FI116074B - Sulfur recovery in a sulfate cellulose process - Google Patents

Description

116074

Sulfur recycling in the sulphate pulp process

The invention relates to a process for recycling sulfur in the sulphate pulp process and further relates to a process for the production of tall oil and magnesium sulphate using recycled sulfur in the sulphate pulp process.

Traditional pulp production methods for raw wood still in use in the 21st century include mechanical pulping processes and chemical pulping processes. Chemical defibration methods include: soda process, sulfite process and sulfate process. These methods have been constantly refined and further developed.

10 In recent years, particular attention has been paid to the energy efficiency and environmental friendliness of processes. For example, the use of additives has been increased in the production of chemical pulp and efforts have been made to minimize emissions of both malodorous sulfur compounds and sulfur dioxide. Emission limit values for air protection are constantly tightening, and today efforts are being made to recover sulfur dioxide from the combustion of odorous sulfur compounds 15 and other process steps. Sulfur dioxide can be removed from the flue gases, for example in gas scrubbers where it is known to convert sulfur dioxide into sodium bisulfite with sodium hydroxide.

The sulfate process is currently the most widely used chemical pulping process; I. In it, the alkaline cooking liquor, or white liquor, contains approximately three parts 20 sodium hydroxide, NaOH, and one part sodium sulfide, Na 2 S. An essential part of the process is the recovery of cooking chemicals and certain by-products. After cooking, the pulp is washed and the fatty acids and resin acids from the wood extractants are removed from the wash liquor, the black liquor. In addition, the recovery includes:; recovery of lighter organic components such as turpentine based on extractant »» · 25 to. In cooking, most of the lignin in the wood is soluble in the cooking liquor, which is burned in a steam boiler, where the sulfur and sodium in the cooking liquor are also recovered.

IV. A large proportion of the black liquor is evaporated from the water, and the residue is then incinerated: ': in a recovery boiler. In the recovery boiler, the organic matter is recovered and the inorganic portion of the black liquor which at this stage consists mainly of sodium carbonate, Na 2 CO 3, and sodium sulfide, Na 2 S, remains. From the boiler, this salt melt is dissolved in water to give a so-called. green liquor. Green liquor is led to causticization, where · '' sodium carbonate reacts with calcium hydroxide in aqueous solution: 116074 2

Na2CO3 + Ca (OH) 2 -> 2NaOH + CaCO3 (1)

The resulting calcium carbonate is further regenerated in the lime kiln by burning it into calcium oxide:

CaCO 3 - »CaO + CO 2 (2) 5 The causticizing white liquor contains mainly sodium hydroxide and sodium sulphide. The composition of this white liquor is of considerable importance for the properties of the pulp to be cooked. As the process is also essentially closed in this respect, i.e. the cooking chemicals are not consumed in the process, it is essential that the sulfur-sodium ratio in the chemical cycle is kept in equilibrium. At present, the biggest problem in equilibrium management is, for example, the excess sulfur introduced into the process. Pulp mills have a number of different sulfur-producing sub-processes, the most important of which is the tall oil production process. In addition, sulfur comes from the wood itself, water, lime kiln fuel oil and magnesium sulphate used in oxygen bleaching. 1

In sulphate pulp cooking, from wood additives, pitch fatty acids and resin acids are saponified to sodium salts, or soaps, and also unsaponifiable organic components remain, depending on the quality of the raw wood used. These soaps are still micellarized and some unsaponifiable matter is dissolved in the micelles. Pulp cooking is usually followed by a washing step in which the black liquor,. The soap micelles rise to the surface of the broth to form a crude paste. The soap formed is decanted from the black liquor, after which it can be separated into tall oil with the help of sulfuric acid '* ··' and heat. Thus, in the plating, the soap separated from the surface of the black liquor is boiled with sulfuric acid, whereupon the water-soluble sodium salts of the fatty and resin acids produce the corresponding acids, which are referred to as crude tall oil. The reaction in the plotting is of the form:; ..: 'H 2 SO 4 + 2 R-COON -> 2 R-COOH + Na 2 SO 4 (3)

The fatty acids, in their acid form, are insoluble in water and thus separate, · · ·, from the leachate. Sulfuric acid is used in the plating mainly for converting the leach into pine oil, but also for neutralizing the black liquor present in the leach, and for 9 »•. *. · 30 to adjust pH to a low enough level to convert fatty acids: to acid form.

In practice, sulfuric acid should be added in such a way that the pH drops below 4. In baking, tall oil is separated from the mother liquor phase, which is returned to the mill's chemical cycle.

