Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
  • D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
  • D21C9/142—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 in a multistage process involving ClO2/Cl2 exclusively

Definitions

• This invention relates to the bleaching of cellulosic materials and more particularly to a process of sequentially treating cellulosic materials with chlorine dioxide and chlorine in a manner whereby the color bodies, lignins and other impurities are more readily removed or brightened
• the bleaching of cellulosic materials, particularly woodpulp has been the subject of extensive study and experimentation. As a result, many processes are known for effecting such bleaching operations and numerous of these processes are used in commercial bleaching opera-: tions.
• Various oxidizing reagents such as chlorine, chlorine dioxide, alkali and alkaline earth hypochlorites, hydrogenperoxide, and the like have been suggested and used in specific processes wherein the reagent is applied to the cellulosic material in one or more steps.
• the more common processes often use more than one reagent and more than one bleaching step.
• Some of these processes involve treating the cellulosic material with a reagent such as chlorine, washing the residual chlorine and solubilized impurities from the treated material with water, neutralizing or extracting the chlorinated lignins with caustic and washing with water, again treating the cellulosic materials with another reagent, such as chlorine, chlorine dioxide or sodium hypochlorite and again washing theresidual reagent and solubilized impurities from the cellulosic material.
• a reagent such as chlorine
• washing the residual chlorine and solubilized impurities from the treated material with water
• neutralizing or extracting the chlorinated lignins with caustic and washing with water
• another reagent such as chlorine, chlorine dioxide or sodium hypochlorite
• a process for the bleaching of a cellulosic material comprising treating cellulosic materials with a bleaching agent selected from the group consisting of chlorine dioxide and chlorine, reacting said bleaching reagent with the cellulosic material until the reagent is at least partially depleted, and treating said cellulosic material, without an intermediate wash, with a second bleaching reagent, said bleaching reagent being other than the first bleaching reagent and being selected from the group consisting of chlorine dioxide and chlorine, reacting said reagent with said cellulosic material.
• a bleaching agent selected from the group consisting of chlorine dioxide and chlorine
• subsequent bleaching stages used in multistage bleaching operations and the particular bleaching reagents used in these stages affect the characteristics of the final bleached pulp, improved results are obtained using the method of this invention as the first stage with other normal bleaching sequences, in second, third or subsequent stages.
• chlorine dioxide is used as the first bleaching reagent and chlorine is used as the second bleaching reagent without an intermediate'wash, followed by a caustic extraction, .a further chlorine dioxide bleach, a further caustic extraction and a final chlorine dioxide bleach
• superior results are obtained when compared to using a chlorine dioxide chlorine mixture, pure chlorine or pure chlorine dioxide the first stage followed by the same sequence.
• the total effect is that lesser amounts of chemicals can be used to obtain comparable brightness levels using the present method than with those previously proposed.
• the present process is applicable to cellulosic materials in general, it is herein more specifically described in relation to the bleaching of woodpulp, such as that ob tained from hemlock, Douglas fir, balsam, cedar, and the like.
• the pulp in an aqueous slurry and to add the reagents to the slurry.
• the prepared pulp is normally in an aqueous slurry which may be acidic, neutral or basic, and is directly bleached therein. Therefore the invention will be more completely described in relation to aqueous slurries of pulp.
• chlorine dioxide or chlorine is added in sequence to the pulp, such that first one reagent is added and reacted with the cellulosic material and then the second reagent is added and reacted without an interaugust mediate washing step.
• chlorine dioxide is added and allowed to react for a period of time suflicient to at least partially deplete the chlorine dioxide, i.e., more than about 50 percent is depleted.
• chlorine is added and reacted with the cellulosic material to complete the bleaching stage. After a reaction of sufiicient time to pro Jerusalem the desired results, the residual reagent is washed from the pulp with water.
• Second, third or more bleaching stages may be applied, using conventional bleaching sequences in the additional bleaching stages.
• caustic, chlorine dioxide, chlorine, hypochlorite, peroxide or mixtures thereof can be used.
