ROLLER MILL FOR GRINDING OF PARTICULATE MATERIAL

Description

[0001] The present invention relates to a roller mill for grinding of particulate material, said roller mill comprising at least one grinding roller which via at least one piston can be thrust against a grinding surface by means of at least one hydraulic cylinder being separately connected via pipe connections to at least two accumulators.

[0002] Roller mills of the aforementioned kind comprise, for example, vertical mills which are known, for example, from Danish patent No. 150818, ring roller mills known, for example, from the Danish patent application No. 748193 and roller presses known, for example, from international patent No. WO 92/13 639.

[0003] In roller mills the grinding of particulate material takes place according to a method whereby a grinding roller is subjecting a bed of material to compaction. In vertical mills the material is distributed across a rotating grinding table and directed by virtue of the rotation of the grinding table to a grinding gap between the grinding table and a stationary, rotating grinding roller which is rolling upon the grinding table. In ring roller mills the material is deposited on a rotating grinding ring and directed by virtue of the latter's rotation to a grinding gap between the grinding ring and a stationary, rotating grinding roller which is mounted inside the grinding ring, and which is rolling hereon. In roller presses which typically comprise two oppositely rotating grinding rollers, the material is led by virtue of the force of gravity into a grinding gap between the two grinding rollers. In order to generate a desired grinding pressure, the grinding roller - for roller presses most frequently just one of the rollers - is connected to, for example, a hydraulic tensioning system which comprises one or several hydraulic cylinders, depending on the design of the grinding system. To ensure appropriate cushioning of the roller system, the hydraulic cylinders are normally connected to one or several gas accumulators. For practical reasons, the hydraulic cylinders and the gas accumulators are normally connected via pipe or duct connections. It is established practice to use one or several identical gas accumulators for each hydraulic cylinder. If several gas accumulators are used for each hydraulic cylinder, the precharging pressure in the gas accumulators will normally be the same.

[0004] For all types of roller mills it is essential to ensure presence of an even and stable grinding bed in order to ensure trouble-free and steady-state operation. Roller mill installations normally comprise vibration equipment which will monitor and cut out the mill if the vibration level exceeds a predetermined level.

[0005] The vibrations in roller mills may be ascribable to several different circumstances. They may be due to operation with materials which exhibit variations in terms of fineness and/or quantity, which will often give rise to high vibration levels due to irregularity of operation. The vibrations may also be ascribable to the fact that the grinding roller is operating directly on the grinding table or it may be due to the presence of large lumps of material, which would involve substantial accelerations of the grinding parts. Such vibrations will not normally have any characteristic frequency spectrum, unless, for example, a fixed frequency is involved, corresponding to the number of segments, feed frequency or the like. Usually, the frequencies involved for such vibrations will be at a relatively low level.

[0006] Observations have also been made of another type of vibration which is due to intrinsic oscillations in the roller system, and, quite often, such vibrations are at a considerably higher level. These vibratory oscillations may be linked to a very narrow frequency range within the range 5-25 Hz dependent on mill type and size. For the individual mill, the frequency involved will also be so narrow that quite often it will only deviate 1-2 Hz from occurrence to occurrence. Any such vibration event often occurs in connection with changes in the operation of the roller mill, caused for example by an increase in the fineness rate, changes in the grinding bed thickness or similar changes.

[0007] It is the objective of the present invention to provide a roller mill by means of which vibrations which are due to intrinsic oscillations in the roller system are eliminated or at least substantially reduced.

[0008] This is achieved by a roller mill of the kind mentioned in the introduction, being characterized in that at least one accumulator is designed for a lower eigenfrequency, whereas at least one accumulator is designed for a higher eigenfrequency viewed in relation to a characteristic eigenfrequency of the roller system.

