FI94443B - Method for regulating the sulphide balance of a sulphate cellulose factory - Google Patents

Description

94443

Method for adjusting the sulfur balance of a sulphate pulp mill

The invention relates to a method for adjusting the sulfur balance of a sulphate pulp mill, in which sulfur is removed from the black liquor in the form of a gas containing sulfur compounds by heat treatment at a temperature higher than the cooking temperature and by adjusting the heat treatment temperature and / or delay time.

When preparing cellulose by the sulfate cooking method, the wood is treated with a cooking solution containing sodium hydroxide and sodium sulfide. In this case, the lignin in the wood separates from the actual cellulose fiber. The mixture of cooking liquor and cellulose pulp obtained in the cooking is washed, whereby the fibers are further passed to bleaching and / or other post-treatment and the remaining waste broth, i.e. black liquor, is passed to further processing. Black liquor is rich in chemicals that can be reprocessed to return to the cooking process, thus saving the amount of new chemicals fed. Likewise, the black liquor 20 is rich in organic matter, which is obtained by burning energy. Black liquor is usually concentrated by exploiting the steam generated at different stages of the process and the thermal energy it contains, which * | evaporates the water from the black liquor. The concentrated black liquor of this ice is fed to a recovery boiler, where it is incinerated and the melt formed in the lower part of the recovery boiler is used to form water or lean white liquor to remove the liquor, which is further reduced by lime. by causticization it is converted into white liquor, i.e. cooking liquor.

30 Sulfur enters the process mainly for the production of bleaching chemicals and for the production of tall oil by tall oil digestion. in the form of sulfuric acid required for columning, in which case • • m sulfur in its various forms enters the cooking process. Some of the sulfur is removed as various odorous gases in pulp cooking, black • · *: .. J5 lye is concentrated in the evaporator and, of course, soda ash * * · • · · *% · »· 2 94443 in the combustion of black liquor. As gaseous sulfur emissions are considered to be harmful to the environment, efforts must be made to keep them to a minimum due to both sulfur deposition and odor nuisance. If the sulfur emissions 5 of the pulp mill are reduced, which is achieved e.g. by raising the dry matter content of the lye and collecting the odorous gases of the mill together for incineration for disposal, the result is the accumulation of sulfur in the lye cycle. Therefore, it should be possible to remove the desired amount of sulfur from the cycle in order to maintain the desired sulfur content in the lye-10 cycle. It is known, inter alia, from FI publication 85515 to remove sulfur from black liquor in gaseous form by heating the black liquor above the cooking temperature and maintaining it for at least 20 minutes before the last concentration step, after which the black liquor is allowed to swell, releasing vapor containing gaseous sulfur compounds. According to the publication, the separated sulfur is passed to an incineration or other disposal plant and the black liquor is further passed to a final concentrate, where it is further removed by evaporation of water. The solution proposed by that publication, although apparently workable, is very large and expensive and. its energy economy is very poor.

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U.S. Pat. No. 2,711,430, on the other hand, is known per se, i.e. by heating the black liquor to a temperature ΙΓ above the cooking temperature olla and allowing it to remain there for a sufficient time, it is obtained by expanding sulfur as gaseous compounds. with water vapor. The amount of sulfur that differs depends on both the temperature and, in particular, the delay time, in which case it can be adjusted as desired by changing these.

It is an object of the present invention to provide a method which avoids the disadvantages of the known solutions and which allows the sulfur balance of a pulp mill to be adjusted as desired as cost-effectively as possible and by 9 9 * ·· · • · ♦. · · • · • · 3 94443 effectively. The process according to the invention is characterized in that the heat treatment is carried out in connection with or immediately after the last concentration step.

