CA2067296C

CA2067296C - Process for bleaching of lignocellulose-containing pulp

Info

Publication number: CA2067296C
Application number: CA002067296A
Authority: CA
Inventors: Lennart Andersson; Jiri Basta; Lillemor Holtinger; Jan Hook
Original assignee: Eka Nobel AB
Current assignee: Nouryon Pulp and Performance Chemicals AB
Priority date: 1991-04-30
Filing date: 1992-04-27
Publication date: 1997-03-04
Anticipated expiration: 2012-04-27
Also published as: CA2067296A1; SE470065C; DE69227640T2; SE9101301D0; SE470065B; DE69213080D1; EP0511695B1; EP0511695B2; NO180495C; EP0511695A1; ATE141971T1; EP0679760A1; JP2592747B2; FI921886A0; EP0679760B1; NO921671L; MX9201966A; FI111964B; FI921886A; DE69227640D1

Abstract

The present invention relates to a process for delig-nification and bleaching of chemically digested ligno-cellulose-containing pulp, wherein the pulp is acid treated at a pH of between about 1 and about 6, whereupon a water-soluble compound containing an alkaline earth metal is added at a pH of between about 1 and about 7 before the pulp is treated with a chlorine-free bleaching agent. The chlorine-free bleaching agent includes peroxide compounds, ozone, oxygen and sodium dithionite in an optional sequence or mixture. After the treatment according to the invention, the pulp may be finally bleached to the desired brightness, suitably with a chlorine-free bleaching agent, such as ozone, to completely avoid formation and discharge of AOX.

Description

Process for bleachinq of liqnocellulose-containinq PUlP
This invention relates to a process for delignification and bleaching of chemically digested lignocellulose-containing pulp, wherein the pulp is acid treated at a pH of between about 1 and out 6, whereupon a water-soluble compound or mixture of compounds containing an alkaline earth metal or metal is added at a pH of between about 1 and about 7 before the pulp is treated with a chlorine-free bleaching agent. The initial acidic treatment removes the trace metals of the pulp, whereas the subsequent addition of alkaline earth metal ions in aqueous solution returns the ions to the positions in the pulp where they have a particularly beneficial effect on the preservation of the cellulose chains and, con-sequently, on the viscosity, as well as on the con-sumption of bleaching agent in the subsequent bleaching step. After the treatment according to the invention, the pulp may be finally bleached to the desired bright-ness, suitably with a chlorine-free bleaching agent, such as ozone, to completely avoid formation and discharge of AOX.
Backqround In the production of chemical pulp of high bright-ness, wood chips are first cooked to separate the cellu-lose fibres. Part of the lignin holding the fibrestogether is thus degraded and modified, such that it can be removed by subsequent washing. However, in order to obtain sufficient brightness, more lignin has to be removed, together with brightness-impairing (chromophoric) groups. This is frequently effected by delignification with oxygen, followed by bleaching in several stages.
For environmental reasons, it has become increas-ingly common to treat chemical pulp with chlorine-free bleaching agents already in the first bleaching steps.
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~ - 2 - 20 67296 The big advantage is the drastic reduction in the dis-charges of chlorinated organic substances detrimental to the environment, owing to the combined effect of a smaller amount of chlorine-containing bleaching agents and lower content of lignin, which is the organic sub-stance primarily reacting with the chlorine.
It is known to use chlorine-free bleaching agents, such as hydrogen peroxide, peracetic acid or ozone, already in the prebleaching. However, the delignifi-cation and consumption of the bleaching agent becomesless effective than with chlorine-containing bleaching agents, unless the pulp is pretreated. Thus, a hydrogen peroxide treatment in an alkaline environment is dis-turbed by the presence in the pulp of ions of certain metals, such as Mn, Cu and Fe. These metal ions cause degradation of hydrogen peroxide, thereby reducing the efficiency of the peroxide treatment and increasing the consumption of peroxide. According to CA 1,206,704, this can be counteracted by pretreating the pulp with an acid, such as sulphuric acid or nitric acid, whereby the con-centration of all types of metal ions is reduced. How-ever, by this treatment also metal ions, for example, Mg, which are advantageous to the peroxide treatment dis-appear, which ions stabilize the peroxide and increase the selectivity of the peroxide.
CA 575,636 discloses the addition of magnesium sul-phate to stabilize alkaline peroxide solutions. However, the addition is made directly to the bleaching liquor and in alkaline environment insoluble magnesium hydroxide precipitates. Furthermore, US 4,222,819 discloses the addition of magnesium ions to acidic peroxide solutions, but also in this case the addition is made directly to the bleaching liquor. None of the related methods makes possible diffusion of the magnesium ions into the pulp to E