[0009] The aforementioned eigenfrequency for the accumulators refers to a vibratory system which consists of an equivalent mass for the oil in the pipe between the hydraulic cylinder and the accumulator and a spring defined by the precharging pressure in the accumulator. The terms "lower" and "higher" eigenfrequency must be interpreted in relation to a characteristic eigenfrequency for the roller system, at which, during operation, the roller system may be subjected to resonance vibrations which are so severe that it will normally be necessary to stop the roller mill in order to prevent structural damage.

[0010] As a result hereof, it will be possible to prevent mill vibrations which are caused by intrinsic oscillations in the roller system due to the fact that any attempt by the roller to oscillate at the characteristic eigenfrequency of the roller system will cause it to be locked hydraulically, while the roller will retain its free movability at other frequencies. The elimination of such oscillations will, at the same time, make it possible to prevent the substantial mechanical load rates on the various mill components, thereby increasing their durability. Also, a stoppage of mill can be avoided, thereby ensuring a general optimisation of the operation. This means that it will be possible to operate the mill on a continuous basis, thereby widening the range of options for varying the operating parameters, such as capacity and fineness.

[0011] In the following, the invention is described in further details with reference to the drawing, the only figure of which is a schematic illustration of a roller mill.

[0012] In the figure is seen a roller mill 1 which comprises a grinding roller 3 and a grinding surface 5 which, dependent on the type of mill, may be made up of a grinding table, a grinding ring or an additional grinding roller. In the gap between the grinding roller 3 and the grinding surface there is a draw-in of the material to be ground in the form of a grinding bed which is shown schematically by means of a spring 4. The grinding roller 3 is connected via a piston 6 to a hydraulic cylinder 7 which in turn is connected via pipe connections 8 and 10 to two gas accumulators 9 and 11.

[0013] The roller system itself which comprises the grinding roller 3, the oil in the pipes 8, 10, the cylinder 7 and the accumulators 9, 11 can normally be regarded as a total mass in a vibratory system in which the spring is made up partly by the grinding bed 4 and partly by the gas in the accumulators 9, 11. Viewed in relation to the physical movement of the grinding roller 3, the oil masses in the cylinder-, accumulator- and pipe systems will normally migrate at different velocities, being a function of the dimensions for the transmission ratio between the cylinder 7 and the grinding roller 3 and of the ratio between the diameters of the individually incorporated components in the hydraulic system. In systems involving use of long pipes with small cross-sectional areas, the acceleration of the ingoing oil masses will, subject to an acceleration of the grinding roller, be greater than that occurring in systems with short pipes with considerable cross-sectional areas. So, the equivalent masses of oil and components may thus in many instances constitute a significant part of the total equivalent mass of the vibratory system.

[0014] The grinding bed 4 in the vibratory system can be considered to be a spring. This is due to the fact that a uniform bed of material which is impacted by the roller will be more or less compacted, dependent on the grinding force between the grinding roller 3 and the grinding surface 5. If a grinding roller 3 is impacting a uniform bed of material at a given grinding force, a reduction in the force of the grinding roller will lead to an increase in the thickness of the grinding bed 4. Similarly, an increase in the grinding force will lead to a reduction in the thickness of the grinding bed 4. Due to the close correlation between the change in the grinding bed thickness and the applied grinding force, the grinding bed 4 can be considered to be a spring with a given spring constant. However, the calculated spring constant will depend on the initial thickness of the grinding bed 4 since a substantial initial thickness of the grinding bed 4 will normally result in low spring rigidity and conversely. This is due to the fact that the compression function expressed by the grinding bed thickness as a function of the grinding force subject to compaction of material is not linear, but exponential.

[0015] It is also evident that a reduction in the grinding pressure will only result in a corresponding increase in the grinding bed thickeness when the mill is in operation, with fresh material being supplied to the roller gap. If the mill is at standstill, the elimination of the load exerted upon the roller will only lead to an increase in the grinding bed thickness which corresponds to the elastic springback. Within "minor" variations in the grinding bed thickness, the system may, however, be considered to be approximately linear. The addition of fresh material to the grinding gap must be uniform and of sufficient size to allow any change in the grinding pressure to produce a given change in the grinding bed thickness.

