Title of Invention

PROCESS FOR BLEACHING LIGNOCELLULOSE-CONTAINING NON-WOOD PULP

Abstract

The present invention refers to a process for bleaching lignocellulose-containing pulp comprising pulp obtainable from lignocellulose-containing materials selected from the group consisting of non-wood with a peroxide-containing compound, where the pulp prior to the bleaching is treated with chlorine dioxide, characterised in that the pulp after the treatment with chlorine dioxide is treated in a separate stage with a chelating agent at a pH up to about 2.9 and washed between the chlorine dioxide treatment and chelating agent treatment.

Full Text

Process for bieachinc lianoceliuiose-Gontaininq non-wood pulp The present invention relates to a process for bleaching Jignocellulose-containing pulp comprising pulp obtainable from lignocellulose-containing material selected from the group consisting of non-wood material with a peroxide-containing compound, where the pulp prior to the bleaching is treated with chlorine dioxide whereby the pulp after the treatment with chlorine dioxide is treated in a separate stage with a chelating agent at a pH up to about 2.9 and washed between the chlorine dioxide treatment and chelating agent treatment. Background The present invention refers to the bleaching of pulp originating from non-wood material, in regions lacking significant wood sources and/or regions where non-wood material is available abundantly, e.g. India and China, the use of non-wood material for producing paper is not only economically feasible but also to a great extent used as raw material for producing paper and board. If the non-wood pulp is intended for products where a certain ievei of brightness is needed, such as for the production of paper, the non-wood pulp has to be bieached subsequent the pulping process. In order to reduce the content of environmentally harmful compounds in the bleach plant effluents certain bleaching chemicals should be avoided specifically the use of gaseous chlorine, albeit chlorine is an exceptionally selective and effective bleaching chemical Other bleaching chemicals which could be used instead of gaseous chlorine would, for example, be chlorine dioxide and peroxide-containing compounds. Therefore, one advantage with the present invention is to bleach a pulp originating from non-wood puip in a cost effective way, using environmentally friendly bleaching chemicals, i.e. without using gaseous chlorine, and, furthermore, to proyide a bleached non-wood pulp having high brightness and acceptable viscosity. Chlorine dioxide and peroxide-containing compounds have been extensively applied in bleaching sequences intended for bleaching hardwood and softwood pulp. An immense number of different bieaching sequences have been disclosed for bleaching wood puip comprising chlorine dioxide stages and peroxide stages. When bleaching pulp originating from non-wood material we have surprisingly found that of all innumerable sequences comprising inter alia chlorine dioxide and peroxide-containing compound, a specific bleaching process according to the claims renders exceptional results on pulp originating from non-wood material. Although just the specific positioning of the treatment stages of the present invention per se provides improved results on pulp originating from non-wood pulp, the present bieaching sequence of the invention was even more surprising as the invention is contradictory to the common general knowledge specifically with regard to bleaching of puip from wood materials. According to common knowledge when applying a separate complexing agent treatment in a bleaching sequence on softwood or hardwood prior to a peroxide bleaching stage, it has been demonstrated that a pH between 5 and 7 in the complexing agent treatment, renders optimal properties of hardwood and softwood puip, e.g. EP-A2-402335. In view of these facts it was very surprising to find out that a pH significantly lower than between 5 to 7 of the separate chelating agent treatment located prior to a peroxide stage renders the best bleaching properties while bleaching pulp originating from non-wood material. What is more, the non-wood pulp bleached according to the present invention is liable to minimal brightness reversion. Stilt further advantages are apparent below. Yet another advantage is to obtain a non-wood pulp having a high brightness using a compact and cost effective bleaching process containing only three treatment stages, washing stages not included. A further advantage with the present bleaching sequence is that a dewatering or washing stage after