SE530462C2 - Process for the recovery of sulfur in a chemical pulp process comprising the oxidation of bisulfite to bisulfate, which is then used for the digestion of tall oil or as a raw material in the production of magnesium sulfate - Google Patents

Description

530 452 2 control is a major problem in the form of an excess of sulfur that enters the process as sulfuric acid. Pulp factories have olika your different sub-processes where sulfur is formed. One of the most important is the tall oil process. In addition, sulfur enters the process from, for example, the wood itself, from the water, from fuel oil in the lime kiln and from magnesium sulphate which is used in oxygen bleaching.

Of the substances that are released from the wood during the sulphate pulp cooking, the fatty acids and resin acids are saponified into sodium salts, ie. soap. There are also unsaponifiable organic constituents, which depend on the type of tree used in the wood. Thereafter, the soaps are further micellarized, dissolving a certain amount of unsaponified substances in the micelles. Normally the mass cooking is followed by a washing step. The solution separated during this step, black liquor, contains soap micelles, which rise to the surface of the liquor in the form of black soap. The soap formed is separated from the black liquor by decantation, after which it can be broken down into tall oil with sulfuric acid and heat. The sliver is thus an acid treatment, in which the soap separated from the surface of the black liquor is boiled together with sulfuric acid. The corresponding acids are prepared from the water-soluble sodium salts of the fatty and resin acids and the mixture of acids is called crude tall oil. The reactions during the digestion are described by the following compounds: H, so ,, + za-cooNa -> 2R-cooH + Na, so, <4) In practice, the addition of acid should be such that the pH drops below 4. Tall oil as formed during the cleavage is separated from the aqueous phase of the pulp press, which is returned to the pulp mill's chemical circulation system.

Residual acid from, for example, a chlorine dioxide reactor, for example from the well-known Matheson process, can be used as a cleavage acid, which in fact also often happens.

Since the aqueous phase formed during the digestion is returned to the chemical circulation system, dissolved sulfur will also be introduced into the chemical circulation system. Together with the residual acid, a not insignificant amount of sulfur will enter the process, since the residual acid contains sodium sulphate or sodium squisulphate in a significant amount in addition to sulfuric acid. The residual acid also contains chlorine compounds, ie. sodium chloride and sodium chlorate, which cause corrosion problems. Depending on the tree species and place of growth, the tall oil digestion can add sulfur to the process at a rate of 2 to 4 kg per tonne of pulp. The sulfur losses in a modern pulp mill, ie. the sulfur that is not recycled but is removed from the chemical recovery can be covered with sulfur-containing chemicals formed in the production of the bleaching chemicals. The current amount even exceeds the sulfur consumption in the pulp mill. In particular, a significant excess of sulfur is added to the process during the splitting of tall oil.

Additional sulfur enters the process during the step to add a soluble magnesium compound during the oxygen depletion, usually magnesium sulphate.

The excess sulfur must therefore be removed from the process. If excess sulfur is removed from the process in the form of sodium sulfate, the amount of sodium lost must be covered by adding fresh lye, which increases the cost of removing sulfur. When controlling the chemical balance in a sulphate factory, it is important to control the proportion between sulfur and sodium, ie. sul fi diteten. Pine soap splitting is an important cause of imbalance in the sulfidity of a sulfate process. If excess sulfur is removed in the form of, for example, gas ygaska, sodium will be lost at the same time, which must be covered by adding fresh lye to the system.

In a modern sulphate factory, sulfur-containing gases are collected from many sources. The amount of sulfur compounds formed in this way has generally been about 2 kg per tonne of pulp, but when using, for example, evaporation of black liquor, the amount can rise to 5 kg per tonne or even 10 kg per tonne of pulp. The sulfur compounds can be burned and the sulfur dioxide formed can be absorbed in a NaOH solution, giving a sodium bisul solution.

WO 94/1 157 1 describes acidification of soap by means of a large excess of sodium bisulate solution in relation to bulk soap with the aim of recovering sulfur for reuse in tall oil production. However, it is necessary to further acidify the soap with the help of sulfuric acid to complete the cooking. A bisul solution is in itself not very acidic but has a pH slightly below 5. When the acid is split, the pH needs to be lower than 4.

The sodium bisul solution has also been used to partially replace sulfuric acid in the final step of a chemical pulp bleaching process to break down residual chloride or, depending on the type of step, hydrogen peroxide, and for final acidification. Acidification with sulfur dioxide also improves the dewatering during the drying of the pulp.