such an extent, that a pulp of high brightness and strength can be obtained.
The invention The invention provides a process in which ligno-cellulose-containing pulp is treated under conditions whereby the metal ions harmful to the subsequent bleach-ing are effectively removed and the profile of alkaline earth metals is restored before the pulp is bleached in a chlorine-free bleaching step.
Broadly the invention relates to a process for bleaching of chemically digested lignocellulose-contain-ing pulp, wherein the pulp is acid treated at a pH in the range from about 1 up to about 6, whereupon a water soluble compound or water soluble mixture of compounds containing an alkaline earth metal or metal is added at a pH in the range from about 1 up to about 7 and in an amount of from about 0.01 kg, suitably 0.5 kg, up to about 10 kg/ton of dry pulp, calculated as alkaline earth metal, and the pulp subsequently is treated with a chlorine-free bleaching agent.
In a particular embodiment of this aspect of the invention there is provided a process for delignification and bleaching of chemically digested lignocellulose-con-taining pulp, characterised in that the pulp is bleached and delignified during an acid treatment at a pH in the range from about 1 up to about 6, whereupon a water-soluble chemical containing magnesium is added to a pH in the range from about 1 up to about 7 and in an amount of from about 0.01 up to about 10 kg/ton of dry pulp, cal-culated as magnesium, and that subsequently the pulp isdelignified and bleached with a chlorine-free bleaching agent comprising hydrogen peroxide, at a pH of from about 8 up to about 12.
r In another embodiment of the invention there is pro-vided a process for delignification and bleaching of chemically digested lignocellulose-containing pulp, characterized in that the pulp is acid treated at a pH in the range from about 1 up to about 6, whereupon a water-soluble mixture of a magnesium compound and a calcium compound is added at a pH in the range from about 1 up to about 7 and in an amount of from about 0.5 up to about 10 kg/ton of dry pulp, calculated as magnesium and calcium, and that subsequently the pulp is delignified and bleached with a chlorine-free bleaching agent comprising hydrogen peroxide at a pH of from about 8 up to about 12.
In this latter aspect of the invention the water soluble mixture of a magnesium compound and a calcium compound is preferably added in an amount of from about 2 to about 10 kg/ton of dry pulp calculated as magnesium and calcium.
Acid treatment is an effective process to eliminate metal ions from lignocellulose-containing pulps. At the same time it is known, that ions of alkaline earth metals, especially when in their original positions in the pulp, have a positive influence on the selectivity of the delignification as well as on the stability and con-sumption of chlorine-free bleaching agents, such as per-oxides, ozone and oxygen. The present process presentsan economic solution to the problem of creating a suit-able trace-metal profile for the subsequent chlorine-free bleaching, in that non-desirable metal ions are eliminated while supplied ions of alkaline earth metals essentially recover the positions in the vicinity of the cellulose chains previously occupied by ions of alkaline earth metals. This is achieved by adding the compound containing an alkaline earth metal at such a pH and such a temperature that the compound is dissolved in water, thus enabling the diffusion required to obtain the 20672~6 intended effect. Furthermore, an advantage of the pre-sent process is that the pH adjustment between the treat-ment with acid and addition of alkaline earth metal ions becomes very limited or may be left out altogether, which is advantageous to process technique and economy.
Chlorine-free bleaching agents include inorganic peroxide compounds, such as hydrogen peroxide and sodium peroxide, organic peroxide compounds, such as peracetic acid, as well as ozone, oxygen and sodium dithionite.
Suitably, hydrogen peroxide (P), oxygen(O) and ozone (Z) are used in an optional sequence or mixture. Preferably, use is made of hydrogen peroxide or mixtures of hydrogen peroxide and oxygen (PO). The sequence P-Z or (PO)-Z is especially preferred.
In the treatment with a chlorine-free bleaching agent in an alkaline environment, pH is suitably adjusted by adding to the pulp an alkali or an alkali-containing liquid, such as sodium carbonate, sodium hydrogen carbon-ate, sodium hydroxide, oxidized white liquor or magnesium hydroxide slurry. Suitably, the magnesium hydroxide slurry is taken from the chemical handling system in the production of sulphite pulp with magnesium as base, i.e., magnefite pulp.
The acid treatment suitably is carried out with an acid. The acids used are inorganic acids, suitably sul-phuric acid, nitric acid, hydrochloric acid or residual acid from a chlorine dioxide reactor, either separately or in an optional mixture. Preferably, sulfuric acid is employed.
Compounds containing an alkaline earth metal relate to water-soluble chemicals containing magnesium or cal-cium, or mixtures of such chemicals. Use is suitably made of magnesium-containing compounds, such as magnesium sulphate or magnesium chloride, or calcium-containing compounds, such as calcium chloride or calcium oxide.

