FI62362C - LIGNINAVLAEGSNING OCH BLEKNING AV LIGNOCELLULOSAMASSA MED OZON - Google Patents

Description

I-ra1 ADVERTISEMENT / 07ί9 JSSTa Β ί11) UTLÄGGNINGSSKRIFT θ26θΖ • g | pc (4S) ry ^ ^ (51) K ».lk? /Lnt.a.3 D 21 C 9/16 FINLAND — FINLAND pi) fi ^ iiaww —Γκ »βηι» Ν «ι 7Ti500 (22) HakamIspUvi - AnaSknlngtdig 11.05-77 (23) Alkupiivt — GlMghatsdag 11.05-77 (41) Tullut (ulktaakal - Blhrtt affancHg 03.12-77

Patent and register registered (44) Date of publication. -

Patent- och registrerttyrelaan '' AraOkan utlagd och utl.akrlftan publkermd 31.08 .82 (32) (33) (31) Pyydatty atuolkaui-Begird prtorket 02.06.76 16.07.76 usa (us) 692062, 705869 (71) International Paper Company, 220 East l * 2nd Street, New York, New York 10017, USA (72) Arthur W. Kempf, Warwick, New York, Richard B. Phillips, Tuxedo Park,

New York, USA (US) (7M Berggren Oy Ab (Lignin removal and bleaching of 5M lignocellulosic pulp with ozone)] - Lignin cellulose and leaching with lignocellulosic pulp

The present invention relates generally to a process for bleaching lignocellulosic pulps and removing lignin therefrom. More particularly, the invention relates to a process for bleaching lignocellulosic pulps and removing lignin therefrom with ozone.

In recent years, considerable efforts have been made in the pulp and paper industry to develop chlorine-free or less chlorine-containing bleaching processes. One achievement in this chain of efforts has been the development and implementation of general oxygen bleaching systems, particularly for the low density oxygen bleaching process described in U.S. Patent No. 3,832,276.

However, oxygen bleaching is only a partial solution to chlorine-free bleaching treatment, as oxygen bleaching itself is not capable of producing enough white and high-quality pulps. One way that has been investigated by several researchers is to use ozone as a bleaching agent either alone or after the oxygen bleaching step, as ozone 2 62362 is capable of bleaching pulps to very white and does not contain chlorine. Recent publications related to ozone bleaching of wood chips include Rothenberg et al. / Pin 58 (8) 1975: 1827; N. Soteland and K. Kringstad / Norsk Skogindustri (2) 1968: 1); R. B. Secrist and R.P. Singh'in / Pin 54 (4) 1971: 581 /; Z. Osawa and C. Schuerch / Tappi 46 (2) 1963: 79 / publications. U.S. Patent Nos. 396,325, 1,760,042, 1,957,937, 2,466,633, 3,049,394, 3,149,906, 3,318,657, 3,352,642, 3,451,888, 3,663,357, relating to ozone bleaching of fibrous materials. 3,806,404 and 3,829,357, and Canadian Patent Nos. 902,861, 966,604 and 970,111.

The above-mentioned publications and patents generally describe gaseous ozone bleaching of fibrous materials with a density of 15-75%. Reference sources indicate that the optimum conditions for ozone bleaching are in the density range of 30-60%. There are many benefits to working in this tightness range. These advantages include relatively short reaction times, from 1 minute to 60 minutes, and efficient utilization of ozone. There are also numerous disadvantages associated with these high-density processes for bleaching pulp with ozone. These include: (1) the need to use expensive process equipment, e.g., high-density presses, pulp grinders, and high-density storage pumps; (2) the risk of mass fires in these relatively dry conditions; and (3) the difficulty of degassing the pulp after ozone treatment, which can lead to serious problems with the operation of the scrubber.

In an attempt to avoid the difficulties associated with high-density ozone bleaching, several researchers have experimented with ozone bleaching in the context of low-density pulp. If it is possible to successfully use low density paper pulp in ozone bleaching, the above-mentioned difficulties are avoided.

Thus, Osawa and Schuerch (Tappi 46 (2) 1963: 79) compared the ozone bleaching of sulphate cellulose with a density of 1% and 50%, respectively, at neutral pH. They concluded that the efficiency of the ozonation is at its maximum when carried out in the gas phase in the case of fibers which are clearly above the fiber saturation point,

SRQ. 19-20% in density, and is much lower when the fibers are dissolved in water.

