NZ274852A - Pulp bleaching process comprising steps qpa in that order (ie decontamination, alkaline hydrogen peroxide bleaching, non peroxygenated acid) - Google Patents

Abstract

Process for bleaching a chemical pulp through a sequence of processing steps comprising, in this order, a step for decontaminating the pulp by removing its transition metals, an alcaline hydrogen peroxide processing step and a processing step using a non peroxygenated acid.

Description

New Zealand Paient Spedficaiion for Paient Number £74852 * New Zealand No. 274852 International No. PCT/EP94/03591 Priority Dat»(s): Ife Comptet# Specification Filed: ci*u' ®.

PuWlcatton Date: P.O. Journal No: 2%'WjW NO DRAWINGS NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Process for bleaching a chemical pulp Name, address and nationality of applicant(s) as in international application form: SOLVAY INTEROX (SOCIETE ANONYME), a Belgian company of Rue du Prince Albert 33, B-1050 Brussels, Belgium 274852 Process for bleaching * «pal paper puIp The invention relates to a process for bleaching a chemical paper pulp.

It is known to treat raw chemical paper pulps, 5 obtained by cooking cellulosic materials in the presence of chemical reactants, using a sequence of delignifying and bleaching processing steps involving the use of oxidizing chemical products. The first step of a standard sequence for the bleaching of chemical pulp has the 10 objective of completing the delignification of the raw pulp as found after the cooking operation. This delignifying first step is conventionally performed by treating the raw pulp with chlorine in an acidic medium or with a chlorine-chlorine dioxide combination, as a mixture or in 15 sequence, in order to react with the residual lignin of the pulp and give rise to chlorolignins, which may be extracted from the pulp by dissolving these chlorolignins in an alkaline medium in a subsequent processing step.

For various reasons, it turns out to be useful, 20 in certain situations, to be able to replace this delignifying first step by a treatment which no longer requires the use of a chloro reactant.

For about the last twenty years, it has been proposed to precede the first processing step using 25 chlorine or the chlorine-chlorine dioxide combination by a step involving oxygen gas in an alkaline medium, (Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, Vol. 19, New York 1982, page 415, 3rd paragraph and page 416, 1st and 2nd paragraphs) . The degree of 30 delignification which is obtained by this treatment with oxygen is, however, not sufficient if the aim is to produce chemical pulps of high whiteness whose mechanical properties are not degraded.

It has been proposed (Patent Application 35 EP-Al-0,456,626) to bleach kraft pulps using hydrogen peroxide according to a sequence Q P or O Q P (the abbreviation Q representing a step with an agent for sequestering metal ions, the abbreviation P a step using 2>4852 hydrogen peroxide and the abbreviation 0 a step using oxygen) . It is, however, difficult to obtain a final whiteness greater than 85eISO with this process without having recourse to additional steps using chloro 5 reactants such as chlorine dioxide.

The invention overcomes these drawbacks of the known processes, by providing a new process for delignifying and/or bleaching chemical paper pulps which enables high levels of whiteness to be achieved without exces-10 sively degrading the cellulose and without using chloro reactants.

To this effect, the invention relates to a process for bleaching a chemical paper pulp using a sequence of processing steps comprising at least the 15 following steps, carried out in the order: Q P A where the abbreviation Q represents a step of decontamination of the pulp in respect of its transition metals, the abbreviation A represents a step with a non-20 peroxygenated acid and the abbreviations P represent a step with alkaline hydrogen peroxide.

