FI60733B - SAET OCH ANORDNING FOER ATT FOERBAETTRA BLEKNINGSFOERLOPPET VID BLEKNING AV CELLULOSAMASSA - Google Patents

Description

τ, -ΐΒ ^ .Ι rDl / 44. PUBLICATION SC \ n 77 B UTLÄGGN INGSSKRIFT 60 '^ 3 C (45) Patent ayir.rt lly 53 03 1982 Paterit o: ··· -Party V> v ^ (51) Kv.ik.3 / int.ci. 3 D 21 C 9/10 FINLAND - FINLAND (21) Pitenttlhekeimi »- P * tencei» ekninS 902/73 (22) Hakemlspllvl - Arafiknlngideg 22.03 * 73 * * (23) Alkupllvl - GHtlfhetsdig 22.03.73 (41) Become Public · k * l - Bllvlt ofTentll * 01.10.73

National Board of Patents and Registration, _ (44) Date of publication of the published publication. -

Patent and registration authorities Aniöken Utltfd och ucl.tkriften publtcerad 30.11. oi (32) (33) (31) Privilege claimed - Begird prloritet 30.0 3.72

Sweden-Sweden (SE) ^ 251/72 (71) Mo och Domsjö Aktiebolag, Fack, S-891 01 Örnsköldsvik 1,

Sweden-Sweden (SE) (72) Göthe Norberg, Husum, Hans Olof Samuelson, Gothenburg, Sweden-Sweden (SE) (7 * 0 Berggren Oy Ab (5h) Method and device for improving the pulp bleaching process - Sätt och anordning för att förbättra blekningsförloppet This invention relates to a process for bleaching pulp with oxygen gas and to an apparatus for carrying out the process, and on a technical scale bleaching with oxygen gas has proved difficult to obtain as good results as on a laboratory scale without a number of disadvantages such as hygiene difficulties, additional chemical and heat losses. and the very large investment required for the equipment required between the digester used to dissolve the wood and the oxygen gas reactor itself.

It has now proved possible to avoid the above drawbacks and to obtain good recovery of chemicals and organic matter from both the cooking process and the bleaching process and, in addition, to obtain a pulp with very good strength properties at a given lignin content if the pulp from a continuous cooking process can be is impregnated with bleach waste liquor and then impregnated with the chemicals used in oxygen bleaching while the pulp is still under pressure.

Accordingly, the invention relates to a method of improving a pulp bleaching process using an oxygen-containing gas, in which 60733 pulp obtained by dissolving wood under pressure in a continuous digester is almost completely released from cooking liquor by passing the pulp under pressure through a scrubbing zone. The method according to the invention is characterized in that the washed pulp under pressure is subjected to one or more impregnation steps, in which the pulp is impregnated with the alkali required for oxygen bleaching and possibly with anti-pulping agents.

The process according to the invention can be used in connection with both alkaline and neutral and acidic cooking methods in which the cooking liquid contains sodium as the main metal cation. Examples of such cooking methods are acid sulfite soup, bisulfite soup, neutral sulfite soup and alkaline sulfite soup, as well as multi-stage sulfite soup as well as sulfate soup, polysulfide soup, hydrogen sulfide soup, sodium hydroxide soup and oxygen soup, soda soup. The method has proven to be particularly suitable in connection with sulphate cooking and polysulphide cooking. In the process according to the invention, the bulk of the pulp obtained from wood by continuous cooking is removed from the cooking waste liquor under pressure of the digester by displacing it with bleaching waste liquor from oxygen bleaching, after which the pulp, which is still under pressure, is impregnated with oxygen bleaching chemicals. When the pulp and cooking liquor do not come into contact with the surrounding atmosphere, there are no hygienic disadvantages in the form of toxic and foul-smelling gases and no risk of splashing of toxic and eye-hazardous liquids. Furthermore, the heat content of the cooked pulp and the waste liquor is utilized so as to achieve a reduction in the total heat demand of the required process steps. In addition, the need for alkali and the need to add substances that protect the pulp against decomposition, so-called inhibitors, becomes smaller than in hitherto known processes e.g. because the losses of alkali and inhibitors have been reduced to a minimum. In addition, the method enables the combined recovery of rational cooking waste liquor and bleaching waste liquor in an apparatus of simple construction.

