Peroxide bleaching of cellulose pulp

Abstract

A method of controlling viscosity in peroxide bleaching of a cellulose pulp, comprising treating the cellulose pulp with disodium 3,3'-((2-ethylhexyl)azanediyl)dipropionate as a chelating agent to chelate metals present in the pulp; peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to peroxide bleaching of a cellulose pulp. More particularly, the invention relates to a method for controlling the viscosity of a cellulose pulp during peroxide bleaching.BACKGROUND OF THE INVENTION

[0002] The purpose of bleaching of cellulose pulp is to remove residual lignin still present in the pulp after delignification of a lignocellulosic material. Bleaching may be carried out, e.g., with oxygen-containing chemicals including hydrogen peroxide. However, heavy metals, especially copper ions, included in the pulp may cause problems in peroxide bleaching process. Heavy metals react with cellulose breaking it down and thus decreasing viscosity of cellulose.

[0003] WO 99 / 25919 A1 discloses a process for bleaching chemical pulp in which pulp is peroxide bleached and treated with a chelating agent to bind heavy metals to a chelate complex. The chelating agent is selected from N-bis-[(1,2-dicarboxylethoxy)-ethyl]-amine, N-bis-[(1,2-dicarboxylethoxy)-ethyl]-aspartic acid and N-tris-[(1,2-dicarboxylethoxy)-ethyl]-amine and alkali metal and earth alkali metals salts thereof.

[0004] WO 96 / 37654 A1 discloses a method of delignifying / bleaching a delignified / prebleached lignin-containing cellulose in which the cellulose pulp is treated in one and the same stage with a complexing agent, molybdenum-containing substance and peroxide bleaching agent. The complexing agent may be ethylene diamine tetraacetic acid (EDTA) or diethylene triamine pentaacetic acid (DTPA). The pH during the treatment is 2-7.

[0005] US 5658429 discloses a process for delignifying and bleaching chemically digested lignocellulose-containing pulp in which pulp is treated with a complexing agent at a pH ranging from 3.1. to 9.0. The complexing agent removes the metal ions detrimental to the subsequent ozone bleaching. After ozone bleaching, the pulp is bleached with a peroxide-containing compound at a pH from about 8 up to about 12. The amount of complexing agent is sufficient to maintain a high pulp viscosity. The complexing agent is primarily chosen from nitrogenous polycarboxylic acids. EDTA and DTPA are mentioned as suitable complexing agents.

[0006] US 6706143 B1 discloses a method for bleaching a digested kraft pulp in which the pulp is treated with a chelating agent followed by peroxide bleaching. EDTA and DTPA and alkali and alkaline earth salts thereof are mentioned as suitable chelating agents. Substantial improvement in viscosity retention is reported.

[0007] EP 0402335 A2 discloses a process for bleaching chemically delignified lignocellulose-containing pulp, adapted to render more efficient a peroxide-containing treatment step, by treating the pulp with a complexing agent before the peroxide step, characterised in that the trace metal profile of the pulp is altered by treatment with a complexing agent, there being no sulphite present, at a pH in the range from 3.1 up to 9.0 and at a temperature in the range from 10°C up to 100°C, whereupon, in a subsequent step, the treatment with a peroxide-containing substance is carried out at a pH in the range from 7 up to 13, this two-step treatment being carried out at an optional position in the bleaching sequence applied to the pulp.

[0008] EP 1375734 A1 discloses A process for preparing bleached sulfite chemical pulp, comprising the following steps: delignification of chips of a lignocellulosic material in a sulfite pulping process until the kappa number (residual lignin content) of said material has reached values below 50, and bleaching of the fibrous material thus obtained by means of a bleaching sequence which is performed exclusively with chlorine-free reagents and comprises at least a first bleaching step with a chlorine-free oxidant in the presence of a base.

