RU2401350C2 - Method for production of chemical-mechanical wood mass - Google Patents

Abstract

FIELD: textile industry. ^ SUBSTANCE: wood mass is deaerated. For this purpose it is steamed at the temperature of 80-100C for removal of air from wood chips. Then steamed chips are impregnated with alkaline impregnating solution. At the same time at the stage of impregnation temperature of impregnating solution is maintained below temperature of steaming. Under these conditions alkali penetrates into wood chips. Produced mass of wood chips is ground down to desired freeness. Impregnation is carried out at excessive pressure. ^ EFFECT: reduced amount of alkali used for impregnation, increased light scattering of mass without increase in content of non-ground fibres and increased specific volume of mass. ^ 14 cl, 1 dwg, 4 tbl, 2 ex

Description

The present invention relates to a method for producing chemical-mechanical pulp from wood raw materials containing wood chips, in accordance with the restrictive part of paragraph 1 of the claims.

In accordance with this method, wood chips containing raw materials are brought into contact with an alkaline impregnating solution under conditions under which the solution penetrates the wood chips, impregnating it, and the wood chips impregnated with the impregnating solution are milled to produce mechanical wood pulp.

The present invention also relates to chemical-mechanical pulp in accordance with paragraph 14 of the claims.

Chemical-mechanical pulp is obtained by grinding wood chips treated with chemical reagents, usually an alkaline solution, to the desired cadness. Chemical treatment simplifies grinding and improves the properties of mechanical pulp. During chemical cooking, wood chips are also treated with alkaline solutions to facilitate fiber separation.

To obtain both mechanical and chemical wood pulp, it is especially important that the chemical reagents are absorbed by the mass well and evenly. Uneven absorption leads to a high content of unmilled fibers in the wood pulp, which, in turn, reduces the strength potential.

Two mechanisms, namely penetration (penetration) and diffusion, ensure the absorption of wood and liquid chemicals. Penetration occurs as a result of the pressure difference, and diffusion, respectively, as a result of the difference in concentrations. At the beginning of the impregnation stage, the difference between the pressure prevailing inside the wood chips and, accordingly, the pressure outside is the factor that triggers the penetration process until the pressure is equalized. When the whole wood chips are filled with liquid, diffusion enters into action, as a means by which chemical reagents move into the wood chips and the reaction products from it.

There are two types of penetration: natural and forced. With natural penetration, the pressure difference is created by capillary forces, while with forced penetration, the pressure difference is created by external pressure or by creating a vacuum inside the wood chips.

Alkaline impregnating solutions expand the walls of the fibers and, therefore, narrow the capillaries. This reduces longitudinal penetration. However, at the same time, alkaline solutions also dissolve the fiber components, as a result of which penetration is accelerated.

Diffusion is much slower than penetration. An increase in temperature increases the diffusion rate, but it is known that the reaction rate increases much faster than the diffusion rate. So, according to literature, for example, during the sulfate cooking process, calculations using diffusion coefficients and activation energy show that the diffusion rate at a temperature of 170 ° C is approximately 4 times higher than at a temperature of 100 ° C. At the same time, the reaction rate increases by a factor of 900 (Talton, J. The diffusion of sodium hydroxide in wood at high pH as a function of temperature and degree of pulping, M.Sc. Thesis, North Carolina State University, 1986, 45 s).

With an increase in the reaction rate, alkali consumption also increases.

The amount of alkali and the size of wood chips, in turn, affect the diffusion. In order to avoid the formation of more waste, with an increase in the thickness of wood chips, the amount of alkali should increase. As the amount of alkali increases, the alkaline gradient between wood chips and the surrounding solution also increases (with the same liquid / wood ratio), and thus the diffusion rate increases.

It should also be noted that the addition of alkali for impregnation is the most important factor affecting the yield of chemical-mechanical pulp (HMM). The loss of fiber mainly occurs as a result of the destruction of acetic acid, as well as the dissolution of lignin and acidic polysaccharides, all of which are caused by exposure to alkali. The addition of alkali has a greater effect on deciduous wood than on conifers, and especially affects aspen wood. If the amount of alkali (for example, the amount of NaOH) increases from 0.5% to 3%, the yield from aspen wood decreases linearly from approximately 95% to 89% in a typical process.

