FI74750C - Process for preparing bleached cellulose pulp from lignin-containing raw material. - Google Patents

Description

74750

A method of making bleached cellulosic pulp from a lignin-containing feedstock

The present invention relates to a process for producing bleached cellulose pulp from a lignin-containing raw material according to the preamble of claim 1. According to this method, the raw material is first treated with a broth containing oxidizing ingredients, followed by pulp bleaching.

The production of fully bleached chemical pulp today takes place by methods that require the use of sulfur- and chlorine-containing chemicals. These cause environmental damage, the minimization of which is a significant cost factor in the industry. The ever-tightening environmental protection requirements are likely to increase these costs in the near future, and it is therefore understandable that efforts to achieve the least polluting pulp production methods have been of increasing interest in recent years.

Efforts have been made to achieve low-pollution pulp production by using as closed processes as possible. In addition, attempts have been made to find chemicals that are less harmful to the environment and that would still allow for a better closure of the process.

The closure of the process in the first stage of conventional pulp production, i.e. in cooking, has already been implemented quite well with current cooking methods. This applies in particular to the chemical cycle of the power process, i.e. sulphate cooking. However, the formation of malodorous sulfur-containing gases that disturb the environment is still problematic here.

In the case of the second stage of pulp production, ie bleaching, the situation is difficult in terms of closing the process. The current 74750 bleaching methods are largely based on the use of chlorine and its compounds, the 100% recycling process of bleaching effluents containing new chlorine compounds has proven to be very cumbersome mainly due to corrosion problems. The various harmful chlorine compounds formed during bleaching are thus necessarily released into the environment in large quantities.

The use of chemicals that are as environmentally friendly as possible, as well as closed-loop chemicals, in pulp production has received increasing attention since the possibilities of current pulp production methods to meet increasing environmental requirements have begun to prove limited. However, this path has so far not been reached in industrial applications, although much is being researched around the world today. The high cost of new methods has very often been cited as a reason for their abandonment.

The least polluting pulp production is best achieved when the chemicals used contain only carbon, hydrogen and oxygen. However, many of the new methods tested do not meet this condition, but may include, for example, nitrogen, which is an unpredictable element in terms of environmental impact and can prove to be very difficult.

The first stage of pulp production, ie cooking with chemicals containing only carbon, hydrogen and oxygen, has been studied mainly in the so-called using organosolv methods. They are based on the use of organic solvents. There is usually a substance called a catalyst that promotes the decomposition and conversion of lignin to a soluble form. The weaknesses of these organosolv methods are their relatively poor delignification ability and the difficulty of making pulp from softwood. In addition, the catalysts often contain undesired elements, such as chlorine or sulfur, which already calls into question the non-contamination of the process with higher amounts of catalyst.

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However, it has been found that the solubility of lignin in certain organosilizing solvents, carboxylic acids, can be significantly increased if a lignin oxidizing chemical such as hydrogen peroxide is used instead of the catalyst. If hydrogen peroxide is added to a liquid carboxylic acid such as acetic or formic acid, the lignin is oxidized to a form that is soluble in the carboxylic acid. You see, hydrogen peroxide forms with carboxylic acids the so-called peroxyacids, which in an acidic system have a strong effect on lignin but much less on other components of wood. Thus, selective delignification occurs and the wood fibers can be made to separate from each other, i.e. pulp is generated.

The use of a carboxylic acid / peroxide mixture in the production of paper and board pulp is disclosed in SU patent 761647 from 1980. According to the patent, the cooking of wood raw material takes place at normal pressure at a temperature below 100 °. The patent publication does not provide information on the wood species used or the bleaching of the pulps obtained. Em. Due to the high amount of peroxide required for cooking according to the patent, a subsequent patent (SU patent 821614, 1981) has resulted in a two-step process in which wood chips are first treated without peroxide with a carboxylic acid at a temperature of 90 ° to 95 ° C, followed by acid / peroxide -reading. Again, this patent publication does not provide further information on the types of wood used or the possible bleaching of the pulps. Another two-step cooking process in which the raw material is treated with a carboxylic acid is disclosed in U.S. Patent 3,458,394. This process uses a cooking broth containing acetic acid and chlorine dioxide. The lignin separated and reacted by the peracetic acid formed in the broth is removed from the pulp in a second step with a dilute alkali solution. The publication shows pulp yields of 75.6 ... 86.0%.

