CHLORINE DIOXIDE PULP BLEACHING PROCESS USING SEQUENTIAL CHLORINE ADDITION

Abstract: A high-efficiency wood pulp bleaching process to produce wood pulps with high brightness and with high levels of chlorine substitution. The process comprises reacting chlorine and chlorine dioxide with wood pulp at a pH of about 7-10 for about 5-40 minutes. Next, the remaining chlorine is added and the mixture brought to a pH of 4 or less, where it is allowed to react for 30 or more minutes to complete the two-step high/low pH sequential chlorine addition bleaching process.

Description: Description CHLORINE DIOXIDE PULP BLEACHING PROCESS USING SEQUENTIAL CHLORINE ADDITION Technical Field The present invention relates to the bleaching of pulp and more particularly to an improved process for bleaching wood pulp with chlorine dioxide and chlorine in a manner whereby the wood pulp is subjected to a two-step high pH/low pH bleaching stage wherein chlorine dioxide and chlorine are used in the first step and chlorine is used in the second step. Background Art 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, hypochloriteε, oxygen, hydrogen peroxide, and chlorine dioxide. The number of stages required in a particular bleaching process is dependent upon the nature of the unbleached pulp as well as the end use to which the pulp will be put. A εulfate or kraft pulp is today most typically bleached in a five-stage sequence which is designated as (CD) (EO)DED. In the (CD) (EO)DED designation, D denotes chlorine dioxide, C denotes elemental chlorine, E denotes caustic extraction, and 0 denotes oxygen gas. 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 , and a second and final bleaching stage (D~) . In the conventional (CD) (EO)DED multi-stage bleaching process, each of the two chlorine dioxide bleaching stages is carried out in a one-step process at a final 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) (E0)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. Chlorine dioxide used at the mill site normally contains only about 10% chlorine in the one-step chlorine dioxide bleaching stage presently used in the pulp and paper industry. Existing technology does not allow for more than about 20% chlorine substitution without detrimentally affecting wood pulp quality. This is a significant concern in view of the high cost of chlorine dioxide in contrast to the relatively less expensive chlorine. The present invention solves this well-known deficiency in state of the art chlorine dioxide bleaching by making it possible to perform higher levels of chlorine substitution through a two-step sequential addition of chlorine at controlled high/low pH levels during the chlorine dioxide bleaching process. The advantages are a reduced use of chlorine dioxide and a significant reduction in chemical costs for the wood pulp bleaching process. Disclosure of the Invention In accordance with the present invention, applicants provide an improved process for bleaching wood pulp in aqueous suspension using chlorine dioxide and chlorine in a two-step high pH/low pH bleaching stage wherein chlorine dioxide and chlorine are used in the first step and chlorine is used in the second step. The chlorine dioxide and chlorine comprise between about 15-80% chlorine on an available basis. The novel process comprises first subjecting the aqueous wood pulp suspension to a first bleaching step by mixing it with chlorine dioxide and chlorine and maintaining the mixture at a pH between about 7-10 for about 5-40 minutes. Next, the mixture is subjected to a second bleaching step by mixing it with the remaining chlorine and maintaining the mixture at a pH of 4 or less for at least 30 minutes. This novel process can be used in any D 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. All of the chlorine dioxide and approximately 0-40% of the chlorine charged are added initially in the first step of the bleaching process, and the remainder of the chlorine is added at the second stage of the bleaching process. The novel bleaching process provides for increasing chlorine substitution up to 60% with nominal effect on wood pulp quality. It is therefore an object of the present invention to provide a more efficient v/ood pulp bleaching process. It is another object of the present invention to increase chlorine substitution in the wood pulp bleaching process. It is still another object of the present invention to reduce the chlorine dioxide requirements