AU650962B2 - Process for preparing bleached paper pulp in high yield - Google Patents

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

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P/00/011 28/5i91 Regulation 3.2(2)

AUSTRALIA

Patents Act 1990 F77

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT nber: dged: PROCESS FOR PREPARINC, BLEACHED PAPER PULP IN HIGH YIELD Application Nun

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Invention Title: 0000 Oat at The following statement is a full description of this invertion, including the best method of performing it known to u 14 to the ISO standards of the nanpr ntr-

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1 PROCESS FOR MANUFACTURING BLEACHED PAPER PULP IN HIGH YIELD 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.

To obtain a pulp, the wood chips are subjected to separate or combined actions of mechanical, chemical or thermal origin.

The chemical pulps are obtained by chemical grinding of the wood. Their yield is generally less than 50 Yield is understood as meaning the weight of the pulp in the dry state relative to the weight of the starting material in the dry state.

The pulps of the mechanical type or high-yield pulps are pruduced by mechanical grinding of the wood chips, for example in a grinder or in a disc grinder or refiner. The yield is generally above 85 However, the mechanical characteristics, especially the resistance to breaking, to tearing and to bursti are poor.

To improve these mechanical characteristics, the wood chips have been subjected to a thermal treatment with steam at a temperature from 100°C to 140 0 C prior to grinding. These pulps are termed thermomechanical pulps (TMP). However, their whiteness is 2 poor.

The wood chips have also been treated with a sodium sulphite solution having an acidic or basic pH, depending on the type of wood. The resulting pulps, which are also termed chemicomechanical pulps, have good mechanical characteristics.

It has also been possible to substantially increase 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 boiling of the material at a temperature of 100 0 C or above, under the pressure of saturated water vapour in the presence of sulphite, in particular sodium sulphite Na 2 SO3, or sodium bisulphite NaHSO 3 The advantage of these CTMP pulps is that they have an improved mechanical resistance and that the yield remains generally above 85 mostly at least approximately, and thus similar to the yield of the pulps of purely mechanical origin.

While the pulps which have been treated in this manner, in particular the chemicomechanical pulps (CMP) and the chemicothermomechanical pulps (CTMP), have improved mechanical resistance, their bleaching, which the quality required for paper demands, remains a problem.

3 To Obtain better bleaching of these pulps, Patents FR 1,389,308 and EP-A-293,309 have proposed to treat wood chips with combined solutions of alkali metal borohydride and sulphite, at an acidic or alkaline pH.

These treatments allow pulps to be obtained after grinding which have good mechanical properties and an improved 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 is caused especially by the sulphite treatment which, while it improves the mechanical properties of the resulting refined pulps, brings about a poor response to bleaching with hydrogen peroxide in these pulps.

It seems therefore that obtaining pulps which have good mechanical properties and simultaneously a sufficient whiteness for use in the production of paper with a high degree of whiteness is problematic.

To solve this problem, Patent EP-A-239,583 describes a process in which the wood chips undergo three successive treatments before grinding, namely a treatment with a solution of a sequestering agent for metal ions, a treatment with a solution of a hydrogen peroxide stabiliser, such as sodium silicate and magnesium salts, and a treatment with an alkaline peroxide solution.

This gives, after grinding, pulps of improved 1 4 whitei.ss. However, this whiteness, which remains below ISO, especially in the case of coniferous wood, is still 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.

Moreover, 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 in the case of coniferous wood.

It is therefore the object of the present 0 invention to provide a process which allows, after refining pulps to be obtained 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 o0o 0woods, making them suitable for use in the production of paper with a high degree of whiteness.

The process according to the invention is a process for producing paper pulps from wood chips in high yield. It consists in treating the chips successively before refining with a solution of at least one reducing agent, then with an alkaline hydrogen peroxide solution.

More particularly, the process according to the invention comprises the following steps: q-? soaking the wood chips with the solution of reducing agent(s), heating the chips which have been soaked in step at a temperature of between 60 and 160C, pressing the chips so as to extract at least 20 of the solution of reducing agent(s), soaking the chips obtained in step (c) with the alkaline solution containing hydrogen peroxide, heating the chips which have been soaked in step at a temperature of between 50 and 120°C, and refining the chips which have been treated in this manner.

