Abstract:
In a slurry dewatering process, slurry is pumped into a porous tube at a sufficiently low velocity for laminar flow conditions 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 predetermined 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:
BACKGROUND TO THE INVENTION 
     This invention relates to a slurry dewatering process and apparatus which uses a tubular filter press. 
     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 &#34;extruded&#34; from the tube in the form of an elongate &#34;sausage&#34; under the effects of pressure exerted on it by incoming slurry. Because of its highly compact nature, the &#34;sausage&#34; cannot easily be dried further and it is usually necessary to break up the &#34;sausage&#34; to increase its specific surface area. This is clearly not desirable. 
     It is an object of this invention to provide a slurry dewatering process and apparatus which has a high throughput and which has the facility for easy removal and subsequent drying of accumulated solids. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a slurry dewatering process which includes a cake deposition stage and a cake dislodgement and removal stage, wherein the cake deposition stage comprises the steps of: pumping 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 an annular layer of cake on that internal surface, and controlling the deposition of the solids so that the cake layer which is formed is even or nearly even thoughout; 
     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 discrete particles and conveying the dislodged cake downstream in the tube. 
     A second aspect of the invention provides a tubular filter press apparatus for dewatering slurries which includes at least one porous tube, means for introducing slurry which is to be dewatered into the tube, means for controlling the slurry flow velocity in the tube during a cake deposition stage such that laminar flow conditions prevail and for establishing a sufficiently high pressure in the tube 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 an annular layer of cake on that surface, means for controlling the deposition of solids on the internal tube surface so that the cake layer is even or nearly even throughout, means for decreasing the pressure in the tube and for increasing the velocity of flow in the tube once a predetermined thickness of cake has been deposited on the internal tube surface, and means for dislodging the cake from the internal tube surface in the form of discrete particles, the dislodged cake then being conveyed downstream in the tube by the increased velocity flow. 
     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 drawings in which: 
     FIG. 1 shows an apparatus according to the invention in the cake formation mode; 
     FIG. 2 shows cake removal by spray cleaning; 
     FIG. 3 shows cake removal by external rollers; 
     FIG. 4 shows a system for cake capture; 
     FIG. 5 shows cake removal using an internal bobbin; 
     FIGS. 6 and 7 show a filter tube array in inoperative and operative conditions respectively; and 
     FIG. 8 shows how the array of FIGS. 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. FIG. 1 illustrates cake deposition. 
     Referring to FIG. 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 1000 kPa. 
     Filtrate permeates outwardly through the walls of the filter tubes as indicated by the arrows 18 in FIG. 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 &#34;blow-down&#34;, 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 2 m/s. 
     Alternatively, in a case where the valve 48 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 FIG. 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 FIG. 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. FIGS. 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. 
     FIG. 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 1 mm and are operated at between 1 to 5 MPa and a water flow rate of 2 to 3 l/min at a distance of approximately 10 mm from the tubes. 
     In the FIG. 2 case, the internal tube pressure should be low, typically about 10 kPa. 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. 
     FIG. 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 disloding 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 FIG. 4 differs from that of FIG. 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 FIG. 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 FIG. 5 can also be used in cases where the tubes are not flexible, the bobbin in this case stil causing a pressure reduction which will dislodge the cake. Accordingly, the technique of FIG. 5 is also applicable to dislodgement of cake from rigid, perforated tubes. 
     Yet another technique for dislodging the cake from flexible tube walls involves rapid pulsing of the flow in the tubes. When the tubes are subjected to full flow, they have a circular shape as is explained below with reference to FIGS. 6 and 7. However, if the flow is stopped or reduced, the tubes will deform under the weight of the slurry which they contain and this deformation will dislodge cake from the internal tube walls for downstream conveyance in the bulk slurry flow. 
     Note that the technique of FIG. 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 3×10×5 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. FIGS. 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. 
     FIG. 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 FIGS. 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 flexibility of the sheets enables them, when filled with slurry during operation, to assume the shapes seen in FIG. 7 defining an array or curtain of parallel, flexible tubes 40 of circular cross-section. FIG. 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). FIG. 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 40 mm 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 FIG. 6. It should also be noted that the array need not necessarily hang vertically is in FIG. 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 which 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 8.5 mm wide seams to form twenty tubes of length 10 m and diameter 25 mm. The total filtration area per array was 15.7 m 2 . 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 FIG. 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. 
     
         __________________________________________________________________________             Test No.Parameter    Units             1   2   3   4   5__________________________________________________________________________(a)  Feed slurry        g/l  13,9                 43,2                     53,6                         6,4 10,0  suspended solids(b)  Filtrate suspended        g/l  --  Less                     Less                         0,13                             0,10  solids              than                     than                 0,05                     0,05(c)  Cake total solids        mass %             30,5                 35,7                     38,8                         29,9                             30,0  (dry solids/wet cake)(d)  Operation during  cake deposition:  Blow-down operation             Used                 Used                     Used                         Used                             Used  Frequency  min. 5   6   5   8   5  Duration   s    20  10  10  30  10(e)  Filtration rate:  Start batch        l/m.sup.2 h             113 --  --  278 300  End batch  l/m.sup.2 h             67  69  38  89  43  Average  Volume processed        l    669 223 332 1580                             1800  Batch time min. 30  12  22  55  47  Operating pressure        kPa. 500 500 450 400 450(f)  Cake dislodging and  conveying:1.  Roller        Used                 Used                     Used                         Used                             Not used    Roller outside    diameter mm   21,3                 21,3                     21,3                         21,3                             --    Internal tube        mm   2,6 2,6 3,1 1,9 --2.  Water Spray   Not Not Not Not Used             used                 used                     used                         used    Nozzle orifice        mm   --  --  --  --  0,9    diameter    Operating        MPa  --  --  --  --  3,5    pressure    Spray angle        deg. --  --  --  --  80    Conveying fluid        l/s/tube             0,75                 0,75                     0,75                         1,0 0,75    rate                          (Pulsed)    Cleaning head        m/min             5   5   5   6,5 5    traverse rate(g)  Cake distribution             Near                 Near                     Near                         Near                             Near  results         even                 even                     even                         even                             even             cake                 cake                     cake                         cake                             cake             distri-                 distri-                     distri-                         distri-                             distri-             bution                 bution                     bution                         bution                             bution(h)  Cake thickness        mm   2 to 3                 2 to 3                     3 to 5                         2 to 3                             2 to 3(i)  Cake dislodging Com-                 Com-                     Com-                         Com-                             Com-  and removal     plete                 plete                     plete                         plete                             plete__________________________________________________________________________ 
    
     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 FIG. 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 15 g/l, a slurry recycling velocity of between 0.05 and 0.1 m/s and a cake conveyance velocity of 1.5 m/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 &#34;sausage&#34; 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.