Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
  • D21D5/00—Purification of the pulp suspension by mechanical means; Apparatus therefor
  • D21D5/02—Straining or screening the pulp
  • D21D5/16—Cylinders and plates for screens
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/18—De-watering; Elimination of cooking or pulp-treating liquors from the pulp
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
  • D21F1/66—Pulp catching, de-watering, or recovering; Re-use of pulp-water
  • D21F1/74—Pulp catching, de-watering, or recovering; Re-use of pulp-water using cylinders

Definitions

• the present invention relates to a method and an apparatus for separating more efficiently than before liquid from liquid/fiber suspension by dividing the suspension into two portions, of which one portion mainly comprises liquid and the other both liquid and fibers.
• the method and apparatus in accordance with the invention may well be applied to thickening of low consistency (0.5 to 5 %) fiber suspensions in pulp and paper industry.
• the pulp and paper industry has processes which are carried out with a pulp of low consistency, even under 1 %. Such processes are, for example, normal and reverse vortex cleaning. Subsequent to the vortex cleaning the pulp suspension is transferred to the next processing stage, which may be either a thickener or a headbox of a paper or drying machine. In any case, the stage following the vortex cleaning is almost always thickening, whereby a thickener is required for the low consistency zone, which thickener must be able to treat large amounts of fiber suspension and also discharge large amounts of liquid from the suspension, even if the thickening of the consistency were apparently low (cf. 0.5 % -> 3 %).
• Thickening is carried out according to the modern techniques by means of different types of disc and drum thickeners and curved filters.
• conventional disc or drum thickeners the discharge of liquid, in other words thickening, is based on gravity (so called gravity deckers), on vacuum thickening (so called vacuum filters) or on pressure thickening (so called pressure filters).
• the thickening is carried out by means of a drum made of perforated plate covered by wire cloth.
• the pressure difference required for the thickening results from the level difference between the pulp in the inlet tower and the pulp in the filtrate chamber.
• Pulp may be filtered either from the inside of the drum to the outside or from the outside to the inside, which latter direction is the most usual.
• the diameter of the drum may be 4 m, of which, for example, 60 % is underwater.
• 10 to 15 % of the inner surface of the drum is operating as a filter surface.
• the maximum pressure difference is about 20 kPa. At the bottom dead center of the drum the pressure difference is zero and it will increase from thereon to its maximum value towards the surface of the inlet tower.
• the filtering surface of the drum is kept clean or open to the flow by moving the surface against the filtrate or by using air to clean it.
• a mill producing 500 tons of 90 % consistency pulp requires a filter with a diameter of 4 m and a length of 7 m and having about 88 m 2 of wire surface, to thicken the pulp from 0,5 % to 1,5 %.
• the pressure difference required for filtering in vacuum and disc filters is carried out with a suction leg.
• These kind of thickeners differ from the gravity deckers in that a pulp layer is formed on them. This means that subsequent to thickening the consistency of the pulp is 8 to 14 %.
• the capacity of a vacuum or disc filter is approximately the same as that of a gravity decker.
• pulp web tends to be formed by suction on the filtering surface in the pulp suspension when the filtering surface is submerged. Filtrate is removed from the web generated on the part of the drum, risen above the surface of the suspension, so as to achieve the consistency of said 8 to 14 % in the discharge. It is clear that when forming a pulp layer on the filtering surface, liquid discharge through the layer considerably slows down.
• a pressure filter differs from the vacuum drum filter in that the filtering pressure difference is carried out by pressurized air.
• the filtering capacity does not substantially differ from that of a vacuum drum filter.
• the thickening method using a curved filter is based on gravity decker filtering.
• the suspension being thickened is pumped onto an inclined filtering surface.
• the thickening capacity is in practice 3 to 5 % and the specific capacity of liquid discharge is approximately the same as that of the drum filters.
• Curved filter types of thickeners are used in the pulp industry where relatively minor thickening and pulp capacities are concerne .
• the above described conventional thickeners are characterized in that thickening is carried out using a very small pressure difference in more or less open equipment and only part of of the filtering surface is utilized.
• a result of the small pressure difference and the partial use of the filtering surface is low liquid discharge capability.
• Due to the open construction and operational principle the pulp and the filtrate may contain air. As is known air in the pulp decisively lowers the infiltration qualities of the pulp.
• the filter surface is a vertically arranged drum and a surface movable relative to it, which is arranged inside the filter surface, is a substantially cylindrical rotor, on the surface of which either spherical protrusions or foil-like members are arranged in order to create local pressure differences.
