FI70440B - FOER FARING FOR CHEMICAL PAPER MACHINERY - Google Patents

Description

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C Patent ::, 73.: ...: tty (45) Pa. t :. at :: i V: 3.;: 1 j. *) 0 1.) 3 6 (51) Kv.lk. «/ lnt.CI.« D 21 C 3/26, 9/16 FINLAND —FINLAND (21 ) Patent application - Patentansökning 782342 (22) Filing date - Ansökningsdig 27 07 78 (FI) '' (23) Starting date - Giltighetsdag 2 7.07.78 (41) Publication - Blivit offentlig 30.01 .79

National Board of Patents and Registration Date of publication and publication. - n,

Patent and registration authorities '' Ansökan utlagd och utl.skriften publicerad 27.03.86 (86) Kv. application - Int. ansökan (32) (33) (31) Privilege claimed - Begärd priority 29.07.77 O5.O7.78 France-France (FR) 7724131, 7820715 (71) Produits Chimiques Ugine Kuhlmann, 25, Boulevard de l'Amiral Bruix, 75116 Paris, France-France (FR) (72) Dominique Lachenal, Grenoble, Christian De Choudens, Gieres,

Pierre Monzie, Grenoble, France-France (FR) (74) Berggren Oy Ab (54) Method for the production of chemical pulps -Förfarande för f ramstä11 Ning av kemikaliska pappersmassor

The invention relates to a new process for the production of pulps, so-called chemical pulps, in which the delignification of the lignocellulosic feedstock is carried out by a two-stage cooking process, the second stage of which is carried out by means of an alkaline peroxide solution.

The new requirements of the French environmental legislation have encouraged paper manufacturers to seek to replace certain particularly polluting pulp production methods with methods that can produce pulps with the same quality characteristics without pollution.

This problem is particularly acute in so far as it concerns a chemical pulp called sulphate pulp, which has a very high level of quality but which is particularly polluting.

In fact, the production of sulphate pulp develops atmospheric pollution by releasing malodorous sulfur compounds (mercaptans, S <^ 2 'lI * * ^ 7nc) into the air. pollute water bodies.

To address these problems, paper research has focused in particular on the use of delignifying agents that are not themselves contaminating and do not develop contaminating products when reacting with lignin. In this way, attention has been drawn to cooking methods which take advantage of the delignifying effect of oxygen in an alkaline medium.

These methods consist essentially of cooking the lignocellulosic material in two steps separated by intermittent defibering, comprising in successive steps alkaline pressure cooking, intermediate fiberization by a plate mill and finally cooking at oxygen pressure in the presence of an alkaline solution. Such a method is described, for example, in Canadian Patent Publication 895,756.

Although these methods give pulps of good quality without causing atmospheric pollution, they have a number of drawbacks: on the other hand, the low solubility of oxygen requires a sufficiently homogeneous transition between the gas phase, the aqueous phase and the cellulose fibers of relatively large sizes of 8 to 10 bar. the use of pressures that cannot be used in non-expensive special equipment other than those used in the conventional sulfate process. This knows that replacing the sulphate process with the oxygen process requires work and investment that is too heavy for small and medium-sized units, on the other hand the quality of pulps obtained by these oxygen processes is slightly worse than that of sulphate pulps, especially oxygen depolymerising on cellulose; most published studies unanimously acknowledge this lower level of mechanical properties and especially the burst coefficient. (Reference may be made in this connection to the following publications: AG Jamieson, O. Samuelson, LA Smedman in TAPPI, vol. 58, no. 2, pp. 68-71; M. Saukkonen and I. Palenius TAPPI, vol. 58, no. 7, pp. 117-120; HM Chang, JS Gratzl, WT McKean, rh Reeves and VE Stockman, TAPPI, Vol 59, No. 8, pp. 72-75).

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In their most recent forms, the above methods allow for lower oxygen pressures and lightening of the technology: omission of intermediate fiber (French Patent 2,256,283), and improved oxygen transfer (French Patent 2,220,620).

What remains, however, is that the investment required to implement them will remain a purely sulphate method. without improving the quality characteristics of the pulps. In addition, the lower level of the tear index is particularly disadvantageous for the use of these pulps in wrapping papers.

These various downsides explain the fact that to date, these methods have been inherently little used in industrial applications.

The object of the present invention is to solve these problems both in terms of the use of the process and in terms of the quality characteristics of the pulps obtained.

