Device for separating solids from suspension

Abstract: Device for separating solids from suspension comprises two filtering belts (5) pervious to water, such as filter wires, travelling in the same direction and being arranged to be supported each by their respective liquid withdraw­ ing surfaces (6) provided with holes or like and which, guided by the surfaces, form therebetween a prefiltering chamber (17) converging in the direction of travel. The prefiltering chamber comprises in the beginnig a supply inlet (12) for supplying suspension to be treated into the chamber, as well as sealing elements defining the chamber at its sides in directions perpendicular to the direction of travel of the belts (5). The sealing elements are formed by rigid walls (1a) rigidly joined to said liquid withdrawing surfaces (6), the prefiltering chamber (17) having a substantially closed housing-like construc­ tion.

Description: The invention relates to a device for separating solids from suspension, the device being more closely defined in the preamble portion of claim 1. The present invention is a so-called twin filtering belt press, wherein the suspension to be filtered is supplied in between two filtering belts travelling in the same direction. The filtrate becomes separated when passing through the belts and the solids in the suspension remain as solid cakes on the surfaces of the filtering belts. Finally the filtering belts are led against each other, at which stage dewatering still takes place by squeezing through the belts. In conventional twin filtering belt presses, the open construction at the prefiltering chamber formed between the converging filtering belts has heretofore prevented the increase of the velocity. For example, there are known solutions, where the filtering belts are supported on the side of the filtrate by means of rollers and edges of the belts are in this case relatively free. At the side a chamber of this type is sealed by means of wedge-like sealing strips, which are in contact with the edges of the filtering belts and hence cause wear of the belts. Further, such a relatively open construction has not allowed to use pressure in the prefiltering stage, and therefore there has not been another alternative than to construct the filtering stage to a big and expensive section. US-A-1,875,075 discloses a solution of one kind to the above-mentioned problem. However, in this construction the walls supporting the prefiltering chamber from the sides are fabricated of a resilient material, as a result of which the pressure can not be elevated in this case either. Moreover, the sealing at the exit end of the prefiltering chamber is not sufficient. From US-A-4244804 there is known a dewatering apparatus for sludge or the like, provided with elements defining a closed housing-like prefiltering chamber construction. The known apparatus does not have any particular means to increase the pressure in the prefiltering chamber. DE-U-1966230 discloses a filtering device for sludges. At both ends of the filtering chamber of the known device there is provided a door which is operated by a hydraulic cylinder and serves only to close the chamber when the filtering operation is started. None of the two doors is located at the inlet of the known filtering device. The purpose of the invention is to eliminate the drawbacks which are due to the above-mentioned technique, and to disclose a solution, with the aid of which the prefiltering stage can be made more effective by utilizing the pressure. By virtue of this, the filtering can be effected on both filtering belts at equal intensity. The aims of the invention which are presented above are achieved with the help of a device having the features disclosed by the characterizing portion of claim 1. Due to a firm, housing construction of the prefiltering chamber, one can take advantage of pressure in filtering, thereby increasing the filtration rate. Some advantageous embodiments of the invention are disclosed in the accompanying dependent claims. According to this, the device may be made constructionally compact by providing the chambers collecting the filtrate coming from the prefiltering chamber, as well as the prefiltering chamber in a single, substantionally closed housing construction. In this housing construction the side walls of the prefiltering chamber and the side walls of the filtrate chambers can be constituted of the same wall, and the dewatering surfaces are fixed between these walls. According to one advantageous embodiment, the width of the dewatering area on the dewatering surfaces defining the prefiltering chamber towards the filtrate chambers and supporting the filtering belt is substantially less than the width of the filtering belt, as a result of which the solid cakes formed on the surfaces of the belts are of smaller width than the belt. This makes it possible to guide the edges of the belts against each other at the exit end of the prefiltering chamber and for realizing the same, both margin areas of the bands can be equipped with a sealing surface located at least in one of the dewatering surfaces and enabling the above-mentioned operation. The invention will be described more closely in the following with reference to the accompanying drawings, wherein Fig. 1 is a sectional side view of a device according to the invention, the section being a longitudinal section taken in the direction of travel of the filtering belts, Fig. 2 is a cross-section of the device along the plane A-A of Fig. 1, and Fig. 3 is a cross-section of the device along the plane B-B of Fig. 1. Fig. 1 shows a twin filtering belt press constructed according to the invention. A mixture of solids and liquid is pumped through an inlet 12 into a prefiltering chamber 17. On the opposite walls of the chamber are located hole or slit plates 6, which converge, in