NZ260982A - Bleaching paper pulp using oxygen, an enzyme, a treatment selected from peroxide, sequestrant and ozone; no chlorine-containing reactants are used - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £60982 r n o o VJ /' ; Priority Date(s): ... A£. .71'.^Y2» Compiete Specification Fi'.d: !■"?*.'f): Class: VPr^'rSiS'^.

Publication Date: .. %.? .?£?.

P.O. Journal, No: .. Xh&7...., MWII Patents Form No. 5 OFFICE .15 \ "t-j-Z-FATSHT A2 iUL^ TccejvSS.

Patents Act 1953 COMPLETE SPECIFICATION PROCESS FOR BLEACHING A PAPER PULP We, SOLVAY, a Societe Anonyme organised and existing under the laws of Belgium of 33 rue du Prince Albert, B-1050 Brussels, Belgium, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 (followed by la) 2 6 0 9 P " « -s' «. ^4. - la - The invention relates to a process for bleaching a paper pulp.

It is known to treat unbleached chemical paper pulps obtained by cooking cellulosic matter in the 5 presence of chemical reactants by means of a sequence of stages of delignifying and bleaching treatment involving the use of oxidizing chemical products. The first stage of a conventional sequence for bleaching chemical pulp is intended to complete the delignification of the un^ 10 bleached pulp in the form in which it is after the ♦ cooking operation. This first delignifying stage is conventionally carried out by treating the unbleached pulp with chlorine in acidic medium or with a chlorine -chlorine dioxide combination, mixed or in sequence, so as 15 to react with the residual lignin in the pulp and to give rise to chlorolignins which can be extracted from the pulp by solubilizing these chlorolignins in an alkaline medium in a subsequent stage of treatment.

For various reasons it is found useful, in some 2-0 situations, to be capable of replacing this first delignifying stage with a treatment which no longer makes use of a chlorine-containing reactant.

It has already been proposed to treat a kraft pulp by means of a first stage of treatment with an 25 enzyme followed by a stage with ozone and a stage with hydrogen peroxide in alkaline medium and another stage with ozone followed by another stage with hydrogen peroxide, so as to carry out the sequence X Z P Z P (European Patent Application EP-Al-0512978) . However, the 30 brightnesses obtained never reach the values of 89-90 °ISO, required to enable the pulp to be marketable directly as a high quality bleached kraft pulp.

To obtain high brightnesses of the order of 89 °ISO and more, it is necessary, when this known process 35 is used, to employ high quantities of ozone, to the detriment of the intrinsic quality of the pulp, in particular of the degree of polymerization of the cellulose chains. 2/^ ^ f*'' D • The invention is intended to overcome the disadvantages of the known processes by providing a process which makes it possible to obtain new bleaching sequences which give rise to high brightness levels while preserving the intrinsic qualities of the pulp.

To this end, the invention relates to a first process for bleaching a paper pulp by means of a sequence of stages of treatment in which the sequence includes at least the three stages 0 X Z in any order (0: oxygen stage; X: stage with an enzyme; Z: ozone stage). The process is particularly suitable for the treatment of paper pulp by means of a sequence of stages of treatment without chlorine (Cl2) (ECF) and preferably free from chlorine-containing reactants of TCF (total chlorine free) type.

Besides the three stages described above, this process advantageously comprises at least one other stage chosen from stages Q and P (Q: treatment with a seques-trant; P: stage with hydrogen peroxide in alkaline medium). These additional stages may be situated at various points of the sequence. They may, for example, be inserted between the stages or be situated at the end of a sequence. In these sequences the stages 0 and X (in any order, but preferably 0 followed by X, optionally with an intermediate stage) are carried out at the beginning of a sequence.

Stage Q is preferably carried out before the first P stage.

A sequence which has given good results in treatments free from chlorine-containing reactants of TCF type is the sequence 0 Q P Z P comprising an X stage. This X stage may precede the sequence so as to produce the complete sequence X 0 Q P Z P. It may also be inserted between any two stages of the sequence, such as, for example, the complete sequences 0 X Q P Z P and 0 Q P X Z P.

