CA1287705C - Process for preparing pulp for paper making - Google Patents

Abstract

PROCESS FOR PREPARING PULP FOR PAPER MAKING
ABSTRACT OF THE DISCLOSURE
This invention relates to a process for producing pulp for paper making. This invention involves the chemical impregnation of wood cellulosic fragments such as chips with an alkaline aqueous liquor having at least one agent acting to provide hydrophilic groups and as an antioxidant capable of protecting the chips against oxidation and which develops hydrophilic groups during the cooking stage. The impregnated chips are cooked using saturated steam in the substantial absence of air at a high temperature and pressure to a K value of at least about 1000 to provide softened chips. The softened chips are refined to produce pulp suitable for paper making. According to a supplementary disclosure, in industrial or semi-industiral applications, the K
value can be substantially lower, in the range of 250 plus.

Description

PR~CESS FOR PREPARING PULP FOR PAPER MAR:[NG
This invention relates to a process for producing pulp for paper making.
In my copending Canadian Application 532,871 filed March 24, 1987 en-titled "Improved Prooess ~or Preparing Pulp for Paper MaXing", I described a process in which the energy saving advantages of explosive decompression are achieved together with good brightness, high yield and good fiber trength. This process involved the -thorough impregnation of wood fragments with an alkaline aqueous liquor including at least one agent acting to provide hydrophilic groups and as an antioxidant to impregnate enough of said agent throughout the wood fragments from acidic hydrolysis and oxidative degradation during subsequent cooling.
This was followed by steam cooking of the impregnated chips with saturated steam in the substantial absence of air at superatmospheric pressure and a temperature within the range of about 170 deg.C to 210 deg.C.
In accordance with the process described in Canadian Application 532,871 the steam cooking step was followed by explosive decompression. It has now been discovered that at high K values (K value or constant is cooking time in minutes mul-tiplied by cooking temperature in degrees Celsius) benefits are obtained even in the absence of explosive pressure release.
Although there is still an advantage in fiber properties if explosive pressure release is used, the improvement in such properties at high K values of about 1000 or more is significantly less than the improvement achi.eved at lower K
values such as 500-600.
In accordance with this invention therefore, 1) The wood fragments, having fibers suitable for paper making, such as chips, are in a form in which thorough chemical impregnation can be achieved in a reasonable time.

2) There is an initial thorough impregnation of the chips or other wood fragments by an alkaline aqueous liquor having at least one agent acting to produce hydrophilic groups and as an antioxidant which is capable of protecting the chips against oxidation and develops hydrophilic groups during the cooking 77~)5 stage. The same chemical may act as both an agent to pro~uce hydrophilic ~roups and as an antioxidant or these functions may be performed by separate chemicals. Preferably at the end of cooking the p~ should not be lower -than about 6.0, so that acids released during cooking will be neutralized. Preferably a swelling agent is also used in the case of high density wood.

3) The impregnated chips are cooked using saturated steam in the substan-tial absence of air at high temperature and pressure -to a K value of at least about 1000 to provide softened chips.
The -term "high temperature" means tempera-ture of about 170 deg.C
to about 210 deg.CO