The invention relates to a non-contaminated, two-stage process for cooking lignocellulosic substances with sodium hydroxide and peroxide. This method has the desired characteristics of efficiency and simplicity and can be implemented with relatively simple, existing industrial equipment. It is able to obtain pulps with good strength properties, good whiteness and as good yield as the classical methods used so far.

The method according to the invention comprises subjecting the cellulosic material in a known manner to a first cooking step in the presence of sodium hydroxide, optionally with added excipients, then defibering the treated material and finally subjecting the defibered material to a second cooking step, characterized in that said second cooking step carried out in the presence of an alkaline peroxide solution. Preferably, this treatment is performed at atmospheric pressure and preferably below 100 ° C.

This method can be applied to any lignocellulosic feedstock: deciduous or coniferous wood, bagasse, Provence 4 70440 cane, straw chips, kenaf, etc. To carry out the first cooking step known per se, the cellulosic material is 30% by weight of sodium hydroxide, based on the weight of the dry plant material originally used, at an elevated temperature of 150-180 ° C.

It has been found that particularly advantageous results are obtained under the same conditions when the second step according to the invention is carried out on lignocellulosic materials which have undergone a first known cooking step in the presence of sodium hydroxide with added excipients such as certain aromatic nitrogen derivatives, certain phenazine-type heterocyclic heterocycles. or cyclic anthraquinone-type ketones. The dosages of the excipients used are very limited, for example 0.01 to 0.2% by weight of anthraquinone, based on the weight of the dry starting material.

The duration of this first cooking varies depending on the values selected for the other parameters and the nature of the wood or plant used.

The cooking must be sufficient to allow the cooked chips to be de-fibrous in a conventional device (e.g. a disc grinder) without consuming much energy and without the risk of damaging the fibers. However, delignification must not be carried out to such an extent that the fibers become detached from each other by simple mechanical agitation.

To this end, soda cooking is stopped when the yield is between 50 and 60%, based on the starting material. The time required to achieve this result is between about 1 and about 4 hours, depending on the values selected for the various parameters.

The cooked chips are then treated in a conventional defibering device, for example in a disc mill, to separate the fibers from each other by mechanical action, under conditions in which they remain undamaged.

The fiber thus fibrous is then subjected to a second cooking step in order to delignify it under particularly advantageous and efficient conditions.

li 5 70440 For this second cooking, an aqueous alkaline solution of peroxide is selected as the delignifying agent. As is known, peroxides are compounds of the general formula R-O-O-X, in which R represents a radical and X represents a metal ion or hydrogen. Within the scope of the present invention, it is possible to use hydrogen peroxide, sodium peroxide and other organic or inorganic peroxides, including sodium and calcium peroxide, sodium perborate, sodium and potassium persulfate, peracetic acid, perpropionic acid, perbutyric acid, perglutaric acid -succinic acid, per-malic acid. According to a preferred embodiment, hydrogen peroxide or sodium peroxide is used. The amount of peroxide used is preferably between 0.1 and 10%, more preferably between 1 and 5% by weight, based on the dry weight of the pulp treated as pure hydrogen peroxide, e.g. As 100% hydrogen peroxide.

The cooking liquid also contains 1-10%, preferably 2-5% by weight of sodium hydroxide, based on the weight of the dry mass.

According to the invention, the material from the defibering step is boiled with this liquid: at a consistency of 5-30, preferably 10-25%, at a temperature between 20-120 ° C, preferably between 60-90 ° C, and the cooking time is 0.5-5 hours, preferably 1-3 hours.

As already mentioned, it is advantageous to operate at atmospheric pressure.

This second cooking step is followed in the usual way by washing with water.

As the examples below show, the pulps obtained by the process according to the invention are chemical pulps whose strength properties, including the tear coefficient, are comparable in all respects to the strength values of sulphate pulps, even when the starting plant material is softwood.

This is due to the choice of peroxide as the delignifying agent, because peroxides have a homogeneous and particularly gentle oxidizing effect of lignin, which does not cause cellulose to depolymerize like oxygen.

Peroxides have so far been used in the treatment of chemical pulps 6,70440 as bleaching agents. Their poor delignifying ability and their instability have prevented their use in the production of pulps, especially chemical pulps.

