The grinding process in ball mills and vertical roller mills differ fundamentally. In a ball mill the comminution takes place by impact and attrition. The comminution in the vertical roller mill takes place by exposing a bed of material to a pressure sufficiently high to cause fracture of the individual particles in the bed, although the majority of the particles in the bed are considerably smaller than the thickness of the bed.It is thus necessary that a stable and consistent grinding bed is formed between the rollers and the table of the vertical roller mill, able to sustain such a pressure without the material being squeezed away from the pressure zone. A stable grinding bed is usually easily obtained in raw material grinding in a vertical roller mill with a high efficiency separator. However, in cement grinding it becomes more difficult to form a stable grinding bed as .

• cement is ground much finer than raw meal

• the feed to a cement mill is often completely dry and is significantly more difficult to grind than raw materials

• requirements to the particle size distribution of the finished product are much more strict when grinding cement than when grinding raw materials

These differences between cement grinding and grinding of raw materials made it a serious challenge to obtain a good performance of a vertical roller mill in cement grinding. Today, however, the vertical roller mill has become an excellent example of a mill design that has overcome the difficult grinding conditions associated with finish grinding of cement clinker and related products.

As shown in Figure 1, the rollers of the OK mill are spherical in shape with a groove in the middle. The table is also curved forming a wedge-shaped compression and grinding zone between the rollers and the table.

This dual-lobed design is optimum for clinker grinding because it supplies two distinct grinding zones – a low pressure zone and a high pressure zone .

As shown in Figure 2. The low-pressure area under the inner lobe de-aerates and consolidates the material to be ground. The proper grinding takes place in the high-pressure zone under the outer lobe. The groove in the middle of the roller facilitates de-aeration of the material without fluidising it.

In order to further ensure a stable operation with low vibrations, the vertical mill is provided with a high efficiency separator. This results in a reduced internal circulation of fine material and a correspondingly higher feed rate. The material on the grinding track will thus become coarser and therefore less prone to fluidisation .

Industrial experience has proven that due to the design of grinding parts and the high efficiency separator the vertical mill, Figure 3, adequately addresses all the difficult grinding conditions associated with fine grinding of cement clinker and related products, enabling a high grinding efficiency and stable operation.

However, despite a generally good performance of vertical roller mills used for grinding of cement and related products the vertical roller mill is still inferior to the ball mill in terms of sensitivity to variations of mill feed quality (fineness) and mill feed rate.