For example, the waste acid from the chlorine dioxide-5 reactor can be and often is used as the column acid. Because the aqueous phase produced during the plating is recycled, the sulfur in its dissolved form also goes into the chemical cycle. However, the waste acid entails a considerable amount of sulfur in the process because the waste acid contains, in addition to sulfuric acid, a considerable amount of sodium sulphate or sodium sesquisulphate. Depending on the species of wood and where it is grown, the columning of tall oil can bring sulfur into the process at 2-4 kg / tonne of pulp. The sulfur losses of a modern pulp mill, that is sulfur that does not go into recycling, but is removed from the chemical cycle, can be replaced by sulfur-containing chemicals from bleaching chemicals that even exceed the sulfur requirements of the pulp mill. In particular, the tall oil plantation adds significantly more sulfur to the process. Thus, excess sulfur 15 must be removed from the process.

If excess sulfur is removed from the process as sodium sulfate, fresh sodium hydroxide has to be replaced, which increases the cost of desulfurization. When controlling the chemical balance of a sulphate pulp mill, it is important to ensure that the sulfur to sodium ratio or sulfide is controlled. The tall oil column is one of the major contributors to sulphate fusion imbalance in the sulphate pulp process.

A: If excess sulfur is removed, for example in the form of fly ash, it will be removed. · · ·. at the same time, sodium which must be replaced by fresh liquor introduced into the process.

: V One solution to this problem would be to use sulfur free acid. Hydrochloric acid is not suitable because of the problems caused by chloride. Organic acids such as formic acid and acetic acid could at least partially replace sulfuric acid.

; However, they are probably too weak acids and also relatively weak; ''; expensive. Nitric acid is also unsuitable for the process as it can produce nitrogen oxides in the digestion plant.

; ': For recycling and reusing sulfur in tall oil production there is a pot.' WO 9411571 discloses high acid overload acidification; la sodium bisulfite solution relative to the amount desired. The acidification may be carried out in one or, preferably, several steps. The bisulfite solution used pre-acidifies the salt at elevated temperature 90-150 ° C and at atmospheric pressure or mild overpressure. However, the soap must be post-acidified with sulfuric acid to complete the cooking. The bisulfite solution as such is not very acidic, with a pH slightly below 5. The pH of the paraffinic acid should be less than 4.

Another step in introducing additional sulfur into the process is the soluble magnesium compound used in oxygen bleaching. Typically, magnesium sulfate is introduced externally into the sulfate pulp process. Instead of magnesium sulphate, for example, acetates or formates could be used to reduce the sulfur load through the choice of chemicals, but in practice they are too expensive.

10 Magnesium hydroxide and magnesium carbonate would be good alternatives in their composition but have the problem of insufficient solubility.

It is an object of the present invention to simply and efficiently reuse recycled sulfur from the pulp mill's own chemical cycle without significantly adding additional sulfur to the process, so that the sulfide process does not become a problem.

Now, we have surprisingly discovered that the cooking of crude tall oil can be accomplished using recycled sulfur from the pulp mill's own chemical cycle. The process according to the invention is based on oxidizing the recycled sulfur to a bisulfate solution by means of an oxidizing agent. The bisulphate solution obtained is considerably more acidic than, for example: bisulphite solution, whereby its acidity is sufficient, for example, to column:. - prefer crude tall oil at one stage, similar to that used when fresh,. · * ·, Sulfuric acid imported from the process balance. In addition, the bisulfate solution can be used to prepare sulfate compounds needed in the process, such as magnesium sulfate for bleaching.

In the process of the invention, the bisulfite obtained from the process cycle can be oxidized to the bisulfate, for example, by hydrogen peroxide acting as an oxidant. In this case; At the same time, the pH of the solution drops substantially. The acid constants of sulfuric acid (H2SO3) are pKal = 1.8 and pKa2 = 6.8, while with sulfuric acid, pKal = -3 and pKa2 = 1, respectively. , 8. Sodium bisulfite can be oxidized to sodium bisulfate, for example, by hydrogen peroxide according to equation (4), as follows: ···: NaHSO3 + H2O2 - »NaHSO4 + H2O (4) 5 116074

Thus, the process according to the invention can convert the process sulfur sulfur from the sodium bisulfite form to a sufficiently acidic bisulfate form to be well suited, for example, to tall oil milling. When bisulfite is used as such, tall oil can only be partially columned and an external acid, such as sulfuric acid, is needed for the final column. The sulfuric acid is not necessarily required in the process of the present invention.

