Apparatus for treating fiber suspension

An apparatus for treating pulp includes a pressure-proof outer casing for a suspension to be treated. A shaft is rotatably arranged in the outer casing and a number of disks are arranged substantially radially on the shaft within the casing. The disks are formed by a number of sectors each having two opposite filter surfaces for allowing liquid from the suspension to be drained therethrough and a liquid compartment between the filter surfaces for the drained liquid or filtrate, so that a substantially annular filter surface is formed. The liquid compartment is in communication with a liquid discharge member. The apparatus further includes members for dividing the annular filter surfaces into two or more portions. The members are stationary relative to the casing, so that the members are used for separating different treatment stages of fiber suspension from one another.

The present invention relates to an apparatus for treating fiber 
suspension, in which the treatment is carried out under overpressurized 
conditions in an airless space. The invention is applied to a new type of 
disc filter treatment apparatus, which is developed to operate in a 
pressurized state. 
There are, in principle, two different types of pulp treatment apparatuses 
known in the prior art. In most cases the most simple apparatus is a 
thickener, screen or washer operating under normal atmospheric pressure, 
whereby the apparatus does not have to be either pressure-proof or 
air-proof and whereby the transfer of liquid is carried out by means of 
reduced pressure. Thus the apparatus may be constructed considerably 
lighter, whereby the manufacturing costs remain relatively low and the 
acquisition of such an apparatus to the mills becomes economic. On the 
other hand due to the small pressure differences having an effect on the 
operation, in other words on the specific capacity, the apparatus has to 
be large so as to achieve a particular total capacity. In some cases it is 
also disadvantageous, particularly if air or like gas is included in the 
pulp to be treated. In order to at least partially eliminate said 
disadvantages pressurized pulp treatment apparatuses have been developed. 
Additionally, by using such apparatuses large filtrate tanks may be 
dispensed with. 
Only drum-type filters, thickeners and screens of the known pulp treatment 
apparatuses have generally been pressurized. The advantages achieved by 
them have been, for example, the space saved at the mills, a specific 
capacity which is considerably higher than that of the unpressurized 
apparatuses and the airlessness of the treatment space of the pulp, 
whereby the quality of the pulp remains better. It may even be considered 
that the capacity of a whole mill could be increased if the old 
unpressurized treatment apparatuses were to be replaced by new type of 
apparatuses operating with larger pressure differences. 
Since the drum-type pulp treatment apparatuses operating with overpressure 
have the same problems as the other drum-type apparatuses, for example, a 
long retention time of the so called cloudy water in the treatment means 
before its possible return back to the means to be cleared. In the 
drum-type apparatuses fiber suspension is normally fed to the outer 
circumference of the filter surface and the filtrate is discharged further 
on through the longitudinal channels of the cylinder, which are connected 
with each sector of the cylinder, and through valve members at the end of 
the cylinder both being inside the filter surface. If the cloudy liquid is 
desired to be returned back to the cylinder to be cleared, it has to be 
taken into consideration that the liquid has flowed long along channels 
which are several meters long to the end of the drum and from there 
further to the feed point. Thereby it is in practice impossible to divide 
the liquid flowing from the channel to cloudy and clear portions, in other 
words to a portion which is returned to the cylinder and a portion which 
is discharged from the means or fed into another stage, because the cloudy 
and the clear filtrate have probably already mixed with each other, 
whereby a lot of clear filtrate has to be returned back to the cylinder. 
Because the retention time of the filtrate is by disc-type apparatuses only 
a fraction of that of a drum-type apparatus, the disc-type pulp treatment 
apparatuses have recently become popular. At the same time, however, a 
higher specific capacity is required from them as well as better 
adjustability and clearer filtrate, whereby the pressurization of the 
apparatuses has become the only possibility. West German Patent 
Application 3.210.200 and U.S. Pat. No. 4,695,381 have distinctively 
illustrated reasons for pressurizing disc filters. When using 
underpressure for the discharge of liquid from the fiber suspension, a 6 
to 10 m high drop leg is required in an unpressurized disc filter to 
create a sufficient underpressure inside the filter sectors. Therefore, 
the filter has to be installed to said level to which the whole amount of 
the pulp to be treated has to be pumped. A second alternative for 
developing underpressure is naturally to use a vacuum pump, which however 
also adds costs. Additionally, the use of underpressure is restricted by 
the temperature of the fiber suspension to be treated, which may not rise 
over 80.degree.-90.degree. C., because due to the underpressure the liquid 
would start to boil on the underpressure side. Also the maximal pressure 
difference is 101 kPa which, as commonly known, cannot be exceeded. 
