CA1284557C - Process for preparing pulp out of lignocellulosic raw material - Google Patents

Abstract

ABSTRACT:
Lignocellulosic raw material is pulped with a cooking liquor containing organic peroxyacids, such as peroxyformic acid or peroxyacetic acid, and the defibred pulp is bleached with an alkaline solution containing hydrogen peroxide, the initial pH of which is at least 10, and which is provided by adding to a sodium hydroxide solution hydrogen peroxide in an amount which calculated as per cents of the dry weight of the matter coming to the treatment corresponds to from 0.20 to 0.80, preferably from 0.25 to 0.70 and most preferably from 0.45 to 0.65 times the kappa number of the pulp obtai-ned from the previous stage of the process.

Description

~28~ 7 Process for preparing bleached pulp out of lignocellulosic raw material The present invention relates to a process for preparing bleached pulp out of lignocellulosic raw material. According to said process, the raw-material is first treated with a pulping liquor containg oxidizing components, whereupon the pulp is bleached.
The preparation of fully bleached pulp i5 nowadays carried out by means of methods which call for the use of sulphur-and chlorine-containing chemicals. These have a detrimental environmental influence, the minimization of which forms an important cost factor in industry. The environmental re-quirements of ever increasing stringency are obviously going to raise said costs even further in the future. It is conceivable that the efforts to provide less contaminating pulping processes during the last few years have been the objects for constant interest.
Non-polluting pulping has been aimed at by using closed processes. Furthermore, there have been efforts to find chemicals, the use of which is accompanied with a diminished adverse environmental influence, when compared with conven-tional pulping chemicals, and which would allow the process to be closed even better.
The closing of the process in the first stage of conventional pulping, i.e. during cooking, has to rather a high degree already been achieved for the present pulping processes.
This is true especially for the chemical circulation of the main process, i.e. the kraft process. In this context, the formation of malodorous sulphur-containing volatile compounds which exhibit an adverse environmental influence is, however, still a problem.
The closing of the process of the second stage of pulping, i.e. bleaching, is difficult. The present bleaching processes are mainly based on the use of chlorine and compounds there-( of, but lO0 % recirculation of bleaching waste-liqours containing chlorine compounds in the process has proved to be very difficult mainly due to corrosion problems. The various detrimental chlorine compounds formed during bleaching are therefore to rather a large extent emitted to the environment.
In pulping, the chemicals which cause a minimum of pollution and which are preferable when it comes to the closing of the process, have met an ever increasing interest, as the possibilities of the present pulping processes for meeting the stricter environmental requirements have been shown to be limited. Industrial applications have not yet been reached by means of this way, although the matter is largely investigated around the world today. The new methods are expensive and this has frequently been mentioned as the rea-son for their not being widely used.
Pulping which causes a minimum of environmental pollution is best reached when the chemicals used therein contain carbon, hydrogen, and oxygen only. Many of the new methods tested do not, however, fulfil this requirement; they may contain e.g. nitrogen which is an unpredictable element when it comes to the environmental influence thereof. It may even prove to be very troublesome.
2s ~he first stage of pulping, i.e. cooking, using chemicals containing carbon, hydrogen, and oxygen only, has mainly been investigated in the connection with so-called organo-solv pulping methods. Said methods are based on the use of organic solvents. These usually contain some compound which is called catalyst, which enhances the degradation of lignin and the conversion thereof into a dissolving form.
The drawback of said organosolv-methods lies in their rela-tively low delignification capacity and the difficulty of pulping softwood. Furthermore, the catalysts oftentimes con-tain undesirable elements, such as chlorine or sulphur.

