Note: Descriptions are shown in the official language in which they were submitted.
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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 (N~vIO) 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 (N~O) 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 ~imlllt~neously 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reduced pressure, water being withdrawn and a first solution being produced which after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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cellulose. 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 2.5%
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~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 (1) pre-shredded cellulose materials are introduced into an aqueous solution of a tertiary amine-
oxide to produce a first suspension having a dry substance consistency of not less than 10%
by mass of cellulose, (2) the first suspension is subjected to grinding, thus a second
suspension being obtained, and (3) the second suspension is transformed into themouldable cellulose solution by supplying heat at reduced pressure. Grinding has the
purpose of further reducing rem~ining 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
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produced. 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 cm2 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 rem~ining 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 fast-
rotating 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 20
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
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NMMO at the required processing temperature of from 60-90~C 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 (e.g. 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 cm2, preferably not less than 100 cm2. It is even
possible to employ plane pieces having a size of more than 1000 cm2 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.
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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
60 to 90~C.
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, SiO2, 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 cm2.
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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 applupliate 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 5
rigidly attached to the bottom of rotatable receptacle 1. Reference number 3 indicates a
gear.
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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 appropl;ate 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 80~C.
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 1). 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
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suspension 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 mm2) instead of pulp sheets and an aqueous
NMMO solution having a content of 78% by mass of NMMO. Moreover, the ground pulpwas 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.
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Table
ExampleMixing timeSuspension Sheet production
2 yes no hard spots, blooming
yes no hard spots, blooming
yes no hard spots, blooming
2 2 yes no hard spots1, blooming
2 5 yes no hard spots1, blooming
2 10 yes no hard spots2, blooming
3 2 yes a lot of hard spots3
3 5 yes no improvement
3 10 yes no improvement4
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 cm2 were mixed for 30 seconds
with 6.23 kg of an aqueous NMMO solution (72%; 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.
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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 beobserved. Only after another 180 seconds of mixing time, a suspension could be produced,
which however was not subjected to sheet production since its consistency (10.6%) 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 l) having a stationary receptacle was fed with 900 l
of NMMO solution (74%, 70 - 80~C). Then sheet pulp which had not been previouslyshredded 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.
