AU718574B2

AU718574B2 - Process for the production of a cellulose suspension

Info

Publication number: AU718574B2
Application number: AU36131/97A
Authority: AU
Inventors: Wilhelm Feilmair; Johann Manner; Ulrich Moderl; Christoph Schrempf
Original assignee: Lenzing AG; Chemiefaser Lenzing AG
Current assignee: Lenzing AG
Priority date: 1996-08-01
Filing date: 1997-07-31
Publication date: 2000-04-13
Anticipated expiration: 2017-07-31
Also published as: CN1077581C; JPH11513076A; BR9702351A; CN1205014A; ID17962A; ATE227751T1; EP0853642B1; EP0853642A1; NO981440D0; DE59708711D1; AT404594B; ATA137996A; ES2186903T3; AU3613197A; WO1998005702A1; CA2233574A1; NO981440L

Abstract

A process is disclosed for preparing a homogeneous suspension of cellulose in an aqueous solution of a tertiary amine oxide. Cellulose is mixed with the aqueous solution of tertiary amine oxide in a mixer which has a container for accommodating the suspension and a mixing tool. This process is characterised by the use of a mixer with a container which rotates during the mixing process.

Description

F;I I WO 98/05702 PCT/AT97/00179 -1- PROCESS FOR THE PRODUCTION OF A CELLULOSE SUSPENSION The present invention is concerned with a process for the production of a homogeneous suspension of cellulose in an aqueous solution of a tertiary amine-oxide, wherein pulp is mixed with the aqueous solution of the tertiary amine-oxide in a mixing device comprising a receptacle for receiving the suspension and a mixing tool. The invention is further concerned with a process for the production of cellulose moulded bodies.

For some decades there has been searched for processes for the production of cellulose moulded bodies able to substitute the viscose process, today widely employed. As an alternative which is interesting for its reduced environmental impact among other reasons, it has been found to dissolve cellulose without derivatisation in an organic solvent and extrude from this solution moulded bodies such as fibres and films. Fibres thus extruded have received by BISFA (The International Bureau for the Standardization of man made fibers) the generic name Lyocell. By an organic solvent, BISFA understands a mixture of an organic chemical and water.

It has turned out that as an organic solvent, a mixture of a tertiary amine-oxide and water is particularly appropriate for the production of cellulose moulded bodies. As the amine-oxide, primarily N-methylmorpholine-N-oxide (NMMO) is used. Other amine-oxides are described e.g. in EP-A 0 553 070 and in US- A 4,196,282. A process for the production of mouldable cellulose solutions is known e.g. from EP-A 0 356 419.

US-A 4,246,221 describes an amine-oxide process for the production of spinnable cellulose solutions, wherein as a starting material among others a mixture of cellulose in liquid, aqueous N-methylmorpholine-N-oxide (NMMO) is used. According to this process, a suspension of shredded cellulose in the aqueous amine-oxide solution is produced in a discontinuously operating mixing device while simultaneously heating the mixture at reduced pressure, water being withdrawn and a first solution being produced which after filtration and after-treatment in an extruder is transformed into a mouldable solution.

According to Example I, a conventional double-armed mixer is used as the mixing device for the production of the cellulose suspension. In this mixer, a suspension having a consistency of about 20% by mass of cellulose is produced. This known process however has the drawback of being discontinuous and difficult to carry out on an industrial scale.

Further, US-A 4,246,221 recommends to use the cellulose and the (solid) amine-oxide hydrate in a ground state to achieve a particularly gentle and fast dissolution of the

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WO 98/05702 PCT/AT97/00179 -2cellulose. However, according to the experience of the applicant, this procedure is disadvantageous, since the cellulose is deteriorated by the grinding process when e.g. local overheating occurs. Moreover it is disadvantageous to use a solid mixture of shredded cellulose and shredded amine-oxide as a starting material for the production of the cellulose solution, since it is known from EP-A 0 419 356 that a suspension of cellulose in an aqueous amine-oxide can be transformed into a mouldable cellulose solution in a faster, more gentle and efficient way by means of the thin-film technique.

