EP0733734A2 - Procédé et tamis d'épuration - Google Patents

Procédé et tamis d'épuration Download PDF

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Publication number
EP0733734A2
EP0733734A2 EP96660005A EP96660005A EP0733734A2 EP 0733734 A2 EP0733734 A2 EP 0733734A2 EP 96660005 A EP96660005 A EP 96660005A EP 96660005 A EP96660005 A EP 96660005A EP 0733734 A2 EP0733734 A2 EP 0733734A2
Authority
EP
European Patent Office
Prior art keywords
straining
screen
straining element
strained
chamber
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP96660005A
Other languages
German (de)
English (en)
Other versions
EP0733734A3 (fr
Inventor
Tapio Järvensivu
Ilkka Roitto
Esa Forsten
Jouni Alho
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Metso Powdermet Oy
Original Assignee
Jylharaisio Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Jylharaisio Oy filed Critical Jylharaisio Oy
Publication of EP0733734A2 publication Critical patent/EP0733734A2/fr
Publication of EP0733734A3 publication Critical patent/EP0733734A3/fr
Withdrawn legal-status Critical Current

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    • 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
    • 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
    • D21D5/04—Flat screens

Definitions

  • the present invention relates to a method according to the preamble of claim 1 for straining coating mixtures and slurries used in the paper industry.
  • the invention also concerns a screen capable of implementing the method.
  • Vibrating flat screens are conventionally used for straining slurries, whereby straining occurs through a flat wire cloth with the help of the hydrostatic pressure imposed by a slurry layer a few centimeters thick.
  • the wire cloth is kept unclogged by means of vibration that simultaneously improves the straining capacity of the screen apparatus by virtue of decreasing the apparent viscosity of the slurry.
  • a benefit of vibrating screens is their good selectivity as compared to pressure screens. Due to the square holes of the wire cloth employed as the straining element, the screen performs particularly efficient in straining of elongated foreign matter particles such as fibers detached from the web.
  • the hole sizes of the wire cloth may be 50 um and up.
  • the straining element conventionally comprises a cylindrical slotted drum.
  • the drum is typically made from wire of approx. 1 mm dia. which is first wound into a spiral and then the wire turns are attached in a suitable manner equidistantly spaced from each other.
  • the slot width is 75 - 300 um. Owing to the screen fabrication method, the slot will have an elongated shape. For instance, the above-described type of screen drum actually has only a single, spiralled slot.
  • Pressure screens are used for straining both slurries and coating mixtures when installed in the coating circulation.
  • the straining surface is kept clean by mechanical scraping or backwashing.
  • the screen is washed or the accumulating rejects are removed by way of tapping a suitable amount of the fluid being strained away from the screen, generally at intervals of a few hours.
  • the capacity of a pressure screen per unit straining area is multifold as compared to a vibrating screen, because the stiff structure of the screen drum allows use of a several times higher pressure head than in a vibrating screen.
  • the proportion of holes on the screen drum is smaller than 10 % of the total area of the drum. Because the straining slot has an elongated shape, long and thin fibers can pass through the slots of the screen.
  • a problem hampering vibrating screens is their low straining capacity per unit area, particularly with regard to their open area which may amount up to 30 - 40 % of the straining element total area.
  • the strength of the wire cloth is a limiting factor of straining capacity and no significant pressure head can be imposed on the screen in order to improve the straining capacity of the wire cloth.
  • approx. 1 m 2 of wire cloth surface is required to strain 1 liter of slurry per second.
  • Due to the low capacity of vibrating screens per unit area a large footprint is required for these screens.
  • up to ten vibrating screens are required for straining pigment slurries. Arranging such a high number of screens in a mill building may be extremely awkward making the use of vibrating screens even impossible.
  • a major problem hampering both vibrating and pressure screens is the clogging of the screen during use.
  • the screen units have to be dismantled for cleaning and maintenance at regular intervals, and moreover, gradual clogging causes degradation of straining capacity.
  • straining apparatuses disclosed in DE patent application 30 10 952 and EP patent application 0 616 072 resemble vibrating screens.
  • the straining element comprises a perforated plate, whereby the plate is kept clean and the straining capacity improved with the help of a blade adapted to rotate close to the plate.
