EP0561786A1

EP0561786A1 - Bleaching chemical pulp using chlorine/chlorine dioxide then ozone

Info

Publication number: EP0561786A1
Application number: EP91916878A
Authority: EP
Inventors: Ted Yuan Tsai
Original assignee: International Paper Co
Current assignee: International Paper Co
Priority date: 1990-08-14
Filing date: 1991-08-14
Publication date: 1993-09-29
Anticipated expiration: 2011-08-14
Also published as: CA2089477A1; EP0561786B1; JP3093263B2; JPH06503857A; FI930625A0; DE69128287T2; FI930625A; EP0561786A4; DE69128287D1; WO1992003609A1; ES2109273T3

Abstract

L'invention concerne des améliorations dans la préparation de pâte chimique de bois cellulosique utilisée dans la fabrication du papier, et en particulier l'amélioration de la qualité d'une pâte kraft qui a été soumise à un blanchiment au dioxyde de chlore/chlore puis à une ozonation avant de passer dans un étage d'extraction dans la séquence de blanchiment. La pâte améliorée de la présente invention se caractérise par une réduction des restes de chlore liés organiquement dans la pâte de bois et des halogénures organiques adsorbés (AOX) dans l'effluent, et posséde une bonne brillance et une bonne viscosité. Les avantages de la présente invention peuvent être obtenus au moyen d'une séquence de blanchiment qui utilise un minimum d'étages, à savoir (D->C)ZED ou DcZED qui s'accompagne d'investissements relativement plus faibles que ceux qui étaient nécessaires jusqu'à présent.The invention relates to improvements in the preparation of chemical pulp from cellulosic wood used in papermaking, and in particular to the improvement of the quality of a kraft pulp which has been subjected to chlorine dioxide / chlorine bleaching and then to ozonation before going to an extraction stage in the bleaching sequence. The improved pulp of the present invention is characterized by a reduction of organically bound chlorine residues in the wood pulp and adsorbed organic halides (AOX) in the effluent, and has good gloss and good viscosity. The advantages of the present invention can be obtained by means of a bleaching sequence which uses a minimum of stages, namely (D-> C) ZED or DcZED which is accompanied by relatively lower investments than those which were needed so far.

Description

B EACHING CHEMICAL PULP USING CH__ORINE/CH--C«IE DIOXIDE THEN OZONE

Field of the Invention

The present invention relates to delignification and bleaching of chemical cellulosic wood pulp fibers for use in papermaking and to the production of a pulp having reduced halogen concentrations, and good brightness and vis¬ cosity. Background of the Invention

Chlorine-based chemicals, such as chlorine, chlo¬ rine dioxide, and hypochlorite, have been used in pulp bleaching for several decades, and continue to be used for removing lignin and bleaching the pulp to high brightness. In general terms, the extent of bleaching, hence the degree of brightness obtained, is determined by the type of pulp being bleached and the proposed end use of the paper product. For example, kraft pulps intended for use in making fine writing papers require different bleaching circumstances which will produce the desired brightness in the final paper product. In all instances, however, where chlorine-based bleaching agents have been employed in the prior art, there are produced chlorinated organics and organic halogens (i.e. chlorides). The compounds are gener¬ ally insoluble in an aqueous medium and substantial quanti¬ ties are swept from the pulp with the effluent from one or more of the stages of the bleaching sequence. A relatively smaller percentage of such chlorinated organics and organic chlorides remain in the pulp and eventually appear in the paper product.

In chlorine-based bleaching processes, the "C" factor of the pulp is employed as a measure of the chlorina- tion charge required for a specific pulp. By definition, the "C" factor refers to the effective chlorination charge and is equal to the chlorine dioxide plus the chlorine in the charge (expressed in terms of effective chlorine) , divided by the Kappa number of the pulp. Generally, it is stated in the prior art that higher "C" factors produce brighter pulps, but that a "C" factor of 0.22 provides maximum brightness in a pulp without unacceptable degrada¬ tion of the cellulosic fibers (e.g. reduced strength) . Thus, it has been common heretofore when seeking maximum brightness of the pulp, (in the range of 70-85% GE) to use large "C" factors, i.e. larger quantities of chlorine- containing bleaching agent. Invariably such quantities of chlorine result in presently undesirable or unacceptable levels of chlorinated organics and/or organic chlorides.

