AU642652B2 - Bleaching of lignocellulosic pulp - Google Patents

Description

COMPLETE SPECIFICATION FOR OFFICE USE Application Number: Lodged: Class: Int. Class: 6% 2 Complete Specification Lodged: Accepted: Published: Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventors: Address for Service: ELDERS RES(URCES NZFP LIMITED NZFP House, 110 Snmonds Street, Auckland, New Zealand Allan James KERR ind ;bert Neil JONES SMITH SHELSTON BEADLE 207 Ri.versdale Road (P 0 Box 410) Hawthcrn Victoria 3122 Australia (Attorney Code SA) Complete Specification for the invention entitled: BLEACHING OF LIGNOCELLULOSIC PULP The following statement in a full description of ,his invention, including the best method of performing it known to us: Page 1 Our Ref: #5659 JC:BC:B 3eld m This invention relates to the bleaching of chemically digested lignocellulosic pulp, and more particularly but not solely to the bleaching of pulp digested by alkaline processes such as the kraft (sulphate), polysuiphide, or soda pulping processes. The invention may also be applied to neutral or alkaline sulphite pulping, and to pulping by any of these processes wherein additives such as anthraquinone for example are used during digestion.

i0 For many uses of pulp and paper products there is a requirement or market preference for bleached grades.

These have been traditionally produced by subjecting the partially delignified pulp from the digester to a further delignification process or sequence involving treatment with chlorine and/or chlorine dioxide, and caustic extraction.

More recently, there has been an increasing interest in the use of alternative bleaching sequences due to recognition of the undesirable environirental consequences of discharging chlorine containing waste compounds from the pulpmill into water bodies. These wastes have been found to he mutagenic in some cases, and to cause adverse effects on aquatic organisms.

Efforts have been made to eliminate chlorine com- 2

I

pounds altogether and little or no distinction has been made between chlorine and chlorine dioxide.

A typical prior art bleaching sequence based upon chlorine and chlorine dioxide is: chlorinationcaustic extraction-chlorine dioxide treatment- caustic extraction- chlorine dioxide treatment, which may be abbreviated to C-E-D-E-D. The pulp is washed between each stage principally with water, which may contain spent pulping liquor, or other effluents to conserve water consumption. Counter current washing is often used to advantage in both reducing water consumption and effluent volume.

Oxygen gas delignification in the presence of sodium hydroxide as an active alkali has been extensively investigated but this has not been found to be an alternative on its own because if delignification is advanced beyond about 50%, depolymerisation and degradation of the cellulose component of the pulp becomes excessive, with an unacceptable reduction in pulp and paper properties.

However, oxygen gas delignification is now becoming widely used as a partial bleaching sequence Sin pulpmills, and this reduces the amount of chlorine needed in the following stages considerably, to perhaps half that otherwise required for the same 3 result. A bleaching sequence based upon a combination of oxygen and chlorine or chlorine dioxide stages is for example the sequence; oxygen delignificationtreatment with a mixture of chlorine dioxide and chlorine-caustic extraction-chlorine dioxide treatment-caustic extraction-chlorine dioxide treatment, which may be abbreviated to O-D/C-E-D-E-D.

Another example, of a shorter sequence which may be used and wherein following activation with a mixture of chlorine dioxide and chlorine a single oxygen delignification stage is used, followed by a final chlorine dioxide stage is: D/C-O-D. Although this short sequence has lim.ted application it can often yield consistent bleaching of pulp from suitable wood species provided relatively high chlorine charges are used in the D/C stage.

It should be mentioned that in stages where a mixture of chlorine dioxide and chlorine are used, they may be introduced to the pulp either together or sequentially, and in the latter case the chlorine dioxide is usually added first.

It is also known to activate pull by treatment with nitrogen dioxide gas prior to oxygen delignification, and ozone and hydrogen peroxide for example have also been proposed as an alternative to 4 oxygen. Various additives have also been proposed as sLabilizing or complexing agents during these bleaching steps, including polycarboxylic acids, polyphosphates and magnesium compounds for example.

There are many variants on bleaching sequences and the abovementioned are given as examples only, and not intended to be in any way comprehensive, as there are many more. In all cases the pulp is washed between each stage, which is the common practice in bleaching processes, except in the case of sequential addition of chlorine dioxide and chlorine within the same stage. As a general rule the wash effluent from stages not containing chlorine are returned to the pulpmill chemical recovery system and most of the effluent containing chlorine compounds is discharged as waste. The effluent from chlorine containing bleaching stages is typically not recovered as it is a potential cause of serious corrosion problems in the pulpmill, and this of course is one of the fundamental conflicts between commercial considerations and care of the environment, especially the resultant pollution of the receiving body of water into which the mill effluent discharges.