  • a stiff plate is difficult to perforate so that a sufficiently small size of holes can be attained combined with a high proportion of the open area formed by the perforating holes, and thus, large permeable area with respect to the total area of the plate, these apparatuses are hampered by low selectivity and, due to the small proportion of open area, also by inferior efficiency particularly with regard to their selectivity.
  • the proportion of the permeable area in the perforated plate is approx. 10 % and, because the flow resistance of the screen increases rapidly with smaller hole sizes, the holes of the perforated plate usually must have a diameter of 0.5 mm or larger.
  • These apparatuses are designed for straining of paper web and cellulosic fibers and removal of foreign matter from pulped wastepaper.
  • the perforated screen plate used therein must have large holes to provide easy passage for paper web fibers.
  • equipment intended for straining of suspensions containing paper web fibers are not suitable for highly selective straining applications such as straining of coating mixes or filler slurries.
  • the goal of the invention is achieved by performing straining under a pressure head through a straining element having a hole size smaller than 300 um and improving the flow the material passed through the element by virtue of arranging a turbulent flow on the ingoing side of the straining element.
  • a turbulent flow is arranged on the ingoing side of the straining element and the surface of the wire cloth is kept clean by means of a rotary blade adapted to the ingoing side of the element.
  • the screen according to the invention is characterized by what is stated in the characterizing part of claim 6.
  • the invention offers significant benefits.
  • the screen according to the present invention acts as a pressure screen combining the advantages of a vibrating screen and a conventional pressure screen.
  • High selectivity is attained, because the straining element can be a similar wire cloth as is used in a vibrating screen.
  • the wire cloth is supported by a grooved polymer plate and a support wire or fabric, whereby the pressure head rating of the screen is widely improved over a conventional vibrating screen.
  • the straining element may also be a perforated plate, the fabrication of a small-hole plate with a sufficiently large proportion of open area using concurrent manufacturing methods is expensive as compared to the costs of a wire cloth straining element.
  • Slurries such as coating mixes and filler slurries used in papermaking are thixotropic and pseudoplastic as to their flow properties, whereby such materials are characterized by significant increase of fluidity under mixing of such materials.
  • This property is utilized in the invention by generating a turbulent flow on the ingoing side of the straining element of the screen by means of an agitating rotor.
  • the agitation of the material to be strained decreases essentially the viscosity of the material and simultaneously the agitation increases the straining capacity of the small-hole screen.
  • the cleaning of the wire cloth is accomplished by means of an agitating rotor instead of vibration, the wire cloth will not be subjected to stress by vibration. With no need for vibration, the stresses imposed by vibration on the straining element are also eliminated.
  • the screen Owing to the support structures of the wire cloth, the screen can be operated with a pressure head as high as that used in conventional pressure screens.
  • the straining capacity of the screen according to the invention per unit area of the straining element is larger than in conventional pressure screens operated at equal pressure head. Additionally, as the present screen can be operated at an essentially higher pressure head than is possible with a conventional vibrating screen, the straining capacity at a constant open area can be increased in linear proportion to the elevated pressure head. Since the straining capacity of a screen is proportional to the pressure difference over it, the capacity of the present screen may be easily elevated tenfold over that of a conventional vibrating screen, whereby a single screen apparatus will be is sufficient for applications conventionally requiring the use of several screens in parallel. A particularly effective contribution to straining capacity of the present screen is imparted by the agitation of the material being strained, thus permitting a screen according to the present invention to bestow essential reduction of straining costs due to the smaller number and size of screens required.
  • the wire cloth is kept unclogged and clean by means of a rotor invoking a turbulence that elevates the foreign matter away from the wire cloth surface.
  • the movement of the rotor parallel to the wire cloth surface improves the separation of elongated foreign matter particles such as fibers.
