CRYOGENIC GRINDING APPARATUS AND PROCESS

Abstract

A cryogenic grinding apparatus is provided. The apparatus includes a material charge, a low-temperature embrittling device for embrittling the charged ground material by supplying liquid low-temperature gas, a mill for grinding the low-temperature embrittled ground material in an atmosphere containing the low-temperature gas, and a separating device. The separating device has a wind classifier, a removal device to remove the ground material from the mill-output volumetric flow, and a directing device to feed the removed ground material into a classifier-input volumetric flow and to discharge excess mill-output gas of the mill-output volumetric flow. In addition, a cryogenic grinding process is provided in which the ground material is removed from the volumetric flow, excess grinding gas is discharged, removed ground material is feed into a classifier-input volumetric flow of a wind classifier and a classifier-input volumetric flow is classified for separating off a classifier-output volumetric flow containing the fine material.

Claims

1. A cryogenic grinding apparatus for producing fine material from a material to be ground comprising: a material charge for charging the material to be ground; a low-temperature embrittling device connected to said material charge and configured for embrittling the charged material to be ground at a low temperature by supplying a liquid low-temperature gas; a mill connected to said low-temperature embrittling device and configured for grinding the embrittled material to be ground in an atmosphere containing the supplied liquid low-temperature gas; a separating device configured for separating the fine material from a mill-output volumetric flow leaving the mill; said mill-output volumetric flow containing the liquid low-temperature gas and ground material; said separating device including a wind classifier configured for separating off a classifier-output volumetric flow containing the fine material from a classifier-input volumetric flow containing the ground material; a removal device connected to the mill and configured for removing the ground material from said mill-output volumetric flow; and, a directing device configured for feeding the removed ground material into said classifier-input volumetric flow and to discharge excess mill-output gas of said mill-output volumetric flow.

2. The cryogenic grinding apparatus of claim 1, wherein: said directing device includes a line network; said line network is arranged to feed said classifier-input volumetric flow with the removed ground material and with a feed gas fraction of mill output gas; said gas fraction of the mill output gas is necessary for at least one of creating and maintaining a desired low-temperature gas fraction in an atmosphere in the wind classifier; and, said line network is arranged to discharge said excess mill-output gas into surroundings.

3. The cryogenic grinding apparatus of claim 1, further comprising: a plurality of gas-directing actuators; said directing device having a detection device to detect at least one parameter of an atmosphere prevailing in said wind classifier; a controller configured to control the plurality of gas-directing actuators in response to an output from said detection device so as to feed a feed gas fraction of mill-output gas into the classifier-input volumetric flow and to discharge remaining gas into the surroundings; and, said gas fraction of said mill-output gas being necessary for at least one of creating and maintaining at least one setpoint parameter value of said atmosphere prevailing in said wind classifier.

4. The cryogenic grinding apparatus of claim 3, wherein said detection device is configured to detect at least one of a temperature of said atmosphere prevailing in said wind classifier and a content of oxygen in said atmosphere prevailing in said wind classifier.

5. The cryogenic grinding apparatus of claim 3, wherein: said directing device is configured to smoothen said classifier-input volumetric flow over time and to set said classifier-input volumetric flow to a desired setpoint volumetric flow value.

6. The cryogenic grinding apparatus of claim 2, wherein: said line network includes a connecting line and a gas-excess outlet line; said connecting line connects said removal device with said wind classifier; said gas-excess outlet line branches off from said connecting line; said connecting line feeds the removed ground material and the feed gas fraction into the classifier-input volumetric flow; and, said gas-excess outlet line discharges the excess mill-output gas into the surroundings.

7. The cryogenic grinding apparatus of claim 6, further comprising: a blower; and, an air filter arranged in said gas-excess outlet line.

8. The cryogenic grinding apparatus of claim 6, further comprising: a classifier circulating line; said wind classifier having a classifier wheel output and a classifier input side; said classifier wheel output being connected to said classifier circulating line; said classifier circulating line leading back to said classifier input side and opening out into said connecting line; and, a classifier-circuit outlet line branching off from said classifier circulating line to discharge excess fractions of gas contained in a classifier output volumetric flow into the surroundings.

