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.
~ -2- 2 1 95409 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.
~ _3_ 2 1 9540~
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:
~ _4- 2 1 954~9 (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 _ _ _ ... . , .. , . ... . , _ _ . ,, ., . ,,, . .... ,, _ , . , . _ _ _ ..
~ _5_ 21 95409 (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 = _ _ _ _ .. . _ ... . . .. _ .... . . _ _ . . _ . . . . . .
~ -6- 2 ~ 95409 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.