GB2372223A

GB2372223A - Pressure filtration using a diaphragm

Info

Publication number: GB2372223A
Application number: GB0103840A
Authority: GB
Inventors: Michael Shane Sullivan
Original assignee: Svedala Ltd
Current assignee: Svedala Ltd
Priority date: 2001-02-16
Filing date: 2001-02-16
Publication date: 2002-08-21
Also published as: GB0103840D0; WO2002066139A1

Abstract

A pressure filtration apparatus comprises a filter medium 21, a diaphragm (Fig 1,13) movable under fluid pressure between an operative (Fig 2, 13) and an inoperative position, and fluid pressure supply means that urges the diaphragm toward the operative position when feed, such as a slurry, is supplied to the apparatus. Preferably the fluid pressure supply means is provided with pressure release means to reduce pressure exerted on the diaphragm as feed is supplied. Preferably the feed cavity 28 expands according to the feed volume and may help to reduce the foaming properties. Keeping the feed cavity volume to a minimum may pressurise the feed on entry to the cavity 28 and allows a degree of pre-filtration which may induce filtercake formation from the upper end of the filter medium to the other. The fluid used to apply the pressure may form part of a hydraulic reservoir (Fig 9, 46) and during operation may be contained within a pressure chamber (Fig 2, 18). Filtercake may collect on a cylindrical filter medium 21 and may be removed by a blast of air from a discharge inlet (Fig 8, 31).

Description

Title: A Filtration Method and Pressure Filtration Apparatus.

Description of Invention This invention relates to a filtration method of the kind in which the unfiltered liquid, slurry or the like (henceforth referred to as the"feed") is forced against the filter medium by pressure exerted thereupon by a flexible member such as a diaphragm. The invention relates also to a pressure filtration apparatus.

Such filtration methods are used in a number of industrial applications, and are particularly well suited to the separation of relatively fine solids from liquid effluent as are found in the industrial minerals field, such as the ceramic glaze industry. The manufacture of many products gives rise to an array of finely ground minerals which do not lend themselves readily to separation and/or disposal by other methods. Specifically, the traditional"settling" process gives rise to a thick sludge which must then be disposed of as landfill and is rapidly becoming prohibitively expensive, and could be prohibited altogether by legislation in the near future.

Pressure filtration, on the other hand, allows a high degree of particulate separation to be achieved as the feed is forced through the filter medium under fluid (usually hydraulic) pressures of up to 150 bar. This pressure filtration can be effected using a number of different types of apparatus, which differ principally in the orientation and configuration of the diaphragms and filter media. In brief, these can be subdivided into arrangements having generally planar diaphragms and filter media, disposed either vertically or horizontally, with a third type having the diaphragm and filter medium disposed in a tubelike manner, with the filter medium being disposed inwardly of, but generally concentrically with, a substantially cylindrical diaphragm.

It is to this latter type of apparatus-known in the art as a"tube press"that the present invention particularly relates, although it will be appreciated from the following that application of the invention is not limited in any way to this type of apparatus, and that it could equally be used with other pressure filtration systems.

Pressure filtration methods consist essentially of three stages: an initial feed-filling stage, during which the feed is introduced to the apparatus, a pressure filtration stage during which the feed is forced against the filter medium and a discharge state during which the filtered solids (known collectively as the"cake") are removed from the system.

During the feed-filling stage, the feed enters a cavity defined by the diaphragm and filter medium, with pressure then being exerted upon the feed by the diaphragm during the subsequent pressure stage. Conventionally, this filter pressure is applied at two levels-a first, low, pressure to initiate the filtration process and a second, much higher, pressure to remove most of the remaining liquid, thus finalising production of the cake. During the discharge stage, the pressure exerted on the cake by the diaphragm is reduced, thus allowing the cake to be removed from the surface of the filter medium.

For the sake of convenience, the position of the diaphragm when 'retracted'shall hereinafter be referred to as its'inoperative position' ; the term 'operative position'shall be used to indicate the filtration position that the diaphragm adopts when under pressure from the pressure fluid.

The cake, which by then is quite hard and surface dry, is then able to be transported without difficulty for immediate storage or disposal. Such pressure filtration methods, hereinafter referred to for convenience as being"of the kind specified"thus offer industry many advantages, especially where the separation and recovery of fine solids is required.