Part of the yield loss occurs already during the impregnation process, and part only at the next grinding stage at high temperature.

In practice, to ensure maximum impregnation, a large amount of alkali should be used. Penetration of the alkaline solution into the wood chips is slow, and the alkali tends to be absorbed by the outer parts of the wood chips, resulting in an insufficient amount of alkali for the inside of the wood chips. At the grinding stage, this is expressed in a higher content of unmilled fibers.

Alkali has a negative effect on the light-scattering properties of chemical-mechanical pulp. The light-scattering properties deteriorate with an increase in the amount of alkali, and with the same degree of grinding, light scattering is always less with a larger amount of alkali. In addition, whitening further reduces light scattering. To produce high-quality printing paper, good light-scattering properties and whiteness values of wood pulp are required. You can get coniferous HMM with a high degree of whiteness, even up to 88% according to the international classifier of standards (ISO).

The present invention is the elimination of at least some of the disadvantages associated with the known technology, and the proposal of a new method for producing chemical-mechanical wood pulp.

The present invention is based on the fact that upon receipt of a chemical-mechanical pulp, the impregnation of wood chips is carried out at an overpressure.

Richardson and LeMahieu disclosed a method for producing high-quality wood pulp, whereby aspen chips are impregnated before grinding at a temperature of approximately 75 ° C and a pressure of 4 bar using a mixture of sodium hydroxide and sodium sulfite (Tappi 1965 (48), no 6, p. 344 -346). In accordance with this article, an increase in alkali concentration can increase the strength of wood pulp, and, at the same time, reduce energy consumption. However, a disadvantage of the known solution is that the addition of alkali impairs the whiteness of the wood pulp.

In connection with the present invention, it was unexpectedly found that by combining effective chip deaeration, preferably by steaming, and pressure impregnation, it is possible to increase the efficiency of impregnation and significantly reduce the amount of alkali. When compared with conventional impregnation without applying pressure at the same degree of grinding, a significantly lower amount of alkali can be used to achieve the same or even lower content of unmilled fibers, and at the same time, the light scattering of the mass is increased.

The method in accordance with the present invention is carried out in equipment where the following installations are arranged in series: installation of chip deaeration, installation of impregnation of chips and installation of grinding chips, the installation of impregnation contains a closed vessel, inside which the impregnation can be carried out at an overpressure.

More specifically, the method in accordance with the present invention is mainly characterized by the features contained in the characterizing part of claim 1.

Wood pulp in accordance with the present invention is characterized by the features contained in the characterizing part of paragraph 14 of the claims.

The present invention provides significant advantages. Thus, preliminary tests have shown that the use of pressure impregnation can reduce the amount of alkali by 50% or more. In addition, the present invention improves the light-scattering properties of wood pulp: during testing, light scattering unexpectedly turned out to be higher than that of thermomechanical wood pulp (TMM) (alkali content 0%). It should also be noted that it became possible to increase light scattering without increasing the content of unmilled fibers. With the same degree of grinding, the specific volume also increases.

In laboratory tests, pressure impregnation provided a penetration rate of more than 95%. For the control samples used in the tests, the maximum degree of penetration was 63-74%. During the control tests, it was also possible to reduce the alkali concentration from the usual level of 0.8-1.2% / Adt (Adt is a ton of air-dry mass), even to the level of 0.25% / Adt without increasing the content of unsplit fibers. For all experimental samples, the light scattering was clearly higher than for the control sample.

Pressure impregnation allows to improve the quality of chemical-mechanical refiner wood pulp for various end use.

Below the present invention will be discussed in more detail using a detailed explanation and with reference to examples of application below.

The accompanying drawing shows in a simplified form the equipment used in the method in accordance with the present invention.

As indicated above, the method in accordance with the present invention includes the following three steps:

- firstly, the removal of at least most of the air present in the wood raw materials containing wood chips (i.e. deaeration of wood chips);

- secondly, the impregnation of the raw material thus obtained with an alkaline solution at excess pressure to effectively absorb alkali with wood chips;

- and, thirdly, the grinding of treated wood chips to achieve pre-set values sagasti.

