SE530461C2 - Process for the recovery of sulfur in a chemical pulping process, in particular a sulfate process, as well as the use of recycled products obtained by the process for producing tall oil and magnesium sulfate using the recovered sulfur for the process - Google Patents

Description

530 451 2 sulfide. The composition of white liquor has a significant effect on the properties of the pulp to be boiled. Since the process is in principle closed also in this respect, ie. Since the cooking chemicals are not consumed during the process, it is important to keep the chemical cycle's ratio between sulfur and sodium in balance. In the current situation, balance control has a major problem in the form of a surplus 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 the 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 mass cooking, the fatty acids and resin acids are saponified into sodium salts, ie. soap. Unsaponifiable organic constituents also remain, depending on the type of tree used. 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 decomposed into tall oil with sulfuric acid and heat. During the digestion, the soap separated from the surface of the black liquor is thus 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 cleavage are described by the following relationships: Hzso, + zn-cooNa -> .an-coon + Naäso. <3) In its acid form, the fatty acids are insoluble in water, and are therefore excreted from the soap. During the digestion, sulfuric acid is consumed mainly for the conversion of soap into tall oil but also for neutralizing the black liquor that accompanies the soap and for regulating the pH to a level that is low enough for the fatty acids to be converted to acid form.

In practice, the addition of acid should be such that the pH drops below 4. Tall oil formed during the digestion 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 can be used as cleavage acid, which in fact also happens. Since the aqueous phase formed during the digestion is returned to the chemical recovery, sulfur in loose form will also be introduced into the chemical recovery. Together with the residual acid, a not insignificant amount of sulfur will enter the process, since the residual acid contains sodium sulphate or sodium sesquulphate in a significant amount in addition to the sulfuric acid. 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 replaced with sulfur-containing chemicals that are formed during 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 in the tall oil digestion. The excess sulfur must therefore be removed from the process.

If the excess sulfur is removed from the process in the form of sodium sulfate, the removed sodium must be covered with fresh lye, which increases the cost of removing the sulfur.

When controlling the chemical balance in a sulphate pulp mill, it is important to control the proportion between sulfur and sodium, ie. sul fi diteten. Pine soap splitting is a particularly important reason for the imbalance in the sulphate in a sulphate 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.

One way to solve this problem would be to use a sulfur-free acid. Hydrochloric acid is not suitable for this purpose due to problems caused by the chloride. Such organic acids as formic acid and acetic acid might be able to replace sulfuric acid to some extent, but they are probably too weak acids and also relatively expensive. Nitric acid is also not suitable for the process in that it can cause nitric oxide formation during combustion in the recovery boiler. .

WO 94/1 157 1 describes acidification of soap by means of a large excess of sodium bisul solution in relation to the amount of soap for the purpose of recovering sulfur for reuse in tall oil production. The acidification can be carried out in a single step or preferably in several steps. The bisul solution used acidifies the soap at an elevated temperature in the range 90 to 150 ° C at atmospheric pressure or a slight overpressure. 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 acids are broken down, the pH needs to be lower than 4.

Additional sulfur is added to the process during the addition of a soluble magnesium compound in connection with the oxygen bleaching. Magnesium sulphate is usually added from the outside to the process. It is also possible to use acetates or formats instead of magnesium sulphate, which makes it possible to reduce the sulfur load by means of the choice of chemicals, but in practice these alternatives are too expensive. Magnesium hydroxide and magnesium carbonate are suitable alternatives in terms of their composition, but they have the disadvantage that they are too sparingly soluble.

The object 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 can be avoided.

In the drawing Fig. 1 shows the pH decrease during the oxidation process in an oxidizing treatment, Fig. 2 is a diagram of the apparatus used in the treatment according to the invention, Fig. 3 is a diagram of an application of the invention in the recovery of sulfur, Fig. 4 is an example. Fig. 2 shows the change in pH, Fig. 5 shows the sulfur-sodium proportions in a comparison in fi g 3, Fig. 6 the pH change which occurs in Example 4 during the oxidation process, Fig. 7 shows the oxidation in Example 4 as a function of time and Fig. The pH change occurring in Example 4 during the oxidation process.