Factories that do not produce chlorine dioxide with the Mathieson process still use sulfur dioxide as a reducing agent in the form of, among other things, sulfuric acid to break down residual chlorine dioxide and peroxide as above and to acidify the final pulp. Due to the risks involved in handling commercially available sulfur dioxide in liquid form, it is desirable to produce sulfur dioxide directly in the plant.

FI 64408 describes a process in which a bisul solution is treated with an acidic residual solution obtained from chlorine dioxide production and contains both sulfuric acid and sodium sulphate, which in practice are in the form of an acidic sodium bisulphate solution. The residual solution also contains chlorine compounds that can cause corrosion problems in the process equipment. The process further includes evaporating and crystallizing the solution remaining after sulfur dioxide formation to precipitate sodium sulfate. The remaining sulfuric acid, which contains chloride, is returned to the chlorine dioxide production process. The residual solution, which contains chloride, can also cause corrosion and cleaning problems in the finishing equipment.

The purpose of the invention is to achieve a simple and efficient recycling process for sulfur recovered from a chemical pulp process, especially from the pulp mill's chemical circulation system, without the need to add additional sulfur from the outside to a significant extent, whereby problems with the process sulfur can be avoided. The invention also relates to a process for recycling sulfur with which the sulfur-sodium balance in a process can be further improved.

The invention also provides an improved process for the recovery of sulfur in such a way that in the treatment steps it is possible to avoid corrosive treatment liquids or to use less corrosive liquids instead, for example acid solutions containing chlorides.

The invention also provides additional alternatives to treatment solutions and treatment reagents used in pulp processes and sulfur recovery processes, whereby the treatment solutions and treatment reagents make it possible to replace, for example, previously used solutions containing chlorides and / or other corrosive substances.

The invention will now be explained in more detail with reference to the drawing, Fig. 1 shows the pH reduction during the oxidation process during an oxidizing treatment, Fig. 2 a diagram of an embodiment of the invention for recycling sulfur and Fig. 3 a diagram of another embodiment of the invention. for sulfur recovery.

In connection with the present invention, it has surprisingly been found that it is possible to treat a bisulfite solution produced from sulfur recovered in a pulp production process, in particular chemical pulp, for example a sulphate process, in such a way that the sulfur can be returned to the process can be used very efficiently and even if desired further in the treatments according to the Uppflïlllíllgeïl- The invention provides a process for recovering sulfur in a process for the production of pulp, in particular chemical pulp, the sulfur produced from sulfur recovered from the process cycle using an oxidant is converted to bisulfate, which is then used further as recycled acid in various steps in the process.

Thanks to the invention, the bisul solution produced from recycled sulfur can be used for a wider range of purposes. Sulfur and also sodium are recycled very efficiently in the process, ie. The process makes it possible to reduce the amount of sulfur compounds added to the process and at the same time reduce the consumption of sodium hydroxide.

It also becomes easier to control the balance between sulfur and sodium in the circulation system of the chemicals and in the whole process.

The process also makes it possible to use and produce treatment agents which contain less corrosive substances, for example chlorides.

In the present context, the expression "then also used as recovered acid in various stages of the process" or "then also used in various stages of the process" means that the bisulfate product obtained from the treatment is used as a reagent, for example as an acidic agent. ("Acid source") in one or more process steps and / or as a raw material for the preparation of one or more of the reagents for use in the process, for example magnesium sulphate used in the oxygen depletion or bleaching of pulp or chlorine dioxide used in the bleaching of pulp. The products obtained from these treatments can be used continuously or as needed for the stated purposes. The process according to the invention may, in addition to the treatment, also include other process steps for recycling / use of the products obtained from the treatment.

The invention relates to a process for producing chemical pulp produced in the process can be used in, for example, the manufacture of paper and board. The terms "pulp production process" and "process" in the present context thus refer in a broad sense to the process for pulp production, preferably chemical pulp, in particular a sulphate process, including the various pulping processes, subsequent pulping processes, for example PTOCGSSÛT for manufacturing. of paper or paperboard, as well as in a known manner other processes related to a pulping process, for example the chlorine dioxide process, and various process steps relating to the subsequent processing of non-pulp products formed and obtained from the specified processes, such as the to the sulphate process normally belonging to the process step for acid treatment of pine soap separated from the pulp, ie. the cleavage treatment with which tall oil is produced. Products obtained by the treatment according to the invention can thus be used in optional production and / or treatment steps where they are suitable for use.

