A PROCESS AND A SPINNING LINE UNIT FOR WET SPINNING OF CELLULOSE FIBERS FROM AN ALKALINE SPIN BATH

Abstract

A process for forming a fiber tow, involves a wet spinning procedure comprising the steps of: dissolving cellulose pulp N in an alkaline aqueous solvent to form a cellulose spin dope composition, spinning the cellulose spin dope composition in a coagulation having a p H of more than 7.0, preferably a pH of at least 10, to produce a fiber tow, and passing the produced fiber tow through a sequence of consecutive stretching and washing steps in which the formed fiber tow is washed with a washing liquid by a counter-current flow washing procedure.

Claims

1. A process for forming a fiber tow, said process involving a wet spinning procedure comprising the steps of: dissolving cellulose pulp in an alkaline aqueous solvent to form a cellulose spin dope composition, spinning the cellulose spin dope composition in a coagulation bath having a pH of more than 7.0, preferably a pH of at least 10, to produce a fiber tow, and passing the produced fiber tow through a sequence of consecutive stretching and washing steps in which the fiber tow is washed with washing liquid having gradually lower alkalinity, preferably by a counter-current flow washing procedure.

2. The process according to claim 1, wherein the step of dissolving the cellulose pulp is performed in a cold alkaline aqueous solvent at a temperature of 0° C. or lower.

3. The process according to claim 1, wherein the process comprises passing the produced fiber tow through at least five consecutive washing steps, preferably at least seven consecutive steps, more preferably at least ten consecutive steps.

4. The process according to claim 1, wherein at least one washing step is performed by spraying, preferably the flow of spray washing liquid in at least some of the washing steps with spraying, or preferably in each washing step with spraying, is at least 5 kg washing liquid per kg of cellulose spin dope composition supplied to the coagulation bath, more preferably at least 8 kg washing liquid per kg of cellulose spin dope composition.

5. The process according to claim 1, wherein the alkali content, calculated as ppm by weight of NaOH, in the produced fiber tow is lowered gradually during the washing procedure to less than 50 ppm wt NaOH, calculated on dry fiber tow.

6. The process according to claim 1, wherein the coagulation bath comprises sodium hydroxide and sodium carbonate and/or sodium sulfate, preferably said coagulation bath comprises 3-10 wt. % sodium hydroxide, preferably said coagulation bath comprising 10-28 wt. % of at least one of sodium carbonate and sodium sulfate.

7. The process according to claim 2, wherein the cold alkaline aqueous solvent comprises 0.4-1.2 wt. % zinc (as Zn), calculated on the total weight of the cellulose spin dope composition.

8. The process according to claim 1, wherein the cellulose spin dope composition comprises cellulose, cellulose carbamate or another derivative of cellulose in the range of 4-12 wt. %, preferably 5-10 wt. %, calculated on the total weight of the cellulose spin dope composition.

9. The process according to claim 1, wherein the cellulose spin dope comprises urea or cellulose carbamate which at least partially is hydrolyzed in the coagulation bath and in the consecutive washing steps.

10. The process according to claim 8, wherein any ammonia formed from the hydrolysis of cellulose carbamate or urea is collected and discharged from the spinning step.

11. The process according to claim 1, wherein stretching of the fiber tow is performed and controlled by regulating the speed at which the fiber tow travels from the coagulation bath and through the consecutive washing steps.

12. The process according to claim 1, wherein the speed of the fiber tow as of washing step 1 is substantially maintained constant or is gradually increasing in one or more following washing steps at least up to and including the washing step after which the hydroxide concentration of the fiber tow is lower than 0.3 wt. %.

13. The process according to claim 1, wherein the stretching is controlled to a degree resulting in a total stretch in the range of 30-80%, preferably controlled to a degree resulting in the tensile strength of the produced fiber becoming at least 15 cN/tex.

14. The process according to claim 1, wherein stretching of the fiber tow is performed between the coagulation bath and the first washing step and wherein the fiber tow is kept in a stretched condition during at least part of the process thereafter.

15. The process according to claim 1, wherein stretching of the fiber tow is performed between the coagulation bath and the first washing step and wherein stretching is continued during at least two, more preferably at least three, consecutive washing steps, preferably said at least two consecutive washing steps during which stretching is continued being among the four washing steps being arranged immediately after the coagulation bath, preferably the portion of the total stretch being performed in the at least two washing steps being at least 25%, more preferably at least 40% of the total stretch.

16. The process according to claim 1, wherein a stretched condition, meaning that any substantial slack in the fiber tow is avoided, is maintained during at least 50% of the washing procedure, preferably the fiber tow is subjected to stretching for elongation in at least two positions, of which a first position being between the coagulation bath and the first washing step and a second position being in at least one consecutive washing step, still more preferably said second position of stretching for elongation being inside the first washing step or between the first and second washing steps.

