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
  • D21C9/1057—Multistage, with compounds cited in more than one sub-group D21C9/10, D21C9/12, D21C9/16
• • 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
  • D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
  • D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
  • D21C9/144—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 and other bleaching agents in a multistage process
• • 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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• the present invention relates to a process for bleaching of ligno-cellulosic pulps for reduced formation of chloro-organic substances while preserving the final brightness and strength properties of the pulp, where,in a first treatment stage, only chlorine dioxide is used at a low charge factor, whereupon, in a second stage, the treatment with hydrogen peroxide is carried out under alkaline conditions with increased charging of peroxide. After the two-stage treatment the pulp can be finally bleached to the desired brightness in one or more chlorine dioxide stages.
• ligno-cellulosic pulp refers to chemical pulps from softwood and/or hardwood, delignified according to the sulphite, sulphate, soda, or organosolv process, or modifications and/or combinations thereof. Before the bleaching sequence with chlorine-containing chemicals according to the invention, the pulp may also have been delignified in an oxygen stage.
• Bleaching of chemical pulps is mainly carried out with chlorine-containing bleaching agents, such as chlorine, chlorine dioxide and hypochlorite, and in addition to this in intermediate extraction stages with alkali and hydrogen peroxide and/or oxygen.
• the chlorine-containing substances react with the lignin and render it soluble, either in the same stage or in the subsequent alkaline extraction stage.
• a normal bleaching sequence for softwood treated in accordance with the sulphate process is (C + D) E1 D E2 D wherein
• the amount of chloro-organic compounds formed is reduced by a combination of a changeover from molecular chlorine to chlorine dioxide and/or reduced charge factor according to formulae 1 and 2, respectively.
• US 3,720,577 and DE 2,754,949 relate to the use of 100% chlorine dioxide in the first bleaching stage.
• a charge factor of 2.7 is used in Example 1 which is a far too high value as stated above.
• the D stage is followed by a peracetic acid stage and optionally another D stage.
• the peracetic acid reduces the viscosity of the cellulose and yields acetic acid as a by-product, which substantially increases the COD-value (Chemical Oxygen Demand) which is another important measure of the discharges to the environment.
• COD-value Chemical Oxygen Demand
• AOX a low charge factor is used in the D stage but there is no indication that the subsequent extraction stage must be reinforced with hydrogen peroxide to reach acceptable concentrations of AOX.
• these documents do not indicate the importance of using a substantially higher proportion of the chlorine dioxide in the final bleaching to get maximum benefit from the bleaching capacity and still produce a pulp with low impact on the environment.
• US-A-3 655 505 relates to a two-stage bleaching sequence for chemical pulps, where the pulp is bleached with gaseous chlorine dioxide and, after an intermediate washing stage, with a peroxygen compound.
• EP-A-0 429 767 is prior art under Art. 54(3) EPC.
• EP-A-0 429 767 has a priority of 17.11.89 and was published 05.06.91 it relates to bleaching in a first stage which comprises ClO2 followed by an alkaline extraction stage reinforced with oxygen.
• the dosage of the chlorine-containing bleaching agent can be reduced by increasing the temperature in the extraction stage to at least 85°C and reinforcing the extraction with at least 0.8% by weight of oxygen and at least 0.3% by weight of hydrogen peroxide.
• the present invention relates to a treatment process by which the discharge of AOX is greatly reduced, and the brightness and the strength properties after the final bleaching are simultaneously preserved.
• This treatment is carried out in two stages in which the pulp is first bleached with chlorine dioxide at a low charge factor, and a reinforced treatment with hydrogen peroxide and, optionally, oxygen is carried out in the second stage.
• This two-stage treatment results in a bleaching process which is far less detrimental to the environment in that the amount of chlorine-containing chemicals in the bleaching process is reduced.
• the invention thus relates to a process for treating ligno-cellulosic pulp as disclosed in the appended claims.
• the invention relates to a process in the prebleaching of pulp, in which the combination of a low charge factor, a high proportion of chlorine dioxide, and reinforcement of the extraction stage with hydrogen peroxide jointly give the intended AOX reduction.
• Chlorine dioxide as the only chlorine-containing bleaching agent of stage 1, relates to chlorine dioxide produced by conventional techniques, without external addition of chlorine, so-called technical chlorine dioxide.
• the chlorine dioxide may contain chlorine formed during the production and dissolved in the absorption water.
