FR2666597A1 - MOLECULAR CHLORINE-FREE PROCESS FOR BLEACHING PAPER PULP. - Google Patents

Abstract

The invention relates to a chlorine-free process for bleaching chemical pulps, in particular kraft pulp, using a first step of treating the pulp with an enzyme of a microorganism acting on hemicellulose. Then the treated pulp is extracted, treated with chlorine dioxide, re-extracted and reprocessed with chlorine dioxide to result in a degree of whiteness of at least 88.

Description

i

  The present invention relates to a new process for

  More specifically, the chemical pulp bleaching

  ment, it relates to a process which does not require the use of a conventional bleaching step with molecular chlorine during the bleaching of the chemical pulp, for example

kraft pasta.

  Wood is a complex material, composed of cellulose, hemicellulose and lignin, as well as many other minor components Lignin is associated, and perhaps even linked by covalent bond, to a cellulose matrix

  and / or hemicellulose (polymers of different polysacchari-

(especially xylans).

  To reduce the lignin content of the chemical pulp and thereby reduce the brownish color conferred by the presence of lignin, the pulps are subjected to a series of chemical treatments called "bleaching" The operation

  laundering involves a series of steps, which represent

  try at least two or three different types of treatment,

  some of which are often repeated in the operation glo-

  bale These treatments include the following:

  C Chlorination Reaction with light chlorine

  (molecular) acidic E Extraction with alkalis Dissolution of the reaction products using Na OH D Chlorine dioxide Reaction with C 102 in acidic environment O Oxygen Reaction with oxygen

elementary under high pres-

alkaline medium

  Eo Extraction / oxygen Extraction with alkalis in pre-

  sence of oxygen under increased pressure CD Chlorine / C 102 Reaction with mixtures of chlorine and chlorine dioxide H Hypochlorite Reaction with hypochlorite in alkaline solution P Peroxide Reaction with peroxide in basic medium

  Eop Reaction / oxygen / peroxide Alkali extraction in pre-

  sence of oxygen and peroxide under increased pressure

  Ep Extraction / peroxide Pre-alkali extraction

  sence of a peroxide To obtain a paste having the desired degree of whiteness (usually 88 or more), bleaching has hitherto been commonly used in the context of

  of a five or six step operation, in which the processing

  molecular chlorine was in a representative way

  tive performed in the first stage; the most frequent sequences

  common are CEHDED and CEDED (for a more detailed description

  increase in conventional bleaching processes, see Hand-

  book for Pulp and Paper Technologists, by G A Smook

  (1982), TAPPI, Atlanta, GA, especially pages 153-173).

  One of the problems associated with conventional bleaching processes using chlorine lies in the possibility of obtaining undesirable byproducts from the point of view of pollution. The effluents of conventional operations of

  bleaching may contain organic chlorine compounds

  res, some of which are known to present different

  toxic, mutagenic and possibly carcinogenic effects.

  Elimination of the chlorine treatment step should lead to the production of a lower amount

unwanted effluents.

  Recently, the use of enzymes, for example fungal enzymes, to degrade and eliminate lignin from pasta, has aroused some interest. Hemicellulases, for example xylanases and mannanases, represent a

interesting class.

  Examples of these enzymes include those of

  mushrooms Aureobasidium pullulans, Streptomyces sp, Schi-

  common zophyllum, Sporotrichum sp and Trichoderma spp,

  as well as Bacillus sp These hemicellulases are partly

  shown to reduce the amount of chlorine needed to whiten chemical pulps However, these enzyme treatments have not succeeded in eliminating the use of

  molecular chlorine in representative blanch processes

  chastise. It has been found, according to the invention, that an enzyme acting on hemicellulose (in English: hemicellulose-acting enzyme, HAE), or an enzymatic system comprising one or more enzymes of this kind, can completely replace elemental chlorine. in the first step of a conventional chemical pulp bleaching operation, five or six

  steps; it is thus possible to have a whitening process

  chili without elemental chlorine, which makes it possible to obtain a bleached pulp having the desired whiteness and quality. Thus, the present invention relates to an in vitro process for bleaching chemical pulp, characterized in that it consists:

  a) treating the dough with an enzymatic system

  health on hemicellulose in vitro for a time sufficient to make the lignin component soluble; b) extracting the paste obtained in step a); c) treating the paste obtained with a first solution containing only chlorine dioxide; d) extracting the paste obtained in step c); and e) treating the dough obtained with a second

  solution comprising only chlorine dioxide.

