WO1991004778A1

WO1991004778A1 - Method and device for filtering of suspensions

Info

Publication number: WO1991004778A1
Application number: PCT/SE1990/000639
Authority: WO
Inventors: Kent Strid; Rolf Oswaldsson
Original assignee: Kent Strid; Rolf Oswaldsson
Priority date: 1989-10-04
Filing date: 1990-10-04
Publication date: 1991-04-18
Also published as: SE501068C2; NO178785C; US5149449A; JPH04501981A; ES2084038T3; ATE135250T1; FI912678A0; CA2042357C; FI92801C; NO178785B; CA2042357A1; SE8903267L; EP0452435A1; NO912097L; NO912097D0; EP0452435B1; SE8903267D0; AU637417B2; AU6533090A; FI92801B

Abstract

In a method for filtering a suspension through a stationary filter (1a, 1b) having vertical filter element arranged in a pressure vessel (A, B) and for re-conditioning the filter after filtering , the filtrate is discharged from the bottom of the filter element (27) during the main part of the filtering. After filtering unfiltered suspension is discharged in a known manner from the pressure vessel (A, B) later to be returned to the inlet (2a, 2b) for suspension. Thereafter, back-wash is performed with another medium than filtrate, an outlet (12a, 12b) for material deposited on the filter element being opened for removal of said material together with the back-washing medium. Preferably, before back-wash, drying of the material deposited on the filter element (27) takes place by means of a gas medium circulating within the filtering plant.

Description

METHOD AND DEVICE .'FOR FILTERING OF SUSPENSIONS

The present invention concerns a method for filtering a suspen sion, e.g. filtering for separating liquor and lime sludge in a causticising process, and a device for performing the method.

For performing the filtration a stationary filter is used bein arranged in a pressure vessel, and after each filtration a re-conditioning of the filter is made including back-washing th filter before start of the next filtration. The filtering takes place through at least one substantially vertically arranged filter element covered with filter cloth surrounding at least one filter channel through which flows filtrate obtained during filtration, for instance white liquor or green liquor, while filtered material, for instance lime sludge, is deposited on th filter cloth. The pressure vessel is provided with at least inlet means for the suspension to be filtered, outlet means for filtrate obtained during filtration, outlet means for material obtained during filtration, and inlet means for media for -re-conditioning the filter.

According to prior art utilizing a stationary filter, separatio of white liquor and lime sludge in a causticising process is performed in a so called tube filter, i.e. depending perforated tubes covered with filter cloth which have their filter outlets directed upwards. This tube filter is divided in separate parts having separate white liquor outlets from each part. While part of the tube filter are back-washed, which takes place with liquid already filtered, i.e. white liquor, filtering takes place in the other parts. This involves, a.o., a bad ratio of utilization of the filter. Further, during the entire process a continuous discharge of material removed by the back-washing, i.e., lime sludge, takes place from the pressure vessel. This brings about a.o. the drawback, that the lime sludge gets a great content of unfiltered liquid including a large content of chemical8.

SUBSTITUTE The object of the present invention is to provide a new and efficient method and an improved device for filtering a suspension. This has been achieved by the method stated in claims 1-18 and the device stated in claims 19-32.

The drawbacks mentioned above have widely been eliminated by th method according to the present invention. By discharging filtrate from the bottom of the filter it is assured that the filter elements are emptied of filtrate when back-washing takes place. Further, since back-washing according to the present invention is made with another medium than filtrate, preferably water, it is achieved that material removed from the filter cloth is not mixed with a washing medium having a large content of chemicals, as is the case when filtrate is used as back-wash liquid. At the same time a dilution of the removed filter cake is obtained already in the filtering vessel which in turn results in that the mixture of washing liquid and removed filte cake self-drains out through an outlet provided for this purpos without any need for a scraper mechanism or other emptying means. In the drying step, preferably taking place before the back-wash, a gas medium is preferably used which is circulating at least between the pressure vessel and a separator for separa ting the gas medium from the filtrate, but preferably also between a tank for collecting filter cake and washing medium. B circulating the gas medium, mainly air and water vapour, is achieved that the oxygen contents of the gas medium is so low that oxidation of the filtrate is avoided. In all, in improved filtering result having high capacity and efficiency is obtaine with a method according to the invention as compared to the prior art.