In the past decade, there has been a growing concern about the environmental impact of chlorinated organ¬ ic compounds in bleach plant effluent. Also, public concern for the disposal of paper containing organically bound chloride has been increasing. Undesirable chlorinated organics such as dioxin have been detected in the exhaust gases of incinerators burning municipal wastes containing chloride, including, for example, paper products made from chlorine bleached pulps. West German environmental regula- tions, for example, propose restricting the total chloride residue for packaging material including wood pulp to less than 200 ppm. The allowable adsorbed organic halogens (AOX) discharged in the effluent per ton of wood pulp are proposed to be restricted to 2.0 kg or less in some of the European countries. More stringent regulations are expected in the near future.

Several options have been proposed or practiced to reduce or eliminate chlorinated organics in the bleaching process. The most straightforward method is to substitute non-chlorine bleaching chemicals, such as oxygen, peroxides, ozone, peracetic acid, etc. for chlorine-based bleaching chemicals. Unfortunately, no chlorine-free bleaching proc¬ ess has been developed with the ability to produce accept¬ able pulp properties (such as brightness or viscosity) at an acceptable bleaching cost.

Another option to reduce the discharge of chlori¬ nated organic compounds is to reduce the chlorine usage in the first stage of the bleaching process. Two alternatives that produce no significant degradation of pulp properties have been commercialized for this purpose. These are (a) extended delignification in the cooking stage and (b) oxygen delignification. These alternatives, with proper extrac¬ tion, reduce the lignin content of brown stock going into the bleach plant. They do not, however, reduce the chlori¬ nated organic compounds in the bleached pulp and effluent to sufficiently low levels. A third option to reduce the generation of chlori¬ nated organics in a bleaching process is to substitute chlorine dioxide for chlorine. Chlorine dioxide is a rela¬ tively strong oxidant compared to chlorine; to achieve the same degree of delignification, it requires only about thirty-eight weight percent chlorine dioxide on the pulp compared with one hundred weight percent of chlorine. However, these prior art processes are of the DEDED type wherein the chlorination stage (D) is followed by the con¬ ventional extraction (E) and additional chlorination (D) stages. The pulp and the discharge effluents resulting from this prior art option contain higher concentrations of chlorinated organics than are acceptable and/or desirable. Processes using both oxygen delignification and chlorine dioxide substitution have been suggested but do not achieve the regulated concentrations of chlorine-containing residues in either the pulp or the effluent. Summary of the Invention

The present invention comprises a method for the treatment of kraft pulp in preparation for its use in paper- making including the steps of contacting the pulp while dispersed in an aqueous medium with an initial charge of a chlorine-based bleaching agent selected from the group consisting of chlorine dioxide, and mixtures of chlorine dioxide and chlorine, wherein the total chlorine dioxide p esent in the charge represents between about 100% and about 50% of the total charge, expressed as effective chlorine, the remainder of the charge being chlorine, the bleaching agent producing a substantial quantity of chlorine- containing moieties that are insoluble in an acid 5 medium,

thereafter, and prior to any intervening treatment of said pulp other than a wash, contacting the pulp with ozone, the ozone being present in an amount which provides a ratio of "C" factor to %

10 ozone, based on dry weight of pulp, of between about 0.03 and about 0.6 for a period of time sufficient to cause the ozone to oxidize substan¬ tial quantities of the chlorine-containing moie¬ ties and render the same soluble in the acid

15 medium of the pulp, and

thereafter, separating the solubilized chlorine- containing moieties from the pulp,

whereby there is obtained a pulp having substan¬ tially lower chlorine content, essentially no 20 tetrachlorodibenzodioxin, essentially no tetra- chlorodibenzofuran, and substantially equivalent brightness and viscosity as the same pulp which has been bleached employing conventional chlorine-based bleaching sequences,

25 a cellulosic kraft pulp suitable for papermaking purposes, the pulp being bleached through the use of chlorine dioxide or a mixture of chlorine dioxide and chlorine, being essentially free of tetrachlorodibenzodioxin or tetradibenzofuran,