The invention also relates to a second process which applies to the bleaching of paper pulps by means of a sequence of chlorine-free stages of TCF or ECF (elemental chlorine free) type and in which the sequence includes at least the stages X 0 or 0 X followed by a P stage and including a Q stage at any point of the sequence before the P stage and in which X, 0, P and Q have the same meanings as above.

Among the sequences which have yielded good results in the bleaching of paper pulps of ECF type and preferably of TCF type the following four sequences may be reported: 0 X Q P, 0 X/Q P, X 0 Q P and X/Q 0 P.

In all the sequences referred to above, the symbol / means that the stage immediately preceding the symbol and that following the symbol may optionally be combined into a single stage.

The sequences in which the Q stage immediately precedes the P stage are preferred in the treatments of ECF or preferably TCF type.

Another set of preferred stages in the treatments of ECF or preferably TCF type are those in which the Q stage immediately follows the X stage.

Finally, sequences which are very particularly preferred are those in which, simultaneously, the X stages are followed by a Q stage and the P stages are preceded by a Q stage. The process is particularly suitable for the treatment of chemical paper pulps.

A chemical paper pulp is intended to denote pulps which have undergone a delignifying treatment in the presence of chemical reactants such as sodium sulphide in alkaline medium (kraft or sulphate cooking), sulphur dioxide or a metal salt of sulphurous acid in acidic medium (sulphite or bisulphite cooking). Semichemical pulps such as those in which the cooking has been carried out with the aid of a salt of sulphurous acid in neutral medium (neutral sulphite cooking also known as NSSC cooking) can also be bleached by the process according to the invention, as can the pulps obtained by processes employing solvents, such as the Organosolv, Alcell <R), Organocell (R> and Asam pulps described in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A18, 1991, pages 568 and 569. . "'1 ( / f.T'l The invention applies particularly to the pulps which have undergone a kraft cooking or a sulphite cooking. All types of wood employed for the production of chemical pulps are suitable for making use of the process 5 of the invention and, in particular, those employed for the kraft and sulphite pulps, namely coniferous woods such as, for example, the various species of pine and fir and deciduous woods such as, for example, beech, oak, eucalyptus and hornbeam.

According to the invention the stage with oxygen is performed by bringing the pulp into contact with gaseous oxygen at a pressure of between 20 and 1000 kPa in the presence of an alkaline compound in a quantity such that the weight of alkaline compound in relation to 15 the weight of dry pulp is between 0.5 and 5.0 %. The temperature of the stage with oxygen must be generally higher than 70°C and, preferably, higher than 80°C. It is also appropriate that this stage should usually be lower than 130°C and, preferably, lower than 120°C. The dura-20 tion of the treatment with oxygen must be sufficient for the reaction of oxygen with the lignin present in the pulp to be complete. However, it must not exceed this reaction time too much, or it will induce degradation in the structure of the cellulose chains of the pulp. In 25 practice it will be longer than 30 minutes and, preferably, longer than 40 minutes. It will also usually be shorter than 120 minutes and, preferably, shorter than 80 minutes. The oxygen treatment of the pulp can also take place in the presence of a protective agent for cel-30 lulose, such as soluble ' magnesium salts, or organic sequestering agents such as polycarboxylic or phosphonic acid. Magnesium salts are preferred, in particular magnesium sulphate employed in a proportion of 0.02 to 1 % by weight relative to the dry pulp. 35 The pulp consistency during the O stage is generally not less than 8 % by weight of solids content and, preferably, not less than 10 %. This consistency usually does not exceed 30 % by weight of solids content and, preferably, 25 %.

In an alternative form the 0 stage may also be performed in the presence of hydrogen peroxide (Op stage). The quantity of hydrogen peroxide which may be incorporated into the 0 stage is generally not less than 0.2 g H202 per 100 g of dry pulp and, in most cases, not less than 0.5 g. Similarly, 2.5 g of H202 per 100 g of dry pulp and, in most cases, 2 g will not usually be exceeded.

According to the invention the Q stage consists in treating the pulp with at least one sequestering agent in acidic medium, such as an inorganic phosphate or polyphosphate in acidic medium such as, for example, an alkali metal pyrophosphate or metaphosphate, an organic polycarboxylate or aminopolycarboxylate such as, for example, tartaric, citric, gluconic, diethylenetriamine-pentaacetic and cyclohexanediaminetetraacetic acid and their salts, poly-a-hydroxyacrylic acid and its salts and an organic polyphosphonate such as ethylenediamine-(tetramethylenephosphonic), diethylenetriaminepenta-(methylenephosphonic) or cyclohexanediaminetetra(methylenephosphonic) acid and their salts. Diethylenetriamine-pentaacetic acid (DTPA) has given excellent results.