The process according to the invention is based on the surprising discovery that recycled sulfur is oxidized to a bisulphate solution with the aid of an oxidant. It is surprising that the bisulfate solution obtained is considerably more acidic than, for example, a bisulfite solution and has an acidity which is sufficient to break down soap into crude tall oil already in one step. Crude tall oil can thus be boiled using recycled sulfur obtained from the pulp mill's chemical circulation system. The bisulphate solution can also be used to prepare sulphate compounds needed in the process, for example magnesium sulphate used in bleaching.

In the present context, the expression “then also used as recycled acid at different stages in the process” or “then also used at different stages in the process” means that the bisulphate product obtained from the oxidizing treatment is reused as a reagent, for example as an acid agents ("acid coals") in one or more process steps and / or as a raw material for the preparation of one or more reagents for use in the process, for example magnesium sulphate used in oxygen delignification or pulp bleaching.

The process according to the invention can utilize the sulfur recovered from the process, in particular from sulfur-containing malodorous gases. According to a preferred embodiment, at least a part of the sulfur, preferably the foul-smelling gases recovered from the process, for example a sulphate process, is combusted, so that the constituent sulfur compounds are converted to sulfur dioxide, after which at least a part of the sulfur dioxide is used to to prepare bisulate, preferably an aqueous solution of bisulfite, by means of an alkaline substance, preferably sodium hydroxide.

The invention relates to a process for producing chemical pulp. Pulp produced during 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 a process for the production of the pulp in question, preferably a sulphate process, including the various pulp production steps, subsequent pulp processing steps, for example manufacturing processes. of paper or paperboard, as well as in a known manner other processes related to a pulping process and various process steps relating to the subsequent processing of products other than pulp formed and obtained in connection with the specified processes, such as the The sulphate product obtained from the treatments according to the invention can thus be used in optional production and / or treatment steps where they are suitable. to use.

The term "acidic treatment" generally refers to a treatment carried out with any acidic agent, usually an aqueous solution of an acidic agent having a pH below 5, preferably below 4, for example 2.

By "recovery" is meant that at least a portion of the sulfur-1 or bisulfate product treated by the process is returned to the process cycle. 10 15 20 25 30 35 40 SBC! 461 5 “Bisul wash” means that the sulfur dioxide obtained from the sulfur combustion is treated with an alkali, usually sodium hydroxide or an agent containing sodium hydroxide, to form a bisul solution.

The bisulphate product obtained in the treatment process according to the invention is used in various steps in the process to completely or partially replace substances which would otherwise need to be added from the outside to the process cycle, for example to completely or partially replace an acid, for example sulfuric acid, which is supplied from the outside to a acid treatment step in the process.

According to a variant, the process comprises a step in which pine soap is extracted and subjected to an acidic treatment to form pine oil, the acidic treatment being carried out using a bisul solution obtained by an oxidizing treatment and applied alone or together with a second acid. substance. The treatment can be carried out using known methods.

According to another variant, the bisulate solution obtained in the oxidizing treatment can in turn be used as a 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 oxidant used in the process of the invention may be Optional OXIDHIIt of common types, for example hydrogen peroxide, oxygen, air, ozone and for example organic acids, for example peracetic acid or permyric acid. Preferably oxygen, air or hydrogen peroxide are used, especially hydrogen peroxide, which is also the most convenient to use. When desired, it is also possible to use a peroxide-containing solution obtained from any other oxidation process step, for example a 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, bisulite can be oxidized in whole or in part, preferably in part, so that the amount of bisulite 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 bisul solution may continue 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.

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.