It is an object of the present invention to provide an improved filtration method of this general kind.

According to a first aspect of the present invention, there is provided a filtration method of the kind specified, characterised in that the diaphragm is urged towards its operative position during the feed-filling stage.

The diaphragm may be urged towards its operative position by fluid pressure. Preferably, the fluid used to apply said pressure forms part of a hydraulic fluid reservoir.

Conveniently, the pressure exerted on the diaphragm by the incoming feed effects movement of the diaphragm towards its inoperative position.

The fluid pressure preferably is decreased as the amount of feed located in the feed cavity increases.

In this way, the volume of the incoming feed is able to displace a substantially equivalent volume of pressure fluid on the other side of the diaphragm. Thus, the feed cavity is able to expand in accordance with the amount of feed located therein.

As will be appreciated, this keeps the size of the feed cavity relatively small, with this having been found by the applicants to greatly reduce any foaming which is often found with conventional pressure filtration methods.

Although the applicants do not wish to limit the scope of protection by detailing the manner by which this benefit is obtained, it is thought that foaming is caused by mixing of the incoming feed with the relatively large volume of air disposed within the feed cavity, the size of which, in conventional systems, is maximised by retracting the diaphragm as far as possible towards its inoperative position.

In accordance with the invention, the fluid pressure preferably is decreased to a predetermined level, the predetermined level being lower than that at which the low pressure filtration stage is carried out, but greater than the level at which the discharge stage is carried out.

Preferably, the feed cavity increases in size as the feed is supplied thereto. This, as explained above, allows the size of the feed cavity to be kept relatively small, thus reducing the possibility of feed/air mixing occurring, which can give rise to foaming of the feed, and thus to a friable laminated filtercake.

Conveniently, the urging of the diaphragm towards its operative position during the feed-filling stage effects a pre-filtration of the feed, allowing a reduction in the duration of the high pressure filtration stage. The filtercake formation may be induced at a first position on the filter medium, the formation continuing towards a second position as the feed is supplied to the cavity.

According to a further aspect of the present invention, there is provided pressure filtration apparatus having a filter medium, a diaphragm movable under fluid pressure between operative and inoperative positions, and fluid pressure supply means, characterised in that the fluid pressure supply means is provided with pressure release means which is effective gradually to reduce the pressure exerted on the diaphragm as feed is supplied to the apparatus.

According to a still further aspect of the present invention, there is provided pressure filtration apparatus having a filter medium, a diaphragm movable under fluid pressure between operative and inoperative positions, and fluid pressure supply means, characterised in that means is provided to urge the diaphragm towards its operative position when feed is supplied to the apparatus.

The invention will now be described in greater detail, but by way of example only, by reference to the accompanying drawings, of which: Figure 1 is a partial cross-section of a tube press, with the diaphragm shown in a"rest"position ; Figure 2 shows a similar view, with the diaphragm urged towards an operative position, and thus towards the filter medium, in preparation for entry of the feed; Figure 3 shows the incoming feed and initial filtercake formation; Figure 4 shows the continued formation of filtercake, and the gradual displacement of the diaphragm; Figure 5 shows the press subsequent to the feed-filling stage, but prior to the pressure filtration stages; Figure 6 shows the effect of the low and high pressure filtration stages; Figure 7 shows the resulting filtercake in contact with the filter medium, the diaphragm being shown in a fully retracted (inoperative) position; Figure 8 is a more complete sectional view of the press, illustrating the discharge stage; and Figure 9 is a schematic illustration of the valve and pump arrangement used to effect the various filtration stages.