The invention preferably relates to a process for bleaching a chemical paper pulp using a sequence of steps according to which the processing steps are free of 25 chloro reactants and the sequence comprises the following steps, carried out in the order: Q P A P Chemical paper pulp is understood to denote pulps which have undergone a delignifying treatment in the 30 presence of chemical reactants such as sodium sulphide in an alkaline medium (kraft cooking or cooking with sulphate), sulphurous anhydride or a metal salt of sulphurous acid in an acidic medium (cooking with sulphite or with bisulphite) . This is thus also 35 «•«•» refer to chemico-mechanical pulps and semi- chemical pulps, for example those in which the cooking has been performed using a sulphurous acid salt in a neutral medium (cooking with neutral sulphite, also known as NSSC cooking) which may also be bleached by the 374852 process according to the invention, as well as the pulps obtained by processes using solvents, for example such as the ORGANOS OLV, ALCELL®, ORGANOCELL® and ASAM pulps described in Ullmann's Encyclopedia of Industrial 5 Chemistry, 5th Edition, Vol. A18, 1991, pages 568 and 569.

The invention particularly addresses pulpB which have undergone a kraft cooking or a cooking with sulphite. All the types of wood used for the production 10 of chemical pulps are suitable for the implementation of the process of the invention, and in particular those used for the kraft pulps and the sulphite pulps, namely resiniferous woods, for example such as the various species of pine and of fir, and leaf-bearing woods, for 15 example such as beech, oak, eucalyptus and hornbeam.

According to the invention, the first step is a step of decontamination of the pulp in respect of its transition metals (step Q).

According to a first variant of the invention, 20 step Q consists in treating the pulp with at least one sequestering agent such as an inorganic phosphate or polyphosphate, for example such as an alkali metal pyrophosphate or metaphosphate, an organic polycarboxy-late or aminopolycarboxylate, for example such as 25 tartaric acid, citric acid, gluconic acid, ethylene-diaminetetraacetic acid, diethylenetriaminepentaacetic acid, cyclohexanediaminetetraacetic acid and their salts, poly-a-hydroxyacrylic acid and its salts or an organic polyphosphonate such as ethylenediaminetetramethylene-30 phosphonic acid, diethylenetriaminepenta (methylenephos-phonic) acid, cyclohexanediaminetetramethylenephosphonic acid and their salts. Ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) gave excellent results.

The sequestering agent is generally used in step Q in an amount of at most 1.5 g of sequestering agent per 100 g of dry pulp. Usually, this amount is at most 1.0 g of sequestering agent per 100 g of dry pulp. It is often at least 0.01 g/100 g of dry pulp. i 7 4 8 5 2 In addition to the sequestering agent, there may also be added, according to this first variant, a small amount of acid at step Q. The amount of acid used to do this will generally be adjusted to impose a pH of 5 approximately 5 on the medium.

Step Q may, according to a second variant, also consist of a treatment with an acid free of a sequestering agent, followed by an addition of soluble magnesium salt in an amount such that the weight ratio of 10 the amount of Hg to that of Mn present in the pulp is at least 30. Furthermore, it is generally not useful for the amount of Mg to exceed that for which the Mg/Mn weight ratio is equal to 104.

Acid is understood to denote inorganic anhydrides 15 or acids such as sulphurous anhydride and sulphuric acid, sulphurous acid, hydrochloric acid, phosphoric acid and nitric acid or their acidic salts, as well as organic acids such as carboxylic acids or phosphonic acids or their acidic salts. Sulphurous anhydride or alkali metal 20 bisulphites or alkaline-earth metal bisulphites are especially suitable. Bisulphite is understood to denote the acidic salts of sulphurous acid corresponding to the formula Me(HS03)n, in which Me symbolizes a metal atom of valency n, n being a integer equal to 1 or 2. 25 According to this second variant, the amount of acid to be used will depend on the type of wood and on the amount of metal impurities that it contains. In general, an amount of acid will be used such that the pH of the pulp is at least approximately 5 and preferably 30 approximately 5.5. Similarly, the amount of acid will be adjusted often so that the pH does not exceed 7 and preferably does not exceed 6.5. When step Q is free of sequestering agent, the pH will be adjusted in order to render the medium substantially more acidic, that is to 35 say of pH at most equal to 5 and preferably at most equal to 4.5. In order not to degrade the pulp, descending below pH 2.0 and preferably below pH 2.5 will generally be avoided.

