The present invention relates to a method for pressure filtering of the kind set forth in detail in the preamble of patent claim 1. Also, the invention relates to a device used to carry out said method according to the preamble of the first device claim.

The invention relates particularly to solutions of prob¬ lems of loosening and discharging the filter cake from such pressure filters. For this purpose it has already been sug¬ gested that the filter cloths be subjected to vibrations. DE-C 3 341 636 and DE-A 3 527 735 describe already known pressure filters, in which the filter cloths are suspended from slewing brackets and springs which are raised on the pressure plates. The pressure chambers are opened up one at a time and the filter cake is loosened by means of a movable vibration device, which directly influences the suspension rod of the respective filter cloths. However,- these constructions have a drawback. The discharge is very time-consuming, since the filter chambers are emptied one at a time.

In order to empty all the filter chambers simultaneously it is suggested in Applicant's patent application PCT/SE87/ 000321, that the suspension rods of the filter cloths be movably suspended in a horizontal direction in the longi¬ tudinal direction of the pressure filter on two flexibly or elastically suspended guide rails, which are provided with vibration devices in order to vibrate all of the suspension rods and the corresponding filter cloths at the same time. The positions of the suspension rods when the pressure plates are brought apart are obtained by means of flexible and/or pivotable distance means fastened to the ends of the sus¬ pension rods, which distance means connect the suspension rods to each other and to ^' the movable gable plates of the pressure filter. Such a pressure filter allows a very fast loosening of the filter cakes from the filter cloths and

consequently short filtering cycle times.

This already known pressure filter functions very satis¬ factorily for most types of materials, with an excellent cake loosening effect. However, as far as certain quite ad¬ hering or in other ways difficult to loosen types of mate¬ rials are concerned vibrations of filter cloths produced in this already known pressure filter are not adequate, since they do not always result in a reliable and comp¬ lete discharge of the filter cakes. Consequently, portions of or even entire filter cakes may remain in one or seve¬ ral of the filter chambers after a terminated filter cake discharge. Such remaining filter cake portions are parti¬ cularly found in the upper parts of the filter chambers. This results in an inferior filtering capacity, because during the next filtering cycle portions of the avail¬ able filter chamber space already are occupied by the re¬ maining filter cake residues. Such remaining filter cake residues usually will remain also during the subsequent filtering cycles and increasingly stick to the walls of the filter chamber, i.e. the filter cloths. Thus, the de¬ terioration of the filtering capacity of the pressure fil¬ ter will be permanent.

Said remaining filter cake residues can at least be part¬ ly removed by spraying a rinsing fluid onto the filter cloths, at least during certain filtering cycles. The fil¬ ter cake residues will then be brought back to a contai¬ ner or the like for a liquid suspension of filter mate¬ rials. The filter cake residues which have been loosened by washing will then form a circulating load on the pres¬ sure filter, which worsens the filtering capacity of the system. Also, large loosened and washed away filter cake residues may lead to shutdowns and/or other disturbances in the rinsing liquid circulation system.

Due to the above-mentioned drawbacks of already known pres¬ sure filters the purpose of the present invention is to suggest a method of efficiently and completely loosening the filter cakes from the filter cloths in the filter chambers during the cake discharge period. An additional purpose of the invention is to suggest a device to carry out said method.

In accordance with the invention this purpose is achieved by proceeding according to what is set forth in the charac¬ terizing clause of patent claim 1 when using a pressure filter of the kind described above. A device for carrying out this method is characterized by the features set forth in the characterizing clause of the first device claim.

The method according to the present invention advantageous¬ ly can be combined with a vibration of all of the filter » cloths at the same time according to what is set forth in PCT/SE87/000321. The main part of the filter cakes can then be loosened and discharged according to the known method by means of vibrations, while the method according to the present invention is used to loosen possibly remaining fil¬ ter cake residues.

In a first embodiment of the invention the filter cakes are subjected to shearing forces, because the lower part of the filter cloths are moved laterally in their plane direction. When the filter cloths are moved laterally, folds are formed where the filter cakes or the filter cake residues are lo¬ cated and consequently substantial shearing forces are pro¬ duced in those parts of the filter cakes located closest to the filter cloths and the filter cakes are loosened from the filter cloths.

