FI118572B - Process for bleaching lignocellulosic pulp - Google Patents

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

118572

The present invention relates to a process for bleaching a pulp containing lignocellulose. markedly improved brightness and significantly reduced kappa number after bleaching with a chlorine-free bleaching agent such as hydrogen peroxide. The brightness enhancing effect is highly selective, i.e. the viscosity of the pulp remains relatively high.

Non-chlorinated bleaching agents have long been used to bleach mechanical pulps. In recent years, chlorine-free bleaching agents, such as hydrogen peroxide and ozone, have also begun to be used to bleach chemical pulps in the early stages. It has been found necessary to pre-treat the pulp directly after cooking and optional oxygen delignification to avoid deterioration of pulp properties and excessive consumption of bleach. Massan • · ·

M1 pre-treatment mainly consists of acid treatment and treatment of * ·, · [. metal complexing agent or alkaline earth metal blocking agent.

i M

. * parasites, optionally in combination. The strongly acidic pretreatment removes both the beneficial and harmful metal ions from their original position in the pulp. Treatment with suitable complexing agents removes mainly the harmful metal ions, while at the same

ML V * remain. Treatment with alkaline earth metal salts:, ·. retains or reintroduces preferred metal ions.

• · · «·» ·

EP-A-0 415 149 discloses the use of peroxomonomeric sulfuric acid, prior to treatment with oxygen and / or peroxide. The pulp may be pretreated in the process steps of removing heavy metals and organic impurities. In the Examples, the mass 118572 2 is then treated in the presence of DTPA at pH 2 before being treated with peroxomonomeric acid and oxygen. Such acid washing also removes those metal ions, primarily alkaline earth metals, which are necessary for the subsequent efficiency of bleaching with peroxy-diphenyl compounds or ozone.

EP-A-0402335 describes a process for improving the efficiency of initial alkaline peroxide treatment. The mass is pretreated with a complexing agent at pH 3.1-9 at a temperature of 26-100 ° C. The only peroxide-containing compounds described in the examples are hydrogen peroxide and its mixture with oxygen. The purpose of the pretreatment according to EP-A-0402335 is to prepare a pulp for alkaline peroxide treatment.

WO-A-9215752 describes a process in which a pulp is treated with a biphasic sequence. In the first step, peroxomonomor sulfuric acid is added, i.e. Caronic acid (an inorganic acid containing sulfur). In the Caro acid treatment, a complexing agent may be present. In the second step, the pulp is bleached with hydrogen peroxide. When peracid • *;, · S is present simultaneously with the complexing agent, i.e..

:: in the same treatment step, the effect of the complexing agent · * ·. · is greatly reduced because the complexing agents are degraded in the presence of peracids.

• • t · • · · t * ·

As environmental standards continue to tighten, there is a growing,. the need for completely chlorine-free delignification and bleaching of pulp containing lignocellulose. In order to obtain («» ** | * full strength bleach with constant strength properties;; * · masses with a reasonable number of steps and a moderate consumption of bleaches, it has become * necessary to consider the use of strong and due to the use of difficult to control bleaches with good *: delignification and / or bleaching properties.

As a result of the present invention, there is provided a process wherein the lignocellulosic pulp is delignified and bleached as defined in the appended claims, whereby a good delignification and bleaching effect is obtained even before bleaching with a peroxide-containing compound, ozone or sodium dithionate.

In the process of the invention, the lignocellulosic pulp is bleached with a peroxide-containing compound, ozone or sodium dithionite, wherein the pulp in the pre-bleaching sequence is first treated with a complexing agent at pH 3.5 to about 11 and at 26 to about 100 ° C and then delignified with organic peracid or between 50 ° C and about 140 ° C, and wherein the pulp is washed prior to bleaching at a pH of at least about 4.

The process of the invention has enabled delignification of the pulp after treatment with the complexing agent without adversely affecting the conditions optimized by the treatment with the complexing agent in the following chlorine-free bleaching, taking into account the favorable and harmful metal ions.

• *;, {* Thus, alkaline earth metal ions, especially when they are !? in their original positions in the pulp, is known to have a favorable effect on the selectivity of bleaching and chlorine-free • · i · *; consumption of bleaching agents such as peroxide-containing compounds and · »· · ozone.

r »« 4. , In the invention, the organic acid of peracid or its salts * *! *! *, * set and inorganic peracids and their salts. The organic ft · * \ * peracid is aliphatic peracid, aromatic ie: ': peracid or salts thereof. Peracetic acid or perroic acid is suitably used. The salts suitably use sodium * as a cation, since such salts are generally cheap, and sodium is naturally present in the chemical balance of the pulp mill. Peracetic acid or its salts are preferred, being advantageous for both preparation and use. In addition, peracetic acid is only slightly corrosive. Possible waste water containing eg. peracetic acid degradation products, can be used without light for washing or can be recycled to a chemical recovery system.