17. The process according to claim 1, wherein stretching for elongation of the fiber tow is performed between the coagulation bath and the first washing step at a certain degree of stretching, preferably stretching for elongation is continued during at least three consecutive washing steps, still more preferably the stretching for elongation is continued between and/or within those at least three consecutive washing steps at a total degree of elongation being in the range of 0.7 to 1.2 times the stretching being performed between the coagulation bath and the first washing step, preferably added degree of stretching in each step is lower than the added degree of stretching in the step between the coagulation bath and the first washing step, more preferably wherein the added degree of stretching decreases in each consecutive step.

18. The process according to claim 1, wherein the largest part of the total stretch for elongation of the produced fiber tow is performed in a stretching step between the coagulation bath and the first washing step of the counter-current flow washing procedure, when comparing the elongation of the stretching step between the coagulation bath and the first washing step to the elongation of other individual stretching steps between and/or within washing steps of the counter-current flow washing procedure, preferably at least 40% of the total elongation is performed between the coagulation bath and the first washing step of the counter-current flow washing procedure.

19. The process according to claim 1, wherein the largest part of the total stretch for elongation of the produced fiber tow is performed in a stretching step between the first and the second washing step and/or within the first or the second washing step.

20. The process according to claim 1, wherein the alkaline aqueous solvent comprises zinc, wherein the alkalinity is reduced in the fiber tow gradually during the washing procedure, and wherein zinc diffuses out from the fiber tow and into the washing liquid during the washing procedure.

21. The process according to claim 20, wherein zinc diffuses out from the fiber tow and precipitates in the washing liquid in at least one washing step, wherein the washing liquid is suspended to keep the precipitated zinc dispersed in the washing liquid, and wherein the precipitated zinc is transported together with the washing liquid to at least one upstream, as seen with regard to the transport direction of the fiber tow, washing step in dispersion without settling.

22. The process according to claim 21, wherein the precipitated zinc following the counter-current washing liquid is dissolved at increasing alkalinity of the washing liquid in at least one upstream washing step and is at least partially recycled and reused in the step of dissolving cellulose pulp in an alkaline aqueous solvent to form a cellulose spin dope composition.

23. The process according to claim 1, wherein the washed fiber tow is subjected to a drying operation in which the fiber tow is dried in an unconstrained fashion, substantially free from sharp bends, to allow free shrinkage substantially without tension in the fiber direction, to produce a washed and dried fiber tow.

24. The process according to claim 1, wherein the process also comprises crimping of the fiber tow, preferably said crimping is performed subsequently to drying of the fiber tow.

25. The process according to claim 24, wherein mechanical crimping is performed subsequent to the drying of the fiber tow.

26. The process according to claim 24, wherein the process comprises cutting the dried and crimped fiber tow into staple fibers.

27. The process according to claim 1, wherein the fiber tow is treated with an acid in an acid treatment step subsequently to the last washing step in the washing procedure.

28. The process according to claim 1, wherein one or more surface active agents are supplied during the process to decrease cohesive forces between single filaments in the fiber tow, wherein said one or more surface active agents preferably are supplied to the fiber tow subsequently to the washing procedure, preferably subsequent to a possible acid treatment step and preferably before the drying procedure.

29. A spinning line unit for forming a fiber tow, said spinning line unit comprising a dissolver unit arranged for dissolution of cellulose pulp in an alkaline aqueous solvent, preferably in a cold alkaline aqueous solvent at a temperature of 0° C. or lower, to form a cellulose spin dope composition; a spinning unit arranged for spinning the cellulose spin dope composition in an alkaline aqueous coagulation bath having a pH of more than 7.0, preferably a pH of at least 10, to produce a fiber tow; and a washing line, preferably a counter-current flow washing line, comprising several washing units arranged in sequence for washing of the produced fiber tow in consecutively lower alkalinity.

30. The spinning line unit according to claim 29, wherein the washing line comprises at least five washing units, preferably at least seven washing units.

31. The spinning line unit according to claim 29, wherein the spinning line unit also comprises one or more stretch control units, preferably at least for control of the stretching for elongation between the coagulation bath and the first washing unit, more preferably for controlling the stretching for elongation also between and/or within several of the washing steps.

32. The spinning line unit according to claim 29, wherein the spinning line unit comprises a fiber tow drying unit adapted to perform a drying operation in which the fiber tow is dried in an unconstrained fashion, substantially free from sharp bends, to allow free shrinkage substantially without tension in the fiber direction, for the production of a washed and dried fiber tow, preferably the spinning unit comprising a crimping unit for crimping the washed fiber tow, still more preferably the spinning line unit further comprising a fiber cutting unit for cutting the washed and dried fiber tow into staple fibers.

