Classifications

• • 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
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1031—Pulse, dynamic, displacement processes
• • 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
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0021—Introduction of various effluents, e.g. waste waters, into the pulping, recovery and regeneration cycle (closed-cycle)
  • D21C11/0028—Effluents derived from the washing or bleaching plants
• • 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
  • D21C9/02—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents
  • D21C9/06—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents in filters ; Washing of concentrated pulp, e.g. pulp mats, on filtering surfaces

Definitions

• the present invention relates to a continuous-working treatment of a fiber pulp with a chemical solution by forming from a pulp suspension a pulp bed on a filter surface and by feeding the bed through one of several chemical treatment steps in which a chemical solution is displaced through the bed.
• a process of this type is used for the bleaching of a cellulose material it is in general called displacement bleaching or dynamic bleaching.
• the fiber When it is desired to produce a cellulose fiber having a high degree of brightness, the fiber has to be bleached after the actual defibration process. For example, in order to obtain a sulfate pulp with a high degree of brightness, it normally has to be bleached in five steps by using chlorine and chlorine dioxide as the bleaching chemical and sodium hydroxide as the extraction chemical.
• the pulp bed Before the removal of the above- mentioned chemical solution, and together with it the formed reaction products, from the bed by displacing them with the chemical solution used in the subsequent step, or in the case of the last bleaching step with washing water, the pulp bed is directed through a so-called stationary step, with a sufficient retention time, in order that the concentration profile pro ⁇ cuted in the bed after the displacement should have time to level out under the effect of diffusion.
• the retention time of the above-mentioned stationary step should be in the same order as the retention times in the conventional method.
• the object of the present invention is to avoid the above- mentioned disadvantage by using a displacement process which requires no stationary step for achieving an even chemical treatment throughout the pulp bed.
• the bleaching process based on the displacement method is in many respects similar to the washing process based on the dis ⁇ placement method, but there are between them also great dif ⁇ ferences which it is important to take into account expediently in the further development of the bleaching process.
• both processes there occurs a similar mixing of liquids due to dis ⁇ persion and to the fact that the liquid flow in the fiber bed is laminar.
• the displacement never occurs ideally as a plug, but there always occurs mixing between the liquids, which is disadvantageous in both the wash ⁇ ing process and the bleaching process.
• This disadvantageous mixing can be reduced in both processes by carrying out several successive displacements according to the countercurrent prin ⁇ ciple.
• the higher the number of displacement steps used the higher the respective equipment costs.
• the displacement is based on the cross-flow method, in which the liquid flow through the bed during the displacement is perpendicular to the travel direc ⁇ tion of the bed.
• pulp fibers are compressible, the consistency of the pulp bed is not constant throughout the bed; it increases exponen ⁇ tially towards the filter surface. This profile is, of course, dependent on the type of the pulp concerned and on how large a pressure difference is used to produce the liquid flow. Normal ⁇ ly it rises from a consistency value of approx. 2 % to approx. 20 %, being on average approx. 10 %.
• this phenomenon does not have a great significance, but in bleaching, in which it is important that all the fiber layers in the bed receive a sufficient quantity of active bleaching chemicals, it is of especially great significance, in particular since, in addi ⁇ tion, the concentration of the active chemicals in the solution decreases continuously as the solution flows toward the filter surface. This is due to the above-mentioned dispersion phenome ⁇ non and the fact that chemicals are consumed as they react with the pulp layers through which they flow.
• the pulp layers which are at a greater distance from the filter surface are treated with a chemical solution which has a great excess of chemicals as compared with the requirement.
• the layers close to the filter surface do not receive a sufficient amount of active chemicals, or receive no active chemicals, during the displacing, and so the bleaching of the pulp bed is quite uneven.
• the object of the present invention is to provide this kind of pulp bed treatment, but in such a manner that through losses of active chemicals there is not caused a higher consumption of chemicals than in a normal bleaching process.
• a pulp bed is formed from a clean washed pulp to be bleached, and the washing liquid present in it is displaced by using as the displacing liquid the first bleaching solution (which con ⁇ tains as the active chemicals, for example, chlorine and/or chlorine dioxide), which contains active chemicals in excess, preferably approx. double the amount required in this step for the complete chemical treatment of the whole pulp bed.
• the first bleaching solution which con ⁇ tains as the active chemicals, for example, chlorine and/or chlorine dioxide
• the active chemicals in excess preferably approx. double the amount required in this step for the complete chemical treatment of the whole pulp bed.
• the filtrate displaced from the bed during these displacements is divided into two fractions so that the first fraction con ⁇ tains the above-mentioned washing liquid and the first dis ⁇ placed bleaching solution which has reacted with the bed and contains in the main only spent bleaching chemicals and bleach ⁇ ing reaction products.
• the second fraction of the filtrate con ⁇ tains most of the bleaching chemicals which have not been spent during the displacement.
• This solution fraction is recovered and reinforced with fresh bleaching chemicals in the same amount as is the consumption of the active chemical in the step, before it is reused in the same chemical treatment step.
• one bleaching step contains not only one displacement but displacements with at least two solutions. Respectively, the filtrates are divided into at least two fractions.
• the bleaching steps described above may be included several in succession in the process.
• the recovery of the above-mentioned active chemicals effective ⁇ ly displaced through the bed is important in order that as little as possible of the solution of the step, containing ac ⁇ tive chemicals, should become mixed with the active chemical solution used in the subsequent treatment step.
