NZ242356A

NZ242356A - Process for preparing bleached paper pulp comprising: treating chips with a reducing agent, then alkaline hydrogen peroxide, followed by refining

Info

Publication number: NZ242356A
Application number: NZ242356A
Authority: NZ
Inventors: Michel Devic
Original assignee: Atochem Elf Sa
Priority date: 1991-04-17
Filing date: 1992-04-14
Publication date: 1994-06-27
Also published as: IE75202B1; NO921335L; EP0509905A1; IE921176A1; ES2089436T3; NO921335D0; FR2675518B1; CA2066316C; JP2588495B2; EP0509905B1; ATE140283T1; DE69212051D1; FI921671A0; FR2675518A1; JPH06220788A; NO300225B1; AU1489792A; AU650962B2; FI921671A; DE69212051T2

Abstract

Manufacture of high-yield paper pulp from wood chips. The process consists in successively treating the chips, before grinding, with a solution containing at least one reducing agent, and then with an alkaline hydrogen peroxide solution. Application to the manufacture of paper pulp with a high degree of whiteness and good mechanical characteristics.

Description

<div class="application article clearfix" id="description"> New Zealand Paient Spedficaiion for Paient Number £42356 242356 f. Priority Date(o): Complete Specification Filsd. 2. Class: DS.WIOS'.-'^.O.I*- ;tloakzh-iy . 2 7 "juii 199^1 ;Publication Date: ;P.O. Journal, No1 /<?£/.•■•■ ;itj I su~i 'j $ i j ^ ;n.z. patent c^-k ;NEW ZEALAND PATENTS ACT, 1953 ;No.: ;Date: ;COMPLETE SPECIFICATION PROCESS FOR PREPARING BLEACHED PAPER PULP IN HIGH YIELD ;HOE ;1992 ;We, ELF ATOCHEM S A, a French body corporate, of 4 Cours Michelet, La Defense 10, 92091 La Defense, France, ;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 page la) ;242356 ;- lA- ;The present invention relates to a process for the production of bleached paper pulps in high yield which have good mechanical characteristics, in particular chemicomechanical and chemicothermomechanical properties. ;5 To obtain a pulp, lignocellulose, generally wood, ;chips may be subjected to separate or combined actions of mechanical, chemical or thermal nature. ;Chemical pulps are obtained by chemical grinding of the wood. Their yield is generally less than 50 %. ;10 Yield is to be understood as meaning the weight of the pulp in the dry state relative to the weight of the starting material in the dry state. ;Pulps of mechanical type or high-yield pulps are produced by mechanical grinding of wood chips, for example ;15 in a grinder or in a disc grinder or refiner. The yield is generally above 85 %. However, the mechanical characteristics of the product, especially the resistance to breaking, to tearing and to bursting, are generally poor. ;20 To improve these mechanical characteristics, wood chips have been subjected to a thermal treatment with steam at a temperature from 100°C to 140"C prior to grinding. These pulps are termed thermomechanical pumps (TMP). However, their whiteness is poor. ;25 Wood chips have also been treated with sodium sulphite solution having an acidic or basic pH, depending ;242 356 ;- 2 - ;on the type of wood. The resulting pulps, which are termed chemicomechanical pulps, have good mechanical characteristics. ;It has also been possible to substantially increase 5 the resistance of the mechanical pulps by subjecting the wood chips to a treatment with the aid of one or more chemicals combined with heating and mechanical grinding processes. These pulps are termed chemicothermomechanical pulps (CTMP). This treatment consists of non-destructive 10 boiling of the material at a temperature of 100'C or above, under the pressure of saturated water vapour in the presence of sulphite, in particular sodium sulphite Na2S03, or sodium bisulphite NaHS03. ;The advantage of these CTMP pulps is that they have 15 improved mechanical resistance and that the yield remains generally above 85 %, usually at least 90 % approximately, and thus is similar to the yield obtained for pulps of purely mechanical origin. ;While pulps which have been treated in this manner, 20 in particular chemicomechanical pulps (CMP) and chemicothermomechanical pulps (CTMP), have improved mechanical resistance, their bleaching, which the quality required for paper demands, remains a problem. ;To obtain better bleaching of these pulps, French 25 Patent FR 1,389,308 and European Patent EP-A-293,309 have proposed the treatment of wood chips with combined ;24 2 3 f ;- 3 - ;solutions of alkali metal borohydride and sulphite, at an acidic or alkaline pH. ;This treatment enables