As can be seen from the above, wood pulp is obtained by the chemical-mechanical method. In accordance with the present invention, the production of chemical-mechanical pulp means essentially a method that includes both chemical and mechanical defibration, as described above. A method of producing a chemical-mechanical pulp and a method for producing a chemical-thermomechanical pulp are chemical-mechanical methods. According to the chemical-mechanical method (HMS), wood raw materials are processed in a refiner at normal atmospheric pressure, while in the chemical-thermomechanical method (HTMS), wood raw materials are processed in a refiner under pressure. Since large quantities of chemicals are used when using HMS, its yield is usually less (less than 90%) than when using HMS. In both cases, the chemical treatment of wood is usually carried out using sodium sulfite (sulfonation), while hardwood can also be treated with sodium hydroxide. In this case, the usual amount of chemicals in the HTMS is approximately 0-4% sodium sulfite and 1-7% sodium hydroxide, and the temperature is approximately 60-120 ° C. In CMS, in contrast, the amount of chemicals is 10-15% sodium sulfite and / or 4-8% sodium hydroxide (based on dry wood), and the temperature is 130-160 ° C and 50-100 ° C, respectively.

According to the chemical-mechanical method, the chips can also be impregnated with an alkaline peroxide solution. The amount of peroxide is generally 0.1-10% (by weight of dry wood pulp), usually about 0.5-5%. The amount of alkali supplied, such as sodium hydroxide, is about the same, i.e. about 1-10% by weight.

The present invention relates generally to a chemical-thermomechanical method, in which the chips coming from the impregnation are defibrated using a method for producing refined mechanical pulp under pressure.

The starting material for the method in accordance with the present invention is wood chips containing material from soft or hard wood. In particular, deciduous chips obtained from birch (mainly wood from the genus Betula (birch) or wood from the genus Populus (poplar) or a mixture thereof are used. Examples of suitable wood species from the genus Betula are B. pendula ( drooping birch) and B. pubescens (fluffy birch), and examples of tree species from the genus Populus are mainly the following: R. tremula (common aspen), P. tremuloides (American aspen), P. balsamea, P. balsamifera (balsam poplar ), R. trichocarpa (poplar hairy), P. heterophylla (poplar variegated), P. d eltoides (deltoid poplar) and P. grandidentata (large-toothed poplar). Aspen (European aspen, P. tremula; American aspen, P. tremuloides), aspen from different species, hybrid aspen (e.g. P. tremula × tremuloides , P. tremula × tremula, P. deltoides × trichocarpa, P. trichocarpa x deltoides, P. deltoides x nigra, P. maximowiczii × trichocarpa) and other species created by genetic technology, along with poplars are considered the most preferred.

In addition to the wood species of the genera Betula and Polulus, other hardwoods, such as eucalyptus and a mixture of tropical hardwoods, can be used as raw materials. Among coniferous trees, mention should be made of spruce (Picea abies) and pine (Pinus silvestris), as well as other species belonging to the genera Picea and Pinus.

In accordance with one application example, the resulting chemical-mechanical pulp contains up to 100% softwood fibers. However, the present invention allows to obtain a chemical-mechanical pulp consisting of a mixture of hard and soft wood fibers and containing at least 5% softwood fibers, for example, it can contain 50-99% hardwood fibers and 1-50% softwood fibers . The use of softwood fibers, especially spruce fibers, allows to increase the specific volume, strength characteristics and hardness of wood pulp.

The size of the wood chips of wood raw materials is generally about 20-50 mm × 1-10 mm, usually about 35-40 mm × 3-5 mm.

First, air is removed from the wood chips as much as possible. It is generally desirable to remove at least 70%, in particular approximately 80-100% of the air contained in the chips. Usually the air is in a gaseous state. As can be seen from the attached process flow diagram, this deaeration can be carried out by steaming the chips in the evaporator 1. In accordance with the method, the technological scheme of which is shown in the drawing, the raw material chips are fed through a screw conveyor 2 into the steaming hopper 1, into which steam is supplied or from one feed nozzle or, as shown in the drawing, from several nozzles 3a-3c for uniform processing of wood chips with steam in the hopper.