In an alternative form, the Q stage may also consist of a treatment with an acid free from a seques-trant. An acid is intended to denote the inorganic anhydrides or acids such as sulphur dioxide and sulphuric, sulphurous, hydrochloric and nitric acids and their acidic salts, as well as organic acids such as carboxylic or phosphonic acids or their acidic salts. Sulphur dioxide or alkali or alkaline-earth metal bisulphites are suitable. A bisulphite is intended to denote the acidic salts of sulphurous acid corresponding to the formula Me(HS03)n, in which Me symbolizes an atom of metal of valency n, n being an integer which has the value of 1 or 2.

When a sequestrant is present, a small quantity of acid may also be added to the Q stage.

The quantity of acid to be used in the Q stage according to the invention depends on the type of wood V e. /" \ i ( — 6 — S'Q »J ^ j and on the quantity of metallic impurities which it contains. In general, a quantity of acid will be used such as to make the pH of the pulp higher than approximately 5 and preferably approximately 5.5. Similarly, the quantity of acid will be frequently adjusted in order that the pH should not exceed 7 and preferably should not exceed 6.5. When the Q stage is free from sequestrant the pH will be controlled so as to make the mixture appreciably more acidic, that is to say lower than pH 5 and, preferably, lower than 4.0. In general, in order not to degrade the pulp, coming down below pH 1.0 and, preferably, below pH 1.5 will be avoided.

When it is present, the sequestrant is generally used in the Q stage in a quantity of less than 1.5 g of sequestrant per 100 g of dry pulp. In most cases this quantity is less than 1.0 g of sequestrant per 100 g of dry pulp.

The Q stage is generally performed at a pressure close to atmospheric pressure and at a sufficient temperature to ensure an efficient consumption of the acid and/or the sequestrant and, at the same time, not too high so as not to degrade the cellulose and not to burden the energy cost of the means of heating used in the said stage. In practice a temperature of at least 40°C and, preferably, of at least 50°C is suitable. Similarly, it is advantageous that the temperature should not exceed 100°C and, preferably, should not exceed 90°C. The best results have been obtained at approximately 50°C.

The duration of the Q stage must be sufficient to ensure a complete reaction. Although longer durations have no effect on the degree of delignification of the pulp or on its intrinsic strength qualities, it is not advisable to prolong the duration of the reaction beyond that needed to complete the reaction, so as to limit the capital costs and the energy costs of heating the pulp. In practice the duration of the pretreatment can vary within wide proportions depending on the type of equipment employed, the choice of the acid, the temperature and the pressure, for example from approximately 15 9 f : r\ ^ n A L» t- / minutes to several hours. Periods of at least 10 minutes and, preferably, of at least 15 minutes are generally sufficient. Similarly, it is important that the pretreat-ment periods should not exceed 60 minutes and, prefer-5 ably, should not exceed 40 minutes. A period of approximately 30 minutes has given excellent results.

The Q stage is generally performed at a pulp consistency of at least 2 % of solids content and, preferably, of at least 2.5 % of solids content. In most 10 cases this consistency does not exceed 15 % and> preferably, does not exceed 10 %. The consistency of approximately 5 % of solids content has given excellent results.

According to the invention the P stage of treat-15 ment is a stage with alkaline hydrogen peroxide. The nature of the alkali must be such that the latter exhibits a good efficiency of extraction of the oxidized lignin at the same time as a good solubility. An example of such an alkali is sodium hydroxide in aqueous solu-20 tion. The quantity of alkali to be used must be sufficient to maintain the pH above 10 and, preferably, above 11. The quantity of alkali must also be adjusted to ensure a sufficient consumption of the peroxide at the end of the reaction. In practice, quantities of alkali of 25 between 1 and 3 g of alkali per 100 g of dry pulp are suitable. In addition to these quantities of alkali a quantity of hydrogen peroxide which is higher than 0.3 g H202/100 g of dry pulp and, preferably, higher than 0.5 g/100 g of dry pulp will be employed. It is also 30 appropriate that the quantities of hydrogen peroxide should be generally less than 5.0 g H202/100 g of dry pulp and, preferably, less than 4.0 g/100 g of dry pulp.