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. In this case, only water remains as an additional reaction product. At the same time, the pH of the solution drops considerably. Sulfuric acid (HzSOa) has the acid constants pKal = 1.8 and pKa2 = 6.8, while sulfuric acid correspondingly has pKal = -3 and pKa2 = 1.8. Sodium bisulfate can be oxidized to sodium bisulfate, for example with the aid of hydrogen peroxide according to the reaction formula (4): NaH 50, + H 2 O, a bisulphate form which has sufficient acidity for the bisulphate to be suitable for use in, for example, digestion of tall oil.

When bisulfite alone is used, 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 the addition of acid. It is known that pine soap can be broken down 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 itself is not 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 lower the pH low enough for the fatty acids, which are weak acids, to turn into acid form and separate. 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, most of the fatty acids are in acid form, not in the form of salt.

In the process according to the invention, the hydrogen peroxide consumption for stoichiometric oxidation of bisulfite has a mass ratio of 1: 3 H 2 O 2 / NaHSO 3, whereby the peroxide consumption is in the order of 0.33 kg H 2 O, per kg NaHSO 2. However, it is not necessary to oxidize all the bisulte in order to achieve a sufficiently low pH, but also a partial oxidation may be sufficient.

The pH decrease achieved as a result of the oxidation reaction will now be described by means of an example shown in fi g 1. In the uren gure, the pH of a bisulfite 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 being above 0.1, preferably above 0.2, times the amount required for stoichiometric oxidation of bisul fi t. Therefore, the bisulfate solution becomes more acidic than the bisulfate solution, which makes it much more suitable for, for example, tall oil spraying. This makes it possible to significantly reduce the amount of sulfuric acid added from the outside to the process for this use in the plant. Soap splitting by means of sulfuric acid which in its entirety was added from the outside to the process would form about 2 to 4 kg / Adt of sulfur during the process, whereby the amount of sulfur would decrease significantly at least in this respect using the method according to the invention.

The equipment needed for the oxidation reactions in carrying out the invention is simple, an example being shown in fi g 2. The equipment essentially comprises a mixing vessel 1, in which the reaction between the bisulphate recovered sulfur from the sulfur process S and the oxidant takes place. The equipment may also contain an oxidant dosing device 2 and an intermediate storage container 3 for the bisulphate which is formed as a reaction product and from which bisulphate liquid can be passed on to the desired process site, for example pine soap cleavage or magnesium sulphate production.

It is also possible to prepare the bisulphate solution obtained with the aid of the invention already in the bisulate scrubber, so that sulfuric acid and bisulphate mixed with bisulphite solution are formed. In this case, the materials in the equipment must be resistant to all types of corrosion caused by the oxidant and the oxidation reaction SO2 -> SO3. However, this alternative embodiment is more expensive than the previously described embodiment. The bisulfate solution prepared will also contain a certain amount of sulfuric acid.

Figure 3 shows an example of the recovery and reuse of sulfur in an sulphate process. The bisulphate solution is prepared according to the invention by oxidizing the sodium bisulate obtained from the bisulate scrubber 4 in the mixing vessel 1 by means of an oxidant, which is introduced into the mixing vessel 1. The sodium bisulphate produced in the mixing vessel is fed, for example, to a reactor a reactor 6 for the production of 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 evaporator 8 and from there to a recovery boiler 9. Crude soap is separated from the liquid coming from the washing net 11 and goes to the evaporator 8 during the production of crude oil, to a boiling reactor 5, where it reacts to crude tall oil with the aid of bisulfate. The crude tall oil 12 is removed from the process in the tall oil boiling reactor 5 and the remaining sulfuric acid mother liquor 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 d. From the recovery boiler 9, an aqueous solution of sodium carbonate and sodium sul fi d, green liquor, is passed to a causticisation step 13, where sodium carbonate is converted to calcium carbonate. The resulting mixture of sodium hydroxide and sodium salt vitd, 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 dimethyldimpercaptan, are burned to sulfur dioxide, after which they are passed to the sulfur scrubber. 40 530 451 8 recycled. In the bisulate scrubber 4, sulfur dioxide is absorbed in a NaOH solution, whereby sodium bisulte is formed. The sodium bisulfate obtained in the bisulate scrubber is then passed to the mixing vessel 1 to be oxidized according to the invention.