  • the separation of other shapes of foreign matter is enhanced, because the rotor generates a flow vector orthogonal to the accept material flow via the holes of the screen, whereby the passage through the wire cloth will be impeded for particles to be classified as reject.
  • the turbulence serves to prevent build-up of the material being strained onto the interior surfaces of the screen.
  • an essential variable which is characteristic of the present invention is the hole size of the straining element that in this context refers to the largest diagonal dimension of the hole, e.g., its diameter.
  • the shape of the holes can be varied depending on the manufacturing method of the straining element. To achieve sufficient selectivity, the hole size may maximally be 300 um, but conventionally the hole size seldom exceeds 200 um. While the hole size is advantageously in the range 50 - 120 um, the actual design value of hole size is among other factors dependent on the shape of the holes.
  • the screen shown therein is assembled onto a planar base 1.
  • Straining occurs in a screen chamber 2 formed by rotation-symmetrical, relatively flat chamber that is divided into two subchambers by means of a straining element 3.
  • the top part of the chamber is closed by a lid 4.
  • the infeed channel 5 exits into the upper subchamber of the screen chamber 2, that is, to an infeed chamber 8 enclosed from above by the lid 4 and from below by the straining element 3.
  • Beneath the straining element 3, delimited by said straining element and the bottom of the screen chamber 2 is located a discharge chamber 9, wherefrom a discharge channel 6 exits.
  • the gas vent channel 11 is connected to a lattice 10 supporting the lid.
  • a reject discharge nozzle 24 To the circumference of the screen chamber 2 is adapted a reject discharge nozzle 24.
  • an agitating rotor 12 To the end of the shaft 7 is mounted an agitating rotor 12 arranged at a distance from the upper surface of the straining element 3.
  • the shaft 7 is mounted to the planar base 1 of the screen apparatus on bearings 13 and is rotated by means of an electric motor 14 driving a V-belt and pulley transmission chain mounted on the planar base 1.
  • the straining element 2 is shown therein in more detail.
  • the element comprises five members.
  • the frame of the element is formed by a perforated support plate 17.
  • the support plate 17 is a circular, essentially flat plate having a round hole 21 at its center.
  • the circumference of the round hole 21 and the outer rim of the plate 17 are provided with holes 22 and 23 to accommodate fixing screws, and the holes perforating the support plate 17 are made to the bottoms of annular grooves machined to the support plate.
  • the support plate 17 Onto the support plate 17 is adapted a support wire fabric 19 having relatively large mesh holes, and onto the support wire fabric 19 is placed the straining wire cloth 20 proper made from a woven material similar to that of the wire of a vibrating screen, in which interwoven wires form holes through which the material to be strained can pass through the wire fabric.
  • the support wire fabric 19 and the straining wire cloth 20 are attached to the support plate 17 at their outer rims and at the inner rim about the center hole 21 with the help of clamp rings 16 and 18, whereby the support wire fabric 19 serves to prevent the straining wire cloth from sagging into the grooves of the support plate 17.
  • the function of the screen apparatus described above is as follows.
  • the material to be strained is fed into the screen via the infeed channel 5 under a pressure head, whereby the material first enters the infeed chamber 8, wherefrom it passes via the straining element 3 into the discharge chamber 9 and therefrom further via the discharge channel 6 into, e.g., the coating mix circulation of a coater station.
  • the major portion of the material entering the infeed chamber 8 can pass through the straining wire cloth 20, whereupon the actual reject separation of oversize particulate matter as well as clumps and fibers from accepts takes place.
  • the accept portion passed through the wire cloth 20 can subsequently flow through the coarse-mesh support fabric 19 and the holes of the support plate 17 into the discharge chamber.
  • the screen according to the present invention is basically a pressure screen in which the fluid being strained is passed through the wire cloth with the help of a pressure head.
  • a pressure head can be readily accomplished with the help of a feed pump by controlling the volume flow rate to a sufficiently high value.
  • the material to be strained is fed into infeed chamber forming the upper space above the wire cloth so that the infeed flow exits into the chamber at the center of the straining element about the rotor shaft, whereby the material flow takes place radially toward the perimeter of the chamber. Actual cleaning of the wire cloth is performed by means of the driven rotor 12.
  • the driven rotor 12 When rotated at a tangential speed of approx. 5 - 20 m/s, the driven rotor 12 causes turbulence which detaches oversize particulate matter from the surface of the wire cloth 20.