With the methods according to the above patents, it is possible to chemically fiberize the wood, i.e. to obtain 4,74750 pulp. As such, however, this product is not suitable for the production of most grades of paper and board because its brightness is not high enough. However, raising the brightness of the pulp to the required level by conventional bleaching methods containing chlorine chemicals results in the formation of chlorine compounds that pollute the environment, thus losing the environmental benefit provided by the carboxylic acid / peroxide process.

Even chlorine-free bleaching methods are already known. Thus, particularly high expectations have recently been placed on ozone bleaching, in which the bleaching chemical is mainly an ozone-containing gas dissolved in a suitable medium. Hydrogen peroxide has long been used in the bleaching of cotton, linen and wool-based textiles. It has also been applied to some extent to the bleaching of mechanical pulp and pulp from sulphite and sulphate processes. In the latter applications, however, it has not achieved wider use because it does not achieve as high a brightness or viscosity as multi-stage chlorine bleaching. An example of these methods is the solution presented in FI patent publication 68685. It shows a two-stage bleaching process of sulphite pulp, in the first stage of which hydrogen peroxide (0.2 to 3.0% by weight) and peracid made from an organic carboxylic acid (0.1 to 5.0% by weight) are used, and in the second stage, alkaline peroxide treatment is carried out by adding aqueous alkali to the broth from the first stage.

The object of the present invention is to obviate the drawbacks associated with known pulping and bleaching methods and to provide a completely new method for producing fully bleached pulp in substantially two steps directly from a lignin-containing raw material. The invention is based on the fact that in the first step a cooking broth containing peroxyacids derived from carboxylic acids is used and in the second step the obtained defibered pulp is treated with an alkaline peroxide solution.

More specifically, the solution according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

Preferably, the peroxyacid-containing solution used in the first step of the process is obtained by adding hydrogen peroxide to an organic carboxylic acid such as formic, acetic, propionic and butyric acid.

Despite its two-stage nature, the process according to the invention forms an integrated delignification system, the stages of which operate synergistically. In this connection, it should be noted that the single-stage peroxide bleaching used in the process cannot increase the brightness of, for example, unbleached pulp cooked by the sulphate process to a level as high as is now possible for carboxylic acid / peroxide pulp. As can be seen from the examples below (Examples Ia and Ib), the brightness of the pulp according to the invention is about twice as high as that obtained by treating the unbleached sulphate pulp with an alkaline peroxide solution.

The invention provides considerable advantages. Thus, fully bleached pulp can only be obtained with delignification chemicals containing carbon, hydrogen and oxygen. Only the sodium or the corresponding alkali or alkaline earth metal in the base to be added in the second step is included as the inorganic material. No polluting chemicals are used at all. The pulps obtained have a high brightness and a sufficiently high viscosity. The pulp yield is completely comparable to that of bleached sulphate pulp.

The hardware required in the method is simple. Both the acidic and alkaline steps can be performed at normal pressure because the reaction temperatures do not need to be raised above the boiling point of the mixture at any stage. Low temperatures mean significant savings in energy consumption.

The two-step delignification method now presented can be used to delign any type of lignocellulosic material. It is particularly well suited for hardwood and softwood chips normally used in pulp production. The delignification of hardwood chips goes somewhat better than that of pine and spruce chips. Annual plants, grass, straw and bagasse can also be used as raw material.

The first or peroxyacid step of the process is preferably carried out using a mixture of a liquid carboxylic acid and hydrogen peroxide. Under the action of hydrogen peroxide, carboxylic acids are known to be oxidized to peroxoacids, which form the active agent of the broth.