The temperature of the P stage must be adjusted so as to remain at least equal to 50*0 and, preferably, 35 to 60°C. It also must not exceed 100°C and, preferably, 95°C. A temperature of 70°C has given excellent results.

The duration of the P stage must be sufficient for the bleaching reaction to be complete. However, it must not exceed this reaction time too much or else it n ir ■<- n » fea V' will induce a reversal of the pulp brightness. In practice it will be set at a value of at least 60 minutes, and, preferably, at least 90 minutes. In most cases it will also have not to exceed 300 and, preferably, 200 5 minutes. A combination of the temperature and duration conditions of approximately 70°C and of approximately 120 minutes has given good results.

The consistency of the P stage is generally chosen lower than or equal to 40 % of solids content and, 10 preferably, lower than 30 % of solids content. It will not often be lower than 5 % and, preferably, not lower than 8 %. A consistency of 12 % has given good results.

According to the invention the Z stage of the treatment sequence is a stage with ozone. It consists in 15 subjecting the pulp to a gas stream consisting of a mixture of ozone and oxygen originating from an electrical ozone generator fed with dry gaseous oxygen. A generator with an output of between 50 and 100 1/hour and, preferably, between 70 and 90 1/hour is advan-20 tageously employed in the laboratory. The quantity of ozone used may be easily adjusted by varying the duration of sweeping the pulp with the stream of ozone/oxygen mixture. Periods of from 1 minute to 10 minutes are generally sufficient to use a quantity of ozone of 25 between 0.2 and 1 g per 100 g of dry pulp. On an industrial scale arrangements will be made to adjust the output of the ozone generators and the treatment period to fix the quantity of ozone used on the pulp to values similar to those produced in the laboratory. 30 The ozone treatment is preferably carried out in an acidic medium. pH values of 0.5 to 5 are suitable and, preferably, from 1.5 to 4. A pH of 3 has given very good results.

The consistency in the ozone treatment stage will 35 be chosen with a value of at least 2 % of solids content and, preferably, of at least 10 %. It will generally not exceed 50 % of solids content and, preferably, 45 %. A consistency of 40 % has given excellent results.

The temperature of the ozone treatment stage must / . ^ f1 n p 9 remain relatively low or else it will result in considerable degradation of the mechanical strength properties of the treated pulp. Generally this temperature does not exceed 50°C and, preferably, 35°C. It will neverthe-5 less be at least 2°C and, in most cases, at least 10°C. The ambient temperature of 22°C has given good results.

An advantageous alternative form of the process according to the invention consists in preceding the ozone treatment with a mechanical treatment for opening 10 the pulp (called "fluffing" in the literature), intended to increase the surface of contact between the pulp and the ozone. This operation is particularly useful when the consistency of the pulp during the ozone treatment is at least 15 % of solids content.

A P stage is intended to denote stages with hydrogen peroxide in alkaline roedium as described above, as well as alkaline extraction stages reinforced with a small quantity of hydrogen peroxide (Ep stages) and stages involving the use of oxygen mixed with hydrogen 20 peroxide (Eop stages).

According to the invention the X stage of treatment with at least one enzyme consists in treating the pulp with a composition containing at least one enzyme.

An enzyme is intended to denote any enzyme 25 capable of promoting delignification, by the stages of treatment which are subsequent to the stage of treatment with the enzyme, of an unbleached chemical paper pulp originating from the cooking operation or of a chemical paper pulp which has already been subjected to one or 30 more stages of bleaching with oxidizing reactants such as oxygen, hydrogen peroxide and ozone.

An alkalophilic enzyme will preferably be employed, that is to say an enzyme whose maximum efficiency lies in the alkaline pH range and most particularly at a 35 pH of 7.5 and above.

A category of enzymes which are highly suited for the process according to the invention are hemicellu-lases. These enzymes are capable of reacting with the hemicelluloses to which the lignin present in the pulp is :> bound.

