AU671337B2

AU671337B2 - Process for bleaching of lignocellulose-containing pulp

Info

Publication number: AU671337B2
Application number: AU55831/94A
Authority: AU
Inventors: Jiri Basta; Ann-Sofie Hallstrom; Magnus Linsten
Original assignee: Eka Nobel AB
Current assignee: Nouryon Pulp and Performance Chemicals AB
Priority date: 1992-11-27
Filing date: 1993-11-25
Publication date: 1996-08-22
Anticipated expiration: 2013-11-25
Also published as: FI118572B; CA2149649C; FI952551A0; JPH08503749A; CZ285246B6; CZ132895A3; BR9307520A; DE69304342T2; WO1994012722A1; DE69304342D1; ATE141972T1; EP0670929B2; NO952074L; MX9307413A; ES2091121T3; NZ258273A; NO306737B1; PL309190A1; NO952074D0; AU5583194A

Abstract

The present invention relates to a process for bleaching of lignocellulose-containing pulp, where the pulp is first treated with a complexing agent, then delignified with an organic peracid or a salt thereof, and subsequently bleached with a peroxide-containing compound, ozone or sodium dithionite. Before bleaching, the pulp is washed at a pH of at least about 4. Suitably, delignification is performed with the strongly oxidizing peracetic acid, so that a considerable increase in brightness as well as a considerable reduction of the kappa number are obtained after said bleaching. The brightness-increasing effect is very selective, i.e. the viscosity of the pulp is maintained to a relatively great extent. By a suitable choice of pH in all the stages before the chlorine-free bleaching, the advantageous metal ions are retained in the pulp, while at the same time the pH adjustment between the stages is minimized and the resulting spent bleach liquors are advantageously used internally, e.g. for washing the pulp.

Description

Process for bleaching of lignocellulose-containinα pulp The present invention relates to a process for bleaching of lignocellulose-containing pulp, in which the pulp is first treated with a complexing agent, then delignified with a per- acid or a salt thereof, and subsequently bleached with a per¬ oxide-containing compound, ozone or sodium dithionite. Suit¬ ably, delignification is carried out with the strongly oxidis¬ ing peracetic acid, giving a considerable increase in bright¬ ness and a considerable reduction of the kappa number after bleaching with a chlorine-free bleaching agent, such as hydro¬ gen peroxide. The brightness-increasing effect is highly sele¬ ctive, i.e. the viscosity of the pulp is maintained to a com¬ paratively great extent. Background of the Invention Chlorine-free bleaching agents have long been used for bleaching mechanical pulps. In recent years, it has become in¬ creasingly common to bleach also chemical pulps with chlorine- -free bleaching agents, such as hydrogen peroxide and ozone, even in the first stages. It has been considered necessary to pretreat the pulp immediately after digestion and an optional oxygen-delignifying stage so as to avoid deteriorated pulp properties and an excessive consumption of the bleaching agent. Pretreatment of the pulp primarily involves acid treat¬ ment and treatment with a complexing agent or salts of alkali- ne-earth metals, optionally in combination. Strongly acid pre¬ treatment removes desirable as well as undesirable metal ions from the original positions in the pulp. Treatment with suit¬ able complexing agents primarily removes the undesirable metal ions, while the desirable ones are largely retained. Treatment with salts of alkaline-earth metals maintains or reintroduces the desirable metal ions.

EP-A-0 415 149 discloses the use of peroxomonosulphuric acid before treatment with oxygen and/or peroxide. The pulp can be pretreated in process stages where heavy metals and organic impurities are removed. In the Examples, the pulp is thus treated in the presence of DTPA at a pH of 2 before treatment with peroxomonosulphuric acid and oxygen. Such an acid wash removes also those metal ions, primarily of alkali¬ ne-earth metals, that are necessary for an efficient subse- quent bleaching with peroxide-containing compounds or ozone. With increasingly stringent environmental standards, there is a growing need for completely chlorine-free processes for delignifying and bleaching lignocellulose-containing pulps. To produce fully bleached pulps with unaltered strength properties in a reasonable number of stages and with a reason¬ able consumption of the bleaching agents, it has become neces¬ sary to consider using also powerful, and hence difficultly- controlled, bleaching agents having a high delignifying and/or bleaching capacity.

Description of the Invention

The invention provides a process in which a lignocellu¬ lose-containing pulp is delignified and bleached under the conditions disclosed in the appended claims, whereby a good delignifying and bleaching effect is obtained even before bleaching with a peroxide-containing compound, ozone or sodium dithionite.