Hydrogen peroxide, ozone or, for example, organic peracids such as peracetic acid or permuric acid may be used as the oxidizing agent in the process of the invention. Sulfite can also be oxidized by oxygen, but it usually fails directly without the use of a catalyst, which complicates the required process equipment. The most preferred oxidant of the invention is hydrogen peroxide, which is also the easiest to use. Alternatively, a peroxide solution from another process step, such as a residual peroxide filtrate from pulp bleaching, may also be used for oxidation.

It is known that tall oil can be columned with an externally introduced acid such as sulfuric acid or waste acid from the chlorine dioxide process to provide a sufficiently low pH. The pH of bisulfite obtained directly from the process is not sufficient as such. By oxidizing the bisulfite solution according to the invention, it is possible to reach this pH range. It is important that the pH is lowered sufficiently so that fatty acids which are weak acids can be acidified and separated. A solution of bisulfate '' below pH 2 would yield most of the fatty acid to the acid form.

When the pH is about 2 units lower than the pKa of the fatty acid, most of the fatty acid is in the acid form and not in the form of a salt.

* · »'.. In the process of the invention, hydrogen peroxide is consumed by the stoichiometry of bisulfite.

I I

'* For oxidation to 25 in a weight ratio of 1: 3 H2O2: NaHSO3, ie peroxide consumption in the order of 0.33 kg H2O2 / kg NaHSO3. Preferably, however, the bisulfite need not be completely oxidized to achieve a sufficiently low pH, but partial acidification is sufficient.

';;; The drop in pH resulting from the oxidation reaction of the invention is shown in Figure 1. As shown in the figure, the pH of a 10 g / l bisulfite solution decreases: Y: rapidly from its initial value of just over four when hydrogen peroxide is added. Bi-: '; the pH of the sulphite solution drops significantly even with a small amount of peroxide. The pH of the resulting bisulfate solution can be adjusted to a level suitable for cooking tall oil, from 6116074 to a pH <2, preferably pH <1.5, wherein the amount of hydrogen peroxide is greater than 0.1, preferably more than 0.2 times the stoichiometric amount required for bisulfite oxidation. Thus, the bisulfate solution is more acidic than the bisulfite solution, so it is much better suited for use in, for example, tall oil extraction. Thus, 5 mills can substantially reduce the amount of sulfuric acid introduced into the process, for example for this purpose. Salting the filter entirely with sulfuric acid introduced into the process would produce about 2-4 kg / Adt of sulfur into the process, so that at least in this respect, the amount of sulfur would be significantly reduced using the process of the invention.

When operating according to the invention, the equipment required for the oxidation reaction is simple, as shown, for example, in Figure 2. The apparatus essentially comprises a mixing tank 1 in which the reaction between the sulfur-containing bisulfite entering the sulfate process, S, and the oxidant takes place. In addition, the apparatus may include, for example, a dosing device 2 for the oxidant, and an intermediate storage tank 3 for the bisulfate formed as a reaction product, from which the bisulfate solution may be recycled to a desired process point such as tall oil milling or magnesium sulfate.

Alternatively, the bisulfate solution provided in accordance with the invention may already be prepared in the bisulfite scrubber itself, whereby sulfuric acid and bisulfate are also formed in the bisulfite solution. However, materials resistant to corrosion and oxidation reaction SO2 -> SO3 caused by the oxidant should be used in the process equipment. However, this embodiment becomes more expensive in the above manner. The resulting bisulfate solution also contains some sulfuric acid.

. * · ·. The sulfur recycling, recovery and reuse of the sulfur cellulose process of the invention is exemplified in Figure 3.

The bisulfate solution is prepared according to the invention by oxidizing in a mixing tank 1:. In the actor 6, magnesium sulfate is prepared from the exogenous magnesium starting material, and sodium bis: sulfate prepared from the recycled sulfur according to the invention. The resulting magnesium sulfate is used in oxygen bleaching 7. The oxygen bleaching filtrate is further fed to evaporation plant 8 and through it to a recovery boiler 1 1 6074 7 9. In the production of crude tall oil, The tall oil boiling reactor 5 discharges crude tall oil 12 from the process and the remaining sulfur-containing mother liquor is fed through an evaporator 5 8 to a recovery boiler 9 for burning organic compounds and sulfur. Sulfur compounds are burned into sodium sulphide in a baking soda 9. From the recovery boiler 9, an aqueous solution of sodium carbonate and sodium sulfide, green liquor, is conducted to causticize 13, whereby the sodium carbonate is converted into calcium carbonate and the resulting sodium hydroxide-sodium sulfide solution, white liquor, is recycled to pulp cooking 10.

