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Powder and Bulk Engineering, June 1994 53
ri
BrimVernon U.S. StonewareCorp.
Ball mills can produce finely ground high-purity materi- als,
such as chemicals, pharmaceuticals, semiconductor and capacitor raw
materials, paints and pigments, com- puter and microwave
components, and dental porcelain. This article discusses mill
types, mill linings, and grind- ing media, then provides detailed
information to help you understand ball mill operating factors.
ball mill reduces particles to a required size and can also
disperse or mix materials when size reduction is a sec- A ondary
need. The mill can handle a range of applica-
tions; this article concentrates on high-purity ball milling
applications.
The required equipment typically consists of a cylinder (a hori-
zontal cylindrical vessel) fitted with shafts. The cylinder’s
length exceeds its diameter, and the cylinder rotates about its
longitudinal axis. The unit can provide wet or dry milling and can
be charged with different grinding media, which contacts and
reduces the material during milling.
Looking at some ball mill basics - including mill types, mill
linings, and grinding media - will lay the groundwork for un-
derstanding ball mill operating factors.
Mill types Various production-size ball mill types are available
for high- purity milling applications. The types are based on
cylinder capacity.
For capacities up to about 60 gallons, the mill can consist of a
one-piece ceramic cylinder reinforced at either end by dome-shaped
steel heads that are held in place by steel tie rods (Figure
la).
For capacities over about 60 gallons, the mill can consist of a
three-piece ceramic cylinder that is ground and lapped to form a
leak-tight seal. The cylinder is further reinforced and protected
by a flanged steel casing.
Another type, consisting of a steel cylinder with a ceramic
brick or other lining mortared or bolted in place (Figure lb),
handles capacities from about 10 to 2,000 gallons.
Each type has a loading-unloading port located along the
cylinder wall. The port can be equipped with a solid cover for
sealing the cylinder during mill rotation or, for wet or dry dis-
charging, a screen- or grid-like discharge cover that retains the
grinding media while discharging the ground material.
The mill is also available in a lab or limited-production size,
called a jar mill (Figure 2). The unit consists of one or more
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pairs of motor-driven friction rolls mounted on steel frames;
the rolls hold and rotate one or more jars. Each jar is typically
ce- ramic but can also be made from unlined stainless steel or rub-
ber- or polyethylene-lined steel. For loading and discharging
material, the jar has either a tapered neck or a full opening.
The
Powder and Bulk Engineering, June 1994
jar is typically available in sizes from X pint to about 6
gallons, but a jar larger than 6 gallons can be unwieldy and
require spe- cial handling equipment, such as a hoist or hydraulic
lift table.
Mill linings A mill lining can be the cylinder material itself,
as with a ce- ramic cylinder, or a separate lining material
fastened to the cylinder interior. Linings can be ceramic, rubber,
polyurethane, or metal.
Ceramic. Ceramic lining is formulated of silicon oxide and alu-
minum oxide. The proportion of each oxide in the ceramic can vary,
and you can choose a ceramic consisting primarily of one oxide or
the other based on which is least likely to contaminate your
material. For instance, when milling high-purity silica raw
materials, using a lining made primarily of silicon oxide ce- ramic
is best because any material worn off the lining won’t af- fect the
silica’s purity.
Rubber. Abrasion-resistant low-ash rubber lining is used for
milling contaminant-sensitive materials. The rubber’s re- siliency
helps the rubber resist abrasion, adding only negligible rubber
contamination and virtually no organic contamination to the milled
material. After milling, igniting the milled material in an
oxidizing atmosphere can remove any rubber contamina- tion in the
batch, leaving only a trace of oxide contamination.
Low-ash rubber lining can be applied in production-size mills
and jar mills for wet- or dry-milling alumina, titanates, zir-
conates, nuclear ceramics, high-temperature refractories, and other
materials.
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Polyurethane. Polyurethane lining has the greatest wear resis-
tance and highest tensile and tear strength of any synthetic rub-
ber and provides toughness, resiliency, and load-bearing
capability. The lining resists oils and greases and isn’t affected
by oxygen, ozone, or weathering. Polyurethane lining can be used
with aqueous salt solutions, dilute mineral acids and greases,
aliphatic hydrocarbons, and other nonpolar organic compounds.
Powder and Bulk Engineering, June 1994
.
Metal. Abrasion-resistant steel, stainless steel, or another
metal lining can be used when some metal contamination is accept-
able or when the milled material will undergo further treatment to
remove metal contaminants. Metal lining is best for wet- milling
ceramic fenites in both solvent and water dispersions and is
suitable for milling any paint except the finest white or pastel
colors.
Grinding media Grinding media for high-purity ball milling is
typically high- density alumina ceramic spheres or cylinders or
zirconia cylin- ders . Both a re long wearing with hard, nonporous,
chip-resistant surfaces and are nonmagnetic and nonconduc- tive.
Other standard-density media - including flint pebbles, steel
balls, and porcelain balls - are typically used for applica- tions
with less stringent purity requirements.
