FI126694B - Chemical-mechanical pulp and process for producing chemical-mechanical pulp - Google Patents

Description

Chemimechanical pulp and process for its preparation

The present invention relates to a process for the preparation of a chemimechanical pulp from wood chips consisting of chips according to the preamble of claim 1. According to such a method, the chips consisting of wood raw material are contacted with an alkaline impregnation solution under conditions where the solution penetrates the chips to impregnate the chips and the chips treated with the impregnating solution are ground to produce a mechanical pulp.

The invention also relates to a chemimechanical pulp according to claim 13.

Chemimechanical pulp is prepared by grinding chips, which have been pre-treated with chemicals, typically alkaline liquid, to the desired drainage. Chemical treatment promotes grinding and improves mechanical pulp properties. Also in chemical cooking, chips are pretreated with alkaline solutions to improve defibration.

In the production of both mechanical and chemical pulps, good, even chemical absorption of chips is essential. The uneven absorption results in high pulp content and reduced strength potential.

The absorption of fluid and chemicals into wood is accomplished by two mechanisms, namely penetration and diffusion. Penetration occurs due to differential pressure and diffusion respectively at the prevailing concentration differences. At the beginning of the impregnation, the difference between the internal and external pressures of the chips is the factor driving the penetration until the differential pressures have leveled out. When the entire chips are filled with liquid, diffusion is the material transfer mechanism by which chemicals can be transferred to the chips and reaction products are out of the chips.

Penetration is natural and forced. In natural penetration, the differential pressure is created by capillary forces, whereas in forced penetration, the differential pressure is achieved by external pressure or by creating a vacuum in the chip.

Alkaline impregnation solutions swell the fiber walls, thus narrowing the capillaries. This reduces longitudinal penetration. On the other hand, alkaline solutions also dissolve the fiber components and thus accelerate penetration.

Diffusion is much slower than penetration. Raising the temperature increases the diffusion rate, but it is known that the reaction rate increases much faster than the diffusion rate. Thus, there is information in the literature that, for example, for diffusion coefficients and activation energy, the diffusion rate at 170 ° C has been calculated for sulphate soup to be about 4 times higher than at 100 ° C. At the same time, the reaction rate has increased 900-fold (Taiton, J. The diffusion of sodium hydroxide in wood at high pH as a function of temperature and pulping, M.Sc. Thesis, North Carolina State University, 1986, 45 s).

At the same time, the increase in reaction rate leads to an increase in alkali consumption.

Diffusion, in turn, is influenced by the dose of alkali and the size of the chips. When increasing the thickness of the chips, the alkali dose must be increased to prevent the rejection amount from increasing.

Increasing the alkaline dose increases the alkaline gradient between the chip and the surrounding solution (liquid / wood ratio same) and thus increases the diffusion rate.

It should further be noted that the dosage of alkali to be used for impregnation is the most important factor affecting the yield of the chemimechanical pulp, CTMP. The loss of fiber is mainly due to the cleavage of acetic acid, the dissolution of lignin and acidic polysaccharides by the action of alkali. The effect of alkaline application on hardwood is greater than that of coniferous wood and it is particularly large on wound. As the dose of alkali (e.g., NaOH) rises from 0.5% to 3%, the yield decreases linearly from about 95% to 89% with a conventional wound.

Part of the yield loss occurs already during impregnation, some only in the subsequent refining subject at high temperature. In practice, a high dose of alkali must be used in order to make impregnation as complete as possible. The penetration of the alkali solution into the chips is slow and the alkali tends to wear on the outer parts of the chips while the inside is free of alkali. This is reflected in the high pulp content of the pulp during grinding.

Particularly problematic is the effect of alkali on the scattering of the finished chemimechanical pulp. As the alkaline dose increases, the scattering decreases and is always smaller at the same grinding level for a higher alkaline dose. Bleaching further reduces scattering. For high-quality printing papers, scattering and brightness are key qualities required of the pulp. It is possible to produce hardwood CTMP pulps up to high lightness up to 88% ISO.