The term "acid treatment" or "acid treatment" generally refers to a treatment SUN is carried out with an optional acidic agent, usually an aqueous solution of an acidic agent with a pH below 6, preferably below 4, for example 2.

"Recycling" means that at least part of the sulfur or of the products treated by the process is returned to the process cycle.

By "bisul wash" is meant that sulfur dioxide obtained from sulfur combustion is treated with an alkali, usually sodium hydroxide or an agent containing sodium hydroxide, to form a bisul solution. The bisul solution used in the treatment is usually an aqueous solution of sodium bisulphite in a manner known in the art. Thus, the bisulfate solution obtained in the treatment normally has the form of aqueous solution. If necessary, the solutions can be concentrated, for example preferably by evaporation, in order to obtain a concentration and / or volume which is suitable for the intended purpose.

The process according to the invention can utilize the sulfur recovered from the process, in particular sulfur contained in the malodorous gases (for example TRS gases). According to a preferred embodiment, at least a part of the sulfur, preferably the malodorous gases recovered from the process, for example a sulphate process, is burned, so that the constituent sulfur compounds are converted to sulfur dioxide, after which at least part of the sulfur dioxide in a known manner prepare bisulfite, preferably an aqueous solution of sodium bisulfate, with the aid of an alkaline substance, preferably sodium hydroxide.

According to a further embodiment, the above-mentioned sodium bisulate solution is used in the process, wherein a) at least a part of the obtained sodium bisulphite solution is subjected to an oxidizing treatment which oxidizes at least a part of the bisults to bisulphate, after which the bisulthal further reaction mixture is used. ! And / or b) at least a portion of the resulting sodium bisul solution may undergo an acid treatment which converts the bisult to sulfur dioxide for further use in the process, the mother liquor remaining after the treatment being able to be used further in the process if desired. Accordingly, the bisul solution obtained in the treatment of the process according to the invention for recycling sulfur can be used as an acidifier in one or more acid treatment steps in the process, being used alone or together with another acidifier.

According to a preferred embodiment, the process comprises a process step in which pine soap is extracted and subjected to an acidic treatment to form tall oil, the acidic treatment being carried out using a bisul solution obtained with the treatment and added alone or with a second acidic substance. The treatment can be carried out using known methods.

According to a second preferred embodiment, the bisulfate solution obtained in the treatment can be used as raw material for the production of sulphates used in the process, for example in the oxygen delignification step (bleaching step) for the production of magnesium sulphate, which is used as a reagent.

The invention thus provides new and very useful alternatives for the recovery of sulfur in pulp production processes, for example in the sulphate process.

According to the invention, the bisulphate solution from the treatment is further used as an acidifier in acid treatments at various stages in the process, preferably in tall oil digestion.

Furthermore, the invention uses the bisulphate solution from the treatment as a raw material in the preparation of reagents, preferably magnesium sulphate, which are used in the process.

The treatments according to the invention and the embodiments regarding the recovery of the products obtained in the treatment in other process steps in the process will now be explained in more detail.

The treatment process according to the invention is based on the discovery that in a pulping process, especially for chemical pulp, recycled sulfur is oxidized with an oxidant so that a bisulfate solution is formed, which can be used in the process. The recovered sulfur is thus oxidized to a bisulfate solution with the aid of an oxidant.

It has surprisingly also been found that crude tall oil can be boiled using recycled sulfur obtained from the process circulation system for chemicals, for example in a pulp mill. The bisulfate solution obtained by the process according to the invention is considerably more acidic than, for example, a bisulfate solution and has an acidity which is sufficient to split soap into crude tall oil already in one step. In a factory, this enables a significant reduction in the amount of sulfuric acid that must be added from the outside to the process for the purpose in question or for any other acid treatment.

The bisulfate solution can also be used to prepare reagents needed in the process, for example sulfate compounds, especially magnesium sulfate used at bleach SBU 462 nmg.

During the treatment, the oxidation takes place using an oxidant. The oxidant used in the process according to the invention can be any oxidant of common types, for example hydrogen peroxide, oxygen, air, ozone or for example organic peracids, for example peracetic acid or permyric acid. Preferably oxygen, air or hydrogen peroxide are used. When desired, it is also possible to use for the oxidation a peroxide-containing solution obtained from another process step, for example an filtrate containing residual peroxide obtained from the pulp bleaching.

