EP0074633A2 - Method of operating a ball mill, and such a ball mill - Google Patents

Abstract

1. An agitator ball mill comprising a mill container (12) surrounding a mill space (34) in which at least one agitator element (32) rotatable relative to the mill container (12) is arranged, and further comprising at least one lock means (60) in a line (68) for feeding milling balls (36) from a reservoir (64) into the mill space (34) during operation, characterized in that a valve (70) and upstream of if a nozzle means (74, 78) are provided in the line (68) between the reservoir (64) and the mill space (34) through which a fluid may be discharged under pressure in the direction of the desired ball flow to transport the milling balls (36) into the mill space (34).

Description

The invention relates to a method for operating a ball mill, in particular for the digestion of biomass, in which the ground material is fed to a grinding chamber containing grinding balls and is ground in the grinding chamber by the rotary movement of at least one rotating grinding body and the resulting movement of the grinding balls.

The invention further relates to a ball mill which is particularly suitable for carrying out the method and has a grinding container surrounding a grinding chamber in which at least one grinding body which is rotatable relative to the housing is provided.

Ball mills have become increasingly important in a number of industrial processes in recent years. An example of this is the paint industry, where it is particularly important to distribute the pigments in the paints as evenly as possible in order to achieve a good dispersion effect.

Another example of the advantageous use of ball mills is the disintegration of biomass from fermentative processes for the production of enzymes in biochemical plants. Here it comes first of all to achieve the best possible digestion of the cells of the biomass, ie to release as high a percentage of the cell content as possible, such as enzymes, in order to be able to isolate them in the subsequent further processing steps.

The effects of ball mills in general and in particular the digestion of biomass are influenced by a number of different parameters. For a given mill size, this includes in particular the stirring speed, the throughput and the size of the grinding balls used, which generally consist of glass, metal or aluminum oxide.

In a special embodiment, the gap width between the grinding element and the grinding container is also important for the grinding effect. In DE-OS 28 11 899, a split ball mill is therefore proposed, in which the gap width can be changed in that the distance between the grinding element and grinding container is fixed by means of an interchangeable spacer ring, which can be exchanged between the grinding processes by the gap width adapt to the respective requirements.

Another important influencing variable is the degree of ball filling, which is generally defined as the ratio between the actually used and the maximum possible ball filling quantity. In principle, an increased degree of ball filling generally leads to an improvement in the grinding effect. However, it should be noted that increasing the degree of ball filling also causes problems, that set limits. The wear on the mill and on the balls increases with the degree of filling. Energy consumption increases because the grinder rotates harder. In connection with this is the corresponding increase in the frictional heat in the ball mill, which can lead to cooling problems. Finally, especially in the case of biomass, an optimal digestion can be achieved at which a maximum digestion
is achieved, but at the same time the cells are not damaged so much that the subsequent processing is thereby made more difficult.

In the known ball mills, the degree of filling can be determined before the start of a grinding process by filling in a precisely defined amount of balls. However, this degree of filling is not retained during the grinding process. Rather, it changes due to the wear of the balls and the wall and stirring parts in contact with them. In a ten-hour long-term test, wear of 2.5% was found, for example. On the one hand, this changes the grinding behavior of the ball mill during the grinding process. Perhaps more importantly, however, there is no possibility in the known ball mills or in the known methods of operating ball mills to vary the degree of ball filling during the operation of the ball mill in such a way that the optimum operating conditions are achieved which vary greatly depending on the product .

The object of the present invention is therefore to provide a method for operating a ball mill and a corresponding ball mill with which a uniform and optimized grinding action is achieved during the entire grinding process.

This object is achieved in a method of the type mentioned at the outset by regulating the degree of ball filling in the grinding chamber during the grinding process in order to adapt to the respective operating conditions.

The device according to the invention of the type mentioned at the outset is characterized by means by which the degree of ball filling in the grinding chamber is controlled during operation of the ball mill and can be changed continuously.

