Roller presses are known i. a. from international patent application No. WO 92/00141. There are many different configuration options for the drive system of a roller press.

In one embodiment it may comprise two motors which drive separate rollers, either directly or via separate gear units.

In a second embodiment it may comprise one motor used either for driving one roller directly or for driving one or both rollers via a gear unit. Also the drive system often includes a safety coupling. For variability of the roller gap width, power to one or both rollers may be transmitted through a cardan shaft.

Sometimes, the operation of roller presses may entail operational problems manifested by torsional vibrations of the rollers. This may result in substantial torque variations in the drive system and, as a consequence hereof, overloading of the gear unit or cut-out of the safety coupling or main motor. However, this tendency seems to increase with an increase in the roller peripheral velocity, and, therefore, it is often a limiting factor for the roller peripheral velocity with which a roller press can be operated. Indeed, a frequently attempted method for enhancing the capacity of a roller press installation relates to enhancements of the roller pheripheral velocity.

For previously known roller presses this seems achievable only up to a roller peripheral speed of about 1. 5 m/s.

Therefore, it is the objective of the present invention to provide a roller press which can also be driven at a roller peripheral velocity in excess of 1. 5 m/s without generating any undesirable torsional vibrations in the drive system.

This is achieved by means of a roller press of the kind mentioned in the introduction, and being characterized in that the drive system is designed so that its lowest natural torsional frequency system exceeds a frequency expressed by the formula :

v 1-vu2 System D port _ 1 Pas where V roller peripheral velocity D roller diameter H grinding bed thickness Pout density of pressed cakes Pin bulk weight of feed to roller press It will thus be possible to design the roller press so that it can be driven at roller peripheral velocities in excess of 1. 5 m/s. This is due to the fact that the roller press is designed with the lowest natural frequency, which for a given -roller diameter and a given roller velocity exceeds the excitation frequency calculated on the basis of the above formula.

The roller press according to the invention can thus be utilized in connection with methods for grinding of particulate material, such as cement raw materials, cement clinker and similar materials, where the roller peripheral velocity is greater than 1. 5 m/s. According to the invention it is preferred that the roller peripheral velocity is greater than 5 m/s, preferably greater than 8 m/s. It is also preferred that the particulate material be subjected to a pressure of more than 50 MPa by subjecting the material once to pressing action between the rollers of the roller press.

The invention will now be explained in further details with reference to the drawing, being diagrammatical, and with its only figure showing a side view of a roller press.

In the figure is seen a roller press comprising two rollers 1 and 2. Particulate material which is to be ground in the roller press is directed from the top via a not shown shaft into the gap between the rollers.

The material being fed to the roller press behaves more or less like a liquid as it is drawn into the gap between the rollers by the roller surfaces. At a nip point the material is subjected to compaction until it behaves as a solid. The angle 8 between the roller surfaces at the nip point on each roller is termed the nip angle, and the angle a measured between the horizontal plane through the roller axis and the nip point is termed the nip angle.

It is a basic rule that where L length of compacting zone R1, D, radius, diameter of roller 1 R2, D radius, diameter of roller 2 The quantity of material which passes through the rollers, assuming that the gap is full, is the product of the grinding bed thickness. H, roller width W, the roller peripheral velocity V and the density of the pressed material pout. The density increases as the material is subjected to pressing action and compaction between the two rollers. The ratio F between the density of the pressed

material Pout and the bulk density of the feed pi, constitutes the compaction ratio. <BR> <BR> <BR> <BR> <BR> <BR> <P> F = Pout<BR> <BR> <BR> <BR> <BR> #in The grinding bed thickness H can be expressed geometrically as a function of the compaction ratio F, the nip angle 8, the compaction length L and the nip angles al and a2 : We will then have : For roller presses it is a basic rule that al=a2 and D1=D2.

Hence the equation is reduced to the following : The tendency towards torsional vibrations in the roller shafts occurs in case of uneven feeding to the press, choke feeding of the roller press or feeding of overly fine material to the roller.

It is a fundamental rule for constant grinding pressure and roller velocity that the grinding bed thickness is determined primarily by the feed characteristics, the feed rate and, in part, by the roller surfaces. If the particle distribution and the composition of material being fed to the press is constant and if the feed rate is constant, the

grinding bed thickness will be constant. In connection with roller press grinding it will be possible to measure the maximum grinding bed thickness for a given type of feed.

This constant maximum grinding bed thickness is achievable only in case of extremely low roller velocity (<0. 05 m/s) and optimization of shaft feeding.

The conditions for achieving a constant maximum grinding bed thickness are never present when grinding is performed by traditional industrial roller presses. Essentially, this is ascribable to variations in the composition of the material being fed to the press due to segregation in feed bin or poor blending of fresh feed and recirculated material and feed rate variations around a given setpoint. An increase in the fineness of the feed will reduce the grinding bed thickness, and the grinding bed thickness will be reduced in case of decreased feed rate.

When torsional vibrations on roller presses are observed, this occurs at a frequency which corresponds to the natural torsional frequency of the roller system. This natural torsional frequency can either be measured directly or it can be calculated.

Excitation of the roller occurs in connection with deviations from steady-state operation. In steady-state operation the grinding bed thickness is less than the maximum grinding bed thickness. The greater the sudden deviation from the maximum grinding bed thickness, the greater the excitation intensity of the system. On condition of sufficient and frequent excitation, the system can be made to oscillate at its natural torsional frequency.

The excitation mechanism relates to the time it takes the material to pass through the compaction zone between the rollers, i. e. the angle of engagement a, the length L of the nip zone and the roller peripheral velocity.

The length L of the nip zone is : <BR> <BR> <BR> <BR> <BR> a D-a<BR> <BR> <BR> 2. 2 Thus, the time T for the passage of the material through the compaction zone will be : L D a T =-=-.- V V 2 Where V Roller peripheral velocity The frequency F is 1/T.

Based on the above equations, the frequency F can be expressed as : where V roller peripheral velocity D roller diameter H grinding bed thickness density of pressed cakes bulk weight of feed to roller press Assuming that the excitation frequency is the frequency achieved when the grinding bed thickness H is increased to a maximum, the above-mentioned equation can be used to estimate the excitation frequency for a given roller diameter at a given roller velocity.