4) The softened chips are preferably washed and then, without undue delay, and preferably immediately, refined to provide pulp.
The steps of this process will now be considered in greater detail.
The Wood Fraqments The starting material will normally be chips in which the fibers are of a length suitable for paper making. Shavings could also be used but sawdust would be undesirable except as a minor part of the total furnish as the fibers are partially cut.
The chips should also, as is well known, be suitable in the sense of bei.ng substantially free from bark and foreign matter.
It is desirable for the purposes of this invention that coarse chips be avoided as otherwise the subsequent impregnation may deposit chemicals only on the chip surface, unless impregnation is carried out for a very long time. ~nother problem with coarse chips is that cooking would not be complete. It is best to use shredded or thir. chips or ordinary chips having length 2.5 to 3.75 cm and width 1 to 2 cm and thickness 1 to 9 mm. Examples of chips that may he used include black spruce and industrial soEtwood chips which are 75% spruce, 20% fir and 5%
aspen. This process is applicable also to hardwoods, jack pine, larch and o-ther softwoods.
Impreqnation The purpose of impregnation is to protect the chips against oxidation during cooking and during transfer from the cooking ~2l537~
vessel to the refiner. It is also an objective to provide a positive increase in strength by deve].oping hydrophylic groups on the fiber surface during steam treatment. 'rhis will then provide additional sites for hydrogen bonding.
The preferred anti-oxidant i.s sodium sulphite Na2S03 which also ~orms hydrophilic groups, and whi.ch is ava.ilable at a low cost. It is used to provide a concentration of absorbed chemical of about 1 to 10%. Concentrations below 4% would be used where brightness protection is unimportant and high streng-th is not required. Where, however, brightness is important the sodium sulphite should be at least 4%. If physical properties are important these will be improved by using a concentration of at least 4% sodium sulphite and will be further improved as the concentra~tion is further increased towards 10%. The concentratipn of the solution is preferably about the same as percent of chemical to be absorbed where there are equal quantities of chips and liquor. For example, a ton of chips of.50% consistency mixed with one ton of 8% solution will result in 8% absorbed on the pulp. Of importance is thorough impregnation to distribute the antioxidant evenly rather than depositing it just on the surface. Other antioxidants that can be used are potassium sulphite or magnesium sulphite.
Ammonium sulphite could be used if cooking conditions are not severe or with a buffer. Complexing agents such as . ethylene 2S diamine tetracetic acid (EDTA), sodium diethylene triaminepentacetate (DTPA), sodium tripolyphosphate (~PF)and other complexing agents known in the art as bei.ng usable under alkaline conditions may be added to minimize the catalytic effect of metals such as iron on oxidative degradation.
It is desirable also to use a swelling agent to assist the antioxidant or hydrophilic agent in penetrating the wood and this contributes also to softening the chip. This is of particular value in the case of high density wood. Suitable swelling agents are sodium or potassium hydroxide and ammonium hydroxide which will contribute also to providing hydrophilic groups. Other swelling agents that can be used and which may be desirable as auxiliary swelling agents for high density wood are sodium - 4 ~