But now, surprisingly, the process according to the invention has revealed that they are particularly advantageous cooking agents for obtaining pulps comparable to sulphate pulp, as it has been shown that their instability does not prevent the delignification of lignocellulosic materials. In fact, when they decompose, oxidative groups are formed, which in turn act on lignin. However, in order to reap this advantage under the best conditions, this fragmentation must not take place too quickly.

The operating conditions of the method according to the invention have been selected according to this requirement.

The use of peroxide as a delignifying agent offers another considerable advantage, since the peroxide also provides pulp bleaching, which makes it possible to use shorter sets of bleaching steps or sets of steps which can be carried out with less bleaching agents, in particular chlorine-containing agents.

The process according to the invention can be carried out efficiently using an alkaline peroxide solution as described above without any other additive.

However, in order to make the implementation in industry more flexible and to lead to the best possible results, it may be advantageous to add to this solution additives known per se as peroxide auxiliaries.

In a preferred embodiment of the invention, the peroxide treatment is carried out in the presence of stabilizers which retard the well-known decomposition reactions of the peroxides.

For this purpose, any of the compounds known for their stabilizing effect can be used. The best known of these compounds is sodium silicate. However, using silicates has it

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70440 that it causes silicon oxide to form in the plant's combustion and generation circuits, which must be removed by known silicate-containing effluent or cooking liquid treatments.

More preferably, other stabilizing agents may be used, including magnesium sulfate, some buffering agents, sodium carbonate, sodium phosphate, certain organic compounds such as pyrimidinetrione, barbituric acid, acetanilide itself, stable ones, and other peripherals are particularly effective. already on the market.

The improvement in the efficiency of the stabilizers on the market makes it possible to further improve the economic conditions for the use of the method by making it possible, for example, to operate at a higher temperature using lower amounts of peroxide.

In the case where the starting plant material is rich in metal cations, the decomposition reactions of the peroxides are accelerated by the trace amounts of transition metal ions in the pulp, and this results in a peroxide loss which adversely affects the economics of the process.

These trace metals can be largely removed by either treating the pulp prior to its peroxide treatment with an acid, for example, a dilute solution of sulfur dioxide, chlorine, chlorine dioxide, sulfuric acid, hydrochloric acid, oxalic acid, gluconic acid, etc., or compensating for one of these metal ions. in the presence of sodium salts of an agent, for example ethylenediaminotetraacetic or 1-3-diamino-2-tetraacetic or nitrilotriacetic acid, which can advantageously be carried out within the scope of the present invention. However, it is often more advantageous to supplement the effect of this pretreatment by also adding peroxide stabilizing agents to the cooking liquid.

In the variant in which aromatic nitrogen derivatives or phenazine-type heterocyclic nitrogen compounds or anthraquinone-type cyclic ketones are used as sodium hydroxide auxiliaries in the first stage of cooking, the amounts of peroxides used in the second stage are particularly small, between 0.1-1.7%, preferably between 0.3-0.7 70440% by weight of peroxide by weight of dry, fibrous material, which represents a significant advantage over the price of the pulps obtained, given the relatively high price of peroxides available on the market.

The invention is illustrated by the following examples, which are of course not limited to the invention and which have been carried out on a number of hardwood and softwood species selected from the paper wood species most commonly used in France for the production of chemical pulps.

Example 1

In a four-liter autoclave, 400 g of industrial beech chips are immersed in about 4 times their volume of liquid containing 17% by weight of sodium hydroxide, calculated on the dry weight of the chips. The autoclave is then placed in a rotating boiler, which is heated by circulating a heat exchange medium whose temperature is controlled.

The temperature of the autoclave is thus raised to 170 ° C in 90 minutes and maintained at this value for 90 minutes. At the end of this period, the autoclave is cooled and liquid is drained from the chips. The chips are then defibered by passing them once through a Sprout Waldron-type Disc mill (disc C 2976) and the resulting material is washed. The yield is 53.5% and the Kappa number (standard AFNOR NF T 12018) is 50.

The fibrous material is then mixed with a liquid containing the hydrogen peroxide soup reactants and then thickened to a consistency of 20% by weight. The composition of the cooking liquid is such that, at a consistency of 20%, the reactants are in contact with the pulp as follows: sodium hydroxide: 3% - sodium silicate (38 ° Be commercial solution): 5% - 100% hydrogen peroxide 5% (these% reactants are expressed by weight of the dry matter to be treated).