According to the process of the invention, peracetic acid can be prepared by the reaction of acetic acid with hydrogen peroxide to give the so-called acetic acid. equilibrium peracetic acid, by distillation of equilibrium peracetic acid to remove hydrogen peroxide, acetic acid and sulfuric acid, or by reacting the acetic anhydride and hydrogen peroxide directly in the bleaching step to give a so-called. in situ peracetic acid. Typical equilibrium peracetic acid contains about 42% peracetic acid and about 6% hydrogen peroxide, i.e. the weight ratio of peracetic acid to hydrogen peroxide is about 7: 1. When equilibrium peracetic acid is used in the process of the invention, the weight ratio of peracetic acid to hydrogen peroxide may range 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 amount of peracetic acid or its salts added should be in the range of about 1 kg to about 100 kg per tonne of dry weight, calculated as 100% peracid or its salt. Suitably, this amount is in the range of 2 kg to 45 kg / ton dry mass and; in the recommended range of 3 kg to 25 kg / ton dry mass * · · calculated as 100% peracid or its salt.

• · «

The delignification with peracid or its salts is conveniently carried out in a pH range of about 2.5 to about 12. In preferred embodiments where the delignification is carried out with family acid, the pH is suitably in the range of 3 to 11 and recommended • · * ·. * · *. usually in the range of 5.5 to 9. Delignification with the other peracids or salts thereof mentioned above occurs within the normal pH range of the corresponding bleaching agents, which are well known to those skilled in the art.

118572 5

Manganese ions, for example, have a particularly deleterious effect on bleaching in the pulp with chlorine-free bleaching agents such as alkaline peroxide compounds or ozone. Thus, compounds which form strong complexes with various manganese ions are used primarily as complexing agents. Such suitable complexing agents include organic nitrogenous compounds, mainly nitrogenous polycarboxylic acids, nitrogenous polyphosphonic acids, and nitrogenous polyalcohols. Preferred nitrogen-containing polycarboxylic acids are diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA), of which DTPA and EDTA are particularly preferred. Diethylenetriaminepentaphosphonic acid is the preferred nitrogenous polyphosphonic acid. Other compounds such as polycarboxylic acids, suitably oxalic acid, citric acid or tartaric acid, or phosphonic acids may also be used as complexing agents. Other useful complexing agents are organic acids formed when the pulp is treated with, for example, chlorine-free bleaching agents.

pH is a crucial factor in the treatment of complexing agents for the removal of harmful residual metal ions. while maintaining the preferred alkaline earth metal ions.

The appropriate pH range depends, e.g. the type and amount of • residual metal ions in the incoming mass. In the method of the invention, the concept of * * · * ···. The complexing agent should be carried out at a pH in the range of about 3.5 to about 11, suitably in the range of 3.5 to 10, and preferably in the range of 4.5 to 9. A pH in the range of 5 to 9 is particularly recommended. · · ··· · va.

Temperature is a very important factor in the complexing concept of • · * *. · **; lymphocytes to remove harmful residual metal ions.

• · · *. Thus, the concentration of manganese ions decreases as the complexing treatment temperature increases, which increases the brightness and reduces the kappa number. For example, as the temperature rises from 20 ° C to 90 ° C, the viscosity also suddenly increases significantly.

The treatment with the complexing agents is carried out at a temperature of 26 ° C to about 120 ° C, suitably 26 ° C to about 100 ° C, preferably 40 ° C to 95 ° C and most preferably 55 ° C to 90 ° C.

118572

The amount of complexing agent added depends on the type and amount of residual metal ions in the incoming mass. This amount is also influenced by the type of agent and the treatment conditions with the complexing agent such as temperature, residence time and pH. However, the amount of complexing agent added should range from about 0.1 kg to about 10 kg / ton dry weight, calculated as 100% complexing agent. Suitably, the amount is in the range 0.3 kg to 5 kg / ton dry mass, and preferably in the range 0.5 kg to 1.8 kg / ton dry mass calculated as 100% complexing agent.

In preferred embodiments, when both delignification with peracid and treatment as complexing agent are performed at near neutral pH, the need for pH adjustment is minimized. As a result, the spent liquids from the bleaching and treatment steps can also be used internally for washing. This results in a small total amount of wastewater, which allows for a «significantly more closed system with mass •: *; mill.

• · · »• · • * • ·

Bleaching with a Peroxide Containing Compound, Ozone or:. sodium dithionite can be performed as an optional sequence or mixture. The peroxide-containing compound is a suitable inorganic peroxide compound, such as hydrogen peroxide or peroxomonohydride (Caronic acid). Preferably, the peroxide-containing compound is hydrogen peroxide or a mixture of hydrogen peroxide and oxygen.