The process according to the invention makes it possible to completely or partially avoid using residual acid from the chlorine dioxide production step 16, which means a reduced supply of sulfur from outside to the process.

According to one 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 containing the recovered bisulphate according to the invention as a part, for example a mixture of sulfuric acid and bisulphate. This sulfuric acid can also be produced from sulfur dioxide obtained by recovery with the aid of hydrogen peroxide, whereby the sulfur dioxide is first oxidized to sulfur trioxide and then to sulfuric acid. 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 performed in two or two steps. In the two-step process, the first step may consist of 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 acids. Alternatively, the first step may involve the use of a sodium bisulfite solution obtained directly from the process or carbon dioxide, the sodium bisulf 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 bisul solution according to the invention are used in both steps in a two-step pine soap cooking.

If the amount of sulfur used for cleavage is 2 to 4 kg s / Adt, the stoichiometric amount of peroxide will be roughly the same, ie. 2 to 4 kg. However, the amount is not necessarily exactly stoichiometric, as shown by fi g 1. Half of this amount is preferably sufficient, above 0.15 g H 2 O / g NaHSO 2. In particular, the amount of hydrogen peroxide added above is above 0.20 g HQO / g NaHSO 4.

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. Recycled sulfur can be used in the form of sulfur dioxide, bisulate or salt, which is converted to magnesium sulphate by means of an oxidant.

Sulfur-free raw materials of a magnesium compound can be, for example, magnesium hydroxide, magnesium oxide or magnesium carbonate. The preferred source of magnesium is technically MgO. According to an embodiment of the invention, the magnesium compound is dissolved in the bisulphate liquid obtained from the sulphate process using the reaction (5), which gives magnesium sulphate: ZNaI-ISO, + Mg (OH) 2 -> NaQSO, + MgSQ, + 2H2O (5) An alternative way to prepare magnesium sulphate is to carry out the reaction in the gas scrubber, where gas containing SO2 is washed with water containing an oxidant, for example hydrogen peroxide, and Mg (OH) 2. In this case, the range of peroxide consumption is 0.28 to 0.33 kg / kg MgSO 4.

Another alternative way of preparing magnesium sulphate is to first dissolve the magnesium compound in a bisulphate solution and then oxidize the magnesium sulphate thus formed using, for example, hydrogen peroxide. Magnesium salt is in itself a reducing compound, which can not be used in bleaching processes because it unnecessarily consumes other chemicals. »Due to the fact that MgSO 2 contains 26.6% sulfur, the process according to the PPfm 1 to 3 kg per tonne of pulp. The invention provides new and very useful alternatives for the recovery of sulfur in a pulp process, for example a sulphate process.

The invention also makes it possible to use bisulphate liquid from an oxidizing treatment as an acidifier in acid treatment in various process steps, preferably in pine oil splitting. The invention also makes it possible to use the bisulphate liquid obtained in the treatment as a raw material in the preparation of reagents used in prOCGSSGH, preferably magnesium sulphate.

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

Example 1 Investigation of a sulphate process from which excess sulfur is removed in the form of gas ash. The sodium that comes with the sulfur must then be covered by adding fresh liquid to the process. The liquid consumption is 2.5 kg NaOH per kg sulfur removed. If the amount of sulfur removed is 4 kg per tonne of pulp, the equivalent NaOH consumption will be 10 kg per tonne of pulp.

This is to be compared with the sulphate process according to the invention. If the amount of sulfur is reduced by recycling the sulfur contained in the bisulnate during pine soap splitting, the amount of hydrogen peroxide needed to oxidize this amount of sulfur exceeds 4 kg.

