FI102195B - Bleaching of lignocellulosic material with dioxiranes - Google Patents

Description

102195

This invention relates to a process for bleaching a chemical pulp.

In papermaking, wood is usually converted to pulp by chemical means and then applied to a wire to remove water 10 to form paper.

Sulfate pulp cooking is the most widely used form of chemical pulp cooking. The pulp is relatively dark in color and, in order to produce commercially acceptable paper, it usually needs to be bleached before being converted to paper.

Different bleaching agents are used. They include chlorine, chlorine dioxide, hypochlorites, hydrogen peroxide and oxygen. 20 Usually more than one of these chemicals is needed and they are used alone or as a mixture in a cycle. Various bleaching compounds are denoted by the letter in the art: chlorine is C, chlorine dioxide is D, caustic extraction is E, hypochlorite is H, hydrogen peroxide is P and oxygen is O.

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The above compounds include chlorine and certain chlorine-containing compounds, and the use of chlorine and chlorine compounds in pulp bleaching is a concern. Chlorine and chlorine-containing compounds react with pulp constituents, mainly lignin, fatty acids and rosin acids, to produce chlorinated organic compounds. Some of these chlorinated organic compounds are of environmental concern and some large pulp producing countries have limited the amount that can be released from the bleaching process into water bodies. It is reasonable to assume that most countries will enact legislation that restricts the release of such compounds.

2 102195

Elimination of elemental chlorine and similar compounds from the bleaching process would eliminate the production of chlorinated organic compounds in wastewater, but efforts here have not been fully effective.

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The use of elemental chlorine and similar compounds in bleaching can lead to the formation of tetrachlorinated dioxins (TCDD) and furans (TCDF) contained in pulp and pulp mill effluent. However, the formation of these two toxins can be eliminated by reducing the amount of chlorine used. Unfortunately, this is difficult to accomplish with existing technology without sacrificing some pulp quality or causing other problems for cellulose cooking.

Oxygen removal of lignin is a process in which the pulp is treated at an oxygen pressure of about 689,476 kPa, at an elevated temperature of about 80-120 ° C and for an extended period of time of about 30-90 minutes. This method of deoxygenation of lignin tends to produce a weak mass and the removal of lignin is extended too far, i. Up to about 10 for the Kappa number. In this respect, the Kappa number of the pulp is a measure of the whitenability of the pulp. It is a figure indicating the number of milliliters of 0.1 N potassium permanganate solution that can be consumed under standard conditions per gram of mass (oven-dried per-25 bases), corrected for 50% permanganate consumption.

U.S. Patent 4,439,271 to O. Samuelson discloses a process using nitrogen dioxide treatment prior to lignin deoxygenation. The method slows down the degradation of cellulose depolyme-30 during oxygen bleaching. However, the transfer of nitrogen-containing compounds from bleaching effluent raises other problems and these nitrogen compounds cannot be recovered by existing chemical recovery systems for sulphate pulp cooking.

35 U.S. Patent 4,404,061 to J.J. Cael, it is described that pulps produced from standard time pulp cooking methods can be bleached with monoperoxysulfate to produce papermaking pulps and bleached pulps. U.S. Patent 4,756,800 to E. L. Springer also discloses a process for preparing salts of monoperoxy sulfuric acids and for the simultaneous preparation of pulp. Neither patent describes the strength properties of fully bleached pulps.

There is a need for a bleaching process that produces good quality pulp and does so to reduce environmental impact.

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Dioxiranes are known compounds of the general structural formula:

O - O

R 1 and R 2 wherein R 1 and R 2 may be aliphatic or aromatic groups and may be linked to form cyclic compounds. The dioxirane compounds and their preparation have been described, for example, by R.W. Murray and R. Jeyaraman, Journal of Organic Chemistry, 5 £ 20947-20853 (1985).

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The present invention is a process for bleaching chemical pulps without the 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.

Thus, in its broadest aspect, the present invention is a method for bleaching a chemical pulp, comprising contacting the chemical pulp with dioxirane 35.

