DEWATERING SLURRIES

Abstract: In a slurry dewatering process, slurry is pumped into a porous tube at a sufficiently low velocity for laminar flow condi­ tions to exist. The pressure is sufficiently high for liquid in the slurry to permeate outwardly through the tube wall and for solids in the slurry to be deposited in the form of an annular cake on the tube wall. The deposition of solids is controlled so that the cake thickness is substantially uniform throughout. When a predeter­ mined thickness of cake has been deposited, the pressure inside the tube is decreased and the flow velocity is increased and the cake is dislodged from the tube wall in flake form and is conveyed downstream for collection on a screen or the like.

Description: This invention relates to a slurry dewatering process and a tubular filter press apparatus for use therein. In one known slurry dewatering apparatus, slurry which is to be dewatered is pumped at low velocity into a rigid, perforated tube. The liquid in the slurry permeates outwardly through the perforations in the tube wall and the suspended solids build up inside the tube to form a solid cylindrical plug which is "extruded" from the tube in the form of an elongate "sausage" under the effects of pressure exerted on it by incoming slurry. Because of its highly compact nature, the "sausage" cannot easily be dried further and it is usually necessary to break up the "sausage" to increase its specific surface area. This is clearly not desirable. EP-A-O 122 439 discloses a method of cross-flow filtration in which the flow of filtrate is regulated so as to maintain a constant flux rate which is at a level below the ordinary (i.e. unregulated) start-up flux rate, this permitting increased overall (i.e. time averaged) fluxes to be obtained. The method relies on shear forces arising from the circulation velocity of the filtrate (i.e. on a turbulent flow mode for the filtrate) continually to erode the sludge layer of solids deposited on the walls of the filter medium, so as to balance the deposition rate and establish an equilibrium sludge layer thickness. It is observed that the solid matter to be separated should comprise cohesionless particles. According to one aspect of the present invention there is provided a slurry dewatering process for dewatering a slurry composed of a suspension of solid particles having cohesive properties which is characterized in that it includes a cake deposition stage and a cake dislodgement and removal stage, wherein the cake deposition stage comprises the steps of: pumping the slurry which is to be dewatered into a porous tube at a sufficiently low velocity for laminar flow conditions to prevail in the tube and at a sufficiently high pressure for liquid in the slurry to permeate outwardly through the tube and for solids in the slurry to be deposited on the internal tube surface to form a cohesive annular layer of cake on that internal surface the rate of outward permeation of liquid decreasing continuously with time as the annular layer is deposited, and controlling the deposition of the solids so that the cake layer which is formed is even or nearly even over the length of the tube, and wherein the cake dislodgement and removal stage comprises the steps of: decreasing the pressure and increasing the flow velocity in the tube when a predetermined cake thickness has been deposited on the internal tube surface, dislodging the cake from the internal tube surface in the form of flakes composed of discrete particles cohering together by creating a localized zone of further reduced pressure within the tube sufficient to separate the cake from the internal surface of the tube, causing that localized zone to move along the length of the tube, and permitting flow through the tube to convey the dislodged flakes downstream and out of the tube. According to a further aspect of the invention there is provided a tubular filter press apparatus for dewatering a slurry composed of a suspension of solid particles in a liquid, said apparatus including at least one porous tube, a pump for pumping slurry into said tube and a back pressure valve downstream of said tube, characterised in that, in order to dewater a slurry wherein the solid particles have cohesive properties, (A) control means are incorporated which regulate said pump and back pressure valve such that (i) slurry initially enters said tube at sufficiently low velocity for laminar flow conditions to prevail and at sufficiently high pressure for liquid in the slurry to permeate outwardly through the tube and for solids in the slurry to be deposited on the internal tube surface to form thereon an annular layer of cohesive cake which is even or nearly even throughout, and thereafter such that (ii) once a predetermined thickness of cake has been deposited, slurry enters the tube at decreased pressure and increased velocity; and (B) there are incorporated means for creating within the tube during said step (ii) a zone of further reduced pressure