The purpose of the steaming is to remove air from the wood chips. At the same time, steam remains in wood chips.

The steaming can be carried out, for example, in the continuous evaporator 1 shown in the drawing, in which the wood chips move through the steaming bin 1, where it is in contact with saturated or close to saturated steam for about 0.5-20 minutes, in particular approximately 1-10 minutes. Steaming can be carried out at overpressure, but usually steaming at normal atmospheric pressure fully meets the requirements. Elevated temperatures are often used, for example, approximately 50-100 ° C, in particular approximately 80-100 ° C, depending on the degree of saturation of the vapor used.

Instead of steaming, deaeration can be carried out at low pressure / in vacuum, or the efficiency of steaming can be improved by vacuum treatment.

The treated chips are removed from the steaming hopper through the outlet 4, after which the chips are most preferably compressed in the screw press 5. After this step, usually at least 95%, preferably at least 98% of the air is removed and, at the same time, also removed part of the steam.

For the sake of completeness, it should be mentioned that steaming chips is used both to produce sulfate pulp and to obtain chemical-mechanical refiner wood pulp. However, before the advent of the present invention, it was never proposed to use a combination of steaming wood chips and impregnation under pressure to obtain a chemical-mechanical pulp.

After the deaeration step, the wood chips go through the impregnation step 6. According to a preferred embodiment of the present invention, the steamed wood chips still at the steaming temperature are sent to the impregnation step which is carried out in the absorber 6. The temperature of the impregnation solution used in the impregnation step is maintained lower than the steam temperature in the steaming step.

In practice, the impregnation step is carried out in a closed vessel, i.e. in a pressure vessel installed after the steam treatment apparatus. The absorber shown in the drawing mainly contains an elongated absorber, the longitudinal axis of which extends essentially vertically and which has upper and lower parts, and wood chips coming from the deaeration unit can be fed into the upper part of the absorber and removed through its lower part. In the absorber in accordance with the present invention, it is usually possible to create an absolute pressure of at least 1.5 bar, preferably about 1.5-15 bar.

If wood chips are quickly fed into the impregnation vessel, before the stage of impregnation begins, its temperature can drop as much as possible by approximately 10-20 ° C.

The absorber 6 contains an upper separator 7. Through its inlet 7a, the chips are fed into the absorber, and in the separator the liquid is separated from the wood chips. This liquid is returned to the output stream of the steaming hopper 1.

According to a preferred embodiment of the present invention, steamed wood chips are fed to the impregnation step together with impregnating chemicals, the impregnating chemicals being fed through separate inlets 10a-10c to a pipe 11 connecting the outlet 4 of the steam treatment apparatus 1 and the inlet 7a absorber 6. To create pressure in the pipeline, pumps 20, 21 or similar devices are most suitable.

At the stage of impregnation, an impregnating solution containing an aqueous solution of alkali is used; the solution may also contain sulfonating chemicals. An aqueous solution of either an alkali metal hydroxide, such as NaOH or KOH, or an alkaline earth metal hydroxide, such as magnesium hydroxide Mg (OH) ₂ or calcium hydroxide, or a mixture thereof, is usually used. If desired, this solution also contains, for example, sulfite compounds, such as sodium sulfite. The amount of alkali metal hydroxide is usually about 2-12 kg / Adt (Adt is a ton of air-dry wood pulp), however, preferably a maximum of about 6 kg / Adt, more preferably a maximum of about 4 kg / Adt. Alkaline earth metal hydroxides are used in (molar) appropriate amounts. The pH of the solution is approximately 9-11. The consumption of the sulfite compound is about 1-20 kg / Adt, for solid wood most preferably at most 3 kg / Adt.

In addition to pure impregnation solutions, aqueous solutions containing alkaline compounds, such as cooking liquid obtained from pulping, for example white or green liquor, can also be used. The temperature in the impregnation step is approximately 30-95 ° C, preferably 40-90 ° C, which can be achieved at least in part due to the heat from the chips. Typically, the temperature in the impregnation step is lower than the temperature of the deaeration step. In accordance with the present invention, the absolute pressure in the impregnation step is about 1.5-15 bar, preferably about 2-10 bar. As a result, an overpressure of at least about 0.5 bar is used in the impregnation. The ratio between wood and liquid (in parts) is usually from about 1:20 to 1: 4, in particular from about 1:15 to 1: 6.

