Classifying – separating – and assorting solids – Sifting – With liquid treatment
Patent
1994-01-12
1995-08-15
Bollinger, David H.
Classifying, separating, and assorting solids
Sifting
With liquid treatment
209350, 162 55, B07B 108
Patent
active
054411572
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of separating a suspension containing relatively fine particles and relatively coarse particles by spraying the suspension against a side of a rotating filter medium, so that a fine fraction of the suspension containing said fine particles passes through the filter medium, while a coarse fraction of the suspension containing said coarse particles does not pass through the filter medium. The present invention also relates to a device for such a separation.
A method and a device of these kinds are known from WO 90/06395. The known device comprises circular vertical rotatable discs covered with liter media, stationary spray nozzles adapted to spray one side of each disc with high pressure jets of the fibre suspension to be separated, and stationary cleansing spray nozzles adapted to spray the discs with cleansing liquid for cleansing the filter media. During operation of the known device the discs are sprayed with the fibre suspension and the cleansing liquid while the discs are rotated, whereby coarse particles and liquid developing on the discs flow by gravity down the discs to outlets situated below the discs.
The known device according to WO 90/06396 has proved especially usable and advantageous for fractionating fibre suspensions, since clogging of the filter media is efficiently prevented during operation. However, the known device has a relatively limited capacity with regard to the flow each disc is capable of receiving. Thus, a too thick layer of coarse particles is not allowed to have time to develop on the discs during operation. Thick layers of coarse particles deteriorate the separation efficiency, since they prevent the high pressure jets of fibre suspension from hitting the filter media directly.
To provide a quick removal of coarse particles from the discs of the known device also at the ascending sides of the discs, the rotational speed of the discs is relatively low. The low rotational speed of the discs allows the coarse particles and liquid to leave the discs by gravity, though the coarse particles are subjected to an upwardly directed entrainment effect at the ascending sides of the discs, when the filter discs are rotated. The discs of the known device, which has a diameter of about three metres, are therefore rotated relatively slowly during operation, at a few revolutions per minute at most. In consequence, the filter media are completely cleansed by the cleansing spray nozzles only a few times per minute.
The object of the present invention is to provide an improved method of separating a suspension which allows an increase in the flow capacity per unit of area of the filter medium and which improves the separation efficiency.
This object is obtained by a method of the kind stated initially, which primarily is characterized by rotating the filter medium at a speed such that coarse particles and liquid developing on said side of the filter medium are removed from the filter medium substantially as a result of the centrifugal force said coarse particles and liquid are subjected to by the rotating filter medium. Since the coarse particles are quickly removed from the filter medium by centrifugal force, the capacity of the filter medium can be increased, compared with the known method described above. In other words, the filter medium can be loaded with a larger flow per unit of area. In addition, the separation efficiency can be improved, since the relatively high rotational speed which has to be imparted to the filter medium, in order to provide sufficiently strong centrifugal forces, has the consequence that the filter medium passes stationary cleansing spray nozzles at a higher rate than previously, so that the filter medium is cleansed more frequently.
The capacity of the filter medium can be further increased by spraying the suspension against said side of the filter medium in at least two spray zones, which are spaced from each other in the direction of rotation of the filter medium, passing a portion of the filter medium successively throu
REFERENCES:
patent: 5124029 (1992-06-01), Fjallstrom et al.
Bollinger David H.
Celleco Hedemora AB
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