Classifying – separating – and assorting solids – Sifting – With liquid treatment
Patent
1999-05-14
2000-12-05
Walsh, Donald P.
Classifying, separating, and assorting solids
Sifting
With liquid treatment
209283, 209305, 209306, 209273, 209281, B07B 122
Patent
active
061554278
DESCRIPTION:
BRIEF SUMMARY
This invention relates to pressure screens and more particularly to a screen for removing contaminants from a suspension of paper making pulp.
Closed pressure screens in which a flat or cylindrical screen is used to separate a suspension of paper-making pulp into an accepts pulp fraction and a reject fraction have long been used for paper pulp cleaning. Such pressure screens commonly employ a generally cylindrical perforated screening member, which may form an aperture pattern made up of either holes or slots. A rotating impulse member is positioned to operate adjacent a surface of the screen, which is commonly, but not always, an inner inlet surface, to maintain the stock suspension in a state of agitation and to provide pressure impulses by which the screening function is aided. The rotating-member may comprise a drum-type rotor in which protuberances or foil-shaped sections are mounted on the outer surface and move adjacent to a screen surface, or foils may be mounted on generally radially extending arms for rotation adjacent the screen surface.
Commonly, the stock suspension to be screened is brought in at or adjacent an axial end of a cylindrical screen and, during screening, moves axially along the inlet surface, as stated above, commonly the inner surface of the screen cylinder. At the same time, a rejects fraction is created by the inhibiting or screening out of undesirable material which does not pass through the apertures or openings in the screen, and this undesirable material also moves axially along the screen surface until it reaches the end of the screen axially opposite the inlet end, where it is directed to a rejects accumulation chamber and to a rejects outlet. Examples of pressure screens made and operated according to this principle may be seen in the following patents which are assigned to the same assignee as this application: Chupka et al., U.S. Pat. No. 4,328,096 issued May 4, 1986, Martin et al., U.S. Pat. No. 4,851,111 issued Jul. 25, 1989, and Weber, U.S. Pat. No. 4,166,028 issued Aug. 28, 1979.
With the increase in the quantity of contaminants in waste paper, and the need to remove such unwanted material from a pulp furnish, it has become necessary to improve the efficiency of the pressure screen in order to remove more of the contaminants and unwanted material and to increase the capacity. Screening efficiency may be defined as the percentage of contaminant removed from the stream of the feed suspension by the screening operation.
Conventionally, the stock suspension enters at one end of the screen or enters at the center of the screen and flows in opposite directions over the screen. The multiple foils perform the well known impulse and screening function such that the fibers are accepted through the perforated or slotted screen while the larger or longer material which is unable to go through such perforations is retained within the screening zone. It is also known that such screens cause the stock consistency to increase between the feed end of the screening zone and the reject end of the zone while, at the same time, the contaminant concentration increases as it gathers and flows along the screen inlet surface toward the reject end.
Two principle factors reduce the ability of any screen to retain contaminants. These are: 1) higher velocity of stock through the screen apertures causes more of the contaminants to flow through the apertures; 2) higher concentrations of the contaminants within the screening zone increases the probability that contaminants will flow through the screen apertures.
The conventional screening apparatus has a gradient of velocity of the stock through the apertures over the length of the screening zone, this in an axial length in the case of a cylindrical screen. The highest velocity is at or adjacent the point of entry of the stock suspension into the screening zone where the consistency is the lowest and the contaminants concentration is also the lowest. On the other hand, the lowest stock velocity is at the reject end of the screening zone where th
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Miller Jonathan R
Thermo Black Clawson Inc.
Walsh Donald P.
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