sufficient in use to dislodge deposited cake from the internal surface of the tube in the form of flakes and means adapted to move that localized zone along the length of the tube, whereby in use dislodged cake is conveyed downstream and out of the tube by the slurry flow through the tube. Preferably the process and apparatus also include the facility for capturing the dislodged cake i.e. for separating it from the conveying liquid BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be explained in more detail, by way of example, with reference to the accompanying schematic drawings in which: Figure 1 shows an apparatus according to the invention in the cake formation mode; Figure 2 shows cake removal by spray cleaning; Figure 3 shows cake removal by external rollers; Figure 4 shows a system for cake capture; Figure 5 shows cake removal using an internal bobbin; Figures 6 and 7 show a filter tube array in inoperative and operative conditions respectively; and Figure 8 shows how the array of Figures 6 and 7 can be suspended. DESCRIPTION OF EMBODIMENTS The various stages in the process of the invention are: cake deposition, cake dislodgement and removal and cake capture. Figure 1 illustrates cake deposition. Referring to Figure 1, the slurry 10 which is to be dewatered is stored in a feed tank and is pumped by means of a pump 12 in the direction of the arrow 14 into a parallel array of microporous flexible filter tubes indicated schematically by the reference numeral 16. A return line 22 extends from the downstream ends of the filter tubes and includes a back-pressure valve 47. The valve 47 is either closed completely, in which case the apparatus operates in a dead-end mode, or is maintained slightly open to permit low velocity flow through the tubes. Instead of a partially open valve, a flow restricting orifice can be provided. It is essential during cake deposition that the slurry be pumped into the tube array under laminar flow conditions. The flow velocity is therefore kept extremely low, possibly as low as 0,01 to 0,05 m/s. The pressure of the slurry in the tubes 16 is maintained at a high level by the action of the valve 47 and is typically in the range 50 to 1000kPa. Filtrate permeates outwardly through the walls of the filter tubes as indicated by the arrows 18 in Figure 1. The solids accumulate on the internal surfaces of the tubes 16 to form a cake layer 20. It is important that the cake layer be kept as even as possible as it builds up. In general, the cake thickness will be fairly uniform throughout because of the low pressure gradient in the tubes 16 resulting from the low velocity flow. Even if a section of a tube does develop relatively thin cake layer, the permeability will be greater in that section and the cake deposition will accordingly be higher to rectify the situation and restore even cake distribution. Cake evenness can be promoted further by periodically opening the valve 47 fully for short bursts, typically for 5 seconds every 5 to 10 minutes. This technique, which is referred to subsequently in this specification as "blow-down", has the effect of preventing excessive cake build-up at the upstream end of the tubes, of preventing more build-up from taking place in some tubes than in others and of evening out the cake distribution. The blow-down technique is used in dead-end operation of the apparatus i.e. during operation when the valve 47 is normally completely closed. During blow-down the velocity of the slurry is typically increased to 0,5 to 2m/s. Alternatively, in a case where the valve 47 is kept slightly open or a restrictive orifice is provided, the continuous, low velocity circulation of slurry via the return line 22 (as illustrated in Figure 1) is sufficient to promote even cake deposition. Once again, the effect of low velocity, laminar flow is to prevent excessive cake build-up at the upstream end of the tubes. As stated above, it is important that cake deposition be as even as possible. Uneven cake deposition can result in the formation of solid plugs which can be difficult to dislodge. The thickness of the cake during the cake deposition stage of Figure 1 is continuously monitored, and the deposition stage is usually terminated when the cake thickness reaches about 25% of the internal diameter of the tubes. A simple technique for monitoring the cake thickness involves monitoring the filtration rate, the assumption being that there is an approximate relationship between cake thickness and filtration rate. The filtration rate can, for instance, be monitored by observing the rate of fall of the slurry in the storage tank using level sensors or the like. Once the cake deposition stage has been terminated, the cake dislodgement and removal stage is commenced. Figures 2 to 5 each illustrate a different cake dislodgement and removal technique contemplated within the scope of the present invention. In each of the techniques, which are described in detail below, the pressure in the tubes is reduced