FI87371C - Process for making mechanical pulp - Google Patents

Description

87571

The present invention relates to a method for producing a mechanical pulp according to the preamble of claim 1.

According to such a method, the wood raw material is decomposed into chips, which, after pretreatment, are ground to the desired permeability.

Mechanical pulping processes are playing an increasingly important role in increasing the degree of processing of wood-containing printing papers. The main advantages of mechanical pulps are very high wood yield and good optical printing properties. The most notable disadvantages, in turn, are the high energy demand, the poor strength properties of the masses and the post-emergence.

The hot milling process (TMP) has been specially developed for the production of good printing paper. In the normal TMP process, the chips are preheated to a temperature of 100 to 135 ° C, after which they are ground to the desired permeability. The preheating processing time is relatively short, i.e. usually no more than a few minutes. By raising the preheating temperature, the strength properties of the pulp are improved. At the same time, however, the mass yield deteriorates due to partial acid hydrolysis.

Methods for producing mechanical pulp are also known in which the chips are defibered in a depressurized or pressurized state using mildly delignifying chemicals such as sodium sulfite. The chemicals are absorbed into the chips before grinding. In the case of sodium sulphite, the absorption has been carried out under temporal conditions (pH approx. 10). These chemimechanical methods (CTMP) bring about an improvement in the strength properties of the pulps, but the energy consumption generally increases compared to the conventional TMP process and the yield of the pulp decreases.

Typical of the pretreatments performed in the CTMP and TMP processes are that they change the chemical properties of chip 2 87571; chemically dissolving the lignin or correspondingly softening it at a high temperature. However, as can be seen from the above, these treatments lead to an increase in the energy consumption of grinding.

The object of the present invention is to eliminate the drawbacks associated with the prior art and to provide a completely new method for producing a mechanical pulp, in particular a hot mill.

Our invention is based on the idea that, during the pretreatment, at least part, preferably as much as possible, of the air is first removed from the chips, after which water is absorbed into the chips under acidic conditions before grinding.

More specifically, the method according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

As discussed above, known processes for the production of mechanical or chemimechanical pulp seek to affect the chemical composition of wood by breaking covalent bonds or melting lignin at high temperatures. In contrast, in the process of our invention, the conditions are such that chemical degradation of lignin or carbohydrates is unlikely to occur. We are not fully aware of the mechanism that results in less energy being consumed in the grinding of chips in our method. It is possible that due to the mild processing conditions, the physiological and biochemical structure of the wood at the cellular level is not damaged, allowing water to naturally pass between the fibers in the cellular network, affecting the hydrogen bonds that hold the fibers.

The method according to the invention comprises two steps: a deaeration step and an impregnation step.

According to a preferred embodiment of the invention, the air-firing step is carried out with steam, preferably water steam. In particular, 3 87571 nothing under unpressurized conditions and steam at about 100 ° C is used. The treatment is continued until at least a substantial part of the chips has reached a temperature of about 90 ... 100eC. Such treatment ensures that at least most of the air contained in the chips has been removed. Since it is desired to avoid softening and melting of the lignin during evaporation, which is detrimental to grinding, the aim is to avoid too high steam temperatures and at the same time to use relatively short evaporation times (usually about 10 to 30 minutes). The duration of steam required varies depending on the amount of chips and the dimensions of the chips, as well as the temperature and amount of steam.

According to an alternative embodiment, the chips are treated in the first step of the process under reduced pressure to remove air. In this case, the chips are treated at normal temperature or at elevated temperature, e.g. at a pressure of about 0.1 to 0.9 ata.

The chips are soaked with water under acidic conditions. The absorption is carried out in the liquid phase, in which case the acidic conditions mean that the pH of the liquid phase is less than or equal to 6. The liquid phase of the absorption may consist of water or a mixture of water and another liquid. In addition, it may contain additives which affect the properties of the pulp to be prepared, which are absorbed into the chips at the same time as water.

According to the invention, water with an initial pH of about 1 to 5, preferably about 2 to 4, is preferably used for the absorption of the chips. If the pH is too high, the absorption will not be sufficiently efficient, while too low pH values will cause corrosion problems in the absorption equipment and grinders.

Cold water is preferably used for the absorption, e.g. the plant's normal raw water from about 5 to 20 ° C, which has been acidified with mineral acid. Suitable mineral acids are, for example, hydrochloric, nitric and sulfuric acids. The latter is considered advantageous because it causes the least environmental problems. Sulfuric acid in wastewater can be easily neutralized, for example with lime, to obtain harmless gypsum.