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carbonate, sodium bicarbonate, zinc chloride, sodium chloride, magnesium chloride, sodium bromide, calcium isocyanate, Schweitzer's solution, cupriethylenediamine (C.E.D) tetraethylammonium hydroxide, dimethyldibenzyl-ammonium hydroxide. The conoentra-tion of swelling agent and conditions of swelling must be controlled in such a way as to avoi.d any dissolution of the hollocellulose. Thus, the pexcentage of swelling agent in the impregnatlng solution will be in the range of about 1 to 4% depending on the agent and the conditions.
The impregnating solution must be alkaline and have enough free hydroxyl to be able to neutralize the liberated wood acids such as formic acid and acetic acid. Normally the starting pH is about 8 or higher and the final p~ after steam cooking should be at least 6 or higher. Sodium carbonate (Na2C03) or sodium bicarbonate (NaHC03) can conveniently be used to keep the pH on the alkaline side.
The time of impregnation at atmospheric pressure in holding tanks typically ranges from about 12 hours to 24 hours at a temperature of about 30 deg.C to 60 deg.C. Approximately equal weights of chips and of aqueous impregnating solution can be used. For industrial purposes, however, the time may be shor-tened to minutes or seconds by impregnating chips previously preheated by steam to temperatures varying from 85 deg.C to 95 deg.C and compressed with compression ratio 2 and higher (preferably between 4 and 6) with cool liquor solution. Under these conditions, penetration will be achieved in a shorter time, but penetration is what predominantly occurs. There is no significant cooking.
In the examples, 150 grams of chips were mixed in plastic bags with 150 g. of an aqueous solution of the specified concentration of the chemiaals indicated in the examples. The time of impregnation was 2~ hours and the temperature of impregnation was 60 deg. C. The foregoing is applicable only on a laboratory scale. In industry the impregnati.on time would be shortened as described above.
The temperature of cooking should be within the range of about t70 deg.C to 210 deg.C and preferably within the range 180-195 deg.C. These temperatures correspond wi-th a pressure of 7.9 atmospheres for 170 deg.C. and 15.5 atmospheres ~or 200 deg.C. It is these high pressures which make a very important contribu-tion to ensuring excellent penetration of the chips by the satura-ted steam.
The cooking may be preceded by steam flushing under low pressure steam at lOO deg.C for a short perlod such as one minute. This is a matter of convenience, in that with a ba-tch reactor the coo~ing vessel is initially open to the atmosphere, to eliminate air.
This air would be disadvantageous in that it would result in oxidation if it were trapped in the cooking vessel. Additional antioxidant may if desired be added at this stage. Steam flushing is desirable with a batch reactor but would not be necessary for a continuous reactor.
This preliminary treatment is then followed by cooking for about 5 to 7 minutes and preferably about 6 minutes.
As previously stated, the cooking is conducted at a high K
value where explosive decompression is not employed. A K value of about 1000 or more may be used.
Refining It is desirable immediately to refine the chips. Otherwise if the chips are stored some oxidation will occur with resultant loss of brightness. The rapidity with which this will occur depends on how much residual antioxidant is pxesent at that time and on the temperature of the chips and the extent of exposure to oxygen. Preferably, therefore, refining is carried out immediately so that it is unnecessary to incur the cost of excess antioxidant. In any event, undue delay should be avoided. Such delay is regarded as being undue if oxidation takes place to an extent that will materially affect brightness.
Refining the examples described below was conducted at a 10%
consistency level according to ~APPI standards using an atmospheric laboratory re:Einer. The refining energy reported is the industrial energy obtained by dividing the PFI energy by factor 3.5.
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that at K values in the range 500 - 600 steam cooking followed by explosion leads to about a 30-35% improvement in properties (burst, breaking length) when compared with steam cooking without explosion. On the other hand, when the steam cooking is performed at E~ values o~ about 1000 the positive effect of explosion is only about 5% or less when compared with steam cooked chips without explosion. This is shown by the example tabulated in TABLE 2 which shows a comparison of the paper properties (refined by a beater PF1) of steam cooked aspen pulps with and without explosive pressure release. In TABLE 2, the ~ value is 1330.
Bleachinq The process of this invention is particularly suitable for bleaching with hydrogen peroxide. The formula of chemicals used for bleaching may also include sodium hydroxide, a substance such as magnesium sulphite and a complexing agent such as sodium diethylene triaminepentacetate (DTPA). The improvement in brightness achieved at different concentrations is shown below in TABLE 3 . The data in TABLE 3 was ob-tained with pulps that had been subject to explosive pressure release but it is to be expected that similar results would be obtained in the absence of explosive pressure release but practicing the process described herein. It appears that up to about 4% hydrogen peroxide achieves progressive substantial improvements in brightness following which further additions are of questionable cost effectiveness.
TABLE 4 provides a further illustration of the effect of bleaching the products of the improved explosion process with hydrogen peroxides. Again it is to be expected that similar results will be applicable with the process of this invention.

CONDITIONS OF BLE~C~IING THE IMPROVED
EXPLOSION PULPS WIT~I PEROXIDE
PULP 8% Na2503 BLACK SPRUCE SPRUCE ASPEN

5 190 deg.C; 4 min FIR
DTPA 0.5 0.5 0.5 0.5 Mg~0q(%) 0,05 0.05 0.05 0.05 Na2SiO3 (%) 1.0 1.00 0.5 0.5 NaOH (%) 2.0 2.0 4.0 4.0 10 H202 (%) 4.0 4.0 4.0 4.0 (deg.C) TIME (MIN) 150 150 150 150 CONSISTENCY (%) 20 20 25 20 INITIAL 46.5 (52.5)* 49.3(55.3)* 52.4 70(68)*
FINAL 72.9 (78.9) 75.9 (81.9) 79.5 87(85) BRIGHTNESS
GAIN (%) 26.4 26.6 27.1 17(17) *(after defibration in the Sunds Defibrator Various other factors involving bleaching conditions have been investigated. It was found that under conditions similar to those of TABLE 3 increasing the concentration of sodium silicate improved the increase of brigh-tness up to about 3~ sodium silicate, following which it dropped off. The gain of brightness increased wlth sodium hydroxide concentration up to peak at about 4% NaOH. Increase of consistency progressively improved the gain of brightness within the range investigated, which was up to 30%.
Increasing the time improved the gain of brightness within the range investigated which was up to 4 hours.
The preferred bleaching conditions are 3-5% sodium silicate;
0 to 0.1% magnesium sulphate, time 1 hour to 4 hours; temperature 50 deg.C to 90 deg.C, consistency 10 to 35%. DTPA 0 to 0.5%.
These condi-tions should give a good aomprornise between cost and effecti.veness. The most important chemical additives are the hydrogen peroxide and the sodium hydroxide.

