FI96782B - Production of cellulose pulp - Google Patents

Description

This invention relates to the improvement of various sub-processes of pulp production, both of the processes themselves and of the equipment associated therewith.

In sulphate cooking, the wood is treated with a white liquor containing NaOH and N 2 O 2 S, whereby the lignin dissolves and the cellulose fibers are released. A mixture of cellulosic fibers (pulp) and 10 cooking chemicals is treated with water to give a black liquor. The black liquor is concentrated by evaporation. The concentrated black liquor is burned in a recovery boiler and the chemical melt thus obtained, containing mainly Na 2 S and Na 2 CO 3, is dissolved in water to give green liquor. The green liquor is causticized with quicklime (CaO) to a white liquor containing NaOH. Another product of the causticization reaction is mesa, which consists mainly of CaCO 3. The white liquor is fed to the digester and the lime is calcined in a lime kiln to 20 quicklime for re-causticization.

The pulp removed from the digester can be bleached if necessary. In order to reduce adverse environmental impacts, the pulp industry is seeking to replace traditional Chlorine Bleaching with 25 other options, one of which is the use of ozone. In ozone bleaching, the pulp from the digester is pre-bleached with oxygen under temporal conditions. The pulp is then bleached with ozone, followed by alkali extraction. In the oxygen bleaching and alkali extraction steps, 30 oxidized white liquors are used as alkali. After each treatment, the pulp is washed to remove chemicals from the pulp, e.g. with a countercurrent wash. the washing liquid of the last washing stage, in which the chemicals have been enriched, is led to the chemical recovery process;

It is an object of the present invention to increase the efficiency of pulp production and to reduce the environmental impact of the mill. This is achieved by using two cooking liquors with different sulfur contents in the production of pulp. The chemicals are recovered together and then made into cooking liquors of different sulfur contents. According to the invention, for example, sulfur-rich cooking liquor is used at the beginning of cooking and low-sulfur cooking liquor in later stages of cooking.

The invention also relates to a pulp mill, a digester, a soda ash plant and a white liquor bleaching plant, in which two lye streams with different sulfur contents are produced and used.

The essential features of the invention appear from the appended claims.

The invention achieves a low number of pieces, good pulp strength and yield. On the other hand, the invention makes it possible to minimize sulfur emissions from a pulp mill.

By means of the invention, a considerably low number of pieces is achieved, due to which the load on the bleaching plant is reduced and thus also its environmental disadvantages.

According to the present invention, less potent bleaching chemicals, such as oxygen and ozone, can be used in the cooking for pulp: better strength than before, so that bleaching can be used. Thus, the invention enables the use of more environmentally friendly bleaching chemicals.

The improved pulp yield produced by the invention provides significant economic benefits in terms of lower wood consumption.

According to the invention, the sulfur-rich conditions are concentrated: at the beginning of the cooking, in which case sulfur is no longer present after

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-------------- 96782 3 as abundantly as traditionally. The odor nuisance caused by the various leaks is greatly reduced because the abundant use of sulfur is limited to a smaller factory area.

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

Figure 1 shows a flow chart of pulp cooking and bleaching. Figure 2 shows an embodiment of the invention for producing melts of different sulfur contents.

Figures 3-4 show alternative embodiments for making melts.

Figure 5 shows an embodiment of the invention for the preparation of oxidized white liquor.

Figure 6 shows a structure of the bottom of a recovery boiler according to the invention.

According to Figure 1, the wood chips 1 are fed into the digester, as is the cooking liquor. According to the invention, high-sulfur white liquor 2 is fed to the beginning of the cooking, and as the cooking progresses, low-sulfur liquor 3 is added. Alternatively, sulfur-rich, non-caustic green liquor can also be added to the beginning of the cooking. The sulfur-rich cooking liquor preferably has a sulfidity of about 40% or more and the sulfur-poor cooking liquor about 30% or less. This way of cooking is achieved. low piece number and good pulp quality. At the end of the cooking, the mass is separated from the waste liquor by washing e.g.

• with countercurrent washing liquid 4c from the bleach. The waste water and waste water 5 are led to chemical recovery processes. The unbleached pulp 6 is in turn fed to the silencer.