The object of the present invention is to eliminate the drawbacks of the prior art and to provide a completely new solution for the production of a chemimechanical pulp.

The invention is based on the idea that in the manufacture of a chemimechanical pulp, the impregnation of the chips is carried out under excess pressure.

Richardson and LeMahieu have disclosed a method for producing superfine, which is impregnated with a mixture of sodium hydroxide and sodium sulfite at a temperature of about 75 ° C and 4 bar before milling the chips [Tappi 1965 (48), no. 6, pp. 344-346]. according to the article, the strength could be increased while reducing energy consumption. However, the known problem was that the addition of alkali reduced the bleaching of the pulp.

US5164042A discloses a process for making pulp from wood chips comprising lignocellulosic wood. In the method, the material is treated with steam to remove air from the material and heat it. The heat-treated material is mixed with a liquid containing chemicals. At the impregnation station, the heat-treated material is impregnated with the above-mentioned chemicals, after which the material is purified at the processing station. According to that publication, the steam-treated material is mixed with the above-mentioned liquid in portions such that the resulting mixture of material and liquid is assumed to be a flowable pumpable composition that can be pumped from the mixing station to the impregnation station.

It has been surprisingly found in the present invention that by combining the effective deaeration of the chips, preferably by steaming, with pressure impregnation, the impregnation can be enhanced and the alkali dose significantly reduced. Compared to the same freeness level, a substantially lower alkali dose achieves the same or even lower stick content and, accordingly, the mass scattering is higher than conventional non-pressurized impregnation.

The method according to the invention can be carried out in an apparatus having in series a chips venting unit, a chipping impregnation unit and a chipping milling unit, wherein the impregnating unit comprises a closed vessel in which the impregnation treatment can be carried out under excess pressure. More specifically, the method according to the invention is essentially characterized by what is stated in the characterizing part of claim 1.

The pulp of the invention, in turn, is characterized by what is claimed in claim 13.

The invention provides significant advantages. Thus, pilot experiments have shown that pressurized impregnation can reduce the alkaline dose by up to 50% or more. The invention can increase the scattering of pulp; unexpectedly, the scattering in the experiments has been even higher than the TMP reference (0% alkaline), and it is significant that the scattering could have been increased without an increase in the stick content. At the same freeness level, the bulk also improves.

By applying impregnated impregnation, it has been possible to achieve penetration rates of up to 95% in laboratory tests with a maximum penetration rate of 63-74% at the reference points. Pilot runs have also been able to lower the alkaline concentration from the usual level of 0.8-1.2% / Adt up to the level of 0.25% / Adt with no increase in stick concentration. At all test points, the scattering has been clearly higher than the reference.

Pressurized impregnation provides a better opportunity for qualitative development of chemical pulp for various end-use applications.

The invention will now be further explored by means of a detailed description with reference to the embodiments shown below.

The figure below is a simplified schematic diagram of the apparatus used in the method of the invention.

As shown above, the process according to the invention comprises three steps: - first, at least most of the air contained in the wood raw material is removed, - the thus obtained raw material is impregnated, i.e. impregnated, with an alkaline solution under high pressure so that the alkali is effectively absorbed into the chips. , and - the processed chips are ground to a preselected drainage rate.

The pulp is thus chemically produced. By chemomechanical pulping, the present invention generally refers to a process that includes both a chemical and a mechanical pulping step as described above. The chemimechanical processes are the CMP and CTMP processes, in which the CMP process processes the wood raw material under normal pressure, whereas the CTMP process produces a pressure pulp. The yield of the CMP process is generally lower (less than 90%) than the CTMP process due to its higher chemical dosage. In both cases, the chemical treatment of wood is traditionally carried out with sodium sulphite (sulphonation treatment), whereby hardwood can also be treated with sodium hydroxide. A typical chemical dosage is then about 0-4% sodium sulfite and 1-7% sodium hydroxide in the CTMP process and the temperature is about 60-120 ° C. In the CMP process, the chemical dosage is 10-15% sodium sulfite and / or 4-8% sodium hydroxide (dosages calculated from dry wood) and the temperature is 130-160 and 50-100 ° C, respectively.