In very recent developments an awareness has grown that the various polychlorinated compounds 5 formed during bleaching sequences involving chlorine are not only discharged with effluent to present an environmental problem, but also remain to some extent in the bleached pulp itself. Furthermore, they have been found to migrate from containers made from the bleached pulp to foodstuffs which the containers may hold. Some of these compounds are polychlorinated dibenzo-p-dioxins (PCDD'S), and although the amounts present in the pulp are very small, these compounds are thought by many to be amongst the most toxic known to man. They are at the very least undesirable from a marketing viewpoint as the public can reasonably be expected to avoid pulp and paper products containing them where there is a choice, irrespective of whether there is a known health risk or not.

Iy recent years, bleaching technology has come a long way, and oxygen delignification in particular, although unable to completely replace bleaching sequences using chlorine compounds has played a major 2 role in this progress. Further improvements however must be economically as well as technically viable before they can be used commercially and any modifications to bleaching technology which can offer savings in either capital or operating costs is of great commercial significance.

6 -7- There is therefore a need to provide an economic method for manufacture of bleached pulp and paper which eliminates or at least minimises the production of polychlorinated compounds during bleaching, or which at least offers the public a useful choice in their selection of bleached pulp and paper products.

It is an object of this invention to provide an improved method for bleaching chemically digested lignocellulosic pulp or to at least provide the public with a useful choice.

According to one aspect of this invention there is provided a method for delignifying, at least in part, chemically digested lignocellulosic pulp, the method including a first oxygen delignification stage followed by a washing stage, then followed by a stage having the combined steps, without washing between, of an activation step wherein the pulp is treated with an activating agent rendering the pulp more responsive to subsequent delignification treatment, followed by (ii) an alkaline extraction step with oxygen delignification.

According to another aspect of this invention there is provided an improved method for producing bleached chemically digested lignocellulosic pulp, said method including a first oxygen delignification stage followed by an activation stage wherein the G:AM:3659.RE 20 93 GN:AM:03:5659.RE3 20 8 93 FsY311[111JIPZq$JBs~l -r -I IL a~s~--IIL~ partially delignified pulp is treated with at least one activating agent (as defined herein), then followed by a second oxygen delignification stage, said method being characterised in that the pulp is not washed between said activation stage and said second oxygen delignification stage.

According to another aspect of this invention there is provided an improved method for producing bleached chemically digested lignocellulosic pulp, said method including a first oxygen delignirication stage followed by an activation stage wherein the partially delignified pulp is treated with chlorine dioxide and/or chlorine, then followed by d second oxygen delignification stage, said method being characterised in that the pulp is not washed between said activation stage and said second oxygen delignification stage.

According to another aspect of this invention there is provided a bleached chemically digested lignocellulosic pulp free from or substantially free from polychlorinated dibenzo-p-dioxins, said pulp being further delignified after chemical digestion by Susing a bleaching aequence including a first oxygen delignification stage followed by an activation stage wherein the partially delignified pulp is treated 8 4 I LCC IIC with at least one activating agent (as defined herein), then followed by a second oxygen delignification stage, and wherein the pulp is not washed between said activation stage and second oxygen delignification stage.

According to another aspect of this invention there is provided a bleached chemically digested lignocellulosic pulp free from or substantially free from polych.orinated dibenzo-p-dioxins, said pulp being further delignified after chemical digestion by using a bleaching sequence including a first oxygen delignification stage followed by an activation stage wherein the partially delignified pulp is treated with chlorine dioxide and/or chlorine, then followed by a second oxygen delignification stage, and wherein the pulp is not washed between said activation stage and said second oxygen delignification stage.

The invention which includes the foregoing will now be described by way of example only, with reference to the bleaching of kraft (sulphate) pulp. The invention may also be used with other chemically digested lignocellulosic pulps such as those produced by polysulphida, soda, and neutral or alkaline sulphite processes.

In this specification we refer to Kappa Number 9 which describes the degree of delignification of lignocellulosic pulp as determined by TAPPI Etandard Test T-236, the test being known and widely used in the pulp and paper industry.

We also refer to the consistency of pulp which is the oven dry weight of pulp fibre present in a pulp slurry, expressed as a percentage of the total weight of the slurry. The fluid component of the slurry is typically aqueous, being water often with pulpmill chemicals and dissolved organic materials other than fibre. Typically in the industry, and for the purposes of tle present invention high consistency is from 20% to 40%, medium consistency is :rom 6% to 20%, and low consistency is up to 6%.