Those skilled in the art will appreciate that the oxidation conditions and the amount of oxidant used can be selected in a known manner based on the oxidant used and the degree of oxidation desired.

The bisulfate formed during the oxidizing treatment lowers the pH of the bisul solution, which has a very beneficial effect on the continued use of the solution in the process. Thus, bisulfite can be oxidized in whole or in part, preferably in part, so that the amount of bisulate formed in the reaction mixture provides a suitable pH for the intended purpose in each individual case.

According to one embodiment, the oxidation of the bisult solution is allowed to proceed until the reaction solution has reached a desired pH. The degree of oxidation required can be determined, for example, as a desired pH of the reaction solution. The determination can be carried out in a manner known to those skilled in the art by calculation and / or experiment, depending on which oxidant is used.

The pH decrease obtained as a result of the oxidation reaction is illustrated by an example shown in Fig. 1. In the figure, the pH of a bisul solution with a content of 10 g / l drops rapidly from its initial value slightly above 4 when hydrogen peroxide is added. The pH of the bisul solution decreases considerably even with a small amount of peroxide.

The pH of the resulting bisulfate solution can be adjusted to a level suitable for boiling tall oil in the range below 2, preferably below 1.5, the amount of hydrogen peroxide when using hydrogen peroxide being above 0.1, preferably above 0.2, times the amount required for stoichiometric oxidation of bisul fi t.

In the process according to the invention, the bisulate obtained from the process cycle can be oxidized to bisulphate with, for example, hydrogen peroxide as oxidant according to the reaction funnel (5).

In this case, only water remains as the excess reaction product. At the same time, the pH of the solution drops considerably. Sulfuric acid (H2SO3) has the acid constants pKal = 1.8 and pKa2 = 6.8, while sulfuric acid correspondingly has pKa1 = -3 and pKa2 = 1.8.

In the process according to the invention, for example, the hydrogen peroxide consumption for stoichiometric oxidation of bisulate has a mass ratio of 1: 3 HQOJNaHSOæ, whereby the peroxide consumption is in the order of 0.33 kg H2O2 per kg NaHSO2.

However, it is not necessary to oxidize all the bisulte in order to achieve a sufficiently low pH, but also a partial oxidation is sufficient.

If oxygen or air is used as the oxidant, the reaction is preferably carried out under an overpressure, for example above 6 bar, especially about 10 bar, in an autoclave and at elevated temperature. Oxygen or air can also advantageously be passed through a mixer with the aim of achieving a good contact between the gas and the liquid. A pressurized oxidation of this kind gives a rapid reaction.

It is also possible to prepare the bisulphate solution obtained with the aid of the invention already in the bisulphite scrubber, so that sulfuric acid and bisulphate are also formed in the bisulphate solution. In this case, the materials in the equipment must be resistant to all types of corrosion caused by the oxidant and the oxidation reaction SO, -> SO3. The bisulfate solution prepared will also contain a certain amount of sulfuric acid, so that the solution becomes slightly more acidic.

The process according to the invention thus makes it possible to convert the sulfur recovered from the process from a sodium bisulphate forin into a bisulphate form which has sufficient surlite for the bisulphate to be suitable for use in, for example, tall oil digestion.

When using only bisul fi t, the tall oil is only partially decomposed and acid, for example sulfuric acid, must be added from the outside to complete the decomposition. The process according to the invention does not require that acid is added and if acid still needs to be added, the amount will be less than when, for example, using a bisul solution.

It is known that pine soap can be split with an externally supplied acid which gives a sufficiently low pH, for example sulfuric acid or residual acid from a chlorine dioxide process. The pH of the bisul fit obtained directly from the process is not in itself sufficient for the purpose. By oxidizing a bisul solution to bisulphate in accordance with the invention, this pH range can be achieved. It is important to keep the pH low enough for the fatty acids, which are weak acids, to pass into the acid forum and be separated. A bisulfate solution with a pH below 2 would convert most of the fatty acids to acid form. If the pH is about 2 units below the pKa value of the fatty acid, which is usually in the order of 4, most of the fatty acids are in acid form, not in the form of salt.

According to an embodiment of the invention, the cleavage is carried out in a single step using a sodium bisulfate solution prepared according to the invention as the cleavage acid, the oxidant being used in a stoichiometric amount. In some cases, it may be beneficial to use an excess of the oxidant. An alternative is to only partially oxidize the bisulfate. The cleavage can also be carried out using a mixture, for example a mixture of sulfuric acid and bisulfate. It is previously known to produce sulfuric acid by oxidizing sulfur dioxide directly to sulfur trioxide. The process according to the invention makes it unnecessary to invest in a separate sulfuric acid plant.

