Method of bleaching pulp with chlorine-free chemicals wherein a complexing agent is added immediately after an ozone bleach stage

• US 6,010,594 A
• Filed: 09/15/1997
• Issued: 01/04/2000
• Est. Priority Date: 03/03/1993
• Status: Expired due to Fees

• Current Assignee: Andritz Oy (Andritz AG)
• Original Assignee: Ahlstrom Corporation
• Inventors: Stromberg, Bertil, Henricson, Kaj
• Primary Examiner(s): ALVO, MARC S
• Application Number:
  US08/929,082
• Time in Patent Office: 841 Days
• Field of Search: 162/37, 162/40, 162/55, 162/60, 162/65, 162/72, 162/78, 162/82, 162/76, 162/90, 162/24
• US Class Current: 162/55
• CPC Class Codes: D21C 9/1042 Use of chelating agents
• CPC Class Codes: D21C 9/1057 Multistage, with compounds cited in more than one sub-group D21C9/10, D21C9/12, D21C9/16
• CPC Class Codes: D21C 9/153 with ozone
• CPC Class Codes: D21C 9/163 with peroxides

Abstract

Kraft pulp is bleached to an acceptable brightness (e.g. 86 ISO or more) without using chlorinated organic compounds, yet the strength of the produced pulp remains commercially acceptable. In the manufacture of pulp, the pulp which is cooked and oxygen delignified to a kappa number of 14 or less, and then is bleached with ozone at a medium consistency. After the ozone treatment, heavy metals are removed from the pulp, and then the pulp is led to oxygen and/or peroxide treatment, which is followed by a second ozone bleaching stage. The sequence (ZT) (EOP) (ZP) is preferred.

Claims

1. A method of processing cellulose pulp having a consistency between 5-18% containing undesirable heavy metals without the use of chlorine bleaching chemicals, comprising the steps of:

(a) cooking and delignifying comminuted cellulosic fibrous material to produce cellulose pulp having a consistency between 5-18% and a kappa no. of about 14 or less and containing undesirable heavy metals;

(b) bleaching the pulp having a consistency between 5-18% from step (a) using a (ZT) stage having a Z-portion and a T-portion, said (ZT) stage practiced by (i) if the pH of the pulp is not within the range of 2-7, adjusting the pH so that it is within the range of 2-7;

(ii) bleaching the pulp with ozone in the Z-portion, and then removing gas from the pulp, and then adding alkali and complexing agent; and

(iii) in the T-portion substantially immediately after the degassing, alkali addition, and complexing agent addition following ozone bleaching in substep (ii), without intervening washing, removing undesirable heavy metals from the pulp by using the complexing agent in the T-portion;

(c) after step (b) washing or thickening the pulp; and

(d) after step (c), bleaching the pulp in at least one additional stage with peroxide alone, or peroxide in combination with ozone or oxygen, to produce a bleached pulp.

2. A method as recited in claim 1 wherein step (d) comprises practicing an alkaline stage in which peroxide, or peroxide and oxygen, are added.

3. A method as recited in claim 2 wherein step (d) further comprises washing the pulp after the alkaline stage and then practicing a (ZP) stage which comprises a Z-portion followed by a P-portion with no intermediate washing.

4. A method as recited in claim 1 wherein during the practice of substep (ii) about 2-10 kg of ozone per air dried ton of pulp is utilized.

5. A method as recited in claim 1 wherein during the practice of step (b) the pH of the pulp is always between about 2-7, and the temperature is between about 40-70°

C.

6. A method as recited in claim 1 wherein during the practice of substep (iii) the pH is about 4-7.

7. A method as recited in claim 2 wherein during the practice of the alkaline stage about 10-20 kg of peroxide is added per air dried ton of pulp.

8. A method as recited in claim 1 comprising the further step between steps (b) and (c) of treating the pulp with a filtrate having a pH which is acidic enough to prevent reattaching of heavy metals.

9. A method as recited in claim 8 wherein said further step is practiced in part by adding acid to filtrate.

10. A method as recited in claim 3 wherein step (d) is practiced to produce a pulp having a brightness of at least about 86 ISO.