the chelating agent treatment stage even can be omitted without significantly impairing the pulp properties after the peroxide stage. The above advantages are obtained by the present bleaching process according to the claims. More specifically, the invention refers to a process for bleaching lignocellulose- containing pulp comprising puip obtainable from lignoceilulose-containing material selected from the group consisting of non-wood with a peroxide-containing agent, where the puip prior to the bleaching is treated with chlorine dioxide, whereby the pulp after the treatment with chlorine dioxide is treated in a separate stage with a chelating agent at a pH up to about 2.9 and washed between the chlorine dioxide treatment and chelating agent treatment. Invention The present process is designed to affectively bleach lignoceiluiose-containing pulp comprising pulp obtainable from lignoceilulose-containing materials selected from the group consisting of non-wood materials. Often, non-wood material is referred to as annual plant material such as agricultural crops and residues from agricultural crops. The word non-wood is a common definition in the technical field of the present invention and embraces in principle all lignoceilulose-containing material other than those characterised as wood material, e.g. softwood and hardwood. Materials falling within the term non-wood are e.g. bast fibres, grain hairs, grasses and leaf fibres. Examples of grasses are bamboo, corn, stalk, esparto, papyrus, reed (reed straw grass), different types of straw from wheat, rye, oat, rice, etc; sugar cane and bagasse, just to mention a few. Specimens of bast fibres are e.g. flax (seed flax), hemp, jute, kenaf, ramine, rosella and mixtures thereof, whereas cotton, cotton linters, kapok and mixtures thereof are suitable grain hair materials. Lastly, aloe, sisal and abaca are examples of materials falling under the class of leaf fibres. The present invention is preferably applied on lignocellulose-containing materials selected from the group consisting of reed and straw, suitably the group consisting of reed and wheat straw. One difference of non-wood compared to wood is the substantially higher amount of silica and ash in non-wood. Thus, non-wood pulp can be differentiated from wood pulp with respect to the content of silica. Accordingly, the bleaching process is suitably applied on lignocellulose- containing pulp comprising pulp containing at least 500 ppm Si/dry fibre, preferably at least 80G ppm Si/dry fibre, more preferably at least 1200 ppm Si/dry fibre, most preferably at least 1500 ppm Si/dry fibre. The silica content of the non-wood material was determined by XRF analysis. According to a preferred embodiment of the present invention the pulp used in the process comprises at least 75 % by weight of non-wood puip, suitably at least 85 % by weight. Preferably the pulp consists essentially of pulp obtainable from non-wood materials. The non-wood pulp is suitably obtained by applying chemical puiping processes. Chemical non-wood pulp relates to pulp digested in cooking processes such as the sulphate, sulphite, soda or organosol process or modifications of these processes with optional additives such as anthraquinone, e.g. soda anthraquinone (AQ) pulping process. Preferably, the fjgnoceilulose-containing non-wood pulp is produced by an alkaline pulping process. The chemical pulp obtainable from lignocellulose-containing materials selected from the group consisting of non-wood materials may be subjected to one or more oxygen treatment stages subsequent the pulping process. The inventive process can be applied to chemically digested non-wood pulp having an initial kappa number, i.e. the kappa number after digestion but before oxygen cielignification, in the range from about 5 Up to about 40, suitably from 8 up to 30, and . preferably from 8 up to 18. The referred kappa number is measured according to the SCAN- C 1:77 Standard Method. The chlorine dioxide added to the non-wood puip can be produced according to conventional techniques. Usually, bleaching plants designed to bleach non-wood pulps have a significantly smaller production rate of bleached puip than conventional bieaching plants for bieaching wood materials like hardwood and softwood. As the production rate of a bieaching plant decreases, the cost per tonne bleached puip normally increases. Accordingly, in order to provide a bleaching process for non-wood material which is economically feasible, the bleaching chemicals used should preferably be produced at a minimum of cost. Therefore, the chlorine dioxide applied