Peroxoic acid reacts with lignin, and the oxidation products formed from this reaction are soluble in the carboxylic acid solution. Different carboxylic acids have different peroxoacid formation ability, so their oxidizing ability varies. In principle, however, no carboxylic acid can be excluded from the process. Both aliphatic and aromatic carboxylic acids are suitable for use. In practice, the most suitable acids are formic and acetic acid, of which the former more easily forms peroxyacid, and is preferred in that sense. After the first stage, formic acid gives a darker mass than acetic acid, while the mass produced using the latter has a lower viscosity. In addition to the above-mentioned acids, propionic and butyric acid can also be used.

The formation of peroxyacids can be promoted by a known catalyst, e.g. sulfuric, phosphoric and boric acids, but is not necessary or always even desirable. It has been found that, for example, sulfuric acid may have a carbohydrate-degrading effect in cooking, which is reflected in the reduced viscosity value of the pulp.

40 to 100%, preferably 70 to 100%, carboxylic acid can be used in the process, the liquid / solid ratio being 2 to 10: 1. In the case of pulp production, the usual ratio of 8: 1 can be used, but it is more preferable to use a lower liquid ratio, e.g., 4: 1, because this reduces the amount of hydrogen peroxide required and promotes the delignification reaction. Pulp yield and brightness are improved at the same time. From the point of view of regeneration, it would be most suitable to use formic acid as a water azeotrope with a boiling point of 107 ° C of about 80%, but from the point of view of delignification, it is again most advantageous to use as little acid as possible.

Alternatively, peroxoacids can be formed in solution in another manner known per se, e.g. by reacting the corresponding aldehydes with a molecular oxygen or oxygen-ozone mixture. In this case, the carboxylic acids required for dissolving the reaction products are formed as decomposition products of peroxyacids. Peroxoacids can be formed in the broth prior to the addition of the cellulosic feedstock.

As the peroxide, for example, an industrially prepared 50% hydrogen peroxide solution can be used. However, the concentration of peroxide may be higher or lower than said, i.e. e.g.

30 ... 90% by weight. The amount used varies widely depending on how far the delignification of the acidic phase is to be taken. It is desirable that the chips be made to fiber before the second stage. In the examples shown, the amount of peroxide has varied between 5 and 60%, based on the dry weight of the chips, but these are not absolute limits. It has been found that, especially in the case of birch raw material, delignification already occurs when using a broth to which about 1% by weight of hydrogen peroxide has been added. Due to the relatively high cost of peroxide, it is preferable to use the smallest possible amounts. The advantage of the process is shown by the fact that using as little as 20% hydrogen peroxide already gives a birch pulp with a kappa number of about 20. By carrying out the carboxylic acid pretreatment described below for the raw material, peroxide consumption can be further reduced without increasing the pulp count.

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Both carboxylic acid and peroxide always come with water, as does wood chips. As mentioned above, the water content of the acid is important, especially for the dissolution of oxidized lignin, so it should be kept as low as possible.

The acid peroxide treatment can be performed at any temperature between room temperature and the boiling temperature of the system used. However, high temperatures are not desirable because peroxoacids decompose as the temperature is raised, thereby losing the delignifying ability of the cooking solution. Thus, for example, the oxidizing ability of a pure formic acid / peroxide mixture is completely lost within 1 hour at a temperature of 80 ° C. The oxidizing ability of the acetic acid / peroxide mixture is maintained for quite a long time even at 95 ° C, but this is due to the fact that the rate of peroxoic acid formation is slower with acetic acid than with formic acid. The choice of cooking temperature thus depends on the acid used. In the case of formic acid it is preferably about 70-90 ° C and in the case of acetic acid slightly higher. For propionic and butyric acid, even higher temperatures can be observed.

It is also possible to carry out the cooking by first heating the broth alone at a higher temperature for some time, e.g. about 90 ° C, after which the actual cooking is carried out at a lower temperature, e.g. 70-75 ° C.

The cooking time can also vary within very wide limits depending on the temperature used. At room temperature, the treatment takes several days, while near the boiling point of water the time can be as long as half an hour. Too long a processing time at high temperature drops the viscosity of the pulp. Depending on the possible pretreatment, the acid used and the cooking temperature, the cooking time is 2 to 10 hours, preferably 4 to 6 hours.