The material thus impregnated with the reactants is then placed in a vessel where it is boiled for 2 hours at atmospheric pressure of 90 ° C.

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temperature. After this treatment, the pulp has a Kappa number of 25 and a total cooking yield of 49.5%.

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The strength properties of the pulp are determined according to AFNOR standards after sorting in a Werverk laboratory sorter (0.15 mm slits) and ground in a JOKRO mill to 40 ° SR and the results are shown in Table I, which is also marked for comparison with approximately similar Kappa numbers. the strength values of the sulphate mass and the mass obtained by the two-step sodium-oxygen method.

As can be seen, the pulps obtained by the process of the invention are approximately equivalent to conventional sulphate pulps, with a better degree of whiteness than these.

Table I

BEECH

The sulfate-2-step process of the invention is the nitric acid process

Eg 1__

Kappa number AFNOR NF T 12018 25 20 22

Yield calculated as dry.

by weight of plant material,% 9.8 9

Unbleached pulp _ whiteness,% Jö 11 40

Breaking length at 40 ° SR, _n, n m, AFNOR NF Q 03 004 7060 6500 6500

Burst coefficient at 40 ° SR 4.0 4.1 4.1 AFNOR NF Q 03 014

Tear coefficient at 40 ° SR _Qr.

AFNOR Q 01 011 750 780 670

Example 2

Industrial oak chips were treated as described in Example 1 except that the conditions were as follows: in the first cooking stage: sodium hydroxide 19% in the second cooking stage: hydrogen peroxide 4% magnesium sulphate 0.05%

cooking temperature 80 ° C

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The yield and Kappa number are determined. The resulting pulp is sorted as above and then bleached in a 5-step conventional bleaching series under the following conditions:

Content of reactants as% of dry weight 1 - chlorine 5.5% 2 - sodium hydroxide 2.7% 3 - chlorine dioxide 1.5% 4 - sodium hydroxide 1% 5 - chlorine dioxide 0.5%

The properties of the bleached pulp were determined according to the same AFNOR standards and are shown in Table II, which, for comparison, also shows the corresponding values of the conventional bleached sulphate pulp.

As can be seen, the pulp according to the invention is to the same degree equivalent to the sulphate pulp, which can thus be replaced without disadvantage in, for example, printing and writing papers.

Example 3

Example 2 is repeated using industrial ash chips, except for the following conditions: in the first cooking stage: sodium hydroxide 17/5% in the second cooking stage: sodium hydroxide 4%

The raw pulp is sorted and bleached as in Example 2 using the same amounts of bleaching agent. The results are shown in the following Table II, in which, for comparison, the results of the sulphate method are also indicated.

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Table II

OAK ASH

Unbleached pulp Sulphate of the invention Process according to the invention Sulphate

Ex. 2___ _

Kappa chapter 27 28 25 22

Yield% 47.5 46 48 47

Bleached pulp

Whiteness% 88.2 83.3 88.5 88.4

Breaking length n 5800 5600 6500 7000 40 SRr, m

Burst coefficient, 3.6 3.6 4.4 4.7 at 40 ° SR

Tear coefficient at 40 ° SR 960 950 102 ° 850

Example 4

Proceed as in Example 2, using industrial chips made from small, unpeeled ash beetle containing 12% by weight of bark, except for the following conditions: in the second stage of cooking: hydrogen peroxide 5% sodium hydroxide 4%

The raw pulp is sorted and then bleached as in Example 2, using the same amounts of bleaching agent. The results are shown in Table III. The tear coefficients obtained are much higher than those obtained by sulphate cooking or two-stage soda-oxygen cooking, which is probably due to the known specific effect of hydrogen peroxide on the shell components, which is the reason for the reduction in tear coefficient generally observed in previously known methods. This thus represents an additional advantage of the present invention in the increasingly common case of cooking chips made from unpeeled wood.

Example 5

Proceed as in Example 2, using industrial chips made from small, unpeeled oak wood containing 16% by weight of bark, except for the following conditions: 70440 in the first cooking stage: sodium hydroxide 20% in the second cooking stage: sodium hydroxide 4% in 5-stage bleaching: 1 to 6% chlorine sodium hydroxide solution 3% 3 - chlorine dioxide 1,5% 4 - sodium hydroxide solution 1% 5 - chlorine dioxide 0,7%

The results are shown in Table III. The same advantages as in Example 4 are noted.

