Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2 1 95409
PROCESS AND PLANT FOR S~REDDING CELLULOSE MATERIAL
The invention is concerned with a process and a plant for
shredding a cellulose material, particularly for shredding
pulp.
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 among other reasons for its reduced
environmental impact, it has been found to dissolve cellulose
without derivatisation in an organic solvent and extrude from
this solution moulded bodies, e.g. fibres, films and other
moulded bodies. 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 appropiate 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. A
process for the production of mouldable cellulose solutions
is known e.g. from EP-A - 0 356 419. The production of
cellulose moulded bodies using tertiary amine-oxides is
generally referred to as amine-oxide process.
In EP-A - 0 356 419, an amine-oxide process for the
production of spinnable cellulose solutions is described,
wherein among other substances a Sll~pPnc; nn of cellulose in
liquid, aqueous N-methylmorpholine-N-oxide (NMMO) is used.
This process consists in transforming the suspension in a
thin-film treatment apparatus in a single step and
continuously into a mouldable solution. Finally, the
mouldable solution is spun into filaments in a forming tool
such as a spinneret and the filaments are conducted through a
precipitation bath.
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As mentioned above, as a starting material for the production
of the mouldable cellulose solution a suspension of cellulose
in aqueous tertiary amine-oxide is used. This suspension is
produced by shredding cellulose and introducing it within a
mixer into the aqueous amine-oxide solution.
From WO 94/28217, a process for the production of a premix
based on shredded cellulose, wherefrom a mouldable cellulose
solution can be produced, is known. As a starting material,
pulp in rolls is used. The pulp is shredded in a cutting
device and the shredded pulp is blown by means of air into a
pulp separator, wherein the shredded pulp is separated into
two fractions: into a first fraction exhibiting a particle
size of not more than 2,54 mm, and into a dust fraction. The
dust fraction is entrained by the transport air and separated
from the latter in a filter. Afterwards, the filtered dust
fraction is ~ in~d again with the first fraction and used
to produce the cellulose suspension.
This known process has the disadvantage that as a starting
material pulp in rolls and not pulp in sheet form is used.
Pulp in sheet form is more difficult to process, since each
of the pulp bales is packed by means of metal strips and
these metal strips cause deformations of the outer edges of
the sheets. These deformations imply a certain mechanical
~h~Sirln of the sheets among each other, making it difficult
to attain a uniform feeding to the shredding device. ~owever,
when the feeding is not uniform, the driving element of the
shredding device is not uniformly charged. On the other hand,
the use of sheet pulp would be more convenient, since in
comparison to roll pulp it is more easily available and may
be more easily transported.
Another drawback of the process known from the above-
mentioned WO 94/28217 consists in that the shredded pulp has
to be separated into two fractions, which naturally implies a
more complex technical plant.
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Thus it is the object of the invention to provide a process
and a plant able to overcome the above-mentioned drawbacks.
The process according to the invention for shredding a
cellulose material is characterized in that the shredding is
carried out in at least two steps by
(1) pre-shredding the cellulose material, thus producing
particles of different size fractions, and
(2) transporting the particles produced by means of the pre-
shredding without separation of any of the size fractions
to a device wherein the particles are further shredded.
It has been shown that by means of an at least two-step
shredding, a less deteriorating shredding of the cellulose
and a uniform charge on the shredding devices can be
attained. Noreover, it is possible to process sheet pulp.
Pulp mixtures may also be employed.
As the cellulose material, preferably pulp is used.
In the process according to the invention, the particles
produced by the pre-shredding are transported most preferably
by means of a gas.
The process according to the invention is preferably carried
out continuously.
Further, it has been shown that by means of the process
according to the invention, a shredded cellulose material
which is particularly appropiate for the production of a
suspension of cellulose in an a~ueous tertiary amine-oxide
for the amine-oxide process can be conveniently produced.