~Z~ 5 In order to protect brightness, stability and prevent reversion, the pulp should be washed, preferably with a solution of sodium metabisulphite (for example a 2~ solution) o.r a solu-tion of water saturated with sulphur dioxide. ~'hese solutions will provide sulphur dioxide which will reaat with and neutralize the excess of hydrogen peroxide.

377~
SUPPLEMENTAR~ DI SCIIOSURE
The principal disclosure was prepared on the hasis of laboratory experimentation. lt has now been found that on an industrial or semi-industrial se-tting involving press impregnation of cellulosic fragments such as wood ahips, cellulosic fragments as bagasse, flax ekc., continuous cooking and industrial refining, satisfactory paper properties are obtainable with much lower K constant values than originally expected.
The term "cellulosic fragments" includes wood chips bu-t also other cellulosic fragments such as bagasse or flax. Although tests carried out were conducted with wood fragments, those skilled in the art will appreciate that other cellulosic fragments could be processed under the same production concept.
In general -terms, the method which has now been ~ound to provide satisfactory results when applied on an industrial or semi-industrial scale can be defined as a process for making paper making pulp, comprising the steps of:
a) thoroughly impregnating cellulosic fragments having fibers suitable for papermaking with an alkaline aqueous liquor including at least one agent acting to provide hydrophilic groups and as an antioxidant to impregnate enough of said agent throughout the cellulosic fragments to protect the cellulosic fragments from acidic hydrolysis and oxidative degradation during subsequent cooking;
b) steam cooking the impregnated fragments with saturated steam in the substantial absence of air at superatmospheric pressure and a temperature within the range of about 170'C to 210-C and to a ~ value of more than about 250 to provide softened chips;
c) without undue delay that ~ould result in brightness loss, refining the softened and defibrated fragments to provide pulp.
Reference should be made briefly to some -terminology used throughout this disclosure.
First, reference is made to "explosion pulp". This term is o~
used to generally refer to the pulp produced by cooking in saturated steam at high pressure and high temperature from about 170 deg.C to about 210 deg.C. It may but does no-t necessarily have to be produced by explosive release of -the pressure in the reactor.
Another -term used in the disclosure is that of "slow release'~. This term is intended -to contrast the quick release, or "explosive" release of pressure in the reactor such as referred to at the outset or in my Canadian Patent No. 1,230,208 referred to above. The quick or explosive release of pressure usually takes place for up to about 3 seconds. The term "slow release", on the other hand refers to a gradual release of temperature which usually takes place for 1 to 2 minutes but may take place for a period as short as about 3.1 seconds and as long as 10 minutes or even more.
Another term used herein is "high pressure" and this refers to pressures prevailing in a reactor or digester during the cooking in saturated steam as set forth above. It is of the magnitude of about 7.4 atm for 170 deg.C up to 15.5 atm for 200 deg.C and higher for higher temperatures.
Finally, the term "high temperature" within the context of the present invention refers to about 170 deg.C. to about 210 deg.C.
The new finding is significant as it provides still further savings when utilizing the invention on an industrial or semi-industrial scale.
The advantages obtained with the present invention on industrial scale are apparent from the examples which are tabulated in TABLE 5. The conditions and results of explosive release pulp preparing method of a laboratory test or example are compared with those of semi-industrial trials of explosive release and of the explosion pulp wi-th slow pressure release (within one minute. interval) to go from high to atmospheric pressure.
The figures of TABLE 5 show that the resulting qualities of the aspen pulps of the laboratory and semi-industrial methods are virtually the same despite -the fact that the K-value, which is ~215~7~

important from the standpoint of energy consllmption of the process, is si~nificantly lower in the semi-indu~trial proces~.

