WO2007063171A1

WO2007063171A1 - Process for producing chemi-mechanical pulp

Info

Publication number: WO2007063171A1
Application number: PCT/FI2006/000399
Authority: WO
Inventors: Kai Erik Johannes Vikman; Maija Pitkänen; Pirita Suortamo; Ole Nickull; Auli Laurila-Lumme
Original assignee: M-Real Oyj
Priority date: 2005-12-02
Filing date: 2006-12-01
Publication date: 2007-06-07
Also published as: ZA200804758B; BRPI0619143A2; RU2401350C2; UY29982A1; CN104674583A; FI126694B; AR058265A1; CN101341288A; CA2631767C; AU2006319052B2; FI20051251A; JP2009517560A; BRPI0619143B1; CA2631767A1; RU2008125038A; JP4823317B2; FI20051251A0; SE0801296L; AU2006319052A1; SE534004C2

Abstract

A method of producing a chemi-mechanical pulp from a raw material comprising wood chips. According to the method, first, the wood raw material is steamed at an elevated temperature to remove air from the wood chips, then, at the impregnation stage the steamed wood chips are brought into contact with an alkaline impregnation solution under conditions in which the alkali penetrates into the wood chips and, after that, to produce the pulp, the wood chips which are treated with the alkali are refined to a desired drainability. According to the present invention, the impregnation stage is carried out at overpressure, in which case it is possible to decrease the alkali dose of the impregnation and thus increase the scattering of the pulp, without increasing the sliver percentage. At the same freeness level, the bulk is improved, too.

Description

Process for producing chemi-mechanical pulp

The present invention relates to a method of producing chemi-mechanical pulp from a wood raw material comprising wood chips, according to the preamble of Claim 1.

In a method such as this, the wood chips comprising the wood raw material are brought into contact with an alkaline impregnation solution in conditions where the solution penetrates into the wood chips to impregnate the wood chips, and the wood chips which are impregnated with the impregnation solution are refined to produce a mechanical pulp.

The present invention also relates to chemi-mechanical pulp according to Claim 14.

Chemi-mechanical pulp is produced by refining wood chips which have been prepared with chemicals, typically with an alkaline liquid, to a desired drainability. The chemical treatment facilitates the refining and improves the properties of the mechanical pulp. In a chemical cooking, too, the wood chips are prepared with alkaline solutions to facilitate the defibring.

In the production of mechanical as well as chemical pulp it is essentially important that the chemicals are both well absorbed and evenly absorbed into the pulp. An uneven absorption results in a high percentage of slivers in the pulp and in turn a decreased strength potential.

Two mechanisms, namely penetration and diffusion, enable the absorption of the liquid and the chemicals into the wood. Penetration takes place as a result of the differences in pressure, and diffusion, correspondingly, as a result of the differences in concentration. At the beginning of the impregnation stage, the difference in the pressures prevailing inside the wood chips and, correspondingly, those outside them is the factor which drives the penetration, to the point at which the differences in pressure are evened out. When a whole wood chip becomes filled with liquid, diffusion takes over as the means by which the chemicals move into the wood chips and the reaction products out of them.

There are two kinds of penetration: natural and forced. In natural penetration, the pressure difference is generated by capillary forces, whereas in forced penetration the pressure difference is generated by an outside pressure or by generating a vacuum inside the wood chips.

Alkaline impregnation solutions swell the fibre walls and thus narrow the capillaries. This decreases the longitudinal penetration. However, at the same time, alkaline solutions also dissolve the components of the fibre, which accelerates the penetration.

Diffusion is considerably slower than penetration. Raising the temperature increases the diffusion speed, but it is known that the speed of the reaction increases considerably faster than does the speed of the diffusion. Accordingly, the literature mentions, for example, that during the process of sulphate cooking, calculations using diffusion coefficients and activation energy show that the diffusion speed at 170 ⁰C is approximately 4 times higher than at 100 ⁰C. At the same time, there is a 900-fold increase in the reaction speed (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).

When the reaction speed increases, the consumption of alkali increases as well.