pulps to be obtained after grinding which have good mechanical properties and improved 5 whiteness. The whiteness obtained, however, is still not sufficient, and the refined pulps must undergo a bleaching treatment with hydrogen peroxide. This problem of insufficient bleaching results especially from the sulphite treatment which, while it improves the mechanical 10 properties of the resulting refined pulps, brings about a poor response to bleaching with hydrogen peroxide in these pulps. ;The obtaining of pulps which have good mechanical properties and simultaneously a sufficient whiteness for 15 use in the production of paper with a high degree of whiteness has thus previously proved problematic. ;With a view to overcoming this problem, European Patent EP-A-239,583 describes a process in which wood chips undergo three successive treatments before grinding, namely 20 treatment with a solution of a sequestering agent for metal ions, treatment with a solution of a hydrogen peroxide stabiliser, such as sodium silicate and magnesium salts, and treatment with an alkaline peroxide solution. This gives, after grinding, pulps of improved whiteness. 25 However, this whiteness, which remains below 80° ISO, especially when coniferous wood is used, is still ;/ ' k l ''' it ' - ;/"< ;/■■v ;'V* 18 APR 1994 - 4 - insufficient to be suitable for all types of use. It is therefore generally necessary to make provision for a traditional bleaching step of the refined pulp with hydrogen peroxide, which increases the production costs. 5 Treatment with hydrogen peroxide before refining, without sulphite treatment, also allows the mechanical resistance to be improved, but the properties are still not good enough, in particular when coniferous wood is used. According to the present invention, there is 10 provided a process for preparing bleached paper pulp from lignocellulose chips, which process comprises treating the chips with a solution of at least one reducing agent, subsequently treating the chips with an alkaline hydrogen peroxide solution, and then refining the treated chips. 15 Using the process of the present invention, it is possible to obtain, after refining, pulps in high yield which have good mechanical properties (resistance to breaking, tearing, bursting) combined with a high degree of whiteness, in particular above 80° ISO in coniferous woods, 20 making them suitable for use in the production of paper with a high degree of whiteness. More particularly, the process according to the invention comprises the following steps: (a) soaking wood chips in the solution of 25 reducing agent(s), (b) heating the chips which have been soaked in 18 APR . f ^ ly rii d 5 step (a) at a temperature of from 60 to 160°C, (c) pressing the chips to extract at least 20 % by weight of the solution of reducing agent(s), (d) soaking the chips obtained in step (c) in the 5 alkaline solution containing hydrogen peroxide, (e) heating the chips which have been soaked in step (d) at a temperature of from 50 to 120°C, and (f) refining the chips. 10 An additional step in which the chips are soaked with a complexing or sequestering agent for metal ions can, if appropriate, be provided before step (d), for example using an aqueous 40 % strength solution of DTPA (sodium salt of diethylenetriamine pentaacetic acid). 15 The lignocellulose materials which can be treated by the process according to the invention comprise coniferous wood such as fir, pine, spruce, etc., hardwood such as poplar, eucalyptus wood, etc ... in the form of chips as they are used customarily in the paper pulp 20 industry. Annual plants such as bagasse, straw, cut into pieces of a few centimetres can also be treated. The chips are generally subjected to preparatory treatments, which are customarily used in the paper pulp industry, for example, washing, sorting, preheating of the 2 5 steam. According to the process of the invention, the 18 APR 2423 - 6 - chips are first treated in a solution of one or more reducing agents (step a). This treatment may be effected by soaking the chips in the solution. Means conventionally used in the paper industry can be used for this soaking 5 step. Before proper soaking of the chips, the chips, which may have been previously been softened by wetting and/or oven-steaming, may be compressed by means of a screw press so as to remove part of any gases and/or liquid present in the chips. When the chips have expanded after pressing, 10 they