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 blea ⁇ ched 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. Background
• the amount of active chlo ⁇ rine is indicated, which is the oxidation potential multi ⁇ plied by the proportion of chlorine in the molecule.
• 1 kg of chlorine dioxide corresponds to 2.63 kg active chlorine.
• a normal bleaching sequence for softwood treated in accordance with the sulphate process is
• El alkaline extraction stage, optionally with addition of peroxide (EP) and/or oxygen (EPO and EO, respectively)
• E2 alkaline extraction stage, optionally with addition of peroxide (EP)
• the (C + D) and El stage is defined as a prebleaching sequence.
• the sequence D E2 D is called final bleaching.
• final bleaching may also be rest ⁇ ricted to one or two D stages.
• the above-mentioned oxygen delignification is normally regarded as part of the diges ⁇ tion process.
• the amount of active chlorine used/required in the (C + D) stage is defined as the charge factor CF according to
• 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.
• 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 l 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
• 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.
• 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, op ⁇ tionally, 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. 6
• 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 addi- tion of chlorine, so-called technical chlorine dioxide.
• the chlorine dioxide may contain chlorine formed during the production and dissolved in the absorp ⁇ tion 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 prefer ⁇ red.
• the first stage is carried out by treatment with technical chlorine dioxide at a charge factor of up to 2.0, pre ⁇ ferably 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 hydro ⁇ gen 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 concen- tration 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 sul ⁇ phite, sulphate, soda or organosolv method, or modifica- tions 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 be ⁇ tween the amount of chlorine dioxide charged in the pre ⁇ bleaching 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 carbohydra- tes, 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.
• Example 1 use is made of an oxygen-delignified sulphate pulp of softwood, with kappa number 17.0, visco ⁇ sity 1040 dm 3 /kg, and brightness 33.3% ISO after the oxygen stage.
• the kappa number, viscosity and brightness of the pulp as well as the amount of AOX were determined throug ⁇ hout according to SCAN Standard Processes.
• the amount as well as the proportion of chlorine dioxide are calculated as active chlorine.
• 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 El 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 El stage: Final pH 11.0+/-0.3; 90°C; 60 min; 2 kg H 2 0 2 /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 H 2 0 2 /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.
• 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 hydro ⁇ gen peroxide in the El stage, the discharge of AOX from the entire bleaching sequence, and quality properties.
• the bleaching sequence was D El 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 chlori ⁇ ne, was used in the final bleaching.
• the final viscosity of the pulp varied in all tests within the range of from 916 to 920 dm 3 /kg, which shows that the process according to the invention does not adversely affect the strength properties (measured as vis ⁇ cosity) .
• This example illustrates the effect of different charges of hydrogen peroxide in step 2 (El) on the forma ⁇ tion 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/dm 3 and a brightness of 30.0% ISO.
• the amount of chlorine dioxide is calculated as active chlorine.
• Bleaching sequence DO El DI E2 D2, where DO El makes up the prebleaching sequence and DI E2 D2 the final bleaching sequence.
• the treatment conditions in DO and the stages of the final bleaching sequence were kept constant throughout test 1 to 5.
• the treatment conditions were: DO: 100% chlorine dioxide; Charge factor 1.0 corresponding to 27 kg of active chlorine/ton of dry pulp El: 0-20 kg hydrogen peroxide/ton of dry pulp; Reinforced with oxygen. DI: 25 kg of active chlorine/ton of dry pulp E2: 2 kg hydrogen peroxide/ton of dry pulp D2: 7 kg of active chlorine/ton of dry pulp
• DO 100% chlorine dioxide
• E2 2 kg hydrogen peroxide/ton of dry pulp
• D2 7 kg of active chlorine/ton of dry pulp

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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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• 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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