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Soteland and Kringstad / Norsk Skogindustri (2) 1968: 1 / confirmed the results of Osawa and Schuerch by treating mechanical pulps with 2.25% ozone (w / w) in oxygen at neutral pH. They found that "preliminary studies showed that ozone passes through a mechanical pulp slurry in water (at a concentration of, for example, 0.1-0.2%) without being used to a greater extent". They found that the addition of 10% (v / v) of a suitable organic solvent such as methyl or ethyl acetate, acetic acid or acetone to the mixture was necessary to catalyze the ozone reaction with the pulp. Similarly, Osawa and Schuerch found that the presence of nitromethane, or methyl acetate, catalyzes the ozone reaction with pulp.

It has now been completely unexpectedly found that, contrary to the results of Osawa et al. And Soteland et al. 7 and a temperature of 0-70 ° C, to react with a gaseous mixture selected from the group consisting of ozone / oxygen, ozone / air and combinations thereof. Ozone containing a gas mixture with an ozone content of 0.1-20% by weight of oxygen or air is blown in bubbles into the pulp slurry by stirring the mixture of pulp, water and gas at a rate of about 0.01-5.0 horsepower days / tonne of pulp. The above reaction conditions eliminate the need to use an organic solvent to catalyze the reaction according to the results of Osawa et al. And Soteland et al. The process of the present invention results in efficient utilization of ozone, high reaction rate, improved whiteness, and substantial removal of lignin from unbleached and partially bleached pulps.

Figure 1 is a logarithmic plot of the decrease in permanganate as a function of horsepower days / tonne of mass.

The lignocellulosic pulp fibers used can be either unbleached or partially bleached, for example after basic oxygen bleaching. If oxygen bleaching has previously been carried out in the presence of a base, it can be carried out in connection with either high-density or low-density pulp. An example of such a low density oxygen / base bleaching process is disclosed in U.S. Patent No. 3,832,276.

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Bleached or unbleached paper pulp can be either paper pulp or silk pulp produced by chemical, chemimechanical or mechanical pulp processes. Examples of such processes are the kraft paper process, the sulfite process, the neutral semi-chemical sulfite process, i.e. the grinding process. Specific lignocellulosic materials subject to said pulp processes may include hardwoods, softwoods, esparto papers, etc.

Although the bulk density of the process used may be 1% to about 10% by weight *, it has been found advisable to use a density of 1% to 5%, with a density of 3% being particularly preferred.

The pulp slurry is bleached in a suitable reactor equipped with mixing equipment. The reactor can be either pressurized or unpressurized and should be equipped to supply the ozone-containing mixture in the form of a finely divided gas jet.

The pulp slurry is acidified using either an acid, for example sulfuric acid, acetic acid, etc., or an effluent from an earlier or later bleaching step, such as chlorine dioxide, hydrogen peroxide, peracetic acid, sodium hypochlorite, etc. in connection with chlorination, at a pH between about 1 and about 7. However, a pH of about 3 or 5 has been found to be preferred. Although the bleaching and lignin removal process can be performed at a slurry temperature of about 0 ° C to about 70 ° C, it is recommended to operate at a temperature of about 20 ° C to about 30 ° C.

The ozone-containing gas stream is introduced into a reaction vessel containing acidified pulp slurry. The finely divided gas stream may contain about 0.1 to 20% by weight of ozone; most preferably, however, it contains about 2% to about 3% ozone. The ozone-containing gas stream is introduced into the pulp slurry through a sprayer, for example a porous plate or a sprayer ring, which contains a number of holes.

When using the process of the present invention, it has been found advantageous to use an ozone-containing gas stream having substantially all bubbles having a diameter of about 3.3 mm or less.

It is essential that the pulp slurry be agitated as it contacts and reacts with the ozone-containing gas stream. Stirring can be performed in a number of different ways, with mechanical stirring being most preferred, e.g. by means of a multi-blade pot mixer or a turbine mixer. In order to increase the lignin removal rate and whiteness of the present process, it is essential to consume about 0.01 to about 5.0 horsepower days of energy per ton of pulp. Mixing energies of about 0.1 to about 1.0 horsepower day / ton of pulp provide the best balance between bleaching result and power consumption.

