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

Definitions

• This invention relates to separation of a fiber fraction from a liquid containing fibers and small particles.
• the treatment is carried out in several steps, at which the process water is supplied and drained.
• the papermaking pulp is transported as fiber suspension at a pulp concentration of 3 to 4 % from the pulp mill to the paper mill.
• the process water in the paper mill thus, originates mainly from the pulp mill.
• the surplus water from the papermaking is re-used later on at the pulpmaking. This implies that at the making of pulp and paper very great amounts of water are used which mainly circulate in the manufacturing process. From this complicated process results totally a small surplus of water, which must be cleaned before it can be discharged to the recipient.
• Inreased re-circulation of water in the mill results in an increased build-up of contaminations in the process.
• contaminations such as size lumps, particles of printing ink, ash and other matter (dispersed or solved) affect the process negatively. Even if it was possible to remove a large portion of the heavily contaminated process water, it is normally not defensible economically, because it often contains too many prime fibers.
• Another object of the present invention is to increase the fiber separation efficiency of liquor screens in chemical pulp mills. These screens separate fibers from the waste liquor, before it is to be combusted. The separation efficiency of these screens is often much too low, especially in the case of short-fibrous hardwood pulps.
• Fig. 1 is a flow chart showing an embodiment of the invention
• Fig. 2 shows the effect of re-circulation of fibers according to the invention.
• Fig. 1 The embodiment shown in Fig. 1 comprises a conventional screen means 1 with an inlet 2 and a first outlet 3 for a first fraction and a second outlet 4 for a second fraction. There is further a return line 5 with a central valve 6 between the second outlet 4 and inlet 2.
• Fig. 1 the volume flow is indicated by the following definitions:
• V c return flow
• a liquid which can be process water containing fibers and contaminations, is fed as inject (Vj) through the inlet 2 to the screen means 1.
• the liquid is allowed to flow along a screen member with screen apertures whereby a first fraction (V f - V r ) passes through the screen apertures and is taken out through the first outlet 3, and a second fraction (V r - V c ), which does not pass through the screen apertures, is taken out through the second outlet 4.
• the first fraction (V f - V r ) contains mainly small particles in the form of dispersed contaminations and fine material from the papermaking pulp
• the second fraction (V r - V c ) contains mainly long usable fibers.
• the fiber concentration of the ingoing liquid is very low and thereafter increases along the screen member during the course of fractionation. This implies that the strength of the fiber network increases much along the screen member, from the inlet to the outlet for concentration fibers.
• the energy supply from the rotor of the screen produces turbulence, which breaks up the weak fiber network at the beginning of the screening zone, which results in deteriorated separation of fibers.
• the strength of the fiber network is much higher, because the fiber concentration has been multiplied, and the efficiency of the separation process is high.
• the fiber network formed on the surface of the screen member due to the flocculation tendency of the fibers has a very important role in the separation mechanism. It is, thus, not only the apertures of the screen member which separate the fibers from the liquid.
• the concentration of this fraction can be increased at the inlet end of the screen member.
• the flow of the second fraction preferably is increased 2-5 times, and 50 - 80% of this fraction is recycled to the inlet.
• the differences in concentration decrease strongly along the length of the screening zone with the help of the long fiber re-circulation according to the invention. This renders it possible to optimize the supplied de-flocculation energy along the screening zone so that the efficiency of the process increases, i.e. it is thereby possible to substantially decrease the share of long usable fibers in the first fraction.
• An increased fiber concentration in the ingoing liquid increases the network formation in the ingoing fraction, and the probability that network forming fibers shall pass through the apertures of the screen barrier decreases.
• Fig. 2 shows by means of a theoretical model based on mill data the effect of recirculation of a varying portion (V c ) of the second fraction (V r + N c ) to the inlet when the totally separated flow is 5 and 10 %, respectively.
• the flow division (FS V/ V f ) is 0,05 and 0,10, respectively. It appears from Fig. 2, that it is possible to increase the efficiency of the recovery of usable fibers from about 80 % to about 90 % by using a re-circulation factor (CF) of 3-4.
• the fiber concentration in the inlet has simultaneously trebled from 0,2 to 0,6.
• the invention can also be used for increasing the efficiency of liquor screens in processes for the production of chemical pulp.
• Liquor screens are used, for example, in a sulphate mill for recovering fibers from black liquor before its combustion.

Landscapes

• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Paper (AREA)
• Separation Of Solids By Using Liquids Or Pneumatic Power (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=20412375

Family Applications (1)

Country Status (7)

Families Citing this family (2)

Family Cites Families (5)

• 1998
  • 1998-08-26 SE SE9802870A patent/SE9802870L/ not_active Application Discontinuation
• 1999
  • 1999-06-09 AU AU46698/99A patent/AU750899B2/en not_active Ceased
  • 1999-06-09 WO PCT/SE1999/001003 patent/WO2000012811A1/en not_active Application Discontinuation
  • 1999-06-09 CA CA002341535A patent/CA2341535A1/en not_active Abandoned
  • 1999-06-09 JP JP2000567786A patent/JP2002523651A/ja not_active Withdrawn
  • 1999-06-09 BR BR9913254-0A patent/BR9913254A/pt not_active IP Right Cessation
  • 1999-06-09 EP EP99930090A patent/EP1108084A1/de not_active Withdrawn

Non-Patent Citations (1)

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