While in the known ball mills a relatively low ball filling level of maximum 80% had to be used due to the above-mentioned limitations, it has surprisingly been found that a control and regulation of the filling level taking place during operation, i.e. without interrupting the grinding process, significantly higher ball filling levels and so that improved grinding properties can be achieved without excessive wear and tear or the ground material and damaged being heated and damaged too much. For example, the grinding process can be controlled so that a relatively low degree of ball filling is used in the start-up phase of the mill that the starting resistance is relatively easily overcome by the mill motor. Then the fill level is slowly increased to the optimum fill level for normal operation.

The regulation of the degree of filling can, as in the example described above, be designed so that the degree of filling is varied during the operation of the ball mill. According to a particularly preferred embodiment of the method according to the invention, however, the ball mill is operated in such a way that the degree of ball filling is kept essentially constant during most of the grinding process. In the context of the present invention, it was found that the area of optimal ball filling, in particular in the disintegration of biomass, is relatively narrow. For example, a change in the ball filling level of 5 can result in a 20% change in the enzyme content of the product emerging from the ball mill. If you compare this numerical value with the above-mentioned fact that a 2.5% wear on the ball filling and mill is possible after ten hours of operation and with the additional requirement that the ball mill should be operated continuously for as long as possible without interruption, you can see That a longer operation in the optimal range is only possible if the filling volume which is reduced due to wear is compensated in such a way that the degree of ball filling remains constant.

The degree of ball filling can be controlled manually. However, it is particularly preferably carried out automatically, with the power according to a preferred embodiment Inclusion of the ball mill motor is used as a measure for controlling the degree of ball filling. It has been found that the load on the motor and thus its power consumption depends very much on the degree of ball filling. With a certain ball mill and a certain type of regrind (and otherwise the same boundary conditions as temperature etc.), the current consumption of the motor can be clearly assigned to the ball filling level.
The current consumption fluctuates, for example, by 20% in the limit range which is essential for an optimal grinding effect in a particular ball mill if the ball filling is changed by only 1%. On the basis of this knowledge, it is sufficient to determine the power consumption of the motor corresponding to a certain degree of filling for a specific application, in order then to use this value as a setpoint for keeping the ball filling degree constant.

According to a preferred embodiment of the method according to the invention, the control of the degree of ball filling according to the invention can be accomplished by adding or removing grinding balls during the grinding process. For this purpose, only relatively minor measures are necessary on the corresponding ball mill, so that existing ball mills can also be relatively easily converted to this method. Balls can be added in relatively small quantities, so that sensitive dosing is possible. A removal of balls must be provided, in particular, if operating states can occur in which the degree of ball filling, for example due to momentary overloading of the machine, is to be reduced relatively quickly. It is particularly advantageous if the device for removing balls from the same control device. how to control the addition of balls, which enables fast and accurate control of the ball filling level according to the respective operating conditions.

Another preferred method proposal is to be used in particular in connection with ball mills specially designed for this purpose and proposes to control the volume of the grinding chamber during the grinding process in such a way that the desired control of the degree of ball filling is achieved. Such a solution allows a particularly quick reaction to the prevailing conditions by a correspondingly rapid change in the grinding chamber volume and thus the degree of filling.

• Figur 1 eine erfindungs-gemäße Kugelmühle mit Einrich_- tungen zur Zugabe und Entnahme von Kugeln während des Mahlvorganges im Schnitt;
• Figur 2 eine Ausführungsform der erfindungsgemäßen Kugelmühle mit einem mit Hilfe eines Balges variablen Mahlraum im Schnitt;
• Figur 3 eine erfindungsgemäße Kugelmühle mit zwei zwecks Variation des Mahlraumvolumens gegeneinander verschiebbaren Gehäuseteilen im Schnitt.