• · • t · • · · im ·. ···. When hydrogen peroxide is used as the bleaching agent, the pulp may be processed at a pH of about 7 to about 13, suitably at a pH of from 8 to 12, and preferably at a pH of 9.5 to 11.5. Bleaching with other • * *. * 1 with the above bleaching agents occurs at normal pH ranges of the corresponding 118572 7 substances well known to those skilled in the art.

The process of the invention may also include a bleaching step with a peroxide-containing compound or ozone prior to delignification with peracid or its salts.

The process of the invention is carried out comprising a washing step prior to bleaching with a peroxide-containing compound, ozone or sodium dithionate, wherein the washing is carried out at least at about pH 4. The wash effectively removes the complexed residual metal ions which have a detrimental effect on the subsequent bleaching of the peroxide-containing compound, ozone or sodium dithionite, mainly manganese ions but also e.g. copper and iron ions. Since the pH of the washing step is at least 4, the alkaline earth metal ions, mainly magnesium and calcium ions, which are favorable for the next chlorine-free bleaching, remain in the pulp. Suitably, the pH of the washing step is in the range of 5 to about 11, preferably in the range of 6 to 10.

"" * · Washing prior to bleaching with a peroxide-containing compound, · · with ozone or sodium dithionite, is suitably carried out. After treatment with the complexing agent and before delignification with peracid or its salts. , while perh ·· ♦ / ··. when any remaining peracid or peroxide-containing compound can be used in the following bleaching agent. When washing needs to be particularly effective, it is possible to use a washing step. after treatment with complexing agents • · · * · * * and after delignification with peracid or its salts.

·

»K

• · • · ··· *. The washing liquid may be fresh water, optionally * · · added with a pH adjusting chemical, or waste water from one or more * * · * · bleaching or extraction stages to obtain a suitable pH for the washing step. The wash liquid may also be 118572 8 other types of potentially purified wastewater, provided that it has low levels of harmful metal ions such as manganese, iron and copper.

Washing means methods whereby the liquid consumed in the pulp suspension is replaced more or less completely, e.g. to reduce the concentration of residual metal ions dissolved in the suspension. Washing methods may also include increasing the pulp concentration, e.g., by suction or squeezing, but also decreasing the pulp concentration, e.g., by diluting the wash liquid. Washing is also meant combinations and sequences in which the pulp concentration is alternately increased and decreased one or more times. In the process of the invention, a washing method is selected which, in addition to removing the dissolved organic matter, also removes the residual metal ions released by treatment with the complexing agent, while taking into account what is appropriate from a process technology and economical point of view.

Washing efficiency can be defined as the amount of liquid phase replaced compared to the liquid phase in the pulp suspension prior to washing. The overall benefit ratio of washing is calculated by all. the sum of the efficiency of the washing stages. Thus, the dewatering of the pulp suspension after the treatment step e.g. from 10% to 25% i. to mass concentration gives a washing efficiency of 66.7%. After a scrubbing step, where the pulp is first diluted • to »3% and then dewatered to 25%, it reaches. . · 96.9% total washing efficiency for soluble impurities. In the process of the invention, the washing efficiency should be at least about 75%, suitably in the range of 90% to 100%, and in the preferred range of 92% to 100%. Highly recommended - ««% ·. ** ·. this efficiency range is 96% to 100%.

* ·· * · ♦ ··· Using the method of the invention, chlorine-free bleaching. conditions can be optimized to achieve high brightness, kappa number reduction and viscosity with minimum consumption of chlorine-free 1 1 8572 9 bleach. This is possible without the use of additional chemicals such as stabilizers and preservatives in chlorine-free bleaching. The remaining bleaching chemicals, such as hydrogen peroxide and alkali, can advantageously be used directly in the bleaching step, the peracid step or any other suitable step, so as to achieve an optimum combination of process technology and production economics.

It is also within the scope of the invention that delignification with peracid or its salts can be enhanced by the addition of one of the peroxide-containing compounds described above. Such enhancement is suitably performed with hydrogen peroxide or a mixture of hydrogen peroxide and oxygen.

Ligno-cellulosic or lignocellulosic pulp refers to pulps containing fibers which have been chemically or mechanically separated, or recycled fibers. The fibers may be softwood or hardwood. Chemical pulp refers to pulps that have been cooked by sulphate, sulphite, soda or organosolvent. Mechanical pulp means pulp that is «♦ ·: · produced by grinding wood chips in a disk refiner (pulp) or! · Grinding logs in a pulverizer (grinder). Lignocellulosic pulp: * ·. · The other pulp also denotes pulps obtained by modifications or combinations of the above methods.

