EP0258352B1 - High temperature, high consistency bleaching - Google Patents

Abstract

The bleaching chemicals are added to the pulp immediately after heating of the raw pulp thickened to more than 20% by weight and immediately prior to a dispersing process. This is carried out in a high-speed, high-efficiency disperser, where the heated, thickened pulp is forced through a plurality of narrow slits in a disperser lining and is exposed to strong shearing, impact and kneading stresses. In this way, the dispersing process is reduced to a time period less than 3 seconds, preferably about 1 second or less. As a result, the total time required for reductive bleaching is reduced to less than 15 seconds and no further measures are required for displacing and excluding air from the system. The method can also be advantageously used with oxidative bleaching at reduced reaction time requirements. A high dispersing temperature in the region of 40 DEG C. to 110 DEG C., preferably about 95 DEG C. is possible and there are essentially simultaneously carried out the bleaching and dispersing actions without the need for separate dispersing stages and large reaction towers and pumps.

Description

The invention relates to the high-temperature, high-consistency rapid bleaching of pulp obtained from waste paper for the production of paper, the bleaching chemicals being added to the fabric immediately before a mixing operation.

The subject matter of the invention further relates to an oxidizing peroxide bleach, as well as to a reducing bleach made with dithionite as bleaching agent.

Both types of bleaching mentioned today are usually long-lasting, temperature-consuming processes, for which complex structures, for example high bleaching towers, have to be built as reaction vessels, since the bleaching process takes 1-2 hours.

According to a method common today, e.g. worked with the dithionite as a bleaching agent in such a way that a thin substance suspension with a 5% (weight) substance content is heated to 60 ° C. and this material is mixed with 1-2% dithionite in a twin-shaft mixer. This mixture is transferred to a bleaching tower in which there is an upward flow and where the bleaching material remains for over 60 minutes.

Another method known with dithionite as a bleaching agent is the so-called MC, i.e. Medium consistency method in which a consistency of 15% (weight) fiber is used. In this case too, 1-2% dithionite is added to the material. A mixer pump, the so-called, serves as the mixer

MC pump that mixes the bleach quickly and well. Nevertheless, with this method, an air extraction must be ensured, for which purpose the MC pump is set up so that an air extraction takes place at the same time during the mixing process. Since this is a reducing bleaching process, it is necessary to work without the presence of air, which would otherwise cause the bleaching agent to be decomposed by oxygen. In this case too, the bleached material will have a temperature of about 60 ° C and the bleaching time will be about 60 minutes. Because of the higher concentration, a slightly smaller bleaching tower can be used with this method than was necessary with the previously mentioned thin material method.

As far as oxidizing bleach with added peroxide is concerned, a 15-30% consistency has already been used. The chemicals were added to the suspension in a slow running disperser.

The reducing dithionite bleach at fabric densities above 15% has not yet been achieved. There is a US-A-2,963,395, in which material densities of 20-50% and temperatures of 38-110 ° C are proposed for dithionite bleaching. However, the air must be largely removed and an airtight reaction vessel must be used.

It was therefore known that in the experiments using high-temperature, high-consistency rapid bleaching with dithionite as bleaching agent, it was necessary to remove or keep the air away, which was difficult to achieve in spite of expensive means, the expenditure exceeding the expected savings.

A bleaching process can also be carried out in refiners. For example, US-A-3,467,574 offers a possibility of bleaching so-called chips from cellulose production with the aid of a refiner treatment. However, the mixing effect of a refiner is not sufficient for high demands.

The process is also very complex chemically and therefore expensive.

The French FR-A-1 239 047 shows a device which looks similar to the known disperser, but the bleaching process described there is significantly different, e.g. due to the low temperature of 40 ° to 60 ° C not comparable to a high temperature high consistency quick bleach.

It is an object of the present device to realize high-temperature, high-consistency rapid bleaching, in which savings can be achieved compared to the previous bleaching methods with approximately the same results. The response times should be shortened. It should be worked in a consistency of over 15%.

This object is achieved in that the bleaching chemicals are added to the substance immediately after the thickened fiber material has been heated, that the mixing process is carried out in a high-speed, short-term intensive mixer of the type of a disperser in which the mixture is passed through many narrow spaces Slots of the disperger set are forced, whereby the material is simultaneously subjected to strong shear and impact loads, and the mixing process takes place within three seconds.

It has been found to be particularly advantageous if the mixing process takes place within one second if possible.

