FI68432C - FOERFARANDE FOER FRAMSTAELLNING AV RAFFINERAD TRAEMASSA. - Google Patents

Description

68432

METHOD FOR THE PRODUCTION OF GRINDED WOOD PULP - FÖRFARANDE FÖR FRAMSTÄLLNING AV RAFFINERAD TRÄMASSA

The present invention relates to a process for producing pulverized wood pulp from wood chips, in which process softwood chips and hardwood chips are used so that the obtained ground pulp contains a mixture of both softwood and hardwood material, and in which

It is already known that small amounts of hardwood can be added to softwood when producing wood pulp mechanically by the hot milling method. The addition of hardwood has a positive effect on the optical properties of the pulp, but the use of hardwood has been limited by the fact that as the amount of hardwood increases, the strength properties of the pulp deteriorate rapidly.

It is further known that when using hardwood for pulp production, hardwood chips are impregnated with chemicals before grinding. A prerequisite for achieving a sufficiently uniform absorption result has been that the treatment has been strong enough. An active factor in chemical treatment is a base such as NaOH, which reacts with the carbohydrates in the wood to swell and soften the structure of the wood. At the same time, however, the base darkens the wood and lowers the fines content and light scattering coefficient of the resulting ground pulp. Sulphite, e.g. Na 2 O 2, has also been used in the chemical treatment to prevent darkening of the wood. Sulfite further reduces the amount of fines and the light scattering coefficient of the pulp, which determines the opacity of the paper made from the pulp.

When using pulp with hardwood treated in addition to softwood as described above for the production of printing paper, the dustiness of the paper on the one hand and the opacity of the paper on the other hand become critical factors. These critical factors limit the amount of hardwood in the pulp to 10-15%.

The object of the present invention is to provide a solution which makes it possible to increase the proportion of hardwood in the ground pulp for the production of printing paper to a substantially higher height. The invention is characterized in that an alkali hydroxide solution with a concentration in the range of 0.1 to 0.5 M is used in the pretreatment of the hardwood chips, that the pre-treated hardwood chips are mixed with the softwood chips and that the resulting chip mixture is then ground to a pulp.

It is essential in the invention that the pretreatment of the hardwood chips with a 0.1-0.5 M time hydroxide solution is very mild compared to the way the hardwood has been treated so far. Such a slight pre-treatment is made possible by the fact that the pre-treated hardwood chips according to the invention are ground into a pulp mixed with softwood chips. Slightly treated hardwood fibers have viscoelastic properties close to softwood fibers, so that the conditions for mixing and simultaneous grinding of hardwood and softwood exist and the proportion of hardwood in the mixture can be increased up to 70%. Preferably, the proportion of hardwood in the mixture to be ground is in the order of 30-60%.

In the previously known methods using hardwood chips, the strongly pretreated hardwood is ground separately into a pulp and only then mixed with the pulp obtained from softwood. In the case of a slight pretreatment according to the invention, grinding the hardwood separately would present difficulties due to the uneven absorption of the alkali hydroxide. However, there are no difficulties when the slightly pretreated hardwood is ground according to the invention together with the softwood, since then the larger and more flexible softwood fibers keep the grinder's blade gap stable and equalize the stress peaks on the hardwood fibers. The whole mixture thus receives a uniform friction treatment.

A particular advantage of the mild alkaline pretreatment according to the invention is that the good optical properties of the hardwood fibers are maintained and the binding of the fibers is improved. The method according to the invention can thus be used to produce pulp with a high proportion of hardwood which, in terms of its bonding capacity and opacity, meets the requirements set by offset printing paper.

68432

In the process according to the invention, the soluble amount of organic matter is so low that it is not necessary to wash the pulp. This significantly reduces the plant's investment costs. In addition, the amounts of chemicals and vapors required for pretreatment are even lower, which lowers the operating costs of the process. Overall, the invention provides a simple and flexible process in which the relationship between the amounts of hardwood and softwood can be varied within wide limits and in which the investment costs remain lower than in the corresponding known methods.

The alkali hydroxide solution used in the pretreatment of hardwood chips can be, for example, a sodium hydroxide solution containing 5-20 g of NaOH / l. When the sodium hydroxide concentration is low, the addition of sulfite may not be necessary. However, a solution containing, for example, 5-50 g of Na 2 SO 4 / l can also be used in the pretreatment.

To enhance the absorption of alkali, the chips are treated with water vapor before absorption. The alkaline solution used for absorption is cold. To enhance the reaction between the absorbed alkali and the wood, the absorbed chips are also treated with hot water vapor. Evaporation follows immediately after the impregnation step and, according to the invention, can take place at normal pressure in the temperature range 60-100 ° C. However, it is possible that the evaporation temperature is also higher, e.g. 100-130 ° C, and that a slight overpressure is used in the evaporation.

The invention will now be described in more detail with reference to the accompanying drawings, in which

Figure 1 shows in diagrammatic form a process for the production of ground pulp according to the invention and

Figures 2-5 show some physical properties of ground wood pulps with different CSF values in some exemplary cases.

