the expense. The selection of the grinding media depends on several factors,
some of which are interrelated. In general,
high-density media give better results, and the media should be denser than the
material to be ground. Also, highly viscous materials require media with higher
density to prevent floating. Smaller media cannot easily break up large particles,
but are more efficient when ultrafine particles are desired. The harder the
media, the lesser the contamination, and, consequently, the longer the wear.
Contamination is another
important consideration. The material resulting from the wear of the media
should not affect the product or should be removed through the use of a
magnetic separator, chemically, or in a sintering process. Cost can also be a
factor. Media that may be 2-3 times more expensive may wear better (sometimes
5-6 times longer), and therefore could be worth the extra cost in the long run.
Additional factors include pH (some strong acid or basic materials may react
with certain metallic media) and discoloration (e.g., white material should
4. A research attritor being used with a cryogen.
Cryomilling is a variation of mechanical
milling in which powders are milled in a cryogen slurry (usually liquid
nitrogen) or at a cryogen temperature under processing parameters such that a
nano-structured microstructure is attained.1
shows a research attritor being used with a cryogen at Advanced Ceramics
Research in Tucson, Ariz. One can readily see the effects of the lower
temperatures on the equipment, which illustrates the point that special
precautions need to be taken for personnel protection.
5. Vapor recovery system and special mill covers must be used with cryogenic
Figure 5 shows some of the special equipment
that must be used with cryomillng. This photo, supplied by Advanced Ceramics
Research, shows the vapor recovery system and special mill covers that that are
used with this type of milling. Cryomilling takes advantage of both the
extremely low cryogen temperature and the benefits that are provided with
conventional mechanical milling. The extremely low milling temperature in
cryomilling suppresses the material’s recovery and recrystallization, leading
to finer grain structures and more rapid grain refinement.
A lot of work is being done with this
technique to produce a much stronger aluminum metal. Cryomilling, which yields
a grain size as small as 30 nµ, encourages the formation of nano-scale oxides
and nitride particles. This results in a metal that is 2-3 times stronger than
the starting metal. The Marine Corps is working to develop a 30- to 40-ton tank
that will offer the same level of protection as a 70-ton
Studies are also underway to develop cryomilled ceramics for the
Navy. James Murday, Ph.D., head of the chemistry division of the National
Research Laboratory in Washington, D.C., has reported on a ceramic
nanocomposite coating that has been used to coat the drive shaft on four
After four years in service, inspections showed
no visible damage. Technical cost modeling done by Schoenung, He, Tang and
Witkin at the University of California in Davis and Irving has shown that
cryomilling has the potential to be commercialized to fabricate nano-structured
Ceramic nanoparticles have enabled the economical manufacturing of
ceramic parts. According to R. W. Siegel of Rensselaer Polytechnic Institute,
“The ability to net shape form ceramics into final parts has become a reality
in recent years, owing to advancements made in the scaled-up production and
processing of ceramic nanoparticles. With the availability now of tonnage
quantities of nanophase ceramic powders with their unique rheological and
mechanical properties, it is possible to directly form ceramic parts in a
sinter-forging mold under sufficient pressure and temperature to yield final
parts with all of the definition and precision of the original mold. Nanophase
ceramics such as titania and alumina, made from the consolidation of ceramic
nanoparticles, have been shown in the laboratory to be readily formable into
small samples, and studies of their mechanical behavior indicate that a
significant degree of ductile behavior in compression is exhibited in these
ultrafine grain size materials."4
Ensuring High-Quality Results
With increasingly higher standards required
by the ceramic industry, fine grinding/particle size reduction has become one
of the most important factors for success. Over the years, attritors have
proven to be an excellent and reliable means to achieve these milling tasks.
additional information regarding media milling, contact Union Process Inc.,
1925 Akron Peninsula Rd., Akron, OH 44313; (330) 929-3333; fax (330) 929-3034;
e-mail firstname.lastname@example.org; or visit www.unionprocess.com.
note: This article is based on a paper given at the Ceramic Manufacturers
Association (CerMA) conference held May 2008 in Columbus, Ohio.