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

Definitions

• the invention relates to a process for the production of semi-bleached kraft pulp without bleach containing chlorine.
• semi-bleached or medium degrees of whiteness are synonymous with whitenesses between 60 and 80 (ISO).
• the bleaching of kraft pulps requires chlorine or chlorinated compounds, even if only medium levels of white are desired.
• High levels of whiteness generally require the use of multi-stage bleaching sequences. Starting with a chlorination step, medium whiteness is usually bleached in a further 2 steps (alkaline extraction and hypochlorite treatment). If higher or highest whiteness levels are to be achieved, treatment with chlorine dioxide must be added to these three stages. Hydrogen peroxide is only used in small quantities, for example to stabilize the white content or to reduce the need for chlorine dioxide.
• Common bleaching sequences today are e.g. B.
• Chlorinated compounds are mutagenic, such as e.g. B. chloroacetone or toxic, such as. B. chlorophenols.
• the biodegradability of the chlorinated compounds is limited, so that even simple techniques such. B. the implementation with SO2 can not solve the fundamental problem of persistence in the environment.
• the lignin In the sulfate or power process, the lignin is largely dissolved under strongly alkaline conditions. The high temperature and alkalinity lead to strong condensation and dark brown discoloration of the lignin, which is very difficult to bleach. Under relatively mild conditions, only chlorine reacts very selectively with the remaining lignin, while hydrogen peroxide in the strongly condensed Kraft lignin only leads to a moderate increase in the white content.
• the pulp can be brightened, for example, with 2% H2O2 from a white content of 22 (ISO) to 28. Significantly higher amounts of peroxide provide hardly any better whiteness levels ((1) Degussa company publication "Hydrogen peroxide, use in pulp bleaching, 1987).
• the upstream washing also improves the whiteness by a few points.
• the object of the invention is a process for the production of semi-bleached kraft pulp without the use of chlorine or chlorinated compounds. It should also be possible to use pulps with a higher residual lignin content.
• the invention relates to a process for the production of semi-bleached kraft pulp by delignification and bleaching, in which the kraft pulp is acid-washed after alkaline digestion and then delignified with oxygen under alkaline conditions, characterized in that a peroxide stage at 60-90 ° C. , 8 - 25% consistency of the pulp, a residence time of 1 - 8 hours using 0.5 - 4% H2O2, 0.2 - 2.5% NaOH and 0.05 - 3% water glass followed, the percentages based are based on the total weight of the pulp
• the acidic wash is preferably carried out using an inorganic mineral acid, in particular sulfuric acid, at a pH of ⁇ 4 and> 0.5 and a temperature of 30-60 ° C within 1-60 minutes.
• an inorganic mineral acid in particular sulfuric acid
• the treatment with oxygen takes place in the presence of 0.05-1% known Mg salts, in particular magnesium sulfate, 0.5-7% NaOH and an oxygen pressure of 0.1-0.5 MPa.
• Mg salts in particular magnesium sulfate
• the consistency of the pulp in this step is 5 - 25%, and the dwell time is 20 - 120 minutes.
• 0.01-0.8% of a generally known complexing agent can also be used in the peroxide stage.
• the process according to the invention enables bleaching of kraft pulp to medium white contents by using oxygen and hydrogen peroxide even with pulps with a higher residual lignin content.
• the process can easily be put into practice in plants with an existing oxygen level after the installation of a washing system.
• the wastewater of the 02 stage can be returned to the evaporation plant in countercurrent, so that only the low wastewater pollution of the peroxide stage has to be conducted through a wastewater treatment plant.
• a spruce kraft pulp with a ground whiteness of 4.1 (ISO) and a degree of digestion of Kappa 30 is subjected to an acidic treatment with H2SO4 at pH 2.0 and 45 ° C for 5 minutes.
• the pulp is washed and subjected to an oxygen delignification with 2.5% NaOH and 0.5% MgSO4 at 0.4 MPa pressure at 100 ° C for 90 minutes.
• a pulp with a kappa number of 14 whiteness (ISO) is obtained after the 02 stage 34.8.
• the bleaching with hydrogen peroxide results in the following whiteness at 12% consistency, 75 ° C and 4 hours residence time: H2O2% 1 2nd 3rd 3rd NaOH% 0.5 0.8 1.0 1.2 Water glass% 1.0 1.0 1.9 - White content (ISO) 66.3 70.2 75.8 75.2
• the strength of the pulp after bleaching, despite the more intensive delignification at 20 SR, is 8.8 km tear length and 10.4 mN m2 / g tear resistance, compared to the original values of 9.8 km and 11.5 mN m2 / g.
• a pine kraft pulp with a whiteness of 22.8 (ISO) and a kappa number of 46 is subjected to an intensive oxygen delignification at 115 ° C, 90 minutes residence time and 12% consistency after an acid wash to eliminate the heavy metals.
• the oxygen pressure is 0.5 MPa
• the lignin content is reduced to kappa 15.
• the white content (ISO) after the 02 stage is 33.8.
• Peroxide bleaching at 75 ° C, 20% consistency and 5 hours residence time gives the following white. content: H2O2% 1 2nd 3rd 2nd 3rd NaOH% 0.5 0.8 1.0 1.0 1.2 Water glass% 1.0 1.0 1.0 - - White content (ISO) 65.2 69.1 74.3 69.5 74.1
• the strength of the pulp was unbleached at 20 SR 8.9 km tear length and 11.9 mN m2 / g tear resistance.
• the tear length changed to 8.0 km and the tear resistance to 10.1 mN m2 / g (at 20 SR).
• a eucalyptus kraft pulp with a white content of 26.4 and a kappa number of 16 is treated at 60 ° C for 5 minutes at pH 2 (adjusted with H2SO4) and then washed. Then with 2.0% NaOH at 100 ° C and initially 0.3 MPa oxygen pressure delignified in an open tower for 90 minutes. The lignin content after washing is Kappa 8.
• the whiteness according to the O2 level is 41.4 ISO.
• the peroxide bleach provides the following white contents: H2O2% 1 2nd 3rd 4th NaOH% 0.5 0.8 1.0 1.0 Water glass% 1.0 1.0 - 1.0 White content (ISO) 70.2 74.0 78.8 79.3
• Examples 1-3 show that kraft pulp with sufficient white contents can be obtained using the process according to the invention.
• the combination of the known acid wash before the oxygen stage with a subsequent hydrogen peroxide bleaching produces a synergistic effect.
• the increase in white content is greater than the sum of the increases in the individually carried out combination of oxygen or hydrogen peroxide stage with the washing stage.
• the resulting pulps since they are produced without the use of chlorine-containing compounds, cannot contain chlorinated organic compounds.

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• Engineering & Computer Science (AREA)
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Citations (3)

• 1988
  • 1988-01-28 DE DE3802401A patent/DE3802401A1/de not_active Withdrawn
  • 1988-11-30 EP EP88119939A patent/EP0325731A1/de not_active Ceased
  • 1988-12-12 AU AU26798/88A patent/AU607026B2/en not_active Ceased
  • 1988-12-12 ZA ZA889289A patent/ZA889289B/xx unknown
  • 1988-12-13 NO NO88885525A patent/NO885525L/no unknown
• 1989
  • 1989-01-09 FI FI890100A patent/FI890100A/fi not_active IP Right Cessation
  • 1989-01-23 BR BR898900259A patent/BR8900259A/pt unknown
  • 1989-01-26 PT PT89533A patent/PT89533A/pt not_active Application Discontinuation
  • 1989-01-26 NZ NZ227748A patent/NZ227748A/en unknown

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