According to a second embodiment of the invention the fil¬ ter cakes are subjected to sudden acceleration forces by

lifting the rail guides which support the filter cloths and letting them fall against stationary stops. When the falling filter cloths suddenly are braked particularly the larger remaining residues of the filter cakes are loosened. Thus, this method is principally different from the al¬ ready known method with vibrations to loosen the filter cakes. Large amounts of remaining filter cake residues have a pronounced suppressing influence on the amplitude of the vibrations. A lower vibration amplitude of course lessens the cake loosening effect of the vibrations.

Also, the vibration amplitude can be different in diffe¬ rent parts of the pressure filter. The vibration amplitude can be smaller particularly in the central filter cells due to various damping effects. Such a difference in mo¬ tion amplitude does not exist when the guide rails are lif¬ ted and then fall against stationary stops.

In accordance with another advantageous embodiment of the invention the suspension means of the filter cloths are subjected to blows, preferably in an upward direction, e.g. by means of a blow device and/or a drop hammer, acting di¬ rectly or indirectly. These blow devices produce an effect, which resembles a lifting and a falling of the guide rails.

According to an additional advantageous embodiment of the invention a presence of possibly remaining filter cakes is detected by means of load sensors, which transmit a sig¬ nal to the process control unit. The process control unit may be designed, in case remaining filter cake residues are present, to transmit a control signal to start one or several of the above-mentioned cake loosening processes and/or to transmit a warning signal and/or interrupt the filtration process in favor of a manual cleaning. If such a load sensor is used, one may confine oneself to using

said above-mentioned cake loosening processes at those occa¬ sions when there really are remaining cake residues present. In this way the mechanical stresses on the suspension means of the filter cloths are reduced.

The invention will now be described in more detail by means of a few embodiments, reference being made to the attached, partly schematic drawings, in which:

Fig. 1 shows a pressure filter according to the invention, provided with blow devices;

Fig. 2 a schematic cross section of a pressure filter in a closed position;

Fig. 3 a section which corresponds to Fig. 2 but showing an open position and also showing remaining filter cake re¬ sidues;

Fig. 4a and 4b a detail of an embodiment showing eccentric lifting devices and stationary stop surfaces; Fig. 5 a cross section of a filter according to the inven¬ tion showing means for moving the lower part of the fil¬ ter cloths;

Fig. 6 a detail of the above-mentioned moving means; and Fig. 7 a cross section of a guide rail and a blow device fastened to the guide rail.

Fig. 1 shows a pressure filter 1, known per se. This pres¬ sure filter corresponds to the pressure filter, which is described in our copending PCT patent application PCT/SE87/ 000321, reference expressly being made to this specification as regards its design and function, with the exception of those instances which are directly connected with the pre¬ sent invention. The pressure filter comprises mainly a frame 2 , which supports a fixed gable plate 3, a movable gable plate 4 and pressure plates 5,6 arranged between said gable plates. The pressure plates can be pressed against each other and brought apart from each other by means of

hydraulic cylinders 7. Pressure plates 3,4,5,6 form in pairs between each other pressure chambers 8, in each of which two filter cloths 9 are placed.

The filter cloths are supported by suspension rods 10, which are movable in a horizontal direction on two flexib¬ ly or elasticly supported guide rails 12. The rails are at their two ends supported by flexible or elastic supporting elements 14, e.g. rubber blocks or helical springs. Elements 14 are carried by supports 16, which can comprise separate posts fixed to the floor or a portion of frame 2 or consoles projecting from the frame. Guide rails 12 are connected to each other at their ends by means of transversal yokes 18, which support vibrators 20. The pressure filter is also provided with an inlet conduit A for material and outlet conduits B,C,D,E for filtrate as well as an inlet conduit F for a pressure fluid.

Fig. 2 and 3 show the pressure plates and the filter cloths in detail. Hollow suspension rods 10 are connected to rin¬ sing water inlets G and are provided with upwardly directed positioning ribs 22 and rinsing nozzles 24. The filter cloths are provided with a divisible inlet sleeve 26 and rods 28 inserted in the lower hems. The rods on the respective sides of each pressure plate 5,6 are connected at their ends by rubber bands 30. Fig. 3 illustrates also how residues of filter cake 32 stick to filter cloths 9 in various man¬ ners, when the pressure filter is in an open position.

