Ceramic Industry article about Modern Milling Techniques

1. special section | materials handling/powder processing
MODERN
Wet Milling and Material Processing
for about a 65% (of total capacity) fill of
product, which allows room for the grind-
ing media to rise and fall for impact.
Especially with smaller units, the labor
and time required to batch and discharge
the machine can become problematic and
inefficient. Because of this inefficiency,a typ-
ical ball mill will only be available for grind-
ing around 75% of the time, which does
not take into consideration both scheduled
maintenance and emergency breakdowns.
Continuous ball mills are in use
around the world, but they tend to be very
large units suitable only for high-volume
slurries and non-critical milling applica-
tions. Mills like this are commonly used in
large ceramic tile factories to prepare slip
for spray drying. Advancements in vertical
ball mills have been made in recent years
but also lack efficiency when compared to
other methods.
Another challenge involves finer
and finer particle size demands. Tradi-
tional ball mills are generally limited to
a finished particle size of approximately
➤ Processes in a variety of industries now routinely demand
5 microns, which can be achieved only
particle sizes with an absolute maximum of well below 1 micron. with great attention to media size.
by Carl D. Yerger, President, Custom Milling & Consulting, Inc., Fleetwood, Pa. Grinding media for ball mills ranges
in size from 10-90 mm for most tradi-
he world of material process- Traditional Milling tional applications. Typical ball mills use
T ing and size reduction demands
faster and more efficient meth-
ods to yield increasingly smaller
sizes and narrower particle size distribu-
tions. Processes in a variety of industries
The ceramic industry is no different from
others in its search for modern manufac-
turing techniques. The traditional ball
mill has been the primary method used
to mill ceramic materials for hundreds of
high-alumina or porcelain balls for their
density, but wear is relatively high. Mod-
ern mono-size yttria-stabilized zirconium
media is not a practical alternative due
to the high cost per charge, as well as the
now routinely demand particle sizes with an years, and is still considered to be one of frequent re-charge rate. Simply put, it is
absolute maximum of well below 1 micron. the best mixers ever invented. difficult for the traditional ball mill to put
Manufacturers are challenged to inte- Ball mills are generally batch units, enough energy into the media charge to
grate a repeatable method of production which means that all ingredients are reduce particles to the sizes and distribu-
that is capable of this level of size reduction charged into the chamber and the mill is tions now required.
while controlling labor and energy costs. run unattended for a specified amount In all milling processes, the ultimate
The system must also be robust enough to of time (or number of revolutions). The performance of the mill is highly depen-
handle the most abrasive of raw materials. design of a typical ball mill only allows dent on the media size and distribution.
Above: The horizontal media mill has a closed chamber that receives the pumped pre-mixed slurry in one end and discharges the milled product from
the opposite end.
C E R A M I C I N D U ST RY ➤ S e p t e mb e r 2 0 1 1 29

2. MODERN WET MILLING
Table 1. Effect of different media sizes on the number of cycles required.
1.7-2.1 mm 1.0 mm 0.6 mm 0.3 mm
Theoretical Passes Recirculation Time (Min) d50 (µ) d50 (µ) d50 (µ) d50 (µ)
0.0 0 2.606 2.606 2.606 2.606
1.2 15 2.310 1.959 0.978 0.427
2.4 30 1.985 1.413 0.550 0.336
3.6 45 1.637 1.059 0.552 0.316
4.7 60 1.411 0.820 0.454 0.297
5.9 75 1.271 0.655 0.440 0.273
7.1 90 1.094 0.477 0.367 0.260
8.3 105 0.950 0.423 0.291 0.251
9.5 120 0.862 0.381 0.284 0.244
10.7 135 0.778 0.376 0.277 0.232
11.8 150 0.673 0.346 0.268 0.221
13.0 165 0.602 0.329 0.260 0.218
14.2 180 0.573 0.307 0.258 0.214
Table 1 shows the effect of different media particle impact and shear. At the exit end to two mills. When nano-sized particles
sizes on the number of cycles required. of the chamber, media is retained in the are specified, such as those required in
mill by a screen (also called a separator or ceramic inks used in inkjet applications,
Horizontal Media Mills filter) that is of such size that slurry par- the mills are typically setup in high-flow
The horizontal media mill has been used ticles can pass through. It is important to recirculation.
