Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With classifying – separating or screening of pulp
Patent
1997-05-20
1999-10-19
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With classifying, separating or screening of pulp
209270, 209288, D21D 506
Patent
active
059683155
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an apparatus and a process for screening a fibre suspension such as papermaking stock in a pressurized screen especially in connection with pumping said liquid. The invention also relates to the use of the apparatus in a process for the production of paper or board.
The invention relates principally to the removal of impurities from fibre slurries used in the pulp and paper industries. The invention presents an improvement of commonly used pressurized screens, by which the loss of good material together with rejects is minimized so that the cleaning process becomes more efficient, compact and energy efficient and the apparatus is made less complicated. The invention further allows the integration of the pumping, cleaning and stock distribution functions with the screening, whereby the papermaking process is further simplified.
2. Prior Art
In known screens, presented among others in U.S. Pat. No. 3,363,759, a fibre slurry is fed into a space limited at least in one direction by a screen plate through which the fibre suspension is forced, and which prevents fibre bundles and other bigger particles from passing through the screen together with the accepted fraction, i.e. the accept. Due to the mechanism of screening, also a part of the good fibres stop at the screen plate, forming a fibre mat which gradually thickens and prevents the flow through the screen. According to the above mentioned US Patent the fibres collecting at the screen plate are brought back into the suspension by means of turbulence generating bumps in the surface of a rotor.
The temporary hold of fibres at the screen plate also makes the liquid component of the suspension pass the screen faster than the fibres, which causes the suspension being screened to become gradually more concentrated in the screening zone. This gradually decreases the functionality of the screening, until the screening has to be interrupted in order to prevent excessive concentration and a resulting clogging of the screen.
Removing the fraction which is not to pass the screen, i.e. the reject, from the screen, also requires a sufficient volume flow so that the flow speed in reject pipes can be kept sufficiently high, and the separation of solids and consequent clogging of the piping can be avoided.
Due to these phenomena a significant portion, typically 5 to 30%, of the good fibres screened are rejected when screening in a conventional screen. In order to recover these fibres, the reject is re-diluted and cleaned in a second cleaning stage in another screen, the reject of which can be cleaned in a third screen, and so on. The last cleaning stage, normally, comprises an open screen, from which the reject can be removed at a high consistency.
Screening in a multiple of screens is obviously disadvantageous in view of the need for space, investment, energy consumption, cleanliness and also in view of process controllability.
In previously used open screens it was common practice to dilute the goods to be screened in the screening zone. Internal dilution has been attempted, also for pressurized screens and the resulting difficulties have been resolved in various ways. U.S. Pat. No. 3,437,204 avoids stratifications and poor mixing, when diluting through a rotor, by feeding dilution water through a screen plate. EP Patent Application 0233517, aiming at the washing of recycled wastepaper, also introduces water through an opening in the screen plate. Both these solutions lead to consistency differences at the accept side, which would require further equalizing before use in a paper machine. There are also other drawbacks, such as the fast draining of dilution water and consequently an excessive need for such water, complicated screen maintenance, etc.
A problem with most pressurized screens of the prior art is that the rotors have to generate pressure pulses for keeping the screen plate open and permitting the fibres to pass. These pressure pulses are conducted further in a pape
REFERENCES:
patent: 3363759 (1968-01-01), Pounder
patent: 3437204 (1969-04-01), Pounder
patent: 3814244 (1974-06-01), Young
patent: 4441999 (1984-04-01), Frykhult
patent: 4749474 (1988-06-01), Young
Nguyen Dean T.
POM Technology Oy AB
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