In the chemimechanical process, the chips can also be impregnated with an alkaline peroxide solution (APMP process). The dosage of the peroxide is generally 0.1 to 10% (by dry weight), typically about 0.5 to 5%. An alkali such as sodium hydroxide is fed in the same amount, i.e. about 1-10% by weight.

In particular, the invention relates to a CTMP process in which the chips obtained from impregnation are defibrated by a pressure pulverization process.

The starting material for the process according to the invention is chips consisting of softwood or hardwood material. In particular, hardwood chips produced from birch (generally of the species Betula) or Populus or a mixture thereof are used. Examples of suitable Betula tree species include Be support pendula and Betula pubescens, and Populus species include, in particular, P. tremula, P. tremuloides, P. balsamea, P. balsamifera, P. trichocarpa, P. heterophylla, P. deltoides and P. grandidentata. Aspen (Maatia aspen, P. tremula; so-called Canadian aspen E. tremuloides), aspen species crossed with various root wounds hybrid acids (e.g., 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 genetically engineered species and poplar are considered particularly preferred .

In addition to Betula and Populus species, other hardwood species such as eucalyptus and mixed tropical hardwood can be used as raw materials. Conifers include six (Picea ah ie s) and pine (Pinus silvestris) as well as other Picea. Tree species of the genus Pinus.

According to one embodiment, a chemimechanical pulp consisting 100% of softwood fibers is produced. However, the invention may provide a chemimechanical pulp consisting of a mixture of hardwood and softwood fibers containing at least 5% softwood fibers, for example, it may contain 50-99% hardwood fibers and 1-50% softwood fibers. Coniferous fibers, especially spruce fibers, can be used to increase pulp mass, strength properties and stiffness.

Wood chips are usually about 20-50mm x 1-10mm, typically about 35-40mm x 3-5mm.

The wood chips are first vented as completely as possible. Usually, at least 70%, especially about 80-100%, of the air contained in the chips is removed. In the case of the process diagram shown, this deaeration is effected by steaming the chips in the evaporator 1. In the process of the figure, the feedstock chips are fed by means of a screw conveyor 2 to a steaming silo 1. The purpose of steaming is to drive the air out of the chips. At the same time, steam remains in the chips. The steaming can be carried out, for example, in the continuous steamer 1 shown in the figure, where the chips pass through the steaming silo 1 according to the figure, where they are contacted with saturated or nearly saturated steam for about 0.5-20 minutes, especially about 1-10 minutes. Steaming can be done under excess pressure, but normally normal-atmospheric steaming is quite sufficient. In particular, the operation is carried out at elevated temperature, which onesim. about 50-100 ° C, especially about 80-100 ° C, depending on how saturated the steam is. Instead of steaming, the deaerating can be done under vacuum / vacuum or the steaming can be enhanced by vacuum treatment. The treated chips from the steaming silo are removed through outlet 4, after which the chips are preferably pressed in a plug screw 5. After this step, typically at least 95%, preferably at least 98% of the air has been removed, with some of the steam being removed. For the sake of completeness, it should be noted that the steaming of chips is used in the manufacture of both sulphate pulp and chemimechanical pulp. Prior to the present invention, however, it has not been proposed to combine steaming and pressurized impregnation of chips in the manufacture of chemimechanical pulp.

After the venting step, the chips are subjected to impregnation treatment 6. In a preferred embodiment of the invention, the steamed chips are introduced into a impregnation step substantially at the steaming temperature in the impregnator 6, whereby the temperature of the impregnating solution is kept at the temperature of the steaming. In practice, the impregnation step is carried out in a sealed vessel, i.e. a pressure vessel arranged downstream relative to the evaporator. The absorber shown generally comprises an elongate absorber having a substantially vertical longitudinal axis having an upper portion and a lower portion, wherein the chips from the deaeration unit can be fed to the upper portion of the absorber and removed from the lower portion of the absorber. According to the invention, an absolute pressure of at least 1.5 bar, preferably about 1.5 to 15 bar, can generally be applied to the impregnator.

