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
  • D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
  • D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
  • D21C9/142—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 in a multistage process involving ClO2/Cl2 exclusively

Definitions

• the present invention relates to the bleaching of pulp and more particularly to an improved process for bleaching wood pulp with chlorine dioxide and chlorine in a manner whereby the wood pulp is subjected to a two-step high pH/low pH bleaching stage wherein chlorine dioxide and chlorine are used in the first step and chlorine is used in the second step.
• the main objectives of wood pulp bleaching are to increase the brightness of the pulp and to make it suitable for the manufacture of printing and tissue grade papers by removal or modification of some of the constituents of the unbleached pulp, including the lignin and its degradation products, resins, metal ions, non-cellulosic carbohydrate components, and various types of flecks.
• the bleaching of chemical wood pulp is normally carried out in multiple processing stages utilizing elemental chlorine, caustic soda, hypochlorite ⁇ , oxygen, hydrogen peroxide, and chlorine dioxide. The number of stages required in a particular bleaching process is dependent upon the nature of the unbleached pulp as well as the end use to which the pulp will be put.
• a ⁇ ulfate or kraft pulp is today most typically bleached in a five-stage sequence which is designated as (CD) (EO)DED.
• D denotes chlorine dioxide
• C denotes elemental chlorine
• E denotes caustic extraction
• 0 denotes oxygen gas.
• the multi-stage process in essence comprises a chlorination step (CD) , a first oxidative extraction stage (EO) , a first bleaching stage (D, ) , a second caustic extraction stage (E , and a second and final bleaching stage (D ⁇ ) .
• each of the two chlorine dioxide bleaching stages is carried out in a one-step process at a final pH of about 3.8 for three hours at 70° centigrade. It is commonly known that pH has an important bearing on brightness and strength properties as well as the chemical species present in the wood pulp mixture, and this particular pH has heretofore been considered optimal for each of the two chlorine dioxide bleaching stages in the (CD) (EO)DED sequence. It should also be appreciated that although the (CD) (E0)DED sequence has been specifically addressed, the one-step chlorine dioxide bleaching stage can be used in any D- stage for most other three, four, five, or six-stage bleaching processes known to those familiar with the art of wood pulp bleaching.
• Chlorine dioxide used at the mill site normally contains only about 10% chlorine in the one-step chlorine dioxide bleaching stage presently used in the pulp and paper industry.
• Existing technology does not allow for more than about 20% chlorine substitution without detrimentally affecting wood pulp quality. This is a significant concern in view of the high cost of chlorine dioxide in contrast to the relatively less expensive chlorine.
• the present invention solves this well-known deficiency in state of the art chlorine dioxide bleaching by making it possible to perform higher levels of chlorine substitution through a two-step sequential addition of chlorine at controlled high/low pH levels during the chlorine dioxide bleaching process.
• the advantages are a reduced use of chlorine dioxide and a significant reduction in chemical costs for the wood pulp bleaching process.
• applicants provide an improved process for bleaching wood pulp in aqueous suspension using chlorine dioxide and chlorine in a two-step high pH/low pH bleaching stage wherein chlorine dioxide and chlorine are used in the first step and chlorine is used in the second step.
• the chlorine dioxide and chlorine comprise between about 15-80% chlorine on an available basis.
• the novel process comprises first subjecting the aqueous wood pulp suspension to a first bleaching step by mixing it with chlorine dioxide and chlorine and maintaining the mixture at a pH between about 7-10 for about 5-40 minutes. Next, the mixture is subjected to a second bleaching step by mixing it with the remaining chlorine and maintaining the mixture at a pH of 4 or less for at least 30 minutes.
• This novel process can be used in any D stage of the (CD) (EO)DED bleaching sequence as well as in any D bleaching stage of other three, four, five, six, and seven-stage bleaching sequences. All of the chlorine dioxide and approximately 0-40% of the chlorine charged are added initially in the first step of the bleaching process, and the remainder of the chlorine is added at the second stage of the bleaching process.
• the novel bleaching process provides for increasing chlorine substitution up to 60% with nominal effect on wood pulp quality.