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2o57296 -5a -Use is preferably made of magnesium sulphate or magnesium chloride, the use of magnesium sulphate being especially preferred. The combination of temperature and pH at the addition of the compound containing an alkaline earth metal is always so chosen that the compound is in aqueous solution when contacted with the pulp.
In the process according to the invention, the acid treatment is carried out at a pH of from about 1 up to about 6, suitably from 1.5 up to 5, preferably from 2 up to 4. It is especially preferred that the acid treatment is carried out at a pH of from 2 up to 3. When magnesium is the alkaline earth metal in the compound containing an alkaline earth metal, or a magnesium compound and a cal-cium compound mixture is employed, the addition is made at a pH in the range from about 1 up to about 7, suitably in the range from 2 up to 6, preferably in the range from 2 up to 4. It is especially preferred that the addition of magnesium compound or mixture of magnesium and calcium compound is made at a pH of from 2 up to 3.
When the chlorine-free bleaching agent is hydrogen peroxide, the pulp is suitably treated at a pH of from about 8 up to about 12, preferably at a pH of from 10 up to 12. Treatment with the other chlorine-free bleaching agents mentioned above, is carried out within the normal pH ranges for each bleaching agent, which are well-known to the person skilled in the art.
The treatment according to the invention is prefer-ably carried out with a washing step between the acid treatment and addition of alkaline earth metal ions, such that the trace metals that are harmful to the treatment with a chlorine-free bleaching agent are removed from the pulp suspension.

-- - 5b- 20~7296 The realization of the acid treatment, a compound containing an alkaline earth metal and a chlorine-free bleaching agent, can be carried out at an optional position in the bleaching sequence, e.g. immediately after digestion of the pulp or after an oxygen step. The process according to the invention is preferably applied to pulp that has been delignified in an oxygen step prior to the treatment.
According to a particular embodiment of the invention the pulp is subjected to bleaching and delignifying treatment, during the acid treatment.
Bleaching and delignifying chemicals active within the pH
range suitable in the acid treatment, are, for example, chlorine dioxide, ozone, peracetic acid and/or an acid peroxide-containing compound. Suitably, a combination of acid treatment and bleaching and/or delignifying treatment takes place in an ozone step.
Lignocellulose-containing pulps relate to chemical pulps of softwood and/or hardwood digested according to the sulphite, sulphate, soda or organosolv process, or modifications and/or combinations thereof. Use is suit-ably made of softwood and/or hardwood digested according to the F

