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
• • 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
• • 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
  • D21D5/18—Purification of the pulp suspension by mechanical means; Apparatus therefor with the aid of centrifugal force
  • D21D5/24—Purification of the pulp suspension by mechanical means; Apparatus therefor with the aid of centrifugal force in cyclones

Definitions

• the present invention relates to a method and apparatus for the treatment of fiber suspensions in the pulp and paper industry.
• the invention relates in particular to intensifying the so-called post-screening or secondary screening of bleached pulp or pulp which is in the bleaching stage.
• the hydrocyclone plant requires quite a large space, mainly due to the large number of cyclones.
• the use of cyclones further has a third drawback, i.e. a remarkable increase in the volume of the internal circulation waters and the effluent load of the pulp mill.
• the consistency of the pulp suspension to be cleaned must be of the order of tenths of a percent in order to enable a hydrocyclone to work efficiently and separate well enough the acceptable fiber fraction from knot particles and sticks.
• a further drawback of a hydrocyclone cleaning plant should be mentioned, i.e. the fact that bleaching plants usually employ washers, the washing boxes of which require a consistency of about 1.2 %, at least 1.0 %, of the pulp to be fed into the boxes. Also the drying machine, which is the subsequent treatment phase in the process following the bleaching, requires the same feed consistency of about 1.0 - 1.2 %.
• a hydrocyclone plant does not work efficiently enough at the consistency mentioned, it requires a consistency of about 0.5 %. This means that pulp must be thickened after a hydrocyclone plant from a consistency of 0.5 % to a consistency of 1.0 - 1.2 %.
• a screen must be used in a secondary stage for screening, the screen must have larger screen openings for recovering fiber material; then for example the expanded polystyrene pieces mentioned will pass through the secondary screen as such. Thus, it is almost impossible to separate non-desirable material of for example the type mentioned above from the accepted fiber material.
• a hydrocyclone accepts light reject, for example expanded polystyrene bits and also other light pieces, such as plastics particles, with the fiber material.
• a so-called reverse hydrocyclone disclosed for example in U.S. patent no. 4,155,839 has been developed for solving the problem mentioned.
• a characteristic feature of the cleaner in question, and of its function in particular, is that it is capable of separating light rejects, for example plastics, and acceptable fiber material from each another efficiently and precisely although the specific gravities of the materials do not differ much.
• a cleaner of this type works so that it discharges the heavier fiber fraction via its narrower end at the tip of a cone which in the conventional position of the apparatus is the lower end, and the lighter reject via a discharge opening located centrally at the top end.
• Reversed hydrocyclones have, however, been erected also the other way around, i.e. the tip of the cone upwards.
• a severe problem which may be solved with the method and the apparatus according to the present invention is connected with the so-called secondary screening plant described above which is capable of preventing by means of the state-of-the-art narrow slot screening for example plastics pieces from ending up to the end product.
• the state-of-the-art secondary screening plant comprises a hydrocyclone as a device subsequent to the narrow-slot screen, it accepts also light reject, i.e. for example plastics particles, with the fiber material.
• the light reject mentioned remains circulating in the screen circulation until it is ground into so small pieces that the narrow-slot screen accepts it. As a consequence the light reject ends up to the end product and in the worst case causes clearly perceivable quality problems.
• reject from the so-called primary screen is taken to a pressure screen, and reject from this pressures screen is further treated in a so-called first hydrocyclone step.
• the method and the apparatus described above may still be developed further so that accept from the first hydrocyclone step mentioned is taken either to a second, reversed hydrocyclone step or to a pressure screen.
• the method and the apparatus described above may still be developed further so that the accept from the reverse hydrocyclone step or the pressure screen mentioned is brought to the feed of the secondary screen and the reject is removed to a second reverse hydrocyclone step.
• the reject from the so-called primary screen is introduced into the first hydrocyclone step, the accept from which is treated in the reverse hydrocyclone step, the accept from which is returned to the feed of the primary screen and the reject is removed from the process or guided to a suitable further treatment point.
• the methods and the apparatus according to the embodiments described above improve the cleaning result of the screening plant remarkably because different screening apparatus, such as pressure screen, hydrocyclone and reverse hydrocyclone, are used in a way to utilize their best properties, only.
• the apparatus in question provide excellent supplement to each other so that the final reject contains only particles, which really should be rejected.
• the apparatus according to the invention is essentially less expensive for the pulp mill to purchase and to maintain; thus, considering also the improved efficiency/accuracy of the screening plant, a superior product is provided compared with both prior art methods and apparatus.
• Fig. 1 illustrates schematically a prior art apparatus
• FIG. 2 illustrates schematically another prior art apparatus