in the inks and coatings industries for over note that the screen plays no role in par- With the onset of digital technology
30 years. Best described as a “high-energy ticle size distribution. in inkjet ceramic decoration, specially
continuous ball mill,” the machine’s processed inks are critical to success.
design has undergone continuous changes Benefits The higher resolutions created by today’s
since its introduction to keep up with the Determined by product type and particle non-contact digital printing technol-
demands for finer particle sizes and nar- size and distribution, these milling systems ogy offers unlimited image variation for
rower particle distributions. are unique to each application so design ceramic tile production. The digital ink-
The horizontal media mill has a closed and setup are crucial for success. Equip- jet printer is quickly becoming the stan-
chamber that receives the pumped pre- ment manufacturers must be able to supply dard on the modern glaze line, using spe-
mixed slurry in one end and discharges the latest in wear-resistant materials as part cially formulated and milled pigmented
the milled product from the opposite of a successful installation. Selection of the ceramic inks in submicron particle sizes.
end. The mill uses a mechanically sealed proper construction materials is based on
shaft that is cantilevered from a heavy factors such as vehicle system, product end System Integration
bearing housing. This shaft can be fit- use and the abrasive nature of the raw mate- In today’s competitive environment, it’s
ted with a variety of agitators, includ- rials. Materials of construction for milling not just about selecting the right wet
ing discs, pegs or disc/pin combinations, wearing parts might include various types grinding mill—integration to the overall
depending on the particular process. The of ceramics, urethane and urethane coated, system is important as well. Whereas a
chamber is typically charged with grind- high polymer plastics, and composites. ball mill installation is fairly straightfor-
ing media to 60-90% of the net shell These milling systems are versatile ward and uncomplicated, the successful
volume. The high energy imparted by and can be configured for one- or multi- installation of a high-energy attrition mill
the agitators on the grinding beads cre- pass operation through outfitting with requires careful consideration of a num-
ates frictional heat, which is removed by the proper tanks and pumps. In addi- ber of factors:
chilled water circulating in the cooling tion, duplex systems with dual milling • Premix equipment selection for slurry
jacket of the chamber. heads on a common frame combine the preparation
As the milling shaft rotates, the media best of both by passing through a sec- • Properly sized chiller system to remove
is fluidized by the agitators positioned on ond chamber on the same machine that BTU energy from the grinding process
the shaft. The peripheral agitator speed for can be charged with media of a different • Pump systems sized for optimum flow
this type of mill is typically 8-17 m/sec. The size than the first chamber. Duplex mill- and resistance to abrasive ceramic
ceramic slurry passes through the length ing (sometimes referred to as “cascad- materials
of the chamber under pump pressure ing”) offers the advantage of operating • Feed and receiving tanks equipped with
while being milled through media-parti- two grinding chambers with one control mixers for adequate suspension and
cle impact and shear, as well as particle- system in a reduced footprint compared batch turnover
30 S e p t e mb e r 2 0 1 1 ➤ W W W. C E R A M I C I N D U S T RY. C O M

3. • Process plumbing designed to minimize
waste and optimize product yield
• Controls and instrumentation packages
are crucial for process control and auto-
mation, as well as data acquisition and
product repeatability
• If post milling filtration is required, the
selection of a filter system is crucial to
upstream mill efficiency
• Moving materials to and away from
milling system in the most efficient
manner possible
High-energy media (attrition) mills
are commonly used in the manufacture
of inks, coatings, agricultural, nutraceu-
ticals and pharmaceuticals, electronics,
and many other fine-particle products.