In US-A 4,416,698 it is also recommended to those skilled in the art to grind the cellulose, up to a particle size of less than 0,5 mm.

According to DD-A 226 573, the starting material for the production of a solution is a NMMO-containing cellulose suspension having a low consistency of not more than by mass of cellulose. This cellulose suspension is homogenized in a stirring vessel.

Subsequently, the consistency is increased to 12.5% by mass by means of centrifugation or squeezing, the suspension is dried to a water content of from 10 15% by mass (based on NMMO) and transformed into a clear solution in an extruder comprising a degassing zone at temperatures ranging of from 75 to 120 0

C.

The process according to the above-mentioned DD-A 226 573 has the disadvantage that it is necessary to increase the consistency from 2.5% by mass to 12.5% by mass after homogenisation, before the production of a solution can be actually started. This requires a separate, additional operation step. Moreover, when squeezing or centrifugating it is difficult to achieve a homogeneous squeezing degree, which is a prerequisite for a constant solution composition.

From WO 95/11261 of the applicant a process is known according to which preshredded cellulose materials are introduced into an aqueous solution of a tertiary amineoxide to produce a first suspension having a dry substance consistency of not less than by mass of cellulose, the first suspension is subjected to grinding, thus a second suspension being obtained, and the second suspension is transformed into the mouldable cellulose solution by supplying heat at reduced pressure. Grinding has the purpose of further reducing remaining pulp particles and splicing them into single fibres.

As grinding means, conventional high-consistency-mixers, dispergers and refiners are proposed.

From WO 94/28217, a process for the production of a pre-mixture from shredded cellulose and aqueous amine-oxide is known wherefrom a mouldable cellulose solution may be WO 98/05702 PCT/AT97/00179 -3produced. As a starting material, pulp in rolls which first is pre-shredded in a shredder is used. In WO 94/28217, it is pointed out that when shredding the pulp attention should be paid to squeeze its edges of cuts as little as possible, since this renders later mixing with the aqueous amine-oxide solution more difficult. For this purpose, a special shredder is recommended wherein pulp pieces having a size of typically not more than 15 cm 2 are produced. However, as a by-product of shredding, significant amounts of pulp dust are produced.

After being shredded in a shredder, the pre-shredded pulp is further reduced in a fan having propeller blades and transported by air to a screen wherein the pre-shredded pulp is separated from the air stream. The screen separates all pulp particles having a particle size of not less than 2.54 mm. In the remaining air stream however, a significant amount of dust having a particle size of less than 2.54 mm is still present. To avoid the loss of this pulp it is collected in a filtrating device and finally recombined with the bigger pulp particles.

To produce the suspension, the shredded cellulose and the amine-oxide solution are introduced into a horizontal cylindrical mixing chamber comprising a rotor having stirring elements which exhibit an axial clearance. The mixture is stirred in the mixing chamber by rotating the rotor at a rate of from 40 to 80 revolutions per minute. On the wall of the cylindrical mixing chamber, fast-rotating refiner blades whereby the pulp particles are shredded are preferably provided. The combined action of the stirring blades and the fastrotating refiner blades finally yields a homogeneous mixture comprising up to approximately 13% by mass of cellulose, based on the mass of the suspension.

This known process however is disadvantageous in that it requires a complex technical arrangement and is time-consuming to finally produce the homogeneous suspension from the pulp rolls. In addition, the known process is discontinuous and takes more than minutes of mixing time per batch. Moreover, it would be desirable for economical reasons to produce cellulose suspensions having a higher consistency and use even bigger pulp pieces as starting products.