33. The spinning line unit according to claim 29, wherein the spinning unit also comprises means for enabling zinc to diffuse out from the fiber tow and precipitate in a washing liquid, and for precipitated zinc to follow the washing line, preferably counter-current washing flow line, for recycling and reuse of zinc in the dissolver unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] FIG. 1 A illustrates a spinning line according to a first embodiment.

[0071] FIG. 1 B illustrates a spinning line according to a second embodiment.

[0072] FIG. 2 illustrates a spinning line according to a further embodiment.

[0073] FIGS. 3a and 3b illustrate stretching of tow at different alkalinity.

[0074] FIG. 4a illustrate titer of fibers sampled at different locations along a spinning line.

[0075] FIG. 4b illustrate the composition of the washing liquids of the spinning line referred to in FIG. 4a.

[0076] FIG. 5 illustrate the composition of the washing liquids in a test of stretching for elongation.

[0077] FIG. 6a is a micrograph of a fiber cross section of a fiber washed in pure water.

[0078] FIG. 6b is a micrograph of a fiber cross section of a fiber washed gradually with consecutively lower alkalinity.

[0079] FIG. 7 illustrates a trial set-up for testing fiber adhesion.

[0080] FIG. 8 is a photograph of fibers washed with gradually decreasing alkalinity, to the right in FIG. 8 and marked with a “B”, and fibers immediately washed with pure water, to the left in FIG. 8 and marked with “BW”.

[0081] FIG. 9a is a photograph showing to the left: fibers dried as fiber tow in unconstrained manner and free from sharp bends, and to the right: staple fibers dried in randomly wrinkled state.

[0082] FIG. 9b illustrates the strengths of the fibers of FIG. 9a.

[0083] FIG. 10 illustrates schematically mechanical crimping using a stuffer box crimper.

[0084] FIG. 11a is a picture of a dried fiber tow before crimping (to the right) and after crimping (to the left).

[0085] FIG. 11b illustrates the strength of fiber tows before and after mechanical crimping.

[0086] FIG. 12 shows the washing efficiency for trials made in which the washing technologies immersion and spraying are compared.

EXAMPLES AND DETAILED DESCRIPTION OF THE DRAWINGS

[0087] In FIG. 1A there is shown part of a spinning line 1 according to a first embodiment. In this case the coagulation bath 2 comprises at least three spinning positions or spinning heads 3. Each spinning head 3 comprises a plurality of spinnerets and each spinneret comprises a plurality of capillaries. The fiber tows from the spinning heads/positions are combined, side by side, into a flat common fiber tow. The produced fiber tow is led into the washing procedure 4, preferably operated according to the counter current flow principle. As depicted in FIG. 1A there may be up to a number of n washing steps, where n may be at least 5, preferably at least 7, and up to 10 or above 10. Vn refers to the speed of the tow over each godet 5 in each specific washing step. In the washing procedure water is flown into the final washing step. The washing liquid is then led through each washing step in a counter-current flow when comparing with the fiber tow. The washing liquid led out from the first washing step (as numbered with reference to the processing of the fiber tow) has a higher alkalinity. As notable in FIG. 1A, in each respective washing step the fiber tow, still in a condition of tension, is contacted with the washing liquid in each respective washing step by means of a pump driven wash bath liquid circulation flow, sprinkling or spraying wash bath liquid onto the fiber tow. Press rollers are applied to the outgoing tow from each washing step to reduce the amount of wash liquid entrained (carried over) by the tow from one washing step into the next washing step.

[0088] In FIG. 1B there is shown another embodiment. As notable in FIG. 1B, in each washing step the fiber tow, still in a condition of tension, meaning the fiber tow is either experiencing stretching for elongation, or is at least not slacking, is led down into the washing bath and then up from the bath and led into the next washing bath. Press rollers are applied to the outgoing tow from each washing step to reduce the amount of washing liquid entrained by the tow from one washing step into the next washing step.

[0089] In FIG. 2 there is shown a schematic view of a spinning line according to still another embodiment. As notable, in this case the coagulation bath comprises one spinning head. From the spin dope led into the coagulation bath a fiber tow is produced. The spun fiber tow is then led to a counter-current washing procedure which functions as disclosed above. Between some, or even between each, washing step, and also between the coagulation bath and the first washing step, a stretching for elongation is performed. As mentioned above, the stretching for elongation may be performed in different ways and with different magnitude in the different steps. Subsequent to the last washing step, an avivage step may be arranged, in which step surface active agents are added to the fiber tow. The tow may pass an avivage bath or being sprayed or receiving the agents by means of a kiss roll or in some other way. Thereafter, drying may be performed, then crimping and finally cutting of the produced fiber. Crimping may also be performed before drying or in a semi dry state (not indicated in FIG. 2). It should be noted that also other steps may be involved, such as an acid addition step arranged directly after the final washing step and/or a bleaching step.

