Classifying – separating – and assorting solids – Sifting – With liquid treatment
Patent
1995-06-05
1997-12-02
Nguyen, Tuan
Classifying, separating, and assorting solids
Sifting
With liquid treatment
210331, B01D 3321
Patent
active
056926197
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for filtering suspensions, such as fibre pulp suspensions.
2. Description of the Prior Art
WO 91/12063 discloses an apparatus for filtering suspensions, such as fibre pulp suspensions, including a container for the suspension to be filtered, inlet means for supplying the suspension to the container, at least one annular, substantially vertical disc arranged in the container, wall means connected to the disc and forming a filtrate chamber at the center of the disc, and means for rotating the disc about a horizontal axis extending centrally through the disc. There are means dividing the disc into a number of segments disposed in series around the disc. Two side walls of filter material cover the disc at mutual sides thereof, each segment and said side walls of filter material defining a cell for receiving filtrate. Each cell communicates with the filtrate chamber. Means are provided for controlling the volume of the suspension in the container during operation, such that said side walls of filter material are partly above the suspension in the container, while the filtrate chamber is at least partly submerged in the suspension in the container, whereby a fine fraction of the suspension is forced through the filter material into the cells and further into the filtrate chamber, and a coarse fraction of the suspension is created outside the filter disc. There are means for maintaining a pool of fine fraction in the filtrate chamber. Fine fraction discharge means is provided for discharging said fine fraction of the suspension from the filtrate chamber, and coarse fraction discharge means is provided for discharging said coarse fraction of the suspension from the apparatus.
In this prior art filtering apparatus each cell functions as a dropleg at the ascending part of the rotating disc, when at least a part of the cell is above the pool of fine fraction in the filtrate chamber, so that a negative pressure is created in the cell. Thus, the pressure difference between the outside and the inside of the cell is increased by said created negative pressure in the cell. The increased pressure difference gives the advantages that the flow of fine fraction through the filter material covering the cell is increased, and a compressed mat of particles (mainly coarse particles) can be formed on said filter material resulting in an increased particle concentration of the coarse fraction of the suspension.
Furthermore, the prior art apparatus can be operated such that each cell contains a volume of fine fraction, when at least a part of the cell has been displaced up above the suspension in the container, while the outlet of the cell still opens into the pool of fine fraction. The advantage of this is that fine fraction is also sucked from the mat of particles which covers the filter material of the cell above the suspension, whereby the particle concentration of the coarse fraction is further increased.
However, the capability of the prior art apparatus for producing a concentrated coarse fraction primarily depends on the height of the column of fine fraction in each cell, i.e. the height of the suction leg, as seen at the ascending part of the disc. Thus, the higher column of fine fraction there is trapped in the cells, the more concentrated coarse fraction can be obtained. The prior art apparatus is deficient in this respect, because during rotation of the disc the fine fraction trapped in each ascending cell has to escape through the outlet of the cell before the cell descends back into the suspension, so that the created mat of particles on the side walls of the cell is not rewetted with fine fraction. In consequence, the outlet of the cell has to be large enough to allow a sufficient flow of fine fraction from the cell. This limits the radial extension of the cell, and as a result the column of fine fraction in the cell is limited. The radial extension of the cells cannot be increased by simply increasing the diameter of
REFERENCES:
patent: 3591009 (1971-07-01), Luthi et al.
patent: 4136028 (1979-01-01), Toivonen
patent: 5124029 (1992-06-01), Fjallstrom et al.
Beloit Technologies Inc.
Campbell Raymond W.
Donovan Paul F.
Nguyen Tuan
Veneman Dirk J.
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