A method according to the invention enables in a simple manner continuous filtering and re-conditioning cycle in a unit inclu¬ ding at least two pressure vessels. According to the present invention, while the filtering process according to the .above proceeds in one pressure vessel the re-conditioning process as stated above is performed in the other pressure vessel. The invention enables in an advantageous manner co mmon sources fo feeding the suspension and discharging the filtrate, a common source for the gas medium for drying the material on the filte cloth, a common source for media for back wash, and a comman source for the material taken from the filter element. Further features and advantages of the present invention will be appa¬ rent from the following discription of an embodying example, reference being made to the accompanying drawings, wherein

Fig. 1 shows an example of a filtering unit including two pressure vessels in which filters are arranged,

Fig. 2 shows more in detail an embodiment of a filter comprising a plurality of filter elements,

Fig. 3 is a side view of one of the filter elements,

Fig. 4 is a section at an enlarged scale through the filte element according to Fig. 3,

Fig. 5 shows a filter element according to Fig. 3 in a disassembled state and without a surrounding filter cloth, and

Fig. 6 shows a development of the filtering unit shown in

Fig. 1.

In the following embodying example the invention will, for a simplifying reason, be referred to one field of use, viz., filtering of liquor suspension in a causticising process and specifically white liquor and lime sludge. It should be appreci ated however that also filtering of other suspensions and withi other fields are within the scope of the invention.

In the filtering unit shown in Fig. 1 are arranged two pressure vessels A and B in which are arranged stationary filters la, lb respectively. As appears from the figure the pressure vessels are equally shaped and provided with the same surrounding equip¬ ment. Therefore, corresponding details of the two pressure vessels have been given the same reference numerals, the reference numerals for details concerning vessel A being followed by "a" and for details concerning vessel B being followed by "b" .

In the following description it is assumed that filtering of white liquor and lime sludge takes place in one vessel, for instance vessel A having filter la, and that re-conditioning of filter lb in vessel B takes place after finished filtering in this vessel. Filtering in one vessel suitably can take place while re-conditioning of the filter in the other vessel takes place. Thus, each vessel works discontinuously with alternating filtering and re-conditioning, wheras, by means of a unit as the one shown in Fig. 1 having at least two pressure vessels, a continuously working filtering unit is enabled.

It should be observed however, that the present invention includes the use of only one discontinuously working pressure vessel and that the filtering process only as well as the re-conditioning process only involve novelty and substantial advantages compared to the prior art.

The filters la and lb are only schematically shown in Fig. 1, while in Fig. 2-5 is shown a suitable embodiment of the filters which will be closer described hereinafter. However, the filters are generally of a kind comprising at least one substantially vertical filter element covered with filter cloth surrounding at least one filter channel. At filtering the filtrate (white liquor) is pressed through the filter cloth and flows downwards to the bottom of the filter, while filtered material (lime sludge) deposits on the filter cloth and builds up a layer thereon. After having reached a certain pressure difference over the filter cloth and the lime sludge layer, the filtering process is discontinued and the lime sludge layer is removed from the filter cloth during the re-conditαonmg process.

