Cooling crushing material before crushing

Abstract

To provide a method and facility to crush crushing material without applying a liquefied gas to a mill.SOLUTION: Crushing material 2 made from material having Polyamides, is passing through a liquid bath 6 filled with liquid nitrogen 7 to cool the crushing material 2 before entering a mill 10 for crushing the crushing material 2. The crushing material 2, for example, is moved through the liquid bath 6 by exciting mechanical vibrations in the liquid bath 6 by a vibrational motor 28 coupled to the liquid bath 6 and / or an ultrasonic resonator 26 attached to the liquid bath 6. The invention, for example, allows crushing even materials being difficult to crush by reaching temperature of -150°C or less before entering the mill 10 while avoiding direct cooling by directly introducing liquid nitrogen into the mill 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The subject of the present invention is a method for grinding crushed material by pre-cooling to low temperatures, for example below -150°C, without applying liquefied gas to the respective mill. [Background technology]

[0002] Grinding or milling is used to prepare ground materials with a defined particle size distribution, such as powders. Grinding is frequently used for crushed materials, i.e., materials that are provided in pieces with a specific particle size distribution. Crushed materials include both materials with a fairly uniform particle size distribution, such as powders or pellets, and materials with a less uniform particle size distribution, such as materials that are provided in discrete pieces of irregular shapes and sizes.

[0003] Materials that are difficult to grind exist, such as soft, medium-hard, brittle, hard-brittle, fibrous, or temperature-sensitive materials. Such materials are either not grindable or suffer degradation during grinding, for example, due to the temperature increase caused by mechanical processing. Solutions are known for some specific cases. For example, DE 100 13 742 A1 discloses that it can be advantageous to supply liquefied gas into the mill during grinding or milling to grind animal and plant parts used in traditional Chinese medicine. This can lead to uneven cooling of the ground material, resulting in uneven particle distribution after grinding, and thermal distortion in the mill due to the uneven distribution of the liquefied gas in the mill. Summary of the Invention [Problem to be solved by the invention]

[0004] It is therefore an object of the present invention to at least partly overcome the drawbacks known from the prior art. This object is solved by the features of the independent claims. The dependent claims are directed to preferred embodiments of the invention. [Means for solving the problem]

[0005] The method for crushing crushed material according to the present invention comprises: providing a quantity of said crushed material to a material inlet into a bath of liquid nitrogen; transporting the crushed material from the material inlet to a material outlet of the liquid bath by exciting mechanical vibrations in the liquid bath; Guiding the crushed material directly from the material outlet through a chute to a mill; comminuting the crushed material in the mill into crushed material; Includes.

[0006] The term "crushed material" is understood to include materials with a uniform particle size distribution, such as powders or pellets, as well as materials with a non-uniform particle size distribution, such as materials present in small pieces with irregular shapes and sizes. Furthermore, the materials are preferably difficult to crush under normal conditions, especially at temperatures above -120°C. In particular, these materials include plastic materials such as polyamides, thermoplastic materials, ferritic metals, ferritic-martensitic metals, iron-based casting alloys, tin, and / or zinc. Crushed materials made from materials containing at least one polyamide are preferred.

[0007] The term liquid bath is understood as a container partially filled with liquid nitrogen, preferably at an angle of maximum + / - 45° with the material inlet positioned higher (relative to the horizontal plane) than the material outlet in order to facilitate transport through the bath from the material inlet to the material outlet. It is inclined towards the horizontal plane. Nevertheless, it is equally possible to transport the crushed material through the tank if the tank is at the same height as the horizontal plane. The term horizontal plane should be understood as a plane perpendicular to the force of gravity.

[0008] The mechanical vibration excitation can be performed, for example, by stimulating mechanical vibration of the entire liquid bath, for example, by inducing rhythmic movement of the entire liquid bath, for example, by using a vibration table to which the liquid bath is attached. Alternatively, ultrasonic excitation can be used to introduce ultrasound waves into the liquid nitrogen. The material to be crushed is placed at the bottom of the liquid bath.

[0009] The term direct guidance of the crushed material from the material outlet through the chute to the mill is to be understood in particular as meaning that no dosing means are placed between the material outlet and the mill, meaning that all crushed material passing through the material outlet goes directly to the mill and is delayed by the transit time in the chute.