[0009] Unwanted metal ions can be removed from peroxide bleaching solution with different chelating agents, like EDTA and DTPA. However, these chelating agents are more effective in lower pH and thus chelation is typically done prior to peroxide bleaching step, which increases the total needed bleaching steps.

[0010] Peroxide bleaching is carried out in alkaline conditions. In these conditions, silica originally present in a lignocellulosic material is dissolved from the pulp. Silica forms water-insoluble precipitates negatively affecting the bleaching.

[0011] The suitability of cellulose pulps in various application depends on the viscosity of the pulps, among others. In certain applications high viscosity cellulose pulps are desired, while in some applications low viscosity pulps are more efficient.

[0012] We have now found that the viscosity of bleached pulps can be controlled by using a chelating agent. Also, the negative influence of impurities, such as heavy metals and silica, on peroxide bleaching efficiency of pulp is avoided.BRIEF DESCRIPTION OF THE INVENTION

[0013] An object of the present invention is to provide a method of controlling viscosity in peroxide bleaching of a cellulose pulp, comprising treating the cellulose pulp with disodium 3,3'-((2-ethylhexyl)azanediyl) dipropionate as a chelating agent to chelate metals present in the pulp, peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

[0014] It was found that the present invention provides an efficient method for the production of peroxide bleach pulp with both low viscosity and high viscosity in a controlled manner. This is achieved by using a chelating agent which stabilizes the peroxide bleaching process and protects the pulp from undergoing adverse effects due to presence of impurities. In addition, when low viscosity bleached pulps are desired, the viscosity may be reduced in a controlled manner by means of a copper compound after using the chelating agent during the bleaching.

[0015] High viscosity bleached pulp is desired, for example, for the production of paper and packaging products. Dissolving bleached pulp is a high-grade cellulose with a high content of cellulose and with a high cellulose purity exhibiting both high and low viscosity.DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention provides a method of controlling viscosity in peroxide bleaching of a cellulose pulp, comprising treating the cellulose pulp with disodium 3,3'-((2-ethylhexyl)azanediyl) dipropionate as a chelating agent to chelate metals present in the pulp, peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

[0017] In an embodiment of the method, the cellulose pulp is obtained using an organosolv pulping method. In organosolv pulping, lignin is extracted from a lignocellulosic material by using an organic solvent as a cooking liquor. A major portion of lignin, hemicellulose and extractives dissolves into the cooking liquor while cellulose is recovered in a solid fraction. Various organic solvents including alcohols and acids and combinations thereof have been used in the organosolv pulping. A cooking liquor may contain, for example, formic acid and acetic acid, furan compound(s) including furfural, and water.

[0018] Any lignocellulosic material suitable for organosolv pulping can be used in the invention. The lignocellulosic material may be, for example, softwood or hardwood, such as birch. Herbaceous plants, such as common reed or reed canary grass pulp, may also be used. Further, agricultural waste materials, such as straw, is applicable. In an embodiment, the lignocellulosic material is biomass of birch, wheat straw, rice straw or bagasse, without limiting thereto.

[0019] In the present invention, 3,3'-((2-ethylhexyl)azanediyl)dipropionate having a formula below is used as a chelating agent.

[0020] The chelating agent stabilizes hydrogen peroxide by chelating metal ions catalyzing radical formation. The agent also prevents formation of silica-lignin-extractives agglomerates by enhancing the solubility of extractives. The amount of the chelating agent added the cellulose pulp varies in the range of about 0.002 wt-% to about 1 wt-% calculated as 100% pure compound, based on the weight of dry cellulose pulp. In an embodiment, the chelating agent is added in an amount of 0.012 wt-% based on dry cellulose pulp.

[0021] In an embodiment, the method of the invention involves one peroxide bleaching stage in which the treatment of the cellulose pulp with the chelating agent and the peroxide bleaching are carried out simultaneously at a pH in the range of about 9.5 to about 13. In this embodiment, high viscosity bleached pulp is obtained which is suitable for the productions of paper and packaging products. Test B of Table 1 describes this embodiment.