[0016] The pressurized gas in the accumulators will constitute the spring or the other springs incorporated in the vibratory system. However, the rigidity of the accumulators will normally be negligible viewed in relation to the rigidity of the grinding bed.

[0017] Theoretically, the mass of the vibratory system can therefore be considered to comprise the grinding roller with associated components as well as the equivalent oil mass, whereas the spring may be considered to consist exclusively of the grinding bed.

[0018] The eigenfrequency for such a simple vibratory system will be as follows:


where k is the spring rigidity and m is the mass.

[0019] During mill operation, the vibratory system may be made to oscillate by means of an arbitrary impulse triggered, for example, by an irregularity in the grinding bed. If the system is largely undamped, i.e. if it consists solely of a mass with a spring it will be able to maintain the oscillations at its eigenfrequncy during prolonged periods of time. Since the grinding bed has only a certain degree of compressibility, this will impose a restriction on the oscillating amplitude.

[0020] If for a given mill modifications are made to the accumulator system, for example by changing the cross-sectional areas of the pipe connections and/or the pipe lengths, this will also lead to changes of the equivalent mass, and thus of the eigenfrequency of the vibratory system. For a known mill which is accessible to the applicant of the present patent application, it has thus been ascertained that an eigenfrequency of 17 Hz for the vibratory system will be changed to an eigenfrequency of 14 and 8 Hz, respectively, if the accumulator system is changed to a gradually increasing oil equivalent mass.

[0021] Regardless of the eigenfrequency of the vibratory system, the vibrations at the preset eigenfrequency may reach a level which is so high that it will necessitate a stoppage of the mill. The vibrations at the preset eigenfrequency may be merely a number of short oscillations, but may also extend over a prolonged vibratory period without any tendency towards damping.

[0022] In normal mill operation, minor variations in the grinding bed thickness will trigger equivalent changes in the pressure of the hydraulic cylinder and accumulators. This is the underlying basis of a spring system. In the situations where vibrations occur at the eigenfrequency of the vibratory system it has, however, been ascertained that there is a counter-phase between the pressure in the cylinder and on the gas side of the accumulator. This is due to the fact that even the equivalent oil pipe masses and the spring rigidity of the gas can be considered to be a vibratory system with an eigenfrequency in instances where the system comprises several accumulators which are connected in parallel to the cylinder. Calculations have demonstrated that the eigenfrequency of the single accumulators will normally be lower than the eigenfrequency of the vibratory system comprising the grinding roller.

[0023] To ensure effective locking of the roller at a given eigenfrequency, the eigenfrequency of each accumulator must be adjusted so that a change in the pressure at the given frequency will not generate a net oil flow from the hydraulic cylinder to the accumulators, and this requirement will be met when the oilflow to and from the single accumulators has been balanced in such a way that they only just cancel each other out. In other words, the amount of oil in the hydraulic cylinder must be kept constant at the indicated eigenfrequency.

[0024] The adjustment of the accumulators may be done by setting at least one accumulator so that its eigenfrequency is lower than the characteristic eigenfrequency of the roller system, whereas at least a second accumulator is set to have an eigenfrequency which is higher than that characteristic eigenfrequency. Hence, there will be a phase difference between the (at least) two accumulator systems in terms of pressure and movement. If the (vibration) movement is assumed to be undamped, the (at least) two accumulator systems will in accordance with the general theory of vibrations be in direct counter-phase. As a consequence hereof, an increase in the pressure in the hydraulic cylinder will cause the oil from the hydraulic cylinder to flow to the accumulator(s) the eigenfrequency of which exceeds the eigenfrequency of the roller system, while, at the same time, oil from the accumulator(s) with an eigenfrquency which is less than the eigenfrequency of the roller system will flow from these accumulators into the hydraulic cylinder.

