The present invention relates to a filter for continuous filter ing of a suspension. The filtering takes place under pressure o under vacuum or under a combination of pressure and vacuum.

As previously known the filter includes a vβssle containing the suspension to a certain level and a space with gas positioned above the suspension, an essentially horisontal centre axle rotatably arranged in the vesεle, filter discs arranged at a distance from each other along the centre axle essentially perpendicular to this, said filter discs consisting of filter sectors with surrounding filter cloth and internal channels for filtrate and gas obtained during the filtering, connection mean in the centre axle between the filter sectors and an outlet fro the vesεle, removing means for each filter disc arranged in the gas space for the removal of filter cake obtained on the filter cloth during the filtering, and means for receiving the filter cake material removed by said removing means .

It is previously known to arrange as said removing means m su filters a so called doctor, i.e. a scraper or cutter principal m form of a planer. In this known filters the scraper is arranged immediately above the suspension level where the filt sectors descend into the suspension, i.e. at the end of the rotation of the filter sectors in the gas space. In these filters the scrapers are therefore positioned substantially horisontally . Such a positioning of the scrapers, however, results in several disadvantages. Thus, scraped-off material will shuffle over the scraper with accompanying risk for deposits on the scraper. Moreover, this positioning of the scraper means a limitation of the utilization of the filtering surfaces of the filter discs since the suspension level in the filter vessle can not be higher than the position for the scraper. The positioning of the scrapers furthermore means tha

CORRECTED

the receiving device must be formed relatively wide, at least in the upper part and then taper downwardly to circular outlet. This results in a relatively large distance between the filter diβcs. Furthermore, there is a great risk that scraped-off material is built up in the downwardly directed cone from rectangular crosssection at the scrapers to circular cross-section at the discharge of scraped-off material through the bottom the vessle.

By the present invention the above disadvantages have been eliminated. The filter is in this respect characterized in that the removing means consists of an essentially vertically arranged scraper for removing at least an outer layer of the filter cake along an essentially vertical line for essentially vertically sliding and falling down of the scraped-off filter cake material along the scraper. By this positioning of the scraper, scraped-off material will essentially drag-free slide and fall along the scraper and therefrom. The scraper is accord¬ ingly not under load. Moreover, the scraped-off material falls in a controlled form, principally hour-glass effect. Futhermore the risk is eliminated for scraped-off material to be built up as is the case when using previously known horizontally positioned scrapers. Another advantage with the positioning of the scrapers according to the invention is, that the suspension level in the filter can drastically be raised so that the filte¬ ring surface can be optimally used resulting in high and optimal capacity for each filter disk. Another advantage with the positioning of the scrapers according to the invention is that by this a filter construction having considerably less distance between the filter disks on the center axle has been made possible, which makes possible an essentially smaller filter dimension than earlier in order to obtain the same capacity.

Preferably the scraper is arranged somewhat inclined in such a way that the lower end of the scraper is positioned further away

from a vertical plane through the center axle than the upper end of the scraper. By this positioning the falling down of scraped- off filter cake material from the scraper is still more facili¬ tated .

In a preferred embodiment said receiving device consists of an essentially vertically arranged outlet pipe. Due to the form of the vertical scraper, this outlet pipe below the scraper can have smaller diameter than corresponding receiving device in previously known filters and this outlet pipe does not need to be conical. This contributes to above mentioned less distance between the filter discs.

Preferably only an outer layer of the filter cake on the filter cloth is by the scraper removed for each revolution of the filter discs and the remaining filter cake, so called precoat layer, serves beside the filter cloth as filter. This precoat layer becomes in course of time too tight and must be removed. Conventionally a blow off of the entire precoat layer takes place by blowing air into one of the channels in the center axle via a filtrate valve arranged at the outlet end of the axle and the entire precoat layer from a sector falls off the filter cloth. This results in that a large amount falls at the same time down into the tapered receiving devices and results in great risk for clogging. The internal blowing causes also that the cloths of the sectors are blown up immediately ahead of the scraper resulting in great risk for the cloth to. be damaged by the scraper. Moreover an uncontrolled blow-off of the precoat ^' layer is obtained since the blown gas rushes through the filter cloth where it most easily can pass, i.e. where the layer first releases the gas rushes most easily out and accordingly leaves certain sector cloths unblown. In a preferred embodiment of the invention there is also a device for intermittent removal of the precoat layer and this device is positioned above the receiving device preferably formed as an outlet pipe and alongside the substantially vertical scraper. The precoat removal is preferably carried out by a substantially vertically positioned

spray pipe for high pressure washing. The spray pipe is positioned immediately ahead of the scraper and the wash liquid is supplied to the wash pipe in a separate conduit. By this arrangement a controlled flushing iε obtained and flush-off material falls controlled and in less amount per time unit down into the receiving device compared with above mentioned blowing of the precoat layer in a previously known manner. In other words, the intermittent removal of the precoat layer in accordance with the present invention is operating only the material that at the moment is fed to the removal device is removed, i.e. a sm^ll layer per time unit. Nor is there any risk for the cloth to be damaged by the scraper when using this removal according to the invention. This shape and positioning of the removal device contributes also to the compact construc- tion of the filter.