• Figure 1 is a graph of D, brightness versus percent available chlorine on pulp as chlorine dioxide for one- step D-stage bleaching, conventional one-step chlorine- chlorine dioxide mixture bleaching at maintained pH levels of 4 and 2 (as taught by the Rapson and Reeve patents referenced herein) , and the novel two-step chlo ine-chlo ine dioxide method of the present invention (chlorine-chlorine dioxide mixtures contain 60% chlorine on an available chlorine basis, (CD)E kappa equals 5.1, and temperature is 70 centigrade) ;
• Figure 2 is a graph of D 2 brightness versus D, charge for the subsequent bleaching of the pulps of Figure 1 through E 2 and D 2 stages (D 2 charge of 0.3% chlorine dioxide on pulp and temperature of 70° centigrade) ;
• Figure 3 is a graph of the final D 2 viscosity measurements for the pulps of Figures 1 and 2 versus D. charge illustrating higher brightness and viscosity for the method of the invention when compared to conventional bleaching methods used in the D, stage;
• Figure 4 is a graph of D, brightness versus D, charge (percent available chlorine on pulp as chlorine dioxide) for conventional one-step D stage bleaching and the method of the invention ((CD),., is 60% chlorine, 40% chlorine dioxide on an available basis and (CD)E kappa equals 5.2. Reaction temperatures are 70° centigrade and 50° centigrade for (CD)., and 70° centigrade for conventional D stage bleaching) ;
• Figure 5 is a graph of D 2 brightness versus D. charge (percent available chlorine on pulp as chlorine dioxide) for the pulps of Figure 4 after E 2 and D ⁇ stage bleaching (D 2 charge is 0.2% C10 2 on pulp, and temperature is 70° centigrade)
• Figure 6 is a graph of D, brightness versus D, charge (percent available chlorine on pulp as chlorine dioxide) for conventional one-step D stage C10 2 bleaching and the method of the invention ((CD)E kappa equals 4.6, ( CD --hl s 60 * chlorine, 40% chlorine dioxide on an available basis, and temperature is 70° centigrade);
• Figure 7 is a graph of D, and D 2 brightness versus percent available chlorine on pulp as chlorine dioxide for conventional one-step D stage C10 2 bleaching and the novel two-step (CD). , bleaching process of the present invention wherein (CD) comprises 60% Cl 2 and 40% C10_ on an available basis, temperature is 70° centigrade, and (CD)E kappa equals 4.9;
• Figure 8 is a graph of D, brightness versus D, charge for conventional one-step D stage bleaching and the novel two-step (CD). , bleaching process of the present invention at 70 centigrade for 3 hours with (CD)E kappa equals 5.0 and wherein (CD) is 60% Cl 5 and 40% CIO- on an available basis;
• Figure 9 is a graph of D 2 brightness versus D, charge for the pulps of Figure 7 wherein the D_ charge is 0.3% C10 2 on the pulp and temperature is 70° centigrade;
• Figure 10 is a schematic representation of two different process systems for a wood pulp bleaching plant which incorporate the chlorine dioxide with the sequential chlorine addition wood pulp bleaching process of the present invention.
• Rapson to bleach wood pulp using an aqueous solution of chlorine dioxide and chlorine, treating the wood pulp with alkali, and then treating the wood pulp with chlorine dioxide.
• the process of Rapson provides for chlorine dioxide to constitute about 20-95% of the total available chlorine of the solution in the first (CD) stage.
• Canadian Patent No. 1,120,660 to Reeve discloses a bleaching sequence which comprises subjecting the wood pulp to a first bleaching step with a chlorine dioxide and chlorine solution and, after a period of time from about 5 seconds to about 10 minutes, applying an aqueous chlorine solution to the pulp suspension in a second bleaching step which is allowed to proceed for about 10-60 minutes.
• the first bleaching step is used usually effected at a pulp suspension pH of about: 1-6
• the second bleaching step is usually effected at a pulp suspension pH of about 0.7-3.
• Applicants' process provides for the use of chlorine-chlorine dioxide bleaching in stages other than the first (CD) stage, preferably the D stage designation in any 3-7 stage bleaching sequence, whereas Rapson and Reeve both refer specifically to the first (CD) stage of bleaching.