to between 10 and 100 kPa and the flow velocity is increased to between 1 and 3 m/s, the exact parameters of pressure and velocity which are used being dependent on the particular dislodgement technique, the particular slurry being dewatered and on the nature of the cake particles which are produced during the dislodgement stage. Figure 2 shows a technique which makes use of high pressure fluid sprays 24 which are directed against the exterior surfaces of the tubes in the array. The spray nozzles are carried by a header (not shown) which moves along the length of the tubes in the direction of bulk flow. Although less preferred it is also possible to move the header in the direction opposite to that of the bulk flow. In one embodiment, the sprays are water sprays having a spray angle of 60° to 80° and a nozzle aperture of 1mm and are operated at between 1 to 5 MPa and a water flow rate of 2 to 3 1/min at a distance of approximately 10mm from the tubes. In the Figure 2 case, the internal tube pressure should be low, typically about 10kPa. The fluid sprays have the effect of deforming the tubes to some extent to dislodge the accumulated cake from the internal tube surfaces. The dislodged cake, which is dislodged in the form of small flakes, is conveyed downstream by the bulk slurry flow in the tubes. Figure 3 shows a technique in which the cake is dislodged using opposing rollers 26 the spacing between which is chosen to pinch the tubes almost closed. The result is a localised zone of high flow velocity in the pinched zone and deformation of the tube in the region at and downstream of the rollers. The venturi effect created by the pinched zone gives rise to a low pressure in the tubes just downstream of the rollers. The cake is sucked off the internal surfaces of the tubes and is conveyed downstream by the bulk flow. The pressure reduction at the pinched zone may be sufficient to suck ambient air inwardly through the tube walls to assist in dislodging the cake. Even if the roller spacing is not small enough to create a significant pressure reduction in the tubes, the local tube deformation and consequential increased flow velocity can be sufficient to cause cake dislodgement. Note that the rollers need not be round and that pinching of the tubes can be achieved by other non-rolling bodies applied to opposite sides of the tubes. The technique illustrated in Figure 4 differs from that of Figure 3 in that it is not the bulk slurry flow which transports the dislodged cake downstream in the tubes, but flow of process filtrate which is pumped from a tank 48 by means of a pump 50. Note that any other suitable liquid could be used to transport the dislodged cake instead of the process filtrate, the creation of which is described in more detail below. In Figure 5, there is again a zone of reduced pressure which is created in this case by means of a bobbin 52 which is fed on a wire 54 through each tube in the array in the bulk flow direction. The bobbin has a diameter which is slightly less than the internal tube diameter. The reduction in cross-sectional area available to flow results in a velocity increase and a pressure reduction. In the illustrated case, the tubes are flexible tubes which deform slightly just downstream of the bobbin. The combined effects of tube deformation and pressure reduction dislodge the cake from the tube walls for conveyance downstream by the bulk flow. It will be appreciated that the technique of Figure 5 can also be used in cases where the tubes are not flexible, the bobbin in this case still causing a pressure reduction which will dislodge the cake. Accordingly, the technique of Figure 5 is also applicable to dislodgement of cake from rigid, perforated tubes. Note that the technique of Figure 2 can be used simultaneously with the other techniques described above for cake dislodgement. An important advantage of the invention is the fact that the cake is dislodged from the tube walls in the form of fine flakes as opposed to a solid plug. In a typical application of the invention, the flakes will have dimensions of 3 x 10 x 5 mm, ,although this will of course depend on the actual properties of the suspended solids in the slurry. The fact that the cake is dislodged in flake form facilitates drying of the cake after the cake capture stage. Cake which is dislodged from the tube walls and removed from the tubes is captured during the cake capture stage of the process. Figures 2 and 3 show how the bulk flow is returned during this stage along a line 28 to a screening device 30 which includes a mesh screen 32. The screen 32 is located above the feed tank and the dewatered cake collects on top of the screen while filtrate passes through into the feed tank. The mesh aperture may typically be about 1 mm. Figure 4 illustrates an alternative cake collection system in which there is a perforated endless conveyor belt 56 passing around rollers 58. The bulk flow with entrained cake is deposited onto the belt and the filtrate passes