In summary, it can be said that in the state of the art the step of producing the cellulose suspension, i.e. the starting product for the production of the mouldable cellulose solution, has not yet been satisfactorily developed. In the present case, conventional pulpers such as those used in a number of embodiments in the viscose technique or in paper industry cannot be satisfactorily employed, since they only permit the production of cellulose suspensions having a consistency of not more than about 11.5%, as the applicant has found. The reason for this is that due to the extensive swelling of pulp in 60-78% aqueous iii WO 98/05702 PCT/AT97/00179 -4- NMMO at the required processing temperature of from 60-90C the flowability of the suspension drops to such a point that the substance mixture pulp/NMMO/water is no more subjected as a whole to a sufficient shearing and mixing.

Thus it is the object of the present invention to further develop the amine-oxide process in this respect, providing a process not comprising the disadvantages mentioned above with regard to WO 94/28217, wherein it is much easier to produce a homogeneous cellulose suspension from pulp and an aqueous amine-oxide solution which is usable as a starting product for the amine-oxide process.

The process according to the invention for the production of a homogeneous suspension of cellulose in an aqueous solution of a tertiary amine-oxide, wherein pulp is mixed with the aqueous solution of the tertiary amine-oxide in a mixing device comprising a receptacle to receive the suspension and a mixing tool, is characterized in that a mixing device having a receptacle which rotates during mixing is used.

Mixers having rotating receptacles are known Ullmann, Enzyklopadie der Technischen Chemie, 4th edition, volume 2, pp. 290-291). The invention is based on the finding that when using such a mixer it is not necessary to pre-shred pulp in such a way as known from WO 94/28217, being nevertheless possible to produce a homogeneous suspension wherein the pulp is virtually broken off into its individual fibres. If for instance sheet pulp or pulp. in rolls is used, it is not necessary to pre-shred this starting material to pieces having a surface of less than 20 cm 2 preferably not less than 100 cm 2 It is even possible to employ plane pieces having a size of more than 1000 cm 2 as a starting material, introducing them directly into the mixer used according to the invention.

Since it is possible to process pieces of such big sizes directly in the mixer, it is not necessary to pre-shred the material to such small particles as known from WO 94/28217.

Thus the production of pulp dust during pre-shredding is prevented, and the complex arrangement for separating and recovering dust becomes unnecessary. Moreover, it is also easier to break off the pulp into its individual fibres, since the pulp used according to the invention has significantly less edges of cuts. Accordingly, the deterioration of the pulp caused by shredding is reduced.

As another advantage of the process according to the invention it has been shown that it is possible to produce cellulose suspensions having a consistency of more than 13%, using for this purpose even a concentrated amine-oxide solution having 78% by mass of NMMO.

L f WO 98/05702 PCT/AT97/00179 All these effects are not achievable in the opposite case, i.e. when using a mixer having a fixed, i.e. non-rotating receptacle and a rotating mixing tool.

A preferred embodiment of the process according to the invention consists in that the mixing device employed comprises a mixing tool which is not moved during mixing. The mixing tool may be provided as a paddle, jib or spiral.

Preferably, the mixing tool is provided and arranged in the receptacle such that during mixing it prevents the formation of a coat on the inner surfaces of the receptacle, i.e. on the walls, the bottom and the lid. Most simply, it is provided as a scraper.

Further, it has proven convenient for the mixing device employed to comprise a mixing tool which rotates during mixing. The mixing tool may be eccentrically arranged.

Conveniently, the mixing tool rotates in opposite direction to the rotating receptacle.

The receptacle may be designed such that it comprises a symmetrical axis wherearound it rotates. In another preferred embodiment, the process according to the invention is carried out in a mixing device having a symmetrical axis which during mixing is inclined towards the horizontal level.

The aqueous solution of the tertiary amine-oxide used in the process according to the invention contains of from 60 to 82% by mass of amine-oxide, preferably NMMO.

