Clarification apparatus

Abstract: Liquor-clarifying apparatus comprises a tank 1, inlets 2 for suspension to be clarified, outlets 3 for clarified water, outlet 4 for sedimented solids, and a plurality of settling assemblies each comprising a stack 6 of downwardly inclined plates 7 mounted around a vertical collection duct 39. This duct generally comprises a downwardly extending duct 18 that leads directly into an upwardly extending duct 27. The plates 6 are preferably conical or pyramidal and are preferably self-spacing and capable of being clipped together by a push fit. A downwardly extending skirt 40 may be provided. Orifices 25 allow clarified liquid to pass from between the plates into the collection duct and preferably these orifices are elongated.

Description: The present invention relates to clarification apparatus for increasing the rate of gravitational separation of particulate solids from an aqueous or other liquid suspension. Such clarification apparatus is used in a range of industrial applications where large volumes of water must be clarified for either disposal or recirculation. When only limited space is available it is very desirable for the apparatus to be compact. In many industrial processes, sedimentation occurs in large tanks, with the clarified liquor being taken from the top and the sediment from the bottom. Such tanks occupy a large horizontal surface area and it is well known to reduce the required horizontal surface area of the tanks, per unit throughput, by dividing the tank into a plurality of sedimentation cells, each cell being defined by a pair of inclined, substantially parallel, adjacent settling plates. The solids flow down the upper surface of the lower plate in each pair. Generally a plurality of the plates are arranged vertically as a stack, and each stack of plates and its associated collection apparatus can be regarded as an assembly within the overall apparatus. In some systems the clarified liquor flows along the lower surface of the uppermost plate and into a collection duct at the end of the plates, e.g., as in GB-A-2,150,039. In GB-A-2,170,419 the suspension is fed into the centre of a series of open cones and both the clarified liquor and the sediment move downwardly cocurrently between adjacent pair of cones and it is proposed that both the sediment and the clarified liquor can flow downwardly to the bottom of a vessel from which the clarified liquor is then taken upwardly through a riser pipe while the sediment is removed from the base of the tank. Unfortunately there is a very high risk of separated solids being resuspended into the clarified liquor. In order to minimise this risk it is well known to arrange for the solids and the clarified liquor to move counter-current in each cell with the solids sedimenting from the lower part of each adjacent pair of plates and the clarified liquor rising to the upper part of each pair, and being taken off from this upper part. In general terms, liquor-clarifying apparatus of this general type comprises a tank having at least one inlet for suspension to be clarified, at least one first outlet for the clarified liquor, at least one second outlet for separated solids, a plurality of settling assemblies, and collector means for collecting clarified liquor within the settling assemblies and for leading it to the first outlet or outlets, and in this apparatus each settling assembly comprises a stack of inclined substantially parallel settling plates positioned in use to discharge the separated solids from the lower part of each plate downwardly towards the second outlet or outlets, a substantially vertical collection duct which is part of the collector means and which extends up through the stack, and orifices opening into the duct from the top of each cell defined by each pair of adjacent plates for passage of clarified water from each cell into the duct. Although the principle is very well known, and although many alternative constructions have been described in the literature, in practice they have proved useful only for a very limited number of commercial applications. One problem is that it is necessary to provide a large number of the parallel plates, if the apparatus is to have a large throughput, and this has led to very complex piping and engineering difficulties which has rendered the systems relatively uneconomic where space is very limited. Another, and potentially fatal, problem has been the tendency for blockages to occur, especially in the collectors for removing the clarified water. In GB-A-5340/1891 various settling arrangements are shown and in Figure 5 of this document apparatus is shown comprising a tank containing a stack of individually mounted, downwardly inclined, plates that appear to be conical and that are arranged whereby clarified water is collected upwardly between each pair of plates and is removed upwardly from the tank while sludge collects in the base of the tank. In GB-A-635591 and US-A-2570304 various arrangements of inclined plates are described and, in Figure 5, a tank is shown containing a plurality of stacks of conical plates individually mounted on a central duct provided with orifices, whereby clarified water can be collected upwardly and removed through piping from the top of the apparatus. Spacing supports can be provided on the inclined surfaces. The collector means can include horizontal pipework leading to outlets from the tank. It is mentioned in US-A-2570304 that the plates can be pyramidal instead of conical. In US-A-3718257 is it admitted that this apparatus has not provided acceptable dewatering of the sludge and is proposed to put a secondary unit beneath the main unit, for dewatering the sludge. The drawings show the provision of a very large number of cylindrical stacks, each with its associated pipe for the collection of clarified water from the top. In GB-A-879349 inclined plates are welded to an upflow