[0090] In the processes shown in FIGS. 1 and 2, the spin or coagulation bath liquid may contain water, sodium carbonate (Na.sub.2CO.sub.3) or sodium sulfate (Na.sub.2SO.sub.4) or a mixture thereof, sodium hydroxide (NaOH) and a minor amount of zinc containing salts. When the thin (e.g. from 50 to 70 μm in diameter) dope jets extruded from the spinneret capillaries come into contact with the spin bath liquid, water and some hydroxyl ions is diffusing out from the jets while sodium and carbonate ions, and/or sulfate ions in case of sodium sulfate being present in the spin bath, diffuses into the jets due to the differences in osmotic pressure (concentration differences). Due to the change in chemical composition within the dope jets cellulose does not stay in solution any longer and precipitates in the form of a more or less oriented network of nanofibrils. To which extent the nanofibrils are oriented, parallel with the longitudinal direction of the fiber, depends on the design of the spinneret capillaries and the draft ratio applied in the coagulation bath, that is, on the ratio V.sub.0/V.sub.exit. V.sub.0 is the speed of the tow exiting the coagulation bath and V.sub.exit being the exit speed of the dope jets from the spinneret capillaries (volumetric flow rate of dope divided by the total capillary cross-sectional area). By the coagulation process the liquid jets leaving the spinneret capillaries are transformed into soft and highly swollen gel filaments that are drawn upwards through the spin bath by means of a godet roller and by the action of buoyancy. The coagulation process is thus very different to the viscose process where the spin or coagulation bath contains sulfuric acid (H.sub.2SO.sub.4). In the viscose process the sodium hydroxide in the spin dope is neutralized by the acid. The cellulose precipitates very quickly and form rather compact and solid filaments instantaneously at the spinneret exit. This also means that most of the water in the spin dope (dope contains about 85% water) end up in the acidic bath and that only about 120 wt. % spin bath liquid based on dry cellulose is entrained with the fiber tow from the spin bath. For the process according to the present invention, coagulating the dope jets into an alkaline spin bath, the corresponding figure may exceed 1000 wt. % spin bath.

[0091] In the process, sodium carbonate and/or sodium sulfate is picked up by the filaments. At the same time some water and hydroxyl ions from the spin dope is transferred to the coagulation bath liquid. It has been found that the coagulation bath level may slowly increase or decrease depending on how much spin bath liquid that is squeezed out from the tow at the godet roller and fed back into the spin bath. The press roller force is preferably adjusted so that the coagulation bath level stays constant or so that an over flow of coagulation bath liquid is obtained, see FIG. 1A. In this way only salt (Na.sub.2CO.sub.3 and/or Na.sub.2SO.sub.4) must be continuously fed to the coagulation bath to keep the salt concentration constant. It can be mentioned that if hydrated forms of sodium salt are fed to the coagulation bath, to keep the sodium salt ion concentration constant, the over flow will be higher. The concentration of NaOH and Zn in the coagulation bath will then be lower if not deliberately adjusted by adding NaOH and Zn to the coagulation bath.

[0092] Maximum stretching for elongation, and thus maximum fiber tenacity, is obtained by performing the stretching for elongation of the fiber tow when the fiber tow is in an alkaline state, having a relatively high content of sodium hydroxide. Such stretching for elongation makes the nanofibrils orient themselves in the longitudinal direction of the fiber. However, if the fibers are not maintained in a stretched state, such “stretched state” meaning the fibers are being further elongated, or at least kept at such a tension that there is no substantial slacking of the fiber tow, under further reduction of alkalinity the induced orientation will relax to a certain degree, thereby reducing the fiber tenacity.