The alkali dose and the size of the wood chip, in turn, have an effect on the diffusion. In order to avoid generating more reject, the alkali dose must be increased when the thickness of the wood chip is increased. When the alkali dosage is raised, the alkali gradient between the wood chip and the surrounding solution increases as well (liquid/wood-ratio the same), and thus the diffusion speed increases.

It should also be noted that the dosage of the alkali which is dosed for the impregnation is the most important factor affecting the yield of the chemi-mechanical pulp (CTMP). The fibre loss mainly results from the breaking down of the acetic acid, and the dissolving of the lignin and the dissolving of acidic polysaccharides, all of which are caused by the alkali. The influence of the alkali dosage is greater for hardwood than for softwood, and for aspen it is especially large. When the alkali dose (for instance the amount of NaOH) is raised from 0.5 % to 3 %, the yield from aspen wood decreases linearly from approximately 95 % to 89 % in a conventional process.

Part of the yield loss takes place already during impregnation, part not until the following refining stage at a high temperature.

In practice, a large alkali dose must be used to ensure maximum impregnation. The penetration of the alkali solution into the wood chip is slow and the alkali tends to be consumed in the outer parts of the wood chip, leaving insufficient alkali for the inner part of the chip. In the refining stage, this is expressed as a higher percentage of slivers.

The effect of the alkali on the scattering qualities of the chemi-mechanical pulp is especially problematic. The qualities of the scattering fall with increasing alkali dosage and, when level of refining is the same, the scattering is always smaller when the alkali dosage is larger. Also, bleaching decreases the scattering further. To make high-class printing paper it is prerequisite that the scattering and the brightness properties of the pulp are good. It is possible to produce hardwood CTMP pulps to a high brightness, even up to an ISO brightness of 88 %.

The purpose of the present invention is to eliminate at least some of the disadvantages associated with the known technology, and to provide a novel solution for producing chemi-mechanical pulp.

The present invention is based on the principle that when producing chemi-mechanical pulp, the impregnation of the chips is carried out at overpressure.

Richardson and LeMahieu have presented a method of producing a super-groundwood pulp, in which method aspen chips are impregnated at a temperature of approximately 75 ⁰C and a pressure of 4 bar, using a mixture of sodium hydroxide and sodium sulphite, before the refining of the chips [Tappi 1965 (48), no 6, p. 344-346]. According to the article, increasing the concentration of the alkali, the strength of the pulp could be increased and, at the same time, the energy consumption reduced. However, a problem in the known solution is that the addition of alkali decreases the bleachability of the pulp.

Associated with the present invention we have unexpectedly discovered that when combining an effective deaeration of the chips, which is most suitably carried out by steaming, and a pressurized impregnation, it is possible to make the impregnation more effective and reduce the alkali dose significantly. When compared at the same freeness level, it is possible to use a significantly smaller alkali dosage to achieve the same or even lower sliver percentage and, at the same time, the scattering of the pulp is rendered higher than in a conventional unpressurized impregnation.

The method according to the present invention is carried out by arranging in series equipment comprising the following: the chip deaeration unit, the chip impregnation unit and the chip refining unit, in which case the impregnation unit comprises a closed vessel within which the impregnation treatment can be carried out at overpressure.

More specifically, the method according to the present invention is mainly characterized by what is stated in the characterization part of Claim 1.

The pulp according to the invention is, again, characterized by what is stated in the characterizing part of Claim 14.

Considerable advantages are obtained by means of the invention. Thus, with pilot experiments it has been demonstrated that using pressurized impregnation it is possible to decrease the alkali dose by 50 % or more. In addition, the invention increases the scattering of the pulp: in the experiments the scattering has unexpectedly been even higher than in the TMP reference (0 % of alkali). Also noteworthy is that it has been possible to increase the scattering without increasing the sliver percentage. At the same level of freeness, the bulk is improved, too.

Pressurized impregnation has produced a degree of penetration exceeding 95 % in laboratory experiments. In the reference points used in the experiments, the maximum degree of penetration has been 63-74 %. hi pilot test runs, it has also been possible to decrease the alkali concentration from the conventional level of 0.8-1.2 %/Adt (air dried tonne of pulp) even to a level of 0.25 %/Adt without the sliver percentage rising. In all test points, the scattering has been clearly higher than in the reference.

