Method for comminuting heat-sensitive feedstock

Abstract

A method for comminuting heat-sensitive feedstock, particularly thermoplastics, rubber, caoutchouc, and elastomers, to a particle size of less than 500 m, preferably less than 425 m. Process steps are provided to achieve an economic material processing without the use of cryogenic comminution. First, a precomminution of the feedstock to a size smaller than 4 mm is performed in a rotating comminuting device through which a first process gas PG.sub.1 flows. Then, a fine grinding is performed of the precomminuted feedstock to a size smaller than 500 m, preferably smaller than 425 m, in a rotating fine grinding device through which a second process gas PG.sub.2 flows. Whereby the temperature of the precomminuted feedstock in the outlet of the fine grinding is regulated in a second control circuit by adding water to the precomminuted feedstock before and/or during and/or after the fine grinding.

Claims

1. A method for comminuting heat-sensitive feedstock to a particle size of less than 500 m, the method comprising: precomminuting the feedstock to a size smaller than 4 mm in a rotating comminuting device through which a first process gas stream flows; regulating a first temperature of the feedstock in an outlet of the rotating comminuting device in a first control circuit by adding water to the feedstock before and/or during and/or after the precomminution; after the precomminuting of the feedstock, fine grinding the feedstock to a size smaller than 500 m in a rotating fine grinding device through which a second process gas stream flows; and regulating a second temperature of the feedstock in an outlet of the rotating fine grinding device in a second control circuit by adding water to the feedstock before and/or during and/or after the fine grinding, wherein the feedstock is removed by suction from the outlet of the rotating comminuting device, and wherein a third temperature of the feedstock in the suction area is regulated in another control circuit by adding water in the suction area and/or in the outlet of the rotating comminuting device.

2. The method according to claim 1, wherein a metering of the feedstock to the rotating comminuting device is regulated in a third control circuit as a function of the energy consumption of the rotating comminuting device.

3. The method according to claim 1, wherein a metering of the feedstock to the rotating fine grinding device is regulated in another control circuit as a function of the energy consumption of the rotating fine grinding device.

4. The method according to claim 1, wherein the feedstock is temporarily stored in an intermediate storage facility before entering the rotating fine grinding device, and wherein a temperature of the feedstock in the intermediate storage facility is monitored in another control circuit.

5. The method according to claim 4, wherein the temperature of the feedstock in the intermediate storage facility is at most 75 C.

6. The method according to claim 4, wherein the temperature of the feedstock in the intermediate storage facility is at most 60 C.

7. The method according to claim 1, wherein the first temperature and/or the second temperature are at most 70 C.

8. The method according to claim 1, wherein the third temperature is at most 45 C.

9. The method according to claim 1, wherein the third temperature is at most 35 C.

10. The method according to claim 1, wherein the first process gas stream and/or the second process gas stream are constant in terms of volume.

11. The method according to claim 1, wherein the feedstock includes thermoplastics, rubber, caoutchouc, and/or elastomers.

12. The method according to claim 1, wherein the first temperature and/or the second temperature are at most 65 C.

13. The method according to claim 1, wherein the first temperature and/or the second temperature are at most 55 C.

14. The method according to claim 1, wherein the third temperature is at most 40 C.

15. A method for comminuting heat-sensitive feedstock to a particle size of less than 500 m, the method comprising: precomminuting the feedstock to a size smaller than 4 mm in a rotating comminuting device through which a first process gas stream flows; regulating a first temperature of the feedstock in an outlet of the rotating comminuting device in a first control circuit by adding water to the feedstock before and/or during and/or after the precomminution; after the precomminuting of the feedstock, fine grinding the feedstock to a size smaller than 500 m in a rotating fine grinding device through which a second process gas stream flows; and regulating a second temperature of the feedstock in an outlet of the rotating fine grinding device in a second control circuit by adding water to the feedstock before and/or during and/or after the fine grinding, wherein the feedstock is removed by suction from the outlet of the fine grinding device, and wherein a temperature of the feedstock in the suction area is regulated in another control circuit by adding water in the suction area and/or in the outlet of the rotating fine grinding device.

16. The method according to claim 15, wherein the temperature of the feedstock in the suction area is at most 45 C.

17. The method according to claim 15, wherein the temperature of the feedstock in the suction area is at most 40 C.

18. The method according to claim 15, wherein the temperature of the feedstock in the suction area is at most 35 C.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein the sole FIGURE illustrates a flowchart according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

(2) The course of the method of the invention emerges from the flowchart shown in the FIGURE. The starting material for the method of the invention in the present exemplary embodiment is rubber waste, for example, peeled rubber as it accumulates during used tire processing, with pieces about 10 mm to 20 mm in size. This feedstock is supplied to a rotating comminution device 2 for precomminution via a metering station 1, where the feedstock is precomminuted to a particle size of at most 4 mm, preferably at most 2 mm. The precomminution occurs in a knife mill or cutting mill whose electrically driven rotor is equipped with blades. The rotor is surrounded by a screen basket, which keeps the feedstock within the active zone of the blades until comminution to below the hole diameter of the screen has occurred. After sufficient comminution of the feedstock, the material particles in process gas stream PG1 are removed from the precomminution. During the comminution, only part of the driving power is used for material comminution; the remaining part is converted to heat energy, which is the cause of the rise in the feedstock temperature.

