US3802958A - Chlorination of cellulose pulp - Google Patents

Abstract

A process is provided for delignifying cellulose pulps that have previously been subjected to treatments with chlorine, or chlorine dioxide, or a mixture thereof, and with alkali, and is intended to be carried out as a third or subsequent stage in the multi-stage bleaching of cellulose pulp, which comprises delignifying the cellulose pulp with a mixture of chlorine and chlorine dioxide comprising from about 10 to about 50 percent chlorine dioxide calculated as available chlorine, at a pH within the range from about 1 to about 3.5 and at a pulp consistency within the range from about 3 to about 15 percent, and a temperature within the range from about 40* to about 70* C., for a time within the range from about 30 to about 300 minutes.

Description

Croon et a1.

Apr. 9, 1974 CHLORINATION OF CELLULOSE PULP [75] Inventors: Ingemar Liss-Albin Croon,

Salgvagen; Sten Oskar Dille'n, Strandvagen, both of Sweden [73] Assignee: M och Domsjo Aktiebolag,

Ornskoldsvik, Sweden [22] Filed: Apr. 8, 1 971 [21] Appl. No.: 132,367

Related US. Application Data [63] Continuation-in-part of Ser. No, 774,171, Nov. 7,

1968, abandoned.

[52] US. Cl. 162/89, 8/108 [51] Int. CL, D21c 9/12 [58] Field of Search 162/89, 88, 87; 8/108 [56] References Cited UNITED STATES PATENTS 3,560,330 2/1971 Ahlborg 162/89 X 3,433,702 3/1969 Jack et a1 162/88 X 3,652,388 3/1972 Croon et al.... 162/89 X 3,536,577 10/1970 Rapson 162/89 X 2,166,330 7/1939 Vincent 162/87 X OTHER PUBLICATIONS Rydholm, Pulping Processes, 1965, p. 839. Scott Paper Company Handbook, 1st ed., 1956, p. 95.

Primary E.\'aminerS. Leon Bashore Assistant Examiner-Arthur L. Corbin [5 7 ABSTRACT A process is provided for delignifying cellulose pulps that have previously been subjected to treatments with chlorine, or chlorine dioxide, or a mixture thereof, and with alkali, and is intended to be carried out as a third or subsequent stage in the multi-stage bleaching of cellulose pulp, which comprises delignifying the cellulose pulp with a mixture of chlorine and chlorine dioxide comprising from about 10 to about 50 percent chlorine dioxide calculated as available chlorine, at a pH within the range from about 1 to about 3.5 and at a pulp consistency within the range from about 3 to about percent, and a temperature within the range from about to about C., for a time within the range from about 30 to about 300 minutes.

7 Claims, N0 Drawings This application is a continuation-in-part of Ser. No. 774,171, filed Nov. 7, 1968, and now abandoned.

This invention relates to a process for delignifying Chlorine is considered to be the most effective and least expensive of the delignifying agents that are available. However, it has a tendency to decompose the cellulose, and therefore it is necessary to keep the amount Up to 130 C. in hot alkali refining.