Turning back to the filtering unit shown in Fig. 1, assuming that the suspension comprises white liquor and lime sludge, arrow C shows feeding thereof through conduit 2 which branches into conduit 2a to vessel A and 2b to vessel B. In conduits 2a and 2b are inserted valves 3a, 3b respectively. From the bottom of filters la and lb filtrate conduits 4a, 4b, respectively, having valves 5a, 5b, respectively, lead to a filtrate separato 6 for separating filtrate, white liquor and the gas (air or air-vapor-mixture) brought by the white liquor to the separator 6. The white liquor is discharged from the separator through outlet conduit 7 out of the filtering unit (arrow D) and the ga is brought from the separator through conduit 8 including a compressor 23 either through branch conduit 8a having valve 9a back to vessel A or through branch conduit 8b having valve 9b back to vessel B to be used during the re-conditioning process. Thus, the gas is circulating in a closed system bringing about the advantage that oxidation of white liquor is avoided since the contents of oxygen of the gas is mainly consumed. For the re-conditioning process is also arranged a feeding conduit 10 (arrow E) for back-wash liquid (water) either to vessel A through branch conduit 10a having valve 11a or to vessel B through branch conduit 10b having valve lib. The lime sludge removed during the re-conditioning process is discharged from the bottom of vessel A through valve 12a and from the bottom of vessel B through valve 12b, and the lime sludge is discharged t a tank 13, for instance a so called lime sludge stirring work, common to both vessels. As will be closer described hereinafter the lime sludge is suitably mixed with a back-wash liquid befor discharge from the pressure vessels, and lime sludge diluted with back-wash liquid is discharged out of the filtering unit through conduit 14 (arrow F) . The filtering unit shown also includes drainage conduits 15a and 15b from the respective pressure vessel having a valve 16a, 16b, respectively, said drainage conduits leading to a return conduit 17 which, through a pump well 18, leads to the feeding conduit 2 for the white liquor lime sludge suspension. Further, from a point before valve 5a a conduit 19a having a valve 20a leads from filtrate conduit 4a to return conduit 17, and in a corresponding manner conduit 19b having a valve 20b is arranged between filtrate conduit 4b and return conduit 17. Further, from a point before valve 5a a conduit 21a having a valve 22a leads from filtrate conduit 4a to tank 13 and in a corresponding manner a conduit 21b having a valve 22b. Further, between gas conduit 8 of sepa¬ rator 6 and tank 13 is arranged a conduit 26. Finally, between air branch conduit 8a and water branch conduit 10a between points before valve 9a and after valve 11a is arranged a condui 24a having a valve 25a, and m a corresponding manner a conduit 24b having a valve 25b is arranged between a air branch conduit 8b and water branch conduit 10b.

The purpose of arranging the different conduits and valves will now be closer described in connection with the functional description of a filtering process in pressure vessel A and a re-conditioning process in pressure vessel B.

Before start of the filtering process lime sludge deposited on the filter cloth during a previous filtration has been removed from vessel A and the vessel has substantially entirely been drained from white liquor lime sludge suspension remaining from the previous filtration. When starting the filtration valve 3b is closed and the white liquor lime sludge suspension is fed into a vessel A through conduit 2a and open valve 3a. During a short initial phase filtrate valve 5a is kept closed while valv 20a is open. The purpose hereof is to discharge the first white liquor penetrating through the filter cloth back to the incomin white liquor lime sludge suspension, and this takes place through conduits 19a and 17. Thereafter valve 20a is closed an filtrate valve 5a is opened, suitably just before closure of valve 20a, and is kept open during the entire filtering proces All remaining valves connected to vessel A are kept closed, i. valves 9a, 11a, 12a, 16a, 22a and 25a. Thus, during the filter ing process itself only valve 3a for feeding the white liquor lime sludge suspension and valve 5a for discharging white liqu are open. Filtering preferably takes place with the entire filter la submerged in the suspension whereby the entire filte is used for filtering, and filtering proceeds until an adapted pressure difference over the filter element is obtained. Suitably, this pressure difference is at least principally defined by the pressure of the incoming white liquor lime slud suspension. It may also, however, at least to a certain degree be determined by a vacuum within the filter or a level differ¬ ence between the separator 6 and the filter. The white liquor discharged as previously mentioned through conduit 7 from sepa ator 6.

In pressure vessel B the filtering has terminated after the above mentioned adapted pressure difference has been reached, and vessel B shall now be re-conditioned for renewed filtering as stated above for vessel A. At termination of the filtering process in vessel B, in a corresponding manner as described above for vessel A, valves 3b and 5b are open and valves 9b, lib, 12b, 16b, 22b and 25b are closed. When starting the re-co ditioning process valve 3b is closed. Valve 9b is opened and a air-vapour mixture is introduced from separator 6 via conduit having compressor 23 and conduit 8b to the inner of the vessel to dry the lime sludge deposited on the filter cloth, whereby the lime sludge is further relieved from white liquor and, thus also from chemicals. Just after the valve 9b has been opened, valve 16b is opened to discharge from the vessel unfiltered white liquor lime sludge suspension which is returned through conduit 17 to the incoming white liquor lime sludge suspension. After performed drying of the lime sludge layer drainage valve 16b is closed and soon thereafter filtrate valve 5b and air valve 9b. Now valve lib is opened for back-washing of the lime sludge layer with back-wash liquid (water) from conduit 10, 10b communicating with the filtrate channels of the filter.