are generally soaked with the reducing solution. Apparatus which may be mentioned as particularly suitable for industrial continuous soaking of the chips is the system PREX, by SUNDS-DEFIBRATOR, the system IMPRESSAFINER, by BAUER, the conical HYMAC screw press, as 15 well as other forms of apparatus which allow chips to be compressed and to expand once or several times, in the presence of a soaking solution with, preferably, the highest possible compression ratio. According to the invention, the chips are generally 2 0 allowed to expand in the presence of the smallest amount possible of the solution in which the chips are soaked. The reducing solution which is used in the process according to the invention contains one or more reducing agents which can be selected from amongst alkali metal 25 sulphites or bisulphites or mixtures of sulphur dioxide and an alkaline agent such as sodium hydroxide. Sodium 1 3 ^ ""?4 242 3 - 7 - borohydride (NaBH^ , pure or in alkaline solution, can also be used. Zinc or sodium hydrosulphite (Na2S2C>4), pure or obtained by reacting the alkaline sodium borohydride solution with sodium sulphite (Na2S03) or sodium bisulphite 5 (NaHSC>3) can also be used. It is also possible to use thiourea dioxide or formamidinesulphinic acid as reducing agent. For economic reasons, the reducing agent is preferably selected from amongst alkali metal sulphites, 10 such as sodium sulphite, sodium borohydride in alkaline solution, such as the product B0R0LR, marketed by MORTON International, which comprises 12 % by weight of sodium borohydride and 40 % by weight of sodium hydroxide, sodium hydrosulphite or sodium dithionite, and mixtures of sodium 15 sulphite and of the alkaline sodium borohydride solution BOROLr. The reducing agent is generally used in an amount of 0.1 to 10 % by weight relative to the weight of the chips in the dry state and, more particularly, in an amount 20 of 0.5 to 6% in the case of sodium sulphite, in an amount of 0.5 to 1.5 % in the case of an alkaline sodium borohydride solution B0R0LR, and in an amount of 0.5 to 2.5 % in the case of sodium hydrosulphite. The pH of the reducing solution used according to 25 the invention is suitably between a value greater than 7 and 13, depending on the nature of the lignocellulose 2423 - 8 - material of the chips and the pH is preferably from 11 to 12.8 in the case of hardwood and from 9.5 to 11 in the case of soft wood. If necessary, an alkaline agent such as NaOH, KOH, 5 Na2C03, K2C03, MgO and CaO can be added to bring the pH of the reducing solution to the desired value. A complexing or chelating agent for metal ions can be added, according to the invention, to the reducing solution. This agent can be, for example, sodium 10 tripolyphosphate, sodium tetrapyrophosphate, a sodium salt of nitrilotriacetic acid, ethylenediaminetetraacetic acid and diethylenetriamine pentaacetic acid. The sodium salt of diethylenetriamine pentaacetic acid (DTPA) in the form of an aqueous 40 % strength solution is preferably used, in 15 an amount of, preferably, 0.2 to 0.5 % of this solution. The temperature of the reducing solution during soaking is suitably from 10 to 100°C, depending on the stability of the reducing agent, a high temperature favouring the speed of soaking, and a low temperature 20 favouring the good preservation of the reducing properties of the solution. In most cases, a temperature from 20 to 60°C is preferred and, in particular, a temperature from 20 to 40'C is preferred when the reducing solution is obtained by 25 mixing Na2S03 and the alkaline sodium borohydride solution BOROLR. - 9 - Immediately after soaking with the reducing solution, the chips may be heated at a temperature of 60 to 160°C, preferably 75 to 120°C (step b). The duration of heating depends on the temperature 5 and the nature of the wood and varies from 1 min to 3 hours. The preferred duration is 15 to 60 min at 90°C and 5 to 30 min at 120°C. The soaked chips are preferably transported and heated with the exclusion of atmospheric oxygen, for 10 example using an atmosphere of steam or of an inert gas such as nitrogen or carbon dioxide. The chips which have been soaked in the reducing solution and then heated may be pressed by means of a screw press or by another means so as to extract part of the 15 reducing solution after the reaction. The chips are generally subjected to this pressing step since the effectiveness of the process increases with elimination of this solution. Elimination of at least 20 % by weight of the reducing solution is particularly desirable. 