Step Q is generally carried out at a pressure in 274852 the region of atmospheric pressure and at a sufficient temperature to ensure efficient consumption of the acid and/or of the sequestering agent and, at the same time, not too high a temperature in order not to degrade the 5 cellulose and put a strain on the energy costs of the means of heating used in the said step. In practice, a temperature of at least 40°C and preferably of at least 50 °C is especially suitable. Similarly, it is advantageous for the temperature not to exceed 100°C and 10 preferably not to exceed 95°C. The best results were obtained at 50-55°C.

The duration of step Q must be sufficient to ensure complete reaction. Although longer durations have no influence on the degree of delignification of the pulp 15 or on its intrinsic strength qualities, it is not advised to prolong the duration of the reaction beyond that which is necessary to complete the reaction, in order to limit the capital costs and the energy costs for heating the pulp. In practice, the duration of the pretreatment may 20 vary within wide proportions depending on the type o£ equipment used, the choice of acid, the temperature and the pressure, for example from 15 minutes approximately to several hours. Durations of at least 10 minutes and preferably of at least 15 minutes are generally suffi-25 cient. Similarly, the pretreatment durations generally do not exceed 90 minutes and preferably do not exceed 60 minutes. A duration of approximately 30 minutes gave excellent results.

Step Q is generally carried out at a pulp consis-30 ?tency of at least 2 % solids content and preferably of at least 2.5 % solids content. Usually, this consistency does not exceed 15 % and preferably does not exceed 10 %. A consistency of approximately 3 % solids content gave excellent results.

According to the invention, the preferred sequence is a sequence QPAP. In this sequence, the symbols P denote steps using alkaline hydrogen peroxide. The nature of the alkali should be such that it has a good efficiency for extraction of the oxidized lignin, in 274 8 52 conjunction with good solubility. One example of such an alkali is sodium hydroxide in aqueous solution. The amount of alkali to be used should be sufficient to maintain the pH at a valve of at least 10 and preferably 5 of at least 11. The amount of alkali should also be adjusted to ensure a sufficient consumption of the peroxide at the end of the reaction. In practice/ when step P is carried out in the absence of stabilizing agents, or when sodium silicate is present as stabilizing 10 agent, amounts of alkali of at least 1 and of at most 3 g of alkali per 100 g of dry pulp are especially suitable. In the case where a stabilizing agent giving rise to an acidic reaction in an aqueous solution is employed, the amounts of alkali will be increased in order to maintain 15 a pH of at least 10. In this case, amounts of alkali which may range up to at most 8 g/100 g of dry pulp may be employed.

In addition to these amounts of alkali# an amount of hydrogen peroxide of at least 0.3 g HjOj/lOO g of dry 2 0 pulp and preferably of at least 0.5 g/100 g of dry pulp will be used. It is also generally appropriate for the amounts of hydrogen peroxide not to exceed 5.0 g Ha02/ 100 g of dry pulp and preferably not to exceed 4.0 g/ 100 g of dry pulp.

The temperature in the steps P should be adjusted in order to remain at least equal to 50 °C and preferably equal to 70 °C. It should also not exceed 150 °C and preferably not exceed 135°C. A temperature of 90°C gave excellent results. 3 0 An advantageous variant o£ the process according to the invention consists in carrying out at least one of the steps P of the sequence at a high temperature, of at least 100°C. According to this variant, the temperature of this step P is preferably at least 110°C. It generally 35 does not exceed 140°C and preferably does not exceed 135°C.

The duration of the steps P should be sufficient for the bleaching reaction to be as complete as possible. It cannot, however, exceed this reaction time too greatly 274852 without running the risk of inducing a retrogradation of the whiteness of the pulp. In practice, it will be set at a value of at least 60 minutes and preferably of at least 90 minutes. It also usually must not exceed 600 minutes 5 and preferably not exceed 500 minutes.

The consistency of the steps P is generally chosen to be less than or equal to 50 % solids content by weight and preferably less than or equal to 40 % solids content. It will often not be less than 5 % and prefer-10 ably not less than 8 %. A consistency of 10 % gave good results.