Suitably, back-wash liquid is supplied in such an amount that the lime sludge becomes self-draining so that the diluted lime sludge can self-flow down into tank 13 through valve 12b, which is opened just after valve lib has been opened. Thereafter, valve lib is closed and soon after that valve 12b. During a short while thereafter valve 22b is opened for drainage of diluting liquid in the filter through conduit 21b down to tank 13. Vessel B is now prepared for filtering. Concerning drying of the lime sludge layer an adapted difference pressure of the filter elements is obtained by means of the compressor 23. It is to be observed that filtering, drying, back-wash and diluting can be regulated entirely independent of each other. Regulating one of these processes, thus, does not affect the others, at least not when two vessels are arranged as shown in Fig. 1.

Control of the valves for a correct timely opening and closing can take place by any conventional control system.

With reference to Fig. 1 has been described above a filtering unit comprising two filter vessels, whereby entirely or at leas partly depending on overlapping between a filtering process in one filter vessel and a re-conditioning process in the other filter vessel is obtained a continuously working filtering unit. Of course, the unit may comprise more filtering vessels. It should be observed however, that the invention includes the use of one filter vessel alone which works discontinuously with alternating filtering and re-conditioning. One single such filter vessel with its surrounding equipment and in an embodi¬ ment as described in Fig. 1 is obtained if for instance vessel and its connections are removed from Fig. 1. Further shall be observed the substantial existing novelties concerning the filtering process alone as well as the re-conditioning process alone, namely, as concerns the filtering process, firstly that during this process there is only feeding of suspension and discharge of filtrate, while all remaining connections to the pressure vessel are closed, and that filtering takes place all over the filter, and, concerning the re-conditioning process, that drying of material deposited on the filter cloth takes place within the pressure vessel before the material is removed from the filter cloth. Advantageous is also that back-washing can be performed with sufficient dilution of the material to achieve a condition of self drainage of the diluted material from the pressure vessel.

As concerns back-wash, this may, although not shown in Fig. 1 b performed with a combination of gas and liquid or with only gas It is, for instance, possible to provide a spraying device abov the filter.

An example of a suitable filter to be used in connection with the present invention is shown in Fig. 2-5. In Fig. 2 is shown pressure vessel, for instance a pressure vessel A, which includes a plurality of filter elements 27, of which one is shown in Fig. 4. The filter element is kept together between gables 28 and 29 by means of pull rods 30. In Fig. 3 is shown a section through a filter element and as shown it comprises a folded plate 31 surrounded by filter cloth 32. Between the filter cloth 32 and plate 31 are formed filtrate channels 33. Particularly if a filter element is relatively wide it is suitable to provide a central clamping frame 34 for tightening the cloth and for parting the filter surface. In Fig. 5 is illustrated the simplicity of disassembling the filter element and to change filter cloth. Of course, Figs. 2-5 show only an embodying example of a filter suitable to be used in connection with the present invention.

The embodiment of a filtering unit shown in Fig. 6 differs from the one shown in Fig. 1 primarily in that filtrate outlets are provided also at the top of filters la and lb. In practice, to conduits 10a, 10b between filters la, lb and valves 11a, lib ar connected conduits 35a, 35b respectively, leading to the filtrate separator 6. In conduits 35a, 35b are provided valves 36a, 36b, respectively. In the embodiment of Fig. 6 the pump well 18 - apart from the connection with conduit 17 - is connected to conduit 8 between the separator 6 and the compressor 23 by means of a conduit 37. Hereby is obtained a more complete circuit for the gas medium circulating in the plant. Discharge of filtrate through conduits 35a, 35b is utilized primarily in the beginning of the filtering process in order thereby to avoid the problem which may arise when filtrat discharge takes place only through conduits 4a, 4b, namely that the filter cake becomes substantially thicker in the lower part of the filter than in the upper part thereof.