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 for example using an aqueous 40 strength solution of DTPA (sodium salt of diethylenetriamine pentaacetic acid).

The lignocellulose materials which can be treated by the process according to the invention comprise coniferous wood such as pine, spruce, etc., hardwood such as poplar, eucalyptus wood, etc in the form of chips as they are used customarily in the paper pulp industry. Annual plants such as bagasse, straw, etc cut into pieces of a few centimetres can also be treated.

l 6 These chips undergo preparatory treatments which are customarily used in the paper pulp industry, for example, washing, sorting, preheating of the steam, etc According to the process of the invention, these chips are first treated with a solution of one or more reducing agents (step This treatment is effected by soaking the chips in the solution. The conventional means of the paper industry can be used for this soaking step. Before proper soaking of the chips, the latter, which have previously been softened by wetting and/or oven-steaming, are compressed by means of a screw press so as to remove part of the gases and/or liquid present in the chips. When the chips have expanded after pressing, they are 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 well as all apparatus which allow the chips to be compressed and to expand once or several times, in the presence of the soaking solution with, preferably, the highest possible compression ratio.

According to the invention, the chips must expand in the presence of the smallest amount possible of the solution in which the chips must be soaked.

The reducing solution which is used in the 7 process according to the invention contains one or more reducing agents which can be selected from amongst alkali metal sulphite or bisulphite or a mixture of sulphur dioxide and an alkaline agent such as sodium hydroxide. Sodium borohydride (NaBH 4 pure or in alkaline solution, can also be used. Zinc or sodium hydrosulphite (Na 2 SzO) pure or obtained by reacting the alkaline sodium borohydride solution with sodium sulphite (Na 2

SO

3 or sodium bisulphite (NaHSO 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 the alkali metal sulphites, such as sodium sulphite, sodium borohydride in alkaline solution, such as the product BOROL', 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 sulphite and of the alkaline sodium borohydride solution BOROL.

The reducing agent is used at 0.1 to 10 by weight relative to the weight of the chips in the dry state and, more particularly, at 0.5 to 6 in the case of sodium sulphite, at 0.5 to 1.5 in the case of the alkaline sodium borohydride solution BOROLR, and at to 2.5 in the case of sodium hydrosulphite.

The pH of the reducing solution is, according to the invention, between a valve greater than 7 and 13, depending on the 8 nature of the lignocellulose material of the chips and the pH is preferably between 11 and 12.8 in the case of hardwood and between 9.5 and 11 in the case of soft wood.

If necessa.ry, an alkaline agent such as NaOH, KOH, NazCO 3 KzCO3, 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 tripolyphosphate, sodium tetrapyrophosphate, the sodium salts of nitrilotriacetic acid, ethylenediaminetetraacetic acid and diethylenetriamine O pentaacetic acid. The sodium salt of diethylenetriamine pentaacatic acid (DTPA) in the form of an aqueous 40 strength solution is preferably used, in an amount of, preferably, 0.2 to 0.5 of this solution.

The temperature of the reducing solution during soaking can be between 10 and 100 0 C, depending on the stability of the reducing agent, a high temperature favouring the speed of soaking, and a low temperature favouring the good preservation of the reducing properties of the solution.

In most cases, a temperature from 20 to is preferred and, in particular, a temperature from 20 to 40°C in the case of the reducing solutions which are obtained by mixing Nae.SO 3 and the alkaline sodium borohydride solution BOROLR.

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Immediately after soaking with the reducing solution, the chips are heated at a temperature of to 160*C, preferably 75 to 1201C (step b).

The duration of heating depends on the temperature and the nature of the wood and varies from 1 min to 3 hours. The preferred duration is 15 to min at 90 0 C and 5 to 30 min at 120 0

C.

The soaked chips are preferably transported and heated with the exclusion of atmospheric oxygen, for example using an atmosphere of steam or of an inert gas such as nitrogen or carbon dioxide.

The chips which have been soaked with the reducing solution and then heated are pressed by means of a screw press or by another means so as to extract part of the reducing solution after the reaction. The chips are subjected to this pressing step since the effectiveness of the process increases with elimination of this solution. An elimination of at least 20 of the reducing solution is particularly desirable.

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 temperat'nre of between 20 and 1600C.

