FI57796B - SULFIDITETSKONTROLL - Google Patents

Description

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Sweden-Sweden (SE) 7506965-8 (71) Mo och Domsjö Aktiebolag, Fack, S-891 01 Örnsköldsvik 1, Sweden-Sweden (SE) (72) Sture Erik Olof Noreus, Husum, Lars Tage Uhlin, Örnsköldsvik,

Sweden - Sweden (SE) (7 * 0 Berggren Oy Ab (5 * 0 Sulfidity control - Sulfidity control)

The chemical losses that occur when dissolving cellulosic material with sodium-containing solutions are usually replaced, especially in the sulfate process, by the addition of sodium sulfate. As a result of the measures taken to reduce air pollution, a situation may arise in which sulphidity, i.e. the ratio of sulfur to sodium in the treatment fluids rises above the desired level.

Sulfidity can be adjusted, for example, by the use of sulfur-free replacement chemicals, for example sodium hydroxide or sodium carbonate, but such methods generally lead to increased costs.

Regardless of the type of substitute chemicals used, their costs become significant, and there is a desire to reduce these costs as much as possible by using low-value waste solutions from other methods in cases where these solutions contain sodium and / or sulfur. One conceivable method that can be utilized for this purpose is the preparation of chlorine dioxide by reducing chlorate with sulfur dioxide in an acidified environment with sulfuric acid and recovering 2 57796 of the waste solution thus obtained, the so-called waste acid described in Swedish patent publication 319 962 * This method involves a considerable improvement in environmental conditions because the waste solution was previously often removed untreated to the discharge water.

In modern plants, improved recovery can lead to a situation where waste acid is the only chemical that replaces sodium and sulfur. Since the sulfur-sodium ratio in waste acid is higher than, for example, in sodium sulphate, the previously mentioned difficulties with regard to sulphidity become more pronounced. For equilibrium reasons, the sulfur that enters the waste acid is removed into the atmosphere, as a result of which the water pollution problem becomes an air pollution problem. Other difficulties that become more topical as the chemical cycle increases in pulp mills are those related to corrosion. The chloride content of process fluids becomes higher the lower the chemical losses of the plant can be considered when the chloride supply is constant. In addition, the use of waste acid as a replacement chemical results in a higher amount of chlorine-containing compounds in the process, since this also contains a smaller amount of these compounds in the form of, for example, sodium chloride and sodium chlorate.

It has long been known that considerable gains in pulp yield can be obtained in the production of sulphate pulp if elemental sulfur is added to a conventional cooking liquor to obtain a white liquor polysulphide. Higher yields, without compromising the quality of the finished product, are highly desirable in a situation where the intake of wood raw material is reduced.

The previously described chemical imbalance difficulties become even greater if, in addition to waste acid, elemental sulfur is introduced into the treatment to obtain a polysulfide-containing cooking liquid. In this case, excessive amounts of sulfur are released into the outside air. Thus, in order for polysulfide soup to be used successfully, elemental sulfur must be returned to processing.

Several ways have been proposed to adjust the chemical balance and reuse sulfur in the polysulfide manufacturing process. Very interesting is the method proposed in the above-mentioned Swedish patent publication 319 962, according to which the waste acid is mixed with white or green liquor to obtain hydrogen sulphide. Sulfur 3,5796 hydrogen can be converted to either sulfur or sulfur dioxide, or both. However, one significant disadvantage of this method is the handling of large amounts of toxic hydrogen sulfide. Hydrogen sulphide must be oxidised in this process by burning it to sulfur dioxide. If sulfur dioxide is used in the production of chlorine dioxide, great demands are placed on the completeness of said oxidation process, since the chlorine dioxide process is very sensitive to reducing agents. When sulfur dioxide is used to treat pulp in connection with its bleaching, great demands are also placed on the completeness of the oxidation process because otherwise hydrogen sulphide is formed in the bleaching plant.

It is known to affect the chemical balance by using oxidized white liquor as an extractant in pulp bleaching, whereby the extract is discharged into a collecting vessel or discharge water. The disadvantage of this is that in addition to sulfur, sodium is also removed from the system.