in the present process is preferably produced by using a chlorine dioxide plant with a production rate of chlorine dioxide in the range from about 0.1 tonne CiC2/day up to about 12 tonnes C102/day. Such a chlorine dioxide production plant may consist of one or several production units. Advantageously, the pH of the produced chlorine dioxide should be preferably below 2, i.e. lower than in conventional processes. Examples of such suitable processes are SVP Pure™ or its modifications, The pulp consistency in the chlorine dioxide treatment siage is not critical and may be from about 1 up to 40% by weight, preferably from 3 up to 30% by weight and more preferably from about 5 up to 15 % by weight. The pH in the chlorine dioxide stage may vary considerably, but is normally in the range of 1.5 up to 5, suitably from 2 up to 3. Commonly, the pulp is treated with chlorine dioxide from 3 minutes up to 2 hour at temperatures ranging from 20 up to 90 °C. In order to obtain a sufficiently bleached pulp chlorine dioxide is suitably added at an amount corresponding to kappa factors from 0.05 up to 0.6, preferably from 0.1 up to 0.5 and more preferably from 0.2 up to 0.4. The kappa factor is the amount of active chlorine in % of dry pulp, divided by the kappa number of the pulp entering the bleaching stage. Subsequent the chlorine dioxide treatment stage, the non-wood pulp is washed. The term washing, as used in this process, relates to methods for displacing, more or less completely, the spent liquor in the pulp suspension to reduce its content of e.g. dissolved trace, metal ions in said suspension. The washing method may bring about an increase in the pulp concentration, for example by sucking off or pressing. The washing methods may also entail a reduction of the pulp concentration, e.g. by dilution with washing liquid. Washing also embrace combinations and sequences where the pulp concentration is alternately increased and reduced. The pH during the washing stage is suitably so chosen that no additional make-up chemicals, such as acids or alkaline compounds, have to be added into the subsequent chelating agent treatment. As indicated above, the chlorine dioxide stage is carried out at an acid or acidic pH. Ideally, the chlorine dioxide stage and the chelating agent stage can be carried out without any make-up chemicals added, as both stages can be performed at similar pH ranges. Hence, a further advantage with the present invention is that less make¬up chemicals need to be added, since the chlorine dioxide treatment and the chelating agent treatment are carried out at comparable pH ranges. After the washing treatment, the non-wood pulp is treated with a chelating agent in a separate, stage. According to the present invention, it has been shown that not oniy a specific pH range is of importance for the effectiveness of the process, but also the location of the addition of the chelating agent is important for achieving excellent bleaching results. By washing a non-wood pulp after a chlorine dioxide stage and subsequently treating the pulp with a chelating agent in a separate stage at a pH of up to about 2.9, followed optionally by a washing stage, the non-wood pulp suspension is rendered suitable for a subsequent bleaching with a I peroxide-containing compound. Suitable chelating agents are nitrogenous organic compounds, primarily nitrogenous polycarboxylic acids, nitrogenous polyphosphonic acids and nitrogenous polyalcohois. Preferred nitrogenous polycarboxylic acids are diethylenetriaminepentaacetic acid (DTPA), ethyienediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA). EDTA is the preferred nitrogenous polycarboxylic acid. Also other compounds can be used as chelating agents, such as polycarboxylic acids, suitably oxalic acid, citric acid or tartaric acid, or phosphonic acids like diethyienetriaminepeniaphosphonic acid. Further, such organic acids as are formed during the pulp treatment with, inter alia, chlorine-fee bleaching agents can also be used as chelating agents. The treatment with the chelating agent is suitably carried out at a pH in the range from about 1.5 up to 2.9, preferably from about 2 up to about 2.9, and most preferably from about 2.5 up to about 2.9. The temperature in the treatment with the chelating agent may have an influence on the bleaching efficiency. The temperature in the treatment with the chelating agent is suitably in the range from about 20 up to about 120 "C, preferably from about 40 up to about 100 °C, and most preferably from about 55 °C up to about 90 "C. The added amount of chelating agent depends, inter alia, on