In accordance with prior art there is also in the present invention a device for intermittent washing of the filter cloth. According to a preferred embodiment of the invention the device for said intermittent removal of the precoat layer and the device for the intermittent washing are one and the same device, which in a constructive point of view is advantageous .

In previously known filters of the kind described in the introductory part, the connection in the center axle between the filter sectors and the outlet from the vessle consists of a number of channels corresponding to the number of disk sectors . When the number of disk sectors is for instance 20, which often is the case, there are accordingly in the center axle 20 channels meaning that the center axle must be made with relatively large diameter and also is complicated and expensive. According to the invention the connection in the center axle between the filter sectors and the outlet consists of one channel common for the filtrate and the gas from all filter sectors in the pressure vesεle and preferably the center axle

consists of a tube. This means of course an essentially cheaper manufacture cost for the center axle and moreover it can also be made with considerably less diameter.

5.. By the configuration of the center axle as a tube another essen¬ tial advantage is furthermore achieved. In a preferred embodi¬ ment of the invention a filtrate separator is as previously known connected to the outlet of the pressure vessle and arranged for separating filtrate and gas. In previously known 0 filters there are sealing problems in the transition section from the center axle. This transition section has been obtained with a so called filtrate valve having a wearing disk as a sealing between the rotating center axle and the outlet pipe to the filtrate separator. When the filtrated liquid which flows in 5 the channels through the outlet is fairly cleaned the liquid wears less in the center of the wearing disc and the axle, while the outer part of the sealing is in unfiltrated medium which forces its way in between the sealing surfaces and wears the sealing element at the periphery. For this reason it has been 0 necessary to change wear disk rather often and furthermore there is a great risk for wear of the end of the center axle. Moreover, this transition section from rotating axle to stationary tube is complicated and expensive. These disadvan¬ tages have been eliminated in a preferred embodiment of the 5 invention and in consequence of the fact that the center axle is a tube, wherein for the connection between the vesεle outlet and the filtrate separator the separator is provided with a tube having a dimension corresponding to the center tube of the vesεle and wherein a coupling including a sealing is arranged 0 for interconnection of the tubes. Besides that such a connection is essentially cheaper than above mentioned filtrate valve no sealing problems exist.

Beside above mentioned advantages achieved by a filter according 5 to the invention also the following can be mentioned:

- the number of filter sectors is limited to what can be inser¬ ted into the filter via the openings for the assembly,

- a εmall number of filter εectors and filter discs means small manufacture and assembly errors, - more straight filter sectors can be manufactured,

- the filtering can be driven a longer time since the scraper can come closer to the filter disc without any risk that it will grip in and damage the filter cloth,

- lower manufacture costs, - lower maintenance costs,

- lower shut down time,

- the diameter of the filter discs can be decreased with main¬ tained filtering surface owing to the fact that the center axle has less diameter, - the filtering surface of the filter discs can be used opti¬ mally since the suspension level can be raised in the filter vesεle,

- maximum capacity per installed filter εurface is achieved,

- the volume of the filter can compared with previously known filters of the kind in question and with at least as large capacity be made essentially less and to an essentially lower cost, wherein for instance the distance between each filter disc can be decreased with about 35% meaning also shorter center axle, the diameter of the vessle can be made about 15% less and the diameter of the filter discs about 12% less.

The invention is in the following described more in detail in an embodiment with reference to enclosed schematic drawing, wherein

fig. 1 shows a lateral view, partly cut-up, of a filter according to the invention,

fig. 2 shows a vertical section of the filter, and

fig- 3 shows in larger scale a view from above of certain elements included in the illustrated filter.

The filter shown on the drawing, which in this embodiment is a pressure filter, includes as previously known pressure filters a substantially cylindrical pressure vessle 1, a center axle 2 rotatably journaled in the pressure vessle and a number of filter discs 3 arranged at a distance from each other on the center axle. Each filter disc consists of a number of disc sectors (not shown) , for instance having the configuration shown in US patent No. 4695381. As usual, the filter sectors consists of a surrounding filter cloth and interior channels which are connected to the center axle for discharging the filtrate and gas obtained during the filtering operation to a filtrate separator 4 for separating filtrate and gas. The gas is returned to the pressure vesεle 1 via gas conduit 5 provided with a compressor 6. Moreover, the filter includes a removal device or scraper 7 for removing the filter cake 8 (see fig. 3) obtained on the filter cloth during the filtering operation, a device 9 for removing filter cake material on the cloth positioned inside scraped-off filter material, so called precoat layer, a device 12 for receiving the removed filter cake material and a device for washing the filter cloth. The filter vessle has furthermore an inlet 10 for supplying suspension and maintaining the suspension at a certain level 11 in the pressure vessle. .

