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. 

Principle of operation

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 abrasion.

The balls may be of metal, porcelain or stones (pebble
mill).

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 mill). 

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.

Advantages

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

Disadvantages

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

Applications

Ball mills are applicable to a wide variety of materials      

Large ones being used for grinding ores prior to manufacture of pharmaceutical chemicals

Small versions for the final grinding of drugs or for grinding suspensions.

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