of the wood pulp bleaching process and to thereby reduce chemical costs. It is yet another object of the present invention to allow for chlorine substitution in the D bleaching stage at a level greater than the conventionally accepted 20% limit without a detrimental effect on wood pulp quality. Description of the Drawings Some of the objects of the present invention having been stated, other objects will become evident as the description proceeds, when taken in connection with the accompanying drawings, in which: Figure 1 is a graph of D, brightness versus percent available chlorine on pulp as chlorine dioxide for one- step D-stage bleaching, conventional one-step chlorine- chlorine dioxide mixture bleaching at maintained pH levels of 4 and 2 (as taught by the Rapson and Reeve patents referenced herein) , and the novel two-step chlo ine-chlo ine dioxide method of the present invention (chlorine-chlorine dioxide mixtures contain 60% chlorine on an available chlorine basis, (CD)E kappa equals 5.1, and temperature is 70 centigrade) ; Figure 2 is a graph of D 2 brightness versus D, charge for the subsequent bleaching of the pulps of Figure 1 through E 2 and D 2 stages (D 2 charge of 0.3% chlorine dioxide on pulp and temperature of 70° centigrade) ; Figure 3 is a graph of the final D 2 viscosity measurements for the pulps of Figures 1 and 2 versus D. charge illustrating higher brightness and viscosity for the method of the invention when compared to conventional bleaching methods used in the D, stage; Figure 4 is a graph of D, brightness versus D, charge (percent available chlorine on pulp as chlorine dioxide) for conventional one-step D stage bleaching and the method of the invention ((CD),., is 60% chlorine, 40% chlorine dioxide on an available basis and (CD)E kappa equals 5.2. Reaction temperatures are 70° centigrade and 50° centigrade for (CD)., and 70° centigrade for conventional D stage bleaching) ; Figure 5 is a graph of D 2 brightness versus D. charge (percent available chlorine on pulp as chlorine dioxide) for the pulps of Figure 4 after E 2 and D~ stage bleaching (D 2 charge is 0.2% C10 2 on pulp, and temperature is 70° centigrade) ; Figure 6 is a graph of D, brightness versus D, charge (percent available chlorine on pulp as chlorine dioxide) for conventional one-step D stage C10 2 bleaching and the method of the invention ((CD)E kappa equals 4.6, ( CD --hl s 60 * chlorine, 40% chlorine dioxide on an available basis, and temperature is 70° centigrade); Figure 7 is a graph of D, and D 2 brightness versus percent available chlorine on pulp as chlorine dioxide for conventional one-step D stage C10 2 bleaching and the novel two-step (CD). , bleaching process of the present invention wherein (CD) comprises 60% Cl 2 and 40% C10_ on an available basis, temperature is 70° centigrade, and (CD)E kappa equals 4.9; Figure 8 is a graph of D, brightness versus D, charge for conventional one-step D stage bleaching and the novel two-step (CD). , bleaching process of the present invention at 70 centigrade for 3 hours with (CD)E kappa equals 5.0 and wherein (CD) is 60% Cl 5 and 40% CIO- on an available basis; Figure 9 is a graph of D 2 brightness versus D, charge for the pulps of Figure 7 wherein the D_ charge is 0.3% C10 2 on the pulp and temperature is 70° centigrade; and Figure 10 is a schematic representation of two different process systems for a wood pulp bleaching plant which incorporate the chlorine dioxide with the sequential chlorine addition wood pulp bleaching process of the present invention. Best Mode for Carrying Out the Invention It has previously been suggested in U.S. Patent No. 3,536,577 to Rapson to bleach wood pulp using an aqueous solution of chlorine dioxide and chlorine, treating the wood pulp with alkali, and then treating the wood pulp with chlorine dioxide. The process of Rapson provides for chlorine dioxide to constitute about 20-95% of the total available chlorine of the solution in the first (CD) stage. Also of interest, Canadian Patent No. 1,120,660 to Reeve discloses a bleaching sequence which comprises subjecting the wood pulp to a first bleaching step with a chlorine dioxide and chlorine solution and, after a period of time from about 5 seconds to about 10 minutes, applying an aqueous chlorine solution to the pulp suspension in a second bleaching step which is allowed to proceed for about 10-60 minutes. Reeve discloses that the first bleaching step is used usually effected at a