The remaining calcium carbonate is further regenerated in the lime kiln to 14 kcal oxide. The main source of sulfur dioxide is the combustion of odorous gases 15, whereby the reduced sulfur compounds, mainly H2S, methylmercaptan, dimethylmercaptan and dimethyldimethylcaptan, are incinerated as sulfur dioxide and further recycled to the bisulphite scrubber 4. In the bisulfite scrubber 4, the sulfur dioxide is absorbed into the NaOH solution to form sodium bisulfite. The sodium bisulfite from the bisulfite scrubber is further introduced into the mixing tank 1 for oxidation according to the invention.

The process according to the invention avoids, for example, the use of waste acid from the production of chlorine 20 20 either partially or completely and thus negatively. reduce external sulfur entering the process.

In one embodiment of the invention, the plating is carried out in a single step using sodium bisulphate, a stoichiometric solution of the bisulphate prepared in accordance with the invention, in some cases. It may also be advantageous to use an excess of an oxidizing agent, alternatively, the bisulfate may be only partially oxidized, or a mixture of the recycled bisulfate of the invention in one portion, e.g. a mixture of sulfuric acid and bisulphate, may also be used. · From carbon dioxide with hydrogen peroxide by first oxidizing sulfur dioxide to sulfur dioxide _ ;; It is known to prepare sulfuric acid by oxidizing sulfur dioxide directly to sulfur dioxide. The procedure of the invention is avoided. '. however, investing in a separate sulfuric acid plant.

In one embodiment of the invention, the columning of tall oil is carried out in two or more stages. In the two-step process, a bisulfate solution prepared according to the invention can be used in the first step and another acid such as, for example, fresh sulfuric acid or a waste acid or a mixture of acids in the chlorine dioxide reactor. Alternatively, in the first step, a sodium bisulfite solution or carbon dioxide directly obtained from the process may be used, and only in the latter step may the sodium bisulfate solution of the invention be used. In this case less bisulfate and thus peroxide is needed than the one-step process. Alternatively, the bisulphate solution characteristic of the invention is used in both stages of tall oil cooking.

If the amount of sulfur entering the plating is 2-4 kg S / Adt, the required amount of stoichiometric peroxide should be approximately the same, i.e. 2-4 kg. However, the amount need not be quite stoichiometric, as shown in Figure 1. Preferably already about half of it, more than 0.15 g H 2 O 2 / g NaHSCb is sufficient. Most preferably, the amount of hydrogen peroxide to be added is greater than 0.20 g H 2 O 2 / g NaHSO 3.

The magnesium compound used in oxygen bleaching can be prepared by utilizing sulfur recovered in the process cycle of the sulfate process, eliminating the need to use exogenous sulfur loading magnesium sulfate. Recycled sulfur can be utilized as sulfur dioxide, bisulfite or sulfite, which is reacted to magnesium sulfate with the aid of an oxidizing agent.

. ···. As the sulfur-free raw materials for the magnesium compound, for example, magnesium hydroxide, magnesium oxide or magnesium carbonate may be used. Preferably • ·. . the source of magnesium is technical MgO.

I I * • · »

Ml · t <: According to one embodiment of the invention, the magnesium compound is dissolved in a bisulphate solution obtained from the sulphate process according to reaction equation (5) to give magnesium sulphate. 1 2 NaHSO 4 + Mg (OH) 2 -> Na 2 SO 4 + MgSO 4 + 2 H 2 O (5)

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t '': An alternative way of making magnesium sulfate is to carry out the reaction ', already in a scrubber, where the SO 2 -containing gas is directly washed with an oxidant, for example »I · • ;; hydrogen peroxide, and water containing Mg (OH) 2. The peroxide consumption is then between 0.28 and 0.33 kg / kg MgSO 4.

* *; A further alternative way of making magnesium sulfate is to first dissolve the magnesium sulfate compound in a bisulfite solution and oxidize the resulting magnesium sulfite 9116074 to magnesium sulfate, for example, with hydrogen peroxide. Magnesium sulphite as such is a reducing compound which is not useful in the bleaching process because it consumes other chemicals unnecessarily.

Since MgSO 4 contains 26.6% sulfur, the method 5 according to the invention reduces the amount of sulfur by 0.26-0.8 kg / tonne if the amount of MgSO 4 to be bleached is 1-3 kg / tonne pulp.