68432

The starting materials for the pulverized wood pulp production process according to Figure 1 are hardwood and softwood chips (aspen and spruce), which are stored in silos 1 and 2. Hardwood chips are passed from silo 1 to washing step 3, from where the chips are further processed. In the pretreatment, the chips are pre-evaporated (step 4) and the chips are impregnated at room temperature with a dilute sodium hydroxide solution, preferably containing about 10 g of NaOH / l (step 5). Absorption can take place, for example, by passing the chips by means of a conveyor through a container containing sodium hydroxide solution so that the residence time of the chips in the container is about 5 minutes. After treatment with sodium hydroxide, the chips are transferred to a separate evaporation chamber (step 6), which preferably has a temperature of about 90 ° C and normal atmospheric pressure. The steam required for steaming is obtained from the grinding process. After steaming, the pre-treated hardwood chips are passed together with the softwood chips passed through the washing step 7 to a mixing tank 8, where the chips are mixed with each other, e.g. in a ratio of 1: 1.

The mixing step is followed by grinding the chip mixture into a pulp by the hot milling method, which represents a technique known per se. The chip mixture is transferred from the mixing tank 8 to two successive grinding steps 9, where it is ground into a pulp 10 by means of plate grinders. The finished pulp is available, for example, as a starting material for the production of offset printing paper.

Figures 2-5 show some physical properties of ground pulps with different CSF values (Canadian Standard Freeness). Figures 2 and 3 are tensile and tear indices describing the strength properties of the pulp, Figure 4 is a light scattering coefficient of the pulp describing the optical properties of the pulp, and Figure 5 is a question of the proportion of long fibers in the pulp describing the binding capacity of the pulp. The four curves shown in each figure represent four different masses according to the following table.

5 68432 I NaOH Six Aspen Birch 1 (g / 1) (¾) (X) (X) _____i_________1__________1_______________________________ 0 I 0 50 50 X * 5 50 50 □ I 10 60 20 20 O 1 10 50 50

The first vertical column of the table describes the pretreatment of the hardwood chips used to make the pulp, showing the NaOH content of the solution used to soak the chips. The top horizontal figure in the table, in which the NaOH content is marked as 0, represents a mass in the preparation of which no pretreatment with alkali was carried out and which is therefore not in accordance with the invention. The other three masses, on the other hand, are all made according to the invention. Looking at the figures, it can be seen that the pulps prepared according to the invention have at least a satisfactory level of all properties. The pulp, which was not pretreated with alkali, has a higher light scattering coefficient than the pulps prepared according to the invention, but instead has a lower bonding capacity and strength properties than said pulps.

The following is a table comparing the properties of pulps obtained from softwood chips and hardwood chips pretreated according to the invention by separate rubbing and by the mixture rubbing according to the invention.

6 68432

Mixtures CSF Density Tensile index Tear index Burst index.

(ml) (kg / m3) (Nm / g) (mNrn / g) (kParn / g)

Aspen / Spruce 50/50

Seoshierto 73 467 45.1 8.7 2.53

Separate rotation 123 + 75 450 36.4 6.8 2.12 "123 + 128 434 36.2 7.8 2.05

Aspen / Spruce 70/30

Seoshierto 88 414 34.9 7.0 1.89

Separate rotation 123 + 75 453 34.8 6.8 1.96

Seoshierto 125 399 31.9 «7.1 1.78

Separate rotation 123 + 128 414 33.1 6.4 1.77 Starting masses

Aspen CTMP 123 452 35.0 4.3 1.64

Six TMP 128 395 38.9 10.1 2.43 75 445 43.2 10.2 2.65

It can be seen from the table that with a 70% hardwood fraction, mixture and separate rubbing achieve essentially the same result in terms of pulp quality. In contrast, when the proportion of hardwood is 50%, mixture rubbing achieves substantially better pulp processability and strength properties than stand-alone rubbing.

It will be clear to a person skilled in the art that the various embodiments of the invention are not limited to the example given above but may vary within the scope of the appended claims.

Claims (5)

7 68432 A process for producing pulverized wood pulp from wood chips, which process uses softwood chips and hardwood chips in such a way that the obtained pulverized pulp contains a mixture of both softwood and hardwood material, and in which solution with a concentration in the range of 0.1 to 0.5 M, that the pretreated hardwood chips are mixed with the softwood chips and that the resulting chip mixture is then ground to a pulp. Process according to Claim 1, characterized in that a sodium hydroxide solution with 5 to 20 g of NaOH / l is used in the pretreatment. Process according to Claim 2, characterized in that a solution of 5 to 20 g of NaOH / l and 5 to 50 g of Na 2 SO 3 / l is used in the pretreatment. Process according to one of the preceding claims, characterized in that the hardwood chips are steamed in connection with the pretreatment at normal pressure at a temperature of 60 to 100 ° C. Method according to one of the preceding claims, characterized in that the hardwood chips and the softwood chips are mixed together in such a way that the proportion of hardwood chips in the mixture is at most 70%, preferably in the range from 30 to 60%.