The hemicellulases used in the process according to the invention are preferably xylanases, that is to say hemicellulolytic enzymes capable of breaking the xylane bonds which constitute a major part of the interface between lignin and the remainder of the carbohydrates. An example of xylanase in accordance with the process according to the invention is 1,4-fS-D-xylane xylanohydro-lase, EC 3.2.1.8.

The preferred xylanases in the processes according to the invention may be of diverse origins. They may, in particular, have been secreted by a wide range of bacteria and of fungi.

The xylanases of bacterial origin are particularly advantageous. Among the xylanases of bacterial origin the xylanases secreted by the bacteria of the genus Bacillus have given good results.

Xylanases originating from bacteria of the genus Bacillus and of the pumilus species have given excellent results. Among these, the xylanases originating from Bacillus pumilus PRL B12 are very particularly advantageous.

The xylanases from Bacillus pumilus PRL B12 in accordance with the invention may originate directly from a strain of Bacillus pumilus PRL B12 or else from a host strain of a different microorganism which has previously been genetically manipulated to express the genes coding for the degradation of the xylanes of the Bacillus pumilus PRL B12.

A purified xylanase which does not contain any other enzymes will preferably be employed. In particular, it is important that the xylanase in accordance with the process according to the invention should not contain any cellulase in order not to destroy the cellulose polymeric chains of the pulp.

The process in accordance with the invention applies to the delignification of any kind of chemical pulp. It is suitable for delignifying kraft pulps and sulphite pulps. It is particularly well suited for the treatment of kraft pulps.

The examples which follow are given with the aim of illustrating the invention without, however, limiting its scope.

Examples 1R and 2R (not in accordance with the invention) A sample of hardwood pulp which had undergone a kraft cooking (initial brightness 33.4 °ISO measured according to ISO standard 470-1977(F) , kappa number 12.3 measured according to SCAN standard Cl-59 and degree of 10 polymerization of 1370 expressed as number of glucose units and measured according to SCAN standard C15-62) was treated by following a sequence of 5 stages 0 Q P Z P in the following conditions: 1st stage: stage with oxygen (0 stage): pressure, bar: 6 NaOH content, g/lOOg dry pulp: 2.5 MgS04-7H20 content, g/lOOg dry pulp: 0.5 temperature, degrees C: 120 duration, min: 90 consistency, % by weight of solids content: 15 2nd stage: stage with DTPA (Q stage): DTPA content, g/lOOg dry pulp: 0.2 temperature, degrees C: 50 duration, min: 30 consistency, % by weight of solids content: 5 3rd stage: stage with hydrogen peroxide (P stage) : H202 content, g/lOOg dry pulp: 1.5 NaOH content, g/lOOg dry pulp: 1.8 38°Be Na silicate content, g/lOOg of dry pulp: 2.5 MgS04-7H20 content, g/lOOg dry pulp: 0.5 temperature, degrees C: 70 duration, min: 120 consistency, % by weight of solids content: 12 4th stage: stage with ozone (Z stage): 35 Example 1R Example 2R 03 content, g/lOOg dry pulp: 0.35 0.52 pH: 3.0 3.0 temperature, degrees C: 22 22 duration, min: 2.25 3.5 ^ /T"* ' 1 ' J V. consistency, % by weight of solids content: 40 40 5th stage: stage with hydrogen peroxide (P stage): H202 content, g/lOOg dry pulp: 2.0 NaOH content, g/lOOg dry pulp: 1.6 38°Be Na silicate content, g/lOOg dry pulp: 3.0 MgS04-7H20 content, g/lOOg dry pulp: 1.0 temperature, degrees C: 80 duration, min: 240 consistency, % by weight of solids content: 30 The kappa number of the pulp was measured at the end of the first P stage before it was subjected to the Z stage. Its degree of polymerization and its brightness were determined at the end of the sequence, after treat-15 ment.