To illustrate the above problems, the conditions prevailing in a very modern sulfate plant are described below (see Figure 1). Pulp production takes place in the usual way, but bleaching is started on the subject of oxygen gas, the waste liquor of which is returned to the chemical recovery system upstream through the screening and washing department. Due to the equilibrium, oxidized white liquor is used as the acid stage alkali and, to protect the environment, it is also used as a replacement chemical in the flue gas scrubber. The waste acid from the production of chlorine dioxide is fed to a chemical recovery system. Environmental conditions also cause the use of a relatively large amount of chlorine dioxide in further bleaching.

In kitchen 1, the cellulosic material is treated with white liquor, i.e. with a chemical solution containing mainly sodium sulfide and sodium hydroxide. The relationship between sulfur and sodium, the so-called sulfide, is obtained from the quotient Na2S / (NaOH + Na2S), the concentrations being calculated as NaOH. In order to obtain a good cooking result, the sulfidity calculated according to the above formula must be between 0.3 and 0.6. The sulphidity of the white liquor used is 0.4. After the cooking is completed, the pulp is released from the cooking liquid in succession in scrubbers 2, screeners 3 and finally in an oxygen gas bleaching step 4. The latter step also comprises the final washing of the pulp with water or condensate. The resulting cooking liquid (black liquor) is passed to an evaporator 5, where the water is removed so that the dry content of the black liquor is "57796 about 65 X. The black liquor is then burned in a recovery boiler 6, recovering the cooking chemicals as a melt of sodium carbonate and sodium sulphide. through an electrostatic precipitator 7 and via line 22 to scrubber 8, where most of the sulfur dioxide formed in the combustion is recovered. back to the digester via line 24. A small portion of the white liquor from the causticizer is removed via line 25 and oxidized in reactor 10, where sodium sulfide is converted to sodium thiosulfate. white liquor is used as an alkali in oxygen gas bleaching via line 27 and for cleaning flue gases in scrubber 8 via line 26. Depending on the equilibrium conditions prevailing in the factory, the oxidized white liquor can also be fed to the bleaching plant 11 via line 28. The lye used in the flue gas scrubber is led via line 20 to a causticizing device 9.

Chlorine dioxide and sulfur dioxide water are also used in the bleaching, both of which are produced in a special device 12 and fed to the bleaching plant 11 via lines 36 and 37, respectively. Chlorine dioxide is prepared by reducing sodium chlorate with sulfur dioxide in a sulfuric acid-containing environment. Sulfur dioxide is obtained by burning sulfur, which also produces sulfur dioxide water. The waste solution from the production of chlorine dioxide is passed to the chemical circuit before the recovery boiler via line 29.

Both lines 13 and 14 mean the supply of sulfur in the form of elemental sulfur and sulfuric acid and the introduction of sodium in the form of sodium chlorate, respectively. Replacement chemicals are passed through line 15 in addition to those obtained with waste acid.

The quantities of waste products from the plant depicted in Figure 1 are shown in the following table: 5 57796

Supply of chlorine dioxide- Losses of pur- Alkaline removal- Atmospheric din production pictogram oxidized- occur as a result of work na white losses per day S kg per tonne of pulp 16 2 1 13

Na kg per tonne of mass 10 730

As can be seen from the table, 13 kg of sulfur is discharged into the open air, which in the case of a 300,000 tonne plant corresponds to about 7,800 tonnes of sulfur per year.

Replacing part of the sodium hydroxide introduced into the ordinary bleaching plant with oxidised white liquor to correspond to about 25 kg of NaOH per tonne of pulp and discharging the bleaching waste liquor into the discharge water provides the following equilibrium:

Supply of chlorine dioxide- Losses of pur- Alkaline removal- Atmospheric din production figurative oxidized- occur due to work na white losses per day S kg tonnes 16 2 8 6 per mass

Na kg per tonne 10 7 24 0 per mass

If 21 kg of sodium per tonne of mass is added in the form of a non-hazardous substitute chemical, this measure will roughly halve the sulfur emissions to ambient air compared to the previous case, while the sulfur emissions to the discharge water will have multiplied.

_ The method described above provides a very low effluent, about 10 kg BS per tonne of pulp, acid consuming substances in terms of cooking, screening waste and chemical recovery.

In contrast, the effluent from the bleaching plant constitutes a relatively significant source of impurities. In order to reduce this, it has been proposed to return the bleach waste solution or parts thereof back to the chemical recovery system. However, the danger of such a process is that the chloride is enriched so that the chloride content becomes too high in the chemical circuit. Various methods for removing chloride from the chemical cycle have been proposed, but no practical and preferred methods have been provided to date.