The period for lignin removal and bleaching according to the process of the present invention is from about 1 minute to about 60 minutes, with the most preferred period being from about 10 minutes to about 20 minutes. During the reaction time, it is recommended to feed about 0.1 to about 5.0% by weight of ozone according to the oven-dried pulp slurry to the pulp slurry.

To achieve a final Elrepho whiteness of 80 or more when using hardwood pulp in accordance with the present invention, the bleaching sequence could include a first step in which oxygen is used in combination with a base in a low density unbleached pulp, i. less than 10% (U.S. Patent No. 3,832,276), followed by ozone bleaching in accordance with the present invention and finally a peroxide or peracetic acid step. To bleach softwood to a final Elrepho whiteness of 80 or more, the bleaching sequence could include oxygen bleaching as a first step, followed by ozone bleaching as a second step in the present process, followed by an extraction step with sodium hydroxide or water, a second ozone bleaching step and then peroxide -acid, chlorine dioxide or hypochlorite stage. In certain cases, a second extraction step followed by a P, Pa, D or H step may be recommended. Of course, according to the desired result, a fairly large number of bleaching steps containing the ozone process of the present invention could be used in combination with other bleaching chemicals in addition to the above-mentioned steps and chemicals.

By using the low density ozone bleaching process of the present invention - which does not require additives, protective equipment or catalysts as previously used - several advantages are achieved over the previously used high density ozone process. These are: (1) lower equipment costs; (2) increased efficiency and ease of oxygen recovery (because in a high-density reactor, the pulp fibers would be led from the reactor to the oxygen recovery system); (3) a minimum hazard for pulp pieces (because the low-density process operates at a high moisture content, whereas in the high-density process, the fibers have a very low moisture content); (4) improved control of the amount of whitening (because the low-density process is in no way dependent on the degree of rubbing, unlike high-density mass bleaching); and (5) the ability to perform the ozone bleaching step at a lower temperature than is possible with high density ozone bleaching.

To more clearly illustrate the nature of the present invention, the following examples are provided to illustrate the invention. It is to be understood, however, that this is done by way of example only, without the intention of limiting the scope of the invention or the scope of the appended claims.

Examples 1-3 160 grams of oven dried unbleached hardwood pulp prepared by a kraft paper process was placed in a 20 liter plexiglass reactor equipped with baffles and diluted with water to produce a pulp slurry having a density of 1%. The reactor contained a centrally mounted three-bladed propeller agitator operating at 1/30 horsepower at 1200 rpm. A nozzle made of sintered glass with a diameter of about 10 cm, equipped with a series of holes with a diameter of 70-100 microns, was centered on the bottom of the reactor. A combination of ozone and oxygen 3 was introduced into the pulp slurry, pH 7 and temperature 20 ° C, which was sprayed through the plate at a rate of 0.35 m / h. The gas mixture contained 2.5% by weight of ozone in oxygen. The reaction took place over certain periods of time according to the ozone uses shown in Table I.

7 623 ^ 2 TABLE I Unbleached control 1 23

Ozone use,% - 1.0 3.0 5.0

Permanganate, amount 9.5 6.9 3.8 2.6

Viscosity, cp 31.8 19.0 15.0 10.1

Whiteness% (EI) 33.4 41.1 54.3 65.6

Time, min, - 4.6 15.0 24.4 HV days / tonne - 0.178 0.583 0.949 (mass)

The results in Table I indicate that the process of the present invention can be used over a wide range of ozone uses. In Example 3, substantial lignin removal was achieved after only a reaction time of 24.4 minutes.

Examples 4-7

The procedure of Examples 1-3 was repeated using a different hardwood sulphate cellulose pulp. The only other differences in conditions included the use of ozone in an amount of 3.5% by weight and a total gas flow rate of 0.11 m / h. The reaction was carried out for certain periods of time in connection with the amounts of ozone in Table II.

TABLE II Unbleached control

Ozone use,% - 0.2 0.6 0.8 1.0

Permanganate, amount 9.7 8.9 7.8 6.7 6.4

Viscosity, cp 23.8 23.6 20.8 15.0 14.1

Whiteness% (EI) 30.6 35.4 38.3 42.2 41.6

Time, min. - 3,5 10,4 15,1 17,0 HV days / tonne (mass) - 0,136 0,404 0,587 0,660

The results presented in Table II show that there is a linear relationship between lignin removal and ozone use. The data presented in Table II also indicate the applicability of the present invention with ozone use of 1% and below.