Claims

1. Method for filtering a suspension through a stationary filter arranged in a pressure vessel and for re-conditioning t filter after performed filtration, said filter comprising at least one substatially vertical filter element (27), which is covered with filter medium (32), on the outside of which material is deposited during filtration while filtrate flowing through the filter medium is discharged through at least one filtrate channel on the inside of the filter medium, said pressure vessel being provided with at least inlet means (2a, 2 for suspension to be filtered, outlet means (4a, 4b) for filtrate, outlet means (12a, 12b) for material deposited on the filter medium and inlet means (10a, 10b) for at least one mediu for back-wash of the filter element from the filtrate channel through the filter medium to the pressure vessel, during filtration only feeding of suspension through said inlet means for suspension and discharge of filtrate through said outlet means for filtrate taking place, whereas remaining inlets and outlets are kept closed, c h a r a c t e r i z e d i n

- that during the main part of the filtration filtrate is discharged from the filtrate channel at least at the bottom of the filter,

- that after filtration unfiltered suspension is discharged fr the pressure vessel in a manner known per se in order to later be returned to the inlet for suspension, and

- that after discharge of unfiltered suspension back-wash is performed with another medium than filtrate, the outlet for material deposited on the filter medium being opened for remova thereof together with the washing medium.

2. Method according to claim 1, wherein the supply of suspension is interrupted before unfiltered suspension is discharged, said pressure vessel also being provided with inlet means (8a, 8b) for a gas medium, c h a r a c t e r i z e d i n that in connection with discharge of unfiltered suspension a ga medium is introduced into the pressure vessel for drying of material deposited on the filter medium during filtration and for displacement of filtrate out of the filter element and the filtrate conduit.

3. Method according to claim 2, c h a r a c t e r i z e d i that during at least a substantial portion of the drying discharge of filtrate is continuing, whereafter the back-wash i performed .

4. Method according to claim 2 or 3, c h a r a c t e r i z e i n that filtrate is supplied to a separator (6) for separatin filtrate and gas medium brought along therewith.

5. Method according to any of the preceding claims, c h a r a c t e r i z e d i n that back-wash is performed wit only liquid.

6. Method according to any of claims 1 - 4, c h a r a c t e r i z e d i n that back-wash is performed wit liquid as well as gaseous medium.

7. Method according to claim 5 or 6, c h a r a c t e r i z e i n that back-wash is performed with such dilution of the material deposited on the filter medium, that self drainage is achieved through said outlet means for material deposited on th filter medium.

8. Method according to any of the preceding claims, c h a r a c t e r i z e d i n that discharge of unfiltered suspension is approximately total.

9. Method according to any of the preceding claims, c h a r a c t e r i z e d i n that at start of filtration th filtrate is discharged from the filtrate channel at the upper end of the filter.

10. Method according to any of the preceding claims, c h a r a c t e r i z e d i n that at start of filtration th first filtrate is returned to the incoming suspension.

11. Method according to any of claims 4 - 10, c h a r a c t e r i z e d i n that the gas medium for drying is taken from the separator.

12. Method according to any of claims 4 - 11, c h a r a c t e r i z e d i n that the gas medium is circulated in a closed system including the pressure vessel an the separator.

13. Method according to any of claims 4 - 12, c h a r a c t e r i z e d i n that before being fed to the pressure vessel the gas medium for drying of material on the filter cloth is given a pressure rise (23) defining the pressur difference over the filter element (27) during drying.

14. Method according to any of the preceding claims for achieving a continuous filtering and re-conditionmg process in one unit including at least two pressure vessels (A,B), c h a r a c t e r i z e d i n that while the filtering proces takes place in one pressure vessel (A) the re-conditioning process takes place in the other pressure vessel (B) .

15. Method according to claim 14, c h a r a c t e r i z e d l n that the suspension is supplied to the pressure vessels (A,B) from the same source and that the filtrate is discharged from the pressure vessels to a common filtrate separator (6) .

