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
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/003—Pulping cellulose-containing materials with organic compounds
• • 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
  • D21C9/1005—Pretreatment of the pulp, e.g. degassing the pulp
• • 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
  • D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
  • D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone

Definitions

• the invention relates to a process for bleaching pulps in aliphatic monocarboxylic acids, e.g. Sulphate pulps or pulps obtained by means of other pulping processes for cellulose-containing raw materials can be considered as starting material.
• aliphatic monocarboxylic acids e.g. Sulphate pulps or pulps obtained by means of other pulping processes for cellulose-containing raw materials
• cellulose obtained by processes that use organic solvents instead of environmentally harmful inorganic chemicals as disintegrants are also suitable as feedstock.
• Acetic acid in particular has proven to be advantageous in this regard.
• Both pressure-free processes for the production of cellulose from lignocellulosic starting materials by means of acetic acid in the presence of catalytically active amounts of mineral acids and non-catalytic processes for the production of cellulose from lignocellulosic materials by means of acetic acid have become known, cf. US-A-3 553 076, according to which temperatures of 150 to 205 ° C are used. Under optimal conditions using these methods such.
• B. with coniferous wood as a starting material residual lignin content of 3.4 to 6.8 wt .-%, which corresponds to kappa numbers of 20 to 40, achievable.
• the digestion liquid contains, besides concentrated acetic acid, still small amounts of a mineral acid used as a catalyst, e.g. B. hydrochloric acid.
• acetic acid is used both in the cooking stage for breaking up shredded wood or annual plants as a starting material and in a subsequent bleaching stage in the presence of hydrogen peroxide.
• ozone has also been used successfully as a bleaching agent in acetic acid, cf. EP-A-0 325 891.
• the process according to DE-A-34 45 132 is characterized by high consumption of bleaching chemicals at comparatively low degrees of whiteness.
• the task is derived, in order to obtain pulps suitable for paper production or also for the production of products from regenerated or chemically modified cellulose, to specify a sequence of process steps in a process of the type specified at the outset which are a bleaching sequence without the use of chlorine-containing chemicals enables.
• This object is achieved in a process for bleaching cellulose in aliphatic monocarboxylic acids of the type mentioned at the outset, which is characterized by a weight ratio of the starting material to a bleaching solution of aqueous acetic acid between 1 to 6 to 1 to 30 and a water content in the bleaching solution of 5 to 50 wt .-% with the addition of nitric acid at a weight fraction of 0.5 to 5.0 wt .-% / atro des Starting material at a temperature of 60 to 140 ° C and a pressure of 1 to 6 bar and a residence time of 0.1 to 6 h, then washing or extracting with water or the bleaching solution, then
• the method according to the invention thus includes a treatment in three successive bleaching stages, the lignin content of the pulp overall being able to be reduced to less than 1% by weight.
• the treatment according to the invention in the first bleaching stage with the addition of nitric acid can also be designed in such a way that nitrogen oxides, advantageously together with a suitable carrier gas, or inorganic nitrates are used in addition to or instead of nitric acid. Gaseous oxygen can also be added to intensify this reaction.
• acetic acid is preferably used as the simplest (unsubstituted) representative of a C2-aliphatic monocarboxylic acid.
• the pulp which may have been digested and extracted, is mixed with a concentrated aqueous acetic acid solution and with the addition of nitric acid at a temperature between 60 to 140 ° C., preferably 100 to 120 ° C. and at a pressure between 1 to 6, preferably 1 to 4 cash, treated.
• the reaction times are 0.1 to 6, preferably 0.5 to 2 hours.
• the acetic acid content in the concentrated aqueous acetic acid solution should be at least 50% by weight.
• the weight ratio of pulp to solvent should be between 1: 6 and 1: 30, preferably between 1: 8 and 1: 12.
• the amount of nitric acid added is expediently 0.5 to 5% by weight / dry cellulose, preferably 1 to 2% by weight.
• the raw pulp obtained is expediently treated with a concentrated aqueous acetic acid solution in order to extract the soluble degradation products.
• the extracted cellulose obtained from the first bleaching stage is then subjected to a further bleaching treatment by means of ozone-containing gases with an ozone content in the carrier gas of approximately 5 to 10% by weight.
• This reaction can be carried out in consistency ranges from only 3% by weight solids content up to over 60% by weight, preferably between 30 and 50% by weight solids content in anhydrous acetic acid or concentrated aqueous acetic acid solution.
• the pulp is squeezed or further acetic acid solution is added.
• the reaction is carried out at a pressure between 1 and 2 bar, but pressures of 1 to 12 bar can be used especially in the medium consistency range from 8 to 16% by weight solids content.
