Paper making and fiber liberation – Processes and products – Reclamation – salvage or reuse of materials
Patent
1996-12-06
1998-05-19
Czaja, Donald E.
Paper making and fiber liberation
Processes and products
Reclamation, salvage or reuse of materials
162189, 162264, 210928, D21F 166
Patent
active
057530807
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a method of recovering fibers from white water using a vat with a rotating filter surface movable into and out of the vat, typically disc filters.
The invention is described more in detail below as applied to the recovery of fines in the white water circulation of a paper machine. However, the invention may also be applied to other corresponding methods and systems.
The filters used in the recovery of fines from the white water circulation of a paper machine are mainly disc filters. These filters usually have a large number (e.g. six to thirty) of discs mounted on a horizontal shaft with the surface of the disc, which serves as the filtering surface, located essentially perpendicular to the center shaft of the filter. Vacuum is provided inside the disc by connecting the disc to the suction end of the filter. The screen surfaces are rotated while immersed in the mixture to be filtered. The pulp to be filtered is collected onto the filter surfaces and it is removed from the surface of the disc at a point above the surface of the pulp to be filtered.
In the feeding systems presently used, the fiber recovered from the white water and the auxiliary pulp, having longer fibers than the white water, are mixed in a static mixer prior to feeding the pulp to the supply vat of the disc filter. Long-fibered auxiliary pulp is mixed with the white water because the fines contained in the white water are so fine that it is difficult to separate them alone from the white water.
The feed consistency is controlled by a control circuit and the adjustment of the ratio of auxiliary pulp and white water aims at producing a mixture of normal feed consistency (approx. 0.6 to 1.3%) and the mixture is then filtered with the disc filter. The main part of the cumulative solids content of the filtrates is collected immediately following the beginning of the filtering process when no fiber layer has formed onto the filter wire to prevent passing of the fines through the wire. The filtrates obtained are divided in three fractions: very clear, clear, and cloudy filtrate.
The filtrates are commonly utilized so that the cloudy filtrate, which has the highest solids content, is recirculated to the supply of the filter, to be introduced with the white water and the auxiliary pulp. The clear filtrate is used in the detaching and washing jets of the filter and possibly also for the same purpose at a paper machine. The very clear filtrate is discharged from the process.
Prior art fiber recovering systems require high-capacity filters because the share of the recirculated cloudy filtrate is quite large. This also increases the amount of the auxiliary pulp required.
Finnish patent application no. 802528 discloses a fines recovery system. The characterizing feature of the system is that it comprises two separate filter apparatus interconnected by a flow duct. The first filter apparatus treats with a coating pulp, introduced from outside the system, either the cloudy filtrate received from the filter apparatus 8 and supplied together with the coating pulp and/or the clear filtrate received from the filter apparatus 6 or the cloudy filtrate supplied with the coating pulp and/or the clear filtrate received from the second filter apparatus; and the clear filtrate received from the second filter apparatus and supplied separately to the first filter. This means that the coating pulp brought to the fines recovery system comes at no stage in direct contact with the circulation water to be cleaned but the pulp is at first used in the treatment of the filtrate from the second filter apparatus in order to recover the fines in the filtrate.
According to the system of the publication, the thickened pulp received from the first filter apparatus and containing the fines recovered therein, is brought to the second filter apparatus to serve as the so-called auxiliary pulp. The whole volume of the circulation water to be treated, or at least a part of it, is supplied to the second filter apparatus wit
REFERENCES:
patent: 5290454 (1994-03-01), Dorica et al.
Ahlstrom Machinery Oy
Czaja Donald E.
Leavitt Steven B.
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