The pressure vessle shown on the drawing presents, however, several essential novelties compared with previously known filters .

Thus, for scraping off the filter cake obtained on the filter cloth during the filtering operation the scraper 7 is substantially vertically arranged in the gas space above the suspenεion level, whereby scraped-off material falls in a controlled form from the scraper and does not load the scraper and furthermore the risk for scraped-off material to be built up on the scraper is eliminated. The scraper can preferably be somewhat inclined outwardly seen from the center axle 2. The receiving device 12 for the εcraped-off material consists of a vertical cylindrical tube having less diameter than previously

known receiving devices formed with a cone, whereby also the risk for scraped-off material to be built up in the receiving device is eliminated. Thiε poεitioning of the scraper and the shape of the receiving device as a cylindrical tube means that the distance between the filter discs can be made essentially less than what haε been earlier the case (for instance about 35% less). The positioning of the scraper also means that the suspension level in the filter can be essentially raised, whereby the filtration surface can be used optimally.

As appears from the drawing the scraper 7 is adjustable about an axle 13.

Having a configuration according to the invention the removal of so called precoat layer and the washing of the filter cloth can be carried out in an advantageous way. A subεtantially vertical¬ ly arranged removing device combined with waεh device iε in form of a εpray tube 9 arranged ahead of the scraper in the rota¬ tional direction of the filter discs and above the outlet pipe 12. The spray tube receives εpray liquid from conduit 14.

Sprayed-off material fallε herewith controlled down into the tube 12 and in an adapted amount per time unit. Moreover, the risk for the filter cloth on the filter discs to be damaged by the scraper is eliminated as diεtinguiεhed from what iε the caεe in previously known filters where the removal of the precoat layer takes place by blowing from inside outwardly through the filter cloth.

Another essential novelty in the filter according to the invention is that the center axle consists of only one channel common for filtrate and gas from all filter sectors in the pressure vesεle as distinguished from previously known filters where the filter axle haε been formed with a channel from each filter sector. As shown on the drawing, the center axle consists of only one cylindrically tube with connections to the filter

sectors. Besideβ a considerably simpler construction and less manufacture costs also an essentially simpler and more reliable . connection 15 to the filtrate separator 4 is obtained compared with the comparatively complicated filtrate valves which are arranged at the center axles formed with several channels in previously known filterε. The connection 15 iε positioned outside the filter vessle adjacent the vessle but can also be positioned at another place, for instance inside the filter vesεle. The connection 15 between the rotating center tube 2 and a tube 16 connected to the filtrate separator 4 is constructive¬ ly simple and allows a simple sealing, preferably a mechanical sealing. As to the center axle it- shall furthermore be mentioned that it can be manufactured with considerably less diameter than center axles including a number of channels as in previously known filterε.

As to filter discs and filter sectors the following advantages are obtained in a filter according to the invention. The number of filter sectors can be limited to what is posεible to insert in the filter though assembly openings to the filter vesεle and a εmall number of filter sectors per filter disc means small manufacture and assembly errors and that more straight sectorε can be manuf ctured. The diameter of the filter discs can be decreased with maintained filtering surface owing to the fact that the center axle has leεε diameter. Moreover, the filtering surface of the filter discε can be used optimally since the suspension level can be raised in the filter vesεle. Thuε, maximum capacity per installed filter surface iε obtained. The ^" leεε number and more straight filter sectors having less diameter furthermore means that the scraper can scrape-off the filter cake closer to the filter cloths without riεk for the εcraper to graεp in the filter cloth. The precoat layer therefore doeε not need to be changed aε often aε in earlier known filters reεulting in longer operation times. Owing to the thinner precoat layer, the larger, is also easier to remove.

Aε to the filer vessle the volume can be considerably decreased with maintained capacity. Thus, the filter vessle can for instance have about 15% lesε diameter and filter diεcε have about 12% less diameter; particularly however the length of the filter vessle can be considerably decreased by the fact that the distance between two filter discs can be essentially decreased, for instance for about 35%, meaning also a shorter center axle. A smaller vessle also means that thinner plate for the vesεle can be used. The smaller size also gives shorter maintenance time meaning lesε risk for sedimentation in the vesεle.

A filter for continouε filtering under pressure has been shown on the drawing and described above. It shall, however, be noted that the filter can be a vacuum filter and also be constructed for filtering during both pressure and vacuum. Nor is the invention limited to the configurations of different elements in the filter which have been described above in connection with the shown embodiment, but the invention can be varied in many ways within the frame of the following claims.