METHOD FOR RECOVERING SOLVENT AND CELLULOSE IN THE PRODUCTION OF CELLULOSIC SPUN-BONDED NONWOVEN FABRICS

Abstract

A plant (1) for the production of spunbonded nonwoven (8), comprising a spinning solution production (3), a spinning system (2), a device (6) for the delivery of coagulation liquid, at least one conveying device (7, 9) for depositing the spunbonded nonwoven (8), and a collecting device (13) for the spunbonded nonwoven (8), wherein at least one discharge device (14) is provided between the device (6) for the delivery of coagulation liquid and the collecting device (13) for the spunbonded nonwoven (8).

Claims

1. A plant for the production of a spunbonded nonwoven, comprising: a spinning solution production, a spinning system, a device for the delivery of coagulation liquid, at least one conveying device for depositing the spunbonded nonwoven, and a collecting device for the spunbonded nonwoven, and at least one discharge device, wherein the at least one discharge device is provided between the device for the delivery of coagulation liquid and the collecting device for the spunbonded nonwoven.

2. The plant according to claim 1, wherein the at least one discharge device is conductively connected to the spinning solution production.

3. The plant according to claim 1, wherein the at least one discharge device is provided at the conveying device in the area of deposition of the spunbonded nonwoven.

4. The plant according to claim 1, further comprising a drying device, wherein the at least one discharge device being provided between the conveying device and the drying device.

5. The plant according to claim 1, further comprising a drying device, wherein the at least one discharge device being provided between the drying device and the collecting device.

6. The plant according to claim 1, wherein the at least one conveying device is multipart and the at least one discharge device is provided between two parts of the at least one conveying device.

7. The plant according to claim 6, wherein a portion of the at least one conveying device further comprises a washing device, wherein the at least one discharge device being provided upstream of the washing device.

8. The plant according to claim 6, wherein a portion of the at least one conveying device further comprises a washing device, wherein the at least one discharge device being provided downstream of the washing device.

9. The plant according to claim 1, further comprising a shredding device, wherein an input of the shredding device is connected to the at least one discharge device.

10. The plant according to claim 9, wherein an output of the shredding device is connected to the spinning solution production.

11. The plant according to claim 8, further comprising a suspension container provided between the shredding device and the spinning solution production.

12. A method of recycling solvent or cellulose in a plant for the production of a spunbonded nonwoven, comprising: extruding spinning dope filaments from a spinning system, depositing and collecting the filaments, in a drawn state, to form a spunbonded nonwoven, wherein the spunbonded nonwoven is deposited during a start-up of the plant, during a shutdown of the plant, or in case of operational problems of the plant is discharged prior to collection, wherein at least (a) the solvent is removed from the spunbonded nonwoven or (b) the spunbonded nonwoven is supplied to the spinning solution production.

13. The method according to claim 12, wherein the spunbonded nonwoven is shredded before it is supplied to the spinning solution production.

14. The method according to claim 12, wherein that the discharged spunbonded nonwoven is mixed with solvent before it is supplied to the spinning solution production.

Description

[0048] For a better illustration of the invention, the essential features are depicted based on preferred embodiments of the method according to the invention, with reference to the following figures:

[0049] FIG. 1 schematically shows a plant according to the invention.

[0050] FIG. 2 shows the viscosity of a spinning dope from fresh pulp in comparison to reused cellulose from a spunbonded nonwoven.

[0051] FIG. 1 shows a plant 1 according to the invention, with the process stages also being explained with reference to the plant 1. By means of the spinning system 2, extruded and drawn spinning dope filaments 5 are deposited to form a spunbonded nonwoven 8 and are discharged at several discharge devices 14 via an interruption of the main process flow. During normal operation, the filaments 5 can be sprayed with a coagulation liquid from a device 6 for the delivery of coagulation liquid in order to regenerate the cellulose and to stabilize the shape of the filaments 5 before the process air 4 and the filaments 5 impinge on the conveying device 7 in the form of a conveyor belt and the spunbonded nonwoven 8 is formed. The spunbonded nonwoven 4 is then washed in the main process flow in a washing device 10, optionally solidified in a solidification system 11, dried in the drying device 12 and wound up into rolls in the collecting device 13.

[0052] It has been shown that during start-up, shutdown or in case of production problems, for example, in case of a failure of the device 6 for the delivery of coagulation liquid or of the washing device 10, more than 500 ppm NMMO, in some cases more than 5000 ppm NMMO, in extreme cases more than 50,000 ppm NMMO, remain in the spunbonded nonwoven during the production of spunbonded nonwovens from lyocell spinning dope. Without the method according to the invention, the solvent-laden spunbonded nonwoven would be transported to the collecting device 13 via the solidification system 11 and the drying device 12. Over an extended period of time, the NMMO accumulates in the water cycle of the solidification system 11 and may cause corrosion of plant parts and solidification nozzles. During prolonged operation, the NMMO may accumulate at the drying device 12 and cause fires in the drying device 12. In all cases, NMMO is lost, which, in addition to the above-mentioned safety problems, is neither economically nor environmentally acceptable.