Example Final Kappa Final No. brightness number DP °IS0 before Z 1R 88.8 6.4 830 2R 91.0 6.4 810 Examples 3 and 4: (in accordance with the invention) The same sample of pulp as in Examples 1R and 2R was bleached according to the sequence 0 X Q P Z P by using in the X stage a quantity of xylanase originating from Bacillus pumilus PRL B12 corresponding to 10 XU/g of dry 25 pulp. The unit XU (Xylanase Unit) is defined as being the quantity of xylanase which, in the conditions of the test, catalyses the release of reducing sugars equivalent, in reducing power, to 1 micromole of glucose per minute The operating conditions were as follows: 1st stage: stage with oxygen (0 stage): pressure, bar: 6 NaOH content, g/lOOg dry pulp: 2.5 MgS04-7H20 content, g/lOOg dry pulp: 0.5 ,r" \ - ^ ,T < •* L D temperature, degrees C: 120 duration, min: 90 consistency, % by weight of solids content: 15 2nd stage: stage with the xylanase enzyme (X stage): 5 xylanase content, XU/g dry pulp: 10 temperature, degrees C: 50 duration, min: 90 consistency, % by weight of solids content: 5 3rd stage: stage with DTPA (Q stage): DTPA content, g/lOOg dry pulp: 0.2- temperature, degrees C: 50 duration, min: 30 consistency, % by weight of solids content: 5 4th stage: stage with hydrogen peroxide (P stage): 15 H202 content, g/lOOg dry pulp: 1.5 NaOH content, g/lOOg dry pulp: 1.8 38®Be Na silicate content, g/lOOg of dry pulp: 2.5 MgS04- 7H20 content, g/lOOg dry pulp: 0.5 temperature, degrees C: 70 duration, min: 120 consistency, % by weight of solids content: 12 5th stage: stage with ozone (Z stage): Example 3 Example 4 03 content, g/lOOg dry pulp: 0.28 0.43 pH: 3.0 3.0 temperature, degrees C: 22 22 duration, min: 2.25 3.0 consistency, % by weight of solids content: 40 40 6th stage: stage with hydrogen peroxide (P stage): H202 content, g/lOOg dry pulp: 2.0 NaOH content, g/lOOg dry pulp: 1.6 38°Be Na silicate content, g/lOOg dry pulp: 3.0 MgS04-7H20 content, g/lOOg dry pulp: 1.0 temperature, degrees C: 80 duration, min: 240 consistency, % by weight of solids content: 30 The results obtained were: r:, ~.-y /..■ n * \ . —»» i i k * 9 Example Final Kappa Final No. brightness number DP °IS0 before Z 3 89.6 .5 920 4 91.5 .5 870 Examples 5 and 6: (in accordance with the invention) The same sample of pulp as in Examples 1R, 2R, 3 and 4 was bleached by means of the sequence X 0 Q P Z P by using in the X stage the same quantity of xylanase originating from Bacillus pumilus PRL B12 as in Examples 10 3 and 4.