... i This makes the non-polluting character of the method questionable when larger amounts of catalysts are employed.
However, it has been discovered that the dissolving of lignin in certain organosolv-solvents, the carboxyli~ acids, may be significantly increased when a lignin-oxidizing chemical, e.g. hydrogen peroxide, is used instead of said catalyst. If hydrogen peroxide is added to a liquid carboxylic acid, such as acetic or formic acid, lignin is oxidized to dissolve in carboxylic acid. Hydrogen peroxide forms together with carboxylic acids peroxyacids, which in an acid system have a strong influence on lignin but much less influence on the other components of wood. A selective delignification is thus achieved by means of which the wood fibres may be separated from each other, i.e. pulp is obtained.
The use of a mixture of carboxylic acid/peroxide in the preparation of paper and paperboard pulp has been disclosed in the SU Patent Specification 761647 published in 1980.
According to said patent, cooking is carried out at normal pressure in a temperature below 100C. No data as to the species of wood nor to the bleaching of the pulp are given.
Due to the huge amount of peroxide required in the pulping in accordance with said patent, a later patent tSU Patent Specification 821614, published in 1981) arrived at a process in two stages, wherein the wood chips are initially treated with carboxylic acid without peroxide at a tempera-ture of 90C to 95 C, whereupon the acid/peroxide treatment is conducted. No more detailed data as to the wood species used, or of the bleaching of the pulp are given. Another pulping process in two stages, wherein the raw material is treated with carboxylic acid, is disclosed in US Patent Specification No. 3 458 394. A pulping liquor containg acetic acid and chlorine dioxide is used in said process.
The lignin separated and reacted due to the action of the peracetic acid formed in the liquor is removed from the pulp during the second stage of the pulping by means of a dilute alkali solution. According to the publication, the yield is from 75,6~% to 86,0 %.

According to the processes disclosed in said patents it is possible to obtain chemical defibration of the wood, i.e.
to obtain pulp. However, the product as such is not suitable for the preparation of most of the paper and paperboard qualities due to the unsufficient degree of brightness. The brightness of the pulp has to be raised to a required level by means of conventional bleaching methods employing chlorine chemicals. This results, however, in the formation of polluting chlorine compounds, whereby the advantage over conventional pulping offered by the carboxylic acid/
peroxide-process is lost.
Chlorine-free bleaching methods are known per se. Thus, during the recent years great expectations have been focused on ozon bleaching employing as its main bleaching chemical an o~on containing gas dissolved in a suitable medium.
Hydrogen peroxide has been used in the bleaching of cotton, linen and wool based textiles for long. It has also to some extent been used in the bleaching of mechanical pulp and for the bleaching of pulp from sulphite and sulphate proces-ses. In last mentioned applications, it has not been used toany larger extent, since it is not possible to obtain as high a brightness or viscosity as in multistage chlorine bleaching. Said methods are exemplified by the solution mentioned in the FI Patent Specification No. 6868~. Said publication discloses a bleaching process in two stages for bleaching of sulphite pulp, in the first stage of which hydrogen peroxide (0,2 - 3,0 % by weight) and peracid pre-pared out of an organic carboxylic acid (0,1 - 5,0 ~ by weight) is employed, and in the second stage of which an alkaline peroxide treatment is conducted by adding aqueous alkali to the liquor obtained from the first stage.
One ob~ect of the present invention is to offset the draw-backs related to the known pulping and bleaching processes and to provide an entirely novel process for preparing pulp directly out of lignocellulosic raw material essentially in two stages. The invention is based upon using in a first stage a pulping liquor containing peroxyacids derived from carboxylic acids and subsequently treating the defibrated pulp thus obtained with an alkaline solution of peroxyacids in a second stage.
Accordingly, the process in accordance with the comprises the combination of defibering the cellulosic raw material by means of a cooking liquor containing as active agent organic peroxyacids, and bleaching the defibred pulp with an alkal~i-ne solution containing hydrogen peroxide, the pH of which is at lea5t lO in the beginning of the treatment, and which is provided by adding hydrogen peroxide to an alkali solution.
The amount of hydrogen peroxide (calculated as per cents of the dry weight of the matter coming to the treatment) cor-responds to from 0.20 to 0.80, preferably from 0.25 to 0.70 and most preferably from 0.45 to 0.65 times the kappa number of the pulp obtained from the first stage of the process.
The liquor employed in the first stage containing peroxyacids is preferably provided by adding hydrogen per-oxide to an organic carboxylic acid, e.g. formic, acetic, propionic, or butyric acid.
The process according to the invention forms, in spite of the provision of two stages, an integrated delignification system, the stages of which function synergetically. In this context it should be mentioned that it is not possible to raise the brightness of, e.g. unbleached pulp from the kraft process to as high a level as that of the carboxylic acid/peroxide-pulp by separately employing the peroxide bleaching process of the present invention. As shown in the examples lA and lB hereinafter, the brightness of the pulp in accordance with the invention is about twice as high as that of the unbleached kraft pulp after treatment with an alkaline peroxide` solution.
By means of the invention, significant advantages are achieved. Thus, fully bleached pulp is obtained by using 39L5~

