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
  • D21C9/144—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 and other bleaching agents in a multistage process
• • 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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
• • 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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• This invention relates to a process for bleaching a chemical pulp.
• wood is converted into a pulp, usually by chemical means, and then spread on a wire to be dewatered to form the paper.
• Kraft pulping is the most widely practiced form of chemical pulping in North America. The pulp is relatively dark in colour and to produce a commercially acceptable paper, it is generally bleached before being formed into paper.
• bleaching agents include chlorine, chlorine dioxide, hypochlorites, hydrogen peroxide and oxygen. Typically, more than one of these chemicals is required and they are used singly or in admixture in sequence.
• the various bleaching compounds are designated by letter; chlorine is C, chlorine dioxide is D, caustic extraction is E, hypochlorite is H, hydrogen peroxide is P and oxygen is O.
• the above compounds include chlorine and certain chlorine containing compounds and there are concerns about the use of chlorine and chlorine compounds in bleaching pulp.
• Chlorine and chlorine containing compounds react with pulp constituents - mainly lignin, fatty acids and resin acids - to produce chlorinated organic compounds.
• Some of these chlorinated organic compounds are of concern environmentally and some major pulp producing countries have restricted the amount that can be discharged from the bleaching process into waterways. It is reasonable to assume that most countries will introduce legislation restricting the discharge of such compounds.
• Oxygen delignification is a process of treating pulp under oxygen pressure, about 6.9 x 105 N/m2 (100 psig), at elevated temperature, about 80° to 120°C, and for extended periods of time, about 30 to 90 minutes.
• This oxygen delignification process tends to produce weak pulp if delignification is extended too far - i.e. down to about 10 Kappa number.
• the Kappa number of a pulp is a measure of the bleachability of the pulp. It is a number indicating the number of milliliters of 0.1 Normal potassium permanganate solution that can be consumed under standard conditions by one gram of pulp (oven dried basis) corrected to 50% consumption of permanganate.
• the dioxiranes are known compounds of the general structural formula: in which R1 and R2 may be aliphatic or aromatic groups and may be linked to form cyclic compounds.
• the dioxirane compounds and their preparation are described, for example, in the Journal of Organic Chemistry, 50 2847-2853 (1985) by R.W. Murray and R. Jeyaraman.
• the present invention is a process of bleaching a chemical pulp that comprises contacting the chemical pulp with a dioxirane.
• the dioxirane may be used with known bleaching agents either in combination or in sequence.
• the pulp is desirably one having a kappa number in the range of 15 to 60 if derived from a softwood species and 5 to 25 if derived from a hardwood species.
• the dioxirane is contacted with the pulp preferably at a temperature in the range of about 5° to 50°C for a time up to about 90 minutes.
• the dioxirane is used in amount sufficient to provide an active oxygen charge, i.e. one active oxygen atom per dioxirane molecule, of from 0.2 to 1.0% based on the oven-dried pulp.
• the dioxirane has the structure: where R1 and R2, which may be the same or different, is each an aliphatic or aromatic group and may be linked together to form a ring.
• One dioxirane compound that has proved useful is dimethyldioxirane but any dioxirane compound is appropriate, for example, those compounds where R1 and R2 are each alkyl groups with 1 to 4 carbon atoms.
• a subsequent bleaching step subsequent that is to the dioxirane treatment then, when the dioxirane-treated pulp has a residual lignin of less than 10 Kappa number, that subsequent bleaching of the pulp may be carried out by chlorine dioxide (D) alone or by the sequence chlorine dioxide, caustic extraction, chlorine dioxide (DED).
• D chlorine dioxide
• DED chlorine dioxide
• the sequence may be set out as D1ED2.
• Chlorine dioxide in the D1 stage is used in the range 0.5 to 1%, preferably about 0.8%
• caustic in the E stage is in the range 0.2 to 0.7%, preferably about 0.4%
• chlorine dioxide in the D2 stage is in the range 0.2 to 0.7%, preferably about 0.4%. All the percentages are expressed as weight of oven-dried pulp. If chlorine dioxide alone is used, then it is in the range of 0.5 to 1% by weight of the oven-dried pulp, preferably about 0.8% of the oven-dried pulp.