sulphate process, preferably sulphate pulp of hardwood.
The treatment according to the invention can be applied to lignocellulose-containing pulps having an initial kappa number within the range from about 5 up to about 40, suitably 7 up to 32, preferably from 10 up to 20. Here, the kappa number is determined according to the standard method SCAN-C 1:77.
In the process according to the invention, the acid treatment is carried out at a temperature of from about 10 up to about 95C, suitably from 20 up to 80~C and prefe-rably from 40 up to 80C, and for a period of time of from about 1 up to about 120 min, suitably from 10 up to 120 min and preferably from 20 up to 40 min. The compound containing an alkaline earth metal is added at a tempera-ture of from about 10 up to about 95C, preferably from 40 up to 80C, and for a period of time of from about 1 up to about 180 min, preferably from 20 up to 180 min and prefe-rably from 30 up to 120 min. When the chlorine-free bleaching agent is hydrogen peroxide, the pulp is treated at a temperature of from about 30 up to about 100C, preferably from 60 up to 90C, and for a period of time of from about 30 up to about 300 min, suitably from 60 up to 240 min. In the acid treatment and in the addition of alkaline earth metal ions, the pulp concentration may be from about 3 up to about 35% by weight, preferably from 3 up to 15% by weight. When the chlorine-free bleaching agent is hydrogen peroxide, the pulp concentration may be from about 3 up to about 50% by weight, suitably from 3 up to 35% by weight and preferably from 10 up to 25% by weight. Treatment with the other chlorine-free bleaching agents mentioned above, is carried out within the normal ranges of temperature, time and pulp concentration for each bleaching agent, which are well-known to the person skilled in the art.
The amount of compound containing an alkaline earth metal charged, lies in the range from about 0.01 up to about 10 kg/ton of dry pulp, calculated as alkaline earth metal, suitably in the range from 0.5 up to 5 kg/ton of dry pulp, calculated as alkaline earth metal, and prefera-bly in the range from 2 up to 4 kg/ton of dry pulp, calculated as alkaline earth metal.
In preferred embodiments employing hydrogen peroxide as the chlorine-free bleaching agent, the amount of hydrogen peroxide, lies in the range from about 2 up to about 50 kg/ton of dry pulp, calculated as 100% hydrogen peroxide. The upper limit is not critical, but has been set for reasons of economy. The amount of hydrogen peroxi-de suitably lies in the range from 3 up to 30 kg/ton ofdry pulp and preferably from 4 up to 20 kg/ton of dry pulp, calculated as 100% hydrogen peroxide.
After the acid treatment, a compound containing an alkaline earth metal and a chlorine-free bleaching agent, the pulp can be used for direct production of paper with a lower demand of brightness. Alternatively, the pulp may be finally bleached to the desired higher brightness, by treatment in one or more steps. Suitably, the final bleaching is also carried out with chlorine-free bleaching agents of the type mentioned above, optionally with intermediate alkaline extraction steps, which may be reinforced with peroxide and/or oxygen. In this way, the formation and discharge of AOX is completely eliminated.
Suitably, the final bleaching is carried out with ozone in one or more steps. By the treatment according to the invention, the lignin content has been reduced to a sufficiently low level before any chlorine-containing bleaching agents are used. Therefore, chlorine dioxide and/or hypochlorite may well be used in one or more final bleaching steps without causing formation of large amounts of AOX.
Moreover, use of the process according to the inven-tion means that the brightness and kappa number of the resulting pulp is higher and lower, respectively, than with the processes in which a compound containing an alkaline earth metal is not added at all or is added at a higher pH. In a process for bleaching chemical pulps, the aim is a high brightness as well as a low kappa number, the latter meaning a low content of undissolved lignin. At the same time, the consumption of the chlorine-free bleaching agent should be as low as possible meaning lower treatment costs. In the process according to the inven-tion, these objects are met, as is apparent from theExamples. Furthermore, the strength of the pulp, measured as viscosity, is sufficient, which means that the pulp contains cellulose chains which are long enough to give a strong product.
The invention and its advantages are illustrated in more detail by the Examples below which, however, are only intended to illustrate the invention and are not intended to limit the same. The percentages and parts stated in the description, claims and examples, refer to percent by weight and parts by weight, respectively, unless anything 1 ~ t d e se lS s~a e .
Example 1 Sulphate pulp of softwood having a kappa number of 17, a brightness of 35% ISO and a viscosity of 970 dm3/kg was treated with sulphuric acid at a pH of 2Ø The pulp was treated at a temperature of 60C for 30 min, the pulp concentration being 10% by weight. After washing the pulp with water, magnesium was added in the form of an aqueous solution containing MgSO4, to give a concentration of magnesium in the pulp of at least 500 ppm. In the tests, the pH at the time of the addition was varied between 2.3 and 11.5 by addition of sulphuric acid. Then, the pulp was bleached with hydrogen peroxide at a temperature of 90C, the residence time and pulp concentration being 180 min and 15% by weight, respectively. The final pH was 11.5, and the addition of hydrogen peroxide was 15 kg/ton of dry pulp, calculated as 100% hydrogen peroxide. For comparati-ve purposes, magnesium was added directly to the hydrogen peroxide step under the conditions stated above, in accordance with the prior art. To provide a further comparison, the pulp was also treated with only sulphuric acid and hydrogen peroxide under the conditions stated above. The kappa number, viscosity and brightness of the pulp were determined according to SCAN Standard Methods, and the consumption of hydrogen peroxide was determined by iodometric titration. The test results appear from the Table below.