The following illustrates, by way of example, certain features of the invention, without, however, limiting the invention thereto:

Example 1.

10 Consider the sulphate pulp process from which excess sulfur is removed in the form of fly ash. In this case, the sodium leaving the sulfur must be replaced by fresh liquor introduced into the process. The broth requires 2.5 kg NaOH / kg sulfur to be removed.

If the amount of sulfur to be removed is 4 kg / tonne mass, the corresponding need for NaOH is 10 kg / tonne mass.

This is compared to the sulfate pulp process of the invention. If the excess sulfur is reduced by recycling the sulfur contained in the bisulfite through a tall oil column, a little more than 4 kg of hydrogen peroxide is needed to oxidize that amount of sulfur.

...: At current chemical prices, the price of hydrogen peroxide becomes € 2.1.

*. ·. If the sulfur is removed as fly ash, the cost of the sodium required is € 3.2, i.e., * * * '*'. '20 has a cost effect of 1.5 times that of the process of the invention.

Example 2.

• ·

The sodium bisulphite solution of the sulfate pulp process, NaHSO 3, at a concentration of 43 g / l ... and pH 4 was oxidized by addition of 50 wt.% Hydrogen peroxide solution. Figure 4 shows'; shows the change in pH of the solution as a function of peroxide addition. In a bisulfite solution to which 11 g / l H 2 O 2 was added was readily soluble in 12 g / l Mg (OH) 2, the resulting solution contained 24.9 g / l MgSO 4. The solution also contained sodium sulfate.

•; * In a non-oxidized bisulfite solution, magnesium hydroxide appeared to dissolve *: / slightly more slowly. However, when the magnesium bisulfite thus obtained was oxidized by the addition of 11 g / L hydrogen: '': peroxide, the same result was obtained as the oxidation of the bisulfite before dissolution.

ίο 116074

The prepared magnesium sulfate solutions work in oxygen bleaching and peroxide bleaching in exactly the same way as exogenous, pure magnesium sulfate.

No undesirable effects were observed with the included sodium sulfate.

Example 3.

Comparison of the sulfur-sodium balance of one sulfate cellulose process in two different cases, A and B. In the first case, A, sulfur was not recycled and all necessary sulfuric acid was introduced into the process and sodium bisulfite was removed. In another case of the invention described above, B, the bisulfite was oxidized with hydrogen peroxide to the bisulfate and the required amount of exogenous sulfuric acid was added.

Figure 5 shows the two cases A and B and their sulfur-sodium balances at different stages. Step 1 describes the amount of sulfur present with sulfuric acid, Step 2 describes the required Na substitution (sodium hydroxide), Step 3 describes the sulfur in the process with oil, wood and MgSO 4, Step 4 illustrates e.g. removal of ash salt, fly ash, sulfur dioxide and soda ash, step 5 depicts washing losses and step 6 is other removal.

In case B, bisulfite was recovered using recycled white liquor, whereby no new sodium was introduced into the system with Na bisulfate. The sulfur that came with the bisulfate went into circulation normally, so it doesn't appear on the balance sheet.

The procedure of Case B resulted in the reduction of sulfuric acid entering the system by replacing 2kg sulfur or 6.1kg sulfuric acid from bisulfite by oxidation with the bisulfate solution prepared.

Figure 5 shows how the need for NaOH substitution will decrease significantly. When hydrogen peroxide was used as the oxidant, the peroxide consumption depended on the degree of oxidation of the bisulfite. For 100% oxidation of bisulfite, 2.1 kg H 2 O 2 and 1.1 kg H 2 O 2 respectively are required at an oxidation rate of 50%.

»* 116074 π

Example 4.

Sodium bisulfite solution of the sulfate pulp process, NaHSO3, at 250 g / L was taken in 41 and placed in an 8 L autoclave. The autoclave was equipped with a gas recirculating mixer using oxygen as the gas. The autoclave temperature-5 was adjusted to 80 ° C and supplied with pure oxygen so that an overpressure of 6 bar was created in the autoclave. The solution was sampled at initial t = 0 and several other samples as the oxidation proceeded. Samples were measured for pH and analyzed for sulphite and sulphate ions. Figure 6 shows the evolution of pH over time as a function of oxidation, and Figure 7 shows concentrations of sulfate and sulfite ions, respectively.