The inventive process comprises bleaching of lignocellu¬ lose-containing pulp with a peroxide-containing compound, ozone or sodium dithionite, wherein, before said bleaching, in a sequence is first treated with a complexing agent at a pH in the range of from 3.5 up to about 11 and at a temperature in the range of from 26°C up to about 100°C and then delignified with an organic peracid or salts thereof at a temperature in the range of from 50°C up to about 140°C, and that the pulp, before said bleaching, is washed at a pH of at least about 4. The inventive process has made it possible to delignify the pulp after treatment with a complexing agent without ad¬ versely affecting the conditions, optimised by the treatment with a complexing agent, for the subsequent chlorine-free bleaching, taking into consideration the desirable and unde¬ sirable metal ions. Thus, ions of alkaline-earth metals, espe¬ cially when in their original positions in the pulp, are known to have a favourable effect on the selectivity in bleaching and the consumption of chlorine-free bleaching agents, such as peroxide-containing compounds and ozone.

In the invention, peracid or salts thereof include orga¬ nic peracids or salts thereof. As organic peracid, use is made of aliphatic peracids, aromatic peracids or salts thereof. Suitably, peracetic acid or performic acid is used. Sodium is suitably used as cation in the salts, since such salts norm¬ ally are inexpensive and sodium occurs naturally in the chemi¬ cal balance in the pulp mill. Peracetic acid or salts thereof are preferred, being advantageous in terms of production and use. In addition, peracetic acid has limited corrosiveness. Any wastewater containing, inter alia, the degradation prod¬ ucts of peracetic acid can be easily used for washing or recy¬ cled to the chemical recovery system. According to the present process, peracetic acid can be produced by reacting acetic acid and hydrogen peroxide, giving what is known as equilibrium peracetic acid, by distilling equilibrium peracetic acid to remove hydrogen peroxide, acetic acid and sulphuric acid, or by reacting acetic acid anhydride and hydrogen peroxide directly in the bleaching stage, giving what is known as in situ peracetic acid. A typical equilibrium peracetic acid contains about 42% of peracetic acid and about 6% of hydrogen peroxide, i.e. the weight ratio of peracetic acid to hydrogen peroxide is here about 7:1. When equilibrium peracetic acid is used in the present process, the weight ratio between peracetic acid and hydrogen peroxide can be in the range of from about 10:1 to about 1:60, suitably from 7:1 to 1:15 and preferably from 2.8:1 to 1:2.

The added amount of peracid or salts thereof, should be in the range of from about 1 kg up to about 100 kg per tonne of dry pulp, calculated as 100% peracid or salt thereof. Suit¬ ably, this amount lies in the range of from 2 kg up to 45 kg per tonne of dry pulp, preferably in the range of from 3 kg up to 25 kg per tonne of dry pulp, calculated as 100% peracid or salt thereof.

Suitably, delignification with peracid or salts thereof is carried out at a pH in the range of from about 2.5 up to about 12. In preferred embodiments where delignification is carried out with peracetic acid, the pH suitably lies in the range of from 3 up to 11, preferably in the range of from 5.5 up to 9. Delignification with the other peracids or salts thereof mentioned above is carried out within the normal pH ranges for the respective bleaching agents, these being well- known to those skilled in the art. In the pulp, manganese ions, inter alia, have a particu¬ larly adverse effect on the bleaching with chlorine-free blea¬ ching agents, such as alkaline peroxide compounds or ozone. Thus, compounds forming strong complexes with various manga- nese ions are primarily used as complexing agents. Such suit¬ able complexing agents are nitrogenous organic compounds, primarily nitrogenous polycarboxylic acids, nitrogenous poly- phosphonic acids and nitrogenous polyalcohols. Preferred nitrogenous polycarboxylic acids are diethylenetriaminepenta- acetic acid (DTPA) , ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) , DTPA and EDTA being especially preferred. Diethylenetriaminepentaphosphonic acid is the pre¬ ferred nitrogenous polyphosphonic acid. Also other compounds can be used as complexing agents, such as polycarboxylic acids, suitably oxalic acid, citric acid or tartaric acid, or phosphonic acids. Further, such organic acids as are formed during the pulp treatment with, inter alia, chlorine-free bleaching agents can also be used as complexing agents.

The pH in the treatment with a complexing agent is of decisive importance in removing the undesirable trace metal ions while at the same time retaining the desirable alkaline- earth metal ions. A suitable pH range depends, inter alia, on the type and the amount of trace metal ions in the incoming pulp. In the inventive process, however, the treatment with a complexing agent should be carried out at a pH in the range of from 3.5 up to about 11, suitably from 3.5 up to 10, and preferably from 4.5 up to 9. A pH within the range of from 5 up to 7 is especially preferred.

The temperature in the treatment with a complexing agent is of considerable importance for removal of the undesirable trace metal ions. Thus, the content of manganese ions decrea¬ ses with increasing temperature in the treatment with a com¬ plexing agent, which gives an increase in brightness and a reduction of the kappa number. For instance, when the tempera- ture is increased from 20°C to 90°C, the viscosity is also found, surprisingly, to increase noticeably. The treatment with a complexing agent is carried out at a temperature of from 26°C up to about 120°C, suitably from 26°C up to about 100°C, preferably from 40°C up to 95°C, and most preferably from 55°C up to 90°C.