The invention is explained in more detail below with reference to an embodiment shown in the drawing, the advantages of the device according to the invention also being discussed in more detail. Show it:

• Figure 1 is a fiction, modern-ball mill with Einrich _- obligations to the addition and withdrawal of balls during the grinding process in section;
• Figure 2 shows an embodiment of the ball mill according to the invention with a grinding chamber variable with the aid of a bellows in section;
• 3 shows a ball mill according to the invention with two housing parts which can be displaced relative to one another for the purpose of varying the grinding chamber volume.

1 shows the essential parts of a ball mill 10, the parts which are not essential for the invention, such as the machine stand, the drive motor and the operating parts, being omitted in the cross section shown for reasons of clarity.

The part shown consists essentially of the grinding container 12, which is designed in the ball mill shown here as a circular cylinder, which is closed with a base plate 14 on one side. On the side of the grinding container 12 opposite the base plate there is the grinding container end on the motor side, which in its entirety is provided with the reference number 16. It essentially consists of a grinding chamber cover 18 which is screwed via a connecting flange 20 to a corresponding flange 22 with screws not shown in the drawing. A seal 24 is located between the two flanges. A shaft bushing 26 is provided in the center of the grinding chamber cover 18, which is not described in detail here, since it can completely correspond to the bushings used in the known ball mills.

With the drive shaft 28 penetrating the shaft bushing 28, the agitator shaft 30 is connected, which carries agitator disks 32 and can rotate in the grinding container 12 driven by the motor, not shown.

The grinding container 12 and the agitator shaft 30 with the stirring disks 32, which can also be generally referred to as entrainment bodies, limit the grinding chamber 34, which in operation contains the grinding balls 36, preferably of about 1-3 mm in diameter, and the material to be ground 38.

The grinding container 12 is surrounded by a cooling jacket 40 with two connecting pieces for cooling liquid 44 and 46.

A product inlet 48 is provided for feeding the regrind, and the regrind flows after the grinding process from the separation gap 50, the ring channel 52 and the product outlet 54. The separating gap is formed by a separating disk 53 rotating with the agitator shaft 30 and a stationary separating disk 55 and is so narrow that the balls conveyed on with the product cannot pass through it and thus remain in the grinding chamber 34.

In this respect, the ball mill shown here corresponds to one of a number of known ball mill types and other types could also be used for the invention. The peculiarity of the ball mill shown is that there are devices for the continuous and controlled change of the ball filling level in the grinding chamber, a lock device 60 for adding and a drain valve 62 for removing grinding balls being provided in the embodiment shown in FIG.

The lock device 60 comprises a storage container 64 which is connected to the grinding chamber 34 via a line 68. In the case shown, the product inlet 48 is connected to line 68. On the side of the line 68 facing away from the grinding chamber 34 there is a valve which, in the embodiment shown, is designed as a rubber bellows valve 70 which can be actuated with compressed air via a line 72. A nozzle device 74 with a valve 76, which can be a conventional flap valve, and an injector nozzle 78, which is arranged in the center of the storage container 64 and directed towards the line 68, opens into the storage container 64. The reservoir 64 can be closed tightly with a lid 80.

The entire lock device is designed to be pressure-stable to such an extent that it can easily accommodate the pressures occurring in operation in ball mills of, for example, 0.5 bar to 2 bar.

The drain valve 62 is connected via a line 82 to the grinding chamber 34 and also has a compressed air line 84 for actuating the rubber bellows not shown in the figure in the valve 62. The compressed air lines 84 and 72 are connected to the same control device 73, which controls the function of the lock device 60 and the drain valve 62.

• . der Flußrichtung der Mahlkugeln 36 durch die Injektordüse 78 injiziert. Dadurch wird ein hinreichend schnelles und gut dosierbares Zugeben von Mahlkugeln und damit bei konstantem Volumen des Mahlraums 34 eine Erhöhung des Kugelfüllgrades erreicht.