delignification chemicals containing carbon, hydrogen and ( oxygen only. The sodium or corresponding alkali or earth alkali metal present in the alkali added in the second stage is the only inorganic material to be added. Environ-mentally polluting chemicals are not used at all. The pulps obtained exhibit high brightness and sufficiently high viscosity. The yield is comparable to that of bleached kraft pulp .
The apparatus required by the process is simple. The acid stage as well as the alkaline one may be conducted at normal pressure, since there is no need for raising the reaction temperatures above the boiling point of the mixture. The low temperatures result in huge savings in energy consumption.
The present delignification process in two stages may be used for delignification of all kinds of lignocellulosic materials. It is particulary well suited for chips of hardwood and softwood commonly employed in pulping. De-lignification of hardwood proceeds somewhat better than that of spruce and pine chips. Annual plants, grass, straw and bagasse may also be used as raw materials.
The first stage of the process, i.e. the peroxyacid stage, is preferably conducted by using a mixture of some liquid carboxylic acid and hydrogen peroxide. As known, due to the influence of hydrogen peroxide, the carboxylic acids are oxidized to peroxyacids which form the active agents of the cooking liquor.
The peroxyacid reacts with lignin, and the oxidization products formed from this reaction dissolve in the solution of carboxylic acids. The various carboxylic acid have a different capability of forming peroxyacids, i.e. their oxidative capacity differs. In principle, no carboxylic acid may be excluded from the process. Aliphatic as well as aromatic acids are suitable. In practice, formic and acetic acids are particulary preferred, of which the first mentio-~l~8~S~

ned acid more easily forms peroxyacid, and it is thereforein this sense preferred. The pulp obtained when using formic acid in the first stage is darker than that obtained by using acetic acid, on the other hand, the viscosity of the pulp is lower when the last-mentioned acid has been employed.
In addition to aforementioned acids, propionic and butyric acids may also be used.
The formation of peroxyacids may be enhanced by using some known catalysts, e.g. sulphuric, phosphoric or boric acids, but this is neither necessary nor always even preferable.
It has been discovered that, eOg., sulphuric acid may degrade the carbohydrates during the cooking, which leads to a decreased viscosity of the pulp.
In the process, the concentration of the carboxylic acid lS employed is in the range of 40...100 %, preferably in the range of 70...100 %, the liquid-to-wood ratio being from 2:1 up to 10:1. The ratio 8:1 normally used in pulping may be used, it is, however, preferable to use a smaller liquid-ratio, e.g. 4:1, because said ratio makes it possible to decrease the required amount of hydrogen peroxide and enhance the progress of the delignification reaction. Pulp yield and brightness of the pulp are also improved by a smaller liquid-to-wood ratio. On one hand, when it comes to recovery of the formic acid, it would be preferable to use an azeotropic formic acid-water solution, containing 80 % of formic acid, but on the other hand, when it comes to de-lignification, it is preferable to use an acid containing as small an amount of water as possible.
Alternatively, the peroxyacids may be formed in the solution in some other known manner, e.g. by reacting the corresponding aldehydes with molecular oxygen or an oxygen-ozone mixture. In this case, the carboxylic acids needed for the dissolution of the reaction products are formed as decomposition products of the peroxyacids. The peroxyacids may also be formed in the cooking liquor prior to the addition of the cellulosic raw material.

The peroxide used may comprise, e.g., an industrially prepared hydrogen peroxide solution, the concentration of which is 50 %. The concentration of the peroxide may, how-ever, be higher or lower, i.e. from about 30 ~ up to 90 % by weight. The amount used varies within a large range depending on the extent to which the delignification is to be conducted during the acid stage. It is preferable to achieve a de-fibration of the chips before the second stage. The amount of peroxide used in the examples has been varied from 5 ~ up to 60 % based on the oven dry (o.d.) weight of the chips, but said values are no absolute limits. It has been ascer-tained that, especially for birch raw material, delignifica-tion is achieved when using a liquor containing l % by weight of hydrogen peroxide. Due to the relatively high price of the hydrogen, it is preferable to use as small amount thereof as possible. The usefulness of the process is demonstrated by the fact that by employing 20 % of hydro-gen peroxide a birch pulp is obtained, the kappa number of which is on the order of about 20. By carboxylic acid pre-treatment of the raw material as described hereinafter, itis possible to further decrease the consumption of the hyd-rogen peroxide without letting the kappa number of the pulp rise to a hlgher level.
Water present in the carboxylic acid, in the peroxide as well as in the chips, is introduced into the cooking liquor.
As mentioned above, the water content of the acid influences especially the dissolution of the oxidized lignin. Thus, it should be kept as low as possible.
The acid peroxy-treatment may be conducted at any tempera-ture between ambient temperature and the boiling point of the system used. However, high temperatures are not suitable due to the fact that the peroxyacids are decomposed when the temperature is raised, whereby the deligni~ic tion capacity of the cooking liquor is lost. Thus, the oxidizing capacity of a pure formic acid/peroxide mixture disappears altogether within an hour when the temperature is 80C. The oxidizing capacity of the mixture of acetic acid and peroxide is conserved rather a long time even at 95C, but this is due to the fact that the rate of peroxyacid formation is slower for acetic acid than for formic acid. Accordingly, the cooking temperature to be selected depends on the acid used.
For formic acid the temperature is preferably in the range of about 70- 90C, and for acetic acid somewhat higher.
For propionic and butyric acids still higher temperatures may be used.