The added amount of complexing agent depends on the type and amount of trace metal ions of the incoming pulp. Also, this amount depends on the type of agent used as well as the conditions in the treatment with the complexing agent, such as temperature, residence time and pH. However, the added amount of complexing agent should be in the range of from about 0.1 kg up to about 10 kg per tonne of dry pulp, calculated as 100% complexing agent. Suitably, the amount lies in the range of from 0.3 kg up to 5 kg per tonne of dry pulp, and preferably in the range of from 0.5 up to 1.8 kg per tonne of dry pulp, calculated as 100% complexing agent.

In preferred embodiments where the delignification with peracid as well as the treatment with a complexing agent are carried out at a close to neutral pH, the need of pH adjust¬ ment is minimised. As a result, also the spent liquors from the bleaching and treatment stages can be used internally for washing. This gives a small total volume of wastewater, enabl¬ ing a considerably more closed system in the pulp mill. Bleaching with a peroxide-containing compound, ozone or sodium dithionite can be carried out in an optional sequence or mixture. The peroxide-containing compound consists of inor¬ ganic peroxide compounds, such as hydrogen peroxide or peroxo¬ monosulphuric acid (Caro's acid). Preferably, the peroxide- containing compound is hydrogen peroxide or a mixture of hyd¬ rogen peroxide and oxygen.•

When hydrogen peroxide is used as bleaching agent, the pulp can be treated at a pH of from about 7 up to about 13, suitably at a pH of from 8 up to 12, and preferably at a pH of from 9.5 up to 11.5. Bleaching with the other bleaching agents indicated above takes place within the normal pH ranges for the respective agents, these being well-known to those skilled in the art.

The inventive process may also include a bleaching stage with a peroxide-containing compound or ozone before the delig¬ nification with peracid or salts thereof.

The inventive process is carried out with a washing stage before the bleaching with a peroxide-containing com¬ pound, ozone or sodium dithionite, such that the washing is carried out at a pH of at least about 4. Washing efficiently removes the complexed trace metal ions that have an adverse effect on the subsequent bleaching with a peroxide-containing compound, ozone or sodium dithionite, primarily manganese ions but also ions of e.g. copper and iron. Because the pH in the washing stage is at least about 4, the alkaline-earth metal ions that have a favourable effect on the subsequent chlorine- free bleaching, primarily magnesium and calcium ions, are re¬ tained in the pulp. The pH in the washing stage lies suitably in the range of from 5 up to about 11, preferably in the range of from 6 up to 10.

Washing before the bleaching with a peroxide-containing compound, ozone or sodium dithionite, is suitably carried out after the treatment with a complexing agent and before the delignification with peracid or salts thereof. In this way, the complexed trace metal ions are efficiently removed, while at the same time the remaining peracid or peroxide-containing compound, if any, can be used in the subsequent bleaching stage. When washing has to be particularly effective, it is possible to have a washing stage after the treatment with a complexing agent as well as after the delignification with peracid or salts thereof.

The washing liquid may be fresh water, optionally with an addition of a pH-adjusting chemical, or wastewater from one or more bleaching stages or extraction stages, in such a way that a suitable pH in the washing stage is obtained. Further¬ more, the washing liquid may consist of other types of optio¬ nally purified wastewater, provided it has a low content of undesirable metal ions, such as manganese, iron and copper. Washing relates to methods for displacing, more or less completely, the spent liquid in the pulp suspension to reduce its content of, inter alia, dissolved trace metal ions in said suspension. The washing methods may entail an increase of the pulp concentration, e.g. by sucking-off or pressing, but also a reduction of the pulp concentration, e.g. by dilution with washing liquid. Also, washing relates to combinations and sequences in which the pulp concentration is alternately increased and reduced, one or more times. In the present process, a washing method is chosen, which removes not only dissolved organic substance, but also the trace metal ions released in the treatment with a complexing agent, while tak¬ ing into account what is suitable in terms of process tech¬ nique and economy. Washing efficiency may be given as the amount of liquid phase displaced, as compared with the liquid phase present in the pulp suspension before washing. The total washing efficie¬ ncy is calculated as the sum of the efficiency in each washing stage. Thus, after a treatment stage, dewatering of the pulp suspension from, say, 10% to 25% pulp concentration gives a washing efficiency of 66.7%. After a subsequent washing stage in which the pulp is first diluted to 3% and then dewatered to 25%, the total washing efficiency is 96.9% with regard to sol¬ uble impurities. Washing efficiency should, in the present process, be at least about 75%, suitably in the range of from 90% up to 100%, and preferably in the range of from 92% up to 100%. It is especially preferred that the washing efficiency lies in the range of from 96% up to 100%.