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In o-ther words, TABLE 5 shows that labora-tory resul-ts obtained with the K value or constant 760 can be reproduced ln an actual semi-industrial setting using as low a K-constant as about 193.
The examples of T~BLE 5 ~urther show that, with other conditions belng equal, if the the K-value is about 200 (193.6 and 195 in the examples shown), when the pressure release is slow rather than a sudden, explosive pressure release, the quality of the resulting pulp is reduced.
~owever, when the K-value was increased, the results obtained in semi-industrial setup with other pulps were as is shown in TABLE 6 relating to tests in which a comparison is made of a method utilizing explosion with sudden pressure release with that of the "slow pressure release", both at a higher K-value of 585. It can be seen on comparing the quality parameters that at a sufficiently high value of K, in the example shown, 585, the sudden pressure release is not necessary.

____ ___________________________________________________________ Explosion with fast Explosi.on with slow pxessure .releasepressure release _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ . _ _ _ _ _ _ _ _ _ _ _, _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Na2SO3(%) 8 8 NaOH (%) 0-5 0 5 Solution/chips 3 3 Impregnation Press, 30 sec. Press, 30 sec.

15 Cooking Continuous StakeBatch Pilot Plant Technology 2 Reactor Reactor Time (min) 3 3 20 Temperature (deg.C) 195 195 K (deg.C x min) 585 585 Break length (km) 9.1 9.1 Tear (mN.m2/g) 6.6 6.5 Burst (kPa.m2/g) ~.8 ~.8 30 Yield (%) 89 87 Brightness (~) 55 56 _________________________________________________________________ TABLE 7 shows the result of other examples, wherein the tests were directed to establish the lower practical limit of the K value in semi-industrial or industrial conditions utilizing the ~2~ S
present inven-tion. ~lere it can be seen that reasonably close quality parameters are s-till obtained when the K value is about 250.

COMPARISON OF SEMI-INDUSTRIAL TESTS AT K V~L.UE 2~

Explosion with fast Explosion with slow pressure release pres 5 ure release ______~__________________________________________.___________~__ 10 Na2SO3 (%) 8 8 NaOH (%) 0 5 0.5 Solution/Chips 3 3 ImpregnationPress, 30 secPress, 30 sec CookingContinuous StakeBatch Pilot Plant Technology 2 Reactor Reactor Time (min) 1.25 1.25 Temperature (deg.C) 195 195 K (deg.C x mi.n) 244 244 Break length (km) 8.4 8.0 20 Tear (mN.m2/g) 6.3 6.2 Burst (kPa.m2/g) 5.1 4.9 Yield 90 89 Brightness (%) 57 57 ___________________________________________________ . ____________ It is believed that the lower limit of the K value for semi-industrial and industrial applications of the present invention is as low as 250 but below that value the ~uality parameters drop rather substantially, as shown in TABLE 5.
TABLES 5, 6 and 7 also show that the remaining conditions of the method are virtually the same as disclosed in the principal disclosure. This also applies to the selection of characteristics which are not shown in TABLES 5, 6 or 7 such as the type of cellulosic fragments or impregnation.

Claims (23)

1. A process for making paper making pulp comprising the step of (1) thoroughly impregnating cellulosic fragments having fibers suitable for paper making with an alkaline aqueous liquor including at least one agent acting to provide hydrophilic groups and as an antioxidant to impregnate enough of said agent throughout the cellulosic fragments to protect the cellulosic fragments from acidic hydrolysis and oxidative degradation during subsequent cooking;
(2) steam cooking the impregnated chips with saturated steam in the substantial absence of air at superatmospheric pressure and a temperature within the range of about 170 deg.C. to 210 deg.C. and to a K
value of more than about 1000 to provide softened chips, the K-value being equal to time of cooking in minutes multiplied by the cooking temperature in degrees Celsius;
(3) without undue delay that would result in brightness loss, refining the softened chips to provide pulp.

2. A process as in claim 1 in which the temperature of steam cooking is in the range 180 deg.C. to 195 deg.C.

3. A process as in claim 1, in which the aqueous liquor used for impregnating is at a pH of at least 7.5 and the final pH
following steam cooking is at least 6.