20 The reducing solution can be eliminated in one or more pressing steps. The chips can also be rinsed with water after pressing. Pressing can be effected at a temperature of from 20 to 160°C. 25 All, or part of, the reducing solution which is extracted during the pressing step can be recycled to the 18 ,n?~> - 10 - reducing solution used in step (a). After soaking in the solution of reducing agent(s) and after pressing, the chips are treated by soaking (step d) with an alkaline hydrogen peroxide 5 solution. The oxidant, hydrogen peroxide, is suitably used in an amount of 0.1 to 10 % by weight relative to the weight of the chips to be treated in the dry state, depending on the desired degree of whiteness; in general, the amount of 10 hydrogen peroxide is between 1 and 5 % by weight. More than 5 % by weight is not generally used for economical reasons. To the H202 solution there are added one or more alkaline agents such as NaOH, KOH, Na2co3, K2C03, MgO, Cao, 15 in such a way that the initial pH of this solution is suitably from 8 to 12.5. The preferred agent is NaOH. The optimum amount of alkaline agent depends on the nature of the wood and is proportional to the amount of H202. This amount, expressed in NaOH, can vary from 0.5 % to 10 % and, 20 preferably, 0.5 to 6 %, by weight relative to the weight of the chips to be treated in a dry state. One or more agents which stabilise hydrogen peroxide in alkaline medium can be added to the alkaline H202-soluti°n to reduce decomposition of this solution and 25 hence increase the effectiveness of the treatment. Stabilisers which may be mentioned are sodium silicate, 18AP??-X3} 242356 - n - generally in the form of a 40 % strength solution, used at 0.5 % to 5 % by weight relative to the weight of the dry chips, magnesium salts such as magnesium chloride, magnesium nitrate, magnesium carbonate and mixed salts at 5 0.01 to l % of magnesium, and polylactones of poly(a-hydroxyacrylic) acid, used at 0.02 to 2 % by weight. It is also possible to add, to the hydrogen peroxide solution, one or more complexing or sequestering agents for metal ions such as, for example, 10 diethylenetriaminepentaacetic acid and ethylenediaminetetraacetic acid in the form of sodium salts. They may be used in an amount of 0.1 to 1 % by weight relative to the weight of the chips to be treated in a dry state. 15 The temperature of the alkaline hydrogen peroxide solution during soaking is suitably between 10 and 80°C, and preferably not higher than 40°C, so as to reduce decomposition of the peroxide. After the step in which the chips are soaked with 20 the alkaline hydrogen peroxide solution, the chips are heated, in a step (e), at a temperature of from 50°C to 120"C, preferably from 60 to 80°C, generally for a period of 10 min to 5 hours, depending on the nature of the wood, the temperature and the amount of hydrogen peroxide. In 25 general, this period can reach 5 hours at 60°C, and is 2 to 3 hours at 70"C and 1 to 2 hours at 90°C. Heating is 242356 - 12 - generally provided by steam in a saturated atmosphere. At the conclusion of the two treatments with reducing and oxidising solution, the chips are ground or refined by passing through one or more refiners under 5 atmospheric pressure or under pressure of a few bars of water vapour. Refining is generally carried out in two refining steps, the first step being under pressure and the second under atmospheric pressure. The refining conditions depend 10 on the characteristics required for the paper pulp. The refined pulp is generally neutralised with an acid to eliminate the alkalinity which results from the treatment with the alkaline hydrogen peroxide solution. This neutralisation is preferably carried out to a pH of 15 5.5 to 6 using dilute sulphuric acid or gaseous SO2. The pulp is subsequently treated in the conventional manner (e.g. straining, screening, recycling of rejects). The Examples which follow are given by way of 20 example. The amounts of reactants in these Examples are given in per cent by weight relative to the wood chips in the dry state. Coniferous wood chips (fir) or hardwood chips (poplar) are used in the Examples. The chips were taken from an industrial plant for the production of 25 mechanical paper pulp and comply with the standards of this industry. 