• the present method in the first stage of a bleaching operation, this process can also be used in a second or later stage or as the only bleaching stage. However, the present method is preferably used as the first stage of a multistage bleaching operation.
• Either chlorine dioxide or chlorine can be used as the first added reagent, chlorine dioxide being preferred.
• the addition of the first bleaching reagent is preferably made in suflicient quantity so that a neutral to acidic pulp slurry is formed. Under such conditions, the bleaching action of the reagent is more effective.
• the second added reagent is either chlorine dioxide or chlorine, depending on which reagent was used first.
• the chlorine dioxide may contain a small amount of chlorine due to the simultaneous release of chlorine during most chlorine dioxide generation processes, when it is stated herein that the cellulosic material is treated with chlorine dioxide, such a treatment is with a material which is primarily or substantially all chlorine dioxide but may contain some chlorine.
• the chlorine used i primarily or substantially all chlorine, but may contain some chlorine dioxide.
• the bleaching time for each reagent in the process of this invention can vary, depending on the bleaching temperature, concentration of the reagents used, the specific characteristics desired in the bleached pulp and the percentage pulp dispersed in the aqueous solution.
• sufficient time is provided after each addition so that the reagent is reacted with the pulp, thereby at least partially depleting it prior to the addition of the second reagent.
• the reagent is reacted to substantially or to almost completely deplete it prior" to the next addition.
• At lower temperatures and lower concentrations of reagent when the ultimate bleaching capacity of the reagent is to be utilized, longer bleaching times are used.
• the total bleaching or reaction time for the process of this invention can be up to about three hours or more. Preferably, the reaction time will be from about five minutes to about two hours. Of this reaction time, it is preferred to react the first reagent for a shorter time than the second reagent.
• the reaction time for each reagent has been found to be preferably in a time ratio of a major proportion of the reaction time for the second reagent and a minor proportion of the reaction time for 'the first reagent, and more preferably, a reaction time ratio of the first reagent to the second reagent of about 1:2.
• the temperature at which the solution is maintained during the bleaching operation can be varied from about five degrees centigrade up to about 100 degrees centigrade, but it is normally preferred to effect the bleaching operation in the temperature range of about 10 to 50 degrees centigrade.
• the speed of the bleaching reaction is accelerated at the higher temperatures.
• the amount of total available chlorine added can vary considerably depending upon the particular characteristics desired to be obtained as a result of the bleaching operation. Normally, I to about 10 percent of available chlorine, by weight of the cellulosic material, as measured by the percent consistency, is added and reacted with the cellulosic material in the present first stage bleaching process, the exact amount depending on the particular bleaching requirements. Available chlorine is a measure of the oxidizing power of the reagent using chlorine, as the standard. As such, one part by weight of chlorine dioxide is equivalent to 2.63 parts by weight of chlorine. It has been found to be preferable to add 2 to 90 percent of the total available chlorine in the first phase addition and more preferably to add 20 to percent of the total available chlorine based on the total available chlorine required in the bleaching stage.
• Examples 1-3 These examples illustrate a comparison between the results obtained by the two phase chlorine dioxide-chlorine bleaching process of the present invention used as a first stage (Example 1) compared to a mixture of chlorine dioxide and chlorine used as the first stage (Example 2) and the use of chlorine alone as the first stage bleach (Example 3).
• the comparative examples were run in a five-stage bleaching operation, wherein the first stage of Example 1 was the method of the present invention, wherein the first stage of Example 2 utilized a mixture of chlorine dioxide and chlorine and wherein the first stage of Example 3 used chlorine.
• the remaining bleaching stages for each example follow the same sequence of caustic extraction, chlorine dioxide addition, caustic extraction and final chlorine dioxide addition.
• TAPPI Standard Testing Methods wherein the General Electric (G.E.) Brightness was measured according to Standard Method T 217 m-48 and wherein the one hour heat-aged brightness test was carried out in a mechanical convection oven at 105 degrees centigrade for the indicated time:
