CA1111607A

CA1111607A - Pulping procedure

Info

Publication number: CA1111607A
Application number: CA323,926A
Authority: CA
Inventors: Douglas W. Reeve; W. Howard Rapson
Original assignee: Erco Industries Ltd
Current assignee: Erco Industries Ltd
Priority date: 1978-03-30
Filing date: 1979-03-21
Publication date: 1981-11-03
Also published as: AR220185A1; ATA239479A; JPS54134104A; CS231164B2; NZ189914A; PT69361B; AT376249B; FI791065A; AU4557479A; ZA791174B; ES479062A1; AU535076B2; PL214517A1; PL116704B1; FR2421242B1; SE7902741L; FR2421242A1; PT69361A; DE2911334A1

Abstract

IMPROVED PULPING PROCEDURE
ABSTRACT OF THE DISCLOSURE
The adverse effects of dissolved organic material in the aqueous phase of pulp suspensions during bleaching using mixtures of chlorine dioxide and chlorine are overcome by applying the chlorine dioxide and chlorine serially to the pulp in two steps without an intermediate wash. An aqueous solution of chlorine dioxide or an aqueous solution of chlor-ine dioxide and chlorine may be used in the first step and chlorine is used in the second step.

Description

The present invention relates to the bleachiny of cellulosic fibrous material, more particularly ~o the bleaching of wood pulpo In multistage pulp bleaching and purification pro-cesses wherein wood pulp is subjected to a plurality ofbleaching and caustic extraction stages, typically using mixtures o~ chlorine dio~ide and chlorine in the ~irst bleaching stage, chlorine dioxide in subsequent bleaching stages and sodium hydroxide in the caustic extraction stages, and the pulp is washed intermediate each such bl~aching and caustic extraction stage, the volum~ of effluents from the multistage bleaching process and the overall fresh water requirement are decreased by effecting countercurrent wash-ing operations within the multistage process and by using effluents from the bleach plant to wash the unbleached pulp prior to its passage from the digester to the hleach plant, preferably utilizing the procedure described in U.S.Patent No. 4,039,372(J32).
One unforeseen problem which arises when the latter operations are used is that, to achieve the same final pulp brightness, chemical consumption in the first bleaching stage is increased when compared with bleaching effected in the absence of such use of effluents fromthe bleach plant. The increase in chemical consumption has been found to result from the presence o~ dissolved organic materialin the aqueous phase of the dilute pulp suspension.
The dissolved organic material consumes some bleaching chemicals, thereby, increasing chemical consumption.

The term "dissolved organic material" as used herein refers to bleaching chemical-consuming orga~ic material dissolved in the aqueous phase of the pulp suspen-sion and is measured in terms of total organic carbon (TOC).
It has now surprisingly been found that improved first stage bleaching may be effected in the presence of dis-solved organic material by utilizing a serial addition ofbleaching chemicals without intermediate washing between the , .

separate bleaching chemic~l ad~i-tion steps, to re~ulk ereby in a decreased chemical consumption as compared with the application of the bleaching chemicals all at one time.
In accordance with the present invention, 5 therefore, there is provided a process for the bleaching of cellulosic ~ibrous material in an aqueous suspension con-taining dissolved organic material, wherein a first bleaching step is effec~ed using chlorine dioxide or a mixture of chlorine dioxide and chlorine and, without an intermediate washing step, a second bleaching step is effected using chlorine.
By utilizing this serial bleaching operation, the effect of the dissolved organic material on the bleaching chemical requirement is decreased and often negated and a more efficient bleaching chemical usage thereby is achieved, when compared with the use of mixtures of chlorine dioxide and chlorine in the bleaching stage.
The bleaching process of the present invention is applicable to any cellulosic fibrous material but has particular application to the bleaching of wood pulp, preferably wood pulp produced by the kraft process, i.e. wood pulp produced by digestion of wood chips in a pulping liquor containing sodium hydroxide and sodium sulphide as the active pulping chemicals.
The serial addition of bleaching chemicals in accordance with this invention preferably is effecte~ in the first bleaching stage of a multistage bleaching and caustic extraction operation wherein the above-described washing operations are used, since the dissolved organic material exerts its greatest effect in the first bleaching stage. The invention, however, is broadly applicable to the bleaching of any pulp where dissolved organic material is presenl:. For example, the serial bleaching operation may be effected following an oxygen-alkali delignification.