999 9

Examples of such masses are thermomechanical, chemimechanical and kernitermomechanical. Suitably. The lignocellulosic pulp is chemically boiled pulp, preferably sulphate pulp. Particularly popular • ♦ · *. tent is a lignocellulosic pulp consisting of • ·! : i from softwood sulphate pulp.

99 · · • 99 9 9 9 · 9 ·· *. The process according to the invention can be applied to pulps with a yield of up to about 90%, suitably in the range of 30% 9 · · '' - 80% and preferably in the range of 45% - 65%.

118572 10

The method of the invention can be carried out at an optional point in the bleaching sequence, e.g. immediately after the pulp is produced. When applied to the chemical pulp, the process of the invention is preferably delignified in the oxygen phase prior to treatment with the complexing agent.

The process of the invention can be applied to chemical pulps having an initial number in the range of about 2 to about 100, suitably 5 to 60 and preferably 10 to 40. The number of songs is then measured by the SCAN-C 1:77 standard method.

In the process of the invention, treatment with the complexing agent should be carried out for a period of from about 1 minute to about 960 minutes, suitably from 15 minutes to 240 minutes and preferably from 35 minutes to 120 minutes. When treated with the complexing agent, the pulp concentration may be from about 1% to about 60% by weight, suitably from 2.5% to 40% by weight, preferably from 3.5% to 25% by weight and most preferably from 5.5% to 25% by weight.

In the process of the invention, delignification with peracid should * * be performed at a temperature in the range of about 50 ° C to about 140 ° C and * conveniently in the range of about 50 ° C to about 120 ° C. Preferably, the temperature is · 50 ° C to about 100 ° C and more preferably 50 ° C to 90 ° C.

* t '·· 50 ° C - 80 ° C is particularly recommended.

: Delignification with peracid should be performed over a period of about 1 minute to about 960 minutes, suitably 10 minutes to 270 minutes and preferably 30 minutes. (»(Minutes to 150 minutes.) The concentration of pulp in the deli" *. * Niferation with peracid may be from about 1% by weight to about 70% by weight, suitably 3% by weight to 50% by weight, preferably 8% by weight. % by weight - 35% by weight and most preferably 10% by weight - 30 · ***: weights.

* · * T # · * * ··· When hydrogen peroxide is used as the bleaching agent, the pulp should be * · treated at a temperature of about 30 ° C to about 140 ° C and suitably about 30 ° C to about 120 ° C. Preferably, the pulp is treated at 11857211 at a temperature of about 30 ° C to about 100 ° C, and more preferably at 60 ° C to 90 ° C, and for a period of time ranging from about 5 minutes to about 960 minutes, suitably 60 minutes to 420 minutes and preferably 190 minutes to 360 minutes. When hydrogen peroxide is used as the bleaching agent, the mass concentration may be from about 1% to about 70% by weight, suitably from 3% to 50% by weight, preferably from 8% to 35% by weight and most preferably from 10% to 30% by weight. The treatment with the other bleaching agents mentioned above takes place at normal temperature, time, and mass concentration ranges of these bleaching agents well known to those skilled in the art.

In preferred embodiments using hydrogen peroxide as the bleaching agent, the amount of hydrogen peroxide added during the bleaching step should be in the range of about 1 kg to about 60 kg / ton dry weight, calculated as 100% hydrogen peroxide. The cap is not critical, but is set for economic reasons. Suitably, the amount of hydrogen peroxide is in the range of 6 kg to 50 kg / ton dry mass and preferably 13 kg to 40 kg / ton dry mass based on 100% hydrogen peroxide.

* · '* · In preferred embodiments using hydrogen • • ·, ·: oxide as a bleach, the delignification step added •; whereas the proportion of peracid in the total amount of peracid and hydrogen peroxide added during the delignification and bleaching steps should be equal; · '; less than about 60% w / w. Then the amount of peracid «s ·» is calculated as 100% hydrogen peroxide. When peracetic acid is calculated as hydrogen peroxide, 1 kg of peracetic acid corresponds to 0.45. kg hydrogen peroxide.

In preferred embodiments using ozone iii as a bleaching agent, the amount of ozone may be in the range of about 0.5 i * ": kg to about 30 kg / ton of dry pulp, suitably in the range of 1.

»M

*. kg - 15 kg / ton dry mass, preferably 1.5 kg - 10 a "**. kg / ton dry mass and most preferably 1.5 kg - 5 * · f '* · kg / ton dry mass.