collector duct arranged in a tank. In one embodiment the plates are conical. In another embodiment the tank is rectangular and the plates are flat roofs welded to a central collecting duct. In GB-A-1064918 and US-A-3306456 stacks of plates are provided wherein each plate has an open neck portion and the plate is inclined downwardly from that neck portion in an arrangement which is, overall, generally conical but which includes a complex configuration that includes, inter alia, a downwardly extending step in the inclined portion at a position radially outwards from the radius of the neck portion, whereby engagement of these steps in adjacent plates cause them to be self-spacing. They are mounted on a duct containing orifices, whereby clarified water is collected downwardly in the duct. Another arrangement of individually mounted conical plates is shown in US-A-2793186. Although conical collectors are described in most of those specifications, it is also well known (for instance US-A-3951818 and 4747196 and WO-A-84/01724) to provide a stack of elongated chrevons having collector means including horizontal collectors by which clarified liquor is removed horizontally between the plates and is fed into a vertical collection duct. The known constructions have not proved satisfactory partly because, in practice, unwanted sedimentation occurs at one or more places in the apparatus so as to prevent its efficient operation and partly because of the complexity of the apparatus. The construction of most of the apparatus mentioned above is extremely complex and does not lend itself to convenient manufacture and easy access to all the component parts. GB-A-1,064,918 mentions that the cones in that can be self-spacing but the cones in that are of very complex shape and are arranged to discharge the sludge through small ports instead of discharging off the bottom edge of the cones. U.S.-A-4,747,196 describes a system of clipping together the horizontal and upflow collectors and of resting the elongated chevron plates on the horizontals. Neither of these proposals provides a convenient way of constructing and assembling conical or pyramidal collectors of simple shape. It would therefore be desirable to provide a clarification apparatus that is less liable to blockage by sedimentation, especially in the collector means for carrying the clarified liquid away from the stacks, and to provide apparatus that can be of very simple construction and yet can have a very large number of collector plates, arranged in a plurality of stacks. It would also be desirable to provide such apparatus that provides improved clarification at a given rate of throughput, or improved throughput at a given rate of clarification, than is obtainable using present apparatus of similar volume. In particular it would be desirable to provide such apparatus in which the settling assemblies can easily be installed in a tank that is already provided with the necessary inlets and outlets, since this facilitates the provision of the apparatus in confined spaces, such as underground. The invention provides novel clarifying apparatus that comprises a plurality of settling assemblies, novel settling assemblies for incorporation in this clarifying apparatus, a novel method of clarifying a suspension, and novel settling members that can be used in the novel settling and clarifying apparatus. A settling assembly according to the invention can be fitted substantially vertically in a clarifier tank for clarifying a liquid suspension into clarified liquid and separated solids, and this assembly comprises a stack of inclined substantially parallel settling members each of which has a neck portion and each adjacent pair of which defines a substantially parallel sided cell, a substantially vertical collection duct extending through the neck portions, and orifices distributed around and opening into the duct from the top of each cell for passage of clarified liquid from each cell into the duct, and in this apparatus each settling member has the form of a hollow truncated 4-sided pyramid extending from the neck portion and the members in each pair are spaced apart by spacer means on the neck portion of at least one of the members of the pair or fitted between the neck portions of the members. Liquor clarifying apparatus according to the invention comprises a tank, at least one inlet for suspension to be clarified, at least one first outlet for clarified liquor, at least one second outlet for separated solids and a plurality of settling assemblies as defined above. Each stack can be assembled on a central collection duct by alternating plates and spacers that fit between the neck portions of adjacent members, or by sliding over the duct members that have spacers integral with their neck portions. Thus each member may have an integral spacer extending upwardly (or downwardly) or some of the plates may have such spacers extending upwardly and downwardly while the other members have no spacers integral with the neck. These spacers take the form of substantially vertical necks on the uppermost edges of the members. The assembly may therefore be formed merely by sliding the members, and if necessary the spacers, over the central duct. A novel settling member for use in the invention comprise a hollow, truncated 4-sided pyramid the inclined surfaces of which extend down from a neck portion, a neck that extends vertically from the neck portion, and orifices through the necks. The settling members are referred to below as plates. The substantially vertical collection duct can be a preformed pipe or other duct on to which the plates are fitted, e.g., as a tight sliding fit. It is particularly convenient for the substantially vertical collection duct to be defined by the integral vertical necks on the upper edges of the plates since it is then unnecessary to provide a preformed duct. Thus it is preferred that the stack