the type and amount of inorganic compounds in the non-wood pulp to be treated. The amount is further affected by the type of chelating agent as well as the conditions in the treatment such as temperature, residence time and pH. The added amount of chelating agent is suitably from about 0.1 kg up to about 10 kg per tonne pulp, calculated as 100% chelating agent Preferably, the amount is from 0.3 kg up to 5 kg per tonne puip and even more suitably from 0.5 kg up to 2.5 kg per tonne dry puip, calculated as 100% chelating agent The pulp concentration during the treatment with the chelating agent may range from about 1% by weight up to about 60% by weight, suitably from 2,0% up to 40% by weight, preferably from 3.0% up to 25% by weight, and most suitably from 5.0% up to 15% by weight The treatment period with the chelating agent is commonly from about 1 minute up to about 2 hours, preferably from 20 minutes up to 1 hour. The chelating agent stage is preferably carried out essentially in absence of chlorine dioxide, as essentially most of the chlorine dioxide is washed away by the washing stage. If exceptionally an acid is added to the treatment with the chelating agent, an add readily available in the pulp miil may be employed. Examples of suitable acids are inorganic acids such as sulphuric acid, nfejc add, hydrochloric acid or residual acid from a chlorine dioxide reactor, either added separately or in mixtures. After the chelating agent treatment the non-wood puip may be washed. However, bieaching efficiency is not greatly influenced if a washing treatment after the chelating agent treatment is omitted. Therefore, according to a preferred embodiment of the present invention there is no washing stage after the separate chelating agent treatment stage and before the treatment with a peroxide-containing compound. Washing efficiency may be given as the amount of liquid phase removed, as compared with the liquid phase present in the pulp suspension before washing. The total washing effi¬ciency is calculated as the sum of the efficiency in each washing stage. Thus, after a treatment stage, dewatering of the pulp suspension from, say, 10% to 25% pulp consistency and subsequent dilution with water to 10% pulp consistency gives a washing efficiency of 66.7%. Ater a subsequent washing stage in which the pulp is further diluted to 3% and then dewatered to 25%, the total washing efficiency is 96.9%. Washing efficiency specifically in the washing stage after the chelating agent treatment can be as low as about 25% without significantly impairing the pulp properties. A suitable washing efficiency range is form about 25% up to 100% and more preferably from about 25% up to about 75%. Subsequent the treatment wih a chelating agent the non-wood pulp is bleached with a peroxide-containing compound which can be an inorganic peroxide compound such as hydrogen peroxide or peroxomonosulphuric acid (caro's acid) or an inorganic peroxide compound like aliphatic peracids and aromatic peracids and salts thereof. Suitable organic peroxides are peracetic acid and performic acid. Preferably, the peroxide-containing compound is hydrogen peroxide or a mixture of hydrogen peroxide and peracetic acid commonly referred to as equilibrium peracetic add. Most preferred is hydrogen peroxide, if an equilibrium peracetic acid is used in the peroxide stage the pH should preferably be held in the acidic range, e.g. from 3 to 6.5 and the alkaline range, e.g. from 8 to 12 respectively. By altering the pH, the bleaching and deiignirlcation power of both the peracetic acid and the hydrogen peroxide is effectively used. When hydrogen peroxide is used the pH of the pulp suspension is suitably above 7, preferably at a pH in the range of from about 7 up to about 13. More preferably, the pH lies in the range from 8 up to 12. The most preferred pH range is from 9.5 up to 11.5. The temperature of the hydrogen peroxide bleaching is from about 30 4C up to about 130 °C, preferably from 50 3C up to 100 "C, and more preferably from 50 °C to 90 °C. The length of the hydrogen peroxide treatment may be from about 3 minutes up to 960 min, suitably from 10 minutes up to 360 minutes, and preferably from 60 minutes up to 240 minutes. The hydrogen peroxide treatment is normally carried out at atmospheric pressure, however, the hydrogen peroxide bleaching stage may be pressurised, especially when oxygen is used together with hydrogen peroxide. Suitably the pressure is in the range from 1.5 bar up to around 7 bar, preferably from 2 up to 5.5 bar. The pulp consistency typically ranges from 1% up to 70 % by