After treatment with a complexing agent, a delignification peroxide, and subsequent bleaching with a peroxide-containing compound, ozone or sodium dithionate, the pulp may be used directly for the production of paper. Alternatively, the pulp may be bleached to the desired higher brightness in one or more steps. The final bleaching is also conveniently carried out with the chlorine-free bleaching agents described above, optionally using intermediate extraction steps which may be enhanced with peroxide and / or oxygen. In this way, the formation and removal of AOX is completely eliminated. In the final bleaching it is also possible to use chlorine-containing bleaching agents such as chlorine dioxide, but to obtain very limited AOX formation and removal because the process of the invention has significantly reduced the lignin content of the pulp.

In the following, the invention and its advantages will be described in detail by the following examples, which, however, are intended only to illustrate the invention without limiting it in any way. In the Explanation, Claims and Preliminary. the percentages and parts shown in the marks refer to percentages by weight and parts by weight, respectively, unless otherwise stated i.i *. The pH values given in the specification, claims, and examples * *:: mean the pH at the end of each treatment, unless otherwise stated.

In the following examples, pulp kappa number, viscosity and brightness were determined by the SCAN standard methods C 1:77 R,; . ·. C 15-16: 62 and C 11-75: R respectively. The consumption of hydrogen peroxide and peracetic acid was measured by titration with sodium thiosulphide. • potassium permanganate and sodium · · ·:: thiosulfate.

«· · • · • ♦ ··»

Example 1 Oxygen-delignified softwood pulp with a kappa number of 16.0, a brightness of 37.1% ISO and a viscosity of 1010 dm 3 / kg, 118572 13 was treated with EDTA according to the invention, delignified with peracetic acid. and bleached with hydrogen peroxide to illustrate the importance of pretreatment on pulp properties following the process of the invention. The pulp was treated with 2 kg EDTA / ton dry pulp at 90 ° C, 60 min residence time, 10 wt% pulp concentration and various ρΗ values. The amount of peracetic acid added was 22.4 kg / ton dry weight, calculated as 100% peracetic acid. For delignification with peracetic acid, the pH was 5.5-5.9, the temperature was 70 ° C, the treatment time was 60 min and the pulp concentration was 10% by weight. The pulp was then bleached with hydrogen peroxide at a temperature of 90 ° C, a residence time of 240 min and a mass concentration of 10% by weight. The amount of hydrogen peroxide added was 25 kg / ton dry weight calculated as 100% hydrogen peroxide and the pH was 10.7-11.6. For comparison, pulp was treated with 2 kg EDTA / ton dry pulp at 25 ° C for 30 min at pH 6 and 2 (tests 5 and 6). For further comparison, the pulp was treated at about pH 2 without complexation wave (Test 7) and without any pretreatment (Test 8). After each step, the pulp was washed with delonated water at pH 6.0. In this case, the pulp was first dewatered to a pulp concentration of 25% and then diluted to a pulp concentration of 3% by weight. After a few minutes, water was removed from the pulp to 25% by weight. In this case, the overall linen efficiency was about 97%. The results with hydrogen peroxide: after bleaching are shown in the following table.

• · 1 • · »

»I I

9 0 4 • · • · 9 94 · • · 1 • · · · 1 • · • 1 · «• fs 4 • · • 1 · 1 · • • mm • 994 · • 1 · • ·· 4 4 118572 14

TABLE I

Properties of pulp after H202

Test pH Kappa- Viscose. Brightness _count_dm3 / kg_% ISO_ 1 1.9 6.1 790 69.1 2 6.4 4.7 890 81.3 3 9.2 5.0 875 77.4 4 12.1 6.5 800 68.3 5 6.0 5.0 850 77.0 6 2.0 6.2 785 68.5 7 2.0 6.4 752 67.5 8 --- 6.5 800 65.0

The table shows that treatment of the softwood pulp with a complexing agent in a separate step, and at elevated temperature and pH 3.5 to about 11, results in a substantial reduction in kappa number and a significant increase in brightness and high viscosity.

Example 2 The oxygen-delignified softwood sulphate pulp used in Example 1 was treated with EDTA, delignified with peracetic acid and bleached with hydrogen peroxide for washing; : *: to illustrate the individual steps in the sequence • · · ·. The conditions in the complexing agent treatment were as in Example 1 except that the pH was 5.7 (Test 1). Olot; , *. delignification with peracetic acid and bleaching with hydrogen oxide were as in Example 1. In Test 1, the mass »· · • * · *. was washed according to Example 1 after both EDTA treatment and after peracetic acid delignification. For comparison ················································· 60 (Test 2). In the test 2 masses

4M

After treatment with the complexing agent, water was removed to a mass concentration of up to 25% by weight. In Test 2, no washing or dehydration was performed after the deletion of peracetic acid. Washing efficiency in Test 1 was about 97% and in Test 2 about 74%. The results after bleaching with hydrogen peroxide (H2O2) are shown in the following table.