The consistency of the final step P may, as a variant, be advantageously chosen from high values of at least 25 % solids content by weight and preferably 30 % 15 or more. In this variant, the consistency of the other steps P will generally be at most 20 %. It will also usually be appropriate for the consistency of the other steps P to be at least 8 %.

According to the invention, step A is a step of 20 treatment with a non-pexoxygenated acid. Non-par oxygenated acid is understood to denote any inorganic or organic acid in which the molecule does not contain a peroxy group -O-O-. All the usual inorganic or organic acids used in aquorus solution, alone or as a mixture, 25 are suitable. Strong inorganic acids such as, for example, sulphuric acid, phosphoric acid, nitric acid or hydrochloric acid are especially suitable. Sulphuric acid in aqueous solution gave excellent results.

The operating conditions of step A should be 30 determined in each particular case as a function of the type of paper pulp and of the species of wood which are used. It is generally appropriate to set the choice of the acid and the amount used in order to impose on the medium a pH which is substantially lower than 7, for 35 example from at least approximately 1 to at most approximately 6.5. Especially advantageous pH values are those of at least approximately 2.5 and at most approximately 5.5. The temperature and the pressure are not critical, atmospheric pressure and temperatures from 274852 at least 10°C to at most 100°C being generally suitable. The duration o£ the pretreatment may vary within wide proportions according to the type of equipment used, the choice of acid, the temperature and the pressure, for 5 example from 30 minutes approximately to several hours.

In step A, there may be introduced, in addition to the acid, a magnesium salt which is soluble in an aqueous acidic medium. The amount of magnesium which may be added generally does not exceed 0.4 g of magnesium per 10 100 g of dry pulp and preferably does not exceed 0.1 g/ 100 g of dry pulp. When an Mg salt is introduced, it is in an amount of at least 0.005 g/100 g of dry pulp.

According to another variant of the invention, step A may consist of a simple washing of the pulp using 15 an acidic aqueous solution. In the case of a sequence QPAP, this acidic washing may come between the two steps P.

Another variant of the process according to the invention consists in repeating a second time the steps 20 P and A in order to achieve the complete sequence Q P A P A P.

In another variant of the process according to the invention, the sequence may be preceded by a step using oxygen (step 0). This step using oxygen is carried 25 out by placing the pulp in contact with oxygen gas at a pressure of at least 20 kPa and of at most 1000 kPa, in ohe presence of an alkaline compound in an amount such that the weight of alkaline compound relative to the weight of dry pulp is at least 0.5 and at most 5 %. 30 A specific embodiment of the process consists in recycling all or some of the effluents of the second step and/or of the third step P into a step P located earlier in the sequence. A similar recycling of the effluents of the second acidic step, when this is present, into the 35 first acidic step of the sequence may also be performed. Recycling of the effluents of the steps P into a prior step P and recycling of the effluents of an acidic step into a prior acidic/step may also be combined within the same sequence. These recyclings have the advantage of 2 7 4 8 52 economizing on the chemical products used by recovery of the reactant not consumed.

The temperature of the step using oxygen should generally be 70°C or more and preferably at least 80°C.

It is also appropriate for this temperature not to exceed 130°C and preferably not to exceed 120°C.

The duration of the treatment with oxygen should be sufficient for the reaction of the oxygen with the lignin contained in the pulp to be complete. It cannot, 10 however, exceed this reaction time too greatly without running the risk of inducing degradations in the structure of the cellulose chains of the pulp. In practice, it will be 30 minutes or more and preferably at least 40 minutes. It usually will not exceed 120 minutes and 15 preferably will not exceed 80 minutes.

Treatment of the pulp with oxygen may also be carried out in the presence of a cellulose-protecting agent such as soluble magnesium salts or organic sequestering agents such as polycarboxylic acids or phosphonic 20 acids. Magnesium salts are preferred, in particular magnesium sulphate heptahydrate used in am amount of 0.02 to 1 % by weight relative to the dry pulp.