The amount of impregnating chemicals can be adjusted depending on the chips intended for processing, and if necessary can be increased.

The duration of the impregnation is about 1-240 minutes, preferably about 5-120 minutes, more preferably about 10-60 minutes.

At the stage of impregnation, the wood is impregnated with alkali to the maximum extent possible. Usually at least 85%, preferably at least 90%, more preferably at least 95% of the pore volume of wood chips should be filled with an impregnating solution.

The impregnation can be carried out in one or several stages, and at least one of the stages of impregnation is carried out at an overpressure. In accordance with a preferred embodiment of the present invention, the hot chips are first impregnated under overpressure under the indicated conditions, after which the impregnation process continues in an open container or vessel at the same or different temperature. About 10-80% of the impregnation time can be carried out under pressure. In the example described below, the duration of processing under pressure and the duration of processing without applying pressure are equal, with a total duration of 40 minutes.

Wood chips coming after the impregnation step are removed through the outlet 6a. 6b indicates the release of the absorber. With this release, waste accumulated at the bottom of the device can be removed. After this, the chips are fed to step 12 of grinding the usual chemical-mechanical pulp, which can be carried out, for example, in refiners equipped with corrugated disks. The wood raw materials are milled to preset values of friability of 50-500 ml according to Canadian Standard CSF (CSF - Canadian Standard Freeness), more preferably approximately 90-150 ml CSF.

The drawing shows how in practice the chip stream obtained after the impregnation step can be further processed before grinding. Accordingly, the excess impregnation solution is first removed in the screw press 13, after which the exposure to chemicals can continue in the reaction hopper 14 until the wood chips for grinding are transported using screw conveyors 15a and 15b. The reaction time in the reaction hopper 14, if such a hopper is used, is usually about 0.1-10 hours.

In the screw press 13, it is possible to separate impurities and unsuitable for grinding fibrous material, and they are removed through the filter 16 into the pipeline to remove waste. The liquid phase 17 formed in the screw press may be returned to the conduit 10, possibly in combination with a fresh water supply.

Positions 22 and 23 indicate the pumps installed to supply the liquid phase. As shown in the drawing, the impregnation solution is most preferably returned, and the concentration of alkali in it can be controlled (increased) by supplying fresh alkali.

It should be noted that the two most important features of the present invention are that good deaeration is achieved before connecting the impregnation solution to the chips, and that the impregnation is carried out under pressure. Together, these two features ensure the effective penetration of the impregnating solution into the wood chips. The residence time and temperature in the pressure vessel are selected so that the diffusion time and reaction time can be controlled. There should be enough time for diffusion; the reaction rate should not be too high.

The chemical-mechanical pulp described above has exceptionally good properties. As indicated in the introduction, the light-scattering properties of wood pulp are improved, and this improvement is achieved without increasing the content of unmilled fibers. Therefore, with the same degree of grinding, the light-scattering properties of wood pulp in accordance with the present invention are at least 5%, even 10% better compared to a control sample treated with a large amount of alkali. At the same time, the content of unmilled fibers in the wood pulp in accordance with the present invention is lower than the content of unmilled fibers in the control sample TMM, and unexpectedly even lower than in the control sample treated with a large amount of alkali. With the same degree of grinding, the specific volume also increases, up to 5%.

It is noteworthy that the light scattering of HTMM obtained from aspen can exceed 45 m ² / kg, and the content of unmilled fibers can be less than 0.3%. Accordingly, it is possible to obtain wood pulp from birch having a light scattering of more than 45 m ² / kg and an unmilled fiber content of less than 1.5%. The present description provides only examples of the properties of mechanical wood pulp, and it should be noted that within the framework of the present invention, the manufacturer of mechanical wood pulp can easily obtain the desired level of light scattering or the content of unmilled fibers and using the present invention to achieve a significant improvement in one of the parameters.

Pulp in accordance with the present invention can be used for the manufacture of paper and cardboard products.