It is also possible to carry out the cooking such that the cooking liquor is heated for some time at a higher tempera-ture, e.g. at about 90C, whereupon the actual cooking is conducted at a lower temperature of, e.g., from 70C up to 75C.
Cooking time may also vary within a large range depending on the temperature. At ambient temperature the treatment lasts for several days, whereas it may be as short as half an hour near to the boiling point of water. Too long a treatment time at high temperature lowers the viscosity of the pulp.
Depending on the optional pretreatment, the acid used and the temperature of the cooking, the cooking time is from 2 hours up to 10 hours, preferably from 4 hours up to 6 hours.
It is preferable to impregnate the chips with the treatment liquor either ln vacuo or by means of pressure before starting the actual treatment. The impregnation may be conducted at ambient temperature. It has been discovered that a delignifying pretreatment of the chips decreases considerably the amount of hydrogen peroxide needed in the peroxyacid treatment stage, thus reducing the actual cooking time. This is true especially for birch raw material. The pretreatment may be conducted at ambient temperature or preferably at increased temperature by means of some chemi-cal, e.g. an alkali solution or preferably formic acid as taught by the SU Patent Specification No. 821614. The acid treatment is carried out, e.g., at the boiling point of the acid, whereat prolonging of the pretreatment time lowers the kappa number. As pretreatment acid spent liquor of the peroxyacid cooking may be used. In an alkaline pretreatment the alkali solution used may be the same as in the second phase of the process. By means of the alkali treatment it is possible to reach an extremely high brightness.
A preferred embodiment of the first stage of the invention comprises pulping the chips first in peroxyformic acid, thereafter in formic acid and subsequently again in peroxy-formic acid. Delignified pulp (kappa number on the order oflO) is easily obtained by said procedure. Peroxyacetic acid pulping may also be conducted in a similar manner.
After the first phase, the defibrated pulp is preferably washed with water such that the pulp obtained is at least approximately neutral. The acid of the spent liquor obtained from this phase may be re-used after recovery. During reco-very the acid is separated from the solid substance, e.g., by means of distillation. Formic acid and water form an azeotropic mixture which boils already at 107C. This azeo-tropic mixture contains about 80 ~ of formic acid and may bere-used as such.
The second phase of the present pulping process, the alkaline peroxide treatment, may be conducted by using a water-soluble alkali and hydrogen peroxide. The amount of the alkali depends on the amount consumed during the treat-ment. The pH value of the treatment liquor shall initially exceed lO, the required amount being dependent upon the amount required for reaching said pH level. It has been ascertained that acceptable results are reached already at pH values slightly in excess of lO. Higher pH values may of course also be employed, but when it comes to the bleach-ing result or the economy of the chemicals, it is not sen-sible to increase the alkalinity to a high level. It has further been ascertained that the amounts of alkali and hydrogen peroxide are somewhat interdependent. The more .