The overall bleaching stage in which the serial bleaching operation of this invention is effected utilizes a quantity of chlorine dioxide in the first step thereof which constitutes about 20 to about 95%, preferably about 40 6~'7 to about 90~, of the total available chlorine used in the stage.
The term "total available chlorine" is used herein in its normal meaning in the bleaching art and refers to the total 5 bleaching power of the solution, chlorine dioxide having a bleaching power which is 2.63 times that of chlorine on a weight basis.
The term "bleaching stage" as used herein refers to a pulp bleaching operation effected between other pulp treatments, usually washings. The term "bleaching step"
10 as used herein refers to a pulp bleaching treatment effected within a bleaching stage.
The first bleaching step of the serial application of bleaching chemicals without an intermediate wash may ba effected using an aqueous solution of chlorine dioxide. When 15 reference is made herein to "an aqueous solution of chlorine dioxide", it will be understood that such solutions include technical chlorine dioxide solution which may contain small quantities of dissolved chlorine, up to but less than 5 of the total available chlorine of the solution.
When the first step of the serial bleaching appli-cation is effected using an aqueous solution of chlorine dioxide and the chlorine dioxide constitutes about 40 to about 90% of the total available chlorine used in the bleaching stage, it has been found that, in the presence of 25 dissolved organic material, the improvement in pulp properties attained as compared with mixtures of chlorine dioxide and chlorine applied in the bleaching stage is greater than the improvement in pulp properties attained in the absence of dissolved organic material.
The results, indicating an enhanced synergism by using an aqueous solution of chlorine dioxide in the first step of the bleaching sequence in the presence of dissolved organic material, are entirely unexpected.

In accordance with a first embodiment of the 35 invention, therefore, a cellulosic fibrous material is bleached in an aqueous suspension using chlorine dioxide and chlorine wherein the chlorine dioxide pxovides from about 40 to about 90% of the total available chlorine of the chlor-ine dioxide and chlorine. The process is characterized by the presence of dissolved orgallic material in the aqueous suspension, and the hleaching is effected in - two steps without an intermediate washing step, the first bleaching step using an aqueous solution of chlorine dioxide, as defined above, i.e. optionally containing chlorine in an amount less than 5% of the total available chlorine of the aqueous chlorine dioxide solution, and the second bleaching step using chlorine.
The first bleaching step also may be effected using an aqueous solution of chlorine dioxide and chlorine.
The term "aqueous solution of chlorine di.oxide and chlorine"
is distinguished from the term "aqueous solution of chlorine dioxide" in that the former term refers to solutions con-taining chlorine in an amount of at least 5~ of the total available chlorine of the solution whereas the latter term, as noted above, refers to solutions which may be chlorine free but which also may contain chlorine in an amount less than 5~ of the total available chlorine.
In one preferred embodiment, an aqueous solution of chlorine dioxide and chlorine containing chlorine in an amount of about 5 to about 10~ of the total available chlorine of the solution is used in the first bleaching step-o~ a serial bleaching stage, wherein chlorine dioxide con-stitutes about 20 to about 95% of the total available chlorine in the bleaching stage and dissolved organic material is present~
It has been found that, when an a~ueous solution of chlorine dioxide and chlorine of this type is used and dissolved organic material is present, the improvement in pulp properties attained as compared to the pulp properties when a mixture of chlorine dioxide and chlorine is used in the bleaching stage is not significantly differenk from the improvement in pulp properties observed when the first bleaching step is effected using an aqueous solution of chlorine dioxide.