TABLE I
pH at the Kappa Viscosity Brightness Residual H2O
addition number step 2 step 2 step 2 of Mg step 2 (dm3/kg) t% ISO) (kg/ton) 2.3 9.1 903 61.0 1.5 4.7 9.2 910 60.0 1.0 9.5 9.8 930 56.1 0.9 11.5 10.0 940 52.2 0.2 ---- * 9.8 890 54.1 0.5 2.3 ** 9.9 875 48.2 0.0 15 * Magnesium added directly to the alkaline hydrogen peroxide step.
** No magnesium added.
As is apparent from the Table, the treatment accord-ing to the present invention with MgSO4 at a pH in the range from about 2 up to about 6 is essential to give maximum increase in brightness and maximum reduction of the kappa number, as well as minimum decrease in viscosity and minimum consumption of hydrogen peroxide. Furthermore, the importance of the magnesium ions for the increase in brightness appears from the comparison at a pH of 2.3, where, in the final test, the peroxide treatment was preceded only by acidic treatment.
Example 2 Oxygen-delignified sulphate pulp of softwood having a kappa number of 13.7, a brightness of 37.1% ISO and a viscosity of 1057 dm3/kg, was treated in a first step with 15 kg of sulphuric acid/ton of dry pulp at a pH of 1.9.
The pulp was treated at a temperature of 50C for 60 min, the pulp concentration being 10% by weight. After washing the pulp with water, in a second step between 0.1 and 1.5 kg of magnesium/ton of dry pulp was added in the form of an aqueous solution containing MgSO4. Magnesium was added at a pH of 4.1, a temperature of 50C for 60 min and with a pulp concentration of 3.5% by weight. Then, the pulp was bleached with hydrogen peroxide at a temperature of 90~C, the residence time and pulp concentration being 240 min and 10% by weight, respectively. The final pH was 11.5 and the addition of hydrogen peroxide was 20 kg/ton of dry pulp calculated as 100% hydrogen peroxide. For comparative purposes, the pulp was also treated with only sulphuric acid and hydrogen peroxide under the conditions stated above. The kappa number, viscosity and brightness were determined according to SCAN Standard Methods. The results after bleaching with hydrogen peroxide appear from the Table below.
TABLE II
Amount of Amount of Kappa no. Viscosity srightness Mg added Mg in pulp step 3step 3 step 3 in step 2 after step 2 (kg/ton) tppm) (dm3/kg) (% ISO) 0 3,7 9.6 900 51.5 0.1 130 8. 2 870 61. 8 0.3 250 7.8 864 65.1 0.75 370 7.7 855 65.9 1.5 480 7.6 845 67. 3 As is apparent from the Table, acid treatment followed by addition of dissolved magnesium and bleaching 25 with hydrogen peroxide according to the present invention, positively influence the pulp as regards kappa number, viscosity and brightness.
Example 3 The oxygen-delignified sulphate pulp of softwood used in Example 2, was treated in the sequences D - EOP-Z - P (test 1) and D - EOP - Z - Mg - P (test 2), where the conditions in each step were equal in both sequences.
D and EOP relate to a conventional chlorine dioxide step and a conventional alkaline extraction step reinforced with hydrogen peroxide and oxygen, respectively. Z relates to an ozone step with a pH of 2.3. Mg relates to the addition of 1 kg of magnesium/ton of dry pulp, in the form of an aqueous solution containing MgSO4. Magnesium was added at a pH of 4.1, a temperature of 50C for 30 min, the pulp concentration being about 3% by weight. P relates to a hydrogen peroxide step, where the pulp was treated at a temperature of 80C for 120 min. The final pH was about 11.5 and the addition of hydrogen peroxide was 5 kg/ton of dry pulp calculated as 100% hydrogen peroxide. The visco-sity and brightness were determined according to SCAN
Standard Methods. The results after bleaching with hydro-gen peroxide appear from the Table below.
TABLE III
Test Amount of Amount of Mg Viscosity Brightness Mg added in pulp after addition (kg/ton) (ppm) (dm3/kg) (% ISO) 1 0 16 820 86.3 2 1.0 255 872 88.4 As is apparent from the Table, addition of dissolved magnesium after an initial acid treatment with ozone and bleaching with hydrogen peroxide according to the present invention, positively influence the pulp as regards viscosity and brightness.
Example 4 The oxygen-delignified sulphate pulp of softwood used in Example 2, was treated in the sequences Stepl -- Z - P2 (test 1) and Stepl - Pl - Z - Mg - P2 (test 2), where the conditions in each step were equal in both sequences. Stepl relates to treatment with EDTA at a pH of 5. Z relates to an ozone step with a pH of 2.3, the pulp concentration being 10% by weight. Mg relates to the addition of 1 kg of magnesium/ton of dry pulp, in the form of an aqueous solution containing MgSO4. Magnesium was added at a pH of 4.1, a temperature of 50C for 30 min, the pulp concentration being about 3% by weight. P2 relates to a hydrogen peroxide step, where the pulp was treated at a temperature of 80C for 120 min. The final pH was about 11.5 and the addition of hydrogen peroxide was 5 kg/ton of dry pulp calculated as 100~ hydrogen per-oxide. For comparative purposes, the pulp was also treated in the sequence Stepl - P1 - Z - (PMg) (test 3). (PMg) relates to the addition of magnesium in the second alkali-ne hydrogen peroxide step under the conditions stated above, in accordance with the prior art. The viscosity and brightness were determined according to SCAN Standard Methods, and the consumption of hydrogen peroxide was determined by iodometric titration. The results after the second hydrogen peroxide step appear from the Table below.
TABLE IV
10 Test Amount of Amount of Viscosity Bright- H22 con-Mg added Mg in pulp ness sumed in P2 (kg/ton) (ppm) tdm3/kg) (% ISO) (% of added) 1 0 9 794 80.9 76 2 1.0 300 856 83.9 28 3 1.0 120 809 81.6 60 As is apparent from the Table, addition of magnesium within the present pH range before bleaching with hydrogen peroxide, positively influence the pulp as regards visco-sity and brightness and reduces the consumption of hydro-gen peroxide.

