Classifying – separating – and assorting solids – Sifting – With liquid treatment
Reexamination Certificate
2000-03-24
2001-08-14
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Sifting
With liquid treatment
C209S017000, C209S155000, C209S268000, C209S393000
Reexamination Certificate
active
06273266
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 199 13 515.0, filed on Mar. 25, 1999, and German Patent Application No. 199 16 038.4, filed on Apr. 9, 1999, the disclosures of which are expressly incorporated by reference herein in their entireties.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for wet screening of stock suspensions in a pressure grader, and to a pressure grader screen for wet screening of stock suspensions.
2. Discussion of Background Information
An important use of the above-noted process and device is the grading of stock suspensions. In the known process, the fibers contained in the suspension are to pass through the screen, while the undesired solid components are rejected at the slit and conducted out of the screen again. Because the apertures have an essentially elongated shape, e.g., slots or slits, fibrous particles pass through more readily than flat particles, even if both types are present in a similar order of magnitude. With such grading technology, it is therefore possible to achieve a very good separation effect of non-fibrous disruptive substances from stock suspensions. However, it is a prerequisite that the shape of the slot be extremely precise over the entire screen surface. Another important use is the separation of different fibers, in particular fractionation, e.g., based on fiber length or flexibility.
Screens or revolving screens are known, e.g., from published Patent Application DE 33 27 422 A1, in which the grading slots are formed by essentially parallel identical profiles connected to transverse holding ribs. The profiles are shaped and arranged so that, when the screen is in operation, the approach surfaces of the profiles have an oblique contour and a break-away edge behind. This causes the suspension flow sweeping past to undergo a deflection that leads the suspension flow away from the grading slot so that eddies are formed that assist in keeping the grading slots clear and promoting the passage of the fibers through the screen.
In almost all processes of this type, a clearer is moved past close to the screen to produce pressure and suction impulses on one side of the screen. The brief pressure and suction impulses cause the flow direction to reverse in the grading aperture. In this way, the grading aperture is kept clear in that fiber clumps or particles that cannot pass through are conveyed back again against the normal flow-through direction. Pressure and suction impulses can also be introduced into the suspension directly, e.g., via membranes.
The above-mentioned devices have proven to be effective in many applications. However, it is still not always possible to reliably keep the grading slots clear and, at the same time, to conduct the desired high underflow quantity through the screen slots.
In U.S. Pat. No. 4,898,665, a screen device is described in which step-shaped flow barriers (“obstacles”) are installed in the inflow area to each screen aperture, which barriers stem the approaching suspension. They also cover the screen apertures partially or completely and produce turbulence in the flow. The eddies thus produced at the end of the flow barrier are intended to guide the flow directly into the screen apertures. However, the flow barriers can also lead to stagnation points on the screen surface at which disruptive deposits settle.
SUMMARY OF THE INVENTION
The present invention provides a process and a screen device of the type generally discussed above, in which the screen is more effectively kept clear, and in which a good cleaning effect and a high screen throughput occur.
The present invention provides a process in which the inflow side of the screen is formed so that a flow direction of a break-away eddy is reversed by a back flow, which rinses a part of the screen lying between the screen aperture in question and a subsequent downstream aperture.
The process of the instant invention also provides that, as it exits from the screen aperture, the back flow is turned so that its flow direction runs in a direction opposite to a circumferential flow of the break-away eddy, and the back flow completely rinses the part of the screen lying between the screen aperture in question and the subsequent downstream screen aperture.
The pressure grader screen of the instant invention provides faces having a back end with extensions, each of which covers an adjacent grading slot and a part of a front end of an adjacent face of an adjacent grading slot. The extensions can extend over a length of at least approximately 0.5 mm of the front face of the adjacent face of the adjacent grading slot, and an underside of the extension facing and the adjacent face can be oriented an angle to each other of between approximately 3°-45°, and preferably between approximately 5°-25°.
In accordance with the features of the present invention, the flow situated in the inflow area of the screen apertures can be advantageously developed. With the aid of the deflection produced, the return flow is not “caught” by the break-away eddy, but flows against its direction directly past the following downstream face of the screen. Here, this area of the screen is formed so that this face is rinsed up to the next following downstream screen aperture. It can also be the case that, during the change in the flow-through direction, microeddies are formed in the inflow area whose rotation direction is opposite to that of the break-away eddy or that the rotation direction of the break-away eddy is even reversed. Due to this feature of the invention, fibers that might adhere to the screen surface are approached by a flow from another side, promoting their detachment from the screen surface. Such residues originate from the pre-flow phase. The same is also true for disruptive substances to be separated, in particular when they tend to stick (e.g., “stickies”).
Slot-shaped screen apertures may be particularly suitable for the process, which enable a possible advantage for the process to be achieved particularly effectively. In fact, it has been found that the content of long fibers in the screen underflow (accepted stock) is higher than with conventional screens. Thus, a specific problem of slot grading, namely the loss of long fibers, is more readily solved. However, other shapes that can cause a reverse eddy, e.g., round holes with deflection elements, are also conceivable.
The additional expense required for the shaping of the profile bars can be held within narrow limits. As already mentioned and also known per se, it is possible to produce such devices by fixing profile bars arranged next to one another by fastening elements, e.g., rings. In this way, it can then be sufficient to select a different profile to carry out the invention. While this may necessitate dispensing with standard profiles, this dispensing can be taken into account with the large number of the screens to be produced. The new profiles can generally be used without alteration for screens of extremely different sizes. If drawn profiles, for instance, of steel or a steel alloy, are used as a blank for the bars, it is also possible to produce specific cross-section shapes precisely and economically.
Of course, the profile required for the invention can also be produced by subsequent machining of the surface on the already produced screens.
The flow through the cylindrical screens of the type under consideration here is either centrifugal or centripetal. The meaning of the terms that are familiar per se is as follows: centrifugal relates to radially outwardly directed flows (i.e., inward to outward); and centripetal related to radially inwardly directed flows (i.e., outward to inward). The position of the screen clearers is also not always determined thereby. As is known, there is screen clearing on the accepted stock side and screen clearing on the inflow side. The invention is used advantageously when a clearer is on the inflow side.
The present invention is directed t
Beauchaine Mark J.
Greenblum & Bernstein P.L.C.
Voith Sulzer Papiertechnik Patent GmbH
Walsh Donald P.
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