A method for reducing the viscosity of dissolving pulp

Abstract

A method for reducing the viscosity of dissolving pulp is disclosed. The method for reducing the viscosity of dissolving pulp comprises: a) producing cellulose pulp by a kraft cooking with a prehydrolysis step or a soda cooking with a prehydrolysis step, wherein the obtained cellulose pulp has a cellulose content of at least 88%, b) bleaching the obtained cellulose pulp, wherein the bleaching comprises at least one acidic peroxide stage, wherein the cellulose pulp is treated with a mineral acid and a hydrogen peroxide, and at least one other bleaching stage.

Description

[0001] A METHOD FOR REDUCING THE VISCOSITY OF DISSOLVING PULP

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method for reducing the viscosity of dissolving pulp .

[0004] BACKGROUND

[0005] Dissolving wood pulp is bleached pulp that has a high cellulose content . Viscosity is one of the most important properties of dissolving pulp . A typical commercial dissolving pulp has an intrinsic viscosity of less than 700 ml / g and most of the cases the viscosity is in the range of 400 - 600 ml / g . In one typical method, the final viscosity of the dissolving pulp is controlled by bleaching with ozone . However, the ozone stage requires high investment costs , especially in an existing bleaching plant . Thus , the inventors have recogni zed the need for an effective and low-cost method for reducing the viscosity of dissolving pulp .

[0006] SUMMARY

[0007] A method for reducing the viscosity of dissolving pulp is disclosed . The method for reducing the viscosity of dissolving pulp comprises : a) producing cellulose pulp by a kraft cooking with a prehydrolysis step or a soda cooking with a prehydrolysis step, wherein the obtained cellulose pulp has a cellulose content of at least 88 % , b) bleaching the obtained cellulose pulp, wherein the bleaching comprises at least one acidic peroxide stage , wherein the cellulose pulp is treated with a mineral acid and a hydrogen peroxide , and at least one other bleaching stage . DETAILED DESCRIPTION

[0008] The present disclosure relates to a method for reducing the viscosity of dissolving pulp, wherein the method comprises : a) producing cellulose pulp by a kraft cooking with a prehydrolysis step or a soda cooking with a prehydrolysis step, wherein the obtained cellulose pulp has a cellulose content of at least 88 % , b) bleaching the obtained cellulose pulp, wherein the bleaching comprises at least one acidic peroxide stage , wherein the cellulose pulp is treated with a mineral acid and a hydrogen peroxide , and at least one other bleaching stage .

[0009] The prehydrolysis step may be used to minimi ze hemicellulose content in the cellulose pulp by heating wood chips in between 120 ° C - 170 ° C with steam or water, to de-esterify the acetyl groups present in the hemicellulose . After the prehydrolysis step, the cellulose pulp may be cooked with kraft or soda cooking process using white liquor or other alkali source . Viscosity after the cooking is in the range of 800 - 1500 ml / g . All wood species which are used for paper pulp production can be used in this method .

[0010] After cooking, the cellulose pulp may be subj ected to oxygen delignification . The goal of oxygen delignification is to reduce the lignin content of unbleached pulp before using more expensive bleaching chemicals . After bleaching the commercial dissolving pulp has an intrinsic viscosity of less than 700 ml / g and most of the cases the viscosity is in the range of 400 - 600 ml / g .

[0011] In one embodiment , the cellulose pulp is produced by a kraft cooking with a prehydrolysis step . In one embodiment , the cellulose pulp is produced by a soda cooking with a prehydrolysis step .

[0012] In one embodiment , the viscosity of the cellulose pulp after cooking is in the range of 800 - 1500 ml / g. In one embodiment, the viscosity of the cellulose pulp after bleaching is less than 700 ml / g, or in the range of 400-600 ml / g.

[0013] In one embodiment, the cellulose pulp has a cellulose content of at least 90%, or at least 92%, or at least 95%.

[0014] In one embodiment, after kraft cooking and before bleaching the cellulose pulp is subjected to oxygen delignification.

[0015] In one embodiment, the mineral acid is hydrochloric acid or sulphuric acid.

[0016] In one embodiment, the mineral acid is used in amount of 1 - 5 kg / adt, or 2 - 4 kg / adt. The unit "kg / adt" should be understood in this specification, unless otherwise stated, as kg / air dried ton of pulp.

[0017] In one embodiment, the hydrogen peroxide is used in amount of 1 - 20 kg / adt, or 2-10 kg / adt, or 2 - 6 kg / adt, or 3 - 5 kg / adt.

[0018] In one embodiment, the mineral acid and the hydrogen peroxide are fed into the reactor simultaneously. In one embodiment, the mineral acid is first fed into the reactor and then hydrogen peroxide is fed into the reactor. In one embodiment, the hydrogen peroxide is first fed into the reactor and then the mineral acid is fed into the reactor.

[0019] In one embodiment, the bleaching consists of acidic peroxide stage and one other bleaching stage. In one embodiment, the bleaching consists of acidic peroxide stage and two other bleaching stages. In one embodiment, the bleaching stages can be carried out in any order .

