A METHOD OF FINELY CRUSHING PARTICLES OF MATERIAL IN A CENTRIFUGAL MILL AND APPARATUS FOR PERFORMING THE METHOD

Abstract: A method and an apparatus for finely crushing particles of material in a centrifugal mill or the like, wherein at least one rotor (13) projects the particles against stationary impact surfaces (20) in the mill, the comminution taking place substantially in vacuum. To obtain fractions with very small grains (below 10 mu ) with a low energy consumption the particles are accelerated in vacuum to such a high impact energy against the impact surface(s) (14) of the rotor (13), that a first crushing will take place before the particles are projected against the stationary impact surfaces (20) of the mill.

Description: A method of finely crushing particles of material in a centrifugal mill and apparatus for performing the method The present invention relates to a method of finely crushing particles of material in a centrifugal mill or the like, where in at least one rotor projects the particles against stationary impact surfaces in the mill, the comminution taking place substantially in vacuum. The invention also relates to an apparatus for performing the method. Background of the invention Mechanical comminution of solid particles may be classified according to the manner of operation of the apparatus that performes the crushing. Crushing or milling may be performed according to two main principles either with pressure that ruptures the particles, as for instance by means of jaw crushers, pendulum mills, tower mills, roller mills and the like, or with kinetic energy that causes bursting of the particles. Typical examples of the latter category are hammer mills, pin mills and jet mills. There is a great demand on the market for finely crushed material especially in the filler industry as fillers e.g. in plastics and paper industry. Another large product area is finely milled quartz and feldspar for the ceramic industry. Todays technology for producing fine grained materials are very energy and cost demanding, especially when narrow tolerances in grain size and particle sizes below 30_a are desired. Sizes down to 45_ ι may be obtaine with ball mills, but since the grinding balls usually consist of iron it is not possible to obtain iron-free milling. This type of milling therefore is not useable for materials which are to be used as whitening agents in the paper and plastic industries, since even small proportions of iron destroys the whiteness. In order to arrive at OMPI particle sizes of 20 /_, either conventional milling and air stream separation or a jet mill is used. The problem with air-separated material is that too much of too coarse fractions will be included, which is mostly not acceptable. With jet mills it is possible to arrive at particle sizes around 10 i and even smaller but the jet mill has a low efficiency and a very high energy consumption (350-700 kWh/ton) . The amount of energy consumed in crushing by means of kinetic energy or impact energy may be divided into three main groups, viz. energy which is consumed for elastic deformation of the grains or particles and which is lost, supplied energy which bursts the grains, and energy for the operation of surrounding equipment etc. At low velocities, a substantial portion of the supplied energy will be used up for the elastic deformation of the particles, which gives a low efficency. Furthermore, the elastic deformation will cause the particles to bounce instead of bursting which will cause heavy wear. At high velocities, on the other hand, the energy consumed for elastic deformation amounts only to a small portion of the total energy, whereas the high impact energy, if it can be applied as "instantaneously" as possible, gives very high stress concentrations and provides an efficient crushing. This means that a high particle velocity and instantaneous impact force should be chosen. The jet mill which has proved to be especially suitable for crushing or milling into fractions with very small grains utilizes a high particle velocity and uses pressurized air for accelerating the grains to about 100 m/s, and the high velocity grains collide with other grains resulting in the grains bursting each other. However, due to the very high energy consumption per ton, low efficiency and very high production costs, the jet mill has so far only been used in "exclusive" connections, for instance in the chemical industry and the pharmaceutical industry. It has also been suggested (German patent specification 387,995) to use disintegrators or stamp mills, the milling tools of which operate in an air-void space in order to obtain a fine grinding, preferably together with a dispersing agent. The object and important characteristics of the invention The object of the invention is to provide a comparatively simple method