[0010] As mills, basically any mill suitable for grinding the crushed material after cooling can be used, although the following mills are preferred: pin mills, wide chamber mills, cutting mills, Fitz mills and / or hammer mills.

[0011] Preferably, the temperature of the liquid nitrogen and the rate of transport through the liquid bath from the material inlet to the material outlet are controlled so that the temperature of the crushed material as it passes through the material outlet is below -150°C, in particular below -170°C, or even below -180°C.

[0012] The present invention allows for the supply of crushed material to the mill at temperatures below -150°C, which depends on the speed of transport through the liquid bath, while ensuring uniform cooling by immersion of the crushed material in liquid nitrogen. At the same time, the direct application of liquid nitrogen to the mill is avoided, reducing thermal stress on the mill. This means that the crushed material entering the mill has a uniform temperature. In this way, materials that cannot be crushed or cannot be crushed sufficiently at higher temperatures can be crushed.

[0013] In particular, the present invention can be used to grind raw materials used in additive manufacturing processes. Raw materials used in additive manufacturing processes have high requirements for uniform particle size distribution to ensure consistent quality in products manufactured by additive manufacturing. For example, when polyamide or polyamide-containing materials are used for additive manufacturing, prior art methods do not allow acceptable grinding because thermoplastic materials maintain their plasticity at temperatures below -70°C. The present method allows for the preparation of ground materials from polyamides, including materials with high uniformity of particle distribution, thereby preserving the quality of parts manufactured by additive manufacturing from these materials.

[0014] Preferably, the refilling of the liquid nitrogen into the bath is controlled to maintain a predetermined fill level at a predetermined location within the bath. The level is measured at a predetermined location within the bath and controlled to be constant at this location. In the case of a horizontal bath, the same water level is constant throughout the bath, but in the case of an inclined bath, the water level varies based on the angle of inclination. Controlling the amount of liquid nitrogen into the bath based on the liquid nitrogen level allows for easy control, and by adjusting the water level relative to the material outlet, it is ensured that liquid nitrogen does not enter the chute and subsequently the mill.

[0015] Preferably, the rate at which the crushed material is transported from the material inlet to the material outlet is regulated by at least one of the following means: a) Adjusting the frequency of said vibration. b) adjusting the amplitude of said vibrations; c) Adjusting the inclination of the liquid tank relative to a horizontal plane perpendicular to gravity.

[0016] Increasing the frequency and / or amplitude of the vibration increases the speed, while decreasing the frequency and / or amplitude decreases the speed. The inclination is preferably determined so that the material inlet is positioned higher than the material outlet relative to the horizontal. The inclination can be determined by the inclination angle. Increasing the inclination angle increases the speed, while decreasing the inclination angle decreases the speed. Measures a) to c), especially a) and b), allow for easy adjustment of the speed at which the crushed material is transported from the material inlet to the material outlet. This makes it possible to control the amount of crushed material fed to the mill after cooling without using specific dosing hardware such as a star valve.

[0017] Preferably, the speed is controlled based on the temperature of the crushed material leaving the bath at the material outlet or the temperature of the crushed material as it leaves the mill as a controlled variable. The temperature of the crushed material leaving the bath can be measured, for example, by measuring the temperature in a chute, while the temperature of the crushed material leaving the mill can be easily measured in the outlet area of ​​the mill. Preferably, the speed is controlled based on the temperature in the chute.

[0018] Preferably, the liquid nitrogen is subcooled before being fed to the bath, which allows the temperature of the ground material to be further reduced as it enters the mill, and also allows the length of the bath, i.e., the distance between the material inlet and the material outlet, to be reduced while still allowing the temperature of the ground material leaving the bath at the material outlet to be the same as if it had passed through a longer bath filled with non-subcooled liquid nitrogen at the same transport speed.

[0019] Preferably, the crushed material is fed to the bath by a dosing means and a star valve. The dosing means allows for controlling and metering the amount of crushed material fed to the bath and then to the mill. The star valve prevents leakage of nitrogen gas from the atmosphere above the liquid nitrogen in the bath.

[0020] Preferably, the mechanical vibration is generated using a mechanical vibration motor.