• the ozone treatment is expediently carried out at a temperature between 0 to 70.degree. C., preferably 20 to 40.degree. C. and proves to be particularly advantageous in the acetic acid phase because the formation of hydroxyl radicals is suppressed, as a result of which an attack on the cellulose molecule and an associated deterioration occur mechanical characteristics of the pulp only take place to a reduced extent.
• Reaction kinetics is further important that on the one hand ozone is more soluble in acetic acid than in water and on the other hand the decomposition products resulting from the bleaching process with ozone are also relatively readily soluble in acetic acid and are therefore relatively easy to remove by washing or extracting with acetic acid.
• ozone carriers e.g. Air, nitrogen, oxygen or other carrier gases.
• gases containing ozone are produced in commercially available devices.
• the reaction times are 0.01 to 2, preferably 0.05 to 1 h.
• the ozone is transported to the cellulose fiber from the gas phase via diffusion through the liquid film attached to the fiber or, in the case of suspension of the cellulose, in a continuous liquid phase from the ozone fraction dissolved in the liquid phase by means of suitable distributing devices.
• the type of aftertreatment depends on the intended use of the pulp obtained and can consist in further extraction, bleaching or washing in the acidic, neutral or alkaline range. Both water and organic liquids can be used as solvents.
• the process according to the invention works without halogen-containing additives or bleaching agents and also leads to a residual lignin content of less than 1.0% by weight in the case of cellulose from the softwood feedstocks which are difficult to delignify.
• a spruce wood pulp is used which has been broken down under the following conditions: temperature 170 ° C., pressure 9 bar, reaction time 4 h, acetic acid concentration 87% by weight, liquor ratio 1/5.
• the first bleaching stage is carried out with four different concentrations of nitric acid consumed from 1 to 4% by weight / atro pulp. For each concentration, 344.8 g of dry cellulose moistened with acetic acid (120 g dry cellulose with a dry matter content of 34.8% by weight) with 267.2 g of water, 1788.0 g of glacial acetic acid and a previously calculated volume of 100% nitric acid (Density 1.52 g / ml), according to the above Concentrations stirred well. The suspension is placed in a 4 liter stirred vessel to which a reflux condenser is connected. The contents of the stirred vessel are heated through a double jacket by means of heat transfer liquid brought to a temperature of 120 ° C.
• the pulp is removed from the mixing vessel and dehumidified using a Büchner funnel.
• the pulp is washed by opening three times in 1.5 l of 87% strength by weight acetic acid at 70 ° C. by means of an agitator and dehumidifying via the Buchner funnel.
• the pulp is freed of further solvent in a centrifuge.
• a quantity of cellulose obtained from the first bleaching stage corresponding to 80 g atro (dry content 35-38% by weight), is added to a 4 liter glass flask and connected to a rotary evaporator adapted for ozone bleaching.
• the rotating glass flask is held in a water bath heated to 20 ° C.
• the ozone is generated in a commercial generator from pure oxygen.
• the concentration is 105 g ozone / m3 gas under normal conditions of 0 ° C and 1013 mbar.
• a steady stream of 60 1 / h of the ozone / oxygen mixture is fed to the slowly rotating piston.
• the amount of ozone to be added is determined beforehand on the basis of the kappa number of the pulp and determined by different gassing times.
• the gases that emerge from the round bottom flask are introduced into an acidic potassium iodide solution and the unused ozone is determined by titration with sodium thiosulfate and starch as an indicator.
• the consumption of ozone during the reaction is calculated.
• the pulp is washed extensively with water and splinters are removed in a laboratory sorter (slot width 0.15 mm).
• the moist pulp is put into one Polyethylene bag added and immersed in a water bath preheated to 70 ° C for 1 h. The pulp is then washed extensively with water and dewatered in a centrifuge.
• the pulp is examined for its optical and mechanical properties.
• a comparison of the characteristic values obtained for the bleached pulp shows that the use according to the invention of increasing amounts (1 to 4% by weight / atro pulp) of nitric acid leads to a significant reduction in the amounts of ozone to be used with comparable qualities.
• the characteristic values when using 0% by weight of nitric acid and 2.67% by weight of ozone serve as a comparative example.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Polysaccharides And Polysaccharide Derivatives (AREA)
• Agricultural Chemicals And Associated Chemicals (AREA)
• External Artificial Organs (AREA)

Applications Claiming Priority (2)

Publications (2)

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ID=6426710

Family Applications (1)

Country Status (6)

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Family Cites Families (6)

• 1991
  • 1991-03-08 DE DE4107356A patent/DE4107356C1/de not_active Expired - Lifetime
• 1992
  • 1992-02-14 AT AT92102469T patent/ATE131886T1/de not_active IP Right Cessation
  • 1992-02-14 DE DE59204715T patent/DE59204715D1/de not_active Expired - Fee Related
  • 1992-02-14 EP EP92102469A patent/EP0503303B1/de not_active Expired - Lifetime
  • 1992-02-25 FI FI920832A patent/FI920832A7/fi not_active Application Discontinuation
  • 1992-03-03 NO NO92920829A patent/NO920829L/no unknown
  • 1992-03-05 CA CA002062325A patent/CA2062325A1/en not_active Abandoned

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