which, while involving a low energy consumption, may provide fractions with very small grains (below 10 -) . This object has been attained in that the particles are accelerated in vacuum to such a high impact energy against the impact surface or surfaces of the rotor that a first crushing takes place before the particles are being projected against the stationary impact surfaces of the mill. The invention also relates to an apparatus for comminuting solid particles of material and which is characterized by at least one passive or active accelerator operating in vacuum, provided opposite to the rotor and designed to impart to the particles such a high impact energy against the impact surface(s) of the rotor, that a first crushing will take place before the particles are projected against the stationary impact surfaces of the mill. Description of the drawings Fig. 1 is a diagrammatic section through a centrifugal mill according to the invention and intended for laboratory purposes. Fig. 2 is a section taken on the line II-II in fig. 1. Fig. 3 is a partly broken side view of a centrifugal mill with twin rotors according to another embodiment of the invention. Fig. 4 is a section taken on the line IV-IV in fig. 3. Fig. 5 is a side view of a complete installation for the recirculation of the particles of material. The centrifugal mill according to the invention consists of a housing 11 which may be sealed airtight and which contains a crushing chamber 12 wherein there is rotatably arranged a rotor 13 which is provided at each end with an impact surface 14. The rotor is journalled in a heavy bearing 15 and is driven by a motor 16 by means of a suitable transmission 17. Opposite to the path of movement of the impact surfaces 14 there is provided at least one supply channel 18 through which particles of material that are to be finely divided are supplied intermittently. The opening 19 of the supply channel 18 in the housing 11 is provided at some distance from an impact surface 20 which is oriented in such a way that it extends substantially in parallel with the rotor impact surface 14 when this is disposed below the outlet opening 19. In the embodiment shown in Figs. 1 and 2, the impact surface 20 is disposed outside of the path of rotation of the impact surface 14. Any particles projected to the side of the impact surface 20 will be arrested by the following impact surfaces 20' and 20". Below the impact surfaces there is provided, in the bottom 21 of the crushing chamber, an outlet opening 22 through which the material which has been projected against the impact surfaces 14 and 20 and crushed thereby leaves the crushing chamber 12. The outlet opening 22 may be connected to a reσirculation system of the kind illustrated in Fig. 9 which returns the particles to the supply channel 18. The outlet opening may also be connected to a feeding out device (not shown) for batchwise removal of the crushed material from the apparatus. By the aid of a passive or active accelerator 28, the particles of material are given a very high impact velocity against the rotor impact surface or surfaces 14. The "passive" accelerator consists of a vertical tube 18 included in the air-void system and having such a length (for instance 3 m), that the impact velocity against the impact surface 14 of the rotor together with the rotor's own velocity will be so large, that the particles will be crushed a first time at this impact and the second time when they are projected against the stationary impact surfaces. By the aid of a device not shown a vacuum is maintained in the accelerator 28, the crushing chamber 12 and spaces communicating therewith, so that the crushing chamber is practically air-void (the air should be evacuated to at least 90%) . Thanks to this arrangement the rotor 13 with the impact surfaces 14 will not provide any fan action, and interfering shock waves and uncontrolled turbulency is avoided. The particles instantaneously hit by the impact surface will be crushed by the impact against the surface and will be projected tangentially out of the path of movement of the rotor and against one or several stationary impact surfaces 20 where further crushing takes place. By means of a dosing device 23 provided with a rotor 24 a suitable amount of particles of material is fed to the impact surfaces 14 synchronously with their passing of the outlet opening 19. Practical tests with a laboratory mill has shown that it is feasible to give the rotor a peripheral velocity of 180 m/s. However, for commercial operation, the upper practical limit for the peripheral velocity of the rotor would probably be about 400 m/s. In order to accelerate one ton of particles to a velocity of 180 m/s, 4,5 kWh are consumed, and at four circulations the theoretical energy consumption would be 36 kWh/ton. To this is added the energy