is assembled by sliding the neck portions of the plates and the spacer means over the duct or by forming the duct by interengagment of spacer means that are vertical necks on the plates, and thereby self-spacing the plates in the stack. Preferably the vertical necks are provided with push inter-engaging fit means for securing the plates one above the other by push inter-engagement of the fit means, i.e., merely by pushing them together. It is often preferred for the inter-engaged pyramids to be maintained under vertical compression so as to minimise the risk of them separating from one another. When the upward duct extends through the length of the stack convenient means for providing this vertical compression comprises the upwardly extending duct being provided with means for exerting vertical compression on the stack. Suitable means is a nut threaded externally on to the lower or upper end of the duct. The assembly of the invention can be made of any corrosion resistant material, for instance corrosion resistant metal or, preferably, plastics material. When the stack is merely fitted together by push inter-engagement it is particularly preferred that the plastics material should have a density less than the density of the fluid (and therefore generally below 1) so as to provide vertical compresion on the stack and to eliminate any tendency for the plates to sink. Pyramid shaped plates in the invention generally have an inclined wall length (ie the linear dimension up the inclined part) of 5-30, often 10-20, cm. A preferred width dimension at the lowermost edge of the pyramid is about 15cm. They can be vertically spaced, e.g., about 1cm apart but the spacing is preferably greater, e.g., 2 to 10cm, often about 5cm. As explained below, the lowermost edges of the plates (or the lowermost edge of the lower plate and the lowermost edge of a skirt from the upper plate) preferably define a horizontal open slot and the vertical dimension of the slot is usually at least 1cm, ofter 2-5cm. Each pair of adjacent plates defines an inclined clarification cell and particles settle out on the upper surface of the lower plate and are discharged downwards from the lowermost part of that cell. The angle of the plates (and therefore of each cell) to the horizontal is usually in the range 35 to 75°, often 50 to 70°, preferably about 60°. The suspension that is to be clarified enters the tank via the inlet or inlets, which is or are usually at or near the top of the tank. The amount of gravitational settlement of the particulate solids material towards the second outlet is increased by the large settlement area provided by the plurality of settling plates. The suspension that is to be clarified enters from the tank into each cell at the lowermost part of the cell and thus flows countercurrent to the solids that are discharging downwardly from that part. There may be some degree of entrainment of the solids that are discharging from the upper cells as they pass across the lowermost parts of the lower cells, by suspension that is flowing into the lower cells. However this is generally acceptable. In order to minimise the entrainment of solids by the suspension as they discharge from the lower parts of the cells down towards the second outlet it is preferred that the stacks should be arranged in such a way as to allow free fall of solids from the outer edges of each stack down to the second outlet for the separated solids. This second outlet is generally in the base of the tank. Typically therefore there must be an adequate free space between each adjacent stack to allow substantially free fall of solids and in practice this generally necessitates that there should be a free space of at least 2cm, and usually 4 to 10cm, between the closest portions of each stack. These dimensions are appropriate when the inclined distance between the upper and lower parts of each cell is, as is usually preferred, not more than about 25cm, typically 5 to 20cm and when the vertical spacing between the plates is typically 0.5 to 10cm, often around 5cm, since these dimensions will then provide a sufficient quiescent zone for the removal of the volumes of solids that are likely to discharge from each cell. However if the dimensions of the cell are considerably greater, and in particular if the inclined distance between the upper and lower parts is much greater, it may be necessary to provide a larger free space between each adjacent stack. Each pair of inclined settling plates defines a downwardly inclined, substantially parallel-sided, cell having a horizontal open slot defined by the lowermost edges of the plates. Despite the general desirability of avoiding intermixing of the discharging solids with inflowing suspension that is to be clarified, it is desirable to arrange the plates so as to cause the suspension to enter each cell at its lowest possible level upwardly through the blanket of solids settling within the cell. This promotes upflow clarification and entrapment of particles and of microflocs of particles in a manner similar to a conventional sludge blanket effect. Preferably therefore each plate has a skirt that extends downwardly from its lowermost edge so as to define with the lowermost edge of the lower plate a horizontal open slot situated at substantially the lowest possible level of the cell. For instance if the vertical spacing between the inclined plates is Xmm, the vertical dimension of the slot that is defined between the bottom of the skirt on the upper plate and the lowermost edge of the lower plate is preferably less than 3/4 of X and is most preferably around 1/4 to 2/3 X, generally around ½ X. The positioning of this slot means that the suspension that is to be clarified tends to be forced to flow into each cell at the lowest possible level and hence to rise upwardly through the solids already contained in the cell to result in improved clarification and