By using the inventive process the conditions for the chlorine-free bleaching, are optimised such that a high brightness, kappa number reduction and viscosity are achieved with a minimum consumption of the chlorine-free bleaching agent. This is possible without the use of any auxiliary che¬ micals, such as stabilisers and protective agents, in the chlorine-free bleaching. The remaining bleaching chemicals, such as hydrogen peroxide and alkali, are advantageously used directly in the bleaching stage, in the peracid stage or in some other suitable stage, such that an optimum combination of process technique and production economy is obtained. It is also within the scope of the invention, that the delignification with peracid or salts thereof can be reinforc¬ ed by an addition of such peroxide-containing compounds as have been indicated above. Suitably, such reinforcement is carried out with hydrogen peroxide or a mixture of hydrogen peroxide and oxygen.

Lignocellulose-containing pulp relates to pulps contain¬ ing fibres that have been separated by chemical or mechanical treatment, or recycled fibres. The fibres may be of hardwood or softwood. Chemical pulp relates to pulps digested according to the sulphate, sulphite, soda or organosolv process. Mecha¬ nical pulp relates to a pulp produced by refining chips in a disc refiner (refiner mechanical pulp) or by grinding logs in a grinder (groundwood pulp) . Lignocellulose-containing pulp also relates to pulps produced by modifications or combina¬ tions of the above-mentioned methods or processes. Such pulps include thermomechanical, chemimechanical and chemi-thermo- mechanical pulps. Suitably, the lignocellulose-containing pulp consists of chemically digested pulp, preferably sulphate pulp. It is especially preferred that the lignocellulose-con¬ taining pulp consists of sulphate pulp of softwood.

The inventive process can be applied to pulps with a yield of up to about 90%, suitably in the range of from 30% up to 80%, and preferably in the range of from 45% up to 65%. The inventive process can be carried out in an optional position in the bleaching sequence, e.g. immediately after the production of the pulp. When the inventive process is applied to chemically digested pulp, this is preferably delignified in an oxygen stage before the treatment with a complexing agent. The inventive process can be applied to chemically dige¬ sted pulps having an initial kappa number in the range of from about 2 up to about 100, suitably from 5 up to 60, and prefer¬ ably from 10 up to 40. The kappa number is then measured according to the SCAN-C 1:77 Standard Method. In the inventive process, the treatment with a complex¬ ing agent should be carried out for a period of time of from about 1 min up to about 960 min, suitably from 15 min up to 240 min, and preferably from 35 min up to 120 min. The pulp concentration in the treatment with a complexing agent may be from about 1% by weight up to about 60% by weight, suitably from 2.5% by weight up to 40% by weight, preferably from 3.5% by weight up to 25% by weight and most preferably from 5.5% by weight up to 25% by weight.

In the inventive process, delignification with peracid should be carried out at a temperature of from 50°C up to about 140°C, and suitably from 50°C up to about 120°C. Prefe¬ rably the pulp is delignified at a temperature of from 50°C up to about 100°C and more preferably from 50°C up to 90°C. A temperature in the range of from 50°C up to 80°C is especially preferred. Delignification with peracid should be carried out for a period of time from about 1 min up to about 960 min, suitably from 10 min up to 270 min, and preferably from 30 min up to 150 min. The pulp concentration in the delignification with peracid may be from about 1% by weight up to about 70% by weight, suitably from 3% by weight up to 50% by weight, prefe¬ rably from 8% by weight up to 35% by weight and most preferab¬ ly from 10% by weight up to 30% by weight.

When the bleaching agent used is hydrogen peroxide, the pulp should be treated at a temperature of from about 30°C up to about 140°C, and suitably from about 30°C up to about 120°C. Preferably the pulp is treated at a temperature of from about 30°C up to about 100°C and more preferably from 60°C up to 90°C, and for a period of time of from about 5 min up to about 960 min, suitably from 60 min up to 420 min, and prefer¬ ably from 190 min up to 360 min. When the bleaching agent used is hydrogen peroxide, the pulp concentration may be from about 1% by weight up to about 70% by weight, suitably from 3% by weight up to 50% by weight, preferably from 8% by weight up to 35% by weight and most preferably from 10% by weight up to 30% by weight. Treatment with the other bleaching agents indicated above takes place within the normal ranges as to temperature, time and pulp concentration for the respective agents, these being well-known to those skilled in the art. In preferred embodiments using hydrogen peroxide as bleaching agent, the amount of hydrogen peroxide added in the bleaching stage should be in the range of from about 1 kg up to about 60 kg per tonne of dry pulp, calculated as 100% hyd¬ rogen peroxide. The upper limit is not critical, but has been set for reasons of economy. Suitably, the amount of hydrogen peroxide lies in the range of from 6 kg up to 50 kg per tonne of dry pulp, and preferably in the range of from 13 kg up to 40 kg per tonne of dry pulp, calculated as 100% hydrogen peroxide. In preferred embodiments using hydrogen peroxide as bleaching agent, the share of peracid added in the delignify¬ ing stage to the total amount of peracid and hydrogen peroxide added in the delignifying and bleaching stages, should be less than about 60% on a weight-to-weight basis. Here, the amount of peracid has been recalculated as 100% hydrogen peroxide. When recalculating peracetic acid as hydrogen peroxide, 1 kg of peracetic acid is equivalent to 0.45 kg of hydrogen peroxi¬ de. In preferred embodiments using ozone as bleaching agent, the amount of ozone may be in the range of from about 0.5 kg up to about 30 kg per tonne of dry pulp, suitably in the range of from 1 kg up to 15 kg per tonne of dry pulp, preferably from 1.5 kg up to 10 kg per tonne of dry pulp and most prefer- ably from 1.5 kg up to 5 kg per tonne of dry pulp.