Quite surprisingly, the process according to the invention gives a pulp which, at a certain degree of delignification, has better strength properties than pulps illuminated by previously known oxygen bleaching processes. This is especially the case when the impregnation takes place in two steps, so that an aqueous solution containing magnesium ions, e.g. magnesium sulphate, magnesium chloride, magnesium acetate 3 60733 or complex magnesium salts, is added to the first step and the alkaline solution has a pH so high as to give sparingly soluble precipitation of magnesium compounds, e.g. magnesium hydroxide and magnesium carbonate, is added to the second step. In practice, it has proved particularly suitable to add bleach waste liquor containing magnesium ions to the first stage, to which, for example, magnesium sulphate, magnesium carbonate or magnesium oxide has been added. When sparingly soluble magnesium salts, e.g. carbonate or oxide, are used, they must be in contact with the bleach waste liquor for such a long time that almost complete dissolution takes place before the magnesium-containing waste liquor is fed to the impregnation step. The time required for this depends on e.g. grain size and mixing efficiency. About 5-60 minutes is normally sufficient time.

The total charge of the magnesium compounds is preferably adjusted so that the total amount of magnesium dissolved and precipitated remaining in the pulp after the last impregnation step is 0.05-1% y, preferably 0.05-0.3%, calculated as MgO of the dry weight of the pulp. It has proved particularly suitable to select the conditions in the impregnation steps in such a way that part of the magnesium is precipitated, e.g. as hydroxide, while part is dissolved, preferably as complex magnesium compounds, in which the magnesium is complexed with organic acids in the waste liquor and at least two carboxyl groups. In order to obtain the optimum result, at least 0.03% »preferably 0.05-0.1% of magnesium, calculated as MgO of the dry weight of the pulp, must be present as a soluble magnesium salt after the last impregnation step. In the first impregnation step, the major part, and preferably the entire amount of magnesium present, should be dissolved so as to provide uniform impregnation and maximum power in oxygen bleaching. In certain cases it may be appropriate to use other complexing agents in the impregnation step in addition to the bleach waste liquor, e.g. polyphosphates or suitable amounts of nitrogenous polycarboxylic acids, especially those which form a relatively weak complex with magnesium, such as iminodiacetic acid or nitrilic acid For certain pulps, in particular those containing cobalt, it is advantageous to add a smaller amount, e.g. 0.1 kg, of ethylenediamine per ton of pulp to the pulp.

* 60733

The washing prior to the impregnation step can be carried out in a known manner in a continuous digester or in some pressure washing equipment, such as, for example, one or more pressurized diffusers. The washing takes place according to an embodiment by displacing the cooking waste liquor with the waste liquor obtained from oxygen bleaching, i. with an aqueous solution containing waste liquor from oxygen bleaching. This waste liquor may thus also contain other solutions added as condensate, e.g. in connection with the washing of oxygen-bleached pulp and other washing liquids obtained, for example, from washing after other bleaching steps or by regenerating adsorbents to which organic matter has been adsorbed in a manner known per se. The degree of washing is controlled so that most of the organic matter in the cooking liquor is displaced and fed to the chemical recovery or returned to the digester, while a smaller portion, 1-20 kg, preferably 1-10 kg per 100 kg of dry pulp, is passed to oxygen bleaching. The washing preferably takes place in several steps, the first washing step resulting in a lye with the highest dry matter content, which is passed to an evaporator and then incinerated, while the lye from the last washing step is used as a component of the cooking liquid. The liquors can also be wet-burned under pressure, which can be carried out in a manner known per se without prior evaporation. In order to achieve good temperature economy, it is expedient to wash the bleach waste liquor from the oxygen bleaching reactor on a countercurrent basis, whereby a high content of organic matter is obtained in the bleach waste liquor. The special advantage of maintaining the overpressure in the impregnation zone is that it is possible to transfer the pulp in a simple manner through the impregnation zones and from the last impregnation zone to the oxygen bleaching plant. In addition, it has been shown to provide more uniform oxygen bleaching with less carbohydrate degradation when impregnated with alkali under pressure. In impregnation, the pressure should be 3-30 kg / cm, 2 preferably 5-20 kg / cm.