The essential idea of the invention is that the desired amount of sulfur is removed from the black liquor only after or in connection with the last concentration step, in which case the entire operation can be carried out in the already existing equipment or with very small changes. Surprisingly, it has been found that when concentrating the lye to more than 80% dry matter, the residence time and temperature of the liquor can be adjusted so that sulfur can be removed in the expansion tank after the last concentration step, with the last concentrator reactor operating at the same time. The invention is particularly well suited for the solution according to FI patent 73474, in which the lye is finally concentrated under pressure at a pressure higher than atmospheric pressure and at a temperature higher than the boiling point of the atmospheric pressure of the liquor, respectively. In this solution, the separation of sulfur from the liquor can be achieved easily and simply by simply adding a separate delay tank between the concentration reactor and the expansion tank, where the liquor can be heated if necessary. for example with hot steam. In this case, the implementation of the method · * · ··; investment is quite cheap and easy to apply. The invention is also applicable to the solution according to FI application ·· · .ϊ 894049, in which a solution has been applied in order to obtain a high dry matter content, in which ash is fed to the black liquor only after the last concentration reactor and the subsequent expansion tank and boiler. in 30 separate ash mixing tanks placed between them. Whereas: this solution is already applied at the highest possible level of ··· ·. ·; ·. In order to obtain the concentration of the substance in the black liquor, the decontamination can be carried out in the above-mentioned manner or the expansion tank after the final concentration can be used as a heat treatment and delay tank and the sulfur-containing gases and steam can be separated ♦ »· • · • · · • ·· • ♦ _______________ —— ————— ..... va 94443 4 only in the last ash mixing tank, where the black-head is also expanded.

The advantage of the solution according to the invention is that it can also be implemented with already existing equipment 5 without significant additional investments, while the energy efficiency remains high. At the same time, the sulfur balance of the entire pulp cooking process can be easily and simply adjusted, and the recovered sulfur can be suitably treated to be returned as sulfuric acid to the bleaching process or pine-10 oil column to replace externally imported sulfuric acid, further making the whole process more economically viable.

The invention is described in more detail in the accompanying drawings, in which Fig. 1 schematically shows an apparatus in which the method according to the invention can be advantageously implemented, Fig. 2 schematically shows a second embodiment of the apparatus according to Fig. 1 and Fig. 3 schematically shows another apparatus for implementing a second embodiment of the method.

In the solution shown in Figure 1, the black liquor feed f ·; is fed along channel 1 to the mixing tank 2, from where it is further fed to channel 3 via channel 3.

ϊ * Ash and possibly make-up chemicals are further fed into the mixing tank 2 via channel 5. In the concentration reactor 4, the liquor is concentrated by either direct or indirect heating by supplying steam from the channel 6, 30, whereby the condensate formed is removed through the channel 7 and. if desired, the effluent separating from the lye can be removed from the top of the lightening reactor via the channel 8 along the valve 32 * 9. through. The concentrated black liquor is fed through the channel 9 forward by a 9 V pump 10, whereby the valve 11 directs • · ..35 e.g. part of the concentrated liquor forward to the delay tank ·· * 9 • · • 9 5 94443 12 and part is returned to the concentration reactor 4. The black liquor temperature in the concentration reactor 4 is typically about 150 ° to 170 ° C and a pressure equivalent thereto, whereby the viscosity of the lye decreases so that it can be pumped forward despite a dry matter content of 80%. Delay time, e.g.

20 - 60 min. in the delay tank 12, it is adjusted so that the sulfur begins to separate from the lye, inter alia, as dimethyl sulfide and other gaseous compounds. In the delay tank, the temperature drops slightly relative to the concentration reactor temperature 10 as the pressure drops, but can be heated, for example, by hot steam fed through duct 13 and valve 33, which can be taken from a suitable point in the pulp process, e.g. However, the temperature of the liquor is kept higher than the cooking temperature 15 and the pressure in the delay tank is then corresponding.

From the delay tank 12, the liquor is further led along the channel 14 through the valve 15 to the expansion tank 16, where the liquor is allowed to expand and separates sulfur-containing gases and vapors which are discharged from the tank top 20 through the valve 17 to the duct 18. The discharge of gases and steam is controlled by a pressure regulator 19. 17. From the expansion tank 16, the liquor is led along the channel 20 by means of a pump 21 through a valve 22 to be injected into a recovery boiler 23, where it is burned in a known manner. If necessary, some of the liquor can be • • «• ϊ returned to the expansion tank 16 via the valve 24.