After treatment with a complexing agent, delignification with peracid and subsequent bleaching with a peroxide-contain¬ ing compound, ozone or sodium dithionite, the pulp can be used for direct production of paper. Alternatively, the pulp may be finally bleached to a desired higher brightness in one or more stages. Suitably, final bleaching is also carried out by means of such chlorine-free bleaching agents as are indicated above, optionally with intermediate extraction stages which can be reinforced by peroxide and/or oxygen. In this way, the forma- tion and discharge of AOX is completely eliminated. It is also possible to use chlorine-containing bleaching agents, such as chlorine dioxide, in the final bleaching and yet obtain a very limited formation and discharge of AOX, since the lignin con¬ tent of the pulp has been considerably reduced by the present process.

The invention and its advantages will be illustrated in more detail by the Examples below which however, are only in¬ tended to illustrate the invention without limiting the same. The percentages and parts stated in the description, claims and Examples, refer to percent by weight and parts by weight, respectively, unless otherwise stated. Furthermore, the pH values given in the description, claims and Examples refer to the pH at the end of each treatment, unless otherwise stated.

In the Examples below, the kappa number, viscosity and brightness of the pulp were determined according to the SCAN Standard Methods C 1:77 R, C 15-16:62 and C 11-75:R, respect¬ ively. The consumption of hydrogen peroxide and peracetic acid were established by titration with sodium thiosulphate, and potassium permanganate and sodium thiosulphate, respectively. Example 1

Oxygen-delignified sulphate pulp of softwood having a kappa number of 16.0, a brightness of 37.1% ISO and a viscos¬ ity of 1010 dm³/kg was treated with EDTA in accordance with the invention, delignified with peracetic acid, and bleached with hydrogen peroxide, in order to illustrate the importance of the pretreatment for the pulp properties after the present process. The pulp was treated with 2 kg EDTA per tonne of dry pulp at a temperature of 90°C, a residence time of 60 min, a pulp concentration of 10% by weight and varying pH. The amount of peracetic acid added was 22.4 kg per tonne of dry pulp, calculated as 100% peracetic acid. In the delignification with peracetic acid, the pH was 5.5-5.9, the temperature 70°C, the treatment time 60 min, and the pulp concentration 10% by weight. Subsequently, the pulp was bleached with hydrogen peroxide at a temperature of 90°C, a residence time of 240 min and a pulp concentration of 10% by weight. The addition of hydrogen peroxide was 25 kg per tonne of dry pulp, calculated as 100% hydrogen peroxide, and the pH was 10.7-11.6. For com- parison, the pulp was treated with 2 kg EDTA per tonne of dry pulp at 25°C and for 30 min at a pH of 6 and 2 (tests 5 and 6) . For further comparison, the pulp was treated at a pH of about 2 in the absence of a complexing agent (test 7) and without any pretreatment whatsoever (test 8) . After each stage, the pulp was washed with deionised water at a pH of 6.0. At this, the pulp was first dewatered to a pulp concen¬ tration of 25% and then diluted to a pulp concentration of 3% by weight . After a few minutes, the pulp was dewatered to a pulp concentration of 25% by weight. Consequently, the total washing efficiency was about 97%. The results after bleaching with hydrogen peroxide appear from the Table below.