• the filter surface has, however, been of smooth surfaced, perforated plate material.
• the present invention overcomes or minimizes the above mentioned problem in such a way that members creating micro turbulence of high intensity are arranged on the filter surface itself, and that the size of the openings of the filter surface is diminished in such a way that the diameter of the openings (perforations) is about 0.15- 0.2 mm.
• the movement of the suspension which is due to the members of the rotor causing pressure differences and which is parallel to the filter surface, is disturbed by the members of the filter surface only to the extent that the fiber matting moving along the surface of the filter surface and disturbing the thickening is broken and mixed to the rest of the suspension stabilizing the consistency.
• the purpose is not to generate a slight micro turbulence on the filter surface, but to fluidize the whole suspension in the thickening space between the rotor and the filter surface, as it can be noted from said application.
• the reason for this is that thickening pulp having the consistency of more than 5 % (so called medium consistency zone) requires quite different conditions and operations from what is required when operating in the consistency zone of about 1 %, because, for example, the liquid in the suspension may be separated from the fibers in a quite different way at a low consistency compared with operating at a consistency of more than 5 %.
• a pulp having the consistency of more than 5 % forms especially when being thickened with the consistency increasing thereby, large pulp cakes having strong bonds between the fibers, which bonds must be broken, for one thing just to allow the flow of the pulp further in the thickening apparatus.
• the pulp tends to be fluidized completely in the whole thickening space, because otherwise it would not be possible to remove liquid from medium or high consistency pulp. Fluidization in the whole space requires, however, a great amount of energy, so it should be avoided, if possible.
• a filter surface and a rotor provided with above described rather high protrusions generate a strong turbulence extending throughout the whole treatment zone of pulp, the approximate intensity of which turbulence is at the whole treatment zone, however, low.
• the number of the ribs or protrusions on the filter surface is relatively low, in other words they are located far from each other on the filter surface.
• the number of the ribs or protrusions on the filter surface is relatively low, in other words they are located far from each other on the filter surface.
• the thickening method in accordance with the invention is characterized in that micro turbulence is generated on the filter surface by means of the filter surface to disperse the fiber matting thickening tending to form on the surface or to prevent the formation of fiber matting on the filter surface.
• the thickening apparatus in accordance with the present invention is characterized in that unevennesses are provided on the filter surface and are shaped and/or spaced and/or arranged and/or dimensioned such that, in operation and together with the kinetic speed of the fiber suspension, they generate micro turbulence of high intensity, so that any fiber matting otherwise tending to form at close proximity to the filter surface is dispersed or prevented from forming so that the thickening process is optimized.
• Fig. 1 is a schematic illustration of the basic construction of a thickening apparatus in accordance with the present invention
• FIGs. 2 a...c are schematic illustrations of a preferred embodiment of an apparatus in accordance with the invention.
• Fig. 3 is a schematic illustration of a variation of the embodiment in accordance with Fig. 2;
• Fig. 4 is a schematic flow diagram of an apparatus in accordance with the invention when testing the used test arrangement.
• Figs. 5 - 11 are illustrations of a group of appropriate groove alternatives to be applied in a filter plate in accordance with the invention.
• Figs. 2 a...c illustrate fragmentary details of the filter cylinder arrangement 9 in accordance with one embodiment, which arrangement is made of substantially planar smooth metal or other material plate 21 prior to forming, which are covered throughout with small apertures or perforations 22 having a diameter of advantageously 0.1 - 0.25 mm, or with perforations of a size having similar thickening qualities, but having a different form.
• the plate is, after being covered with apertures, rolled to form a cylindrical filter drum. In some cases, mostly when the starting consistency of the suspension being thickened is higher, it is possible to use a larger perforation/ opening size.
• the plate 21 has grooves or recesses 24 formed in the surface 23 on the side of the suspension on the plate 21, advantageously low or shallow grooves the shape of which may greatly deviate from what is shown in the drawing.
• the machined grooves should only be about 0.2 - 1.0 mm.
• Fig. 2 c is an alternative arrangement with larger grooves 24 which in effect form what appear as projections 23 with sloping walls.
• the apparatus described above in connection with different embodiments operates in such a way that the fiber suspension of low consistency is fed under pressure to the apparatus, whereby it flows from the direction of one end of the apparatus advantageously to the thickening space between the rotor 10 and the filter cylinder 9 and whereby due to the effect of the advantageously tangential feeding method and rapidly rotating rotor 10 the fiber suspension is caused to effect a circulating movement in the thickening space between the filter cylinder 9 and the rotor 10.