The Figures show a generally cylindrical tube press 10 having a steel outer casing 11 and a vertically moveable duplex stainless steel inner candle 12. The casing 11 is generally cylindrical, as is the candle 12, and is provided on its inner surface with a flexible (in this case rubber) diaphragm 13 which is securely attached to the casing 11 at its upper and lower ends, in generally conventional manner. A number of radially extending conduits 14 are provided in upper and lower parts of the casing 11, the bores communicating, via fluid lines 15, with a fluid port 16. The fluid port 16 is connected to a valve arrangement (described in more detail hereinafter), by which a hydrolubric fluid such as water may be supplied or withdrawn from the fluid circuit defined by the conduits 14, lines 15, and an outwardly facing surface 17 of the diaphragm 13. From this, it will be appreciated that the supply of pressurised fluid to the thus defined pressure chamber 18 will cause the diaphragm to flex outwardly (i. e. to the right of Figure 1). The fluid lines 15 are connected to the outer casing 11 in generally conventional manner, by a nut and bolt arrangement 19, and a bored support block 20. The inner candle 12, which is moveable vertically by a conventional hydraulic arrangement, is provided about its outer surface with a generally cylindrical filter medium 21 which, in practice, may be made up of three or more layers, such as backing mesh, backing felt and the filter cloth itself. For convenience, however, the filter medium is shown as a one-piece construction. The filter medium 21 is pinned or otherwise attached to angled support parts 22 of the candle, in generally conventional manner. This attachment may be releasable, to facilitate replacement of the filter medium. Disposed radially inwardly of the filter medium 21 is a support member 23 provided with a number of radially extending filtrate escape apertures 24 through which the liquid filtrate passes, subsequent to and during the pressure filtration stage. Towards the base of the candle 12 is located a filtrate drain 25, through which the liquid is passed for further filtration, processing, or disposal.

The candle 12, which is held in fluid sealing engagement with the outer casing 11 by upper and lower polyurethane seals 26, is provided with a feed input port 27, through which the feed is introduced to a feed cavity 28 defined by the inwardly facing surface 29 of the diaphragm 13 and the filter medium 21.

Figure 1, as explained above, shows the flexible diaphragm 13 in a "rest"condition, in that no fluid pressure (positive or negative) has been applied thereto.

Figure 2, on the other hand, shows the diaphragm 13 in an operative condition, in which it is urged towards the filter medium 21, under the action of pressurised fluid contained within the pressure chamber 18. This state, in which low pressure hydraulic fluid is contained within the pressure chamber 18, is the"pre-fill"stage of the filtration method of the present invention. It will be appreciated by those skilled in the art that the position of the diaphragm

in this "pre-fill" stage differs markedly from prior art methods, in which the diaphragm is fully retracted under vacuum away from the filter medium 21.

Figure 3 illustrates the effect of the incoming feed on the diaphragm 13.

As shown, the incoming feed pressure, regulated by a feed control valve, causes the diaphragm 13 to be displaced away from its operative position, to make room for the incoming feed and the resulting filtercake. The ensuing reduction of fluid pressure in the pressure chamber 18 is permitted by opening a pressure relief valve in communication with the fluid port 16, the effect being that the feed cavity 28, in which the feed is located, increases gradually in size in accordance with the amount of feed located therein. As will be appreciated, this minimising of the feed cavity volume has the effect of exerting pressure on the feed as soon as it enters the feed cavity 28, and the applicants have found that this has the surprising effect of inducing filtercake formation very shortly after the feed enters the cavity. This, it will be appreciated, differs clearly from prior art methods, in which substantially no filtration pressure is applied to the feed until the later pressure filtration stage is initiated. Thus, the present arrangement allows for a degree of"pre-filtration"to occur as soon as the feed enters the feed cavity.

Moreover, the reduction of the feed cavity volume keeps the amount of air located therein to a minimum, and the applicants have found that this greatly reduces the amount of"foaming"which otherwise occurs as a result of mixing of the feed and air upon entry of the feed to the cavity. Such foaming, which had previously been thought to constitute an inherent and inevitable feature of pressure filtration, is thought to cause air to become entrapped with the feed, leading to filtercake lamination and hence to a highly friable (dusty) product which is difficult and inconvenient to store and transport.

As shown in Figure 3, the rapid filtercake formation results in liquid effluent (filtrate) being forced through the filter medium 21, and hence through the filtrate drain apertures 24 to the filtrate drain 25.

As shown in Figure 4, continued input of feed to the feed cavity results in continued filtercake formation, with the cake"growing"from one (upper) end of the filter medium to the other. As the filtercake formation continues, the flow of filtrate is similarly augmented, with the diaphragm 13 being moved further and further away from the filter medium 21 as the pressure within the feed cavity 28 increases.