4. A process as in claims 1, 2 or 3, in which the cellulosic fragments are chips.

5. A process as in claims 1, 2 or 3, in which the cellulosic fragments are shredded chips.

6. A process as in claims 1, 2 or 3, in which the aqueous liquor used for impregnating includes a swelling agent.

7. A process as in claims 1, 2 or 3, in which the aqueous liquor used for impregnating includes a swelling agent selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate and ammonium hydroxide in an amount of 1 to 3% of the aqueous liquor.

8. A process as in claims 1, 2 or 3, in which the hydrophilic and antioxidant agent is sodium sulphite.

9. A process as in claims 1, 2 or 3, in which the hydrophilic and antoxidant agent is selected from the group consisting of sodium sulphite, potassium sulphite and magnesium sulphite in an amount of 1-10% absorbed by the cellulosic fragments.

10. A process as in claims 1, 2 or 3, in which the aqueous liquor comprises a complexing agent selected from the group consisting of ethylene diamine tetraacetic acid, sodium diethylene triaminepentacetate and sodium tripolyphosphate.

11. A process as in claims 1, 2 or 3, in which the time of cooking is in the range 5 to 7 minutes.

12. A process as in claims 1, 2 or 3, in which the time of cooking is about 6 minutes.

13. A process as in claim 1, in which the temperature of cooking is in the range 180 deg.C. to 195 deg.C. and the time of cooking is in the range 5 to 7 minutes.

14. A process as in claims 1, 2 or 3, in which the resultant pulp is bleached with hydrogen peroxide.

15. A process as in claims 1, 2 or 3, in which the resultant pulp is hardwood having a brightness of at least 60 without bleaching.

16. A process as in claims 1, 2 or 3, in which the resultant pulp is softwood having a brightness of at least 55 without bleaching.

17. A process as in claims 1, 2 or 3, in which the resultant pulp is hardwood bleached with less than 5% hydrogen peroxide to a brightness of at least 85.

18. A process as in claims 1, 2 or 3, in which the resultant pulp is softwood bleached with less than 5% hydrogen peroxide to a brightness of at least 80.

19. A process as in claims 1, 2 or 3, in which the resultant pulp is bleached using 3-5% hydrogen peroxide and 3-5 sodium hydroxide.

20. A process as in claims 1, 2 or 3, in which the resultant pulp is bleached using 3-5% hydrogen peroxide, 3-5%
sodium hydroxide, 0.5 to 3% sodium silicate 0 to 0.1% magnesium sulphate 0-1% diethylene, triamine pentacetate at a temperature of 50 deg.C. to 90 deg.C, time 1 hour to 4 hours and a consistency of 10 to 35%.

21. A process as in claims 1, 2 or 3, in which the resultant pulp is bleached using 3-5% hydrogen peroxide and 3-5%
sodium hydroxide, and in which the pulp is washed with a washing solution which will neutralize excess hydrogen peroxide to obtain a final pH of about 5.5.

22. A process as in claims 1, 2 or 3, in which the resultant pulp is bleached using 3-5% hydrogen peroxide and 3-5%
sodium hydroxide, and in which the pulp is washed with a washing solution which will neutralize excess hydrogen peroxide to obtain a final pH of about 5.5, and in which such washing solution comprises sulphur dioxide.

23. A process as in claims 1, 2 or 3 in which the hydrophilic and antioxidant agent is sodium sulphite in the amount of about 4-8% absorbed by the cellulosic fragments.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

SD24. A process for making paper making pulp, comprising the steps of:
a) thoroughly impregnating cellulosic fragments having fibers suitable for papermaking with an alkaline aqueous liquor including at least one agent acting to provide hydrophilic groups and as an antioxidant to impregnate enough of said agent throughout the cellulosic fragments to protect the cellulosic fragments from acidic hydrolysis and oxidative degradation during subsequent cooking;
b) steam cooking the impregnated fragments with saturated steam in the substantial absence of air at superatmospheric pressure and a temperature within the range of about 170°C to 210°C and to a K value of more than about SD250 to provide softened fragments, the K-value being equal to time of cooking in minutes multiplied by the cooking temperature in degrees Celsius;
c) without undue delay that would result in brightness loss, refining the softened and defibrated fragments to provide pulp.