9. The cryogenic grinding apparatus of claim 8, further comprising: a blower; and, an air filer arranged in a circulating line portion leading to said classifier-circuit outlet line.

10. The cryogenic grinding apparatus of claim 3, wherein said plurality of gas-directing actuators includes a controlled throttle valve in at least one of said gas-excess outlet line and said classifier-circuit outlet line.

11. The cryogenic grinding apparatus of claim 8, wherein: said removal device is configured for separating gas from said ground material; said mill has a mill output and a mill-outlet line attached to the mill output; said removal device has an inlet, a gas outlet, and a particle discharge; said inlet is connected to said mill-outlet line; said gas outlet opening out into the connecting line upstream of a branch into the gas-excess outlet line; and, said particle discharge opening out downstream thereof into the connecting line.

12. The cryogenic grinding apparatus of claim 6, wherein said removal device is a cyclone separator.

13. The cryogenic grinding apparatus of claim 12, wherein said line network has a circulating line which branches off from said gas-excess outlet line and opens out into the connecting line downstream of the branch of said gas-excess outlet line.

14. The cryogenic grinding apparatus of claim 1, further comprising: a coarse-material return line opening out upstream of said mill; said wind classifier having a coarse-material discharge; and, said coarse-material discharge being connected to said coarse-material return line.

15. The cryogenic grinding apparatus of claim 14, wherein: said mill is a whirlwind mill; and, said mill-output volumetric flow is blown out by said whirlwind mill.

16. The cryogenic grinding apparatus of claim 15, wherein: said mill has a mill-temperature control device; and, said mill-temperature control device variably controls an amount of low-temperature gas supplied per period of time from a low temperature gas source of said mill directly via at least one of a low-temperature gas feeding line and the low-temperature embrittling device to keep a temperature below a desired setpoint temperature value during cold grinding.

17. A cryogenic grinding process for producing fine material from a material to be ground, the method comprising the steps of: charging the material to be ground; low-temperature embrittling the charged material to be ground subsequent to the charging by supplying liquid low-temperature gas; grinding low-temperature-embrittled material to be ground subsequent to the low-temperature embrittling in an atmosphere containing the supplied liquid low-temperature gas; separating the fine material from a volumetric flow subsequent to the grinding; said volumetric flow including ground material and a grinding gas containing the liquid low-temperature gas; said separating including substeps of: removing the ground material from the volumetric flow; discharging excess grinding gas; feeding the removed ground material into a classifier-input volumetric flow of a wind classifier; feeding a feed gas fraction of the excess grinding gas that is necessary for at least one of creating and maintaining a desired low-temperature gas fraction in an atmosphere in said wind classifier; wind classifying the classifier input volumetric flow for separating off a classifier-output volumetric flow containing the fine material in an atmosphere in the wind classifier; and said classifier input volumetric flow containing at least a desired low-temperature gas fraction.

18. The cryogenic grinding process of claim 17, wherein said cryogenic grinding process is carried out on a cryogenic grinding apparatus as claimed in claim 1.

19. The cryogenic grinding process of claim 17, wherein: said charged material to be ground is a precomminuted material to be ground; and, said precomminuted material is rubber granules for producing powdered rubber.

20. The cryogenic grinding process of claim 19, wherein said supplied liquid low-temperature gas is liquid nitrogen.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0040] The invention will now be described with reference to the single figure of the drawing (FIG. 1) which is a schematic illustration of a cryogenic grinding apparatus according to an exemplary embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0041] FIG. 1 shows a cryogenic grinding apparatus 100 for carrying out a cryogenic grinding process. The cryogenic grinding apparatus 100 is suitable in particular for producing powdered rubber from rubber granules with rubber-granule particle diameters of 0-6 mm. The cryogenic grinding apparatus 100 has a coarse material charge, which is denoted by 1 and at which the rubber granules, or some other material that is difficult to grind, is charged. From the coarse material charge 1, the material to be ground passes to a low-temperature embrittling device 2, advantageously formed as a cooling screw, at which it is for example charged via a rotary-vane feeder.