[0020] In one embodiment, the other bleaching stage is P, Eop, Ep, Eo, D, H, C, or A stage. In one embodiment, the bleaching stage is a combination stage D / C or A / D.

[0021] The bleaching stage "P" should be understood in this specification, unless otherwise stated, as an alkaline stage with hydrogen peroxide (H2O2 ) . The bleaching stage "Eop" should be understood in this specification, unless otherwise stated, as an alkaline extraction bleaching stage using sodium hydroxide with subsequent addition of oxygen and hydrogen peroxide solution as a reinforcing agent . The bleaching stage "Ep" should be understood in this specification, unless otherwise stated, as an extraction stage using NaOH with subsequent addition of H2O2 solution as a reinforcing agent . The bleaching stage "Eo" should be understood in this specification, unless otherwise stated, as an extraction stage using NaOH with subsequent addition of gaseous oxygen as a reinforcing agent . The bleaching stage "D" should be understood in this specification, unless otherwise stated, as a chlorine dioxide stage (CIO2 ) . The bleaching stage "Z" should be understood in this specification, unless otherwise stated, as an ozone stage (O3 ) . The bleaching stage "Px" should be understood in this specification, unless otherwise stated, as an acidic peroxide stage , wherein cellulose pulp is treated with a mineral acid and a hydrogen peroxide . The bleaching stage "A / Z" should be understood in this specification, unless otherwise stated, as an acidic wash stage followed by ozone stage . The bleaching stage "Da" should be understood in this specification, unless otherwise stated, as a chlorine dioxide stage with an acidic stage . The bleaching stage "Do" should be understood in this specification, unless otherwise stated, as a first chlorine dioxide stage . The bleaching stage "A" should be understood in this specification, unless otherwise stated, as an acidic wash stage . The bleaching stage "C" should be understood in this specification, unless otherwise stated, as a chlorine stage (CI2 ) . The bleaching stage "H" should be understood in this specification, unless otherwise stated, as a hypochlorate , NaOCl , Ca (OCl ) 2 stage . The bleaching stage "D / C" should be understood in this specification, unless otherwise stated, as a chlorine and chlorine dioxide stage . The bleaching stage "A / D" should be understood in this specification, unless otherwise stated, as an acidic wash stage followed by chlorine dioxide stage .

[0022] In one embodiment , the bleaching is performed in the following sequence : Px-Eop, or Px-D, or Px-Ep, or any combination of Px-stage and D-stage and alkaline peroxide stage . The bleaching sequence "Px-Eop" should be understood in this specification, unless otherwise stated, as the bleaching sequence of acidic peroxide stage ( Px) and alkaline extraction bleaching stage using sodium hydroxide with subsequent addition of oxygen and hydrogen peroxide solution as a reinforcing agent (Eop) . The bleaching sequence "Px-D" should be understood in this specification, unless otherwise stated, as the bleaching sequence of acidic peroxide stage ( Px) and chlorine dioxide stage ( D) . The bleaching sequence "Px- Ep" should be understood in this specification, unless otherwise stated, as the bleaching sequence of acidic peroxide stage ( Px) and extraction stage using NaOH with subsequent addition of hydrogen peroxide solution as a reinforcing agent (Ep) . The expression "bleaching sequence" should be understood in this specification, unless otherwi se stated, as a combination of individual bleaching stages .

[0023] In one embodiment , the pH during the acidic peroxide stage is in the range of 1 . 5 - 5 . 5 , or 2 - 4 . 5 . In one embodiment , the temperature during the acidic peroxide stage is in the range of 50 - 100 °C, or 60 - 95 ° C . In one embodiment , the process time in the acidic peroxide stage is 20 - 240 min, or 60 - 120 min . In one embodiment , the pulp consistency during the acidic peroxide stage is in the range of 5 - 15% , or 7 - 12 % , calculated as dry content .

[0024] In one embodiment , the viscosity decrease of the pulp in the acidic peroxide stage is at least 50 ml / g, or at least 100 ml / g, or at least 120 ml / g when calculated following the formula of : viscosity decrease in the acidic peroxide stage (ml / g) = viscosity before the acidic peroxide stage (ml / g) - viscosity after the acidic peroxide stage (ml / g) .

[0025] In one embodiment , the viscosity decrease of the pulp in the acidic peroxide stage is in the range of 100 ml / g - 150 ml / g . In one embodiment , the viscosity decrease of the pulp in the acidic peroxide stage is in the range of 100 ml / g - 200 ml / g . Viscosity can be measured according to standard I SO 5351 : 2010 but not limited to this method and all same type of viscosity measurement methods can be used as a reference .

[0026] The acidic peroxide stage as described in this application has the added utility of reducing the viscosity of dissolving pulp . Further, the inventors surprisingly found out that the bleaching sequence of acidic peroxide stage and alkaline extraction bleaching stage using sodium hydroxide with subsequent addition of oxygen and hydrogen peroxide solution as a reinforcing agent efficiently reduced the viscosity of dissolving pulp . Further, the acidic peroxide stage as described in this application has the added utility of reducing the content of metals in the dissolving pulp .