Dioxirane can be used with known bleaching agents either in combination or in section, 4 102195

The pulp is preferably one with a Kappa number in the range of 15-60 if derived from softwood species and 5-25 if derived from hardwood species. The dioxirane is preferably contacted with the pulp at a temperature of about 5-50 ° C for up to about 90 minutes. Dioxirane is preferably used in an amount sufficient to provide an active oxygen charge, i. one active oxygen atom per molecule of dioxirane, 0.2-1.0% based on the oven-dried mass. Dioxirane has the structure: 10 ° \ 7 ° 15 "V2 wherein R 1 and R 2, which may be the same or different, are each an aliphatic or aromatic group and may be linked together to form a ring.

One dioxirane compound that has been shown to be useful is dimethyldioxirane, but any dioxirane compound is suitable, for example, those compounds wherein R 1 and R 2 are each alkyl groups having 1 to 4 carbon atoms.

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If a later bleaching step is used, it is after the dioxirane treatment when the Kappa number of the residual lignin in the dioxirane-treated pulp is less than 10, so that the subsequent bleaching of the pulp can be carried out with chlorine dioxide (D) alone or in a phase with chlorine dioxide, caustic extraction, chlorine dioxide. When bleaching is performed using the DED cycle, the cycle can be set to D- | ED2. Chlorine dioxide is used in the Dj step in the range of 0.5-0.1%, preferably about 0.8%, caustic E in step 35 is in the range of 0.2-0.7%, preferably about 0.4%, and chloride oxide D2 ~ is in the range of 0.2-0.7%, preferably about 0.4%. All percentages are expressed as the weight of the oven dry mass. If chlorine dioxide is used alone, it is in the range of 0.1 to 1% by weight of the oven-dried mass, preferably about 0.8% of the oven-dried mass.

When the residual lignin content of the dioxirane-treated pulp corresponds to a Kappa number greater than 10, subsequent bleaching can be performed using the cycle: chlorine, caustic extraction, chlorine dioxide (CEDED). Expressed in this section, the section CE- | D-jE2D2 uses a substantially reduced amount of chlorine charge in step C, in step E- | the caustic charge is 60% of that used in step C, 0.5-1.2% chlorine dioxide is used in step D1, the caustic charge in step E2 is 50% of that used in step D1 and 0.25 is used in step D2. -0.6% chlorine dioxide charge. The figures are based on the weight of the oven dry mass. The chlorine used in step C is usually used in combination with various amounts of chlorine dioxide.

The invention is illustrated in the examples, which are given to illustrate and not to limit the invention.

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Example 1

Aspen pulp (50 g oven dry weight) prepared by the sulphate method to a Kappa number of 16.4, 39% pulp consistency, was treated with a solution of dimethyloxirane (DMD) in acetone (280 ml, 4.59 g / l concentration, i.e. 0.55% active oxygen by oven dry mass) at 25 ° C for one hour. The dimethyloxirane-treated pulp was then bleached with chlorine dioxide (0.8% by weight of the oven-dry pulp) at 70 ° C for three hours.

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From another sample of the same aspen sulphate pulp (100 g oven dry weight) lignin was removed with molecular oxygen at 10.7% pulp consistency using sodium hydroxide 4% by weight kiln dry pulp), magnesium sulphate (0.5 wt% 35% kiln dry pulp), molecular oxygen ( 689.476 kPa) at 115 ° C for one and a half hours. This molecular oxygen-depleted lignin pulp was then bleached with elemental chlorine (1.8% by weight of available chlorine from the oven-dry pulp) at 20 ° C for one hour at 3 102195% pulp, extracted with sodium chloride (1.1% by weight of oven-dried pulp). from pulp) at 74 ° C for two hours at 12% pulp consistency and further deprotected with chlorine dioxide (0.5% by weight of oven dry pulp) at 74 ° C for three 5 hours at 6% pulp consistency. This bleaching cycle is called OCED.