TABLE I Pulp properties after the H₂0₂ stage Test pH Kappa Viscosity Brightness number

(dm³/kg) (% ISO)

TABLE I (cont. ) Pulp properties after the H₂0₂ stage Test pH Kappa Viscosity Brightness

It is evident from the Table that the treatment of soft¬ wood pulp with a complexing agent in a separate stage and at an elevated temperature and at a pH in the range of from 3.5 up to about 11, results in a substantial reduction of the kappa number and a considerable increase in brightness, as well as a high viscosity. Example 2

The oxygen-delignified sulphate pulp of softwood used in Example 1 was treated with EDTA, delignified with peracetic acid and bleached with hydrogen peroxide, in order to illus¬ trate the effect of washing between the separate stages in the sequence. The conditions in the treatment with a complexing agent were as in Example 1, except that the pH was 5.7 (test 1) . The conditions in the delignification with peracetic acid and the bleaching with hydrogen peroxide were as in Example 1 throughout. In test 1, the pulp was washed in accordance with Example 1, both after the treatment with EDTA and after the delignification with peracetic acid. For comparison, the pulp was treated with 2 kg EDTA per tonne of dry pulp at a pH of 7, a temperature of 25°C and a pulp concentration of 8% by weight and for 10 min (test 2) . In test 2, the pulp was dewatered after the treatment with a complexing agent to a pulp concen¬ tration of 25% by weight. There was no washing or dewatering after the delignification with peracetic acid in test 2. Wash- ing efficiency was about 97% in test 1 and about 74% in test 2. The results after bleaching with hydrogen peroxide (H₂0₂) appear from the Table below. TABLE II Pulp properties after the H₂0₂ stage Remaining

It is evident from the Table that the inventive treat¬ ment of softwood pulp with a complexing agent at a high tem- perature followed by washing, gives a much greater reduction of the kappa number and a much higher increase in brightness with a low consumption of hydrogen peroxide and an essentially maintained strength of the pulp, than does treatment at room temperature followed by dewatering. Example 3

The oxygen-delignified sulphate pulp of softwood used in Example 1 was treated with EDTA, delignified with peracetic acid and bleached with hydrogen peroxide, in order to illus¬ trate the effect of peracetic acid and separate stages in the sequence according to the invention. The conditions in the treatment with a complexing agent were as in Example 1, except that the pH was 5.7. The conditions in the delignification with peracetic acid were as in Example 1, except that 11.2 kg of peracetic acid was added per tonne of dry pulp. The condi- tions in the bleaching with hydrogen peroxide were as in Exam¬ ple 1. In test 1, the pulp was treated with EDTA, delignified with peracetic acid and bleached with hydrogen peroxide. In test 2, the pulp was delignified with peracetic acid in the presence of EDTA, whereupon the pulp was bleached with hydro- gen peroxide. In the delignification in the presence of EDTA in test 2, the pH was 5.1, the temperature 90°C and the treat¬ ment time 1 h. In test 3, the pulp was treated with EDTA, whereupon it was delignified and bleached with peracetic acid in the presence of hydrogen peroxide. In the delignification and the bleaching in test 3, the temperature was 70°C for 1 h, whereupon it was raised to 90°C and kept there for 4 h, the pH being 11.1. For comparison, the pulp was treated with EDTA and bleached with hydrogen peroxide (test 4) . After each stage, the pulp was washed in accordance with Example 1. The results after the bleaching with hydrogen peroxide (H₂0₂) appear from the Table below.

TABLE III Pulp properties after the H₂0₂ stage Remaining

It is evident from the Table that softwood pulp treated in accordance with the present invention in separate stages will show a substantial reduction of the kappa number and a considerable increase in brightness with a low consumption of hydrogen peroxide and an essentially maintained pulp strength. Example 4

The oxygen-delignified sulphate pulp of softwood used in Example 1 was treated with EDTA, delignified with peracetic acid and bleached with hydrogen peroxide, in order to illus¬ trate the effect of pH in washing on the brightness of the pulp after the bleaching stage. The conditions in the treat¬ ment with a complexing agent were as in Example 1, except that the pH was 5.7. The conditions in the delignification with peracetic acid were as in Example 1, except that the pH was 6.1. The conditions in the bleaching with hydrogen peroxide were as in Example 1. After each stage, the pulp was washed in accordance with Example l, except that the pH was varied in the washing after the treatment with a complexing agent. The results after the bleaching with hydrogen peroxide (H₂0₂) appear from the Table below.

TABLE IV

Test EH Brightness after the H-0_? stage (% ISO)

1 2.4 65.9 2 3.5 72.0

3 4.1 79.0

4 6.6 82.6

5 9.8 81.4

6 10.7 80.9 It is evident from the Table that softwood pulp treated in accordance with the present invention shows a substantial increase in brightness. Example 5 The oxygen-delignified sulphate pulp of softwood used in Example 1 was treated with EDTA, delignified with peracetic acid and bleached with hydrogen peroxide, in order to illus¬ trate the effect of the weight ratio between peracetic acid and hydrogen peroxide in the equilibrium peracetic acid and the washing efficiency between delignification and bleaching. The conditions in the treatment with a complexing agent were as in Example 1, except that the pH was 5.7. The conditions in the delignification with peracetic acid were as in Example 1, except that the pH was 5.2 to 6.3. The conditions in the blea- ching with hydrogen peroxide were as in Example 1, except that the amount of added hydrogen peroxide was 30 kg per tonne of dry pulp, calculated as 100% hydrogen peroxide. In tests 1 to 4, the pulp was washed as in Example 1, i.e. with a washing efficiency of about 97%. The washing efficiency was about 67% in test 5. There was no washing or dewatering after the delig¬ nification with peracetic acid in test 6. The results after the bleaching with hydrogen peroxide (H₂0₂) appear from the Table below.