Table III

Ash beech, 12% bark Oak, 16% bark

The sulfate 2-step of the invention.

Example 4 Example 5 netel- _ _ _ _ me

Raw mass

Kappa chapter 28 27 21 30 28

Yield% 44 43 44 42.5 41.5

Bleached pulp

Whiteness% 90 88 88 89 88

Breaking length at 40 ° SR, m 6400 7000 6500 4600 «00

Burst coefficient at 40 ° SR 4.5 4.3 4.5 4.3 3'6

Tear coefficient at 40 ° SR 950 780 830 960 83 °

As mentioned above, to date, the pulps used in wrapping papers proposed to replace sulphate pulps have not been satisfactory due to their insufficient tear index. These are mainly softwood pulps for liners.

In the following Examples 6-8, the invention has been carried out using ipicea and Pinus martimus chips.

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Example 6

Example 1 is repeated using industrial epicea chips, except for the following conditions: in the first cooking stage: sodium hydroxide 22% in the second cooking stage:

cooking temperatures ila 80 ° C

cooking time 90 min

The resulting mass is sorted in a WEVERK laboratory sieve (0.30 mm slits) and ground in a JOKRO mill to 20 ° SR. The strength properties of the pulp are indicated in Table IV. It compares them with the strength properties of the sulphate pulp and the pulp obtained by two-stage soda-oxygen cooking. The comparison shows that the method according to the invention is very suitable for the production of pulp for liners.

Example 7

Example 6 is repeated except for the following conditions: in the first cooking step: sodium hydroxide 23% in the second cooking step: sodium silicate is replaced by sodium carbonate.

The amounts of reactants are as follows: (by weight based on the weight of the dry treated material: hydrogen peroxide 2% sodium peroxide 4% sodium carbonate 4%

A mass suitable for liners is obtained, the strength properties of which are shown in Table IV.

Table IV

14 70440 _IPICEA______

The Sul-2-step of the invention - The fatal nat-method according to the invention meneron-oxygen method (silicate method (with carbonate))

Example 6 Example 7

Kappa number 77 70 78 76

Yield% 49 49 51 49

Breaking length at 20 ° SR, m 5650 6100 6200 6200

Burst coefficient at 20 ° SR 4.1 5'1 4'4 4> 1

Tear coefficient at 20 ° SR 1000 950 930 1050

Example 8

Example 1 is repeated using Pinus naritimus industrial chips, except for the following conditions: in the first cooking stage: sodium hydroxide 23% in the second cooking stage: sodium hydroxide 4%

cooking temperature 80 ° C

cooking time 90 min

A mass suitable for liners is obtained, the strength values of which determined at 20 ° SR are shown in Table V, where they are compared with the strength values of sulphate mass and soda-oxygen mass.

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Table V

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Pinus maritimus

Oxygen soup method according to the invention Soup

Eg 8

Kappa number 60 65 60

Yield% 48.5 49 48.5

Breaking length at 20 ° SR, m 6700 5700 6800

Burst coefficient at 20 ° SR 4.6 4'6 5 '°

Tear coefficient at 20 ° SR 1160 1000 1140

Example 9 400 g of industrial Pinus maritimus chips are treated under exactly the same conditions as in Example 1, but using the following amounts of reactants: Stage I: sodium hydroxide 19.5% anthraquinone 0.08% Stage II: sodium hydroxide 3% 100% hydrogen peroxide 0.5% magnesium sulphate 0.5%

It should also be noted that the defibering of the material between the cooking steps can be performed in a relatively low energy consuming device, taking into account the known effect of anthraquinone on delignification.

The properties of the obtained mass are shown in Table VI.

This pulp is then bleached in a conventional 5-stage bleaching series using the following amounts of reactants (by weight on a dry weight basis): 1 - chlorine 6,3% 2 - sodium hydroxide 3,1% 3 - chlorine dioxide 1% 4 - hydrogen peroxide H 2 O 2 0,5 %

NaOH 1% sodium silicate 3% 5-chlorine dioxide 0.5%

The properties of the bleached pulp, determined according to the same AFNOR standards, are shown in Table VI.