[0021] Preferably, the mechanical vibration is ultrasonic vibration. Preferably, ultrasonic vibration generated by an ultrasonic resonator, i.e., ultrasonic waves in a liquid bath, can be easily applied. Furthermore, the control of the ultrasonic excitation in terms of its frequency and / or amplitude allows for easy adjustment of the transport speed of the crushed material from the material inlet to the material outlet.

[0022] Preferably, the evaporated liquid nitrogen is sucked through the chute, which reduces the ingress of evaporated liquid nitrogen into the mill and prevents the mill from becoming too cold, which reduces icing on the mill and reduces thermal distortion in any thermoplastic parts of the mill, such as bearings and / or shock absorbers.

[0023] According to a further aspect of the present invention, there is provided an apparatus for crushing crushed material, the apparatus comprising a liquid tank to which liquid nitrogen can be supplied, the liquid tank having a material inlet at a first end through which the crushed material can be fed into the liquid tank, and a material outlet at a second end opposite the first end through which the crushed material can be fed into a chute that guides the crushed material directly to a mill for crushing the crushed material, and mechanical excitation means for mechanically exciting the liquid tank. Preferably, the apparatus is operated according to the method of the present invention.

[0024] Preferably, the installation further comprises a control device (16) in which at least one of the following parameters is adjustable: a) The excitation frequency of said mechanical excitation means. b) the excitation amplitude of said mechanical excitation means; c) The inclination of the liquid tank relative to a horizontal plane perpendicular to gravity.

[0025] This allows the control device to control the transport speed of the crushed material from the material inlet to the material outlet by adjusting at least one of the parameters a) to c).

[0026] Preferably, the facility further comprises at least one of the following sensors: a) a first temperature sensor for measuring the temperature within the chute; b) a second temperature sensor measuring the temperature in the exit region of said mill; However, at least one of the temperature sensors is connected to the control device.

[0027] This allows the controller to control the transport rate based on the temperature measured by one of the temperature sensors.

[0028] Preferably, the facility further includes a subcooler for subcooling the liquid nitrogen, which can reduce the temperature of the liquid nitrogen to the normal boiling point of −196° C. or lower.

[0029] Preferably, the equipment further comprises a dosing system and a star valve capable of supplying a predetermined amount of grinding material to the liquid tank.

[0030] Preferably, the mechanical excitation means comprises an ultrasonic resonator, in particular an ultrasonic resonator coupled to at least one wall or floor of the bath, which allows direct excitation of ultrasonic waves in the liquid nitrogen in the bath.

[0031] The details and features explained for the method according to the invention are applicable to the installation according to the invention, and the advantages realized by the method according to the invention are also realized by the installation according to the invention.

[0032] It should be noted that the individual features specified in the claims may be combined with one another in any desired, technically reasonable manner to form further embodiments of the present invention. This specification, particularly in conjunction with the drawings, further explains the present invention and specifies particularly preferred embodiments of the present invention. Particularly preferred variants and technical fields of the present invention will now be described in more detail with reference to the accompanying drawings. It should be noted that the exemplary embodiments shown in the figures are not intended to limit the present invention. The drawings are schematic and may not be to scale. [Brief explanation of the drawings]

[0033] [Figure 1] An example of equipment for crushing crushed materials. [Figure 2] Schematic diagram of the liquid tank. DETAILED DESCRIPTION OF THE INVENTION

[0034] 1 shows a schematic diagram of an installation 1 for crushing crushed material 2. The term crushed material 2 in the context of this specification should be understood to include crushed material 2 having a particle size of up to several centimeters. The term crushed material 2 includes both materials with a narrow size distribution (e.g., regularly shaped), such as pellets and powders, and materials with a wide size distribution (e.g., irregularly shaped).

