DE19910796A1 - 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 and foreign matter. The fiber suspension is generally free of flexible foreign material. 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.

When processing paper fibers for later use in the Paper production is often required to use a wet screening process Part of the undesirable substances, so-called contaminants, from the paper fiber suspension remove. Such methods are also called "sorting". The unwanted components retained on one sieve while another Fraction which does not contain these contaminants or to a considerably lesser extent, the Sieve together with a large part of the paper fibers can pass.

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.  

If wet screening processes are used on a fiber suspension, which both Contains cellulose fibers from chemical digestion as well as wood pulp tends to be the particularly valuable cellulose fibers together with the contaminants held back on the sieve. As an undesirable side effect of the sorting goes a particularly valuable component of the fiber suspension with the reject, while of all the less valuable components, namely the wood pulp fibers reach the accepted fraction in an increased concentration.

The invention is therefore based on the object of an economical method create with a mixture of chemically digested pulp fibers and mechanically disrupted pulp fibers can be sorted out, and at the same time the proportion of chemically digested pulp fibers in the cleaned Fabric is as high as possible.

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 increased cellulose fiber content that can be formed simultaneously by Interfering substances are largely exempt. Such a fraction is ideal for Outer layers of multi-layer paper or cardboard. The overflow can into the Middle layers are used, provided the content of impurities is not too high.

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. Those skilled in the art are familiar with how such settings are made in pressure sorters. 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, which is divided into the run A and the overflow R. For easier identification of the relationships, reference lines to the curves 3 , 4 , 5 and 6 are drawn. Line 7 shows the theoretical limit, namely that the proportion of wood pulp does not change in wet screening.

Claims (11)

1.Procedure for sorting out impurities from a paper fiber suspension (S) which contains both chemically and mechanically disintegrated fibers, this fiber suspension being subjected to wet screening, in which the impurities are rejected on a screen element and discharged as an overflow (R), while A large part of the paper fibers passes through the screen element into the passage (A), characterized in that an average flow velocity of at least two m / sec is set in the openings of the screen element. 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 along with mechanically digested short fibers and that mechanically in the overflow digested long fibers are enriched together with the contaminants. 7. The method according to any one of the preceding claims, characterized, that the paper fiber suspension is largely free of flexible contaminants. 8. The method according to any one of the preceding claims, characterized, that the wet screening is carried out in a pressure sorter. 9. The method according to claim 8, characterized, that the pressure sorter contains a cylindrical screen element. 10. The method according to any one of the preceding claims, characterized, that the pass (A) in paper or cardboard manufacturing to form a Outer layer is used. 11. The method according to any one of the preceding claims, characterized, that the overflow (R) in paper or cardboard manufacture to form a Inner layer is used.