There is a specific operating speed for most efficient
grinding. At a certain point, controlled by the mill
speed, the load nearest the wall of the cylinder breaks
free and it is so quickly followed by other sections
to form a cascading, stream containing several layers
of balls separated by material of varying thickness.
The top layers in the stream travel at a faster speed
than the lower layers thus causing a grinding action
between them. There is also some action caused by the
gyration of individual balls and secondary movements
having the nature of rubbing or rolling contacts occur
inside the main contact line.
It is important to fix the point where the charge,
as it is carried upward, breaks away from the periphery
of the Mill. We call this the "break point",
or "angle of break" because we measure it
in degrees. It is measured up the periphery of the Mill
from the horizontal.
|There are four factors
affecting the angle of break:
- Speed of Mill
- Amount of grinding media
- Amount of material
- In wet grinding, the consistency or viscosity
The smaller the Mill the faster in RPM it must run
to attain critical speed.
For most grinding and dispersing problems, we strive
to attain the cascading, sliding action described earlier,
and to accomplish this we have found that the most desirable
angle of break ranges from 50 to 60 degrees from the
The lower range is recommended for most wet grinding
operations like paints and soft dry materials, and the
higher break point (which provides a more severe grinding
action) for most dry materials and wet grinding such
hard products as enamel frit and glaze.
It is also known that the grinding action in a larger
mill is more severe than in the smaller sizes and, consequently,
we are of the opinion that the angle of break should
be lower for the larger Mills than for the smaller.
For the most efficient results, the Mill should be
at least half filled with grinding media. Some prefer
to go a little beyond the halfway mark to compensate
for wear. There is no objection to this and we have
been suggesting a limit of about 5 percent.
In steel ball grinding, many especially in the paint
industry, are satisfied to run with a smaller ball charge
ranging as low as one-third the volume of the mill.
They find the smaller charge gives them the required
grind within allowable limits of grinding time and the
extra space gives them more loading room.
Grinding media should be periodically checked. Reduction
in the quantity and size of the grinding media will
result in poor grinding. We suggest a maximum schedule
of once every six months. In some cases, where abrasive
materials are involved, once a month is not too often
and, in a few cases, even shorter intervals are indicated.
|There are three types of grinding media that
are most commonly used:
- Flint Pebbles
- Porcelain Balls - regular and high density
- Steel and other metal Balls
|Flint Pebbles - These are the oldest
type of grinding media in use and they continue to be
extremely popular. They can be used with all types of
Porcelain Balls - This is a pure white ceramic material
with a dense, highly vitrified body that will not chip
or crack in service. They have been immensely improved
in quality in recent years and are used exclusively
in many industries.
High Density Media - This is the latest grinding media
developed for Ball Mills. They are made with high alumina
oxide content and have a density 40 to 50% greater than
the regular porcelain balls. They are also fired at
higher temperature making them
harder and more abrasion resistant.High density media
are available in various shapes including spheres, cylinders
and ovals resembling the natural flint pebbles.
Steel Balls - Steel balls are unquestionably doing
a faster grinding job than any of the other commercially
available media. They have proven especially valuable
in the paint industry.
|The following types
of metal balls are commonly used in Ball Mills:
- High Carbon - High Manganese Steel .
- Cast Nickel Alloy
- Stainless Steel
- Chilled Iron
- Forged Low Carbon Steel
- Other, more special types include bronze or brass,
aluminum, tungsten carbide, etc.
|The following general
rules should be carefully adhered to regardless of the
type media used.
- There should be enough material in the batch to
cover the grinding media.
- Grinding time must be watched carefully to avoid
- Excessive buildup of heat should be avoided. In
paint grinding, this may lower the operating viscosity
beyond the critical point.
- The smallest grinding media should be employed.
These not only reduce the danger of overheating but,
as is well known, the smaller grinding media provide
faster and better results.
- When using extenders, their abrasive nature may
cause excessive wear. To avoid this, some operators
are able to hold out the extenders until the grinding
is almost completed and then add them for the final
|Probably the most common cause for faulty
operation and complaints has been due to the size of grinding
media. It is strongly recommended that the smallest feasible
grinding media be used in all cases. The optimum size
of media should not change with Mill size. If the laboratory
small Ball successfully grinds a sample batch in a lab
Mill, the same size grinding media will do the best job
in a production Mill whether the Mill is one foot or eight
feet in diameter.
|Small grinding media
are recommended because:
- They provide many more grinding contacts per revolution
than larger media. This results in much quicker grinding
- They provide smaller voids, limiting the size of
particles or agglomerates which can exist there.
- They do not create excessive energy which cannot
- Oversized grinding media frequently develop more
grinding energy than is needed for the job. This excess
merely builds up heat and wears down the media and
lining, introducing contamination in the batch.
- The chief disadvantage of the smallest size grinding
media is that discharging takes somewhat longer due
to increased surface tension in the smaller voids.
Almost invariably, however, the reduced grinding time
realized by smaller media more that offsets this disadvantage.
|Clogging of material in the Mill makes further
operation harmful. This is generally caused by moisture
of fat, as in oily seeds. Possible remedies include:
- Taking the material out and thoroughly dry it.
- Adding a dry filler to absorb the excessive moisture
while the batch is being ground.
- Adding a few pieces of steel angle, bar, or chain
which can slide along the Mill surface and scrape
off any materials starting to pack.
- If the material is packing due to particle size
alone, grinding should be stopped prior to this point.
The material should then be screened and tailings
returned to the mill.