In displacement with bleach waste liquor, the temperature is not very critical, but can be in the range of 170-50 ° C, in which case it should be noted, however, that the high temperature causes rapid displacement. When cooking the pulp using time-consuming cooking methods, e.g. sulphate cooking, work is mainly in the range 150-80 ° C, while when cooking in the acidic range, e.g. according to the sulphite process, lower temperatures, e.g. 11G-50 ° C are used.

5 60733 Almost the same temperature ranges used to displace waste liquor can also be used in impregnation zones. However, when impregnating with alkali, it has proved suitable to use a lower temperature, e.g. 130-70 ° C for alkali cooked pulps. The pulps cooked in an acidic medium have a suitable temperature in the range of 100-30 ° C, preferably 60-140 ° C. The process according to the invention can be carried out with a combination of devices comprising a) one or more tanks for washing the pulp (displacement zone), b) one or more tanks for impregnating the pulp with inhibitors (impregnation zone), c) one or more tanks for impregnating the pulp with alkali (impregnation zone) equipment for transporting pulp and solutions from one tank to another.

According to the present embodiment, the above-mentioned tanks a), b) and c) form zones in a continuous digester. In this case, the mass is transferred between the different zones without interference without any technical transport or closing devices.

According to the present embodiment, the impregnation is performed in a continuous digester. A prerequisite for this is, of course, that the kettle is equipped with a continuous washing zone. The impregnation thus takes place in one or more impregnation zones located after the washing zone. Equipment of this type is particularly suitable for a new factory building, provided that local conditions do not constitute an obstacle. According to another suitable embodiment, when bleaching a pulp with oxygen with an already existing continuous digester, the impregnation is carried out in a separate apparatus outside the digester, e.g. in one or more pressurized diffusers. In this case, too, the impregnation equipment is connected immediately after the washing zone, which can be carried out either directly in the digester or in the washing apparatus between the digester and the impregnation apparatus. Washing and impregnation equipment can also be built together, i.e. they may consist of a single tank in which the pulp first passes through the washing zone and then through one or more impregnation zones.

In impregnation zones built inside a continuous digester, the pulp is normally fed from top to bottom in the same way as in the washing zones in continuous digesters. When using a separate impregnation apparatus, it may be expedient to allow the pulp to run in the opposite direction instead of 6 60733, i. from the bottom up through the impregnation equipment, whereby the transfer takes place using the pressure of the digester. In certain cases, it may be appropriate to place a mechanical device, e.g. a stirrer, to dry the pulp before or in the impregnation zone. In this case, the impregnation can be accelerated. On the other hand, the use of such a device brings with it certain difficulties and e.g. the risk of clogging of the screens increases. In such a case, methods known per se can be used to keep the screens free of interfering blockages. By doubling the number of screens and driving alternately through different screens, difficulties can be avoided.

