BLEACHING OF LIGNOCELLULOSIC MATERIAL WITH DIOXIRANES

Abstract: A process of bleaching a chemical pulp with a dioxirane produces a pulp of good quality and reduces environmental concerns in so doing. A chemical pulp is contacted with a dioxirane, and either with or without known bleaching agents, in combination or sequence.

Description: BLEACHING OF LIGNOCELLULOSIC MATERIAL WITH DIOXIRANES This invention relates to a process for bleaching a chemical pulp. In the manufacture of paper, 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. A variety of bleaching agents is used. They 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. In the art 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. T TE SHEET The elimination of elemental chlorine and its related compounds in the bleaching process would eliminate the production of chlorinated organic compounds in the effluent but efforts along this line have not been wholly effective. The use of elemental chlorine and its related compounds in bleaching may lead to the formation of tetrachlorinated dioxins (TCDD) and furans (TCDF) which are contained in the pulp and in the pulp mill effluent. However, the formation of these two toxins can be eliminated by reducing the amount of chlorine applied. Unfortunately, this is difficult to do with existing technology without sacrificing some pulp quality or introducing further problems into the pulping process. Oxygen delignification is a process of treating pulp under oxygen pressure, about 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. In this regard 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. United States Patent 4,439,271 to O. Samuelson discloses a process using nitrogen dioxide pretreatment before oxygen delignification. The process retards the depolymerization of cellulose during oxygen bleaching. However, the emission of nitrogen-containing compounds from the bleaching effluent may pose another problem and these nitrogen compounds cannot be recovered by existing kraft pulping chemical recovery systems. U.S. Patent 4,404,061 to J.J. Cael discloses that pulps produced from a standard alkaline pulping process can be bleached with monoperoxysulphate to produce papermaking pulps and bleached pulps. Also, U.S. Patent 4,756,800 to E.L. Springer discloses a method of producing salts of monoperoxysulphuric acid and simultaneously bleaching pulp. Neither patent describes the strength properties of the fully bleached pulps. There is a need for a bleaching process that produces pulp of good quality and which reduces environmental concerns in so doing. The dioxiranes are known compounds of the general structural formula: in which R^ 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 20847- 20853 (1985) by R.W. Murray and R. Jeyaraman. The present invention is a process for the bleaching of chemical pulps without use of elemental chlorine or by using elemental chlorine at a lower level than in the prior art, thus avoiding the production of the toxins TCDD and TCDF. Accordingly, in its broadest aspect, 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. Preferably 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 Rl 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 R^ and R2 are each alkyl groups with 1 to 4 carbon atoms. If a subsequent bleaching step is used, 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) . When the bleaching is carried out using the DED sequence the sequence may be set out as D j ED2. Chlorine dioxide in the D j 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%, and 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. When the dioxirane-treated pulp has a residual lignin content corresponding to a Kappa number greater than 10 the subsequent bleaching may be carried out using the sequence: chlorine; caustic extraction; chlorine dioxide; caustic extraction; chlorine dioxide (CEDED). In this sequence, expressing the sequence as CEιDιE2 D 2' a substantially reduced amount of chlorine charge is used in stage C, in stage E^ the caustic charge is 60% of that in the C stage, in stage D^ 0.5 to 1.2% chlorine dioxide charge is used, in stage E2 the caustic charge is 50% of that used in D^ 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. The invention is illustrated in the examples, which are included for purposes of illustration not limitation. Example 1 Aspen pulp (50g oven-dried weight), produced by the kraft process to a Kappa number of 16.4, at 39% pulp consistency, was treated with the dimethyldioxirane (DMD) solution in acetone (280mL, 4.59g/L concentration, i.e. 0.55% active oxygen by weight on oven-dried pulp) at 25*C for one hour. 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 (lOOg 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 β 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 β C for two hours at 12% pulp consistency, and further delignified with chlorine dioxide (0.5% by weight on oven-dried pulp) at 74 β 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 β 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. The results, shown in Table 1, illustrate that aspen kraft pulp treated with DMD to a level of 80% delignification has better mechanical properties than that treated with molecular oxygen to a level of 67% delignification. This fully bleached pulp at 86.8% Elrepho, which was DMD-treated, has similar zero-span tensile strength to that of a pulp, at 89.9% Elrepho, bleached by a conventional CED bleaching process. S»τ i TABI---- 1 Physical and cptical prcperties of aspen kraft pulp bleached with d---τet-lγldiox----ane or molecular oxygen or by a αxwεnfcional blβ-i iing process. Aspen Kappa Degree of Zero Stan Elrepixi Pulp Ifcπber Delig. (%) Tensile (km) Brightness (%) Unbleached 16.4 - 16.3 40.6 EMD- Treated 3.4 80 17.0 59.4 EM-HD Bleached - - 17.4 86.8 0 2 -Delig. 5.4 67 10.6 70.7 COED Bleached 10.0 91.5 CED Bleached - - 18.4 89.9 TITUTE SHEET In these Examples Elrepho Brightness was measured with a Technobrite (trade mark) Micro TB-IC apparatus at 457 nm. The standard test is set out in TAPPI T-452 om- 87 and CPPA E.l. Zero span tensile strength was measured according to TAPPI T231 cm-85 using Pulmac Instruments* The Troubleshooter (trade mark) zero span tester. Example 2 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 β 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 β C for three hours. A fifth sample of the same aspen kraft pulp (lOOg 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 β 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 β C for three hours. In this particular example, 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. On the other hand 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. T-ABE-S 2 Zero span strengths and viscosities of EM>-treated, molecular-oxygen- deligni ied pulps and those produced try a conventional process. Aspen Kappa Degree of Zero-J-pan Elrej-ho Pulp Nmfcer Delig. (%) Tensile (km) Brightness (%) 16.3 40.6 63 15.0 54.0 15.9 91.4 63 12.7 66.7 12.6 91.0 18.4 89.9 Example 3 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 (lOOg 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 (100 psig) at 115*C for one and a half hours. The resulting σxygen- delignified pulp was thoroughly washed with a large amount of deionized water. The results, shown in Table 3, illustrate that the DMD- treated pulp has a better yield than that bleached by molecular oxygen to the same degree of delignification. This illustrates that DMD has better selectivity toward lignin than does molecular oxygen. TABLE 3 Pulp yields after EM) treatment and oxygen delignification. Aspen Kappa Degree of Yield % Pulp Natfcer Delig. (%) Unbleached Pulp Unbleached 16.4 EM>Treated 5.3 68 98.0 θ2-Delignified 5.4 67 92.7 Example 4 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 β 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 cptical prcperties of spruce kraft pulps bleached with EMD or by a conventional process. Spruce Kappa Degree of Zero Span Viscosity Elreph-) Pulp Ni-irber Delig. (%) Tensile (km) (mRa.s) Brightness (%) Un¬ bleached 32.0 19.4 35.0 32.7 EMD- Treated 18.8 41 18.8 23.0 40.2 E D-CED 18.3 18.3 82.5 Bleached CED 19.1 25.0 82.3 Bleached Thus, 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-treafeed 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.

