Process for cleaning the filtering medium of a filtering centrifuge

A process is used to clean a filtering centrifuge. A base layer of filtered matter remains on the filtering medium of the centrifuge after the removal or repeated removal of a portion of a solid cake of filtered matter by a peeling blade and discharge apparatus. The process removes the base layer of solid filtered matter by initially penetrating and flooding the base layer with a liquid and then strongly braking or rapidly accelerating the drum of the filtering centrifuge to rapidly reduce or increase the rotational speed of the drum, respectively.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for cleaning the base layer of filtered 
matter from the filtering media in filter centrifuges which remains after 
peeling and discharge operations. 
2. Description of the Related Art 
It is known to blast off the base layer of filtered matter by exposure to a 
compressed gas. While the product residues on the surface of the filter 
medium are largely removed in this manner, product particles that have 
penetrated into the filter medium remain. In addition, the gaseous 
blasting medium must then be separated from the solids. 
Given the fact that the lower areas of the filter are particularly wet in 
view of the prevailing capillary forces, there are strong adhesive and 
bonding effects present, which may interfere with blasting off the base 
layer. 
It is further known to remove the base layer by a high pressure liquid 
spray. This, however, requires relatively large volumes of the liquid to 
achieve the desired effect. As a rule, however, large quantities of a 
liquid are undesirable, as they must again be separated from the solid 
material or even evaporated. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a process for an 
especially thorough cleaning of a filter medium in a filtering centrifuge 
by removing the remaining base layer of filtered matter, without 
additional equipment investments and with only small quantities of a 
liquid. 
This object is attained by using the process of the present invention. 
The invention relates to a filtering centrifuge having a rotating drum, a 
filtering medium and a peeling blade and discharge apparatus. During the 
centrifuging operation, a suspension flows into the drum and is separated 
into solid filtered matter and a filtrate. The solid filtered matter 
accumulates on the filtering medium, and the filtrate flows through the 
filtering medium and exits the rotating drum. 
A cake of solid filtered matter accumulates on the filtering media of 
filter centrifuges during operation. The filter cake is removed in batches 
by a peeling and discharge apparatus. The filtering resistance of the 
filtering medium tends to increase as the base layer, which remains after 
peeling, becomes more dense after each peeling process It is therefore 
necessary to completely remove the base layer after a certain number of 
peeling and discharge processes from the filter medium. 
The invention is based on the discovery that the adhesion of the solid 
components of the base layer among themselves and on the filter medium may 
be reduced by applying a liquid to achieve a saturating penetration and 
overflow of the base layer. If a strong braking force is applied to the 
drum, mass inertia shear forces are generated between the filter medium 
and the solid residues and the liquid to cause a largely complete 
separation of the base layer from the filtering medium. 
Further objects, features and advantages of the present invention will 
become apparent from the detailed description of preferred embodiments 
which follows, when considered together with the attached figures of the 
drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a filter centrifuge 1' configured with a siphon or reversible 
flow apparatus. A drum 3 having a horizontal rotating shaft 2 is disposed 
in a housing 1. The drum includes a drum bottom 4, a full jacket 5, a bowl 
6, an annular rim 7 and a filtering medium 8. The full jacket 5 and 
filtering medium 8 define a filtrate collector space 9 which is in fluid 
communication with bowl 6 through passage opening 10 in drum bottom 4. A 
liquid feeder pipe 11 and a peeling tube 11a open into the bowl 6. 
A discharge apparatus 14 pivots about a pivot axis 13 and carries a peeler 
blade 15 that extends over the axial length of the drum. Discharge 
apparatus 14 projects into the internal space 12 of drum 3. Peeler blade 
15 is followed by a discharge funnel 16 which opens into a discharge pipe 
17 equipped with a discharge screw (not shown). A suspension feeder pipe 
18 further projects into inner space 12 of drum 3. 
A base layer 19 of solid filtered material accumulates on filtering medium 
8. The thickness of base layer 19 is determined by the radially outermost 
pivoting position of the peeler blade 15 during a peeling and discharge 
operation. In the position shown in the figures, the peeler blade is 
retracted from the surface of the base layer. 
In operation, suspension feeder pipe 18 fills drum 3 with a suspension. 
During centrifuging, filtered matter is deposited on filtering medium 8 
and a solid layer or cake of filtered matter accumulates thereupon. The 
filtrate flows through filtering medium 8, filter collector space 9 and 
passage opening 10 into bowl 6. The filtrate may be drawn off with peeler 
tube 11a that may be pivoted to the bottom of the bowl. The solid layer 
deposited on filtering medium 8 may be subsequently peeled off by pivoting 
peeler blade 15 into contact therewith and discharged through discharge 
funnel 16 and discharge pipe 17. Base layer 19 remains on filtering medium 
8, which--especially following several peeling processes--becomes more 
dense, thereby increasing the filtering resistance of the filtering 
medium. 
In the operating phase, in which base layer 19 is to be removed from 
filtering medium 8, liquid flows into bowl 6 through liquid feeder pipe 11 
and passes through passage opening 10 into filtrate collector space 9, 
whereupon it flows through filtering medium 8 and the base layer radially 
from the outside of the drum to the inside, until the base layer is 
flooded. Preferably, a predetermined number of peeling and discharge 
operations are performed so the base layer remains capable of absorbing 
the liquid to saturation. However, it is not necessary to saturate the 
base layer to practice the invention. Further, it is not necessary to 
increase the mass or weight of the base layer with liquid. In practicing 
the invention, it is sufficient to make use only of the adhesion forces 
between the liquid layer and the surface of the base layer. 