Said prior publications illustrate a solution for eliminating or minimizing 
said problems, in which the pressure difference over the filter surfaces 
may be increased to the value of 300-400 kPa without any limitations as to 
the temperatures of the fiber suspensions to be treated. The apparatus 
comprises conventional filter disc units mounted on the shaft, which units 
are arranged inside a casing which may be pressurized. The pressure 
difference may be effected by a blower by which a desired overpressurized 
gas layer is generated in the upper part of a pressure vessel. The 
apparatus according to said U.S. patent includes also a control system by 
which the size of the gas layer is maintained as desired, in other words 
substantially the same as in conventional disc filters. Thus the only 
difference in the arrangement of said U.S. patent compared with the 
conventional disc filters is the pressure vessel operating as an outer 
casing of the apparatus and the overpressure created in the filter by a 
blower, by which arrangements a drop leg or a vacuum pump may be avoided 
and a higher pressure difference may be created over the filter surfaces, 
but there are now extra components, i.e. a blower, a pressure control 
system and a discharge and circulation system for air which has got into 
the filtrate, so altogether the apparatus has become even more 
complicated. Additionally, it has not been possible to eliminate from the 
apparatus the most well known defect in the disc filters, which is the 
considerably short length of the formation a pulp cake, about 180-200 
degrees of the whole circumference of the filter disc, which considerably 
limits the specific capacity of the apparatus. On the other hand, this 
kind of guiding of pressurized air or like gas to communication with fiber 
suspension is not desirable in all cases, because the increase in the 
content of air in the suspension disturbs many processing stages by 
causing, for example, slime problems and making the pulp transfer by 
pumping more difficult. 
It has been possible to eliminate or minimize the defects of the 
arrangements according to the above mentioned publications by the 
apparatus in accordance with the invention, in which the treatment of pulp 
is carried out in a closed pressurized apparatus, which has no gas space 
and no gas discharge and pressure control systems typical of the apparatus 
according to the above mentioned U.S. patent. Also the apparatus is 
characterized in that the filter surface in the apparatus can be utilized 
almost completely. 
The apparatus in accordance with the present invention is characterized in 
that the annular surface formed by a sequence of sectors with filter 
surfaces is divided into two or more portions separated from each other by 
members which are stationary relative to the casing of the apparatus, 
which members separate different treatment stages of fiber suspension from 
each other.

According to FIGS. 1 and 2, the pulp treatment apparatus 1 in accordance 
with the present invention mainly comprises discs 5, which are formed by a 
number of adjacent sectors 4 having two opposite filter surfaces and a 
liquid compartment therebetween, said discs being arranged radially on a 
shaft 3, all being located within a substantially pressure-proof casing 2. 
The sectors 4 are mounted on the shaft 3 in such a way that inside shaft 3 
each sector has its own discharge/inlet duct 6 for liquid, which duct 
communicates with a liquid compartment 7 of each sector, which is defined 
in a known way by filter surfaces. According to the invention, the fiber 
suspension to be treated is fed in a pressurized manner into the interior 
of the casing 2 so that the space within the casing and between the discs 
is filled with suspension to be treated. 