• the active chemical solution of the directly subsequent treatment step is not used for displacing the active chemical solution of the step con ⁇ cerned, but a solution as neutral as possible with respect to both treatment steps is used, for example preferably pure water at a suitable temperature.
• a chlorine solution is displaced directly with a sodium hydroxide solution, owing to dispersion the solutions become mixed with each other in the boundary zone. Thus they partly cancel each other's positive effect in terms of bleaching and, in addition, disadvantageous secondary reactions are produced. Owing to the drastic pH change, for example, the chlorine solution may be converted to a hypochlorite solution.
• the filtrate which is removed from the fiber bed through the filter surface during the chemical treatment by displacing it with the active chemical solution and the neutral solution used in the step changes with respect to its chemicals content as the pulp bed proceeds during the progress of the displacement. Its first fraction contains mainly chemicals used and spent in the previous step and the reaction products produced. The frac ⁇ tion then changes more slowly or faster, according to the strength of the dispersion caused by the bed, to a filtrate which mainly contains the active chemicals used in the treat ⁇ ment step in question, which chemicals have become non-active when reacting with the fiber material of the pulp bed, and reaction products which have been formed.
• This fraction then in a corresponding manner changes to a fraction which in the main contains active chemicals used in the treatment step which have not been spent through reaction and correspond to the excess of active chemical which is used in the treatment step.
• This frac ⁇ tion for its part changes, when the bed proceeds, in a cor ⁇ responding manner to a fraction which in the main contains chemicals which are derived from the neutral solution which is used as a displacement solution in this step immediately after the active chemical solution.
• this solution as is normally recommendable, is pure water, the above-mentioned fraction which contains active chemicals changes to nearly pure water before it again changes to a solution which contains spent chemicals used in the subsequent treatment step and the formed reaction products, unless the step in question is the last treatment step.
• the third filtrate fraction above which contains active chemi ⁇ cals which have not been spent in the reactions in the ' treat ⁇ ment step, are recovered as carefully as possible and are re ⁇ used together with fresh added chemicals as a chemical solution in the same treatment step.
• the portion of this solution which can be recovered is, of course, the active chemical solution quantity of the step minus the quantity of fresh chemicals.
• the maximum recovery of active chemicals is obtained when it is possible to adjust the removal of the filtrate fractions so that the maximum concentration of active chemicals in both of the "tails" is the same, i.e. the concentration of active chem ⁇ icals in the filtrate is the same at both division points of the filtrate fractions.
• the filtrate could be divided into different fractions simply by dividing the filtrate chamber behind the screen surface by means of partitions sealing tightly to the screen. In this case it would, however, not be possible in all operating situations to achieve maximal recovery of chemicals, since the location of the chemical profile of the filtrate in the equipment used shifts when the ratio between the displacement liquid flow rate and the filter surface travel speed changes. In addition to this, the location of the chemical profile between the chemi ⁇ cals spent in the reaction and the still active chemicals is affected by the proportion of the used active chemicals spent in the treatment of the bed. This proportion depends, among other things, on how much residual lignin or a similar chemicals-consuming constituent is present in the pulp bed which is being treated.
• the filtrate chamber is divided by means of partitions which leave a uniformly wide slit in relation to the filter surface, through which slit part of the filtrate fraction can pass into the preceding or subsequent filtrate fraction.
• the most recommendable embodiment is that the principal vari ⁇ ables of the process are in the same order as when the equip ⁇ ment is used for washing pulp.
• the thickness of the pulp bed is 20 -100 mm, preferably approx. 50 mm.
• the pressure difference for accomplishing the liquid displacement is 1 - 4 meters of water head.
• the travel speed of the screen is 0,2 - 1 m/s, preferably approx. 0.5 m/s.
• the total quantity coefficient of the displacement of one treatment step is >1.5, preferably >2.
• the quantity coefficient of the active chemical solution is approx. one-half of these values.
• the total retention time of the fiber bed in one treat ⁇ ment step will be 10 - 50 seconds, normally approx. 20 seconds. Since the most preferred embodiment of the present invention is to use in each displacement bleaching step a high quantity co ⁇ efficient, i.e. a coefficient approx. twice that in the dis ⁇ placement bleaching processes used so far, and in addition to divide each step into at least two partial steps, the use of this bleaching process requires that there be available a method and equipment required for implementing the method, en ⁇ abling several successive displacement steps to be implemented in one and the same apparatus and that, when steps are added their marginal costs are relatively low and also the equipment costs calculated per effective screen surface are economical.
• the surface layer of the fiber bed is treated with an approx. double amount of chemicals as compared with the bottom layer.
• This non-uniform distribution does not, however, affect the uniform chemical treatment of the bed, since, if we examine the individual fiber layers of the bed, through each layer there flow active chemicals the amount of which approaches infinite in ratio to the amount that the fiber layer concerned spends in order to achieve a complete reaction.
• the present process requires a maximally rapid transfer of chemicals, which is based on a forced, directed flow not only around the fibers but also through them, because the retention time is so short that no significant transfer of material can take place through diffusion. For this reason it is also neces ⁇ sary to aim at the system remaining a two-phase system, which requires that the pressure in the system will not be below the total equilibrium pressure of the gases and vapors. In this case a maximum proportion of the gases and vapors remains dis ⁇ solved in the liquid phase, and no disturbing gas bubbles or gas phase is formed in the bed during the treatment.
• the bleaching process becomes simple, since it is possible to carry out several bleaching steps in one apparatus.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Treatments For Attaching Organic Compounds To Fibrous Goods (AREA)
• Communication Cables (AREA)
• Management, Administration, Business Operations System, And Electronic Commerce (AREA)
• Medicines Containing Antibodies Or Antigens For Use As Internal Diagnostic Agents (AREA)