8 6P.3 6 2

Examples 8-12

The procedure used in Examples 1-3 was repeated using the same hardwood sulphate pulp. The density of the pulp was 1% and the temperature of the pulp slurry was 20 ° C. The ozone content was 2.6% by weight of oxygen. 1% ozone by weight of pulp was applied for 4.5 minutes at a flow rate of 0.35 m / h. A spray plate with holes 25-50 microns in size was in use. Each of the experiments contained in the examples was performed at a pH according to Table III below.

TABLE III

Bleached control 8 9 10 11 12 pH - 3.0 4.9 7.1 9.1 11.9

Permanganate, 9/5 5.1 6f2 73 7fl 8.7

Viscosity, cp 31.8 16.6 18.5 19.9 23.7 28.7

Whiteness% (EI) 33.4 55.1 47.7 41.8 41.9 36.8 HV days / ton _ 0.178 0.178 0.178 0.178 0.178 (mass) '' ''

The results shown in Table III indicate that although the process of the present invention can be used with both acidic and basic pH values, the most suitable range for the pH number to achieve maximum lignin removal and whiteness is between about 3 and about 5.

Examples 13 to 14

Sulfate pulp made from hardwood, initially bleached with oxygen in the presence of base at a density of 4.5%, was bleached mainly according to the method used in connection with Examples 1-3. However, instead of a propeller agitator, a turbine-bladed agitator operating at 1/30 horsepower at 540 rpm was used. The temperature of the pulp slurry was 20 ° C. The ozone content was 3.5% by weight of oxygen and the flow rate of the combination of oxygen and ozone was 0.11 m 2 / h. A spray plate with a sleigh size of 25-50 microns was in use. The reaction was performed at mass densities of 1% and 2%, as shown in Table IV below.

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TABLE IV

Unbleached Oxygen bleach control 13 14

Ozone use% _ _ 1010 by mass ''

Mass density - - 1.0 2.0 pH - - 3.03.2

Permanganate, amount 9.5 6.1 2.4 2.7

Viscosity, cp 31.8 18.3 13.3 13.4

Whiteness% (EI) 33.4 46.2 72.3 66.4

Time, min - - 17.7 37.0 HP days / tonne (mass) - - 2.32 2.32

The results in Table IV show that significant lignin removal and bleaching occurs using the process of the present invention after the pulp has first been bleached with oxygen.

Examples 15-18

The procedure used in Examples 1-3 was repeated using the same hardwood sulphate pulp. The density of the pulp was 1%, the slurry temperature was 20 ° C and the pH was 7. Ozone was fed for 14.0 minutes at a flow rate of 0.35 m-Vh using a spray plate with a hole size of 70-100 microns. The ozone content was 2.6% by weight of oxygen and 3% ozone was used according to the weight of the oven-dried pulp.

TABLE V Unbleached control

Ozone use in% by mass - 3.0 3.0 3.0 3.0

Permanganate, amount 9.5 7.7 5.4 4.4 4.0

Viscosity, cp 31.8 21.9 16.1 15.6 11.53

Whiteness% (EI) 33.4 39.1 46.0 47.6 57.5

Permanganate, reduction,% - 18.9 43.2 53.7 57.9 HV days / tonne (mass) - 0.010 0.108 0.544 1.837

Mixing, rpm - 300 700 1200 1800

From Table V and Figure 1, it is apparent that satisfactory ozone bleaching can be achieved using 0.01 to 1.8 HV days / pulp ton of pulp blending energy. The drawing indicates a decrease in the amount of permanganate between 1 and 2 HV days / tonne of mass. Thus, mixing energies greater than 1 HV day / ton of pulp provide only a small additional benefit. These results indicate that the ozone treatment is highly dependent on the appropriate mixing of the mixture of gas, liquid, and pulp for close contact with ozone at the reaction sites of the pulp fibers.

Example 19

The procedure used in Examples 1-3 was repeated using hardwood sulfate cellulose pulp having the following physical properties: permanganate content 9.5, viscosity 31.8, cp and brightness (E1) 33.4. The density of the pulp was 1%, the temperature of the pulp slurry was 20 ° C and the pH of the slurry was 3. The ozone content was 3.3% by weight of oxygen. 1% ozone, measured by weight of the pulp, was fed for 4.5 minutes at a low flow rate of 0.35 m 2 / h with an agitation of 1200 rpm (corresponding to about 0.178 horsepower days / ton of pulp). 62% of the ozone fed was consumed. A spray shrink plate with a hole size of 25-50 microns was in use.