The purpose of bleaching is to increase the brightness of the pulp in terms of its reflectivity to light, and the brightness values used in this specification are in accordance with the test method ISO 3688.

2U In describing bleaching sequences in this specification we use the following abbreviations: C Means a stage using chlorine.

0 M ns a first oxygen delignification stage which may optionally be pre:ceded by an activation step, and/or which may be 10 c' I conducted in the presence of additives in addition to alkali.

E Means an alkaline extraction stage the usual alkali being sodium hydroxide.

Eo Means an oxygen delignification stage o her than a first stage.

D Means a chlorine dioxide stage.

D/C Means a stage using a mixture of chlorine dioxide and chlorine.

±0 Means that any stages enclosed within the brackets are conducted without the usual intermediate washing step therebetween.

O (x Means a first oxygen delignification stage followed by a combination of an activation and an oxygen delignification stage without the usual washing step therebetween, followed by a plurality of unspecified final bleaching stages.

In the manufacture of bleached grades of chemically digested lignocellulosic pulp such as kraft (sulphate) pulp for example the lignocellulose, such as wood chips are first digested in either a batch or continuous digester to a Kappa Number of 25 to For bleached grades of high brightness (near reflectivity) the delignification must be progressed 11 to a Kappa Number of about 5 to 10. As a first stage oxygen delignification will only lower the Kappa number by 40% to 50% to about Kappa Number 18 without deterioration of the carbohydrate component of the pulp further delignification processef must be employed.

It is necessary to conduct the further delignification processes in a most cost effective way in order to provide bleached grades economically in competition with mills still using chlorine as their main bleaching chemical.

In the prior art when consideration has been given to eliliination of polychlorinated compounds from p' ipmill effluents, less attention has been given to the differences in chemical action between chlorine and chlorine dioxide than might have been.

The principal chemical action of chlorine is chlorination while that of chlorine dioxide is oxidation.

The formation of polychlorinated compounds becomes less likely therefore, with increased substitution of chlorine by chlorine dioxide. Anrher advantage of using chlorine dioxide is that overall the bleaching system contains significantly less chlorire and it becomes feasible to return more bleach plant effluent to the pulpmill chemical recovery. Increased use of 12 chlorine dioxide however increases costs, since compared with chlorine, chlorine dioxide requires about three times the energy cost to produce. In the present invention this increase in chemical cost is offset by a reduction in bleach plant capital and operating cost so that overall the economics remain much the same, while the formation of polychlorinated compounds is greatly reduced or almost eliminated altogether.

We have found that after an initial oxygen delignification stage the next activation stage and extraction stage may be combined by entirely eliminating the normal washing stage therebetween and that high grade bleached pulp may be produced by the new method.

The advantages of the new method are that the significant capital cost of the washing plant and the operating cost and repairs and maintenance are eliminated, and there is no wash effluent stream from 2G within the new combined stages. The savings may be applied to offset higher chemical costs, allowing a preference to use a.'.ternatives such as chlorine dioxide rather than chlorine in the appropriate stages of the bleaching sequences.

It will be appreciated that where we refer to 13chlorine dioxide in this invention we mean technical grade chlorine dioxide as produced in the pulpmill chemical plant, which is substantially free from chlorine although it will usually contain small amounts.

Examples of bleaching sequences which may be used in the present invention are: 1. O (C Eo) D E D 2. O (C Eo) D Eo D 3. O (D/C Eo) D E D 4. O (D/C Eo) D Eo D O (D Eo) D E D 6. O (D Eo) D Eo D 7. 0 (x Eo) Chlorine when used in the present invention is restricted to the combined second stages as shown in th= sequences 1 to 4 above. In this position we have found that good results can be obtained from small amounts, to advantage in meeting the overall objective of minimising the use of chlorine and the formation of unwanted polychlorinated compounds in the pulp product and the effluent.

More preferably, chlorine dioxide is used in the combined second stages of the invention as shown in sequences 5 and 6, these examples being preferred fo=ms of the invention. The chlorine dioxide, being the normal technical grade as produced in the 14 chemical plant, while not being entirely free from chlorine will contain minor amounts only. Some of the added advantages of sequences 5 and 6 are that due to the high efficacy of the chlorine dioxide during the activation step of the combined (D Eo) stages all of the effluent may optionally be returned to the chemical recovery system without contributing an unacceptable chloride load to the plant.

Sequence 7 shows the present invention in the broadest form, which is a first oxygen delignification stage, followed after washing of the partially delignified pulp by a combined activation and oxygen delignification stage without washing therebetween, followed by an unspecified number of 13 final bleaching stages. The bleaching sequence numbers identified above are used throughout this specification.

