Liquid purification or separation – Processes – Separating
Patent
1991-05-01
1993-11-23
Dawson, Robert A.
Liquid purification or separation
Processes
Separating
210392, 210393, 210396, 210402, 210404, 210406, 210797, 210798, B01D 3306
Patent
active
052641384
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for treating pulp, which method and apparatus are especially suitable for treating fiber suspensions in the wood processing industry. The method and apparatus are particularly designed for thickening said suspensions, in other words, in filtering liquids from fiber suspensions in the pulp and paper industry.
The invention relates more specifically to a so-called drum cylinder, the basic construction of which comprises a vessel, into which suspension being thickened is supplied and a wire-surfaced cylinder rotating in the vessel. The ends of the cylinder have been sealed so as to prevent the cylinder from being filled with suspension. The cylinder has under its wire surface filtrate compartments, into which liquid from the suspension in the vessel passes due to the hydrostatic pressure thereon, whereby the fibers in the suspension are transferred with the liquid to the wire surface and are thickened forming a fiber matting thereon. The cylinder in this kind of filter based on hydrostatic pressure can be submerged in the suspension in the vessel only slightly below its axial level. When the cylinder rotates slowly, approximately 1 to 3 rpm, a fiber matting is slowly generated on the wire surface. The formation speed of the matting naturally depends not only on the thickness of the generated matting, but also on how deep in the vessel the filtration takes place. The filtration reaches its maximum speed slightly before the bottom dead centre, because the hydrostatic pressure is then close to its maximum and a thick matting does not yet disturb the filtration. After the bottom dead centre the formation of the matting slows down, until the formation, of course, ceases completely, when the matting rises from the suspension.
As the fiber matting descends into the vessel when the cylinder rotates the filtration begins, in other words, the discharge of air and initial filtrate from the filtrate compartments without vacuum begins. The fiber matting is removed from the cylinder at a stage, when it has passed the top dead centre and approaches the surface of the suspension. The matting may be removed either mechanically with wipers or by injecting either liquid or compressed air through the wire surface. The liquid which has flowed into the filtrate compartments may be discharged from the apparatus, for example, via the shaft of the apparatus or by utlizing some other arrangement. The length of a filtration period based on hydrostatic pressure in an apparatus according to the above description is approximately 140.degree. of a rotation of a cylinder and the proportion of the initial filtration is approximately 30.degree..
The above described basic type of a drum filter has been considerably improved over the past decades. One significant improvement worth mentioning is subjecting the suspension in the vessel to suction applied through the compartments, whereby the filtration of liquid improves considerably and thus the capacity of the apparatus increases significantly. Most usually the suction is carried out by a suction leg in such a way that each of the filtrate compartments is connected by means of a separate filtration tube to a valve connected to the suction leg in the shaft of the apparatus, the operation of which valve is controlled in such a way that the fiber matting is subjected to suction in a sector of approximately 240.degree.. Approximately 30 filtrate compartments have been arranged on the cylinder, each of which compartments has been connected with a separate filtrate tube to the shaft tube of the cylinder surrounding the valve. Since the fiber matting generated this way is significantly more tightly attached to the filtrate surface than the matting generated in a conventional way, the thickened fiber matting must be removed by separate apparatuses from the filtrate surface. At this point it must be noted that the suction leg may be replaced by some other apparatus creating vacuum, such as a vacuum pump.
One alternative method for removing the fibe
REFERENCES:
patent: 2070074 (1937-02-01), Young
patent: 3363774 (1968-01-01), Luthi
patent: 3530992 (1970-09-01), Turner
patent: 4147634 (1979-04-01), Wegener
patent: 4255264 (1981-03-01), Madsen
patent: 4608171 (1986-08-01), LaValley
Heino Jukka
Kohonen Raimo
Savolainen Erkki
A. Ahlstrom Corporation
Dawson Robert A.
Kim Sun Uk
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