TABLE II

Characteristic of pulp Post-H2O2 Residue Test Kappa- Viscose. Lightness H202 _count_dm3 / ka_% ISO_ka / tn 1 4.7 885 81.3 9.2 2 6.0 760 70.7 0

The table shows that the treatment of the softwood sponge according to the invention with a high temperature complexing agent followed by washing yields a much greater reduction in kappa number and a much greater brightness increase with low hydrogen peroxide consumption and pulp strength substantially the same as treatment at room temperature followed by dewatering.

•

Example 3 • # 1 "- '" -' -f · · • »« • a »*:: The oxygen-delignified coniferous sulphate mass used in Example 1 was treated with EDTA, the peracetic acid was delignified. and bleached with hydrogen peroxide to illustrate the effect of peracetic acid and the separate steps in the sequence of the invention. Olot complexing agent-:. *. treatment were as in Example 1 except that the pH was a · a. ··· ', 5.7. The conditions for peracetic acid delignification were as *. in Example 1 except that 11.2 kg of peracetic acid were added per ton of dry pulp. The conditions for hydrogen peroxide in bleaching were as in Example 1. In Test 1, pulp was treated with EDTA, delignified with peracetic acid and bleached; with hydrogen peroxide. In assay 2, the pulp was delignified with peri-• ♦ acid in the presence of EDTA, followed by bleaching with hydrogen peroxide. In the presence of EDTA 118572 16 dellification, in Test 2, the pH was 5.1, the temperature was 90 ° C and the treatment time was 1 h. In Test 3, the pulp was treated with EDTA followed by delignification and bleaching with peracetic acid in the presence of hydrogen peroxide. In delignification and bleaching of Test 3, the temperature was 70 ° C for 1 hour, after which it was raised to 90 ° C and held for 4 hours, pH 11.1. For comparison, the pulp was treated with EDTA and bleached with hydrogen peroxide (Test 4). After each step, the pulp was washed as in Example 1. The results after bleaching with hydrogen peroxide (H2O2) are shown in the following table.

TABLE III

Characteristic of pulp Post-H2O2 Residue Test Kappa- Viscose. Brightness H202 _count_dm3 / ka_% ISO_kq / tn 1 5.0 910 79.7 10.4 2 8.7 800 74.1 0 3 6.1 840 76.2 9.0, 4 7.5 890 74.0 6 , 6 ·· ♦ · »• ·!»: * The table shows that, according to the invention, separate! : The coniferous pulp treated in the steps yields a substantial · ·!, ** reduction in kappa number and a significant increase in brightness with low hydrogen peroxide consumption and substantial retention of pulp strength.

:. ·. Example 4 • · · · «« · ··· «· ·» ·

The oxygen-delignified softwood sulfate pulp mass used in Example 1 was treated with EDTA, delignified with peracetic acid and bleached with hydrogen peroxide to illustrate the effect of washing pH on the whiteness of the pulp after the bleaching step * "*. were * * as in Example 1 except that the pH was 5.7 The conditions for delignification with peracetic acid were as in Example 1 except for 1 857217 that the pH was 6.1 The conditions for hydrogen peroxide in bleaching were as in Example 1. After each step, the pulp was washed except that the pH of the washes was varied after treatment with the complexing agent The results after bleaching with hydrogen peroxide (H2O2) are shown in the following table.

TABLE IV

Test_pH_Valeness after HQ (% 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

The table shows that the brightness of the softwood pulp treated according to the invention is substantially increased.

Example 5 • · * »*!. *. * The oxygen delignified softwood sulphate used in Example 1 was treated with EDTA, delignified with peracetic acid, · · · pol and bleached with hydrogen peroxide in the ratio of peracetic acid to: to illustrate the effect of washing efficiency. The conceptual conditions for the complexing agent were as in Example 1, except that the pH was 5.7. Being delignified by peri-acacia i · i *. polla were as in Example 1 except that the pH was 5.2- · ·; 6.3. The conditions for hydrogen peroxide bleaching were as in Example 1 except that the amount of hydrogen peroxide added was 30 kg / tonne * ·, dry mass calculated as 100% hydrogen peroxide. In tests 1- **. '*, The mass was washed as in Example 1, i.e. the washing efficiency was' * * about 97%. In test 5, the washing efficiency was about 67%. In Test 6, after delignification with peracetic acid, 18 1 1872 were not subjected to any washing or dewatering. The results after hydrogen peroxide bleaching (H2O2) are shown in the following table.

TABLE V

Test PAA: H202 Washing benefit. brightness

PAA trace PAA trace H202 trace. _% _% ISO_% ISO

1 11.5: 1 97 54.1 78.0 2 5.0: 1 97 57.0 81.2 3 2.1: 1 97 60.0 82.7 4 0.6: 1 97 63.1 84 , 0 5 2.1: 1 67 60.0 79.5 6 2.1: 1 60 60.0 78.0

The table shows that treating softwood pulp according to the invention provides a significant increase in lightness after peracetic acid delignification and after hydrogen peroxide bleaching.