[0093] To obtain fibers with high tenacity the tow is stretched for elongation between godet 0 and 1 (speed of godet 1 is suitably higher than that of godet 0, see FIG. 2). The stretching for elongation is assumed to orient the nanofibrillar cellulose structure in the direction of (along) the fiber tow. It has been found that the tow is more stretchable if the alkalinity of the tow is high (high amount of NaOH in the tow), as it is between godet 0 and 1 (FIG. 2) for instance. This is illustrated in FIGS. 3a and 3b showing bench scale stretching of tow with different alkalinity. The coagulation bath held at 28° C. contained 20 wt. % sodium carbonate, 5.6 wt. % NaOH and 0.56 wt. % Zn and the speed of godets 0 (V0) and 1 (V1) was the same as the extrusion speed (V.sub.exit) from the spinneret. The spin dope used contained 6 wt. % cellulose, 7.5 wt. % NaOH and 0.76 wt. % Zn and was extruded through a spinneret having 300 capillaries with the diameter 55 μm. The fiber tow was stretched between godet 1 and 2 (V2>V1) after passing a bath in which the concentration of NaOH was adjusted to 0, 2.2, 3.5 and 4.6 wt. %. FIG. 3a shows that the maximum possible stretch ratio (V2/V1), before the fiber tow breaks, increases with wt. % NaOH in the bath preceding the stretching for elongation. It has, however, been found that if the stretched tow is cut to staple fibers at this stage (directly after godet 2), meaning that the tension in the fibers is released, the orientation of the nanofibrils is lost to a large extent, resulting in less strong fibers. By gradually reducing the alkalinity of the fiber tow under maintained tension, meaning that any slacking in the fiber tow is substantially avoided, it has been found that the fiber orientation remains high and becomes a permanent feature of fiber. A possible explanation is that nanofibrils are gradually coming closer to each other (decreased swelling) and become bonded to each other by the formation of hydrogen bonds as the alkalinity is gradually decreased in the consecutive washing steps (1−n).

[0094] To provide one example, tow samples were taken at different positions along the spinning line. The washing unit was designed according to FIG. 1A with 10 washing steps (n=10). The number of capillaries in the spinning head was 13500 (capillary diameter was 55 μm). The spin bath was operated at 29° C. and contained 18 wt. % sodium carbonate, 5.3 wt. % sodium hydroxide and 0.5 wt. % Zn. The spin dope used contained 6 wt. % cellulose, 7.5 wt. % NaOH and 0.76 wt. % Zn. The wash water flow rate to spin dope flow rate ratio was unity (Qw/Qd=1) and the ingoing wash water temperature was 20° C. The stretch ratio V1/V0 was 1.4. The speed of the godets 2 to 10 were kept constant and equal to the speed of godet 1, that is, V1=V2=V3=V4=V5=V6=V7=V8=V9=V10. Tow samples were taken directly after godets 1, 3, 4, 6, 7, 9, and 10, see FIG. 1A and FIG. 2, and were further washed freely in water. After drying the tow samples freely in air, the titer (dtex) was measured on single fibers extracted from the tow samples. FIG. 4a show the measured titer variation while FIG. 4b show the concentration of NaOH and Na.sub.2CO.sub.3 in the relevant wash liquids. The theoretically calculated fiber titer based on the spinning conditions used and assuming zero relaxation was 1.30 dtex. From the graphs in FIGS. 4a and 4b it can be concluded that in order to completely avoid relaxation of the orientation induced by stretching between godet 0 and godet 1, and thus to obtain a final fiber titer of 1.3 dtex, the fiber tow should be kept under tension (not slacked) at least up to the washing step in which the wash liquid has a NaOH concentration of about 0.3 wt. %.

[0095] Regarding partitioning of the stretching for elongation over the counter-current washing process, the individual speed regulation of the godet rollers (0−n in FIG. 2) may be seen as important for several reasons: 1) differentiated stretching for elongation along the washing sequence may be used to optimize fiber properties (tenacity and elongation), 2) to avoid possible slack in the fiber tow, 3) to avoid unwanted high tension in the fiber tow due to shrinkage. Individual speed regulation thus ensures a close control of tow tension in (along) the washing process.

[0096] In one test trial fibers were spun according to FIG. 2 using 12 washing steps (n=12). The stretching for elongation was partitioned as shown in table 1 below.

TABLE-US-00001 TABLE 1 Total stretch Tenacity Elong. Titer V1/V0 V3/V0 V5/V0 V7/V0 (%) (cN/tex) (%) (dtex) 1.400 1.414 1.428 1.457 45.7 16.9 9.0 1.3 1.200 1.296 1.387 1.456 45.6 18.9 10.2 1.3

[0097] Stretching was done in 4 steps as indicated in the table 1 to virtually the same total stretch. The alkalinity in the wash baths is shown in the FIG. 5.

[0098] From the table 1 and the FIG. 5 it is seen that the final fiber properties can be affected by distributing the stretch so that stretching is performed at different alkalinity. In this case it is seen that fiber properties improves if a part of the stretching for elongation is performed at lower alkalinity.

[0099] With reference to the perspective of avoiding precipitation of zinc in fibers, it has been found that zinc can precipitate inside the fibers in the form of Zn(OH).sub.2 if alkalinity is not lowered in a gradual fashion during washing. Zinc may be harmful to the aquatic environment and should therefore be minimized in the final fiber. Important is also that zinc following the fiber is lost and cannot be recycled in the process thus generating extra cost.