According to an embodiment of the invention, the pine soap splitting is carried out in two or two steps. In a two-step process, the first step may consist in preparing a bisulfate solution according to the present invention, while the second step comprises some other acid, for example fresh sulfuric acid or residual acid from a chlorine dioxide reactor or a mixture of SBÜ 462 9 acids. Alternatively, the first step may involve the use of a sodium bisulfate solution obtained directly from the process or carbon dioxide, the sodium bisulfate solution according to the invention being used only in the second step. In this case, less bisulfate, and thus peroxide, is consumed than in a one-step process. Alternatively, the properties of the bisulfite solution according to the invention are used in both steps in a two-step pine soap boiling.

A magnesium compound for use in oxygen bleaching can be prepared using sulfur recovered in a sulfate process cycle. Thus, it is not necessary to use externally added magnesium sulphate, which would increase the sulfur load.

Sulfur-free raw materials of a magnesium compound can be, for example, magnesium hydroxide, magnesium oxide or magnesium carbonate. A preferred source of magnesium is technically MgO.

According to one embodiment of the invention, the magnesium compound is dissolved in, for example, the bisulfate liquid obtained from the sulfate process using the reaction formula (6), which gives magnesium sulfate: 2 NaHSO, + Mg (OH) 2 -> NaQSO, + MgSO 4 + 21-120 (6) An alternative way of preparing magnesium sulphate is to carry out the reaction already in the scrubber, where gas containing SO 4 is washed with water containing an oxidant, for example hydrogen peroxide, and Mg (OH) 2. In this case, the peroxide consumption will be in the range 0.28 to 0.33 kg / kg MgSO 4.

Due to the fact that MgSO4 contains 26.6% sulfur, the process according to the invention emits sulfur in an amount of less than 0.26 to 0.8 kg / ton of pulp, while the amount of MgSQ, which is added in oxygen bleaching, is 1 to 3 kg. per ton mass. Since both the sodium bisulfate and sodium bisulfate solution contain water and water is also formed during the reaction, 50%, preferably 70% of the water can be evaporated from the two solutions, thereby lowering the pH of the solution.

By using the bisulfate solution obtained in the treatment, bisulfate can be used in a stoichiometric amount or in excess to form sulfur dioxide. In the case of an excess of sodium bisulphate, the excess is generally greater than 40%, preferably greater than 60% and especially greater than 70%. Preferably, an excess of the sodium bisulfate solution is used to complete the reaction (7), the remaining mother liquor also being very suitable for use in acid treatments, for example in pine soap splitting.

Bisulfate can also be used in conjunction with sulfuric acid added to the process. The recovery and reuse of sulfur in a pulp process, for example a sulphate process, according to the invention is described in the following in the exemplary form by means of a number of preferred embodiments, which are shown in fi g 2 and 3.

The equipment needed for the oxidation reactions in the practice of the invention is simple. The equipment essentially comprises a mixing vessel, in which the reaction between the bisulphate recovered sulfur from the sulphate process and the oxidant takes place. The equipment may also contain an oxidant dosing device and an intermediate storage container for the bisulfate which is formed as a reaction product and from which bisulfate solution can be passed on to the desired process site, for example pine soap splicing or magnesium sulphate production.

In the embodiment according to fi g 2, the bisulphate solution according to the invention is prepared by oxidizing the sodium bisulite obtained from the bisulate scrubber 4 in the mixing container 1 by means of an oxidant, which is introduced into the mixing container 1. The sodium bisulfate prepared in the mixing container is fed, for example. 5 for boiling crude tall oil a or to a reactor 6 for producing magnesium sulphate. In the reactor 6, magnesium sulphate is produced from a magnesium reactant which is fed to the process from the outside and sodium bisulphate which is produced from recycled sulfur according to the invention. The magnesium sulphate thus produced is used in oxygen bleaching 7. The filtrate from the oxygen bleaching is then led to an evaporation plant 8 and from there to a recovery boiler 9. Raw soap is separated from the liquid coming from the laundry 11 and goes to the evaporation plant 8 during the production of crude tall oil. , the crude soap being led to a boiling reactor, where it reacts to crude tall oil with the aid of bisulphate. The crude pine oil 12 is removed from the process in the tall oil boiling reactor 5 and the remaining sulfur-containing pulp pressurized water is passed through the evaporator 8 to the recovery boiler 9 for the combustion of organic compounds and sulfur. In the recovery boiler 9, the sulfur compounds are burned to sodium sul fi cl.