• Fig. 3 illustrates schematically an apparatus according to a preferred embodiment of the invention
• Fig. 4 illustrates schematically an apparatus according to another preferred embodiment of the invention
• Fig. 5 illustrates schematically an apparatus according to a third preferred embodiment of the invention.
• Fig. 6 illustrates schematically an apparatus according to a fourth preferred embodiment of the invention.
• a prior art arrangement comprises mainly a bleaching tower 2, a subsequent hydrocyclone plant 4, a thickener 6 and a washer, most usually a drum washer 8, and a bleaching tower 10 of the subsequent bleaching stage.
• Fiber suspension to be treated is discharged from the tower 2 to a pipe line in which the suspension is diluted to a consistency of about 0.5 % and supplied further to the hydrocyclones 4.
• the accepted material is fed directly to the thickener 6, which thickens the pulp to the feed consistency of the subsequent washer 8.
• the pulp is supplied to the washer and the washed pulp is discharged from the washer at the bleaching consistency of the bleaching tower.
• the reject from the hydrocyclones is removed from the apparatus either to be destroyed or to be guided to a previous treatment stage.
• FIG. 2 illustrates another prior art arrangement (U.S. 5,571,384) comprising a bleaching tower 20, a screen 22, a hydrocyclone/cleaning plant 24, a washer 26 and a bleaching tower (not illustrated) of the subsequent, usually the last bleaching stage.
• the pulp bleached in the tower 20 is discharged from the bleaching tower 20 to a pipeline.
• the pulp either has been diluted while being discharged or it is diluted in the pipeline to a screening consistency of > about 1.0 - 1.2 %; from the pipeline the pulp is supplied to the screen 22.
• the accept from the washer 22 is fed directly into the washer 26 and the reject is supplied to a hydrocyclone plant 24, which is remarkably smaller than prior art hydrocyclone plants, at a consistency diluted to be suitable for the cyclones.
• the accept from the hydrocyclones 24 is recirculated to the feed of the screen 22 mentioned above or directly to the washer 26.
• the reject in turn is either destroyed or returned to a previous suspension treatment stage.
• the apparatus according to a preferred embodiment of the invention illustrated in Fig. 3 comprises a so-called primary screen 30 and the accept from this is directed by prior art means in a so-called main treatment line 30a to the process for further use.
• the reject from the primary screen 30, i.e. the coarse fraction is in this embodiment transported in a line 30r to a second step pressure screen 32, a so-called secondary screen; the accept from the secondary screen is returned in line 32a to the feed of the primary screen 30, and the reject, the coarser fraction, is directed in line 32r to a first hydrocyclone step 34.
• the accept from the secondary screen may be returned to the main line 30a to a location after the primary screen (illustrated with a broken line).
• the accept, i.e. the lighter fraction, from the first hydrocyclone step 34 is guided in line 34a to a pressure screen 36 and the reject, i.e. the heavier fraction, is taken in line 34r to a second hydrocyclone step 38.
• the accept from the second hydrocyclone step 38 is returned in line 38a to the feed of the first hydrocyclone step 34 and the reject is discharged from the process.
• the accept from the pressure screen 36 i.e.
• the fraction which has passed through the screen surface is guided in line 36a to the feed of the pressure screen 32 of the second screening step and the reject, i.e. the coarser fraction, in line 36r to a reverse hydrocyclone 40.
• the heavier fraction from the hydrocyclone 40 is recirculated in line 40a to the feed of the pressure screen 36 as the accept from the reverse hydrocyclone step 40, whereas the lighter fraction obtained from the cleaner 40 is removed (line 40r) as reject from the process, or is guided to a suitable further treatment.
• the screening member serving as the primary screen 30 is a slotted cylinder having a slot width of 0.15 mm.
• a cylinder of this kind accepts, or allows passage therethrough of, only fiber material suitable for further use. All larger fiber particles, sand, sticks, and other impurities, such as for example plastics, entrain to the reject.
• the reject is conveyed to the secondary screen 32 preferably comprising a slotted cylinder having a slow width of 0.18 mm. In principle also this cylinder should accept only acceptable fiber fraction but, only to be sure, the accept from this screen is returned to the feed of the primary screen for rescreening.
• the reject from the secondary screen is conveyed to the first hydrocyclone step 34 comprising a required number of cleaners connected in series.
• the task of the first hydrocyclone step is to separate from the reject of the secondary screen, on one hand, the acceptable fiber fraction to the accept and, on the other hand, the heavy fraction, such as sand, to the reject.
• the heavy reject is treated in the secondary hydrocyclone step 38 in order to recover usable fiber material and to return it to the first hydrocyclone step; after this almost only sand or other heavy impurities are removed from the step 38 as the reject.
• the accept from the first hydrocyclone step 34 is conveyed at a very low consistency (of the order of 0.4.
• the pressure screen 36 which has a very small slot size, for example 0.12 mm.
• the purpose is to separate from the accept of the first hydrocyclone step almost all the usable fiber fraction and to return it to the feed of the secondary screen.
• Reject from the pressure screen 36 which may be taken either continuously or intermittently depending on the amount of the reject, is treated in the reverse hydrocyclone 40 the main task of which is to separate the light reject from the fiber-containing material.