In the combined activation and oxygen delignification stage, the activating agent need not be chlorine, chlorine dioxide, or a combination of these two, but may be an activating agent selected from nitrogen dioxide, ozone, hydrogen peroxide, and/ or mixtures thereof.

Prior to the combined stages and after the first oxygen delignification stage it is important that the 15 partially delignified pulp be effectively washed to avoid excessive chemical usage in the following combined stages. In the more preferred form of the invention using chlorine dioxide prior to the Eo delignification in the combined stages shown in this specification enclosed in brackets, the activation treatment is preferably conducted under acidic conditions, but preferably not below pH 2.5, and more preferably within the range pH 3.0 to pH although pH outside this range may be used if desired. To establish and maintain this pH ra' r the pH may be monitored in the combined stages between the D and the Eo stage and a suitable acid added after the washing step of the first O st Sulphuric is a suitable acid although alternative acids may be used, but hydrochloric is not recommend as it introduces additional chlorine atoms to the system.

All stages of the delignification sequences used in the invention will normally be conducted at medium consistency, although high consistency first stage oxygen delignification may optionally be used if desired to suit a particular pulpmill installation.

Consistency is not a constraint so far as the 16 p~~pp~~pp i M 040 WbAI1I I l( e~I chemical reactions are concerned of course, and given suitable equipment consistencies outside of the medium consistency range which we have defined as from 6% to 20% may be used.

The bleaching sequences employed in this invention have been found particularly suitable for use with chemically digested lignocellulosic pulp derived from Pinus species, more particularly Pinus radiata, especially when digested by the kraft isulphate) pulping process.

Examples of the use of the invention in bleaching kraft pulp produced from Pinus radiata will now be described using data obtained from laboratory experiments, the chemical usage being expressed by weight on oven dry pulp in all cases.

Studies were also conducted to compare chlorine and chlorine dioxide in the activation stages, with the chlorine activations selected to match the chloride generating potential of the chlorine dioxide levels used. Surprisingiy, we found that the chlorine dioxide gave superir- results at the same chloride generating level. In other words, chlorine dioxide generates less chloride for the same bleaching results, and since it is the amount of chloride present in the effluent which limits its 17 I return to the pulpmill chemical recovery system we have found that there is a si'-iificant advantage in using chlorine dioxide where possible. Apart from the greater efficacy found with chlorine dioxide the coimparative chloride generating potential of equivalent amounts of chlorine and chlorine dioxide in the bleach plant reactions are:

CIO

2 4H 5e Cl 2H 2 0 Cl 2 2e 2 Cl- (e electrons) Therefore: 1 Kg of CIO 2 produces 0.52 kg of chloride.

1 Kg of Cl2 produces 1 kg of chloride.

There is therefore a multiple advantage in using chlorine dioxide in that: It is more effective than chlorine for an equivalent amount.

It produces less chloride than chlorine for an equivalent amount.

It does not of itself chlorinate and give rise to the emounts of polychlorinated compounds produced when chlorine is used.

A disadvantage is that it require more energy to produce than chlorine does.

18 I- 1, c I- i Advantage may be taken of this situation in the present invention by offsetting the higher cost of chlorine dioxide against the savings due to the absence of washing in the combined activation and extraction stages used in the invention. It will be appreciated that some chlorine may be used, but the invention allows this to be minimised, or if desired eliminated altogether as indicated in the examples.

The number and nature of the final bleaching stages following the combined activation and delignification stages is optional as indicated by sequence 7 herein. and this will depend to som': extent upon the circumstances and equipment prevailing in a particular pulpmill, and the brightness required in the bleached pulp product.

We have also found that the chemical charges may be varied over wide limits, and adjusted during the process to obtain optimum results. By way of example on y, and not intended to be limiting we indicate a range of conditions and chemical charges which may be used in preferred bleaching sequences of the invention employing chlorine dioxida. The first oxygen delignification stage may be conducted under conditions well known in the art either in a pressure 19 vessel or at atmospheric pressure using a high or medium consistency system.

Depending upon the apparatus employed and the wood species the conditions will 1,e dttermined to produce near 50% delignification of the brown stock pulp with the minimum of degradation thereof. With Pinus radiata pulp produced by the Kraft process we have found that delignification with oxygen for between 40 and 80 minutes at a temperature of between 80 and 130 degrees C with between 20 and 40 kg/tonne of sodium hydroxide and between 0 and 15 kg/tonne of magnesium sulphate provides suitable conditions in the 0 stage. As mentioned hereinbefore we have found good washing following this stage to be important to the outcome of the following stages and to economy of chemical consumption.