[0022] In an embodiment, cellulose pulp that is treated with the chelating agent, which can also be an alkaline peroxide stage, is washed with water, or preferably an alkaline water, to remove the chelated metals from the pulp. The alkaline agent may be sodium hydroxide, for example. The pH of the chelating agent-containing bleaching stage is in the range of 10.7 to about 13. The washed pulp can be then subjected to a peroxide bleaching stage at a pH in the range of about 9.5 to about 13. In this embodiment, high viscosity bleached dissolving pulp with high cellulose purity is obtained.

[0023] In a further embodiment, the method involves two peroxide bleaching stages. In this embodiment, cellulose pulp is first treated with the chelating agent and then washed with an alkaline agent to remove the chelated metals from the pulp. After the washing, the pulp is subjected to a first peroxide bleaching stage at a pH in the range of about 10.7 to about 13. After the first peroxide bleaching stage, a second peroxide bleaching is carried out at a pH in the range of 10.7 to about 13 during which a copper compound is added. The viscosity of the bleached pulp is decreased in a controlled way. Low viscosity bleached dissolving pulp is obtained. Test E of Table 1 describes this embodiment.

[0024] The bleaching stage(s) is / are carried out during of 1 to 6 hours. In an embodiment, the bleaching of the cellulose pulp is performed for 4 hours.

[0025] In an embodiment, the copper compound is a water soluble copper salt, such as CuSO 4 .

[0026] In an embodiment, the copper compound as a pure copper is added in an amount of about 0.0001% to about 0.01%, calculated as pure copper, based on the weight of dry cellulose pulp.

[0027] Thus, in an aspect the invention provides a method for retaining the viscosity of a peroxide bleached cellulose pulp in which the cellulose pulp is treated with disodium 3,3'-((2-ethylhexyl)azanediyl)dipropionate as a chelating agent to chelate metals present in the pulp, followed by peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

[0028] In another aspect, the invention provides a method for decreasing the viscosity of a peroxide bleached cellulose pulp in which the cellulose pulp is treated with disodium 3,3'-((2-ethylhexyl)azanediyl)dipropionate as a chelating agent to chelate metals present in the pulp, the chelated metals are removed by washing the pulp with an alkaline agent, the washed pulp is peroxide bleached in a first peroxide stage at a pH of about 10.7 to about 13, followed by a second peroxide bleaching stage at a pH of about 9.5 to about 13 in which a copper compound is added.

[0029] The following example illustrates the invention without limiting the invention thereto.Example 1

[0030] Birch pulp was peroxide bleached at a temperature of 85°C for about 4 hours. The chelating agent was added to a first bleaching stage. The chelating agent is washed out before a second bleaching stage is carried out. The conditions under the peroxide bleaching are given in Table 1.

[0031] Viscosity is measured according to ISO standard 5351.

[0032] CuSO 4 is added as a water soluble salt. The amount of the copper salt is given as pure copper based on the weight of dry cellulose pulp.

[0033] The amount of the chelating agent is given as 100% pure compound based on the weight of dry cellulose pulp. Table 1. Chemicals and bleaching conditionsTest ATest BTest CTest DTest ETest FH 2 O 2 , %555555NaOH, %8.358.358.358.358.358.35Cu, %--0.0001-0.00010.001Chelating agent, %0.0120.0120.012---Consistency, %101010101010pH; beginning12.112.112.112.312.012.0pH; end12.110.710.810.810.810.9Cellulose yield, %85.286.985.287.386.985.3Viscosity, mg / l1127111810201085969716

[0034] The results show that the addition of the chelating agent during the peroxide bleaching prevents the decrease of the viscosity of the pulp compared with peroxide bleaching without the chelating agent. The results further show that copper decreases the viscosity of the pulp when added during the peroxide bleaching.