[0035] The material to be crushed 2 is introduced into a dosing system 3 and introduced into a liquid tank 6 through a star valve 4. This dosing system 3 allows the amount of material to be crushed 2 to be controlled, which is fed into a material inlet 5 of the liquid tank 6 filled with liquid nitrogen 7. The material to be crushed 2 enters the liquid nitrogen 7 and is then crushed in the liquid tank 6. The liquid nitrogen 7 is transported in the liquid bath 6 to a material outlet 8 of the liquid bath 6 by exciting mechanical vibrations in the liquid nitrogen 7. In this embodiment, the liquid bath 6 includes mechanical excitation means 29, i.e., an ultrasonic resonator 26 that excites ultrasonic waves in the liquid nitrogen 7. The ultrasonic resonator 26 is provided on or in a floor 27 of the liquid bath 6. Alternatively or additionally, the liquid bath 6 is provided with a vibration motor 28 as the mechanical excitation means 29 for exciting mechanical vibrations in the liquid bath 6.

[0036] The cooled crushed material 2 leaves the liquid bath 6 at a material outlet 8 and enters a chute 9 which conveys the cooled crushed material 2 directly into a mill 10. By the term direct conveyance, it is understood that no means are provided downstream of the liquid bath 6 for controlling the amount of crushed material 2 entering the mill 10, in particular no star valve is provided between the material outlet 8 and the mill 10. In the mill 10, the cooled crushed material 2 is ground into crushed material 11 which can be recovered from a crushed material outlet 12.

[0037] Liquid nitrogen 7 is supplied to the liquid bath 6 via a supply line 13 connected to a liquid nitrogen reservoir 14. The liquid nitrogen 7 extracted from the liquid space of the reservoir 14, which has a temperature of -196°C, is supplied to a subcooler 15 where it is subcooled, i.e., its temperature is further reduced. The flow of liquid nitrogen 7 downstream of the subcooler 15 is controlled by a control device 16. The flow of nitrogen is controlled by the control device 16 based on data from a level controller 25 in the liquid bath 6 to maintain a predetermined level of liquid nitrogen 7 in the liquid bath 6. This level 7 is predetermined so that liquid nitrogen 7 does not enter the chute 9. As a safety measure, the temperature in the chute 9 can be measured to detect a situation in which liquid nitrogen 7 enters the chute 9, resulting in a very short temperature drop.

[0038] The water level controller 25 is preferably a temperature sensor, an ultrasonic water level sensor, or a temperature switch. The position of the water level controller 25 can be adapted to the height required to cool the particular material 2 being ground.

[0039] The controller receives data from a first temperature sensor 17 measuring the temperature in the chute 9 and / or a second temperature sensor 18 measuring the temperature in the outlet region 19 of the mill 10. The temperatures measured by the first temperature sensor 17 and / or the second temperature sensor 18 are used as control variables in the controller 16 to control the transport speed of the crushed material 2 in the liquid bath 6. The speed is adjusted by adjusting the frequency and / or amplitude of the mechanical vibrations excited in the liquid bath by controlling the ultrasonic resonator accordingly. Additionally or alternatively, the tilt angle 23 can be adjusted by the controller 16, as will be explained with reference to FIG. 2 below. FIG. 2 schematically shows the liquid bath 6 inclined at the tilt angle 23 with respect to a horizontal plane 20 perpendicular to gravity 24.

[0040] The evaporated liquid nitrogen 7 is sucked in by a fan 21 through a filter section 22. The fan 21 is connected to a chute 9 between the material outlet 8 of the liquid tank 6 and the mill 10. Suction of the evaporated liquid nitrogen reduces the amount of evaporated liquid nitrogen 7 that enters the mill 10. This prevents the mill 10 from being cooled by this evaporated liquid nitrogen 7 and reduces the risk of ice formation in the mill 10. Furthermore, due to the higher temperature of the mill 10, the bearings of the mill 10 are protected from rotating at excessively low temperatures, avoiding thermal embrittlement of the bearings.

[0041] According to the present invention, the crushed material 2, for example made from a material comprising polyamide, passes through a liquid bath 6 filled with liquid nitrogen 7 to cool the crushed material 2 before entering a mill 10 for grinding the crushed material 2. The crushed material 2 is moved through the liquid bath 6 by exciting mechanical vibrations in the liquid bath 6, for example, by a vibration motor 28 coupled to the liquid bath 6 and / or an ultrasonic resonator 26 attached to the liquid bath 6. The present invention allows the crushed material 2 to be cooled to -15°C before entering the mill 10, while avoiding direct cooling by, for example, introducing liquid nitrogen directly into the mill 10. By reaching temperatures below 0°C, it is possible to grind even difficult-to-grind materials. [Explanation of symbols]