weight, suitably from 3% up to 59% by weight and most preferably from 9% up to 30% by weight The amount of added hydrogen peroxide depends on the type of non-wood puip to be treated and on the conditions under the peroxide bieaching as well as the desired brightness. Usually the amount of added hydrogen peroxide is from about 1 kg up to about 60 kg per tonne dry puip, calculated as 100% hydrogen peroxide. The upper limit is not critical, but has been set from economical reasons. Suitably, the amount of hydrogen peroxide lies in the range from 6 kg up to 50 kg per tonne dry pulp, preferably from 13 kg up to 40 kg per tonne dry puip. Other bieaching agents may be added to the hydrogen peroxide bleaching stage such as oxygen. The bleaching process of the present invention can be carried out in an optionai position in the bleaching sequence, e.g. immediately after the pulping process, such as a chemical digestion process, or after an initial oxygen deiignifi cation treatment said oxygen treatment suitably comprising one or more stages. If the non-wood pulp after chemical pulping is subjected to oxygen damnification the kappa number of the pulp after oxygen delignification is suitably below 20, preferably below 10. The non-wood pulp may after the treatment with a peroxide-containing compound further be bleached in one or more bleaching stages, although a high brightness of the non- wood pulp (above 85 %ISO) can be obtained already after the treatment with an oxygen reinforced peroxide-compound. Preferably, the bleaching agent used in the bieaching stage or stages after the treatment with a peroxide-containing compound are selected from elementally chlorine free bleaching agents, such as chlorine dioxide and peroxide-containing bieaching agent like inorganic peroxide, e.g. hydrogen peroxide, peroxomonosulphuric acids or organic peracid, usually aliphatic peracids, aromatic peracids or salts thereof. Common organic peracids are peracetic acid and performic acid. Ozone as well as sodium dithioniie are also examples of usable bleaching agents. According to one preferred embodiment of the present invention the pulp is bleached by a process where no gaseous chlorine is used. The invention is further illustrated by the following examples which, however, shouid not be construed as limiting the scope of the invention. The percentages and parts stated in the description, claims and examples refer to percent by weight and part per weight, respectively, unless otherwise stated. The pH values given in the description, claims and examples refer to the pH at the end of each treatment stage, unless otherwise stated. The amount of added chlorine dioxide is calculated as active chlorine. Furthermore, to all hydrogen peroxide stages was added 300 ppm of magnesium sulphate calculated as 100% iV!g2+ on dry pulp and 30 kg of hydrogen peroxide per tonne dry puip, unless otherwise indicated. In the examples below the kappa number, viscosity and brightness of the pulp were determined according to SCAN standard methods C1:77 R, C 15-16:62 and C 11-75:R, respectively. The consumption of hydrogen peroxide were established by iodometric titration with sodium thiosulphate. The reverted brightness was measured on sheets after heat treatment at 120 3C for 16 hours. Tne content of magnesium and manganese in the pulp was measured on pulp entering the final P stage with JCP technique. The content of silica in the puip was measured with XRF technique. Example 1 A non-oxygen delignified soda anthraquinone (AQ) pulp of reed having a kappa number of 10.2 and a brightness of 35.1 % ISO, an amount of silica of 2000 ppm and a viscosity of 1171 dm3/kg was in all teste bleached with chlorine dioxide (D), treated with EDTA (Q) and bleached with hydrogen peroxide' (P), in order to illustrate the importance of treating the reed pulp with a chelating agent in a separate stage, the location of the separate chelating agent treatment stage and the pH during the chelating agent stage. The reed pulp was in test 1 first treated with 30 kg chlorine dioxide per tonne dry pulp at a temperature of 60°C, a residence time of 30 min, a pulp consistency of 10 % by weight and a pH of 2.1. Thereafter, the pulp was treated with EDTA (Q-stage). The amount of added EDTA was 2 kg per tonne dry pulp, the pH was 2.8, the temperature 60aC, the residence time 30 min, and the puip consistency 10 % by weight. Subsequently, the pulp was bleached with hydrogen peroxide (P-stage) at a pH of 10.5, a residence time of 240 min, a temperature of 90°C, and a pulp consistency of 10 % by weight. The addition of hydrogen peroxide was 30 