The degree of ball filling of the ball mill shown in FIG. 1 is now checked during operation, ie while the agitator shaft 30 is rotating, in that, as required, 60 grinding balls are fed in via the lock device or 62 grinding balls are discharged via the drain valve. For this purpose, the reservoir 64 is filled with grinding balls and sealed airtight with the lid 80. If the degree of filling is now to be increased, the valve 76 of the nozzle device 74 and the rubber bellows valve 70 are preferably opened at the same time. Since in the preferred embodiment shown the storage container 64 is arranged vertically above the grinding chamber 34, the balls fall down into the grinding chamber. To accelerate this process, a liquid or a gas is particularly preferred, but not necessarily, through the nozzle device 74

• . the direction of flow of the grinding balls 36 injected through the injector nozzle 78. A sufficiently quick and easily metered addition of grinding balls and thus an increase in the ball filling degree with a constant volume of the grinding chamber 34 are achieved.

The rubber bellows valve 70 is particularly well suited for this purpose, since it still closes reliably even with the contamination with the grinding balls 36, which contamination can hardly be avoided here. The product inlet 48 can also be provided separately from the line 68. The actuation of the valves 76 and 70 and thus the supply of grinding balls can be carried out both manually and automatically in the device according to the invention. In the automatic embodiment, corresponding actuators and an electronic circuit are provided, by means of which the valves are actuated when an increase in the degree of ball filling is necessary. Such devices are known and are not described in detail here.

If the ball filling degree is to be reduced at a certain moment, the drain valve 62 is simply opened so that, driven by the pressure prevailing in the ball mill and due to the force of gravity, a small amount of the material to be ground 38 leaves the grinding chamber 34 together with the grinding balls 36 therein . Since such a ball drain is rarely necessary in practical operation, the drain valve 62 can also be omitted. In any case, it only has to be operated rarely, so that the associated loss of regrind can be tolerated.

Figure 2 shows another embodiment of the ball mill according to the invention, the corresponding parts with the same reference numerals, but provided with a dash. The main difference compared to the previously described embodiment is that here a variation in the degree of ball filling is not caused by addition or Removal of grinding balls 36 ', but is achieved by variating the volume of the grinding chamber 34'. For this purpose, a bellows 86 is provided on the inner surface of the base plate 14 ', which can consist of a suitable rubber-elastic material or of metal. Its interior is sealed off from the grinding chamber 34 'and connected via the connecting piece 88 to an external source, not shown in the drawing, for a pressure medium, preferably hydraulic oil, so that the bellows 86 is reduced or enlarged in volume by the supply or removal of hydraulic oil can be. In the preferred embodiment shown, a cover cover plate 90 is located on the surface of the bellows on the grinding chamber side, which is sealed with the aid of an annular seal 92, for example an O-ring seal, against the inner wall 93 'of the cylindrical part of the grinding container 12. This prevents ground material 38 'and grinding balls 36' from being able to penetrate into the gap existing between the bellows 86 and the inner wall 93 'and then no longer being conveyed in the desired uniform manner towards the product outlet 54'. However, other embodiments of the invention are also conceivable, in which a bellows is used in the grinding chamber, which lies more in the product flow, so that a seal of the ones shown here is used. Art is not necessary in such cases.

FIG. 3 finally shows a further embodiment of the invention, in which, as in the exemplary embodiment according to FIG. 2, the change in the degree of ball filling is achieved by a corresponding change in the grinding chamber volume. Here are the corresponding components with the same reference characters in Figures 1 and 2, but with two dashes.

In the exemplary embodiment shown, the grinding container 12 "comprises two housing parts 94 and 96, which are sealed off from one another with the aid of an annular seal 98 and are guided concentrically to one another by the guide rings 100. The housing part 96 facing away from the shaft bushing 26 is relative to that by means of hydraulic cylinders 102 Movable housing part 94. The hydraulic rods 104, crossbeams 106 and 108 and corresponding articulated flanges 110 and 112 serve as mechanical connecting parts. Although this embodiment requires a largely new construction of the grinding chamber compared to the known ball mills, it enables a particularly rapid and sensitive adjustment of the degree of filling to the respective operating conditions.