cellulose pulp that has previously been subjected to 5 Of chlorine at the minimum that give effective detreatments with chlorine, or chlorine dioxide, or a mixhghificahoh- The chlorination thus is usually carried t th f, d th ith lk li; d more i out in such a manner that there is a small excess of l l t a ltit process f bl hi ll l chlorine present at the conclusion of the chlorination. pulp which includes as a third or later treatment stage, This excess is rather important to control. If there is too following treatment with chlorine, or chlorine dioxide, high an excess remaining, the tendency of the or mixture th f d i alkali, a d li ifi ti chlorination stage to decompose cellulose is increased, stage with a mixture of chlorine and chlorine dioxide at and if there is no excess, if the eXCeSS is $00 Small, a temperature i hi the range f about 4 t about the delignification of the cellulose is incomplete. If the 70 C, fo a i i hi the range f about 30 to chlorination must be applied in two stages, the risk of about 200 minutes and at a pulp consistency within the decomposition of the cellulose is increased in the secrahge f about 3 percent to about 15 percent, and stage, and therefore the arrangement of the treat- Th bl hi f ll l l b i d b either ment stages is usually such that the chlorination is rethe sulfate or sulfite process is a multi-stage procedure, Stricted to one Stage, and a Substitute hypochlorite 0F and usually includes as separate steps in any of a larger Chlorine dioxide stage is used instead, if Possiblenumber of sequences of one or more of each, The Chlorination, Stage is normally Performed a, a hl i i Stage, with a relatively low pulp consistency, from about 1 to b. an alkali treatment or alkali extraction stage, about 4 Percent, and at low temperatures, from about c. a hypochlorite treatment stage, and 5 to about C., for short treatment times, from about d. a chlorine dioxide treatm t t 1/3 to about 2 hours. The quantity of chlorine used var- These treatments can be carried out in almost any or- 25 ieS With the properties of the unbleached P p. but is der, and two or more of such stages are frequently insufficient n y to g e n XCeSS Within the range from eluded, but usually not in succession, but interleaved about 0.1 to about 0.5 percent chlorine, based on the with one 01' more f th oth st weight of the dry pulp, at the conclusion of the treat- In general, sulfate pulps require more treatment menistages for complete bleaching than sulfite pulps. Also, Chlorine dioxide can also be used in place of chlorine in the case of sulfate pulps, chlorination, alkali treatin e Chlorination g in this event, the amount of ment and chlorine dioxide treatment are almost always chlorine dioxide used is sufficient to supply an amount necessary while the hypochlorite treatment can someof chlorine within the range set out above sufficient to times be omitted, whereas in the case of sulfite pulps, ensure an excess of from 0.1 to 0.5 percent chlorine at either the hypochlorite or the chlorine dioxide treatthe end of the treatment. ments can be omitted, and the bleaching can be re- The alkali extraction, stage E, is designed to remove duced to as little as three or four stages. However, all the lignins that have been made alkali-soluble in the multi-stage bleaching processes employ a chlorination chlorination stage, and is carried out at a relatively high stage and an alkali treatment stage, and the reason is pulp consistency. This is normally at from about 4 perthat these stages are required for delignification. Some cent to about 18 percent; when using presses for dewasulfate pulps are more difficult to delignify than others, tering, up to 30 percent. The temperature can be rather and in the case of these pulps, one chlorination stage low, from 25 to 30 C., but in the so-called hot alkali may be insufficient to delignify the pulp sufficiently. refining processes, for producing pulp-withahigh alpha Hence, a complementary delignification stage can be cellulose content, temperatures of up to 130C. can be carried out in an additional chlorination stage, or in a employed. The treatment time can be very short, for hypochlorite treatment stage, or in a chlorine dioxide just several minutes, up to from 4 to 5 hours. The treatment stage. amount of alkali charged to the system varies according The conditions under which the various treatment to the properties of the pulp which it is desired to prostages are carried out in conventional bleaching procduce. In the case of paper pulps, it is usually carried out esses are summarized in Table A, which follows: in such a manner that the pH at the conclusion of the TABLE A i Pulp 4 consist- Tempera- BHGY, 1TB Stage Treatment percent C.) Time Amount 0 Chlorination 14 5-25 %-2 hours... Enough C1; or C102 to provide an end of treatment 0.1 to 0.5% excess C12 based on (Cl: or C10: pulp weight. E Al lt aii di r aetion 4-18 25-80 Several min- Enough NaOH to give a pH 8-12.51 at end of treatment.

(NaOH) ggelsrgo 4-5 H l iii) H hm vi iii, fihhiilfif2.ififiliifi iifiieSffirifiiifiiob"zi'i hiiiiififfioii t ifi fi r pH 8.5, usually 10-11. 1 Ca 001): excess alkali D Gi le? dioxide 4-15 50-90 2-5 hours Enough to provide 0.5 to 5% 01:, based on pulp weight, with an excess of from 0.1

(C102). to 0.5% C 2 at end of treatment.

treatment is within the range from about 8 to about l2.5, generally about ll.

The hypochlorite treatment, stage H, is also carried out at relatively high pulp consistencies, normally from about 4 percent to about 18 percent. The temperatures are usually rather higher than in the chlorination, and range from about 25 to about 60 C. The treatment time is also somewhat longer, from 1 hour and upwards, normally from two to four hours. The quantity of hypochlorite charged to the system varies with the type of pulp and the conditions, and is based on the amount of chlorine provided. The amount can range from very small amounts, of the order of 0.2 percent C1 by weight of the pulp, to 3% C1 based on the weight of dry pulp. This treatment is also carried out in a manner to ensure that a small excess of chlorine remains at theconclusion of the treatment, about 0.1 percent by weight of the pulp.

l-Iypochlorite having a pH at about and immediately below the neutral point of 7 strongly decomposes cellulose, and consequently an amount of alkali is also charged to the system to provide a surplus of alkali, and maintain the pH well above 7. The amount of alkali charged as NaOH can be within the range from about 10 percent to about 100 percent by weight of the chlorine charged to the system in this stage, so as to ensure that at the conclusion of the hypochlorite treatment the pH is above 8.5, and is normally within the range from 10 to 11.