30 having a total organic chloride content of less than about 200 ppm, based on pulp, and having a total adsorbed organic halogen content of less than about 2 kg/ton, based on pulp. In accordance with the method of the present invention, a chemical cellulosic wood pulp, e.g. a kraft pulp, is initially contacted with chlorine dioxide as the first-stage of a bleaching sequence. This chlorine dioxide may initially contain up to about 50% chlorine, based on effective chlorine, or in this initial bleaching stage, up to about 50% chlorine may be added to the chlorine dioxide as a substitute for the chlorine dioxide, also based on effective chlorine. Contrary to the prior art, after this initial D stage and prior to any extraction (E) stage, the pulp is processed through an ozonation stage. After the ozonation stage, the pulp is processed through the custom¬ ary extraction, further bleaching, etc. stages. Important¬ ly, and as noted to be contrary to the prior art, the ozona- tion takes place immediately following chlorination and prior to extraction. It has been recognized heretofore that ozone as a strong oxidant tends to degrade the pulp. Also, the art teaches the _ any use of ozone on pulp should be preceded with an acid wash, for example to remove heavy metals known to be harmful to the stability of ozone and/or decomposition products that may cause low pulp properties (e.g. strength) . Contrary to such prior art, the present inventor has found that the ozonation can take place immedi¬ ately following an initial chlorine dioxide/chlorine (D_c) bleaching stage as defined hereinabove. In this regard, it is to be understood that preferably there is a conventional water washing of the pulp between the stages of the bleach¬ ing sequence. This washing is not deemed to be a "stage", but rather is in the nature of a dilution of the aqueous phase of the pulp. For example, the quantity of water commonly used provides a dilution factor of between about 1 and about 3. It has been found that the wash water need not be "clean" or "fresh" water. For example, recirculated water which contains considerable residues from previous usage has been used successfully.

Among the several unexpected advantages provided by the present method, it has been found that the ozone treatment not only provides delignification and bleaching but also oxidizes substantial quantities of the chlorinated lignin residues left over from the chlorination stage. This oxidative activity converts substantial quantities of the insoluble chlorinated organics to inorganic chlorine- containing compounds and/or to chlorinated organics that are soluble in the acid liquor present in the ozonation stage. The oxidation of these chlorinated residues by the ozone thus not only reduces the chlorinated organic content in the wood pulp, but also reduces the adsorbed chlorinated organ¬ ics discharged in the effluent. These converted inorganic chlorides, generally, are not environmentally hazardous and/or are recoverable. The result is a pulp having good brightness, acceptable viscosity, and reduced chlorinated organic content. Notably, such desirable results are ob¬ tained by employing as few as four stages in the bleaching sequence, and employing lower total effective chlorine charges in the chlorination stage than heretofore known to be possible. Specifically, the present inventors, using their improved method, have produced wood pulp which has less than 200 ppm total organic chloride (T0C1) residue. TOC1 as used herein refers to the total organically bound chlorine content of the pulp resulting from the bleaching sequence unless indicated otherwise. T0C1 is essentially insoluble. The effluent discharge from the bleaching process of the disclosed method has less than 2.0 kg adsorbed organic halogens (AOX) per ton of wood pulp. AOX, as the term is used herein refers to the sum of the AOX, i.e. adsorbed organic halogens, of the several stages of bleaching, i.e. AOX of the D_c stage plus AOX of the Z stage plus AOX of the E stage and plus AOX of further D stages, unless indicated otherwise. As noted, the pulp produced by the present invention has a brightness and viscosity that meet or exceed the requirements of industrial papermaking.

Therefore, it is an object of the present inven¬ tion to provide a method for the preparation of kraft cellu- losic wood pulp fibers for use in papermaking that can provide wood pulps with lowered residual chlorine content and effluents with lowered levels of adsorbed organic ha- lides (AOX) while retaining or enhancing the characteris- tics of brightness and pulp viscosity within acceptable ranges.

It is another object of the present invention to provide such an improved method for bleaching kraft cellu¬ losic pulp that is both economical and readily adaptable to the current papermaking industry. These and other objects will be recognized from the present description including the Figures. Brief Description of the Figures

FIGURES 1-5 are graphic representations, computer- generated from mathematical models depicting the relation¬ ship of ozone charge and chlorination factor in obtaining various values of the brightness, viscosity and total chlo¬ ride on pulp.