separation of solids. Each cell must be provided with one or more orifices for removing clarified water from the space contained between each pair of adjacent plates, i.e., the upper part of each separation cell. Thus each cell is not open to the tank at its upper part but the orifices that lead into the substantially vertical collection duct provide the only route for the clarified water out from the upper part of the cell. Since the orifices lead from the cells direct into a substantially vertical collection duct this minimises the risk of blockage. Since the plates are pyramidal the substantially vertical collection duct is a central column surrounded by the plates and provided with orifices through its walls to allow the entry into the duct of clarified liquid from each pyramidal cell. Provided the substantially vertical collection ducts and stacks are sufficiently upright for effective upflow, the exact angle of them is not critical. However it is convenient, for constructional reasons, for each to be as close to the true vertical as is conveniently possible. The orifices for removing clarified liquid from each cell may be conventional and thus they may be spaced apart circular orifices which may all be of the same diameter and may provide the same orifice area at each position in the stack, or the orifice area may vary along the height of the stack. For instance the orifices may be smallest near the base of the stack so that the orifice area is lowest near the base of the stack. A problem with conventional orifices is that each orifice has a small cross section area relative to the cross section area of the adjacent part of the cell from which the orifice is collecting clarified liquid. For instance even if several orifices are distributed around the collection duct the total cross section area of the orifices is generally less than one third, and often less than one tenth, of the cross section area of the cell at that point, i.e., of the periphery of the duct. Accordingly the clarified liquor has to flow from the much wider collection area of the cell into this very narrow orifice area, and this both promotes turbulence and resuspension of settled solids close to the orifice and their redeposition in the collector duct into which each orifice opens. These disadvantages can be eliminated if the orifices have a greater horizontal length than usual. According to a preferred aspect of the invention the orifices at each cell are distributed around the collection duct and have a horizontal orifice length at least half, generally at least two thirds and preferably at least three quarters of the horizontal length of the plates at the position of the orifices, i.e. at the neck portion. By referring to the horizontal orifice length we mean the horizontal length of the opening or openings through which the clarified liquid can pass. Conveniently the orifices comprise one or more substantially horizontal slots but, if desired, they can comprise a closely adjacent row of other orifices, for instance substantially circular orifices. Since the plates are pyramid shaped since it is possible to provide a slot (or a row of orifices) having a length that is a significant amount of the total horizontal length of each neck portion of the pyramid. This achieves particularly linear, non-turbulent flow towards the orifice and avoids redeposition in and blockage of the collector means. The collector means can extend downwardly and out from the bottom of the tank or they can extend upwardly and out from the top of the tank. However it is preferred that the collector means extend upwardly from the tank. When there is upflow, the rate of flow at the bottom of the upflow pipe is derived solely from liquid entering from the cell defined by the lowermost pair of plates, whilst the rate of flow at the top of the upflow pipe is derived from the entire stack of parallel plates, and so it is inevitable that the rate of flow at the top is very much greater than the rate of flow at the bottom. The clarified liquor often carries a small amount of suspended solids into the upflow pipe and the low flow rate at the base of the pipe results in these solids tending to form a sediment in the pipe, causing a blockage at the bottom of the pipe and, effectively, putting the lower pairs of plates out of action. This difficulty may be accentuated by the fact that the suspension tends to have a higher solids content at the bottom of the tank than at the top. It is therefore desirable to ensure that the take-off of clarified liquid from between each lower pair of plates is less than the take-off from between each higher pair of plates, where the solids content is less. This may be achieved by providing smaller orifices from the lower pairs of plates than from the higher pairs of plates. The reduced flow of clarified liquor through these smaller orifices accentuates still further the risk of sedimentation in the collection pipe. In the invention it is preferred that the substantially vertical collection duct should comprise a downwardly extending duct into which open the orifices from at least the lowermost pairs of plates in a stack, and an upwardly extending duct into which the downwardly extending duct leads directly. Thus clarified liquor from at least the lowermost cells flows downwardly through the downwardly extending duct and is then led directly into the upwardly extending duct and it flows upwardly through this either direct to a first outlet or to another part of the collecting means for transmission to a first outlet. Since the flow of liquid in the downwardly extending collection duct is downwards, the flow velocity will be higher at lower parts of the duct than at higher parts, since the lower parts will be carrying the clarified liquid from a greater number of cells, and thus the tendency for sedimentation in the duct is avoided. However this advantage would be lost if, as in