The chlorine dioxide treatment is carried out with the same pulp consistency as the hypochlorite treatment, although in practice a somewhat lower consistency can be used, since the chlorine dioxide is obtained in a more dilute form than the hypochlorite solution. Thus, the pulp consistency during chlorine dioxide treatment can be within the range from about 4 percent to about percent. The temperature is higher, within the range from about 50 to about 90 C., and normally about 70 C. The treatment time is at least 2 hours, and can be from 3 to 5 hours.

The quantity of chlorine dioxide charged to the system is based on the amount of chlorine equivalent to the chlorine dioxide within the range from about 0.5 to about 3% C1 based on the weight of the dry pulp, and sometimes even more than 3 percent, up to about 5 percent. This treatment is also carried out in a manner to ensure a small excess, about 0.l to 0.5% Cl at the end of the treatment.

The usual bleaching process for sulfate pulps includes six stages, in the order C E H D E D, the letters corresponding to the treatment stages set out above. An alkali extraction normally follows one or more stages of delignification, to remove solubilized lignins DED DED D mzmrn muom Uri-1::

Sulfite pulps can be bleached with shorter bleaching sequences, and illustrative sequences include O0 000 mm rnmrn :2: 03:0

It will be noted that in all of the sequences in common use for sulfate and sulfite pulps, the first treatment stage is invariably a chlorination stage, and the second treatment stage is almost invariable an alkali or extraction stage, to remove the lignins and other alkalisoluble extractives. In the single exception noted above, the hypochlorite stage serves as a substitute second chlorination stage, seeking to reduce the risk of cellulose decomposition in the first chlorination stage. Anyhow as pointed out above also this hypochlorite stage comprises an alkali treatment immediately following the chlorination. An alkali extraction is interposed wherever more alkali-soluble extractives are to be removed.

The difficulty of control of the chlorination stage so as to prevent decomposition of the cellulose has drawn the attention of numerous workers in the pulp bleaching art, and many suggestions have been made. The most effective and least expensive alternatives, however, have involved the substitution of a hypochlorite or chlorine dioxide stage for a second chlorination stage, and a reduction in the severity of the first chlorination. Even in this variation, however, it is not possible to prevent decomposition of the cellulose altogether, particularly when the complementary stage is a hypochlorite treatment.

It has recently been proposed that the chlorination be carried out using a mixture of chlorine and chlorine dioxide. The amount of chlorine dioxide can be rather small, up to a major proportion. The chlorination is otherwise carried out in the usual way under the treatment conditions as set out in Table A above. However, the degree of amelioration obtained of the tendency to decompose the cellulose is not under these conditions sufiicient to compensate for the considerable increase in the cost of the chlorination, due to the higher cost of chlorine dioxide as compared to chlorine. Moreover, the amelioration of the decomposition of the cellulose may also be accompanied by an incomplete delignification, as a result of which it becomes necessary to introduce another chlorination stage, or a hypochlorite or chlorine dioxide treatment stage, so as to complete the necessary delignification In this event, there remains the possibility of decomposition of the cellulose during this complementary stage.

Ahlborg, US. Pat. No. 3,560,330, patented Feb. 2, l97l, proposes to delignify shives and specks which have not been sufficiently delignified in the first stage. The first delignification stage is a quite normal one, removing the bulk of lignin. In the second delignification stage, chlorine is added in a large excess, leaving as much as 2 to 10 grams of active chlorine per kilogram of pulp unreacted at the end of the treatment. The high concentration of active chlorine throughout the delignifying stage is claimed to be a prerequisite for an effective bleaching of the shives. The addition of chlorine dioxide in the second delignification stage is a secondary possibility, used to protect the cellulose in a way that has long been known. Such addition of a small amount of C10 is a standard procedure in chlorination (see TAPPI Monograph No. 27, pp -136). This addition does not change the stage into something other than the normal chlorine stage, performed 'at a somewhat higher concentration of chlorine than usual. In the second stage, moreover, the temperature is held below 35 C., and preferably between 20 and 30 C.

In accordance with the instant invention, it has been determined that a second chlorination stage in which a mixture of chlorine and chlorine dioxide containing an amount of chlorine dioxide within the range from about to about 50 percent by weight of the mixture calculated as available chlorine is used at a pH within the range from about 1 to about 3.5 can be arranged to follow as a third or later stage a first chlorination stage, and a second alkali extraction stage, with good control of the decomposition of the cellulose pulp, as well as an at least substantially better delignification than in prior pulp treatment processes, and in many cases, a substantially complete delignification.