When the chips are rapidly fed to the impregnation vessel, their temperature will drop to a maximum of about 10-20 ° C before the impregnation step begins.

The absorber 6 is provided with an upper separator 7 through which the inlet nozzle 7a is fed to the absorber and where the liquid is separated from the chips. This liquid is recycled to the effluent stream of the steaming silo 1.

In a preferred embodiment, the steamed chips are fed to the impregnation step together with the impregnation chemicals, wherein the impregnating chemicals are supplied from separate feeders 10a-10c to a conduit 11 that connects the evaporator 1 outlet 4 and the impregnator 6 feed 7a. Preferably, the pipeline includes pumps 20, 21 or the like for generating pressure.

The impregnation step uses a impregnating solution comprising an aqueous solution of an alkaline agent, optionally containing sulfonating chemicals. Typically, an aqueous solution of an alkali metal hydroxide such as NaOH or an alkaline earth metal hydroxide such as Mg (OH) 2 is used, optionally containing, for example, sulfite compounds such as sodium sulfite. The concentration of alkali hydroxide is typically about 2 to 12 kg / Adt, preferably not more than about 6 kg / Adt, particularly preferably not more than about 4 kg / Adt. The solution has a pH of about 9-11 The sulfite compound is used in an amount of about 1-20 kg / Adt, preferably no more than 3 kg / Adt for hardwood.

In addition to pure solutions, aqueous solutions of mixtures of alkaline agents, such as a pulp cooking broth such as white or green liquor, may also be used for impregnation. The temperature of the impregnation step is about 30-95 ° C, preferably about 40-90 ° C, which can be achieved, at least in part, by the heat supplied through the chips. Generally, the temperature during impregnation is lower than that of the deaeration step. According to the invention, the pressure of the impregnation step is about 1.5 to 15 bar, preferably about 2 to 10 bar, absolute. This means that at least about 0.5 bar overpressure is used for impregnation. The wood / liquid ratio (w / w) is generally about 1:20 to about 1: 4, especially about 1: 15.1: 6.

The dosage of the absorption chemicals can be adjusted according to the chips to be treated and increased as necessary.

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

During impregnation, the chips are impregnated as completely as possible with the base.

In general, it is sought to fill the impregnation solution with at least 85%, preferably at least 90%, particularly preferably at least 95%, of the pore volume of the chips.

The impregnation may be one or more steps, with at least one impregnation step being carried out under excess pressure. According to one preferred embodiment, hot chips are first impregnated under excess pressure under the above conditions, after which impregnation is continued in an open container or container at the same or different temperatures. About 10-80% of the total impregnation treatment can be performed over time under pressure. In the embodiment shown below, the pressure and non-pressure treatment duration was equal to a total time of 40 minutes.

The chips from the impregnation step are removed through the discharge connection 6a. Reference numeral 6b denotes an absorber discharger that can be used to remove the fraction on the bottom of the device. The chips are then introduced into a conventional chemimechanical pulp mill 12, which can be performed, for example, on mills with grooved grinding blades. The wood raw material is ground to a preselected drainage of 50 to 500 ml CSF, particularly preferably about 90 to 150 ml CSF.

Scheme 1 illustrates how in practice the impregnation chips stream can be subjected to further processing prior to grinding, whereby the impregnation solution is first removed in the screw press 13, after which the chemicals can be further reacted in the reaction silo 14 before transferring the chips to the screw conveyors 15a and 15b. The reaction time in the reaction silo 14, if used, is typically about 0.1 to 10 hours.

In the screw press 13, the chips can be used to separate impurities and non-refining fibrous material, which through the screen 16 is removed to the reject channel. The liquid phase 17 from the screw press may be recycled, possibly in combination with fresh water supply, to pipeline 10. Reference numerals 22 and 23 refer to pumps arranged to supply the liquid phase. As shown in the figure, the impregnation solution is preferably recycled in the process and its alkali concentration can be adjusted (increased) by the fresh addition of alkali.