Pressurized impregnation makes it possible to improve the quality of chemi-mechanical refiner pulp for different end-use applications.

In the following, the invention will be examined more closely with the help of a detailed explanation and with reference to examples of applications that are presented below.

The accompanying figure presents a simplified drawing of the equipment used in the method according to the present invention

As explained above, the method according the present invention comprises three stages, which are

- first, at least the most part of the air included in the wood raw material comprising wood chips is removed (i.e. the wood chips are deaerated),

- second, the raw material thus obtained is impregnated with an alkaline solution at overpressure in order that the alkali is efficiently absorbed into the wood chips, and

- third, the treated wood chips are refined to a predetermined drainability.

As can be seen from the above, the pulp is produced by means of a chemi-mechanical process. In the present invention, chemi-mechanical pulp production means in general a process which comprises both a chemical and a mechanical defϊbring stage, as described above. The CMP and the CTMP processes are chemi-mechanical processes. In the CMP process, the wood raw material is refined at normal atmospheric pressure, whereas in the CTMP process a pressure refiner mechanical pulp is produced. Because a higher dosage of chemicals is used in the CMP, its yield is generally smaller than that of the CTMP process (less than 90 %). In both cases, the chemical treatment of the wood is traditionally carried out with sodium sulphite (sulphonation treatment), in which case hardwood can be treated with sodium hydroxide, too. In this case, a typical chemical dosage in the CTMP process is approximately 0-4 % of sodium sulphite and 1-7 % of sodium hydroxide, and the temperature is approximately 60-120 ⁰C. By contrast, in the CMP process, the chemical dosage is 10-15 % of sodium sulphite and/or 4-8 % of sodium hydroxide (calculated from dry wood), and the temperature is 130-160 °C and, correspondingly, 50-100 ⁰C.

In a chemi-mechanical process, the chips can be impregnated with an alkaline peroxide solution, too (APMP process). The dosage of peroxide is generally 0.1-10 % (of the dry pulp weight), typically approximately 0.5-5 %. The amount of the alkali feed, such as sodium hydroxide, is about the same, i.e. approximately 1-10 weight %.

The present invention relates especially to the CTMP process, in which the chips coming from the impregnation are defibred using the pressure refiner mechanical pulp method.

The initial material in the process according to the present invention is chips comprising soft- or hardwood material. In particular, hardwood chips are used in the production, the chips of which are prepared from birch (generally, a wood species of the Betula genus) or a wood species of the Populus genus or a mixture of them. Examples of suitable wood species of the Betula genus are B. pendula and B. pubescens, and of the wood species of the Populus genus especially the following: P. tremula, P. tremuloides, P balsamea, P. balsamifera, P. trichocarpa, P. heterophylla, P. deltoides and P. grandidentata. Aspen (the European aspen, P. tremula; Quaking aspen, P. tremuloides), aspen species crossbred from different stock aspens, hybrid aspens (for instance P. tremula x tremuloides, P. tremula x tremula, P. deltoides x trichocarpa, P. trichocarpa x deltoides, P. deltoides x nigra, P. maximowiczii x trichocarpa) and other species generated by gene technology, along with poplars, are considered to be particularly preferable.

Besides wood species of the Betula and Populus genera, other hardwood species, too, can be used as raw material, such as eucalyptus and mixed tropical hardwood. Among the coniferous trees, spruce (Picea abies) and pine (Pinus silvestris) and other wood species of t thhee P Piicceeaa a anndd P Piinnuuss e geenneerraa., s shhoouulldd b bee m meennttiioonneedd.. According to one application, chemi-mechanical pulp comprising up to 100 % softwood fibres is produced. However, with the present invention it is possible to produce a chemi- mechanical pulp which consists of a mixture of hard- and softwood fibres, and which pulp comprises at least 5 % softwood fibres, for instance it may comprise 50-99 % hardwood fibres and 1-50 % softwood fibres. It is possible to increase the bulk, the strength properties and the stiffness of the pulp, by using softwood fibres, especially by using spruce fibres.