[0025] An appropriate adjustment or selection of the accumulators will ensure that, subject to a pressure variation in the hydraulic cylinder at the characteristic eigenfrequency of the roller system, the accumulators will cancel each other out in respect of the oil flow to and from the hydraulic cylinder, so that there will only be a mutual flow between these accumulators. This can be achieved by dimensioning the accumulator system in the way described below.

[0026] For a simple vibratory system without damping, the dynamic amplitude for the movement can be calculated on the basis of the following formula:

where

and where
Fo = force amplitude
m = mass


where k is the spring rigidity of accumulator and m is the mass
Ω² = the square of the angular velocity of the applied frequency

[0027] Using as a reference point the fact that the amplitude must be the same for the two accumulator systems with two different eigenfrequency ratings, we have:

[0028] Expressed in terms of spring rigidities and masses, or alternative frequencies, we will instead have:

or

Example:

[0029] At a characteristic eigenfrequency f_o = 17 Hz and f₁ = 10 Hz for one accumulator system with a low eigenfrequency, we have:
f₂ = 21.8 Hz for the second accumulator system.

[0030] If, for example, the same equivalent masses m₁ = m₂ are used for the two accumulator systems, the relationship between the rigidities of the accumulators can be calculated as follows:

[0031] This difference in spring rigidity is attained at different precharging pressures in the two accumulator systems since the substantial rigidity k₂ is obtained at a low precharging pressure.

[0032] Also, different masses can be selected for the two systems, and, therefore, this will lead to a change in the relationship between the rigidities of the two accumulator systems.

[0033] Normally, the small rigidity k₁ is obtained at a normal precharging pressure for operation, where, for example, the precharging pressure is within the range 25-60 per cent of the working pressure. Of course, due consideration must be given to the required accumulator volume for the normal operation, and, therefore, an increase in the total volume will normally be required.

[0034] So, with the selected accumulator configuration it will be possible to obtain a counter-phase between the two accumulator systems at the characteristic eigenfrequency, thereby locking the very piston movement.

Claims

1. A roller mill (1) for grinding of particulate material, said roller mill comprising at least one grinding roller (3) which via at least one piston (6) is capable of being thrust against a grinding surface (5) by means of at least one hydraulic cylinder (7) being separately connected via pipe connections (8, 10) to at least two accumulators (9, 11), characterized in that at least one accumulator is designed for a lower eigenfrequency whereas at least one accumulator is designed for a higher eigenfrequency viewed in relation to a characteristic eigenfrequency of the roller system.

Ansprüche

1. Wälzmühle (1) zum Schleifen von partikelförmigem Material, wobei die Wälzmühle mindestens eine Schleifwalze (3) umfasst, welche mittels mindestens eines Kolbens (6) gegen eine Schleifoberfläche (5) mit Hilfe von mindestens einem hydraulischen Zylinder (7) geschoben werden kann, der über Rohrverbindungen (8, 10) mit mindestens zwei Akkumulatoren (9, 11) separat verbunden ist, dadurch gekennzeichnet, dass mindestens ein Akkumulator für eine niedrigere Eigenfrequenz ausgelegt ist, wobei mindestens ein Akkumulator für eine höhere Eigenfrequenz ausgelegt ist, betrachtet in Bezug auf eine charakteristische Eigenfrequenz des Wälzsystems.

Revendications

1. Broyeur à cylindres (1) pour le broyage de matériau particulaire, ledit broyeur à cylindres comprenant au moins un rouleau de broyage (3) qui, par l'intermédiaire d'au moins un piston (6), est apte à être poussé contre une surface de broyage (5) au moyen d'au moins un cylindre hydraulique (7) étant relié de manière séparée par l'intermédiaire de raccords de tubes (8, 10) à au moins deux accumulateurs (9, 11), caractérisé en ce qu'au moins un accumulateur est conçu pour avoir une fréquence propre inférieure à une fréquence propre caractéristique du système à cylindres, alors qu'au moins un accumulateur est conçu pour avoir une fréquence propre supérieure à une fréquence propre caractéristique du système à cylindres.

Drawing