It is advantageous to absorb the treatment broth into the chips before the actual treatment, either under vacuum or pressure, and

Hl 9 74750 it can be done at room temperature. It has been found that the delignifying pretreatment of the chips significantly reduces the amount of hydrogen peroxide required in the peroxyacid cooking step and shortens the actual cooking time. This is especially the case for birch raw material. The pretreatment can be carried out at room temperature or, preferably, at an elevated temperature with a chemical, for example an alkali solution or especially a carboxylic acid, such as formic acid according to SU patent 821614. The acid treatment is carried out, for example, at the boiling temperature of the acid, whereby extending the pretreatment time reduces the number of pieces in the pulp. As the pretreatment acid, the waste broth of the peroxyacid soup can be used. The same alkali solution as in the second step of the process can be used in the alkaline pretreatment. With the help of alkali treatment, it is possible to achieve a very high degree of brightness.

After the first step, the fibrous pulp is preferably washed with water in order to obtain at least an approximately neutral pulp. The broth acid from this step can be reused after regeneration. In regeneration, the acid is separated from the solid, e.g. by distillation. Formic acid boils with water as an azeotrope already at 107 ° C. This azeotrope contains about 80% formic acid and can be reused as such.

The second step of the present cooking process, the alkaline peroxide treatment, can be performed with a water-soluble alkali and hydrogen peroxide. The amount of alkali depends on how much it is consumed in this treatment. The pH of the treatment solution should be above pH 10 at the beginning of the treatment, the amount of alkali required depending on how much it needs to be added to reach this pH level. It has been found that good results are already obtained at pH values slightly above 10. Of course, even higher pH values can be used, but raising the alkalinity to a high level does not make sense for the bleaching result or the chemical economy.

It has further been found that the amounts of alkali and hydrogen peroxide 10 74750 are to some extent interdependent. The more hydrogen peroxide used, the more alkali is needed. During the treatment, the pH of the solution decreases somewhat, usually by about one unit.

The alkaline peroxide step can also be performed in such a way that the calculated peroxide dose is used in several steps, e.g. 3 to 6 steps. The bleach solution is removed from the pulp • at the end of each step. In conventional bleaching, the pulp is preferably washed between processing steps, but if the bleaching is carried out as so-called displacement bleaching, this need not be done. Multi-stage bleaching has been found to have a beneficial effect on the brightness of the pulp. At the same time, the viscosity of the pulp remains high.

Examples of the alkalis used are hydroxides, carbonates and bicarbonates of alkali and alkaline earth metals. Alkali metal hydroxides and carbonates are preferred, and sodium hydroxide and carbonate are particularly preferred. Mixtures of the above-mentioned substances can also be used as alkali.

Sodium hydroxide is advantageous because it achieves the required pH even in a small amount. Sodium carbonate is again preferred in that it can be obtained by incineration of alkaline phase waste liquor. Since the pulp to be treated in the second stage is approximately neutral, no carbon dioxide is released from the carbonate, which could complicate the treatment.

The treatment temperature can be varied, but in the peroxide bleaching of conventional pulps, for example, the general 80 ° C is also suitable here. The duration of the treatment varies according to the temperature, but at 80 ° C, for example, a suitable time is about 1 hour.

In general, the peroxide residue after this step should be in the order of 0.2% by mass.

The amount of peroxide required depends on the lump number of the pulp entering this stage, i.e. the lignin content. Usually the amount of peroxide is 1 to 20%. Empirically it has been found that I! 74750 hydrogen peroxide must be added to approximately the same percentage by dry weight of the material to be treated as is obtained by multiplying the number of pieces of the mass after the first stage by a number between 0.20 and 0.80, preferably 0.25 ... 0.70 and particularly preferably 0 45 ... 0.65.

Within the scope of the invention, the second step can be carried out in a manner different from that described above. Thus, the required alkali and hydrogen peroxide can be replaced by a peroxide derivative such as metal peroxide, preferably sodium peroxide, which when dissolved in water forms hydroxide and hydrogen peroxide.