The size of the wood chips of the wood raw material is generally approximately 20-50 mm x 1-10 mm, typically approximately 35-40 mm x 3-5 mm.

First, as much of the air in the wood chips as is possible is removed. Generally, the aim is to remove at least 70 %, especially approximately 80-100 % of the air contained in the chips. The air is typically present in gaseous form. As will appear from the accompanying flow sheet, this deaeration can be carried out by steaming the chips in an evaporator 1. In the process according to the figure, the chips of the initial material are fed in by the screw conveyor 2 into the steaming silo 1, into which steam is fed either from one feed nozzle or, as the figure shows, from several nozzles 3a-3c, in order to distribute the steam evenly into the chips in the silo.

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

The steaming can be carried out for instance in a continuous evaporator 1, shown in the figure, in which the wood chips move through the steaming silo 1, where they are brought into contact with the saturated or almost saturated steam for a period of approximately 0.5- 20 minutes, especially approximately 1-10 minutes. The steaming can be carried out at overpressure but generally steaming at normal atmospheric pressure is fully adequate. In particular, an elevated temperature is used, for instance approximately 50-100 °C, especially approximately 80-100 °C, depending on how saturated the steam used is.

Instead of using steaming, the deaeration can be carried out at low pressure/in a vacuum or the steaming can be made more effective with vacuum treatment.

The treated chips are removed from the steaming silo through the outlet nozzle 4, after which the chips are most suitably compressed in the plug screw 5. After this stage, typically at least 95 %, preferably at least 98 %, of the air has been removed, and at the same time, part of the steam has also been removed. For the sake of completeness, it should be mentioned that steaming of the chips is used in the production of both sulphate pulp and chemi-mechanical refiner pulp. However, until the present invention, it had never before been suggested that a combination of the steaming of wood chips and pressurized impregnation could be used in the production of chemi-mechanical pulp.

After the deaeration stage, the wood chips are conducted to the impregnation treatment stage 6. According to a preferred embodiment of the present invention, the steamed wood chips are brought, essentially still at the temperature of the steaming stage, to the impregnation stage which is carried out in the absorber 6. The temperature of the impregnation solution used in the impregnation stage is kept lower than the temperature of the steam in the steaming stage.

In practice, the impregnation stage is carried out in a closed vessel, i.e. a pressure vessel, which is arranged downstream from the steamer. The absorber illustrated in the figure basically comprises an elongated absorber, the longitudinal axis of which is essentially arranged vertically, and which has an upper and a lower part, in which case the wood chips coming from the deaeration unit can be fed into the upper part of the absorber and removed via the lower part of the absorber. In the absorber, according to the present invention, it is generally possible to create an absolute pressure of at least 1.5 bar, preferably approximately 1.5-15 bar.

When the wood chips are fed rapidly into the impregnation vessel, their temperature may fall at maximum approximately by 10-20 ⁰C before the impregnation stage starts.

In the absorber 6, there is an upper separator 7. Through its input nozzle 7a the chips are fed into the absorber and in the separator, liquid is separated from the wood chips. This liquid is recirculated into the reject flow of the steaming silo 1.

According to a preferred embodiment, the steamed wood chips are fed into the impregnation stage together with the impregnation chemicals, in which case the impregnation chemicals are fed in from separate input nozzles 10a- 10c to the pipeline 11 which connects the outlet nozzle 4 of the steamer 1 and the input nozzle 7a of the absorber 6. hi order to create pressure, most suitably pumps 20, 21 or similar devices are arranged in the pipeline.

At the impregnation stage, an impregnation solution is used which comprises an aqueous solution of an alkaline material, which solution optionally comprises sulphonation chemicals. Typically, 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 mixtures thereof, are used. If desired, this solution comprises for instance also sulphite compounds, such as sodium sulphite. The dosage of alkali hydroxide is typically approximately 2-12 kg/Adt (air dried tonne of pulp), preferably, however, at maximum approximately 6 kg/Adt, more preferably at maximum approximately 4 kg/Adt. Alkaline earth metal hydroxides are used in (molarly) corresponding dosages. The pH value of the solution is approximately 9-11. The consumption of the sulphite compound is approximately 1-20 kg/Adt, for hardwood most suitably at maximum 3 kg/Adt.