16. Method according to claim 15, c h a r a c t e r i z e d i n that the gas medium for drying of the material on the filter medium (32) is obtained from the common filtrate separator (6) .

17. Method according to any of claims 14 - 16, c h a r a c t e r i z e d i n that the back-washing medium or media is supplied to the pressure vessels (A,B) from the same source of liquid and/or gas.

18. Method according to any of claims 14 - 17, c h a r a c t e r i z e d i n that the material removed from the filter element is discharged to a tank (13) common to the pressure vessels (A,B) .

19. A device for filtering a suspension through a stationary filter (la, lb) arranged in a pressure vessel (A,B) and for re-conditioning the filter after filtration, said filter comprising at least one substantially vertical filter element (27) covered with filter medium (32) on the outside of which material is deposited during filtering while filtrate flowing through the filter medium is discharged through at least one filtrate channel (33) on the inside of the filter medium, said pressure vessel being provided with at least inlet means (2a, 2b for the suspension to be filtered and outlet means (4a,4b) for filtrate, c h a r a c t e r i z e d i n that a first filtrate outlet (4a, 4b) is connected to the lower end of the filtrate channel.

20. A device according to claim 19, c h a r a c t e r i z e d i n that said first filtrate outlet (4a, 4b) leads to a tank (6) .

21. A device according to claim 19 or 20, c h a r a c t e r i z e d i n that the filtrate channel (33) in its upper end is connected to a second filtrate outlet (35a,35b) .

22. A device according to claims 20 and 21, c h a r a c t e r i z e d i n that the first filtrate outlet (4a, 4b) and the second filtrate outlet (35a, 35b) lead to a common tank.

23. A device according to any of claims 19 - 22, said pressur vessel (A,B) being provided with inlet means (10a, 10b) for at least one medium for back-wash of the filter element from the filtrate channel (33) through the filter medium (32) to the pressure vessel, c h a r a c t e r i z e d i n that the filtrate channel (33) communicates with means (10, 10a, 10b) for supplying another back-wash medium than filtrate.

24. A device according to claim 23, c h a r a c t e r i z e l n that the back-wash medium is water.

25. A device according to claim 23 or 24, said pressure vesse (A,B) being provided with outlet means (12a, 12b) for material deposited on the filter medium, c h a r a c t e r i z e d i that the pressure vessel and said outlet means are adapted for self drainage of material dissolved in the back-wash medium, a in that said outlet means communicates with a collecting vesse (13) .

26. A device according to any of claims 19 - 25, said pressur vessel being provided with inlet means (8a, 8b) for a gas mediu for drying of material deposited on the filter medium, c h a r a c t e r i z e d i n that said inlet means for gas medium communicates with the tank (6).

27. A device according to claim 26, c h a r a c t e r i z e l n that the tank (6) is adapted to separate the gas medium from the filtrate discharged from at least said filtrate outle (4a, 4b) .

28. A device according to claim 26 or 27, c h a r a c t e r i z e d b y a closed system for circulating the gas medium.

29. A device according to claim 28, c h a r a c t e r i z e d l n that the closed system includes the tank (6), the pressure vessel (A,B) and the collecting vessel (13).

30. A device according to claim 29, c h a r a c t e r i z e d l n that the closed system also includes a container (18) for receiving unfiltered suspension discharged from the pressure vessel (A,B), said container communicating with the inlet for suspension .

31. A device according to any of claims 28 - 30, c h a r a c t e r i z e d i n that the closed system also includes a compressor (23) in a conduit (8, 8a, 8b) between the tank (6) and the pressure vessel (A,B) .

32. A device according to any of claims 19 - 31, c h a r a c t e r i z e d by two pressure vessels (A,B) , one common tank (6) for separating gas medium and filtrate and a common collecting vessel (13) for material deposited on the filter medium.

33. A device according to claim 32, c h a r a c t e r i z e d by a common container (18) for receiving unfiltered suspension discharged from the pressure vessels (A,B), said container communicating with said inlet (3a, 3b) for suspension of both pressure vessels.