the prior art downflow collectors, the collected clarified liquor was allowed to move into an area of slack flow at the base of the duct. Accordingly it is important that the downwardly extending duct leads directly into an upwardly extending outlet duct, in the sense that the fluid path between the downflow and upflow ducts is sufficiently short that slack flow of a type that is liable to lead to sedimentation in that path cannot occur. This is in contrast to, for instance, the chamber of GB-A-1,064,918 and the horizontal connection of GB-A-635,591, this horizontal connection inevitably having a considerable length if that apparatus was modified to have a plurality of stacks within a tank. Although it is usually sufficient for each of the downwardly and upwardly extending ducts to be of similar and uniform cross section area along their lengths if desired either or both can be made of variable cross section area. For instance the downwardly extending duct could be tapered or otherwise constricted at the lower end in order to accelerate the linear rate of flow at the lower end to reduce the risk of sedimentation or could be constricted at its upper end so as to control the amount of flow throughout its length. The upwardly extending outlet duct can be spaced apart from the downwardly extending collection duct (provided the connection between them does not permit slack flow, as discussed above), but preferably the ducts are substantially directly adjacent to one another. Preferably there is a common wall separating them and, in particular, it is preferred for the upwardly extending duct to be inside, and most preferably co-axial with, the downwardly extending duct. Thus the clarified liquid flows down the annular or other downwardly extending duct and at the bottom of the duct it reverses its flow and flows substantially immediately upwardly through the central or other upwardly extending duct. The downwardly extending duct must collect the clarified liquid at least at the lowermost part of the stack, since this is where blockage in the collection duct is most liable to occur. For instance it usually collects from the cells providing at least the bottom 10%, and often at least the bottom 25% of the depth of the stack. Frequently it collects from the entire stack in which event the upwardly extending duct does not also collect clarified liquid. If the downwardly extending duct extends over, and collects from, part only (e.g., not more than 50%) of the depth of the stack, the upwardly extending duct above the downward duct will serve as collector from the upper parts of the stack and so will have the orifices (or the horizontal collectors) leading into it from cells at positions above the downward duct. Thus a preferred assembly has a composite substantially vertical collection duct comprising a downwardly extending collection duct that opens at its base into an inner, substantially coaxial, upwardly extending duct and in which the orifices (or the horizontal collection pipes if the orifices are in these pipes) extend through the outer wall of the downwardly extending duct and, if this duct does not extend through the entire length of the stack, through the outer wall of the upwardly extending duct above the downwardly extending duct. The composite duct can be of uniform diameter and so it is possible to control the downwards or upwards flow rates by appropriate choice of the diameter of the inner duct. Two or more downwardly extending ducts may be positioned one above the other in the composite duct and may have differing internal diameters. In the liquor clarifying apparatus of the invention each stack is arranged substantially vertically and parallel to each adjacent stack. The tank can contain as few as, for instance, 4 stacks but preferably it contains at least 10, and often at least 40, stacks. Each stack generally extends from at or near the top of the tank downwards to the lower half of the tank. Often it terminates sufficiently above the second outlet to minimise accumulation of settled solids around the stack. For instance it normally extends through 50 to 90% of the distance from the top of the tank to the second outlet. Each stack often includes above 50, and often above 100, of the parallel plates. The use of pyramid shaped plates is advantageous since it permits a central collector means for each stack and also permit a high plate area in a tank without causing turbulence between stacks. Some or all of the upwardly extending outlet ducts in a tank may lead through other suitable components, such as a horizontal duct into which the upwardly extending ducts open, to a single first outlet from the tank. It is usually preferred for the open, upper outlet end of each upwardly extending duct to serve as one of the first outlets and there are as many first outlets as there are upwardly extending ducts. It is particularly convenient for each of the upwardly extending ducts to lead directly into a launder above the tank. This eliminates the need for horizontal pipe connections typical of much of the prior art and, instead, permits a very simple system for collecting the clarified liquor. If any sedimentation occurs within this collection launder, removal of the sediment can be achieved easily since the launder is situated above the tank and so is readily accessible. Thus, in preferred clarifying apparatus of the invention, there is at the top of the tank at least one launder into the base of which the first outlets lead and from which clarified liquid can be discharged, and each substantially vertical collecting duct discharges up into a first outlet. There may be a weir for controlling the depth of clarified liquid in the or each launder. The invention is useful for the separation of the liquid and solid phases of a wide variety of suspensions. The liquid is usually water and the solid may be an inorganic particulate material, for instance