Claims
- 1. A two-step bleaching process for bleaching wood pulp in the D.sub.1 or D.sub.2 bleaching step in an aqueous suspension using chlorine dioxide and providing high brightness and a high brightness ceiling, comprising the steps of:
- subjecting said aqueous wood pulp suspension to a first bleaching step during said D.sub.1 or D.sub.2 bleaching stage by mixing it with alkali and 10% to 50% of the total chlorine dioxide charge for about 5-40 minutes so that the pH at the end of said first bleaching step is between about 6.0-12.0; and
- adding the remaining portion of the total chlorine dioxide charge and subjecting said mixture to a second bleaching step during the D.sub.1 or D.sub.2 bleaching stage for about 2 or more hours so that the pH at the end of said second step is between about 1.9-4.2.
- 2. A bleaching process according to claim 1 wherein said alkali comprises sodium hydroxide.
- 3. A bleaching process according to claim 1 wherein the end pH of the mixture during said first bleaching step is between about 6.0-7.5.
- 4. A bleaching process according to claim 1 wherein the temperature during said first bleaching step is between about 50.degree.-85.degree. centigrade.
- 5. A bleaching process according to claim 4 wherein the temperature during said first bleaching step is about 70.degree. centigrade.
- 6. A bleaching process according to claim 1 wherein the end pH of the mixture during said second bleaching step is about 3.8.
- 7. A bleaching process according to claim 1 wherein the temperature during said second bleaching step is between about 55.degree.-85.degree. centigrade.
- 8. A bleaching process according to claim 7 wherein the temperature during said second bleaching step is about 70.degree. centigrade.
- 9. A bleaching process according to claim 1 wherein the final consistency of the mixture after the second bleaching step is between about 3-12%.
- 10. A bleaching process according to claim 9 wherein the final consistency of the mixture after the second bleaching step is about 10%.
- 11. A two-step bleaching process for bleaching wood pulp in the D.sub.1 or D.sub.2 bleaching stage in an aqueous suspension using chlorine dioxide providing high brightness and a high brightness ceiling, comprising the steps of:
- subjecting said aqueous wood pulp suspension to a first bleaching step during said D.sub.1 or D.sub.2 bleaching stage by mixing it with alkali and 10% to 50% of the total chlorine dioxide charge for about 5-40 minutes at a temperature of about 70.degree. C. so that the pH at the end of said first bleaching step is between about 6.0-7.5; and
- adding the remaining portion of the total chlorine dioxide charge and subjecting said mixture to a second bleaching step during said D.sub.1 or D.sub.2 bleaching stage at a temperature of about 70.degree. C. for about 2.5-2.9 hours so that the pH at the end of the second step is about 3.8.
- 12. A bleaching process according to claim 11 wherein said alkali comprises sodium hydroxide.
- 13. A bleaching process according to claim 11 wherein the final consistency of the mixture after the second bleaching step is about 10%.
RELATED APPLICATION
This application is a continuation-in-part of application Ser. No. 424,347, filed Oct. 19, 1989, and now abandoned.
US Referenced Citations (2)
Number |
Name |
Date |
Kind |
3884752 |
Campbell et al. |
May 1975 |
|
4274912 |
Carles et al. |
Jun 1981 |
|
Foreign Referenced Citations (1)
Number |
Date |
Country |
1149111 |
Jul 1983 |
CAX |
Non-Patent Literature Citations (2)
Entry |
Macas et al, "The Effect of Chlorine in the Di Stage", J of Pulp & Paper Science, vol. 13, No. 3 May 1987, pp. J106-J110. |
Publication by Teder et al., "Carbohydrate Degradation in Chlorine Dioxide Bleaching", in Tappi, Dec. 1978, vol. 61, No. 12, pp. 59-62. |
Continuation in Parts (1)
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Number |
Date |
Country |
Parent |
424347 |
Oct 1989 |
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