COOLING FRAGMENTED MATERIAL BEFORE MILLING

Abstract

According to the present invention fragmented material 2, e.g. made from a material having Polyamides, is passing through a liquid bath 6 filled with liquid nitrogen 7 to cool the fragmented material 2 before entering a mill 10 for grinding the fragmented material 2. The fragmented material 2 is moved through the liquid bath 6 by exciting mechanical vibrations in the liquid bath 6 e.g. 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 allows to grind even materials being difficult to grind by reaching a temperature of −150° C. and less before entering the mill 10 while avoiding a direct cooling e.g, by introducing liquid nitrogen directly into the mill 10.

Claims

1. A method for grinding fragmented material, comprising: providing an amount of the fragmented material at a material inlet to a liquid bath of liquid nitrogen, transporting the fragmented material from the material inlet to a material outlet of the liquid bath by exciting mechanical vibrations in the liquid bath, directly guiding fragmented material from the material outlet to a mill via a chute; and grinding the fragmented material in the mill to a grinded material.

2. The method according to claim 1, wherein the refill of the liquid nitrogen to the liquid bath is controlled to maintain a predetermined filling level at a predetermined position in the liquid bath.

3. The method according to claim 1, wherein the speed by which the fragmented material is transported from the material inlet to the material outlet is adjusted by at least one of the following measures: a) adjusting the frequency of the vibrations; b) adjusting the amplitude of the vibrations; and c) adjusting the incline of the liquid bath with respect to a horizontal plane being perpendicular to the gravity force.

4. The method according to claim 3, wherein the speed is controlled based on the temperature of the fragmented material exiting the liquid bath at the material outlet or the temperature of the grinded material when exiting the mill as a control variable.

5. The method according to claim 3, wherein the speed is controlled based on the temperature in the chute.

6. The method according to claim 1, wherein the liquid nitrogen is subcooled before being provided to the liquid bath.

7. The method according to claim 1, wherein the fragmented material is provided to the liquid bath by a dosing means and a star valve.

8. The method according to claim 1, wherein the mechanical vibrations are ultrasonic vibrations.

9. The method according to claim 1, wherein evaporated liquid nitrogen is sucked off from the chute.

10. An installation for grinding fragmented material, comprising a liquid bath being providable with liquid nitrogen having a material inlet at a first end through which fragmented material is providable into the liquid bath and a material outlet at a second end opposite the first end trough which fragmented material is providable to a chute directly connected to a mill for grinding the fragmented material, further comprising mechanical excitation means for mechanically exciting the liquid bath.

11. The installation according to claim 9, further comprising a control device by which at least one of the following parameters is adjustable: a) the excitation frequency of the mechanical excitation means; b) the excitation amplitude of the mechanical excitation means; and c) the incline of the liquid bath with respect to a horizontal plane being perpendicular to the gravity force.

12. The installation according to claim 11, further comprising at least one of the following sensors: a) a first temperature sensor measuring the temperature in the chute; and b) a second temperature sensor measuring the temperature in an exit zone of the mill, wherein the at least one temperature sensor is connected to the control device.

13. The installation according to claim 10, further comprising a subcooler for subcooling the liquid nitrogen.

14. The installation according to claim 10, further comprising a dosing system and a star valve by which predeterminable amounts of fragmented material are providable to the liquid bath.

15. The installation according to claim 10, wherein the mechanical excitation means comprise an ultrasonic resonator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] It should be noted that the individual features specified in the claims may be combined with one another in any desired technologically reasonable manner and form further embodiments of the invention. The specification, in particular taken together with the figures, explains the invention further and specifies particularly preferred embodiments of the invention. Particularly preferred variants of the invention and the technical field will now be explained in more detail with reference to the enclosed figures. It should be noted that the exemplary embodiment shown in the figures is not intended to restrict the invention. The figures are schematic and may not be to scale. The figures display:

[0046] FIG. 1 an example of an installation for grinding fragmented material; and

[0047] FIG. 2 a schematical drawing of a liquid bath.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0048] FIG. 1 displays schematically an installation 1 for grinding fragmented material 2. The term fragmented material 2 in the context of this document is to be understood as comprising fragmented material 2 with a particle diameter of up to several centimeters. The term fragmented material 2 comprises both materials having a narrow distribution of dimensions (e.g. being regularly shaped) such as pellets and powder and materials having a broader distribution of dimensions (e.g. being irregularly shaped).