To prevent the hydrogen peroxide degrading effect of heavy metals that may enter the system, some or some peroxide stabilizers may be added. In the peroxyacid step, citric acid can be used, and in the basic peroxide step, for example, diethylenetriaminepentaacetic acid (DTPA) and / or certain magnesium salts, e.g. Mg sulphate. The amount of stabilizer is preferably in the order of a few per mille.

The invention will be examined in more detail by means of the following non-limiting embodiments. The brightness values reported in the examples were determined according to the SCAN C-11 method and the viscosity values according to the SCAN C-15 method, respectively. Part numbers are determined according to the SCAN C-1: 77 method. All percentages given are by weight.

Example la 50 g abs. the dried pine chips (92% dry matter) were placed in a mixture of 250 ml formic acid, 75 ml water and 60 ml 50% hydrogen peroxide solution (corresponding to 60% peroxide by dry weight of the chips). The mixture was soaked in the chips under vacuum for 30 minutes, after which the temperature was raised to 70 ° C in 2.5 hours. The actual cooking was performed at 70.75 ° C for 2.5 hours. The broth was separated from the soft chips by filtration, the chips were washed with some water, and then defibered with a Waring Blendor laboratory mixer. The defibering time was 30 s at the lowest power of the device. The pulp obtained after defibering was washed with water and 0.8% of the sticks were removed. The number of pieces obtained was 11.3.

The pulp dried at room temperature was treated with an alkaline hydrogen peroxide solution at 80 ° C for 2 hours. The pulp consistency was 10% and the NaOH dose was 5% and the hydrogen peroxide dose was 7.3% (peroxide dose = 0.65 x piece number). The pH of the solution was initially 10.3 and dropped to 9.4 during treatment. The stabilizer was 0.2% DTPA. After treatment, the mass was washed and acidified, after which it was dried. The final brightness of the pulp was 89.0%, the viscosity 830 cm 2 / g, and the yield based on wood was 44.4%.

Example Ib (comparative)

The unbleached pine sulphate pulp, having a kappa number of 31.9, was treated at a consistency of 10% at 80 ° C for 30 minutes. The dosage of NaOH was 3% by weight, the dosage of hydrogen peroxide was 20.7% (= 0.65 x number of pieces), and the pH was 10.5. Despite the short time, the peroxide was completely consumed. The pulp obtained had a brightness of 46.5% and a viscosity of 750 cm 2 / g.

Example 2

As in Example 1, but birch chips (90% dry matter) were used instead of pine. In contrast, the cooking took place so that the temperature was raised to 70 ° C in 5 hours, after which the temperature was raised to 80 ° C in one hour, and the cooking was then stopped. The number of sticks was 3.1%, and the number of pieces in the pulp was 5.3. The alkaline hydrogen peroxide treatment took place at 80 ° C and lasted for one hour. The mass consistency was 10% and the dosage was 5% NaOH and 3% hydrogen peroxide (peroxide dose = 0.57 x number of pieces). The pH of the solution was 10.4. The analytical values of the pulp were: brightness 89.0%, viscosity '1050 cm 2 / g.

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Example 3

As in Example 1, but instead of pine, spruce chips (dry matter 93%) were used. · In cooking, the temperature was raised to 80 ° C in 2.5 hours, where it was then allowed to stand for 2.5 hours. Sticks 11.4%, and mass number 14.0. The alkaline peroxide treatment took place at a mass consistency of 10% for 2 hours. The dosages were 5% NaOH and 4.5% hydrogen peroxide (= 0.32 x number of pieces). The pH of the solution was 10.8. The analytical values of the pulp were: brightness 84.3%, viscosity 920 cm 2 / g and pulp yield 43.1% (based on wood).

Example 4

As in Example 3, but instead of formic acid, acetic acid. During cooking, the temperature was raised to 85 ° C in 1.5 hours, where it was allowed to stand for 2.5 hours. The sticks were 5.5%, and the kappa number of the pulp was 18. Alkaline hydrogen peroxide treatment as in Experiment 3, peroxide dose 0.25 x kappa number, pH 10.9. The analytical values of the pulp were: brightness 85.2%, viscosity 740 cm 2 / g and yield 48.8% (based on wood).