In the combined stages which follow the 0 stage, activation with chlorine dioxide within the range 1 to 7 kg/tonne at temperatures between 35 and 105 degrees C for between 7 and 30 minutes is suitable, and during this time the pH of the pulp suspension is preferably maintained within the range pH 2.5 to pH 7, and more preferably between pH 3 and pH 5 by adjustment if necessary with a suitable acid such as sulphuric acid. After activation and without 20 I washing, the pulp is subjected to a second oxygen delignification stage with between 8 and 36 kg/tonne of sodium hydroxide and between 6 and 15 kg/tonne of magnesium sulphate with oxygen. The pulp is now washed ready for final bleaching to obtain a high brightness. While any suitable prior art final bleaching sequence may be used, stages based upon chlorine dioxide activation and oxygen delignification are very much preferred to enable the process to be completed with the minimum of pollution. For example the pulp may be treated in a D stage with between 2.5 and 10 kg/tonne of sodium hydroxide and between 5 and 20 kg/tonne of chlorine dioxide at temperatures between 35 and 85 degrees C and for a time between 45 and 240 minutes. After activation and washing the pulp may be further delignified using between 8 and 22 kg/tonne of sodium hydroxide with oxygen at temperatures between 60 and 100 degrees C for a time between 60 and 120 riinutes.

The pulp may be finally treated with between 2 and kg/tonne of sodium hydroxide and between 4 and 14 kg/tonne of chlorine dioxide at temperatures between and 100 degrees C and for a time between 120 and 240 minutes.

We now provide further examples of the 21 production of bleached chemically digested lignoceilulosic pulp using bleaching sequences in accordance with the invention, wherein following oxygen delignification, combined activation and extraction stages are used without the customary washing stage therebetween.

In the following examples reference is made to Figures 1 and la, Figures 2, and 2a, Figures 3 and 3a, and Figures 4 and 4a, in which: Figures 1 and la show the bleaching sequence 0 (C Eo) D E D which is bleaching sequence 1 using various operating conditions and chemical charges.

Figures 2 and 2a show bleaching sequence 0 (C Eo) D Eo D which is bleaching sequence 2 using various operating conditions and chemical charges.

Figures 3 and 3a show bleaching sequence O (D Eo) D E D which is bleaching sequence 5 using various operating conditions and chemical charges.

Figures 4 and 4a show bleaching sequence O (D Eo) D Eo D which is bleaching sequence 6 using various operating conditions and chemical charges.

Example 1 Referring to Figure 1, this illustrates the use 22 of chlorine as the activating agent in the combined (C Eo) stage in accordance with the invention and bleaching sequence 1. The conditions under which the bleaching sequence was carried out are shown in Table 1, and the chemical charges used are shown in thk diagram itself. Fig"re lb shows a similar diagram in which the results obtained from the bleaching sequence shown in Figure 1 are given. As shown, the Kappa number of the kraft pulp from the digester was 27.4. After the O stage the Kappa number of the pulp was 15.8, giving 42% delignification. The pulp was then washed. After washing, the pulr was then treated in the combined stages firstly by activation with 10.5 kg of chlorine per tonne, and then without washing, subjected to an Eo treatment. The overall delignification obtained to this point was 71% and the pulp brightness was 45.1%. After the comLined stages the pulp was conventionally washed and iurther treated in one of two alternative D stages as shown, the alternatives differing in the chlorine dioxide charge used as shown. Following the usual washing, the pulps were each treated in an E stage before being again washed followed by treatment in a final D stage. Before the final D stage one of the two pulps 23 was again divided giving three alternative D stages as shown. Depending upon the chemical charges used, the brightness range obtained was between 86.4% and 87.8% ISO.

Example 2 Referring now to Figure 2, this illustrates the use of chlorine as the activating agent in the combined (C Eo) stage in accordance with the invention and bleaching sequence 2. The difference between b'eaching sequences 1 and 2 are in the treatments following the combined stages as can be seen in the diagrams. The conditions under which the bleaching sequence was carried out are shown in Table 2, and the c.iemical charges are shown in the diagram itself. The similar diagram shown in Figure 2b records the results obtained from the bleaching sequence illustrated in Figure 2. In the bleaching sequence shown in Figure 2 the Kappa number of the kraft pulp from the digester was 27.4 as previously.

After the O stage th- Kappa number of the pulp was 15.6, giving 43% delignification. The pulp was then washed.