The whiteness of the pulp increased to 55.1 points (E1), representing an increase of 64.9%, and the amount of permanganate decreased to 5.1, representing a decrease of 46.3%.

Example 20

Unbleached hardwood sulphate cellulose pulp slurry with a density of 4.5% by weight was initially bleached with oxygen in the presence of a base. The pulp slurry, which had a density of 1% by weight and a pH of 3, was then bleached with a mixture of ozone and oxygen passing through a spray plate with holes of 25 to 50 microns at a rate of 0.062 m 2 / h. The slurry was agitated with a turbine-blade mixer at a power of 3.26 HV days / ton of pulp. After consuming 1.03% ozone, the pulp with a whiteness of 77.2 (E1) was bleached with 1.1% peracetic acid. During the peracetic acid bleaching step, the pH was initially 8.0, the density 10% and the temperature 70 ° C. This bleaching step lasted for four hours. The bleached pulp from this step was then treated with aqueous sulfuric acid at 50 ° C, pH 4.0 for 30 minutes. The final whiteness of the pulp was 87.9 (El).

This example, as well as the following Examples 21, 22 and 23

11 C Γ · 7 f O

Between each individual bleaching step contained in O: * - the pulp was washed with deionized water until the pH of the filtrate was neutral.

The following Table VI provides relevant information on the properties of the pulp at different stages during bleaching.

TABLE VI

Pulp Unbleached Oxygen- Ozone- (^ OjPa Feature Kraft Paper Coating Bleaching Bleaching

Permanganate, 9.7 6.1 2.3 2.3

Viscosity, cp 23.8 16.7 12.93 12.38

Whiteness, (EI) 30.6 45.1 77.2 87.9

Example 21

An unbleached softwood pulp slurry having a density of 4.5% by weight was initially bleached with oxygen in the presence of a base. The pulp slurry, which had a density of 1% by weight and a pH of 3, was then bleached with a mixture of ozone and oxygen passing through a spray plate with holes of 25 to 50 microns at a rate of

O

0.062 m / h. The slurry was agitated with a turbine-blade mixer at a power of 3.26 HV days / ton of pulp. After consuming 0.93% ozone, the mass with a whiteness of 55 (EI) was extracted with sodium hydroxide. The density of the slurry during the extraction was 10% and the pH was 11. The extraction was performed for 90 minutes at 50 ° C.

The extracted mass with a density of 10% was then treated with 2% peracetic acid for 4 hours at a slurry temperature of 70 ° C at pH 8. The peracetic acid step was followed by a second basic extraction step completely identical to the basic extraction step described.

Finally, the slurry was treated with 1.2% peracetic acid under the same conditions as in the peracetic acid step described. The mass was then treated with sulfuric acid for 30 minutes. During this treatment, the pulp slurry had a density of 6%, a pH of 4 and the treatment was performed at a temperature of 50 ° C. The final whiteness of the pulp was 83.6% Elrepho whiteness.

12 6 2 3 6 2

The following Table VII shows the properties of the pulp at different stages of the bleaching process.

TABLE VII

Pulp Owi- Unbleached Oxygen- Ozone- C ^ O ^ EPaEPa femininity kraft paper bleaching bleaching

Permanganate, 4 levels

Viscosity, cp 34.6 23.6 14.40 10.9

Whiteness,% (EI) 24.6 31.6 55.0 83.6

Example 22

Unbleached hardwood sulphate cellulose pulp with a density of 4.5% by weight was initially bleached with oxygen in the presence of a base. The pulp slurry, which has a density of 2% by weight and a pH of 7, was then bleached with a mixture of ozone and oxygen passing through a spray plate with holes of 70-100 microns at a rate of 0.35 3 m / h. The amount of ozone used was 0.75%, measured by weight of the oven-dried pulp. The slurry was agitated with a three-bladed propeller mixer at a rate of 1 HV day / ton of pulp. The mass was then extracted with hot water. The density of the slurry during the extraction with hot water was 6%. The extraction was performed for two hours at 49 ° C.

The extraction step was followed by a second ozone step similar to the ozone step described above.

The extracted mass having a density of 10% by weight was then treated with 1% hydrogen peroxide and 2% sodium silicate for 5 hours at a slurry temperature of 70 ° C and a pH of 10.5.