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Table VI

Example 9 At the end of Phase I At the end of Phase II After __________________

Kappa number 50.7 32

Yield% 45.3 44

Whiteness - - 88

Breaking length, m - 7020 8410

Burst coefficient - 5.35 5.54

Tear factor - 904 1045

As can be seen, the bleaching of the chemical pulps obtained according to this variant of the process according to the invention provides not only a high degree of whiteness but also a particularly high improvement in strength values, including the tear coefficient.

In order to obtain equally good strength values for the bleached pulp obtained from the same plant starting material by the one-step soda cooking process, about 1.28% anthraquinone should be added to 19.5% sodium hydroxide. The degree of whiteness achieved under these conditions would be about 4 points worse.

It has previously been proposed to expose lignocellulosic substances to a first cooking step with sodium hydroxide to which anthraquinone has been added, followed by a second cooking step with oxygen.

By this means, a high level of whiteness and good strength values are achieved after bleaching. However, the tear coefficient remains much worse than that obtained with the use of peroxide in the second step, as shown in Examples 1-8. This is a major downside, especially when these pulps are used on wrapping papers.

Example 10 400 g of Pinus maritimus chips are treated under exactly the same conditions as in Example 1, except that the amounts of reactants used in the second stage of cooking are now as follows: 17 70440

Hydrogen peroxide 0.3%

Sodium hydroxide 4.5%

A mass with a Kappa number of 35 and a very good whiteness is obtained. The yield is 45%.

It is thus seen that the process according to the invention is able to obtain, in satisfactory yields, chemical, homogeneous crude pulps with satisfactory bulk properties of the pulps, the strength properties of the pulps are at least as good as those of the sulphate pulps, and without causing new pollution problems.

It has the advantage over the non-polluting methods proposed so far in that it is perfectly suitable for treating softwoods to replace sulphate pulps for wrapping papers, which has not been the case, in particular, with methods using oxygen in the second stage of cooking.

Its effectiveness is also remarkable in the treatment of young and unpeeled trees, thanks to the special oxidizing effect of peroxide on the bark constituents.

In addition, its use has considerable advantages over sodium-oxygen methods.

In fact, the process according to the invention, which comprises only one pressure cooking step, can be carried out with conventional equipment, essentially comprising a digester, a device for carrying out mechanical defibering and a simple tower in which the peroxide cooking step is carried out.

In addition, the use of an oxidizing agent in the form of a solution facilitates the installation by eliminating the risk of explosion, the release of large amounts of CO and CO2, and the need to use excess reactant which would cause a certain loss of this substance.

These advantages make it possible to consider the industrial use of the process, both to replace conventional chemical pulping processes, in particular the sulphate process, without overwhelming investment, and to set up new small or medium-sized units suitable for exploiting local lignocellulosic feedstocks economically.

This use is of particular interest in the case of a variant of the process in which excipients such as hydroquinone are added to the first, sodium hydroxide step. In addition, the advantages listed above are those which result in particular from this use, namely: low consumption of fibrous energy, use of small amounts of peroxide - low cost due to low consumption of expensive reactants (peroxide, anthraquinone) to obtain chemical pulps with high levels of bleaching a combination of optical and strength properties that allows them to be used to replace bleached sulphate pulps.

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Claims (9)

Process for the preparation of chemical pulp by delignifying lignocellulosic material, characterized in that: (A) the lignocellulosic material is subjected to a first boiling step carried out at 150-180 ° C and in the presence of an aqueous solution of sodium hydroxide containing 10-30% sodium hydroxide by weight of dry starting material; dry painted material. Process according to claim 1, characterized in that the peroxide is selected from a group comprising hydrogen peroxide and alkali peroxides. Method according to claim 1 or 2, characterized in that the alkali solution of the first stage of the cook contains 0.1-10%, preferably 2-5% by weight of alkali, calculated as hydroxide, by weight of defibrated dry material. Process according to any of claims 1-3, characterized in that the alkaline solution used in the second stage of the cooker contains 0.5-10% by weight, preferably 1-5% by weight peroxide calculated on the weight of defibrated dry material. Process according to any one of claims 1-3, characterized in that the alkaline solution used in the second stage of the cooker contains 0.1-1% by weight, advantageously 0.3-0.7% by weight peroxide calculated by weight defibrillated dry material and that the sodium hydroxide solution of the first stage of the cook contains an auxiliary selected from the group comprising aromatic nitrogen derivatives, heterocyclic nitrogen compounds of phenazine type and cyclic ketones of anthraquinone type. Method according to claim 5, characterized in