  A five-step bleaching process essentially comprising steps a) to e) above is one form

preferred embodiment.

  The invention also relates to a six-step process in which conventional treatment with hypochlorite is

  sandwiched between steps b) and c).

  In the conventional five or six bleaching process

  steps, steps b) to e), as well as optional processing

  hypochlorite, can be used in different

  annuity conditions, among which the presence or absence of certain agents making it possible to achieve the ultimate objectives in terms of whiteness and quality

  of the paste During the practical implementation of the pre-

  invention, and even if the enzyme treatment may not suggest any benefit or effect close to

  that obtained from the chlorine treatment (which it removes

  place), especially when this benefit or effect is measured

  Res by conventional parameters, like the kappa index, it turned out that it was possible to vary or regulate between certain practical and acceptable limits the treatment conditions of steps b) to e) and of the optional treatment to

  hypochlorite, which also makes it possible to obtain a white paste

  chie having the desired properties of whiteness and quality Among these variations in conditions, there may be mentioned for example an increase in the amount of chlorine dioxide used in one of steps c) and e) or in the

  of them; and / or the implementation of one of the extraction steps

  b) and d), or both, with the addition of a peroxide and / or molecular oxygen, this oxygen being present under

  pressure during at least part of the extraction step

  Preferably, one or both of the extraction steps b) and d) are carried out in the presence of peroxide

  and / or added molecular oxygen; for example, oxy-

  gene can be added to one or both of the steps, or alternatively a peroxide and oxygen can be added to either or both of the steps Preferably, oxygen is used in at least the one of the steps

  When oxygen is to be used in only-

  one of the extraction steps, we usually prefer-

  use it in step b) In a form of realization

  Particularly preferred, peroxide and

  molecular oxygen in the two stages b) and d).

  The enzymes acting on hemicellulose (HAE) are intended to act on the hemicellulose matrix to which lignin is linked, so that, after treatment of the

  paste with HAE enzymes, lignin (or chemically lignin

  modified) becomes available and / or can be released

  ble, and can be separated using a suitable extracting agent and removed from the paste Preferred HAE enzymes

  in the context of the invention include xylanases.

  Other HAE enzymes include mannanases. Celluloses can also be present, as long as the celluloses liable to be present do not degrade the cellulose fraction of the pulp too much,

  thereby reducing the quality of the paper.

  The hemicellulases which can be used in the context of the invention can come from any microbiological source The microorganisms which are known to produce HAE enzymes which can be used to extract lignin include Aureobasidium pullulans, Streptomyces sp., Schizophyllum commune, Sporotrichum sp , Trichoderma

  spp, (including T Reesei) and Bacillus sp Some micro-

  organisms may be preferred sources For example, the swarm fungus is a particularly preferred source, which is rich in active xylanases, and there are

  also strains that do not produce an appreciable amount

  of an undesirable cellulose which could have negative repercussions on the final product Although,

  according to the invention, any A pulp can be used

  lulans, preferred sources include those that produce

  feels large amounts of HAE enzyme, especially

  which can be characterized as "enzyme overproducers" The preferred strains are A pullulans Y-2311

  and Y-2311-1 A pullulans Y-2311 and Y-2311-1 are available

  from the ARS Culture Collection (NRRL), such as

  Leathers et al, 1984 Biotechnology and Bio-

  engineering Symp N O 14, pp 225-240, article incorporated here

  for reference Other xylanases which may be preferred

  according to the present invention are the xylanases of Tri-

choderma spp, especially T reesei.

