Ball Mill / Pebble Mill/ Tumbling Mill
The mechanisms of impact and attrition can be combined in
two forms of mill.
In the ball mill the
particles receive impacts from balls or stones and are subjected to attrition
as the balls slide over each other.
The ball mill consists of a hollow cylinder mounted in such
a way that it can be rotated on its horizontal axis with a rotational frequency
depend upon the diameter of the mill ≈ 0.5 r.p.s
The cylinder may be of metal, porcelain or of rubber, to reduce
The balls may be of metal, porcelain or stones (pebble
The cylinder contains balls that occupy 30 to 50 per cent
of the mill volume.
The cylinder may be of metal, porcelain or of rubber, to
reduce abrasion. The balls may be of metal, porcelain or stones (pebble
The ball size being dependent on the size of the feed and
the diameter of the mill. Usually a mill 1 m in diameter will use balls of 75
mm, in practice, the balls are damaged, so that a range of sizes from 20 mm upwards
are used. This gives a better product, since the larger balls crush the feed
and the smaller ones form the fine product.
Importance factors in
the operation of the ball mill
The amount of the
material in the mill is of importance: too much exerting a reduced effect
too little leading to loss of efficiency and to abrasion.
The speed of rotation
At low speeds, the mass of balls will slide or roll over
each other and negligible size reduction will occur
At high speeds, the balls will be thrown out to the wall by
centrifugal force and no grinding will occur
At about two-thirds of the speed at which centrifuging just occurs,
movement takes place as shown in Fig. (c), that is, the balls are carried almost
to the top of the mill and then fall in a cascade (tumble) across the diameter
of the mill.
By this means, the maximum size reduction is effected by
impact of the particles between the balls and by attrition between the balls.
It is capable of grinding a wide variety of materials of differing
character and of different degrees of hardness
It can be used in a completely enclosed form; which makes it
especially suitable for use with toxic materials
It can be used for both batch and continuous operation, and
a classifier can be used in conjunction with the mill, so that particles of suitable
size are removed while oversize particles are returned..
It is equally suitable for wet or dry grinding processes.
Grinding medium is cheap
It can produce very fine powders
The cost of installation and production is low
Wear occurs, principally from the balls, but partially from
the shell and this may result in the contamination of the product; with abrasive
materials this may exceed 0.1%
Soft or sticky materials may cause problems by caking on the
sides of the mill or by holding the balls in aggregates
The ball mill is a very noisy machine, particularly if the casing
is of metal, but much less if rubber is used
Ball mills are applicable to a wide variety of
Large ones being used for grinding ores prior to manufacture
of pharmaceutical chemicals
Small versions for the final grinding of drugs or for
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