In the case of reducing dithionite bleaching, it has surprisingly been found that, when working in this high-temperature, high-consistency range, the intensive and extremely brief intermixing of the reducing bleaching agent and because of the high reaction rate of the bleaching agent, no special measures for displacing air or Oxygen are more necessary, which were previously considered essential.

The high consistency of the fabric saves a great deal of thermal effort.

It is known from the literature that only a low activation energy is required in the case of dithionite bleaching, so that the diffusion process for the bleaching reaction becomes rate-determining.

It was recognized that due to the intensive and extremely brief intermixing of the reducing bleach, the bleaching reaction started the fibers start earlier before the oxygen-sensitive bleach has a chance to come into contact with air for a harmful period of time.

The disperger is particularly well suited for this task as an intensive, short-term bleaching mixer, because here the material is positively guided through many narrow slots under simultaneous strong shear and impact loads. This kneading brings the active chemical extremely quickly into the immediate vicinity of the fiber, where the chemical diffusion into the interior of the fiber then begins and takes place quickly.

It is a further advantage of this method that a disperser is used as the mixer because two process tasks are then carried out in one device: by mixing the use of the bleaching chemicals and by simultaneously dispersing the substance, rendering contaminants harmless, which occurs when processing Waste paper is necessary.

With the use of dispersers as bleach mixers, the previous limit of bleach densities has been raised from approx. 15-18% to approx. 30% and above, which results in a number of reaction-kinetic, energetic and combinatorial advantages.

The high consistency means an increase in the concentration of bleaching chemicals, i.e. according to the law of mass action also an increase in the bleaching product. In the presence of interfering constituents of the fiber, e.g. Heavy metal ions, which cause the bleach to decompose, naturally also have to be expected to concentrate these disruptive components if the consistency of the fabric to be processed is increased. Appropriate measures, e.g. through the so-called complexation, i.e. Addition of complexing agents to be counteracted.

Another advantage of this method is that it is possible to work at a temperature of, for example, 95 ° C., which is also required for hot dispersion to render the abovementioned impurities harmless, so that no additional heating of the fiber material is required. This must be seen as a decisive advantage, since the thermal costs of dithionite bleaching are significant. It goes without saying that the dispersing temperature, which should also be the bleaching temperature, must not be so high that this leads to excessive decomposition of the bleaching agent. This is achieved by - as mentioned above - working in a pressure-free area below 100 ° C.

Another success will be the saving of a larger thin material or MC bleaching tower with the appropriate pump equipment.

The aforementioned combination of dispersion and bleaching effects leads to the saving of an apparatus which is otherwise necessary for one of these effects.

• Ihre niedrige Aktivierungsenergie, d.h. der Reaktionsablauf ist weitgehend durch die Diffusion des Bleichmittels in die Faser bestimmt und weniger, bzw. überhaupt nicht durch katalytische, dissoziative und konzentrative Vorgänge.

The proposed method fully exploits the advantages of a dithionite bleach. The following are the features that distinguish the dithionite bleach:

• Their low activation energy, ie the course of the reaction is largely determined by the diffusion of the bleaching agent into the fiber and less, or not at all, by catalytic, dissociative and concentrative processes.

The bleaching reaction rate is very high, e.g. 80-90% of the bleaching effect can be achieved in an effective bleaching time of 1 minute or less. The reaction is practically complete after 5 minutes. This leads to the fact that instead of a larger bleaching tower for a dwell time of 60 minutes, a much smaller buffer tank can be used due to the high reaction speed. This must be dimensioned for a maximum of 15 minutes. Because of the rapid bleaching agent mixing in a period of approx. 1 second and because of the short and equal bleaching time, no measures are required to displace the air, as was the case in the past. Such a reducing process can also be carried out in the presence of air. There is no significant decomposition of the bleaching agent, so that the achievable white gain for a given bleaching agent use is only slightly below or equal to that of a complex conventional bleaching method.

The mentioned combination of dispersion and bleaching effects can save a separate bleaching stage, since the bleaching effect of the process takes place simultaneously with the dispersion step.

Furthermore, the subject matter of the invention is explained in more detail using a drawing sheet and the description of application examples.

• Figur 1 schematisch eine Anlage zur Durchführung der erfindungsgemässen Schnellbleiche,
• Figur 2 einen partiellen Längsschnitt durch einen Disperger,
• Figur 3 einen photographischen Blick in einen geöffneten Disperger.

Show in the drawing sheet:

• FIG. 1 shows schematically a plant for carrying out the rapid bleaching according to the invention,
• FIG. 2 shows a partial longitudinal section through a disperser,
• Figure 3 is a photographic view of an open disperser.