~ ~1.

s This result is important both from the poin-t of view of an improvement in pulp properties by the use of serial application of chemicals when compared with the use o~
mixtures and from the point of view of the ability to use aqueous solutions of chlorine dioxide and chlorine in the first step of serial bleaching which result directly from chlorine dioxide generating plants which utilize the ERCO
R3 (Trademarks) process.
The ERCO R3 process is widely practised through-out the world and is described in U.S. Patent No. 3,~64,456.The chlorine dioxide and chlorine solution formed by contact of the off-gas mixture of chlorine dioxide, chlorine and steam from the generator with water typically contains about 7% of the available chlorine as chlorine. This solu-tion may be used directly in the first step of the serialbleaching stage of this embodiment of the invention without significantly affecting the improvement which may be attained, when compared with the results obtained when an aqueous solution of chlorine dioxide is employed in the first step.
The necessity to further treat the aqueous solution of chlorine dioxide and chlorine to decrease the chlorine con-tent thereof thereby is avoided.
In accordance with one preferred embodiment of the invention, therefore, a cellulosic fibrous material is bleached in an aqueous suspension using chlorine dioxide and chlorine wherein the chlorine dioxide provides from about 20 to about 95% of the total available chlorine of the chlorine dioxide and chlorine. The process is characterized by the presence of dissolved organic material in the aqueous suspen-sion, and the bleachiny is effected in two steps without anintermediate washing step, the first bleaching step using an aqueous solution of chlorine dioxide and chlorine wherein the chlorine constitutes about 5 to about 10% of the avail-able chlorine of the aqueous solution, and the second step using chlorine.
In another preferred embodiment o~ the invention, an aqueous solution of chlorine dioxide and chlorine con-taining chlorine in an amount greater than about 10~ up to about 30% of the total available chlorine of the solution is used in the first bleaching step oE a serial bleaching stage, wherein chlorine dioxide conS~itutes about ~0 to about 95~
of the total availabl.e chlorine in the bleaching stage and dissolved organic material is present.
It has been found that, when an aqueous solution of chlorine dioxide and chlorine of this t~pe is used and dissolved organic material is present, an improvement in pulp properties is attained as compared to the pulp proper-ties, when a mixture of chlorine dioxide and chlorine isused in the bleaching stage.
While the second step of the serial bleaching in this invention is usually effected using chlorine alone~
small quantities of chlorine dioxide may be included with the chlorine to achieve the well-known effect of protection of the pulp from the effects of overchlorination by the chlorine.
Where the countercurrent washing procedures described above are utilized, higher than normal first stage bleaching temperatures are encountered, usually in excess of about 35C
up to about 70C, for example, about 50 to about 60C, and hence the process of this invention is preferably effected at such temperatures.
The time which elapses between the application of chlorine dioxide or mixture of chlorine dioxide and chlorine in the first step and the application of chlorine in the second step may vary widely relative to the total bleaching time and the temperature at which the bleaching is effected.
The second bleaching agent is not applied until the first bleaching agent is at least partially depleted, usually at least about 30~ depleted and more commonly at least about 50~ depleted.
For example, in a high temperature first stage ble~ching oper-ation, i.e. above about 35C, having about 30 to about 60 minutes total bleaching time, the second bleaching agent may be applied about 5 seconds to about 10 minutes, usually about 30 seconds to about 5 minutes, after the application of the first bleaching agent.