[0042] The low-temperature embrittling device 2, formed as a screw feeder or cooling screw, has in this case along with the input for the coarse material to be embrittled a connection 2a, which is connected to a liquefied gas container, that is, for example a pressure vessel filled with liquid nitrogen. The liquefied nitrogen, or some other inert gas kept in a liquid state, is then injected via the connection 2a into the interior space, for example tubular interior space, of the preferably heat-insulated low-temperature embrittling device 2, through which a feeding screw extends, which feeds the coarse material introduced at the input through the interior space enclosed by the insulating tube to an output at the other end of the tube. The liquefied gas provides an interior temperature that is well within the negative C. range and cools and embrittles the charged material particles to be ground, while at least partially undergoing evaporation. For example, nitrogen has a boiling point of 196 C. at the normal pressure prevailing in the low-temperature embrittling device 2. From the output of the low-temperature embrittling device 2, the embrittled coarse material passes together with the injected liquefied gas, which has now at least partly evaporated but still has a temperature in the at least two-digit negative C. range, to the input or the material charge of a mill 3, preferably formed as a whirlwind mill. In response to an output of a temperature sensor, a valve in a supplementary-gas line, which optionally connects the mill 3 with the liquefied gas container, is in this case controlled.

[0043] Mill 3 has a rotor with a vertical axis of rotation, which bears a plurality of metal plates, which are surrounded at the outer circumference by a grinding path, in order to form a grinding gap between the rotor and the stator bearing the grinding path. The grinding gap in this case preferably runs conically downward, so that its width can be set by way of a height adjustment of the rotor. The coarse material must in this case pass the grinding gap on its way from the mill input to a mill output, and is thereby flung back and forth between the baffle plates on the rotor and the grinding path on the stator, and as a result is ground.

[0044] During the grinding, a low-temperature in the at least two- digit negative C. range still continues to prevail in the mill 3, since the gas atmosphere in the mill is dominated by the low-temperature gas added directly via the supplementary-gas line or via the low-temperature embrittling device 2, and consists at least to the greatest extent of this low-temperature gas.

[0045] The mill housing is in this case also advantageously heat-insulated. The temperature may in this case be controlled such that, depending on whether the recorded temperature in the mill lies below a setpoint value of for example 60 or 70 C., more or less gas is allowed into the mill 3.

[0046] The temperature control of the mill 3 in this case brings about a rising and falling supply of refrigerating medium, and consequently a changing mill-output volumetric flow, in which the material to be ground into fine powder and the low-temperature gas from the gas atmosphere in the mill interior space is contained, which although heated somewhat in the course of the grinding operation, is still well within the two-digit negative C. range.

[0047] In order to separate off the ground material particles contained in the mill-output volumetric flow that have the desired fineness, the diameter of which is therefore below a desired upper grain limit, from the mill-output volumetric flow, a separating device denoted overall by 4 is provided and is connected to the mill via a mill-output line 7 connected to the mill output. The line 7 as well as all other lines of the installation are preferably heat-insulated.

[0048] The actual separation cut, that is, the separation of the fine material fraction below a predetermined grain limit from a coarse material fraction, takes place in this case in a wind classifier 5, the classifier wheel of which defines the upper grain limit. The mill-output volumetric flow cannot, however, be supplied to the wind classifier 5 in this case, because it is unsuitable for this on account of the pulsations induced by the temperature control of the mill 3. This is because the wind classifier 5 requires an at least relatively constant classifier-input volumetric flow, which is accomplished by its classifier wheel rotating at a constant rotational speed. Arranged between the mill output and the classifier input are therefore a ground-material particle removal device 8 and a directing device 6, which remove from the mill-output volumetric flow a feed gas fraction that is suitable for the wind classifier 5 and is smoothened over time, and the ground material to be classified, and feed these into a classifier-input volumetric flow. Excess grinding gas is discharged into the surroundings. The directing device 6 in this case includes a wide variety of lines, sensors, actuators and a controller 12, 12a, 14, 14a, 15, 25, 25a, 26, 27, 28, which will be discussed later, but not the cyclone separator serving as the ground-material particle removal device 8.