Examples

example 1

Example 1

[0030]Birch pulp was peroxide bleached at a temperature of 85°C for about 4 hours. The chelating agent was added to a first bleaching stage. The chelating agent is washed out before a second bleaching stage is carried out. The conditions under the peroxide bleaching are given in Table 1.

[0031]Viscosity is measured according to ISO standard 5351.

[0032]CuSO 4 is added as a water soluble salt. The amount of the copper salt is given as pure copper based on the weight of dry cellulose pulp.

[0033]The amount of the chelating agent is given as 100% pure compound based on the weight of dry cellulose pulp.

Table 1.

Chemicals and bleaching conditions

Test ATest BTest CTest DTest ETest F

H 2 O 2 , %555555

NaOH, %8.358.358.358.358.358.35

Cu, %--0.0001-0.00010.001

Chelating agent, %0.0120.0120.012---

Consistency, %101010101010

pH; beginning12.112.112.112.312.012.0

pH; end12.110.710.810.810.810.9

Cellulose yield, %85.286.985.287.386.985.3

Viscosity, mg / l1127111810201085969716

[0034]T...

FIELD OF THE INVENTION

[0001] The present invention relates to peroxide bleaching of a cellulose pulp. More particularly, the invention relates to a method for controlling the viscosity of a cellulose pulp during peroxide bleaching.BACKGROUND OF THE INVENTION

[0002] The purpose of bleaching of cellulose pulp is to remove residual lignin still present in the pulp after delignification of a lignocellulosic material. Bleaching may be carried out, e.g., with oxygen-containing chemicals including hydrogen peroxide. However, heavy metals, especially copper ions, included in the pulp may cause problems in peroxide bleaching process. Heavy metals react with cellulose breaking it down and thus decreasing viscosity of cellulose.

[0003] WO 99 / 25919 A1 discloses a process for bleaching chemical pulp in which pulp is peroxide bleached and treated with a chelating agent to bind heavy metals to a chelate complex. The chelating agent is selected from N-bis-[(1,2-dicarboxylethoxy)-ethyl]-amine, N-bis-[(1,2-dicarboxylethoxy)-ethyl]-aspartic acid and N-tris-[(1,2-dicarboxylethoxy)-ethyl]-amine and alkali metal and earth alkali metals salts thereof.

[0004] WO 96 / 37654 A1 discloses a method of delignifying / bleaching a delignified / prebleached lignin-containing cellulose in which the cellulose pulp is treated in one and the same stage with a complexing agent, molybdenum-containing substance and peroxide bleaching agent. The complexing agent may be ethylene diamine tetraacetic acid (EDTA) or diethylene triamine pentaacetic acid (DTPA). The pH during the treatment is 2-7.

[0005] US 5658429 discloses a process for delignifying and bleaching chemically digested lignocellulose-containing pulp in which pulp is treated with a complexing agent at a pH ranging from 3.1. to 9.0. The complexing agent removes the metal ions detrimental to the subsequent ozone bleaching. After ozone bleaching, the pulp is bleached with a peroxide-containing compound at a pH from about 8 up to about 12. The amount of complexing agent is sufficient to maintain a high pulp viscosity. The complexing agent is primarily chosen from nitrogenous polycarboxylic acids. EDTA and DTPA are mentioned as suitable complexing agents.

[0006] US 6706143 B1 discloses a method for bleaching a digested kraft pulp in which the pulp is treated with a chelating agent followed by peroxide bleaching. EDTA and DTPA and alkali and alkaline earth salts thereof are mentioned as suitable chelating agents. Substantial improvement in viscosity retention is reported.

[0007] EP 0402335 A2 discloses a process for bleaching chemically delignified lignocellulose-containing pulp, adapted to render more efficient a peroxide-containing treatment step, by treating the pulp with a complexing agent before the peroxide step, characterised in that the trace metal profile of the pulp is altered by treatment with a complexing agent, there being no sulphite present, at a pH in the range from 3.1 up to 9.0 and at a temperature in the range from 10°C up to 100°C, whereupon, in a subsequent step, the treatment with a peroxide-containing substance is carried out at a pH in the range from 7 up to 13, this two-step treatment being carried out at an optional position in the bleaching sequence applied to the pulp.