6 57796 In such a process, the sulfur must also be returned to the chemical cycle, which, as mentioned above, causes a very large emission of sulfur into the atmosphere.

The object of the present invention is to eliminate the above-mentioned drawbacks and to provide a method for reducing the need for fresh chemicals when dissolving a cellulosic raw material with sodium and sulfur-containing solutions without increasing the amount of waste in water or atmosphere. Accordingly, the present invention relates to a process comprising dissolving a cellulosic feedstock with sodium-sulfur solutions, recovering the spent solvent, evaporating and incinerating this liquid, purifying the flue gases formed during combustion, dissolving the resulting melt in water to form sodium sulfide, forming sodium sulfide recovering the dissolving liquid (white liquor) from this solution and recirculating such regenerated liquid to the dissolving step, adding sodium and / or sulfur from methods other than the above sub-methods to compensate for process chemical losses. This process is characterized in that the acidic waste solution obtained from the preparation of ClCl 2 by reduction of an alkali metal chlorate solution is reacted with oxidized white and / or green liquor from a dissolution process to form sulfur dioxide and elemental sulfur and the sulfur dioxide formed is removed and again and the waste solution released from the SC is returned to the dissolution treatment.

It is very suitable according to the present invention that the oxidized white liquor is used in whole or in part as an alkaline absorption liquid for cleaning flue gases and then reacted with an acidic waste solution, and that the removed sulfur dioxide is used directly and / or converted to sulfuric acid to form chlorine dioxide. The removed sulfur dioxide can be used in whole or in part in a pulp mill bleaching plant, for example at some or some extraction and / or bleaching stages.

The elemental sulfur formed can be separated and passed into white liquor and / or green liquor. It is also possible to add additional elemental sulfur from a source outside the pulp mill, which sulfur can be led to white and / or green liquor. It is also possible to convert the elemental sulfur formed into sulfur dioxide, which is used as mentioned above. If desired, the waste solution liberated from sulfur dioxide can be liberated from the chloride ions before being returned to the dissolution treatment so that the chloride ions are removed from the process. This can be done, for example, by evaporation from the sulfur dioxide-liberated waste solution so that the alkali salts, with the exception of the alkali metal chloride, are precipitated and removed from the remaining alkali metal chloride solution, after which the precipitated alkali metal salts are returned to solution after possible redissolution. The resulting chloride solution is reacted with sulfuric acid, in which case the removed hydrogen chloride can be utilized as a reducing agent in the production of chlorine dioxide.

According to the invention, the waste acid from the production of chlorine dioxide is reacted with oxidized white liquor or oxidized white liquor used in flue gas scrubbing. This method provides several advantages. The sulfur dioxide absorbed in the flue gas scrubber is partially evaporated directly and can be used, for example, in the production of chlorine dioxide or for the production of sulfur dioxide for bleaching. The thiosulfate formed in part by the oxidation of white liquor decomposes according to the following formula:

S2032 '+ 2 H + -> H 2 O + SO 2 + S

A significant advantage of the present invention is thus that gaseous sulfur dioxide is obtained directly without bypass via toxic hydrogen sulfide.

A further advantage is that after the elemental sulfur has been removed and introduced into the white liquor to produce polysulphide, the waste solution can be enriched to environmentally harmless chloride ions, for example by repeated evaporation and mixing of salts other than chloride, mainly sodium sulphate. for the manufacture of bleaching chemicals in another factory. The precipitated salt, mainly sodium sulfate, can be returned directly or after dissolution in a suitable liquid for the preparation of white or green liquor. The separated chloride solution can also be utilized in the chlorine dioxide preparation step by adding concentrated sulfuric acid. In this case, the hydrochloric acid is removed as a gaseous substance and can be used directly both as a reducing agent and as a proton source in the production of chlorine dioxide.

8 57796

In order to achieve an even higher polysulphide content in the cooking liquid, sulfur can also be derived from a source outside the cellulose mill in the desired amount. The sulphidity then increases in the chemical cycle and the oxidized white liquor used in the flue gas scrubbing will contain larger amounts of removable sulfur. Said addition of elemental sulfur can therefore take place until a degree of sulphidity is reached in which the amount of sulfur recovered corresponds to the amount of sulfur in the form of elemental sulfur and sulfuric acid which is necessary for the production of chlorine dioxide. If an even higher polysulfide content is required than is required for equilibrium reasons as described above, the recovered excess sulfur can be removed as sulfuric acid and sold.