  • the detached rejects are driven toward the perimeter of the chamber having the rejects discharge nozzle 24 mounted thereto, whereby the rejects can be removed either as a continuous flow, or alternatively, at controlled intervals.
  • the reject fraction can be strained in a secondary screen such as a vibrating screen. Since the rejects fraction with respect to the total volume of material strained usually is relatively small, cyclic discharge of the rejects from the screen chamber will often suffice.
  • the rotor 12 serves to improve flow through the screen. Because the hole size of the straining element must be relatively small, the flow resistance of the straining element to high-viscosity materials such as coating and filler slurries will be high. While a pressure head imposed on the straining element can increase the flow through the screen apparatus, the application of the straining method according to the invention to such high-viscosity materials is essentially based on vigorous agitation with the help of a rotor. Such agitation can significantly reduce the viscosity of slurried materials thus aiding their through the straining element, whereby a screen apparatus with a high straining capacity is achieved without compromising its selectivity against foreign matter.
  • the present invention may have alternative embodiments.
  • the straining cloth or wire can be selected from a number of different fabric types, it is advantageous to use a fabric made from interwoven wires, whereby a wire cloth is provided characterized by large proportion of open area combined with good selectivity against foreign matter.
  • the fabrics and wires employed in the method typically have a hole size of approx. 50 - 120 um, the actual size of mesh holes will be selected according to the application.
  • the support plate and the support fabric must have a design permitting minimum resistance to the flow of the strained material and simultaneously provide support for the straining wire cloth against the stress imposed by the pressure head.
  • the present supported screen construction can be operated at the same pressure head, typically approx. 1 bar, as is imposed in conventional pressure screens over the straining element.
  • the straining element may also comprise a perforated plate made from sheet steel or other suitable material, it will be rather costly by virtue of concurrent manufacturing techniques to manufacture a small-hole perforated plate with an open area of 20 - 30 %.
  • such a plate can be fabricated by means of laser machining, whereby it is possible to achieve a straining element of higher mechanical strength than that of a conventional wire cloth.
  • the straining capacity of a screen is ultimately determined by the open area of the straining element, it is entirely suitable to use a low-cost straining element of smaller open area as long as the straining capacity requirements of the specific application are fulfilled.
  • the operation of the screen apparatus may be controlled by altering the profile and rotational speed of the agitating rotor as well as the pressure head over the straining element.
  • the rotor shape can be varied widely provided that the rotor design preserves its agitating properties to generate a turbulent flow which elevates foreign matter aggregates away from the surface of the straining element.
  • the drive motor of the rotor can be placed so that the shaft 7 need not extend through the straining element, whereby the alternative location of the drive motor in the illustrated embodiment would be above the screen. Owing to the use of a rotatable rotor in the screen apparatus, the optimal shape of the screen chamber obviously is rotation-symmetrical.
  • the infeed of the material to be strained can be arranged from the side of the screen chamber, whereby any clumpy and hard aggregates possibly carried over with the infeed cannot land in the gap between the rotor and the straining element thereby causing damage to the rotor.
  • the infeed nozzle could be place on the short side of the rectangular chamber and the turbulent flow could be accomplished using high infeed velocity or different kinds of rotor wing or propeller agitator designs.
  • the planar shape of the straining element could be replaced by a slightly curved straining plate, or alternatively, a curved support plate on which the wire cloth or other straining fabric is placed. While the latter arrangement offers improved ejection of rejects toward the perimeter of the screen chamber, a more complicated construction results.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Paper (AREA)
  • Separation Of Solids By Using Liquids Or Pneumatic Power (AREA)
EP96660005A 1995-03-24 1996-03-21 Procédé et tamis d'épuration Withdrawn EP0733734A3 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FI951396 1995-03-24
FI951396A FI97983C (fi) 1995-03-24 1995-03-24 Sihtausmenetelmä ja sihti