[0049] The fragmented material 2 is introduced into a dosing system 3 and through a star valve 4 into the liquid bath 6. By this dosing system 3 the amount of fragmented material 2 being provided to a material inlet 5 of a liquid bath 6 filled with liquid nitrogen 7 can be controlled. The fragmented material 2 enters the liquid nitrogen 7 and is transported in the liquid bath 6 to a material outlet 8 of the liquid bath 6 by exciting mechanical vibrations in the liquid bath 6 and, consequently, in the liquid nitrogen 7. In this example the liquid bath 6 comprises mechanical excitation means 29, i.e, an ultrasonic resonator 26 which excites ultrasonic waves in the liquid nitrogen 7. The ultrasonic resonator 26 is provided at or in a floor 27 of the liquid bath 6. Alternatively or cumulatively, the liquid bath 6 is provided with a vibration motor 28 as mechanical excitation means 29 to excite the mechanical vibrations in the liquid bath 6.

[0050] At the material outlet 8 the cooled fragmented material 2 leaves the liquid bath 6 and enters a chute 9 which directly delivers the cooled fragmented material 2 into a mill 10. The term direct delivery is understood such that no means for controlling the amount of fragmented material 2 entering the mill 10 is provided downstream the liquid bath 6, in particular, no star valve is provided between the material outlet 8 and the mill 10. In the mill 10 the cooled fragmented material 2 is grinded to grinded material 11 that can be withdrawn from a grinded material outlet 12.

[0051] The liquid nitrogen 7 is provided to the liquid bath 6 via a supply line 13 being connected to a reservoir 14 of liquid nitrogen. The liquid nitrogen 7 extracted from the liquid space of the reservoir 14 having a temperature of −196° C. is provided to a subcooler 15 in which the liquid nitrogen 7 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 the data of 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 in such a way that no liquid nitrogen 7 enters the chute 9. As a security 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 drop of temperature on a very brief timescale.

[0052] The level controller 25 is preferably a temperature sensor, an ultrasonic level sensor or a temperature switch. The position of the level controller 25 is adaptable to the level being necessary to cool the specific fragmented material 2 to be milled.

[0053] The control device receives data of a first temperature sensor 17 measuring the temperature in the chute 9 and/or the data of a second temperature sensor 18 measuring the temperature in an exit zone 19 of the mill 10. The temperature measured by the first temperature sensor 17 and/or the second temperature sensor 18 is used as a control variable in the control device 16 to control the speed of transport of the fragmented material 2 in the liquid bath 6. The speed is adjusted by adjusting the frequency and/or the amplitude of the mechanical vibrations excited in the liquid bath by controlling the ultrasonic resonator accordingly. Additionally, or instead, an inclination angle 23 can be adjusted by the control device 16 as will be described with reference to FIG. 2 in the following. FIG. 2 schematically displays the liquid bath 6 which is inclined by an inclination angle 23 with respect to a horizontal plane 20 which is perpendicular to a gravity force 24.

[0054] Evaporated liquid nitrogen 7 is sucked off by a fan 21 through a filter unit 22. The fan 21 is connected to the chute 9 between the material outlet 8 of the liquid bath 6 and the mill 10. The suction of the evaporated liquid nitrogen reduces the amount of evaporated liquid nitrogen 7 entering the mill 10. This avoids the cooling of the mill 10 by this evaporated liquid nitrogen 7 reducing 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 turning too cold avoiding thermal embrittlement of the bearings.

[0055] According to the present invention fragmented material 2, e.g, made from a material comprising Polyamides, is passing through a liquid bath 6 filled with liquid nitrogen 7 to cool the fragmented material 2 before entering a mill 10 for grinding the fragmented material 2. The fragmented material 2 is moved through the liquid bath 6 by exciting mechanical vibrations in the liquid bath 6 e.g. 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 allows to grind even materials being difficult to grind by reaching a temperature of −150° C. and less before entering the mill 10 while avoiding a direct cooling e.g. by introducing liquid nitrogen directly into the mill 10.

REFERENCE NUMERALS

[0056] 1 installation for grinding fragmented material

[0057] 2 fragmented material

[0058] 3 dosing system

[0059] 4 star valve

[0060] 5 material inlet

[0061] 6 liquid bath

[0062] 7 liquid nitrogen

[0063] 8 material outlet

[0064] 9 chute

[0065] 10 mill

[0066] 11 grinded material

[0067] 12 grinded material outlet

[0068] 13 supply line

[0069] 14 reservoir

[0070] 15 subcooler

[0071] 16 control device

[0072] 17 first temperature sensor

[0073] 18 second temperature sensor

[0074] 19 exit zone

[0075] 20 horizontal plane

[0076] 21 fan

[0077] 22 filter unit

[0078] 23 inclination angle

[0079] 24 gravity force

[0080] 25 level controller

[0081] 26 ultrasonic resonator

[0082] 27 floor

[0083] 28 vibration motor

[0084] 29 mechanical excitation means