From the recovery boiler 9, an aqueous solution of sodium carbonate and sodium salt, green liquor, is passed to a causticizing step 13, where sodium carbonate is converted to calcium carbonate. The resulting solution of sodium hydroxide and sodium sulfide, white liquor, is returned to the pulp boiling 10. The remaining calcium carbonate is further regenerated in the lime kiln 14, so that calcium oxide is formed. The most important source of sulfur dioxide is the combustion stage 15 for malodorous gases, where reduced sulfur compounds, mainly H28, methylmercaptan, dimethylmercaptan and dimethyldimercaptan, are burned to sulfur dioxide, after which they are passed to the bisulphite scrubber 4. The sulfur dioxide gas from other sources can also be recycled. In the bisulate scrubber 4, sulfur dioxide is absorbed in the NaOH solution, whereby sodium bisulte is formed. The sodium bisultene obtained in the bisulate scrubber is then passed to the mixing vessel 1 to be oxidized according to the invention.

In 3 g 3, the recovery process comprises equipment for combustion of gases containing malodorous sulfur compounds (TRS gases), for example a lime furnace 14 and a bisulte scrubber 4, to which the sulfur dioxide formed during the combustion is passed to form a bisul solution, preferably a sulfur solution. Furthermore, in addition to an oxidation step 1, the equipment may contain an acid treatment step 17, in which sulfur dioxide is formed. In operation, part of the bisulte solution is passed to the sulfur dioxide-forming stage 17, to which an acid treatment solution is added, for example an acid supplied from outside or a bisulphate solution which is oxidized in stage 1 by means of an oxidant and used alone or together with a or fl your acidic substances. The mother liquor can, depending on how it is composed, be recycled to the oxidation step 1, where unreacted bisulate can be oxidized together with fresh bisulfite. Alternatively, the mother liquor can undergo acid treatments in the process, whereby it can be used alone or together with bisul fi t from the oxidation step 1, for example for digestion of pine soap, after which sodium can also be recovered. SVHVGIÖIOXïÖGI-1 50111 formed in step 17 can be led to a process 18 for producing sulfuric acid.

Thanks to the process according to the invention, it is possible to completely or partially avoid using residual acid from, for example, a chlorine dioxide process 16, for example a Mathieson process, or from a process which uses hydrogen peroxide as reducing agent.

In the following, the invention is described with specific details, which, however, are not limiting of the invention.

Example 1 A sodium bisulfate solution NaHSO 3 from a sulphate pulp plant with a concentration of 43 g / l and pH 4 was oxidized with the addition of 50% hydrogen peroxide solution.

Table 1 shows the pH variation of the solution as a function of the peroxide additive. Mg (OH) 2 IÖSTGS without problems in the proportion 12 g / l in a bisul solution to which 11 g / l HQOQ had tiIISaVßS- The final solution then contained 24.9 g / l MgSOA. The solution also contained sodium sulfate.

The magnesium sulphate solution thus prepared functioned in exactly the same way in oxygen bleaching and peroxide bleaching as pure magnesium sulphate supplied from the outside.

No adverse effects of the accompanying sodium sulfate could be observed.

Table 1 Hydrogen peroxide content [g / l] pH 0 3.99 0.9 2.88 2.5 2.35, 0 1.82 7.4 1.33, 2 0.75 11.0 0.68 Example 2 4 1 of a sodium bisulate solution NaHSOf originating from a sulphate pulp mill, with a concentration of 210 g / l was introduced into an autoclave with a volume of 8 1. The autoclave was equipped with a circulating gas stirrer, using oxygen gas as gas. The temperature of the autoclave was regulated to 80 ° C and the autoclave was supplied with pure oxygen until an overpressure of 10 bar prevailed in the autoclave. The consumption of added oxygen and pH were measured. Table 2 shows the pH development 530 462 12 during the progressive oxygen consumption.

Table 2 Oxygen consumption pH (% of theoretical) 0 5.18 59.5 2.22 78.5 1.47 94.3 1.02 98.1 0.92 100 0.88 The pH of the bisul solution decreases rapidly to a value below 3 during the initial stage of oxidation and continues to decrease as the oxidation proceeds. The increased acidity is obviously due to the almost complete oxidation of bisulfite to bisulfate.

