The invention also relates to a vertical mill for carrying out the method according to the invention.

A vertical mill of the aforementioned kind is known, for example, from GB-B- 2214106. This prior art mill functions ideally in accordance with an operating principle in which the material which is ground on the grinding table by the grinding rollers is flung outwards from the rotating grinding table and across the surrounding nozzle ring by means of which finer particles are dispersed and suspended in the conveying gas flowing up through the nozzle ring and are passed to a separator mounted in the top of the mill where the material is sorted into a fine fraction which is discharged from the mill as a finished product and into an insufficiently ground coarser fraction which is returned to the grinding table for renewed grinding.

However, in practice it has emerged that not only does a sorting process take place in the separator, but also in the mill housing itself. In terms of energy usage, this is undesirable since the effectiveness of the separation process in the separator must be considered as being superior to that effected in the mill housing.

It is an object of the present invention to provide a method for grinding of particulate method in a vertical mill whereby the aforementioned disadvantage is reduced.

This is achieved by a method of the kind mentioned in the introduction and being characterised in that the vertical gas velocity, calculated as the average vertical gas velocity over the cross-sectional area lying between the mill housing and the separator, increases from a horizontal plane immediately above the roller or rollers to a horizontal plane at the louvre of the separator.

Preferably, the average vertical gas velocity through the mill housing to the louvre of the separator is at least 7 m/s, preferably at least 9 m/s.

A vertical mill for use in the method is characterised in that the cross-sectional area lying between the mill housing and the separator decreases from a horizontal plane immediately above the roller or rollers to a horizontal plane at the louvre of the separator.

Thus the efficiency of the combined separation process in the mill housing and the separator can be enhanced. This is due to the fact that the separation of fine and coarse material particles will take place to a greater extent in the separator and to a lesser extent in the mill housing when the vertical mill is operated with a vertical gas velocity increasing from at least 7 m/s.

The invention will now be described in detail with reference made to the accompanying figure which is a diagrammatical drawing depicting a vertical section through a vertical mill.

In the figure is shown a vertical mill comprising a grinding table 1 which is rotatable about its central vertical axis and driven via a gear unit 2. Rolling on the grinding table 1 are grinding rollers 3 which are mounted on shafts (not shown) and urged down towards the grinding table 1 by means of known, but now shown, pull or pressure means.

The vertical mill is encased by a mill housing 5 which at its upper end has a built-in separator 6. The separator 6 has in its inlet a louvre 7 which may be adjustable. The separator may also comprise a rotor (not shown) which is fitted inside the louvre and is rotatable about a vertical axis. At its lower end the separator 6 is equipped with an outlet hopper 8 with an outlet 9 extending down towards the grinding table 1.

The grinding table 1 is surrounded by a nozzle ring 10 through which conveying gas flows from a chamber 11 under the nozzle ring and grinding table. Gas is fed to the chamber 11 from a gas supply duct 12.

Fresh, unground material to be fed to the vertical mill is introduced into the vertical mill through a supply duct 13 ending at the top of a feed pipe 14 which is fitted coaxially with the grinding table 1 and at some distance above its central section 15.

The material which is introduced to the vertical mill through the supply duct 13 passes down through the feed pipe 14 to the central part 15 of the grinding table wherefrom, resulting from the rotation of the grinding table in use, the material is flung outwards and ground under the grinding rollers 3.

The ground material is flung onward beyond the nozzle ring 10 and scavenged by the conveying gas which flows up through the nozzle ring 10. Particles which are too heavy to be suspended in the conveying gas and carried up through the vertical mill drop through the nozzle ring and are discharged from the space under the nozzle ring via an

outlet 16 wherefrom the particles are lifted to the supply duct 13 by means of an external conveyor indicated at 17 and returned to the vertical mill for renewed grinding.

Lighter particles of ground material which can be suspended in and carried up through the vertical mill by the conveying gas stream are diverted, as indicated by means of arrows, to the louvre 7 in the inlet of the separator and into the separator 6 in which the material is divided into a fine fraction which is extracted from the vertical mill via a top outlet 18 and a coarse fraction which falls down along the sides in the outlet hopper 8, out through the outlet 9 and down into the feed pipe 14.

In order to reduce the sorting process which takes place in the mill housing and hence to achieve benefits from a reduction of energy usage, the design principe of the vertical mill involves decreasing the free cross-sectional area in a horizontal plane from the mill housing to the louvre of the separator from a horizontal plane immediately above the rollers 3 to a horizontal plane at the louvre 7 of the separator. In the embodiment shown the upwardly decreasing cross-sectional area is obtained by means of a hopper 8 which is designed with an increasing cross-sectional area in the upwards direction towards the separator 6. Alternatively, the wall of the mill housing 5 may be of a conical configuration with a decreasing diameter or the cross-sectional area in the direction towards the separator or the mill housing may be equipped with internal fittings. The latter option would be particularly advantageous for converting existing mills. During the operation of the vertical mill, this will lead to an increase in the gas velocity up through the vertical mill from a horizontal plane immediately above the rollers 3 to a horizontal plane at the louvre 7 of the separator. In order to substantially reduce the sorting process in the mill housing while simultaneously ensuring low energy usage, it is preferred that the vertical gas velocity through the vertical mill is at least 7 m/s, preferably at least 9 m/s.