kg per tonne dry pulp. For comparison, the final pH in the treatment with EDTA was 6.8 (test 2), otherwise the conditions were the same as in test 1. In addition, reed puip was first treated with EDTA at a pH of 5.9 and subsequently bleached with chlorine dioxide at a pH of 2.0 before being further bleached with hydrogen peroxide (test 3). Apart from the slightly different pH levels in the EDTA and chlorine dioxide stage, the conditions were the same in the three treatment stages as in test 1. Finally, the reed puip was treated in a two stage sequence, where the pulp first was treated with 30 kg chlorine dioxide per tonne Ghlorine dioxide and 2 kg EDTA per tonne dry puip and subsequently with 30 kg per tonne of hydrogen peroxide (test 4). The pH in the combined EDTAJchlorine dioxide stage was 3.0. The duration of the treatment was 30 mtn, the temperature 60 3C, and the pulp consistency 10 % by weight The conditions during the hydrogen peroxide stage were the same as in test 1. After each treatment stage in all tests mentioned above the pulp was washed with deionised water according to a standard procedure resulting in a washing efficiency of 96.9%. Ail pH measurements were conducted at the end of the treatment stages, i.e. the final pH. It is evident from table 1 that a lower pH in the G-stage, Le. up to 2.9 (test 1), significantly improves brightness and viscosity compared to a higher pH of 6.8 (test 2). Furthermore, the position of the Q-stage in the sequence is of importance as indicated by comparison between test 1 and test 3, and further that the Q-stage is a separate stage, comparison between test 1 and test 4. Example 2 In this example, a non-oxygen delignrfied soda AQ wheat straw pulp with a initial kappa number of 11.6, an Initial brightness of 42.5 % ISO, a viscosity of 1013 dm3/kg and an amount of silica of 10,000 ppm was bleached according to the sequences described in example l (tests 1, 2 and 4). Thus, the conditions of the treaiment stages in the tests were all identical to the conditions of the stages in example 1 except that the pH in some stages differed somewhat which is indicated in table il and that the wheat straw pulp was treated with 20 kg chlorine dioxide (D-stage) per tonne dry pulp in all tests. The pH in the P stages was about 10 in all tests. After each stage mentioned above the pulp was washed with deionised wafer according to a standard procedure resulting in a washing efficiency of 96.9%. Table If shows that the properties of wheat pulp, a grass pulp, are improved if bleached according to the present invention (test 1). Example 3 For comparison an oxygen deiignmed sulphate softwood pulp having a kappa number of 16.3, a brightness of 35.3, and a viscosity of 1106 dm3/kg was bleached with chlorine dioxide, then treated with EDTA in a separate stage and subsequently bleached with hydrogen peroxide. The softwood pulp was first treated with 25 kg chlorine dioxide per tonne dry pulp, at a pH of 2.1, during 30 min at a temperature of 60°C. The pulp consistency was 10 % by weight The pH in the EDTA treatment stage was 2.8 (test 1) and 6.1 (test 2), respectively. The amount of EDTA added was 2 kg per tonne of dry pulp, the residence time 30 min, the temperature 60 °G and the pulp consistency of 10 % by weight The puip was lastly bleached with 30 kg hydrogen peroxide per tonne dry pulp under similar conditions as in test 1 of example 1. After each stage mentioned above the puip was washed with deiontsed water according to a standard procedure resulting in a washing efficiency of 96.9%. As shown by table 111, when bleaching wood pulps, e.g. softwood pulp, best puip properties are obtained if the pH in the separate Q-stage prior to a peroxide treatment stage is form 5 to 7, here 6.1. Surprisingly, if non-wood pulp is treated with the same compounds in the same order, pulp properties ate significantly improved if pH in the Q-stage is up to 2.9 (se fable I). Example 4 Another batch of the same non-oxygen delignified soda AQ wheat straw puip as used in example 2 was bleached according to the invention. The pulp was first bleached with 20 kg/tonne dry puip of chlorine dioxide (D-stage) at a pH of 2.2 under 30 min. at 60 °C prior to an EDTA treatment stage at a pH of 2,8 (Q-stage), a temperature of 60 °C, under 30 minutes, where 2 kg/tonne dry pulp of EDTA was added. After the chelating agent treatment the pulp was bleached with 30 kg/tonne dry pulp of hydrogen peroxide at a pH of around 10, a temperature of 90 °C during 240 min. The pulp consistency during the D-, Q- and P-stages was 10%, respectively. After the D-stage the pulp