[0027] The term "comprising" is used in this specification to mean including the feature ( s ) or act ( s ) followed thereafter, without excluding the presence of one or more additional features or acts .

[0028] EXAMPLES

[0029] The description below discloses some embodiments in such a detail that a person skilled in the art is able to use the method based on the disclosure . Not all steps of the embodiments are discussed in detail , as many of the steps will be obvious for the person skilled in the art based on this specification.

[0030] Example 1

[0031] In this example, cellulose pulp was bleached in a Px stage. In addition, comparative examples were made by bleaching the cellulose pulp in an A / Z stage and in a Da stage. The cellulose pulp used in these examples was prehydrolysed oxygen-delignified kraft pulp.

[0032] The properties of the prehydrolysed oxygendelignified kraft pulp are presented in the below table:

[0033] Viscosity was measured according to standard ISO 5351:2010. ISO Brightness was measured according to standard ISO 2470-1:2016.

[0034] In the Px stage, the cellulose pulp was treated with sulfuric acid and hydrogen peroxide. In the Da stage, the cellulose pulp was treated with chlorine dioxide and sulfuric acid. In the A / Z stage, the cellulose pulp was treated with sulfuric acid and ozone. The bleaching conditions are presented in the below table.

[0035]

[0036] It can be seen from the above table, that the acidic peroxide stage reduced the viscosity by 141 ml / g, and the decrease in the viscosity is greater in the acidic peroxide stage compered to A / Z and Da stages.

[0037] Example 2

[0038] In this example, cellulose pulp was first bleached in the chlorine dioxide stage (DO) and then further bleached in the acidic peroxide stage. In addition, a comparative example was made by bleaching the cellulose pulp first in the DO stage and then in the Eop stage. The cellulose pulp used in these examples was prehydrolysed oxygen-delignified kraft pulp.

[0039] The properties of the prehydrolysed oxygen-delignified kraft pulp are presented in the below table:

[0040] Viscosity was measured according to standard ISO 5351:2010. ISO Brightness was measured according to standard ISO 2470-1:2016. In the Px stage, the cellulose pulp was treated with sulfuric acid and hydrogen peroxide . In the Eop stage , the cellulose pulp was treated with sodium hydroxide with subsequent addition of oxygen and hydrogen peroxide solution as a reinforcing agent .

[0041] The bleaching conditions are presented in the below table .

[0042] It can be seen from the above table , that the acidic peroxide stage reduced the viscosity by 147 ml / g, and the decrease in the viscosity is greater in the acidic peroxide stage compared to the Eop stage .

Claims

CLAIMS1. A method for reducing the viscosity of dissolving pulp, wherein the method comprises: a) producing cellulose pulp by a kraft cooking with a prehydrolysis step or a soda cooking with a prehydrolysis step, wherein the obtained cellulose pulp has a cellulose content of at least 88%, b) bleaching the obtained cellulose pulp, wherein the bleaching comprises at least one acidic peroxide stage, wherein the cellulose pulp is treated with a mineral acid and a hydrogen peroxide, and at least one other bleaching stage.

2. The method of claim 1, wherein the mineral acid is hydrochloric acid or sulphuric acid.

3. The method of any one of the preceding claims, wherein the mineral acid is used in amount of 1- 5 kg / adt, or 2 - 3 kg / adt.

4. The method of any one of the preceding claims, wherein the hydrogen peroxide is used in amount of 1 - 20 kg / adt, or 2-10 kg / adt, or 2 - 6 kg / adt, or 3- 5 kg / adt.

5. The method of any one of the preceding claims, wherein the other bleaching stage is P, Eop, Ep, Eo, D, H, C, or A stage.

6. The method of any one of the preceding claims, wherein the bleaching is performed in the following sequence: Px-EOP, or Px-D, or Px-EP, or any combination of Px-stage and D-stage and alkaline peroxide stage .

7. The method of any one of the preceding claims, wherein the pH during the acidic peroxide stage is in the range of 1.5 - 5.5, or 2 - 4.

58. The method of any one of the preceding claims, wherein the temperature during the acidic peroxide stage is in the range of 50 - 100 °C, or 60 - 95 °C.

9. The method of any one of the preceding claims, wherein the process time in the acidic peroxide stage is 20 - 240 min, or 60 - 120 min.

10. The method of any one of the preceding claims, wherein the pulp consistency during the acidic peroxide stage is in the range of 5 - 15%, or 7 - 12%, calculated as dry content.

11. The method of any one of the preceding claims, wherein the viscosity decrease of the pulp in the acidic peroxide stage is at least 50 ml / g, or at least 100 ml / g, or at least 120 ml / g when calculated following the formula of: viscosity decrease in the acidic peroxide stage (ml / g) = viscosity before the acidic peroxide stage (ml / g) - viscosity after the acidic peroxide stage (ml / g) .

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