It should be emphasized that good venting prior to the combination of impregnation solution and chips and pressure impregnation is essential in the invention. These together provide effective penetration of the impregnating solution into the chips. The residence time and temperature in the pressure vessel are selected to adjust the diffusion and reaction time. Diffusion must take place, the reaction rate must not be too high.

The chemimechanical pulp described above has exceptionally good properties. As stated in the introduction, the pulp has improved light scattering, which is achieved without increasing the stick content. Thus, at the same level of feenness, the light scattering of the pulp according to the invention is at least 5%, even 10%, better than the high alkaline reference. At the same time, the pulp contents of the pulps according to the invention are lower than the TMP reference and, surprisingly, even lower than the high alkaline reference. At the same freeness level, the bulk also improves by up to 5%

As an example, a wound CTMP pulp may have a scattering of greater than 45 m2 / kg and a stick content of less than 0.3%. Similarly, it is possible to obtain a birch pulp with a scattering of greater than 45 m2 / kg and a content of sticks of less than 1.5%. These are just examples of the properties of the pulp and it should be emphasized that within the scope of the invention the pulp manufacturer is free to choose either the desired scattering level or the stitching level and the invention achieves a significant improvement in the second parameter.

The pulp of the invention can be used to make paper and board products.

Thus, after the above defibration, the pulp is usually bleached, e.g., with hydrogen peroxide, under alkaline conditions to a lightness of about 75-88%. To modify the properties of the starting material, the pulp may, if desired, be mixed with the chemical pulp so as to obtain a resolvable starting material which, however, contains a significant amount (at least 30% by weight) of the chemical pulp. Preferably, the chemical pulp is softwood pulp, which then represents between 1% and 50% of the dry weight of the raw material fibers. However, it is possible to use only chemomechanical aspen pulp.

The pulp is pulverized in a manner known per se to a suitable consistency (typically about 0.1 to about 1% solids) and applied to a wire where it is webbed to form a paper or board web. A filler, such as calcium carbonate, can generally be added to the fiber stock, generally from about 1% to about 50% by weight of the fibers.

The paper web may be surface-glued and / or provided with a coating layer and, if desired, cured. Coating pastes can be used as single coat pastes as well as so-called pre-coat and top coat pastes. Triple coatings are also possible. In general, the coating composition of the 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 additives known per se.

As described above, the webs of the present invention can be used to produce material webs having excellent printability, good smoothness, and high opacity and brightness. Examples of applications include fine papers, coated printing papers and brochure papers, and liner layers for multi-ply paperboard.

The following non-limiting examples illustrate the invention.

Example 1

A CTMP pulp from wound was prepared in the laboratory under the following conditions:

At 100 ° C for 2 to 5 minutes, the steamed aspen chips were impregnated with various amounts of sodium hydroxide at 5 bar (a) at 80 ° C for 20 minutes in a sealed container. The impregnation was then continued for another 20 minutes in an open reaction silo at 80 ° C.

To determine the properties of the pulp, e.g. the following methods: - bulk cm3 / g: EN 20534 - scattering m2 / kg: ISO 9416 - CSF ml: ISO 5267-2 - so-called sticks. Pulmac sticks, sample size 3 g and slit plate for 150 ml CSF mass 0.08 and 325 ml CSF mass 0.10 mm.

Table 1 shows the results when the chips so treated were milled to a filtration level of 150 ml CSF.

Table 1. Aspen. CSF 150 mL

As can be seen from the table, the invention can reduce the dosage of alkali hydroxide, whereby the scattering is clearly increased without an increase in the stick content. Scattering shows an increase of more than 10% over conventional alkali. Surprisingly, the concentration of sticks was even lower than the 1.2% alkaline reference.

Table 2 shows the bulk of the pulps shown above.