The objective of this invention is to remove lignin homogeneously throughout the body of cellulose pulp. An insufficient amount of delignifying agent is charged in the first delignification stage, leaving a substantial amount of lignin to be delignified in a subsequent stage. The charge in the second delignifying stage is sufficient to remove the remaining lignin, leaving no or only a small excess of available chlorine at the end of the treatment. The bleaching sequence includes a chlorinezchlorine dioxide stage, where the percentage of chlorine dioxide calculated as active chlorine is from about 10 to about 50 percent, preferably from 10 to 30 percent. Chlorine dioxide must be present. A pure chlorine stage would cause a severe degradation of the cellulose. The higher the temperature, the larger amount of chlorine dioxide that is needed. A high temperature, from 40to 70 C. is used. Such a high temperature range has not earlier been possible in chlorination, due to cellulose degradation. The high temperature has been made possible by adding a sufficient amount of chlorine dioxide.

In the multistage pulp bleaching process of the invention, the first chlorination stage can be carried out with chlorine, with chlorine dioxide, or with a mixture of chlorine and chlorine dioxide either in the usual way, as shown in Table A, or preferably, under relatively mild conditions, so that delignification is incomplete. The cellulose pulp is then subjected to alkali treatment, and then in the third or some subsequent stage before the bleaching is complete, a continued delignification stage using a mixture of chlorine and chlorine dioxide in accordance with the invention.

The delignification stage of the invention is carried out under relatively severe conditions, unlike the preliminary chlorination stage, whether this stage is a normal chlorination, or a mild chlorination. In the delignification stage of the invention, the pH is within the range from about 1 to about 3.5. At a pH from 4 to 5 hypochlorous acid concentration reaches a maximum, and at a pH above 5, especially at from 6 to 7, the hypochlorous acid activity is at a maximum. This is undesirable, because of the degradation that results.

The pulp consistency is rather high, and is within the range from about 3 to about percent. An ordinary chlorination stage without chlorine dioxide addition is carried out at C. or lower. The temperature in the delignification stage of the invention is rather high,

within the range from about 40 to about 70 C., and the treatment time is within the range from about to about 300 minutes. These temperature and time conditions are not usually employed in a chlorination step, but correspond instead to theconditions of a hypochlorite or chlorine dioxide treatment.

In the delignification stage of the invention, the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture can be within the range from about 10 to about 50 percent chlorine dioxide, preferably within the range from about 10 to about 30 percent chlorine dioxide. The amount is calculated in terms of the amount of available chlorine equivalent to the chlorine dioxide.

The chlorine is employed as chlorine liquid or-chlorine gas, and is added to an aqueous suspension of the pulp in any conventional way. Chlorine dioxide is also added as such, or can be formed in situ. Usually, it is preferable to prepare chlorine dioxide as and when required, and then add it directly to the aqueous pulp mixture. Chlorine and chlorine dioxide should preferably be mixed thoroughly before the addition thereof into the aqueous pulp mixture.

The delignification stage of the invention offers several advantages over prior treatments, among them a lower cost for the chemicals required, and a very much smaller risk of decomposition of the cellulose. It is possible because of the benefits of the delignification stage of the invention to lower the cost for the chemicals re quired during bleaching, because of a reduction in the amounts of the chemicals, and rearrangement of the order of the various treatment stages of the bleaching process, with an unchanged or even improved quality of the pulp, in consequence.

In the process of the invention, the initial chlorination can be the usual chlorination described in Table A. Preferably, however, the chlorination stage is quite incomplete, and the chlorine charge rather small. Thus, the chlorine charge can be within the range from about 0.15 to about 0.25 percent chlorine per kappa number unit of the unbleached pulp, so that at the conclusion of the chlorination a small excess, within the range from about 0.1 to about 0.5 percent chlorine, remains. Chlorine dioxide can be substituted in whole or in part for chlorine, in amounts to supply an equivalent amount of chlorine, and an equivalent excess of chlorine at the conclusion of the process. Thus, mixtures of chlorine and chlorine dioxide can be used, with like effect. The treatment time can be rather small, as low as twentyminutes, up to about two hours. Under these conditions, there is no noticeable decomposition of the cellulose.

The pulp consistency during this incomplete chlorination stage is within the usual range from about 1 to about 4%, and the temperature is also within the usual range from about 5 to about 25 C.

An alkali extraction stage is interposed following the chlorine-chlorine dioxide treatment of the invention to remove the alkali soluble lignin derivatives and other alkali-soluble extractives formed in that stage. This alkali extraction can directly follow the Cl -ClO treatment, or be interposed at a later stage.

The alkali extraction can be under the usual conditions, as shown in Table A. However, lower alkali coricentrations can be used, since a large proportion of alkali-soluble extractives will have been removed in the first alkali extraction. Thus, the amount of alkali can be from 0.5 to 1.5 percent, and at such amounts the pH will be from 8 to 12.5 at the end of the extraction, as in the usual alkali extraction stage.