In order to enable automatic regulation of the degree of filling, a series of preliminary tests are expediently carried out for a specific regrind, with the aid of which it is determined at which degree of ball filling the optimum grinding effect is achieved. In the case of the digestion of biomass from fermentative processes for the production of enzymes, a digestion with ultrasound can be used as a benchmark, which is carried out in parallel on the same sample. The parallel determination of the enzymatic activity on the sample digested with ultrasound on the one hand and on the sample digested with the ball mill on the other hand then enables a very precise determination of the degree of disintegration with the help of the ball mill. In these experiments, one measures the power consumption of the ball mill motor, ie at constant voltage its power consumption. If, with the help of these preliminary tests, the optimal operating conditions and in particular the optimum ball filling degree are determined, the power consumption of the motor under these operating conditions can be used as a yardstick for the ball filling degree. A completely automatic control can then be achieved very easily if one uses the motor current as the actual value for the ball filling level in the manner known in control technology and then changes the ball filling level with the aid of a suitable electronic control and the device according to the invention such that the the current intensity corresponding to the desired ball filling level is reached. Normally, such a regulation would be desirable, in which in the start-up phase a relatively low degree of ball filling is used, which is then continuously increased to the value corresponding to the optimal digestion, which is then kept constant over the vast majority of a grinding process.

Claims (10)

1. A method for operating a ball mill, in particular for the digestion of biomass, in which the ground material is fed to a grinding chamber containing grinding balls and is ground in the grinding chamber by the rotary movement of at least one rotating entraining body and the resulting movement of the grinding balls, characterized in that the Ball filling level in the grinding chamber during the grinding process to adapt to the respective operating conditions. 2. The method according to claim 1, characterized in that the ball filling degree is kept substantially constant at least during the majority of the grinding process. 3. The method according to any one of claims 1 or 2, characterized in that the power consumption of the ball mill is used as a measure for controlling the degree of ball filling. _4th Ball mill, in particular for carrying out the method according to claim 1, with a grinding container (12) surrounding a grinding chamber (34), in which at least one driving body (30, 32) rotatable relative to the grinding container (12) is provided, characterized in that devices (60, 62; 86, 90; 94, 96) are present, by means of which the degree of ball filling in the grinding chamber (34) can be controlled and continuously changed during the operation of the ball mill (10). ₅ . Ball mill according to claim 4, characterized by at least one lock device (60) for adding and / or removing grinding balls (36) during operation. 6. Ball mill according to claim 5, characterized in that the lock device (60) for adding grinding balls (36) includes a pressure-resistant storage container (64) for grinding balls (36) which is connected via a line (68) to the grinding chamber (34) A valve (70) is provided in the line (68) between the pressure-resistant storage container (64) and the grinding chamber (34) and a nozzle device (74, 78) is arranged upstream of the valve (70) through which a fluid can emerge under pressure in the direction of the desired ball flow in order to convey the grinding balls (36) into the grinding chamber. 7. Ball mill according to claim 6, characterized in that the valve (70) is a rubber bellows valve. 8. Ball mill according to claim 4, characterized in that the volume of the grinding chamber (34) is variable. 9. Ball mill according to claim 7, characterized in that in the grinding chamber (34) a bellows (86) is arranged, the interior of which is sealed off from the grinding chamber (84) and connected to an external source for a pressure medium, so that the bellows volume during the operation of the ball mill (10) can be changed hydraulically or pneumatically. 10. Ball mill according to claim 8, characterized in that the grinding container (12) includes at least two mutually displaceable and sealed housing parts (94, 96) which are mutually adjustable such that the volume of the grinding chamber (34) can be changed in a controlled manner.

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