A third sample of the same aspen sulfate pulp (50 g oven dry weight) was also bleached by conventional method 10 using a CED cycle. Elemental chlorine bleaching was performed at 3% pulp consistency and at 20 ° C for one hour using 3.3% available chlorine charge from the oven dried pulp, extraction at 74 ° C for two hours using sodium hydroxide (2.0% by weight in the oven dried pulp) at 12% pulp. 15 and removal of chlorine dioxide (1.0% by weight in oven-dry pulp) lignin at 74 ° C for three hours at 6% pulp consistency.

The results shown in Table 1 show that aspen sulphate 20 pulp treated with DMD to 80% lignin removal level has better mechanical properties than molecular oxygen treated to 67% lignin removal level. This fully bleached pulp of 86.8% Elrephos treated with DMD has a similar "zero span" -25 tensile strength as the pulp of 89.9% Elrephos bleached by the conventional CED bleaching method.

table 1

Physical and optical properties of aspen sulphate pulp bleached with dimethyloxirane or molecular oxygen or conventional bleaching method «7 102195" Zero

Lignii- span "-

Aspen- Kappan aspiration- Elrephomass_number_total (%) lui ♦ (km) brightness (%) 5 Unbleached 16.4 - 16.3 40.6 DMD treated 3.4 80 17.0 59.4 Bleached with DMD-D 10 - - 17.4 86.8

Chili delignified 5.4 67 10.6 70.7 0CED bleached 10.0 91.5 15 CED bleached - - 18.4 89.9 In these examples, Elrepro brightness was measured on Technobrite (trademark) Micro TB-1C- at 457 nm. The Standard 20 test is presented in TAPPI T-452 om-87 and CPPA E.1.

Zero span tensile strength was measured according to TAPPI T231 cm-85 using the Pulmac Instruments' The Troubleshooter (trademark) zero span tester.

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Example 2

A third sample of aspen pulp (50 g oven dry weight) prepared by the sulfate method to a Kappa number of 16.4, 39% pulp consistency, was treated with a solution of DMD in acetone 30 (170 mL, 4.44 g / L concentration, i.e. 0.33 wt%). active oxygen in the oven-dry mass) 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 further stripped of lignin with chlorine dioxide 35 (1.0% by weight of oven-dried pulp) at 74 ° C for three hours at 6% pulp consistency, extracted with sodium hydroxide (0.72% by weight of oven-dried pulp) at 10% pulp consistency: and 74 ° At C for two hours and bleached in a second chloride-8 102195 oxide step (0.5% by weight of oven dry pulp) at 6% pulp consistency and at 74 ° C for three hours.

A fifth sample of the same aspen sulphate pulp (100 g 5 oven dry weight) was stripped of lignin with molecular oxygen at 12% pulp consistency using sodium hydroxide (4.0% by weight of oven dry pulp), magnesium sulphate (0.5% by weight of oven dry pulp), molecular oxygen (689 ° C for one and a half hours. This lignin-depleted pulp with molecular oxygen was further bleached with elemental chlorine (2% by weight of available chlorine from oven dry pulp) at 20 ° C for one hour at 3% pulp consistency, extracted with sodium hydroxide (1.2% by weight of oven dry pulp) at 12% pulp. at a consistency of 74 ° C-15 for two hours and bleached with chlorine dioxide (0.5% by weight of oven dry pulp) at a consistency of 6% pulp and 74 ° C for three hours.

In this particular example, lignin was removed from the aspen sulfate pulp to the same extent (63%) with either DMD bleach or molecular oxygen. The results shown in Table 2 show that fully bleached aspen sulfate pulp produced from DMD treatment followed by DED bleaching without elemental chlorine has better "zero 25 span" tensile strength than molecular oxygen delignification followed by CED bleaching 2. % of the available chlorine charge from the kiln dry mass produced in the elemental chlorine step. On the other hand, the pulp treated with DMD produces a completely white pulp with a zero span tensile strength similar to that produced by a conventional method involving the use of elemental chlorine for bleaching.

