Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
  • D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1042—Use of chelating agents
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C5/00—Other processes for obtaining cellulose, e.g. cooking cotton linters ; Processes characterised by the choice of cellulose-containing starting materials

Definitions

• the present invention relates to a process for bleaching lignocellulose-containing pulp comprising pulp obtainable from lignoceiiulose-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 at er 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.
• 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 level of brightness is needed, such as for the production of paper, the non-wood pulp has to be bleached 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.
• one advantage with the present invention is to bleach a pulp originating from non-wood pulp in a cost effective way, using environmentally friendly bleaching chemicals, i.e. without using gaseous chlorine, and, furthermore, to provide 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 bleaching sequences have been disclosed for bleaching wood pulp comprising chlorine dioxide stages and peroxide stages.
• 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
• the present bleaching sequence of the invention was even more surprising as the invention is contradictory to the common general knowledge specifically with regard to bleaching of pulp from wood materials.
• non-wood pulp bleached according to the present invention is liable to minimal brightness reversion. Still 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 invention refers to a process for bleaching lignocellulose- containing putp comprising memep obtainable from lignocellulose-containing material selected from the group consisting of non-wood with a peroxide-containing agent, 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.
• the present process is designed to effectively bleach lignocellulose-containing pulp comprising pulp obtainable from lignocellulose-containing materials selected from the group consisting of non-wood materials.
• non-wood material is referred to as annual plant material such as agricultural crops and residues from agricultural crops.
• non-wood is a common definition In the technical field of the present invention and embraces in principle all lignocellulose-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.
• 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.
• bast fibres are e.g. flax ⁇ seed flax), hemp, Jute, kenaf, ra ine, rosella and mixtures thereof, whereas cotton, cotton .inters, kapok and mixtures thereof are suitable grain hair materials.
• 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.
• 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.
• non-wood pulp can be differentiated from wood pulp with respect to the content of silica
• the bleaching process is suitably applied on lignocellulose- containing pulp comprising pulp containing at least 500 ppm Si/dry fibre, preferably at least 800 ppm Si/dry fibre, more preferably at least 1200 ppm Si/dry fibre, most preferably at least 1500 pprn Si/dry fibre.
• the silica content of the non-wood material was determined by XRF analysis.
• the pulp used in the process comprises at least 75 % by weight of non-wood pulp, suitably at least 85 % by weight.
• the pulp consists essentially of pulp obtainable from non-wood materials.
• the non-wood pulp is suitably obtained by applying chemical pulping processes.
• Chemical non-wood pulp relates to pulp digested in cooking processes such as the sulphate, sulphite, soda or organosolv process or modifications of these processes with optional additives such as anthraquinone, e.g. soda anfhraquinone (AO) pulping process.
• the lignocellulose-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 delignification, 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 pulp can be produced according to conventional techniques.
• bleaching plants designed to bleach non-wood pulps have a significantly smaller production rate of bleached pulp than conventional bleaching plants for bleaching wood materials like hardwood and softwood.
• the cost per tonne bleached pulp normally increases.
• 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 ClOs/day up to about 12 tonnes ClO 2 /day.
• Such a chlorine dioxide production plant may consist of one or several production units.
• 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 PureTM or its modifications.
• the pulp consistency in the chlorine dioxide treatment stage 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 °Cschreib
• 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,
• washing 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.
• the chlorine dioxide stage is carried out at an acid or acidic pH.
• 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.
• a further advantage with the present invention Is that less makeup chemicals need to be added, since the chlorine dioxide treatment and the chelating agent treatment are carried out at comparable pH ranges.
• the non-wood pulp is treated with a chelating agent in a separate stage.
• a chelating agent in a separate stage.
• Suitable chelating agents are nitrogenous organic compounds, primarily nitrogenous polycarboxylic acids, nitrogenous po ⁇ yphosphonic acids and nitrogenous polyalcohols.
• Preferred nitrogenous polycarboxylic acids are diethylenetriaminepentaacetic acid (DTPA), ethyienediaminetetraacetic acid (EDTA) or n ⁇ trilotriacetic acid (NTA), EDTA is the preferred nitrogenous polycarboxylic acid.
• DTPA diethylenetriaminepentaacetic acid
• EDTA ethyienediaminetetraacetic acid
• NTA n ⁇ trilotriacetic acid
• EDTA is the preferred nitrogenous polycarboxylic acid.
• other compounds can be used as chelating agents, such as polycarboxylic acids, suitably oxalic acid, citric acid or tarfaric acid, or phosphonic acids like diethylenetriaminepentaphosphonic acid.
• organic acids as are formed during the pulp treatment with, inter alia
• 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 8 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 pulp and even more suitably from 0.5 kg up to 2.5 kg per tonne dry pulp, 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.
• an acid readily available in the pulp mill may be employed.