In connection with the invention, the aim is to absorb at least almost as much water in the chips as it can absorb. The dry matter content of the chips is then usually about 25 ... 35% after absorption. The wood material is known to have a near-neutral pH value, but in connection with the impregnation treatment the pH value drops so that after the treatment it is about 3 ... 5. The progress of the absorption can thus be monitored by pH measurements. The absorption time varies depending on the chip / water ratio and the acidity of the water. However, absorption always takes considerably more time than steaming, usually 1 to 24 hours, preferably about 2 to 18 hours. The absorption temperature is not critical.

However, as stated above for evaporation, it is advisable to avoid excessive temperatures, which, as causing polymerization reactions of lignin and other wood raw material components, may lead to an increase in the need for grinding energy. The method according to the invention therefore operates, for example, in such a way that cold water is introduced into the steamed, hot chips, whereby the temperature of the chips is allowed to lower to the level of water during impregnation. However, freezing of wood chips should be avoided, as it may contribute to increasing the energy consumption of grinding. It is also possible to maintain a constant temperature during absorption. The application examples below have been performed at a temperature of about 40eC.

The invention provides considerable advantages. Thus, as mentioned above, grinding the chips consumes 15 to 20% less energy than in a conventional TMP process.

The present invention is thus of great economic importance, as the method can save 10-15% of electrical energy in the production of mechanical pulp (some energy is used for steaming and absorption), which means that at least several hundred million marks a year are saved in Finland.

However, the properties of the pulp produced by the process, in particular their printing properties, are at least as good as those of conventional TMP pulp.

The advantage of the method is even more pronounced in that it does not, in principle, require much expensive equipment investment, as the equipment required for the method in existing plants is already in place.

The invention will now be examined in more detail by means of application examples with reference to the accompanying drawings, in which Figure 1 shows a side view of the basic structure of an apparatus suitable for use in the method according to the invention, Figures 2-4 show specific energy consumption as a function of the tensile index of the luminosity of the pulps, Fig. 6 shows the light scattering coefficient (CSF) of the ground pulps as a function of the tensile index of the pulp of the pulps.

As mentioned above, conventional equipment currently used, e.g. for the production of chemimechanical pulp, can be used in the invention. The structure of the apparatus used in the application examples below is shown in Figure 1. The apparatus comprises a CTMP digester 1 comprising a heat exchanger 2 and a cooking chemical tank 3. The digester has a volume of about 4 m3 and can be filled with about 500 kg of chips at a time. The digester 1 consists of a sealed elongate pressure vessel, the upper end of which is fitted with a chip filling pipe 4 and two liquid supply connections 5 and 6, one of which can also be used to remove gas from the digester. A liquid drain pipe 7 is arranged at the bottom of the pressure vessel 6 8 7 ο 71, which is connected to a heat exchanger 2 via a valve. The drain pipe 7 is connected to a dense grate structure inside the digester 1.

The chips, in turn, can be emptied from the pressure vessel 1 and transferred to grinding by means of a screw conveyor 8.

By means of the heat exchanger 2, water vapor can be supplied to the tank 1 from below 7 both under unpressurized conditions and under pressure, the water vapor passing through the chips and leaving the tank 1 via the second upper connection 5. The water required for absorption is fed to the digester 1 from above via the second upper connection 6, removed through the drain pipe 7 and circulated through the heat exchanger back to the upper connection 6. If desired, the water to be recycled can be heated in the heat exchanger 2 to reach a constant temperature.

By means of the solution tank 3 provided with a stirrer 9, an acidic aqueous solution for impregnation can be prepared. The solution is passed through an outlet pipe 10 at the bottom of the tank to a heat exchanger 2, from where it can be further fed to the digester 1 via an upper connection 5, as described above.

Example 1

Preparation of water-impregnated and reference TMP pulp

The raw material used was United Paperitehtaat Oy, Jämsänkoski spruce chips. The dry matter content of the chips was 45.6%. The dimensions of the chips based on the Williams experimental screening were: mean dimension 18 mm and mean thickness 5.3 mm.

Water-impregnation:

For water absorption, 530 kg (240 kg abs calculated dry) of the above-mentioned chips were transferred to digester 1. To remove air from the chips, the chips were steamed with steam at 100 ° C for 15 min before impregnation with water. 1400 L of cold (about 15 ° C) water was added to solution tank 3 (87371) and its pH was adjusted to 3.4 with sulfuric acid. The solution was sprayed from above into the hot chips. The temperature measured from the solution at the beginning of absorption was 36 ° C. During the recirculation, the chips were always covered with a liquid and the temperature of the solution at the end of the absorption was 38 ° C. At the end of the absorption, the pH of the solution was 4.6, which reduced the dry matter content of the chips from the initial 45.6% to about 33-34. % by weight.