[0100] In FIGS. 6a and 6b there are shown SEM micrographs of an ion polished fiber cross section of a fiber washed in pure water directly after the coagulation bath (FIG. 6a) and of a fiber washed gradually in several wash baths with consecutively lower alkalinity (FIG. 6b). In FIG. 6a there is seen numerous sub-micron particles of zinc hydroxide while the gradually washed fiber does not show any signs of precipitated particles (see FIG. 6b).

[0101] By using a washing principle in which the alkalinity is gradually lowered Zn is obviously following NaOH, without precipitating inside the fiber, and diffusing out from the fiber and into the wash liquid. It has been experimentally shown that the Zn/Na weight ratio is the same (about 0.1) in the outgoing wash liquid as it is in the spin dope when the wash principle of gradually decreasing alkalinity is used, showing that Zn stays in solution with NaOH and thus is completely washed out from the fibers. Then a Zn content lower than 50 mg/kg fiber may be obtained.

[0102] The possible precipitation of Zn inside the fibers thus seems to depend on the concentration gradient over the washing machine/unit. The concentration gradient depends, among other things, on the following factors:

[0103] The ratio of wash water flow to spin dope flow

[0104] The number of washing steps

[0105] The amount of washing liquid that is entrained with the tow which depends on the force applied on the pressure rollers

[0106] Since the fiber tow needs to be washed down to some maximum alkalinity the above factors are not independent. For instance, the ratio of washing water to spin dope has to be increased if the number of washing steps is reduced for the same wash efficiency and the number of washing steps or the washing water to spin dope ratio can be reduced if the force is increased on the pressure rollers (less wash liquid is entrained to the next washing step).

[0107] A problem might also be when Zn is precipitated in the form of zinc hydroxide particles in the wash liquid. Such particles may lead to sedimentation and possible clogging of the connection lines/pipes between the wash steps. By ensuring that the washing liquid is in enough motion (avoiding stagnant areas/zones) sedimentation may be avoided and the Zn particles will then follow the washing liquid flow to higher alkalinity wash steps where Zn is again dissolved. This implies that there is provided means to avoid extracting solid Zn particles from the washing liquid in downstream washing steps for recycling which could be an alternative. Furthermore, dilution of chemicals for economic recycling may also be minimized. Since the recycling of NaOH, Zn and sodium salt may involve energy intensive processes, such as evaporation of water from the washing liquid, the degree of dilution is relevant for process economy. The wash water flow rate to spin dope flow rate ratio (Qw/Qd) should thus be minimized. Counter-current flow washing is in this respect a very efficient process. Qw/Qd is decreasing with increasing number (n) of wash steps. Qw/Qd is also decreasing with decreasing entrainment of washing liquid between the washing steps which in turn depends on the squeezing force of the pressure rollers (see FIG. 2). Process complexity and investment cost increase with n. Pressure roller force can be high downstream in the washing process where alkalinity is low. Pressure roller force upstream in the washing process, where alkalinity is high and the fibers are still soft and swollen, has to be lower to avoid damaging the fibers/tow. In FIG. 4 there is presented one example with n=12 and Qw/Qd=1. In this case the pressure roller force was low in the first 3 washing steps and then gradually increased in step 4 to 12 to be about 10 times higher in step 12 compared to step 1.

[0108] Moreover, with reference to avoiding fiber to fiber adhesion during washing out chemicals it has, according to the present invention, been found that fiber to fiber adhesion can be a problem if the alkalinity of the tow is reduced abruptly by washing at the same time as the tow is under tension, meaning not in a slacking condition. By using counter-current flow washing in which alkalinity is reduced gradually, fiber to fiber adhesion is eliminated or at least minimized.

[0109] One trial set-up is presented in FIG. 7. Bundles of tow (about 20 tows in a bundle) was gathered after V1 according to FIG. 7 and were washed in wash baths 1-5 with gradually decreasing alkalinity according to table 2 below.

TABLE-US-00002 TABLE 2 Bath # 1 2 3 4 5 Na.sub.2CO.sub.3, wt. % 17.6 7.04 1.76 0.44 0 NaOH wt. % 5.4 2.16 0.54 0.135 0

[0110] Samples marked BW in FIG. 8 were directly transferred to wash bath #5 (pure water). Samples marked B in FIG. 8 were first transferred to bath #1 and then to baths #2-5 in turn.

[0111] It may be seen that the individual fiber tows in the bundles are clearly discernible in the samples marked BW to the left while individual tows are hardly discernible in the samples to the right marked B showing that fiber to fiber adhesion can be avoided to a large extent by gradually washing out the chemicals from the tow.