The invention is described by means of the following embodiments.

Example 1

Figure 2 shows an embodiment of the present invention. Reference numerals used in this figure have the same meaning as in Fig. 1.

In addition, according to the invention, a reactor 16 is used, in which the waste acid from the chlorine dioxide production plant 12 and the oxidized white liquor from the reactor 10 are reacted with each other. The waste acid and the oxidized white liquor are fed to the reactor 16 via lines 30 and 31, respectively. In the reaction, the thiosulphate ions in the oxidized white liquor are converted to sulfur dioxide and elemental sulfur according to the following equation: s2 ° 32 "+ 2 H + -► SO2 + S + H2O

The sulfur dioxide is fed directly back to the device 12 via line 17 and the elemental sulfur is separated in device 18 and also returned to device 12 via line 19. The waste solution from reactor 16 and separator 18 is passed to the chemical cycle of the plant via line 32.

With a sulphidity of 0.4, experiments have shown that it is possible to reuse 6 kg of sulfur per tonne of pulp in this way, divided so that half of this sulfur is obtained as sulfur dioxide via line 17 and half of the sulfur as elemental sulfur via line 19. Reuse of sulfur means that the addition of sulfur from the outside can be reduced accordingly. The following equilibrium can be achieved provided the same losses as in the factory illustrated in Figure 1.

9 57796

Supply of chlorine- Losses of decontamination- Removal of alkali- Atmospheric dioxide in the finished water to oxidised- occurring as a result of white na- losses per kg S kg per tonne of mass 10 217

Na kg per tonne of mass 10 730

Sulfur emissions to the atmosphere are thus roughly halved and air pollution can be more easily reduced by using sulfur-free replacement chemicals while introducing more oxidized white liquor to the bleaching plant as shown in the apparatus of Figure 1.

The principle shown in Figure 2 also applies in a plant without a flue gas scrubber 8. In order to have the same equilibrium for sulfur and sodium as in the plant in Figure 2, an oversized recovery boiler must be used in this case with relatively little sulfur dioxide removed by the flue gases. The disadvantage in this case, however, is that the heat content of the flue gases is not utilized.

In the apparatus shown in Figure 2, the waste solution can be treated after the sulfur dioxide and elemental sulfur have been separated at 16 18, in various ways to remove chloride ions from the chemical system. Repeated evaporation of the waste solution from device 18 causes valuable sodium salts to precipitate and sodium chloride solution to remain. The latter can be removed via line 33 (dotted). The precipitated salts are introduced into the chemical system directly or after dissolution in a suitable liquid.

Example 2

In a sulphate plant of the same type as that shown in Fig. 2, the invention is applied as shown in Fig. 3. The plant is equipped with a flue gas scrubber 8 using oxidized white liquor, and the oxidized white liquor used in the flue gas scrubbing was reacted with waste acid in the reactor 16. The used oxidized white liquor (scrubbing liquid) was fed to the reactor via line 20.

The experiments showed that with a degree of sulphidity of 0.4, about 13 kg of sulfur per tonne of pulp could be removed in the form of sulfur dioxide from 10 57796 reactors 16 and lead to chlorine dioxide production via line 17. A further 3 kg of sulfur per tonne of pulp could be obtained in the same way as in the plant of Figure 2. from the separator 18.

Since in this case the losses were the same as in the previous cases, the following ratios were obtained.

Supply of chlorine- Losses of decontamination- Removal of alkali- Atmospheric dioxide in the finished water to oxidized- occurring as a result of na na White losses in white S kg per tonne of pulp 0 2 1 0 -

Na kg per tonne of mass 10 730

As can be seen from this table, a sulfur deficit arises. This means that the sulphidity starts to decrease, which in turn means that the sulphide content in both white liquor and oxidized white liquor decreased. As a result, the amount of recycled sulfur was also reduced because a smaller amount of sulfur as sulfur dioxide and elemental sulfur was obtained from reactor 16. The equilibrium was thus set at a far too low level of sulfide capacity. This problem was solved by allowing elemental sulfur to enter the chemical cycle via line 32, i. only the sulfur obtained as sulfur dioxide in the reactor 16 was returned. It is also possible to solve this problem by passing only a part of the scrubbing liquid through the line 20 to the reactor 16, as shown in the figure, and the remaining scrubbing liquid is recycled according to Fig. 2.