3~7 Thé bleaching process of ~his invention may be effected under any desired consistency conditions, such as those routinely used in bleaching~ generally about 2 to about 6% by weight of pulp, as well as those used in the so-called 5 mediu~ consistency treatments, generally from about 6 to a~out 16% by weight of pulp.
The present invention is effective over a wide range of dissolved organic material concentrations, from about 2% to about 10~ by weight, more commonly about 2%
10 to about 8% by weight TOC (total organic carbon) on pulp.
The countercurrent washing procedure outlined above usually results in a dissolved organic material concentration in the range of about 3.5 to about 6.5~ by weight TOC on pulp, and hence the present invention has particular application there-15 to.
As mentionad above, the serial bleaching stage ispreferably utilized as the first stage of a multistage bleach plant operation. When so utilized and the serial bleaching stage is completed, the pulp may be washed, and then may be 20 subjected to caustic extraction, washed again, and subjected to one or ~ore bleaching and caustic extraction stages to achieve the desired brightness and purity ~f pulp. When the multistage bleach plant operation utilizes the so-called "dynamic bleach-ing" process, as described in Canadian Patent No. 783,483, the 25 washing steps usually are omitted. The causkic extraction is usually effected using aqueous sodium hy~roxide solution and additional bleaching may be effected using a bleaching agent selected from chlorine dioxida, chlorine, hyFochlorite, peroxide and combination~ thereof.
For example, the pulp may be subjected to an EDED
sequence after the initial serial bleaching stage, wherein E
refers to a caustic extraction stage and D re~ers to a b~e~ching stage using an aqueous solution of chlorine dioxide, with washing being effected after each chemical treatment stage. The 35 conditions utiliæed in such subsequent stages are the conven-tional ones.
The invention is illustrated by the following Examples. In the following Examples, reference is made ig3 . 7a - to the accompanying drawings, wherein:
Figures 1, 2 and 4 to 7 are graphical representa-tions of the variation of El Kappa number of pulps with % TOC; and Figure 3 is a graphical representation of the variation of El Kappa number, Dl hrightness and D~ bright-ness of a pulp with ~TOC.