  The enzymatic treatment step of the invention is carried out in vitro, that is to say in the absence of the living organism from which the enzyme can be obtained, for a time sufficient to degrade the lignin contained in the dough, and release it The process in

  in vitro consists in bringing the dough into contact with a quantity

  sufficient to release lignin from an "enzymatic system" comprising an HAE enzyme, preferably an endoxylanase. For softwood pulps, it is preferred

  mixtures comprising xylanases, in particular endoxy-

  lanases, with mannanases Within the framework of the description

  tion and claims, the expression "enzymatic system

  that "includes: a) an HAE enzyme present in purified form; or b) a mixture of purified HAE enzymes; or

  c) one or more HAE enzymes in non-purified form

  trusted, but separate from the microorganism that produced them; for example, a partially purified preparation or a culture supernatant; or d) one or more HAE enzymes in association with the microorganisms which produced them, as long as the

  microorganism that produced the HAE enzymes is no longer via-

corn; or

  e) at least two HAE enzymes from microorganisms

  my different, in purified or unpurified form.

  HAE enzymes which can be used as indicated above can be obtained from the microorganism which produced them naturally, or by

means of recombination.

  When several HAE enzymes are present in the system

  enzyme enzyme, each HAE enzyme can be present in different amounts When the HAE enzyme system

  present in the form of a culture supernatant, the supernatant

  giant can be used as is, but it can be concentrated

  very before use A special concentration process

  Lely convenient involves a simple ultracentrifugation intended to concentrate the supernatant 5-25 times and, in all cases, to recover for use a concentrate containing

  basically all the enzymes that affect the compounds

  fundamental health of wood, that is to say the possible

  celluloses, hemicellulases and all enzymes which affect

  reactively try lignin (here called

"lignolytic enzymes").

  The HAE enzyme system can vary in activity, depending on factors such as the type of enzyme producing organism, culture media and other environmental conditions of the culture. In general, the HAE enzyme system should have an activity

  endoxylanase from 0.05 to 100 U / g (endo activity units

  xylanase per gram of pulp dried to absolute dryness), and preferably from 0.5 to about 10 U / g Doses of enzyme much higher than about 10 U / g usually do not provide significant additional benefits to the bleaching operation One unit of endoxylanase activity is defined by the production of one micromole of reducing sugar per minute, from an oat-spelled xylan substrate at C, during a test of ten minutes The weight of the dough dried in absolute dryness is determined by drying the dough in the oven for two hours at 100 C. Preferably, the enzymatic treatment is carried out at a temperature which improves the enzymatic activity in vitro. temperatures can be between approximately 10-80 ° C, and preferably 30-750 ° C. The p H of the enzymatic treatment step is advantageously approximately 2.5 to 11, depending on the source of the enzymes HAE used for the enzymes. HAE from A swarm, p H should be

  between 3.0 and 6.5, preferably between 3.5 and 5.0.

  The duration of the in vitro treatment varies with certain factors, such as the desired results, the amount of substrate used, the amount of HAE enzymes in the enzyme system, the specific activity of the enzyme system,

  the type of substrate used, the temperature, and the like.

  In general, satisfactory results can be obtained

  after at least 0.5 hour of treatment, and the duration of the treatment

  can be from 1 to 8 hours, more usually

  2 to 4 hours, depending on the dough used.

  The main effect of enzyme treatment is a reduction

  tion of lignin, and the results show that the desired effects of the enzyme treatment are usually obtained in a relatively short time, for example 4 hours, or even in a shorter period of time, the advantages which can be achieved with a longer processing time

being weak, even zero.

  The enzymatic system is preferably obtained from

  from selected cultures of microorganisms, which are grown in the presence of a carbon source Preferred sources of carbon that can be used include xylan, xylose, corn straw, corn cobs, corn husks, kraft hardwood pulp, wheat straw and barley straw, other sources containing hemicellulose, or other sources of carbon producing

  the desired enzymatic activities The sources of car-

  bone particularly preferred from the point of view of the quan-

  xylanase produced include xylan, including beech xylan, xylose and corn cobs.

  processing of softwood pulp, enzyme systems

  ticks containing larger amounts of mannanase

  are preferred and can be obtained for example by crossing

  s of A pullulans in the presence of a production inducer

  mannanase fungal tion, such as mannan, mannose, softwood or softwood pulp The microorganism is grown on the desired carbon source for at least

  1-10 days before sampling of the enzyme system The

  optimal enzyme duction is representative

completed in 1-6 days.

  The present invention can be used to reduce or facilitate the reduction of the lignin content (and / or

  the chemically modified lignin content) of any

  conch of a wide variety of chemical pulps, i.e. pulps which have already undergone a preliminary treatment by any of the numerous techniques making it possible to reduce

  their lignin content The invention is advantageously used.

  edged with any chemical paste which can be

  should apply the indicated procedure of the prior art

  five or six-step chlorine bleaching

  achieve the desired minimum whiteness of 88.