2423 - 13 - The Examples are carried out in a pilot plant for the production of high-yield mechanical pulp. This pilot plant has a capacity of approximately 200 kg/h, operates continuously and comprises: 5 A chip preparation line: storage, washing with water and oven-steaming at a temperature of 90°C, A first screw-type impregnator comprising a compression zone with filtration and an expansion zone where the reactant is injected by means of a metering pump, 10 A buffer capacity which allows a residence time of 5 to 60 min at a temperature of 40 to 100°C to be guaranteed, A second screw-type impregnator comprising a compression zone equipped with a filter and a device for 15 injecting wash water, and a soaking zone where the second reactant is injected by means of a metering pump, A buffer capacity which allows a residence time of 15 to 120 min at a temperature of 40 to 100"C to be guaranteed, 20 A double-disc refiner (40 inches), A HOOPER slotted screen and a storage chest for neutralising the pulp with gaseous S02. After the pulp has been produced, sheets of paper are made with this pulp for measuring the whiteness and the 25 mechanical characteristics according to the ISO standards of the paper industry. ?42 3 5 - 14 - The commercial sodium borohydride solution which is used originates from MORTON International and contains 12 % of NaBH4 and 40 % of NaOH; it is referred to by the name of B0R0LR. 5 The complexing agent for metal ions which is used in the Examples is DTPA (sodium salt of diethylenetriaminepentaacetic acid) in the form of a 40 % strength aqueous solution. EXAMPLE 1 10 Coniferous chips (fir, spruce) are washed and oven- steamed. The chips are soaked continuously with a reducing solution at 20°C which comprises 5.9 % of sodium sulphite, 0.5 % of 40 % strength DTPA solution and 1.5 % for BOROLR sodium borohydride solution, the solution having a pH of 15 10.5. The chips are heated for 30 min at 60"C. The chips are pressed at 60°C to eliminate approximately 0.5 1 of reducing solution per kg of (dry) chips (that is approximately 50 % elimination). The chips are rinsed with 20 2 1 of water per kg of dry chips. The chips are then soaked continuously with an oxidising solution at 20°C which comprises 5 % of hydrogen peroxide, 2 % of sodium hydroxide, 4 % of 40 % strength sodium silicate solution and 0.5 % of 40 % strength DTPA 25 solution. The oxidising solution has a pH of 11.5. The soaked chips are then heated for 1 hour at 90°C 242 3 - 15 - and then refined in a double-disc refiner. The pulp is then screened, and the rejects are returned to the refiner. The refined pulp has a draining index of 70"SR. After the pulp has been neutralised to pH 6 with 5 S02, the following characteristics are obtained: ISO whiteness 82° Resistance to breaking: Breaking length 4500 m Resistance to bursting 2.3 KPa m2/g 10 Resistance to tearing 5.1 mN m2/g. It can be seen that the process according to the invention, which comprises a treatment with a reducing solution containing sodium sulphite and sodium borohydride and a treatment with hydrogen peroxide, allows a pulp to be 15 obtained from coniferous wood which, after grinding, has a high degree of whiteness, above 80° ISO. EXAMPLE 2 The process is carried out as in Example 1, but the duration of heating at 90°c after soaking with the 20 oxidising solution is 2 hours instead of 1 hour. This gives a pulp having the same mechanical characteristics and a whiteness of 83" ISO. EXAMPLE 3 The process is carried out as in Example 1, with 25 the exception that the reducing solution is composed of 5.9 % of Na2S03 and 0.5 % of 40 % strength DTPA solution, and 242356 - 16 - has a pH of 9.6. This gives a pulp having the following characteristics: ISO whiteness 80° ISO Resistance to breaking: 5 Breaking length 4400 m Resistance to bursting 2.2 KPa m2/g Resistance to tearing 5.4 mN m2/g. tocampt.t: a (comparison) The process is carried out as in Example 1, but the 10 chips are not treated with a reducing solution. The first treatment consists in soaking the chips with a solution containing only 0.5 % of 40 % strength DTPA solution. This gives a pulp having the following characteristics: 15 ISO whiteness 78° ISO Resistance to breaking: Breaking length 3200 m Resistance to bursting 1.6 KPa m2/g Resistance to tearing 5.2 mN m2/g. 20 It can be seen that treatment of the chips before grinding with a hydrogen peroxide solution, as described in European Patent EP-A-239,583, and thus without a reducing treatment, does not enable a pulp to be obtained which has a degree of whiteness and mechanical resistance properties 25 which are analogous to those obtained with the process according to the invention. 