• the fiber flow obtained as the accept from the reverse hydrocyclone step 40 at the tip of the cone is conveyed in a thickened state to the feed of the pressure screen 36 through which the fiber fraction is further taken into use.
• slotted cylinders are used in the different screens the slot width of the secondary screen 32 drum is less than 0.25 mm, preferably less than 0.20 mm, and in the screen 36 less than 0.20 mm, preferably less than 0.15 mm.
• the apparatus according to another preferred embodiment of the invention illustrated in Fig. 4 comprises, like the one in Fig. 3, a primary screen 30, a secondary screen 32, and a subsequent first and a second hydrocyclone step 34 and 38.
• the pressure screen 36 of Fig. 3 has been replaced in Fig. 4 with a reverse hydrocyclone step 46, the accept of which, i.e. the heavier fraction, is returned in line 46a to the feed of the secondary screen 32 and the lighter fraction discharged as reject is conveyed in line 46r to a second reverse hydrocyclone step 48.
• the accept from the second reverse hydrocyclone step is conveyed in line 48a to the feed of the first reverse hydrocyclone step and the reject is discharged (line 48r) from the process. It is possible also in this embodiment, and particularly when the secondary screen has been constructed in the required way, to return the accept from the secondary screen also to the main line 30a to a position after the primary screen (illustrated with a dash line).
• the slot width of the cylinder in the primary screen 30 is less than 0.20 mm, preferably about 0.15 mm and the slot width of the cylinder in the secondary screen 32 is less than 0.25 mm, preferably less than 0.20.
• the apparatus according to a third preferred embodiment of the invention illustrated in Fig. 5 comprises, like the previous embodiments, a primary screen 30, a secondary screen 32, and a first hydrocyclone step 34. It differs from the previous embodiments in that the accept from the secondary screen 32 is conveyed in line 32a, instead of the feed of the primary screen 32, to a reverse hydrocyclone 57 and the accept, i.e. the heavier fraction, from the hydrocyclone 57 is conveyed in line 57a to the feed of the primary screen.
• the secondary screen 32 removes large particles to the hydrocyclone step 34 which classifies them according to their specific gravity either to reject (line 34r) or to be returned in line 34a to the feed of the secondary screen 32.
• the accept of the secondary screen contains light material which is taken to the reverse hydrocyclone 57 in which the light reject (line 57r) is separated from the process and the heavier, the fiber fraction is returned to the feed of the primary screen 30. Also in this embodiment, and particularly when the secondary screen 32 and the hydrocyclone step 34 have been constructed in the required way, the accept from the hydrocyclone step 34 may be recirculated to the main line 30a to a point after the primary screen (illustrated with a dash line).
• the apparatus according to a fourth preferred embodiment of the invention illustrated in Fig. 6 comprises a primary screen 30, the reject from which is conveyed by prior art methods in line 3 Or to a first hydrocyclone step 62, the reject from which is removed by a know method in line 62r from the process or it is conveyed to a suitable further treatment step.
• the new approach in the apparatus of this embodiment is to convey the accept from the first hydrocyclone step 62 in line 62a to a reversed hydrocyclone step 64 the heavier fraction from which is returned in line 64a to the feed of the primary screen 30 and the lighter fraction is rejected from the process.
• a second new feature is that the filtrate from the thickening device 66 following the primary screen in the line 30a is treated by flotation in order to recover fiber material.
• the invention preferably employs a screen cylinder disclosed in Finnish patent application no. 955724, comprising a plurality of screen wires provided side by side and secured firmly to support wires provided on the accept space side.
• narrow slots are preferably used the width of which is of the order of ⁇ 0.25 mm, preferably ⁇ 0.20 mm, sometimes even nearly of the order of 0.10 mm.
• the screen element may have the form of a cone, a double cone, or even a plane.
• the rotor in the pressure screen mentioned may be for example the rotor illustrated in U.S. patent no. 5,000,842 in which the surface of the rotor cylinder has been provided with so-called projections to generate turbulence which disrupts fiber floes in the suspension. Also so-called foil rotors may be used in some cases. Further, a screen cylinder either inside or outside the rotor may be used, or even cylinders provided on both sides of the rotor. Further, in some cases, a rotating screen cylinder or cylinders may be used. As may be understood from the above description, a method and apparatus for screening suspension has been developed which, compared with prior art methods and apparatus, is very user-friendly and advantageous for both the process economy and the energy economy. However, it should be noted that the invention has been described above by way of reference to only a few embodiments thereof although the real scope of protection of the invention is much broader and covers more. Thus, the invention is defined by the appended patent claims, only.

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• Mechanical Engineering (AREA)
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Publications (1)

Family

ID=8549909

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Cited By (4)

Citations (5)

• 1997
  • 1997-11-11 FI FI974192A patent/FI103734B/fi active
• 1998
  • 1998-11-11 WO PCT/FI1998/000874 patent/WO1999024662A1/fi active Application Filing
  • 1998-11-11 ID IDW990823A patent/ID22428A/id unknown
  • 1998-11-11 CA CA002278610A patent/CA2278610A1/en not_active Abandoned

Patent Citations (5)

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