3~
Example I
This Example illus-trates the results obt~ined when serial bleaching is effected using chlorine dioxide, ollowed by chlorine as compared with a single mixture applica-tion.
A softwood kraft pulp, having a 20.8 K number, 5 30~9 Kappa number and a viscosity of 31.1 cps., was bleached at 60C and 3.5% consistency for 30 minutes at an applied chlorine value of 6.9% with 70~ of the available chlorine being C1O2 and 30% of the available chlorine being C12, in the presence of dissolved organic material at various levels of 10 % TOC on pulp.
In one set of tests, the chlorine dioxide and chlorine were added as an aqueous solution containing all the chlorine dioxide and chlorine while in another set of tests, the chlorine dioxide was first added as an aqueous 15 solution thereof 30 seconds prior to the addition, without any intermediate washing, of chlorine as an aqueous solution thareof.
For each set of tests, the Kappa number of the pulp was determined after the pulp had been washed, alkali 20 extracted with 2.-/% NaOH on pulp for 2 hours at 70C and 12~--consistency and again washed. The Kappa number deter-minations were plot-ted against ~ TOC on pulp and the resulting graphical representations appear in Figure 1.
In addition spot determinatiorsof K number were 25 made at 6.8% TOC concentration and spot determinations of Kappa and K numbers were made for 9.3% equivalent C12 on pulp. The latter results are reproduced in the following Table I:
TABLE I
30 Total Equivalent C12El Kappa No.El K No.
% on ~ul~ Serial Mixture Serial Mixture 6.9 6.07 7.79 4.60 5.74 g.3 4.22 5.76 3.40 4.65 From the results shown in Figure 1 and Table I, it 35will be seen that the pulp delignification was better in the case of the serial chemical application at the same av~ilable chlorine dosage and that, while in both cases the Kappa number increased with increasing TOC concentration, the increase -- in Kappa number i8 less pronounced in the caseof the serial application, indicatiny more efficient bleaching in the case of the serial application with tlle difference in efficiency increasing with TOC concentration. ~n enhanced synergistic result thus was observed.
The more efficient bleaching obtained when the serial applica-tion of bleaching chemicals is effected permits less overall ~leaching chemical usage for the same pulp pro-perties as attained when mixtures are applied~
Example II
This example illustrates the results obtained when the proportion of the total available chlorine provided by chlorine dioxide is varied from tha-~ in Example I.
A wood pulp having a Kappa number of 34.2, a K
number of 23.4 and a viscosity of 28.8 cps. was bleached at lS 60Cand 3.5% consistency for 30 minutes at an applied chlor-ine value of 7.6~ with 90% of the available chlorine being ClO2 and 10% of the available chlorine being C12, in the presence of dissolved organic material at various levels of ~ TOC on pulp.
In one set of experiments, the chlorine dioxide and chlorine were added as an aqueous solution containing all the chlorine dioxide and chlorine while in another set, the chlorine dioxide was first applied as an aqueous solution thereof one minute prior to the addition, without any inter-25 mediate washing, of chlorine as an aqueous solution thereof.
Following the pulp bleaching, the pulp was washed, alkali extracted under the conditions outlined in Example I
using 3.0% NaOH on pulp and again washed. The Kappa number determinations were plotted against % TOC on pulp and the 30 resulting graphical representations appear as Figure 2.
As may be seen from Figure 2, the results obtained parallel those obtained in Example I. Thus, pulp delignifi-cation was better when serial application of the bleaching chemicals was effected and an enhanced synergism was obtained 35 in the presence of dissolved organic material.
Example III
This Example illustrates the effects attained when ~dditional processing of pulp is effected.
The procedure of Example II was repeated, except that chlorine dioxide provide~ 70% of the ~otal available chlorine and chlorine provided 30~ of the tot~l available chlorine of the first bleaching stage, and further hleaching (Dl), washing, caustic extraction (E2), washing, bleaching 5 (D2) and final washing were effected following the El washing.
The conditions of such further treatments were as follows:
Dl: 1.0% C102 Gn pulp, 0.4% NaOH on pulp, 6% c~nsisbency, 70 70C, 2 hrs.
~: 0~4% NaOH on pulp, 12% consisbency, 70C, 2 hrs.
D2: 0-4% ClO2 on pulp~ 6% c~nsistency, 7QC, 3 hrs.
The El Kappa numbers, and Dl and D2 brightness values were determined and plotted graphically against varia-tions in TOC. The graphical results appear as Figure 3.
As may be seen from Figure 3, in addition to the 15improved pulp properties after El extraction,using serial application of chlorine dioxide and chlorine and enhanced synergism with increasing % TOC levels, as compared with mixtures of chlorine dioxide and chlorine, paralleling the results outlined in Examples I and II, improved brightness 20values were observed at both the Dl and D2 bleaching stages for the pulp to which the chlorine dioxide and chlorine were applied serially in the first bleaching stage, as compared with mixtures application in the first bleaching stage.
Example IV
This Example illustrates the effect of the serial application of an aqueous solution of chlorine dioxide and chlorine followed by chlorine, when compared with the mixtures application of a solution of chlorine dioxide containing all the chlorine.
A series of experiments was carried out on samples of the wood pulp used also in Examples II and III and associa-ted with pulp mill filtrate. The bleaching chemical was added at a value of 7.6% equivalent C12, with 70% of the available chlorine being provided by chlorine dioxide and 35 30% of the available chlorine being provided by chlorine.
In one set of experiments, the chlorine dioxide was applied as an aqueous solution and the chlorine was applied 1 minute after the chlorine dioxicle application without an intermediate wash at varyiny ~TOC levels in the filtrate.
In another set of experimen-ts at varying %TOC
levels in the filtrate t the chlorine dioxide was applied as an aqueous solution of chlorine dioxide containing part of the chlorine in an amount equivalent to 5% of the ov~rall available chlorine, so that ~he chlorine provide~ about 7%
of the available chlorine of the solution, followed by application of the remainder of the chlorine (25% of the overall available chlorine) 1 minute after the aqueous solu-tion application without an intermediate washO
In a further set of e~periments, an aqueous solu-tion of chlorine dioxide and chlorine again was applied, in this case containing chlorine in an amount equivalent to 10%
of the overall available chlorine, so that the chlorine provided about 13% of the available chlorine of the solution.
The remainder of the chlorine (20% of the overall available chlorine) was applied 1 minute after the aqueous solu-tion application without an intermediate wash.
For each set of experiments, the El Kappa number was determined and plotted as a function of %TOC. Results obtained using mixtures of chlorine dioxide and chlorine in a single application were also plotted. The results appear in Figure 4, the dotted lines showing the trend of results for mixtures application and for serial application of chlorine dioxide and chlorine, the actual result points being omitted.
As may be seen from Figure 4~ the results obtained at 7% available chlorine of the chlorine dioxide and chlorine solution provided by chlorine represent a considerable improvement over the results ob-tained using a single mixtures application and are not significantly different from the case where the initially-applied chlorine dioxide solution contains no chlorine.
3i The latter results are important in thatthey demonstrate that an aqueous chlorine dioxide and chlorine solution produced by the ERCO R3 process, typically contain-ing about lOg of chlorine dioxide and ~g of chlorine (i.e.
the chlorine provides about 7% of total availahle chlorine Of the solution), may be used in the first step of ~he serial application to obtain results not significantly different from pure chlorine dioxide solution, without the necessity for purification by way of chlorine removal from the solu-5 tion.
The results obtained at 13% of the available chlorine of the chlorine dioxide solution provided by chlorine represent an improvement over the results obtained using a single mixtures application.
10 Example V
The experiments of Example IV were repeated. In one case (Figure 5~ 17.5~ of the total available chlorine was present in the aqueous solution of chlorine dioxide and chlorine, so that the aqueous solution comprised 80/20 C1O2/C12 available chlorine. Since this experiment was cond~ct~d at a different time f,rom those of Example IV, experiments using a 70/30 mixture and 70~30 serial application were repeated. (The symbol ~ signifies a serial application of bleaching chemi-cals without an intermediate wash).
In another case (Figure 6), the overall available chlorine was provided 40% by chlorine dioxide and 60% by chlorine.
The chlorine dioxide was applied as an aqueous solution also containing chlorine in an amount of 10% of the total available chlorine, corresponding to 20% of available chlorine of the 25 applied solution. Comparative runs at 0% C12 and with mixtures were made.
In a final case (Figure 7), the overall available chlorine was provided 10% by chlorine dioxide and 90% by chlorine~
applied 10% C102/2.5% C12 ~87.5% C12, the chlorine thus 30 constituting 20% of the total available chlorine of the chlorine dioxide,~olution. Comparative-runs at 10%
ClO2 > 90% C12 and lQ/90 mixtures were effected.
, The El Kappa values were determined and the I results are graphically shown in Figures 5, 6 and 7. As seen in Figure 5, and in comparison to the 87% C1O2/13% C12 result of Figure 4, an advantage results frorn the use of an aqueous solut-on containing chlorine dioxide and chlorine in a serial application of bleachiny chemicals, as compared with mixtures, j , although the advantage becomes diminished as the proportion .