[0008] EP 1375734 A1 discloses A process for preparing bleached sulfite chemical pulp, comprising the following steps: delignification of chips of a lignocellulosic material in a sulfite pulping process until the kappa number (residual lignin content) of said material has reached values below 50, and bleaching of the fibrous material thus obtained by means of a bleaching sequence which is performed exclusively with chlorine-free reagents and comprises at least a first bleaching step with a chlorine-free oxidant in the presence of a base.

[0009] Unwanted metal ions can be removed from peroxide bleaching solution with different chelating agents, like EDTA and DTPA. However, these chelating agents are more effective in lower pH and thus chelation is typically done prior to peroxide bleaching step, which increases the total needed bleaching steps.

[0010] Peroxide bleaching is carried out in alkaline conditions. In these conditions, silica originally present in a lignocellulosic material is dissolved from the pulp. Silica forms water-insoluble precipitates negatively affecting the bleaching.

[0011] The suitability of cellulose pulps in various application depends on the viscosity of the pulps, among others. In certain applications high viscosity cellulose pulps are desired, while in some applications low viscosity pulps are more efficient.

[0012] We have now found that the viscosity of bleached pulps can be controlled by using a chelating agent. Also, the negative influence of impurities, such as heavy metals and silica, on peroxide bleaching efficiency of pulp is avoided.BRIEF DESCRIPTION OF THE INVENTION

[0013] An object of the present invention is to provide a method of controlling viscosity in peroxide bleaching of a cellulose pulp, comprising treating the cellulose pulp with disodium 3,3'-((2-ethylhexyl)azanediyl) dipropionate as a chelating agent to chelate metals present in the pulp, peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

[0014] It was found that the present invention provides an efficient method for the production of peroxide bleach pulp with both low viscosity and high viscosity in a controlled manner. This is achieved by using a chelating agent which stabilizes the peroxide bleaching process and protects the pulp from undergoing adverse effects due to presence of impurities. In addition, when low viscosity bleached pulps are desired, the viscosity may be reduced in a controlled manner by means of a copper compound after using the chelating agent during the bleaching.

[0015] High viscosity bleached pulp is desired, for example, for the production of paper and packaging products. Dissolving bleached pulp is a high-grade cellulose with a high content of cellulose and with a high cellulose purity exhibiting both high and low viscosity.DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention provides a method of controlling viscosity in peroxide bleaching of a cellulose pulp, comprising treating the cellulose pulp with disodium 3,3'-((2-ethylhexyl)azanediyl) dipropionate as a chelating agent to chelate metals present in the pulp, peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

[0017] In an embodiment of the method, the cellulose pulp is obtained using an organosolv pulping method. In organosolv pulping, lignin is extracted from a lignocellulosic material by using an organic solvent as a cooking liquor. A major portion of lignin, hemicellulose and extractives dissolves into the cooking liquor while cellulose is recovered in a solid fraction. Various organic solvents including alcohols and acids and combinations thereof have been used in the organosolv pulping. A cooking liquor may contain, for example, formic acid and acetic acid, furan compound(s) including furfural, and water.

[0018] Any lignocellulosic material suitable for organosolv pulping can be used in the invention. The lignocellulosic material may be, for example, softwood or hardwood, such as birch. Herbaceous plants, such as common reed or reed canary grass pulp, may also be used. Further, agricultural waste materials, such as straw, is applicable. In an embodiment, the lignocellulosic material is biomass of birch, wheat straw, rice straw or bagasse, without limiting thereto.

[0019] In the present invention, 3,3'-((2-ethylhexyl)azanediyl)dipropionate having a formula below is used as a chelating agent.