A braking force is applied to reduce the rotational speed and accompanying 
revolutions per minute (rpm) of the drum with a strong negative 
acceleration, so base layer 19, which is preferably saturated with the 
liquid, separates from filtering medium 8 and slides thereover. 
Preferably, the rotational speed of the drum is reduced and a number of 
peeling and discharge operations are carried out prior to the application 
of the braking force. The reduced rotational speed and executed peeling 
and discharge operations create the condition in which the base layer 
remains capable of absorbing the liquid until saturated, but no 
appreciable filtering of the liquid applied occurs. That condition 
minimizes the amount of liquid required to carry out the process. The 
intensity of the cleaning effect depends on the difference in rotational 
speed between the drum and the liquid loaded with the solids of the base 
layer. Recognizing that the cleaning effect depends on the difference in 
rotational speed between the drum and the liquid loaded with the solids of 
the base layer, the invention may also be practiced by applying a positive 
acceleration or accelerating force in lieu of the braking force previously 
described. 
In this process, product residues settled under the filtering medium or on 
its support structure (not shown) may also be flushed away and drawn off 
together with the liquid from the bowl 6 by peeling tube 11a. 
During the reverse flow filtering or the flooding of the base layer by the 
liquid, the rotational speed or rpm of the drum is determined by how 
strongly the residues of the base layer are adhering to the filter medium. 
In stubborn cases, the process according to the invention may be repeated. 
The rotational speed of the rotating drum will depend on the type of 
filtering process being performed. The range of rotational speeds (rpm) of 
the rotating drum when filtering the suspension depends on the diameter of 
the drum. Although any size drum may be used, it is preferable to use 
rotating drums having a drum diameter in the range from about 0.25 meters 
to about 2.50 meters. With respect to the drum diameter, it is 
contemplated to have rotational speeds sufficient to generate 
gravitational forces in the range from about 10 g to the limits of 
material stability of the drum (approximately 1000 g). During peeling, the 
rotational speed must not be in the range of critical rotational speed. 
Preferably, the rotational speed just prior to applying the braking force 
is in the range from about 1/10 to 1/2 of the normal rotational speed. Of 
course, the gravitational force just prior to applying the braking force 
will depend on the kind of solid material being filtered. The braking 
force applied will also depend on the kind of solid material being 
filtered. Preferably, the liquid applied to the base layer is the liquid 
from the suspension. However, it is contemplated to use any liquid which 
has no solubility with respect to the solid particles of the base layer. 
An example of typical parameters of operation according to an embodiment 
of the invention includes: 
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Diameter of a horizontal 
1.60 meters 
rotating drum 
Rotational speed during 
950 rpm 
filtering 
G-factor 800 
Rotational speed just 200 rpm 
prior to applying the 
breaking force 
Rotational speed just 30 rmp 
after applying the 
braking force 
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The discharge of the liquid loaded with the solids of the separated base 
layer may be carried out at a reduced drum speed during which the 
solid-liquid mixture accumulates in the lower region and is suctioned off. 
Resumption of centrifuging accompanied by flow of suspension into the drum 
may also be effective. The flow of the suspension into the drum is resumed 
with the suspension flowing into the mixture that includes the solids of 
the separated base layer and the liquid. After the separated base layer is 
filtered, a normal operating cycle of the dewatering of a batch of the 
suspension results. 
The discharge process is particularly simple in the case of centrifuges 
with a vertical rotating axle and bottom discharge. Following the braking 
of the drum, the mixture of solids of the separated base layer and the 
liquid slowly descends and drops from the drum. 
FIG. 2 shows a filter centrifuge 1" of simple configuration without a 
siphon or reverse filtering apparatus. The parts corresponding to those 
illustrated in FIG. 1 carry the same reference symbols. In a deviation 
from the embodiment of FIG. 1, a liquid feeder pipe 20 opens into inner 
space 12 of the drum and not into the bowl. 
Base layer 19 is removed from filtering medium 8 in the following manner. 
Liquid is applied by liquid feeder pipe 20 from the inside of the drum to 
base layer 19. Preferably, the rotational speed or rpm of drum 3 is 
reduced and a predetermined number of peeling processes are carried out so 
the base layer remains capable of absorbing the liquid until saturation 
but no appreciable filtering of the liquid applied occurs prior to the 
application of the braking force. After saturating and flooding base layer 
19 with the liquid, the process for the complete cleaning of the filtering 
medium as described above is effectuated. 
The components of the separated base layer and liquid may be discharged in 
the same fashion as described above for the filter centrifuge with a 
siphon or reverse flow drum as shown in FIG. 1. The discharge of the 
mixture made up of the components of the separated base layer and the 
liquid may be carried out in a fashion similar to that of the first 
embodiment with a peeler tube (not shown) to suction off the mixture. The 
mixture may also be removed by initiating a new cycle of centrifuging with 
the introduction of a suspension into the drum containing the mixture for 
the subsequent normal operation of the dewatering of a batch of the 
suspension. 
It is understood that the foregoing description is for illustrative 
purposes only. To those skilled in the art it will be apparent that the 
invention is capable of taking various useful forms and the scope of the 
invention is to be determined by the appended claims.