All, or part of, the reducing solution which is extracted during the pressing step can be recycled to prepare the reducing solution which can be used in step After soaking with the solution of reducing agent(s) and after pressing, the chips are treated by soaking (step d) with an alkaline hydrogen peroxide solution.

The oxidant, hydrogen peroxide, is used at 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 hydrogen peroxide is between 1 and 5 by weight. An amount of 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, Na 2

C

3

K

2 C0 3 MgO, CaO, etc., in such a way that the initial pH of this solution is between 8 and 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 Hz02. This amount, expressed in NaOH, can vary from 0.5 to 10 and, preferably, 0.5 to 6 by weight relative to the weight of the chips to be treated in the dry state.

One or more agents which stabilise hydrogen peroxide in alkaline medium can be added to the alkaline HO, 2 -solution to reduce decomposition of this solution and hence increase the effectiveness of the treatment. Stabilisers which may be mentioned are sodium silicate, generally in the form of a 40 strength solution, used at 0.5 to 5 by weight I 1 11 relative to the weight of the dry chips, magnesium salts such as magnesium chloride, magnesium nitrate, magnesium carbonate and mixed salts at 0.01 to 1 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, diethylenetriaminepentaacetic acid and ethylenediaminetetraacetic acid in the form of sodium salts. They are used at 0.1 to 1 by weight relative to the weight of the chips to be treated in the dry state.

The temperature of the alkaline hydrogen peroxide solution during soaking is between 10 and 800C, preferably at a temperature not higher than 40 0

C,

so as to reduce decomposition of the peroxide.

After the step in which the chips are soaked with the alkaline hydrogen peroxide solution, the chips are heated, in a step at a temperature of between 0 C and 120°C, preferably between 60 and 80 0 C, over a period of 10 min to 5 hours, depending on the nature of the wood, the temperature and the amount of hydrogen peroxide. In 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 0 C. Heating is generally provided by steam in a saturated atmosphere.

At the conclusion of the two treatments with i i I i 12 reducing and oxidising solution, the chips are ground or refined by passing through one or more refiners under 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 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 5.5 to 6 using dilute sulphuric acid or else gaseous SO 2 The pulp is subsequently treated in the conventional manner (straining, screening, recycling of rejects, etc.).

The examples which follow and which are given by way of example but without imposing any restriction allow the invention and its advantages to be assessed.

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 plaut for the production of mechanical paper pulp and comply with the standards of this industry.

'1 13 The examples are effected 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: A chip preparation line: storage, washing with water and oven-steaming at a temperature of 90 0

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, A buffer capacity which allows a residence time of 5 to 60 min at a temperature of 40 to 100 0 C to be guaranteed, A second screw-type impregnator comprising a compression zone equipped with a filter and a device for 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 0

C

to be guaranteed, A double-disc refiner (40 inches), A HOOPER slotted screen and a storage chest for neutralising the pulp with gaseous SO 2 After the pulp has been produced, sheets of paper are made with this pulp for measuring the whiteness and the mechanical characteristics according

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14 to the ISO standards of the paper industry.

The commercial sodium borohydride solution which is used originates from MORTON International and contains 12 of NaBH 4 and 40 of NaOH; it is referred to by the name of BOROLR.

The complexing agent for metal ions which is F used in the examples is DTPA (sodium salt of diethylenetriaminepentaacetic acid) in the form of a strength aqueous solution.

EXAMPLE 1 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 of BOROLR sodium borohydride solution, the solution having a pH of 10.5.

The chips are heated for 30 min at 60°C. The chips are pressed at 60 0 C to eliminate approximately 1 of reducing solution per kg of (dry) chips (that is approximately 50 elimination). The chips are rinsed with 2 1 of water per kg of dry chips.

The chips are then soaked continuously with an oxidising solution at 20 0 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 solution. The oxidising solution has a pH of 11.5.

The soaked chips are then heated, for 1 hour w at 90°C 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 708°R.

After the pulp has been neutralised to pH 6 with SO 2 the following characteristics are obtained: ISO whiteness 820 Resistance to breaking: Breaking length 4500 m Resistance to bursting 2.3 KPa m 2 /g Resistaice to tearing 5.1 mN m 2 /g.