TABLE V Test PAA:H₂0₂ Washing efficiency Brightness after PAA after PAA after H₂0₂ £%J ( % ISO) (% ISO)

1 11 . 5 : 1 97

2 5 . 0 : 1 97 3 2 . 1 : 1 97

4 0 . 6 : 1 97

5 2 . 1 : 1 67

6 2.1:1 0 It is evident from the Table that treatment of softwood pulp in accordance with the invention, results in a consider¬ able increase in brightness after delignification with perace¬ tic acid as well as after bleaching with hydrogen peroxide. Example 6

Oxygen-delignified sulphate pulp of softwood having a kappa number of 16.5, a brightness of 36.0% ISO and a viscos¬ ity of 1010 dm³/kg was treated with EDTA in accordance with the invention, delignified with peracetic acid, and bleached with hydrogen peroxide, in order to illustrate the importance of the pretreatment pH for the pulp properties after the pre¬ sent process. The conditions in the treatment with a complex¬ ing agent were as in Example 1, except that the pH was 4.0 (test 1) . The peracetic acid was an equilibrium peracetic acid, with a weight ratio between peracetic acid and hydrogen peroxide of 4:1. The amount of peracetic acid added was 5 kg per tonne of dry pulp, calculated as 100% peracetic acid. The conditions in the delignification with peracetic acid were as in Example 1, except that the pH was 5.8-6.1. The conditions in the bleaching with hydrogen peroxide were as in Example 1, except that the addition of hydrogen peroxide was 35 kg per tonne of dry pulp, calculated as 100% hydrogen peroxide, and the pH was 11.2-12.0. For comparison, the pulp was treated with 2 kg EDTA per tonne of dry pulp under the conditions stated above, except that the pH was 3.0, i.e. outside the pH range of the present invention (test 2) . After each stage, the pulp was washed in accordance with Example 1. The results after bleach-ing with hydrogen peroxide appear from the Table below.

TABLE VI Pulp properties after the H₂0₂ stage

Test pH Kappa Viscosity Brightness number

(dm³/kg) (% ISO)

1 4.0 6.2 860 79.1 2 3.0 7.3 850 64.6

It is evident from the Table that the treatment of soft¬ wood pulp with a complexing agent at a pH of at least 3.5, results in a pulp with superior properties compared to treat¬ ment with a complexing agent at a pH which is more acidic. Example 7

The oxygen-delignified sulphate pulp of softwood used in Example 6 was treated with EDTA, delignified with peracetic acid, and bleached with hydrogen peroxide (tests 1-2) , in order to illustrate the effect of residence time and tempera- ture in the delignification step. The conditions in the treat¬ ment with a complexing agent were as in Example 1, except that the pH was 5.7. The amount of peracetic acid added was 10 kg per tonne of dry pulp, calculated as 100% peracetic acid. In the delignification with peracetic acid, the pH was 6.0-6.5, the temperature 110°C and the pulp concentration 10% by weight, whereas the residence time was varied. The conditions in the bleaching with hydrogen peroxide were as in Example 1, except that the pH was 11.0-11.1. For comparison, the pulp was delignified at 40°C, i.e. outside the temperature range of the present invention (tests 3-4) . After each stage, the pulp was washed in accordance with Example 1. The results after delig¬ nification with peracetic acid appear from the Table below.

TABLE VII Pulpproperties afterdelignification

Test Tempera- Residence Kappa Viscosity Brightness ture time number i_Z£ (min) (dm³/kg) (% ISO)

It is evident from the Table that the treatment of soft¬ wood pulp with peracetic acid at a temperature in the range of from 50°C up to about 140°C, results in a pulp with superior properties compared to treatment at a lower temperature. Example 8

Oxygen-delignified sulphate pulp of softwood having a kappa number of 10.3, a brightness of 41.7% ISO and a viscos- ity of 1000 dm³/kg was treated with EDTA, delignified with two types of peracetic acid, and bleached with hydrogen peroxide, in order to illustrate the effect of peracetic acid of various compositions. The conditions in the treatment with a complex¬ ing agent were as in Example 1, except that the pH was 5.5 and the charge of EDTA was 1.5 kg per tonne of dry pulp. The per¬ acetic acids used were one equilibrium peracetic acid with a weight ratio between peracetic acid and hydrogen peroxide of 4:1 (Equil) , and one distilled peracetic acid essentially free of hydrogen peroxide and acetic acid (Dist) . With both types of peracetic acid, the added amount was 10 kg per tonne of dry pulp, calculated as 100% peracetic acid. The conditions in the delignification with peracetic acid were as in Example l, except that the pH was 6-7. The conditions in the bleaching with hydrogen peroxide were as in Example 1, except that the addition of hydrogen peroxide was 35 kg per tonne of dry pulp calculated as 100% hydrogen peroxide, the pH was 11.5 and the temperature 110°C in tests 2 and 4. After each stage, the pulp was washed in accordance with Example 1. The results after bleaching with hydrogen peroxide appear from the Table below.