Example 6: Oxygen-delignified softwood pulp with a kappa number of · · · · · · · · · · · ·, 16.5, a brightness of 36.0% ISO and a viscosity of 1010 dm 3, was treated with EDTA according to the invention, · · *; peracetic acid and bleached with hydrogen peroxide to pre-

To illustrate the importance of thinking on the properties of pulp after the process of the invention. Feeling reading. '·' With the complexing agent were as in Example 1 except that the pH was 4.0 (Test 1). Peracetic acid was equilibrium • * · *, acetic acid with a ratio of peracetic acid to hydrogen peroxide of 4; : 1. The amount of peracetic acid added was 5 kg / -: * [*! Ton dry weight calculated as 100% peracetic acid The conditions * ·, delignification with peracetic acid were as in Example * Γ *, except that the pH was 5.8-6.1. The conditions for hydrogen peroxide bleaching were as in Example 1, except that the amount of hydrogen peroxide added was 35 kg / ton dry weight based on 100% hydrogen peroxide and the pH was 11.2-12.0. EDTA / ton dry pulp under the previous conditions except that the pH was 3.0, i.e. outside the pH range of the invention (Test 2) After each step the pulp was washed according to Example 1. The results after hydrogen peroxide bleaching are shown in the following table .

TABLE VI

Properties of pulp after H202

Test pH Kappa- Viscose. Brightness _count_dm3 / kg_% ISO_ 1 4.0 6.2 860 79.1 2 3.0 7.3 850 64.6

The table shows that treating softwood pulp with a complexing agent at a pH of at least 3.5 yields a pulp having superior properties to the complexing treatment at a more acidic pH.

. Example 7 • 1 * · * The oxygen-delignified conifer sulfate pulp used in Example 6 was treated with EDTA, delignified with peracetic acid S / ·· and bleached with hydrogen peroxide (tests 1-2) to illustrate the effect of residence time and temperature on delignification. The conditions of treatment with complexing agent a were as in Example 1 except that the pH was 5.7. Added:. the amount of peracetic acid was 10 kg / ton dry weight 100% * · · t] .1 calculated as peracetic acid. Delignif iion peretika haw- »·« *. polla had a pH of 6.0-6.5, a temperature of 110 ° C and a mass concentration of 10% by weight, while the residence time was varied. The conditions for hydrogen peroxide in bleaching were as in Example 1 except '·. that the pH was 11.0-11.1. For comparison, the mass was delignified at 40 ° C, i.e., outside the temperature range of the invention • ·· * (tests 3-4). After each step, the pulp was washed according to Example 118572. The results after peracetic acid digestion are shown in the following table.

TABLE VII

Characteristic of the mass, delignif. After food. Test Temperatures. Slices of cold cut. Kappa- Viscose. brightness

_ ° _C_time min_count_dm3 / kg_% ISO

1,110 30 13.2 1005 52.0 2 110 75 12.9 1000 52.1 3 40 60 12.5 1005 49.7 4 40 120 12.0 1025 49.6

The table shows that the treatment of softwood pulp with peretic acid in the temperature range of 50-140 ° C provides a pulp which has superior properties compared to the treatment at lower temperatures.

. Example 8 Oxygen-undifferentiated softwood sulphate pulp having a kappa number of:; was 10.3, brightness 41.7% ISO and viscosity 1000 dm3 / kg, · · *. * ·· treated with EDTA, delignified with two types of peracetic acid and bleached with hydrogen peroxide. to list. Conditions for treatment with complexing agent:. were as in Example 1, except that the pH was 5.5 and EDTA-***, an increase of 1.5 kg / ton dry mass. The peracetic acids used were one equilibrium peracetic acid with a 4: 1 weight ratio of peracetic acid to hydrogen peroxide per liter (acetic acid) and one distillate of peracetic acid which was essentially free of hydrogen peroxide or acetic acid (distillate). .). For each type of peracetic acid, the addition amount was 10 kg / ton dry mass •% «* * 100% peracetic acid. You are in delignification

peracetic acid were as in Example 1 except that the pH

118572 21 was 6-7. The conditions for hydrogen peroxide in bleaching were as in Example 1 except that 35 kg / ton dry pulp calculated as 100% hydrogen peroxide was added, pH 11.5 and pulp temperature 110 ° C in tests 2 and 4. After each step, the pulp was bleached according to example 1. The results after hydrogen peroxide bleaching are shown in the following table.

TABLE VIII

Pulp's own. H202-vaih.jälk.