Priority Applications (5)

Applications Claiming Priority (2)

Publications (1)

Family

ID=8528196

Family Applications (1)

Country Status (13)

Cited By (5)

Citations (3)

Family Cites Families (1)

• 1989
  • 1989-04-07 FI FI891662A patent/FI85513C/fi not_active IP Right Cessation
• 1990
  • 1990-04-06 ES ES90905528T patent/ES2091240T3/es not_active Expired - Lifetime
  • 1990-04-06 NZ NZ233237A patent/NZ233237A/xx unknown
  • 1990-04-06 DE DE69027770T patent/DE69027770T2/de not_active Expired - Fee Related
  • 1990-04-06 AU AU54201/90A patent/AU647020B2/en not_active Ceased
  • 1990-04-06 EP EP90905528A patent/EP0466752B1/en not_active Expired - Lifetime
  • 1990-04-06 CA CA002049951A patent/CA2049951A1/en not_active Abandoned
  • 1990-04-06 BR BR909007276A patent/BR9007276A/pt not_active Application Discontinuation
  • 1990-04-06 JP JP2505791A patent/JPH05500835A/ja active Pending
  • 1990-04-06 WO PCT/FI1990/000095 patent/WO1990012146A1/en active IP Right Grant
  • 1990-04-06 AT AT90905528T patent/ATE140282T1/de active
  • 1990-04-09 ZA ZA902703A patent/ZA902703B/xx unknown
• 1991
  • 1991-10-04 NO NO913904A patent/NO176485C/no unknown

Patent Citations (3)

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