• the physical tests were carried out according to TAPPI Standard Testing Methods T 200 ts-61, T 205 m-58, T 218 m-59, T 402 m-49 and T 220 m60 wherein the burst strength is in pounds per square inch (p.s.i.) per pound per ream of SOD-M by 40 inch sheets times 100; the tear strength is in grams per pound per ream of 500-24 by 40 inch sheets times 100; the tensile is the breaking length measured in meters of paper, and double folds is per pound per ream of 500-24 by 40 inch sheets times 100.
• G.E. General Electric
• Example 1 which illustrates the process of the present invention
• the two phase addition employed a reaction time of 20 minutes for the addition of the first reagent, which was chlorine dioxide and a reaction time of 40 minutes. for the addition of the second reagent, which was chlorine.
• a comparison of the data obtained illustrates the improved results obtained by the present method.
• Higher and third and fifth stage brightnesses are obtained by the two phase method.
• higher third and fifth stage viscosities are obtained, compared with normal chlorine treatment.
• the higher brightness of the final product obtained both before and after aging for an 18 hour period illustrates the superiority of the present process in producing superior product characteristics.
• Examples 4 through 6 These examples compare the two-phase process of the present invention when used as the first stage of a sixstage bleaching process, I, with the use of a mixture of chlorine dioxide and chlorine in the first stage and the use of pure chlorine in the first stagefgThe comparisons were run using the same composition of kraft pulp as that used in Examples 1 through 3, dispersed in an aqueous phase. The results of the experiments are shown in Table II.
• Example 4 is atypical illustration of the use of chlorine as the bleaching" reagent in tlie firststage.
• Example 5 shows two-phasg chlorine dioxide-chlorine additionof the present invent1on, wherein t chlorine dioxide was reacted for 20 minutes prior to thqaddition of chlorine, which was then reacted for 40 minutes.
• Example 6 illustrates the use of alrnixture of chlorine dioxide and chlorine in the first stage.
• the fourth stage brightness was also 4 to 5 points higher with the two phase method and the final brightness after the sixth stage was again higher than that obtained with chlorine or the mixcaustic extraction was used in the second stage, chlorine dioxide addition was used in tlie third stage, caustic extraction was used in the fourth stage and chlorine dioxide was added in the fifth stage.
• Example 7 shows chlorine dioxide employed as the first added reagent
• Example 8 shows chlorine employed as the first added reagent.
• the bleaching reaction was effected for a total period of 60 minutes, 20 minutes with the first added reagent and 40 minutes with the second added reagent. In both instances 7.7 percent available chlorine wats used.
• the ratio of reagents, based on the available chlorine added, was 85 percent chlorine dioxide to 15 percent chlorine.
• the reactions were run using equal amounts of unbleached -kraft pulp consisting of about 70 percent hemlock and about 30 percent Douglas fir pulp.
• the reaction temperature was 20 degrees centigrade- A comparative analysis of the viscosities and brightness, both before heat-aging and after heat-aging, as shown in these examples, compare favorably with products resulting when either the chlorine dioxide or chlorine is added as the first phase.
• Examples 9-14 These examples show the results obtained when various ratios of chlorine to chlorine dioxide are used in the two phase addition of the present invention. Compared with these changes in chlorine to chlorine dioxide ratios is the use of pure chlorine (Example 9) and pure chlorine dioxide (Example 14).
• Available chlorine used in the first stage of each example was 6.5 percent based on the weight pulp TABLE IV
• Example numbers mar sues I Available 012 ratio, 012.0102 100 0 :20 60:40 40:60 20:80 0:100 0102 applied, percent 0 0. 494 0. 988 1. 48 1. 98 2. 47 C12 applied, percent 650 5. 20 3. 2. 60 1. 30 lstp ase terminal pH... 5.9 4.1 3.2 3.1 2nd hase terminal pH.. 2. 0 2. 1 2. 3 2. 3 2. 4 2. 6 Viscosity, centipoises..-- 147 191 192 179 201 265 N aOH applied, percent 3. 5 3. 5 3. 5 8. 5 3. 5
• a process for the bleaching of woodpulp comprising:
• the total reaction time for said process is from about 5 minutes to about two hours;

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Heterocyclic Carbon Compounds Containing A Hetero Ring Having Nitrogen And Oxygen As The Only Ring Hetero Atoms (AREA)

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Cited By (15)

Citations (4)

• 1965
  • 1965-06-28 US US467697A patent/US3433702A/en not_active Expired - Lifetime
  • 1965-11-06 CA CA944750A patent/CA932908A/en not_active Expired
• 1966
  • 1966-06-28 GB GB28988/66A patent/GB1145339A/en not_active Expired
  • 1966-06-28 DE DE1594861A patent/DE1594861B2/de active Granted
  • 1966-06-28 FR FR67213A patent/FR1484998A/fr not_active Expired
  • 1966-06-28 NO NO00163696A patent/NO126747B/no unknown
• 1967
  • 1967-06-05 BE BE699505D patent/BE699505A/xx unknown

Patent Citations (4)

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