  The invention is particularly applicable to pasta

  treated by the sulfite process or by the sulfite process

  fate (kraft) and is particularly applicable to kraft pasta The pasta which can be particularly used has

  already generally 80% to 97% cleared, more often

  wind of 85 to 95% of their lignin These chemical pulp treatments are usually called "bleaching" operations and are highlighted by a lightening

  of the color of the dough The invention is used quite spe

  especially with hardwood pasta or their mixtures

  holding up to 25% by weight of softwood.

  After treatment of the pulp with an HAE enzymatic system, it can undergo additional bleaching up to the desired level, thanks to known processing steps, of the type usually combined with chlorine treatments of the first step, as long as one provides for two extraction steps, and at least one of these steps includes a

  peroxide and molecular oxygen treatment The pre-

  the first extraction step preferably takes place immediately

  after the enzyme treatment step and before any treatment

  chlorine dioxide The second stage of extraction

  preferably takes place after an intermediate processing step

  diary, such as a chlorine dioxide treatment and before the final step, which can also be a chlorine dioxide treatment For example, the sequences which can be used include the following sequences, without being limited thereto (X indicates a treatment enzymatic, and Eop indicates an extraction in the presence of a peroxide and oxygen, the oxygen being present under increased pressure during at least part of the extraction operation) X-Eop-D 1-Eop-D 2 X-EO-Dj-Eop-D 2 XEop-D 1-Eo-D 2

  Or again, in the conventional process of bleaching

  Six steps (CEHDED), we can consider the following chlorine-free sequence: X-Eop-H-DI-Eop-D 2 X-EO-HD 1-Eop-D 2 X-Eop-HD 1-Eo-D 2 Among the above bleaching sequences, the

  sequence X-Eop-Dj-Eop-D 2 is the most preferred.

  The equipment that is currently used for operations

  conventional five- or six-step bleaching rations may

  can easily be adapted by a person skilled in the art to be used in the chlorine-free process according to the invention.

  has the advantage of reducing the amount of compound

  effluents which may be dangerous, which

  results in lower effluent treatment costs

and pollution problems.

  The extraction steps used in the practical implementation of the invention can be any extraction operations, used in a conventional manner and implemented in a manner essentially compatible with those which can be used in the sequences conventional bleaching, the basic objective of these extractions being to eliminate the degraded and solubilized lignin, as well as

  other materials, coming from a chemical processing step

  prior mique Simple extractions, that is to say those carried out without the addition of bleaching agents or substances affecting bound lignin, such as peroxides

  and oxygen, are typically the extrac-

  However, when extracting pasta that has undergone an enzymatic treatment as in step b), simple hot water (for example 65-1000 ° C.) has also proved to be usable. An alkali extraction, which uses

  a dilute aqueous solution of sodium hydroxide is generated

  Generally desirable when it includes a treatment with peroxides and / or oxygen The concentration of sodium hydroxide will depend on known factors, such as the type and condition of the dough Generally, its quantity will lead to

  a p H from 10.5 to 12.5, preferably from 11 to 12.

  The temperature of the extraction steps preferably does not exceed 900 ° C. and is for example from 55 to 900 ° C. More particularly, the temperature is between 60-80 ° C.,

  especially in the presence of peroxides.

  When using an oxygen extraction, at least part of the extraction step is carried out in the presence of oxygen under a pressure higher than the ambient pressure. The oxygen pressures which can be used

  can range from 70,000 to 700,000 Pa, preferably

  from 240,000 to 410,000 Pa The increased oxygen pressure

  gene should only be present for part of the extraction period, i.e. for less than

  half of the extraction period, and it can be pre-

  smells for less than a quarter of the extraction period.

  Adequate results have been obtained for example with increased pressure during the first ten minutes of a

sixty minutes extraction.

