Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1995-07-13
1997-10-14
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 19, 162 37, 162 38, 162 41, 162 68, 162 84, 162 86, 536 57, D21C 102, D21C 302
Patent
active
056767951
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a process for producing viscose pulp according to a steam prehydrolysis sulfate (Kraft) displacement digestion process.
SUMMARY OF THE INVENTION
Viscose pulps are pulps that are used for the production of rayon, cellophane, carboxymethylcellulose, nitrocellulose, cellulose acetate, textile fibers and special papers. The specific characteristics of viscose pulps are a high purity and a high content of alpha cellulose.
Viscose pulps have a high content of alpha cellulose, a low content of hemicellulose, lignin, ashes and extraction substances. The elimination of hemicellulose in the digestion process is particularly difficult because pentosans are almost as resistant to alkalis and acids as the cellulose itself. The content of alpha cellulose is determined by dissolving pulp in 18% NaOH. Alpha cellulose is that part of cellulose which is not soluble in 18% NaOH. Beta cellulose is denoted that part of cellulose which precipitates at the subsequent dilution of the 18% solution and acidification. Gamma cellulose is denoted that part of the substances dissolved in 18% NaOH which does not precipitate when neutralizing of the solution. Roughly, one may say that alpha cellulose constitutes the cellulose normally present in plants, while beta cellulose is a measure for the cellulose degraded during chemical digestion and gamma cellulose constitutes a measure for the remaining hemicellulose content.
Depending on the end product, the demands as to the alpha cellulose content vary. For rayon, for instance, an alpha cellulose content of 88 to 91 will do. However, viscose pulps that are to be used for cellulose acetate, nitrocellulose or other derivatives must have a substantially higher alpha content, i.e., an alpha content of at least 94 to 98 and less than 1.5% of hemicellulose. Nitrocellulose for explosives usually are produced of linters, since for this purpose an alpha content of above 98% and a hemicellulose content of nearly 0% are required.
In contrast to paper pulp, wherefor a high content of hemicellulose is sought for reasons of tenacity, the hemicelluloses must be removed from viscose pulps. During the production of rayon, for instance, the xylanes react with CS.sub.2 in the xanthonation reaction as rapidly as the cellulose itself, which leads to an elevated consumption of CS.sub.2. Other hemicelluloses react more slowly than cellulose, thus involving difficulties in filtration.
All over the world, viscose pulp is produced primarily according to the sulfite process. With one-step processes, primarily the acidic sulfite process is employed because of its rapid hydrolysis of the hemicellulose as well as of the quite good delignification rate. Yet, bisulfite and neutral sulfite processes are also applied in two- and multi-step processes. In general, the following may be said in respect of sulfite digestion processes: Basically, they are carried out as batch cookings, i.e., discontinuously. The digestion temperature with sulfite processes is about 135.degree. C., with bisulfite processes 160.degree. C. With the heating of the digestion solution to the optimum digestion temperature, the pressure of the SO.sub.2 gas in the digester will increase, excess SO.sub.2 is blown off at an appropriate point of time. Digestion requires a total time of 6 to 8 hours.
Sulfidity, pH and temperature are the critical paremeters in determining the quality of the end product and its yield. Also, the type of base is of influence, in particular, on the rate of diffusion of the digestion chemicals into the chips. The degradation of hemicelluloses, in particular of xylanes and mannanes, primarily was effected by acidic hydrolyses of glucosidic bonds. The degraded hemicelluloses are removed from the pulp with the digestion solution. The degraded celluloses (beta celluloses) must be removed by subsequent alkaline treatment.
The cellulose in viscose pulps basically has a lower average degree of polymerization than in paper pulp. This is due to the a
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Database WPI, Section Ch, Week 8814, Derwent Publications Ltd., London, GB; Class F09, AN 88-097086.
Krotscheck Andreas
Lackner Karl
Schuster Johann
Wizani Wolfgang
Czaja Donald E.
Fortuna Jose
Lenzing Aktiengesellschaft
Voest-Alpine Industriean-lagenbau GmbH
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