coal, or crushed mineral ore or fines, or a pigment. Alternatively, the solid can be organic, for instance sewage particles or a coagulum formed from humic substances. The solids content of the suspension typically is in the range 0.01% to 5%. The particles are preferably flocculated, for instance by the addition of a conventional flocculating agent before entry to the apparatus, so as to promote their separation from the water. The various aspects of the invention provide separating apparatus that can give good solids separation and clarification at a high throughput of suspension even though the space occupied by the apparatus can be very low, and in particular the invention provides apparatus that is very easy to construct and maintain and to keep clear of blockages. It is possible to increase the amount of suspension that can be clarified to any particular clarity by a factor of at least 5, and often 10 to 12 or more, relative to the amount that can be clarified in a tank of the same volume and base area but in the absence of the stacks. The invention is illustrated in the accompanying drawings in which Figure 1 shows a perspective view, part broken away, of an apparatus according to the invention; Figure 2 shows, in enlarged perspective, part of a stack of pyramids of the type shown in Figure 1; Figure 3 shows a modified forms of pyramid; Figure 4 shows, in exploded view, a composite collection duct of the type used in Figure 1; Figure 5 shows, in diagrammatic vertical cross-section, the assembled composite collection duct of Figure 4; Figures 6 and 7 are sections similar to Figure 5 but of different composite collection ducts. The apparatus comprises a tank 1, inlets 2 for suspension to be clarified, outlets 3 for clarified water and outlet 4 for sedimented solids from a sedimentation conical zone 5 positioned beneath the rectangular tank 1. Within the tank 1 there are a plurality of settling assemblies comprising a stack 6 of pyramids 7 mounted around a central composite duct 39. Each settling assembly is positioned such that clarified liquid flowing up through the duct through the centre of the assembly emerges through the corresponding outlet 3. The inlets 2 are elongate slots in the bases of troughs 9 in upper raised surfaces 10 of the tank. The troughs 9 open into a launder 12 and suspension to be clarified is pumped into this launder through pipe 8. It overflows from this launder along the troughs 9 and down through the inlets 2, into the chamber. Liquid that has been clarified and is passing up through the stack 6 emerges from the outlets 3 in launders 13 between the raised parts 10 and that are blanked off at 14 but that open into a launder 15 provided with an outlet pipe 16. A vertically adjustable weir 17 is positioned at the ends of the launders 13 so as to control the depth of clarified liquid held in the launders. Generally the weir 17 is adjusted to be, for instance, 0.5 to 5cm above the base of the launder 13 so as to hold clarified water to this depth in the launder, as this helps maintain uniform flow of clarified water up through the stacks 6. Each stack 6 consists of a plurality of settling plates 7. As shown in the drawings, these plates have the shape of pyramids Each pyramid may consist solely of outwardly inclined surfaces in which event each is preferably a tight sliding fit over a composite duct 39 that can be a downwardly extending duct 18 that is closed at the end 19, as shown in Figure 4. However the pyramids can have vertically extending necks 20 at their uppermost edges and these necks can slide over the duct 18 or they can inter-engage with each other so as to define the duct 18. The inter-engagement may be by means of, for instance, ribs 21 that clip over one another to hold the pyramids rigidly with respect to one another. Suspension flows down through the tank between the adjacent stacks and enters at the slot 22 between the lowermost edges of the lower part 23 of an adjacent pair of plates. The suspension flows upwardly between the inclined plates towards the upper part 24 and as it flows up between the plates solids tend to sediment on to the lower plate 7L while clarified liquor tends to rise up against the upper plate 7U. The cell between these plates is closed at its upper part 24 by a neck 20 or the pipe 18 but orifices 25 are provided in this upper part to allow the clarified water to escape from the cell into the central duct defined by the neck 20 or the pipe 18. Figure 3 shows a pyramidal duct of the same general construction as the ducts illustrated in Figure 2 except that a skirt 40 extends vertically downwards from the lowermost edge 41 of each of the pyramids. The vertical spacing between adjacent pyramids is defined by the vertical height of the neck 20 and the skirt 40 has a vertical dimension less than about 3/4 of the vertical spacing between the pyramids so as to ensure that the slot 23 that is defined between the lowermost edge 42 of the skirt and the lowermost edge of the next adjacent pyramid is at least about 1/4 of the vertical spacing between the pyramids. If the slot is too narrow then excessive mixing of the solids and the incoming suspension may occur. Preferably the vertical dimension of the skirt 40 is about half the vertical length of the neck 20. The top of the outer duct 18, or the duct defined by the necks 20 of the pyramids, is blanked off (for instance as at 19 in Figure 5) so that the flow through this duct has to be downwardly. If the duct is defined by pipe 18 then this is provided with orifices 26 in positions to match those of orifices 25. The area of the orifices can decrease downwardly, for instance the orifices at the top of Figure 4 are very much larger than the orifices at the bottom. As shown in Figure 5, the composite duct 39 is formed of a downflow collection pipe 18 or the vertical necks 20 and a co-axial, smaller, upflow duct 27 fitted in it. This duct is