As can be seen from the example, according to the invention, the access of sulfur to the ambient air can be clearly reduced at the same time as the sulfidity can be adjusted to the desired level without conflicting with the low sulfur emission requirement to the ambient air. According to the invention, chloride removal can also be carried out via line 33, which is very advantageous in this case, since the enrichment of sodium chloride takes place in the liquid used for cleaning the flue gases.

Example 3

As can be seen from Example 2, in certain embodiments of the invention, such large amounts of sulfur can be reused that a sulfur shortage arises. When the original sulfur addition is maintained, the sulfidity decreases to 57796. The present example illustrates the derivation of sulfur from an external source to increase sulfidity in such a case and to provide a polysulfide-containing cooking liquid. Figure 4 illustrates a plant similar to Example 2 with the difference that elemental sulfur separated in device 18 is passed through line 34 to produce a white-headed polysulfide and no sulfur is passed through line 19 to device 12. To increase polysulfide content, elemental sulfur is added from a non-plant source via line 35. The polysulfide content was increased only by gently oxidizing the white liquor with air in reactor 10. This thus only partially oxidized white liquor was removed after passing through only part of the reactor 10 through line 38 and returned to digester 1.

The experiments performed showed that the addition of elemental sulfur from the apparatus 18, sulfur from the outside via line 35, and a polysulfide-containing solution via line 38 gave a polysulfide-sulfur content in white liquor corresponding to 2% of wood. In this case, the pulp yield could be increased by 3%. calculated from wood compared to conventional sulphate soup. The equilibrium situation at the plant is shown in the following table: S kg per tonne of mass Na kg per tonne of mass

Elemental sulfur feed 16 0

Feed through chlorine dioxide production 0 10 Losses to discharge water 4 7

Alkali removal as oxidized white liquor 2 3 „Sulfur removal as S0?; And / or H2SO2j ά 7 0 Atmospheric losses 3 0

The sulphidity level in the plant became higher than in Examples 1 and 2, as a result of which more sulfur was lost to the discharge water and more sulfur could be removed in reactor 16. Here about 23 kg of sulfur was removed as sulfur dioxide, but only 16 kg of sulfur was needed to produce chlorine dioxide. An additional 7 kg of sulfur per tonne of pulp could be used in the extraction stage in the bleaching plant or converted into sulfuric acid for sale.

Claims (11)

1- A process for reducing the need for fresh chemicals without increasing the emissions to the recipient and atmosphere upon digestion of cellulosic material with sodium and sulfur-containing solutions according to a process, comprising digesting the cellulosic material with a sodium- and sulfur-containing solution, recovering it spent digestion liquid, evaporation and burning of this liquid, purification of the flue gases generated during combustion, dissolution of the poured melt in water to form a sodium sulfide-containing solution (green liquor), regeneration of digestion liquid (white liquor) from this solution, and recirculation regenerated liquid for the digestion step, wherein sodium and / or sulfur from processes other than the above-mentioned processes are added to cover the chemical loss in the process, characterized in that the acidic residual solution from production of C102 by reduction of an alkali metal chlorate solution is brought about. react with oxidized white and / or green liquor from the digestion process to form sulfur dioxide and elemental sulfur, that the sulfur dioxide formed is taken out in gaseous form and is used and that the residual solution liberated from SO2 is returned to the digestion process. 2. A process according to claim 1, characterized in that the oxidized white liquor is used wholly or partly as an alkaline absorption liquid in the purification of the flue gases and is then reacted with the acidic residual solution. 3. A process according to claims 1-2, characterized in that the stripped sulfur dioxide is used directly and / or after conversion to sulfuric acid to generate chlorine dioxide. 4. A process according to claims 1-3, characterized in that the stripped sulfur dioxide is wholly or partially added to the bleaching plant of the cellulose factory. 5. A process according to claims 1-4, characterized in that the stripped sulfur dioxide is added to some or all of the extraction and / or bleaching steps in the bleaching plant. 6. A method according to claims 1-5, characterized in that the elemental swath formed is separated and fed to the white liquor and / or the green liquor.