ll hydrogen peroxide is used, the more alkali is needed. During the treatment the pH of the liquor decreases to some extent, usually about one pH unit.
The alkaline peroxide phace may also be accomplished such that the calculated amount of peroxide is used in several portions, e.g. in 3 to 6 portions. The bleaching liquor is removed from the pulp after each phase. When the bleaching is carried out as a conventional bleaching, the pulp is preferably washed between the phases of treatment, but if the bleaching is conducted as a so-called displacement bleaching this is not necessary. ~ multiphase bleaching has been discovered to have a preferable influence on the brightness of the pulp. The viscosity of the pulp remains high at the same time.
The alkali used may be selected from the group consisting of alkali metal and earth alkali metal hydroxides, carbonates and bicarbonates. Alkali metal hydroxides and carbonates are preferred, particulary preferred are sodium hydroxide and sodium carbonate. The alkali may also comprise mixtures of said compounds. It is preferable to use sodium hydroxide because by means of it minor amounts already result in the required pH value. Sodium carbonate on the other hand is suitable in the sense that it may be obtained by calcination of the spent liquor from the alkali phase. Because the pulp treated in the second phase is almost neutral, no carbon dioxide is evolved from the carbonate, which might make the treatment more difficult.
The temperature of the treatment may vary, but e.g. the common temperature, 80C, used in peroxide bleaching of conventional pulps may also be used in this context. The duration of the treatment varies according to the temperature. At 80C the suitable time is about 1 hour.
Generally, a peroxide residue after this stage on the order of 0.2 % of the pulp should be aimed at.

i'7 The required amount of peroxide depends on the kappa number, i.e. the lignin content of the pulp coming to said stage.
Generally, the amount is from 1 % up to 20 % by weight. It has been discovered empirically that the added amount of hydrogen calculated as per cent of the dry weigh of the mat-ter coming to the treatment amounts to about 0.20 to 0.80, preferably 0.25 to 0.70, most preferably 0.45 to 0.65 times the kappa number of the pulp from the first phase.
Within the scope of the invention, the second phase may be accomplished in a different manner also. Thus, the alkali and the hydrogen peroxide required may be substituted by a peroxide derivative., e.g. a metal peroxide, preferably sodium peroxide, which dissolved in water forms hydroxide and hydrogen peroxide~
To prevent heavy metals possibly introduced into the system from exhibiting a hydrogen peroxide decomposing effect one or a few peroxide stabilizers may be added. ~uring the per-oxyacid stage citric acid may be used, whereas e.g. diethy-lene triaminepenta acetic acid (DTPA) or/and one magnesium salt, e.g. Mg sulfate, may be used during the alkaline peroxide stage. The amount of the stabilizer is preferably on the order of a few promilles.
The invention will now be examined in more detail by means of the following non-limitative practical examples. The brightness values indicated in the examples have been deter-mined according to the SCAN C-ll method and the viscosity values correspondingly according to the SCAN C-15 method.
The kappa numbers have been measured by the SCAN C-1:77 method. All of the indicated per cent amounts are expressed by weight.

EXAMPLE lA
50 g pine chips (dry matter 92 %) calculated od., was admixed with 250 ml formic acid, 75 ml water, and 60 ml 50 % hydrogen peroxide solution (corresponding to 60 %
peroxide of the dry weight of the chips). The chips were impregnated with the mixture in vacuo for 30 minutes, whereupon the temperature was raised to 70C within 2.5 hours. The cooking was carried~out at a cooking temperature of 70 C to 75C for 2.5 hours. The cooking liquor was removed from the soft chips by filtration, the chips were washed to some extent with water, whereupon it was defibered with a Waring Blendor laboratory blender. The defibration time was 30 sec using the smallest effect of the apparatus.
After the defibration the pulp was washed with water and the shives (0,8 %) were removed. The kappa number of the pulp was 11.3.
The pulp dried at ambient temperature was treated with an alkaline hydrogen peroxide solution at 80C for 2 hours, at 10 per cent stock, the added amount of NaOH being 5 % and the amount of hydrogen peroxide being 7,3 % (the amount of peroxide = 0.65 x kappa number). The initial pH value was 10.3, and it decreased to a value of 9.4 during the treat-ment. The stabilizing agent comprised 0.2 % of DTPA. After the treatment, the pulp was washed and acid was added, whereupon the pulp was dried. The final brightness of the pulp was 89.0 %, the viscosity 830 cm3/g, and the pulp yield in per cent of the raw material 44.4 %.
EXAMPLE lB (control) Unbleached spruce kraft pulp, the kappa number of which was 31,9, was bleached at 10 per cent stock at 80C for 30 minu-tes. The amount of NaOH was 3 % of the pulp, the amount of hydrogen peroxide being 20.7 % (= 0.65 x kappa number), and the pH value 10.5. In spite of the short treatment time the peroxide was completely exhausted. The brightness of the pulp thus obtained was 46.5 % and the viscosity 750 cm3/g.

