FI52878C - FOERFARANDE FOER BLEKNING AV CELLULOSAHALTIGT MATERIAL. - Google Patents

Description

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Patents and registries. «« «H r., <«> 31.08.π (32) (33) (31) Caught «chairs — Buglrd prloHtut 20.12.73 RuotBi-Sverige (SW) 7317221-5 ^ (71) A. Ahlström Osakeyhtiö, Noormarkku, Finland-Pinland (SP) (72) Prey Viking Sundman, Karhunkatu 14 A, Karhula, Finland-Pinland (SP) (74) Berggren Oy Ab (54) The method - Exemption for the sale of material

The present invention relates to a process for bleaching a cellulosic material with oxygen or an oxygen-containing gas by means of an alkali, in particular in connection with the production of cellulose according to the sulphate process.

Until now, the bleaching of cellulose pulp has usually been carried out in such a way that, after washing, most of the liquid between the fibers has been pressed from the pulp, after which the fiber concentration of the pulp has been about 30%. The amount of alkali required for bleaching (NaOH solution) is then added, stirring, to reduce the concentration to about $ 25. The pulp is then fed to the reactor and disintegrated by means of a ripper placed at the top, according to a known method described in Swedish Patent No. 333-98.

The method previously used has certain drawbacks which the present invention is intended to eliminate, as will be described in more detail below.

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Oxygen is used in bleaching instead of chlorine to act on the lignin in the wood fibers so that it can be removed by dissolving. However, since the bleach also acts on the cellulose, but more slowly, it is desirable that the reaction takes place as selectively as possible, i. so that the lignin is separated as much as possible, while the cellulose is affected as little as possible.

Oxygen bleaching is a process in which alkali, water, heat and oxygen are used to selectively delignify cellulose fibers as selectively as possible. However, the amount of reagent derived (O 2, NaOH) is directly dependent on the size of the phase surface (transfer from liquid to fibers), which can be at most the outermost surface of the fiber. This presupposes that the fibers are not stuck together. If the fibers are flocculated, the total contact surface of the fibers is smaller and the reaction time required to separate the lignin from the fibers is also longer. As a result, excessive bleaching of the fibers occurs on the outermost surface of the floc, where the effect also extends to the cellulose of the fibers and the bleaching becomes uneven. When the fibers are not flocculated, homogeneous bleaching results.

Pulp with a high fiber concentration is very difficult to handle. It is difficult to mix the required amount of alkali, which causes the alkali to be unevenly distributed. It is also difficult to transport to the reaction vessel. Until now, it has been impossible with methods to feed the pulp into a large tank in a form other than flocs.

The method used so far in oxygen bleaching requires long reaction times for the reasons mentioned above, which results in a large dimensioning of the reactor. Different strengths and viscosities are formed for the innermost and outermost fibers of the fiber flocs. If the alkali is unevenly distributed, it may result in the desired reaction not occurring in place.

The amount of oxygen required to delignify the cellulosic fibers is 1-2% by weight. The 1 m 2 reactor contains about 100 kg of wet disintegrated pulp, i.e. 25% fiber concentration 25 kg of dry cellulose. If the reaction takes place at a pressure of 10 kp / cm 2, the corresponding amount of oxygen is about 10 kg, i.e. more than 30 times the required amount. There is thus a large excess of oxygen in the reaction.

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According to the present method, the amount of oxygen can now be adjusted to correspond to the required amount. According to previous methods, it is not possible to mix as much gas into the pulp as would be needed. The gas then forms a continuous phase and a mass-alkali-water dispersed phase. According to the new method, the relationship is the opposite.

The gas forms a dispersed phase and the mass-alkali-water phase a continuous phase.

The object of the invention is to obviate the drawbacks of the hitherto used method by bleaching with surfactants "which form small oxygen gas bubbles in the form of a stable foam when dispersing the fibers. The size of the bubbles is about 30 to 800 μm, so that the oxygen has a very large reaction surface, the amount of oxygen to be dispersed is large, and when measuring the volume of the fiber suspension to which oxygen was introduced at a pressure of 1 ata, it was found that the volume increased by $ 100.

According to the method of the invention, the blowing agent is introduced into the pulp suspension before it is reacted with oxygen gas.

