Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FILTER CAKE REMOVAL BY APPLYING DETACHING
COATING TO FILTER BANl:)S
The present invention relates to a process for removing, or
detaching, the filter cake from circulating filter means, and to an
apparatus for carrying out this process, particularly in connection with
drainage machines with double screen bands which are guided over
transversely positioned rolls.
This process serves for achieving the easy removal of the filter
cake from the filtering means.
1~ During the squeezing operation of the filter cake in drainage
machines it has been found that residues thereof will constantly remain
in the wire mesh of the screen and are returned again into the material
inlet during the rotation of the screen. Because of this effect, a
decrease in efficiency must be expected at all times so that already
when the machine is designed, this material reflux must be taken into
account. Furthermore, the contamination of the draining filtrate cannot
be avoided.
For the purpose of obviating the aforementioned disadvantages
it is known in the art to admix an auxiliary agent or filter aid to the
substance to be filtered in order to facilitate the detachment or removal
of the filter cake. Particularly in the case of chamber filter presses, a
prior rinsing of the screens with an auxiliary separating layer (a pre-
coat) has become known in the art. This operation is referred to as a
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discontinuous coating, as it is carried out in chamber filter presses.
It is an undesirable consequence of this method that, during
the addition to the substance of the auxiliary agent or filter aid, a very
considerable consumption of the latter is observed. The consumption
thereof as such would not present the cardinal problem, but
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undesirable is primarily the change in the pH value of the discharging
or draining filtrate and, respectively, of the filter cake, which is
separated therefrom. Also, a rather significant expenditure with
respect to apparatus is required for this method. It is necessary to
provide for control members or means which continuously mix the
auxiliary separating material into the substance to be filtered. In
order to bring a sufficient amount of the auxiliary separating material
to the upper side and/or the underside of the suspension, i.e. within
the area of contact with the sieve or screen bands, an intimate inter-
mixture of the suspension with the auxiliary separating material is
required. A uniform distance of the filter cake residue from the sieve
or screen band is not achieved thereby.
It is also known in the art to clean the sieves or screens me-
chanically. Used for skimming or stripping were so-called scraper
means which were intended to clean the screens. While the screen
is indeed freed as a result from larger quantities of filter cake resi-
dues, so much of the cake remains in the mesh that an additional
cleaning operation becomes necessary. ;
Based on and starting out with this state of the art in mind,
the present invention was conceived with the object of bringing about a
decrease in the amount of auxiliary separating material to be consumed,
and an improvement regarding the detachability of the filter cake from
the filter means. It is intended that this should be achieved without
control members or means and as nearly as possible in a continuous
manner. Furthermore, it should be possible to adapt the amount of the
auxiliary separating material correspondingly to the degree of moisture.
This object is achieved, in accordance with the present inven-
tion, in that, prior to the inlet of the suspension, an auxiliary
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separating layer is continuously applled to the upper and the lower
sieves or screens, and in that it is composed of, for example, a llme
or ash suspension. In order that it be possible to carry out the inven-
tive process, it is proposed according to a further embodiment of the
present invention that disposed at the reversing rollers ahead of the
material inlet is an application roll with a lime or ash suspension pro-
vided thereon, which brings about the metering and distribution of the
detaching suspension, whereby a surface part of the application roller
dips, or is immersed, into open supply containers, and is covered with
' 10 detaching suspension, thus feeding or transporting the amount adhering
to the application roller to the sieve or screen bands.
The present invention will now be further described hereinafter
on the basis of one embodiment thereof and with reference to the accom-
panying drawings, wherein
Figure 1 is a side view of a drainage machine
with application rolls; and
Figure 2 illustrates a section of an application
roll, at an enlarged scale.
