The present invention relates to a procedure for cleansing the filter cloth in a box filter, in which procedure, after the essentially vertical filtering space located between the filter plates and delimited by the filter cloths has been opened, a cleaning element provided with at least one nozzle is passed through the filtering space to spray the cloth with a jet of a cleansing medium-

Box filters are industrially used for the separation of solid materials from fluids. Such materials include e.g. cement, mineral coal and ore concentrates. Box filters are also used for sludge separation in sewage disposal plants.

The box filter consists of a number of vertical filter plates forming a block inside which the filtering spaces are enclosed between adjacent plates, each filtering space being delimited by a filter cloth. The sludge to be filtered is supplied into the filtering space, where, in order to separate the filtrate, it is subjected to a pressure applied by a pressure medium, e.g. air, supplied into a space behind a separate pressing membrane. After this, the sludge cake thus produced can, if necessary, be washed by means of a washing fluid supplied into the filtering space and removed by applying a pressure again. The sludge cake can also be dried by blowing compressed air through the filtering space. Finally, on completion of the desired treatment of the sludge cake, the filtering space is opened for removal of the cake.

In a known type of box filter, the filtering space is delimited on one side by the pressing membrane and on the other by the filter cloth, which runs as an endless belt around rollers placed above and below the filter plates.

The cake of filtered material is removed from the filtering space by rotating the cloth. To ensure that the cake will -be detached -from the -moving cloth, a scraper may be provided near the lower roller below the filtering plates.

There are also box filters in which the filtering space is delimited on each side by a filter cloth fixed to the filter plate.. Such filters are especially suited for materials which, as a filtered and compressed cake, do not stick fast to the filter cloth but fall down from the fil¬ tering space when the filter assembly is opened. However, in practice it has often been necessary to employ either manual or automatic means to help the cake get detached.

In the former type of known box filter, the movable filter cloth can be cleansed by counter-current wash in the closed filtering space. In the latter type of filter, the fixed filter cloths are cleansed using a washing device which is provided with washing jets and moves vertically through the opened filtering space. As this device cleans the filtering spaces between the filter plates one at a time, the opera¬ tion takes a long time and cannot be performed frequently enough wh efficient use of the filter is aimed at. How¬ ever, neglecting the cleansing leads to a deterioration of the filtering properties and to a reduced useful life of the cloth.

The -object of the present invention is to provide a more effective solution for the attachment of the cake of fil¬ tered material from the filter cloth and for the cleansing of the cloth especially in box filters in which the filter cloth is fixed to the filter plates. The invention uses a cleaning element which is provided with one or more nozzles and moves in the opened filtering space. It is character¬ ized in that a jet of compressed air is blown at the cloth from the cleaning element at the same time when the cloth is scraped to remove the filtered material from it.

For the scraping, the cleaning element is preferably provided with a scrape which is located, relative to the direction of motion of the element, after the air jets.

In addition to compressed air, the procedure of the invention for the cleansing of the filter cloths in a box filter may comprise cleansing with a fluid, which can be sprayed onto the cloth from the same nozzles as the compressed air. In this case, the cleansing operation can be arranged e.g. in such manner that the air spraying and scraping are performed when the cleaning element is moving upwards and the washing when the element is moving downwards in the opened filtering space.

Since a box filter typically comprises a large number of filter plates and a corresponding number of filtering spaces between them, the cleansing is rendered particularly effective by having all the filter cloths cleansed simultaneously using cleaning elements that are passed through the filtering spaces, said elements being connected to each other so as to form a single cleaning device. In this manner, the filter cloths are always cleansed during the removal of filtered material without significantly reducing the capacity of the filter.

