DE19910797A1 - Sorting fibers in a suspension uses a wet sieve action with short fibers passing through the sieve for outer paper/cardboard layers and longer fibers in a fraction over the sieve for the inner layer - Google Patents

Abstract

To separate a fiber suspension (S) into fiber fractions, where the suspension is composed of chemically broken down cellulose fibers and mechanically broken down wood fibers, it is given a wet sieve processing where one fraction (A) passes through the sieve and another fraction (R) travels over the sieve. The sieve openings are set to give a mean flow speed through it of at least 2 m/sec. The mean flow speed through the sieve is pref. at least 3 m/sec. The sieve openings are slits, with a clear width (w) of max. 0.5 mm or max. 0.2 mm. The fraction which passes through the sieve contains the chemically processed cellulose fibers and short wood fibers from the mechanical processing, and the overflow fraction contains long fibers. The wet sieve separation action is within a pressure suspension sorting appts., with a cylinder sieve. When the fiber suspension is used for the prodn. of paper or cardboard, the fraction (A) passing through the sieve is used for the outer layers and the overflow fraction (R) forms the inner layer.

Description

The invention relates to a method according to the preamble of claim 1.

Processes of this type contain at least one wet screening. In doing so, one Sieve the long fibers preferentially retained, while the short fibers hold the sieve relatively can easily happen. In this way, material fractions are to be formed meet special requirements better than the starting mixture. It can also be one separate processing of the political groups should be intended.

As is known, there are several options for producing paper fibers. In particular, a distinction is made between the chemical digestion with which Cellulose fibers are produced that are largely free of lignin and the mechanical digestion, with the help of which the so-called wood pulp is obtained, the are fibers with significant lignin content. The mechanical digestion can also carried out under elevated temperature (TMP) and possibly supported with chemicals become (CTMP). Chemical and mechanical digestion processes are from of considerable economic importance and, depending on the intended use, the Fibers applied to a large extent. Often are made by papermaking and / or Use of paper mixed both types of fibers. This is particularly the case with the usual ones Waste paper suspensions, the composition of which is broad Scattered dimensions. If such a mixture is present, the paper made from it can in many cases not meeting optimal requirements, is not as high quality as if it is e.g. B. from pure chemically digested pulp fibers could.

When using wet screening processes to fractionate one  Fibrous suspension, which contains both pulp fibers from chemical digestion also contains wood pulp, tend to be the particularly valuable cellulose fibers retained on the screen along with the longer wood pulp fibers. This creates a valuable long fiber fraction because of its high strength potential unfavorable formation and surface properties. The flow fraction, on the other hand is extremely short grain.

The invention is therefore based on the object of an economical method create with a mixture of chemically digested pulp fibers and wood fiber fractions can be formed which are optimal have overall fiber technology properties.

This object is achieved by the features mentioned in the characterizing part of claim 1 solved.

The mean flow velocity is the quotient of volumetric Throughput through the openings of the screen element and their free passage area. According to the average flow rate is chosen so high that a strong suction is created, which the pulp fibers despite their relatively long length detected. Chemical pulp fibers are in aqueous suspension much more flexible than mechanically digested pulp fibers. One also speaks from a higher "wet fiber flexibility". The effect described is the usual Flow velocities that are at most approx. 1 m / sec hardly exist.

With the help of the method according to the invention, a simple means Increase in the proportion of chemically digested pulp fibers in the course of the Wet sieve device reached. This can be the case with a mixture of fibers passing through Different types of digestion were generated, can be advantageous because one Continuous fraction with a lot of short wood pulp fibers can be formed at the same time has an increased cellulose fiber content. Such a fraction is ideal for the outer layers of multilayer paper or cardboard. The overflow can into the  Middle layers are driven.

The invention is explained on the basis of a single figure, a diagram. The wood pulp fraction 1 is plotted over the flow fraction 2 , which is set during wet screening. The skilled worker is familiar with how such settings are made in pressure sorters or fractionators. The diagram shows the following curves:

Curve 3 : wood pulp content in the run at an average flow rate of 1 m / sec;

Curve 4 : proportion of wood pulp in the pass at an average flow speed of 2 m / sec;

Curve 5 : proportion of wood pulp in the overflow at an average flow rate of 1 m / sec;

Curve 6 : wood pulp content in the overflow at an average flow rate of 2 m / sec.

The important advantage of the invention is shown graphically in the small amounts of wood pulp which increase the flow velocity Pass. As a result, the wood pulp enrichment, which is undesirable in many cases, succeeds to avoid in the run, i.e. the higher-quality fraction, or at least to to reduce.

The tendency increases at even higher speeds in the screen openings.

At the right edge of the diagram, a sieve symbol is shown with the inlet in the form of the paper fiber suspension S. This is divided into the continuous fraction A and the overflow fraction R. For easier identification of the relationships, reference lines to curves 3 , 4 , 5 and 6 are shown. Line 7 shows the theoretical limit, namely that the proportion of wood pulp does not change in wet screening.

Claims (10)

1. A method for fractionating paper fibers in a paper fiber suspension (S), which contains both chemically digested pulp fibers and mechanically digested pulp fibers, in which the paper fiber suspension is subjected to a wet screening, which produces a continuous fraction (A) and an overflow fraction (R), thereby characterized in that an average flow rate of at least two m / sec is generated in the openings of the screen element used in wet screening. 2. The method according to claim 1, characterized, that produces an average flow velocity of at least three m / sec becomes. 3. The method according to claim 1 or 2, characterized, that the openings are slits. 4. The method according to claim 3, characterized, that the slots have a clear width (w) of at most 0.5 mm. 5. The method according to claim 3, characterized, that the slots have an internal width (w) of at most 0.2 mm. 6. The method according to any one of the preceding claims, characterized,  that the chemically digested pulp fibers in the continuous fraction together with mechanically digested short fibers and that in the Overflow fraction enriched the mechanically digested long fibers become. 7. The method according to any one of the preceding claims, characterized, that the wet screening is carried out in a pressure sorter. 8. The method according to claim 7, characterized, that the pressure sorter contains a cylindrical screen element. 9. The method according to any one of the preceding claims, characterized, that the continuous fraction (A) in paper or cardboard manufacture for Formation of an outer layer is used. 10. The method according to any one of the preceding claims, characterized, that the overflow fraction (R) in the paper or board manufacture for formation an inner layer is used.