  The amount of hydrogen peroxide used generally depends on the characteristics of the pasta, as well as

  how it is used (in one of the two extraction steps

  tion, or in both) In general, the total amount, for the two stages, will not exceed about 0.8% relative to the weight of dry pulp, and the total amount is usually 0.2 to 0.8 % Thus, an amount of 0.8% or less, for example 0.6%, can be used in one extraction step and omitted in the other, it is preferably used in the first step

  extraction, or it can be used differently

  your quantities, the total making 0.8%, in the two extraction stages Preferably, the peroxide is used in the two extraction stages and very particularly in an amount not exceeding approximately 0.3% in each stage, and more particularly from 0.1 to 0.25% in

each step.

  After the first extraction step, the paste obtained is treated with chlorine dioxide or hypochlorite

  depending on whether you have to work with five or six steps.

  After the second extraction step, the pulp is treated with chlorine dioxide The amount of chlorine dioxide

  used in step D 1 may vary depending on the characteristics

  of the dough used and is represented in a

  sentative of 1.2% or less, more particularly 1.0% or less relative to the weight of dry pulp The second treatment with chlorine dioxide is used to bring the degree of whiteness to an acceptable level (in a representative manner 88 or more) In the second treatment at

  chlorine dioxide, the amount of chlorine dioxide generally

  used is less than what we had in

  the first treatment with chlorine dioxide We prefer to use

  read in the second treatment less than half the

  amount of chlorine dioxide used in the first treatment

  so that in general the amount of dioxide

  chlorine in the second treatment is in a way

  Sentative of 0.6% or less, more specifically 0.5% or less In general, the total amount of chlorine dioxide in D 1 and D 2 does not exceed about 2%, and it is

  1.8 t or less when using in an extraction stage

  tion of oxygen or peroxide Preferably, the amount is 1.6% or less, for example when using oxygen in the two extractions More particularly, it is 1.3% or less For example, good results have been obtained with 1% chlorine dioxide in the first treatment and only 0.2% chlorine dioxide in the second treatment, and 0.85% chlorine dioxide in the first treatment and 0.2% in the second treatment when using both oxygen and peroxide extractions Chlorine dioxide treatments are used

  implemented in a conventional manner, for example at temperatures

  erasures from 60 to 80 ° C, more particularly from 65 to 750 C, for 2 to 4 hours, more particularly for 2.5 to

3.5 hours.

  In the five-step process, each of steps b) to e) is conveniently carried out in the same interval of

temperatures, from 60 to 80 'C.

  Treatment with hypochlorite (H), when used

  carried out in a chlorine-free six-step bleaching process, is used in a conventional manner for this treatment. Sodium hypochlorite or

  calcium, preferably sodium hypochlorite The quan-

  The amount of hypochlorite used can also vary widely.

  partly with the characteristics of the pulp, and it is representative of 2% available chlorine or less relative to the weight of dry pulp The durations of the treatment are representative of 0.75 to 2.5

  hours, more particularly 1 to 2 hours, at temperatures

  erasures from 30 to 450 C, more particularly from 35 to 40 'C.

  In general, treatments with chemicals, such as peroxide, oxygen, and hypochlorite, tend to

  be partially non-selective, in the sense that the cellu-

  lose can be attacked and quality is affected

  In general, the quality of a dough

  bleached mique can be indicated by its viscosity, determin

  born by the TAP Pl T 230-OM-82 procedure after the end of the bleaching operation A paste viscosity of at least 8, preferably at least 12, is usually the sign

  of a paste having an acceptable quality These levels of

  cosiness can be easily achieved by the process according to

  the invention, which generally gives a dough having a qua-

* lity at least equal to that of the bleaching process

  chlorine, which it replaces for the pulp considered In the pro-

  assigned according to the invention, the HAE enzymes can be used

  instead of chlorine treatment to give a

  chlorine-free bleaching process which makes it possible to obtain a whiteness degree of at least 88 for the pulp, at the same time as a viscosity of at least 8, and usually 10 or more, and in general the quantity and conditions of treatment, particularly with regard to

  peroxide and oxygen and possibly hypochlo-

  rite, in order to achieve these objectives after treatment with the enzymes HAE of the first stage The amount of chlorine dioxide is also kept at the indicated low levels, which are effective, and it does not adversely affect viscosity or quality In general, the mechanical strength of paper, for example its tear strength or its tensile strength, is approximately the same or slightly higher when HAE enzymes are used instead of chlorine in

the method according to the invention.