In the first alkali extraction, on the other hand, the application after an incomplete first chlorination impliesa 15 to 25 percent saving in the amount of alkali, in this stage, in comparison to the application after a conventional chlorination. In the alkali extraction stage following the chlorine-chlorine dioxide treatment of the invention, the pulp consistency is within the usual range from about 4 to about 18 percent, the temperature within the usual range from about 25 to about 30 C., up to about 130 C. in a hot alkali refining stage, and the treatment time can range from several minutes lulose molecules, and (3) the extinction at 205 nm, i.e., ultraviolet light (which is determined for a solution of the pulp in phosphoric acid containing mg. of pulp per ml. and is a measurement of the quantity of lignin to about 5 hours. 5 remaining in the pulp). The higher the viscosity, the

FOllowing the chlorination and alkali extraction greater the strength of the pulp, and the lower the exstages, the pulp is then subjected to the delignification tinction, the greater the extent of delignification. stage in accordance with the invention. The pulp at this The following Examples represent preferred embodipoint may well be less completely delignified than after ments of the invention. conventional chlorination and alkali extraction proc- 1Q esses, but nonetheless at the conclusion of the deli a EXAMPLE 1 fication stage of the invention, the resultmg pulp is more delignified and has a better quality. This means unbleached Cellulose P p P p from Pirle that the final bleaching of the deli hified pulp can be wood y the sulfate p a ng a appa num er f performed with less chemical consumption and results 5 31 pp number is a measurement of the lignin in better quality of the final bleached pulp. Quality of Content of the p p, and thus of its nsumption of the final bleached pulp is evaluated by the usual critebleachlhg chemlcals- The determination of PP "P ria: (1) brightness, percent SCAN before and after her according to the Standard SCAN-C 1159) was aging at el v t d t r t (2) vi i hi h i chlorinated and treated with alkali as the first two determined for an aqueous copper ethylene diamine Stages of the bleaching p The pulp as th n disolution of the pulp according to SCAN C15;62, d vided into four batches, and the bleaching was continwhieh is a measurement of the mean polymerization ued in a series of steps. The conditions are outlined in degree of the cellulose, i.e., the chain length of the cel-' able l TABLE I BATCH NUMBER Stage Treatment l 2 4 (1)-Chlorination Pulp consistency 3.5% Temperature 20C. Chlorine charge 6.8% Treatment time 1 hour Chlorine residue at end of treatment 0.15%

(2) Alkali Extraction Pulp consistency 8'71 Temperature C. NaOH charge 371" Treatment time 2 hours pH at end of treatment 11.6

3) HOCI CHLORINE (210 Pulp consistency 8% 6% 6% 6% Temperature C. 45 40 40 NaOCl' cl cl Cl Charge 1.7% l.08% 0.775% 1.08% NaOH Clo, c10 C10,

Charge 0.68% 0.47 0.775 0.47 Treatment time (hours) 2 3 3 3 Chlorine excess at end of treatment 0.2% 0.067: 0.02 0.01% pH at end of treatment l0.7 2.1 ll

' (4) Alkali Extraction Pulp consistency None Temperature 50C- NaOH charge V711 Treatment time (hours) 2 pH at end of treatment 11.5

(5) Chlorine Dioxide Pulp consistency 6% Temperature C.

ClO charge (as equivalent C1,) Treatment time (hours) Surplus chlorine equivalent at end of treatment Table i Continued Stage Treatment BATCH NUMBER (6) Alkali Extraction Pulp Consistency 8% Temperature 50C.

NaOH charge 0.7%

Treatment time (hours) 2 pH at end of treatment ll.2-l 1.3

(7) Chlorine Dioxide Pulp consistency 6% Temperature 85C.

ClO charge (as equivalent C1 0.9%"

Treatment time 5 hours Surplus chlorine equivalent at end of treatment ("/2) (LIZ-0.3%

PROPERTIES OF BLEACHED PULPS Brightness, '7: SCAN 93.6 93.7 93.7 93.6

Brightness after ageing l6 hours at 180C, 80% rh 86.8 87.7 87.1 83.2

Brightness after ageing 16 hours at 120C, dry atmosphere 87.7 88.3 88.0 88.5

Viscosity, cp Tappi 44 55 55 59 Extinction at 205 nm, cm 0.35 0.3l 0.30 0.36

based on Cl equivalent dry pulp weight Batch 1 served as a control in which a conventional hypochlorite treatment was used as stage (3) instead of the chlorine-chlorine dioxide treatment in accordance with the invention, which was used for batches 2 to 4. It is apparent from the properties of the finished pulp that pulps of equivalent or improved brightness were obtained using the chlorine-chlorine dioxide treatment of the invention. These pulps also had a distinctly higher viscosity, and a lower extinction. The viscosity for a sulfate pulp up to viscosities up to approximately 60 cp Tappi shows a direct correlation between the strength of paper produced from the pulp, and the magnitude of viscosity. At levels above 60 cp Tappi, increases in the viscosity may improve strength but not to the same extent.

