It is obvious from the positions of the different sizes of

grinding media balls within the charge that mill speed is a major factor. At low speed, all the large (white) balls are found at the periphery of the charge. At high speed, they are in the centre. This applies to all types of grinding media, e.g. pebbles, ball or rods.

This segregation is a result of the fact that at low speed, the force of gravity is stronger than the centrifugal force, so that the grinding media move around the core of the charge. The relative movement of the material being ground is greatest at the top of the charge. As the mill continues to rotate, the smaller particles move. Towards the centre and the larger ones collect at the top. The same effect can be observed by shaking a bucket of gravel for a few minutes; the larger pieces of rock will move to the top.

If mill speed is increased, a cascade effect is obtained. The relative movement of the particles is very small at the top of the charge.

Charge pressure is reduced, so that the smaller particles at and beneath the core are pulled by both gravity and centrifugal force to the periphery of the charge.

The results of grinding operations are strongly dependent on the position of the components of the charge.

Bets results are achieved when all sizes of balls are uniformly distributed throughout the charge, so that there is a maximum of contact between the particles. The total volume of grinding bodies can be minimized, so that mill capacity is utilized more efficiently.

How can this situation be obtained? Is has been shown that mill speed is a major factor. But the lining profile as also vital. If the correct balance between mill speed and lifter bar height is determined, the optimal cascade effect can be generated.

The relation between lifter bar height and mill speed can be described in the following formula:

(1 - Cs/100 ) x A = B

Where

Cs = percent of critical speed

A = plate width and

B = lifter bar height.

Cs = 76.6 / D^0.5

This formula applies to fig.

A 75.In Fig, E75, the height of the lifter bars is too low.

In some cases, the combination of low mill speed and excessively low lifter bars may result in length wise reserved ball segregation. This can occur in log tube mills in which balls are replaced through the feed trunnion.

The reason is that the large balls tend to gather at the toe of the charge, where the flow through the mill causes them to roll towards the discharge end. This problem can be solved by using higher lifter bars at the feed end of the mill, in order the feed end of the mill, in order to reduce the extent of the toe and create a higher cascade. Higher profiles at the feed end also generate greater impact on the material, which is required if the mill is receiving a coarse feed.