The pH of the bisulfite solution rapidly drops to below three during the initial oxidation and continues to decrease as the oxidation progresses. The increase in acidity is clearly due to the almost complete oxidation of bisulphite to bisulphate. The experiment shows that by using oxygen as an oxidant, it is also possible to reach a suitable pH range, whereby the recyclable solution is suitable for reuse in tall oil oil plating.

... Example 5.

A 300 mL sample was taken from a sodium bisulfite solution of sulfate cellulose, NaHSO 3, at a concentration of 44: g / L, which was placed in an open flask with a glass sinter. Oxygen with a flow rate of * · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ((-)) was passed through a glass sinter. the rotameter was 1 1 / min 02. Figure 8 shows the change in pH of the solution as a function of time as oxidation progressed.

• * • * ·

The pH of the bisulfite solution drops rapidly at the beginning of the oxygen supply and equates to slightly below • · · • '·'.

25 »*»> «♦» »»

Claims (5)

116074: \ A process for recycling sulfur in a sulphate pulp process, characterized in that the sulfur recovered in the process cycle is converted, by means of an oxidant, into bisulphate, which is further used as recycling acid at various stages of the process. Process according to Claim 1, characterized in that the sulfur recovered in the process cycle is partially or completely converted into bisulphate. Process according to Claim 1, characterized in that hydrogen peroxide, ozone, organic peracids, oxygen, a peroxide-containing solution from another process step or mixtures thereof, preferably hydrogen peroxide, are used as the oxidizing agent. Process according to claim 3, characterized in that, when the oxidant used is hydrogen peroxide, the amount thereof is more than 0.1, preferably more than 0.2 times the stoichiometric amount required for the oxidation of bisulfite. Process according to Claim 1, characterized in that the sulfur recovered in the process cycle is converted into bisulfate by an oxidant after it has first been converted into the bisulfite form in a bisulfite scrubber. t » Process according to Claim 1, characterized in that the sulfur recovered in the process cycle is converted into bisulfate by means of an oxidizing agent in a bisulfite scrubber. *: ': 7. A process for milling tall oil, characterized in that the milling acid is: "**. · 20 is used at least in part from the bisulfate produced from the sulfur recovered in the sulfate pulp process by converting it to bisulfate according to claim 1. A process according to claim 7, characterized in that the plating is carried out in one or more steps, at least one of which uses the bisulfate, either alone or in combination with other acids. 9. A process according to claim 7 or 8, characterized in that the acid 'or * carbon dioxide' in addition to the bisulfate according to claim 1 is used as the scavenging acid '* * *'. 116074 Process according to claim 7 or 8, characterized in that sulfuric acid or waste acid or bisulphite from the chlorine dioxide process is used as the acid. The process according to claim 7, characterized in that hydrogen peroxide, ozone, organic peracids, oxygen, a peroxide-containing solution from another process step or mixtures thereof, preferably hydrogen peroxide, are used as the oxidizing agent in the preparation of the bisulfate according to claim 1. Process according to claim 11, characterized in that the amount of hydrogen peroxide is greater than 0.15, preferably more than 0.20 g / g of bisulfite. 13. A process for the preparation of magnesium sulfate for use in bleaching a sulphate pulp, characterized in that the sulphate raw material is sulfur recovered in the sulphate pulp process as sulfur dioxide, bisulphite or sulphite, which is reacted with the oxidized magnesium compound. Process according to claim 13, characterized in that the magnesium compound 15 is a sulfur-free magnesium compound, preferably magnesium hydroxide, magnesium oxide or magnesium carbonate, most preferably technical magnesium oxide. Process according to claim 13, characterized in that the oxidant is used hydrogen peroxide, ozone, organic peracids, oxygen, any other pro- *. peroxide-containing solution from the process step or mixtures thereof, preferably hydrogen peroxide. Process according to claim 13, characterized in that the magnesium compound: is dissolved in a bisulfate solution prepared according to claim 1. ; (§ 17. The process according to claim 13, characterized in that the magnesium compound and the oxidant are added to the bisulphate washer. * 18. The process according to claim 14, characterized in that the magnesium compound, *, is first dissolved in a bisulfite solution and then oxidized. • »I t *» • »116074! Ί 7 I: I 1! ---------------------------- ---------------------------! ·. Ί; i; | 0 4-- · —--- 1 ----- -------— ---- 1; ί i 0 0 0 0 0 0 0 1 I Η202, g / g NaHS03 ί 5 Figure 1. »» 116074 h2o2 _i_ 2 hso4 - »- _Ϊ_ hso3 3 ·; hso4 • S * -:.:: 4- hso4 • * FIGURE 2.