Data show that replacement of the hypochlorite treatment stage with the delignification stage of the invention gives an increase in the potential strength properties of the pulp, and an improved delignification. Moreover, there is a marked reduction in the chemical costs for bleaching in the case of batches 2 to 4, as compared to batch 1.

EXAMPLE 2 Using the same starting cellulose pulp as in Example 1, a bleaching operation was carried out in accordance with the stages outlined in Table 1, except that in the first chlorination stage, 5 percent of the chlorine charge was replaced by an equivalent quantity of chlorine dioxide. The results obtained were similar to those shown in Table I, with the exception that the viscosities were higher:

TABLE II Batch No. l 2 3 4 Viscosity, cp Tappi 64 87 76 83 Because of the use of the chlorine dioxide in substitution for chlorine, the cost for the chemicals was somewhat higher than the cost of the process outlined in Table I.

EXAMPLE 3 process and having a kappa number of 31 was used.

This was divided into six batches, which were bleached in accordance with the stages outlined in Table III. Batches 1, 2 and 3 were-controls, and batches 4 to 6 illustrate the use of the chlorine-chlorine dioxide deligni- 5 fication of the invention, as stage (3), in replacement of the hypochlorite stage (3) used for batches l to 3.

TABLE III BATCH NUMBER Stage Treatment 1 2 3 4 5 6 (l) Chlorination Pulp consistency 3.5% 3.5% 3.5%

Temperature 20C. 20C. 20C.

Cl charge 6.8% 6.46% 5.25%

ClO charge 0.34%

Time (hours) I l l/3 Cl excess 0.2% 0.2% 0.1%

' TABLE m mi'iiied BATCH NUM BER Stage Treatment 1 2 3 4 5 6 (2) Alkali Extraction Pulp consistency 8% 8% Temperature 50C. 50C. NaOH charge 3% 2.3% Time (hours) 2 10.9-11.4 pH at end of treatment 10.9-11.4 10.9-11.4

(3) Hypochlorite C1 +C10,

Pulp consistency '871 8% 8% 6% 6% 6% Temperature C. 40 40 40 40 40 40 NaOCl NaOCl' NaOCl' C1 C1 C1 Charge 1.08 1.08 2.32 1.94 1.73 1.51

NaOH NaOH NaOH C10 C10 C10,

Charge 0.43 0.43 0.93 0.22 0.43 0.65 Time (hours) 2 2 2 3 3 3 pH at end of treatment 10.6 10.9 109 1.65 1.7 1.85 Cl equivalent at end of treatment ("/b) 0.05 1 0.10 0.02 0.02 0.02

(4) Alkali Extraction Pulp consistency None 871 Temperature C. NaOH charge 171 Time (hours) 2 pH at end of treatment l111.2

(5) Chlorine Dioxide Pulp consistency 6% 6% Temperature C.

C10 charge (as equivalent C1 1% of dry pulp weight Time (hours) 3 Surplus chlorine equivalent at end of treatment. (1.05 to (1.2071 01 dry pulp weight (6) Alkali Extraction Pulp consistency 87! Temperature 50C.

NaOH charge 0.771 of dry pulp weight Time (hours) 2 pH at end of treatment 11.2-11.3

(7) Chlorine Dioxide Pulp consistency 6% Temperature C.

C10 charge (as equivalent C1,) 0.971 of dry pulp weight Time (hours) 5 Surplus chlorine equivalent at end of treatment 0.2-0.37r dry pulp weight Bleached Pulp Properties Brightness. 71 SCAN 91.9 91.7 93.0 92.9 92.4 92.2 Ditto after ageing for 16 hrs/80C.. 809? rh 85.2 84.9 84.5 85.4 85.3 84.8 Ditto after ageing l6 hrs/120C.. dry atmosphere 85.4 85.1 85.1 86.9 86.2 85.6 Viscosity. cP Tappi 56 89 68 86 90 93 Extinction at 205 nm. em" 0.38 0.40 0.53 0.36 0.36 0.35

based on Cl. equivalent 1 dry pulp weight It is evident from the data that the cost for the chemicompared t bat he l to 3. Batches 4, 5 and 6 gave as leals tn the treatment of batches 4 t0 6 as reduced, and high a viscosity as in the case of batch 2, in which the in pite of hi n impr righ was obtained, as bleaching was conducted with the addition of chlorine dioxide in the chlorination stage. Moreover, the final delignification is more complete, as indicated by the extinction values for batches 4 to 6.