In accordance with the present invention, an aqueous slurry of cellulosic wood fibers as is commonly produced by conventional digestion means in the papermaking industry is processed through sequential stages of a bleach¬ ing operation. Softwoods, hardwoods or mixtures thereof may be processed employing the present invention. Following digestion, the slurry preferably is processed directly to the bleaching sequence disclosed herein. If desired, it may be subjected to such treatment as oxygenation, ozonation or other oxidation prior to entering the present sequence, but with attendant increased expense. In the first stage of the present sequence, the pulp slurry is treated with a chlorine-based agent, i.e. chlorine dioxide or a mixture of chlorine dioxide and chlo¬ rine. Preferably in this stage the oxidant is principally chlorine dioxide. Consistent with the industry practices, the term, D, is employed to represent a bleaching stage employing chlorine dioxide. Herein, D-L is used to represent the first D stage of a sequence. The term D_c, represents a stage employing chlorine dioxide plus chlorine. In such stage, the chlorine may be present in the chlorine dioxide, i.e. a mixture, or the chlorine dioxide may be first added to the pulp and after a brief time period the chlorine is added. The term D→C is at times used to represent this latter chlorine substitution, but herein, the term D_c is deemed to encompass both the mixture and the chlorine sub¬ stitution procedures unless indicated otherwise.

Following the D-^ stage, and with or without an intervening washing, the pulp of the present invention is processed through an ozonation stage, Z. Heretofore, it has been taught in the industry that ozone tends to be decom¬ posed by certain of the heavy metal components of the pulp that conventionally are removed by an additional acid treat- ment stage that comprises subjecting the pulp to an acidic medium. In the present invention the pulp moves from the D-^ stage which is an acidic stage directly to the Z stage without an acidic treatment stage. In the Z stage, the acid pulp is contacted with ozone in a reaction tower, for exam- pie, for a period of time and employing sufficient ozone to effect oxidation of substantial quantities of the chlorinat¬ ed lignins and/or other chlorinated components of-the pulp, in addition to the contribution of the ozone toward further delignification. Many of the oxidized chlorine-based moie- ties are soluble in the environment of the Z stage. Thus, these oxidized chlorine-based moieties are solubilized into the liquor and extracted from the pulp during washing fol¬ lowing the Z stage. Other of the oxidized compounds are soluble in an alkaline solution such as is present in an E stage. Extraction with an alkaline medium following the Z stage thus serves to remove alkaline-soluble oxidized chlo¬ rine-based moieties from the pulp. As noted, a preferred complete sequence will include an extraction stage following the ozonation stage. Such extraction stage may be enhanced with oxygen, E₀, or peroxide, E_p, a combination of oxygen and peroxide, E_0+p, or other enhancer. The pulp may be further processed through any of several selected stages. Preferably, the extraction stage is followed by a further bleaching stage. Chlorine dioxide, i.e. a D₂ stage is preferred as such further bleaching stage. As desired, washes may be employed between selected stages, e.g. between the E and D₂ stages.

To facilitate a further understanding of the invention, the following examples are given pri¬ marily for purposes of illustrating certain more specific details thereof. Southern softwood kraft pulp with Kappa number of

32.3 and an initial viscosity of 30.3 cP was used as the wood pulp slurry for all of the following Examples except as otherwise noted. The series of Examples used various combi¬ nations of the following bleaching stages: O stage: Prechlorination oxygen delignification was conducted in a reactor for 1 hr. at 85°C, with 3% sodium hydroxide, 40-80 psig oxygen pressure and 0.5% magnesium sulfate on the pulp at a consistency of 10%.

D_c stage: In the sequential chlorine dioxide substitution stage, D→C, chlorine dioxide was added to the pulp and mixed; after 20 seconds, chlorine was added and the pulp was well mixed; this stage was conducted in a reactor for 40 min. at 50°C with a consistency of 3-10%. In those instances where the chlorine was present in the chlorine dioxide as a mixture, the chlorine was introduced simultane¬ ously with the chlorine dioxide.

C stage: Chlorination was conducted in a reactor for 30 to 40 min. at 45°C with a consistency of 3%, with chlorine of the desired charge. D stage: Chlorine dioxide addition was conducted at 50 to 70°C with a consistency of 10%;

Z stag The ozone stage was conducted under a pH of about 2-5 at room temperature for 40 to 90 min. with a consistency of 1%; Example 34 was done under alkaline condi- tion at 34% consistency at pH 12.

