Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With classifying – separating or screening of pulp
Reexamination Certificate
2001-12-04
2003-09-02
Griffin, Steven P. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With classifying, separating or screening of pulp
C209S017000, C209S279000, C209S303000
Reexamination Certificate
active
06613191
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. §119 of German Patent Application No. 100 60 822.1, filed on Dec. 7, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a pressurized screen to remove contaminants from a contaminant-containing fibrous paper suspension with a vertical housing that contains at least two mainly axially symmetrical screen elements that are arranged so that the fibrous suspension fed through at least one intake into the housing can flow through them in succession. The upstream first screen element can be rotated and is mounted axially outside and above the chamber in which the second screen element is located. A part of the fibrous paper suspension that has passed through the second screen element is channeled out of the housing through at least one accepts outlet. The housing further includes at least one coarse reject outlet for the overflow of the first screen element and at least one fine reject outlet for the overflow of the second screen element.
The invention also relates to a pressurized screen to remove contaminants from a contaminant-containing fibrous paper suspension with a horizontal housing that contains at least two mainly axially symmetrical screen elements that are arranged so that the fibrous suspension channeled into the housing through at least one intake can flow through them in succession. The upstream first screen element can be rotated, and the part of the fibrous paper suspension that has passed the second screen element is channeled out of the housing through at least one accepts outlet. The housing features at least one coarse reject outlet for the overflow of the first screen element and at least one fine reject outlet for the overflow of the second screen element.
2. Discussion of Background Information
Pressurized screens are used in the processing of fibrous paper suspensions, in order to treat the fibrous suspension in a wet screening. To this end such a pressurized screen contains at least one screen element that is provided with a number of openings. The fibers contained in the suspension should pass through the openings, while the unwanted solid particles are rejected there and conveyed out of the screen again. As a rule, round holes or slits are used as screen openings. In most cases, pressurized screens of the kind considered here are equipped with screen scrapers that feature scraping areas swept across the screen. This is how the clogging of the screen openings is usually prevented in the per se known way.
As both the state of disintegration of the paper fiber material and the volume and structure of the contaminant content change constantly in the course of the pulp processing, it is necessary in many cases to operate pressurized screens in several stages with different openings. In this way it is possible to achieve overall a good straining of most of the contaminants collected. Such contaminants, which differ significantly from the other contaminants due to their sinking action, can thereby also be strained by means of hydro-cyclones. This technique is also known, so that a sequence of hydro-cyclones and pressurized screens is used for pulp screening in general.
A vertical separator for a fibrous suspension that features two screen elements in a single machine is known from DE 197 02 044 C1. With this separator, the pulp stream to be sorted first arrives at the area of a flat coarse screen that is kept clear on the intake side with a scraper. The flow through this flat coarse screen is then conducted into the interior of an axially symmetrical screen basket, through the openings of which the accepts pass, so that the suspended paper fibers can pass through this screen basket into the accepts outlet. This kind of screen should be used in particular whenever the supplied fibrous suspension is mixed with a larger amount of coarse contaminants. This is known to occur very often in waste paper processing, especially with the pulp that comes directly from the pulper or after passing through a cleaning device that removes only the coarsest impurities. However, in certain cases this known screen has the disadvantage that a high degree of wear occurs, particularly in the area of the flat primary screen, and the coarse impurities can lead to breakdowns.
WO 00/5 8549 A1 shows a screen with a vertical housing in which there are two screen elements through which the fibrous paper suspension to be cleaned flows successively. In this the two screen elements are at least partially axially fitted into one another, which facilitates a compact construction. Other versions with two screens flowed through successively are known from EP 0 955 406 A2 and DE 2140 904. The first screen is cylindrical and is set in rotation. However, these devices are unfavorable in many applications. This applies in particular to the processing of coarse pulps. A cylindrical screen has the disadvantage in particular that the oblong impurities with a tendency to spin or wind can easily stick to such a screen and then become firmly fastened as a result of the rotation. They can then only be removed manually after shutting down.
A flat, disk-shaped screen, such as is known from WO 00/52260, for instance, has a simple structure and takes up little space. However, it has only a small screen surface, which is disadvantageous.
SUMMARY OF THE INVENTION
The present invention provides a pressurized screen that is unsusceptible to clogging impurities and at the same time offers good screening quality. It may be particularly useable for disintegrated waste paper that still contains a large proportion of the originally contained, hardly crushed contaminants.
According to the instant invention, the first screen element has a conical form with an opening angle (&agr;) that is between about 10 and 170°. Moreover, in the horizontal orientation, the first screen element is positioned axially outside of and to the side of the chamber in which the second screen element is located.
A pressurized screen of the type of construction according to the invention is particularly suitable for use at the start of paper stock preparation, that is, wherever a relatively large proportion of coarser contaminants is still carried along in the suspension. If waste paper is brought into the pulper or cleaning drum in suspension, for instance, it often has a contaminant content of more than 2% of the solids. Part of this contaminant is relatively coarse and is therefore also rejected to a great extent at a 4 mm perforation.
After entering the pressurized screen according to the invention, the suspension encounters the intake side of the first screen element. Its rotation creates a centrifugal force with which the contaminants are spun from the surface of this screen element, provided they have a higher specific gravity than the suspension surrounding them. Thus not only the tractive forces created due to the pressure difference, but also centrifugal forces act on the solid particles located near the screen intake. The more the longitudinal direction of the screen opening is oriented radially (instead of axially), the stronger the free centrifugal effect and the lower the danger of the particles being caught in the openings. Heavy particles that are rejected at the screen element according to the specification, leave its intake area relatively quickly and are then removed from the housing through the coarse reject outlet. They may possibly also carry with them light contaminants rejected because of their size, particularly plastic foils. The screen is not only less at risk of wear, but it is also not so easily clogged by wire pieces, splinters of glass and small stones.
The centrifugal effect naturally depends on the speeds and the radius at the place in question. The particular advantage of the invention lies in the fact that the first screen element is constructed conically at least in part. A conical form combines the
Rienecker Reimund
Schweiss Peter
Griffin Steven P.
Hug Eric
Voith Paper Patent GmbH
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