SD25. The process of claim SD24, in which the temperature of steam cooking is in the range 180°C to 195°C.

SD26. The process of claim SD24, in which the aqueous liquor used for impregnating in which the aqueous liquor used for impregnating is at a pH of at least 7.5 and the final pH following steam cooking is at least 6.

SD27. The process of claim SD24, SD25 or SD26, in which the cellulosic fragments are wood chips.

SD28. The process of claim SD24, SD25 or SD26, in which the cellulosic fragments are shredded chips.

SD29. The process of claim SD24, SD25 or SD26, in which the aqueous liquor used for impregnating includes a swelling agent.

SD30. The process of claim SD24, SD25 or SD26, in which the aqueous liquor used for impregnating includes a swelling agent selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate and ammonium hydroxide in an amount of 1 to 3% of the aqueous liquor.

SD31. The process of claim SD24, SD25 or SD26, in which the aqueous liquor used for impregnating includes a swelling agent selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate and ammonium hydroxide in an amount of 1 to 3% of the aqueous liquor.

SD32. The process of claim SD24, SD25 or SD26, in which the hydrophilic and antioxidant agent is sodium sulphite.

SD33. The process of claim SD24, SD25 or SD26, in which the hydrophilic and antoxidant agent is selected from the group consisting of sodium sulphite, potassium sulohite and magnesium sulphite in an amount of 1-10% absorbed by the cellulosic fragments.

SD34. The process of claim SD24, SD25 or SD26, in which the aqueous liquor comprises a complexing agent selected from the group consisting of ethylene diamine tetraacetic acid, sodium diethylene triaminepentacetate and sodium tripolyphosphate.

SD35. The process of claim SD24, SD25 or SD26, in which the time of cooking is 1.SD25 to 9 minutes.

SD36. The process of claim SD24, SD25 or SD26, in which the time is about 2 minutes.

SD37. The process of claim SD24, SD25 or SD26, in which the temperature of cooking is in the range of 180 to 195 deg.C and the time of cooking is in the range of 1.5 to 9 minutes.

SD38. The process of claim SD24, SD25 or SD26, in which the resultant pulp is bleached with hydrogen peroxide.

SD39. The process of claim SD24, SD25 or SD26, in which the resultant pulp is hardwood having a brightness of at least 60 without bleaching.

SD40. The process of claim SD24, SD25 or SD26, in which the resultant pulp is softwood having a brightness of at least 55 without bleaching.

SD41. The process of claim SD24, SD25 or SD26, in which the resultant pulp is hardwood bleached with less than 5% hydrogen peroxide to a brightness of at least 85.

SD42. The process of claim SD24, SD25 or SD26, in which the resultant pulp is softwood bleached with less than 5% hydrogen peroxide to a brightness of at least 80.

SD43. The process of claim SD24, SD25 or SD26, in which the resultant pulp is bleached using 3-5% hydrogen peroxide and 3-5%
sodium hydroxide.

SD44. The process of claim SD24, SD25 or SD26, in which the resultant pulp is bleached using 3-5% hydrogen peroxide, 3-5%
sodium hydroxide, 0.5 to 3% sodium silicate 0 to 0.1% magnesium sulphate 0-1% diethylene, triamine pentacetate at a temperature of 50 deg.C. to 90 deg.C, for 1 to 4 hours and at a consistency of 10 to 35%.

SD45. The process of claim SD24, SD25 or SD26, in which the resultant pulp is bleached using 3-5% hydrogen peroxide and 3-5%
sodium hydroxide, and in which the pulp is washed with a washing solution which will neutralize excess hydrogen peroxide to obtain a final pH of about 5.5.

SD46. The process of claim SD24, SD25 or SD26, in which the resultant pulp is bleached using 3-5% hydrogen peroxide and 3-5%
sodium hydroxide, and in which the pulp is washed with a washing solution which will neutralize excess hydrogen peroxide to obtain i a final pH of about 5.5, and in which such washing solution comprises sulphur dioxide.

SD47. The process of claim SD24, SD25 or SD26, in which the hydrophilic and antioxidant agent is sodium sulphite in the amount of about 4-3% absorbed by the cellulosic fragments.