Once a predetermined thickness of pre-filtered filtercake has been produced by the pre-filtration method shown in Figures 1 to 4, the pressure release valve associated with the fluid port 16 is closed, thus preventing any further movement of the diaphragm 13 away from the filter medium 21. This is shown in Figure 5. Simultaneously, input of the feed is halted, by closure of the feed input valve, with the point at which this occurs being selected in accordance with the required thickness of the pre-filtered cake. It will be appreciated that this thickness can be regulated closely, by adjustment of the points at which the hydraulic fluid pressure release valve and the feed input valve are closed. It will also be appreciated that at this point, a degree of pressure remains within the pressure chamber 18, such that the diaphragm is not permitted to return to the position shown in Figure 1. Thus, the outwardly facing surface 17 of the diaphragm 13 remains in contact with the pre-filtered cake, thus keeping the pre-filtered cake 29 in contact with the filter medium 21.

Subsequent to this, as shown in Figure 6, the fluid pressure in the pressure chamber 18 is increased slightly to what is known in the art as the"LP2" (low pressure 2) level, at which increased filtration is effected. Although this marks the commencement of the pressure filtration stage, a much higher pressure (typically 70 to 150 bar) is then required to complete the filtration, to a point at which no further filtrate can be removed by pressing. As shown in Figure 9, the low and high pressure filtration stages are controlled by high and low pressure pumps and valves, each of which is in communication with the hydrolubric fluid reservoir. As will be appreciated, the duration of the high pressure filtration stage, shown in Figure 6, will vary in accordance with the degree of filtration required, and also will depend on the composition of the feed material. Thus, in general, the smaller is the particle size of the solids contained within the feed, or the greater is the solids'aspect ratio, the longer will be the high pressure filtration stage. Similarly, the maximum filtration pressure exerted on the feed will depend on the nature of the feed material, as will be readily apparent to those skilled in the art. By monitoring the output rate of the filtrate effluent through the filtrate drain 25, the point at which no further filtration will take place can be detected, with this indicating that the filtercake has been formed-and dried-as much as the pressure filtration will allow.

At that point, as shown in Figure 7, a vacuum is applied to the pressure chamber 18, by operation of a vacuum valve also in communication with the fluid port 16. As will be appreciated, the application of a vacuum (or near vacuum) to the pressure chamber 18 causes the diaphragm 13 to retract sharply away from the filtercake 30, leaving the filtercake 30 adjacent and in contact with the filter medium 21, with the filtercake 30 thus maintaining a generally cylindrical configuration. In view of the dryness-and hence rigidity-of the filtercake 30, this remains in place despite the withdrawal of the diaphragm 13, and it is therefore necessary to separate the filtercake 30 from the filter medium 21 in order to release the dried solids cake from the candle.

Thus, subsequent to an optional air purge, during which dry air is pumped into the feed cavity 28, and then forced through the cake 30, and/or a cake wash, during which wash fluid is passed through the cake in a somewhat similar manner, the candle 12 is lowered hydraulically, to a position at which the lower angled support parts 22 pass below the bottom of the outer casing 11. At this point, although most-if not all-of the cake remains securely attached to the filter medium 21, the cake is in theory able to fall from the candle, with this being effected by a blast of cake discharge air introduced through a discharge inlet 31 disposed towards the top of the candle 12. This is shown in detail in Figure 8, from which it can be seen that passage of the discharge air out through the filtrate apertures 24 fractures the brittle cake, freeing it from the filter medium 21 and thus allowing it to discharge under gravity from the candle 12. To assist in the fracturing-and hence discharging-process, the outer surface of the filter medium and/or the support member 23 may be provided with a number of generally vertically extending ridges, which induce areas of reduced thickness in the resulting cake.

Referring finally to Figure 9, this shows a schematic illustration of the various pumps and valves which are used to control the filtration method in the manner hereinbefore described.

The tube press 10 is connected, at its upper end, to a feed control valve 41 which regulates the supply of feed from a feed pump 51. The supply and removal of the hydraulic pressure fluid is controlled by a vacuum valve 44, in line with a vacuum pump 54, a high pressure valve 42 in line with a high pressure pump 52, a low pressure valve 43 in line with a low pressure pump 53 and a pressure release valve 45. As shown in Figure 9, the vacuum line, high pressure line, low pressure line and pressure relief line are each connected to a hydraulic pressure fluid tank 46 which contains the hydraulic fluid reservoir.