It can been 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 obtained from coniferous wood which, after grinding, has a high degree of whiteness, above 800 ISO.

EXAMPLE 2 The process is carried out as in Example 1, but the duration of heating at 90 0 C after soaking with the oxidising solution is 2 hours instead of 1 hour.

This gives a pulp having the same mechanical characteristics and a whiteness of 830 ISO.

EXAMPLE 3 The process is carried out as in Example 1, with the exception that the reducing solution is 16 composed of 5.9 of Na 2

SO

3 and 0.5 of 40 strength DTPA solution, and has a pH of 9.6. This gives a pulp having the following characteristics: ISO whiteness 800 ISO Resistance to breaking: Breaking length 4400 m Resistance to bursting 2.2 KPa m 2 /g Resistance to tearing 5.4 mN m 2 /g.

EXAMPLE 4 (comparison) The process is carried out as in Example 1, but the 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: ISO whiteness 780 ISO Resistance to breaking: 0 Breaking length 3200 m Resistance to bursting 1.6 KPa m 2 /g Resistance to tearing 5.2 nN m 2 /g.

It can be seen that treatment of the chips before grinding with a hydrogen peroxide solution, as described in Patent EP-A-239,583, and thus without reducing treatment, does not allow a pulp to be obtained which has a degree of whiteness and mechanical resistance properties which are analogous to those obtained with the process according to the invention.

17 EXAMPLE Washed and oven-dried hardwood chips (poplar, aspen) are soaked continuously with a reducing solution at 20 0 C which is composed of 5 of Na 2

SO

3 1.5 of sodium borohydride solution (BOROLR) 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. Rinsing is effected with 2 1 of water per kg of dry chips.

The chips are then soaked with an oxidising solution at 20 0 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 solution, and has a pH of 11.4 The chips are then heated for 2 hours at and refined in a double-disc refiner under atmospheric pressure.

After screening, the refined pulp has a draining index of 560 SR. The characteristics of this pulp are as follows: o o ISO whiteness 870 ISO Resistance to breaking: Breaking length 3300 m Resistance to bursting 1.3 KPa m 2 /g Resistance to tearing 3.5 mN m 2 /g.

In this example, relatively white hardwood i 1 cl IcS chips are treated. The process according to the invention allows a pulp to be obtained whose degree of whiteness after grinding is very high.

EXAMPLE 6 The process is carried out as in Example but the reducing solution is composed of 5 of Na 2

SO

3 and 0.5 of 40 strength DTPA solution, and has a pH of 11.

The pulp obtained has the following characteristics: ISO whiteness 85.20 ISO Resistance to breaking: Breaking length 2900 m Resistance to bursting 1.20 KPa m 2 /g Resistance to tearing 3.1 mN m 2 /g.

EXAMPLE 7 (comparison) The process is carried out as in Eyample but no reducing agent is added to the first treatment: the first soaking step is effected using a solution which contains only 0.5 of 40 strength DTPA solution.

The pulp obtained has the following characteristics: ISO whiteness 840 ISO Resistance to breaking: Breaking length 2300 m Resistance to bursting 0.9 KPa m2/g Resistance to tearing 2.9 mN m 2 /g.

i 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 obtained with the same chips in Example using the process according to the invention.

EXAMPLE 8 Hardwood chips (poplar) are washed and ovendried at 90 0 C. They are soaked continuously with a reducing solution at 20°C which is composed of 1 of sodium borohydride solution (BOROLR) and of 0.5 of strength DTPA solution. The solution has a pH of 12.7.

The chips are heated for 15 min at 90 0

C.

The chips are pressed at a temperature of i,.0 15 60°C to eliminate approximately 0.5 1 of reducing solution per kg of dry chips (that is approximately of reducing solution eliminated).

The chips are then soaked with an oxidising solution at 201C which comprises 4.25 of hydrogen peroxide and 38 of sodium hydroxide. The solution %oo has a pH of 10.5.

The chips which have been soaked with oxidising solution are heated for 1 hour at 85 0 C. They are refined in a first double-disc refiner under atmospheric pressure and then in a second single-disc refiner, also under atmospheric pressure.