Per- Heat Kappa- Viscose. Brightness Residue Test Acid Status Number H202 _ie_dm3 / ka_% ISO Co / t 1 tl 90 3.2 810 84.4 14.0 2 tl 110 2.8 760 86.9 8.4 3 Tl 90 3.4 750 84, 0 12.1 4 levels 110 3.0 700 86.5 7.2

The table shows that treating softwood pulp according to the invention provides a pulp having excellent properties after bleaching, together with a reasonable consumption of bleaching agents in the sequence.

♦ Example 9 • «« 1 9 ♦ »

Jt · · The oxygen-delignified sulphate pulp used in Example 8: * Γ: was further treated, delignified with peracetic acid and bleached with hydrogen peroxide to effect the difference between the total amount of bleaches and their delignification (step 2) and bleaching (step »· * · 3).

The conditions treated with the complexing agent were as in Example 1 except that the pH was 5.5 and the EDTA addition was 1.5 * ·· kg / ton dry mass. Peracetic acid used was distilled peracetic acid which was essentially free of hydrogen • ••. · Oxide and not acetic acid. The amount of peracetic acid added was between? -80 kg / ton dry weight, calculated as 100% peracetic acid. The conditions for delignification with family 118572 22 acetic 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 / ton dry weight, calculated as 100% hydrogen peroxide. The conditions for hydrogen peroxide bleaching were as in Example 1 except that the pH was 11.5. The total amounts of hydrogen peroxide and peracid acid, calculated as 100% hydrogen peroxide, were 20 kg / tonne dry pulp in tests 1-4 and 40 kg / tonne dry pulp in tests 5-8. The percentage of peracetic acid (100% calculated as hydrogen peroxide) is also expressed as a percentage of the total. After each step, the pulp was washed as in Example 1. The results after hydrogen peroxide bleaching are shown in the following table

TABLE IX

Hydrogen peroxide Characteristic. Post-H2O2 Residue Test Step Step2 Step3 Viscose. Brightness H202 _kg / ton_dm3 / kg_% ISO kg / ton 1 5 15 25 930 79.9 7.1 2 10 10 50 940 79.5 5.0 3 15 5 75 935 77.2 3.3 4 18 2 90 950 73.0 1.5: *! * '5 10 30 25 870 85.3 12.1: i 6 20 20 50 880 84.9 9.3 V ·! 7 30 10 75 890 82.5 5.8 \ X \ 8 36 4 90 895 78.0 3.0 »* * •» · 4

The table shows that, after delignification and bleaching according to the invention, excellent pulp properties are obtained.

4 4 4 4 4 4 «e · 4 4 4 4 4 4 * · 444 4 · ** 4 444 4 4« 44 4 • 44 4 4 4 4 4 4 4 «« 4

Claims (9)

A process for bleaching lignocellulosic pulp with hydrogen peroxide, in which the pulp process is first treated with a complexing agent in the pH range 3.5 - about 11 and in the temperature range 26 ° C - about 100 ° C, characterized in that: a wash is performed after treatment with a complexing agent and prior to delignification with a • · • # ·. organic peracid or its salts at a pH that is at least 4, and that the pulp after the treatment is delignified; is celebrated with an organic peracid or its salts in the temperature range of about 50 ° C - about 140 ° C, and that the mass after • · · ··! · The delignification with the organic peracid or its salts and before the bleaching is washed at a pH of at least about 4, the proportion of peracid added during the delignification step of the total amount of peracid and hydrogen peroxide added during delignification and The bleaching steps are less than about 60% on a weight to weight basis. • · · * • · · • ·. 2. A process according to claim 1, characterized in that the Φ · V. * lignocellulosic pulp is chemically cooked pulp. · · Φ • · * · · 118572 Process according to claim 1 or 2, characterized in that the peracid is peracetic acid. Process according to any of the preceding claims, characterized in that the hydrogen peroxide bleaching is carried out in the presence of oxygen. Process according to any of the preceding claims, characterized in that the complexing agent is a nitrogen-containing organic compound. Process according to any of the preceding claims, characterized in that the mass is delignified at the temperature range 50 ° C - about 120 ° C, preferably 50 ° C - 80 ° C. Process according to any of the preceding claims, characterized in that the amount of complexing agent added is up to 1.8 kg / ton dry mass calculated as 100% complexing agent. Process according to any of the preceding claims, characterized in that the treatment with a complexing agent is preceded by an oxygen step. ··· * ® • * · • · · 9. A process according to any of the preceding claims, characterized in that the delignification with peracid is carried out at a pH in the range 5.5 - 9. • · 9 9 9 9 9 9 9 9 • 9 9 • · 9 • 9 9 9 9 • 9 9 9 9 9 ® 9 9 ® 9 9 99 9 9 9 9 9 9 9 ® 9 9 9 9 9 9 9 9 9 9® 9 9 9 9 99®