The pulp consistency during step O is generally not less than 8 % solids content by weight and preferably 25 not less than 10 %. This consistency usually does not exceed 30 % solids content by weight and preferably does not exceed 25 %.

As a variant, step O may also be carried out in the presence of hydrogen peroxide (step Op) . The amount 30 of hydrogen peroxide which may be incorporated into step O is generally not less than 0.2 g H,Oa per 100 g of dry pulp and is usually not less than 0.5 g. Similarly, 2.5 g HaOa per 100 g of dry pulp will usually not be exceeded and, most often, 2 g will not be exceeded. 35 In an analogous manner, the first step P may also be reinforced by the presence of oxygen gas (step Eop). In this case, the oxygen pressure used will usually be ac least 20 kPa and at most 1000 kPa.

In another variant of the process according to 274852 the invention, an additional enzymatic step consisting in treating the pulp with at least one enzyme (step X) may be incorporated at any point in the sequence of processing steps. This enzymatic treatment may also be 5 performed before or after the step of pretreatment with oxygen.

Enzyme is understood to denote any enzyme capable of facilitating the delignification, by the processing steps after the step of treatment with the enzyme, of a 10 raw chemical paper pulp derived from the cooking operation or of a chemical paper pulp which has already undergone a step of treatment with oxygen.

An alkalophilic enzyme, that is to say an enzyme whose maximum efficiency is situated in the alkaline pH 15 region and most particularly at a pH of 7.5 and higher, will preferably be used.

One category of enzymes which are well suited to the process according to the invention is the hemi-cellulases. These enzymes are able to react with the 20 hamicelluloses to which the lignin present in the pulp is attached.

The hemicellulases used in the process according to the invention are preferably xylanases, that is to say hemicellulolytic enzymes capable of breaking the xylane 25 bonds which constitute a major part of the interface between the lignin and the rest of the carbohydrates. An example of a xylanase in accordance with the process according to the invention is 1,4-/S-D-xylane xylanohydro-lase, EC 3.2.1.8.

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

The xylanases of bacterial origin are particu-35 larly advantageous. Among the xylanases of bacterial origin, xylanases secreted by bacteria of the genus Bacillus gave good results.

The xylanases derived from bacteria of the genus Bacillus and from the species pywH gave excellent 2 7 4 8 5 2 results. Among these, the xylanases derived from Bacillus pntnH in« prl B12 are most particularly advantageous.

The xylanases of Bacllluf* prl B12 which are in accordance with the invention may be derived 5 directly from a strain of Bacillus prl B12 or alternatively from a host strain of a different microorganism which has been genetically manipulated beforehand in order to express the genes coding for the degradation of the xylenes of Bacillus pyri'i ? V? prl B12. 10 A purified xylanase which contains no other enzymes will preferably be UBed. In particular, it is preferable for the xylanase in accordance with the process according to the invention not to contain any cellulase, so as not to destroy the polymeric cellulose 15 chains of the pulp.

An advantageous variant of the process according to the invention consists in carrying out the enzymatic step X in the presence of at least one agent for sequestering metal ions. The agents for sequestering metal ions 20 may advantageously be chosen from the sequestering agents suitable for step Q, which are described above.

The process in accordance with the invention applies to the bleaching of any sort of chemical pulp. It is especially suitable for delignifying kraft pulps and 25 sulphite pulps. It is particularly well suited to the treatment of kraft pulps.

The examples which follow are given with the aim of illustrating the invention, but do not limit the scope thereof.