Claims: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. A process of bleaching a chemical pulp that comprises contacting the chemical pulp with a dioxirane. 2. A process as claimed in claim 1 in which the chemical pulp has a Kappa number in the range 15 to 60 if derived from a softwood species and 5 to 25 if derived from a hardwood species. 3. A process as claimed in claim 1 in which the dioxirane is contacted with the pulp at a temperature in the range about 5* to about 50*C for a time up to about 90 minutes. 4. A process as claimed in claim 1 in which the dioxirane is used in amount sufficient to provide an active oxygen charge of from 0.2 to 1.0% based on the oven-dried pulp. 5. A process as claimed in claim 1 including, when the dioxirane-treated pulp has a residual lignin corresponding to less than 10 Kappa units, bleaching the pulp by chlorine dioxide (D) or by the sequence chlorine dioxide, caustic extraction, chlorine dioxide (DED) . 6. A process as claimed in claim 5 in which the bleaching is carried out using a D]_ E Dj sequence using chlorine dioxide in the D^ stage in the range 0.5 to 1%, caustic in the E stage in the range 0.2 to 0.7% and chlorine dioxide in the D2 stage 0.2 to 0.7%, all percentages being on the weight of the oven-dried pulp. 7. A process as claimed in claim 6 in which the chlorine dioxide in stage D^ is about 0.8%, the caustic in the E stage is about 0.4% and the chlorine dioxide in the D2 stage is about 0.4%. 8. A process as claimed in claim 5 in which the bleaching is carried out with chlorine dioxide in the range 0.5 to 1% by weight of the oven-dried pulp. 9. A process as claimed in claim 8 in which the chlorine dioxide is about 0.8% of the oven-dried pulp. 10. A process as claimed in claim 1 in which, when the dioxirane-treated pulp has a residual lignin content corresponding to a Kappa number exceeding 10, bleaching is carried out using the sequence chlorine, caustic extraction, chlorine dioxide, caustic extraction, chlorine dioxide (CEDED). 11. A process as claimed in claim 10 in which the sequence is CE1D1E2D2 using in stage C an amount of chlorine appropriate for the Kappa number of the pulp after contact with the dioxirane, in stage E^ the caustic charge is 60% of the charge used in the C stage, in stage D^ 0.5 to 1.2% chlorine dioxide charge is used, in stage E2 the caustic charge is 50% of that used in the. D^ stage and in stage D2 0.25 to 0.6% chlorine dioxide charge is used, the percentages being based on the oven-dried pulp. 12. A process as claimed in claim 1 in which the dioxirane has the structure where Rl 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. 13. A process as claimed in claim 12 in which R^ and R are each aromatic groups. 14. A process as claimed in claim 12 in which R^ and are each aliphatic groups. 15. A process as claimed in claim 14 in which the aliphatic groups are alkyl groups with 1 to 4 carbon atoms. 16. A process as claimed in claim 12 in which the dioxirane is dimethyldioxirane. 17. A process for bleaching chemical pulp comprises: (a) contacting the pulp with a dioxirane having the formula in which R^ and R is each an aliphatic or aromatic group and may be linked together to form a ring at an active oxygen charge of 0.2 to 1% by weight of the oven-dried pulp; (b) bleaching the resulting pulp using the sequence chlorine dioxide (D) or chlorine dioxide, caustic extraction, chlorine dioxide (DED) when the pulp has extraction, chlorine dioxide (DED) when the pulp has a residual content corresponding to a Kappa number below 10; (c) when the pulp has a residual lignin content corresponding to a Kappa number greater than 10 bleaching the pulp with the sequence chlorine, caustic extraction, chlorine dioxide, caustic extraction, chlorine dioxide (CEDED) using a significantly reduced amount of elemental chlorine charge. 18. A process as claimed in claim 17 in which the dioxirane is dimethyldioxirane. 19. A process as claimed in claim 17 in which the dioxirane is contacted with the pulp at a temperature in the range of about 5* to 50 β C for a time up to about 90 minutes.

Inventors: LEE CHUNG-LI, LEE, CHUNG-LI
Applicants: PULP PAPER RES INST, PULP AND PAPER RESEARCH INSTITUTE OF CANADA
Application year: 1990
Main classification: D21C9
IPC codes: D21C9