Table 2 Zero Span Tensile Strengths of DMD-Treated Molecular Oxygen Delignified Pulps 35 and Pulps Produced by Conventional Methods 9,102,195 "Zero

Lignii- span "-

Aspen- Kappan withdrawal- Elrephomass_number_table (¾) strong. (km) brightness (¾) 5 Unbleached 16.4 to 16.3 40.6 Treated with DMD 6.0 63 15.0 54.0 DMD-DED 10 Bleached - - 15.9 91.4 02: de - Lignified 6.1 63 12.7 91.0 Bleached with OCED - - 12.6 91.0 15 Bleached with CED - - 18.4 89.9

Example 3

A fourth sample of aspen pulp (50 g oven dry weight) prepared by the sulphate method to a Kappa number of 16.4, 35% pulp consistency, was treated with a solution of DMD in acetone (643 ml, 2.0 g / l concentration, i.e. 0.55 wt.%). -% active oxygen in the oven-dry mass) at 25 ° C for one hour. The resulting mass was then washed throughout with a large amount of de-25 ionized water.

A seventh sample of the same aspen sulphate pulp (100 g oven dry weight) was lignin removed with molecular ha-Pella at a consistency of 12% by mass using sodium hydroxide 30 (4.0% by weight of oven dry mass), magnesium sulphate (0.5% by weight of oven dry mass), molecular oxygen; (689,476 kPa) at 1150 ° C for one and a half hours. The resulting oxygen-depleted lignin-free mass was washed throughout with a large amount of deionized water.

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The results shown in Table 3 show that the yield of DMD-treated pulp is better than that of molecular oxygen: bleached to the same delignification level. This illustrates that DMD has better selectivity for lignin than molecular sieve with oxygen.

Table 3 5 Mass yields after DMD treatment and lignin deoxygenation

Aspen- Kappa- Lignin Unbleached pulp_number_demission level (%) pulp yield% 10 Unbleached 16,4-DMD treated 5.3 68 98.0 02 delignified 5.4 67 92.7

Example 4

Spruce pulp (50 g oven dry weight) prepared by the sulphate method to a Kappa number of 32, 37% pulp consistency was treated with a solution of DMD in acetone (330 ml, 4.6 g / l concentration, i.e. 0.65 wt%). active oxygen from the oven-dry mass) at 25 ° C for half an hour. The DMD-treated pulp was further bleached in a CED cycle. Elemental chlorine bleaching was performed at 3% pulp consistency and at 20 ° C for one 25 hours using 2.8% by weight of available chlorine from the oven-dry pulp, extraction at 74 ° C for two hours using sodium hydroxide (1.7% by weight from the oven-dry pulp). by weight) at 12% pulp consistency and lignin removal with chlorine dioxide (0.8% by weight of oven dry pulp) at 6% pulp consistency and 74 ° C for three hours.

A second sample of the same spruce sulphate pulp (50 g oven dry weight) was bleached by the conventional method using the CED cycle. Bleaching with elemental chlorine was performed at 3% pulp consistency and 20 ° C for 1 hour using 6.4% available chlorine from oven dried pulp, extraction at 12% pulp consistency and 74 ° C for two hours using sodium hydroxide (3% by weight kiln dry pulp 11 102195 ta) and chloride dioxide (1% by weight of the oven dry pulp) bleached at 6% pulp consistency and 74 ° C for three hours.

The results shown in Table 4 show that the DMD-treated pulp can be bleached to the same brightness as bleached by conventional methods. The former uses a significantly lower amount of elemental chlorine compared to what is required in conventional methods. The zero 10 span tensile strengths and viscosity of the bleached pulps in both methods are similar.

Table 4 Physical and optical properties of spruce sulphate bleaches bleached by DMD or conventional methods 15 "Zero

Lignii- span "-

Aspen- Kappan aspiration- Viscosity Elrephomass_number level (%) strong. (km) (mPa.s) _brightness (¾)

Unbleached 20.0 to 19.4 35.0 32.7 EMD treated 18.8 41 18.8 23.0 40.2 25 Bleached with DMD-CED - - 18.3 18.3 82.5 CED Bleached - 19.1 25.0 82.3 30 Thus, the present invention provides a process for treating pulp which can bleach completely 90% of Elrepro from the trace, either without elemental chlorine or with small amounts of elemental chlorine. DMD-treated pulps with a residual lignin content of 5 Kappa or less have pulp strengths equivalent to those of untreated pulps. The strength properties of the fully bleached pulps produced by the process of the invention are similar to those of pulps produced by conventional bleaching processes using a high elemental color charge.