[0049] In the classifier chamber of the wind classifier 5, the introduced ground material particles are sucked up by the classifier wheel rise. Ground material particles that are too heavy sink downward, where they reach a coarse-material discharge denoted by 18, at which the coarse-material fraction is discharged from the wind classifier 5. The risen ground material particles are then either rejected at the classifier wheel located above the classifier chamber, and fall back into the classifier chamber, or pass through the classifier wheel to the classifier output and together with the transporting gas sucked through the classifier wheel enter an after-separating device, which is denoted by 5a. The ground material particles of the fine-material fraction are removed from the classifier-output volumetric flow at the after-separating device 5a and the transporting gas contained in the classifier-output volumetric flow is cleaned and discharged into the surroundings.

[0050] From the coarse-material discharge 18 of the wind classifier 5, a return line 19 leads back to the input side of the mill and opens out there into the downcoming line connecting the coarse-material charge 1 to the low-temperature embrittling device 2. The coarse material segregated at the classifier 5 is supplied once again to the cryogenic grinding process. In this case, the coarse material returned by the classifier 5 is still at a temperature in the two-digit C. range below zero, and therefore no longer has to be cooled as much as the newly charged material to be ground, so that not only is there generally a situation in which the returned coarse material is used, but there is also a saving of liquefied gas and consequently altogether an improved yield of fine material with less use of energy.

[0051] The mill-output volumetric flow discharged by the mill 3 in this case passes through the mill-output line 7 initially into the ground-material particle removal device 8, advantageously into a cyclone separator 8, into the input of which the mill-output line 7 leads. In the cyclone separator 8, a large part of the ground-material particle load of the mill-output volumetric flow, in practice about 98% thereof, is separated off from the mill-output volumetric flow and passes into the product discharge 9, preferably provided with a rotary-vane feeder, of the cyclone separator 8. On the other hand, the transporting gas stream with the remaining, small fraction of ground material particles leaves the cyclone separator 8 at a transporting gas outlet 10. The transporting gas stream, which is then virtually unladen with ground material particles, contains the low-temperature gas injected in the low-temperature embrittling device 2 and also at least in significant parts consists thereof, can then be divided up into a feed gas fraction, which is fed to the classifier-input volumetric flow, and a gas excess, which is discharged via a gas-excess outlet line 14 into the surroundings. For this purpose, the transporting gas outlet 10 of the cyclone separator 8 is connected to a connecting line 12, 12a, which leads to the wind classifier 5 and from which a branch line 14a branches off, which leads to the gas-excess outlet line 14.

[0052] The line branch 12 forms with the mill-output line 7 leading from the mill 3 to the cyclone separator 8 and the line portion 12a connecting to the line branch 12 a line branch 7, 12, 12a altogether connecting the mill 3 and the wind classifier 5, a further circulating line 15, coming from the gas-excess outlet line 14, opening out at the connecting part 13 (for example a T-pipe piece).

[0053] Arranged in the line branch 14a is a blower 16, and also an upstream air filter 17. The blower 16 sucks a part of the grinding gas through the air filter 17 with a predetermined constant suction power, the remaining ground material particles that are contained in the transporting gas of the partial volumetric flow being separated off here. The partial volumetric flow, then completely freed of ground material particles, can then be blown out at the gas-excess outlet 14, without polluting the environment with dust and keeping below the legally prescribed amount of residual dust, or in the case of a closed throttle damper 26 be returned through the circulating line 15 into the connecting line 12, 12a.

[0054] Corresponding controlled or uncontrolled throttles may also be provided in the line portion 12 and the circulating line 15, as indicated by crossbars. Downstream of the connecting part 13 of the circulating line 15, into the line branch 7, 12, 12a connecting the mill 3 and the wind classifier 5, in the line portion 12a that leads to the wind classifier 5 there also opens out a downcoming pipe, which comes from the product discharge 9 of the cyclone separator 8 and by which the classifier-input volumetric flow is laden with the ground material.