Although the manner-and sequence-of operation of the various valves and pumps will be readily understandable to those skilled in the art in the light of the foregoing description, there is set out below, in tabular form, a summary of the status of the various valves during the different stages of the pressure filtration operation.

STAGE VALVE NO. OPEN SHUT COMMENTS READY 41 X 42 X 43 X 44 X Valve open to pull diaphragm back.

45 X PRE-FILL (FIG 2) 41 X 42 X 43 X Valve open until pressure chamber full 44 X 45 X FEED (FIGS 3 & 4) 41 X Valve open for selected time.

42 X 43 X 44 X 45 X Open 1 sec after valve 41.

END OF FEED CYCLE 41 X 42 X 43 X 44 X 45 X Shut 1 sec prior to valve 41.

FILTRATION CYCLE (FIG 7) 41 X 42 X Open after valve 43 43 X Close when valve 42 is opened.

44 X 45 X DISCHARGE 41 X (FIG 8) 42 X 43 X 44 X 45 X [REPEAT SEQUENCE ; GO BACK TO"READY"STAGE] As indicated above, the present invention provides for a much improved pressure filtration method which not only exhibits a more efficient filtercake formation, in view of the pre-filtration which occurs during the feed input, but lamination in the final filtercake is greatly reduced, lessening considerably the friability of the cake, and thus improving its'handle-ability'. The pre-filtration phenomenon allows a reduction in the high pressure filtration time, thus also reducing the amount of high pressure pumping that would otherwise be required to complete the filtration process. As will be appreciated, this not only reduces the filtration cycle time, but also reduces operating costs, and offers potential benefits in relation to the working life of the high pressure components.

Although the invention has been described in detail in relation to a tube press, it will be appreciated by those skilled in the art that the advantages discussed could equally be obtained with other, non-tubular, filtration presses such as, for example, variable chamber or membrane-type filter presses.

In the present specification"comprise"means"includes or consists of and"comprising"means"including or consisting of'.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS 1. A filtration method of the kind specified, characterised in that the diaphragm is urged towards its operative position during the feed-filling stage.
2. A method according to Claim 1 wherein the diaphragm is urged towards its operative position by fluid pressure.
3. A method according to Claim 2 wherein the fluid used to apply said pressure forms part of a hydraulic fluid reservoir.
4. A method according to any one of the preceding claims wherein the pressure exerted on the diaphragm by the incoming feed effects movement of the diaphragm towards its inoperative position.
5. A method according to Claim 2, Claim 3 or Claim 4 wherein the fluid pressure is decreased as the amount of feed located in the feed cavity increases.
6. A method according to any one of Claims 2 to 5 wherein the fluid pressure is decreased to a predetermined level, the predetermined level being lower than that at which the low pressure filtration stage is carried out, but greater than the level at which the discharge stage is carried out.
7. A method according to any one of the preceding claims wherein the feed cavity increases in size as the feed is supplied thereto.
8. A method according to any one of the preceding claims wherein the urging of the diaphragm towards its operative position effects a pre-filtration of the feed, allowing a reduction in the duration of the high pressure filtration stage.
9. A method according to any one of the preceding claims wherein filtercake formation is induced at a first position on the filter medium, the formation continuing towards a second position as the feed is supplied to the cavity.
10. A filtration method substantially as hereinbefore described and/or as shown in the accompanying drawings.
11. Pressure filtration apparatus having a filter medium, a diaphragm movable under fluid pressure between operative and inoperative positions, and fluid pressure supply means, characterised in that the fluid pressure supply means is provided with pressure release means which is effective gradually to reduce the pressure exerted on the diaphragm as feed is supplied to the apparatus.
12. Pressure filtration apparatus having a filter medium, a diaphragm movable under fluid pressure between operative and inoperative positions, and fluid pressure supply means, characterised in that means is provided to urge the diaphragm towards its operative position when feed is supplied to the apparatus.
13. Pressure filtration apparatus substantially as hereinbefore described and/or as shown in the accompanying drawings.
14. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.