After washing, the pulp was th.n treated in the combined stages firstly by activation with 4.4 kg of 24 chlorine per tonne, and then without washing, subjected to an Eo treatment. The overall delignification obtained to this point was 60% and the brightness was 41.1%. After the combined stages the pulp was conventionally washed and further treated in one of two alternative D stages as shown, the alternatives differing in the chlorine dioxide and sodium hydroxide charge used as shown. Following the usual washing, the pulps were each treated in an Eo stage before being again washed and each divided into two alternatives, giving four alternatives for treatment in final D stages, using two chemical charges as tnown. Depending upon the chemical charges used, the brightness range obtained was between 84.8% and 89.0%.8% ISO.

The examples given in Figure 1 and Figure 2 do not show a significant difference in results between use of a E stage or an Eo stage as the final bleaching stages. More importantly, the brightness ranges obtained are similar to those produced by conventional bleaching sequences, and illustrate that a combined (C Eo) stage may be employed in accordance with the present invention to provide high brightness bleached lignocellulosic lp, with advantages in reduced capital and operating cost, and technical 25 advantages in wash effluent management.

Example 3 Referring now to Figure 3, this illustrates the use of chlorine dioxide as the activating agent in the combined (D Eo) stage in accordance with the invention and bleaching sequence 5. The conditions under which the bleaching sequence was carried out are shown in Table 3, and the chemical charges used are shown in the diagram itself. Figure 3b shows a similar diagram in which the results obtained from the bleaching sequence shown in Figure 3 are given.

As shown the Kappa number of the kraft pulp from the digester was 27.4. After the 0 stage the Kappa number of the pulp was 15.8, giving 42% delignification. The pulp was then washed. After washing, the pulp was then treated in the combined stages firstly by activation with 4.0 kg of chlorine dioxide per tonne, and then without washing, subjected to an Eo treatment. Before the Eo treatment the activated pulp was divided into two alternatives so that alternative Eo treatments with differing chemical charge could be used as shown.

The overall delignification obtained to this point was 67% and the pulp brightness was 47.7 and 48.7, respectively, for the two alternatives. After the 26

I

combined stages each of the two pulps was conventionally washed and further divided to be treated in one of two further alternative D stages as shown, the alternatives differing in the chlorine dioxide and sodium hydroxide charges used as shown, giving four alternatives at this point. Following the usual washing, the pulps were each treated in an E stage before being again washed. One of each of the two alternatives was again divided into two alternatives differing in chlorine dioxide and sodium hydroxide charge used, to give six alternatives, and all six pulps were treated in a final D stage as shown. Depending upon the chemical charges used, the brightness range obtained was between 82.6% and 86.3%

ISO.

Example 4 Referring now to Figure 4, this illustrates the use of chlorine dioxide as an activating agent in the 0 combined (D Eo) stage in accordance with the invention and bleaching sequence 6. The conditions under which the bleaching sequence was carried ou:t are shown in Table 4, and the chemical charges used are shown in the diagram itself. The similar diagram shown in Figure 4b records the results obtained from 27 the bleaching sequence illustrated in Figure 4. As before the Kappa number of the unbleached kraft pulp from the digester was 27.4. After the 0 delignification stage the Kappa number of the pulp was 15.6, giving 43% delignification. The pulp was then washed. After washing, the pulp was then treated in the combined stages firstly by activation with 4.0 kg of cnlorine dioxide per tonne, and then wit'.out washing, subjected to an Eo treatment. The overall delignification obtained to this point was 67% and the pulp brightness was 45.6%. After the combined stages the pulp was conventionally washed, and divided into three alternatives. The alternatives were treated in one of thre3 alternative D stages, differing in chlorine dioxide and sodium hydroxide charge used as shown. Following the usual washing, the pulps v:<re each treated in an Eo stage before being again washed. each pulp alternative was again divided into two farther alternatives differing in chlorine dioxide and sodium hydroxide charge used, giving six alternatives in all, and treated in a final D stage. Depending upon the chemical charges used, the brightness range obtained was between 82.7% and 89.9% ISO.

Considering all of the results collectively it 28 is shown that whether chlorine or chlorine dioxide is used as an activating agent, combined stages may be employed in accordance with this invention to produce commercially acceptable bleached pulps of high brightness, with advantages in reduced capital and/or operating cost and improved effluent management.

Considering the results associated with the bleaching sequences shown in Figures 3 and 4, it is shown that the high brightnesses are maintained when chlorine dioxide is used in place of chlorine so hat not only are the advantages of the combined stages retained but the chlorine load in the system is greatly reduced. Furthermore since the functional reaction with chlorine is chlorination, and with chlorine dioxide is oxidLtion, the prospect of polychlorinated compounds such as polychlorinated dibenzo-p-dioxins (PCDD'S) forming is mirimised or eliminated altogether for practical purposes, when compared with conventional chlorine bleaching 2G sequences.