TABLE VIII Pulp prop, after the H₂0₂ stage Remaining Test Perac. Temp. Kappa Viscosity Brightness H₂0₂ acid number C (dmVkg) (% ISO) (kg/tonne)

It is evident from the Table that the treatment of soft¬ wood pulp according to the invention, results in a pulp with excellent properties after bleaching in combination with a moderate consumption of bleaching agents in the sequence. Example 9 The oxygen-delignified sulphate pulp of softwood used in Example 8 was treated with EDTA, delignified with peracetic acid, and bleached with hydrogen peroxide, in order to illus¬ trate the effect of the total amount of bleaching agents and the distribution of the same between the delignifying (Stage 2) and bleaching stages (Stage 3) . The conditions in the treatment with a complexing agent were as in Example 1, except that the pH was 5.5 and the charge of EDTA was 1.5 kg per tonne of dry pulp. The peracetic acid used was a distilled peracetic acid essentially free of hydrogen peroxide and acetic acid. The amount of peracetic acid added was varied between 11 and 80 kg per tonne of dry pulp, calculated as 100% peracetic acid. The conditions in the delignification with peracetic acid were as in Example 1, except that the pH was 6- 7. The amount of hydrogen peroxide added was varied between 2 and 30 kg per tonne of dry pulp calculated as 100% hydrogen peroxide. The conditions in the bleaching with hydrogen per¬ oxide were as in Example 1, except that the pH was 11.5. The total amounts of hydrogen peroxide and peracetic acid calcu- lated as 100% hydrogen peroxide were 20 kg per tonne of dry pulp in tests 1-4 and 40 kg per tonne of dry pulp in tests 5- 8. The share of peracetic acid (calculated as 100% hydrogen peroxide) in stage 2, is also given as a percentage of the total amount. After each stage, the pulp was washed in accor- dance with Example 1. The results after bleaching with hydro¬ gen peroxide appear from the Table below.

TABLE IX Hydrogen peroxide Prop, after H₂0₂ stage Remaining Test Stage 2 Stage 3 Stage 2 Viscosity Brightness H₂0₂

It is evident from the Table that excellent pulp proper- ties are obtainable after delignification and bleaching accor¬ ding to the present invention.

Claims

1. A process for bleaching of lignocellulose-containing pulp with a peroxide-containing compound, ozone or sodium dithionite, c h a r a c t e r i s e d in that the pulp, before said bleaching, in a sequence is first treated with a complexing agent at a pH in the range of from 3.5 up to about 11 and at a temperature in the range of from 26°C up to about 100°C and then delignified with an organic peracid or salts thereof at a temperature in the range of from 50°C up to about 140°C, and that the pulp, before said bleaching, is washed at a pH of at least about .

2. A process according to claim 1, c h a r a c t e ¬ r i s e d in that the washing is carried out after the treat¬ ment with a complexing agent and before the delignification with peracid or salts thereof.

3. A process according to claim 1 or 2, c h a r a c ¬ t e r i s e d in that the lignocellulose-containing pulp is a chemically digested pulp.

4. A process according to any of the preceding claims, c h a r a c t e r i s e d in that the peracid is peracetic acid.

5. A process according to any of the preceding claims, c h a r a c t e r i s e d in that the peroxide-containing compound consists of hydrogen peroxide or a mixture of hydrogen peroxide and oxygen.

6. A process according to any of the preceding claims, c h a r a c t e r i s e d in that the complexing agent is a nitrogenous organic compound.

7. A process according to any of the preceding claims, c h a r a c t e r i s e d in that the pulp is delignified at a temperature in the range of from 50°C up to about 120°C, preferably from 50°C up to 80°C.

8. A process according to any of the preceding claims, c h a r a c t e r i s e d in that the share of peracid added in the delignifying stage to the total amount of peracid and hydrogen peroxide added in the delignifying and bleaching stages, is less than about 60% on a weight-to-weight basis.

9. A process according to any of the preceding claims, c h a r a c t e r i s e d in that the amount of complexing agent added is up to 1.8 kg per tonne of dry pulp, calculated as 100% complexing agent.

10. A process according to any of the preceding claims, c h a r a c t e r i s e d in that the treatment with a complexing agent is preceded by an oxygen stage.