11. A method as recited in claim 1 wherein step (d) is practiced to produce a pulp having a brightness of at least about 86 ISO.

12. A method as recited in claim 1 wherein step (a) is practiced by kraft cooking and then oxygen delignification.

13. A method as recited in claim 1 comprising the further step between steps (a) and (b) of treating the pulp with enzymes.

14. A method as recited in claim 1 wherein delignification in step (a) is practiced by using hot-alkali extraction or enzymes.

15. A method as recited in claim 1 wherein step (a) includes washing after delignification, and step (b) substantially immediately follows step (a) without an intervening metal removal stage.

16. A method as recited in claim 1 wherein delignification in step (a) is practiced by using oxygen.

17. A method as recited in claim 16 wherein step (a) includes washing after delignification, and step (b) substantially immediately follows step (a) without an intervening metal removal stage.

18. A method as recited in claim 1 wherein substep (ii) is performed with the pulp at a consistency of between 5-18%.

19. A method as recited in claim 1 wherein step (c) is practiced to produce two filtrates with different pH values from the same washer, and comprising the further step of utilizing the two different filtrates distinctly from each other during the practice of steps (a)-(d).

20. A method as recited in claim 3 wherein during at least one washing stage during the practice of step (c), or step (d), or both (c) and (d), two filtrates with different pH values are produced from the same washing, and comprising the further step of utilizing the two different filtrates distinctly from each other during the practice of steps (a)-(d).

21. A method as recited in claim 1 wherein during at least one washing stage during the practice of step (c), or step (d), or both steps (c) and (d), two filtrates with different pH values are produced from the same washing, and comprising the further step of utilizing the two different filtrates distinctly from each other during the practice of steps (a)-(d).

22. A method as recited in claim 3 wherein said (ZP) stage is practiced using less than 3 kg/adt of ozone and less than 10 kg/adt of peroxide.

23. A method as recited in claim 2 wherein step (a) is practiced by kraft cooking and then oxygen delignification.

24. A method as recited in claim 23 wherein during the practice of the alkaline stage about 10-20 kg/adt of peroxide is added and about 2-6 kg/adt of oxygen.

25. A method as recited in claim 23 wherein during the practice of substep (ii) about 2-10 kg of ozone per air dried ton of pulp is utilized.

26. A method as recited in claim 23 wherein during the practice of step (b) the pH of the pulp is always between about 2-7, and the temperature is between about40-70°

C.

27. A method as recited in claim 3 comprising the further steps of immediately screening the pulp after the (ZP) stage, and then washing the pulp after screening.

28. A method as recited in claim 27 wherein said screening step is accomplished by diluting the pulp to a consistency of about 0.5-1.5%, and then treating the pulp in a vortex chamber.

29. A method as recited in claim 27 comprising the further step of diluting the pulp to a consistency of about 0.5-4% prior to screening, and wherein said screening step is practiced at said consistency of about 0.5-4%.

30. A method as recited in claim 2 wherein during the practice of the alkaline stage about 10-20 kg/adt of peroxide is added and about 2-6 kg/adt of oxygen.

31. A method as recited in claim 2 wherein the alkaline stage is practiced by maintaining the pH of the pulp within the range of about 9-12, and maintaining the temperature within the range of about 60-95°

C., and wherein the alkaline stage is practiced about 2-8 hours.

32. A method as recited in claim 2 comprising the further step of adding magnesium to the alkaline stage to providing protection for the pulp.

33. A method as recited in claim 3 wherein said (ZP) stage is practiced by adding about 3≧

7 kg/adt of peroxide.

34. A method as recited in claim 3 wherein said (ZP) stage is practiced, while ozone is in contact with the pulp, at a temperature of about 50-80°

C. and with the pH of the pulp about 4-10.

35. A method as recited in claim 3 wherein during the practice of said (ZP) stage while ozone is in contact with pulp the temperature is maintained between about 60-70°

C.

36. A method as recited in claim 3 wherein during the practice of said (ZP) stage while ozone is in contact with pulp the pH of the pulp is maintained within the range of about 6-10.

37. A method as recited in claim 1 wherein enzymes are added to the pulp prior to step (b).