Calculated on the basis of current chemical prices, the cost of hydrogen peroxide will be 2.1 å. If sulfur is removed in the form of fl ash, the cost of the required amount of sodium will be 3.2 ê, ie the cost effect is 1.5 times compared with the process according to the invention.

Example 2 A sodium bisult solution NaHSQ obtained from the sulphate process, with a concentration of 43 g / l and pH 4, was oxidized with the addition of a 50% by weight hydrogen peroxide solution. Fig. 4 shows the pH variation of the solution as a function of the peroxide additive. Without problems, 12 g / l Mg (OH) was dissolved in a bisulfite solution to which 11 g / l H 2 O 2 had been added, whereby the final solution contained 24.9 g / l MgSO 4.

Magnesium hydroxide seemed to dissolve somewhat more slowly in a non-oxidized bisul fi t solution. The oxidation of magnesium bisulate thus obtained with an addition of hydrogen peroxide in a ratio of 11 g / l still gave the same end result as an oxidation of the bisulte before it was dissolved. added pure magnesium sulfate.

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

Example 3 An introduction of the sulfur to sodium ratio in two cases with a sulphate process A and a sulphate process B. In the first case A, sulfur was not recovered, but the total amount of sulfuric acid needed was added from the outside to the process and the sodium bisulte was removed from the process. In the second case B according to the invention described above, bisulate was oxidized to bisulphate with the aid of hydrogen peroxide, the required amount of sulfuric acid being added from the outside.

Fig. 5 shows the two cases A and B and their sulfur-sodium proportions in different steps.

Step 1 shows the amount of sulfur carried by sulfur, step 2 shows the required sodium coverage (sodium hydroxide), step 3 adds sulfur to the process together with oil, wood and MgSO 4, step 4 shows, among other things, the removal of ash salts, gas ash, sulfur dioxide and soda precipitation, while step 5 shows washing losses and step 6 the removal of other substances.

In case B, the bisul was recovered using recycled white liquor. In this case, no fresh sodium was added to the system along with sodium bisulfate. Sulfur carried by bisul fi t entered the cycle in the normal way and is not included in the balance sheet.

With the procedure according to case B, the amount of sulfuric acid introduced into the system was reduced by adding 2 kg of sulfur, i.e. 6.1 kg of sulfuric acid, was covered with a bisulfate liquid prepared by oxidation of bisulate.

Fig. 5 shows how the need to cover losses of NaOH is significantly reduced. The use of hydrogen peroxide as an oxidant depended on the extent of the oxidation of the bisultene peroxide. If the bisultene is oxidized to 100%, 2.1 kg of H2O2 is needed and with 50% oxidation, the need for H2O? consequently 1.1 kg.

Example 4 4 l of sodium bisul solution, NaHSO 3, taken from the sulphate process, had a concentration of 250 g / l 'and was placed in an autoclave with a volume of 8 l. The autoclave was equipped with a circulating gas stirrer, using oxygen as gas. The autoclave temperature 10 15 20 25 30 35 EBÛ 451 11 was regulated to 80 ° C and the autoclave was supplied with pure oxygen until an overpressure of 6 bar prevailed in the autoclave. A sample of the solution was taken at the starting moment t = 0 and a number of additional samples were taken during the oxidation process. The pH of the samples was measured and the salt and sulphate concentrations were analyzed. Fig. 6 shows the change in pH as a function of time during the oxidation process and fi g 7 shows the sulphate and solution concentrations. The pH of the bisul solution rapidly drops below 3 during the first phase of the oxidation and continues to fall during the continued oxidation. The increasing acidity is obviously due to the almost complete oxidation of bisulate to bisulfate. The study shows that the use of oxygen as an oxidant makes it possible to achieve the required pH range, after which the recovered solution can be reused to split pine soap.

Example 5 A 300 ml sample with a concentration of 44 g / l was taken from a sodium bisul solution, N aHSO 1 l 02 per minute according to measurement with a rotameter Fig. 8 shows the variation of the pH of the solution as a function of time as the process progressed.The pH of the bisul solution fell rapidly at the beginning of the oxygen supply and stabilized at a value slightly below 3.