• wood pulp bleaching with chlorine- chlorine dioxide mixtures is typically carried out at 25° centigrade - 60 centigrade in the first stage of a (CD) (EO)DED or (CD) (EO)D sequence.
• the consistency is usually around 3-5%, and bleaching times range from 15 minutes to 1 hour with an end pH of about 1.6-2.5.
• the application of conventional (CD) bleaching procedures in the D-stage of the above sequences or in any other bleaching sequences has little commercial value due to the high viscosity loss (carbohydrate degradation) and poor brightness characteristics of the wood pulp.
• existing (CD) technology is typically only used as a first stage or pre-bleaching stage in the multi-stage bleaching process on wood pulp.
• Pulp slurry is reacted with a chlorine and chlorine dioxide mixture for about 5-40 minutes at a pH of about 7-10.
• caustic is also mixed with the pulp slurry, and the caustic most suitably comprises sodium hydroxide.
• the reaction temperature is between about 40-80° centigrade, most suitably about 70° centigrade, and the optimal pH during the first bleaching step is between about 8-9.5;
• the remaining chlorine is added to the pulp mixture, and the pH is maintained at 4 or less, most suitably a pH of about 2- 3.5 for at least 30 minutes, preferably about 30 minutes - 2.5 hours.
• the reaction temperature is between about 40-80 centigrade, most suitably 70° centigrade.
• Pulp consistency foe both steps is between about 10- 13.5% with the final consistency of the mixture after the second step at most suitably about 10%.
• the chlorine dioxide and chlorine used in the bleaching process comprise between about 15-80% chlorine on an available basis, and preferably all of the chlorine dioxide and one-third of the chlorine are added at the first step, and the remaining two-thirds of the chlorine are added at the second bleaching step.
• the new two-step process developed by applicants has been used to bleach pulp in the D, stage of a (CD) (EO)DED sequence with chlorine-chlorine dioxide mixtures of up to 60% of the chemical charged as chlorine on an available basis with brightness and viscosity characteristics comparable to a conventional chlorine-chlorine dioxide stage (see Figures 1, 2 and 4-9).
• Applicants' process is particularly attractive since the new two-step chlorine-chlorine dioxide bleaching process allows for substituting chlorine for chlorine dioxide and thus results in substantial chemical cost reduction since chlorine is a significantly less expensive chemical.
• applicants* method provides for mixing chlorine, chlorine dioxide and caustic with an aqueous wood pulp slurry for about 5-15 minutes at a pH of about 7-10.
• the optimum pH is between about 8-9.5 for best viscosity, and consistency is between about 10-13.5%.
• the two-step high/low pH bleaching process can be implemented in both a new plant or an existing pulp bleaching plant.
• the optimum design schematic is shown in Figure 10, where CIO- and caustic are added to the first mixer.
• the pulp flows into a J or ⁇ tube ( Figure 10A) or upflow tower ( Figure 10B) with a retention time of approximately 5-40 minutes.
• a second mixer is provided to mix chlorine with the pulp for the second step of the process.
• the pulp can then be discharged directly to a downflow tower.
• the retention time in the downflow tower is 2 or more hours and most suitably between 2.5-3.9 hours.
• the simplest method for implementing the two-step high/low pH bleaching process technology would be to install a mixer on the discharge from the upflow leg of the tower to the downflow leg of the tower.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)

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• 1990
  • 1990-10-17 WO PCT/US1990/005826 patent/WO1991005910A1/en not_active Application Discontinuation
  • 1990-10-17 CA CA002067750A patent/CA2067750A1/en not_active Abandoned
  • 1990-10-17 EP EP90916973A patent/EP0496827A1/de active Pending
  • 1990-10-17 AU AU67303/90A patent/AU6730390A/en not_active Abandoned
  • 1990-10-18 ZA ZA908340A patent/ZA908340B/xx unknown
  • 1990-10-19 PL PL28741690A patent/PL287416A1/xx unknown
  • 1990-10-19 CN CN90109214A patent/CN1051951A/zh active Pending

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