Note: Descriptions are shown in the official language in which they were submitted.
Method for thickening of pulp slurry or suspen~ion and
disk filter ~or carrying out the method
The present invention is concerned with a method
for thickening of pulp slurry or suspension by means of
a rotary disk filter partly submerged in the pulp slurry
or suspension contained in a basin, a positive pressure
prevailing outside the filter ~aces of the disks. The
invention is also concerned with a disk filter for carrying
out the method, which filter comprises a basin, a hood
placed above the basin, and filter disks mounted on a
sha~t, consisting of sectors~ and revolving in the basin.
Pulps are thickened by means of various filters,
e.g. drum or disk filters. An advantage o~ a disk filter
as compared with a drum filter is that its filtering area
is large in relation to the overall volume of space re-
quired by the filter. When a disk filter is used~ an
approximately 2- to 3-~old filtering area can be accommo-
dated within the same floor area as compared with a drum
filter. The construction and operation o~ disk filters
are described, e.g.g in the Finnish Patents 54,o60 and
56,628.
In prior-art disk filters, the filtering pro-
cess is produced by using suction inside the filter disks.
In that case, there is normal atmospheric pressure outside
the disks. However, by means of suction ~ilters, pulps
of a temperature higher than about 80C cannot be filtered
by means of suction filters at a negative pressure of
about 3 to 5 m H20. When higher temperatures are used,
the filtrate starts boiling on the negative-pressure side.
Pulps ground under pressure etc., whose thick-
ness is low, however, frequently occur at a temperature
of about 100 to 105C. Pulps of a low thickr,ess contain
an abundance o~ water~ for which reason their cooling is
dif~icult. The object of the present invention is to
sDlve the above problems.
In accordance with one embodiment of the present
invention, there is provided a method for thickening of pulp
slurry or suspension with a rotary disk filter partly submerged
in the pulp slurry or suspension contained in a basin,
comprising the steps of creating a positive pressure prevailing
outside the filter faces of the disks for filtering~ in
addition to filtering, continuously utilizing pressurized air
for detaching the filtered pulp from the Eilter faces of the
disks by passing the pressurized air into a sector of each
filter disk that is at a detaching location, discharging the
air in the sector through the Eilter face to outside the Eilter
- Eace.
In such a case, filtering can be perEormed, e.g.,
within a temperature range of ~0 to 110C., preEerably 75 to
105C. The overpressure required is <5 m H2O, preferably
2 to 3 m H2O. 'rhereby, from an input thickness of 0.3 to 2~/
an output thickness of 10 to 25% can be reached. The method is
suitable for use for thickening, among other things, of a
low-thickness, hot, poorly filterable pulp slurry, such as,
e.g., oE a pressure-ground pulp at the vortex-cleaner
thickness.
In the method in accordance with the invention, the
pressurized air can, besides for filtering, also be utilized
for detaching the filtered pulp from the filter face. The
system can be constructed closed, whereby the pressure can be
produced easily. The pressure can be generated, e.g., by means
of centrifugal blowers connected in series.
n
2a
In accordance with another embodiment oE the present
.invention, -there is provided a rotary disk -filter for thickening oE
pulp slurry or suspension, the Eilter comprising a basin, a hood
placed above the basin, and filter disks mounted on a shaft, the
filter disks including sectors and revolving in the basin, means
for producing a positive pressure outside the filter aces of the
disks including a longitudinal hollow space inside the shaft
connected with a source of positive pressure and the disk sector,
the disk sector being arranged at the detaching location of each
aisk .
In the equipment .in accordance with the present inven-
tion, the detaching position of the disks can be placed, in the
direction of rotation of the disks, after the topmost position, and
not before that position, which is the case in the disk filters
known at present. In such a case, an area that is up to 90
larger, out of the face of the disks, can be used as drying area.
The effec-tive area is about 240, instead of the present
about 150. For example, at a speed of 1 revolu~ion per
minute, this increases the drying time by 15 seconds,
which increases the output thickness by about 5 %.