After having been neutralised to a pH of 6 with SO 2 the refined pulp has a draining index of 62 0

SR

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and the following characteristics: ISO whiteness 80.90 ISO 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, but a reducing solution composed of 1 of Na 2

SO

3 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 0 SR and the following characteristics: ISO whiteness 81.10 ISO Resistance to breaking: Breaking length 3100 m Resistance to bursting 1.36 KPa m 2 EXAMPLE The process is carried out exactly as in Example 8, but a reducing solution composed of 2 of NazSO 3 1 of sodium borohydride solution (BOROLR) and of 40 strength DTPA solution is used.

The resulting pulp has a draining index of 65 0 SR and the following characteristics: ISO whiteness 81.70 ISO Resistance to breaking: Breaking length 3320 m Resistance to bursting 1.38 KPa m 2 /g r :'I '6-i.f /g 21 EXAMPLE 11 (comparison) 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 and the following characteristics: ISO whiteness 79.80 ISO Resistance to breaking: Breaking length 2900 m Resistance to bursting 1.21 KPa m 2 /g 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 reducing and then 0 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.

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Claims (14)

1. wood chips, Process for producing, in high yield, bleached paper pulp from comprising the following steps: soaking the wood chips with the solution of reducing agent(s), heating the chips at a temperature of between 60 and 1600C, pressing the chips so as to extract at least 20 of the solution of reducing agent(s), soaking the chips with the alkaline solution containing hydrogen peroxide, heating the chips which have been soaked in step at a temperature of between 50 and 1200°C, and refining the chips which have been treated in this manner.

2. Process according to claim 1, characterised in that the reducing agent(s) are selected from the group consisting of sodium sulphite, sodium bisulphite, sodium borohydride, zinc hydrosulphite or sodium hydrosulphite, thiourea dioxide, and a mixture of sulphur dioxide and sodium hydroxide.

3. Process according to claim 2, characterised in that the reducing solution is prepared by mixing sodium sulphite and sodium borohydride.

4. Process according to any one of claims 1 to 3, characterised in that the reducing agent(s) is (are) used in a proportion of 0.1 to 10 by weight relative to the weight of the dry chips.

Process according to any one of claims 1 to 4, characterised in that the pH of the reducing solution is between a value greater than 7 and 13. A O 23

6. Process according to any one of claims 1 to 5, characterised in that the reducing solution contains a complexing or sequestering agent for metal ions.

7. Process according to one of claims 1 to 6, characterised in that the temperature of the reducing solution is between 10 and 1000C.

8. Process according to any one of claims 1 to 7, characterised in that the chips are soaked with the reducing solution and then heated for 1 minute to 3 hours at a temperature of between 60 and 160°C.

9. Process according to any one of claims 1 to 8, characterised in that the chips which have been soaked with reducing solution and heated, are pressed to eliminate at least 20 of the reducing solution.

Process according to any one of claims 1 to 9, characterised in that the chips are soaked with the alkaline hydrogen peroxide solution in a proportion of 0.1 to 10 by weight of hydrogen peroxide relative to the weight of the dry chips.

11. Process according to claim 10, characterised in that the hydrogen peroxide solution has a pH of between 8 and 12.5.

12. Process according to claim 10 or 11, characterised in that the hydrogen peroxide solution contains one or more peroxide-stabilising agents.

13. Process according to any one of claims 10 to 12, characterised in that the temperature of the peroxide solution is between 10 and 800C.

14. Process according to any one of claims 1 to 13, characterised in that the chips which have been soaked with the hydrogen peroxide solution are heated for 10 minutes to 5 hours at a temperature of between 50 and 120C, 24 Process according to any one of claims 1 to 14, characterised in that the wood chips are soaked with the solution of a complexing agent for metal ions, and then pressed before being treated with the hydrogen peroxide- containing solution. DATED this 6th day of May, 1994. ELF ATOCHEM S.A. WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD ~HAWTHORN VICTORIA 3122 S, AUSTRALIA t ABSTRACT The invention relates to the production of paper pulp from wood chips in high yield. The process consists in treating the chips successively before grinding with a solution containing at least one reducing agent and then with an alkaline hydrogen peroxide solution. Application in the production of paper pulps with a high degree of whiteness and good mechanical it 0, characteristics. s o o a *8 0 4.td 0:00 o 6 0 U v ii