It will be appreciated that various prior art additives may be included in the processes described, and that practises desirable for the efficient operation pulpmi:. and bleach plant may also be 29 16 included. For example, the sodium hypochlorite produced from scrubbing of chlorine containing flue gas with sodium hydroxide may be included in one or more of the stages in order to use it conveniently.

Although the chemical charges and operating conditions may be varied over wide limits, and adjusted if necessary during the process to obtain optimum results to meet the objectives of the prjcess, which are bleached pulp of good quality and high brightness, while eliminating or at least minimising the formation of polychlorinated compounds (PCCD'S especially) and their presence in both the pulp and the pulpmill effluent, we will now provide a range of chemical charges and conditions found to be particularly effective in terms of both plant operation and results.

Typically the unbleached brown stock pulp ex the digester which may have a Kappa Number of between and 30, is subjected to delignification in an oxygen delignification stage for between 40 and 80 minutes at temperatures within the range 80 to 130 degrees C with between 26 and 34 Kg/tonne of sodium hydrox.de, and between 6 and 12 Kg/tonne of magnesium sulphate.

The brown stock is subjected to between 41% and 46% 30 delignification during this oxygen delignification stage.

Following the oxygen delignification stage the pulp is conventionally washed.

A combined stage then follows, consisting of two steps. During the first step the pulp is treated with between 3 and 6 Kg/tonne of chlorine dioxide for between 12 and 25 minutes under acidic conditions within the range pH 3 to pH 5 while the temperature is maintained within the range 60 to 105 degrees C.

In order to maintain the desired acidic conditions sulphuric acid may be added if necessary. While maintaining the temperature within the range stated, a second step follows wherein delignification takes 1 5 place during treatment with oxygen and between 21 and 29 Kg/tonne of sodium hydroxide and between 6 and 1:: Kg/tonne of maguesium sulphate for between 45 and minutes. At the completion of the two step combined stage, and following washing the Kappa Number will be about 9, the total delignification now being near 67%, and the residual pH is approximately pH 12. The pulp already has a brightness in terms of its reflectivity of about 45 and is capable of bleaching to a high brightness by any suitable final bleaching sequence.

31 In keeping with the objectives of the invention the following sequence based upon chlorine dioxide activation and oxygen delignification is a preferred sequence. The pulp may first be treated in a chlorine dioxide activation stage with between 4 and 8 Kg/tonne of sodium hydroxide and between 9 and 16 Kg/tonne of chlorine dioxide, at temperature within the range 40 and 70 degrees C, and for a treatment time between 50 and 110 minutes for example. After activation and washing the pulp is further delignified using between 16 and 22 Kg/tonne of sodium hydroxide with oxygen, at temperatures within the range 70 to 90 degrees C for a treatment time between 75 and 110 minutes. After washing the pulp is finally treated with between 2 and 7.5 Kg/tonne of sodium hydroxide and between 4 and 14 Kg/tonne of chlorine dioxide at temperatures within the range to 100 degrees C and for a time between 140 and 220 minutes. The finally washed pulp has a brightness of 2C between 88 and 32 TABLE 1 BLEACHING CONDITIONS TEMPERATURE TIME °C

MINUTES

STAGE

CONSISTENCY

PERCENT

(C Eo)

D

E

D

100 (70 90) 50 80 80 60 (15 60) 75 90 180 (10 TABLE 2 BLEACHING CONDITIONS STAGE TEMPERATURE

°C

O

(C Eo)

D

Eo

D

100 (50 90) 50 80 80

TIME

MINUTES

60 (15 b3) 75 90 180

CONSISTENCY

PERCEN.

(10 33 I TABLE 3 BLEACHING CONDITIONS

STAGE

O

(D Eo)

D

E

D

TEMPERATURE

°C

100 (70 90) 50 80 80

TIME

MINUTES

60 (15 60) 75 90 180

CONSISTENCY

PERCENT

(10 TABLE 4 BLEACHING CONDITIONS STAGE TEMPERATURE

°C

TIME

MINUTES

CONSISTENCY

PERCENT

O

(D EO)

D

D.

D

100 (50 90) 50 80 80 60 (15 60) 75 90 180 (10 The invention has been described by way of example only herein and equivalents and modifications will become apparent to those skilled in the art, and it is 34 c -I recognised that these various modifications are possible and fall within the scope and spirit of the invention as claimed.

The claims form part of the disclosure of this specification.

35

Claims (18)

1. A method for bleaching chemically digested lignocellulosic pulp, the method including a first oxygen delignification stage followed by a washing stage, then followed by a stage having the combined steps, without washing between, of an activation step wherein the pulp is treated with an activating agent rendering the pulp more responsive to subsequent delignification treatment, followed by (ii) an alkaline extraction step with oxygen delignification.