E stage: Extraction was conducted for 60 min. at 70°C with a consistency of 10% and with desired sodium hydroxide charge;

E₀ stage: Extraction enhanced with oxygen was conducted under conditions similar to the E stage with an initial oxygen pressure of 40-50 psig that was gradually reduced to 0 psig;

E_p stage: Extraction enhanced with peroxide was conducted under conditions similar to the E stage with a 0.4-0.6% hydrogen peroxide charge;

^Eo + _p stage: Extraction enhanced with oxygen and peroxide was conducted under conditions similar to the E₀ stage with a 0.4% to 0.6% hydrogen peroxide charge;

Washings: The slurry was washed with distilled water in a screen box or funnel between each stage; the stage labeled (DZ) is a D stage followed immediately by a Z stage without intermediate washing.

EXAMPLES 1-5 Several prior art bleaching sequences employing procedures well known in the art were run on Southern pine softwood kraft pulp having a Kappa number of 32.3. These sequences employed the usual chlorine stages with and with¬ out chlorine dioxide substitution. The results are shown in Table I. As expected, good brightness and viscosity values were noted, except when an oxygenation stage was employed prior to chlorination (Example 3) . This latter Example showed the deterioration of the pulp (low viscosity) that has been noted in the prior art. Of these Examples, only the D→C E_0+pDED produced significant reduction in the total chloride on the pulp, but such reduction was obtained only at the expense of two additional stages. Table 1 Total Organic

Chloride Chloride Pulp Pulp Bleaching Chemical Charge on Pulp on Pulp Brightness Viscosity

Example Sequence (% on Oven Dried (QD) Pulp) (ppm) * (ppm) (GE %) (cP) C Factor

D→C E_0+p D

1 D→CE_0+pD 220 190 86.1 (83.3)** 22.5 0.22

1.66 — 2.9 3 0.8

D→C ^Eo+p D E D

2 D→CE_oJ_DED 195 150 88.5 (86.5) 20.8 0.22

1.66 — 2.9 3 0.8 0.4 0.3

*** D→C E_0+p D

0D→CE_o+pD 215 170 86.8 (84.8) 14.3 0.22 1.15—2 3 0.7 I

C E_Q D

CE₀D 314 300 84.7 (80.3) 23.8 0.22

7.25 4.35 0.7

C E D E D

CEDED 345* 320 86.7 (83.0) 27.0 0.22 7.25 4 0.7 0.5 0.5

Remarks:

* Estimated by the sum of inorganic and organic chlorides ** Reverted brightness (at 105"C for 1 hr) . *** Oxygen pulp with kappa number = 22.3, viscosity = 21.8 cP

EXAMPLES 6-13

In Examples 6-13 the pulp employed was the same as in Examples 1-5. Examples 6-13 involved bleaching sequences using an ozone bleaching stage. Table 2 shows the proper- ties of the pulps of these Examples. Employing ozonation prior to chlorination, Examples 6, 7, 10 and 12, produced pulps of low brightness, except in Examples 7 and 12 where additional and costly extraction and chlorine dioxide stages were added. Examples 8 and 9 (DZED) where ozonation was employed after chlorination, produced pulps of good bright¬ ness and viscosity plus low total chloride on the pulp. Notably, Examples 8 and 9 (DZED) employed 20% and 7% less chlorine dioxide than Example 11 (DZEDED) plus the fact that the DZED sequence with its fewer stages yielded better viscosity values and almost equal brightness values. Fur¬ ther, the brightness obtained by the DZED sequence is almost equal to the brightness obtained in the more costly prior art CEDED sequence (cf. Ex. 10 and Ex. 5) . The DZED (Ex. 8) produced better results than that of ZDED (Ex. 10) at same chemical charge. The DZED produced 84.7% GZ brightness while ZDED produced only 79.0% GZ. The same result has been experienced in the DZEDED (Ex. 11) and ZDEDED (Ex. 12) examples. The DZ sequence also generated less T0C1 than that of ZD sequence.