T?v»nrpi ir tQ 8R (not in accordance with the invention) A sample of resiniferous pulp which has undergone a kraft cooking (initial whiteness 27.9°ISO measured according to ISO standard 2470-1977 (F) , kappa number 26.7 measured according to SCAN standard Cl-59, and degree of 35 polymerization 1680 expressed as the number of glucose units and measured according to SCAN Btandard C15-62) was treated according to a sequence of 2 steps Q P under the following conditions: 274352 1st step: step using DTPA (step Q): DTPA content, g/100 g dry pulp: 0.2 temperature, °C: 90 duration, min: 60 consistency, % solids content by weight: 4 2nd step: step using hydrogen peroxide (step P): HjOj content, g/100 g dry pulp: 5 NaOH content, g/100 g dry pulp: 4 DTMPANa, content, g/100 g dry pulp: 0 to 1 MgS04.lYLfi content, g/100 g dry pulp: 0.5 temperature, °C: 90 and 120 duration, min: 120 consistency, % solids content by weight: 10 where DTMPANa, represents the hept&sodium salt of 15 diethylenetriaminepenta(methylenephosphonic) acid. The DTMPANa, contents and the temperatures used in each of the Examples 1R to 8R were as follows: Example No.

DTMPANa, Temperature g/100 g step P s.c.

°C 1R 0 90 2R 0.25 90 3R 0.50 90 4R 1.00 90 5R 0 120 6R 0.25 120 7R 0.50 120 8R 1.00 120 After each processing step, the pulp underwent a washing with damineralized water at room temperature.

After treatment, the kappa number and the white ness of the pulp were determined. The results are featured in the table below. 274 8 52 Example No.

Final Final whiteness kappa °IS0 number 1R 66.5 11.3 2R 68.2 11.2 3R 68.7 11.2 4R 68.2 11.6 5R 71.2 8.1 6R 77.1 7.6 7R 75.5 7.8 8R 72.9 e.i It can be seen that, despite the considerable amount of hydrogen peroxide used, it was not possible to exceed a whiteness of 78°ISO.

Example 9: (in accordance with the invention) 5 The same pulp sample as in Examples 1R to 8R was bleached according to the sequence Q P A P. The operating conditions used were as follows: 1st step: step using DTPA (step Q): HjS04 (for pH 5) : 0.54 DTPA content, g/100 g dry pulp: 0.2 temperature, °C: 90 duration, min: 60 consistency, % solids content by weight: 4 2nd step: step using hydrogen peroxide (step P): H,02 content, g/100 g dry pulp: 3 NaOH content, g/100 g dry pulp: 4 DTMPANa, content, g/100 g dry pulp: 0.25 temperature, °C: 120 duration, min: 120 consistency, % solids content by weight: 10 3rd step: step using sulphuric acid (step A): H2S04, amount necessary to reach pH: 3 temperature, °C: 90 duration, min: 120 consistency, % solids content by weight: 10 274852 4th step: step using hydrogen peroxide (step P): HjOa content, g/100 g dry pulp: 1 NaOH content, g/100 g dry pulp: 1.7 content o£ Na silicate 38°B6, g/100 g dry pulp: 3 5 MgS04.7HjO content, g/100 g dry pulp: 1 temperature, °C: 120 duration, min: 180 consistency, % solids content by weight: 30 After each processing step, the pulp underwent a 10 washing with damineralized water at room temperature.

At the end of the sequence, after treatment, the kappa number and the whiteness of the pulp were determined.

The results obtained were: Example No.

Final whiteness •ISO Final kappa number 9 84.4 4.4 ^ 1QR: (not in accordance with the invention) The same sample of pulp as in Examples 1R to 9 was bleached according to the sequence O Q P P. The operating conditions used were as follows: 1st step: step using oxygen (step O): pressure, bar: 6 NaOH content, g/100 g dry pulp: 4 MgS04.7HjO content, g/100 g dry pulp: 0.5 temperature, °C: 120 duration, min: 60 consistency, % solids content by weight: 12 2nd step: step using DTPA (step Q): DTPA content, g/100 g dry pulp: 0.12 temperature, °C: 55 duration, min: 30 consistency, % solids content by weight: 3 3rd step: step using hydrogen peroxide (step P): H2Oj content, g/100 g dry pulp: 3 NaOH content, g/100 g dry pulp: 4 274852 DTMPANa? content, g/100 g dry pulp: 0.25 MgS04.7HaO content, g/100 g dry pulp: l temperature, °C: 120 duration, min: 120 consistency, % solids content by weight: 10 where DTMPANa, represents the heptasodium salt of dittthylenetriaminepenta (methylenephosphonic) acid. 4th step: step using hydrogen peroxide (step P): HjOj content, g/100 g dry pulp: 1 NaOH content, g/100 g dry pulp: 1.5 :igS04.7H30 content, g/100 g dry pulp: 1 temperature, °C: 120 duration, min: 180 consistency, % solids content by weight: 30 After each treatment step, the pulp underwent a washing with deminaralized water at room temperature.