other words, the surfaces thereof form a converging chamber therebetween and they support each their respective filtering belt 5 or filtering wire, which slides along the surface of the plate towards the direction of convergence of the chamber. By virtue of the pumping pressure liquid flows first on both sides of the chamber through the wire 5 and the plate 6 into filtrate spaces 7 and 8 located on the opposite side of the plate. The spaces are located one after the other, as seen in the direction of travel of the wire, and they are separated from each other. The prefiltering chamber 17 converges in the direction of travel of the wires in such a fashion that the cakes 18 deposited onto the surfaces of the wires start to become squeezed against each other in the end portion of the chamber and seal the chamber 17 at its exit end. This sealing effect is further ensured by strips 22, shown in Fig. 3, which press the edges of the filtering wire together before the pressing stage. The strips will be described hereinafter in more detail. At the wider supply end of the chamber 17 there is a slit being only as large as the wire is thick and it becomes sealed under the effect of pressure when flexible sealing strips 14 extending along the width of the wires at the sides of the supply inlet are pressed against the wires 5. Both filtering wires 5 form endless wire loops, where the wire situated within the free portion is continously washed by means of shower tubes 11. A partition wall 19 is located between the consecutive filtrate chambers 7 and 8. It is thus possible to withdraw from the device different filtrates having a varying degree of purity. The purest filtrate emerges as a rule from a later filtering stage. The number of filtrate chambers can be one, two or several on both sides of the prefiltering camber 17. The filtrate exits from the filtrate chambers 7 and 8 through filtrate outlets 9, 10. Through these outlets the chambers can be provided with reduced pressure in order to make the filtration more effective. The cake deposited at the filtering stage is squeezed to a higher dryness by means of press rollers 3 located after the prefiltering chamber, this being done at one or several stages. After the press rollers 3 the cake pressed dry falls down into a conveyor 15. The filtrate formed at the pressing stage and the wash water of the wire is collected into a basin 20 and it exits throug an outlet 16. Through an inlet 13, another material to be filtrated can be pumped into the filtering chamber. This material may consist of not easily filterable solids/liquid mixtures, such as biosludge and circulation water. The solids therein will be filtered on top of a pre-coat cake being formed previously on the surface of the wire. Said inlet is located in the direction of travel of the wires 5 after the supply inlet 12. The press rollers 3 act mainly as the driving rollers running the filtering wires 5. The travel of a wire is further enhanced and guided within the free section of the wires by guide rollers 4 and by so-called reversing rollers 2, which quide the free run of the wires to the beginning of the prefiltering chamber 17. The whole device, including its prefiltering chamber, filtrate chambers and rollers, is mounted on a main frame construction 21. Fig. 2 shows the cross-section of the device taken along the plane A-A of Fig. 1, that is, at the location of the prefiltering chamber. The prefiltering chamber 17 and the filtrate chambers 7 on both sides thereof are arranged in a substantially closed housing construction 1. The dewatering surfaces are constituted of slitted plates or hole plates 6, along the surfaces of which the wires 5 pervious to water slide. The prefiltering chamber 17 is defined laterally, that is in the direction of the edges of the wires 5, by rigid walls 1a, which are fixedly joined to said plates 6. The edges of the wires 5 lie at a short distance from the respective wall 1a, this arrangement affording both well-sealed construction of the prefiltering chamber and a construction where edges of the wires are not subjected to wear. Both the opposite, parallel side walls 1a for the prefiltering chamber 17 continue in the direction of the filtrate chambers 7, 8 as the same walls, which form on the side of the filtrate chambers a wall defining the filtrate chamber. The plates 6 are thus fixed between the inner surfaces of the side walls 1a of the housing-like construction 1. To these side walls 1a are perpendicularly joined rear walls 1b defining the chambers. Hence, the afore-mentioned rigid and strong housing-like construction of a rectangular cross-section is formed, within which the prefiltering chamber 17 is situated in the above-mentioned way. Fig. 2 further shows, how the width L₂ of the liquid withdrawing area on the plates 6, that is, the width of the area wherein the holes or slits are situated, is substantially smaller than the width L₁ of the wire 5 lying against it, in which case the edges of the wire 5 extend sideways substantially farther than the corresponding side edge of the liquid withdrawing area L₂ at both edges. Due to this fact the solid cakes 18 formed on top of the filtering wires 5 are slightly wider than the liquid withdrawing area L₂, but their width is still considerably smaller than width L₁ of the corresponding filtering wire 5. The value of L-L₁, wherein L is the total width of the plate 6 restricted by the prefiltering chamber 17, may be 1 to 4 cm. The value of (L₁-L2)/2 is preferably in the range of 5 to 15 cm, depending on the filterability of the solid cake formed by the suspension which is to filterd. Fig. 3 shows the cross-section of the device taken along the plane B-B of Fig. 1, that is, at the location of the exit end of the prefiltering chamber, where the ends of the hole plates or slitted plates 6 guide the wires 5 