Pulp production:

The chips were ground with an Enso-Bauer 411 two-disc grinder as a pressurized two-step process. The first grinding step was preceded by a 1.5 min chip preheating step at an overpressure of 1.8 bar (130eC), the pressure and temperature in the refiner were at the same level as in the preheater. The second grinding step was depressurized, the temperature in the grinder was about 100eC.

The production in the first stage milling was 584 kg / h with TMP and 651 kg / h with water-absorbed TMP. The dry matter content of the pulp after the first stage was 24-25% and the freeness 300-400 ml.

The production of the second stage was 408 kg / h for TMP and 475 kg / h for water-absorbed TMP. In the second stage, mass samples were taken from different load levels of the refiner. The properties of the pulps were tested mainly according to SCAN methods and the sheet properties were measured from 52 g / m2 circulating water sheets. Fineness or permeability values are reported as Canadian Standard Freeness figures.

Mass properties:

Figure 2 shows the specific energy consumption (EOK, MWh / t) of the ground pulps as a function of the fineness (CSF, ml) of the pulps. As can be seen from Figure 2, the grinding of water-absorbed chips consumes significantly less energy when grinding to a constant fineness.

8 87371

Example 2

The second batch of spruce chips was soaked in acidic water as in Example 1, but the soak time here was 2 and 4 hours before rubbing. The comparative experiment was performed as in Example 1. As can be seen from Fig. 3, the specific energy consumption of the water-absorbed chips is clearly lower than in the comparative experiment.

Example 3

The third batch of spruce chips was water impregnated in the same manner as in Example 1, but here water impregnation was also performed with water, the pH of which was adjusted to 2 with concentrated sulfuric acid before impregnation.

It can be seen again from Figure 4 that the rubbing of the chips impregnated with acidic water consumes less energy than the normal hot milling process.

Example 4

In Example 1, test sheets were prepared from the ground pulp to test the properties of the pulp. Figure 5 shows the dependences of the brightness of the reference mass (TMP) and the water-absorbed grit on the tensile index. As can be seen from Figure 5, the brightness of the water-impregnated pulp is at least as good as that of the reference pulp.

Figure 6 shows the dependence of the light scattering coefficient of the fractions on the fineness of the masses (CSF). As can be seen from Figure 6, the light scattering coefficients of the masses here do not differ from each other.

Finally, Figure 7 shows the dependence of the light scattering coefficient of the rubs on the tensile index. As can be seen from this figure, the light scattering of the pulp made of water-impregnated chips is at least as good as that of the control TMP pulp.

Claims (10)

1. A method of producing mechanical pulp of wood-based raw material, according to which process - the raw material is decomposed into wood chips, - the wood is subjected to a pretreatment, in which at least part of the air contained in the wood is removed and the wood is exposed to a wood. impregnation treatment, after which - the chip is ground to the desired drainage capacity, characterized in that - during the impregnation phase, the chip is impregnated with cold water in acidic conditions for a period of time between about 1 and 24 hours. 2. A method according to claim 1, characterized in that the impregnation is carried out with water having a temperature of about 5-20 ° C. 3. A method according to claim 1, wherein the air is removed by means of a meadow treatment, characterized in that the impregnation time is considerably longer than the meadow treatment time. 4. A method according to claim 3, characterized in that the wood is meadow treated without pressure for about 10 - 30 minutes and then it is impregnated with water for about 2 - 18 hours. 5. A method according to claim 3 or 4, characterized in that in the meadow treatment water meadow with a temperature of 100 ° C is used and the treatment is continued until edible minus a substantial part of the chip reaches a temperature of about 90-100 ° C. 6. A method according to claim 1, characterized in that the chip is subjected to a suppression treatment in order to discharge 12 8 7 ύ /! remove the air from it. 7. A method according to any of the preceding claims, characterized in that water is used for impregnating the wood with an initial pH of about 1 - 5, preferably about 2 - 4. 8. A method according to claim 7, characterized in that the impregnation is carried out with cold water acidified with a mineral acid, in particular sulfuric acid. 9. A method according to claim 7 or 8, characterized in that the impregnation of the wood is continued until at least the pH of the wood is at least between 3 and 5 and its dry matter content is about 25 - 35%. 10. A method according to any of the preceding claims, characterized in that the wood is impregnated with at least approximately as much water as it is capable of absorbing.