Ceramic processes using media (attrition)
mills include tape casting, specialty ceramic
coatings and the milling of digital ceramic
inks. As material processing innovations
require finer and finer particle sizes, the Integration is important to the overall system.
use of these systems will grow, especially
where high strength gains through the use
of nano-sized materials are possible.
The special requirements of ceramic OPF Enterprises
materials require the manufacturer to design
P.O. Box 16898, Missoula, Montana 59808
systems capable of withstanding the abra-
sion and wear inherent in most processes. Fax and Phone: 406-493-1102
If the materials of construction and systems
are designed correctly, there is a high prob- Bryan Geary has over 22 years of experience in different
ability that high-energy media mills will manufacturing roles including production operator, laboratory
find new homes in both advanced and tra- technician, laboratory manager, production supervisor, production
ditional ceramics. Industries such as ceramic manager, assistant plant manager, plant manager, and director of
manufacturing. He is an accomplished student and teacher of
tile, brick, refractory coatings, oil and gas manufacturing methods and holds a green belt in lean Six Sigma.
materials, glass, whitewares, pigments, elec-
Cell Phone: (406) 370-8923 e-mail: bryan@ontheplantfloor.com
tronics, polishing compounds, ceramic inks,
and crucible manufacturing may use for-
mulas requiring near- and sub-micron par-
Carl Sorrell has spent 35 years in manufacturing roles including
ticle sizes, opening new avenues to achieving
production operator, front line supervisor, engineering manager,
enhanced material properties. c
maintenance manager, production manager, plant manager, director of
For additional information, contact Custom research and development, and vice president of manufacturing. He
Milling & Consulting, Inc. at 1246 Maid- claims to be an engineer and can produce a Bachelor of Science
encreek Rd., Fleetwood, PA 19522; or email diploma in Ceramic Engineering from the University of Missouri-Rolla
sales@cmcmilling.com. (now the Missouri University of Science and Technology). He holds a
black belt in lean Six Sigma.
Acknowledgment
The author would like to thank OPF Cell Phone: (731) 499-0901 e-mail: carl@ontheplantfloor.com
Enterprises (www.ontheplantfloor.com)
for their assistance with developing this
article. OPF is Custom Milling & Consult-
ing, Inc.’s ceramic consultant.

4. n D
OPF (On the Plant Floor) Enterprises has a rich
history with CMC and CMC equipment.
We have worked together on successful projects
including an installation of Supermill Plus media
mills to produce high output of ceramic slurries in
submicron particle sizes.
The OPF team has almost 60 years of hands-on
experience in the ceramic industries and is proud
to partner with CMC to offer a complete package
to those dealing with ceramics and ceramic Product/Process Development
processes.
Materials are expensive. Time is priceless. Results
Our special areas of expertise include can never arrive soon enough. CMC and OPF staff
provide a winning combination with a proven track
• Ceramic Engineering record in the industrial world.
• Ultra-Fine grinding CMC’s state of the art laboratory and facilities give
you additional means to achieve your goals.
• Raw material selection, batching, mixing
• Small scale milling to <0.2 microns
• Product formulation
• Scale-up estimates and trials
• Ceramic Digital Inkjet
• Formulation development and assistance
• Refractory design, selection, sourcing
• Equipment training and instruction
• Bulk solids storage and transport
• Raw material acquisition and storage
• Scale up from R&D to pilot to full
production • Finished goods storage
• Factory and process design • Full process documentation to meet your ISO
requirements
FROM CONCEPT TO
PRODUCTION WITH CMC AND • Process design
OPF ENTERPRISES • Raw material evaluation
Custom Milling & Consulting, Inc., 1246 Maidencreek Road, Fleetwood, PA 19522
(610) 926-0984 Fax (610) 926-0989 Website: http://www.cmcmilling.com
OPF Enterprises, PO Box 16898, Missoula, MT 59808
Telephone and Fax 406-493-1102 Website: http://www.ontheplantfloor.com