At the end of the sequence, after treatment, the kappa number and the whiteness of the pulp were determined.

The results obtained were as follows: Example No.

Final whiteness °IS0 Final kappa number 10R 82.6 .1 FTHiWPlTP 11 to 14: (in accordance with the invention) The same pulp sample as in Examples 1R to 10R was bleached according to the sequence 0 Q P A P. The operating conditiona uaed were aa follows: 1st step: atep uaing oxygen (atep O): pressure, bar: 6 NaOH content, g/100 g dry pulp: 4 MgSOf.THjO content, g/100 g dry pulp: 0.5 temperature, °C: 120 3 0 duration, min: 60 consistency, % solids content by weight: 12 2nd step: step using DTPA (step 0): DTPA content, g/100 g dry pulp: 0.12 27 4 8 52 temperature, °C: 55 duration, min: 30 consistency, % solids content by weight: 3 3rd step: step using hydrogen peroxide (step P): HaOa content, g/100 g dry pulp: 3 NaOH content, g/100 g dry pulp: 4 DTMPANa, content, g/100 g dry pulp: 0.25 MgS04.7H30 content, g/100 g dry pulp: 1 temperature, °C: 120 duration, min: 120 consiatency, % solids content by weight: 10 where DTMPANa, represents the heptasodium salt o£ diethylenetriaminepenta(methylenephosphonic) acid. 4th step: step with an acid (step A): H3S04 content, g/100 g dry pulp: Ex. 11 : 1.08 (pH 3) Ex. 12 : 0.52 (pH 5) Ex. 13 and 14 : 0 SOj content, g/100 g dry pulp: Ex. 11 and 12 : 0 Ex. 13 : 1.06 (pH 3} Ex. 14 : 0.45 (pH 5) MgS04.7Hj0 content, g/100 g dry pulp: 1 temperature, ° C: 90 duration, min: 120 consistency, % solids content by weight: 10 5th step: step using hydrogen peroxide (step P): HjOj content, g/100 g dry pulp: 1 NaOH content, g/100 g dry pulp: 1.5 content o£ Na silicate 38°Bft, g/100 g dry pulp: 3 MgS04. 7HaO content, g/100 g dry julp: 1 3 0 temperature, °C: 120 duration, min: 180 consistency, % solids content by weight: 30 After each treatment step, the pulp underwent a washing with demineralized water at room temperature. 3 5 At the end of the sequence, after treatment, the kappa number and the whiteness of the pulp were determined.

The results obtained were as follows: 274852 Example No.

Final Final whiteness kappa •ISO number 11 87.5 3.6 12 86.4 4.6 13 CO * r-oo 3.4 14 86.1 4.7 ffiffHiTnPli'rg 15 and 161 (In accordance with the Invention) The same pulp sample as in Examples 1R to 14 was bleached using the sequence 0 X Q P A P, using in step X an amount of xylanase derived from Bacillm* 5 PRL B12 corresponding to 10 XU/g of dry pulp. The unit XU (Xylanase Unit) is defined as being the amount of xylanase which, under the test conditions, catalyses the release of reducing sugars equivalent in reducing power to 1 micromol of glucose per minute.