Table 2

Total Organic

Chloride Chloride Pulp Pulp

Bleaching Chemical Charge on Pulp on Pulp Brightness Viscosity

Example Sequence (% on Oven Dried (QD) Pulp) (ppm) (PPm) (GE %) (cP) C Factor

Z E D

ZED ND (1) ND 78.9 (N.D.)** 16.5

2 2.5 1.5

Z E D E D

ZEDED 75* 57 87.9 (85.2) 14.4

2 2.5 1.5 0.5 0.5

D Z E D

DZED ND ND 84.7 (N.D.) 19.0 0.11

1.37 1.0 2.5 0.7

D Z E D I

DZED 101* 80 86.5 (82.7) 16.8 0.11 1.4 1.0 2.5 1.0 I

Z D E D

10 ZDED ND ND 79.0 (N.D.) 23.5 0.11

1.0 1.37 2.5 0.7

D Z E D E D

11 DZEDED 77* 65 89.0 (85.3) 15.8 0.11

1.37 1.0 2.5 0.7 0.5 0.5

(1) N.D.: not determined.

(2) Kappa NO. of 32.3

* Estimated by the sura of inorganic and organic chlorides.

** Reverted brightness.

EXAMPLES 14-19

Examples 14-16 employed Southern softwood kraft pulp having a Kappa number of 27.3. Table 3 shows certain properties of the resultant bleached pulps. From Table 3 it is noted that enhancement of the extraction stage (e.g. E₀, _p, or E _0+p in the DZED bleaching sequence permits the use of less chlorine dioxide in the D stage and produces a pulp of substantially equivalent brightness and viscosity to the pulp produced using a DZED sequence without such enhance- ment.

Examples 17-19 employed Southern hardwood kraft pulp having a Kappa of 14.9. Table 3 also shows the proper¬ ties of the resultant bleached pulps of these examples. These pulps showed high viscosity and good brightness values as well as relatively low TOCl values employing total chlo¬ rine dioxide amounts less than the total chlorine dioxide amounts employed in obtaining substantially equivalent brightness and viscosity values for softwood pulp. For either softwood or hardwood pulps, the total chlorine diox- ide employed in these Examples 14-19 is substantially less than the total chlorine dioxide employed to obtain equiva¬ lent brightness and viscosity values without employing ozone after chlorination.

TABLE 3

Organic

Pulp Chloride Bleaching Kappa C On Pulp Brightness Viscosity Example Sequence No. Factor % Chemical on Pulp (ppm) (% G.E.) (cP)

14 DZEpD 110 86.3 (ND) 16.2

15 DZE_QD 140 86.0 (ND) 16.2

16 DZE_0+pD 27.3 0.12 1.24 0.6 3%NaOH 0. 110 87.0 (ND)

0.5% H₂0₂

17 DZE₀D 14.9 0.12 0.7 0.4 1.5 0.6 120 85.9 (82.1) 18.6

18 DZE_pD 14.9 0.12 0.7 0.4 1.5 0.6 130 85.8 (81.7) 18.8

19 DZE_0+pD 14.9 0.12 0.7 0.4 1.5 0.6 120 86.5 (82.1) 18.6

EXAMPLES 20-54

A laboratory D_CZED bleaching sequence was studied using a central composite statistically designed experiment which varied D_c stage chlorination factor and fraction of chlorine dioxide charge, and ozone charge in the Z stage. The pulp had a Kappa number of 32.3. The results are shown in Table 4. In Table 4, Examples 24, 29-32, 34, 38-41, 45, 47-49 and 54 represent actual run data which served as the basis for the statistically designed experimental data of the remaining examples. From this data it may be concluded that to obtain a pulp brightness of about 85% GE or higher and a viscosity of at least about 18 cP, the preferred chlorination factor is about 0.12 and the ozone charge is about 1.1%. This holds true for chlorine dioxide which includes up to about 20% chlorine. All of the pulps of Examples 20-38 showed low TOCl and AOX values, each being within present regulatory limits. Most surprisingly, howev¬ er, it was found that by using a lower chlorination factor (e.g. 0.12) which results in substantial savings in the cost of chlorine-based bleaching agent, one can use ozone per¬ centages of about 1% and obtain a three-fold reduction in AOX values.

TABLE 4 (3)

TA3LE 4, continued

% Charge Or anic Cl AOX Pulp

TABL 4. Continued

(1) Kappa number of 32.3.