As can be understood the retention time of the filtrate flowing via the 
ducts from the pulp apparatus in accordance with the present invention is, 
for example, in thickening use very short. When guiding the filtrate 
liquid via the shaft out of the sectors and further out of the treatment 
apparatus, the liquid must, at its maximum, flow only along the length of 
the shaft to reach valve means located outside the apparatus, wherefrom 
the liquid is transferred for further treatment, for instance, to a 
thickening or file separation unit. The purpose of the valve means is to 
control the operation of the apparatus, i.e. to control the flow 
characteristics of the apparatus in such a way that a cloudy, 
fiber-containing filtrate received just in the beginning of the treatment 
operation does not mix with a clear filtrate received after a fiber mat of 
certain thickness is formed on the filtering surface. Thus there is no 
time for the cloudy and clear filtrate to be mixed with each other, and 
the amount of cloudy filtrate, which may possibly be returned to the 
apparatus, hardly differs from the true amount of the cloudy filtrate. In 
other words, there is a certain amount of cloudy filtrate that drains 
through the filtering surface to the filtrate/liquid compartment before 
the fiber mat has grown so thick that no fibers are able to reach the 
filtering surface and to penetrate therethrough into the filtrate 
compartment. After that the drained filtrate is clear. The cloudy filtrate 
should be returned back to the apparatus for recovering the fibers thereof 
or at least the fibers should be collected as well as one is able to. If 
the filtrate has only a little time to spend in the duct i.e. the duct is 
short, then the cloudy filtrate has little time to mix with the clear 
filtrate following it in the duct. The purpose of the valve means is to 
try to direct all the cloudy filtrate for instance back to the apparatus 
and the clear filtrate to some other location. The sooner the filtrate 
reaches the valve means the more accurate is the border between the cloudy 
and the clear filtrate. It is also possible to facilitate the flow of the 
liquid to the valve means by arranging the valve to the middle part of the 
apparatus, in other words to the middle portion of the shaft, whereby the 
above mentioned retention times are halved, as the distance from the 
outermost disc to the valve means is halved. The situation becomes even 
more advantageous when the filtrate liquid or the like is guided outwardly 
from the liquid compartment of the sector to the outer circumference of 
the sector, to which a valve means may be mounted, by which the run of the 
liquid is guided either back to some part of the apparatus, to the 
treatment zone or out of the apparatus. Respectively, it is possible to 
arrange corresponding separate valve means for each sector to the junction 
point of each sector and the shaft, which valve corresponding to the 
previous embodiment guides the flow of the liquid. In the two last 
mentioned cases the retention time of the liquid in the apparatus is 
minimized in such a way that the only factor affecting the retention time 
is the time passing in the flow of the liquid from the end of the sector 
which is opposite relative to the valve apparatus to the valve means. 
When the apparatus in accordance with the present invention is used as a 
thickener, the pressure in the fiber suspension causes the liquid pressing 
through the filter surfaces of the sectors to the liquid compartment 7, 
through discharge duct 6 and finally out from the whole apparatus. Thereby 
the filter surface of the disc filter is almost completely efficiently 
used, i.e. almost all the filtering sectors may be used simultaneously for 
thickening the suspension. In other words the available surface of the 
disc for thickening is about 300.degree., whereby the pressure of the 
suspension causes the liquid to be pressed from the suspension through the 
filter surfaces into the liquid compartments of the sectors. 
Simultaneously the fiber mat starts forming on the filter surfaces and the 
thickness of said mat increases substantially all the time the mat is 
under the pressure of the suspension. The surface of the disc is almost 
completely available to be used for the liquid discharge from the fiber 
suspension on the filter surfaces of the disc with the only exception of 
the part or those sections in which the thickened pulp cake is at a 
particular moment being discharged. 
FIGS. 1 and 2 disclose an embodiment for removing the pulp cake from the 
sectors. On both sides of each filter disc 5, sealing members are arranged 
at least at one location on the side face of disc 5 by which the pressure 
inside the casing is prevented from escaping to the loosening point of the 
pulp cake. The sealing member in the arrangement shown in FIG. 1 comprises 
a sector-like plate 8, in the middle of which there is an opening 9 which 
is larger than the filter surface of the disc sector, through which 
opening 9 the pulp cake is dropped off and transferred to be further 
treated. The plates 8 of two adjacent discs form a space 10 separated from 
the rest of treatment apparatus and opening (in the case of the figure) 
downwardly, through which space the pulp cake is discharged from the 
treatment apparatus. Thus the space inside casing 2 in the area of plates 
8 has no fiber suspension at all. The size, or the circumferential breath, 
of the plates 8 depends on the sectors 4 of the disc 5 so that plate 8 
seals the inner space of casing 2 in all angle positions of the disc to 
prevent the pressure at the suspension inside the apparatus from entering 
in or escaping to the discharge space 10. This space 10 is, of course, 
also sealed on the side of shaft 3 either by a curved or a straight plate 
11. 
FIGS. 3a and 3b shows a disc sector 40 of a preferred embodiment, which 
comprises a middle portion 41 with filter surfaces, the liquid compartment 
7 and edge portions 42-45, which are raised relative to the middle 
portion, i.e., the filter surfaces, an outermost plane of which, relative 
to the wire surface, forms a sealing surface with the surface of the plate 
8 on the disc side shown in the previous figures. Thus the filter surface 
41 of each sector 40 in a way forms the bottom surface of compartment 46, 
in which compartment the pulp cake is formed when the apparatus operates 
as a thickener. The height of the edge portions 42-45 is advantageously 
defined in such a way that even in a maximum thickening process the 
thickness of the pulp cake does not exceed the height of the edge 
portions; in other word, the pulp cake never touches the surface of plate 
8. This is required, because otherwise the friction between the pulp cake 
and plate 8 would rapidly raise the energy consumption of the apparatus. 