[0020] The chelating agent stabilizes hydrogen peroxide by chelating metal ions catalyzing radical formation. The agent also prevents formation of silica-lignin-extractives agglomerates by enhancing the solubility of extractives. The amount of the chelating agent added the cellulose pulp varies in the range of about 0.002 wt-% to about 1 wt-% calculated as 100% pure compound, based on the weight of dry cellulose pulp. In an embodiment, the chelating agent is added in an amount of 0.012 wt-% based on dry cellulose pulp.

[0021] In an embodiment, the method of the invention involves one peroxide bleaching stage in which the treatment of the cellulose pulp with the chelating agent and the peroxide bleaching are carried out simultaneously at a pH in the range of about 9.5 to about 13. In this embodiment, high viscosity bleached pulp is obtained which is suitable for the productions of paper and packaging products. Test B of Table 1 describes this embodiment.

[0022] In an embodiment, cellulose pulp that is treated with the chelating agent, which can also be an alkaline peroxide stage, is washed with water, or preferably an alkaline water, to remove the chelated metals from the pulp. The alkaline agent may be sodium hydroxide, for example. The pH of the chelating agent-containing bleaching stage is in the range of 10.7 to about 13. The washed pulp can be then subjected to a peroxide bleaching stage at a pH in the range of about 9.5 to about 13. In this embodiment, high viscosity bleached dissolving pulp with high cellulose purity is obtained.

[0023] In a further embodiment, the method involves two peroxide bleaching stages. In this embodiment, cellulose pulp is first treated with the chelating agent and then washed with an alkaline agent to remove the chelated metals from the pulp. After the washing, the pulp is subjected to a first peroxide bleaching stage at a pH in the range of about 10.7 to about 13. After the first peroxide bleaching stage, a second peroxide bleaching is carried out at a pH in the range of 10.7 to about 13 during which a copper compound is added. The viscosity of the bleached pulp is decreased in a controlled way. Low viscosity bleached dissolving pulp is obtained. Test E of Table 1 describes this embodiment.

[0024] The bleaching stage(s) is / are carried out during of 1 to 6 hours. In an embodiment, the bleaching of the cellulose pulp is performed for 4 hours.

[0025] In an embodiment, the copper compound is a water soluble copper salt, such as CuSO 4 .

[0026] In an embodiment, the copper compound as a pure copper is added in an amount of about 0.0001% to about 0.01%, calculated as pure copper, based on the weight of dry cellulose pulp.

[0027] Thus, in an aspect the invention provides a method for retaining the viscosity of a peroxide bleached cellulose pulp in which the cellulose pulp is treated with disodium 3,3'-((2-ethylhexyl)azanediyl)dipropionate as a chelating agent to chelate metals present in the pulp, followed by peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

[0028] In another aspect, the invention provides a method for decreasing the viscosity of a peroxide bleached cellulose pulp in which the cellulose pulp is treated with disodium 3,3'-((2-ethylhexyl)azanediyl)dipropionate as a chelating agent to chelate metals present in the pulp, the chelated metals are removed by washing the pulp with an alkaline agent, the washed pulp is peroxide bleached in a first peroxide stage at a pH of about 10.7 to about 13, followed by a second peroxide bleaching stage at a pH of about 9.5 to about 13 in which a copper compound is added.

[0029] The following example illustrates the invention without limiting the invention thereto.Example 1

[0030] Birch pulp was peroxide bleached at a temperature of 85°C for about 4 hours. The chelating agent was added to a first bleaching stage. The chelating agent is washed out before a second bleaching stage is carried out. The conditions under the peroxide bleaching are given in Table 1.

[0031] Viscosity is measured according to ISO standard 5351.

[0032] CuSO 4 is added as a water soluble salt. The amount of the copper salt is given as pure copper based on the weight of dry cellulose pulp.