[0112] The process according to the present invention also presents means for avoiding weak fibers due to dried-in wrinkles by drying fiber tow instead of staple fibers. The regeneration of fibers from directly dissolved cellulose produce stiff and somewhat brittle fibers (both dry and wet moduli are higher than for conventional viscose fibers) due to high crystallinity. High fiber stiffness is assumed to be positive for dimensional stability of garments during washing. However, such fibers are susceptible to dried-in wrinkles forming stress concentrations upon unfolding and stretching. Weak spots along the staple fibers produce lower tenacity, fiber shortening and dust generation during carding/sliver formation/yarn spinning.

[0113] It has now been found that stress concentrations leading to a weakening of fibers due to dried-in wrinkles can be avoided if the fibers are dried in tow form before being cut to staple fibers.

[0114] Several repeated experiments performed show that drying randomly wrinkled fibers, like in the conventional viscose process, produce a reduction in fiber strength compared to unconstrained drying (i.e. under zero tension) of fiber tow substantially free from sharp bends. FIG. 9a shows, to the left: fibers dried as fiber tow in unconstrained manner and substantially free from sharp bends, and to the right: staple fibers dried in randomly wrinkled state. In FIG. 9b there is shown tenacity (strength) in cN/tex of fibers dried in an unconstrained fashion, substantially free from sharp bends, referred to in FIG. 9b as “Free drying of straight tow”, compared to staple fibers dried in a randomly wrinkled state. As illustrated, the fibers dried as a fiber tow in an unconstrained fashion, substantially free from sharp bends, have consistently higher strength than the fibers dried as staple fibers in randomly wrinkled state.

[0115] Moreover, there is also provided means for mechanical crimping without inducing fiber weaknesses. Dried-in wrinkles may be avoided by drying tow as described above. However, straight fibers without crimp are difficult to handle in downstream operations like carding and sliver manufacture due poor web cohesion since straight fibers are less prone to entangle with each other compared to curly/crimped fibers. It has now been found that mechanical crimping using e.g. a stuffer box crimper (see schematic view with crimping principle in FIG. 10) of the dried tow before cutting to staple fibers could be a remedy.

[0116] It has been found that a fiber tow that is dried in an unconstrained state, that is, under no tension during drying; allowing free shrinkage of the tow in the longitudinal direction/fiber direction during drying, produce crimped fibers with maintained strength.

[0117] Several repeated experiments performed show that fiber tow dried in unconstrained manner and substantially without sharp bends, referred to as “Freely dried tow” in FIG. 11b, maintains mechanical properties after crimping, see FIGS. 11a and 11b for comparison of tenacity of un-crimped and crimped tow. In FIG. 11a the right sample is a dried fiber tow before crimping, and the left sample is a dried fiber tow that has been exposed to mechanical crimping by means of a stuffer box, such as the one illustrated in FIG. 10. In FIG. 11b the strength (tenacity) in cN/tex of a number of fiber tow samples that have not been crimped, the left bars in FIG. 11b, are compared to a number of fiber tow samples that have been exposed to mechanical crimping, the right bars in FIG. 11b. As can be seen from FIG. 11b the tenacity is in principle the same before and after the mechanical crimping. The tests described with reference to FIGS. 9a and 9b clearly indicate that “dried-in kinks”, formed during drying, produce weak spots along the fibers, while the tests of FIGS. 11a and 11b clearly indicate that rather sharp folds or bends created in the crimper on already dry fibers (10-20% humidity) do not produce any such weak spots.

[0118] Regarding crimping using a stuffer box crimper it has been found that the dry content of the tow entering the stuffer box cannot be too high, nor too low. The fibers become so stiff and brittle that they break in the stuffer box if the tow has a dry content approaching 100 wt % (very low humidity; as measured gravimetrically by weighing the wet sample before drying and then after drying in an oven at at least 100° C., such as around 105° C., and at least 1 hour, such as 2 hours or more, even up to 24 hours, and where the weight loss is assumed to be evaporated water). If the humidity of the tow is too high (low dry content) the fibers become so soft that the stuffer box become jammed. The best result and smooth processability is found if the dry content of the tow entering the stuffer box is in the range of 80-90 wt. %.

[0119] Moreover, the washing efficiency was also investigated in trials in which immersion was compared with spraying. The washing efficiency WE may be calculated as:

[00001]$W_{E_{\mathrm{NaOH}}}=\frac{{\mathrm{NaOH}}_{\mathrm{IN}}-{\mathrm{NaOH}}_{\mathrm{OUT}}}{{\mathrm{NaOH}}_{\mathrm{IN}}-{\mathrm{NaOH}}_{\mathrm{wL}}}$$W_{E_{\mathrm{Na}2\mathrm{CO}3}}=\frac{\mathrm{Na}2{\mathrm{CO3}}_{\mathrm{IN}}-\mathrm{Na}2\mathrm{CO}{3}_{\mathrm{OUT}}}{\mathrm{Na}2\mathrm{CO}{3}_{\mathrm{IN}}-\mathrm{Na}2\mathrm{CO}{3}_{\mathrm{wL}}}$

This corresponds to the difference in the NaOH concentration (or Na.sub.2CO.sub.3 concentration) between the input tow and the output tow divided by the difference in NaOH concentration (or Na.sub.2CO.sub.3 concentration) between the input tow and the washing liquid. In this context “input tow” implies the tow entering the washing step and “output tow” implies the tow exiting the washing step.