EXAMPLE 4 This Example shows that if a delignification stage inaccordance with the invention is employed, delignification prior to final bleaching can be carried further than in a conventional bleaching process, without the TABLE IV BATCH NUMBER Stage Treatment I l 2 3 4 (l) Chlorination Pulp consistency 3.5% 3.5% 3.5% 3.57: Temperature 20C. 20C. 20C. 20C. 71 Cl 7.22 7.22 5.6 5.6 71 Cl 0.38 0.38 0.30 0.30 Time 1 hour I hour min. 20 min. Clexcess at end of treatment 0.3 0.3 0.1 0.1

(2) Alkali Extraction Pulp consistency 8'71 Temperature 50C. "/1 NaOH 2.8% Time 2 hours pH at end of treatment ll.ll 1.5

(3) Hypoehlorite Cl -ClO Pulp consistency 8% 8% 6% 6% Temperature C. 40C. C. 50C. Charge NaOCl NaOCl Cl Cl 1.9% 1.9% 2.38% 2.38% Charge NaOH NaOH ClO C10.

0.73% 0.73% 0.42% 0.42% Time (hours) 2 2 3 3 Cl excess at end of treatment 0.43 0.43 0.23% 0.23% pH at end of treatment H0 11.0 1.7 l 7 (4) Alkali Extraction Pulp consistency None 8% 8% Temperature 50C. 50C. 72 NaOH l% 1% Time 2 Hours 2 Hours pH at end of treatment l0.95 10.95

(5) CIO Pulp consistency 8% None 8% 8% TemperatureC. 70 85 C10, 1.371 2.0% l.3'7r 2.0% Time 3 hours 5 hours 3 hours 5 hours Cl equivalent at end of treatment (7r) 0.6 0.07 0.20 0.l6

(6) Alkali Extraction Pul consistency 8% I None 8% None Temperature 50C. 50 "X'NaOH 0.7% 0.7% Time 2 hours 2 hours pH at end of treatment TABLE IV Continued BATCH NUMBER Stage Treatment 1 2 3 4 Pulp consistency 671 None 6% None Temperature 85C. 85C. C10, (1.97: 0.9% Time hours 5 hours C1 equivalent at end of treatment ("/t) (1.13-0.14 0.13-0.14

Properties of Bleached Pulp Brightness according to SCAN, '71 93.3 90.6 94.1 92.1 Brightness after ageing for 16 hours at 80C., 11071 86.3 83.5 86.9 85.4 Brightness aftcr ageing for 16 hours at 120C, dry atmosphere 87.3 83.4 89.0 85.2 Viscosity according to Tappi cP 61 S0 65 61 Extinction at 205 nm, em 0.361 0.481 0.293 0.364

' based on C1. equivalent 2 dry pulp weigh\ It is apparent from the data that batch 2, delignified according to the conventional procedure using hypochlorite, and batch 4, delignified in accordance with the present invention, which were finally bleached using only one chlorine dioxide stage, showed a lower degree of brightness than corresponding pulps finally bleached with two chlorine dioxide stages, batches 1 and 3, respectively. The brightness of the pulp batch 4 delignified in accordance with the invention lay at an acceptable level, and the viscosity is as high as that of batch 1, which was bleached with two chlorine dioxide stages in the final bleaching stage, but with conventional delignification including a hypochlorite stage.

Delignification according to the invention followed by final bleaching, using only one chlorine dioxide stage, can be considered as a modification of the generally used sequency C E D E D. The invention thus provides a reduced cost of chemicals, and gives good pulp quality with regard to brightness and mechanical strength. Also, when using the process of the invention,

- it is possible to carry the delignification further with process comprising in a first chlorination and delignifi-.

cation stage chlorinating cellulose pulp with chlorine, with chlorine dioxide, or with a mixture of chlorine and chlorine dioxide under relatively mild conditions, so that delignification is incomplete; in'an alkali treatment stage alkali-treating the chlorinated pulp; and then, as a third or subsequent stage,'before the bleaching is complete, in a second chlorination and delignification stage, chlorinatingthe alkali treated pulp with a mixture of chlorine and chlorine'dioxide comprising from about percent to about 'percent chlorinedioxide 3. A process in accordance with claim 1, which comprises alkali treating the chlorinated pulp subsequent to the second chlorination and delignification stage.

4. A process in accordance with claim 1, which comprises alkali treating the delignified pulp and then treating the alkali treated delignified pulp with a member selected from group consisting of hypochlorite and chlorine dioxide.