Figure 1 illustrates a drainage machine in a side view thareof. ~
It is composed of a stand 1 to which the lower frame 2 is secured. The ~ '
upper frame 3 is pivotally disposed at a perpendicularly positioned arm
of the lower frame 2. Mounted at the end of the lower frame 2 is the
rearward reversing roller 6. Disposed at the upper side of the lower
frame are a plurality of lower rollers 22. Likewise, the corresponding
upper rollers 23 are rotatably mounted on the upper frame 3. The upper
reversing roller 8 is positioned at the end of the upper frame 3.
Finally, also disposed at the stand 1 are the two main rollers 9 and 10
so as to be superimposed, Positioned at the end of the stand 1 are the
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AN 2058
drlve rollers 11 and 12, serving for driving the screen bands 19 and 20.
Accordingly, the upper and lower rollers 22 and 23 thus constitute the
wedge portion 4, and the rollers 9 and 10 constitute the main pressing
or squeezing portion 5. For purposes of guiding the sieve or screen
bands 19 and 20, the carrying rollers 1~L and 13 are secured to the
uppermos-t and lowermost points in a manner known per se.
For the upper sieve or screen 19, the application roller 16 is
provided at the reversing roller 8 in the upper area or portion of the
side ~acing away from the machine. Analogously, an application roller
15 is mounted for the lower sieve or screen 20 at the reversing roller 6
in the lower area or portion of the side facing away from -the machine.
This construction has the purpose of assuring that, prior to the inlet
of the suspension, an auxiliary separating coating or layer is continu- :
ously applied to the upper and/or to the lower screen band 19 and/or
20, this separating layer being composed, for example, of a lime or an
ash suspension. In order that it be pGssible to apply the detaching
suspension 17 onto the screen bands, a certain quantity thereof is
maintained in a supply container 18 so that the application rollers 15
and 16 may be partially immersed therein, i.e., they are uniformly
moistened. The application rollers 15 and 16 are simultaneously driven
because of their contact with the moving screen bands 19 and 20. One
surface portion of each of the application rollers is at all times in
contact with the separating suspension 17. The application rollers 15
and 16 have the same width as the screen bands 19 and 20, and the
supply containers likewise have the identical width, so that detaching
suspension 17 is continuously applied to the screen. Obviously the
case may arise where too general an amount of the separating suspen-
sion 17 is applied to the screen bands 19 and 20, particularly if the
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detaching suspension is somewhat thickened. In order to avoid this
occurrence, a scraping means 21 is provided, as illustrated in Figure
2. It is composed of a strip or ledge which also corresponds to the
length of the application roller. By means of a displacement of the
horizontal position, as indicated by the arrow, it is possible to addi-
tionally regulate the desired amount of the detaching suspension 17.
It is understood that the detaching suspension 17 could be
applied to the screen bands 19 and 20 also in a different manner and
fashion. For example, the detaching suspension 17 could be applied
in a uniform manner by way of a distributing tube 24 which is mounted
transversely to the traveling direction of the screen bands 19 and 20,
and toward which bands the nozzle-like openings 25 are directed.
The arrangement of the distributing tube 24 is advantageously made in
the area or region of the reversing rollers 6 and 8 prior to the inlet of
material. (Not shown in the drawings.) It is also possible, however,
to position the tube within the screen section between the reversing
rollers 6 and 8 and the carrying or supporting rollers 13 and 14. It is
essential that the detaching suspension 17 be applied in a uniform
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distribution onto the screen bands 19 and 20 prior to the inlet of
material.
The use of the present process for purposes of detaching the
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filter cake in circulating filter means assures, in the first place, that
the filtrate and/or the filter cake will retain at all times a desired pH
value. In addition thereto, a small amount of auxiliary separating
layer or coating, which consists for example of a lime or ash suspen-
sion, is sufficient to separate the filter cake during ejection exactly
from the screèn bands. This results in, or produces, at all times a
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clean screen band for the new inlet of material in drainage machlnes
without involving a significant expenditure.
It will be obvious to those skilled in the art that many modifi-
cations may be made within the scope of the present invention without
departing from the spirit thereof, and the invention includes all such
modifications .
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