The experiment shows that the use of oxygen as oxidant makes it possible to achieve the required pH range, after which the recovered solution is suitable for use in the splitting of pine soap.

Example 3 A process solution of sodium bisulfate having a density of 1169 g / l, pH 5.6 and the SO32 "PI" PO01't0 '15% was oxidized with hydrogen peroxide (50% by weight) to sodium bisulfate. After the oxidation the solution contained 263 g / l sodium bisulfate The solution was used to split pine soap. 400 g of pine soap was measured in a two-liter container. When mother liquor was used for the digestion, it was added at this stage. When using sulfuric acid which in the comparison experiment was also added 300 g of water. the container was heated to 100 ° C temperature and the acidic solution used was added until the pH dropped below 3. By using a sodium bisul solution, it covered 50% of the required amount of sulfuric acid.

When the pH had dropped below 3, the sample was boiled for about 15 minutes at 100 ° C temperature. The product was then poured into an extraction funnel. After 30 minutes, three phases had been separated: tall oil, lignin and mother liquor. Tall oil was separated and the volume, density and pH of the solution were measured, after which the yield of tall oil was calculated. In some samples, the phases were separated by centrifugation. These samples are marked with a star.

According to calculations, 50 g of sulfuric acid per 400 g of tall oil are required. If 50% of the sulfuric acid can be replaced with a theoretical amount of sodium bisulfate, the amount will be 6.12 g, and with a solution containing 230 g / l, 266 ml is needed. The bisulphate prepared from the process liquid had a theoretical content of 263 g / l, whereby the required corresponding amount becomes 233 ml.

The amount of pine soap varied slightly from experiment to experiment, but was always slightly over 400 g.

The experiments used two different industrial pine soap samples, a and b. Table 3 below shows for comparison cleavage using sulfuric acid and Table 4 shows cleavage carried out with a mixture of sodium bisulfate and sulfuric acid, where the bisulfate has been prepared by oxidizing the bisul solution from the process using oxygen. In Table 4, the acid proportion refers to the ratio of sulfuric acid obtained from sodium bisulfate to the added sulfuric acid.

Table 3 Sample no. Crude soap 112504, kg Yield Acid number Soap number per ton% mg KOH / g mg KOH / g crude soap 1 a 94.2 es 152 0.6 2 a * 109 68 152 0.4 3 b 104 53 153 0.6 4 b * 106 64 156 0.4 Mean Value 103 65 153 0.5 Table 4 Sample No. Crude Soap Total Acid Pro- Yield _ Purpose 55119131 amount portion% m8 'mg H2so4, Nanso; KOH / g KOH / g kg / ton H2SO4 raw soap a * 122 0.539 75 157 0.9 7 b 137 0.539 74 157 1.5 8 b * 129 0.539 74 158 0.9 Mean- 129 0.539 74 158 1.1 de Den Thus, according to the crude tall oil produced above, it had excellent values for the acid number corresponding to the fatty and resin acid content, which should have a value> 150. The average value of the soap, which corresponded to the amount of soap in the crude tall oil, meets the requirements of high quality industrial tall oil. The yield was much higher than when using sulfuric acid (see Table 3).

Example 4 Corresponding to Example 3 but with the entire sulfuric acid consumption replaced by the bisulfate solution obtained from the process. The salt proportion obviously increases the density of the aqueous phase, thereby improving the separation of the tall oil. As a result, the total oxygen consumption decreases compared with the case with the use of sulfuric acid. The values for the exchange of pine oil correspond to the values given in Table 4. Example 5 Crude pine oil a was treated as in Example 3 with a two-step process, where soap was first pre-neutralized with a sodium bisulfate solution and the final acidification was carried out with sulfuric acid. The results obtained are shown in fi g 5.

Table 5 Sample no. Crude soap Total Acid Pro- Yield «Acid fi l Sole number amount portion% mg mg H, so ,, Na 0.396 76 156 0.8 12 A * 130 0.479 80 153 0.4 13 b 132 0.517 76 156 1.0 14 b * 130 0.523 73 155 0.7 Mean 118 75 1586 1.1 de The results show that a replacement of more than 50% of the sulfuric acid with sodium bisulphate results in a significant reduction in the total acid consumption per tonne of tall oil.