  A preferred process according to the present invention is a process for bleaching kraft hardwood pulp, essentially comprising the steps consisting in:

  a) treating the dough with an enzymatic system

  hemicellulose, from a microorganism

  of xylanase, for a time sufficient to restore

  to dissolve the lignin component; b) extracting the paste obtained in the presence of hydrogen peroxide and oxygen, said oxygen being present at a pressure greater than ambient pressure during at least part of the extraction step; c) treating the paste obtained in step b) with a first solution containing chlorine dioxide;

  d) extracting the dough obtained in step c) by pre-

  sence of hydrogen peroxide and oxygen, the oxygen being present at a pressure higher than ambient pressure during at least part of the extraction step; and

  e) treating the dough with a second compres-

  chlorine dioxide, the second solution containing

  less than half the amount of pre-chlorine dioxide

feel in the first solution.

  Another advantage of the process of the present invention lies in the fact that the yield of bleached pulp is

  equivalent to that of the dough subjected to a standard treatment

chlorine sic.

  The invention will be better understood with reference to the examples

following nonlimiting.

Example 1

  Chlorine-free bleaching procedure A mode is implemented on a hardwood kraft pulp

  bleaching operation which does not involve the use of

  chlorine The sequence used is X-Eop-D 1-Eop-D 2 for the enzyme-treated paste and B-Eop-D 1-Eop-D 2 for the control The conditions used during this sequence are

summarized below.

  Enzymatic treatment: 40 units / g of xylanase from A. pullulans for 3 hours at 45 ° C. in citrate buffer 50

m M, w H 4.8 Consistency 5%.

  Eop: Na OH at 0.5%, H 202 at 0.2%, oxygen pressure

  345,000 Pa, the oxygen being released after 10 minutes.

  Total duration 60 minutes at 70 C, and consistency 10%.

  D 1: chlorine dioxide used in the quantities indicated,

   C, consistency 10% and 180 minutes.

  D 2: chlorine dioxide used in the quantities indicated,

   C, consistency 10% and 180 minutes.

  B: buffer (for control) The dough is incubated for 3

  hours in citrate buffer 50 m M, w H 4.8 to 45 C.

Consistency 5%.

Starting kappa index: 13.5.

TABLE 1

  Echan Kappa index Chlorine dioxide Level of tillon after the pre-whiteness mier Eop D 1 D 2 D 1-Eop D 2 Control 11.9 0.7% 0.3% 69.3 79.1 Control 11.9 1.0 % 0.2% 74.2 83.8 Enzyme 5.2 0.7% 0.3% 77.9 85.8 Enzyle 5.2 1.0% 0.2% 82.3 89.6

Example 2

  Enzymatic / chemical bleaching without chlorine of hardwood kraft pulp A hardwood kraft pulp is prewashed with water until neutral p H then filtered to remove water and bring the pulp to a consistency of about 20% Then we add dough

  to obtain a final consistency of 5% of the enzymes pro-

  from different sources (10 xylanase units / g of absolute dry paste) in a sodium citrate buffer 50 m M, p H 4.8 to 450 C The samples, in 4.5 l glass vials, are mixed by shaking The vials are incubated

  in a water bath of 450 C for 3 hours and periodically-

shaken by hand.

  After three hours of incubation with the enzyme, the

  samples are filtered and washed with water Then the samples

  tillons undergo chemical whitening according to the sequence

Eop-D 1-Eop-D 2-

  Eop: 0.5% sodium hydroxide, 0.2% hydrogen peroxide, oxygen under 345,000 Pa, which is released after 20

  minutes Total time 60 minutes at 700 C, consistency 10 t.

  Dl: chlorine dioxide used in the quantities indicated,

  700 C, consistency 10%, 180 minutes.

  D 2: 0.2% chlorine dioxide, 70 C, 10% consistency,

minutes.

Starting kappa index: 13.5.

  After bleaching, the degree of whiteness of the samples is measured, which is indicated below in Table 2. As can be seen, a good degree of whiteness is obtained for the two enzymatic preparations. The degree of whiteness of the dough which has undergone the enzymatic treatment is significantly higher than that of the control sample which has not undergone any treatment with enzymes.