9 The steam which may be removed from the upper part of the concentrating reactor 4 along the channel 8 f · ', * ·· can be used in previous stages of concentration and evaporation to heat Lipe-30. Likewise, the steam leaving through the duct 18 would contain sulfur. they and their gases can be used in a suitable manner, leading to • 99 · la reprocessing, for example in an incineration plant or a catalyst plant, in order to obtain the best possible form of sulfur, e.g. sulfuric acid. The method according to Fig. 1 can also be carried out without delay.

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* · I 6 94443 tiaa 12 so that the residence time of the liquor in the concentration reactor 4 is extended to 20 to 60 minutes, whereby the sulfur compounds separate from the lye. In this case, a separate delay tank 12 is not required, but the liquor is led directly to the expansion tank 16, where the sulfur compounds are released and can be removed as gaseous compounds. This embodiment is shown in Figure 2, which by the way also corresponds to Figure 1 in its numbering, but lacks a delay tank 12 with its associated parts, because the liquor is fed directly to the expansion tank 16 via the channel 15 after a delay in the concentration reactor 4 ta-10. In this case, the result is that the size of the concentration reactor 4 increases somewhat, but it is not significant.

Fig. 3 shows an apparatus suitable for implementing a second embodiment of the method according to the invention, in which numbers corresponding to the parts corresponding to the parts of the apparatus according to Fig. 1 are also used. In this embodiment, the liquor is fed directly via channel 3 to the concentrating reactor 4. In the apparatus shown in Fig. 3, the process according to the invention can be carried out using two-stage expansion by heating the liquor in the concentrating reactor 4 to a sufficiently high temperature, for example about 190 ° C. When the dry matter-25 content of the lye is suitable, the lye is led to an expansion tank 16, where the temperature of the lye decreases as the pressure decreases in the sauna. The lye remains in the expansion tank for 16 suitable times, i.e. 20-60 minutes, during which the sulfur as gaseous compounds separates from the lye. Some of the gaseous sulfur compounds 30 may be removed along the passage 18 with the steam, if desired, but the remainder is transported with the lye to the ash mixing tank 25 where the lye is expanded to a lower pressure. In connection with the expansion, most of the sulfur-containing gases separate from the liquor with the steam and are discharged through the valve 26 into the duct 27. The pressure control in the ash mixing tank 25 t- * *

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7 94443 is effected by a controller 28 which controls the valve 26. In the ash mixing tank, the lye is mixed with ash from the channel 5 and / or make-up chemicals, which are mixed as efficiently as possible with the se-5 mixer 29 at the bottom of the tank. The lye 23 is further fed from the ash mixing tank 30 through the valve 31 for conventional combustion. Of course, the expansion can also be done in several stages than in two stages, in which case the expansion tank is added as required.

In the foregoing description and drawings, the invention has been shown by way of example only and is in no way limited thereto. The appropriate heat treatment temperature and delay time depend on the equipment used and the sulfur dissipation required, but most importantly, the already existing concentrator reactor equipment with expansion tanks can be used almost as such .

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Claims (5)

94443 r i A method for adjusting the sulfur balance of a sulphate pulp mill, which method removes 5 sulfur in the form of a gas containing sulfur compounds by heat treatment at a temperature above the cooking temperature and adjusting the heat treatment temperature and / or delay time, characterized in that the heat treatment is carried out in a final concentration. Process according to Claim 1, characterized in that the heat treatment is carried out in the last concentration step simultaneously with the concentration, after which the lye is allowed to swell to separate the sulfur-containing gases therefrom. The method of claim 1,; characterized in that the heat treatment is carried out by heating the lye in the last concentration step to a temperature higher than the heat treatment temperature, that the black liquor is then allowed to swell at a lower temperature but above the cooking temperature, and the black liquor is kept at that temperature; that the black liquor is then allowed to swell a second time to separate the sulfur-containing gases from the black liquor. I Method according to one of the preceding claims, characterized in that the heat treatment temperature is at least 140 ° C. Method according to one of the preceding claims, characterized in that the delay time is from 20 to 60 minutes. 94443 I Patentkrav B