The peroxide bleached pulp was then extracted with sodium hydroxide for 2 hours at 49 ° C. The pulp had a density of 10% and a pH of 11.0.

Thereafter, using a pulp concentration of 10% and a temperature of 70 ° C, the pulp slurry was bleached for five hours with 0.5% hydrogen peroxide in the pulp.

The pulp slurry was then treated with sulfuric acid for one hour 6 2 3 6 2 13. The density of the pulp slurry during this treatment was 3% and the pH 4. The final whiteness of the pulp was 82.4% Elrepho.

Table VIII below provides information on the properties of the pulp at different stages of the bleaching process.

TABLE VIII

Pulp own- Unbleached Oxygen- Ozone- Ο-Ο ^ ΕΟ, ΡΕΡ femininity kraft paper rejection bleaching ,,.

_ _ _ _ _ bleaching

Permanganate, amount 8.3 5.0 2.3

Viscosity, cp 23.3 15.4 N.D. 11.2

Whiteness% (EI) 33.3 48.7 62.7 82.4

Example 23

Unbleached hardwood sulphate cellulose pulp slurry with a density of 4.5% by weight was initially bleached with oxygen in the presence of a base. The pulp slurry with a density of 1% by weight and a pH of 3 was then bleached with a mixture of ozone and oxygen passing through a spray plate with holes of 25-50 microns at a rate of 0.11 m3 / h. The amount of ozone used was 0.50%, measured by weight of the oven-dried pulp. The slurry was agitated with a turbine-blade mixer at a rate of 2.02 HV days / ton of pulp. The mass was then extracted with sodium hydroxide. The density of the slurry during base leaching was 10% by weight and the pH was 11.8. The extraction was performed for 90 minutes at 50 ° C.

The extraction step was followed by a second ozone step, which was similar to the ozone step described.

The pulp was then bleached with 0.6% chlorine dioxide, which was used at a pulp density of 11% by weight. This step was performed for two hours at 77 ° C.

Table IX below provides information on the properties of the pulp during the different stages of the bleaching process.

6 2 3 6 2 14

TABLE IX

Pulp Owi- Unbleached Oxygen- Ozone- O2O3EO D

femininity kraft paper reed bleaching bleach

Permanganate, amount 9.5 6.15 2.3 N.D.

Viscosity, cp 23.4 17.14 11.9 14.1

Whiteness% (EI) 33.4 42.7 73.9 81.9

The foregoing terms and expressions have been used in an illustrative but non-limiting sense, and the use of such terms and expressions is not intended to exclude any features or portions thereof, but it is to be understood that various variations are possible within the scope of the present invention.

Claims (9)

15 62 3 6 2 A process for removing lignin from a lignocellulosic pulp and bleaching the pulp, wherein the pulp having a consistency of 1-100% is treated under acidic conditions with a gas mixture containing ozone, characterized in that the lignocellulosic pulp is an aqueous slurry having a consistency of about 1-10% by weight , is reacted at a temperature between 0 ° C and about 70 ° C with a gas mixture consisting of ozone / oxygen, ozone / air or a mixture thereof and having an ozone content of about 0.1 to 20% by weight based on oxygen or air, while the pulp is agitated at a rate of about 0.1-5.0 horsepower / tonne of pulp. A method according to claim 1, characterized in that the gas mixture is in the form of bubbles and essentially all bubbles have a diameter of about 3.3 mm or less. Process according to Claim 1 or 2, characterized in that the ozone content is from about 2.0% by weight to about 3.0% by weight, based on oxygen or air. Process according to any one of the preceding claims, characterized in that about 0.1% to about 5.0% of ozone, based on the weight of the oven-dried pulp, is fed to the pulp stock. Process according to any one of the preceding claims, characterized in that the reaction time is about 1 minute to about 60 minutes. Process according to any one of the preceding claims, characterized in that the agitation power is about 0.1 to 1.0 horsepower day / tonne of lignocellulose pulp. Process according to any one of the preceding claims, characterized in that the consistency of the pulp is from about 2% to about 3% by weight of the oven-dried pulp. Process according to any one of the preceding claims, characterized in that the temperature of the stock is about 20 to 30 ° C. 16 s O 7 C O O / - vJ C Z. Process according to any one of the preceding claims, characterized in that the pH of the pulp stock is about 3-5.