Liquid purification or separation – Plural distinct separators – Filters
Patent
1992-06-30
1994-07-19
Dawson, Robert A.
Liquid purification or separation
Plural distinct separators
Filters
210327, 210347, B01D 3321
Patent
active
053306465
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for filtering liquids, especially for recovering fibres from so-called white water produced in the paper making industry.
2. Description of the Prior Art
A common type of filtering apparatuses for filtering liquids comprises at least one annular hollow filter disc with side walls covered with filter material adapted to be at least partly immersed in a body of liquid to be filtered and arranged substantially transverse to a horizontal axis extending centrally through the filter disc. There is means for rotating the annular hollow disc about said horizontal axis. Shaft wall means is connected to the filter disc and forms a circular cylindrical chamber, which extends centrally through the hollow filter disc coaxial with said axis and which communicates with the interior of the hollow filter disc. There is means for creating a pressure difference between said body of liquid to be filtered and the interior of the hollow filter disc, such that a filtrate of the liquid is forced through the filter material into the hollow filter disc and passed therefrom into said cylindrical chamber. Filtrate discharge means is provided for discharging said filtrate from the cylindrical chamber, and spray nozzle means is provided for cleaning the filter material.
This type of filtering apparatus is of a simple and inexpensive design. However, a drawback to this apparatus is the poor purity of the filtrate when filtering fibre pulp suspensions, which makes the apparatus unsuitable for fibre recovery. As the filter disc is rotated in a body of white water containing fibres, a relatively large flow of cloudy filtrate is passed through the filter material just after the latter has been cleaned by the spray nozzle means. During the further rotation of the filter material, a mat of fibres is built up on the filter material. This mat of fibres constitutes in itself a tight filter medium, with the result that a relatively small flow of clear filtrate is passed through the part of the filter material which is covered with such a mat of fibres. However, said large flow of cloudy filtrate and said small flow of clear filtrate are mixed in the cylindrical chamber resulting in a poor purity of the filtrate leaving the filtering apparatus.
Another type of filtering apparatus known as "vacuum filtering apparatus" includes filter discs with circumferentially spaced filtrate chambers, which are connected to axial filtrate discharge pipes. During rotation of the filter discs, the filtrate discharge pipes are connected in sequence to two droplegs, with the result that the filtrate is divided into a cloudy filtrate and a clear filtrate. However, if a great number of filter discs is used, such as twenty discs, the rotational speed of the filter discs must be low, in order to prevent too much mixing of the cloudy and clear filtrates. This is so because cloudy filtrate created in the filter discs farthest from the droplegs takes some time to flow through the axial discharge pipes to the droplegs. During this time, the filter discs are rotated a bit further, with the result that the cloudy filtrate from said discs farthest from the droplegs is mixed with clear filtrate from the discs close to the droplegs. Thus, when using such a vacuum filtering apparatus for cleaning white water from fibres, the operational capacity of the apparatus is low because of the necessity to keep an extremely low rotational speed. In addition, a vacuum filtering apparatus is very expensive compared to the type of filtering apparatus which is operated without such droplegs.
U.S. Pat. No. 4,123,363 discloses a vacuum filtering apparatus, in which the above-described axial filtrate discharge pipes are replaced by two axial filtrate discharge channels, which are formed by a stationary partition wall extending axially through a hollow shaft, on which the filter discs are mounted. The two filtrate discharge channels are connected to two droplegs, respectively. There is also a third
REFERENCES:
patent: 4123363 (1978-10-01), Koskinen
patent: 4255264 (1981-03-01), Madsen
patent: 4330405 (1982-05-01), Davis et al.
patent: 4728424 (1988-03-01), Miura
patent: 4814093 (1989-03-01), Frykhult
Dawson Robert A.
Ingenjorsfirman R. Frykhult AB
Reitsnyder David
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