The present invention relates to a ring roller mill operating at a subcritical speed which mill comprises at least one grinding ring, at least one roller fitted inside the grinding ring and at least one unit fitted between the roller and the grinding ring.

Such a mill can, for example, be used in connection with the manufacture of cement for grinding of mineral clinker materi¬ als, slag, and similar materials.

In the published European patent application No. 486371 a ring roller mill is described. In this mill the feed material is charged to a space before the grinding path where it is sub¬ jected to centrifuging action. Hereafter the material and the air are carried axially through the mill. The grinding ring in this mill operates at grinding pressure between 10-40 MPa and at supercritical speed, which means that the material subjected to grinding action is retained on the grinding ring all the way round. It is, therefore, possible and necessary to fit scrapers internally in the ring in order to release the material during the passage transversely to the grinding ring. These scrapers can be tilted so that they guide the material forward along the roller in the flow direction.

In a ring roller mill which is operated at subcritical speed, the ground material will only be retained on a part of the grinding ring. In an area on the grinding ring, dependent on the rotational speed of the ring, the surface roughness of the ring and the characteristics of the processed material, the ground material will be detached from the ring, continuing to fall along a downward path towards a draw-in zone in front of the roller.

An even distribution of the material in the draw-in zone and in the grinding bed is of major significance for the operation of the mill. Major variations in the thickness of the grinding bed will cause irregular operation or vibrations in the mill

and substantial fluctuations in the torque of the drive arrangement. An uneven loading of the roller may also cause the wear segments which protect the surface of the grinding ring and the roller to be damaged.

The intensity of grinding pressure applied and the type of material being ground are determining factors in regard to the form that the compacted material will have after its passage through the roller in the mill. A high grinding pressure and/or sticky material will lead to formation of agglomerates which retain their form after the material is discharged from the grinding ring, whereas a lower grinding pressure will result in the dropping material being a relatively loose, crumbling material. Both the size and hardness of the agglomerates will have an effect on the evening-out of the bed of material which is deposited in the draw-in zone and in the grinding bed proper.

A ring roller mill operating at subcritical speed has a lower rate of rotation than a ring roller mill operating at supercritical speed. In order to increase the production in a mill operating at subcritical speed to the same level as in a similar mill operating at supercritical speed it is necessary to increase the grinding pressure. Normally the grinding pressure in a mill operated at subcritical speed will be higher than 50 MPa.

It is the objective of the present invention to provide a ring roller mill in which the agglomerates formed during operation are crushed and distributed along the draw-in zone and the roller in such a way that no undesirable vibrations or skew placement of the roller occur during operation.

This is achieved according to the invention in a surprisingly simple way by mounting internal fittings in the form of impact and distribution units in the mill. These units are placed in a certain distance from the point P on the ring in the path where the compacted material falls off the ring, descending towards the draw-in zone.

During the grinding process the impact and distribution units . will break down agglomerates, while distributing the loosened material down over the draw-in zone in an even layer. The maximum degree of impact is attained when the material hits the surface of the unit at an angle between 60° and 120°, preferably between 70° and 110°.

The units do not need to have a solid surface it might be an advantage that the units have a perforated surface or consists of a grating.

If the material is fed symmetrically via openings at both ends of the grinding ring, agglomeration of material may occur mainly at the middle of the grinding ring and, therefore, the impact and distribution units may be appropriately configured in such a way that they break down agglomerates, while diverting the majority of the material out towards the sides of the grinding ring.

The invention will now be explained in further details with reference to the drawing, where

Fig. 1 shows a cross-section through the ring roller mill

Fig. 2A-D shows different configurations of the impact and distribution units, viewed both from the side where the mate¬ rial hits the plate and cut through lengthways of the beam 8.

The mill in fig. 1 has a grinding ring 1 and a roller 2. The roller 2 and the grinding ring 1 rotate by means of a not shown driving mechanism and the roller 2 is thrust against the grinding ring 1 through a tensioning system. Between the roller 2 and the grinding ring 1, the partially crushed ma- terial constitutes a grinding bed 4 and the space 4a in front of the roller, from which the feed material is drawn in for compaction, is referred to as the draw-in zone.

After the loose material from the draw-in zone has passed under the roller, the material will normally be compacted into a solid mass which is retained on the ring up to a point P where the material starts to drop down towards the draw-in zone.

The position of P will inter alia depend on the speed of the grinding ring and the type of material being ground.

A unit 3a is placed in the mill in the path where the compac¬ ted material descends towards the draw-in zone. The unit may, for example, be fastened by means of a beam 8 crosswise of the mill in such a way that it is possible to adjust the vertical position of the unit and the angle at which the ground materi- al hits the surface of the unit, and this may, for example, be achievable by tilting or turning the unit. The position of the unit and the angle to vertical can be adjusted from the outsi¬ de, which means that the position of the unit can be adjusted while material is being ground in the mill. In order to fulfil its purpose to an optimum the impact unit or the first of the

1 . impact units should be placed at least —x d , preferably more

4 than — x d , away from P, where d is the internal diameter of the grinding ring.

The unit 3a will break down agglomerates, and distribute the loosened material across the roller 2 and the draw-in zone 4a.

The units 3b and 3c show alternative positions in the mill, and, if appropriate, several units can be installed simultane- ous, either side by side or vertically offset, thereby making it possible to enhance the precision of distribution of the ground material.

In fig. 2A is shown a unit configured as a plane plate 6 which does not distribute the material in any specific direction, but merely disperses the material and breaks down agglomera¬ tes.

In fig. 2B is shown a unit configured as a plane plate 6 with end sections 5. The end sections ensure that the dispersion of the material which hits the unit is restricted axially and that the material is directed downwards towards the draw-in zone. The axial extent of such a plate will typically corre¬ spond to the roller width.

In figs. 2C and 2D is shown a unit configured with a raised area 7 which in Fig. 2C has a form which roughly resembles that of a roof ridge. In this configuration, the material is distributed from the middle of the grinding ring and out towards the sides, which is appropriate when there is a ten¬ dency of the material being deposited in the middle of the grinding ring during the grinding process.

The size of a unit, cf. Fig. 2C or 2D, i.e. the axial extent, is a contributory factor in determining what the final finis¬ hed material curve will look like, since the size will be of significance in regard to the number of times the material is recirculated in the mill. A small roof ridge will thus result in a flat particle distribution curve because of the same materials being subjected to several rolling passes without intermediate separation, whereas a wider roof ridge will result in a greater mixture and hence a more effective separa¬ tion and a steeper particle size distribution curve.

When material is to be ground in a ring roller mill according to the invention the material is fed to the mill via one or several inlet ducts through one or both of the stationary units which are installed at the end of the grinding ring 1 and directed to the draw-in zone where it is drawn under the roller 2 and subjected to grinding action. Gradually as the amount of ground material increases, the material is thrust over the edge of the grinding ring 1 where it is collected and either recirculated for renewed grinding in the mill or direc¬ ted, for example while entrained in an airstream, out through the stationary ends at the end of the grinding ring 1 of the mill for renewed processing elsewhere, if appropriate.