(3) According to the invention, in this case thermal damage of the feedstock is prevented by the combined cooling measures. A portion of the excess heat energy is removed by the constant-volume process gas stream PG.sub.1, whose primary task is to provide for the transport of the feedstock to and from the comminution zone. The temperature T.sub.1 of the feedstock at the exit from the precomminution is monitored via a temperature sensor disposed downstream of the precomminution.

(4) If the cooling effect of the self-generated air at the given load of the comminuting device is not sufficient, which can be determined by a rise in the temperature T.sub.1, water, which removes heat from the feedstock by evaporation, is then added as a coolant to the material stream before and/or during the precomminution. The amount of coolant is regulated via a first control circuit R.sub.1 so that the temperature T.sub.1 remains below a predefined setpoint, for example, below 70 C., preferably below 65 C., most preferably below 55 C. The material stream in the outlet from the precomminution in this case has an inherent moisture content of about 3% to 5%.

(5) If these measures are not sufficient to maintain the temperature T.sub.1, then the metering of the comminuting device is regulated in a control circuit R.sub.3 as a function of the electrical current consumption. Reducing the setpoint for the maximum current consumption A.sub.1 of the comminuting device results in a limitation of the feedstock amount supplied to the precomminution and thereby also in a limitation of the heat development.

(6) The precomminuted feedstock is removed via suction from the comminution device 2 and is supplied to a silo 3 for temporary storage. In terms of plant layout, the suction is connected to the material outlet of the comminuting device 2. Water is again supplied in the suction area to reduce the feedstock temperature further to a value T3. The amount of supplied water is determined in a control circuit R5, in which the present material temperature in the suction area is compared with the temperature T3 and if there is a deviation a control signal is output. The temperature of the precomminuted feedstock before its intermediate storage is at most 45 C., preferably at most 40 C., and is preferably between 20 C. and 35 C. The inherent moisture content is 10% to 15%.

(7) As a safety measure to prevent a smoldering fire, it is possible to add water again during the intermediate storage of the precomminuted feedstock in the silo 3 or to flood the silo 3. To this end, the temperature of the precomminuted feedstock is monitored in a control circuit R7 and water is supplied if a limit value T5 is exceeded. The limit value T5 is, for example, at most 75 C., preferably at most 60 C.

(8) The precomminuted feedstock is supplied from the silo 3 to fine grinding in a rotating find grinding device 5 by means of a second metering station 4. The feedstock in this case has a temperature of about 20 C. to 30 C. at an inherent moisture content of 5% to 10%.

(9) The subsequent fine grinding can occur, for example, in a turbo mill whose rotating impact plates produce a highly turbulent vortex field in which the material particles are exposed to high acceleration and impact forces bringing about the comminution. Here as well, the self-generated air of the mill, which flows through the comminution chamber as a constant-volume process gas stream PG.sub.2, contributes a first portion for cooling the feedstock.

(10) If the material temperature exceeds a limit value T.sub.2, which is monitored downstream of the grinding zone at the exit from the fine grinding, then water is supplied as a coolant to the feedstock before the fine grinding, for example, in the feed to the mill, and/or during the fine grinding, for example, in the grinding zone of the mill, and/or after the fine grinding, for example, in the outlet from the mill, in a control circuit R.sub.2 until the temperature T.sub.2 is reached. The temperature T.sub.2 is, for example, at most 70 C., preferably at most 65 C., most preferably at most 55 C.

(11) If this measure is not sufficient, thus slowing down the metering can further affect the temperature development. To this end, the maximum current consumption A.sub.2 of the mill is limited and monitored in a control circuit R.sub.4, as has already been described during the precomminution.

(12) After the fine grinding, a predominant portion of about 90% to 99% of the end product has a particle size of at most 500 m, preferably at most 425 m, at a temperature below T.sub.2 and an inherent moisture content preferably in the range of 0.5% to 2%.

(13) The removal of the sufficiently refined material from the fine grinding occurs again actively via suction, which in terms of plant layout is connected to the material outlet of the fine grinding device. Water is again supplied in the suction area to reduce the material temperature to a value T.sub.4. The amount of supplied water is determined in a control circuit R.sub.6, in which the present material temperature in the suction area is compared with the temperature T.sub.4 and if there is a deviation a control signal is output. The temperature T.sub.4, which in the present exemplary embodiment is at most 45 C., preferably at most 40 C., for example, between 20 C. and 35 C., is therefore used as a reference variable for control circuit R.sub.6. Optionally, an inherent moisture content of at most 1% is achieved by secondary drying of the material in the suction air stream.

(14) In the subsequent process step, the material leaving the mill is screened, whereby particles larger than 500 m, preferably larger than 425 m, are returned to the fine grinding and the sufficiently refined end product is packaged.

(15) The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.