As was mentioned above, the unbroken surface of plate 8 on both sides of 
opening 9 must be at least of the same size as sector 4 of disc 5 to 
prevent the pressure from escaping. When using high treatment pressure, it 
is advantageous to arrange a broader unbroken portion on the plate, 
whereby one sealing surface does not have to bear the stress alone. When 
high treatment pressures are used, it may be necessary to arrange more 
removal and discharge points for the pulp cake on the rim of the disc, 
because the formation speed of a pulp cake is high. Thus by arranging 
several discharge points it is possible to reduce the rotational speed of 
the discs to provide time for the formation of the pulp cake, whereby the 
overall production of a discharge point does not become any lower. 
FIG. 4 shows a preferred embodiment of the apparatus in accordance with the 
present invention, which is mainly aimed to be used for the recovery of so 
called zero fiber, in other words, the fine dry substance which has been 
drawn in with the water removed from the web in a paper machine, 
containing short fibers and other fine particles, such as, filler 
material, or for like purpose. The surface of each rotating disc is 
divided into several operational zone, i.e., portions 21, 22, 23 by plates 
24, 25 and 26 operating as sealing members. Long stock, i.e., pulp formed 
mainly of long fibers, is fed into the forming zone of the basic stock or 
portion 21. The long stock is allowed to thicken, whereby it forms the so 
called basic stock on the filter surfaces of the sectors for instance wire 
surfaces. In other words a fiber layer that operates as a filter medium 
for the actual zero fiber is formed on the filter surfaces. The pulp 
including zero fibers is fed in the apparatus in the beginning of the 
actual thickening zone or portion 22 as shown by arrow 2. The sealing 
member, in other words the plate 24 separates the basic stock forming zone 
or portion 21 from the actual thickening zone or portion 22, which (in the 
case of the figure) is about 250 degrees of the entire disc surface. The 
sealing member 25 separates the pulp cake discharging zone or portion 23 
of the disc from thickening zone or portion 22. Portion 23 is used to 
remove the pulp cake from the filter surface and the cake is discharged 
further on by transfer devices 27, such as, a screw feeder. 
Discharge zone or portion 23 is separated from the basic stock forming zone 
or portion 21 by means of sealing plate 26. As has become clear from the 
above, each sealing member must have an annular breadth of at least the 
same size as the sector of the disc so as to fulfil the minimum 
requirements of sealing. If one desires to improve the sealing, said 
breadth of the sealing surface should advantageously be a multiple of the 
breadth of the disc sector. It may also be seen in the figure that the 
sealing members may be separated relative to one another or, in another 
embodiment they may form a uniform plate surface which has openings for 
the infeed and for the discharge of the pulp cake. 
FIG. 5 shows a scheme for the use of the apparatus of FIG. 4 for filtering 
zero water. Auxiliary pulp, i.e. long stock, is fed from conduit 31 to the 
basic stock forming zone or portion 21 of apparatus 1 of FIG. 4 for 
formation of the fiber mat which later on works as a filtering surface. It 
can be seen that as the filter discs rotate in the direction of the arrow, 
the pulp cakes has been just removed from the filter surfaces of the discs 
at the lowermost position of the filter sectors, whereby the filter 
surfaces are clean before the auxiliary pulp is fed to the portion 21. As 
explained earlier in connection with FIG. 4 the fiber mat is formed on the 
filter surfaces of the sectors before zero water is fed from conduit 32 
for the actual thickening zone (22, FIG. 4), i.e. said zone 22 covers more 
than half of the entire apparatus or the entire disc filter area. It could 
even be said that the apparatus is filled with zero water except the area 
separated by sealing plates 24, 25 and 26 or in other words the area 
between the beginning of sealing plate 25 and the end of sealing plate 24, 
seen in the direction of rotation of the filter disc. 
The initial filtrate 33 from the formation portion of the basic stock is 
guided through the infeed of zero water back to filtering apparatus 1. 