The fibrous material 1, which was prepared for the production of paper from waste paper, comes into a tearing screw apparatus 2, in which the screw is driven by a motor 3. The material is transferred to a heating apparatus 4, the screw of which is driven by a motor 5 and which is heated via a steam line 6. The pulp is heated to approx. 95 ° C in this apparatus. Immediately after leaving the apparatus 4, the heated pulp falls into an input shaft 7 of a fast-running short-term intensive mixer, in this case it is the disperser 11, that is to say the bleaching agent is already added to the thickened and heated fiber pulp in the input shaft 7 Bleaching chemicals through a line 8, which mün in the input shaft 7 det, admitted. The mixing process then begins directly in the area of a screw conveyor 21, which is accommodated in a housing 10 of the disperser 11 and is driven by a motor 9. This screw 21 conveys the material into the chamber 20 of the disperser, in which it is flung radially outwards, so that it comes into the teeth of the stator 18 or the rotor 17 of the disperser and leaves the disperser after this passage. This mixing-dispersing process takes less than 1 second. The material is positively guided over the screw 21, through the chamber 20, and through the toothing of the rotor 17 and the stator 18, it being between the teeth and when passing through the slots, which can be seen particularly well in FIG. 3 between the teeth , subjected to strong shear, impact and kneading stresses. During this passage through the disperger set described, not only is the bleaching agent added to the material to be bleached, as a result of which the bleaching process begins, but also the dispersing step in which contaminants, for example latex glue, are rendered harmless. The known arrangement of the stator 18 and the rotor 17 and their toothing can be seen particularly well from FIGS. 2 and 3. There is also a part of the screw 21 which pushes the mixture into the chamber 20. The rotor 17 is carried and driven by a shaft 19 which is mounted in a housing 12 of the disperser. This shaft 19 is driven by a motor 13. The speed of the rotor is relatively high. Depending on the diameter of the rotor, the speed is between 1000-3600 revolutions per minute. This means that the material passes through the Disperger set at lightning speed in less than 1 second. The dispersed mixture is transferred from the disperser 11 into a buffer tank 14, in which the bleaching process is ended. The bleached material is carried away as finished material through a line 15 for further processing. No special measures need to be taken to keep air away from the material. Thus, the tank 14 is also opened to the atmosphere by a vent 16.

I. Example of a reducing bleach, the dithionite bleach.

A mixture of mostly wood-free, partially coated and colored waste paper was dissolved in a pilot plant at 6% consistency and 45 ° C without the addition of chemicals. After cleaning the thick matter and then sorting it, the fabric was washed using a Variosplit washer (R). The consistency of the washed fabric was 7.6% and its white content R _{457 was} 54.2% MgO.

After dilution to 4.4%, thickening to 30.6% consistency was carried out in a screen press. The white content R ₄₅₇ was now 54.7% MgO. After passing through a disperser without bleaching chemicals, the substance having previously been heated to 95 ° C. in a heating screw, a white content R ₄₅₇ of 52.6% MgO was measured.

• Beim Nullpunkt 61,3% MgO
• Nach 5 Minuten 63,4% MgO
• Nach 15 Minuten 63,5% MgO
• Nach 30 Minuten 63,9% MgO

In the corresponding bleaching test, 1% sodium dithionite Na ₂ S ₂ 0 ₄ and 0.25% complexing agent DTPA were sprayed as a dilute solution with the aid of a nozzle into the heated substance in the input shaft of the disperser after the heating screw. The fiber was then fed into the disperser. After the material had passed through the Disperger set, a sample was taken from. which the white contents R _{457 were} determined after different dwell times. The following white contents R _{457 were} measured:

• At the zero point 61.3% MgO
• After 5 minutes 63.4% MgO
• After 15 minutes 63.5% MgO
• After 30 minutes 63.9% MgO

With the reducing, high-consistency, high-temperature bleaching process using a disperser as an intensive, short-term bleaching mixer, it was possible to increase the white content R ₄₅₇ by more than 11 white points.

11. Example of an oxidizing high-consistency high-temperature bleach, the peroxide bleach.

A mixture of wood-containing and wood-free waste paper was processed in a test center in a pulper at 15% consistency and 50 ° C with the addition of 0.2% complexing agent DTPA, 0.4% sodium hydroxide NaOH, 2% water glass and 0.5% hydrogen peroxide H ₂ 0 ₂ resolved. After cleaning and sorting thick matter, the fabric was washed on the Variosplit washer (R). The white content R _{457 of} the starting material was increased from 60.5 to 64.3% MgO. The fabric density of the washed fabric was 8.5%.