Example 6 - A process solution of sodium bisulate having a density of 1169 g / l, pH 5.6 and SOO 3. When mother liquor was used for the digestion, it was added at this stage, using sulfuric acid which in the comparative experiment was also added 300 g of water. until the pH dropped below 3. By using a sodium bisulfite 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.

In each experiment, about 400 g of pine soap and 233 ml of bisulphate liquid were used for the digestion. .

In the experiments, two different industrial pine soap samples were used, a and b. Table 1 below shows for comparison cleavage using sulfuric acid and Table 2 shows cleavage performed with a mixture of sodium bisulfate and sulfuric acid, where the bisulfate has been prepared by oxidizing the bisul solution from the process. with oxygen. In Table 2, the oxygen ratio refers to the ratio of sulfuric acid obtained from sodium bisulfate 53Ü 451 12 to the added sulfuric acid.

Table 1 Sample no. Crude soap H2SO4, kg Yield Acid number soap number per tonne% mg KOH / g mg KOH / g crude soap 1 a 94.2 68 152 0.6 5 2 a * 109 68 152 0.4 sb 104 es 158 0 , 6 4 b * 106 64 156 0,4 Mean value 103 66 153 0,5 10 Table 2 Sample no. kg / ton HZSO., rasapa 6 a * 122 0,539 75 157 0,9 7 b 137 0,589 74 157 1,5 8 b * 129 0,539 74 158 0,9 15 Mean- 129 0,539 74 158 1,1 de The as above The tall oil produced thus 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 for high quality industrial tall oil. The yield was much higher than when using sulfuric acid (see Table 1).

Claims (15)

530 461 13 Patent claims Process for the recovery of sulfur in a process for the production of chemical pulp, for example a sulphate process, characterized in that the sulfur recovered from the process cycle is converted to bisulphate by means of an oxidant, after which the bisulphate is used as digestive acid for digestion of pine soap or as raw material in the 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 is hydrogen peroxide, oxygen, air, ozone, an organic peracid, 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 oxidant, the amount used is greater than 0.1, preferably greater than 0.2 times the stoichiometric amount for the oxidation of the bisultene. Process according to any one of the preceding claims, characterized in that the sulfur recovered in the process cycle is converted to bisulphate by means of an oxidant after it has first been converted to bisulte in a bisulte scrubber. Process according to one of Claims 1 to 4, characterized in that the sulfur recovered in the process cycle is converted to bisulphate by means of an oxidant in a bisulate scrubber. Process according to Claim 1, characterized in that the cleavage of pine soap takes place in one or more steps, at least one of which uses the bisulphate, which is then used alone or together with other acids. Process according to Claim 7, characterized in that in addition to the bisulphate, any other acid or carbon dioxide is used as the cleavage acid. Process according to Claim 7 or 8, characterized in that in addition to the bisulphate, sulfuric acid or residual acid from the chlorine dioxide process or bisulate is used as the cleavage acid. Process according to Claim 3, characterized in that when hydrogen peroxide is used as oxidant and the bisulphate is used as cleavage acid, the amount of hydrogen peroxide is greater than 0.15, preferably greater than 0.20 g per g of bisul fi t. Process according to Claim 1, characterized in that the bisulphate is used as sulphate raw material, the bisulphate being reacted with the oxidant and a magnesium compound added externally to the process to form magnesium sulphate. Process according to Claim 11, 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. Process according to Claim 1, 1 or 12, characterized in that the magnesium compound is dissolved in the bisulphate solution. Process according to one of Claims 1 to 13, characterized in that the magnesium compound and the oxidant are added to a bisulate scrubber. Process for the digestion of pine soap, characterized in that at least a part of the digestion acid used in the digestion consists of bisulphate produced from the pulp process, for example a sulphate process, recycled sulfur by converting the sulfur to bisulphate with the process according to any one of claims 1 to 6.