The equipment suitable for the impregnation process is in principle similar to the washing zones used in a continuous digester, which are of the type of continuous diffusers previously known. These include e.g. strainers and wires for fluid supply, distribution and discharge. It is further necessary to install lines, pumps and other necessary devices that allow the removed liquid to be returned to the impregnation zone, the chemical replenishment of the removed liquid with e.g. alkali and inhibitors and the supply of these chemicals to the impregnation zone. In addition, devices are needed to remove excess liquid from the impregnation zone and to direct this excess to, for example, a digester or a washing zone. The apparatus must also be equipped with devices which, by adjusting the amount of liquid fed and removed, make it possible to carry out the impregnation at a mass concentration suitable for this process, i.e. at a concentration of 8-25% »preferably at a concentration of 10-18%.

The impregnation step in which the pulp is treated with an aqueous solution containing magnesium ions can preferably be carried out by circulating the aqueous solution through the impregnation zone and adding magnesium compounds, and that when the impregnation is carried out to obtain an excess liquid, this is removed. recycling and led to the washing zone, i.e. used to displace cooking waste liquor. This can be done last in a separate washing zone or by mixing the excess liquid with the bleach waste liquor.

Once the pulp has passed through the impregnation zone, it is fed to an oxygen bleaching reactor. This can take place in such a way that the pulp is fed out in the usual way out of the digester or in separate tanks, e.g. from continuous diffusers tm3., Where the last impregnation takes place. The pulp then comes into contact with the ambient air, after which it is fed by known devices, such as e.g. screw conveyors and / or piston presses or so-called thick-mass pumps, to an oxygen bleaching reactor with shut-off devices of known type. Between the last impregnation zone and the oxygen gas reactor there may be devices for removing water from the pulp in presses, e.g. screw presses, and devices for defibering and flaking the pulp, e.g. in diffusers, in a known manner.

The oxygen bleaching reactor may comprise, for example, a reactor tower with no moving parts other than inlet and outlet devices. A reactor of this type is particularly suitable when bleaching with oxygen gas takes place at a high mass concentration, e.g. 25- ^ 0 i. However, the reactor may have devices of a known type for mixing and conveying the pulp through the reactor. A reactor based on bleaching on a fluidized bed can also be used.

According to the present embodiment, in which the apparatus is simplified, such a high pressure is maintained in the impregnation zone that the alkali-impregnated pulp is fed to an oxygen gas reactor using this pressure. In this case, a conveying device, e.g. a thick pulp pump, can be installed between the impregnation zone and the oxygen gas reactor. Regardless of whether the pump is used or not supplied against the overpressure prevailing in the reactor 2, which is about 3-10 kg / cm. In this case, a rotary shut-off valve, e.g. a rotating click, is installed between the last impregnation zone and the oxygen gas reactor. The main function of this shut-off device is to prevent gas from entering the impregnation zone from the oxygen gas reactor. The above-mentioned thick pulp pumps can also themselves act as shut-off devices. Also in this case, it may be appropriate to allow the pulp to pass between the last impregnation zone and the oxygen gas reactor through devices that remove water from it and defiber it mechanically. If such dewatering is used, it is expedient to return the solution obtained from the pulp to the impregnation zone and, if an excess solution is obtained, it is used to prepare the cooking liquid. Surprisingly, it has been found that these devices do not have to be used and that oxygen bleaching can advantageously be carried out directly from the last impregnation zone to the pulp fed to the oxygen gas reactor. This means reduced investment costs for the equipment and also results in better operating economy, as the maintenance and upkeep costs of the diffusers and presses are saved.

8 60733

By alkali is meant above and hereafter not only alkali metal hydroxides, such as, for example, sodium hydroxide, which are used in the pulp technology, but also other alkali-reactive alkali metal salts, mainly sodium salts and mixtures of such salts. Particularly suitable such salts are sodium carbonate, sodium bicarbonate, sodium sulfide and sodium hydrogen sulfide. It is preferred to use solutions which contain several components. These include green liquor, white liquor, and mixtures thereof, to which the above-mentioned salts may be added. This makes it possible to simplify the recovery of chemicals. It is particularly advantageous from the point of view of chemical balance to use white liquor or a mixture of white liquor with sodium hydroxide and / or sodium carbonate as alkali. With an alkali dosage in which sodium hydroxide and sodium sulfide are the main components, oxygen bleaching is performed at 90-120 ° C. When green liquor, sodium carbonate and sodium hydrogen carbonate are used, a higher temperature, e.g. 100-1A0 ° G, can be advantageously used.