• the subsequent bleaching may be carried out using the sequence: chlorine; caustic extraction; chlorine dioxide; caustic extraction; chlorine dioxide (CEDED).
• a substantially reduced amount of chlorine charge is used in stage C, in stage E1 the caustic charge is 60% of that in the C stage, in stage D1 0.5 to 1.2% chlorine dioxide charge is used, in stage E2 the caustic charge is 50% of that used in D1 stage and in stage D2 0.25 to 0.6% chlorine dioxide charge is used.
• the percentages are based on the weight of oven-dried pulp.
• the chlorine used in the C stage is generally applied in combination with varying amounts of chlorine dioxide.
• Aspen pulp 50g oven-dried weight
• Aspen pulp produced by the kraft process to a Kappa number of 16.4, at 39% pulp consistency
• DMD dimethyldioxirane
• acetone 280mL, 4.59g/L concentration, i.e. 0.55% active oxygen by weight on oven-dried pulp
• the dimethyldioxirane-treated pulp was then bleached with chlorine dioxide (0.8% by weight on oven-dried pulp) at 70°C for three hours.
• a second sample of the same aspen kraft pulp (100g oven-dried weight) was delignified by molecular oxygen at 10.7% pulp consistency using sodium hydroxide (4% by weight on oven-dried pulp), magnesium sulphate (0.5% by weight on oven-dried pulp), molecular oxygen (100 psig) at 115°C for one and half hours.
• This molecular-oxygen-delignified pulp was then bleached with elemental chlorine (1.8% available chlorine by weight on oven-dried pulp) at 20°C for one hour at 3% pulp consistency, extracted with sodium hydroxide (1.1% by weight on oven-dried pulp) at 74°C for two hours at 12% pulp consistency, and further delignified with chlorine dioxide (0.5% by weight on oven-dried pulp) at 74°C for three hours at 6% pulp consistency.
• This bleaching sequence will be referred to as OCED.
• a third sample of the same aspen kraft pulp (50g oven-dried weight) was also bleached by a conventional process using a CED sequence. Elemental chlorine bleaching was carried out at 3% pulp consistency and 20°C for one hour using 3.3% available chlorine charge on oven-dried pulp, extraction at 74°C for two hours using sodium hydroxide (2.0% by weight on oven-dried pulp) at 12% pulp consistency, and chlorine dioxide (1.0% by weight on oven-dried pulp) delignification at 74°C for three hours at 6% pulp consistency.
• Zero span tensile strength was measured according to TAPPI T231 cm-85 using Pulmac Instruments' The Troubleshooter (trade mark) zero span tester.
• a fourth sample of aspen pulp (50g oven-dried weight), produced by the kraft process to a Kappa number 16.4, at 39% pulp consistency, was treated with DMD solution in acetone (170 mL, 4.44g/L concentration, i.e. 0.33% active oxygen by weight on oven-dried pulp) and 80 mL of acetone at 25°C for three hours.
• This DMD treated pulp was filtered and washed with a large amount of distilled water.
• the resulting pulp was then further delignified with chlorine dioxide (1.0% by weight on oven-dried pulp) at 74°C for three hours at 6% pulp consistency, extracted with sodium hydroxide (0.72% by weight on oven-dried pulp) at 10% pulp consistency and 74°C for two hours , and bleached by another chlorine dioxide stage (0.5% by weight on oven-dried pulp) at 6% pulp consistency and 74°C for three hours.
• a fifth sample of the same aspen kraft pulp (100g oven-dried weight) was delignified by molecular oxygen at 12% pulp consistency using sodium hydroxide (4.0% by weight on oven-dried pulp), magnesium sulphate (0.5% by weight on oven-dried pulp), and molecular oxygen (100 psig) at 110°C for one and a half hours.
• This molecular-oxygen-delignified aspen pulp was further bleached with elemental chlorine (2% available chlorine by weight on oven-dried pulp) at 20°C for one hour at 3% pulp consistency, extracted with sodium hydroxide (1.2% by weight on oven-dried pulp) at 12% pulp consistency and 74°C for two hours, and bleached with chlorine dioxide (0.5% by weight on oven-dried pulp) at 6% pulp consistency and 74°C for three hours.
• elemental chlorine 2% available chlorine by weight on oven-dried pulp
• the aspen kraft pulp was delignified to the same extent (63%) by either the DMD bleaching agent or molecular oxygen.
• Results shown in Table 2, illustrate that the fully bleached aspen pulp produced from the DMD treatment followed by DED bleaching without elemental chlorine has better zero span tensile strength than that produced by molecular oxygen delignification followed by CED bleaching with 2% available chlorine charge on oven-dried pulp at the elemental chlorine stage.
• the DMD-treated pulp yields a fully bleached pulp with similar zero span tensile strength to that produced by a conventional process involving the use of elemental chlorine for bleaching.