Conveniently, the process according to the invention is carried out at a temperature of from to It is evident to those skilled in the art that mixtures of different pulps may also be used to produce a suspension. Further, it has been shown that in the mixing device used according to the invention, no demixing of auxiliary agents such as stabilizers, dispersants, auxiliary agents for spinning, reactivity-improving reagents, incorporation media of inorganic or organic nature (barite, activated carbon, SiO 2 CMC, modifiers (polyethylene glycols)) and other polymers such as nylon; dyes contained in the suspension, will occur. This is of vital importance for these auxiliary agents to completely develop their effect.

The invention is further concerned with the use of a mixing device having a rotating receptacle for the production of a homogeneous suspension of cellulose in an aqueous solution of a tertiary amine-oxide, the cellulose used for the production of the suspension best having a particle size of not less than 20 cm 2 ii tE WO 98/05702 PCT/AT97/00179 The invention is further concerned with a process for the production of a mouldable cellulose solution, characterized in that a homogeneous suspension produced according to the process according to the invention is processed into a mouldable cellulose solution while water is evaporated. Conveniently, the production of the cellulose solution is carried out in a thin-film treatment apparatus. Such a process is described for instance in EP-A 0 356 419. An embodiment of a thin-film treatment apparatus is for instance a so-called Filmtruder as manufactured by the company Buss AG (Switzerland). A thin-film treatment apparatus is also described in DE-A 2 011 493.

WO 94/06530 published in 1994 uses the thin-film technique known from EP-A 0 356 419 to produce a mouldable solution from a mixture of cellulose in an aqueous solution of a tertiary amine-oxide. The process is carried out in a Filmtruder, analogously to an embodiment described in EP-A 0 356 419. The object of the process of WO 94/06530 is to save energy, and to achieve this goal it proposes to rotate the rotor more slowly.

The present invention is further concerned with a process for the production of cellulose moulded bodies consisting in that a cellulose suspension produced according to the invention is transformed into a mouldable cellulose solution, which subsequently is processed into films, fibres, membranes or other moulded bodies, in a substantially known manner.

The suspension produced according to the invention may be fed into the Filmtruder directly or by means of an appropriate buffer vessel, such as described in PCT/AT96/00059 of the applicant and in WO 94/28217, by means of a dosing apparatus, and there be transformed into a solution. The suspension produced according to the invention may be transformed into a cellulose solution also using other devices.

A mixing device preferably used in the process according to the invention is schematically shown in Figure 1 and 2 of the attached illustration. An embodiment of such a mixing device is commercially available as a so-called "intensive mixer R 08 type", manufactured by the company Eirich, Germany.

Figure 1 illustrates a section through a schematically shown mixing device comprising a cylindrical receptacle 1 to receive the stock. Receptacle 1 is rotatably arranged, which is indicated by means of reference number 6. Rotation of receptacle 1 is achieved by a stationary engine 2 driving a gear wheel 4 which indents into a ring-shaped counterpiece rigidly attached to the bottom of rotatable receptacle 1. Reference number 3 indicates a gear.

iI I I WO 98/05702 PCT/AT97/00179 -7- Reference number 7 refers to a detachable, stationary lid for the receptacle. Lid 7 exhibits at its edge a guide 13 whereinto receptacle wall 1 glides. Reference number 8 indicates an engine which sets the stationary mixing tool 10 into rotation by means of a gear 9. Mixing tool 10 exhibits at its low end blade-shaped stirring blades 11. To lid 7, another stationary mixing tool 12 is attached which is arranged as an oblong scraper and prevents the formation of a coat on the wall and on other inner surfaces of rotating receptacle 1.

Figure 2 shows a top view of the mixing device used according to the invention wherefrom lid 7 has been taken off. In Figure 2, mixing tool 10 exhibiting mixing elements 11, further mixing tool 12 and receptacle 1 can be seen. The rotation directions of receptacle 1 and mixing tool 10 are indicated by means of arrows. In Figure 2, the rotation directions are opposite to each other.

The mixing device may comprise a heating or cooling device (not shown). The mixing device may be heated or cooled in a conventional way. Naturally, it may be heated also by means of an electric heating.