When using sulphide solutions or sulphide-containing solutions, such as white liquor and / or green liquor, it is advantageous to oxidize the sulphide completely or partially in the oxygen bleaching pretreatment step in a manner known per se. thiosulphate and / or sulphate. The pretreatment step is preferably considered as a separate step, in which the sulphide is oxidized before the sulphide-containing solutions are brought into contact with the pulp to be bleached, but the oxidation can also take place during or after the impregnation step. with sulfide-containing solutions. The oxidative pretreatment step also results in a reduction in oxygen gas consumption during subsequent oxygen bleaching. The oxidation can be performed with air or oxygen at normal atmospheric pressure or higher pressure. A peroxide compound, e.g. hydrogen peroxide, can be used to initiate the reaction. Instead of or in combination with oxidative pretreatment of a sulfide-containing solution, elemental sulfur can be added, which reacts rapidly with sulfide in an alkaline solution to generate polysulfide. This method is particularly suitable for applying the process of the patent application to acid-cooked pulps, e.g. sulphite pulps.

In the impregnation zone where the alkaline impregnation takes place, an excess of alkaline impregnating liquid can advantageously be used. This increases the smoothness of the impregnation. The excess liquid from this step can be added to the cooking process or used to make a new cooking liquid by passing the excess at an appropriate stage to the chemical recovery process. In the production of sulphate pulp, the use of white liquor, possibly supplemented with sodium hydroxide or sodium carbonate, offers the advantage that the excess can be rationally used to dissolve the wood without any chemical intermediate treatment.

The invention is illustrated by the following exemplary embodiment, in which Figure 1 shows an apparatus suitable for carrying out the method according to the invention.

Example 1 Pine wood chips were cooked in a conventional manner in cooking zone 1 in a continuous sulphate digester. The resulting mass, having a number of pieces of 30 after cooking, was continuously passed through a screened washing zone 2. The washing zone was divided into two parts A and B, where the pulp from the cooking zone was treated countercurrently with bleach waste liquor from oxygen bleaching, which was introduced from line 3. The dry matter content of the bleach waste liquor was 1%. Cooking liquor mixed with bleach waste liquor was taken out of line B via line 4. The lye removed via line 4, 2 m 2 per tonne of pulp, was returned to the soup. Cooking liquor was removed from the upper part A via line 5. The cooking liquor liquor had been displaced by the bleach waste liquor and was then somewhat diluted. The organic content of the liquor was 16%. The lye removed via line 5 was passed to evaporation and incineration. The temperature in the washing zone was about 130 ° C and the total residence time of the pulp was 5 hours. Total bleaching liquor liquor introduced into the washing zones 3. ...

the amount was 3 m per tonne of pulp. The pulp was passed from the washing zone 2 to the first impregnation zone 6, where the pulp was impregnated with a recycled liquor, which mainly comprised bleach waste liquor, but which contained part of the cooking waste liquor that accompanied the pulp to the impregnation zone. The amount of dissolved organic matter from the cooking liquor from the washing zone 2 to the impregnation zone 6 was about 30 kg per ton of pulp. The recycle liquor was passed through a buffer tank 7 and mixed with magnesium carbonate dissolved therein in the recycle liquor. The resulting solution was pressed by pump 8 into the impregnation zone 6 from line 9 and the displaced solution was removed from line 10 and returned to the buffer tank 7. Excess solution was removed from line 11 and introduced as displacement liquid via line 3. The amount of magnesium added, calculated as MgO, was 2 kg per ton of pulp. The temperature in impregnation zone 6 was 110 ° C and the pulp concentration was 15 1> The residence time was 60 min. From the impregnation zone 6, the pulp was continuously fed to the impregnation zone 10 60733 12, where the pulp was treated with an alkali solution which otherwise mainly contained bleaching waste liquor from oxygen bleaching. The alkali was added as 35% sodium hydroxide in buffer tank 13 to the extent that it corresponded to 25 kg of sodium hydroxide per ton of dry pulp. The pump 14 pressed the diluted solution obtained in the buffer tank 13 into the impregnation zone 12 via line 15 and the liquid displaced from the impregnation zone was removed via line 16 and returned to the buffer tank 13 where sodium hydroxide introduced via line 18 was added. The temperature in the impregnation zone 12 was maintained at 95 ° C and the pulp concentration was 15%. The residence time in zone 12 was 60 min. Excess liquid was removed via line 17, which was used to displace the cooking liquor or to prepare the cooking liquid.