consumed for recirculation and for the operation of the vacuum pump. Since the crushing takes place in vacuum and at a high velocity of motion the losses will be moderate compared to current technology. The embodiment of Figs. 3 and 4 differs from the above- described embodiment in two important respects: two or more rotors are arranged in parallel one above the other, and the particles are supplied to the first crushing chamber 12 with such a high kinetic energy, that a maximum amount of OMPI crushing material is supplied for each "beat". The necessary- kinetic energy (which, however, is only about 1/10 of the rotational energy of the impact surface) is obtained by means of an "active" accelerator 28 which, as an example, may consists of a centrifuge, the blades of which are preferably rubber coated in order to reduce wear. In the centrifugal mill according to this embodiment the impact surfaces 20 are preferably oriented in such a way, that the particles will fall towards the next rotor 13a which is provided directly below and close to the rotor 13. In order to obtain a high capacity, a supply channel 18 is provided opposite to every other impact surface 20. The impact surfaces 20 have a V-shaped cross-section whereby it will be possible to utilize the second leg of the V by reversing the motor. In this embodiment the two crushing chambers 12 and 12a as well as the collecting chamber 26 and the metering accelerator 28 are connected to one and the same vacuum system. Fig. 5 shows a recirculating device for the centrifugal mill according to the invention and comprising two elevators 29 and 30, a silo 31 and a metering accelerator 28. In this case a practically air-void condition is maintained in the complete installation by means of a pump 32 or the like. The . invention is not limited to the embodiments shown and described since several variations are conceiveable within the scope of the claims. ..OMPi jr., WwI_P_Oυ _

Claims: CLAIMS 1. A method of finely crushing particles of material in a centrifugal mill or the like, wherein at least one rotor (13) projects the particles against stationary impact surfaces (20) in the mill, the comminution taking place substantially in vacuum, c h a r a c t e r i z e d i n that the particles are accelerated in vacuum to such a high impact energy against the impact surface(s) (14) of the rotor (13) , that a first crushing will take place before the particles are projected against the stationary impact surfaces (20) of the mill. 2. A method as claimed in claim 1, c h a r a c t e r i z e d i n that the particles are supplied to the rotor (13) from an accelerator (18) in a measured quantity and at a point where the or each rotor impact surface (14) is substantially parallel with the nearest following stationary impact surface (20) in the direction of rotation of the rotor. 3. A method as claimed in claim 1 or 2, c h a r a c t e r i z e d i n that at least a portion of the particles, after having passed the crushing apparatus, is transported in vacuum via a transport system (29,30) to the accelerator and the crushing apparatus. 4. An apparatus for the comminution of particles of material in a centrifugal mill or the like, and comprising at least one rotor (13) having at least one impact surface (14) projecting radially therefrom, stationary impact surfaces (20) in the mill outside the path of rotation of said rotor impact surface, arid means (32) for generating a vacuum in the mill at least one passive or active accelerator (28) operating in vacuum, provided opposite to the rotor (13) and designed to impart to the particles such a high impact energy against the impact surface(s) (14) of the rotor, that a first crushing will take place before the particles are projected against the stationary impact surfaces (20) of the mill. 5. An apparatus as claimed in claim 4, c h a r a c t e r i z e d i n that at least one dosing device (23) is provided for the controlled supply of the material to at least one position above and opposite to the path of rotation of the rotor impact surface(s) (14). 6. An apparatus as claimed in claim 4 or 5, c h a r a c t e r i z e d i n that the "passive" accelerator (28) consists of a vertical, air-void tube (18) of such a length that the particles will attain, by free fall, the impact velocity necessary for their crushing. 7. An apparatus as claimed in claim 4 or 5, c h a r a c t e r i z e d i n that an "active" accelerator (28) is connected to the inlet of the crushing chamber (12) , said accelerator being adapted, by means of externally supplied energy, to project the particles with increased kinetic energy towards the rotor impact surface(s) (14).

Inventors: BJOERCK CONNY, SIGURDSSON PER ARNE
Applicants: BJOERCK CONNY, SICOMANT AB, SIGURDSSON PER ARNE
Application year: 1982
Main classification: B02C13
IPC codes: B02C13