[0053] It has been shown that, in case of productivities of the spinning system 2 of between 10 kg/h/m and 1000 kg/h/m cellulose throughput, the solvent losses are so high that a new method must be developed for solving the problems which have already been described.

[0054] According to the invention, the gaps between the conveyor belts 7, 9 are used to the extent that discharge devices 14 (not shown in detail) can be installed which allow the solvent-laden spunbonded nonwoven to be discharged. According to the invention, higher flexibility results for the plant operators already from the removal from the main process flow, since they will be able to perform activities on downstream plant parts without having to turn off or throttle the spinning solution production 3 and the spinning system 2 beforehand.

[0055] The discharged spunbonded nonwoven is shredded in a shredding device 15 to form pieces of spunbonded nonwoven and is suspended in the suspension container 16 by addition of liquid 17 to such an extent that it can be conveyed, for example, by a feed pump 18 either via the line 19 to the NMMO recovery (not shown in detail) or via a line 20 to the spinning solution production 3 (not shown in detail). It has been shown that the discharged spunbonded nonwoven can be mixed to a transportable suspension by the shredding step and by mixing with liquid. It is this step which enables the recycling of NMMO or the addition to the spinning solution production.

[0056] The discharge devices 14 may be located downstream of the spinning dope line, downstream of the spinning system 2, upstream or downstream of the transport device 7 and upstream or downstream of the transport device 9 associated to the washing device 10. In addition, discharge devices 14 may be located between the solidification system 11 and the drying device 12. Discharge devices 14 may also be arranged downstream of the drying device 12 and upstream of the collecting device 13 in order to discharge products of poor quality, to supply them to the recycling and to process them once more into the spinning dope. Depending on the process control and the plant layout, other positions for the discharge device 14 are also possible. A plant 1 may therefore have one or several discharge devices 14. The moisture load of the discharged spunbonded nonwovens per kg of cellulose may be from 0.1 kg/kg to 10 kg/kg in the area of the washing and from 0 to 4 kg/kg in the area after drying.

[0057] The discharged spunbonded nonwoven is guided to the shredding device 15 via the discharge device 14. For example, the shredding device 15 may be a mill, preferably a cutting mill, which can shred 10 to 5000 kg/h, preferably 100 to 2000 kg/h, more preferably 200 to 1000 kg/h of spunbonded nonwoven. The shredded pieces of spunbonded nonwoven have a length ranging from 10 μm to 100 mm, preferably from 0.1 mm to 50 mm, more preferably from 1 mm to 10 mm. Depending on the strength and desired degree of shredding of the spunbonded nonwoven, other shredding devices known to a person skilled in the art may also be used.

[0058] The shredded pieces of spunbonded nonwoven are mixed with liquid 17 in the suspension tank 16 and blended to form a transportable suspension. The suspension container 16 may comprise a stirrer for increasing the homogeneity of the suspension. In one embodiment variant, the suspension container may also be heated in order to improve the water absorption of the cellulose and to increase the swelling.

[0059] The liquid 17 used for the suspension may be demineralized water or a solution of the solvent used for the production of the spinning dope and water. If the spunbonded nonwoven is produced from a lyocell spinning dope, the liquid 17 for the suspension may have between 0 and 85% NMMO, preferably 10 to 80% NMMO, more preferably between 20 and 78% NMMO. The prepared suspension has a cellulose content of between 1 to 95%, preferably of 2 to 50%, more preferably of 3 to 30%. Since the suspensions that are prepared are pumped either to the solvent recycling or as an additive to the spinning dope production and the viscosity of the suspension highly depends on the cellulose content and the type of solvent, the cellulose content and the concentration of the solvent must be adjusted depending on the position of the discharge device (moisture content of the spunbonded nonwoven at this point) and the use of the suspension. It has been shown that the cellulose content of the suspension should preferably be below 20% when the suspension is transported to the NMMO recycling, since later purification steps, the solid/liquid separation and, subsequently, the NMMO attenuation can be performed better in the NMMO processing cycles of the lyocell process, which already exist anyway. If the suspension is to be reused for the production of spinning dope, higher cellulose contents of >20% are beneficial, since the cellulose content can be adjusted better in the eventual spinning dope.

[0060] FIG. 2 shows that the viscosity of the spinning dope deviates only slightly from the viscosity of a spinning dope made of fresh cellulose, despite the renewed shredding and dissolving of the spunbonded nonwoven (measured by means of the rheometer Kinexus from the company Malvern Messsystem CP4/40, oscillation measurement at 100° C. with 0.004 and 30 Hz, evaluation with rSpace for Kinexus, both samples measured with the same pulp and moisture contents). Thus, by means of the method according to the invention, spunbonded nonwovens that have already been produced and discharged can be reused. Thus, for example, lyocell spinning dopes with a pulp content of 4 to 17% can be produced from 100%, preferably 1 to 50%, more preferably 2 to 30%, of reused cellulose made of recycled spunbonded nonwoven.