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The pulp is first pre-bleached with oxygen under time conditions. 35 Sulfur-poor white liquor 3a, which has been oxidized to deactivate sulphides, is used as the alkali. After oxygen bleaching, the pulp 1 «ψ 4 96782 is washed again with the washing liquid 4b upstream of the next step. The washed mass 7 is next bleached with ozone under acidic conditions.

5 The pH of the ozone bleaching step is calculated either with a chlorine dioxide plant - which operates, for example, with Mathieson, R8 or R9 process waste acid or an acid obtained therefrom or with an acid obtained from the cooking chemical cycle. In the latter case, the acid can be made, for example, from a sulfur-containing gas 10 obtained from the heat treatment of black liquor or from the combustion of odorous gases. An alternative source of acid is also tall oil cooking acid.

After ozone bleaching, the pulp is washed, as in the previous 15 steps, with the washing liquid 4a from the latter step, whereby part of the acid used to adjust the pH returns to the chemical cycle. The mass 8 is then subjected to alkali extraction. The pH is adjusted, as in oxygen bleaching with a low-sulfur, oxidized white liquor 3a. Some of the white liquor chemicals 20 return to the chemical cycle when the pulp is finally washed in a countercurrent wash. The pulp production process currently in use, which includes cooking, oxygen bleaching and ozone bleaching, achieves approximately a kappa number of 10, at which point the pulp still needs to be bleached with chlorine. With the aid of the present invention, it is possible to achieve a number of pieces of 2, whereby the use of chlorine can be avoided.

»T .1 Preparation of various cooking liquors« «» · 30 As is known, in order to recycle cooking chemicals, the black liquor formed in the cooking is concentrated, burned in a • · * recovery boiler to obtain a melt containing sodium carbonate and sodium sulphide. The melt is dissolved in water to form green liquor. The green liquor reacts with CaO to give a white liquor containing NaOH and Na2S.

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The viscosity of the black liquor can be irreversibly reduced and thus non-evaporatively improved by heat treating the black liquor at a higher temperature than the cooking temperature. The heat treatment releases sulfur-containing gases from the black liquor, and thus by adjusting the temperature and / or delay time of the heat treatment, the sulfur content of the black liquor and thus also the sulfidity of the white liquor can be affected.

The different sulfur-containing cooking liquors required in the pulp mill of the present invention are made from melts having correspondingly different sulfur contents. It is possible to produce such melts in a recovery boiler according to the invention, the lower part of which is divided by a partition into two parts, as shown in Figure 2. The concentrated black liquor 15 is divided into both compartments. Ash from the soda boiler electrostatic precipitators, sulfur gases from the black liquor heat treatment, and other pulp mill sulfur sources, such as digester sulfur-containing gases, are fed to the boiler melt, overhead melt 20, or gases by means of boiler feed means. Alternatively, the sulfur-rich fractions can be mixed with the black liquor fraction entering this compartment before the boiler. Due to the addition of sulfur-rich fractions, a sulfur concentrate melts in the compartment containing these compounds and 25 sulfur-poor melts in one side. These melts are taken to a white liquor plant according to the invention, where they are dissolved separately in their own solvent tanks. The green liquors formed in this case are taken through their own green liquor clarifiers to their own causticization tanks. Causticization thus produces two different white liquors, one rich in sulfide and the other poor in sulfide. Alternatively, for example, sulfur-rich green liquor can be left un causticized, especially if the pH of the broth is desired to be low or the Na 2 CO 3 content of this green liquor is low.

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The white liquors are still separated from the lime formed in the causticization, but the limes thus obtained can be combined at this stage, whereby the washing and burning of the lime in the lime furnace can be carried out in one stage. The honeycombs can alternatively be washed separately and combined only before incineration. Thus, a plant manufacturing cookers with different sulfidities requires relatively small additional investments because causticization tanks, green liquor clarifiers, and melt solvents are simple tanks.