13 ~ 7 of the available chlorine in the chlorine dioxid~ ~olu-tion provided by chlorine increases.
As seen in Figures 6 and 7, in comparison with Figure 5, the advantage attained using serial application o 5 bleaching chemicals declines with increasing overall propor-tions of chlorine. In Figur~ 7, at 10% of the overall available chlorine being provided by chlorine dioxide, there is no significant difference between mixtures and serial application of chlorine dioxide and chlorine, and in -the case 10 of D/C-~C the results are marginally poorer than the D/C case.
Example VI:
This Example illustrates the results attained at another proportion of C1O2 to Cl2 in the bleaching stage.
The softwood kraft pulp used in Example I was 15 bleached at 60C and 3~5% consistency for 30 minutes at an applied equivalent C12 value of 6.9% using 60~ of the avail-able chlorine as C1O2 and 40% of the available chlorine as Cl2, in the presence of dissolved organic material measuring 6.3% TOC on pulp.
After this bleaching step, the pulp was washed, caustic extracted at 12~ consistency for 2 hours at 70C
using 2.8% NaOH and then washed again. The pulp then was tested for its properties.
In one test, an aqueous solution of chlorine di-oxide and chlorine containing all the chlorine dioxide, i.e.
60% of the available chlorine, and 4% C12, as available chlorine (i~e. a solution in which the total available chlor-ine was provided approximately 93% ClO2 and 7% C12) was first applied to the pulp, followed 2 1/2 minutes later without an intermediate wash by an aqueous solution of the remainder of the chlorine, i.e., 36% of the available chlorine.
In a second test, the same aqueous solution of chlorine dioxide and chlorine was used in the first applica-tion but in the second application, applied 2 l/2 minutes after the first application without an intermediate wash, there was used an aqueous solution containing 36% available chlorine provided 4% by chlorine dioxide and 32% by chlorine.