Publications (2)

Publication Number Publication Date
EP0733734A2 true EP0733734A2 (fr) 1996-09-25
EP0733734A3 EP0733734A3 (fr) 1998-06-10

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ID=8543115

Family Applications (1)

Application Number Title Priority Date Filing Date
EP96660005A Withdrawn EP0733734A3 (fr) 1995-03-24 1996-03-21 Procédé et tamis d'épuration

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EP (1) EP0733734A3 (fr)
FI (1) FI97983C (fr)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0791686A1 (fr) * 1996-02-28 1997-08-27 Valmet Corporation Dispositif pour éviter le bouchage d'une buse de coucheuse
EP0916765A1 (fr) * 1997-11-13 1999-05-19 Valmet-Raisio Oy Procédé et dispositif pour la separation des materiaux etrangers de la circulation machine d'un melange d'enduisage dans une station d'enduit
EP1700953A1 (fr) * 2005-03-10 2006-09-13 Sappi Alfeld GmbH Procédé pour le nettoyage et recyclage d'une sauce de couchage
WO2010001153A1 (fr) * 2008-06-30 2010-01-07 Imerys Minerals Limited Processus de revêtement à réutilisation de l’excès de revêtement retiré
US7887872B2 (en) * 2004-09-21 2011-02-15 Metso Paper, Inc. Method and apparatus for separating impurities from machine circulation of coating station
EP2292834A1 (fr) * 2008-08-28 2011-03-09 Hans-Joachim Boltersdorf Triturateur doté d'une tôle de criblage et utilisations d'un tel triturateur
CN103240225A (zh) * 2012-02-09 2013-08-14 美卓造纸机械公司 筛分机

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111608004A (zh) * 2020-06-17 2020-09-01 重庆润民纸业有限公司 一种瓦楞纸制造用纸浆过滤装置及其使用方法

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1111914A (en) * 1965-07-07 1968-05-01 Buckeye Cellulose Corp Removal of gels and small particles from viscose
DE3010952A1 (de) * 1980-03-21 1981-10-08 J.M. Voith Gmbh, 7920 Heidenheim Scheibensortierer zum reinigen von faserstoffsuspensionen
GB2074886A (en) * 1980-04-22 1981-11-11 British Sidac Ltd A backwashable filter
WO1994024365A1 (fr) * 1993-04-16 1994-10-27 Sigurd Fongen Moyen de filtration et de fractionnement de suspensions contenant des fibres, des fragments de fibres, des particules fines et d'autres types de particules.

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1111914A (en) * 1965-07-07 1968-05-01 Buckeye Cellulose Corp Removal of gels and small particles from viscose
DE3010952A1 (de) * 1980-03-21 1981-10-08 J.M. Voith Gmbh, 7920 Heidenheim Scheibensortierer zum reinigen von faserstoffsuspensionen
GB2074886A (en) * 1980-04-22 1981-11-11 British Sidac Ltd A backwashable filter
WO1994024365A1 (fr) * 1993-04-16 1994-10-27 Sigurd Fongen Moyen de filtration et de fractionnement de suspensions contenant des fibres, des fragments de fibres, des particules fines et d'autres types de particules.

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0791686A1 (fr) * 1996-02-28 1997-08-27 Valmet Corporation Dispositif pour éviter le bouchage d'une buse de coucheuse
EP0916765A1 (fr) * 1997-11-13 1999-05-19 Valmet-Raisio Oy Procédé et dispositif pour la separation des materiaux etrangers de la circulation machine d'un melange d'enduisage dans une station d'enduit
US7887872B2 (en) * 2004-09-21 2011-02-15 Metso Paper, Inc. Method and apparatus for separating impurities from machine circulation of coating station
EP1700953A1 (fr) * 2005-03-10 2006-09-13 Sappi Alfeld GmbH Procédé pour le nettoyage et recyclage d'une sauce de couchage
WO2010001153A1 (fr) * 2008-06-30 2010-01-07 Imerys Minerals Limited Processus de revêtement à réutilisation de l’excès de revêtement retiré
EP2292834A1 (fr) * 2008-08-28 2011-03-09 Hans-Joachim Boltersdorf Triturateur doté d'une tôle de criblage et utilisations d'un tel triturateur
CN103240225A (zh) * 2012-02-09 2013-08-14 美卓造纸机械公司 筛分机
EP2626146A1 (fr) 2012-02-09 2013-08-14 Metso Paper Inc. Écran à mailles
CN103240225B (zh) * 2012-02-09 2016-01-06 维美德技术有限公司 筛分机

Also Published As

Publication number Publication date
FI951396A (fi) 1996-09-25
EP0733734A3 (fr) 1998-06-10
FI97983C (fi) 1997-03-25
FI97983B (fi) 1996-12-13
FI951396A0 (fi) 1995-03-24

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