• One example of industrial processes in which a certain amount of chlorine is formed is the reduction of chlorate with chloride.
• Other chlorate reducing agents such as sulphur dioxide and methanol, give but minor amounts of chlorine.
• the chlorine dioxide water from such essentially chlorine-free processes is especially preferred.
• the first stage is carried out by treatment with technical chlorine dioxide at a charge factor of up to 2.0, preferably within the range of from 0.6 to 1.8.
• the most preferred range for the charge factor is from 0.75 to 1.25.
• the pH may lie within the range of from 1 to 4.
• the time and temperature are adjusted such that the concentration of residual chlorine is from 0.1 to 0.5 kg/ton of pulp.
• the pulp concentration may be from 1 to 40% by weight, preferably from 5 to 15% by weight.
• the pulp is treated with hydrogen peroxide at a pH above 7, preferably at a pH in the range of from 7 to 13. Most preferably the pH lies in the range of from 8 to 12.
• the second stage can be reinforced with oxygen up to 10 kg/ton of pulp, but more peroxide remains unreacted after the extraction if no oxygen is used. This peroxide may continue to react with the pulp and may thus improve the properties and the economy.
• Hydrogen peroxide is charged in an amount of from 3.0 kg per ton of dry pulp. The upper limit is not critical and can for economical reasons be set to 20 kg per ton of dry pulp.
• the charge of hydrogen peroxide lies preferably in the range from 4.0 to 10 kg per ton of dry pulp.
• pH at the end of the second stage may lie within the range of from 8.5 to 13, preferably from 10 to 12.
• the residence time may be from about 30 min. to about 6 h at a temperature of from about 50 to 130°C, preferably from 80 to 100°C.
• the pulp concentration may be from 1 to 40% by weight, preferably from 5 to 15% by weight.
• Ligno-cellulosic pulp relates to chemical pulps from softwood and/or hardwood digested according to the sulphite, sulphate, soda or organosolv method, or modifications and/or combinations thereof.
• the method according to the invention is applicable to such bleaching of the treated pulp where the bleaching sequence is preceded by delignification in an oxygen stage.
• the pulp can be finally bleached in known manner to the desired brightness with chlorine dioxide in the sequence D E D, D (EP) D, D D, D, or other combinations of chlorine dioxide stage (D) and extraction stage (E), optionally reinforced with hydrogen peroxide (P).
• a preferred embodiment of the invention is a bleaching sequence, comprising a first stage with chlorine dioxide at a charge factor of up to 2.0, a second stage with hydrogen peroxide at a pH above 7 and with a charge of from 3.0 kg per ton of dry pulp, and at least a third stage with chlorine dioxide, optionally with an intermediate extraction stage. Most preferred is a final bleaching sequence of two chlorine dioxide stages with intermediate extraction, optionally reinforced with hydrogen peroxide.
• the reduction of AOX is improved when the ratio between the amount of chlorine dioxide charged in the prebleaching sequence and the amount of chlorine dioxide charged in the final bleaching sequence, lies within the range of from 1:1 to 1:5, preferably from 1:1 to 1:3.
• the pulp viscosity number which is a measure of the chain length of the carbohydrates, was unchanged.
• the high viscosity implies a high chain length and, thus, a strong pulp.
• the kappa number of the finally bleached pulp is at the same low level as the reference pulp, which means that the amount of undissolved lignin is low.
• the brightness after the final bleaching can be brought to about 89.5% ISO, which is a sufficient and acceptable level for this type of pulp.
• the brightness, measured as % ISO is a standardized technique generally accepted by the cellulose industry.
• This Example is intended to show, for a chemical pulp, the effect of the proportion of chlorine dioxide (D) in the (C + D) stage on the amount of AOX formed in the (C + D) stage, the subsequent E1 stage, and in the final bleaching sequence D E2 D.
• the charge factor was 1.8 throughout.