TABLE 2

  Enzyme Chlorine dioxide Degree of in Dl whiteness Control no enzyme l O 84.9

0.85 85.3

A Pullulans brut l O 89.8

0.85 89.6

  Xylanase from O 89.7 Trichoderma reesei 0.85 90.0 The mechanical strength of the sample paper is also measured. The properties of the paper obtained from the pulp treated with enzymes are practically the same as those of the control pulp. the following properties: burst index, tear resistance, length of

  breaking and endurance in the bending test.

Example 3

Dough yield

  The yield of bleached pulp, when using the pro-

  of the present invention, is compared to the

  yield obtained with a conventional process.

  The following sequences are compared:

  A CDEODED a classic whitening sequence used

  reading 2.6% elemental chlorine.

  B XE Op DE Op D a three hour treatment with enzy-

  mes at p H 4.8, 500 C and consistency 5 t; 1.1% chlorine dioxide is used in steps D.

  In the table below, "Total Active Chlorine Y" represents

  feels the total reactivity of elemental chlorine and reacts

chlorine dioxide activity.

TABLE 3

  CDEODED XE sequence Op DE Op D Chlorine% 2.6 0.0 Chlorine dioxide% 0.5 1.1 Total active chlorine% 3.9 2.9 Final whiteness 88.8 89 Yield% 91.9 + 0 , 7 92.2 + 1.2 As can be seen from Table 3, the enzyme-treated kraft pulp which has been bleached without chlorine

  also gives at least a degree of final whiteness equivalent to

  slow and a performance at least approximately equivalent to

  the pulp having undergone conventional bleaching A paper

  read from the enzyme treated dough has a

  resistance to tearing, for a given breaking length

  born, equivalent to that of a paper obtained from

  pasta that has undergone conventional processing.

Claims (9)

Claims   1 In vitro process for bleaching chemical pulp, in the absence of elementary chlorine, characterized in that it consists   a) treating the dough with an enzymatic system   health on hemicellulose for a time sufficient to make the lignin component soluble; b) extracting the paste obtained in step a); c) treating the paste obtained with a first solution containing chlorine dioxide; d) extracting the paste obtained in step c); and e) treating the dough obtained with a second   solution comprising chlorine dioxide.   2 Method according to claim 1, characterized in that   that a hypochlorite treatment step is interca- between steps b) and c).   3 Method according to claim 1, characterized in that one or both of the extraction steps b) and d) are carried out in the presence of molecular oxygen present   under pressure during at least part of the extraction.   4 Method according to claim 3, characterized in that added hydrogen peroxide is present during one   extraction steps b) and d), or both.   Method according to claim 3, characterized in   that the two stages b) and d) are implemented in pre- oxygen.   6 Method according to claim 4, characterized in that hydrogen peroxide and molecular oxygen   added are present during the two steps b) and d}.   7 Method according to claim 1, characterized in that   that the enzymatic system acting on hemicellulose includes   taking a xylanase produced by a microorganism.   8 Method according to claim 7, characterized in that   that the enzymatic system acting on hemicellulose includes   takes a microorganism culture supernatant.   9 Method according to any one of claims 7   or 8, characterized in that the microorganism is chosen from the group comprising the strain Aureobasidium pullulans   Y-2311, the strain Aureobasidium pullulons Y-2311-1 and Tri- choderma reesei.   Method according to any of claims 1   to 9, characterized in that the paste to be treated is a kraft paste. 11 Method according to claim 10, characterized in   what kraft pulp is a hardwood kraft pulp.   12 Method according to claim 1, for bleaching   kraft hardwood pulp, characterized in that it comprises takes the steps of:   a) treating the dough with an enzymatic system   hemicellulose, from a microorganism   of xylanase, for a time sufficient to restore   to dissolve the lignin component; b) extracting the paste obtained in the presence of hydrogen peroxide and oxygen, said oxygen being present at a pressure greater than ambient pressure during at least part of the extraction step; c) treating the paste obtained in step b) with a first solution containing chlorine dioxide;   d) extracting the dough obtained in step c) by pre-   sence of hydrogen peroxide and oxygen, the oxygen being present at a pressure higher than ambient pressure during at least part of the extraction step; and   e) treating the dough with a second compres-   chlorine dioxide, the second solution containing   less than half the amount of pre-chlorine dioxide feel in the first solution.