In order to loosen such residues of filter cakes 32, which could not be loosened solely by vibrations, sudden accele¬ ration forces are produced by means of drop hammers 34,36, which fall from an elevated position onto pivotally mounted arms 38. Arms 38 abut yokes 18 of guide rails 12 with their opposite ends. However, it is also possible to, in other ways, subject suspension means 10,12 of filter cloths 9 to

preferably upwardly directed blows by means of drop ham¬ mers or other types of hammers.

Fig. 4 shows a detail of one end of the guide rails accor¬ ding to another embodiment of the invention, which includes eccentric lifting means 40 and stop surfaces, onto which the guide rails can drop. In this case supports 16 carry a cross bar 42 on top of elastic support elements 14, which cross bar is designed to carry the guide rails when they are in a not lifted position. A vibrator 20 (not shown) suitably is fastened to the center of the lower side of the cross bar and consequently is protected from blow stres¬ ses. Four guide rods 44 project upwardly fron cross bars 42 or supports 16 and run through guide sleeves 46 fast¬ ened to the guide rails. Between support elements 14 and supports 16 load sensors, e.g. pressure transmitters, are mounted, which are connected to the process control unit' (not shown) of the pressure filter.

Eccentric lifting means 40 are mounted in pairs on shafts 50 and designed to abut the projecting ends of guide rails 12 or yokes 18 which connect them. They are mounted in an arbitrary way on supports 52 (not shown in detail) , e.g. posts erected on the floor. Shafts 50 are designed to be rotated jointly with one or several driving means (not shown in detail) , e.g. by means of chain driving or gear motors. Eccentric means 40 are provided with an elevated part 54, from which matching guide rail 12 or yoke part 18 is to drop onto a semi-elevated part 56, which functions as an anvil or stop surface. The lowest part of eccentric lifting means 40 is designed to normally be directed up¬ wardly and then does not abut guide rails 12. The latter are carried by cross bars 42 during the filtration period and during the vibration. When filter cakes are discharged the eccentric lifting means are rotated one revolution, the guide rails being lifted and then dropping. This lifting

cycle possibly can be repeated, in case load sensors 48 detect remaining residues of filter cakes 32. In case there are no filter cake residues from the start, the pro¬ cess control unit can transmit a signal that a lifting is not to take place.

Fig. 4 b shows an alternative embodiment, in which the ends of the guide rails or the tranversal yokes are de¬ signed to fall onto separate movable anvils 60, which are designed to be moved to a position below the ends of the guide rails or the yokes in a lifted position. This can e.g. be accomplished by means of pull rods 62, the other end of which is mounted on eccentric discs 64 on shafts 50. Eccentric discs 64 preferably are placed outside cor¬ responding eccentric lifting means 40 in order not to interfere with them.

Fig. 5 and 6 show a device designed to loosen residues of filter cakes 32 by agitating the filter cloths and in this way produce shearing forces. This device comprises two longitudinal bars 66 and fingers 68, which project therefrom and are positioned just opposite every second pair of rods 28 in the lower hem of filter cloths 9. Said fingers can be pushed towards the ends of the rods by means of e.g. hydraulic cylinders 70 and thus every second cloth pair will be pushed to the right and every second to the left in the figure. Additional control means possibly can be used to move the bars in a longitudinal direction in order to move the lower part of the cloths in an opposite direction. The means which are used to control the ends of rods 28 can according to the invention also be cont¬ rolled in another way, e.g. manually. This can be done when an alarm signal has been transmitted by load sen¬ sors 48 due to remaining filter cake residues in the fil¬ ter. This device, designed to control the ends of rods 28, advantageously can be combined with the rest of the methods

for filter cake loosening described above.

Fig. 7 shows a device according to the invention designed to produce sudden acceleration forces. In this device guide rails 10 are below provided with a transversal re¬ inforcement flange 72. A few blow devices, so called "rappers", are attached to this flange and designed to jointly produce blows directed upwards against the guide rails. Such blow devices are already known and are used i.a. to loosen adhering material from arched screens and slanting, inoperative screens and in this way to prevent a clogging of the screens.

The present invention is not limited to the illustrated embodiments, but it is possible to combine, in an arbit¬ rary manner, various characterizing features of the in¬ vention, as the latter is set forth in the description and the attached drawings and as it is defined in the following patent claims.