Besides pure solutions, aqueous solutions of compounds of alkaline materials, too, can be used for impregnation, such as cooking liquor obtained from pulp cooking, for instance white or green liquor. The temperature of the impregnation stage is approximately 30-95 °C, preferably approximately 40-90 °C, and which can be achieved at least in part by the heat brought in with the chips. Generally, the temperature of the impregnation stage is lower than the temperature of the deaeration stage. According to the present invention, the pressure of the impregnation stage is approximately 1.5-15 bar, preferably approximately 2-10 bar absolute pressure. As a result, an overpressure of at least approximately 0.5 bar is used in the impregnation. The ratio between wood and liquid (p/p) is generally approximately 1 :20...1 :4, especially approximately 1:15...1:6.

The dosage of the impregnation chemicals can be adjusted, depending on the chips to be treated and, if necessary, it can be increased.

The duration of the impregnation treatment is approximately 1-240 minutes, preferably approximately 5-120 minutes, especially approximately 10-60 minutes.

During the impregnation stage, the wood chips are impregnated with alkali to the maximum degree possible. Generally, at least 85 %, preferably at least 90 %, more preferably at least 95 % of the volume of the pores of the wood chips should be filled with the impregnation solution.

The impregnation can be carried out in one or in several stages, in which case at least one impregnation stage is carried out at overpressure. According to a preferred embodiment, the hot chips are first impregnated at overpressure under the conditions mentioned above, after which the impregnation process is still continued in an open container or vessel at the same or at a different temperature. Approximately 10-80 % of the temporal duration of the impregnation treatment can be carried out under pressurized conditions. In the application described below, the duration times of the pressurized and the unpressurized treatments were equally long, the total duration time being 40 minutes.

The wood chips coming from the impregnation stage are removed through the outlet nozzle 6a. The reference number 6b refers to the discharger of the absorber. With this discharger the residual fraction that has collected at the bottom of the device can be removed. After that, the chips are fed to a refining stage 12 of conventional chemi- mechanical pulp, which can be carried out for instance in refiners equipped with grooved refining blades. The wood raw material is refined to a predefined drainability, which is 50- 500 ml CSF, more preferably approximately 90-150 ml CSF.

The basic drawing 1 shows in practice how the chip flow generated from the impregnation stage can be further processed before the refining. Accordingly, the excess impregnation solution is first removed in the screw press 13, after which the reaction of the chemicals can be continued in the reaction silo 14, before the chips are transported with the screw conveyors 15a and 15b to the refining. The reaction time in the reaction silo 14, if such a silo is used, is typically approximately 0.1-10 hours.

In the screw press 13, it is possible to separate impurities and fibre material which are unfit for refining, and they are removed through the screen 16 into the reject channel. The liquid phase 17 generated from the screw press can be recirculated into the pipeline 10, possibly in combination with the fresh water feed.

The reference numerals 22 and 23 refer to the pumps arranged for the feeding of the liquid phase. As the figure shows, the impregnation solution is most suitably recirculated in the process and its alkali concentration can be adjusted (increased) with fresh alkali feed.

It should be pointed out that the two essential factors in the present invention are that a good deaeration is achieved before the impregnation solution and the chips are put together, and that the impregnation is pressurized. These two factors together allow an efficient penetration of the impregnation solution into the wood chips. The retention time and the temperature in the pressure vessel are chosen in such as way that it is possible to adjust the diffusion time and the reaction time. There must be enough time for the diffusion to take place, the speed of the reaction must not be too high.