As example lA, but birch chips (dry matter 90 %) were used instead of pine. The cooking was carried out departing from the foregoing such that the temperature was raised to 70C
during 5 hours, whereinafter the temperature was raised to 80C within one hour and the cooking was finished. The shives amounted to 3.1 %, and the kappa number of the pulp was 5.3. The alkaline hydrogen peroxide treatment was con-ducted at 80C, the duration being one hour. The stock was 10 % and the amount of NaOH was 5 % and of hydrogen peroxide 3 % (peroxide = 0.57 x kappa number). The pH of the solution was 10.4. Analysis of the pulp: brightness 89.0 %, viscosity 1050 cm3/g.

As example 1, but spruce chips (dry matter 93 %) was used instead of pine. The temperature of the cooking was raised to 80 C within 2.5 hours, and left at said temperature for 2.5 hours. Shives: 11.4 % and the kappa number of the pulp 14Ø The alkaline peroxide treatment was conducted at 10 per cent stock during 2 hours. The added amounts were: 5 %
of NaOH and 4. % of hydrogen peroxide (- 0.32 x kappa number). The pH of the solution was 10.8 %. Analysis of the pulp: brightness 84.3 %, viscosity 920 cm3/g and pulp yield 43.1 % (calculated on the wood).

As example 1, but acetic acid was used instead of formic acid. During the cooking the temperature was raised to 85 C within 1.5 hours, and the cooking liquor was left at said temperature for 2.5 hours. Shives: 5.5 % and the kappa number of the pulp 18. The alkaline peroxide treatment was conducted as in example 3, the amount of hydrogen peroxide 0.25 x kappa number, the pH was 10.9 %. Analysis of the pulp: brightness 85.2 %, viscosity 740 cm3/g and the pulp yield 48.8 % (calculated on the wood).

As example 1, but the pine chips was pretreated before the peroxide cooking for one hour at 90C with 80 ~ formic acid.
The cooking was carried out by raising the temperature with-in 3 hours to 75C, whereupon the cooking was finished.There were no shives. The kappa number was 7.3. The alkaline hydrogen peroxide treatment was conducted at 80C, the dura-tion being two hours at 10 % stock. The amount of NaOH was 5 % and the amount of hydrogen peroxide 4.5 % (= 0.62 x kappa number). The pH of the solution was 10.1. Analysis of the pulp: brightness 8~.0 %, viscosity 900 cm3/g and the pulp yield (calculated on wood) 43.9 %.

As example 2, but the birch chips were pretreated prior to the peroxide cooking for one hour at 1~0C with an alkali solution containing 6 % o~d. wood NaOH. The cooking was car-ried out using the peroxide application rate 20 % per o.d.
wood. The temperature was raised within 4 hours 20 minutes to 75C, the cooking being maintained at 75C to 80C for 2 hours. The amount of shives was 12.6 ~, and the kappa number was 14.5. The alkaline peroxide treatment was con-ducted at 10 per cent stock during 1 hour. The NaOH applica-tion rate was 6 %, the amount of hydrogen peroxide being 8 %
(= 0.55 x the kappa number). The pH of the solution was 10.8. Analysis of the pulp gave: brightness 86.5 %, viscosi-ty 1000 cm3/g and the pulp yield 43.2 % tcalculated on the wood).

As example 6, but the temperature of the alkaline peroxide treatment was 60 C and the duration time 2 hours. Analysis of the pulp gave: brightness 86.5 ~, viscosity 1080 cm3/g and the pulp yield 43.5 ~ (calculated on the wood).

5S'7 As example 2, but the liquor-to-wood ratio was lowered and the amount of hydrogen peroxide was decreased. 50 g birch chips (dry matter 90 %) calculated o.d., was admixed with 200 ml formic acid. Water was not added at all, and the amount of 50 % hydrogen peroxide solution was decreased from 60 ml (in example 2) to 10 ml, which corresponds to 10 % peroxide of the dry weight of the chips. The liquor to wood ratio was thus 4:1 (instead of 8 1 ) o The reaction mix-ture was heated to 78C, maintained at said temperature for 3.5 hours. The kappa number of the pulp was 51.4, the viscosity 1170 cm3/g, the screened yield being 52 9 % and the amount of shieves 7.2 %.
The pulp obtained was bleached with an alkaline solution of hydrogen peroxide such that the calculated amount of hydrogen peroxide, 30.8 % of the pulp (16.3 % o.d. wood, i.e. 0.60 x the kappa number), was added in three portions. The tempera-ture during all stages was 80C~ the reaction time 1 hour, and the initial pH value 10.5. The stabilizer added compri-sed 0.2 % DTPA. The brightness of the pulp was 90.3.
The total consumption of hydrogen peroxide was in this example 10 -~ 16.3 %, i.e. 26.3 % of the o.d. chips.