5. A process in accordance with claim 1, in which the first chlorination and delignification is effected at a pulp consistency within the range from about 1 to about 4 percent, a temperature from about 5 to about 25 C. for from about 1/3 to about 2 hours, with sufficient chlorine, supplied as chlorine, chlorine dioxide, or both, to provide from about 0.1 to about0.5 percent excess chlorine at the end of the chlorination.

6. A process in accordance with claim 1, in which the alkali treatment is effected at a pulp consistency within the range from about 4 to about 18 percent at a temperature from about 25 to about 130 C. for a period of time up to about 5 hours, and with sufficient alkali to afford a pH from about 8 to about 12.5 at the end of the alkali treatment. V

7. A process in accordance with claim 1, in which the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture in the second chlorination and delignification stage is within the range from about 10 to about 30 percent chlorine dioxide, calculated as the amount of chlorine equivalent to the chlorine dioxide. 4 i l LII Page 1 of 4 UNITED STATES PATENT OFFICE (IERHFICATE 6F CORRECTION Patent NO. 3, 2, Dated pri 19 4 Inventods) lngemar Lise-Albin Croon et 21.

It is certified that error appears in the above-identified patent a and that said Letters Patent are hereby corrected as shown below:

Page 1, [75] line 2 after "Salgvagen" insert Domsjoverken line 3 v after "Strandvagen" insert -Hallstavik Column 4, line 43 after "delignification" please insert a period Column 7, line 6 "FOllowing" should be Following-- Q i l -l .Y

Page 2 of 2 UNITED STATES PA'IENT OFFICE @E'llFlCATE 9F CORRECTION Patent No. 958 Dated April 9, 1974 lngemar Liss A1b1n Croon et 211.

It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, Table I, heading (3) Batches l, 2, 3 and 4 should be 1 (3) HOG]; CHLORINE C10 Pulp consis- 8% 6% 6% 670 tency Temperature 45 40 40 60 Naomi d2 d2, ge 1.7% r 1.08% 0.775% rod-5 NaOH (1102 153 01o Charge 0.68% 0.47 0.775 0.47 Treatment 2 3 3 3 time (hours) Chlorine 0.2% 0.069% 0.027% 0.0175

excess at end of treatment pH at end of 10.7 2.1 2.15 2.1

treatment Page 3 of 3 UNITED STATES PATENT OFFICE CERTTFICATE OF CORRECTION Patent No 3, 802, 958 Dated Amf'fl 9 1974 Inventor(s) Ingemar Liss Albin Croon et 211.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, Table I, heading 4, Batch 3 Page 4 of 4 UNITED STATES PATENT OFFICE CER'IIFICATE OF CORREC'UQN Q Patent X0. 3, 802,958 Dated April 9, 1974:

Invent r(5) Ingemar Liss-Albin Croon et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 14, Table IV, heading Batch 2 i should be 0 Pulp consis- 8% 9,6 8 875 tency Temperature 7O 85 c10 1.3% 2.0% 1.3% 2.09% Time 2 3 hours 5 hours 3 hours 5 hours C1 equivalenfi' 0.6 0.07 0,20 0.16 .5 at end of treatment (,0)

Column 15, line 40 "sequency" should be --sequence- 0 ,Zn'gncd and Scaled thisthirtieth D f March 1976 [SEAL] Attest: O

RUTH C. MASON C. MARSHALL DANN Atlesling Officer Commissioner ofPatents and Trademarks

Claims (6)

1. 2. A process in accordance with claim 1, which comprises chlorinating the pulp in the first chlorination and delignification stage with chlorine.
2. 3. A process in accordance with claim 1, which comprises alkali treating the chlorinated pulp subsequent to the second chlorination and delignification stage.
3. 4. A process in accordance with claim 1, which comprises alkali treating the delignified pulp and then treating the alkali treated delignified pulp with a member selected from group consisting of hypochlorite and chlorine dioxide.
4. 5. A process in accordance with claim 1, in which the first chlorination and delignification is effected at a pulp consistency within the range from about 1 to about 4 percent, a temperature from about 5* to about 25* C. for from about 1/3 to about 2 hours, with sufficient chlorine, sUpplied as chlorine, chlorine dioxide, or both, to provide from about 0.1 to about 0.5 percent excess chlorine at the end of the chlorination.
5. 6. A process in accordance with claim 1, in which the alkali treatment is effected at a pulp consistency within the range from about 4 to about 18 percent at a temperature from about 25* to about 130* C. for a period of time up to about 5 hours, and with sufficient alkali to afford a pH from about 8 to about 12.5 at the end of the alkali treatment.
6. 7. A process in accordance with claim 1, in which the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture in the second chlorination and delignification stage is within the range from about 10 to about 30 percent chlorine dioxide, calculated as the amount of chlorine equivalent to the chlorine dioxide.