Therefore, after the grinding described above, wood pulp is usually bleached using, for example, hydrogen peroxide in an alkaline medium to a whiteness of approximately 75-88%.

If desired, you can pre-set the properties of the source material by mixing the mechanical pulp with cellulose, so as to obtain a starting material dissolved in water, containing, however, a sufficient amount (at least 30% by weight) of the chemical-mechanical pulp. Softwood pulp is preferably used as cellulose, and, in this case, its content is 1-50% of the dry residue of the raw material fibers. However, in this way only chemical-mechanical wood pulp from aspen can be used.

The paper pulp is first dissolved to a suitable consistency in a substantially known manner (usually to a solids content of 0.1-1%), after which it is applied to a grid where the fibers are woven together to form a paper or cardboard web. A filler such as calcium carbonate may be added to the pulp, typically in an amount of about 1-50% by weight of the fiber.

The paper web may be surface glued and / or coated and, if desired, calendered. Coating pastes can be used for single coat, primer and topcoat. Three-layer coatings are also possible. Typically, the coating in accordance with the present invention contains 10-100 parts by weight of at least one pigment or mixture of pigments, 0.1-30 parts by weight of at least one binder, and 1-10 parts by weight of other substantially known additives.

By the method described above in accordance with the present invention, a web having excellent printing properties, good smoothness and high opacity and whiteness can be obtained from the pulp. Examples are fine paper, coated printing paper and brochure paper, and the outer layer of laminated paperboard.

The present invention is illustrated by the following non-limiting examples.

Example 1

HTMM from aspen is obtained in the laboratory under the following conditions. Aspen wood chips are steamed at a temperature of 100 ° C for 2-5 minutes, impregnated in a closed container with different amounts of sodium hydroxide at a pressure of 5 bar (a) and a temperature of 80 ° C for 20 minutes. After this, the impregnation is continued for another 20 minutes in an open reaction bunker at a temperature of 80 ° C.

The following characteristics are used, among others, to determine the properties of wood pulp:

- specific volume (cm ³ / g): EN 20534;

- scattering (m ² / kg): ISO 9416;

- creep according to the Canadian standard - CSF (ml): ISO 5267-2;

- wood chips: "Pulmac shives", sample quantity 3 g and a gap between grinding disks of 0.08 mm for a mass with a creep of 150 ml CSF and 0.10 mm for a mass with a creep of 325 ml CSF.

Table 1 shows what happens when the aspen chips treated in this way are milled to a dryness value of 150 ml CSF.

Table 1 The amount of NaOH,% NaOH, g / l NaOH, mol / L Scattering, m ² / kg The content of wood chips,% 1,2 4.8 0.12 53.5 0.43 0.42 1.68 0,042 58.5 0,07 0.33 1.32 0,033 56.4 0.04 0.23 0.92 0,023 59.0 0.14 0 0 0 57.3 0.62

As can be seen from the table, the use of the present invention allows to reduce the amount of alkali hydroxide, and the scattering clearly increases without increasing the content of unmilled fibers. Compared to using the usual amount of alkali, the scattering increases by more than 10%. It was unexpectedly found that the content of unmilled fibers was even less than in the control sample with 1.2% alkali.

Table 2 shows the specific volume of these wood pulps.

table 2 The amount of NaOH,% Specific volume, cm ³ / g 1,2 2.66 0.42 2.70 0.33 2.81 0.23 2.74 0 2.83

You can see that the specific volume is also slightly better at the same CSF level.

The tests were repeated with birch chips. Impregnation under pressure of birch wood chips (80 ° C, 5 bar, 20 min) at a CSF level of 325 ml gave the following scattering values:

Table 3 The amount of NaOH,% NaOH, g / l NaOH, mol / L Scattering, m ² / kg The content of wood chips,% 1,1 4.4 0.11 41.3-42.5 2.24 0.42 1.68 0,042 45.1 1.40

In this case, a significant improvement in light scattering was also obtained, although the content of unmilled fibers during the test remained high. However, it turned out to be one third less than in the control sample.

In the case of birch, a decrease in the amount of alkali did not significantly affect the specific volume.