Claims

1. A process for delignification and bleaching of chemically digested lignocellulose-containing pulp, characterised in that the pulp is bleached and delignified during an acid treatment at a pH in the range from about 1 up to about 6, whereupon a water-soluble chemical containing magnesium is added at a pH
in the range from about 1 up to about 7 and in an amount of from about 0.01 up to about 10 kg/ton of dry pulp, calculated as magnesium, and that subsequently the pulp is delignified and bleached with a chlorine-free bleaching agent comprising hydrogen peroxide, at a pH of from about 8 up to about 12.

2. A process according to claim 1, characterised in that the chlorine-free bleaching agent consists of a mixture of hydrogen peroxide and oxygen.

3. A process according to claim 1, characterised in that the pulp is delignified and bleached with a chlorine-free bleaching agent compris-ing hydrogen peroxide, at a pH of from 10 up to 12.

4. A process according to claim 2, characterised in that the pulp is delignified and bleached with a chlorine-free bleaching agent compris-ing hydrogen peroxide, at a pH of from 10 up to 12.

5. A process according to claim 1, characterised in that the water-soluble chemical con-taining magnesium is added at a pH of from 2 up to 6.

6. A process according to claim 1, characterised in that the pulp is treated with a mixture of said water-soluble chemical containing mag-nesium and a calcium-containing compound.

7. A process according to claim 1, characterised in that the water-soluble chemical con-taining magnesium consists of magnesium sulphate or magnesium chloride.