pulp suspension pH of about: 1-6, and the second bleaching step is usually effected at a pulp suspension pH of about 0.7-3. Applicants' process provides for the use of chlorine-chlorine dioxide bleaching in stages other than the first (CD) stage, preferably the D stage designation in any 3-7 stage bleaching sequence, whereas Rapson and Reeve both refer specifically to the first (CD) stage of bleaching. With reference now to Figures 1-10 of the drawings, it can be seen that applicants have improved upon the processes taught by Rapson and Reeve as well as upon the conventional one-step bleaching stage used in the majority of commercial bleaching mills. More specifically, applicants have invented a new and useful two-stage process for bleaching wood pulp slurries in a chlorine-chlorine dioxide (Cl 2 -C10 2 ) mixture which gives higher brightness and pulp quality (characterized by viscosity) when compared to the conventional one-step Cl 2 -C10 2 mixture technology typically used today in conventional bleaching mills. Presently, wood pulp bleaching with chlorine- chlorine dioxide mixtures is typically carried out at 25° centigrade - 60 centigrade in the first stage of a (CD) (EO)DED or (CD) (EO)D sequence. The consistency is usually around 3-5%, and bleaching times range from 15 minutes to 1 hour with an end pH of about 1.6-2.5. The application of conventional (CD) bleaching procedures in the D-stage of the above sequences or in any other bleaching sequences has little commercial value due to the high viscosity loss (carbohydrate degradation) and poor brightness characteristics of the wood pulp. Thus, existing (CD) technology is typically only used as a first stage or pre-bleaching stage in the multi-stage bleaching process on wood pulp. Although others have proposed chlorine-chlorine dioxide bleaching in either one or two-step stages, such as disclosed in the Rapson and Reeve patents discussed above, it can be seen in Figures 1, 2 and 3 of the drawings that the novel two- step chlorine-chlorine dioxide bleaching process of the present invention provides surprisingly and unexpectedly better brightness and viscosity than has heretofore been achieved. In order to increase the effectiveness of chlorine- chlorine dioxide bleaching of wood pulp, applicants have discovered a new two-step chlorine-chlorine dioxide process which includes the following steps: 1. Pulp slurry is reacted with a chlorine and chlorine dioxide mixture for about 5-40 minutes at a pH of about 7-10. Normally, caustic is also mixed with the pulp slurry, and the caustic most suitably comprises sodium hydroxide. The reaction temperature is between about 40-80° centigrade, most suitably about 70° centigrade, and the optimal pH during the first bleaching step is between about 8-9.5; and 2. After the first bleaching step, the remaining chlorine is added to the pulp mixture, and the pH is maintained at 4 or less, most suitably a pH of about 2- 3.5 for at least 30 minutes, preferably about 30 minutes - 2.5 hours. The reaction temperature is between about 40-80 centigrade, most suitably 70° centigrade. Pulp consistency foe both steps is between about 10- 13.5% with the final consistency of the mixture after the second step at most suitably about 10%. The chlorine dioxide and chlorine used in the bleaching process comprise between about 15-80% chlorine on an available basis, and preferably all of the chlorine dioxide and one-third of the chlorine are added at the first step, and the remaining two-thirds of the chlorine are added at the second bleaching step. It should be appreciated that to calculate total charge such as percent chlorine on pulp, the formula C10 2 •= * 2.63 Cl 2 is used. For example, if a 1% C10 2 charge is placed on wood pulp, it is equivalent to 2.63% available chlorine on the wood pulp. Thus, if one charges 1% available C10 2 on wood pulp, there is actually charged 2.63% available chlorine of which 40% is CK> 2 and 60% is Cl 2 . The new two-step process developed by applicants has been used to bleach pulp in the D, stage of a (CD) (EO)DED sequence with chlorine-chlorine dioxide mixtures of up to 60% of the chemical charged as chlorine on an available basis with brightness and viscosity characteristics comparable to a conventional chlorine-chlorine dioxide stage (see Figures 1, 2 and 4-9). Applicants' process is particularly attractive since the new two-step chlorine-chlorine dioxide bleaching process allows for substituting