The operating conditions were as follows: 1st stage: stage with the xylanase enzyme (X stage): xylanase content, XU/g dry pulp: 10 temperature, degrees C: 50 duration, min: 90 consistency, % by weight of solids content: 5 2nd stage: stage with the oxygen (O stage): pressure, bar: 6 NaOH content, g/100 g dry pulp: 2.5 MgS04-7H20 content, g/lOOg dry pulp: 0.5 temperature, degrees C: 120 duration, min: 90 consistency, % by weight of solids content: 15 3rd stage: stage with DTPA (Q stage): DTPA content, g/lOOg dry pulp: 0.2 temperature, degrees C: 50 duration, min: 30 consistency, % by weight of solids content: 5 4th stage: stage with hydrogen peroxide (P stage): H202 content, g/lOOg dry pulp: 1.5 NaOH content, g/lOOg dry pulp: 1.8 38°Be Na silicate content, g/lOOg of dry pulp: 2.5 MgS04-7H20 content, g/lOOg dry pulp: 0.5 temperature, degrees C: 70 duration, min: 120 -Inconsistency, % by weight of solids content: 5th stage: stage with ozone (Z stage): 2 5 0 ::) 0 "> 12 Example 5 Example 6 0.43 3.0 22 3.0 03 content, g/lOOg dry pulp: 0.31 pH: 3.0 temperature, degrees C: 22 duration, min: 2.08 consistency, % by weight of solids content: 40 40 6th stage: stage with hydrogen peroxide (P stage): H202 content, g/lOOg dry pulp: NaOH content, g/lOOg dry pulp: 38®Be Na silicate content, g/lOOg dry pulp: MgS04- 7H20 content, g/lOOg dry pulp: 15 temperature, degrees C: duration, min: consistency, % by weight of solids content: The results obtained were as follows: 2.0 1.6 3.0 1.0 80 240 30 Example Final Kappa Final No. brightness number DP °IS0 before Z 89.3 .6 900 6 91.2 .6 860 Examples 7 and 8: (in accordance with the invention) The same sample of pulp as in Examples 1R, 2R and 25 3 to 6 was bleached by means of the seguence O Q P X Z P by using in the X stage the same quantity of xylanase originating from Bacillus pumilus PRL B12 as in Exeunples 3 to 6. The operating conditions were as follows: 1st stage: stage with oxygen (0 stage): pressure, bar: 6 NaOH content, g/lOOg dry pulp: 2.5 MgS04-7H20 content, g/lOOg dry pulp: 0.5 temperature, degrees C: 120 • * 6 0 0 0 2 t - lb - duration, min: 90 consistency, % by weight of solids content: 15 2nd stage: stage with DTPA (Q stage): DTPA content, g/lOOg dry pulp: 0.2 temperature, degrees C: 50 duration, min: 30 consistency, % by weight of solids content: 5 3rd stage: stage with hydrogen peroxide (P stage): H202 content, g/lOOg dry pulp: 1.5 NaOH content, g/lOOg dry pulp: 1.8 38°Be Na silicate content, g/lOOg dry pulp: 2.5 MgS04-7H20 content, g/100 g dry pulp: 0.5 temperature, degrees C: 70 duration, min: 120 consistency, % by weight of solids content: 12 4th stage: stage with the xylanase enzyme (X stage): xylanase content, XU/g dry pulp: 10 temperature, degrees C: 50 duration, min: 90 consistency, % by weight of solids content: 5 5th stage: stage with ozone (Z stage) : Example 7 Example 8 03 content, g/lOOg dry pulp: 0.25 0.38 pH: 3.0 3.0 temperature, degrees C: 22 22 duration, min: 2.16 3.0 consistency, % by weight of solids content: 40 40 6th stage: stage with hydrogen peroxide (P stage): H202 content, g/lOOg dry pulp: 2.0 NaOH content, g/lOOg dry pulp: 1.6 38°Be Na silicate content, g/lOOg dry pulp: 3.0 MgS04-7H20 content, g/lOOg dry pulp: 1.0 temperature, degrees C: 80 duration, min: 240 consistency, % by weight of solids content: 30 The results obtained were the following: Example Final Kappa Kappa Final No. brightness number number DP °ISO before X before Z 7 88.8 6.5 3.1 870 8 91.4 6.5 2.5 860 ft '/ 0 ^ rv ™ - - 18 - U ^> ; / . • r

Claims (10)

WHAT WE CLAIM IS:

1. Process for bleaching a paper pulp by means of a sequence of stages of treatment which is free from chlorine-containing reactants of TCP (total chlorine 5 free) type, characterized in that the sequence includes at least the 3 stages 0 X Z in any order (0: oxygen stage; X: stage with an enzyme; Z: ozone stage).

2. Process for bleaching a paper pulp by means of a sequence of stages which are free from chlorine of TCF or 10 ECF (elemental chlorine free) type, characterized in that the sequence includes at least the stages X 0 or O X which are followed by a P stage and including a Q stage before the P stage.

3. Process according to Claims 1 or 2, characterized 15 in that the X stage includes at least one hemicellulase.

4. Process according to Claim 3, characterized in that the hemicellulase is a xylanase.

5. Process according to Claim 3, characterized in that the X stage is performed in the absence of any 20 cellulase.

6. Process according to Claim 4, characterized in that the xylanase originates from a bacterium of the genus Bacillus.

7. Process according to Claim 6, characterized in 25 that the bacterium of the genus Bacillus is of the species pumilus.

8. Process according to Claim 7, characterized in that the bacterium is the Bacillus pumilus PRL B12.

9. Process according to any one of Claims 1 and 3 30 to 8, characterized in that the bleaching sequence is a sequence 0 Q P Z P comprising an X stage.

10. Application of the process according to any one of Claims 1 to 9 to the bleaching of kraft pulps.