2. A method for bleaching chemically digested lignocellulosic pulp as claimed in claim 1, wherein the activating agent is selected from: chlorine dioxide; chlorine; nitrogen dioxide; ozone; hydrogen peroxide and mixtures thereof.

3. A method for bleaching chemically digested lignocellulosic pulp as claimed in claim 1 or 2, wherein the activating agent is selected from: chlorine dioxide; chlorine; and mixtures thereof.

4. A method for bleaching chemically digested lignocellulosic pulp as claimed in any one of the preceding claims, wherein the activating agent contains minor amounts of chlorine relative to the amount of chlorine dioxide. A method for bleaching chemically digested lignocellulosic pulp as claimed in any one of the preceding claims, wherein the activating agent is chlorine dioxide.

6. A method for bleaching chemically digested lignocellulosic pulp as claimed in any one of the preceding c'aims, wherein the activation step is carried out under acidic conditions.

7. A method for bleaching chemically digested lignocellulosic pulp as claimed in claim 6, wherein the acidic conditions range between a pH of 3.0 and

8. A method for bleaching chemically digested lignocellulosic pulp as claimed in claim 6, wherein the chemically digested lignocellulosic pulp to be bleached is selected from pulps digested by kraft (sulphate), polysulphide, soda and neutral or alkaline sulphite processes.

9. A method for bleaching chemically digested lignocellulosic pulp as claimed in any one of the preceding V GNAA M;03:5659.RES 20893 c -1 I -1 23 37 claims, wherein the chemically digested lignocellulosic pulp to be bleached is kraft (sulphate) pulp. A method for bleaching chemically digested lignocellulosic pulp as claimed in any one of the preceding claims, wherein a bleaching sequence accc :ding to sequence is used.

11. A method for bleaching chemically digested lignocellulosic pulp as claimed in any one of claims 1 to 9 wherein a bleaching sequence according to sequence 6 is used.

12. A method for bleac' chemically digested lignocBllulosic pulp as claimed ii any one of the preceding claims, wherein all stages are conducted at medium consistency.

13. A method for bleaching chemically digested lignocellulosic pulp as claimed in any one of claims 1 to 11, wherein the first stage of oxygen delignification is conducted at high consistency.

14. A method for bleaching chemically digested liqnocellulosic pulp as claimed in any one of the preceding claims, wherein the chemically digested lignocellulosic pulp is derived from Pinus Radiata. Bleached chemically digested lignocellulosic pulp substantially free from polychlorinated dibenzo-p-dicxine, said pulp having been bleached after chemical digestion using a sequence including a first oxygen delignificationi stage followed by washing, then followed by an activation stage, then followed, without washing, by an alkaline extraction and oxygen delignification stage.

16. Bleached chemically digested lignocellulosic oulp as claimed in claim 15, wherein the pulp has been treated with an activation agent selected from: chlorine dioxide; chlorine; nitrogen dioxide; ozone; hydrogen peroxide and mixtures thereoi.

17. Bleached chemically digested lignocellulo'ic pulp which has been bleached by a method according to any one of claims 1 to 14.

18. A method for bleaching chemically digested lignocellulosic pulp comprising the steps: a first oxygen delignification step wherein the pulp is treated with oxygen, sodium hydroxide and r GN .4103:5659.RES 20 893 I N -38- magnesium sulphate; a washing step; an acidic activation step wherein the pulp is treated with a chlorine dioxide activating agent; followed without washing by an alkaline extraction step wherein the pulp is treated with sodium hydroxide, with oxygen and magnesium sulphate; a washing step; an alkaline activation step including treatment with chlorine dioxide and csdium hydroxide; a washing step; a second alkaline extraction step wherein the pulp is treated with sodium hydroxide and oxygen; a washing step; a second alkaline activation step including treatment with chlorine dioxide and sodium hydroxide; and a washing step.

19. A method for bleaching chemically digested lignocellulosic pulp substantially as described in any one of sequences 1 to 7. A method ior bleaching chemically digested lignocellulosic pulp substantially as described in any one of Examples 1 to 4.

21. A method for bleaching chemically digested lignocellulosic pulp substantially as described in any one of the drawings.

22. Bleached chemically digested lignocellulosic pulp which has been bleached by the method claimed in any one of claims 18 to 21. DATED 20 August 1993 CARTER SMITH BEADLE Fellows Institute of Patent Attorneys of Australia Patent Attorneys for the Applicant: ELDERS RESOURCES NZFP LIMITED GNAM.03:5659.RES 20 893 I r