chlorine for chlorine dioxide and thus results in substantial chemical cost reduction since chlorine is a significantly less expensive chemical. With the use of the novel two-step bleaching process of the present invention, it is now commercially possible to bleach wood pulp to a comparable brightness as can be achieved in conventional one-step chlorine dioxide bleaching at a significantly reduced commercial cost due to savings achieved by substituting cheaper chlorine for more expensive chlorine dioxide in the wood pulp bleaching process. Method of the Invention To prove the efficacy of the new process generally described above, detailed bleaching experiments were carried out by applicants on southern pine kraft pulp. The furnish was obtained from the decker before the bleach plant, and to insure maximum mixing (CD) stage bleaching was done in plastic Nalgene bottles which rolled on a ball-mill type apparatus for the full reaction time. All other bleaching stages were carried out in sealed polyester bags which were kneaded at various times throughout the bleach to insure proper mixing. Processing parameters used by applicants for the multiple bleaching stages are listed in Table 1 below. Chlorination stage charges were varied to achieve target (CD)E kappa numbers, and ail charges are on OD brownstock pulp. Optimum high/low pH values are 8-9.5 and 2-3.5 respectively. Large batches of (CD)E pulp were made and then divided into individual DED and (CD), ,ED runs for comparison. All comparisons were made on pulps from the same (CD)E batch, and all water used in bleaching and washing was distilled. Chlorine dioxide solutions used in testing were generated on site by acidifying sodium chlorite solution and absorbing the C10 2 gas in cold distilled water. Processing parameters for the bleaching experiments and the analytical methods used in the experiments are as follows: TABLE 1 Stage Charge Time Temperature Consistency End pH CD .17-.22 x Kappa 1 hour 30-40" C 3% <1.8 % Available Chlorine on Pulp (10% CL0 2 Substitution) I l- J Thus, applicants* method provides for mixing chlorine, chlorine dioxide and caustic with an aqueous wood pulp slurry for about 5-15 minutes at a pH of about 7-10. The optimum pH is between about 8-9.5 for best viscosity, and consistency is between about 10-13.5%. Temperature is between about 40-80 centigrade. For the second step in applicants' novel process, chlorine is then added to the pulp slurry which results in a decrease in pH to about 2-3.5. Reaction time is between about 30 minutes to 2.5 hours, reaction temperature is between about 40-80° centigrade, and the final consistency of th-a wood pulp is about 10%. Best brightness and viscosity have been found when all of the chlorine dioxide and one-third of the chlorine to be charged are added in the first step of the novel bleaching process. The other two-thirds of the chlorine are added at the second step and allowed to react for 1-2 hours at 70° centigrade or 1.5 or more hours at 50° centigrade. Temperature and time dependents for efficient chemical consumption and optimum pulp brightness are important, and suitable combinations can be developed according to the needs of any given wood pulp bleaching mill. Although applicants have disclosed the application of the two-step chlorine-chlorine dioxide bleaching stage to the D 1 stage of a (CD) (EO)DED sequence, it should be appreciated that the process can be used in any D or (CD) stage in any three, four, five, six or seven-stage bleaching sequence. Also, although applicants disclose the addition of the remaining chlorine to the pulp in the second step of the new process, additional chlorine dioxide can also be added at this time to the pulp mixture. Process Apparatus 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 10, where CIO- and caustic are added to the first mixer. The pulp flows into a J or ϋ tube (Figure 10A) or upflow tower (Figure 10B) with a retention time of approximately 5-40 minutes. A second mixer is provided to mix chlorine with the pulp for the second step of the process. 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 2.5-3.9 hours. In an existing bleach plant the simplest method for implementing the two-step high/low pH bleaching process technology would be to install a mixer on the discharge from the upflow leg of the tower to the downflow leg of the tower. It will be understood that various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims.