In this way, oxygen can be utilized to form a foam.

Preferably, the pressure is preferably reduced before and during bleaching to inflate the bubbles in a stable foam.

It is often not necessary to add a foaming agent if enough lye is left in the pulp suspension after cooking, which then acts as a foaming agent. A suitable amount of blowing agent is 1 kg / ton of cellulose. A suitable amount of waste liquor is 10-100 kg black liquor (dry matter) / tonne of cellulose.

The invention will now be described in more detail with reference to the accompanying drawings, which show two alternative embodiments for applying the method according to the invention.

In Fig. 1, a kettle marked with the number 1 is provided with a washing zone, in which the lye after cooking is mainly removed from the pulp, which is led to a mixer 3 via a buffer pipe 2. · A fiberizer, for example, can be used as a mixer. Alkali 4, oxygen 5 and blowing agent 6 are added to the pulp. The pulp flows from the mixer through pipe 7 to bleach reactor 8. Some oxygen gas and all blowing agent are added before 4,5878 to the mixer or mixer, while the rest of the oxygen and alkali solution are preferably added to the reactor. recycling. The pulp leaves the reactor 8 via a pipe 9 to a washing diffuser 10, where the pulp is washed.

Between bleaching and washing, the pressure is reduced and the pulp is degassed in a device 11. A pressure relief valve can be placed between the mixer and the reactor, whereby the reactor pressure can be adjusted lower than the digester pressure. As the pressure decreases, the bubbles swell in the foaming mass. If necessary, steam can be introduced into the mass to raise its temperature. To produce the foam, the pulp lye after cooking and washing can be utilized and / or a synthetic blowing agent, e.g. polyoxyalkylene condensate, dodecylbenzenesulfonate and octylene phenoxy polyethoxyethanol, can be added.

Hitherto, in the production of cellulose, attempts have been made to avoid the formation of foam, so that the lye has been removed as accurately as possible. The new method utilizes foam to disperse the fibers. The foam also facilitates the transport of the pulp. Inhibitors can be entered before or after the mixer.

Instead of oxygen gas, air or other gases can be used, e.g. ozone, C + + CIC4 or gas mixtures. According to the new method, the reaction time can be considerably shortened. Now it's only 10-20 min. compared to the previous 30-60 min. Thus, the size of the reactor can be significantly reduced. It can, for example, be made of a pipe of suitable length. Surprisingly, it has been found that the bleaching reaction also takes place at lower temperatures than hitherto believed to be necessary. Good results are obtained at temperatures below 100 ° C, for example 80-90 ° C. The pressure in the bleaching reactor can thus be selected to be lower than hitherto.

The experiments performed showed the applicability of the method up to a fiber concentration of about 15%. In the production of cellulose in a continuous digester, the pulp concentration is approximately 10%, which is a suitable value for the application of the method. The following exemplary embodiments illustrate the new method.

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Example 1 52878

The experimental reactor was filled with cellulose pulp, which was saturated with oxygen gas and heated with steam. The NaOH solution was then allowed to flow by circulating through the pulp. Bleaching experiments were otherwise performed under the same conditions with a pulp containing 1% TEAPOL (Shell's trademark), a synthetic detergent foamed in a blender, and plain pulp.

Before cooking, the pulp had a number of pieces and a viscosity of X = 29.3 η = 1086

After the NaOH solution flowed through the mass under the following conditions, an alkali dose of 6% pressure 4 aty

temperature 110 ° C

pulp consistency 8% time 15 min.

was the number of pieces and viscosity of the mass test 1 with foam test 2, without foam X = 16.5 X - 22.4 η = 890 η = 985

Without foam, it is difficult to achieve a lower number of pieces than 20 - without increasing the consistency by more than 10%. When the consistency of the pulp was increased, bleaching was uneven.

In the experiment carried out without foam and in which the alkali dose was more than 10% and otherwise under the same conditions as in experiments 1 and 2, the following result was obtained: Experiment 3, without foam x = 21.2 η = 900

A comparison of the results of Experiments 1 and 3 shows that a lower number of pieces has been achieved with the same viscosity when foam has been used. Foam-free processing has required more alkali.