242356 17 EXAMPLE 5 Washed and oven-dried hardwood chips (poplar, aspen) are soaked continuously with a reducing solution at 20°C which is composed of 5 % of Na2S03, 1.5 % of sodium 5 borohydride solution (B0R0LR) and 0.5 % of 40 % strength DTPA solution, and has a pH of 11.5. The soaked chips are heated for 30 min at 100 °C under a steam atmosphere. The chips are then pressed at 60°C to eliminate more than 50 % of the reducing solution. 10 Rinsing is effected with 2 1 of water per kg of dry chips. The chips are then soaked with an oxidising solution at 20°C which comprises 5.2 % of hydrogen peroxide, 4 % of sodium hydroxide, 4 % of 40 % strength sodium silicate solution and 0.5 % of 40 % strength DTPA 15 solution, and has a pH of 11.4. The chips are then heated for 2 hours at 90°C and refined in a double-disc refiner under atmospheric pressure. After screening, the refined pulp has a draining 20 index of 56° SR. The characteristics of this pulp are as follows: ISO whiteness 87° ISO Resistance to breaking: Breaking length 3300 m 1.3 KPa m2/g 3.5 mN m2/g. 25 Resistance to bursting Resistance to tearing 242 3 5 6 - 18 - In this example, relatively white hardwood chips are treated. The process according to the invention allows a pulp to be obtained whose degree of whiteness after grinding is very high. 5 EXAMPLE 6 The process is carried out as in Example 5, but the reducing solution is composed of 5 % of Na2S03 and 0.5 % of 40 % strength DTPA solution, and has a pH of 11. The pulp obtained has the following 10 characteristics: ISO whiteness 85.2° ISO Resistance to breaking: Breaking length 2900 m Resistance to bursting 1.20 KPa m2/g 15 Resistance to tearing 3.1 mN m2/g. •ry&mpt.t; 7 (comparison) The process is carried out as in Example 5, but no reducing agent is added to the first treatment: the first soaking step is effected using a solution which contains 20 only 0.5 % of 40 % strength DTPA solution. The pulp obtained has the following characteristics: ISO whiteness 84 ° ISO Resistance to breaking: 25 Breaking length 2300 m Resistance to bursting 0.9 KPa m2/g 2 4 2 3 5 - 19 - Resistance to tearing 2.9 mN m2/g. It can be seen that the hardwood chips which are treated in this comparison Example give, after grinding, a pulp of a whiteness which is substantially below that 5 obtained with the same chips in Example 5 using the process according to the invention. EXAMPLE 8 Hardwood chips (poplar) are washed and oven-dried at 90°C. They are soaked continuously with a reducing 10 solution at 20°C which is composed of 1 % of sodium borohydride solution (B0R0LR) and of 0.5 % of 40 % strength DTPA solution. The solution has a pH of 12.7. The chips are heated for 15 min at 90°C. The chips are pressed at a temperature of 60°C to 15 eliminate approximately 0.5 1 of reducing solution per kg of dry chips (that is approximately 20 % of reducing solution eliminated). The chips are then soaked with an oxidising solution at 20°C which comprises 4.25 % of hydrogen 20 peroxide and 3.8 % of sodium hydroxide. The solution has a pH of 10.5. The chips which have been soaked with oxidising solution are heated for 1 hour at 85"C. They are refined in a first double-disc refiner under atmospheric pressure 25 and then in a second single-disc refiner, also under atmospheric pressure. 242356 - 20 - After having been neutralised to a pH of 6 with SO2, the refined pulp has a draining index of 62°SR and the following characteristics: ISO whiteness 80.9° ISO 5 Resistance to breaking: Breaking length 2800 m Resistance to bursting 1.25 KPa m2/g EXAMPLE 9 The process is carried out exactly as in Example 8, 10 but a reducing solution composed of 1 % of Na2S03, 1 % of sodium borohydride solution (BOROLR) and 0.5 % of 40 % strength DTPA solution is used. The resulting pulp has a draining index of 68°SR and the following characteristics: 15 ISO whiteness 81.1° ISO Resistance to breaking: Breaking length 3100 m Resistance to bursting 1.36 KPa m2/g EXAMPLE 10 20 The process is carried out exactly as in Example 8, but a reducing solution composed of 2 % of Na2S03, 1 % of sodium borohydride solution (BOROLR) and 0.5 % of 40 % strength DTPA solution is used. The resulting pulp has a draining index of 65°SR 25 and the following characteristics: ISO whiteness 81.7° ISO </div>