50 g pine chips were refluxed in 250 ml of 100 % formic acid for three hours. The pretreatment liquor was removed, the peroxyformic acid cooking was subsequently carried out with a cooking liquor of 200 ml 100 % formic acid to which 10 %
hydrogen peroxide had been added. The temperature of the cooking was raised to 80C, the total cooking time being three hours. After the cooking the pulp was washed with water until the washing water was neutral. The kappa number was 31.1 at this stage, the viscosity was 1060 cm3/g, the brightness 19.5 and the yield 43.3. There were scarcely any shives in the pulp.
.

5~7 Test carried out on the pulp showed that it was easily bleached by means of an alkali hydrogen peroxide solution.

As 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 kappa number of the pulp was 62.2 %, the viscosity 1070 cm3/g, the screened yield was 45.8 % and the amount of shives 19.7.
The pulp obtained was bleached as described in example 8, the calculated amount of hydrogen peroxide, 28 ~ of the pulp (12.8 % o.d. wood, i.e. 0.45 x the kappa number), was also this time added in three portions. The brightness of the pulp was 87.0 %.
The total consumption of hydrogen peroxide in this example was 5 + 12.8 %, i.e. 17.8 % of the o.d. chips.

As example 6, but the birch chips were pretreated before the peroxyacid cooking by boiling them for 3 hours in 80 % for-mic acid under reflux conditions. The cooking liquor was re-moved, the peroxyformic acid cooking being subsequently car-ried out employing 85 % formic acid to which 5 % hydrogen peroxide of o.d. wood only had been added. The temperature was raised in 50 minutes to 80C and maintained at said temperature for 1 hour. After the cooking the pulp was was-hed to neutrality with the required amount of hot water.
The kappa number of the pulp was at this stage 14.7 and the viscosity 1180 cm3/g and the yield 42.3 %.
The pulp obtained was bleached with an alkaline solution of hydrogen peroxide such that the calculated amount of hydro-gen peroxide, 8 % of the pulp (3.4 % o.d. wood, i.e. 0.55 x the kappa number), was added in four portions: the duration of the first two stages was 1 hour, the duration of the two 1~

last ones was 2 hours. The temperature during all stages was 80Cr and the initial pH value lQ.7. The stabilizer added comprised 0.2 % DTPA and 0.5 % (calculated on o.d. wood) Mg sulfate. The brightness of the pulp obtained was 90.1 %, and the viscosity 1140 cm3/g.
The total consumption of hydrogen peroxide was in this example 5 + 3.4 %, i.e. &.4 % of the o.d. chips only.

50 g spruce chips (dry matter 92,6 %) calculated per o.d.
wood, were heated in a mixture of 200 ml 100 % formic acid, and 5 ml 50 % hydrogen peroxide. The total time of heating was 2 hours 45 minutes and the maximum temperature 75C.
The spent liquor was filtered off and the somewhat softened chips were refluxed for 3 hours in 250 ml of 100 % formic acid. The cooking li~uor was removed and the chips were defibred in formic acid. Subsequently, an other peroxyacid cooking was conducted, comprising as cooking liquor 200 ml of 100 % formic acid, to which 5 % of hydrogen peroxide had been added. The mixture was heated for 3 hours 30 minutes, the maximum temperature being 75C. ~fter the cooking, the pulp was first washed with formic acid and thereafter with water. The ~appa number of the pulp was 9.0, the ~iscosity 980, the brightness 35.1 and the screened yield 41.4 %. The amount of shieves was only 0.2 %.

The pulp obtained was bleached with an alkaline hydrogen peroxide solution such that the calculated amount of hydro-gen peroxide, 6 % of the pulp (2.5 % o.d. wood), was added in three portions. The duration of the first stage was 1 hour, of the second one 2 hours, and of the third one 3 hours. The temperature was 80 C. The stabilizers were analogous to those employed in example 11. The brightness of the pulp obtained was 90.5, and the viscosity 940.
The total consumption of hydrogen peroxide was in this example 10 + 2.5 ~, i.e. 12.5 % of the o.d. chips.