• (C + D) stage Final pH about 1.5 at 15 and 50% D and 2.5+/-0.3 at 100% D; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp E1 stage: Final pH 11.0+/-0.3; 90°C; 60 min; 2 kg H2O2/ton of pulp; 5 kg oxygen/ton of pulp D stage: Final pH 2.5+/-0.3; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp E2 stage: Final pH 11.0+/-0.3; 60°C; 60 min; 2 kg H2O2/ton of pulp D stage: Final pH 3.4+/-0.2; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp. TABLE I Propn of D in (C + D) (%) AOX (kg/ton of pulp) C + D E1 D E2 D Total 15 1.5 1.4 0.2 3.1 50 1.3 0.75 0.15 2.2 100 0.7 0.15 0.15 1.0
• This Example is intended to show, for the oxygen-delignified pulp, the relationship between the charge factor (CF) according to formula 2, the charging of hydrogen peroxide in the E1 stage, the discharge of AOX from the entire bleaching sequence, and quality properties.
• the bleaching sequence was D E1 D E2 D where the two first stages are included in the prebleaching. 100% chlorine dioxide in varying amounts was used in the D stage of the prebleaching, as shown in the Table. A total of 32 kg chlorine dioxide/ton of pulp, calculated as active chlorine, was used in the final bleaching.
• D stage Final pH 2.5+/-0.3; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp E1 stage: Final pH 11.0+/-0.3, 90°C; 60 min; 5 kg oxygen/ton of pulp D stage: Final pH 2.5+/-0.3; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg of pulp E2 stage: Final pH 11.0+/-0.3; 60°C; 60 min; 2 kg H2O2/ton of pulp D stage: Final pH 3.4+/-0.2; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp.
• the final viscosity of the pulp varied in all tests within the range of from 916 to 920 dm3/kg, which shows that the process according to the invention does not adversely affect the strength properties (measured as viscosity).
• This example illustrates the effect of different charges of hydrogen peroxide in step 2 (E1) on the formation of AOX in the prebleaching and final bleaching and on the final brightness of the pulp with a process according to the invention where the chlorine factor is 1.0.
• the treated pulp consisted of a non-oxygen delignified sulphate pulp of softwood, which, before the treatment, had a kappa number of 26.8, a viscosity of 1329 kg/dm3 and a brightness of 30.0% ISO.
• the amount of chlorine dioxide is calculated as active chlorine.
• Bleaching sequence D0 E1 D1 E2 D2, where D0 E1 makes up the prebleaching sequence and D1 E2 D2 the final bleaching sequence.
• the treatment conditions in D0 and the stages of the final bleaching sequence were kept constant throughout test 1 to 5.
• the treatment conditions were: D0: 100% chlorine dioxide; Charge factor 1.0 corresponding to 27 kg of active chlorine/ton of dry pulp E1: 0-20 kg hydrogen peroxide/ton of dry pulp; Reinforced with oxygen.
• D2 7 kg of active chlorine/ton of dry pulp
• the ratio between the amount of chlorine dioxide charged in the final bleaching sequence and prebleaching sequence, respectively, is 1.19. The results obtained are shown in the Table below.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Polysaccharides And Polysaccharide Derivatives (AREA)
• Chemical Or Physical Treatment Of Fibers (AREA)
• Breeding Of Plants And Reproduction By Means Of Culturing (AREA)
• Immobilizing And Processing Of Enzymes And Microorganisms (AREA)
• Solid-Sorbent Or Filter-Aiding Compositions (AREA)

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Publications (2)

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Cited By (3)

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• 1990
  • 1990-01-31 SE SE9000340A patent/SE9000340L/ not_active Application Discontinuation
• 1991
  • 1991-01-24 BR BR919105959A patent/BR9105959A/pt not_active IP Right Cessation
  • 1991-01-24 EP EP91903930A patent/EP0500813B1/en not_active Revoked
  • 1991-01-24 AU AU72356/91A patent/AU7235691A/en not_active Abandoned
  • 1991-01-24 JP JP3504027A patent/JPH0672385B2/ja not_active Expired - Fee Related
  • 1991-01-24 ES ES91903930T patent/ES2065007T3/es not_active Expired - Lifetime
  • 1991-01-24 AT AT91903930T patent/ATE115208T1/de not_active IP Right Cessation
  • 1991-01-24 WO PCT/SE1991/000051 patent/WO1991011554A1/en not_active Application Discontinuation
  • 1991-01-24 CA CA002074715A patent/CA2074715C/en not_active Expired - Lifetime
  • 1991-01-31 PT PT96630A patent/PT96630B/pt not_active IP Right Cessation
• 1992
  • 1992-07-29 NO NO922995A patent/NO301177B1/no unknown
  • 1992-07-29 FI FI923420A patent/FI923420A0/fi unknown

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