[0120] As notable in two different trial set-ups, presented in FIG. 12, spraying gave an enhanced washing efficiency when compared to immersion. In these trials the total fiber tow had 243,000 dtex (corresponding to 162,000 filament with a titer of 1.5 dtex). This fiber tow was limited to a width of 5 cm giving a theoretical tow thickness of 48,600 dtex/cm.

[0121] The total washing time was 20 seconds for all samples shown in FIG. 12. This time relates to the time during which the tow was immersed in the washing liquid in the immersion case, and the time during which the tow was exposed to the spraying flow in the spraying case, respectively. Moreover, the washing liquid flow was 10,600 kg/h at a spin dope flow of 328 kg/h (=19.7 kg/h dry fiber (cellulose)) in the spraying trial, corresponding to 10,600 kg/h/328 kg/h=32.3 kg washing liquid flow per kg spin dope composition. It should be noted that the given flows can be very different from the ones used in these trials, e.g. lower, but also higher. Furthermore, in the immersion trial a washing liquid volume of 650 liter was used, where said volume was recirculated with the same flow level as in the spraying trial. In the trial, comparatively high liquid flow levels have been used. Hence, lower levels are totally possible to use according to the present invention. Fact is, any type of flow levels may be used according to the present invention.

[0122] As notable, in comparative trials, the washing efficiency in spray washing is kept above 80%, e.g. as high as around 95%, which should be compared to immersion, which presented a washing efficiency at 28% and 51.1%, respectively. Based on these results, according to one embodiment of the present invention, at least one washing step is performed by spraying, preferably all washing steps are performed by spraying.

[0123] To summarize, the process and system disclosed herein provides various preferable means for solving several fiber quality/property and recycling issues. Some examples of such preferable means are summarized below. [0124] 1. Maximum stretching, and thus maximum fiber tenacity, is obtained by stretching the fiber/tow in an alkaline state (high concentration of NaOH in the tow). However, if the fibers are not maintained in a stretched state, meaning that slacking is substantially avoided, under further reduction of alkalinity (washing out NaOH and sodium salt) the induced orientation will relax to a certain degree. [0125] 2. The mechanical properties of fibers might positively be affected if the total stretching for elongation is partitioned into several steps at consecutively lower alkalinity. Individual speed regulation of godet 1 to n is also useful to avoid possible slack in the tow or to reduce unwanted tension along the tow due to possible shrinkage when washing out chemicals from the tow. [0126] 3. In order to achieve improved economy in recycling of chemicals the dilution of chemicals with water after the spin bath should be kept to a minimum. [0127] 4. A quick reduction of alkalinity in the swollen fibers while still being close together will induce unwanted fiber to fiber adhesion resulting in difficulties to separate fibers in downstream operations. Hence, it is preferable to avoid such quick reduction of alkalinity. [0128] 5. The coagulation of fibers from directly dissolved cellulose produces stiff and somewhat brittle fibers (both dry and wet moduli are higher than for conventional viscose fibers) due to high crystallinity. High fiber stiffness is assumed to be positive for dimensional stability of garments during washing. However, such fibers have been found to be susceptible to dried in wrinkles forming stress concentrations upon unfolding and stretching. Such wrinkles are easily formed if fibers are dried in a randomly wrinkled state as would be the case if conventional viscose technology were to be applied (washing and drying of fibers cut to staples). Weak spots along the staple fibers produce lower tenacity, fiber shortening and dust generation during carding/sliver formation/yarn spinning. By drying the fibers in the form of a fiber tow, before being cut, and drying the fibers in an unconstrained manner, such drying in of wrinkles can be avoided, thereby improving fiber strength. [0129] 6. Dried in wrinkles may be reduced or even avoided by drying straight fibers. However, straight fibers without crimp are difficult to handle in downstream operations like carding and sliver manufacture (poor web cohesion). Mechanical crimping without inducing fiber weaknesses is preferable, and preferably such crimping is made after at least partly drying the fiber tow, and before cutting the fiber tow into staple fibers.

[0130] Some or even all of the above listed issues related to fiber production from dissolved cellulose in alkali and coagulation in an alkaline coagulation bath can be tackled by using one or more of the embodiments described herein.