(2) Preceded by an alkaline extraction with 3% sodium hydroxide charge.

(3) Based on predicted data, except examples 24, 29-32, 34, 38-41, 45, 47-49 and 54.

(4) Based on effective chlorine

Table 5 shows the adsorbed organic halogens (AOX) generated in the bleaching stages of the sequences of Exam¬ ples 1-3, 9, 11 and 14. Examples 1 and 2 showed AOX levels in excess of the desired 2.0 kg per ton of wood pulp. A pre-delignification stage, as in Example 3, produced efflu¬ ent within the 2.0 kg level, but Example 3 had high levels of chloride in the wood pulp (see Table 2). Examples 9, 11 and 14 involved stages of chlorine dioxide bleaching fol¬ lowed by ozonation. Example 9 represented a DZED sequence, and produced effluent well below the targeted AOX level. Example 14 showed that pre-delignification with oxygen can improve the AOX level, but at the expense of the cost asso¬ ciated with the additional stage (0) .

Table 5

AOX (kg/BDT) in Effluent

Example Sequence Stages Total

D→C E_0+p D

D→CE_Q+pD 2.20

0.86 1.3 0.04

D→C ^Jo+p D E D

D→CE₀₊pDED 2.23 0.86 1.3 0.04 0.02 0.01

D→C E_0+p D

^0[>-^CEo+p^D 1.27

0.71 0.5 0.06 I t

D Z E D I

DZED 0.43 0.13 0.2 0.06 0.04

D Z E D E D

11 DZEDED 0.63

0.13 0.2 0.06 0.04 0.1 0.1

D Z E D

14 ODZED 0.32 0.17 0.09 0.04 0.02

Experimental data were subjected to regression analysis of pulp brightness, viscosity and TOCl on chlorina¬ tion factor, fraction of chlorine dioxide charge in the D_c stage and ozone charge in the second stage (D_CZED sequence) . The results are depicted graphically in Figures 1-5. From these Figures, at 100% chlorine dioxide in the D_c stage, it may be seen that as one accepts a lower viscosity, equal amounts of ozone can be employed with lower quantities of chlorine dioxide ("C" factor) while still maintaining high brightness and low TOCl values. This relationship holds true when the chlorine dioxide charge includes up to 20% chlorine (see Figs. 4 and 5) , the actual ranges being some¬ what condensed when employing an 80/20 mixture of chlorine dioxide and chlorine. It may be seen from the foregoing that the present invention provides for a method of preparation of kraft cellulosic wood pulp fibers for use in papermaking that reduces the total chloride residue in the wood pulp to less than 200 ppm and the AOX discharge per ton of wood pulp to less than 2.0 kg. In addition, the wood pulp has a bright¬ ness greater than 85% GE and a viscosity above about 14 centipoises. Also, the method is economical and readily adaptable to the current papermaking industry.

From the Examples, it will be noted that the percentage of ozone, based on oven dry weight of pulp, employed varies as a function of the "C" factor, the effec¬ tive ratio of "C" factor to % ozone being between about 0.11 and 0.6, and preferably between about 0.2 and 0.6 for mini¬ mizing the TOCl and AOX values while maximizing the bright- ness and viscosity values of the pulp.

EXAMPLES 55-58 In Tabl' 6, there are shown the results of two runs in which the dioxin content of the pulp and effluent was determined. Southern pine kraft pulp, with 1% reject from cooking, having a Kappa No. of 30.3 and a viscosity of 35 cP was employed in all examples. In Example 55, a con¬ trol run indicative of prior art bleaching sequences that do not employ ozone, but which do include oxygen and peroxide enhanced extraction, there was no detectable tetrachlorodi¬ benzodioxin (TCDD) in the final bleached pulp. In this example, there was detected 2.5 ppt of tetrachlorodibenzofu- ran (TCDF) . In the effluent from this run there was detect¬ ed 2.5 ppt and 30.3 ppt of TCDD and TCDF, respectively,

In Example 58, a D_CZE₀D sequence in accordance with the present invention, there were no dioxins (TCDD or TCDF) detected in either the pulp or the effluent.