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Example 2 52878

Sulfate cellulose pulp with a fiber concentration of 10% was treated in the apparatus of Figure 2 below. The pulp suspension was blown out of the digester 1 at a pressure of 15 kp / cm 2 and a temperature of 90 ° C. In the buffer pipe 2, about 30 kg of NaOH / ton of cellulose, and through the pipe 6, about 30 kg / ton of polyoxyalkylene condensate, as well as octylphenoxypolyethoxyethanol, about 0.5 kg / ton of cellulose and finally through the pipe 5 were introduced into the pulp before the mixer. 10 kg oxygen / tonne of cellulose. The resulting mixture was then stirred in a mixer 3 at 120 ° C to a constant foam, which was fed to the reactor 8 via a pipe 7, after the pressure was first reduced. . . .

set to 3 kp / cm by means of the valve 12 in the pipe 7 · The pressure after the device 11 was then 1 kp / cm. The decanted liquid containing unusable chemicals can be returned to the reactor or digester via return pipes - 13 and 14. Finally, the pulp was washed in a diffuser 10 to obtain a pulp with a viscosity of 900 cm 2 / g and a kappa number of 14, while the corresponding values after digester 1 were 1080 cm 2 / g and 29. Thus, it was found that the viscosity of the pulp was not decreased significantly, but the number of pieces of the mass, on the other hand, has decreased considerably.

Example 3

The sulphate pulp was bleached in the apparatus of Figure 2 under the following conditions:

temperature 95 ° C

pressure 4 aty fiber content 8% inhibitor 0.5% MgSO 4 (calculated on the dry weight of the pulp)

Washing the mass after cooking leaves more or less black liquor in it. No surfactants were added. Samples were taken at regular intervals to monitor the progress of lignin removal. The results are shown in Tables 1 and 2.

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Table 1 52878

Black liquor (dry matter) 25 kg / t mass

Reaction time Part number Viscosity Alkali consumption Oxygen consumption as% by weight% by weight 0 37.0 1054 5 24.7 950 10 20.9 906 15 19.4 880 30 16.3 843 3.27 1.5 - 2.0

Table 2

Black liquor (dry matter) 100 kg / t mass "0 37.0 1054 5 23.1 913 1.87 10 21.0 886 2.46 15 19.0 866 2.71 30 15.0 791 3.57 2 , 0 - 2.5

Claims (2)

8 1. A method for bleaching cellulose-containing material with sulfur or solid gas in the form of an alkali, a mass of a mass suspension in an alkali and an alkali or a sulfur-containing gas for deignifying the mixture at the temperature and mass of the plant, account is stable '• t-gkum, i vilket sa mycket syrre är dispersat i i form av smä bubblor, 1' «» · · att det ududsakligen konsumeras under blekningsreaktionen. A process for bleaching cellulosic material with oxygen or an oxygen-containing gas by means of an alkali, the pulp suspension being treated with an alkali solution and an oxygen or oxygen-containing gas to remove lignin at elevated temperature and pressure, characterized in that the pulp suspension is converted into a permanent foam so much oxygen is dispersed that it is consumed mainly in the bleaching reaction. Process according to Claim 1, characterized in that the blowing agent is added (6) to the pulp suspension before it is reacted (5) with oxygen or an oxygen-containing gas. Process according to Claim 1 or 2, characterized in that oxygen or an oxygen-containing gas is added to the pulp suspension before it comes into contact with the alkali solution. Method according to Claim 1 or 2, characterized in that the bleaching (8) is carried out at a pressure lower than that at which the oxygen or oxygen-containing gas is introduced (5) to expand the gas in the foamable pulp suspension. Method according to Claim 4, characterized in that the permanent foam is formed by mixing, the pressure being adjusted to about 15 kp / cm during mixing (3) and reduced (12) to 3 kp / cm before bleaching (8). Process according to one of the preceding claims, characterized in that waste liquor and / or a synthetic foaming agent, such as polyoxyalkylene condensate, dodecylbenzenesulfonate and octylene phenoxy polyethoxyethanol, is used as the blowing agent. 2. A mixture according to claim 1 is used, which means that the mass (6) of the mass suspension is used to give the reagent (5) with the aid of a solid gas.