The chemi-mechanical pulp described above has extraordinarily good properties. As described in the introduction, the light scattering qualities of the pulp are improved, and has been achieved without increasing the shives percentage. Consequently, at the same freeness level, the light scattering qualities of the pulp according to the present invention are at least 5 %, even 10 %, better compared to the high alkali reference. At the same time, the sliver percentages of the pulps according to the present invention are lower than the sliver percentage of the TMP reference and, unexpectedly, even lower than the sliver percentage of the high alkali reference. At the same freeness level, the bulk is improved, too, by as much as 5 %.

A noteworthy example is that the scattering qualities of CTMP pulp prepared from aspen may be higher than 45 m²/kg and the sliver percentage lower than 0.3 %. Correspondingly, from birch it is possible to generate a pulp exhibiting scattering qualities which are higher than 45 m²/kg and a sliver percentage lower than 1.5 %. These are only examples of the properties of pulp, and it should be pointed out that for a pulp producer, within the limits of the present invention, it is possible to choose freely either a desired level of the scattering or the sliver percentage and, with the help of the present invention, to achieve a substantial improvement in one of the parameters.

The pulp according to the present invention can be used for the production of paper and cardboard products.

Consequently, following the deflbring described above, the pulp is generally bleached, using for instance hydrogen peroxide under alkaline conditions, to a brightness of approximately 75-88 %.

If desired, it is possible to tailor the properties of the initial material by mixing the pulp with a chemical pulp in such a way that a slushable initial material is achieved, one which, however, comprises a significant amount (at least 30 weight %) of chemi-mechanical pulp. Preferably, softwood pulp is used as chemical pulp and, in that case, its percentage is 1-50 % of the dry weight of the fibres of the raw material. However, it is also possible to use only chemi-mechanical aspen pulp.

The paper pulp is first slushed to a suitable consistency in a way which is known per se (typically to a solids percentage of approximately 0.1-1 %), after which it is spread onto the wire, where it is webbed to form the paper or cardboard web. It is possible to add a filler such as calcium carbonate, generally approximately 1-50 weight % of the weight of the fibres, into the fibre slush. The paper web can be surface sized and/or equipped with a coating layer and, if desired, calendered. Coating pastes can be used for single coating, for precoating and for surface coating. Triple coatings are possible, too. Generally, a coating according to the present invention contains 10-100 parts by weight of at least one pigment or a mixture of pigments, 0.1-30 parts by weight of at least one binder, and 1-10 parts by weight of other additives known per se.

In the way described above, it is possible according to the present invention to produce from the pulp material webs which have excellent printability properties, good smoothness and high opacity and brightness. Examples of applications are fine papers, coated printing papers and brochure papers, and liners of multilayer cardboards.

The following non-restrictive examples illustrate the present invention.

Example 1

CTMP pulp of aspen was prepared in the laboratory under the following conditions:

Aspen wood chips, which had been steamed at 100⁰C for a period of 2-5 minutes, were impregnated with different amounts of sodium hydroxide at a pressure of 5 bar (a), at 80 °C for a period of 20 minutes in a closed container. After that, the impregnation was continued for another 20 minutes in an open reaction silo at 80 ⁰C.

The following methods, among others, were used to determine the properties of the pulp: - bulk cm³/g: EN 20534 scattering m²/kg: ISO 9416 CSF ml: ISO 5267-2 — shives: "Pulmac shives", sample quantity 3 g and a gap plate of 0.08 mm for 150 ml of CSF pulp and of 0.10 mm for 325 ml of CSF pulp.

Table 1 shows what happens when the aspen chips treated in this way were refined to a drainability level of 150 ml CSF. Table 1

As the table shows, using the present invention it is possible to decrease the alkali hydroxide dosage, in which case the scattering clearly increases without increasing the sliver percentage. Compared with the case in which a conventional amount of alkali was used, the scattering increases by more than 10 %. Unexpectedly, the sliver percentage was even lower than in the 1.2 % alkali reference.

Table 2 shows the bulk of the pulps described above.

Table 2

It can be seen that the bulk, too, is slightly better at the same CSF level.

The experiments were repeated with birch chips. Pressurized impregnation of birch (80 °C, 5 bar, 20 min) at the CSF level of 325 ml produced the following scattering values:

Table 3

In this case, too, a significant improvement in the scattering was thus achieved, although the sliver percentage remained high in the experiment. However, it was one third lower than in the reference.