Example 6 500 ml of the process solution, which contained 120 g of sodium bisulate, were poured into a reaction vessel equipped with a stirrer. The solution was heated to 80 ° C and 104 g of 95% sulfuric acid was added slowly dropwise over 70 minutes at constant temperature. The sulfur dioxide formed was passed first through methanol and then through water and the dissolved amounts of the sulfur dioxide were analyzed. The sulfuric acid supply was calculated per unit time and the same was said for the sodium bisul fi conversion to sulfur dioxide. The results are shown in Table 6. 530 482 Table 6 Time [min] H 2 SO 4, H 2 SO 4, SO 2 SO 2 SO 2 calculated excess formed formed conversion [mol] [% l [gl [mol] l% l 0 '0 0 0.144 3.8 0, 0593 5.1 0.288 16.5 0.258 22.3 0.432 25.6 0.400 84.7 40 0.576 -0.2 43.5 0.679 58.9 50 0.720 25 59.6 0.930 80.7 60 0.863 50 64.3 1.004 37.0 70 1.007 75 66.3 1.035 89.7 The results show that an almost stoichiometric amount of sulfuric acid and bisulate gives about 60% conversion of the sulfur dioxide.

When sulfur dioxide is produced in stoichiometric amounts, the mother liquor is preferably passed to the oxidation treatment step to be converted to bisulfate. If, on the other hand, sulfuric acid is used, for example 40%, especially more than 70%, the mother liquor can be used for acidification in various places, especially during pine soap blasting.

Claims (15)

536 462 ii: Patent claim Process for the recovery of sulfur in a process for the production of pulp, characterized in that the sulfur produced from the sulfur recovered from the process cycle is converted by means of an oxidant into bisulphate, which is then used as digestive acid for digestion of pine soap or as a raw material in preparation of magnesium sulphate which is used as a reagent in a bleaching step in the process. Process according to Claim 1, characterized in that the sulfur recovered from the process cycle is completely or partially converted to bisulphate. Process according to Claim 1 or 2, characterized in that the oxidant used is hydrogen peroxide, ozone, an organic peracid, oxygen, air, a peroxide-containing solution obtained from another process step or mixtures thereof, preferably hydrogen peroxide, oxygen and / or air. Process according to Claim 3, characterized in that when hydrogen peroxide is used as the oxidant, it is used in an amount of more than 0.1, preferably more than 0.2 times the stoichiometric amount required for the oxidation of the bisultene. Process according to Claim 1, characterized in that the sulfur recovered from the process cycle is converted to bisulfate by means of an oxidant after it has first been converted to bisul fi t in a bisul fi scrubber. Process according to Claim 1, characterized in that the sulfur recovered from the process cycle in a bisulate scrubber is converted to bisulphate by means of an oxidant. Process according to one of the preceding claims, characterized in that the oxidant is used in an amount which gives a bisulfate-containing reaction solution with a pH below 4, preferably below 2, in particular below 1.2. Process according to Claim 1, characterized in that the splitting of pine soap takes place in one or more steps, at least one of which uses the bisulphate, which is used alone or together with other acids. Process according to Claim 8, characterized in that in addition to the bisulphate, an additional acid or carbon dioxide is also used as the cleavage acid. Process according to Claim 9, characterized in that the additional acid is sulfuric acid or bisul fi t. Process according to Claim 3, characterized in that in an intended use as the nitric acid, the oxidation is carried out with hydrogen peroxide, which is used in an amount of more than 0.15, preferably more than 0.20 g per g of bisulate. Process according to Claim 1, characterized in that the residual by-solution obtained after a treatment in which the by-product produced from the sulfur recovered from the process cycle is converted by means of an acidifier other than an acidic residual solution from a chlorine dioxide production process into sulfur dioxide. which is used together with the bisulphate for splitting pine soap. Process according to Claim 1, characterized in that the acidic residual mother liquor as 530 462 in? obtained after a treatment in which the sulfur produced from the sulfur recovered from the process cycle is converted by means of an acidifier, which is other than an acidic residual solution from a chlorine dioxide production process, into sulfur dioxide, which is added to the sulfur to be oxidized, the resulting mixture oxidized to form a bisulfate solution, which is used for the splitting of pine soap. Process according to Claim 1, characterized in that the bisulphate is used as sulphate raw material, the bisulphate being reacted with an oxidant and a magnesium compound added externally to the process to form magnesium sulphate. IN Process according to Claim 14, characterized in that the magnesium compound is a sulfur-free magnesium compound, preferably magnesium hydroxide, magnesium oxide or magnesium carbonate, in particular a technical magnesium oxide, the magnesium compound being dissolved in the bisulphate solution.