This fibrous material was diluted to 3.3% consistency, mixed with a further 0.7% sodium hydroxide NaOH and 0.1% complexing agent DTPA and then thickened to 25.7% consistency by means of a screen press. Its white content R ₄₅₇ was now 65.9% MgO. After passing through a heating screw with a heating time of 5 minutes and a disperser - in each case without the addition of bleaching chemicals - the fabric temperature was 95 ° C, the fabric density 23% and the white content R ₄₅₇ 64.1% MgO.

• beim Nullpunkt 65,6% MgO
• nach 5 Min. Bleichdauer 65,9% MgO
• nach 15 Min. Bleichdauer 67,0% MgO.
• nach 30 Min. Bleichdauer 68,2% MgO
• nach 60 Min. Bleichdauer 68,2% MgO
• Obwohl also der Faserstoff bereits mit Peroxid im Stofflöser vorgebleicht war, konnte hier mit der oxidierenden Hochtemperatur-Hochkonsistenz-Bleiche unter Benutzung eines Dispergers als Intensiv-Kurzzeit-Bleichmischer eine weitere Steigerung des Weissgehaltes R₄₅₇ um 4,1 Weisspunkte von 64,1 auf 68,2% MgO erzielt werden.

In an experiment with the addition of bleaching chemicals, a further 1% hydrogen peroxide H ₂ 0 ₂ was sprayed as a dilute solution into the heated material in the inlet shaft in front of the disperser using a nozzle. After passing through the Disperger sets, samples were taken after various dwell times and the white content R _{457 was} measured. The following results were achieved:

• at the zero point 65.6% MgO
• after 5 minutes bleaching time 65.9% MgO
• after 15 minutes bleaching time 67.0% MgO.
• after 30 minutes bleaching time 68.2% MgO
• after 60 min. bleaching time 68.2% MgO
• So although the fiber already contains peroxide pre-bleached in the pulper, a further increase in the white content R ₄₅₇ by 4.1 white points from 64.1 to 68.2% MgO could be achieved with the oxidizing high-temperature high-consistency bleach using a disperser as an intensive short-term bleach mixer.

Claims (12)

1. High-temperature, high-consistency rapid bleaching of fibrous material obtained from waste paper for producing paper, the bleaching chemicals being added to the material directly before a mixing process, characterised in that the bleaching chemicals are added to the material directly after heating of the thickened fibrous material, that the mixing operation is carried out in a high-speed short-time intensive mixer (11) of the disperser type, in which the mixture is forced through many narrow slots in the disperser fittings (17, 18), the material being exposed simultaneously to strong shearing and impact stresses, and the mixing operation taking place within three seconds.

2. Rapid bleaching according to Claim 1, characterised in that the mixing operation takes place within 1 second.

3. Rapid bleaching according to Claim 1 or 2, characterised in that the bleaching chemicals (8) are sprayed into the introduction shaft of the disperser (11) in front of the screw conveyor (21) of the disperser.

4. Rapid bleaching according to Claim 1 or 2, characterised in that the heating of the fibrous material which precedes the mixing operation in the disperser is carried out up to the temperature which is also necessary for hot dispersion for the shredding and neutralising of impurities.

5. Rapid bleaching according to Claims 1-4, characterised in that peroxide bleach chemicals are mixed with the material (1) to give peroxide bleaching.

6. Bleaching according to Claims 1-4, characterised in that dithionite bleaching chemicals for carrying out reducing bleaching are mixed with the material.

7. Rapid bleaching according to Claim 6, characterised in that it operates in a temperature range of 40-110°.

8. Rapid bleaching according to Claim 7, characterised in that it operates in a temperature range of 95°C.

9. Rapid bleaching according to Claim 6, characterised in that it operates in a consistency range of 20-50% (by weight).

10. Rapid bleaching according to Claim 6, characterised in that it operates in a consistency range of 30% (by weight).

11. Rapid bleaching according to Claim 6, characterised by a bleaching time of less than 15 minutes, and characterised in that the disperser (11) is succeeded by a buffer tank (14), the volume of which corresponds to this bleaching time, and which is discharged into the atmosphere (16).

12. Rapid bleaching according to Claim 6, characterised in that the decomposition of the bleaching agent by the disruptive constituents which build up at a high consistency is counteracted by the addition of complexing agents.

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