In the case of birch, a reduced alkali dose did not have any considerable effect on the bulk. Example 2. Laboratory impregnations

In the laboratory, impregnations using aspen and birch were carried out at normal atmospheric pressure, using alkali doses of 2.5, 5 and 10 kg NaOH/Adt chips. The wood/liquid ratio was 1 :8, and the temperature 80 ⁰C.

Samples were taken from the impregnation solution at the following times: 15 min, 30 min, Ih and 3 h from the beginning of the impregnation process. The organic material which was dissolved in the impregnation solution increased drastically with increasing alkali dose and as a function of the impregnation time. At the same time, the dissolved COD increased drastically.

The table below shows the results, achieved in the example, of the effect of the alkali dose on the fibre loss, in the 40 minute laboratory impregnation of aspen.

It should also be pointed out that the fibre loss was determined from the impregnation solution before the refining. Refining increases the amount of dissolved material and, correspondingly, it increases the fibre loss, the larger the alkali dose the greater the loss.

Table 4

Claims

Claims:

1. A method of producing a chemi-mechanical pulp from a wood raw material comprising wood chips, according to which method - the wood chips are contacted with an alkaline impregnation solution at an impregnation stage under conditions in which the impregnation solution penetrates into the wood chips, and,

- the wood chips treated with the impregnation solution are refined to a desired drainability in order to produce the pulp, c h a r a c t e r i z e d by

- subjecting the wood chips to deaeration, and

- impregnating the thus obtained, deaerated wood chips with an alkaline impregnation solution at overpressure in such a way that the impregnation solution is efficiently absorbed into the wood chips before they are refined.

2. The method according to Claim 1, wherein the wood raw material is steamed at an elevated temperature, especially at approximately 80-100 °C, to remove the air from the wood chips.

3. The method according to Claim 2, wherein

- the steamed wood chips are brought essentially at the temperature of the steaming treatment to the impregnation stage, and

- at the impregnation stage, the temperature of the impregnation solution is kept below the temperature of the steaming treatment.

4. The method according to Claim 2 or 3, wherein the steamed wood chips are fed into the impregnation stage through a screw press.

5. The method according to any of the preceding claims, wherein, at the impregnation stage, a impregnation solution is used which comprises an alkali metal hydroxide, the dose of which is at maximum approximately 6 kg/Adt, preferably at maximum approximately 4 kg/Adt.

6. The method according to any of the preceding claims, wherein an aqueous solution of an alkali metal hydroxide, such as sodium hydroxide, cooking liquor obtained from chemical pulp cooking, for instance white or green liquor, or sulphite compounds, such as sodium sulphite, or mixtures of them, is used as the impregnation solution.

7. The method according to any of the preceding claims, wherein the wood chips are impregnated with the alkaline impregnation solution for approximately 1-240 minutes, preferably approximately 5-120 minutes, especially approximately 10-60 minutes

8. The method according to any of the preceding claims, wherein the wood raw material is refined to a drainability which is 50-500 ml CSF, preferably approximately 90-150 ml CSF.

9. The method according to any of the preceding claims, wherein the temperature of the impregnation stage is approximately 30-95 ⁰C, preferably approximately 40-90 °C.

10. The method according to any of the preceding claims, wherein the pressure of the impregnation stage is approximately 1.5-15 bar, preferably approximately 2-10 bar, absolute pressure.

11. The method according to any of the preceding claims, wherein, at the impregnation stage, at least 85 %, preferably at least 90 %, more preferably at least 95 % of the volume of the pores of the wood chips is filled with the impregnation solution.

12. The method according to any of the preceding claims, wherein the impregnation is carried out in several stages, in which case at least one of the stages is carried out at overpressure and at least one stage at normal atmospheric pressure.

13. The method according to any of the preceding claims, wherein the steaming of the wood chips is carried out at least essentially at normal atmospheric pressure, by using saturated or almost saturated steam.

14. A chemi-mechanical pulp produced using a method according to any of Claims 1-13.