FIELD OF THE INVENTION

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

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 per oxide. 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.

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-[(l,2-dicar- boxylethoxy) -ethyl] -amine, N-bis-[(l,2-dicarboxylethoxy)-ethyl]-aspartic acid and N-tris-[(l,2-dicarboxylethoxy)-ethyl]-amine and alkali metal and earth alkali met als salts thereof.

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

US 5658429 discloses a process for delignifying and bleaching chemi cally digested lignocellulose-containing pulp in which pulp is treated with a com plexing 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 polycarbox- ylic acids. EDTA and DTPA are mentioned as suitable complexing agents.

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 suita ble chelating agents. Substantial improvement in viscosity retention is reported.

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.

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

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.

We have now found that the viscosity of bleached pulps can be con trolled 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

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)azane- diyljdipropionate 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.

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 ef fects 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.

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

The present invention provides a method of controlling viscosity in per oxide bleaching of a cellulose pulp, comprising

- treating the cellulose pulp with disodium 3,3'-((2-ethylhexyl)azane- diyljdipropionate 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.

In an embodiment of the method, the cellulose pulp is obtained using an organosolv pulping method. In organosolv pulping, lignin is extracted from a ligno- cellulosic material by using an organic solvent as a cooking liquor. A major portion of lignin, hemicellulose and extractives dissolves into the cooking liquor while cel lulose 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 com- pound(s) including furfural, and water.

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.

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

The chelating agent stabilizes hydrogen peroxide by chelating metal ions catalyzing radical formation. The agent also prevents formation of silica-lig- nin-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. 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.

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 pref erably 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-con taining 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.

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.

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.

In an embodiment, the copper compound is a water soluble copper salt, such as CuSCH.

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.

Thus, in an aspect the invention provides a method for retaining the vis cosity 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.

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 perox- ide 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.

The following example illustrates the invention without limiting the in vention thereto.

Example 1 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.

Viscosity is measured according to ISO standard 5351. CuS04 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.

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

Table 1.

The results show that the addition of the chelating agent during the per oxide bleaching prevents the decrease of the viscosity of the pulp compared with peroxide bleaching without the chelating agent. The results further show that cop per decreases the viscosity of the pulp when added during the peroxide bleaching.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The inven tion and its embodiments are not limited to the examples described above but may vary within the scope of the claims.