through into the tank 48 referred to previously. The dewatered cake itself is conveyed by the conveyor to a suitable discharge and collection point. Other cake capture systems which are not illustrated may involve the use of hydrocyclones, centrifuges or gravity settling tanks. Reference is now made to Figures 6 to 8 which illustrate a typical array of flexible tubes. Sheets 34 and 36 of flexible filter material are seamed by sewing, bonding or weaving at spaced locations 38. The flexilibity of the sheets enables them, when filled with slurry during operation, to assume the shapes seen in Figure 7 defining an array or curtain of parallel, flexible tubes 40 of circular cross-section. Figure 8 shows how the array or curtain of tubes 40 can be suspended vertically from a horizontal rail 42 supported at its ends by a fixed structure (not shown). Figure 8 also shows upstream and downstream manifolds 44 and 46 are connected to the tube ends. Typically the tubes will have a diameter between 20 and 40mm and it should be noted that the tube array can be formed by securing a series of independent tubes to one another instead seaming flat sheets 34 and 36 as shown in Figure 6. It should also be noted that the array need not necessarily hang vertically as in Figure 8, other orientations also being possible. Furthermore, the invention is also applicable to single tubes which are not combined with other tubes in an array. Any one of a wide variety of different natural and synthetic materials can be used for the tubes or sheets 34 and 36. The process and apparatus described above can be used in a wide variety of applications such as the dewatering of aluminium hydroxide, lime, polyelectrolyte and polyelectrolyte/bentonite slurries resulting from the production of potable water and kaolinite slurries. It is also believed that the process and apparatus could be used to good effect in the dewatering of metallurgical slurries resulting from mining and mineral recovery operations. It will however be appreciated that the invention is limited in its application to slurries in whch the suspended solids have cohesive properties. Solids such as beach sand, which have no cohesive properties, would not form a cake on the tube walls. EXAMPLE 1 In this example a prototype plant was used to dewater a slurry composed of polyelectrolyte/bentonite/surface water and organic matter. The tube array was formed of polyester multi-filament sheets seamed together with 8,5mm wide seams to form twenty tubes of length 10m and diameter 25mm. The total filtration area per array was 15,7m². The blow-down technique was used during the deposition stage. During the cake dislodgement or removal stage, either rollers or water sprays were used. The dislodged cake was conveyed in the tubes by process filtrate as indicated in Figure 4 , the cake being captured on a perforated belt in the manner depicted in that Figure. The results of five separate tests are tabulated in Table 1 below. The example illustrates that a high throughput was obtained and that cake dislodgement and removal from the internal tube walls was successful. EXAMPLE 2 In this example a prototype plant was constructed for dewatering an aluminium hydroxide/polyelectrolyte river silt slurry resulting from the treatment of surface waters to produce potable water. The filter array was identical to that used in the first example. Control of cake evenness during the cake deposition stage was controlled by recircling slurry in the manner depicted in Figure 1. Cake dislodgement was achieved using a double roller device comprising a first pair of rollers and, slightly downstream of the first pair, a second pair of rollers. The rollers of the first pair had a diameter of 33,4 mm while those of the second pair had a diameter of 21,3 mm. The internal spacing created by the rollers was 2 mm in the case of the first pair and 5 mm in the case of the second pair. The cake was captured using a 1 mm square mesh screen. With a feed slurry concentration of 15g/l, a slurry recycling velocity of between 0,05 and 0,1m/s and a cake conveyance velocity of 1,5m/s, good cake distribution and good cake dislodgement and removal were achieved after the cake had attained a thickness of up to 5 mm. Besides the fact that the captured cake is in flaky form, the invention leads to other advantages when compared with the conventional system referred to previously in which a "sausage" or solid plug of cake is formed in the tubes. With the invention, the cake layer which is formed is relatively thin and therefore permits high filtration rates. In the known system, the solid plug detrimentally affects the filtration rate. Furthermore the plug which is formed in the known system can be extremely difficult to remove from the tube while with the invention cake dislodgement is achieved with relatively simple equipment. Also, the cake deposition controls proposed by the present invention are simple in nature and do not require involved manifolds or other pipework.