By means of the following Examples, an appropriate embodiment of the invention is described in more detail. As a mixing device, the "intensive mixer R 08 type" manufactured by the company Eirich, Germany, was used.

Example 1 Pulp sheets (Alicell V-LV) having a length of 800 mm and a width of 150 mm were put into the mixer and an aqueous NMMO solution having a content of 74% by mass of NMMO was added. The amounts were selected in such a way that a cellulose suspension having a content of 13% by mass of cellulose, based on the mass of the suspension, could be obtained. The temperature of the stock was approximately Then the mixer was activated, receptacle 1 being rotated at approximately 30 revolutions per minute and mixing tool 10 at approximately 300 revolutions per minute (opposite to receptacle The whole of the stock present in the mixer could be transformed into a suspension. To determine the degree of splicing of the pulp into individual fibres, samples were drawn after 2, 5 and 10 minutes of mixing time each, and their quality was determined by means of sheet production. To produce a sheet, 25 g of sample (60'C) were mixed with 1000 g of water (65°C) in a 2000 ml beaker glass and stirred for 15 minutes with a magnetical agitator (stirring rod: 5 cm; 500 revolutions per minute). Then the WO 98/05702 PCT/AT97/00179 -8suspension obtained was put on the sheet producer, and by means of fast suction a sheet was produced which was dried in the drying chamber. The appearance of the dried sheet was determined. The results are shown in the Table below. The degree of splicing of the pulp into individual fibres is the higher the less hard spots can be observed in the sheet produced.

From the Table it can be seen that, as already mentioned above, a suspension could be produced from the stock, and further that a suspension is obtained after a mixing time of only two minutes, the sheet produced from this suspension not showing any hard spots, which means that virtually all the pulp has been spliced into its individual fibres.

Example 2 It was proceeded analogously to Example 1, using pulp strips (Alicell V-LV) having a length of 400 mm and a width of 4 mm instead of pulp sheets.

From the Table it can be seen that a suspension could be produced from the stock, and further that a suspension is obtained after a mixing time of only two minutes, the sheet produced from this suspension not showing any hard spots, which means that virtually all the pulp has been spliced into its individual fibres.

Example 3 It was proceeded analogously to Example 1, using ground pulp (cutting mill) (Alicell V- LV; Condux mill; screen holes square, 16 mm 2 instead of pulp sheets and an aqueous NMMO solution having a content of 78% by mass of NMMO. Moreover, the ground pulp was used in such an amount that a cellulose suspension having a content of 15.5% by mass of cellulose, based on the mass of the suspension, could be obtained.

The results of the sheet production are listed in the Table and surprisingly show that, using the mixing device according to the invention, a suspension could be produced using even pulp conventionally shredded (ground) and a NMMO solution having a higher concentration and a significantly increased consistency. Although hard spots were found when producing sheets, they could be spliced into individual fibres by means of a refiner treatment.

I I WO 98/05702 PCT/AT97/00179 -9- Table Example Mixing time Suspension Sheet production 1 2 yes no hard spots, blooming 1 5 yes no hard spots, blooming 1 10 yes no hard spots, blooming 2 2 yes no hard spots 1 blooming 2 5 yes no hard spots 1 blooming 2 10 yes no hard spots 2 blooming 3 2 yes a lot of hard spots 3 3 5 yes no improvement 3 10 yes no improvement 4 Note: 1 occasional expanded strip rests; 2 occasional small open strip rests; 3 scarcely open longer fibres; the sheet releases fine cellulose dust; 4 the sheet, however, is more homogeneous and releases less cellulose dust.

From the suspensions produced according to the invention, dopes of good quality could be produced. It has been shown that the suspensions produced in Example 1 and 2 have even such a good quality that they could be used directly, i.e. without after-treatment in a refiner, to produce a solution.