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Figure 2 shows an embodiment according to the invention of how sulfur-containing compounds can be divided into different compartments of a recovery boiler for the production of different melts. The concentrated black liquor 10 is introduced from the evaporator and heat-treated by pressure heating to form sulfur-containing gases 11, which contain, for example, 40% of the sulfur in the black liquor. The black liquor 12, which has thus been reduced in sulfur content, is divided equally into sections I and II of the recovery boiler bottom. In addition to the sulfur gas 11 formed in the pressure heating 20, other sources of sulfur from the pulp mill, such as fly ash 13 and sulfur-containing gases 14, are introduced into the second compartment 20 to give a sulfur-rich melt 15 in this compartment. It contains about 70% black liquor sulfur and other possible sulfur sources. The low-sulfur melt 16 25 thus contains about 30% of the total sulfur content of the black liquor.

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The black liquor can, of course, be divided in any proportion * between the different parts of the recovery boiler. In this way, the sulfur distribution between the different parts can also be influenced.

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Heat treatment of black liquor is not absolutely necessary to achieve differences in sulphidity. For example, if only the ash from a recovery boiler containing about: 35 30% of total sulfur and 15% of sodium is taken to another *: compartment and the black liquor is divided equally into different compartments, • • m% 9 9 7 96782 on the sulfur-rich side 65% of total sulfur and 55% of total sodium, After 35% sulphide in the original white liquor, white liquors with sulphide theses of about 40 and 30% are now obtained.

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Figure 3 shows another embodiment for the production of white liquors with different sulfur contents. In it, the pre-evaporated black liquor 20 is heat treated in the reactor 21 to reduce the viscosity of the liquor for final evaporation. The black liquor concentrated in the evaporator 10 is divided in two so that one part 23 is led directly to one of the compartments I at the bottom of the recovery boiler. The second part 24 is heat treated to further reduce its sulfur content. The black liquor 25 thus treated is taken to another compartment II, where it is burned to a low-sulfur melt. Soda boiler ash 26, sulfur-containing gases 27 from both heat treatment reactors are added to the soda boiler compartment I to obtain a sulfur-rich melt. In this case, different conditions can be used in different heat treatment steps and thus the breaker-20 range can be controlled.

Figure 4 shows an alternative connection to the embodiment according to Figure 3. The pre-evaporated black liquor 30 is divided in half. The second portion 31 is fed to a heat treatment reactor 32 where the liquor is heated to reduce the viscosity. The lye 33 is then completely concentrated in the evaporator e: 34 and passed to the bottom compartment I of the recovery boiler. One half of the 1 '/ pre-evaporated black liquor 35 is passed to a stronger heat treatment in the reactor 36 to reduce the viscosity to m m • / 30 and to achieve high sulfur separation. The low-sulfur black liquor 37 thus obtained is evaporated in an evaporator 38 1 and introduced into the second compartment II of the bottom of the recovery boiler to obtain a sulfur-free melt 41. In both heat treatments

the resulting sulfur-containing gases 39, ash 40 and other possible sources of sulfur: 1 ·: 35 are discharged into recovery boiler compartment I

to obtain a sulfur-poor melt 42.

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Figure 5 shows an embodiment of the present invention according to which an oxidized soda liquor required in a bleaching plant can be provided. The bottom of the soda boiler 5 is divided into several sections I, II and III. Two sections II and III contain low-sulfur melt and one I contains high-sulfur melt. By maintaining the oxidizing conditions in the second low sulfur melt-containing section III, an oxidized Na 2 S 2 O 4 -containing sulfur-poor melt 50 is obtained. 10 The other two sections I and II have reducing conditions to give a Na 2 S-rich melt 52 and a N 2 S-poor melt 51. oxidation of the white liquor is performed and thus removes one odor source from the pulp mill.

Figure 6 shows an alternative of how the bottom of the recovery boiler can be divided into different compartments I and II to produce melts with different sulfur contents. By arranging the high-sulfur melt-containing compartment I lower than the low-sulfur compartment II, the exhaust gases of the high-sulfur compartment 20 are brought into contact with the sodium-containing gases of the low-sulfur compartment. In this way, the sulfur-containing compounds react with sodium vapors and are recovered at the latest in electrostatic precipitators. This can reduce sulfur emissions from the recovery boiler. Alternatively, the sulfur-rich compartment 25 can be arranged in the middle of the recovery boiler, whereby the sulfur-free part is surrounded by a part. In this case, the exhaust gases of the sulfur-rich part are evenly distributed in the gases of the sulfur-poor part.