• suitable acids are inorganic acids such as sulphuric acid, nitric acid, hydrochloric acid or residual acid from a chlorine dioxide reactor, either added separately or in mixtures.
• the non-wood pulp may be washed.
• bleaching 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 efficiency is calculated as the sum of the efficiency in each washing 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 68,7%.
• the total washing efficiency is 98.9%.
• 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%.
• 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 (care's acid) or an inorganic peroxide compound like aliphatic peracids and aromatic peracids and salts thereof.
• a peroxide-containing compound which can be an inorganic peroxide compound such as hydrogen peroxide or peroxomonosulphuric acid (care'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.
• the peroxide-containing compound is hydrogen peroxide or a mixture of hydrogen peroxide and peracetic acid commonly referred to as equilibrium peracetic scld. Most preferred is hydrogen peroxide.
• 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.
• the bleaching and de ⁇ gnification power of both the peracetic acid and the hydrogen peroxide is effectively 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 up to about 130 °C, preferably from 50 °C 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 980 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.
• 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 pulp to be treated and on the conditions under the peroxide bleaching 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 pulp, calculated as 1 0% 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 pulp. Other bleaching 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 optional position in the bleaching sequence, e.g. immediately after the pulping process, such as a chemical digestion process, or after an initial oxygen delignification treatment said oxygen treatment suitably comprising one or more stages. If the non-wood pulp after chemical pulping is subjected to oxygen delignification 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 %lSO) can be obtained already after the treatment with an oxygen reinforced peroxide-compound.
• the bleaching agent used in the bleaching 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 bleaching 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 perfor ic acid. Ozone as well as sodium dithionite are also examples of usable bleaching agents.
• the pulp is bleached by a process where no gaseous chlorine is used.
• viscosity and brightness of the pulp were determined according to SCAN standard methods C1:77 R, C 15-18:62 and C 11-75: , respectively.
• the consumption of hydrogen peroxide were established by iodomefric titration with sodium thiosuiphate.
• the reverted brightness was measured on sheets after heat treatment at 120 °C for 18 hours.
• the content of magnesium and manganese in the pulp was measured on pulp entering the final P stage with ICP technique.
• the content of silica in the pulp was measured with XRF technique.
• D chlorine dioxide
• Q treated with EDTA
• P hydrogen peroxide-
• the reed pulp was in test 1 first treated with 30 kg chlorine dioxide per tonne dry pulp at a temperature of SOX, 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 80*C, the residence time 30 min, and the pulp 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.
• P-stage hydrogen peroxide
• the final pH in the treatment with EDTA was 6.8 (test 2), otherwise the conditions were the same as in test 1.
• reed memep 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 slightiy different pH levels in the EDTA and chlorine dioxide stage, the conditions were the same in the three treatment stages as in test 1.
• the reed pulp was treated in a two stage sequence, where the pulp first was treated with 30 kg chlorine dioxide per tonne chlorine dioxide and 2 kg EDTA per tonne dry pulp and subsequently with 30 kg per tonne of hydrogen peroxide (test 4).
• the pH in the combined EDTA./ch ⁇ orine dioxide stage was 3.0.
• the duration of the treatment was 30 min, the temperature 80 ⁇ C, and the530p 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 deion ⁇ sed water according to a standard procedure resulting in a washing efficiency of 96.9%. All pH measurements were conducted at the end of the treatment stages, i.e. the final pH. Table I, Reed pulp
• Table t! shows that the properties of wheat pulp, a grass pulp, are improved if bleached according to the present invention (test 1).
• an oxygen delignified sulphate softwood pulp having a kappa number of 16.3, a brightness of 35.3, and a viscosity of 1108 dm 3 /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 80 .
• 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 80 and the pulp consistency of 10 % by weight.
• the pulp 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 pulp was washed with delontsed water according to a standard procedure resulting in a washing efficiency of 96.9%.
• Another batch of the same non-oxygen delignified soda AQ wheat straw pup as used in example 2 was bleached according to the invention.
• the pulp was first bleached with 20 kg/tonne dry pulp of chlorine dioxide (D-stage) at a pH of 2.2 under 30 min. at 60 X prior to an EDTA treatment stage at a pH of 2,8 (G-stage), a temperature of 80 X, under 30 minutes, where 2 kg/tonne dry pulp of EDTA was added.
• G-stage pH of 2,8
• the pulp was bleached with 30 kg/tonne dry memep of hydrogen peroxide at a pH of around 10, a temperature of 90 during 240 min.
• the pulp consistency during the D-, Q- and P-stages was 10%, respectively.
• 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.
• test 1 a considerably lower washing efficiency (test 1) does not significantly impair the brightness of the pulp after the hydrogen peroxide stage.
• 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.

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• 2002
  • 2002-03-28 SE SE0200981A patent/SE0200981D0/xx unknown
• 2003
  • 2003-03-27 AU AU2003216028A patent/AU2003216028B2/en not_active Ceased
  • 2003-03-27 CN CNB038072785A patent/CN1312354C/zh not_active Expired - Fee Related
  • 2003-03-27 WO PCT/SE2003/000512 patent/WO2003083208A1/en not_active Application Discontinuation
• 2004
  • 2004-10-25 CO CO04106973A patent/CO5611215A2/es active IP Right Grant

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