The invention is also concerned with a plant for performing
the process according to the invention, said plant being
characterized by the following features:
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(l)a first device for pre-shredding a cellulose material,
(2)at least one further device for further shredding the
pre-shredded cellulose material, and
(3)a transport duct for pre-shredded cellulose material
connecting the first device and the at least one further
device to each other, provided that in the transport duct
no bifurcation for pre-shredded cellulose material is
provided.
A preferred embodiment of the plant according to the
invention consists in that a drop shaft leading into the
first device for pre-shredding cellulose material is provided
amd that means whereby pulp in sheet form or pulp in rolls
can be introduced into the drop shaft are provided.
Advantageously, in the plant according to the invention two
devices for shredding cellulose material are provided.
It has proven convenient to provide a cyclone within the
transport duct to separate the pre-shredded cellulose
material from the fresh air.
A preferred '~'i L of the invention will be explained in
more detail by means of the attached drawing. The drawing
shows an embodiment of the plant according to the invention
including a two-step shredding of pulp.
As a starting material, pulp in sheet form as well as pulp in
rolls is employed. Reference number 1 denotes pulp in sheet
form (600 mm x 600 mm), which by means of a uullvuyur belt 3
is transported into a direction indicated in the Figure by
means of an arrow above the pulp in sheet form 1. The motor
for running the ~U~ yuL belt is not shown. By means of a
stopping and throwing device indicated in the Figure by
reference num'oers 4 and 5, the pulp sheets 1 are thrown into
drop shaft 6 as required and pre-shredded by granulator 9
_ _ _ ... . , .. , . ... . , _ _ . ,, ., . ,,, . .... ,, _ , . , . _ _ _ ..
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(type Condux CS S00/1000; screen: ~ 0,75 mm). Thus pulp is
fed discontinuously.
Additionally, pulp in rolls (width: 800 mm) is fed to drop
shaft 6. In the Figure it is shown that pulp is drawn from
roll 2a by means of a feeding device 10, consisting
substantially in two transport rollers run by a motor (not
shown) and introduced into drop shaft 6. Thus pulp can be fed
continuously.
In order to avoid an interruption in case of an ocnAcion~l
change of the roll, a second pulp roll 2b, wherefrom pulp can
be drawn while roll 2a is exchanged, may be provided.
Reference numbers 7 and 8 denote metal detectors able to
determine if the pulp includes metal pieces such as residues
of metal strips usually employed for packing the pulp. If
metal pieces are detected, the metal detectors switch off
granulator 9 and uullv~yur belt 3 or the feeding device 10 to
avoid a damage on the blades of granulator 9.
The pulp pre-shredded in granulator 9 enters duct 12
(diameter: 150 mm) and is seized therein by an air stream
produced by fan 11. Conveniently, the air stream of fan 11 is
adjusted to a value of from 20 to 25 m/s, and it may be
charged with pre-shredded pulp of from 0,25 to 2,5 kg/m3 of
air, depending on the particle size.
The pre-shredded pulp is blown by the air stream into cyclone
13, wherein the pre-shredded pulp separates from the
transport gas. The transport gas is conducted across duct 14
into drop shaft 6.
From cyclone 13, the pre-shredded pulp is cnn~uctP~ into the
second granulator 15 (type CS 500/1000; screen: < 20 mm),
wherein the pre-shredded pulp is further shredded to the
desired particle size. Finally, the shredded pulp enters
transport duct 16, wherein the pulp is entrained by the air
= _ _ _ _ .. . _ ... . . .. _ .... . . _ _ . . _ . . . . . .
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stream produced by a fan 17 and may be transported for
instance into a storage tank (not shown).
sy means of the two-step shredding of pulp, not only a better
matching of the shredding performance of the two granulators
is realized, but also a deterioration of the cellulose can be
avoided. Further is has been shown that the dosage of pulp,
for instance into a mixer wherein the shredded pulp is
introduced into the aqueous tertiary amine-oxide and the
cellulose suspension is produced, can be carried out more
easily and with more precision.