Example 2

Laboratory impregnation

In laboratory conditions, aspen and birch are impregnated at normal atmospheric pressure using alkali of 2.5, 5 and 10 kg of NaOH / Adt chips. The wood / liquid ratio is 1: 8 and the temperature is 80 ° C.

Samples of the impregnation solution are taken at the following intervals from the start of the impregnation process: 15 minutes, 30 minutes, 1 hour and 3 hours. The content of organic substances dissolved in the impregnating solution increases significantly with an increase in the amount of alkali and depending on the time of impregnation. At the same time, the content of dissolved chemically consumed oxygen (COD) increased significantly.

The table below shows the results obtained during testing of the effect of a certain amount of alkali on the loss of fiber during aspen impregnation in laboratory conditions for 40 minutes.

It should also be noted that fiber loss was determined for the impregnation solution before grinding. Grinding increases the amount of dissolved material, and accordingly increases the loss of fiber, and the larger the amount of alkali, the greater the loss.

Table 4 The amount of NaOH,% NaOH, g / l NaOH, mol / L Fiber loss,% one 1.43 0,035 2 0.5 0.71 0.017 1,1 0.25 0.36 0.009 0.65

Claims (14)

1. A method of obtaining a chemical-mechanical pulp from wood raw materials containing wood chips, in accordance with which:
wood chips are brought into contact with an alkaline impregnation solution in the impregnation step under conditions under which the impregnation solution penetrates the wood chips, and
grinding wood chips treated with an impregnating solution to the desired values of sagasti to obtain wood pulp, characterized in that:
wood chips are deaerated and
the deaerated wood chips thus obtained are impregnated with an alkaline impregnating solution at an overpressure such that the impregnating solution is effectively absorbed by the wood chips before grinding. 2. The method according to claim 1, characterized in that the wood raw material is steamed at an elevated temperature, in particular at a temperature of about 80-100 ° C, to remove air from wood chips. 3. The method according to claim 2, characterized in that:
steamed wood chips essentially at a temperature of steaming is sent to the stage of impregnation, and
in the impregnation step, the temperature of the impregnation solution is kept lower than the temperature in the steaming step. 4. The method according to any one of claim 2 or 3, characterized in that the steamed wood chips are fed to the stage of impregnation through a screw press. 5. The method according to claim 1 or 3, characterized in that at the stage of impregnation using an impregnating solution containing alkali metal hydroxide, the maximum amount of which is approximately 6 kg per ton of air-dry mass, preferably a maximum of about 4 kg per ton of air-dry masses. 6. The method according to claim 1 or 3, characterized in that the aqueous solution of an alkali metal hydroxide, such as sodium hydroxide, a cooking liquid obtained from pulping, for example, white or green liquor, or sulfite compounds, are used as the impregnating solution. like sodium sulfite, or a mixture thereof. 7. The method according to any one of claims 1 or 3, characterized in that the wood chips are impregnated with an alkaline impregnating solution for about 1-240 minutes, preferably about 5-120 minutes, more preferably about 10-60 minutes. 8. The method according to claim 1, characterized in that the wood raw material is milled to a creep value of 50-500 ml, according to the Canadian standard, preferably about 90-150 ml, according to the Canadian standard. 9. The method according to claim 1 or 3, characterized in that the temperature at the stage of impregnation is approximately 30-95 ° C, preferably approximately 40-90 ° C. 10. The method according to claim 1 or 3, characterized in that the absolute pressure in the impregnation step is approximately 1.5-15 bar, preferably approximately 2-10 bar. 11. The method according to claim 1 or 3, characterized in that at the stage of impregnation of at least 85%, preferably at least 90%, most preferably at least 95% of the pore volume of wood chips is filled with an impregnating solution. 12. The method according to claim 1 or 3, characterized in that the impregnation is carried out in several stages, and at least one of the steps is carried out at excess pressure, and at least one of the steps is carried out at normal atmospheric pressure. 13. The method according to claim 2 or 3, characterized in that the steaming of wood chips is carried out at least essentially at normal atmospheric pressure using saturated or close to saturated steam. 14. Chemical-mechanical pulp obtained by the method according to any one of claims 1 to 13.

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