• a sixth sample of aspen pulp (50g oven-dried weight), produced by the kraft process to a Kappa number of 16.4, at 35% pulp consistency, was treated with DMD solution in acetone (643 mL, 2.0g/L concentration, i.e. 0.55% active oxygen by weight on oven-dried pulp) at 25°C for one hour. The resulting pulp was then washed thoroughly with a large quantity of deionized water.
• a seventh sample of the same aspen kraft pulp (100g oven-dried weight) was delignified by molecular oxygen at 12% pulp consistency using sodium hydroxide (4% by weight on oven-dried pulp), magnesium sulphate (0.5% by weight on oven-dried pulp), molecular oxygen under a pressure of about 6.9 x 105 N/m2 (100 psig) at 115°C for one and a half hours.
• the resulting oxygen-delignified pulp was thoroughly washed with a large amount of deionized water.
• Spruce pulp (50g oven-dried weight), produced by the kraft process to a Kappa number of 32, at 37% pulp consistency, was treated with a solution of DMD in acetone (330 mL, 4.6g/L concentration, i.e. 0.65% active oxygen by weight on oven-dried pulp) at 25°C for half an hour.
• the DMD-treated pulp was further bleached by a CED sequence.
• Elemental chlorine bleaching was carried out at a 3% pulp consistency and 20°C for one hour using 2.8% available chlorine by weight on oven-dried pulp, extraction at 74°C for two hours using sodium hydroxide (1.7% by weight on oven-dried pulp) at 12% pulp consistency, and chlorine dioxide (0.8% by weight on oven-dried pulp) delignification at 6% pulp consistency and 74°C for three hours.
• a second sample of the same spruce kraft pulp (50g oven-dried weight) was also bleached by a conventional process using CED sequence. Bleaching with elemental chlorine was carried out at a 3% pulp consistency and 20°C for one hour using 6.4% available chlorine on oven-dried pulp, extraction at 12% pulp consistency and 74°C for two hours using sodium hydroxide (3.8% by weight on oven-dried pulp), and chloride dioxide (1% by weight on oven-dried pulp) bleaching at 6% pulp consistency and 74°C for three hours.
• Results shown in Table 4, illustrate that the DMD-treated pulp can be bleached to the same brightness as that bleached by a conventional process.
• the former uses a significantly reduced amount of elemental chlorine compared with that required by the conventional process.
• the zero span tensile strengths and viscosity of the pulps bleached by both processes are similar.
• TABLE 4 Physical and optical properties of spruce kraft pulps bleached with DMD or by a conventional process. Spruce Pulp Kappa Number Degree of Delig.
• the present invention provides a process for treating pulp that can be subsequently fully bleached to 90% Elrepho, either without or with only small amounts of elemental chlorine.
• the DMD-treated pulps that have a residual lignin content corresponding to 5 Kappa number or less have pulp strengths equivalent to those of the untreated pulps.
• the fully-bleached pulps produced by the process of the invention have strength properties similar to those produced by conventional bleaching processes using high elemental chlorine charge.
• the bleaching of chemical pulps with the dioxirane compounds can be modified for a range of bleaching conditions, bleaching sequences and combinations with molecular oxygen and other bleaching agents, not specifically exemplified above, for example, hydrogen peroxide, oxygen and monoperoxysulphate.
• the treatment with dioxirane can also be combined with oxygen delignification, being placed before or after the oxygen treatment.
• the use of two oxygen containing compounds can have the virtue of eliminating a wash step needed between stages of a sequence when chlorine and compounds of chlorine are used.
• the pulps used in this invention can be kraft, sulphite, soda, or other types of chemical pulps from hardwood and softwood species.
• the invention extends to chemical pulps and this term is intended to extend to pulps produced by the treatment of wood species with organic solvents to separate the wood components.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Detergent Compositions (AREA)
• Compounds Of Unknown Constitution (AREA)

Applications Claiming Priority (1)

Publications (2)

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ID=4172838

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• 1990
  • 1990-02-16 BR BR909007993A patent/BR9007993A/pt unknown
  • 1990-02-16 AU AU51686/90A patent/AU641640B2/en not_active Ceased
  • 1990-02-16 WO PCT/CA1990/000052 patent/WO1991012369A1/en active IP Right Grant
  • 1990-02-16 DE DE69008870T patent/DE69008870T2/de not_active Expired - Fee Related
  • 1990-02-16 JP JP2503855A patent/JPH0718113B2/ja not_active Expired - Lifetime
  • 1990-02-16 EP EP90903735A patent/EP0515362B1/en not_active Expired - Lifetime
• 1992
  • 1992-08-12 NO NO92923145A patent/NO923145L/no unknown
  • 1992-08-14 FI FI923644A patent/FI102195B/fi active

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