Though the basic stock contains longer fibers there are still shorter 
fibers that are being filtered through the filter surfaces during the 
formation of the basic stock so that in order to collect the shorter 
fibers the filtrate i.e. the cloudy filtrate has to be returned back to 
the apparatus together with the zero water to be treated. The clear 
filtrate received from the filter sectors at the actual thickening zone is 
gathered to conduit 34 and removed from the apparatus. 
If the thickened pulp cake is removed by utilizing water showers, the 
filtrate from conduit 34, i.e. the clear filtrate, may be used for this 
purpose by taking part of it and by feeding it through pump 35 and conduit 
36 back to the treatment apparatus. The removal of the pulp cake may be 
carried out from the inside of the sectors of the disc by guiding the 
water shower along the liquid discharge ducts of the shaft in the opposite 
direction relative to its conventional direction. 
As for the removal of the pulp cake from filter surfaces in case the 
apparatus is used as a thickener, it is possible to use clear filtrate in 
loosening the cake, which filtrate is guided through the filtrate duct or 
the like of the shaft 3 back to the inside of the disc sectors, and 
thereafter the filtrate is pressed through the openings of the filter 
surface, so that it pushes the pulp cake loose from the filter surface and 
at the same time flushes i.e., cleans the filter medium. Naturally, it 
also is possible to blow gas along the same duct network so as to loosen 
the pulp cake. 
It is also easy to arrange means for a liquid shower operating on a 
conventional principle to spray either water from a separate source or 
clear filtrate between the filter surface and the pulp cake. A third 
alternative principle is, of course, different types of scraper 
arrangements, which may be arranged to move into a sliding connection with 
the filter surface, i.e. to wipe the wire surface when it is at the 
discharge opening, and to rise higher when the edge protrusions of the 
sectors pass under the scrapers. 
It is also possible to use the apparatus in accordance with the present 
invention for filtering zero fiber without a separate infeed of auxiliary 
pulp so that the auxiliary pulp is continuously fed together with the zero 
water, whereby zero fibers penetrate the wire surface at the beginning of 
the thickening stage. However, the additional pulp quickly forms basic 
stock on the wire surface, whereby the zero fibers no longer penetrate the 
filter medium. The basic stock is formed in reality so rapidly that even 
this kind of solution is possible, because the amount of cloudy filtrate 
does not increase excessively. The use of auxiliary pulp has been proved 
necessary in filtering zero water, because the filter medium will clog 
immediately at the beginning of the filtering stage without the pulp cake 
having time to be formed, if the filter medium is provided with so small 
perforations that zero fibers do not penetrate the medium. 
FIG. 6 shows yet another embodiment of the apparatus in accordance with the 
present invention, in which an additional sealing element 28 and a pulp 
treatment element 29 have been added to the apparatus as compared to FIG. 
4. The sealing element 28 is similar to previously described sealing 
elements, but the tone or portion 29 may be used, for example, for the 
finishing drying of the pulp cake by blowing drying gas into the 
compartment so that the gas replaces liquid in the pulp cake. Such method 
is used when a high dry substance content is desired and the penetration 
of gas into the pulp does not cause any harm. 
In a corresponding way it is possible to add compartments separated by 
sealing members for different purposes, such as, for example, for washing 
the filter medium or if the apparatus is used as a pulp washer, it is 
possible to carry out all the stages required for the washing in one 
apparatus by arranging a sufficient amount of different zones, in other 
words pulp treatment elements. It is possible to use the apparatus as a 
multi-stage washer, for example in such a way that the fiber suspension to 
be treated is fed in the washing order to the first treatment compartment, 
to which filtrate from the second treatment compartment in the washing 
order is guided as a washing liquid. The suspension is then transferred by 
the rotation of the treatment disc to the second treatment stage, to which 
filtrate of the third treatment stage is guided as a washing liquid. The 
process continues in such way until the fiber suspension reaches a 
sufficient level of cleanliness, after which it is discharged from the 
apparatus. 
It is, of course, possible to arrange the washing also in such a way that 
the filtrate utilized to operate as washing liquid in each washing stage 
is not the filtrate of the immediately following stage, but that of some 
other following stage, whereby the difference in level of cleanliness 
between the washing liquid and the suspension to be treated is greater and 
the washing effect of the liquid is somewhat more efficient. 
As is seen from the above description, a new many-sided pulp treatment 
apparatus is developed which eliminates or minimizes the drawbacks of the 
apparatuses of the prior art techniques, and the above description shows 
only a few embodiments of the apparatus which are not intended to restrict 
the scope of invention from that given in the accompanying claims.