Claims

242356 - 21 -Resistance to breaking: Breaking length 3320 m Resistance to bursting 1.38 KPa m2/g eyampt.k n (comparison) 5 The process is carried out as in Example 8, but without adding reducing agent to the first treatment: the first soaking step is carried out using a solution which comprises only 0.5 % of 40 % strength DTPA solution. The resulting pulp has a draining index of 66°SR 10 and the following characteristics: ISO whiteness 79.8° ISO Resistance to breaking: Breaking length 2900 m Resistance to bursting 1.21 KPa m2/g 15 The hardwood (poplar) chips used in Examples 8 to 11 have a lower initial whiteness than those in Examples 5 to 7. The whiteness of the final pulp is therefore less pronounced than that obtained from Examples 5 to 7, but it can be seen once again that treating the wood chips with a 20 reducing and then with an oxidising treatment allows, after grinding of the wood, a pulp to be obtained which has a very high level of whiteness and improved mechanical characteristics. <.4 23 22 WHAT WE CLAIM IS:

1. A process for preparing bleached paper pulp from lignocellulose chips, which process comprises treating the chips with a solution of at least one reducing agent., 5 subsequently treating the chips with an alkaline hydrogen peroxide solution, and then refining the treated chips.

2. A process according to claim 1, wherein the chips which have been soaked with reducing solution and heated, are pressed to eliminate at least 20 % by weight of the reducing 10 solution.

3. A process according to claim 1, which comprises: (a) soaking wood chips in the solution of reducing agent, (b) heating the chips at a temperature of from 60 to 15 160 °C, (c) pressing the chips to extract at least 20 % by weight of the solution of reducing agent(s), (d) soaking the chips in the alkaline solution containing hydrogen peroxide, 20 (e) heating the chips which have been soaked in step (d) at a temperature of from 50 to 120"C, and (f) refining the chips.

4. A process according to claim 3, wherein the chips are soaked in the reducing solution and then heated for 1 min 25 to 3 hours at a temperature of from 60 to 160°C.

5. A process according to any one of the preceding claims, wherein the reducing agent(s) is/are one or more selected from the group comprising sodium sulphite, sodium bisulphite, sodium borohydride, zinc hydrosulphite, sodium 18 nPr?"^4 23 hydrosulphite, thiourea dioxide, and mixtures of sulphur dioxide and sodium hydroxide.

6. A process according to claim 5, wherein the reducing solution is prepared by mixing sodium sulphate and 5 sodium borohydride.

7. A process according to any one of the preceding claims, wherein the reducing agent(s) is (are) used in an amount of 0.1 to 10 % by weight relative to the weight of the dry chips. 10

8. A process according to any one of the preceding claims, wherein the pH of the reducing solution is a value greater than 7 and less than or equal to 13.

9. A process according to any one of the preceding claims, wherein the reducing solution contains a complexing or 15 sequestering agent for metal ions.

10. A process according to any one of the preceding claims, wherein the temperature of the reducing solution is from 10 to lOO'C.

11. A process according to any one of the preceding 20 claims, wherein the chips are soaked in the alkaline hydrogen peroxide solution in an amount of 0.1 to 10 % by weight of hydrogen peroxide relative to the weight of the dry chips.

12. A process according to claim 11, wherein the hydrogen peroxide solution has a pH of from 8 to 12.5. 2 5

13. A process according to claim 11 or 12, wherein the hydrogen peroxide solution contains one or more peroxide-stabilising agents.

14. A process according to any one of claims 11 to 13, wherein the temperature of the peroxide solution is from 10 to 18 24 80 ° C.

15. A process according to any one of the preceding claims, wherein the chips which have been soaked in the hydrogen peroxide solution are heated for 10 min to 5 hours at 5 a temperature of 50 to 120°C.

16. A process according to any one of the preceding claims, wherein wood chips are soaked in the solution of a complexing agent for metal ions, and then pressed before being treated with the hydrogen peroxide-containing solution. 10

17. A process for preparing bleached paper pulp substantially as described in any one of Examples 1 to 3, 5, 6 and 8 to 10.

18. Bleached paper pulp obtained by the process claimed in any one of the preceding claims. 15 By the authorised agents A J PARK & SON 1,3