Example 4 (Comparative Example) 2.2 kg of pulp sheets (Buckeye V5) having a size of 7 x 7 cm 2 were mixed for 30 seconds with 6.23 kg of an aqueous NMMO solution 65°C) in a conventional laboratory pulper (capacity: 41 liters) having a stationary receptacle. Afterwards further 6.23 kg of NMMO solution were added, so that the cellulose concentration was 15%, and it was mixed for 90 seconds. It was observed that no mixing occurred at the receptacle wall. The pulp was only swollen, and only half of it was pulped.

Thereafter, further 3.0 kg of NMMO solution were added, so that the cellulose concentration dropped to 12.5%, and it was mixed for 40 seconds. No improvement could be observed.

WO 98/05702 PCT/AT97/00179 Then another 3.0 kg of NMMO solution were added, so that the cellulose concentration dropped to 10.6%, and it was mixed for 60 seconds. A slight improvement could be observed. Only after another 180 seconds of mixing time, a suspension could be produced, which however was not subjected to sheet production since its consistency does not seem to be economically interesting for the amine-oxide process.

This result shows that using the conventional laboratory mixing device having a stationary receptacle it was not possible to produce a suspension exhibiting a consistency of more than 11%.

Example 5 (Comparative Example) A conventional pulper (capacity: 1500 1) having a stationary receptacle was fed with 900 1 of NMMO solution 70 80 0 Then sheet pulp which had not been previously shredded was gradually added. After approximately 12 minutes and after reaching a consistency of 11.57%, the process had to be interrupted, since it was not possible to increase the consistency further, as no mixing occurred in the peripheral zones and thus no suspension was obtained.

Claims

1. A process for the production of a homogeneous suspension of cellulose in an aqueous solution of a tertiary amine-oxide wherein pulp is mixed with said aqueous solution of said tertiary amine-oxide in a mixing device comprising a receptacle to receive said suspension and a mixing tool, characterized in that a mixing device having a receptacle which rotates during mixing is used.

2. A process according to Claim 1, characterized in that said mixing device used comprises a mixing tool which is not moved during mixing.

3. A process according to Claim 2, characterized in that said mixing tool is provided as a paddle, jib or spiral.

4. A process according to Claim 3, characterized in that said mixing tool is provided and arranged in said receptacle such that during mixing it prevents the formation of a coat on the wall of said receptacle.

A process according to one of the Claims 1 to 4, characterized in that said mixing device used comprises a mixing tool which rotates during mixing.

6. A process according to Claim 5, characterized in that said rotating mixing device is eccentrically arranged.

7. A process according to one of the Claims 5 or 6, characterized in that said mixing tool rotates in opposite direction to said rotating receptacle.

8. A process according to one of the Claims 1 to 7, characterized in that said receptacle has a symmetrical axis wherearound it rotates.

9. A process according to Claim 8, characterized in that said symmetrical axis is inclined towards the horizontal level.

WO 98/05702 PCT/AT97/00179 -12- A process according to one of the Claims 1 to 9, characterized in that said tertiary amine-oxide is contained in said aqueous solution in an amount of from 60 to 82% by mass.

11. A process according to one of the Claims I to 10, characterized in that it is carried out at a temperature of from 60 to 90 0 C.

12. Use of a mixing device having a rotating receptacle for the production of a homogeneous suspension of cellulose in an aqueous solution of a tertiary amine- oxide.

13. Use according to Claim 12, characterized in that said cellulose used to produce said suspension has a particle size of not less than 20 cm 2

14. A process for the production of a mouldable cellulose solution, characterized in that a homogeneous suspension produced according to Claims 1 to 11 is processed into a mouldable cellulose solution, while water is evaporated.

A process according to Claim 14, characterized in that it is carried out in a thin-film treatment apparatus.

16. A process for the production of cellulose moulded bodies, characterized in that a mouldable cellulose solution produced according to Claim 14 or 15 is processed into films, fibres, membranes or other moulded bodies in a substantially known way.