8. A process according to claim 1, characterised in that the pulp is washed after the acid treatment.

9. A process according to claim 1, characterised in that the chemically digested pulp is a sulphate pulp.

10. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, characterised in that the acid treatment is carried out at a temperature of from about 10 up to about 95°C for about 1 up to about 120 min, that the water-soluble chemical containing magnesium is added at a temperature of from about 10 up to about 95°C for about 1 up to about 180 min and in an amount of from 0.5 up to 5 kg/ton of dry pulp, calculated as magnesium, the treated pulp having a concentration of from about 3 up to about 35% by weight.

11. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein the bleaching and delignifi-cation during the acid treatment is carried out with ozone.

12. A process according to claim 10, wherein the bleaching and delignification during the acid treat-ment is carried out with ozone.

13. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein the bleaching and delignification during the acid treatment is carried out with chlorine dioxide.

14. A process according to claim 10, wherein the bleaching and delignification during the acid treatment is carried out with chlorine dioxide.

15. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein the bleaching and delignification during the acid treatment is carried out with peracetic acid.

16. A process according to claim 10, wherein the bleaching and delignification during the acid treatment is carried out with peracetic acid.

17. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein the bleaching and delignification during the acid treatment is carried out with an acid peroxide-containing compound.

18. A process according to claim 10, wherein the bleaching and delignification during the acid treatment is carried out with an acid peroxide-containing compound.

19. A process for delignification and bleaching of chemically digested lignocellulose-containing pulp, charcterized in that the pulp is acid treated at a pH
in the range from about 1 up to about 6, whereupon a water-soluble mixture of a magnesium compound and a calcium compound is added at a pH in the range from about 1 up to about 7 and in an amount of from about 0.5 up to about 10 kg/ton of dry pulp, calculated as magnesium and calcium, and that subsequently the pulp is delignified and bleached with a chlorine-free bleaching agent comprising hydrogen peroxide at a pH
of from about 8 up to about 12.

20. A process according to claim 19, characterised in that the water-soluble mixture of a magnesium compound and a calcium compound is added in an amount of from about 2 up to about 10 kg/ton of dry pulp, calculated as magnesium and calcium.

21. A process according to claim 19, characterized in that the chemically digested pulp is a sulphate pulp.

22. A process according to claim 19, characterized in that the water-soluble mixture of a magnesium compound and a calcium compound is added at a pH of from 2 up to 6.

23. A process according to claim 19, characterized in that the calcium compound is calcium chloride or calcium oxide.

24. A process according to claim 19 or 23, characterized in that magnesium compound is magnesium sulphate or magnesium chloride.

25. A process according to claim 19, characterized in that the pulp is washed after the acid treatment.

26. A process according to claim 19, characterized in that the chlorine-free bleaching agent consists of oxygen and hydrogen peroxide.

27. A process according to claim 26, characterized in that the pulp, after the treatment, is finally bleached with ozone in one or more steps.

28. A process according to claim 19, 20, 21, 22, 23, 25, 26 or 27, characterized in that the acid treatment is carried out at a temperature of from about 10 up to about 95°C for about 1 up to about 120 min, that the mixture of the magnesium compound and a calcium compound is added at a temperature of from about 10 up to about 95°C for about 1 up to about 180 min and in an amount of from 0.5 up to 5 kg/ton of dry pulp, calculated as magnesium compound and calcium compound, the treated pulp having a concentration of from about 3 up to about 35% by weight, and that the pulp is bleached and delignified with hydrogen peroxide at a pH of from about 8 up to about 12.

29. A process according to claim 24, characterized in that the acid treatment is carried out at a temperature of from about 10 up to about 95°C
for about 1 up to about 120 min, that the mixture of the magnesium compound and a calcium compound is added at a temperature of from about 10 up to about 95°C for about 1 up to about 180 min and in an amount of from 0.5 up to 5 kg/ton of dry pulp, calculated as magnesium compound and calcium compound, the treated pulp having a concentration of from about 3 up to about 35% by weight, and that the pulp is bleached and delignified with hydrogen peroxide at a pH of from about 8 up to about 12.