[0042] 1. Equipment for crushing crushed materials 2 Crushed materials 3. Dosing System 4 Star Valve 5 Material inlet 6 Liquid tank 7. Liquid Nitrogen 8 Material outlet 9 Shoot 10 mils 11 Crushed material 12 Crushed material outlet 13 Supply Line 14 Reservoir 15 Supercooler 16 Control device 17 First temperature sensor 18 Second temperature sensor 19 Exit area 20 horizontal plane 21 Fan 22 Filter section 23 Tilt angle 24 Gravity 25 Water level controller 26 Ultrasonic resonator 27 beds 28 Vibration Motor 29 Mechanical excitation means

Claims

1. A method for grinding a crushed material (2), comprising: feeding a quantity of said crushed material (2) into a material inlet (5) into a bath (6) of liquid nitrogen (7); Transporting the crushed material (2) from the material inlet (5) to the material outlet (8) of the liquid bath (6) by exciting mechanical vibrations in the liquid bath (6); Guiding the crushed material (2) directly from the material outlet (8) through a chute (9) into a mill (10); grinding the crushed material (2) in the mill (10) into crushed material (11); A method comprising:

2. 2. The method of claim 1, wherein the refilling of the liquid nitrogen (7) into the reservoir (6) is controlled to maintain a predetermined fill level at a predetermined position in the reservoir (6).

3. 3. The method according to claim 1 or 2, wherein the speed at which the crushed material (2) is transported from the material inlet (5) to the material outlet (8) is regulated by at least one of the following means: a) Adjusting the frequency of said vibration. b) adjusting the amplitude of said vibrations; c) Adjusting the inclination of the liquid tank (6) relative to a horizontal plane (20) perpendicular to gravity (24).

4. 4. The method of claim 3, wherein the speed is controlled based on the temperature of the crushed material (2) leaving the liquid bath (6) at the material outlet (8) or the temperature of the crushed material (11) as it leaves the mill (10) as a controlled variable.

5. 4. The method of claim 3, wherein the speed is controlled based on the temperature within the chute (9).

6. 3. The method according to claim 1 or 2, wherein the liquid nitrogen (7) is subcooled before being fed into the liquid bath (6).

7. 3. The method according to claim 1 or 2, wherein the crushed material (2) is fed to the liquid tank (6) by means of a dosing means (3) and a star valve (4).

8. The method according to claim 1 or 2, wherein the mechanical vibration is ultrasonic vibration.

9. 3. A method according to claim 1 or 2, wherein evaporated liquid nitrogen is sucked from the chute (9).

10. The apparatus (1) for crushing material (2) includes a liquid tank (6) to which liquid nitrogen (7) can be supplied, the liquid tank (6) having a material inlet (5) at a first end through which the material (2) to be crushed can be supplied to the liquid tank (6), and a material outlet (8) at a second end opposite the first end through which the material (2) to be crushed can be supplied to a chute that guides the material (2) directly to a mill (10) for crushing the material (2), and further includes mechanical excitation means (29) for mechanically exciting the liquid tank (6).

11. 11. The installation (1) according to claim 10, further comprising a control device (16) in which at least one of the following parameters is adjustable: a) The excitation frequency of said mechanical excitation means (29). b) the excitation amplitude of said mechanical excitation means (29). c) Inclination of said liquid reservoir (6) relative to a horizontal plane (20) perpendicular to gravity (24).

12. 12. The installation (1) according to claim 11, further comprising at least one of the following sensors: a) A first temperature sensor (17) for measuring the temperature inside said chute (9). b) A second temperature sensor (18) measuring the temperature in the outlet region (19) of said mill (10). However, at least one of the temperature sensors (17, 18) is connected to the control device (16).

13. 12. The installation (1) according to claim 10 or 11, further comprising a subcooler (15) for subcooling the liquid nitrogen (7).

14. 12. The installation (1) according to claim 10 or 11, further comprising a dosing system (3) and a star valve (4) capable of supplying a predetermined amount of crushed material (2) to said liquid tank (6).

15. 12. Installation according to claim 10 or 11, wherein the mechanical excitation means (29) comprises an ultrasonic resonator (26).

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