From the bottom of the second impregnation step, the treated pulp was fed out via a pipe 19 and transferred at its own pressure to an oxygen gas reactor (not shown in the figure) equipped with a stirrer. The pulp was treated in a reactor with oxygen gas having a partial pressure of 4 kg / cm 2 for 30 minutes at a temperature of 100 ° C. The oxygen bleached pulp was washed with water on a countercurrent basis to recover the bleach waste liquor that was continuously returned to the process. After this bleaching step, a pulp with a kappa number of 14 and a brightness of 44 according to the SCAN standard was obtained, which corresponded to a reduction in the kappa number of 2 by 11 units and an increase in brightness by 4 5i compared to the pulp bleached in the usual way with the same alkali dosage.

Example 2

The pulp was boiled to the same number as in Example 1 and otherwise treated in the same way except that white liquor with a sulphidity of 35% was used instead of sodium hydroxide. The amount of alkali determined by acid titration to pH 10 was calculated as the alkali added. although sulfide and sodium carbonate were also active chemicals. The alkali content of the white liquor, calculated as NaOH, was 130 g / l as determined above. In order to achieve the same number of pieces as when dosing 25 kg of NaOH per ton of mass, the dosage was increased to 28 kg of NaOH as determined as described above. The brightness of the bleached pulp was the same as with pure sodium hydroxide.

Claims (12)