open at its lower end 28 so that the clarified water that flows downwardly along the annulus around duct 27 reverses its flow at the bottom 28 and flows upwardly through the duct 27 to the outlet 3, which is surrounded by a plate 19 that blanks off the top of the outer duct 18. As shown in Figure 2 to 4, the orifices are elongated slots extending over at least three quarters of the horizontal length of the plates at the positon of the orfices, i.e. at top part 24, thus promoting uniformity of flow. In Figure 6, the composite duct 39 comprises a short downflow duct 31, leading at 32 into an inner coaxial upflow duct that has a narrow portion 33 and that leads into an upflow collection portion 34. In Figure 7, the composite duct 39 has two short downflow ducts 35 and 36. The upflow duct consists of a narrow portion 37 leading into a wider portion 38 leading into a collection portion 34. This provides a particularly convenient way of obtaining different flow rates through different orifices even though the orifices are all of the same dimensions. Also, replacement of downflow ducts 31, or 35 and 36, by different ducts having different lengths or radii permits further variation in flow rates. Three or more short downflow ducts may be used if required. To install apparatus according to the invention it is preferred to provide in the desired place of use (for instance in a mine) the tank 1 provided with the necessary inlets and outlets 2 to 5, and subsequently to install in the tank a plurality of settling assemblies, each consisting of a stack 6 of the settling plates mounted around a vertical collection duct. The vertical collection duct may be fitted previously in the chamber, in which event the plates are merely fitted over it, but it is often preferred that an assembly comprising part or all of the vertical collection duct and the stack should be preassembled and inserted into the chamber. In each of the following examples the clarification apparatus used was substantially as shown in Figure 1, comprising a vertical rectangular tank having a height 2.13 metres and cross sectional area 0.21 m² with a sedimentation conical zone positioned beneath the rectangular tank for collection and removal of settled solids. Feed and overflow launders were also fitted at the top of the tank and stacks of pyramids were arranged inside the tank. The central duct formed by each stack of pyramids had square cross sectional area, with the sides of the square base of the pyramids having length 15cm. The slope of the pyramids was 53° to the horizontal. The central collection duct was provided with slots, through which clarified water could escape. EXAMPLE 1 Four stacks of pyramids each comprising 30 pyramids spaced 2.5cm apart, were arranged in the tank, in two pairs, each pair discharging into one of two launders. Two of the stacks were operated with the central duct being a simple, single pipe so that clarified liquid could flow through the slots and into the duct and escaped upwards out of the apparatus (up flow only). The remaining two stacks were operated in accordance with Figure 6 with the top 50% of the pyramids delivering clarified liquids direct into the upflow duct 34 but with the bottom 50% of the pyramids delivering clarified water into the outer downflow duct 31 from which it flowed into the inner upflow duct 33 and 34 to flow upwards out of the apparatus, i.e., the initial removal of clarified water was downflow followed by upflow out of the apparatus The apparatus was operated for a substantial period of time with a feed rate of 15m/hour (i.e., the volume of suspension for clarification entering the tank measured in m³ per hour per m² of tank area). The suspension used for clarification was an aqueous suspension of china clay particles with a solids content 580 mg per litre. The rate of delivery of clarified water from both pairs of stacks was about 15m/ hour and the solids content of the clarified water was found to be 35mg per litre for both pairs of stacks. It was found that over the period of operation solid particles accumulated at the base of the upflow only stacks, to a level of about 8cm whilst the stacks in which the bottom 50% of pyramids delivered into a downflow collection duct did not accumulate any significant amount of solids. Example 2 Two stacks of pyramids having downflow collection of clarified water each comprising 30 pyramids spaced 2.5 cm apart, were arranged in the tank. Aqueous suspension of china clay particles, having a solids content of 490 mg per litre, was fed to the tank at a rate of 20m per hour. The overflow of the clarified water was also at a rate of approximately 20m per hour. The resultant clarified liquid contained a solids content of 51mg per litre. This process was repeated using pyramids having skirts as illustrated in Figure 3. Due to the extra height of the skirts, only 15 pyramids could be arranged in a stack, with spacing of 5 cm (2 inches). In addition, the area of the orifices, per pyramid was twice the area for the apparatus having 30 pyramids, in order to provide a total equal area of the orifices per stack. The same aqueous china clay suspension having solids content of 490 mg per litre was supplied to the clarification apparatus at a rate of 20m per hour, again the overflow rate of clarified liquid was approximately 20m per hour. The solids content of the clarified liquid was even more significantly reduced to 31 mg per litre. Thus, even though the number of pyramids (i.e. the settlment area) was reduced, an even more efficient clarification was achieved. In each of these examples the plant was operated at a flow rate for the suspension approximately 4 or 5 times a suitable flow rate in the absence of a stack, thus illustrating the benefit of the inclusion of the stack. In other processes the flow rates can be up to 10 or more times a suitable flow rate for an apparatus in which there are no stacks.