The invention and its details will be described
in more detail below with reference to the attached
schematical drawings, wherein
Figure 1 is a cross-sectional view of a disk
filter in accordance with the inventiong
Figure 2 is a longitudinal sectional view of
the same filter, as viewed from above,
Figure 3 is a cross-sectional view Or a disk
filter in accordance with a second embodiment, and
Figure 4 is a longitudinal sectional view of
the filter shown in Fig. 3, as viewed from above.
In Figures 1 and 2, the reference numeral 1
denotes the basin into which the pulp to be thickened is
passed through a distribution pipe 2. The disks 4
consisting Or sectors 3 are mounted on a shaft 5. In
each sector there are two opposite filter faces 6 per-
meable to water The sol;d partition walls 16 between
the sectors separate the sectors from each other. The
shaft 5 consists Or a hollow tube~ and inside the shaft
there iæ a stationary, non~rotable hollow tube 14.
A hood 7 is fastened as sealed onto the basin.
Between the disks, at one side of the filter~
there are vertical tubes 8 as chutes, into which the
filtered pulp is gathered from the filter faces. The
lower ends of the vertical tubes 8 are united as a
horizontal trough provided with a screw conveyor 9.
From the discharge screw 9 the pulp is fed into a tank or
into a device, e.g. pulp pump 10, operating as a pressure
lock on the outlet side.
Onto the hollow tube 14, a pipe 12 is connected
towards the detaching position of each disk. The pipe 12
is connected to the rotary disk as gliding ~y means of
a sealing shoe 13. The blow side of the blower 15 is con-
nected to the inside space of the tube 14, and the
suction side of the blower 15 is connected to the inside
space Or the shaft 5.
When the filter operates, its disks rotate in
the direction denoted with the arrow A in ~ig. 1. The
pulp to be thickened is passed into the basin through
the distribution ~ipe 2 ~arrow B). When a disk sector
rises from the pulp slurry ~right side Or the disk 4 in
Fig. 1)~ pulp remains on the sector faces and the filtrate
is pressed by the overpressure into the sectors. From
there the filtrate flows into the open pipe 5, from which
it is drained off (arrows C and C'). Along with the
filtrate, air also flows from outside the disks into
the disks (arrows C').
The blower 15 blows air into the tube 14
(arrow D). From here the pressurized air passes through
the pipes 12 into the sector of each disk that is in the
detaching position (arrows D'). The pressurized air is
discharged through the punched face of the sector to out-
side the sector, at the same time detaching the filtered
pulp placed on the outer face of the sector, the pulp
falling into the tube 8 (arrow E). The pulp is removed by
means of the screw 9 and of the pump 10. The pump 10
operates as a pressur~ lock on the discharge side.
In this way, the air coming from the blower
enters through the sectors in the detaching position into
the hood, whereat it forms the overpressure necessary
there, detaches the pulp mat, and cleanses the wire at
the same time. The air into the blower is sucked from
inside the shaft tube 5 (arrow ~), whereby a closed air
circulation is obtained.
Figures 3 and l~ show a second embodiment of the
invention. In this embodiment, the interior space Or the
shaft 5 is provided with longitudinal compartment walls
17 in thernselves known, rotating along with the shaft.
The sha~t compartments in this way formed are connected
to the sector of each disk placed opposite such compart-
ments. At the compartrnent 19 corresponding to the disk
sector in the detaching position, at one end of the
filter, there is a sealing shoe 18, which permits rota-
tion Or the shaft 5 together with its compartments. The
blo~ side of the blower 15 is, by means of the sealing
shoe 18, connected to the shaft compartment 19 placed
opposite the disk sector at each particular time in the
detaching position. Thereby the pressurized air passes
via this shaft compartment and the corresponding disk
sector and detaches the ~iltered pulp mat from the face
of the disk sector. In the other respects, the construc-
tion and the operation of the filter correspond to what
is illustrated in Figures 1 and 2.