[0033] The amount of the chelating agent is given as 100% pure compound based on the weight of dry cellulose pulp. Table 1. Chemicals and bleaching conditionsTest ATest BTest CTest DTest ETest FH 2 O 2 , %555555NaOH, %8.358.358.358.358.358.35Cu, %--0.0001-0.00010.001Chelating agent, %0.0120.0120.012---Consistency, %101010101010pH; beginning12.112.112.112.312.012.0pH; end12.110.710.810.810.810.9Cellulose yield, %85.286.985.287.386.985.3Viscosity, mg / l1127111810201085969716

[0034] The results show that the addition of the chelating agent during the peroxide bleaching prevents the decrease of the viscosity of the pulp compared with peroxide bleaching without the chelating agent. The results further show that copper decreases the viscosity of the pulp when added during the peroxide bleaching.

Examples

example 1

Example 1

[0030]Birch pulp was peroxide bleached at a temperature of 85°C for about 4 hours. The chelating agent was added to a first bleaching stage. The chelating agent is washed out before a second bleaching stage is carried out. The conditions under the peroxide bleaching are given in Table 1.

[0031]Viscosity is measured according to ISO standard 5351.

[0032]CuSO 4 is added as a water soluble salt. The amount of the copper salt is given as pure copper based on the weight of dry cellulose pulp.

[0033]The amount of the chelating agent is given as 100% pure compound based on the weight of dry cellulose pulp.

Table 1.

Chemicals and bleaching conditions

Test ATest BTest CTest DTest ETest F

H 2 O 2 , %555555

NaOH, %8.358.358.358.358.358.35

Cu, %--0.0001-0.00010.001

Chelating agent, %0.0120.0120.012---

Consistency, %101010101010

pH; beginning12.112.112.112.312.012.0

pH; end12.110.710.810.810.810.9

Cellulose yield, %85.286.985.287.386.985.3

Viscosity, mg / l1127111810201085969716

[0034]T...

Claims

1. A method of controlling viscosity in peroxide bleaching of a cellulose pulp, comprising - treating the cellulose pulp with disodium 3,3'-((2-ethylhexyl)azanediyl)dipropionate as a chelating agent to chelate metals present in the pulp, - peroxide bleaching the cellulose pulp in at least one stage at a pH of about 9.5 to about 13.

2. The method of claim 1, wherein the amount of the chelating agent added the cellulose pulp varies in the range of about 0.002 wt-% to about 1 wt-% calculated as 100% pure compound, based on the weight of dry cellulose pulp.

3. The method of claim 2 wherein the amount of the chelating agent is 0.012 wt-% calculated as 100% pure compound, based on the weight of dry cellulose pulp.

4. The method of any one of the preceding claims, wherein the treatment with the chelating agent and the peroxide bleaching are carried out simultaneously.

5. The method of any one of claims 1-3, wherein the cellulose pulp treated with the chelating agent is washed with an alkaline agent to remove the chelated metals from the pulp followed by a first peroxide bleaching stage at a pH of about 10.7 to about 13.

6. The method of claim 5, comprising further peroxide bleaching stages.

7. The method of claims 5 and 6, wherein after the first peroxide bleaching stage, a second peroxide bleaching is carried out at a pH in the range of about 9.5 to about 13 during which a copper compound is added.

8. The method of claim 7, wherein the copper compound is added in an amount of about 0.0001% to about 0.01% based on dry bleached cellulose pulp.

9. The method of any one of the preceding claims, wherein the cellulose pulp is obtained from organosolv pulping.

10. A method of claim 1 for decreasing the viscosity of a peroxide bleached cellulose pulp, comprising - removing the chelated metals by washing the cellulose pulp with an alkaline agent, - peroxide bleaching the washed pulp in a first stage at a pH of about 10.7 to about 13, - subjecting the peroxide bleached pulp from the first stage to a second peroxide bleaching stage at a pH of about 9.5 to about 13, - adding a copper compound in the second peroxide bleaching stage.

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