High-density alumina ceramic spheres and cylinders (including
radius-end cylinders) are available in sizes from % to 3 inches
with specific gravities from about 3.4 to 3.7 and alumina con- tent
from 85 to 99 percent. They provide faster milling than
standard-density media (such as porcelain balls) and can be used in
unlined or lined mills. The media isn’t affected by most acids or
alkalies and is resistant to mechanical and thermal shock. The
spheres and cylinders have different initial costs, wear rates, and
other characteristics; which is best depends on your
application.
Zirconia cylinders range in size from %to 1% inches. The media
is 1.6 times denser (typically with a specific gravity of 5.5) and
can generally mill twice as fast as high-density alumina ce- ramic
media. The zirconia cylinders have a harder, less porous, more
chip-resistant, and easier-to-clean surface than high-den- sity
alumina ceramic media. The zirconia cylinders also cost more. The
media is very resistant to mechanical and thermal shock.
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discussed here apply to both high-purity and other ball milling
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Rotation speed. The ball mill cylinder’s rotation speed (in
revo- -0 lutions per minute) determines the grinding media’s action
for E to cascade is typically most desirable. Cascading usually
occurs s when the rotation speed causes the media to break away
from the cylinder wall at a 45- to 60-degree angle above the
horizon- ca tal. This action causes the media from the outer edge
to fall and roll in a coherent, mobile mass, like a waterfall. As
shown in d Figure 3, the media at point A will fall to point B,
changing the media’s potential kinetic energy and fracturing the
material’s % particles. 3
Secondary milling action occurs between the media, the
cylin-
mass cascades downward, rotating and rubbing the material
secondary action’s high shear rate provides more complete par-
ticle wetting. ra
-. both wet and dry milling. A rotation speed that causes the
media
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der wall, and the material being milled as the media within
the
and causing further attrition. The secondary action creates in-
tensive disintegration and better dispersion. In wet milling, the
2.
A mill that rotates too quickly creates centrifuging, which
throws individual media clear of the media mass so they move
Understanding ball mill operating factors Several operating
factors affect ball milling efficiency, includ- ing rotation speed,
mill size, milling type (wet or dry), grinding media size, grinding
media quantity, amount of material to be milled, batch viscosity,
and mill cleaning. Most of the factors
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independently until rejoining the media mass at the mill bot-
tom. Unground material stays with the centrifuging media, pro-
ducing an uneven dispersion.
Powder and Bulk Engineering, June 1994
A mill that rotates too slowly causes media to slip. In this
case, the media mass acts as a static mass in relation to the mill
walls, which grooves the mill walls and flattens the media. Media
slip- page can also occur in a mill operating at optimum speed if
the media or material charge is low.
In an unlined mill that rotates slowly, using lifter bars that
are welded or bolted inside the mill cylinder reduces media slip-
page. Using lifter bars also reduces slippage when the media charge
is below 45 percent of the mill volume. But using lifter bars won’t
prevent media slippage with less than a 33 percent media charge in
the mill cylinder.
Calculating critical rotation speed in revolutions per minute is
based on this relationship:
54.2 76.6 Nc= .\jR =-
where N , is critical rotation speed, R is inside radius (in
feet), and D is inside mill diameter (in feet).
Depending on the application and your preference, the mill’s
rotation speed usually ranges from 35 to 115 percent of critical
speed; 60 to 65 percent of critical speed is typical. A variable-
speed drive can help you control rotation speed for reducing a
material to a narrow particle size distribution.
Figure 4 shows the relationship of inside mill diameter to
satis- factory rotation speed for wet-milling applications. The
curve is a rough average based on published data’ for unlined
mills, and the speeds apply with any media shape or density.
Satisfac- tory dry milling rotation speed should be about 2 to 5
rpm faster for the same inside mill diameter.
Generally, a higher rotation speed is used for wet-milling ce-
ramic frits and glazes than for wet-milling paint materials and
dry-milling soft minerals. A higher rotation speed reduces media
slippage and minimizes media wear. An example of how a
higher-than-average rotation speed can be used is milling bronze
and aluminum, which requires critical or near-critical rotation
speeds because the materials are malleable and tend to flake rather
than reduce to powder. The media flies across the cylinder’s top,
striking the opposite wall with hammer-like blows to flake the
malleable material.
Millsize. There is almost no difference in the particle size
reduc- tion you can achieve with a production-size mill or a
lab-size jar mill as long as the milling action is comparable. But
the greater the inside mill diameter, the less milling time is
required. Pro-
duction output depends on mill volume, so base your mill selec-
tion on your volumetric production requirements.
Mill volume varies as the square of the inside mill diameter,
and the horsepower required for rotation varies as the 2.6 power of
the inside mill diameter. Thus an 8-foot-diameter mill requires 40
to 80 times more horsepower than a 2-foot- diameter mill.
Milling type (wet or dry). Whether your material is wet or dry
typically determines whether you use wet or dry milling. If
properly handled, wet milling can be faster than dry milling. For
this reason you may consider adding liquid to a dry material to
form a slurry that can be wet milled, as long as the dry mater- ial
can tolerate the added liquid and you can later dewater or dry the
product if needed.