TABLE 6

DETERMINATION OF DIOXINS

Bleaching Example Sequences Conditions

55 D-CE_0÷DD ^Jo-*-p

1.53%-+2.67% 3% NaOH

0.4% H₂0₂ 0.8%

56 DE₀D? 85.0 (82.4) 0.90 200 Pulp ND 1.762

2.08% 21 0.57.

57 DZΞ_CD D Z D 85.3 (80.0) 0.49 100 Pulp ND ND

Effluent ND ND 1.27% 1.0% 2. 0.8S

53 D_CZED Z 83.0 1.35 200 Pulp ND ND Effluent ND ND

1.0%→0.71% 1.0% 2.5% 0.8%

Remarks:

1. First chlorination stage was conducted at 45^CC for 30 minutes.

2. Eo, E_0+p was conducted in Quantum Reactor at 10% CSC, 75'C for 1 hour at 45 psig 0₂ pressure, gradually reduced to 0 psig.

3. Final D stage was conducted at 10% CSC, 70°C for 2.5 hours.

4. Hot detectable.

EXAMPLE 59 Table 7 presents the results of a bleaching se¬ quence in accordance with the present invention in which the ozonated pulp at a pH of 12.7 was processed directly to an alkaline extraction stage (enhanced with oxygen) without an intervening water washing (ZE₀) . In this Example 59, the GE brightness of the pulp and its viscosity were lower than when the pulp was washed with water between the Z and E stages, but these parameter values were still in a range that is acceptable for certain pulps. The simplification of this process can lead to a capital reduction. The TOCl of the pulp was 128 ppm, well below present regulated stand¬ ards.

TABLE 7 DZED SEQUENCE UNDER ALKALINE OZONATION CONDITIONS

Viscosity TOCl cP ppm

11 128

t

Claims

WHAT IS CLAIMED IS;

Claim 1. A method for the treatment of kraft pulp in preparation for its use in papermaking comprising the steps of contacting said pulp while dispersed in an aqueous medium with an initial charge of a chlorine-based bleaching agent selected from the group consisting of chlorine dioxide, and mixtures of chlorine dioxide and chlorine, wherein the total chlorine dioxide present in said charge represents between about 100% and about 50% of the total charge, expressed as effective chlorine, the remainder of said charge being chlorine, said bleaching agent producing a substantial quantity of chlorine- containing moieties that are insoluble in an acid medium,

thereafter, and prior to any intervening treatment of said pulp other than a wash, contacting said pulp with ozone, said ozone being present in an amount which provides a ratio of "C" factor to % ozone, based on dry weight of pulp, of between about 0.03 and about 0.6 for a period of time sufficient to cause said ozone to oxidize substan¬ tial quantities of said chlorine-containing moie¬ ties and render the same soluble in the acid medium of said pulp,

thereafter, separating said solubilized chlorine- containing moieties from said pulp, and

whereby there is obtained a pulp having substan¬ tially lower chlorine content, essentially no tetrachlorodibenzodioxin, essentially no tetra- chlorodibenzofuran, and substantially equivalent brightness and viscosity as the same pulp which has been bleached employing conventional chlorine- based bleaching sequences.

Claim 2. The method of Claim 1 wherein said separation of said solubilized chlorine-containing moieties from said pulp comprises washing said pulp with an aqueous medium.

Claim 3. The method of Claim 2 and including the further and subsequent treatment of said washed pulp with an alkaline extraction.

Claim 4. The method of Claim 3 including the step of enhancing said extraction by the addition of oxygen, peroxide or a combination thereof.

Claim 5. The method of Claim 1 wherein the total quantity of chlorine dioxide and/or chlorine applied to said pulp is sufficient to provide a "C" factor of greater than about 0.04 and less than about 0.20, based on oven dried pulp.

Claim 6. The method of Claim 3 and including the further and subsequent treatment of said pulp with chlorine dioxide.

Claim 7. A bleaching sequence for kraft pulp consisting of the stages D_CZED in that order.

Claim 8. A cellulosic kraft pulp suitable for papermaking purposes, said pulp being bleached through the use of chlorine dioxide or a mixture of chlorine dioxide and chlorine, being essentially free of tetrachlorodibenzodioxin or tetradibenzofuran, having a total organic chloride con¬ tent of less than about 200 ppm, based on pulp, and having a total adsorbed organic halogen content of less than about 2 kg/ton, based on pulp.