was washed with deionised water according to a standard procedure resulting in a washing efficiency of 96.9%. However, the washing efficiency after the chelating agent treatment was varied as indicated in fable IV, The treatment conditions in the stages corresponded to those in example 1, test 1. *The washing efficiency indicates how much of the total volume of the filtrate from the Q stage has been replaced with deionised water and chemicals in the dilution of the P stage, As shown by table IV, a considerably lower washing efficiency (test 1) does not significantly impair the brightness of the pulp after the hydrogen peroxide stage. Example 5 Another batch of the same non-oxygen deiignified soda AQ wheat straw puip as used in example 4 was used in this example and bleached according to the invention. The pulp was first subjected to a chlorine dioxide treatment followed by a EDTA treatment stage whereafter the pulp was bleached during 150 min. and at a temperature of 110 *C with oxygen reinforced hydrogen peroxide (PO-stage). In the PO-stage 40 kg/tonne dry puip of hydrogen peroxide was applied and the oxygen pressure was 0,5 MPa, The treatment conditions in the D-stage and the Q-stage were the same as in example 4. The pH after the (PO)-stages were about 10. In test 1 the pulp was washed after the D-stage and the PO- stage, however, not after the Q-stage. in test 2, the pulp was washed after the D-stage, the PO-stage and the Q-stage. The washing stages were according to a standard procedure resulting in a washing efficiency of 96.9%. As shown in table V a non-wood pulp can be bleached to a high brightness by using only three treatment stages. The absence of washing after the Q-stage does not significantly impair the final brightness. Claims 1. A process for bleaching ligncceHuiose-containing pulp comprising pulp obtainable from lignoceilulose-cantaining maferiais selected from the group consisting of non-wood with a peroxide-containing compound, where the pulp prior to the bleaching is treated with chlorine dioxide, characterised in that the pulp after the treatment with chlorine dioxide is treated in a separate stage with a chelating agent at a pH up to about 2.9 and washed between the chlorine dioxide treatment and chelating agent treatment, 2. A process according to claim 1, characterised in that the lignocellulose-containing pulp comprises pulp containing at least 500 ppm silica. 3. A process according to claim 1, characterised in that the pulp is obtainable from iignocellulose-containing materials selected form the group consisting of wheat and reed and mixtures thereof. 4. A process according to any of the preceding claims, characterised in that the peroxide-containing compound is hydrogen peroxide. 5. A process according to any of the preceding claims characterised in that the pH in the chelating agent treatment is from about 1.5 up to about 2.5. 6. A process according to any of the preceding claims, characterised in that the chelating agent is selected from the group consisting of nitrogenous organic compounds. 7. A process according to any of the preceding claims, characterised in that the pulp is washed after the treatment with a chelating agent 8. A process according to any of the preceding claims, characterised in that the pulp after the bleaching with a peroxide-containing compound is further bleached in one or more stages to a brightness of at least about 85 % ISO. 9. A process according to any of the preceding claims, characterised in that the puip is bleached without the use of gaseous chlorine. 10. A process according to any of the preceding claims, characterised in that the lignccellulose-containing material is chemically digested. 11. A process according to any of the preceding claims, characterised in that the pulp is produced in an aikaiine process. 12. A process of bleaching lignocellulose-containing pulp substantially as herein described and exemplified.

Documents:

2128-CHENP-2004 CORRESPONDENCE OTHERS 22-08-2012.pdf

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2128-CHENP-2004 AMENDED CLAIMS 28-06-2012.pdf

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2128-CHENP-2004 CORRESPONDENCE OTHERS 18-06-2012.pdf

2128-CHENP-2004 CORRESPONDENCE OTHERS 28-06-2012.pdf

2128-CHENP-2004 FORM-1 28-06-2012.pdf

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2128-CHENP-2004 CORRESPONDENCE OTHERS 15-01-2013.pdf

2128-chenp-2004 correspondence others.pdf

2128-chenp-2004 description (complete).pdf

2128-CHENP-2004 EXAMINATION REPORT REPLY RECEIVED 28-06-2012.pdf

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