Example 5

As in Example 1, but the pine chips were pretreated for 1 hour at 90 ° C with 80% formic acid before peroxide cooking. The cooking took place by raising the temperature to 75 ° C in 3 hours, after which it was stopped. There were no sticks. The number of pieces in the pulp was 7.3. Alkaline hydrogen peroxide treatment was performed at 80 ° C for 2 hours at a mass consistency of 10%. The NaOH dose was 5% and the hydrogen peroxide dose was 4.5% (= 0.62 x number of pieces). The pH of the solution was 10.1. Mass analysis values: brightness 86.0%, viscosity 900 cmVg and mass yield 43.9% (calculated from wood).

Example 6

As in Example 2, but the birch chips were pretreated before peroxide cooking for 1 hour at 100 ° C with an alkaline solution in which 74750 14 was 6% by weight of the chips in NaOH. The soup was done by peroxide raising, which was 20% of the amount of chips. The temperature was raised to 75 ° C in 4 hours 20 minutes, after which the temperature was maintained at 75-80 ° C for 2 hours. The number of sticks was 12.6%, and the number of pieces was 14.5. The alkaline hydrogen peroxide treatment took place at a consistency of 10% at 80 ° C for one hour. The NaOH dose was 6% and the hydrogen peroxide dose was 8% (= 0.55 x number of pieces). The pH of the solution was 10.8. Mass analysis values: brightness 86.5%, viscosity 1000 cm 2 / g and mass yield 43.2% (based on wood).

Example 7

As in Example 6, but the temperature of the alkaline peroxide treatment was 60 ° C and the duration was 2 hours. Mass analysis values: brightness 86.5%, viscosity 1080 cm 2 / g and mass yield 43.5% (based on wood).

Example 8

As in Example 2, but the liquid ratio of the broth was reduced and the amount of hydrogen peroxide was reduced. 50 g abs. the dried birch chips (90% dry matter) were placed in a mixture of 200 ml of formic acid. No water was added at all, and the amount of 50% hydrogen peroxide was reduced from 60 ml (in Example 2) to 10 ml, which corresponds to 10% of the dry weight of the chips. The cooking liquid ratio was thus 4: 1 (instead of 8: 1). The reaction mixture was heated to 78 ° C where it was maintained for 3.5 hours. The pulp had a basis number of 51.4, a viscosity of 1170, a sorted pulp yield of 52.9% and a stick count of 7.2%.

The resulting pulp was bleached with alkaline hydrogen peroxide so that a dose of hydrogen peroxide, 30.8% of the pulp (16.3% of the chips, i.e. 0.60 x number of pieces), was added in three portions. The temperature in all steps was 80 ° C, the reaction time 1 hour, and the starting pH 10.5. 0.2% DTPA was added as a stabilizer. The brightness of the obtained mass was 90.3.

The total consumption of hydrogen peroxide in this example was 10 + 16.3%, i.e. 26.3% by weight of the chips.

II.

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Example 9 50 g of pine chips were pretreated by refluxing in 250 ml of 100% formic acid for three hours. The pretreatment broth was removed, followed by peroxoformic acid cooking with 200 ml of 100% formic acid supplemented with 10% hydrogen peroxide. The cooking temperature was raised to 80 ° C, the total cooking time was three hours. After cooking, the mass was washed with water until the wash water was neutral. The pulp number at this stage was 31.1, viscosity 1060, brightness 19.5 and yield 43.3. There were almost no sticks in the mass.

Experiments on the pulp showed that it could be easily bleached with an alkali-hydrogen peroxide solution.

Example 10

As in Example 8, but the amount of peroxide used was only 5% of the chips, i.e. 5 ml of 50% hydrogen peroxide was added. The pulp had a kappa number of 62.2, a viscosity of 1070, a sorted pulp yield of 45.8 and a stick count of 19.7.

The resulting pulp was bleached as in Example 8, and a dose of hydrogen peroxide, 28% of the pulp (12.8% of the chips or 0.45 x number of pieces) was still added in three batches. The brightness of the obtained mass was 87.0%.