.

1~
In a third test, the chlorine dioxide was ~irst added as an aqueous solution thereof 2 1/2 minutes prior to the addition, without any intermediate washing, of the chlor-ine as an aqueous solution thereof.
In a fourth test, the chlorine dioxide and chlor-ine were added as an aqueous solution thereof containing all the ClO2 a 2 The results obtained are outlined in the following Table II:
TABLE II
Serial Bleach Step 1 Step 2 Kappa No. K No. Viscosity D /C D/C cps.
60/4 0/36 5.23 4.05 26.8 15 60/4 4/32 5.75 4.44 26.7 60/0 0/40 5.68 4.4~ 26.3 60/40 - 7.73 5.87 26.3 The results of the above Table II indicate the improvement in Kappa and K numbers achieved using the serial bleaching sequence in the first bleaching stage.
In summary of this disclosure, the present inven-tion is directed to a bleaching procedure which permits the adverse effects of the presence of dissolved organic carbon to be overcome and chemical usage economies to be effected.
Modifications are possible within the scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A bleaching process for the bleaching of cellulosic fibrous material in an aqueous suspension using chlorine dioxide and chlorine wherein the chlorine dioxide constitutes about 20 to about 25% of the total available chlorine used, which comprises subjecting said suspension to a first bleaching step using a bleaching chemical selected from an aqueous solution of chlorine dioxide and an aqueous solution of chlorine dioxide and chlorine, and without an intermediate washing step, subjecting the suspension to a second bleaching step using chlorine, the suspension containing dissolved organic material present as a contaminant of the suspension in a quantity of about 2 to about 10% by weight total organic carbon on pulp.

2. The process of claim 1, wherein the chlorine dioxide used constitutes from 40 to 20% of the total available chlorine used in the bleaching steps and the first step is effected using an aqueous solution of chlorine dioxide.

3. The process of claim 1, wherein the first step is effected using an aqueous solution of chlorine dioxide and chlorine containing chlorine in an amount of about 5 to about 10% of the available chlorine of that solution.

4. The process of claim 1, wherein the first step is effected using an aqueous solution of chlorine dioxide and chlorine containing chlorine in an amount greater than about 10% up to about 30% of the available chlorine of that solution.

5. The process of any one of. claims 2 to 4, wherein the suspension contains dissolved organic material in a quantity of about 3.5 to about 6.5% by weight of total organic carbon on pulp.

6. The process of any one of claims 2 to 4, wherein the first and second bleaching steps are effected at a temperature of about 35° to about 70°C.

7. The process of any one of claims 2 to 4, wherein the first and second bleaching steps are effected at a temperature of about 50° to about 60°C.

8. The process of any one of claims 2 to 4, wherein the cellulosic fibrous material is wood pulp, the first and second bleaching steps are effected at a temperature of about 35° to about 70°C, the overall bleaching time of the first and second bleaching steps is from about 30 to about 60 minutes, and the second bleaching step is commenced about 5 seconds to about 10 minutes after commencement of the first bleaching step.

9. The process of claim 1, wherein the bleaching steps are effected as the first bleaching stage of a multistage bleaching and purification operation, and the dissolved organic material in the suspension arises from a counter-current flow of wash water with respect to the pulp flow through the multistage operation and utilization of effluents from the multistage operation to wash unbleached pulp prior to passage of the same to the multistage operation.