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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• the present invention relates to a high-yield pulp bleaching process using hydrogen peroxide in an alkaline medium.
• high-yield pulp is understood here the pulp obtained with a yield by weight in the dry state relative to the starting lignocellulosic material, such as wood generally in the form of chips, counted in the dry state, which is greater than approximately 85% and often at least 90%.
• Such pastes are produced by defibering the starting material, wood chips most often, by means of a grinding wheel or a disc shredder, associated or not with a chemical and / or heat treatment.
• thermomechanics chemothermothermics.
• the bleaching consists in discoloring by chemical action the chromophoric groups of the constituents of the wood without dissolving these constituents.
• the chromophoric groups responsible for the brown coloring of the dough are carried mainly by lignin and certain extracts (tannins).
• hydrogen peroxide is an expensive product whose stability decreases as the pH increases. If it is accepted that the pH zone in which bleaching with hydrogen peroxide is normally carried out ranges from approximately 9 to approximately 11 as the recalls "The bleaching of pulp", TAPPI Press, SlNGH ed., Atlanta USA, 1970, p.227, the improvement in this context of the use of hydrogen peroxide is of permanent interest for the industry.
• a first phase is proposed in which the alkalinity conditions are abnormally strong with a view to improving the mechanical quality of the fibers and a second phase under alkalinity conditions normal to correct the bleaching defect resulting from the first phase.
• LACHENAL also describes a two-phase process but in which a quantity of sodium hydroxide much greater than that normally accepted for a first phase is used in the second phase.
• FR-A-2537177 recommends the maintenance of conditions as uniform as possible throughout the laundering by proceeding in a succession of distinct phases.
• a phase is distinguished from the following in that the products which have served in the first, in particular hydrogen peroxide and alkaline agents such as sodium hydroxide, are eliminated from the paste, at least for the most part, for example 90%, normally by washing and / or pressing said paste.
• the energy and investment costs which result from this oppose the advantage of improving the efficiency of use of hydrogen peroxide.
• the process of the present invention comprises two phases but in no way requires the above intermediate operation.
• the term “paste” is understood to mean the assembly consisting of lignocellulosic material counted in the dry state and of the liquid present with it.
• the alkaline agent added to the pulp during bleaching is normally sodium hydroxide in an amount most often between 0.5% and 4%, preferably between 1% and 2%.
• Another alkaline agent than sodium hydroxide can be added provided that it modifies the alkalinity of the paste expected from the sodium hydroxide which it replaces.
• the quantity of hydrogen peroxide H2O2 used, added entirely to the pulp at the start of bleaching, can be between 1% and 6%, preferably between 2% and 4%.
• the total amount of sodium hydroxide used, the sum of the amounts added at the start and during the bleaching process according to the process of the invention, is generally between 1% and 5%.
• bleaching can take place in the presence of sodium silicate, for example from 2% to 6%, and most often from 3% to 5%.
• the pH of the pulp during bleaching thus remains between 8.5 and 11.5, that is to say between usual limits.
• the total duration of the bleaching depends on the choice of the various parameters. It can, for example, be of the order of 4 to 6 hours when, as is the case in the examples presented below, it is ensured that the whiteness of the dough hardly changes over time.
• the time between the start of bleaching and the time when the additional quantity of alkaline agent is added depends essentially on the quantity of peroxide used and the temperature chosen.
• the preferred conditions correspond to a value of said duration not exceeding that separating the time at which the alkalinity of the dough is voluntarily modified and the term of bleaching.
• a paste of fir spruce wheel with a degree of whiteness of 59 ° ISO is bleached with hydrogen peroxide under the following conditions: H2O2: 4%, NaOH: 2.5%, sodium silicate: 3%, DTPA: 0.25%, consistency: 15%, temperature: 60 ° C, until x% of the hydrogen peroxide is consumed, the value of x being different from one example to another, and then 1.5% of sodium hydroxide is added to the dough then the whitening carried out at the end of a total duration equal to 6 hours.
• This example relates to the same paste as in Examples 1 to 6 and is carried out like Example 1, without adding NaOH during bleaching, but with 4% NaOH instead of 2.5% therefore with an equal amount of NaOH the sum of the quantities of NaOH added to the pulp at the start and during the bleaching in Examples 3 to 6 in accordance with the invention.
• the dough has a whiteness equal to 80.7 ° ISO, that is to say similar to that achieved in Examples 1 and 2 and therefore still significantly less than that which the invention allows. .
• the dough is then pressed then brought to a consistency of 15% by adding water and 1.5% NaOH and bleached until a total bleaching time equal to 6 hours using the residual hydrogen peroxide , or 1.4%.
• the bleached pulp has a whiteness equal to ISO 81 ° which does not constitute any progress in this respect compared to the present invention but a certain economic disadvantage compared to the latter.
• the paste then has a whiteness equal to 72.2 ° ISO and the "alkaline” browning is observed.
• a chemicothermomechanical pulp, or CTMP pulp, of spruce fir with a whiteness equal to ISO 61.2 ° is bleached under the following conditions: H2O2: 4%, NaOH: 2.5%, sodium silicate: 3%, DTPA: 0.25%, consistency: 15%, temperature: 60 ° C, until 52.5% of the hydrogen peroxide is consumed.
• Example 13 there is an "alkaline” browning of the already accentuated dough.

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

Priority Applications (2)

Applications Claiming Priority (1)

Publications (2)

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Family Applications (1)

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• 1990
  • 1990-04-30 FR FR9005748A patent/FR2661430B1/fr not_active Expired - Lifetime
• 1991
  • 1991-04-16 NO NO911490A patent/NO177606C/no not_active IP Right Cessation
  • 1991-04-25 JP JP3095601A patent/JPH0726352B2/ja not_active Expired - Lifetime
  • 1991-04-29 NZ NZ237979A patent/NZ237979A/xx unknown
  • 1991-04-29 CA CA002041388A patent/CA2041388C/fr not_active Expired - Fee Related
  • 1991-04-29 AU AU76071/91A patent/AU640775B2/en not_active Ceased
  • 1991-04-29 FI FI912073A patent/FI99153C/fi active
  • 1991-04-29 PT PT97513A patent/PT97513B/pt not_active IP Right Cessation
  • 1991-04-30 US US07/693,629 patent/US5169495A/en not_active Expired - Fee Related
  • 1991-05-22 ES ES91401316T patent/ES2057804T3/es not_active Expired - Lifetime
  • 1991-05-22 EP EP91401316A patent/EP0514608B1/fr not_active Expired - Lifetime

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