• the filter cylinder 9 with small perforations allows the penetration of the filtrate but not the fibers.
• the reference number 30 signifies a circulation tank or tower, from which pulp is pumped by a feed pump 31 to the test, i.e. filter, apparatus 32 though a feed valve 33.
• the pulp may be, if required, led back to the circulation tower 30 directly past the filter apparatus 32 by opening valve 34 and closing valve 33.
• a sample of unthickened pulp may be taken from the conduit at 35 and a sample of thickened pulp from the conduit at 36 downstream of the valve 37.
• a sample of the filtrate may be taken from the conduit 38 downstream of filter valve 39. It is possible to adjust the desired pressure condition in the filter apparatus 32 by valves 37 and 39.
• the surface area of the filter cylinder in the testing apparatus was 0.4 m 2 , of which 10 % open area when the perforation size was 0.2 mm.
• the filter surface in accordance with the invention was produced in such a way that substantially axial grooves were machined on the surface of a smooth perforated plate, the depth of the grooves being about 0.5 mm.
• the measure of the bottom plane of the groove parallel to the rim was about 1.5 mm, one of the side walls of the groove was perpendicular to the filter surface and the other formed a 30° angle with the filter surface.
• the measure of the ridge remaining between the grooves, parallel to the rim was about 0.35 mm. Thus the interval between the ridges is slightly over 3 mm.
• the height of the unevennesses remains 0.2 - 2.0 mm, advantageously between 0.2 - 1.5 mm and that their distance from each other is 1.0 - 10 mm, advantageously 1.0 - 6.0 mm.
• a flow rate which is about four times of that used with the uneven surface, is required with the smooth surface to achieve a certain turbulence level.
• the turbulence level on the uneven surface is at equal speed 2 to 3 times of that on the smooth surface.
• the tests performed show that first of all it is possible to achieve multiple capacity increases by the pressurized thickening method and apparatus compared with the conventional thickeners. Additionally, it may be seen in the results that, as mentioned above, the specific capacity when using an uneven filter plate is more than 80 % higher than when using an even plate. When comparing the power consumption it is seen that the power demand when using an uneven plate is about 80 % of the value when using a smooth plate. Thus the use of a thickening plate in accordance with our invention may be said to be almost 130 % more efficient, if the comparison is based on the power used for thickening a certain amount of pulp. Additionally, the arrangement in accordance with our invention has all the advantages of the pressurized treatment of pulp. Thanks to the pressurized operational principle both the filtrate and the thickened pulp are overpressurized, and this gives great freedom of choice in locating the equipment in the mill and an opportunity to save space. Also the entrance of air to the pulp in the thickening is virtually impossible.
• a filter plate may be used in an apparatus, which plate has unevennesses of one or more different sizes. The most practical arrangement has been carried out, for example so that the smallest unevennesses are at the feed end, the slightly larger in the middle and the largest at the discharge end for the thickened pulp.
• Figs. 5 - 11 illustrate examples of different groove designs that can be applied to to the filter plate in accordance with the present invention.
• the groove designs used in said drawings do not prevent one from using some other kind of groove, the most important thing is to provide means which generate micro turbulence on the filter surface, whereby the quality of the pulp, the consistency, the purity, and the like properties may require also other kind of surfaces.
• both the filter surface and the surface movable relative to the filter surface may be of a form different from cylindrical, said members are only characterized in that they are substantially rotationally symmetrical, cylindrical, conical or spherical, or their form is a combination of those, just to mention a few examples.
• the grooved filter surface mentioned as an example, to either direction, in other words the perpendicular side surface of the groove may be either as the front surface or as the back surface of the ridge between the grooves.

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• Engineering & Computer Science (AREA)
• Life Sciences & Earth Sciences (AREA)
• Wood Science & Technology (AREA)
• Mechanical Engineering (AREA)
• Paper (AREA)
• Filtering Materials (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

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Cited By (5)

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Citations (4)

• 1989
  • 1989-03-02 FI FI890988A patent/FI91171C/fi not_active IP Right Cessation
• 1990
  • 1990-02-28 JP JP2503611A patent/JPH03505759A/ja active Pending
  • 1990-02-28 WO PCT/FI1990/000056 patent/WO1990010110A1/en not_active Application Discontinuation
  • 1990-02-28 EP EP90903595A patent/EP0461138A1/en not_active Ceased
  • 1990-02-28 CA CA002047749A patent/CA2047749A1/en not_active Abandoned
  • 1990-02-28 KR KR1019900702372A patent/KR920700330A/ko not_active Application Discontinuation

Patent Citations (4)

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