Classifying – separating – and assorting solids – Sifting – With liquid treatment
Patent
1994-10-13
1996-08-20
Bollinger, David H.
Classifying, separating, and assorting solids
Sifting
With liquid treatment
209306, 162 55, B07B 104
Patent
active
055470831
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATION
This application is the U.S. National Phase of PCT International Application PCT/FI93/00151 filed Apr. 8, 1993.
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to an apparatus for treating fiber suspension. Said apparatus is especially applicable for screening and also thickening pulp in the wood processing industry. More precisely the inventive concept of the apparatus of the invention is related to the pulse member construction of the screen or thickener being used.
There are in principle two known types of pulse members, which both are generally used and the purpose of which is, as known, to maintain the screen surface clear, in other words, to prevent the formation of a fiber matting on the screen surface. One type is exemplified, for example, by a rotor in accordance with U.S. Pat. No. 4,193,865, in which said rotor is rotatably mounted within a stationary screen cylinder, comprising blades located close to the surface of the screen cylinder. In accordance with said patent the blades form an angle with the axis of the cylinder. The screen surface is subjected to pressure pulses by the moving blades clearing the openings of the screen surface. There are also embodiments that have blades on both sides of the screen cylinder. Consequently, the suspension being treated is fed either to the inside or to the outside of the cylinder and the accept is discharged from the outside or from the inside of the cylinder, respectively.
Another rotor type worth mentioning is, for example, one in accordance with U.S. Pat. No. 3,437,204, in which the rotor is a substantially cylindrical closed body, the surface of which is provided with almost hemispherical protrusions, so called bulges. In said apparatus the pulp is fed to a treatment space between the rotor cylinder and the screen cylinder outside thereof, whereby the protrusions of the rotor, the bulges, act both to press the pulp against the screen cylinder with the leading surface and to draw off the fiber flocs from the perforations of the screen cylinder with its trailing surface. Because this kind of construction has a highly thickening effect on the pulp, there are in the above mentioned arrangement three dilution water connections arranged at different heights on the screen cylinder, so as to make the screening of fiber suspension take place satisfactorily. A corresponding "bump rotor" is disclosed also in U.S. Pat. No. 3,363,759.
Also, other embodiments of the above mentioned cylindrical rotor are known, in connection with which many kinds of protrusions facing the screen cylinder may be used as disclosed in various publications.
DE application 30 06 482 discloses a knot separator, in which on the surface of a cylindrical rotor there are plough-like protrusions, made of plate material, by which the pulp between the rotor and the screen cylinder is subjected to strong mixing forces so as to make fibers pass through the screen cylinder most effectively, and knots, shives and such separate therefrom.
U.S. Pat. No. 3,400,820 discloses a rotor being formed of eccentric portions or lobes. The slope of the fluid contacting surface of the rotor is gradual in the increasing as well as decreasing clearance regions. The slope angle is less than 15.degree. relative to the direction of the movement. Since the slope is equal in both directions the operation of the rotor is far from optimal. The decreasing clearance presses the pulp towards the screen surface such that the openings of the screen cylinder easily clog, whereby the portion having the increasing clearance is not able to create a negative pressure sufficient to draw the fiber agglomerations out of the openings. Yet another drawback of the structure shown in the patent is, on the one hand, its tendency to crush all the impurities like knots etc. between the rotor and the screen surface. On the other hand, if the impurities are hard, such as stone or metal particles, there is a risk that the rotor presses the particles through the screen cylinder.
U.
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Alajaaski Timo
Laakso Kalevi
Laine Antero
Ljokkoi Risto
A. Ahlstrom Corporation
Bollinger David H.
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