1. Sätt att förbättra blekningsförloppet vid blekning av eellu-losASSA med en syre innehällande gas, varvid cellulosamaa, fram-ställd genom uppslutning av ved under övertryck i en kontinuerlig kokare befrias frän taluddelen av kokavluten genom att massing i vilken kokavluten förträng ^ s med blekavlut frän syrgasblekningen, känneteck-nat därav, att den tvättade massan under bibehället övertryck bringas att passera en eller flera impregneringszoner, i vilka pulan impregneras with förgraskekingen erforedellul tertin alkali och A method for improving the bleaching process when bleaching pulp with an oxygen-containing gas, wherein the pulp prepared by dissolving wood under overpressure in a continuous digester releases the major cooking liquor so that the pulp is further pressurized to pass through that the washed pulp, under overpressure, is passed through one or more impregnation zones, where the pulp is impregnated with the alkali required for oxygen bleaching and possibly also with anti-fouling agents. 2. Sät enligt patentkravet 1, känneteck nat där-a v, att förträngningen med blekavlut cajles vid 170-50 ° C, företrädesvis vid 150-80 ° C vid alkalikt kokad massa och vid 110-50 ° C vid surt kokad masass . 15 60733 Process according to Claim 1, characterized in that the temperature during displacement with the bleach waste liquor is 170-50 ° C, more preferably 150-80 ° C, for the alkali-cooked pulp and 110-50 ° C for the acid-cooked pulp. 3. A set of patents 1-2, which can be used at temperatures under impregnation with an alkali of 130-70 ° C to an alkaline temperature range of 100-30 ° C, to a temperature of 60-40 ° C chef masseur. Sät enlist patentkraven 1-3, kännetecknat d ä r a v, att trycket i impregneringsonerna Halles vid ett sädant värde, att den med alkali impregnerade pulp inmatas i syrgasblek-ningreaktorn med användande av detta tryck. Process according to Claim 1 or 2, characterized in that the temperature during the alkali impregnation is 130 to 70 ° C for alkali-cooked pulps and 100 to 30 ° C, preferably 60 to 40 ° C for acid-cooked pulps. Method according to one of Claims 1 to 4, characterized in that the pressure in the impregnation zone is maintained such that the alkali-impregnated pulp can be fed to the oxygen gas reactor by taking advantage of this pressure. 5. Sät enligt patentkravet 4, kännetecknat d ä r- a v, att övertrycket i impregneringszonerna uppgär till 3 account 30 kg / 2 cm. Method according to Claim 4, characterized in. . . 2 that the overpressure in the impregnation zones is 3-30 kg / cm. 6. The provisions of paragraphs 1 to 5 apply to impregnation processes on a continuous basis. Method according to one of Claims 1 to 5, characterized in that the impregnation is carried out in a continuous digester. 7 · Sät enligt patentkraven 1-5, kännetecknat där- a v, att impregneringsprocessen utföres i separat apparatur utanfok kokaren, vilken utgöres av en eller flera under övertryck arbetande dif fusörer. Method according to one of Claims 1 to 5, characterized in that the impregnation is carried out in a separate apparatus outside the digester, which apparatus comprises one or more pressurized diffusers. 8. The provisions of claims 1 to 7, which refer to the use of an impregnating agent with a magnesium ion in the water supply and to the subsequent use of an impregnating agent with an alkaline solution with a high pH value, av sv & rlösliga magnesium-föreningar, säsom magnesiumhydroxid och magnesiumkarbonat, erhälles. Method according to one of Claims 1 to 7, characterized in that the pulp is treated in the first impregnation zone with an aqueous solution containing magnesium ions and then in the subsequent impregnation zone with an alkaline solution having a pH so high that sparingly soluble magnesium compounds such as magnesium hydroxide and magnesium carbonate precipitate. 12 60733 9. Sät enligt patentkravet 8, kännetecknat där- a v, att den magnesium magnesium ingot hattlandland helten or delvis bestar av blekavlut frän syrgasblekning. Process according to Claim 8, characterized in that the aqueous solution containing magnesium ions consists wholly or partly of bleaching liquor obtained from oxygen bleaching. 10. The composition according to claim 9, which comprises a total of magnesium, which has a weight after the impregnation process, having an MgO content of 0.05 to 1% by weight. A method according to claim 9, characterized in that the total amount of magnesium remaining in the pulp after the last impregnation step is 0.05-1% calculated as MgO of the dry weight of the pulp. 11. A composition according to claim 10, comprising about 0.03%> 0.05-0.1% of magnesium in the presence of MgO in the form of a pulp, the addition of which contains a magnesium salt after the impregnation process. Process according to Claim 10, characterized in that at least 0.03%, preferably 0.05-0.1%, of magnesium, calculated as MgO, based on the dry weight of the pulp, is in the form of a soluble magnesium salt after the last impregnation step. Apparatus for carrying out the process according to one of Claims 1 to 11, characterized in that it has A) a pressure vessel for displacing the cooking liquor in the pulp with oxygen bleaching waste liquor, B) a pressure vessel for impregnating the pulp treated in tank A with an anti-pulp solution, C) a pressure tank for processing pulp with alkali-containing solution, D) devices for transporting the pulp and solutions between said containers. Device according to Claim 12, characterized in that the pressure vessels A, B and C are zones in a continuous digester. Patenktrav 11 60733 12. The application of the provisions of claims 1 to 11, which are incorporated herein by reference in their entirety: A bleached pulp with a cellulosic braking effect,