The hot-water boiler plant according to the invention may also comprise 30 at least two separate boilers. According to one embodiment, the boilers have a common heat recovery system and, in addition, may have a common flue gas cleaning system. Alternatively, the boilers can be completely separate. The ash from the common flue gas cleaning system is only returned to the second boiler, whereupon 9 96782 of sulfur-rich melt is naturally formed in this boiler. If the boilers are completely separate, then the ash recovered from them is returned only to another boiler to form a sulfur-rich melt.

In order to control corrosion problems, it is preferable to coat at least a part of the recovery boiler in contact with the high-sulfur melt. A suitable coating is masonry, which can also be extended to an area rich in sulfur gases. Masonry can extend to a height of more than 10 m over 10 molten surfaces.

The various details of the invention may vary and differ from those set forth above by way of example only within the scope of the inventive idea 15 defined by the appended claims.

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Claims (50)

96782 A process for the production of low-grade cellulose and cooking liquors on a factory scale, wherein at least two different cooking liquors are used in the production of pulp, one rich in sulfur-containing chemicals, high in sulphidity and the other low in sulfur-containing chemicals, low in sulphidity 10, characterized in that the chemicals are recovered together and the recovered chemicals are used to make at least two cooking liquors with different sulfur contents from different melts produced in different combustion plants. Process according to Claim 1, characterized in that the cooking liquors with different sulfur contents are produced from melts having correspondingly different sulfur contents. Method according to Claim 2, characterized in that the melts are produced in the same recovery boiler in which at least two different combustion chambers are arranged. A method according to claim 2, characterized in that the melts are made in different boilers with either a common or separate flue gas cleaning system. m • · 9 9 Process according to Claim 2, 3 or 4, characterized in that the green liquors obtained from the melt are causticized separately into sulfur-rich and sulfur-poor white liquor. I I f I · Process according to Claim 2, 3 or 4, characterized in that the green-melt obtained from the sulfur-poor melt is only causticized to white liquor. 35 • »* • · · 96782 Process according to Claim 5 or 6, characterized in that the limes obtained from the causticisations are burned in one and the same lime kiln. Process according to Claim 5 or 6, characterized in that part of the sulfur-free white liquor is oxidized. Process according to Claim 2, 3 or 4, characterized in that part of the melt is oxidized. Process according to Claim 9, characterized in that the oxidized melt is causticized separately to obtain a lye corresponding to the oxidized white liquor. 15 Process according to Claim 8 or 10, characterized in that the oxidized white liquor is used to raise the pH in the pulping processes. Process according to Claim 11, characterized in that the subprocesses are soup, hot alkali extraction, oxygen bleaching or an alkaline stage of bleaching. A method according to claim 11 or 12,; 25 characterized in that at least part of the white liquor used to raise the pH is returned to the cooking chemical circuit by means of countercurrent washing. m Method according to Claim 2, 3 or 4, characterized in that at least one sulfur-rich fraction of the cooking chemical cycle is introduced into the second combustion chamber either separately. as an additional fraction in a melt of the combustion chamber, in a heap or gas space on top of the melt, or mixed with black liquor to obtain meltes of different sulfur content. 35 Process according to Claim 14, characterized in that the sulfur-rich fractions are fly ash from soda ash, tall oil distillate waste acid, chlorine dioxide plant waste acid, sulfur gas obtained from the heat treatment of iron liquor or sulfur-containing exhaust gas from a digester. Method according to Claim 3, characterized in that the flue gases from the various combustion chambers are brought into contact in order to reduce sulfur emissions. Process according to Claim 15, characterized in that part of the sulfur-rich fraction is used for the production of acid. Process according to Claim 17, characterized in that the sulfur-rich fraction is the waste acid from the chlorine dioxide plant 15, the waste acid from the tall oil digester or the sulfur-containing gas. Process according to Claim 17, characterized in that the acid is used to lower the pH in the pulping processes. Process according to Claim 19, characterized in that the subprocess is ozone bleaching. Process according to Claim 19 or 20, characterized in that at least part of the acid used to lower the pH is returned to the cooking chemical circuit by washing. 22. A pulp mill in which at least two cooking liquors 30 with different sulfur contents are used in the production of pulp, characterized in that it is provided with means for producing cooking liquors from melts with different sulfur contents. Factory according to Claim 22, characterized in that, for the production of cooking liquors, the factory is provided with a recovery boiler with at least two different melting chambers. 