and the cake 18 squeezed between the wires at the exit end of the filtering chamber 17 in between the press rollers 3. In this area, both opposite plates 6 are provided with strips 22 protruding from their elsewhere flat surface and having a cross-section of a circle segment. The strips are situated laterally outside the dewatering area L₂, but within the width L₁ of the wires in both margin areas of the wire 5. The strips 22 act as sealing strips, whose arcuate surfaces raise the edges of the wires 5 off from the level of the dewatering surfaces of the plates 6 and press the edges of the opposite wires against each other at the location of the margin area of the wire situated a short distance inwardly from the outer edge of the wire 5. Because the cake formed by the against each other squeezed cakes 18 at the exit end of the prefiltering chamber 17 is of less width than the wires to a sufficient extent, it will be easily entrapped between the wires in the above-mentioned way in such a fashion that the cake does not become spread to the edges of the wires. The cake is denoted by a hatching between the wires 5 in Fig. 3. The edges of the wires 5 can be easily guided against each other at the exit end of the prefiltering chamber 17 in the above-described fashion. The above-described solution also affords a well-sealed construction to the exit end of the prefiltering chamber 17, because the area being subject to the filtering pressure is closed laterally as well. Fig. 3 further shows, how within the area of the strips 22 the side walls 1a of the prefiltering chamber are rigidly fixed to the longitudinal edges of the plates 6. The strips 22 and the plates 6 supporting them continue to as close to the press rollers 3 as possible. At the rollers 3 the cake is solid to such an extent that the sealing at the sides is no more necessary. The closed construction of the device makes it possible to design it so that it is operative in an upright position as shows in Fig. 1. The flow of the solids takes place downwards in this case, but an opposite flow direction is also possible. A horizontal construction and a horizontal flow direction are possible as well. The invention is applicable to the filtration of all suspensions containing solid substances, such as suspensions containing fibres, also irrespective of whether the solids or the filtrate is the more valuable material to be recovered. Various waste sludges and slurries of pulp and papermaking industry can be mentioned as examples. The invention can also be used as a web forming device, the raw material of which is a fibrous suspension. After the prefiltering chamber, various aftertreatment devices and constructions well known in the art may be provided without departing from the scope and spirit of the invention.

Claims: Device for separating solids from suspension, the device comprising two filtering belts (5) pervious to water, such as filter wires, travelling in the same direction and being arranged to be supported each by their respective liquid withdrawing surfaces (6) provided with holes or like and which, guided by the surfaces, form therebetween a prefiltering chamber (17) converging in the direction of travel, the prefiltering chamber comprising in the beginning a supply inlet (12) for supplying suspension to be treated into the chamber, as well as sealing elements defining the chamber at its sides in directions perpendicular to the direction of travel of the belts (5), characterized in that the sealing elements are formed by rigid walls (1a) rigidly joined to said liquid withdrawing surfaces (6), the prefiltering chamber (17) having a substantially closed housing construction, and that the supply inlet is provided with flexible sealing strips (14) extending along the width of the belts and adapted to be pressed against the belts (5). Device as claimed in claim 1, characterized in that the prefiltering chamber (17) and filtrate chambers (7, 8) on the other side of the liquid withdrawing surfaces are arranged in the same substantially closed housing construction (1). Device as claimed in claim 2, characterized in that in the housing construction (1) the side walls (1a) of the prefiltering chamber continue in the direction of the filtrate chambers as the same walls defining the filtrate chambers, in which event the liquid withdrawing surfaces (6) are fixed between the inner surfaces of said walls (1a). Device as claimed in any of the preceding claims 1-3, characterized in that the width (L₂) of the liquid withdrawing area on the liquid withdrawing surfaces (6) is substantially smaller than the width (L₁) of the corresponding filtering belt in a direction perpendicular to the direction of travel of the belts (5) in such a fashion, that the edges of the belt (5) extend sideways substantially farther than the edge of the liquid withdrawing area (L₂). Device as claimed in claim 4, characterized in that at the exit end of the prefiltering chamber (17) at both margin areas of the belts (5) is provided a sealing surface (22) on at least one of the liquid withdrawing surfaces, said sealing surface being positioned to guide the margin area of the filtering belt (5) against the margin area of the opposite belt (5). Device as claimed in claim 5, characterized in that the sealing surfaces (22) are provided at least at one of the margin areas of the belts (5) on both opposite liquid withdrawing surfaces (6) to form protrusions from their main plane. Device as claimed in any of the preceding claims, characterized in that in the filter chamber (17) is provided at least one supplementary inlet (13) being situated in the direction of travel of the belts (5) after the supply inlet (12).

Inventors: HAUTALA JOUKO, HAUTALA, JOUKO
Applicants: TAMPELLA OY AB, OY TAMPELLA AB
Application year: 1989
Main classification: B01D33
IPC codes: B01D33, B01D37