Table 2

As can be seen, at the same CSF level, the bulk is also slightly better.

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

Table 3. Birch mass. CSF 325 mL

In this case, too, a significant improvement in scattering was obtained, even though the concentration of sticks remained high in this experiment. However, it was a third smaller than the reference.

In the case of birch, lowering the alkaline dose did not significantly affect the sweat.

Example 2. Laboratory impregnations

In the laboratory, impregnations were carried out at normal pressure with wound and birch at doses of 2.5, 5 and 10 kg NaOF / ADt chips. Wood: liquid ratio 1: 8, temperature 80 ° C.

The impregnation solution was sampled 15 min, 30 min, lh and 3 h after the start of impregnation. The amount of organic matter dissolved in the impregnation solution increased sharply as the alkali dose increased and as a function of the impregnation time. At the same time, the dissolved COD increased sharply.

The table below summarizes the results of the Example on the effect of an alkali dose on fiber loss in a 40 minute laboratory wound impregnation.

At the same time, it should be noted that the fiber loss is determined from the impregnation solution prior to grinding. Correspondingly, the milling increases the dissolved matter determinant with the higher the alkali dose.

Table 4. Aspen chips

Claims (13)

A process for preparing the chemical mechanical pulp of a wood-shaped wood raw material, according to which the process - the wood pieces are contacted in an impregnation step with an alkaline impregnation solution under conditions where the impregnation solution penetrates the wood-piece, the largest part of the wood raw material is removed from the wood raw material. air containing it by steam treatment, which is carried out at a temperature of 50-100 ° C, and - the chips treated with the impregnation solution are ground to a desired drainage capacity to produce the pulp, characterized by a combination, in which - an impregnation solution is used in the impregnation step. , which impregnation solution contains alkali metal hydroxide, the dose of which is a maximum of about 6 kg / Adt, and - the raw material thus obtained is impregnated with an alkaline impregnation solution under pressure at a temperature of 30 - 95 ° C in such a way that impregnation the bonding solution can be effectively absorbed into the chip pieces before the chip pieces are ground. 2. A process according to claim 1, characterized in that the wood raw material is steam-treated at elevated temperature, especially at a temperature of about 80-100 ° C, to vent the chips. Method according to claim 2, characterized in that - the steam-treated chips are substantially brought to the impregnation stage at the steam treatment temperature and - in the impregnation step, the temperature of the impregnation solution is kept below the steam treatment temperature. Method according to claim 2 or 3, characterized in that the steam-treated chips are fed into the impregnation step via a screw press. Process according to one of the preceding claims, characterized in that in the impregnation step an impregnation solution containing alkali metal hydroxide is used, the dose of which is a maximum of about 4 kg / Adt. Process according to any one of the preceding claims, characterized in that an aqueous solution of an alkali metal hydroxide such as sodium hydroxide, boiling liquid obtained from chemical cellulose boiling such as green liquor or white liquor or a sulphite compound such as sodium sulfite is used as an impregnation solution. . Process according to one of the preceding claims, characterized in that the wood raw material is ground to a drainage capacity of 50 - 500 ml CSF, preferably about 90 - 150 ml CSF. Method according to one of the preceding claims, characterized in that the temperature of the impregnation step is about 40 - 90 ° C. Method according to one of the preceding claims, characterized in that the pressure of the impregnation stage is about 1.5 - 15 bar, preferably about 2-10 bar, absolute pressure. Method according to one of the preceding claims, characterized in that in the impregnation step at least 85%, preferably at least 90%, most preferably at least 95%, of the volume of the pores of the chip pieces are filled with impregnation solution. Method according to one of the preceding claims, characterized in that the impregnation is carried out in a plurality of steps, wherein at least one of the steps is carried out at overpressure and at least one step at normal air pressure. Method according to one of the preceding claims, characterized in that the steam treatment of the chips is carried out at least substantially at normal air pressure using saturated or almost saturated water vapor. Chemical mechanical pulp, characterized in that it is made according to a process according to any one of claims 1-12.