The total consumption of hydrogen peroxide in this example was 5 + 12.8% or 17.8% by weight of the chips.

Example 11

As in Example 6, but the birch chips were pretreated before the peroxyacid soup by boiling for 3 hours in 80% formic acid under a vertical condenser. The broth was removed, followed by a peroxomoric acid soup in which only 5% hydrogen peroxide was added to 85% formic acid. The temperature was raised to 80 ° C in 50 minutes, where it was maintained for 1 hour. After cooking 16,74750, the pulp was washed with so many portions of hot water that the wash water was neutral. The pulp number at this stage was 14.7 and the viscosity 1180, and the yield was 42.3%.

The resulting pulp was bleached with alkaline hydrogen peroxide so that a dose of hydrogen peroxide, 8% by weight (3.4% of the chips, i.e. 0.55 x number of pieces), was added in four batches: the first two steps lasted 1 hour and the last 2 hours. The temperature in all steps was 80 ° C and the initial pH was 10.7. DTPA 0.2% by weight + 0.5% Mg sulfate was added as a stabilizer. The resulting mass had a brightness of 90.0% and a viscosity of 1140.

The total hydrogen peroxide consumption in this example was only 5 + 3.4% or 8.4% by weight of the chips.

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Claims (12)

17,74750 Claims: 'n u u A process for preparing a bleached cellulosic pulp from a lignin-containing cellulosic raw material, which process comprises first treating the raw material with a cooking broth containing oxidizing ingredients and then bleaching the pulp, characterized in that a) the cellulosic raw material is fiberized with an organic , in particular peroxyacids derived from formic, acetic, propionic and butyric acids, and (b) the defibered pulp is bleached with an alkaline solution containing hydrogen peroxide having a pH of at least 10 at the beginning of the treatment and obtained by adding hydrogen peroxide to the alkali solution in a percentage of dry matter corresponds to the number of pieces of the mass obtained from step a multiplied by a number of 0.20 to 0.80, preferably 0.25 to 0.70 and particularly preferably 0.45 to 0.65. Process according to Claim 1, characterized in that the solution used in step a is obtained by adding to 40 to 100% by weight, preferably 70 to 100% by weight, of an organic carboxylic acid, such as formic acid. , acetic, propionic or butyric acid, at least 1% by weight, preferably about 5 to 20% by weight, of hydrogen peroxide from the amount of dry cellulosic raw material. Process according to Claim 1 or 2, characterized in that the liquid / solid ratio in step a is kept between 2 and 10: 1, preferably between about 4 and 8: 1. Process according to Claim 2 or 3, characterized in that the regenerated waste liquor of the α-step is used as the carboxylic acid. Process according to Claim 4, characterized in that about 80% of the formic acid-water azeotrope obtained from the distillation of the α-step waste broth is used as formic acid. Process according to one of the preceding claims, characterized in that, before step a, the raw material is treated with a carboxylic acid, such as formic or acetic acid, preferably at the boiling point of the acid. Process according to Claim 4, in which formic acid is used in the pretreatment, characterized in that the waste liquor containing formic acid obtained from step a is used as the pretreatment acid. Process according to one of Claims 1 to 5, characterized in that, before step a, the raw material is pretreated with an aqueous solution of the alkali used in step b, preferably at a temperature of about 100 ° C. Process according to Claim 1, characterized in that an alkaline solution containing sodium hydroxide, sodium carbonate or a mixture thereof is used in step b. A method according to claim 1, characterized in that the peroxide dose calculated for use in step b is added in several steps, preferably 3 to 6, wherein the bleaching solution is removed from the pulp at the end of each step, after which the pulp is optionally washed. Method according to one of the preceding claims, characterized in that in step a the temperature of the cooking broth is first maintained at a higher temperature, preferably at about 90 ° C, after which the actual cooking is carried out at a lower temperature, preferably from about 70 to 75 ° C. C. Process according to one of Claims 2 to 11, characterized in that citric acid is added as a peroxide stabilizer in step a and diethylenetriaminepentaacetic acid (DTPA) and / or magnesium sulphate in step b. Il 74750 19