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

• the present invention relates to the bleaching of pulp and more particularly to an improved process for bleaching wood pulp with chlorine dioxide in a manner whereby the wood pulp is subjected to a 2-step high pH/ low pH bleaching stage which results in a substantial decrease in the usage of chlorine dioxide required to brighten wood pulp.
• wood pulp bleaching As is well known in the wood pulp bleaching art, the main objectives of wood pulp bleaching are to increase the brightness of the pulp and to make it suitable for the manufacture of printing and tissue grade papers by removal or modification of some of the constituents of the unbleached pulp, including the lignin and its degradation products, resins, metal ions, non-cellulosic carbohydrate components, and various types of flecks.
• the bleaching of chemical wood pulp is normally carried out in multiple processing stages utilizing elemental chlorine, caustic soda, hypochlorites, oxygen, hydrogen peroxide, and chlorine dioxide. The number of stages required in a particular bleaching process is dependant upon the nature of the unbleached pulp as well as the end use to which the pulp will be put.
• a sulfate or kraft pulp is today most typically bleached in a five stage sequence which is designated as (CD) (EO)DED.
• the multi-stage process in essence comprises a chlorination step (CD) , a first oxidative extraction stage (EO) , a first bleaching stage (D,) , a second caustic extraction stage (E 2 ) , and a second and final bleaching stage (D_) .
• each of the two chlorine dioxide bleaching stages is carried out in a one-step process at an end pH of about 3.8 for three hours at 70° centigrade. It is commonly known that pH has an important bearing on brightness and strength properties as well as the chemical species present in the wood pulp mixture, and this particular pH has heretofore been considered optimal for each of the two chlorine dioxide bleaching stages in the (CD) (EO)DED sequence. It should also be appreciated that although the (CD) (EO)DED sequence has been specifically addressed, the one-step chlorine dioxide bleaching stage can be used in any D stage for most other three, four, five, or six-stage bleaching processes known to those familiar with the art of wood pulp bleaching.
• a shortcoming of the one-step chlorine dioxide bleaching stage presently used in the pulp and paper industry is that approximately 30% of the chlorine dioxide is lost to the formation of the unreactive species chlorite and chlorate, and this is very undesirable in view of the relatively high cost of chlorine dioxide.
• the present invention solves this well-known deficiency in state of the art chlorine dioxide bleaching by significantly reducing the chlorine dioxide loss during the chlorine dioxide bleaching process.
• the advantages of the reduced loss of chlorine dioxide are a very significant reduction in the cost of the wood pulp bleaching process as well as the reduction of pollution levels.
• applicant provides an improved process for bleaching wood pulp in an aqueous suspension using chlorine dioxide which substitutes a two-step bleaching stage for the conventional one-step bleaching stage known to those familiar with the wood pulp bleaching art.
• the novel process comprises first subjecting the aqueous wood pulp suspension to a first bleaching step by mixing it with an aqueous solution of chlorine dioxide and maintaining the mixture at a pH between about 5-10 for about 5-40 minutes. Next, an acid or acid gas is introduced into the mixture in order to bring the pH down to a pH between about 1.9-4.2, and the mixture is then subjected to a second bleaching step at the reduced pH for 2 or more hours, most suitably between about 2.5-3.9 hours.
• This novel process can be used in the D, or D 2 stage of the (CD) (EO)DED bleaching sequence as well as in any D bleaching stage of other three, four, five, six, and seven-stage bleaching sequences.
• the operating temperature during the novel process should be between about 55-85°C, and the pulp's final consistency should be between about 3-12%.
• Figure 1 is a graph of the effect of pH on chlorate and chlorite formation in chlorine dioxide bleaching of kraft pulp (reprinted from "The Bleaching of Pulp", Ed. R. P. Singh, p. 137) ;
• Figure 2 is a graph of D.. brightness for the pulp of Figure 2 when the D.. charge is varied on the pulp for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention
• Figure 3 is a graph of D_ brightness versus chlorine dioxide charge for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention wherein the D_ charge is 0.2% C10 2 on pulp;
• Figure 4 is a graph of D, and D_ brightness versus chlorine dioxide charge for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention
• Figure 5 is a graph of D, brightness versus percentage (%) chlorine dioxide on the pulp (D. charge) for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention
• Figure 6 is a graph of D_ brightness for the pulp of Figure 5 when the D ? charge is 0.2% chlorine dioxide on the pulp for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention
• Figure 6(a) is a graph of final brightness versus C10_ charge for the conventional one-step process and the novel two-step bleaching process of the present invention using a (CD) (EO)D sequence. Reverted brightness is also shown after 24 hours at 105°C;
• Figure 7 is a graph of D. viscosity versus D, pH for the conventional one-step bleaching process and high pH for the novel two-step bleaching process of the present invention
• Figure 8 is a graph of total organic chlorine (TOC1) or (AOX) in D, plus E 2 effluents versus chlorine dioxide charge in D. for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention
• Figure 9 is a graph of chlorate formed in the D, stage versus end pH
• Figure 10 is a graph of chlorate formed versus D. charge and CE kappa number for conventional bleaching
• Figure 11 is a graph of chlorate formed versus D. charge and CE kappa number for the novel two-step high/low pH bleaching process of the present invention.
• Figure 12 is a graph of chlorate formed as a percentage (%) of chlorine dioxide converted to chlorate versus percent (%) chlorine dioxide in D.. for the conventional one-step bleaching process and the novel two-step high/low bleaching process of the present invention
• Figure 13 is a graph of D.. pulp brightness versus the percentage of chlorine dioxide on the pulp (D. charge) for the conventional one-step bleaching process and the novel two-step high/low pH bleaching process of the present invention (wherein the middle line is the calculated brightness due to reduced chlorate formation) ;
• Figure 14 is a graph of chlorate formation versus D- brightness for the conventional one-step bleaching process and the novel two-step high/low pH bleaching process of the present invention.
• Figure 15 is a schematic representation of two (2) different process systems for a wood pulp bleaching plant for incorporating the two-step high/low pH bleaching process of the present invention. Best Mode for Carrying Out the Invention