96782 « Factory according to Claim 22, characterized in that the factory is equipped with at least two different recovery boilers for the production of cooking liquors. Plant according to Claim 22, characterized in that the plant is equipped with at least two causticizing plants for the causticization of different sulfur-containing green liquors formed from the melt of the recovery boiler. 10 Plant according to Claim 25, characterized in that the limes obtained from different causticizing plants are combined and burned in the same lime kiln. Plant according to Claim 26, characterized in that the limes are washed together before incineration. Plant according to Claim 23 or 24, characterized in that the at least one combustion chamber is provided with a system 20 for feeding sulfur-containing fractions. A plant according to claim 23, characterized by; that the bottom of the recovery boiler is provided with at least one partition. : 25 • * · '· / 30. A plant according to claim 23 or 24, characterized in that the exhaust gases from the combustion chambers are brought into «contact with each other. ··· • · · • · · Plant according to Claim 23, 24 or 30, characterized in that the at least two different combustion chambers have a common heat recovery system. A plant according to claim 23, 24 or 30, characterized in that the at least two different combustion chambers have a common heat recovery system and a common flue gas cleaning system. 96782 33. A recovery boiler plant in a pulp mill, characterized in that it comprises means for producing melt with at least two different sulfur contents into cooking liquors for further processing separately for the same digester. 5 A recovery boiler plant according to claim 33, characterized in that it comprises one recovery boiler, the melts being made in the same boiler. A recovery boiler plant according to claim 34, characterized in that its bottom is divided by a partition / partitions into at least two combustion chambers. A recovery boiler plant according to claim 35, characterized in that the at least two different combustion chambers have a common heat recovery system. A recovery boiler plant according to claim 35, characterized in that the at least two different combustion chambers 20 have a common heat recovery system and a common flue gas cleaning system. A recovery boiler plant according to claim 35 or 37, characterized in that the sulfur emissions of the recovery boiler '; In order to reduce this, the gases of the more sulfur-containing melting chamber are purified with the gases of the lower sulfur melting chamber. • * · • · e A recovery boiler plant according to claim 33, characterized in that it comprises at least two different recovery boilers, the melts being produced in different boilers. • · · * · # ’· *; A recovery boiler plant according to claim 39, characterized in that the boilers have a common flue gas cleaning system. ··· '· 35 * A recovery boiler plant according to claim 40, characterized in that it comprises means for returning the fly ash separated from the flue gases 96782 to a sulfur-rich molten boiler. A recovery boiler plant according to claim 39, characterized in that the boilers have separate flue gas cleaning systems. A recovery boiler plant according to claim 42, characterized in that the ash 10 separated from the different flue gases is returned to the sulfur-rich molten boiler. A recovery boiler plant according to any one of claims 35 to 43, characterized in that the combustion chamber of the sulfur-rich melt is provided with a system for feeding sulfur-containing fractions, such as fly ash, to the combustion chamber melt, the melt-on-site or the gas chamber. A recovery boiler plant according to any one of claims 35 to 44, characterized in that it comprises means 20 for maintaining oxidizing conditions in a part of the boiler, whereby oxidized white liquor is obtained from the melt under these conditions by separate causticization. . white liquor. ^ 25 Soda ash boiler plant according to one of Claims 33 to 45, characterized in that the parts of the soda ash boiler which are in contact with the sulfur-containing melt or with the gases in its vicinity are protected against corrosion. 30 47. Boiler plant according to Claim 46, characterized in that the corrosion protection is masonry. 48. White liquor mill of a pulp mill, characterized in that it is equipped with equipment for dissolving, clarifying and causticising melts from a recovery boiler plant with ** at least two different sulfur contents: - 96782 sex separately for high sulfur, high sulphide and for producing sulfur-poor, low sulfidity white liquor for the same digester. 49. A plant according to claim 48, characterized in that it further comprises means for combining and then burning the granules formed in the causticising in a lime kiln together. A plant according to claim 49, characterized in that it further comprises means for washing the bees from the causticisations together before incineration. .s e • * • '• ψ e * «e> ·. * «* F • 9 • · I f π 96782