Bleaching of chemical pulps with dioxirane compounds can be modified in the range of bleaching conditions, bleaching cycles, and combinations of molecular oxygen and other bleaching agents, for which bleaching agents are not specifically exemplified herein, for example, hydrogen peroxide, oxygen, and monoperoxysulfate. The dioxirane treatment can also be combined with lignin deoxygenation before or after the oxygen treatment. The use of two oxygen-containing compounds can have advantages in eliminating the washing step required between the steps of the cycle when chlorine and chlorine compounds are used.

The pulps used in this invention may be sulfate, sulfite, soda, or other types of chemical pulp from hardwood and softwood species. The invention encompasses chemical pulps and the term is intended to encompass pulps prepared by treating wood species with organic solvents to separate wood components.

Claims (14)

A method for bleaching a chemical pulp, characterized in that the chemical pulp is contacted with dioxirane. Method according to Claim 1, characterized in that the Kappa number of the chemical pulp is in the range from 15 to 60 when derived from softwood species and from 5 to 25 when derived from 10 hardwood species. Process according to Claim 1, characterized in that the dioxirane is contacted with the pulp at a temperature in the range from about 5 to 50 ° C for up to 90 minutes. Process according to Claim 1, characterized in that it comprises, when the residual lignin of the pulp treated with dioxirane corresponds to less than 10 Kappa units, bleaching the pulp with chlorine dioxide (D) or in the section chlorine dioxide, caustic extraction, chlorine dioxide (DED). Process according to Claim 4, characterized in that the bleaching is carried out using a Dx E D2 cycle using chlorine dioxide in the Di stage in the range from 0.5 to 1%, caustic in the E stage in the range from 0.2 to 0.7% and chlorine dioxide In step D2 0.2-0.7% with all percentages by weight of the oven-dried pulp. Process according to Claim 5, characterized in that it contains about 0.8% of chlorine dioxide in step D, about 0.4% of the caustic in step E and 0.4% of chlorine dioxide in step D2. Process according to Claim 4, characterized in that the bleaching is carried out with chlorine dioxide in the range from 0.5 to 1% by weight of the oven-dried pulp. 102195 Process according to Claim 7, characterized in that it contains chlorine dioxide in an amount of about 0.8% by weight of the oven-dried pulp. Process according to Claim 1, characterized in that the bleaching is carried out using the sequence chlorine, caustic extraction, chlorine dioxide, caustic extraction, chlorine dioxide 0 to 2ϋ2 using in step C an amount of chlorine suitable for the Kappa number after contact with dioxirane in step 10. Εχ the caustic charge is 60% of the charge used in step C, in step Όχ the charge of chlorine dioxide is 0.5-1.2%, in step E2 the caustic charge is 50% of that used in step Dx and in step D2 0.25-0, 6% chloro-15 silica charge based on percent oven-dried pulp. A process according to claim 1, characterized by a) contacting the pulp with a dioxirane of formula 0-0 v / l wherein Rx and R2 are each an aliphatic or aromatic group and which can be coupled together to form a ring in active oxygen loading of 0.2- 1% by weight of oven-dried pulp,: (b) bleaching of the pulp obtained using the cycle chlorine dioxide (D) or chlorine dioxide, caustic extraction, chlorine-dioxide (DED), when the residual pulp content of the pulp is less than 10, (c) when the pulp has a residual lignin content corresponds to a Kappa number greater than 10, from pulp bleaching in the section chlorine, caustic extraction, chlorine dioxide, caustic extraction, chlorine dioxide 102195 (CEDED) using a significantly reduced amount of elemental chlorine input. Process according to Claim 10, characterized in that and R 2 are each an aromatic group. Process according to Claim 10, characterized in that R 1 and R 2 are each aliphatic groups. Process according to Claim 12, characterized in that the aliphatic groups are alkyl groups having 1 to 4 carbon atoms. 13 102195 Process according to Claim 10, characterized in that the dioxirane is dimethyldioxirane.