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
  • D21C9/144—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 and other bleaching agents in a multistage process

Definitions

• a chemical pulp can be bleached with chlorine dioxide in the first stage and peracetic acid in the second stage to a brightness of 60% or above.
• the addition of a chlorine dioxide third stage results in a fully bleached pulp having a brightness of 80% or above in just 3 stages, compared to the usual 5 or 6 stage sequences.
• the present invention is directed to a process for the bleaching of wood pulps and, more specifically, to a two or three-stage sequence for bleaching sulfate or sulfite pulps.
• the bleaching of chemical wood pulp is normally carried out in multiple stages utilizing elemental chlorine, caustic soda, hypochlorites and chlorine dioxide.
• the number of stages required in a bleaching sequence depends upon the nature of the unbleached pulp, i.e., sulfite or sulfate, low or high lignin content, etc., and also upon the end use to which it is to be put, i.e., newsprint furnish, dissolving pulp, writing and bond papers.
• a sulfate or kraft pulp is usually bleached in either 5 or 6 stages which is designated as CEDED or CEHDED.
• C denotes elemental chlorine
• E denotes caustic extraction
• D denotes chlorine dioxide
• H denotes hypochlorite bleaching.
• the use of the three-stage bleaching sequence namely, chlorine dioxide-peracetic acid-chlorine dioxide (D-Pa-D), results in a number of advantages.
• One the brightness achieved in just 3 stages is comparable to that achieved in a conventional 5-stage sequence. This should reduce the capital costs of new bleach plants considerably.
• Two the pulps are of better quality, i.e., higher viscosity, and higher bleached yield, i.e., less pulp shrinkage, when compared to pulp bleached by a fivestage process.
• the present process will be advantageous with respect to heat economy and water requirements, since all stages can be carried out at the same temperature and pulp consistency.
• an unrefined, digested sulfate or. sulfite pulp is treated with either a solution of chlorine dioxide in water or with gaseous chlorine dioxide.
• the consistency of the pulp can be from about 10% to about 20%, based on the weight of the oven-dried pulp, and preferably between. 10% and
• the concentration of the chlorine dioxide can be from about 0.5% to about 3.0%, based on the weight of the oven-dried pulp. It is preferable that the concentration of chlorine dioxide be 1.5% for hardwood kraft pulps, 2% for softwood kraft pulps and 1% or below for sulfite pulps, all percentages being based on the weight of the oven-dried pulp.
• the reaction can be carried out at a temperature between about C. and about 90 C., with a temperature of 70 C. being preferred, and at an initial pH between about 5.0 and 6.0. When, for example, 1.5% chlorine dioxide and a, temperature of 70 C. are employed, a typical reaction time is two hours.
• the pulp consistency can be from about 10% to about with a pulp consistency above 30% being preferred.
• gaseous chlorine dioxide having a concentration of from about 0.5 to about 3.0%, based on the weight of the oven-dried pulp, the reaction time is from about 5 to 10 minutes and the reaction takes place at approximately 20 C.
• the partially bleached pulp of the first stage is subjected in the second-stage to bleaching with peracetic acid.
• concentration of the peracetic acid can be from about 0.5% to about 2.0%, based on the weight of the oven-dried pulp, and the pulp consistency can be from about 10% to about 20%, and preferably between 10% and 15%.
• the peracetic acid second stage which can take from about 30 minutes to about minutes, the inital pH must be within the range of pH 8.0 to 12.0 and preferably between pH 8.0 and 10.0.
• the pH can be adjusted by adding NaOH and/or Na CO
• the initial pH in the second stage must be adjusted to 8.0 or above to obtain optimized bleaching results.
• the pulp After the first two stages, namely, chlorine dioxide (D) followed by peracetic acid (Pa) the pulp has a brightness of 60% or above and is suitable for use as newsprint furnish.
• the pulp is treated in a third stage with chlorine dioxide, either in solution or in the gaseous state.
• the concentration of the chlorine dioxide in solution can be from about 0.25% to about 2.0%, preferably 1%, and the pulp consistency can be from'about to about preferably between 10% and 15%, all percentages beingbased on the weight of the oven-dried pulp.
• the reaction temperature can be from about 50 C. to about 90 C., with 70 C. being preferred and the reaction time can be from minutes to 120 minutes.
• EXAMPLE 1 Onehundred grams, oven-dried basis (O.D.), of hardwood pulp, prepared by the kraft process, was treated in the first stage as follows.
• Chlorine dioxide (solution) 1.5% on 0.13. pulp.
• Reaction temperature 70 C.
• Pulp consistency 10.0%.
• the pulp sample was then thoroughly washed with water and was then centrifuged.
• the semi-bleached pulp (D-Pa) of the second stage was then bleached with chlorine dioxide (D) in third stage as follows.
• the fully bleached pulp of third stage was then washed with water and centrifuged.
• the evaluation of the fully bleached pulp is presented in Table 3 below.
• EXAMPLE 2 One hundred grams, oven-dried basis (O.D.), of hardwood pulp, prepared by the kraft process, was treated in the first stage with gaseous chlorine dioxide, rather than with chlorine dioxide in solution as in Example 1. The conditions in each of the three stages are listed below.
• the pulp was washed with water and then centrifuged between each of the stages. The pulp was then evaluated. See Table 5, No. 3.
• a two-stage process for bleaching chemical pulps, without previous chlorine or hypochlorite treatment, to a brightness of 60% or above which comprises:
• chlorine dioxide in the first stage is chlorine dioxide in solution, with a pulp consistency, based on the weight of the oven-dried pulp, of from about 10% to about and the reaction temperature is about 70 C. for a period of from about to about 120 minutes.
• a three-stage process for bleaching chemical pulp, without previous chlorine or hypochlorite treatment, to a brightness of 80% or above which comprises:
• chlorine dioxide in the first and third stages is chlorine dioxide in solution, with a pulp consistency of from about 10% to about 20%, based on the weight of the oven-dried pulp, and the temperature is about 70 C. for a period of from about 30 minutes to about 120 minutes.

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

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Family Applications (1)

Country Status (7)

Cited By (6)

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• 1970
  • 1970-05-25 US US00040403A patent/US3720577A/en not_active Expired - Lifetime
  • 1970-12-11 CA CA100,405A patent/CA939854A/en not_active Expired
• 1971
  • 1971-05-07 JP JP3036971A patent/JPS5411402B1/ja active Pending
  • 1971-05-17 DE DE2124324A patent/DE2124324C3/de not_active Expired
  • 1971-05-21 FI FI1408/71A patent/FI54620C/fi active
  • 1971-05-24 SE SE7106635A patent/SE375564B/xx unknown
  • 1971-05-25 FR FR7118898A patent/FR2093715A5/fr not_active Expired

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