Claims: Claims What is claimed is: 1. A bleaching process for bleaching wood pulp in an aqueous suspension using chlorine dioxide and chlorine wherein said chlorine dioxide and chlorine comprise between 15-80% chlorine on an available basis, comprising the steps of: subjecting said aqueous wood pulp suspension to a first bleaching step by mixing it with chlorine dioxide and chlorine and maintaining the mixture at a pH between about 7-10 for about 5- 40 minutes; and subjecting said mixture to a second bleaching step by mixing it with chlorine and maintaining the mixture at a pH of 4 or less for at least 30 minutes. 2. A bleaching process according to claim 1 wherein caustic is also mixed with said aqueous wood pulp suspension during said first bleaching step. 3. A bleaching process according to claim 2 wherein said caustic comprises sodium hydroxide. 4. A bleaching process according to claim 1 wherein the pH of the mixture during said first bleaching step is between about 8-9.5. 5. A bleaching process according to claim 1 wherein said first step reaction time is about 5-15 minutes. 6. A bleaching process according to claim 1 wherein the temperature during said first bleaching step is between about 40-80 centigrade. 7. A bleaching process according to claim 6 wherein the temperature during said first bleaching step is about 70° centigrade. 8. A bleaching process according to claim 1 wherein the pH of the mixture during said second bleaching step is about 2-3.5. 9. A bleaching process according to claim 1 wherein said second step reaction time is about 30-150 minutes. 10. A bleaching process according to claim 1 wherein the temperature during said second bleaching step is between about 40-80 centigrade. 11. A bleaching process according to claim 10 wherein the temperature during said second bleaching step is about 70° centigrade. 12. A bleaching process according to claim 1 wherein consistency of the mixture during the first bleaching step and the second bleaching step is between about 10- 13.5. 13. A bleaching process according to claim 12 wherein the final consistency of the mixture after the second bleaching step is about 10%. 14. A bleaching process according to claim 1 wherein said bleaching process is used in any D stage in a three- stage sequence process. 15. A bleaching process according to claim 1 wherein said bleaching process is used in any D stage in any three, four, five, six, or seven-stage sequence process. 16. A bleaching process according to claim 15 wherein the five-stage sequence process is a (CD) (EO)DED process and the bleaching process is used in the D. stage and followed by conventional extraction and bleaching stages. 17. A bleaching process for bleaching wood pulp in an aqueous suspension using chlorine dioxide and chlorine wherein said chlorine dioxide and chlorine comprise between 15-80% chlorine on an available basis, comprising the steps of: subjecting said aqueous wood pulp suspension to a first bleaching step by mixing it with caustic, chlorine dioxide and chlorine, maintaining the mixture at a pH between about 8-9.5 and a temperature of about 40-80° centigrade for about 5-15 minutes; and subjecting said mixture to a second bleaching step by mixing it with chlorine and maintaining the mixture at a pH between 2-3.5 at a temperature of about 40-80° centigrade for 2.0-3.5 hours. 18. A bleaching process according to claim 17 wherein said caustic comprises sodium hydroxide. 19. A bleaching process according to claim 17 wherein consistency during the first bleaching step and the second bleaching step is between about 10-13.5. 20. A bleaching process according to claim 17 wherein the final consistency of the mixture after the second bleaching step is about 10%. 21. A bleaching process according to claim 17 wherein said bleaching process is used in any D stage in a three-stage sequence process. 22. A bleaching process according to claim 17 wherein said bleaching process is used in any D stage in any three, four, five, six or seven-stage sequence process. 23. A bleaching process according to claim 22 wherein the five-stage sequence process is a (CD)(E0)DED process and the bleaching process is used in the D, stage followed by conventional extraction and bleaching stages.

Inventors: CHANG HOU-MIN, JAMEEL HASAN, SEGER GEOFFREY EUGENE, CHANG, HOU-MIN, JAMEEL, HASAN, SEGER, GEOFFREY, EUGENE
Applicants: UNIV NORTH CAROLINA, NORTH CAROLINA STATE UNIVERSITY
Application year: 1990
Main classification: D21C9
IPC codes: D21C9