• Chlorine dioxide bleaching of kraft pulps is typically carried out at an end pH of 3.8 for 3 hours at 70° centigrade. It is commonly known that pH has an important bearing on brightness and strength properties as well as the chemical species present in the mixture. As shown in Figure 1 of the drawings, the formation of chlorate increases as the pH of the solution is decreased. Below pH 5 a major loss of oxidizing power occurs since the chlorate formed is inactive as a bleaching agent. Conversely, as the pH is increased, the conversion of chlorine dioxide to the chlorite anion is increased which is also inactive toward lignin. The sum of chlorite plus chlorate is lowest at end pH 3.8 which is found to be optimal for chlorine dioxide bleaching. However, formation of chlorite is not actually lost oxidizing capability since acidifying the chlorite solution forms chlorous acid which is known to be very reactive toward lignin.
• Pulp is mixed with sodium hydroxide and subsequently mixed with chlorine dioxide in a conventional manner.
• the pH is maintained between about 6 and 7.5 for optimum brightness and viscosity although beneficial results are also found in a pH range of about 5-10.
• Reaction time is varied between about 5-40 minutes, and the reaction temperature is between about 55-85° centigrade, most suitably about 70° centigrade.
• the pulp mixture is acidified to an optimum end pH of 3.8 with sulfuric acid, hydrochloric acid, or other suitable acid. Although a pH of 3.8 is optimal for brightness, end pH values of 1.9-4.2 have been recorded with substantial brightness gains over conventional bleaching methods.
• Final consistency of the pulp is between about 3-12%, most suitably about 10%, and reaction time in this second step is 2 or more hours, most suitably between about 2.5 and 3.9 hours.
• Reaction temperature is between about 55- 85° centigrade, and most suitably about 70° centigrade.
• Pulp viscosity measurements were made using TAPPI standard T 230 os-76. Earlier experimental work has indicated that chlorine dioxide at a pH of less than 5 reacts selectively with lignin, and at a pH greater than 7 chlorine dioxide reacts with the carbohydrate and lignin in the pulp vigorously, which in turn degrades the cellulose chain. As shown in Figure 7, pulp viscosity depends heavily on the pH of the reacting mixture. Pulp viscosity decreases slowly from pH 6 to 7, then falls rapidly at pH values higher than 7. The decrease in viscosity at the high pH for the two-step high/low pH bleaching process is not significant because of the low reaction time in the high pH step. From viscosity and brightness data obtained, a pH of 6-7.5 and a pH of 3.8 is optimal for the high pH and low pH, respectively, in the two-step high/low pH bleaching process.
• TOCl (AOX) measurements in applicant's tests were made on both the D, and E 2 for one data set. The values were added together and are shown in Figure 8 of the drawings. Surprisingly, conventional bleaching TOCl values were parabolic versus an increasing C10 2 charge while TOCl values with the high/low pH bleaching method varied only slightly. A greater decrease in TOCl from bleaching with the two-step high/low pH bleaching process can be realized by substituting the chlorine dioxide saved in the D.. stage back into the chlorination stage (CD) of the multi-stage bleach sequence. This would result in a decrease in TOCl (AOX) in effluents from the bleach plant.
• Chlorate (CIO ' ) is a well known herbicide, and discharge of chlorate from paper mills has been gaining more attention from environmentalists now that possible detrimental effects on various microalgaes have been observed. Thus, improving the efficiency of chlorine dioxide bleaching by lowering chlorate production may have a favorable impact on both economic and environmental issues. Conversion of chlorine dioxide to chlorate can be lowered by the two-stage high/low pH bleaching method for most chemical charges on pulp. At very high chemical charges (or lower lignin concentrations) , chlorate formation is independent of whether the new or conventional bleaching method is used, because a brightness ceiling is reached.
• Equation 1 2C10 2 + 20H ⁇ > C10 3 ⁇ + C10 2 " + H 2 0
• Equation 2 2HCK> 2 > H + + HCIO + C10 3 ⁇
• Equation 1 is not a very prominent reaction in bleaching carried out at pH 7 since only a small concentration of hydroxyl ions are present. Under typical bleaching conditions, the pH starts around 5 and drops to less than 4 by the end of the bleaching process. At pH 5, less than 1% hydroxyl ions would be present for reaction, and at pH 4 only 0.1% exist. Supporting evidence for this observation is shown in Figure 9 of the drawings. The trend indicated shows that as the pH is increased up to 9, the formation of chlorate decreases. The major pathway for chlorate formation is Equation 2 above. In principle, chlorous acid reacts with itself to form chlorate and hypochlorous acid. This is a biomolecular reaction which is considered to be slow at low concentrations. Chlorous acid, as stated above, is very reactive toward lignin. Chlorous acid oxidizes lignin and is reduced to hypochlorous acid according to Equation 3:
• Equation 3 HC10 2 + LIGNIN > HCIO + OXIDIZED LIGNIN
• FIG. 10 shows a plot of D. charge of chlorine dioxide versus % chlorine dioxide converted to chlorate for conventional chlorine dioxide bleaching. As the lignin concentration is increased (low chemical charge or higher kappa number) less chlorate is formed. Likewise if a high concentration of chemical is present (low kappa number) , the higher the formation of chlorate. The same trend also holds true for the two-step high/low pH bleaching process as can be seen in Figure 11.
• Chlorate measurements were found to be 351 ppm and 423.3 ppm as available chlorine for the high/low pH bleaching process and normal bleaching, respectively, at a charge of 0.6% on pulp for a 17% reduction. Subtraction yields a savings of 72.3 ppm available chlorine, which corresponds to only 17% of the total savings realized of 423.3 ppm.
• Figure 13 of the drawings demonstrates this effect by replotting Figure 6 with the calculated savings due to chlorate reduction. It is apparent that a decrease of chlorate is not sufficient to explain the total C10 2 savings. A change in lignin structure and/or greater solubilization of the lignin may be possible explanations for the total savings in the C10 2 observed in the tests.
• Chlorate formation in the D 2 stage is identical for either bleaching process since they are carried out identically.
• the two-step high/low pH bleaching process can be implemented in both a new plant or an existing pulp bleaching plant.
• the optimum design schematic is shown in Figure 15, where C10 2 and caustic are added to the first mixer.
• the pulp flows into a J or U tube ( Figure 15A) or upflow tower ( Figure 15B) with a retention time of approximately 5-40 minutes.
• a second mixer is provided to mix the acid for pH adjustment of the wood pulp.
• the pulp can then be discharged directly to a downflow tower.
• the retention time in the downflow tower is 2 or more hours and most suitably between about 2.5- 3.9 hours.
• the high/low pH bleaching process reduces chlorine dioxide usage by as much as 24% in the D.. stage;
• chlorinated organic material characterized by TOCl can be decreased by the use of the high/low pH bleaching process if the C10_ saved is substituted into the CD stage;
• the high/low pH bleaching process can be easily implemented in either a new mill or an existing mill.
• chlorate at acidic bleaching conditions is due to the biomolecular reaction of chlorous acid with itself. Formation of chlorate can be reduced by lower bleach chemical charges or higher kappa number pulps.