Dry milling can cause the material to pack as it becomes finer,
unless you remove the fines or use a grinding aid to draw mois-
ture out of the material. It’s also more difficult to discharge the
milled material after dry milling. Unless the material is ex-
tremely free-flowing, the mill must be rotated to collect the ma-
terial without removing the media.
Grinding media size. Your milling conditions determine the media
size. Effective milling typically requires the smallest media that
will do the job because smaller media provide more contact per mill
revolution. The smaller media voids also hold only smaller
particles, ensure more physical contact between
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the media and particles, and decrease the distances over which
shear forces have to act. These factors produce more uniformly
milled fine particles.
Powder and Bulk Engineering, June 1994
0 0 73 2.
Larger media has greater impact energy and can generate ex-
cessive heat in the mill if the energy isn’t efficiently con- sumed
by the milling action. However, this extra energy is useful for
milling large or tough particles or a thixotropic wet mixture
(which liquefies when vibrated and solidifies when left
standing).
Grinding media quantity. Grinding media should fill from 45 to
55 percent of the mill’s total volume to minimize excessive media
wear. A charge below 45 percent tends to slip along the cylinder
wall rather than cascade, unless the mill has lifter bars.
A high-density alumina ceramic media charge is typically 45 to
50 percent. For instance, the Porcelain Enamel Institute recom-
mends a 50 to 55 percent charge of high-density ceramic media for
wet-milling porcelain enamel frit and recommends that the frit
charge (in pounds) be equal to 3.5 to 4.5 times the mill’s total
volume (in gallons) for high-density alumina ceramic media.
Amount of material to be milled. For wet milling, the rule of
thumb is to fill the material slurry to about 1 inch above the
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media charge level. The minimum material charge should be 25
percent of the mill’s total volume; a 30 to 40 percent material
charge is best. For dry milling, a material charge of 25 percent 0
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of the mill’s total volume is typically best. This charge
permits the media charge (about 50 percent) to effectively contact
the
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If you use a higher material charge for either wet or dry
milling,
load, but the milling time will be longer. Thus to regulate
charge with care, because undercharging the material can ex-
cessively increase the milling temperature and increase media
you can achieve greater capacity at lower cost per unit mill
milling time to fit shift changes or other factors, you can
reduce or increase the material charge. However, change the
material %
wear.
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rn Batch viscosity. Wet-milling a solid in a liquid to reduce
the
cosity for your media size and density. This will help you
wear, such as 20 percent per year. For example, the pigment
particle size or make a dispersion requires optimum batch
vis-
achieve maximum production with only reasonable media
content of a paint milled with high-density media should be
increased 25 percent over the pigment content when milled with less
dense media, because of the dense media’s greater impact energy.
However, too high a batch viscosity will im- pair the media’s free
movement and cause the media to cling together in a mass that
rotates with the mill, producing no milling action.
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62 Powder and Bulk Engineering, June 1994
You also need to increase the batch viscosity when using a
large-diameter mill, because the higher viscosity provides a
heavier bodied cushion that prevents wear from the greater media
weight. Higher viscosity also compensates for the larger mill’s
greater horsepower because some of the power is trans- formed into
heat that lowers batch viscosity.
The relation of batch viscosity to media size and density can be
seen in the example of milling paint. Using a Stormer viscome- ter
(with a 52.5-millimeter-sweep flat paddle) to measure the paint
viscosity gives the data in Table I, which correlates media size
and density with viscosity in Krebs units (KU). To achieve the most
efficient milling with porcelain balls, the batch should be similar
to a free-flowing slurry with a 70- to 90-KU (600- to
1,100-centipoise) viscosity. With a higher density media such as
high-density alumina ceramic spheres or cylinders, a viscos- ity
higher than 110 KU (2,200 centipoise) is best.
Mill cleaning. Some ball mills require an initial cleaning in
ad- dition to regular cleaning. If your material must be protected
from contamination, initially clean a ceramic- or steel-lined
cylinder by adding a charge of fine sand (or another inexpen- sive
material, such as a scrap product) and 50 percent water with the
media charge. Then run the mill for 1 to 2 hours, dis- charge the
batch, and thoroughly rinse the mill. If the cylinder isn’t clean
after one treatment, repeat the process.
Regular, effective cleaning reduces batch contamination for both
wet- and dry-milling applications. For instance, to clean a wet or
dry material from the mill, dump solvent into the mill, run the
mill for 1 minute, and then immediately discharge the solvent to
prevent solids from settling out. Several cleanings may be
necessary. For water-based emulsions or ceramic slips, use water
with an aerosol wetting agent or a small amount of detergent to
promote draining. But be careful: Running the mill for more than 1
minute during cleaning can excessively wear the media and lining.
PBE
Reference 1. Data researched by U.S. Stoneware Corp., East
Palestine, Ohio.
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Brian Vernon is sales manager at U.S. Stoneware Corp., 700 East
Clark Street, East Palestine, OH 44413; 216/426-4.500. He has a BS
in business computers and an MBA in business ad- ministration, both
from Clarion University of Pennsylvania, Clarion. Pa.