The operating conditions were as follows: 1st Step: step using oxygen (step O): pressure, bar: 6 NaOH content, g/100 g dry pulp: 4 MgS04.7H20 content, g/100 g dry pulp: 0.5 temperature, °C: 120 duration, mint 60 consistency, % solids content by weight: 12 2nd atep; step using enzyme (step X): HaS04 for pH: 7 xylanaae content, XU/g dry pulp: 10 temperature, °C: 50 duration, min: 120 consistency, % solids content by weight: 3 3rd step: step using EDTA (step Q): H2S04 for pH: 5 EDTA content, g/100 g dry pulp: 0.4 temperature, °C: 50 duration, min: 30 consistency, % solids content by weight: 3 4th step: step using hydrogen peroxide (step P):

Claims (8)

- 18 - 274852 H30a content, g/100 g dry pulp: 4 NaOH content, g/100 g dry pulp: 4 DTMPANa, content, g/100 g dry pulp: 0.25 MgS04.7HaO content, g/100 g dry pulp: 1 5 temperature, °C: 120 duration, min: 120 consistency, % solids content by weight: 10 where DTMPANa7 represents the heptasodium salt of diethylenetriaminepenta(methylenephosphonic) acid. 10 4th step: step using sulphuric acid (step A): H3SO< for pH 3 temperature, °C: 90 duration, min: 120 consistency, % solids content by weight: 10 15 5th step: step using hydrogen peroxide (step P): HaOa content, g/100 g dry pulp: 1 (Ex.15) and 2 (Ex.16) NaOH content, g/100 g dry pulp: 1.5 content of Na silicate 38°B€, g/100 g dry pulp: 3 MgS04.7Ha0 content, g/100 g dry pulp: 1 20 temperature, °C: 120 duration, min: 180 consistency, % solids content by weight: 30 After each processing step, the pulp underwent a washing with demineralized water at room temperature, 25 except between steps X and Q which were performed by simple acidification to pH 5 using sulphuric acid and by incorporation of EDTA to the pulp harvested after step X, without dilution or reconcentration. At the end of the sequence, after treatment, the 30 kappa number and the whiteness of the pulp were determined. The results obtained were as follows: Example No. Final Final whiteness kappa °xso number 15 89.6 3.0 16 91.7 2.6 - 19 -CIAIMS 27 4 8 52- 10 15 20 25 30

1. Process for bleaching a chemical paper pulp ualng a aequence of processing ateps comprising at leaat the following steps, carried out in the order: Q P A where the abbreviation Q represents a step of decontamination of the pulp in respect of its transition metals, the abbreviation A represents a step with a non-peroxygenated acid and the abbreviation P represents a step with alkaline hydrogen peroxide.

2. Process for bleaching a chemical paper pulp using a sequence of steps according to Claim 1, characterized in that the processing steps are free of chloro reactants and the sequence coaqprises the following steps, carried out in the order: Q P A P

3. Process according to either of Claims 1 and 2, characterised in that step Q consists of a treatment with an acid free of a sequestering agent, followed by an addition of soluble magnesium salt in an amount such that the weight ratio of the amount of Mg to that of Mn present in the pulp is at least 30.

4. Process according to either of Claims 1 and 2, characterized in that step Q consists of a treatment with a sequestering agent.

5. Process according to Claims 1 to 4, characterized in that at least one of the steps P of the sequence is performed at a temperature of 100°C or more.

6. Process according to any one of Claims 1 to 5, characterized in that a step using oxygen gas precedes the sequence.

7. Process according to any one of Claims 2 to 6, characterized in that the final step P is performed at a pulp consistency of at least 25 % solids content by weight.

8. Process according to any one of Claims 2 to 7, characterized in that the final two steps A P are repeated at the end of the sequence, such that the sequence of processing steps is QPAPAP • vi J « w U •* ABSTRACT 274 8 5 Process for bleaching a chemical paper puIp Process £or bleaching a chemical paper pulp using a sequence of processing steps consisting in carrying out, in order, a step of decontamination of the pulp in respect of its transition metals, a step using alkaline hydrogen peroxide and a step with a non-peroxygenated acid. No figure.