Landscapes

• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Polysaccharides And Polysaccharide Derivatives (AREA)
• Noodles (AREA)

Applications Claiming Priority (3)

Publications (3)

Family

ID=23682296

Family Applications (1)

Country Status (12)

Cited By (1)

Families Citing this family (7)

Citations (3)

Family Cites Families (2)

• 1990
  • 1990-10-17 DE DE69026105T patent/DE69026105T2/de not_active Revoked
  • 1990-10-17 ES ES90915570T patent/ES2085358T3/es not_active Expired - Lifetime
  • 1990-10-17 CA CA002069322A patent/CA2069322A1/en not_active Abandoned
  • 1990-10-17 WO PCT/US1990/005825 patent/WO1991005909A1/en not_active Application Discontinuation
  • 1990-10-17 AU AU65376/90A patent/AU6537690A/en not_active Abandoned
  • 1990-10-17 EP EP90915570A patent/EP0496782B1/de not_active Revoked
  • 1990-10-18 NZ NZ235754A patent/NZ235754A/xx unknown
  • 1990-10-18 ZA ZA908341A patent/ZA908341B/xx unknown
  • 1990-10-19 PL PL90287415A patent/PL164745B1/pl unknown
  • 1990-10-19 MX MX022931A patent/MX172853B/es unknown
  • 1990-10-19 CN CN90109504A patent/CN1052157A/zh active Pending
  • 1990-10-19 AR AR90318139A patent/AR243945A1/es active

Patent Citations (3)

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events