[0055] Furthermore, the transporting gas outlet 10 of the cyclone separator 8 is also followed by a return line 11, which leads back from there to the mill input and opens out between the low-temperature embrittling device 2 and the mill 3 at the product charge on the upper side of the mill 3.

[0056] Along with the line network 12, 12a, 14a, 15, configured as a multiply branched circuit 12, 12a, 14a, 15 with a circuit outlet 14, between the removal device 8 and the wind classifier 5, the cryogenic grinding apparatus has on the classifier side a similar circulating line arrangement with a controlled outlet.

[0057] From the classifier wheel output of the wind classifier 5, a circulating line 24a, 23 leads through an after-separating cyclone separator 20, an air filter 21 and a blower 22 back to the classifier inlet side, to be specific into the connecting line 12, 12a. Downstream of the blower a classifier outlet line 24 branches off, which leads into the surroundings and in which a throttle damper 27 is arranged.

[0058] The throttle damper 26 in the gas-excess outlet 14 and the throttle damper 27 in the classifier outlet 24 are in this case set by controller 28 in response to an output of a recording device 25, 25a, which is formed by an O2 sensor 25a downstream of the classifier wheel outlet and a temperature sensor 25 in the interior of the wind classifier 5.

[0059] Depending on whether or not an excessive temperature or an excessive oxygen content is reported, the controller opens the classifier outlet 24 and closes the gas-excess outlet 14, so that cold grinding gas can flow in a replenishing sense into the classifier circuit, or closes the classifier outlet 24 and opens the gas-excess outlet 14, so that all of the grinding gas is discharged into the surroundings. It goes without saying that intermediate positions of the two throttle dampers 26, 27 are possible. The further throttle dampers, indicated by crossbars, can of course likewise be controlled in an open-loop and/or closed-loop manner by the controller 28.

[0060] Downstream of the cyclone separator 8, the mill-output volumetric flow discharged by the mill 3 under pressure surges and density fluctuations is divided in respect of its transporting gas fraction between a number of alternative routes, depending on the position of the throttle dampers: firstly, the route directly to the wind classifier 5 via the line portion 12 and the further line portion 12a may be chosen, also the route to the wind classifier 5 via the line branch 14a leading to the gas-excess outlet 14, the return line branch 15 and the line branch 12a may be chosen; on the other hand, excess transporting gas may be blown out into the surroundings via the line branch 14a leading to the partial-stream outlet 14 and the partial-stream outlet 14, or it may reach the inlet side of the mill 3 via the return line 11.

[0061] Depending on the momentary pressure conditions that are caused by the pressure resistance of the classifier wheel of the wind classifier 5, and also the suction power of the blower 16, a further blower 22, and the mill 3, and also on account of the position of the throttle damper 26 in the gas-excess outlet 14 and a further throttle damper 27, the greater part of the transporting gas volumetric flow is then sucked directly to the wind classifier 5 or is sucked to the input side of the mill 3, for instance when an acceleration of the mill 3 is commencing on account of a rising temperature there, or is sucked in by the blower 16 and blown out at the partial-stream outlet 14. In this case, the further throttle valves, indicated by crossbars, may also open or close the respective line branches or open or close them to a certain degree.

[0062] The separating device 5a arranged downstream of the wind classifier 5 in terms of flow has in turn a cyclone separator, which is denoted by 20. At the cyclone separator 20 of the after-separating device 5a, the classifier-output volumetric flow, which is then only laden with the fine material, is freed of by far the predominant majority of the fine material particles, which are removed at the product discharge of the cyclone separator 20. The residual gas stream is then sucked by the blower 22 through an air filter 21 and thereby freed of the residual fine material particles, in order to be able to be blown away into the surroundings in a cleaned form through the classifier outlet line 24.

[0063] It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.