Claims: 1. A slurry dewatering process for dewatering a slurry composed of a suspension of solid particles having cohesive properties which is characterized in that it includes a cake deposition stage and a cake dislodgement and removal stage, wherein the cake deposition stage comprises the steps of: pumping the slurry (10) which is to be dewatered into a porous tube (16) at a sufficiently low velocity for laminar flow conditions to prevail in the tube and at a sufficiently high pressure for liquid in the slurry (10) to permeate outwardly (18) through the tube and for solids in the slurry to be deposited on the internal tube surface to form a cohesive annular layer of cake (20) on that internal surface the rate of outward permeation of liquid decreasing continuously with time as the annular layer (20) is deposited, and controlling the deposition of the solids so that the cake layer (20) which is formed is even or nearly even over the length of the tube (16), and wherein the cake dislodgement and removal stage comprises the steps of: decreasing the pressure and increasing the flow velocity in the tube (16) when a predetermined cake thickness has been deposited on the internal tube surface, dislodging the cake from the internal tube surface in the form of flakes composed of discrete particles cohering together by creating a localized zone of further reduced pressure within the tube sufficient to separate the cake (20) from the internal surface of the tube (16), causing that localized zone to move along the length of the tube, and permitting flow through the tube to convey the dislodged flakes downstream and out of the tube. 2. The method claimed in claim 1 characterized in that the tube is flexible and the cake is dislodged from the internal surface of the tube in flake form by applying a local inward deformation to the tube, thereby to create a localized venturi effect therein, and causing or allowing the local inward deformation to move along the length of the tube. 3. The method claimed in claim 2 characterized in that the local inward deformation is applied to the tube by means of external liquid sprays (24) directed locally against the exterior surface of the tube. 4. The method claimed in claim 2 characterized in that the local deformation is applied to the tube by means of external pinching means (26) forced locally against the exterior surface of the tube. 5. The method claimed in claim 1 characterized in that the tube is flexible or rigid and the cake is dislodged from the internal surface of the tube in flake form by causing or allowing a bobbin (52) smaller in diameter than the tube to pass along the length of the tube, thereby creating a localized venturi effect in the vicinity of the bobbin. 6. A tubular filter press apparatus specifically designed for carrying-out the process of claim 1 said apparatus including at least one porous tube (16), a pump (12)for pumping slurry (10) into tube (16) and a back pressure valve (47) downstream of tube (16) characterised in that, in order to dewater a slurry wherein the solid particles have cohesive properties, (A) control means are incorporated which regulate pump (12) and back pressure valve (47) such that (i) slurry (10) initially enters tube (16)at sufficiently low velocity for laminar flow conditions to prevail and at sufficiently high pressure for liquid in the slurry to permeate outwardly through the tube and for solids in the slurry to be deposited on the internal tube surface to form thereon an annular layer of cohesive cake (20) which is even or nearly even throughout, and thereafter such that (ii) once a predetermined thickness of cake has been deposited, slurry (10) enters the tube (16) at decreased pressure and increased velocity; and (B) there are incorporated means (24, 26, 52) for creating within the tube during said step (ii) a zone of further reduced pressure sufficient in use to dislodge deposited cake from the internal surface of the tube in the form of flakes and means (54) adapted to move that localized zone along the length of the tube, whereby in use dislodged cake is conveyed downstream and out of the tube by the slurry flow through the tube. 7. The filter press apparatus claimed in claim 6 characterized in that the means which create a localized zone of further reduced pressure within the tube comprise means (24, 26) for applying a local, inward deformation to the tube. 8. The filter press apparatus claimed in claim 7 characterized in that the tube is flexible and the means for applying a local, inward deformation to the tube comprises external liquid sprays (24) directed against the external surface of the tube. 9. The filter press apparatus claimed in claim 7 characterized in that the tube is flexible and the means for applying a local, inward deformation to the tube comprises means (26) for applying a localized pinching action to the tube. 10. The filter press apparatus claimed in claim 6 characterized in that the means which create a localized zone of further reduced pressure within the tube comprise a bobbin (52) having an external diameter less than the internal diameter of the tube, the bobbin being passable along the length of the tube.

Inventors: BUCKLEY, CHRISTOPHER ANDREW, TREFFRY-GOATLEY, KEVIN
Applicants: THE WATER RESEARCH COMMISSION
Application year: 1987
Main classification: B01D29
IPC codes: B01D29, B01D37, B01D35, C02F11