The invention also relates to a box filter in which the filter cloths can be cleansed by applying the procedure described above. The filter comprises filter plates known in themselves and placed side by side in an essentially vertical position, filter cloths located between the plates, filtering spaces enclosed between the plates and delimited by the filter cloths, and elements for cleansing the filter cloths, said elements being introduced into the filtering spaces after they have been opened and comprising at least one nozzle through which a jet of cleansing medium can be directed onto the cloth, and it is characterized in

that the nozzle communicates with a source of compressed air used as cleansing medium and that the cleaning elements also comprise at least one scraper for xemoving the filtered material from the filter cloth.

In order that the cleaning elements should also wash the filter cloths, the nozzle may additionally be connected to a pipeline supplying- a cleansing fluid. In this case, a preferable arrangement is such that the cleaning elements move up and down in the filtering space, spraying the cloths with a jet of compressed air while moving in one direction and washing and scraping them while moving in the other direction.

In the following, the invention is described by the aid of an example with reference to the drawing attached, wherein:

Fig. 1 presents a box filter as provided by the invention, in a situation where the filter cloths are being cleansed.

Fig. 2 presents a magnified view of adjacent filter plates and cleaning elements placed above thera _f with the box filter in the closed position.

Fig. 3 shows one end of a cleaning element in lateral view.

Fig. 4 presents a cleaning element in the act of spraying the cloths with compressed air and scraping them in an opened filtering space between filter plates, and similarly.

Fig. 5 presents a cleaning element in the act of washing the cloths in an opened filtering space.

Fig. 1 is a schematic representation of the box filter of the invention, which consists of vertical filter plates 1 -and filtering paces 2.between the plates. The figure shows

the filter in a situation where the cleansing of the filter cloths is in progress. A cleaning element 3, which is connected via interconnecting hoses 4 to a compressed air supply line 5 and to a washing fluid supply line 6. The bar-shaped cleaning elements 3 are attached at their ends to supporting beams 7 on each side of the block of filter plates 7. The beams 7 are raised and lowered by means of chains 8 or similar mechanisms.

The filter plates 1 are surrounded by the filter cloths 9 as shown in Fig. 2 in such manner that the filtering spaces 2 between the plates are delimited on either side by the filter cloths. The sludge or similar material to be filtered is supplied into the filtering spaces 2, where it forms a solid cake as the liquid is filtered off through the filter cloths 9 into draining channels 10 beginning in the spaces behind the cloths.

As shown in Figs 2 and 3, the bar-shaped cleaning elements 3 consist of a tube 11, which is connected to the aforesaid interconnecting lines 4 supplying compressed air or washing fluid, nozzles 12 directed obliquely downwards from the tube toward the cloths 9, and scrapers 13 placed on top of the tube and jutting out on both sides of the cleaning element. During the filtering and possible subsequent washing and drying of the material cake, the cleaning elements 3 are held above the closed filtering spaces 2 as shown in Fig. 2.

After the filtering, when the filtered material 14 is to be removed, the filtering space 2 is opened as shown in Fig. 4 and the cleaning element 3, supported by the connecting beam and the chain 8, is moved in the filtering space in the downward direction. The jets 15 of compressed air blown at the cloths 9 from the nozzles 12 cause most of the filtered material 14 to be detached and fall down, and the scrapers 13 following the jets remove the remnants of the

material from the filter cloths, thus performing a prelimi¬ nary rough cleaning of the cloths 9. In the next stage, shown in Fig. 5, the cleaning element is moved in the filtering space in the upward direction while the jets 16 of cleansing fluid supplied via the nozzles 12 wash the filter cloths 9. The cleansing fluid may be e.g. water.

Since the cleaning elements 3 working in adjacent filtering spaces 2 are connected by a beam 7 as shown in Fig. 1 to form a single cleaning unit, all the filtering spaces can be cleaned simultaneously as a single operation, during which the cleaning elements 3, supported by the beams 7 and chains 8, are first moved from the starting position above the filter plates 1 to a position below the plates and then back up to the starting position.

It is obvious to a person skilled in the art that different embodiments of the invention are not restricted to the example presented above, but that they may instead be varied within the scope of the following claims.