Claims: A settling assembly that can be fitted substantially vertically in a clarifier tank (1) for clarifying a liquid suspension into clarified liquid and separated solids, the settling assembly comprising a stack (6) of inclined substantially parallel settling members (7) each of which has a neck portion (43) and each adjacent pair of which defines a substantially parallel sided cell, a substantially vertical collection duct (39) extending through the neck portions (43), and orifices (25; 26) distributed around and opening into the duct (39) from the top of each cell for passage of clarified liquid from each cell into the duct (39), characterised in that each settling member (7) has the form of a hollow truncated 4-sided pyramid extending from the neck portion (43) and the members (7) in each pair are spaced apart by spacer means (20) on the neck portion (43) of at least one of the members of the pair or fitted between the neck portions of the members in the pair. An assembly according to claim 1 in which the spacer means comprise substantially vertical necks (20) on the neck portions (43) of the settling members. A settling assembly according to claim 1 or claim 2 in which there are spacer means on each settling member (7) comprising a substantially vertical neck (20) extending upwardly from the neck portion (43) of that member. An assembly according to claim 2 or claim 3 in which the substantially vertical collection duct (39) is defined by the necks (20). An assembly according to any of claims 2 to 4 in which there are inter-engaging fit means (21) on the necks (20) whereby the settling members (7) can be secured one above the other in the stack by push inter-engagement of the fit means (21). An assembly according to any preceding claim that has been assembled by sliding the neck portions (43) of the members (7) and the spacer means (20) over the duct (39) or by forming the duct (39) by inter engagement of spacer means that are vertical necks (20) on the members (7), and thereby self spacing the members (7) in the stack. An assembly according to any preceding claim including means for keeping the stack under compression, thereby preventing the members (7) from separating from each other. An assembly according to claim 7 in which the members (7) are formed of a plastics material having density below 1. An assembly according to any preceding claim in which the orifices (25; 26) have a horizontal length greater than half the horizontal lengths of the upper-most edges of the members (7) adjacent to the orifices (25; 26). An assembly according to any preceding claim in which the substantially vertical collection duct (39) comprises a downwardly extending duct (18) and an upwardly extending duct (27) into which the downwardly extending duct (18) leads directly, and in which the orifices (25, 26) from the cells defined by at least the lower-most pairs of members open into the downwardly extending duct. An assembly according to claim 10 in which the downwardly extending duct (18) opens at its base into an inner, substantially coaxial, upwardly extending duct (27). An assembly according to claim 10 or claim 11 in which the downwardly extending duct (18) extends over part only of the depth of the stack and the orifices (25; 26) from the cells providing at least the bottom 10% of the depth of the stack open into the downwardly extending duct (18) and the orifices (25; 26) from cells above the downwardly extending duct (18) open into the upwardly extending duct (27). An assembly according to any of claims 10 or 11 in which the downwardly extending duct (27) extends along substantially the entire length of the upwardly extending duct (18). An assembly according to any preceding claim in which each upper settling member (7U) in a cell has a skirt (40) extending downwardly from its lower most edge to define with the lower most edge of the adjacent lower settling member (7L) an open, parallel sided, slot that has a vertical dimension 1/4 to 3/4 of the vertical distance between the members in the cell. An assembly according to any preceding claim in which each cell has a vertical depth of 0.5 to 10cm and an inclined wall length of 5 to 30cm. An assembly according to claim 15 in which each cell has a vertical depth of 2 to 10cm and in which the lowermost edge of the upper settling member in a cell defines with the adjacent lower settling member an open slot having a vertical depth of 1 to 5cm. An assembly according to any preceding claim in which the angle to the horizontal of the inclined surfaces is from 50 to 70°. A liquor-clarifying apparatus comprising a tank (1), at least one inlet (2) for suspension to be clarified, at least one first outlet (3) for clarified liquor, at least one second outlet for separated solids and a plurality of settling assemblies, characterised in that the settling assemblies are assemblies according to any of claims 1 to 17 and each substantially vertical collection duct (39) discharges up into a first outlet (3). Apparatus according to claim 18 in which there is, at the top of the tank (1), at least one launder (13) into the base of which a plurality of first outlets (3) lead and from which clarified liquid can be discharged. Apparatus according to claim 19 including a weir for controlling the depth of clarified liquid in the or each launder (13). Apparatus according to any of claims 18 to 20 in which the at least one first inlet (2) is positioned at or near the top of the tank and the second outlet (4) is in the base of the tank. Apparatus according to any of claims 18 to 21 containing at least 10 of the settling assemblies each comprising a stack (6) of at least 50 of the inclined substantially parallel pyramidal settling members (7). Apparatus according to any of claims 18 to 22 in which each settling assembly is an assembly as defined in claim 15 or claim 16 and there is a free space of 2 to 10cm between adjacent stacks. A process in which a suspension is clarified by feeding it to the at least one inlet (2) of an apparatus according to any of claims 18 to 23 and withdrawing separated solids from a second outlet (4) and clarified liquor from the at least one first outlet (3). A process according to claim 24 in which the suspension is a suspension of sewage particles. A process according to claim 24 or claim 25 in which the suspended particles are flocculated by the addition of a flocculating agent before entry to the apparatus. A settling member for use in an assembly according to any of claims 1 to 17 comprising a hollow truncated 4-sided pyramid the inclined surfaces of which extend down from a neck portion (43), and a neck (20) that extends substantially vertically from the neck portion (43) and orifices (25) distributed around and extending through the neck. A settling member according to claim 27 in which the neck (20) extends substantially vertically upwardly from the neck portion (43). A settling member according to claim 27 or claim 28 in which the orifices (25) are distributed around the neck and have a horizontal length greater than half the horizontal length of the upper most edges of the member adjacent to the orifices. A settling member according to any of claims 27 to 29 in which the neck portions include inter-engaging fit means whereby two of the members can be secured one above the other by push inter engagement of the fit means.

Inventors: MOIR JOHN, MOIR, JOHN
Applicants: ALLIED COLLOIDS LTD, CIBA SPECIALTY CHEMICALS WATER TREATMENTS LIMITED
Application year: 1989
Main classification: B01D21
IPC codes: B01D21