Claims (24)

1. Process for preparing bleached pulp from ligno-cellulosic raw material, which comprises A) defibering the cellulosic raw material by means of a cooking liquor containing as active agent organic peroxyacids, and B) bleaching the defibred pulp with an alkaline solution containing hydrogen peroxide, the pH of which is at least 10 in the beginning of the treatment, and which is pro-vided by adding to an alkali solution hydrogen peroxide in an amount which calculated as per cent of the dry weight of the matter coming to the treatment corresponds to from 0.20 to 0.80 times the kappa number of the pulp obtained from stage A of the process.

2. Process as claimed in claim 1, wherein the solution used in stage A is provided by adding to an organic carboxylic acid, the concentration of which is 40 to 100% by weight, at least 1% by weight of hydrogen peroxide calculated on oven dry (o.d.) wood.

3. Process as claimed in claim 1, wherein the liquor-to-wood ratio is maintained at a value in the range of 2:1 - 10:1.

4. Process as claimed in claim 2 or 3, wherein regenerated spent liquor from stage A is used as carboxylic acid.

5. Process as claimed in claim 2 or 3, wherein the regenerated spent liquor from stage A is used as carboxylic acid and is an azeotropic mixture of formic acid and water, the concentration of which is about 80 %, obtained by distilling the spent liquor obtained from stage A.

6. Process as claimed in claim 1, 2 or 3, wherein the raw material is pulped during stage A with peroxyacid cooking liquor in two separate stages and treated between said stages with the corresponding carboxylic acid at the boiling point thereof.

7. Process as claimed in claim 1, 2 or 3 wherein the raw material is pretreated before stage A with carboxylic acid.

8. Process as claimed in claim 1, 2 or 3 wherein the raw material is pretreated before stage A with spent liquor containing formic acid obtained from stage A.

9. Process as claimed in claim 1, 2 or 3 wherein prior to stage A the raw material is pretreated with the aqueous alkali solution used in stage B.

10. Process as claimed in claim 1, wherein the alkali solution used in stage B contains sodium hydroxide, sodium carbonate or a combination thereof.

11. Process as claimed in claim 1, wherein the calculated amount of peroxide in stage B is added in a plurality of por-tions, the bleaching liquor is removed after the termination of each stage, and the pulp is optionally washed after each stage.

12. Process as claimed in claim 1, 2 or 3, wherein the temperature of the cooking liquor at stage A is initially maintained at a higher temperature, whereupon the actual cooking is conducted at a lower temperature.

13. Process as claimed in claim 2, wherein in stage A citric acid and in stage B diethylenetriamine pentaacetic acid (DTPA) and/or magnesium sulfate are added as stabilizers for the peroxide.

14. Process as claimed in claim 1, wherein the organic peroxy-acid used in stage A is a peroxyacid derived from formic, acetic, propionic and butyric acid.

15. Process as claimed in claim 1, wherein in stage B the amount of hydrogen peroxide added to the alkali solution calculated as per cent of the dry weight of the matter coming to treatment corresponds to from 0.25 to 0.70 times the kappa number of the pulp obtained from stage A of the process.

16. Process as claimed in claim 1, wherein in stage B the amount of hydrogen peroxide added to the alkali solution calculated as per cent of the dry weight of the matter coming to treatment corresponds to from 0.45 to 0.65 times the kappa number of the pulp obtained from stage A of the process.

17. Process as claimed in claim 2, wherein the organic carboxylic acid is formic, acetic, propionic or butyric acid.

18. Process as claimed in claim 2, the concentration of the organic carboxylic acid is 70 to 100% by weight.

19. Process as claimed in claim 2, wherein the amount of hydrogen peroxide added to the organic carboxylic acid is about 5 to 20% by weight calculated on o.d. wood.

20. Process as claimed in claim 3, wherein the liquor-to-wood ratio is maintained at a value in the range of 4:1 - 8:1.

21. Process as claimed in claim 1, 2 or 3, wherein the raw material is pretreated before stage A with formic or acetic acid at a temperature corresponding to the boiling point of said acid.

22. Process as claimed in claim 1, 2 or 3, wherein prior to stage A the raw material is pretreated with the aqueous alkali solution used in stage B at a temperature of about 100°C.

23. Process as claimed in claim 11, wherein the calculated amount of peroxide in stage B is added in 3 to 6 portions.

24. Process as claimed in claim 1, 2 or 3, wherein the temperature of the cooking liquor at stage A is initially maintained at about 90°C, whereupon the actual cooking is conducted at about 70° to 75°C.